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Research article

Case study of financial leasing model driven by fuzzy logic control for alternative fuel vehicles operation


  • Received: 10 August 2022 Revised: 26 September 2022 Accepted: 27 September 2022 Published: 18 October 2022
  • Over the past decade, the alternative fuel vehicle industry in the world has sprung up with huge speed. For example, the annual output has increased from less than 2000 vehicles to now 3,500,000 vehicles in China. It enjoys more than 50% of the market share worldwide in the global market. A spurt of progress in the alternative fuel vehicle industry has built a foundation for carbon peaking and carbon neutrality goals. Financial leasing has unique advantages which not only can provide guarantees for this industry in many aspects concerning related equipment, systems and infrastructures but also offer financial support for green projects. Nevertheless, financial leasing firms are encountering a string of problems to solve, such as selecting optimal green projects and cooperative businesses, designing transaction structures, and controlling project risks. This study contains several main sections: connecting the incremental alternative fuel vehicle investment and purchase project of a leading regional enterprise; building the structure of the financial leasing project; and analyzing the project leasing property using a fuzzy logic model, the financial structure and the repayment capacity of the project main company so as to comprehensively evaluate the feasibility of the project. This paper aims to provide a reference for future financing of alternative fuel vehicle operation enterprises with a real case study. The case study results show that our introduced fuzzy logic method can obtain the satisfying performance and traffic allocation.

    Citation: Junlin Zhu, Hua Wang, Lin Miao, Zitong Yu. Case study of financial leasing model driven by fuzzy logic control for alternative fuel vehicles operation[J]. Mathematical Biosciences and Engineering, 2023, 20(1): 894-912. doi: 10.3934/mbe.2023041

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  • Over the past decade, the alternative fuel vehicle industry in the world has sprung up with huge speed. For example, the annual output has increased from less than 2000 vehicles to now 3,500,000 vehicles in China. It enjoys more than 50% of the market share worldwide in the global market. A spurt of progress in the alternative fuel vehicle industry has built a foundation for carbon peaking and carbon neutrality goals. Financial leasing has unique advantages which not only can provide guarantees for this industry in many aspects concerning related equipment, systems and infrastructures but also offer financial support for green projects. Nevertheless, financial leasing firms are encountering a string of problems to solve, such as selecting optimal green projects and cooperative businesses, designing transaction structures, and controlling project risks. This study contains several main sections: connecting the incremental alternative fuel vehicle investment and purchase project of a leading regional enterprise; building the structure of the financial leasing project; and analyzing the project leasing property using a fuzzy logic model, the financial structure and the repayment capacity of the project main company so as to comprehensively evaluate the feasibility of the project. This paper aims to provide a reference for future financing of alternative fuel vehicle operation enterprises with a real case study. The case study results show that our introduced fuzzy logic method can obtain the satisfying performance and traffic allocation.



    Poroviscoelastic models of multi-component mixtures are often utilized in biological applications to describe the flow of fluids within the pores of a deformable solid skeleton, see e.g., [1,2,3,4]. Skeleton viscoelasticity is often due to the complex structures including extracellular matrix, collagen and elastin that are present in biological tissues. Specifically, our work focuses on the bio-fluid-mechanical response of poroviscoelastic media to non-smooth data, since this aspect is crucial in understanding the mechanisms leading to tissue damage in the optic nerve head, and consequent vision loss, associated with glaucoma [5,6,7,8].

    In the absence of viscoelasticity, we have recently shown that time irregularities in the volumetric and/or boundary sources of linear momentum lead to a blow-up in the solution of poroelastic models [1,4]. Interestingly, the blow-up can be prevented by including structural viscoelasticity [4]. From the application viewpoint, examples of time-irregularities in the data are discontinuities in intraocular pressure, which acts as a boundary traction for the optic nerve head tissue, or discontinuities in the gravitational force, which acts as a volumetric source of linear momentum. The intraocular pressure exhibits rapid changes every time we rub our eyes or we change posture [9], whereas rapid changes in the gravitational acceleration are experienced by jet pilots and astronauts during flights [10,11]. Since tissue viscoelasticity has been shown to decrease with age and/or disease conditions, the solution blow-up identified by our theoretical work led to hypothesize that rapid changes in intraocular pressure and gravitational acceleration, even if within physiological ranges, could damage the optic nerve head tissue if its viscoelasticity was pathologically reduced.

    It is important to emphasize that our previous work was built on the assumption that the poroviscoelastic medium under consideration was made of incompressible components. The incompressibility assumption is quite common in biological applications, since tissues are mostly made of water. However, compressibility is always present in real tissues, and this leads to wonder whether and to what extent compressibility of the mixture components would affect the tissue response to non-smooth data. The present work aims at addressing this question.

    Proceeding as in [4], we assume: (i) a one-dimensional (1D) geometry; (ii) negligible inertial terms in the linear momentum balance equation; (iii) a Kelvin-Voigt model for the effective stress tensor and (iv) a constant hydraulic permeability of the porous medium. Then, we express the fluid pressure and the solid stress as functions of the sole solid phase displacement, and we obtain an initial-boundary value problem (IBVP) of pseudo-parabolic type for the solid phase displacement. For this equivalent formulation we are able to construct an analytical solution and prove the well-posedness of weak solutions. Moreover, we recover analytical formulas for fluid pressure and discharge velocity, and discuss their regularity in terms of the regularity of the data. Finally, we analyze the behavior of all the solutions for various continuous and discontinuous boundary loads, which are of particular interest in understanding how changes in intraocular pressure would impact the bio-fluid-mechanics of ocular tissues.

    The main conclusion of our analysis is that the compressibility of the components of a poroviscoelastic mixture does not give rise to unbounded responses to non-smooth traction data. Interestingly, compressibility provides an additional mechanism that limits the maximum of the discharge velocity when the imposed boundary traction is irregular in time. This mechanism originates from the capability of the system to store potential energy as its components are elastically compressed, thereby delaying the transmission of irregularities in the linear momentum from the solid to the fluid. As a result, the fluid has the time to accommodate for sudden changes, resulting in bounded velocities.

    Our results fit well with other poroviscoelastic studies motivated by geomechanical applications, where viscoelasticity was found to play a crucial role in the response of the medium to impulsive loads. Specifically, the studies focused on evaluating the consequences that different choices for the viscoelastic models would have on the medium response to external loads. For example, in [12], Schanz and Cheng considered a Kelvin-Voigt viscoelastic model and investigated the consequences of adopting it for the bulk compression modulus, the shear modulus and the compression modulus of the solid material. In [2,3], Huang et al. utilized the quasi-linear viscoelastic theory to study the response of articular cartilage under compression and tension experiments. However, these studies did not consider how the mechanical responses of the poroviscoelastic mixture would change depending on the level of compressibility of the mixture components, which is the main focus of the present work.

    The outline of this article is as follows. In Section 2 we describe the compressible poro-visco-elastic model under consideration and discuss the finite compressibility of the components of the mixture. Section 3 introduces the energy identity associated with the system. In Section 4 we present an equivalent form of the fluid-solid mixture system, written solely in terms of the solid phase displacement. To simplify the theoretical analysis, in Section 5 we reduce the poroviscoelastic model into dimensionless form. Section 6 studies the well-posedness and regularity of solution for the IBVP introduced in Section 4, and provides analytical formulas for the elastic displacement, fluid pressure and discharge velocity. In Section 7 we compute and analyze the behavior of the solid displacements, the fluid pressures and the discharge velocities associated with different continuous and discontinuous boundary sources. We also display the energies, as well as the dissipation and source terms associated with the various cases. We conclude the article with Section 9, where we discuss our results and draw our final conclusions.

    In this paper, we focus on a viscoelastic, compressible Biot model in one spatial dimension. Let x and t denote the spatial and temporal coordinates, respectively. In the case where inertial terms are negligible, displacements are infinitesimal and external sources of linear momentum and mass are absent, the balance equations to be solved in the spatial interval (0,L) and in the temporal observational interval (0,T] can be written as:

    σx=0, (2.1a)
    ζt+vx=0, (2.1b)

    where σ is the total stress, ζ is the fluid content and v is the discharge velocity. Equations (2.1) are complemented with the following constitutive laws:

    σ=αp+σ0, (2.2a)
    σ0=θux+ηt(ux), (2.2b)
    ζ=c0p+αux, (2.2c)
    v=Kpx. (2.2d)

    Equation (2.2a) expresses the fact that the total stress is the sum of a contribution due to the interstitial fluid pressure (or pore pressure) p and one due to the effective stress σ0, which is assumed to be characterized by viscoelastic behavior of Kelvin-Voigt type (see Eq (2.2b) where u denotes the solid phase displacement). Equation (2.2c) expresses the fact that the fluid content is altered by changes in fluid pressure and solid deformation. Equation (2.2d) is Darcy's law relating the discharge velocity v with the pressure gradient by means of the hydraulic permeability K. The Biot coefficient α, the storage coefficient c0, the material parameters θ and η and the permeability K are assumed to be given positive constants.

    In the classical Biot's theory, the material parameter θ can be expressed as θ=K(4/3)G, where K is the drained bulk compression modulus and G is the shear modulus. In the following, we will simply refer to θ and η as the elastic and viscoelastic parameters, respectively [12]. In addition, we will use the notation K=K0 to emphasize the fact that the permeability is assumed to be a given constant.

    The problem must be equipped with appropriate boundary conditions. In the following, we will assume that the boundary located at x=0 is fixed and impermeable, namely:

    u(0,t)=0, (2.3a)
    v(0,t)=0, (2.3b)

    and that the boundary located at x=L experiences an external stress P (a compressive stress if P is positive, a tensile stress if P is negative) that is supported entirely by the solid component of the mixture (condition of exposed pores [13]), namely:

    p(L,t)=0, (2.3c)
    σ(L,t)=P(t). (2.3d)

    Finally, we complete the formulation of the problem by prescribing the following initial conditions:

    u(x,0)=0, (2.4a)
    p(x,0)=0. (2.4b)

    We notice that (2.4a) and (2.4b) also imply the following initial conditions for the dilation of the solid material and the discharge velocity, respectively:

    ux(x,0)=0, (2.5a)
    v(x,0)=0. (2.5b)

    Remark 1. The case of incompressible mixture components can be obtained by setting c0=0 and α=1 in the model described above, as detailed in [14,15]. The study of analytical solutions for the incompressible model was addressed in [1,4].

