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Theory article

Analytical solutions and asymptotic behaviors to the vacuum free boundary problem for 2D Navier-Stokes equations with degenerate viscosity

  • In this paper, we constructed a new class of analytical solutions to the isentropic compressible Navier-Stokes equations with vacuum free boundary in polar coordinates. These rotational solutions captured the physical vacuum phenomenon that the sound speed was C1/2-Hölder continuous across the boundary, and they provided some new information on our understanding of ocean vortices and reference examples for simulations of computing flows. It was shown that both radial and angular velocity components and their derivatives will tend to zero as t+ and the free boundary will grow linearly in time, which happens to be consistent with the linear growth properties of inviscid fluids. The large time behavior of the free boundary r=a(t) was completely determined by a second order nonlinear ordinary differential equation (ODE) with parameters of rotational strength ξ, adiabatic exponent γ, and viscosity coefficients. We tracked the profile and large time behavior of a(t) by exploring the intrinsic structure of the ODE and the contradiction argument, instead of introducing some physical quantities, such as the total mass, the momentum weight and the total energy, etc., which are usually used in the previous literature. In particular, these results can be applied to the 2D Navier-Stokes equations with constant viscosity and the Euler equations.

    Citation: Kunquan Li. Analytical solutions and asymptotic behaviors to the vacuum free boundary problem for 2D Navier-Stokes equations with degenerate viscosity[J]. AIMS Mathematics, 2024, 9(5): 12412-12432. doi: 10.3934/math.2024607

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  • In this paper, we constructed a new class of analytical solutions to the isentropic compressible Navier-Stokes equations with vacuum free boundary in polar coordinates. These rotational solutions captured the physical vacuum phenomenon that the sound speed was C1/2-Hölder continuous across the boundary, and they provided some new information on our understanding of ocean vortices and reference examples for simulations of computing flows. It was shown that both radial and angular velocity components and their derivatives will tend to zero as t+ and the free boundary will grow linearly in time, which happens to be consistent with the linear growth properties of inviscid fluids. The large time behavior of the free boundary r=a(t) was completely determined by a second order nonlinear ordinary differential equation (ODE) with parameters of rotational strength ξ, adiabatic exponent γ, and viscosity coefficients. We tracked the profile and large time behavior of a(t) by exploring the intrinsic structure of the ODE and the contradiction argument, instead of introducing some physical quantities, such as the total mass, the momentum weight and the total energy, etc., which are usually used in the previous literature. In particular, these results can be applied to the 2D Navier-Stokes equations with constant viscosity and the Euler equations.



    For simplicity, we consider Poisson equation with a Dirichlet boundary condition as our model problem.

    Δu=f,inΩ, (1)
    u=g,onΩ, (2)

    where Ω is a bounded polygonal domain in R2.

    Using integration by parts, we can get the variational form: find uH1(Ω) satisfying u=gonΩ and

    (u,v)=(f,v),vH10(Ω). (3)

    Various finite element methods have been introduced to solve the Poisson equations (1)-(2), such as the Galerkin finite element methods (FEMs)[2, 3], the mixed FEMs [15] and the finite volume methods (FVMs) [6], etc. The FVMs emphasis on the local conservation property and discretize equations by asking the solution satisfying the flux conservation on a dual mesh consisting of control volumes. The mixed FEMs is another category method that based on the variable u and a flux variable usually written as p.

    The classical conforming finite element method obtains numerical approximate results by constructing a finite-dimensional subspace of H10(Ω). The finite element scheme has the same form with the variational form (3): find uhVhH1(Ω) satisfying uh=IhgonΩ and

    (uh,vh)=(f,vh),vhV0h, (4)

    where V0h is a subspace of Vh that satisfying vh=0 on Ω and Ih is the kth order Lagrange interpolation operator. The FE method is a popular and easy-to-implement numerical scheme, however, it is less flexible in constructing elements and generating meshes. These limitations are mainly due to the strong continuity requirements of functions in Vh. One solution to circumvent these limitations is using discontinuous approximations. Since the 1970th, many new finite element methods with discontinuous approximations have been developed, including the early proposed DG methods [1], local discontinuous Galerkin (LDG) methods [8], interior penalty discontinuous Galerkin (IPDG) methods [9], and the recently developed hybridizable discontinuous Galerkin (HDG) methods [7], mimetic finite differences method [10], virtual element (VE) method [4], weak Galerkin (WG) method [19, 20] and references therein.

