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Immersed hybrid difference methods for elliptic boundary value problems by artificial interface conditions

  • We propose an immersed hybrid difference method for elliptic boundary value problems by artificial interface conditions. The artificial interface condition is derived by imposing the given boundary condition weakly with the penalty parameter as in the Nitsche trick and it maintains ellipticity. Then, the derived interface problems can be solved by the hybrid difference approach together with a proper virtual to real transformation. Therefore, the boundary value problems can be solved on a fixed mesh independently of geometric shapes of boundaries. Numerical tests on several types of boundary interfaces are presented to demonstrate efficiency of the suggested method.

    Citation: Youngmok Jeon, Dongwook Shin. Immersed hybrid difference methods for elliptic boundary value problems by artificial interface conditions[J]. Electronic Research Archive, 2021, 29(5): 3361-3382. doi: 10.3934/era.2021043

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  • We propose an immersed hybrid difference method for elliptic boundary value problems by artificial interface conditions. The artificial interface condition is derived by imposing the given boundary condition weakly with the penalty parameter as in the Nitsche trick and it maintains ellipticity. Then, the derived interface problems can be solved by the hybrid difference approach together with a proper virtual to real transformation. Therefore, the boundary value problems can be solved on a fixed mesh independently of geometric shapes of boundaries. Numerical tests on several types of boundary interfaces are presented to demonstrate efficiency of the suggested method.



    In this paper we propose the immersed hybrid difference (IHD) methods for elliptic boundary value problems. Let us consider an elliptic boundary value problem on a multiply connected domain such that

    Δu=fin   Ω+, (1.1a)
    λu+μν+u=0on   Γ, (1.1b)
    u=0on   Γ0. (1.1c)

    Referring to Fig. 1 let Ω=ΩΩ+, Γ=Ω+Ω and Γ0=Ω. The vector ν± denotes the unit outward normal vector on Γ from Ω±, respectively. We assume the source f is smooth enough in Ω+ and the inner boundary Γ is smooth for the sake of concise regularity analyses. The case (λ,μ)=(1,0) corresponds to the Dirichlet condition and (λ,μ)=(0,1) corresponds to the mixed boundary value problem with the Neumann interior boundary condition. It will be called the Neumann condition from here on since we are interested in treating the interior boundary condition. When λμ0 it is called the Robin condition. The boundary condition on the outer boundary is assumed to be the homogeneous Dirichlet condition for simplicity. For exterior domain problems the outer boundary condition can be replaced by some absorbing boundary condition, which has many physical applications such as the heat conduction and wave propagation problems.

    Figure 1.  Configuration of the problem domain.

    The finite element and finite difference methods work very well for the boundary value problems if a mesh is constructed properly. Many researchers have studied and developed numerical methods for various problem by using finite difference (FD) and finite element (FE) approaches, to name a few, the (compact) finite difference [20,30], conforming and non-conforming FE (see [4] and references therein), mixed FE [1,6], discontinuous Galerkin (DG) [2,7,10] and hybridized DG (HDG) [9]. For these methods, boundary fitting mesh generation is the most effort taking part in implementation when the boundary of a domain is geometrically complicated. For curved interfaces high order methods fail to achieve the optimal order of convergence since the boundary conditions are imposed on a non-physical boundary with the usual elements. Thus, we need additional treatments (e.g. isoparametric curvilinear elements [3,5,19]) to recover the optimal order of convergence. On the other hand, various immersed methods for elliptic boundary value problems and relate problems have been introduced and developed to handle the computational domain involving complex geometries or curved interfaces/boundaries. For example, the boundary integral method (BIM) was introduced by Mayo [24,25]. Recent work by Marques et al. [23] and the kernel-free boundary integral method [32,34,33] are inspired by the BIM. Some of other well established methods are the immersed boundary method [27], the immersed interface method [21,22], the explicit-jump immersed interface method [31], the ghost fluid method [11,12], the immersed boundary smooth extension method [29], the matched interface and boundary method, and other methods [8,28,13]. These methods commonly reduce the efforts of mesh generation for the interface problem, because robust and efficient structured grids are used instead of a fitted mesh generation. The differences are how they enforce the boundary conditions and how they produce the solution in the domain.