    The mathematical system described in Section 2 satisfies an energy identity that helps provide a physical interpretation of the solutions, assuming they exist. To this end, let us multiply (2.1a) by u/tL2(0,L); integrating over (0,L) and using the constitutive laws (2.2a) and (2.2b) and the boundary conditions (2.3a), (2.3c) and (2.3d), we obtain

    θ2ddtux2L2(0,L)η2utx2L2(0,L)+αL0p2utxdx=P(t)ut(L,t). (3.6)

    Multiplying (2.1b) by pL2(0,L), integrating over (0,L) and using the constitutive laws (2.2c) and (2.2d) and the boundary conditions (2.3b) and (2.3c), we obtain

    c02ddtp2L2(0,L)+K0px2L2(0,L)+αL0p2utxdx=0. (3.7)

    Subtracting (3.6) from (3.7) we get the following evolution equation for the total energy stored in the poroviscoelastic system

    ddtEtot(t)+D(t)=F(t),t(0,T], (3.8)

    where the energy functional Etot=Etot(t), the dissipation functional D=D(t) and the force term F=F(t) are defined as:

    Etot(t):=c02p(,t)2L2(0,L)+θ2ux(,t)2L2(0,L), (3.9a)
    D(t):=K0px(,t)2L2(0,L)+η2utx(,t)2L2(0,L), (3.9b)
    F(t):=P(t)ut(L,t). (3.9c)

    Remark 2. The physical units of Etot are Joules per unit area, namely J m2. This is due to the fact that we are considering a one-dimensional problem in space and, as a consequence, all the problem variables are assumed to be constant on every plane perpendicular to the chosen direction x. Mathematically, Etot is obtained by integrating in x between 0 and L the energy density εtot defined as

    εtot(x,t):=c02p(x,t)2+θ2|ux(x,t)|2.

    The units of εtot are J m3. Analogously, the units of D and F are J m2 s1.

    Since Etot0 and D0, in absence of forcing terms (i.e., P=0) the energy decreases in time. It is important to emphasize that the terms proportional to the storage coefficient c0 and the elastic parameter θ contribute to the total energy in the form of potential energy, so that we can write

    Ec0(t):=c02p(,t)2L2(0,L),Eθ(t):=θ2ux(,t)2L2(0,L)withEtot=Ec0+Eθ.

    Conversely, the terms proportional to K0 and η contribute to dissipate energy via viscous effects within the fluid and solid components. We notice that F does not have a definite sign since it depends on the boundary terms. The energy identity (3.8) will be very useful in interpreting the results presented in Section 7.

    Now we express problem (2.1)–(2.4) solely in terms of the solid displacement. Combining (2.1a) and (2.3d), we obtain that the total stress is given by

    σ(x,t)=P(t), for all  x(0,L] and for all t(0,T],

    and, moreover, the fluid pressure p and the discharge velocity v can be written in terms of the solid displacement u as:

    p(x,t)=1αP(t)+1ασ0(u(x,t)), (4.10a)
    v(x,t)=K0αxσ0(u(x,t)), (4.10b)

    where σ0=σ0(u(x,t)) is given by (2.2b). Let us now derive the problem satisfied by u. Integrating Eq (2.1b), where ζ is given by (2.2c), over the space interval [0,x] and taking the boundary conditions (2.3) into account, yields

    c0x0ptdy+αut+v=0

    Now we substitute p and v by expressions (4.10) and note that

    x0σ0dy=θu+ηut

    by virtue of (2.3a). Then we obtain

    c0xP(t)+c0(θut+η2ut2)+α2utK0(θ2ux2+η3utx2)=0.

    The boundary conditions are (2.3a) and σ0|x=L=P(t) (coming from (2.3c) and (4.10a)). The initial conditions are (2.4a) and, on using (2.4b), (4.10a), (2.2b) and (2.5a),

    η2utx(x,0)=P(0)

    or equivalently

    ut(x,0)=1ηxP(0)+u1

    u1 being an arbitrary constant. Note that u1=0 if and only if the compatibility condition with (2.3a)

    ut(0,0)=0 (4.11)

    is satisfied. Summing up:

    Find u=u(x,t) such that:

    c0η2ut2+(α2+c0θ)utK0θ2ux2K0η3utx2=c0xP(t)in (0,L)×(0,T], (4.12a)
    u(0,t)=0in (0,T], (4.12b)
    θux(L,t)+η2utx(L,t)=P(t)in (0,T], (4.12c)
    u(x,0)=0in (0,L), (4.12d)
    ut(x,0)=1ηxP(0)+u1in (0,L). (4.12e)

    Remark 3. The solution u(x,t) of the problem (4.12) is the sum u1(x,t)+u0(x,t) of the solution u1(x,t) of (4.12) where P=0, and of the solution u0(x,t) of (4.12) where u1=0. In the first case, a perturbation is generated at time zero, with a certain initial velocity of propagation (u10): then u1(x,t) measures how the solid displacement changes through the medium when no disturbance is created at the boundary (P=0). On the other hand, if there is no ''initial impulse" (i.e., u1=0) and a disturbance is generated at the boundary (P0), then the corresponding change of the solid displacement is represented by u0(x,t). Therefore, the compatibility condition (4.11) simply means to consider the response of the system to the sole external stress P.

    Remark 4. We assume throughout the article that c0>0 and η>0, i.e. the system is characterized by finite compressibility and structural viscoelasticity.

    Remark 5. The IBVP (4.12) has a markedly different character compared to the linear poroviscoelastic system studied in [4] because of the second-order time derivative on the left-hand side of (4.12a).

    Remark 6. If we define the following quantities:

    ˜ρ:=1K0c0ηα2+c0θ,˜σ:=1α2+c0θσ01K0x0ut(y,t)dy,˜f:=1K0c0α2+c0θxP(t),

    then the partial differential equation (4.12a) that describes the dynamics of the solid phase displacement can be written as

    ˜ρ2ut2=˜σx+˜f.

    Such an equation can be formally interpreted as a linear momentum balance equation for a single phase solid material whose dynamics is equivalent to that of the fluid-solid mixture under the assumptions of Section 2. In particular, we see that the finite compressibility of the mixture components, corresponding to c0>0, provides:

    (i) an inertia-like term for the equivalent solid, even though inertial terms were neglected for the original solid phase within the mixture;

    (ii) an additional term in the stress tensor introducing nonlocal effects in space;

    (iii) a volumetric forcing term that results from the load applied as a boundary condition.

    In order to simplify the theoretical analysis, in this section we reduce the 1D model (4.12) into dimensionless form. With this aim, for any variable Y, we define the corresponding non-dimensional variable by

    ˆY:=Y[Y],

    where [Y] is a suitably chosen scaling factor that has the same units as Y. The selection of the scaling factor is not unique and, in general, not trivial. In this article we generalize the choice made in [4], by introducing the following scaling factors:

    [x]=L, (5.1a)
    [t]=L2(α2+c0θ)θK0, (5.1b)
    [η]=θ[t], (5.1c)
    [σ]=[σ0]=[P]=Pref, (5.1d)
    [u]=LPrefθ, (5.1e)
    [u1]=[u][t], (5.1f)
    [v]=K0PrefαL, (5.1g)
    [p]=Prefα, (5.1h)
    [D]=[F]=K0P2refL(α2+c0θ), (5.1i)
    [Etot]=[Ec0]=[Eθ]=[D][t]=LP2refθ. (5.1j)

    We also define the non-dimensional quantity

    γ:=c0θα2+c0θ. (5.2)

    Note that 0<γ<1. We recover the same definitions of the scaling factors as in [4] by setting α=1 and c0=0 in (5.1).

    Remark 7. For notational simplicity we will drop the 'ˆ' notation for the dimensionless variables and instead use the same symbols adopted for the dimensional variables.

    The linear 1D model for a poroviscoelastic mixture in dimensionless form reads:

    γη2ut2+ut2ux2η3utx2=γxP(t)in (0,1)×(0,T], (5.3a)
    u(0,t)=0in (0,T], (5.3b)
    ux(1,t)+η2utx(1,t)=P(t)in (0,T], (5.3c)
    u(x,0)=0in (0,1), (5.3d)
    ut(x,0)=1ηxP(0)+u1in (0,1). (5.3e)

    Once u(x,t) is known, the functions σ, p and v can be computed as follows:

    σ(x,t)=P(t), (5.4a)
    p(x,t)=P(t)+ux+η2utx, (5.4b)
    v(x,t)=(2ux2+η3utx2)=(γη2ut2+ut+γxP(t)). (5.4c)

    Lastly, the dimensionless energy equation is written again in the form (3.8) where

    Etot(t)=Ec0(t)+Eθ(t), (5.5a)
    Ec0(t)=12γ1γp(,t)2L2(0,1),Eθ(t)=12ux(,t)2L2(0,1), (5.5b)
    D(t)=11γpx(,t)2L2(0,1)+η2utx(,t)2L2(0,1), (5.5c)
    F(t)=P(t)ut(1,t). (5.5d)

    We make the following assumption on the boundary traction in (5.3).

    Assumption 8. P(t) is a piecewise smooth function on [0,T].

    We recall that a function P(t) is piecewise smooth on [0,T] if both P and its derivative P are continuous on [0,T], except possibly at a finite number of points in (0,T), where they have simple jump discontinuities.

    Define

    U(t)=1ηet/ηP(t)=1ηt0exp(tsη)P(s)ds. (6.6)

    Using Assumption 8 we have that U(t) is absolutely continuous on [0,T] and

    ηU(t)=P(t)U(t),U(0)=0,U(0)=P(0)η. (6.7)

    Now we introduce the change of variable

    w(x,t)=u(x,t)+xU(t), (6.8)

    and note that w solves the following auxiliary IBVP with homogeneous boundary data:

    γη2wt2+wt2wx2η3wtx2=f(x,t)in (0,1)×(0,T], (6.9a)
    w(0,t)=0in (0,T], (6.9b)
    wx(1,t)+η2wtx(1,t)=0in (0,T], (6.9c)
    w(x,0)=0in (0,1), (6.9d)
    wt(x,0)=φ(x)in (0,1). (6.9e)

    where the volumetric source and initial datum are given by:

    f(x,t)=(1γ)xU(t), (6.10a)
    φ(x)=u1. (6.10b)

    We shall prove the existence and uniqueness of the solution w(x,t) for a very general class of data f(x,t) and φ(x). For sake of exposition we write H=L2(0,1), and define the real Hilbert space

    V={vW1,2(0,1):v(0)=0} (6.11)

    endowed with the equivalent norm vV=v/xL2(0,1), due to Poincaré's inequality.

    Remark 9. Sobolev's Embedding Theorem gives W1,2(0,1)C0[0,1] so that v(0)=0 holds in a strong sense for every vV.

    Now we make the following assumptions on the functions f(x,t) and φ(x):

    fL2(0,T;H),φH (6.12)

    and write w(t)=w(,t), w(t)=w(,t)/t, etc. We then define weak solutions of (6.9) as follows.

    Definition 10. [Weak solution of problem (6.9)] A function w:[0,T]V is a weak solution of the auxiliary problem (6.9) if:

    D1 wW1,2(0,T;V) and wL2(0,T;V);

    D2 for every vV and for t pointwise a.e. in (0,T)

    γηw(t),vV×V+(w(t),v)H+(ηw(t)+w(t),v)V=(f(t),v)H (6.13)

    or, equivalently,

    ddt(γηw(t)+w(t),v)H+(ηw(t)+w(t),v)V=(f(t),v)HvV; (6.14)

    D3 w(0)=0 and w(0)=φ.