    One obvious disadvantage of discontinuous finite element methods is their rather complex formulations which are often necessary to ensure connections of discontinuous solutions across element boundaries. For example, the IPDG methods add parameter depending interior penalty terms. Besides additional programming complexity, one often has difficulties in finding optimal values for the penalty parameters and corresponding efficient solvers. Most recently, Zhang and Ye [21] developed a discontinuous finite element method that has an ultra simple weak formulation on triangular/tetrahedal meshes. The corresponding numerical scheme can be written as: find uh˜Vh satisfying uh=IhgonΩ and

    (wuh,wvh)=(f,vh),vhV0h, (5)

    where ˜Vh is the DG finite element space and w is the weak gradient operator. The notion of weak gradient was first introduced by Wang and Ye in the weak Galerkin (WG) methods [19, 20]. The WG methods allow the use of totally discontinuous functions and provides stable numerical schemes that are parameter-independent and free of locking [17] in some applications. Another key feature in the WG methods is it can be used for arbitrary polygonal meshes. The WG finite element method has been rapidly developed and applied to other problems, including the Stokes and Navier-Stokes equations [11, 18], the biharmonic [14, 13] and elasticity equations [12, 17], div-curl systems and the Maxwell's equations and parabolic problem [23], etc. The introduction of the weak gradient operator in the conforming DG methods makes the scheme (5) maintain the simple formulation of conforming finite element method while have the flexibility of using discontinuous approximations. Hence, the programming complexity of this conforming DG scheme is significantly reduced. Furthermore, the scheme results in a simple symmetric and positive definite system.

    Following the work in [21, 22], we propose a new conforming DG finite element method on rectangular partitions in this work. It can be obtained from the conforming formulation simply by replacing by w and enforcing the boundary condition strongly. The simplicity of the conforming DG formulation will ease the complexity for implementation of DG methods. We note that the conforming DG method in [21] is based on triangular/tetrahedal meshes. Then in [22], the method is extended to work on general polytopal meshes by raising the degree of polynomials used to compute weak gradient.

    In this paper, we keep the same finite element space as DG method, replace the boundary function with the average of the inner function, and use the weak gradient arising from local Raviart-Thomas (RT) elements [5] to approximate the classic gradient. Moreover, the derivation process in this paper is based on rectangular RT elements [16]. Error estimates of optimal order are established for the corresponding conforming DG approximation in both a discrete H1 norm and the L2 norm. Numerical verifications have been performed on different kinds of quadrangle finite element space. In particular, super-convergence phenomenon have been observed for Q0 elements.

    The rest of this paper is organized as follows: In Section 2, we shall present the conforming DG finite element scheme for the Poisson equation on rectangular partitions. Section 3 is devoted to a discussion of the stability and solvability of the new method. In Section 4, we shall prepare ourselves for error estimates by deriving some identities. Error estimates of optimal order in H1 and L2 norm are established in Section 5. In Section 6, we present some numerical results to illustrate the theory derived in earlier sections. Finally in section 7, we conclude our major contributions in this article.

    Throughout this paper, we adopt the standard definition of Sobolev space Hs(Ω). For any given open bounded domain KΩ, (,)s,K,s,K, and ||s,K are used to denote the inner product, norm and semi-norm, respectively. The space H0(K) coincides with L2(K), and the subscripts K in the inner product, norm, and semi-norm can be dropped in the case of K=Ω. In particular, the function space H10(Ω) is defined as

    H10(Ω)={vH1(Ω):v|Ω=0},

    and the space H(div,Ω) is defined as the set of vector-valued functions q, which together with their divergence are square integrable, i.e.

    H(div,Ω)={q[L2(Ω)]d:qL2(Ω)}.

    Assume that the domain Ω is of polygonal type and is partitioned into non-overlapping rectangles Th={T}. For each TTh, denote by T0 its interior and T its boundary. Denote by Eh={e} the set of all edges in Th, and E0h=EhΩ the set of all interior edges in Th. For each TTh and eEh, denote by hT and he the diameter of T and e, respectively. h=maxTThhT is the meshsize of Th.