    The immersed hybrid difference method was developed by the first author [14] for the elliptic interface problems, and this idea is extended to solve boundary value problems. In the immersed boundary approach, instead of solving the equation on a boundary fitting mesh, we consider the extended problem of it on the whole domain by treating the inner boundary as an immersed interface. To do that, we introduced artificial interface conditions maintaining ellipticity of the weak formulation and imposing the given boundary condition simultaneously. Then, the immersed interface type numerical method solves the problem on the uniform rectangular mesh. Similar to the existing immersed methods, our approach reduces mesh generation efforts compared to the standard finite element or finite difference approaches. Also, there is no numerical integration to solve the problem and the number of global degrees of freedom can be reduced via embedded static condensation since our approach is based on the hybrid difference method. On the other hand, there are drawbacks in our approach compared to the existing immersed methods. The condition number of the local system can be large when the penalty parameter is very small, and the extended problem requires extra computational cost. If the size of Ω is small compared to that of Ω+ the additional computational cost can be ignored, and this kind of situation arises commonly in physical applications such as wave scattering problems.

    The hybrid difference (HD) method is a finite difference version of the hybridized discontinuous Galerkin method and it was introduced by the authors and their colleagues for the elliptic, Stokes and Navier-Stokes equations [15,16,17,18]. Recently, the immersed boundary approach was applied to the HD method to develop the IHD method by the first author [14] for elliptic interface problems. The key feature of the IHD method is the introduction of the virtual to real transformation (VR-T) to find the finite difference stencil on interface cells, which refer to the cells that contain a portion of the given interface. The VR-T defines a relation between two locally extended sub-solutions on interface cells. In this paper we treat boundary Γ as an immersed interface, therefore, we need a mesh generation of the whole domain Ω+Ω, and the boundary conditions are imposed weakly by artificial interface conditions which contain a penalty parameter. The technique reminds us the Nitsche trick somehow.

    The immersed hybrid difference method is consisting of two kinds of finite differences. Let us consider a decomposition of the domain into rectangular cells, Th so that Ω=RThR. The cell difference approximates the local PDE on RTh,

    Δu=fon     R. (1.2)

    The intercell difference patches the local solutions together on intercell edges by approximating the flux continuity,

    [[νu]]e=νu|e+νu|e=0on   e=RR. (1.3)

    The continuity of u is guaranteed by assuming u is single valued on cell edges. Here, ν and ν are the outward unit normal vectors from R and R, respectively. In the IHD the boundary conditions on Γ0 appears explicitly, however, the boundary condition on Γ is included in the VR-T and it does appear explicitly.

    The paper is organized as follows. In §2 the artificial interface conditions are derived, which will replace the boundary conditions on Γ in (1.1), and some related analyses are provided. In §3 one dimensional immersed hybrid difference is introduced and the artificial interface conditions in §2 is used in deriving the virtual to real transformation. The two dimensional extension of the immersed hybrid difference are discussed in §4. In §5, numerical examples are provided to justify the theory and to demonstrate efficiency of the proposed method. Lastly, some concluding remarks are given at the end of the paper.

    In this section we extend the given boundary value problem (1.1) in a multiply connected domain to the immersed boundary value problem in a simply connected domain by introducing artificial interface conditions. For simplicity of discussion on solution regularity we assume that the interior boundary is smooth enough.

    Let us consider the Dirichlet condition.

    Δu+=fin   Ω+, (2.1a)
    u+=gDon   Γ, (2.1b)
    u+=0on   Γ0. (2.1c)

    By solving another Dirichlet condition in Ω

    Δu=0in   Ω, (2.2a)
    u=u+on   Γ, (2.2b)

    one obtains a solution

    u(x)={u+(x),xΩ+,u(x),xΩ.

    By the theory of elliptic operators in [26] we have the following lemmas.

    Lemma 2.1. Suppose ΩCr+1,1 for r0. Then,

    νuHs1/2(Γ)CgDHs+1/2(Γ)

    with r1sr+1.

    Lemma 2.2. Suppose Ω+ is Lipschitz. Then,

    ν+u+L2(Γ)Cu+H1(Γ)+Cu+H1(Ω+)+CfL2(Ω+),

    and

    u+H1(Γ)Cν+u+L2(Γ)+Cu+H1(Ω+)+CfL2(Ω+).

    We convert the above decoupled problems (2.1) and (2.2) into an approximating coupled problem with suitable interface conditions, and it will be called the immersed boundary value problem from here on.

    Let

    uϵ(x)={u+ϵ(x),xΩ+,uϵ(x),xΩ,

    be the solution of the coupled problem;

    Δu+ϵ=fin   Ω+, (2.3a)
    Δuϵ=0in   Ω, (2.3b)

    with the exterior boundary condition u+=0 on Γ0, and with the interface conditions

    u+ϵ=uϵ,ϵν+u+ϵ+ϵνuϵ=u+ϵ+gDon   Γ. (2.4)

    Here, ν+ and ν are the unit outward normal vectors on Γ from Ω+ and Ω, respectively, and 0<ϵ<1.