    Remark 11. Condition [D1] implies that wC0([0,T];V) and wC0([0,T];H), and thus condition [D3] is well defined. The Dirichlet boundary condition in (6.9) at x=0 is included in the regularity requirement that w(t)V, whereas the boundary condition at x=1 is satisfied in a weak sense.

    Lemma 12. Let w be a weak solution of (6.9).Then there are constants Ci's, depending only on γ, η and T, such that the following estimates hold for t pointwise in [0,T]:

    γηw(t)+w(t)2HC1(φ2H+f2L2(0,T;H)), (6.15a)
    w(t)2VC2(φ2H+f2L2(0,T;H)), (6.15b)
    w(t)2HC3(φ2H+f2L2(0,T;H)). (6.15c)

    Proof. Using (γηw+w)L2(0,T;V) as multiplier in (6.14), we get

    ddtγηw+w2H+(ηw+w,γηw+w)V=(f,γηw+w)H.

    Integrating in time over (0,t) and using the given initial conditions, we obtain

    γηw+w2H+η(γ+1)2w2V+t0(w2V+γη2w2V) ds=γ2η2φ2H+t0(f,γηw+w)H ds.

    Using Cauchy-Schwarz and Young's Inequalities for the last term on the right-hand side, we obtain the following estimate:

    γηw+w2H+η(γ+1)2w2V+t0(w2V+γη2w2V) dsγ2η2φ2H+12f2L2(0,T;H)+12t0γηw+w2H ds. (6.16)

    Now, dropping the second and third terms on the left-hand side of (6.16) and using Gronwall's Inequality, estimate (6.15a) follows. Similarly, by dropping the first and third terms on the left-hand side and using (6.15a), we get (6.15b).

    Lastly, we use triangle inequality and the embedding VH to write

    γηwHγηw+wH+wHγηw(t)+w(t)H+12w(t)V.

    Then, estimate (6.15c) follows from (6.15a) and (6.15b).

    Lemma 13. Let w be a weak solution of (6.9).Then there is a constant C, depending only on γ, η and T, such that

    wL(0,T;V)+wL2(0,T;V)+wL2(0,T;V)C(φH+fL2(0,T;H)). (6.17)

    Proof. From (6.15b) it immediately follows

    wL(0,T;V)C2(φH+fL2(0,T;H)).

    In addition, Eqs (6.16) and (6.15a) give

    γη2T0w2V dsγ2η2φ2H+12f2L2(0,T;H)+12T0γηw+w2H ds(γ2η2+T2C1)φ2H+12(1+TC1)f2L2(0,T;H)

    so that

    wL2(0,T;V)C(φH+fL2(0,T;H)) (6.18)

    for a suitable C=C(γ,η,T).

    Lastly, using the definition of a weak solution, we have that for vV

    γη|w,vV×V|wHvH+ηw+wVvV+fHvH12(wV+ηw+wV+fH)vV.

    This implies that

    w(t)Vη+12(wV+wV+fH)

    from which, using estimates (6.15b) and (6.15a), we obtain

    wL2(0,T;V)C(φH+fL2(0,T;H))

    for a suitable C=C(γ,η,T).

    The following corollary is an immediate consequence of Lemma 13.

    Corollary 14. (Uniqueness and continuous dependence on data) The weak solution to problem (6.9) is unique and depends continuously on the data.

    The IBVP (6.9) can be solved formally using separation of variables. If we look for solutions of the form w(x,t)=T(t)X(x), then the associated regular Sturm-Liouville Problem is

    {X+λX=0,0<x<1X(0)=0, X(1)=0, (6.19)

    with eigenvalues and eigenfunctions given by:

    λn=(nπ+π2)2, n0, (6.20)
    Xn(x)=sin((nπ+π2)x), n0. (6.21)

    Remark 15. The sequence of functions {2Xn(x)} forms a Hilbert space basis for H, whereas the sequence of functions {2λnXn(x)} forms a Hilbert space basis for V (see [17]).

    The solution w of (6.9) has the expansion

    w(x,t)=n=0Tn(t)Xn(x), (6.22)

    where Tn(t) can be recovered using the data. Similarly, we use the basis {Xn(x)} to represent φ and f as follows:

    φ(x)=n=0φnXn(x), (6.23a)
    f(x,t)=n=0fn(t)Xn(x), (6.23b)

    where φn and fn(t) are the Fourier coefficients of φ and f(,t) with respect to Xn(x), respectively. Parseval's identity (consequence of Remark 15) provides the following relations:

    φ2H=12n=0φ2n, (6.24)
    f(,t)2H=12n=0|fn(t)|2, (6.25)
    f2L2(0,T;H)=12T0n=0|fn(t)|2dt. (6.26)

    Note that Tn(t) satisfies the following ordinary differential equation (ODE) for all n0:

    γηTn(t)+(1+ηλn)Tn(t)+λnT(t)=fn(t), (6.27)

    and initial conditions

    Tn(0)=0andTn(0)=φn. (6.28)

    The characteristic equation for the homogeneous counterpart of (6.27) is given by

    γηΛ2+(1+ηλn)Λ+λn=0.

    Since the discriminant of the equation is

    (1+ηλn)24γηλn=(1ηλn)2+4(1γ)ηλn>0,

    then for each n0 the characteristic equation has two real, negative, distinct roots r1n=Λ1n and r2n=Λ2n, where:

    Λ1n=12γη(1+ηλn(1+ηλn)24γηλn), and (6.29a)
    Λ2n=12γη(1+ηλn+(1+ηλn)24γηλn). (6.29b)

    Remark 16. Λ1n and Λ2n satisfy the following relations:

    0<Λ1n<Λ2nΛ1n+Λ2n=1+ηλnγη,Λ1nΛ2n=λnγη,1γ=γ(ηΛ1n1)(ηΛ2n1)Λ1n=1η(1γη2)1λn+O(1λ2n),Λ2n=λnγ+1γγη+O(1λn)(as n)Λ1n=λn1+ηλn+O(γ),Λ2n=1+ηλnγηλn1+ηλn+O(γ)(as γ0)

    Therefore the solution for the homogeneous ODE (6.27) is given by T0n(t)=aneΛ1nt+bneΛ2nt. The particular solution is obtained from variation of parameters formula. The Wronskian is given by W(t)=(Λ2nΛ1n)eΛ1nteΛ2nt, so that the particular solution has the following form

    Tpn(t)=1γηt0eΛ1n(ts)eΛ2n(ts)Λ2nΛ1n f(s) ds. (6.30)

    We introduce the following notation

    Gn(t):=exp(Λ1nt)exp(Λ2nt)Λ2nΛ1n (6.31)

    and then write the solution to (6.27) as

    Tn(t)=aneΛ1nt+bneΛ2nt+1γη(Gnfn)(t) (6.32)

    where we used the following formula for convolution

    (Gnfn)(t)=t0Gn(ts)fn(s)ds.

    Now we use (6.32) back into (6.22) and impose the initial conditions. We have:

    w(x,0)=an+bn=0, andwt(x,0)=Λ1nanΛ2nbn=φn

    which yields an=bn=φn/(Λ2nΛ1n). In conclusion, we obtain:

    Tn(t)=Gn(t)φn+1γη(Gnfn)(t), and (6.33a)
    w(x,t)=n=0[Gn(t)φn+1γη(Gnfn)(t)]Xn(x). (6.33b)

    We note that the terms Gn(t) given in (6.31) satisfy the following estimates.

    Lemma 17. There exists a constant C>0 such that for all n0 and for all t[0,T] we have:

    0Gn(t)Cλn, (6.34a)
    |Gn(t)|C. (6.34b)

    Proof. Since 0<Λ1n<Λ2n, then 0<exp(Λ2nt)exp(Λ1nt)1 and thus for all t[0,T] we have

    0Gn(t)<1Λ2nΛ1n

    and we get (6.34a) since the sequence λnΛ2nΛ1n=γηλn(ηλn+1)24γηλnγ as n. Moreover, we have that for all t[0,T]

    |Gn(t)|=Λ2nexp(Λ2nt)Λ1nexp(Λ1nt)Λ2nΛ1n2Λ2nΛ2nΛ1n

    and (6.34b) follows since Λ2nΛ2nΛ1n1 as n.

    Now we can state and prove our well-posedness result.

    Theorem 18. (Well-posedness of problem (6.9)) For every fL2(0,T;H) and φH there is a unique weak solution of (6.9), in the sense of Definition 10. Moreover, the solution depends continuously on the data.

    Proof. Uniqueness and continuous dependence of weak solution have already been proved, see Corollary 14, so that it remains to prove existence, i.e. that w(x,t) given in (6.33b) satisfies conditions (D1)–(D3) of Definition 10.

    (D1) (A) First we show that wL2(0,T;V). For all t[0,T], we have

    w(t)2V=12n=0λnT2n(t)n=0λn|Gn(t)φn|2+n=0λn|1γηt0Gn(ts)fn(s)ds|2.

    Using Lemma 17, we obtain*

    *In what follows, for notational convenience, C will denote possibly different constants depending only on γ,η,T.

    w(t)2VCn=0|φn|2λn+Cn=01λn(t0|fn(s)|ds)2Cn=0|φn|2+Cn=0T0|fn(s)|2dsC(φ2H+f2L2(0,T;H)). (6.35)

    Therefore wL(0,T;V)L2(0,T;V).

    (B) The weak derivative of w with respect to time is given by

    w(t)=n=0Tn(t)Xn(x)=n=0[Gn(t)φn+1γη(Gnfn)(t)]Xn(x),

    and we obtain the following estimate

    w(t)2H=12n=0(Tn(t))2n=0|Gn(t)φn|2+n=0|t0Gn(ts)fn(s)ds|2.

    Again by Lemma 17, we obtain

    w(t)2HCn=0|φn|2+Cn=0(t0|fn(s)|ds)2Cn=0|φn|2+Cn=0T0|fn(s)|2dsC(φ2H+f2L2(0,T;H)). (6.36)

    Thus wL(0,T;H)L2(0,T;H). In order to show that wL2(0,T;V), we first use the Monotone Convergence Theorem to write

    w2L2(0,T;V)=12T0n=0λn|Tn(s)|2ds=12n=0T0λn|Tn(s)|2ds. (6.37)

    We use multiplier Tn in (6.27) and the initial conditions (6.28) and obtain the following estimate:

    γηTnTn+(1+ηλn)T2n+λnTnTn=fnTn  
    γη2ddt(Tn)2+(Tn)2+ηλn(Tn)2+λn2ddt(Tn)2=fnTn  
    γη2(Tn(t))2γη2φ2n+t0(Tn)2 ds+t0ηλn(Tn)2 ds+λn2(Tn)2=t0fnTn ds  
    t0ηλn(Tn)2 dsγη2φ2n++12T0|fn(s)|2 ds+12T0|Tn(s)|2 ds. (6.38)

    Now we use (6.38) back into (6.37), and take advantage of estimate (6.36) to obtain

    w2L2(0,T;V)Cn=0φ2n+Cn=0T0|fn(s)|2ds+Cn=0T0|Tn(s)|2ds=C{φ2H+f2L2(0,T;H)+w2L2(0,T;H)}C{φ2H+f2L2(0,T;H)}. (6.39)

    and this gives the desired conclusion that wL2(0,T;V).