    For any interior edge eE0h, let T1 and T2 be two rectangles sharing e, we define the average {} and the jump [[]] on e for a scalar-valued function v by

    {v}=12(v|T1+v|T2),[[v]]=v|T1n1+v|T2n2, (6)

    where v|Ti,i=1,2 is the trace of v on Ti, n</italic>1 and n</italic>2 are the two unit outward normal vectors on e, associated with T1 and T2, respectively. If e is a boundary edge, we define

    {v}=v|eand[[v]]=v|en. (7)

    We define a discontinuous finite element space

    Vh={vL2(Ω):v|TQk(T),TTh}, (8)

    and its subspace

    V0h={vVh:v=0onΩ}, (9)

    where Qk(T),k1 denotes the set of polynomials with regard to quadrilateral elements. The weak gradient for a scalar-valued function vVh is defined by the following definition

    Definition 2.1. For a given TTh and a function vVh, the discrete weak gradient dvRTk(T) on T is defined as the unique polynomial such that

    (dv,q)T:=(v,q)T+{v},qnT,qRTk(T), (10)

    where n is the unit outward normal on T, RTk(T)=[Qk(T)]2+xQk(T), and {v} is defined in (6) and (7).

    The weak gradient operator d as defined in (10) is a local operator computed at each element. It can be extended to any function vVh by taking weak gradient locally on each element T. More precisely, the weak gradient of any vVh is defined element-by-element as follows:

    (dv)|T=d(v|T).

    We introduce the following bilinear form:

    a(v,w)=(dv,dw),

    the conforming DG algorithm to solve the problems (1) - (2) is given by

    Conforming DG algorithm 1. Find uhVh satisfying uh=IhgonΩ and

    a(uh,vh)=(f,vh),vhV0h, (11)

    where Ih is the kth order Lagrange interpolation.

    We will prove the existence and uniqueness of the solution of equation (11). Firstly, we present the following two useful inequalities to derive the forthcoming analysis.

    Lemma 3.1 (trace inequality). Let T be an element of the finite element partition Th, and e is an edge or face which is part of T. For any function φH1(T), the following trace inequality holds true (see [20] for details):

    φ2eC(h1Tφ2T+hTφ2T), (12)

    where C is a constant independent of h.

    Lemma 3.2 (inverse inequality). Let Th be a finite element partition of Ω that is shape regualr. Assume that Th satisfies all the assumptions A1-A4 in [20]. Then, for any piecewise polynomial function φ of degree n on Th, there exists a constant C=C(n) such that

    φTC(n)h1TφT,TTh. (13)

    Then, we define the following semi-norms in the discontinuous finite element space Vh

    |||v|||2=a(v,v)=TThdv2T, (14)
    v21,h=TThv2T+eE0hh1e[[v]]2e. (15)

    We have the equivalence between the semi-norms |||v||| and v1,h, and it is proved in the following lemma.

    Lemma 3.3. For any vVh, the following equivalence holds true

    C1v1,h|||v|||C2v1,h, (16)

    where C1 and C2 are two constants independent of h.

    Proof. It follows from the definition of dv, integration by parts, the trace inequality, and the inverse inequality that

    dv2T1=(dv,dv)T1=(v,dv)T1+{v}n,dvT1=(v,dv)T1(v{v})n,dvT1vT1dvT1+(v{v})nT1dvT1dvT1(vT1+h12T1(v{v})nT1). (17)

    For any eT1, e=T1T2, we have

    (v{v})|en1=v|T1n112(v|T1+v|T2)n1=12(v|T1n1+v|T2n2)=12[[v]]e.

    Then we can get

    (v{v})n2T112eT1[[v]]2e. (18)

    Substituting (18) into (17) gives

    dv2T1C2dvT1(vT1+eT1h12e[[v]]e),

    this completes the proof of the right-hand of (16).

    To prove the left-hand of (16), we consider the subspace of RTk(T) for any TTh

    D(k,T):={qRTk(T):qn=0onT}.