    Let us introduce function spaces for the extended solution u=u+u.

    Hk(ΩΩ+)={uC(Ω):u±=u|Ω±Hk(Ω±)},Hk0(ΩΩ+)={uHk(ΩΩ+):u|Γ0=0}.

    The extended solution uH10(ΩΩ+) of (2.1) and (2.2) satisfies the variational form, for vH10(ΩΩ+),

    (u,v)Ω+(u+,v+)Ω+ν+u++νu,vΓ=(f,v+)Ω+. (2.5)

    Moreover, the solution uϵH10(ΩΩ+) of the immersed boundary value problem (2.3) and (2.4) satisfies the variational form, for vH10(ΩΩ+),

    (uϵ,v)Ω+(u+ϵ,v+)Ω+ν+u+ϵ+νuϵ,vΓ,=(uϵ,v)Ω+(u+ϵ,v+)Ω++1ϵ(uϵgD),vΓ=(f,v+)Ω+. (2.6)

    Subtracting (2.5) from (2.6) we have

    ((uϵu),v)Ω+1ϵ(uϵgD),vΓ=ν+u++νu,vΓ.

    Taking v=uuϵ one obtains that

    uϵgDL2(Γ)ϵν+u++νuL2(Γ)ϵ(gDH1(Γ)+u+H1(Ω+)+fL2(Ω+)). (2.7)

    Let us consider the the problem with the Neumann boundary condition only on the interior boundary.

    Δu+=fin   Ω+, (2.8a)
    ν+u+=gNon   Γ, (2.8b)
    u+=0on   Γ0. (2.8c)

    The extension of u+ to the whole domain is made by considering an additional Dirichlet condition.

    (ϵu)=0in   Ω, (2.9a)
    u=u+on   Γ. (2.9b)

    Then, the total solution uH10(ΩΩ+) satisfies the following variational problem, for vH10(ΩΩ+),

    (ϵu,v)Ω+(u+,v+)Ω+gN+ϵνu,vΓ=(f,v+)Ω+. (2.10)

    The immersed boundary approximation of the problems (2.8) and (2.9) is obtained by introducing appropriate interface conditions as follows.

    Δu+ϵ=fin   Ω+, (2.11a)
    (ϵuϵ)=0in   Ω, (2.11b)
    u+ϵ=0on   Γ0, (2.11c)

    with the interface conditions

    u+ϵ=uϵ,ν+u+ϵ+ϵνuϵ=gNon   Γ. (2.12)

    Then, uϵH10(ΩΩ+) satisfies the variational problem, for vH10(ΩΩ+),

    (ϵuϵ,v)Ω+(u+ϵ,v+)Ω+gN,vΓ=(f,v+)Ω+. (2.13)

    Subtracting (2.13) from (2.10) one obtains that

    (ϵ(uuϵ),v)Ω+((u+u+ϵ),v+)Ω+=(ϵνu,v)Γ. (2.14)

    Take v=uuϵ, and using the duality of the Sobolev space, Lemma 2.1 and the trace theorem we have

    (u+u+ϵ)2L2(Ω+)ϵνuH1/2(Γ)vH1/2(Γ)ϵuH1(Ω)u+u+ϵH1(Ω+).

    Notice that u is independent of ϵ. The Poincaré inequality yields that

    (u+u+ϵ)L2(Ω+)ϵuH1(Ω). (2.15)

    Let us consider the the problem with the Robin boundary condition on the interior boundary (we consider only the case μλ0 and μ0 in (1.1b)). Without loss of generality assume μ=1 and λ0.

    Δu+=fin   Ω+, (2.16a)
    ν+u++λu=gRon   Γ, (2.16b)
    u+=0on   Γ0. (2.16c)

    As in the Neumann case the extension of u+ to the whole domain is made by considering an additional Dirichlet condition.

    (ϵu)=0in   Ω, (2.17a)
    u=u+on   Γ. (2.17b)

    Then, uH10(ΩΩ+) satisfies the following variational problem, for vH10(ΩΩ+),

    (ϵu,v)Ω+(u+,v+)Ω+gRλu+ϵνu,vΓ=(f,v+)Ω+. (2.18)

    The immersed boundary approximation of the problems (2.16) and (2.17) is obtained by introducing appropriate interface conditions as follows.