    (C) Left to show that wL2(0,T;V). For any v(x)=n=0cnXn(x)V and for t[0,T], we use (6.27) to define the linear functional w(t) as follows

    w(t),vV×V=12n=0cnTn(t)=12γηn=0Tn(t)cn12γηn=0λn(ηTn(t)+Tn(t))cn+12γηn=0fn(t)cn=1γη(w(t),v)H1γη(ηw(t)+w(t),v)V+1γη(f(t),v)H.

    Hence, by virtue of the embedding VH, we obtain that

    |γηw(t),vV×V|w(t)HvH+ηw(t)+w(t)VvV+f(t)HvHC{w(t)V+w(t)V+f(t)H}vV.

    This shows that w(t)V and

    w(t)VC(w(t)V+w(t)V+f(t)H)

    or, equivalently,

    w(t)2VC(w(t)2V+w(t)2V+f(t)2H).

    In conclusion, using the estimates in part (A) and part (B), we obtain that wL2(0,T;V).

    (D2) Now we show that w satisfies condition (D2). Since the {2λnXn} is a basis in V, it suffices to consider the test function v=Xn. For t pointwise almost everywhere in (0,T), we use (6.27) and obtain

    γηw(t),vV×V+(w(t),v)H+(ηw(t)+w(t),v)V=γη2Tn(t)+12Tn(t)+ηλn2Tn(t)+λn2Tn(t)=12fn(t)=(f(t),v)H.

    (D3) As stated in Remark 11, condition (D1) implies that wC0([0,T];V) and wC0([0,T];H). Moreover, solution w(x,t) satisfies the initial conditions (D3) of Definition 10 by virtue of (6.28). This concludes the proof of our well-posedness theorem.

    We have established the existence and uniqueness of the weak solution w to the auxiliary problem (6.9). Now we examine its regularity.

    Proposition 19. (Regularity of w and wxx) Under the hypotheses in Theorem 18, the following is true:

    w(x,t)C0(Q), where Q=[0,1]×[0,T], (6.40a)
    wxxL(0,T;H). (6.40b)

    Proof. (A) The result follows from Lemma 17, the fact that |Xn(x)|1 and Weierstrass Test. Indeed, for all x[0,1] and t[0,T], and every n0, we have

    |Tn(t)Xn||Gn(t)φnXn(x)|+1γη|(Gnfn)(t)Xn(x)|.

    Now, due to Lemma 17, we have

    n=0|Gn(t)φnXn(x)|n=0|Gn(t)φn|Cn=01λn|φn|Cn=0(1λ2n+φ2n)=C(n=01λ2n+φ2H)<,

    and

    n=0|(Gnfn)(t)Xn(x)|n=0|(Gnfn)(t)|n=0t0|Gn(ts)fn(s)|dsCn=0T01λn|fn(s)|dsCn=0T0(1λ2n+|fn(s)|2)dsC(n=01λ2n+f2L2(0,T;H))<.

    Applying Weierstrass Test, we obtain that the series n=0 Tn(t)Xn(x) converges absolutely and uniformly in Q=[0,1]×[0,T]. Moreover, we note that Gn(t)φnXn(x)+1γη(Gnfn)(t)Xn(x)C0(Q), for every n0, and thus w(x,t)C0(Q).

    (B) The second order weak derivative in space of w is given by

    2wx2=n=0Gn(t)φnλnXn(x)n=0(Gnfn)(t)λnXn(x).

    Then we use estimate (6.34a) in Lemma 17 and obtain

    2wx2(,t)2Hn=0|Gn(t)φnλn|2+n=0|(Gnfn)(t)λn|2Cn=0φ2n+Cn=0|t0|fn(s)|ds|2Cn=0φ2n+Cn=0t0|fn(s)|2ds=C(φ2H+f2L2(0,T;H))

    so that wxxL(0,T;H).

    If the data are more regular with respect to x the weak solution w(x,t) enjoys stronger regularity properties.

    Proposition 20. (Regularity of wt, wxx and wtxx) In addition to the hypotheses in Theorem 18, we make the following ones:

    fL2(0,T;V)andφV.

    Then the following is true:

    wt(x,t)C0(Q), where Q=[0,1]×[0,T], (6.41a)
    wxxL(0,T;V), (6.41b)
    wtxxL2(0,T;H). (6.41c)

    Proof. (A) For all x[0,1] and t[0,T], and every n0, we have

    |Tn(t)Xn||Gn(t)φnXn(x)|+1γη|(Gnfn)(t)Xn(x)|.

    Now, due to Lemma 17, we have

    n=0|Gn(t)φnXn(x)|n=0|Gn(t)φn|Cn=0|φn|Cn=0(1λn+λnφ2n)=C(n=01λn+φ2V)<,

    and

    n=0|(Gnfn)(t)Xn(x)|n=0|(Gnfn)(t)|n=0t0|Gn(ts)fn(s)|dsCn=0T0|fn(s)|dsCn=0T0(1λn+λn|fn(s)|2)dsC(n=01λn+f2L2(0,T;V))<.

    Applying Weierstrass Test, we obtain that the series n=0 Tn(t)Xn(x) converges absolutely and uniformly in Q=[0,1]×[0,T] hence wt(x,t)C0(Q).

    (B) Like in the proof of (6.40a), we have

    2wx2(,t)2Vn=0λn|Gn(t)φnλn|2+n=0λn|(Gnfn)(t)λn|2Cn=0λnφ2n+Cn=0λn|t0|fn(s)|ds|2Cn=0λnφ2n+Cn=0t0λn|fn(s)|2ds=C(φ2V+f2L2(0,T;V))

    so that wxxL(0,T;V).

    (C) Since the second order weak derivative in space of w is given by

    3wtx2(x,t)=n=0Tn(t)λnXn(x)

    then

    3wtx22L2(0,T;H)=12T0n=0λ2n|Tn(s)|2ds (6.42)

    In order to estimate the right hand side, we use multiplier λnTn on the ODE (6.27) that Tn solves to obtain

    ηλ2nT2n=λnT2n12(γηλnT2n+λ2nT2n)+λnfn(t)Tn12(γηλnT2n+λ2nT2n)+λnfn(t)Tn.

    Now we integrate from 0 to t and use the initial conditions (6.28), to get

    ηt0λ2n|Tn(s)|2ds12(γηλn|Tn(t)|2+λ2n|Tn(t)|2)+12γηλnφ2n+t0λnfn(s)Tn(s)ds12(γηλnφ2n+t0λn|fn(s)|2ds+t0λn|Tn(s)|2ds). (6.43)

    Thus, using (6.43) back into (6.42), we obtain

    3wtx22L2(0,T;H)C(n=0λnφ2n+T0n=0λn|fn(t)|2dt+T0n=0λn|Tn(t)|2dt)=C(φ2V+f2L2(0,T;V)+w2L2(0,T;V))

    and the assertion follows from estimate (6.17).

    Now we are in a position to return to our original linear 1D problem (5.3). The solid displacement solution u(x,t) of (5.3) is the sum

    u(x,t)=xU(t)+w(x,t) (6.44)

    where we recall that

    U(t)=1ηet/ηP(t)

    and w(x,t) is the unique weak solution of the auxiliary problem (6.9) with special data (6.10). From the Fourier expansions

    n=01λnXn(x)=12,n=0(1)nλnXn(x)=x2,0x1,

    as well as the Fourier series (6.23) of φ and f with respect to the basis {Xn}, their Fourier coefficients are given by:

    φn=2u1λn, (6.45a)
    fn(t)=2(1)nλn(1γ)U(t). (6.45b)

    To summarize, we can write the solution u as a sum

    u(x,t)=u1(x,t)+u0(x,t), (6.46)

    where:

    u1(x,t)=u1n=02λnGn(t)Xn(x), (6.47a)
    u0(x,t)=xU(t)+1γγηn=02(1)nλnGn(t)U(t)Xn(x). (6.47b)

    Remark 21. The solid displacement solution u(x,t)C0(Q) since the physical data (6.10) satisfy f L2(0,T;V) and φH. Notice, however, that our special φ belongs to V if and only if u1=0, i.e. φ=0 and u1(x,t)0.

    By virtue of (6.46), the discharge velocity (5.4c) is given by

    v(x,t)=v1(x,t)+v0(x,t) (6.48)

    where:

    v1(x,t)=u1n=02λn(Gn(t)+ηGn(t))Xn(x), (6.49a)
    v0(x,t)=1γγηn=02(1)n[Gn(t)U(t)+η(Gn(t)U(t))]Xn(x). (6.49b)

    Remark 22. It should be stressed that the Fourier coefficients of v1(x,t) are O(n), hence the component of the discharge velocity due to the presence of an initial impulse u1 always exibits a blow-up whatever boundary traction P(t) is considered.

    In this section, we analyze the behavior of the solutions obtained for model (5.3) in the case where the compatibility conditions are satisfied (i.e., u1=0), and the boundary traction P(t) is characterized by continuous or discontinuous waveforms. To graphically represent the model solutions we proceed as follows: (1) we define the numerical values of model parameters (in dimensional form) consistently with the experimental data illustrated in [16]; (2) we perform the non-dimensionalization of the model equations according to the procedure described in Section 5; (3) we compute the model solutions in non-dimensional form; (4) we multiply the non-dimensional input data and model solutions by the scaling factors introduced in (5.1) and plot the obtained results for subsequent analysis. For each considered waveform of P(t), we provide the non-dimensional expressions of the solutions u, p and v. These latter expressions allow us to compute the non-dimensional energies Eθ, Ec0 and Etot, the non-dimensional dissipation functional D and the non-dimensional force term F using the expressions (5.5). The resulting non-dimensional energies are then multiplied by the scaling factor [Etot] introduced in (5.1j) and the same is done for the non-dimensional dissipation and force terms that are multiplied by the scaling factor [D] introduced in (5.1i).

    Table 1 reports the numerical values of model parameters (in dimensional form) that are used in the next sections. The values for L, T, θ, K0 and Pref have been chosen consistently with the experimental data illustrated in [16]. To observe the asymptotic behavior of the modeled system, T has been taken equal to 105s in the example illustrated in Section 7.1. In this example, the corresponding value of η is 4.85109Nsm2. The value of the Biot coefficient α has been set equal to 0.9, whereas in [16] it was equal to 1 since both mixture components were assumed to be incompressible. The values of the viscoelastic parameter η and of the compressibility parameter c0 (which were not present in the model studied in [16]) have been set equal to θτe and 1/Pref, respectively, where τe is an elastic time constant that has been set equal to T/20.

    Table 1.  Numerical values of model parameters utilized in the numerical simulations.
    symbol value units
    L 0.81103 m
    T 105,104 s
    θ 0.97106 Nm2
    K0 2.91016 m4N1s1
    Pref 6104 Nm2
    c0 1.67105 m2N1
    η 4.85109,4.85108 Nsm2

     | Show Table
    DownLoad: CSV

    The numerical values of the parameters (in dimensionless form) that are used in the computations discussed in the next sections are reported in Table 2. These values have been obtained by applying the scaling procedure described in Section 5 to the values in Table 1. With a slight abuse of notation, the symbols used to denote the dimensionless parameters are the same ones that we used for the corresponding parameters expressed with their physical units.