    Note that D(k,T) is a dual space of [Qk1(T)]2 [13]. Thus, for any v[Qk1(T)]2, we have

    vT=supqD(k,T)(v,q)TqT. (19)

    Using the integration by parts, Cauchy-Schwarz inequality, the definition of D(k,T) and dv, we get

    (v,q)T=(v,q)T+v,qnT=(dv,q)T{v},qnT=(dv,q)TdvTqT,

    where we have used the fact that q</italic>n|T=0 in the definition of D(k,T). Combining the above result with (19), one has

    vTdvT. (20)

    We define the space De(k,T) as the set of all q</italic>RTk(T) such that all degrees of freedom, except those for q</italic>n|e, vanish. Note that De(k,T) is a dual space of [Qk(e)]2 [13]. Thus, we know

    [[v]]e=supqDe(k,T)[[v]],qneqne. (21)

    Following the integration by parts and the definition of d, we can derive that

    (dv,q)T=(v,q)Tv,qne+{v},qne.

    Together with (20), we obtain

    |[[v]],qne|=2|(dv,q)T(v,q)T|2|(dv,q)T|+2|(v,q)T|C(dvTqT+vTqT)CdvTqT.

    Substituting the above inequality into (21), by the scaling argument [13], for such qDe(k,T), we have qTh12qne, then

    [[v]]eCdvTqTqneCh12dvT. (22)

    Combining (20) and (22) gives a proof of the left-hand of (16).

    Lemma 3.4. The semi-norm |||||| defined in (14) is a norm in V0h.

    Proof. We shall only verify the positivity property for ||||||. To this end, assume |||v|||=0 for some vV0h. By Lemma 3.3, it follows that v1,h=0 for all TTh, which means that v=0 for all elements TTh and [[v]]=0 for all edges eE0h. We can derive from v=0 for all TTh that v is a constant in each T. [[v]]=0 on each eE0h implies v is a continuous function. This two conclusions and v=0 on Ω show that v=0, which completes the proof of the lemma.

    The above two lemmas imply the well posedness of the scheme (11). We prove the existence and uniqueness of solution of the conforming DG method in Theorem 3.1.

    Theorem 3.1. The conforming DG scheme (11) has and only has one solution.

    Proof. To prove the scheme (11) is uniquely solvable, it suffices to verify that the homogeneous equation has zero as its unique solution. To this end, let uhVh be the solution of the numerical scheme 11 with homogeneous data f=0, g=0. Letting vh=uh, we obtain

    a(uh,uh)=0,

    which leads to uh=0 by using Lemma 3.4. This completes the proof of the theorem.

    In this section, we will derive an error equation which will be used for the error estimates. For any q</italic>H(div,Ω), we assume that there exist an interpolation operator Πh satisfying ΠhqH(div,Ω)RTk(T) on each element TTh and

    (q,v)T=(Πhq,v)T,vQk(T). (23)

    For any wH1+k(Ω) with k1, from Lemma 7.3 in [20], we have the estimate of Πh as follows.

    Πh(w)wChkw1+k. (24)

    Moreover, it is easy to verify the following property holds true.

    Lemma 4.1. For any qH(div,Ω),

    TTh(q,v)T=TTh(Πhq,dv)T,vV0h. (25)

    Proof. ΠhqH(div,Ω) implies that Πhq is continuous across each interior edge. Since vV0h, we know that {v}=v=0 on Ω. Then

    TTh{v},ΠhqnT=0. (26)

    By the definition of Πh and d and the equation (26), we have

    TTh(q,v)T=TTh(Πhq,v)T=TTh(Πhq,v)T+TTh{v},ΠhqnT=TTh(Πhq,dv)T.

    This completes the proof of the lemma.

    Before establishing the error equation, we define a continuous finite element subspace of Vh as follows

    ˜Vh={vH1(Ω):v|TQk(T),TTh}. (27)

    so as a subspace of ˜Vh

    ˜V0h:={v˜Vh:v|Ω=0}. (28)

    Lemma 4.2. For any v˜Vh, we have

    dv=v.

    Proof. By the definition of d and integration by parts, for any q</italic>RTk(T), we have

    (dv,q)T=(v,q)T+{v},qnT=(v,q)T+v,qnT=(v,q)T,

    which gives

    (dvv,q)T=0,qRTk(T).

    Letting q be dvv in the above equation yields dvv=0, which completes the proof of the lemma.