    Δu+ϵ=fin   Ω+,(ϵuϵ)=0in   Ω,u+ϵ=0on   Γ0,

    with the interface conditions

    u+ϵ=uϵ,ν+u+ϵ+λu+ϵ+ϵνuϵ=gRon   Γ.

    Then, uϵH10(ΩΩ+) satisfies the variational problem, for vH10(ΩΩ+),

    (ϵuϵ,v)Ω+(u+ϵ,v+)Ω+gRλu+ϵ,vΓ=(f,v+)Ω+. (2.19)

    Subtracting (2.19) from (2.18) one obtains that

    (ϵ(uuϵ),v)Ω+((u+u+ϵ),v+)Ω++λ(uuϵ),vΓ=ϵνu,vΓ.

    By a similar argument as in the Neumann condition

    (u+u+ϵ)L2(Ω+)+λuuϵ2L2(Γ)ϵuH1(Ω).

    In this section we introduce the immersed hybrid difference method for the immersed boundary value problems.

    We begin with the simplest one dimensional method. Consider the domain [0,1] with two interface points, Γ1Ik1 and Γ2Il, where it is divided into N-cells so that [0,1]=N1j=0Ij with Ij=[η2j,η2j+2]. The non-interface cell contains one cell-point and two intercell points, and the interface cells Ik1 and Il contain an additional point that is the interface point as in Fig. 2.

    Figure 2.  One dimensional cell configuration: cell-point, intercell point.

    We consider the one dimensional elliptic Dirichlet condition on two disjoint intervals Ω+=(0,Γ1)(Γ2,1): find u+ such that

    u+xx=fonΩ+,u+=g1 on Γ1,u+=g2 on Γ2,u+(0)=u+(1)=0. (3.1)

    In addition to the above problem we consider an auxiliary problem such that

    uxx=0onΩ=(Γ1,Γ2),u=g1 on Γ1,u=g2 on Γ2. (3.2)

    From here on we call u=u+u the exact solution.

    The reason to consider the auxiliary problem is that one wishes to solve the problem (3.1) on a non-matching grid with an extra cost of solving the problem in (3.2). The numerical method on non-matching grids will be called the immersed (interface) hybrid difference method. By following the argument in the previous section we are going to solve the following coupled problem instead of (3.1) and (3.2). Let uϵ=u+ϵuϵ be the solution of the coupled problem;

    Dxxu+ϵ=fin   Ω+, (3.3a)
    Dxxuϵ=0in   Ω, (3.3b)

    with the exterior boundary condition u+(0)=u+(1)=0, and the interface conditions

    u+ϵ=uϵ,ϵν+u+ϵ+ϵνuϵ=u+ϵ+g1on   Γ1, (3.4a)
    u+ϵ=uϵ,ϵνu+ϵ+ϵν+uϵ=u+ϵ+g2on   Γ2. (3.4b)

    Then the equations (3.3) with the interface conditions (3.4) can be solved by an immersed (interface) numerical methods. The advantage of this immersed approach will become more clearer for higher dimensional problems since the problem can be solved on a uniform mesh, which is independent of the geometric shape of an interface. In our approach the interior boundaries Γ1 and Γ2 are treated as immersed interface.

    For the sake of simplicity we use u for uϵ from here on as long as there is no danger of being confused. The total exact solution u=u+u satisfies

    u(x)={u+(x),xΩ+u(x),xΩ.

    We are going to use the finite difference method. Then, the approximation

    Dhxxu2k1=u2k2u2k1+u2k2(hk1/2)2

    with hk1=η2kη2k2 is not appropriate for approximation of uxx on the interface cell since u is not smooth enough on the interface cell Ik1=[η2k2,η2k]. It is inevitable to consider extensions of u± on interface cells. The interface cell Ik1 is meant by the cell with which Ik1Ω+ and Ik1Ω.

    Now, we consider an extension ˜u of u on interface cells.

    ˜u(x)={˜u+(x),x[0,η2k],˜u(x),x[η2k2,η2l+2],˜u+(x),x[η2l,1],

    with reference to Fig. 2. Here, ˜u+=u+ on Ω+ and ˜u=u on Ω, and ˜u is multiply valued on the interface cells Ik1=[η2k2,η2k] and Il=[η2l,η2l+2]. Then, the finite differences based on ˜u± yield good approximations for derivatives as follows; on Ik1

    Dhxx˜u±2k1=˜u±2k2˜u±2k1+˜u±2k2(hk1/2)2,Dhx˜u±2k1=˜u±2k˜u±2k2hk1, (3.5a)
    hν+˜u±2k2=3˜u±2k24˜u±2k1+˜u±2khk1,hν˜u±2k=3˜u±2k4˜u±2k1+˜u±2k2hk1. (3.5b)

    on the interface cell Ik1. Here, the unit normal vector ν+ on the cell interface represents the normal vector from the neighboring right cell and ν represents that from the left cell. Notice that we use the notations, ν and ν+ for the normal derivatives on the genuine interface.