    Table 2.  Numerical values of model parameters in dimensionless form.
    symbol value
    L 1
    T 2.5, 0.25
    η 1.26101, 1.26102
    γ 0.95

     | Show Table
    DownLoad: CSV

    For convenience, we recall here the formulas that we use to compute the solid displacement, the fluid pressure and the discharge velocity in dimensionless form:

    u0(x,t)=xU(t)+1γγηn=02(1)nλnGn(t)U(t)Xn(x), (7.1a)
    p0(x,t)=n=02(1)nλnBn(t)Xn(x), (7.1b)
    v0(x,t)=n=02(1)nBn(t)Xn(x), (7.1c)

    where

    Bn(t)=1γγη[(GnU)(t)+η((GnU)(t))]. (7.1d)

    Recalling formula (6.31) and using the fact that (GnU)=GnU, we obtain the following simplification for the coefficients Bn(t)

    Bn(t)=1γγη(1ηΛ1nΛ2nΛ1neΛ1ntU1ηΛ2nΛ2nΛ1neΛ2ntU). (7.1e)

    Let t[0,T). We consider the following boundary source

    P(t)=H(tt)={0ift<t1iftt. (7.2)

    Figure 1 gives a graphical representation of P(t) for t(0,T).

    Figure 1.  The step pulse at t=t.

    Replacing (7.2) into (6.6) we obtain

    U(t)={0,t<t1ettη,tt  and  U(t)={0,t<t1ηettη,tt. (7.3)

    We can now compute the convolution needed in (7.1a)

    Gn(t)U(t)={0,t<t,1η(Λ2nΛ1n)[e(tt)/ηeΛ1n(tt)Λ1n1/ηe(tt)/ηeΛ2n(tt)Λ2n1/η],tt. (7.4)

    We observe that for tt,

    (Gn(t)U(t))=1η(Gn(tt)(GnU)(t)),

    and therefore the coefficients (7.1d) present in the expansions of pressure and discharge velocity become

    Bn(t)=1γγηGn(tt), for any tt.

    Then displacement, pressure and discharge velocity are all zero for t<t, and have the following representations for tt:

    u(x,t)=xn=02(1)nλneΛ2n(tt)(ηΛ1n1)eΛ1n(tt)(ηΛ2n1)η(Λ2nΛ1n)Xn(x), (7.5a)
    p(x,t)=1γγηn=02(1)nλnGn(tt)Xn(x), (7.5b)
    v(x,t)=1γγηn=02(1)nGn(tt)Xn(x). (7.5c)

    Note that all three series in the expressions (7.5) converge absolutely and uniformly on [0,1]×[t,T], and therefore p,vC0([0,1]×[t,T]). Moreover, we observe that p(x,t)0 and v(x,t)0, as tt.

    Remark 23. Note that if t=0, then this is the case of a continuous constant boundary source P(t)=1, for all t0. Then the solid displacement, the fluid pressure and the discharge velocity have the following representations for t0:

    u(x,t)=xn=02(1)nλneΛ2nt(ηΛ1n1)eΛ1nt(ηΛ2n1)η(Λ2nΛ1n)Xn(x), (7.6a)
    p(x,t)=1γγηn=02(1)nλnGn(t)Xn(x), (7.6b)
    v(x,t)=1γγηn=02(1)nGn(t)Xn(x). (7.6c)

    All three series in (7.6) converge absolutely and uniformly on [0,1]×[0,T], and therefore p,vC0([0,1]×[0,T]).

    Figure 2 illustrates displacement, fluid pressure and discharge velocity (left, middle and right panel, respectively) as a function of t. Displacement and velocity are evaluated at x=1 (right boundary) whereas the pressure is evaluated at x=0 (left boundary). Within the observational time interval, the displacement decreases monotonically till it reaches an asymptotic value of approximately 50μm. Conversely, pressure and discharge velocity exhibit a nonmonotonic behaviour, characterized by a rapid increase, attaining a maximum value of approximately 2.3kPa and 0.002μms1, respectively, followed by a monotonic decrease that approaches zero asymptotically The behavior of the simulated solutions are consistent with those reported in [12,16].

    Figure 2.  Left panel: solid displacement u at x=L as a function of t. Middle panel: fluid pressure p at x=0 as a function of t. Right panel: discharge velocity v at x=L as a function of t. The applied boundary traction is a step pulse of amplitude Pref at t=0.

    Figure 3 illustrates displacement, fluid pressure and discharge velocity (left, middle and right panel, respectively) as a function of x and t. We see that, after an initial transient due to structural viscoelasticity, the solid displacement tends to a linear behavior along the domain length, being maximum (in absolute value) at x=L. Mathematically, this behavior is due to the fact that, for long times, the viscoelastic terms in the series in (7.6a) become negligible with respect to the linear elastic component. Both pressure and discharge velocity distributions decrease as time +, in agreement with the fact that definition (6.31) implies that lim for all n\geq 0 .

    Figure 3.  Top panel: solid displacement u as a function of x and t . Middle panel: fluid pressure p as a function of x and t . Bottom panel: discharge velocity v as a function of x and t . The applied boundary traction is a step pulse of amplitude P_{ref} at t^* = 0 .

    Figure 4 illustrates the energies per unit area \mathcal{E}_\theta , \mathcal{E}_{c_0} and \mathcal{E}_{tot} (left, middle and right panels, respectively) as a function of t . In agreement with the previous analysis for the displacement and pressure profiles, we see that \mathcal{E}_\theta tends to a constant value as time increases because the deformation of the mixture tends to become uniform in all the domain. At the same time, \mathcal{E}_{c_0} decreases in time following the decrease of the pressure, being significant only during the initial transient. As a result, the total potential energy \mathcal{E}_{tot} of the mixture tends to coincide with \mathcal{E}_\theta , as demonstrated by the right panel of Figure 4.

    Figure 4.  Simulated profiles of \mathcal E_{\theta} (left), \mathcal E_{c_0} (middle) and \mathcal E_{tot} as a function of t . The applied boundary traction is a step pulse of amplitude P_{ref} at t^* = 0 .

    Figure 5 illustrates the dissipation \mathcal{D} and force term \mathcal{F} (left and right panel, respectively) as a function of t . The temporal profiles of both functions rapidly decay to zero as time increases. This is consistent with the fact that the domain of the mixture tends to deform in a uniform manner as time gets large so that its time variation becomes rapidly negligible.

    Figure 5.  Left panel: dissipation as a function of t . Right panel: force term as a function of t . The applied boundary traction is a step pulse of amplitude P_{ref} at t^* = 0 .

    We assume that the external traction applied at the right boundary x = L has a jump discontinuity at t^* = 0.25 T .

    Figure 6 illustrates the computed displacement, fluid pressure and discharge velocity (left, middle and right panel, respectively) as a function of t . Displacement and velocity are evaluated at x = L (right boundary) whereas the pressure is evaluated at x = 0 (left boundary). We notice that the three graphs are the translation of the corresponding graphs in Figure 2. In particular, we see that u , p and v are continuous at t = t^* , where their value is equal to zero.

    Figure 6.  Left panel: solid displacement u at x = L as a function of t . Middle panel: fluid pressure p at x = 0 as a function of t . Right panel: discharge velocity v at x = L as a function of t . The applied boundary traction is a step pulse of amplitude P_{ref} at t^* = 0.25 T .

    Figure 7 illustrates the displacement, fluid pressure and discharge velocity (left, middle and right panel, respectively) as a function of x and t .

    Figure 7.  Top panel: solid displacement u as a function of x and t . Middle panel: fluid pressure p as a function of x and t . Bottom panel: discharge velocity v as a function of x and t . The applied boundary traction is a step pulse of amplitude P_{ref} at t^* = 0.25 T .

    Figure 8 illustrates the energies per unit area \mathcal{E}_\theta , \mathcal{E}_{c_0} and \mathcal{E}_{tot} (left, middle and right panels, respectively) as a function of t .

    Figure 8.  Simulated profiles of \mathcal E_{\theta} (left), \mathcal E_{c_0} (middle) and \mathcal E_{tot} as a function of t . The applied boundary traction is a step pulse of amplitude P_{ref} at t^* = 0.25 T .

    Figure 9 illustrates the dissipation \mathcal{D} and force term \mathcal{F} (left and right panel, respectively) as a function of t . We notice that both \mathcal D and \mathcal F are discontinuous at t = t^* where they experience a finite jump.

    Figure 9.  Left panel: dissipation as a function of t . Right panel: force term as a function of t . The applied boundary traction is a step pulse of amplitude P_{ref} at t^* = 0.25 T .

    Let P\left(t\right) be the dimensionless unbounded ramp pulse of unit-slope starting at t = 0 , represented in Figure 10

    \begin{equation} P\left( t\right) = t H\left(t\right) = t, \ t\geq 0. \end{equation} (7.7)
    Figure 10.  The unbounded ramp pulse.

    In this case, we have

    \begin{equation} U(t) = t - \eta \left( 1 - e^{-\frac{t}{\eta}} \right) \ \ \text{and}\ \ U^{\prime }(t) = 1 - e^{-\frac{t}{\eta}}, {\qquad t \geq 0, } \end{equation} (7.8)

    so that:

    \begin{align} & G_{n}\left( t\right) \ast 1 = \frac{1}{(\Lambda_{2n}-\Lambda_{1n})} \left[ \frac{1 - e^{-\Lambda_{1n} t}}{\Lambda_{1n}} - \frac{1 - e^{-\Lambda_{2n} t}}{\Lambda_{2n}} \right], \ t \geq 0, & \end{align} (7.9a)
    \begin{align} & G_{n}\left( t\right) \ast e^{-\frac{t}{\eta}} = \frac{1}{(\Lambda_{2n}-\Lambda_{1n})} \left[ \frac{e^{-t/\eta} - e^{-\Lambda_{1n} t}}{\Lambda_{1n}-1/\eta} - \frac{e^{-t/\eta} - e^{-\Lambda_{2n} t}}{\Lambda_{2n}-1/\eta} \right], \ t \geq 0. & \end{align} (7.9b)

    Summing the two above expressions we obtain

    \begin{align} & G_{n}\left( t\right) \ast U^\prime(t) = \frac{1}{(\Lambda_{2n}-\Lambda_{1n})} \left[ \frac{1 - e^{-\Lambda_{1n} t}}{\Lambda_{1n}} - \frac{1 - e^{-\Lambda_{2n} t}}{\Lambda_{2n}} - \frac{e^{-t/\eta} - e^{-\Lambda_{1n} t}}{\Lambda_{1n}-1/\eta} + \frac{e^{-t/\eta} - e^{-\Lambda_{2n} t}}{\Lambda_{2n}-1/\eta}\right], \ t \geq 0. & \end{align} (7.10)

    Note that the above expression can be rewritten as

    G_{n}\left( t\right) \ast U^\prime(t) = \frac{1}{\Lambda_{1n}\Lambda_{2n}} + \frac{1}{(\Lambda_{2n}-\Lambda_{1n})}\Big(\frac{e^{-\Lambda_{1n} t}}{\Lambda_{1n}(\eta \Lambda_{1n} - 1)}- \frac{e^{-\Lambda_{2n} t}}{\Lambda_{2n}(\eta \Lambda_{2n} - 1)}\Big) + \frac{\eta^2\gamma}{1-\gamma} e^{-\frac{t}{\eta}}.