    Let eh=Ihuuh, where Ih is the kth order Lagrange interpolation, uHk+1(Ω) with k1 is the exact solution of the Poisson equations (1) - (2), and uhVh is the numerical solution of the scheme (11). The following estimate of the Lagrange interpolation operator Ih holds true.

    IhuuChk+1uk+1, (29)
    IhuuChkuk+1. (30)

    It is obvious that ehV0h and Ihu˜Vh. We have the following lemma:

    Lemma 4.3. Denote eh=Ihuuh the error of conforming DG method arising from (11). For any vhV0h, we have

    a(eh,vh)=lu(vh), (31)

    where

    lu(vh)=TTh(IhuΠhu,dvh). (32)

    Proof. Since Ihu˜Vh, we have dIhu=Ihu. Using the property (25), we can derive

    TTh(dIhu,dvh)T=TTh(Ihu,dvh)T=TTh(IhuΠhu+Πhu,dvh)T=TTh(IhuΠhu,dvh)T+TTh(Πhu,dvh)T=lu(vh)TTh(u,vh)T=lu(vh)+(f,vh).

    By the definition of the scheme (11), we have

    TTh(dIhuduh,dvh)T=lu(vh).

    This completes the proof of the lemma.

    The goal of this section is to derive the error estimates in H1 and L2 norms for the conforming DG solution uh.

    Theorem 5.1. Let uHk+1(Ω) with k1 be the exact solution of the Poisson equation (1) - (2), and uhVh be the numerical solution of the scheme (11). Let eh=Ihuuh, there exists a constant C independent of h such that

    |||eh|||Chk|u|k+1. (33)

    Proof. Letting vh=eh in (31), and by the definition of ||||||, we have

    |||eh|||2=lu(eh). (34)

    From the Cauchy-Schwarz inequality, the triangle inequality, the definition of ||||||, (24), and (30), we arrive at

    lu(vh)=TTh(IhuΠh(u),dvh)TTThIhuΠh(u)TdvhT(TThIhuΠh(u)2T)12(TThdvh2T)12=(TThIhuu+uΠh(u)2T)12|||vh|||(TThIhuu2T+uΠh(u)2T)12|||vh|||Chk|u|k+1|||vh|||.

    Then, we have

    lu(eh)Chk|u|k+1|||eh|||. (35)

    Substituting (35) to (34), we obtain

    |||eh|||2Chk|u|k+1|||eh|||,

    which completes the proof of the lemma.

    It is obvious that ˜V0hV0h. Let ˜uh˜Vh be the finite element solution for the problem (1)-(2) which satisfies ˜uh=Ihg on Ω and

    (˜uh,v)=(f,v),v˜V0h. (36)

    For any v˜V0h˜Vh, we have dv=v, i.e.

    (duh˜uh,v)=0,v˜V0h. (37)

    In the rest of this section, we derive an optimal order error estimate for the conforming DG approximation (11) in L2 norm by adopting the duality argument. To this end, we consider the following dual problem that seeks ΦH10(Ω) satisfying

    (Φ)=uh˜uh,inΩ. (38)

    Assume that the dual problem satisfies H2-regularity, which means the following priori estimate holds true

    Φ2Cuh˜uh. (39)

    In the following of this paper, we note εh=uh˜uh for simplicity.

    Theorem 5.2. Assume uHk+1(Ω) with k1 is the exact solution of the Poisson equation (1) - (2), and uhVh is the numerical solution obtained with the scheme (11). Furthermore, assume that (39) holds true. Then, there exists a constant C independent of h such that

    uuhChk+1|u|k+1. (40)

    Proof. First, we shall derive the optimal order for εh in L2 norm. Consider the corresponding conforming DG scheme defined in (11) and let ΦhV0h be the solution satisfying

    a(Φh,v)=(εh,v),vV0h. (41)

    Since IhΦ˜Vh, it follows from (37) that

    (duh˜uh,IhΦ)=0,dIhΦ=IhΦ,

    which gives

    (duh˜uh,dIhΦ)=0. (42)

    Setting v=εh in (41), then by the definition of εh and (42), we have

    εh2=a(Φh,εh)=TTh(dΦh,dεh)T=TTh(d(ΦhIhΦ),duh˜uh)T|||ΦhIhΦ|||(|||uhIhu|||+(Ihu˜uh)).