    Now, we consider how to obtain ˜u from u, that is, how to obtain the extended values {˜u2k2,˜u+2k1,˜u+2k} from the existing values {˜u+2k2,˜u2k1,˜u2k} (see Fig. 3). From here on, {˜u2k2,˜u+2k1,˜u+2k} are called the virtual values, and {˜u+2k2,˜u2k1,˜u2k} are called the real values. It is time to introduce a relation between the real and virtual values, which is called the virtual to real transformation (the VR-T). To obtain the VR-T we use the artificial interface conditions in (3.4) and the governing differential equation

    ˜u+(Γ1)=˜u(Γ1), (3.6a)
    ϵ˜u+x(Γ1)+˜u+(Γ1)=ϵ˜ux(Γ1)+g1, (3.6b)
    ˜u+xx(η2k1)=˜uxx(η2k1)+f2k1. (3.6c)
    Figure 3.  An illustration of the extended solutions ˜u±(x) in the interface cell Ik1: the solid lines are the original u±(x) and the dotted lines are extensions.

    To impose the conditions in (3.6) the Lagrange interpolations, ˜u=˜u2k2ϕ1+˜u2k1ϕ2+˜u2kϕ3 and ˜u+=˜u+2k2ϕ1+˜u+2k1ϕ2+˜u+2kϕ3 are used, where ϕ1,ϕ2,ϕ3P2(Ik)=span{1,x,x2} are the Lagrange basis. In the VR-T the first two conditions corresponds to the 1-d version of the interface conditions in (2.4), and the third condition is called the consistency condition, which is derived from the governing differential equation. In view of Fig. 3 the VR-T yields the following linear relation between the virtual values and the real values

    [˜u2k2,˜u+2k1,˜u+2k]T=M[˜u+2k2,˜u2k1,˜u2k]T+L[0,g1,f2k1]T.

    Then, the virtual values in the finite differences (3.5) can be rewritten in terms of all real values via the VR-T. For example, let us look at Dhxx˜u2k1, hν+˜u+2k2 and hν˜u2k in (3.5a) and (3.5b). The finite differences in real values are obtained as follows.

    Dhxx˜u2k1=˜u2k2˜u2k1+˜u2k2(hk1/2)2=(1+m13)˜u2k(2m12)˜u2k1+m11˜u+2k2(hk1/2)2l12g1+l13f2k1(hk1/2)2,hν+˜u+2k2=3˜u+2k24˜u+2k1+˜u+2khk1=(34m21+m31)˜u+2k2+(4m22+m32)˜u2k1+(4m23+m33)˜u2khk1+(4l22+l32)g1+(4l23+l33)f2k1hk1hν˜u2k=3˜u2k4˜u2k1+˜u2k2hk1=(3+m13)˜u2k+(4+m12)˜u2k1+m11˜u+2k2hk1+l12g1+l13f2k1hk1,

    where mij and lij are the (i,j) components of matrices M and L, respectively.

    Theorem 3.1. Let Γ1[η2k2,η2k] and δ=2Γ1η2k1hk1. The matrix M for the Dirichlet condition is nonsingular if |δ3δ|4ϵhk1, where ϵ is the penalty parameter, 0<ϵ<1.

    Proof. The matrix M is derived by solving the homogeneous jump and consistency conditions, that are,

    [[u]]Γ1=0,ϵ[[ux]]Γ1=u+(Γ1),[[uxx]]η2k1=0. (3.7)

    Consider the function d(x)=u+(x)u(x). Using dP2(Ik1), the conditions in (3.7) yield that dxx=0, dx=K and d=K(xΓ1) for a constant K=u+(Γ1)ϵ on Ik1.

    To show that M is an injection let us assume that the virtual values are zero so that u2k2=u+2k1=u+2k=0 (that is, 1<δ0 with Γ1=η2k1+δhk12 as in Fig. 3). Then, d=K(xΓ1) yields that

    u+2k2=K(η2k2Γ1), u2k1=K(η2k1Γ1), u2k=K(η2kΓ1).