    The time derivative of expression (7.10) is given by: for t \geq 0 ,

    \begin{eqnarray} \left(G_{n}\left( t\right) \ast U^{\prime }\left( t\right)\right)^\prime & = & \frac{1}{(\Lambda_{2n}-\Lambda_{1n})} \Big( e^{-\Lambda_{1n} t} - e^{-\Lambda_{2n} t} \\ &+& \frac{(1/\eta) e^{-t/\eta} - \Lambda_{1n} e^{-\Lambda_{1n} t}}{\Lambda_{1n}-1/\eta} - \frac{(1/\eta) e^{-t/\eta} - \Lambda_{2n} e^{-\Lambda_{2n} t}}{\Lambda_{2n}-1/\eta}\Big) \\ & = & \frac{1}{(\Lambda_{2n}-\Lambda_{1n})} \Big(\frac{-e^{-\Lambda_{1n} t}}{\eta \Lambda_{1n} - 1}+ \frac{e^{-\Lambda_{2n} t}}{\eta \Lambda_{2n} - 1}\Big) - \frac{\eta \gamma}{1-\gamma} e^{-\frac{t}{\eta}}. \end{eqnarray} (7.11)

    Note that

    \begin{align*} & \eta \left( G_{n}\left( t\right) \ast U^{\prime }\left( t\right) \right) ^{\prime } = G_{n}\left( t\right) \ast \exp \left( -t/\eta \right) = G_{n}\left( t\right) \ast 1-G_{n}\left( t\right) \ast U^{\prime }\left(t\right), & \end{align*}

    and the coefficients B_n(t) become

    \begin{align} B_{n}\left( t\right) = \frac{1-\gamma }{\gamma \eta } G_{n}\left( t\right) \ast 1 = \frac{1-\gamma }{\gamma \eta } \int_{0}^{t}G_{n}\left( s\right) ds. & \end{align} (7.12)

    Then the displacement, the pressure and the discharge velocity have the following representations for t \geq 0 :

    \begin{align} & u_0(x, t) = - x(t-\eta) + \frac{1-\gamma}{\gamma \eta} \sum\limits_{n = 0}^{\infty} \frac{2( -1) ^{n}}{\lambda _{n}} \left[ \frac{1}{\Lambda_{1n} \Lambda_{2n}} \right. & \\ & \left. + \frac{1}{\Lambda_{2n}-\Lambda_{1n}} \left( \frac{e^{-\Lambda_{1n} t}}{\Lambda_{1n} (\eta \Lambda_{1n} -1)} - \frac{e^{-\Lambda_{2n} t}}{\Lambda_{2n} (\eta \Lambda_{2n} -1)} \right) \right] X_n(x), &\end{align} (7.13a)
    \begin{align} & p_0(x, t) = \frac{1-\gamma}{\gamma \eta} \sum\limits_{n = 0}^{\infty} \frac{2( -1)^{n}}{\lambda_n} \frac{1}{(\Lambda_{2n}-\Lambda_{1n})} \left[ \frac{1 - e^{-\Lambda_{1n} t}}{\Lambda_{1n}} - \frac{1 - e^{-\Lambda_{2n} t}}{\Lambda_{2n}} \right] X^\prime_n(x), & \end{align} (7.13b)
    \begin{align} & v_0(x, t) = \frac{1-\gamma}{\gamma \eta} \sum\limits_{n = 0}^{\infty} 2( -1)^{n} \frac{1}{(\Lambda_{2n}-\Lambda_{1n})} \left[ \frac{1 - e^{-\Lambda_{1n} t}}{\Lambda_{1n}} - \frac{1 - e^{-\Lambda_{2n} t}}{\Lambda_{2n}} \right] X_n(x). & \end{align} (7.13c)

    Figure 11 illustrates the displacement, fluid pressure and discharge velocity (left, middle and right panel, respectively) as a function of t . Displacement and velocity are evaluated at x = L (right boundary) whereas the pressure is evaluated at x = 0 (left boundary). Unlike the case presented in Section 7.1, here the external pressure load continues to increase linearly with time, thereby inducing a continuous increase in displacement, pressure and velocity as time goes by. Over the observational time interval, the magnitude of the external load is smaller than what considered in Section 7.1 and this leads to a smaller (absolute) value of the maximum displacement, which is about 10\; \mathrm{{\mu m}} in this case.

    Figure 11.  Left panel: solid displacement u at x = L as a function of t . Middle panel: fluid pressure p at x = 0 as a function of t . Right panel: discharge velocity v at x = L as a function of t . The applied boundary traction is an unbounded ramp pulse.

    Figure 12 illustrates the displacement, fluid pressure and discharge velocity (left, middle and right panel, respectively) as a function of x and t . The displacement profile exhibits an approximately bilinear variation with respect to temporal and spatial coordinates, whereas pressure and velocity display a nonlinear behavior in the space-time domain. All dependent variables tend to increase in magnitude as a function of time at any spatial position of the mixture, the discharge velocity being closer to reach a stationary condition than the pressure.

    Figure 12.  Top panel: solid displacement u as a function of x and t . Middle panel: fluid pressure p as a function of x and t . Bottom panel: discharge velocity v as a function of x and t . The applied boundary traction is an unbounded ramp pulse.

    Figure 13 illustrates the energies per unit area \mathcal{E}_\theta , \mathcal{E}_{c_0} and \mathcal{E}_{tot} (left, middle and right panels, respectively) as a function of t . Interestingly, \mathcal{E}_\theta exhibits a nonlinear increase with respect to time in accordance with the fact that deformation is not constant in space. \mathcal{E}_{c_0} follows a similar pattern because of the nonlinear trend of the pressure, but has a much smaller value, so that the \mathcal{E}_{tot} almost coincides with \mathcal{E}_\theta for all times.

    Figure 13.  Simulated profiles of \mathcal E_{\theta} (left), \mathcal E_{c_0} (middle) and \mathcal E_{tot} as a function of t . The applied boundary traction is an unbounded ramp pulse.

    Figure 14 illustrates the dissipation \mathcal{D} and forcing term \mathcal{F} (left and right panel, respectively) as a function of t . Dissipation increase with time is characterized by two markedly different slopes. In a first time interval, approximately equal to T/5 , dissipation increases rapidly and is mainly determined by the fluid component of the mixture. In the remaining part of the observational time interval, dissipation increases less rapidly and is mainly determined by the structural viscoelasticity of the mixture. The forcing term increases linearly with time in accordance with the trend of the temporal variation of solid displacement at the right boundary of the domain, as shown in Figure 12.

    Figure 14.  Left panel: dissipation as a function of t . Right panel: force term as a function of t . The applied boundary traction is an unbounded ramp pulse.

    Let P^{\left(\varepsilon \right) }\left(t\right) be the dimensionless ramp pulse of unit amplitude and finite rise time ( = \varepsilon > 0 ) starting at t = 0

    \begin{equation} P^{\left( \varepsilon \right) }\left( t\right) = \dfrac{1}{\varepsilon }\left[ tH\left( t\right) -\left( t-\varepsilon \right) H\left(t-\varepsilon \right) \right] = \left\{ \begin{array}{lll} 0 & \text{if} & t\leq 0 \\ t/\varepsilon & \text{if} & 0 \lt t\leq \varepsilon \\ 1 & \text{if} & t \gt \varepsilon, \end{array} \right. \end{equation} (7.14)

    represented graphically in Figure 15.

    Figure 15.  Bounded ramp pulse.

    Using the linear superposition principle, the solid displacement, fluid pressure, and discharge velocity are given by:

    \begin{equation} u_{0}^{\varepsilon }\left( x, t\right) = \left\{ \begin{array}{lll} \dfrac{1}{\varepsilon }u_{0}\left( x, t\right) & \text{if} & 0\leq t\leq \varepsilon \\ & & \\ \dfrac{1}{\varepsilon }\left[ u_{0}\left( x, t\right) -u_{0}\left( x, t-\varepsilon \right) \right] & \text{if} & t \gt \varepsilon \end{array} \right. \end{equation} (7.15a)
    \begin{equation} p_{0}^{\varepsilon }\left( x, t\right) = \left\{ \begin{array}{lll} \dfrac{1}{\varepsilon }p_{0}\left( x, t\right) & \text{if} & 0\leq t\leq \varepsilon \\ & & \\ \dfrac{1}{\varepsilon }\left[ p_{0}\left( x, t\right) -p_{0}\left( x, t-\varepsilon \right) \right] & \text{if} & t \gt \varepsilon . \end{array} \right. \end{equation} (7.15b)
    \begin{equation} v_{0}^{\varepsilon }\left( x, t\right) = \left\{ \begin{array}{lll} \dfrac{1}{\varepsilon }v_{0}\left( x, t\right) & \text{if} & 0\leq t\leq \varepsilon \\ & & \\ \dfrac{1}{\varepsilon }\left[ v_{0}\left( x, t\right) -v_{0}\left( x, t-\varepsilon \right) \right] & \text{if} & t \gt \varepsilon . \end{array} \right. \end{equation} (7.15c)

    where the expressions of u_{0}\left(x, t\right) , p_{0}\left(x, t\right) and v_{0}\left(x, t\right) are given by (7.13).

    Notice that p_{0}^{\varepsilon }\left(x, t\right), v_{0}^{\varepsilon }\left(x, t\right) \in C^{0}\left(Q\right) since p_{0}\left(x, t\right), v_{0}\left(x, t\right) \in C^{0}\left(Q\right) and p_{0}\left(x, 0\right) = v_{0}\left(x, 0\right) = 0 .

    Figure 16 illustrates the displacement, fluid pressure and discharge velocity (left, middle and right panel, respectively) as a function of t . Displacement and velocity are evaluated at x = L (right boundary) whereas the pressure is evaluated at x = 0 (left boundary). We see that the displacement increases in magnitude almost linearly during the increase in time of the externally applied pressure. Then, it rapidly tends to stationary conditions once the external applied pressure becomes constant. The asymptotic value of the displacement is the same as in the case of the step pulse illustrated in Section 7.1. The profile of fluid pressure increases in time and the externally applied pressure increases; once the solid deformation attains stationary conditions, the fluid pressure decreases. A similar trend is shown by the discharge velocity. The maximum value of pressure is the same as in the case of a step pulse external pressure whereas the maximum value of the discharge velocity is slightly smaller and coincides with the value of the velocity at t = T/2 that is obtained in the case of an unbounded ramp external pressure (cf. Figure 11, right panel).