    Then, by the Cauchy-Schwarz inequality, (33) and (39), we obtain

    εh2Ch|Φ|2hk|u|k+1Chk+1|u|k+1εh,

    which gives

    εhChk+1|u|k+1. (43)

    Combining the error estimate of finite element solution, the triangle inequality and (43) yields (40), which completes the proof of the theorem.

    In this section, we shall present some numerical results for the conforming discontinuous Galerkin method analyzed in the previous sections.

    We solve the following Poisson equation on the unit square domain Ω=(0,1)×(0,1),

    Δu=2π2sin(πx)sin(πy)in Ω (44)
    u=0on Ω. (45)

    The exact solution of the above problem is u=sin(πx)sin(πy). Uniform square grids as shown in Figure 1 are used for computation.

    Figure 1.  The first three grids used in the computation.

    We first use the Pk conforming discontinuous Galerkin spaces (8) to compute the test case (44)-(45), where Pk denotes the set of polynomials of 2 variables of degree less than or equal to k. The weak gradient is computed locally using rectangular RTk polynomials. The errors and the order of convergence of the conforming DG approximations are listed in Table 1. Optimal order of convergence is achieved in every case, which is consistent with our theory. In particular, a superconvergence of order O(h2) was observed in the discrete H1 norm for P0 elements. Furthermore, the results obtained with P0 elements seems to be slightly better than that obtained with P1 elements.

    Table 1.  Error profiles and convergence rates for test case (44)-(45) obtained with uniform grids and Pk conforming DG spaces.
    level uhQhu0 rate |||uhQhu||| rate #Dof
    by P0 conforming discontinuous Galerkin elements
    6 0.1996E-02 1.97 0.8887E-02 1.98 1024
    7 0.5013E-03 1.99 0.2228E-02 2.00 4096
    8 0.1255E-03 2.00 0.5574E-03 2.00 16384
    by P1 conforming discontinuous Galerkin elements
    6 0.2427E-02 1.97 0.1027E+00 1.02 3072
    7 0.6100E-03 1.99 0.5105E-01 1.01 12288
    8 0.1527E-03 2.00 0.2546E-01 1.00 49152
    by P2 conforming discontinuous Galerkin elements
    5 0.1533E-03 3.00 0.2042E-01 2.03 1536
    6 0.1915E-04 3.00 0.5061E-02 2.01 6144
    7 0.2394E-05 3.00 0.1260E-02 2.01 24576
    by P3 conforming discontinuous Galerkin elements
    5 0.7959E-05 4.00 0.1965E-02 3.00 2560
    6 0.4971E-06 4.00 0.2451E-03 3.00 10240
    7 0.3140E-07 3.98 0.3059E-04 3.00 40960
    by P4 conforming discontinuous Galerkin elements
    4 0.1055E-04 4.97 0.1421E-02 4.05 960
    5 0.3314E-06 4.99 0.8735E-04 4.02 3840
    6 0.1057E-07 4.97 0.5417E-05 4.01 15360
    by P5 conforming discontinuous Galerkin elements
    2 0.2835E-02 6.24 0.1450E+00 5.49 84
    3 0.4532E-04 5.97 0.4718E-02 4.94 336
    4 0.7115E-06 5.99 0.1478E-03 5.00 1344

     | Show Table
    DownLoad: CSV

    The same test case is also computed using the Qk conforming DG finite element space, where Qk denotes the set of polynomials of 2 variables defined on Ω, and for each variable, the degree of the variable is at most k. Table 2 illustrates the numerical performance of the corresponding conforming DG scheme. It can be seen from numerical computing that, in this case, the results obtained with the Q1 element are more accurate than those obtained with Q0(=P0) elements (see Table 1). All numerical results converge at the corresponding optimal order, which is consistent with the theory.