    The Taylor expansion yields that

    u+(Γ1)=u+2k1+δhk12Dxu+2k1+δ2h2k18Dxxu+2k1=δhk12K(η2k2Γ1)hk1+δ2h2k18K(η2k2Γ1)(hk1/2)2=δ2δ2K(η2k2Γ1)=δ(1δ)(1+δ)4Khk1

    where δ=Γ1η2k1hk1/2. The condition, K=u+(Γ1)ϵ yields that K(hk1(δδ3)+4ϵ)=0. If δδ34ϵhk1, K=0. Then, the real values satisfy u+2k2=u2k1=u2k=0, and M is injective.

    Now, let us consider the other case so that the virtual values are {u2k2,u2k1,u+2k} (that is, 0δ<1). Let u2k2=u2k1=u+2k=0. Then, d=K(xΓ1) yields that

    u+2k2=K(η2k2Γ1), u+2k1=K(η2k1Γ1), u2k=K(η2kΓ1).

    The Taylor expansion yields that

    u(Γ1)=u2k1+δhk12Dxu2k1+δ2h2k18Dxxu2k1=δhk12K(η2kΓ1)hk1δ2h2k18K(η2kΓ1)(hk1/2)2=K2(δ+δ2)(η2kΓ1)=Khk14(δδ3),

    where δ=Γ1η2k1hk1/2. The condition, K=u(Γ1)ϵ yields that K(hk1(δδ3)+4ϵ)=0. Then, M is injective under the assumption.

    Theorem 3.2. Let Γ1[η2k2,η2k] and δ=2Γ1η2k1hk1. The matrix M for the Neumann condition is nonsingular with the penalty parameter 0<ϵ<1.

    Proof. The matrix M is obtained by solving

    [[u]]Γ1=0,(u+xϵux)(Γ1)=0,(u+xxϵuxx)(η2k1)=0. (3.8)

    Let us consider d(x)=u+(x)ϵu(x). Using dP2(Ik1), the conditions in (3.4) yield that dxx=0, dx=0, hence u+=ϵu+K for some constant K. Now, using (u+u)(Γ1)=0 we have u+(Γ1)=u(Γ1)=K1ϵ.

    To show that M is an injection let us assume that the virtual values are zero so that u2k2=u+2k1=u+2k=0 (that is, 1<δ0 with Γ1=η2k1+δhk12 as in Fig. 3). Then,

    u+2k2=K, u2k1=Kϵ, u2k=Kϵ.

    The Taylor expansion yields that

    u+(Γ1)=u+2k1+δhk12Dxu+2k1+δ2h2k18Dxxu+2k1=δhk12Khk1+δ2h2k18K(hk1/2)2=K2(δ2δ)

    Using u+(Γ1)=K1ϵ, we have K(δ2δ21ϵ)=0. Since δ2δ21ϵ0, K=0, which implies u+2k2=u2k1=u2k=0. The injectiveness of M is proved. The proof for the remaining case, 0δ<1, can be obtained by a similar way.

    Remark 1. In our approach we use the penalty parameter ϵ=hα with α>1. Therefore, the case |δ3δ|=4hα1 (for Theorem 3.1) can be avoided if the genuine interface Γ1 is not located somewhere very close to the cell center or the cell interfaces.

    Now, let us introduce the hybrid difference method. Let U be the approximate solution of ˜uϵ and U+ and U are those of ˜u+ϵ and ˜uϵ by the immersed hybrid difference method. Then the hybrid difference method can written as follows. The PDE (3.1) at the cell interior point and the flux continuity at the intercell points derives the following hybrid difference equations:

    DhxxU+2j+1=f2j+1,IjΩ+, (3.9a)
    (hν+hν+)U2j+2=0, (3.9b)
    DhxxU2j+1=f2j+1,IjΩ, (3.10a)
    (hν+hν+)U+2j=0, (3.10b)

    and on the interface cell Ik1 and Il (see that η2k1Ω and η2l+1Ω+ in Fig. 2),

    DhxxU2k1=f2k1,DhxxU+2l+1=f2l+1. (3.11)

    Note that the intercell equations related to (3.11) are contained in (3.9b) and (3.10b). The VR-T does not appear explicitly in the hybrid difference formulation, and it is contained implicitly when evaluating the finite differences in the interface cells.

    Now we consider a higher order method based on the cell configuration in Fig. 4. It should be noted that the interior cell points {η4k+1,η4k+2} of the cell [η4k,η4k+3] are the Gaussian points.

    Figure 4.  The P3-interface cell configuration. The cell interior points are Gaussian points for the interval [η4k,η4k+3].