    Figure 16.  Left panel: solid displacement u at x = L as a function of t . Middle panel: fluid pressure p at x = 0 as a function of t . Right panel: discharge velocity v at x = L as a function of t . The applied boundary traction is a bounded ramp pulse with \varepsilon = 0.5 T .

    Figure 17 illustrates the displacement, fluid pressure and discharge velocity (left, middle and right panel, respectively) as a function of x and t . The spatial variation of the displacement becomes linear after the external pressure ceases to increase, whereas, in the same time interval, both fluid pressure and discharge velocity exhibit a spatial decrease.

    Figure 17.  Top panel: solid displacement u as a function of x and t . Middle panel: fluid pressure p as a function of x and t . Bottom panel: discharge velocity v as a function of x and t . The applied boundary traction is a bounded ramp pulse with \varepsilon = 0.5 T .

    Figure 18 illustrates the energies per unit area \mathcal{E}_\theta , \mathcal{E}_{c_0} and \mathcal{E}_{tot} (left, middle and right panels, respectively) as a function of t . Both \mathcal{E}_\theta and \mathcal{E}_{c_0} increase in time until the external pressure increases. Then, \mathcal{E}_\theta becomes constant since deformation is constant in time, whereas \mathcal{E}_{c_0} decreases because of the decrease of fluid pressure. As in all previously considered examples, the contribution of \mathcal{E}_{c_0} is much smaller than that of \mathcal{E}_{\theta} so that the \mathcal{E}_{tot} almost coincides with \mathcal{E}_\theta .

    Figure 18.  Simulated profiles of \mathcal E_{\theta} (left), \mathcal E_{c_0} (middle) and \mathcal E_{tot} as a function of t . The applied boundary traction is a bounded ramp pulse with \varepsilon = 0.5 T .

    Figure 19 illustrates the dissipation \mathcal{D} and forcing term \mathcal{F} (left and right panel, respectively) as a function of t . Both terms exhibit an increase with respect to time during the increase of the externally applied pressure. Then, they both experience a sudden decay once the externally applied pressure becomes constant. Dissipation tends to an asymptotic value that is much smaller than the value attained at t = T/2 whereas the force term tends to zero since the boundary displacement is almost constant in time after t = T/2 .

    Figure 19.  Left panel: dissipation as a function of t . Right panel: force term as a function of t . The applied boundary traction is a bounded ramp pulse with \varepsilon = 0.5 T .

    In this section we investigate the dependence of the solution of model (4.12) on the compressibility parameter c_0 . In terms of the dimensionless equation system (5.3), this amounts to analyzing the solutions as a function of the quantity \gamma defined in (5.2). We denote by c_{0, ref} = 1.67 \cdot 10^{-5} \mathrm{{m^2 N^{-1}}} the reference value of the compressibility parameter. This value has been used in all the computations illustrated in Section 7. Then, we let c_0 assume the following values:

    c_0 = \left[0.001, \, 0.01, \, 0.1, \, 1, \, 10 \, 100, \, 1000 \right] c_{0, ref}

    and we compute the solid displacement and discharge velocity at x = L , the fluid pressure at x = 0 and the energies \mathcal E_{\theta} , \mathcal E_{c_0} and \mathcal E_{tot} , as functions of time in the interval [0, \, T] , T = 10000 \mathrm{{s}} being the width of the observational window considered in Section 7. All the other model parameters have been set equal to the values adopted in Section 7. The applied boundary traction is a step pulse of amplitude P_{ref} at t^* = 0 .

    Figure 20 shows the profiles of solid displacement (left panel), fluid pressure (middle panel) and discharge velocity (right panel) as functions of time and of the compressibility parameter c_0 . We notice that compressibility has a significant impact on the quantitative values attained by the solution variables. In particular we see that increasing c_0 gives rise to:

    Figure 20.  Left panel: solid displacement u at x = L as a function of t . Middle panel: fluid pressure p at x = 0 as a function of t . Right panel: discharge velocity v at x = L as a function of t . The applied boundary traction is a step pulse of amplitude P_{ref} at t^* = 0 . The compressibility parameter c_0 varies in the range [10^{-3}, \, 10^3] c_{0, ref} , where the reference value c_{0, ref} is set equal to 1.67 \cdot 10^{-5} \mathrm{{m^2 N^{-1}}} as in all the computations illustrated in Section 7.

    ● an increase of the magnitude of the solid displacement;

    ● a decrease of pressure and velocity;

    ● a right shift of the peak of pressure and velocity.

    These behaviors are indicative of the fact that the compressibility of the mixture components allows the body to deform more under the same pressure load, thereby reducing the internal level of fluid pressure and limiting the impact on the fluid velocity. We also notice that decreasing c_0 has a much more significant impact than increasing c_0 on solution range variation. From the theoretical viewpoint, this is due to the fact that \gamma \rightarrow 1^- for large values of c_0 and, as a consequence, the ratio (1-\gamma)/(\gamma \eta) , that characterizes the mathematical form of the solution expressions (7.5), tends to 0. From the physical viewpoint, decreasing the value of c_0 implies that the body cannot significantly deform upon applying a pressure load, thereby inducing higher levels of fluid pressure whose gradients lead to larger fluid velocities.

    Figure 21 illustrates the discharge velocity computed by the model studied in this work and the discharge velocity computed by the model studied in [4]. The principal difference between the two models is that no compressibility was included in [4]. We see that:

    Figure 21.  Discharge velocity v at x = L as a function of t . The applied boundary traction is a step pulse of amplitude P_{ref} at t^* = 0 . The compressibility parameter c_0 varies in the range [10^{-3}, \, 10^3] c_{0, ref} , where the reference value c_{0, ref} is set equal to 1.67 \cdot 10^{-5} \mathrm{{m^2 N^{-1}}} as in all the computations illustrated in Section 7. The black solid line represents the discharge velocity computed by the model studied in [4].

    ● the velocity predicted by the model of [4] is a (very sharp) upper bound for all the velocities predicted by the model studied in the present work, when c_0 \rightarrow 0^+ ;

    ● for increasing values of c_0 , the velocities predicted by the present model differ substantially from the upper bound velocity yielded by the model in [4]. In particular, we see that increasing c_0 gives rise to a decrease and to a right shift of the peak in the velocity profile.

    Figure 22 shows the profiles of \mathcal E_{\theta} , \mathcal E_{c_0} and \mathcal E_{tot} as a function of t and of c_0 . We see that increasing c_0 has the effect of increasing substantially and monotonically the magnitude of \mathcal E_\theta because the deformation profile gets larger as the components become more compressible, in accordance with Figure 20, left panel. On the contrary, \mathcal E_{c_0} exhibits a nonmonotonic dependence on c_0 . Specifically, for c_0 < c_{0, ref} we see that increasing c_0 gives rise to an increase of \mathcal E_{c_0} with a right shift of its peak; then, for c_0 \geq c_{0, ref} , we see that increasing c_0 leads to a significant monotonic decrease of \mathcal E_{c_0} . For the theoretical viewpoint, this is due to the fact that the ratio \gamma/(1-\gamma) \to + \infty as c_0 \rightarrow +\infty and the pressure profile tends to decrease in accordance with Figure 20, middle panel. The physical meaning of these results can be appreciated by observing that the predicted total energy is mainly determined by \mathcal E_{\theta} for large values of c_0 , since larger deformations can occur for more compressible components, whereas the contribution to the total energy given by \mathcal E_{c_0} becomes more significant for smaller values of c_0 , since larger pressures develop for less compressible components.

    Figure 22.  Simulated profiles of \mathcal E_{\theta} (left), \mathcal E_{c_0} (middle) and \mathcal E_{tot} as a function of t . The applied boundary traction is a step pulse of amplitude P_{ref} at t^* = 0 . The compressibility parameter c_0 varies in the range [10^{-3}, \, 10^3] c_{0, ref} , where the reference value c_{0, ref} is set equal to 1.67 \cdot 10^{-5} \mathrm{{m^2 N^{-1}}} as in all the computations illustrated in Section 7.

    The analysis presented in this work shows that the solutions of a poroviscoelastic model with compressible components remain bounded even in the case when the imposed boundary traction is irregular in time. In particular, given a certain functional form for the boundary traction and a certain level of structural viscoelasticity, the discharge velocity attains a maximum value that is lower when compressibility is higher. By investigating the dynamics of the energy functionals characterizing the system, we showed that this limiting effect is due to the capability of the system to store potential energy as its components are elastically compressed, thereby delaying the transmission of irregularities in the linear momentum from the solid to the fluid. As a result, the fluid has the time to accommodate for sudden changes, resulting in bounded velocities. This mechanism is very different from that provided by structural viscoelasticity, whose limiting effect on the discharge velocity is due to increased viscous dissipation, as shown in [4].

    Ultimately, this work elucidates the specific role that compressibility plays in the control of fluid flow through complex deformable porous structure, which finds numerous applications in science and engineering. The work presented here offers many future directions of research. For example, it would be very interesting to investigate how the findings concerning the role of compressibility would translate to a more realistic three-dimensional setting. Furthermore, different viscoelastic models could be considered and the specific roles of fluid and solid viscosities could be investigated and compared.

    Dr. Bociu has been partially supported by NSF CAREER DMS-1555062. Dr. Guidoboni has been partially supported by the award NSF DMS-1853222. Dr. Sacco has been partially supported by Micron Semiconductor Italia S.r.l., SOW nr. 4505462139.

    The authors declare there is no conflict of interest.