    Table 2.  Error profiles and convergence rates for test case (44)-(45) obtained with uniform grids and Qk conforming DG spaces.
    level uhQhu0 rate |||uhQhu||| rate #Dof
    by Q1 conforming discontinuous Galerkin elements
    6 0.4006E-03 1.99 0.2389E-02 1.99 4096
    7 0.1003E-03 2.00 0.5982E-03 2.00 16384
    8 0.2510E-04 2.00 0.1496E-03 2.00 65536
    by Q2 conforming discontinuous Galerkin elements
    6 0.2360E-04 2.99 0.3186E-02 1.99 9216
    7 0.2953E-05 3.00 0.7976E-03 2.00 36864
    8 0.3692E-06 3.00 0.1995E-03 2.00 147456
    by Q3 conforming discontinuous Galerkin elements
    5 0.1413E-04 4.08 0.1650E-02 2.97 4096
    6 0.8676E-06 4.03 0.2072E-03 2.99 16384
    7 0.5398E-07 4.01 0.2593E-04 3.00 65536
    by Q4 conforming discontinuous Galerkin elements
    3 0.2226E-02 4.59 0.5414E-01 3.52 400
    4 0.9610E-04 4.53 0.3723E-02 3.86 1600
    5 0.3279E-05 4.87 0.2392E-03 3.96 6400

     | Show Table
    DownLoad: CSV

    To test the superconvergence of P0 DG element, we solve the following 2nd order elliptic equation on the unit square domain Ω=(0,1)×(0,1),

    Δu+u=fin Ωu=0on Ω,

    where f is chosen so that the exact solution is not symmetric,

    u=(xx2)(yy3). (46)

    Uniform square grids as shown in Figure 1 are used for numerical computation. The numerical results are listed in Table 3. Surprising, for this problem, the H1-like norm of error superconverges at 1.5 order, and the L2 error has one order of superconvergence. But we do not yet know if such a superconvergence exists in general.

    Table 3.  Error profiles and convergence rates for test case (46) obtained with uniform grids and P0 conforming DG spaces.
    level uhQhu0 rate |||uhQhu||| rate #Dof
    by P0 conforming discontinuous Galerkin elements
    3 0.8265E-02 1.06 0.4577E-01 1.14 16
    4 0.2772E-02 1.58 0.1732E-01 1.40 64
    5 0.7965E-03 1.80 0.6331E-02 1.45 256
    6 0.2142E-03 1.90 0.2290E-02 1.47 1024
    7 0.5564E-04 1.94 0.8213E-03 1.48 4096
    8 0.1419E-04 1.97 0.2928E-03 1.49 16384

     | Show Table
    DownLoad: CSV

    To test further the superconvergence of P0 DG element, we solve the following 2nd order elliptic equations on the unit square domain Ω=(0,1)×(0,1),

    (au)=fin Ωu=0on Ω,

    where a=1+x+y and f is chosen so that the exact solution is not symmetric,

    u=(xx3)(y2y3). (47)

    Uniform square grids as shown in Figure 1 are used for computation. The numerical results are listed in Table 4. Surprising, again, the H1-like norm of error superconverges at 1.5 order, and the L2 error has one order of superconvergence for this problem.

    Table 4.  Error profiles and convergence rates for test case (47) obtained with uniform grids and P0 conforming DG spaces.
    level uhQhu0 rate |||uhQhu||| rate #Dof
    by P0 conforming discontinuous Galerkin elements
    3 0.4929E-02 0.97 0.5371E-01 0.80 16
    4 0.1917E-02 1.36 0.2401E-01 1.16 64
    5 0.6004E-03 1.67 0.9407E-02 1.35 256
    6 0.1682E-03 1.84 0.3507E-02 1.42 1024
    7 0.4457E-04 1.92 0.1275E-02 1.46 4096
    8 0.1148E-04 1.96 0.4576E-03 1.48 16384

     | Show Table
    DownLoad: CSV

    In this paper, we establish a new numerical approximation scheme based on the rectangular partition to solve second order elliptic equation. We derived the numerical scheme and then proved the optimal order of convergence of the error estimates in L2 and H1 norms of the conforming DG method. Numerical experiments are then present to verify the theoretical analysis, and all numerical results converging at the corresponding optimal order. Comparing with existing numerical methods, the confoming DG method has the following two characteristics: 1. The formulation is relatively simple. The stabilizer s(,) is no longer needed, and the boundary function ub is omitted, which is replaced by the average of internal function u0; 2. The projection operator Qh used in the traditional WG method is replaced by the Lagrange interpolation operator Ih, which makes the theoretical analysis much easier. As can be seen from the numerical examples in Section 6, this method reduces the programming complexity while ensuring the optimal order of convergence.



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