    In this case we have the four real and four virtual values in the interface cell. The real values are {U4k,U4k+1,U+4k+2,U+4k+3} and the virtual values are {U+4k,U+4k+1,U4k+2,U4k+3}. The VR-T is derived by solving the following four independent equations by considering U,U+P3(Ik)=span{1,x,x2,x3}.

    U+(Γ)=U(Γ)U+(Γ)+ϵU+x(Γ)=ϵUx(Γ)+gD(Γ)U+xx(η4k+1)=Uxx(η4k+1)+f4k+1U+xx(η4k+2)=Uxx(η4k+2)+f4k+2}

    For the Neumann condition we use the following VR-T,

    U+(Γ)=U(Γ) (3.12a)
    U+x(Γ)=ϵUx(Γ)+gN(Γ) (3.12b)
    U+xx(η4k+1)=ϵUxx(η4k+1)+f4k+1U+xx(η4k+2)=ϵUxx(η4k+2)+f4k+2.} (3.12c)

    For the Neumann VR-T we can use U+xx(η)=Uxx(η)+f instead of U+xx(η)=ϵUxx(η)+f since Dxxuϵ=0, and the former makes the convergence behavior more regular according to our numerical test in Section 5. For the Robin condition only the equation (3.12b) will be changed to

    U+x(Γ)+λU(Γ)=ϵUx(Γ)+gR(Γ).

    In this section the one dimensional immersed method is revised to accommodate the two dimensional case. Since the immersed hybrid difference method for the non-interface cell is trivial, we mainly discuss on the IHD on interface cells and related two dimensional VR transformation.

    The hybrid difference method for the equation (1.1) is composed of two kinds of finite differences, the cell finite difference and the intercell finite difference as follows. With reference to the cell configuration in Fig. 5 the cell finite difference is

    ΔhU+(η4)=U+(η5)2U+(η4)+U+(η3)(h1/2)2U+(η8)2U+(η4)+U+(η1)(k1/2)2=f+(η4) (4.1)
    Figure 5.  The Q2-grid: |R1|=h1×k1, |R2|=h2×k1, |R3|=h1×k2.

    on the cell R1, and the interface finite differences are

    [[hνU+]]η5=3U+(η5)4U+(η4)+U+(η3)h1+3U+(η5)4U+(η6)+U+(η7)h2=0,[[hνU+]]η8=3U+(η8)4U+(η4)+U+(η1)k1+3U+(η8)4U+(η11)+U+(p13)k2=0. (4.2)

    In the above finite differences U+(η1) and U+(η3) are virtual values, and they should be replaced by the real values {U(η1),U(η3),U+(η4),U+(η5),U+(η8)} via the VR transformation.

    In Fig. 6, two grid types are displayed. The left one is the Q2-grid cell, the others are the Q3-grid cells. Note that the interior grid points are the Gauss points on the corresponding dashed line. If an interface passed through a cell, this cell is called the interface cell. The interface cells are divided into two types. It is called the coherent interface cell if the interface and the lines of integration (the red lines) intersect, and the incoherent interface cell has no intersection. In Fig. 6, the left and center ones are coherent and the right one is incoherent. In the IHD method, an incoherent cell is regarded as a regular cell which an interface does not pass through at all. Now we introduce two dimensional VR transformations for interface coherent cells. Firstly, we consider the constitutive equations on the Q2-coherent grid cell. Since the Q2-grid cell contains five real and virtual values, respectively, we need the constitutive equations composed of five equations as follows.

    Figure 6.  The coherent Q2-grid (left) and Q3-grid cells (center), and the incoherent interface cell (right). There is no rule for the choice of points τi, and a quasi-uniformly distributed set of points along the interface will be reasonable.

    The VR-T for the Dirichlet condition (2.3):

    U+(τ1)=U(τ1)U+(τ2)=U(τ2)} (4.3a)
    U+(τ1)+ϵν+U+(τ1)=ϵνU(τ1)+gD(τ1)U+(τ2)+ϵν+U+(τ2)=ϵνU(τ2)+gD(τ2)} (4.3b)
    ΔU+(η3)=ΔU(η3)+f(η3):consistency

    The VR-T for the Neumann condition (2.11):

    U+(τ1)=U(τ1)U+(τ2)=U(τ2)} (4.4a)
    ν+U+(τ1)=ϵνU(τ1)+gN(τ1)ν+U+(τ2)=ϵνU(τ2)+gN(τ2)}ΔU+(η3)=.(ϵU)(η3)+f(η3):consistency (4.4b)

    These are simple and natural generalizations of the one dimensional VR transformations.