    [1] W. Wang, L. Feng, Y. Li, F. Xu, Q. Deng, Role of financial leasing in a capital-constrained service supply chain, Transp. Res. Part E: Logist. Transp. Rev., 143 (2020), 102097. https://doi.org/10.1016/j.tre.2020.102097 doi: 10.1016/j.tre.2020.102097
    [2] H. Yaqoob, Y. H. Teoh, F. Sher, M. A. Jamil, D. Murtaza, M. A. Qubeissi, et al., Current status and potential of tire pyrolysis oil production as an alternative fuel in developing countries, Sustainability, 13 (2021), 3214. https://doi.org/10.3390/su13063214 doi: 10.3390/su13063214
    [3] S. Beggs, S. Cardell, J. Hausman, Assessing the potential demand for electric cars, J. Econom., 17 (1981), 1–19. https://doi.org/10.1016/0304-4076(81)90056-7 doi: 10.1016/0304-4076(81)90056-7
    [4] L. Capuano, Us energy information administration's international energy outlook 2020 (ieo2020), US Department of Energy: Washington DC, 2020.
    [5] BP Energy Outlook, 2019 edition, London, United Kingdom, 2019. Available from: https://www.bp.com/content/dam/bp/business-sites/en/global/corporate/pdfs/energy-economics/energy-outlook/bp-energy-outlook-2019.pdf.
    [6] U. Jahn, D. Mayer, M. Heidenreich, R. Dahl, S. Castello, L. Clavadetscher, et al., International energy agency pvps task 2: analysis of the operational performance of the iea database pv systems, in Sixteenth European Photovoltaic Solar Energy Conference, Routledge, (2020), 2673–2677. Available from: https://www.researchgate.net/publication/324727795.
    [7] M. A. Ghadikolaei, P. K. Wong, C. S. Cheung, J. Zhao, Z. Ning, K. Yung, et al., Why is the world not yet ready to use alternative fuel vehicles? Heliyon, 7 (2021), e07527. https://doi.org/10.1016/j.heliyon.2021.e07527
    [8] S. Dey, G. C. Dhal, Controlling carbon monoxide emissions from automobile vehicle exhaust using copper oxide catalysts in a catalytic converter, Mater. Today Chem., 17 (2020), 100282. https://doi.org/10.1016/j.mtchem.2020.100282 doi: 10.1016/j.mtchem.2020.100282
    [9] S. Ramalingam, S. Rajendran, P. Ganesan, Performance improvement and exhaust emissions reduction in biodiesel operated diesel engine through the use of operating parameters and catalytic converter: a review, Renewable Sustainable Energy Rev., 81 (2018), 3215–3222. https://doi.org/10.1016/j.rser.2017.08.069 doi: 10.1016/j.rser.2017.08.069
    [10] M. de Almeida D'Agosto, Air pollutant and greenhouse gas emissions (ghg), in Transportation, Energy Use and Environmental Impacts, Elsevier, (2019), 227–257. https://doi.org/10.1016/C2016-0-04814-3
    [11] T. Chen, X. Zhang, J. Wang, J. Li, C. Wu, M. Hu, et al., A review on electric vehicle charging infrastructure development in the UK, J. Mod. Power Syst. Clean Energy, 8 (2020), 193–205. https://doi.org/10.35833/MPCE.2018.000374 doi: 10.35833/MPCE.2018.000374
    [12] A. Ziegler, Individual characteristics and stated preferences for alternative energy sources and propulsion technologies in vehicles: a discrete choice analysis for germany, Transp. Res. Part A: Policy Pract., 46 (2012), 1372–1385. https://doi.org/10.1016/j.tra.2012.05.016 doi: 10.1016/j.tra.2012.05.016
    [13] H. Su, Y. Hu, H. R. Karimi, A. Knoll, G. Ferrigno, E. De Momi, Improved recurrent neural network-based manipulator control with remote center of motion constraints: experimental results, Neural Networks, 131 (2020), 291–299. https://doi.org/10.1016/j.neunet.2020.07.033 doi: 10.1016/j.neunet.2020.07.033
    [14] W. Qi, H. Su, A. Aliverti, A smartphone-based adaptive recognition and real-time monitoring system for human activities, IEEE Trans. Hum.-Mach. Syst., 50 (2020), 414–423. https://doi.org/10.1109/THMS.2020.2984181 doi: 10.1109/THMS.2020.2984181
    [15] M. Beise, K. Rennings, Lead markets and regulation: a framework for analyzing the international diffusion of environmental innovations, Ecol. Econ., 52 (2005), 5–17. https://doi.org/10.1016/j.ecolecon.2004.06.007 doi: 10.1016/j.ecolecon.2004.06.007
    [16] H. Su, A. Mariani, S. E. Ovur, A. Menciassi, G. Ferrigno, E. De Momi, Toward teaching by demonstration for robot-assisted minimally invasive surgery, IEEE Trans. Autom. Sci. Eng., 18 (2021), 484–494. https://doi.org/10.1109/TASE.2020.3045655 doi: 10.1109/TASE.2020.3045655
    [17] W. Qi, H. Su, A cybertwin based multimodal network for ecg patterns monitoring using deep learning, IEEE Trans. Ind. Inf., 18 (2022), 6663–6670. https://doi.org/10.1109/TII.2022.3159583 doi: 10.1109/TII.2022.3159583
    [18] D. Bolduc, N. Boucher, R. Alvarez-Daziano, Hybrid choice modeling of new technologies for car choice in Canada, Transp. Res. Rec., 2082 (2008), 63–71. https://doi.org/10.3141/2082-08 doi: 10.3141/2082-08
    [19] H. Su, W. Qi, Y. Schmirander, S. E. Ovur, S. Cai, X. Xiong, A human activity-aware shared control solution for medical human–robot interaction, Assembly Autom., 2022 (2022). https://doi.org/10.1108/aa-12-2021-0174
    [20] R. A. Daziano, D. Bolduc, Incorporating pro-environmental preferences towards green automobile technologies through a bayesian hybrid choice model, Transp. A: Transport Sci., 9 (2013), 74–106. https://doi.org/10.1080/18128602.2010.524173 doi: 10.1080/18128602.2010.524173
    [21] D. S. Bunch, M. Bradley, T. F. Golob, R. Kitamura, G. P. Occhiuzzo, Demand for clean-fuel vehicles in california: a discrete-choice stated preference pilot project, Transp. Res. Part A: Policy Pract., 27 (1993), 237–253. https://doi.org/10.1016/0965-8564(93)90062-P doi: 10.1016/0965-8564(93)90062-P
    [22] G. Ewing, E. Sarigöllü, Assessing consumer preferences for clean-fuel vehicles: a discrete choice experiment, J. Public Policy Mark., 19 (2000), 106–118. https://doi.org/10.1509/jppm.19.1.106.16946 doi: 10.1509/jppm.19.1.106.16946
    [23] G. O. Ewing, E. Sarigöllü, Car fuel-type choice under travel demand management and economic incentives, Transp. Res. Part D: Transp. Environ., 3 (1998), 429–444. https://doi.org/10.1016/S1361-9209(98)00019-4 doi: 10.1016/S1361-9209(98)00019-4
    [24] D. A. Hensher, M. J. Beck, J. M. Rose, Accounting for preference and scale heterogeneity in establishing whether it matters who is interviewed to reveal household automobile purchase preferences, Environ. Resour. Econ., 49 (2011), 1–22. https://doi.org/10.1007/s10640-010-9420-3 doi: 10.1007/s10640-010-9420-3
    [25] J. Farrell, G. Saloner, Installed base and compatibility: innovation, product preannouncements, and predation, Am. Econ. Rev., 76 (1986), 940–955. Available from: https://www.researchgate.net/publication/243776630.
    [26] R. D. Luce, Preference, utility and subjective probability, in Handbook of Mathematical Psychology, 1965. Available from: https://www.imbs.uci.edu/files/personnel/luce/pre1990/1965/LuceSuppes_Book%20Chapter_1965.pdf.
    [27] M. Achtnicht, German car buyers' willingness to pay to reduce co2 emissions, Clim. Change, 113 (2012), 679–697. https://doi.org/10.1007/s10584-011-0362-8 doi: 10.1007/s10584-011-0362-8
    [28] B. Oztaysi, S. C. Onar, C. Kahraman, M. Yavuz, Multi-criteria alternative-fuel technology selection using interval-valued intuitionistic fuzzy sets, Transp. Res. Part D: Transp. Environ., 53 (2017), 128–148. https://doi.org/10.1016/j.trd.2017.04.003 doi: 10.1016/j.trd.2017.04.003
    [29] W. L. Moore, M. B. Holbrook, Conjoint analysis on objects with environmentally correlated attributes: The questionable importance of representative design, J. Consum. Res., 16 (1990), 490–497. https://doi.org/10.1086/209234 doi: 10.1086/209234
    [30] A. Matviychuk, Bankruptcy prediction in transformational economy: discriminant and fuzzy logic approaches, Fuzzy Econ. Rev., 15 (2010), 21–38. https://doi.org/10.25102/fer.2010.01.02 doi: 10.25102/fer.2010.01.02
    [31] M. Bublyk, A. Kowalska-Styczeń, V. Lytvyn, V. Vysotska, The ukrainian economy transformation into the circular based on fuzzy-logic cluster analysis, Energies, 14 (2021), 5951. https://doi.org/10.3390/en14185951 doi: 10.3390/en14185951
    [32] R. Draeseke, D. E. A. Giles, A fuzzy logic approach to modelling the new zealand underground economy, Math. Comput. Simul., 59 (2002), 115–123. https://doi.org/10.1016/S0378-4754(01)00399-8 doi: 10.1016/S0378-4754(01)00399-8
    [33] H. Sattar, I. S. Bajwa, R. ul Amin, J. Muhammad, M. F. Mushtaq, R. Kazmi, et al., Smart wound hydration monitoring using biosensors and fuzzy inference system, Wireless Commun. Mobile Comput., 2019 (2019). https://doi.org/10.1155/2019/8059629
    [34] K. S. Mayer, M. S. De Oliveira, C. Müller, F. C. C. De Castro, M. C. F. De Castro, Blind fuzzy adaptation step control for a concurrent neural network equalizer, Wireless Commun. Mobile Comput., 2019 (2019). https://doi.org/10.1155/2019/9082362
    [35] N. A. Kheir, M. A. Salman, N. J. Schouten, Emissions and fuel economy trade-off for hybrid vehicles using fuzzy logic, Math. Comput. Simul., 66 (2004), 155–172. https://doi.org/10.1016/j.matcom.2003.11.007 doi: 10.1016/j.matcom.2003.11.007
    [36] K. V. Singh, H. O. Bansal, D. Singh, Feed-forward modeling and real-time implementation of an intelligent fuzzy logic-based energy management strategy in a series–parallel hybrid electric vehicle to improve fuel economy, Electr. Eng., 102 (2020), 967–987. https://doi.org/10.1007/s00202-019-00914-6 doi: 10.1007/s00202-019-00914-6
    [37] S. Safari, A.R. Erfani, A new method for fuzzification of nested dummy variables by fuzzy clustering membership functions and its application in financial economy, Iran. J. Fuzzy Syst., 17 (2020), 13–27. https://doi.org/10.22111/IJFS.2020.5108 doi: 10.22111/IJFS.2020.5108
    [38] S. Ahmadi, S. Bathaee, Multi-objective genetic optimization of the fuel cell hybrid vehicle supervisory system: fuzzy logic and operating mode control strategies, Int. J. Hydrogen Energy, 40 (2015), 12512–12521. https://doi.org/10.1016/j.ijhydene.2015.06.160 doi: 10.1016/j.ijhydene.2015.06.160
    [39] R. Batley, J. Toner, M. Knight, A mixed logit model of uk household demand for alternative-fuel vehicles, Int. J. Transport Econ., 31 (2004), 1000–1023. https://doi.org/10.1400/16901 doi: 10.1400/16901
    [40] A. Körner, C. Tam, S. Bennett, J. Gagné, Technology roadmap-hydrogen and fuel cells, International Energy Agency (IEA), Paris, France, 2015. Available from: https://www.aeh2.org/images/stories/PDF/DOCS_SECTOR/technologyroadmaphydrogenandfuelcells.pdf.
    [41] J. I. Yellott Jr, The relationship between luce's choice axiom, thurstone's theory of comparative judgment, and the double exponential distribution, J. Math. Psychol., 15 (1977), 109–144. https://doi.org/10.1016/0022-2496(77)90026-8 doi: 10.1016/0022-2496(77)90026-8
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