    Since there are twelve real and virtual variables, respectively, (see, the center in Fig. 6) in the coherent Q3-cell grid, we need a set of 12-constitutive equations as follows.

    ν+U+(τ1)=ϵνU(τ1)+gN(τ1)ν+U+(τ2)=ϵνU(τ2)+gN(τ2)}ΔU+(η3)=.(ϵU)(η3)+f(η3):consistency (4.4b)

    Here, p=τ1,τ2,τ3,τ4 and q=η4,η5,η8,η9. The conditions (4.5) and (4.6) are used to derive the VR transformations for numerical computation in Section 5.

    In order to define the VR transformation, the number of equations required is ((k+1)24) which is equal to the number of grid points in a Qk-grid cell. It is natural to consider for the 4(k1) distinct interface points and the (k1)2 interior Gaussian points of a cell.

    In order to impose the constitutive equations, we need binomial spaces for U and U+ satisfying the following properties:

    (P1): The degree of freedom of Qk(R)=(k+1)24.

    (P2): Pk(R)Qk(R)Qk(R)

    (P3): Qk2(R)ΔQk(R).

    Here, Qk(R) is the space of binomials of degree k for each variable, and Pk(R) is that of the total degree k. In order to obtain the optimal convergence rate, the condition (P2) is required. The condition (P3) guarantees independent consistency equations. Since U and νU become functions which have the number of degrees of freedom 2k and (2k1), respectively, in the corresponding variable on a slanted interface, the (2k2)-collocations on U and νU on the interface are independent. If the interface is parallel to the coordinate axes then both U and νU degenerate to polynomials of degree k. Since 2k2k+1 for k3, they are independent only when k3.

    The following two binomial spaces are likely to be most commonly used,

    Q2(R)=span{1,x,y,x2,y2},

    Q3(R)=span{1,x,y,x2,xy,y2,x3,x2y,xy2,y3,x3y,xy3}.

    It is easy to see that Q3 satisfy the properties (P1)–(P3) and Q2 satisfies P1(R)Q2(R) instead of (P2).

    Remark 2. Let the extended solutions satisfy ˜u±Ck+1(R) for an interface cell R with a smooth enough interface. Then,

    infU±Qk(R)˜u±U±,RChk+1u±k+1,,R,

    for k3, and

    infU±Q2(R)˜u±U±,RCh2u±2,,R.

    In this section, we present numerical experiments in one and two dimensions to illustrate efficiency of the IHD method. We also performed numerical experiments by changing α which determines the penalty parameter ϵ (see, Remark 1).

    We perform numerical tests for a one-dimensional problem.

    Ex. 1. Let us consider the differential equation, uxx=f on Ω+=[0,47][57,1], and the exact solution satisfies

    u(x)={ex1,0x47,sin(πx),57<x1.

    Then, the extended solution satisfies u(x)=(e471)(57x)+sin(57π)(7x4) on [47,57].

    The immersed boundary value problem solves the extended problem,

    {Dxxu+ϵ=ex,0x47,Dxxu+ϵ=π2sin(πx),57x1,Dxxuϵ=0,47<x<57,

    with u(0)=u(1)=0. The interface conditions are imposed as

    u+ϵ=uϵ,u+ϵ+ϵν+u+ϵ=ϵνuϵ+e471 at x=47,u+ϵ=uϵ,u+ϵ+ϵν+u+ϵ=ϵνuϵ+sin(57π) at x=57,

    for the Dirichlet condition, and the interface conditions for the Neumann condition are

    u+ϵ=uϵ,ν+u+ϵ=ϵνuϵ+e47 at x=47,u+ϵ=uϵ,ν+u+ϵ=ϵνuϵπcos(57π) at x=57.

    The penalty parameter is taken to be ϵ=hα with α=1,...,4.

    The computation is made on the cell configuration in Fig. 4. According to the standard theory of approximation the best possible order of convergence is of O(h4+hα) when uC4(Ω+) with ϵ=hα. Here, h represents the scale of a mesh. Fig. 7 shows that all the numerical results for the Dirichlet condition concord well with the expected convergence. For the Neumann problem convergence behaves a bit erratically, and convergence stays around O(h3) while the expected convergence is of O(h4) when α=4 (see Fig. 8). It is interesting to see the scaled condition number ((condition number)×h3) stays constant with respect to the penalty parameter ϵ=hα. It is clear that the condition number for the VR-T heavily dependent on the penalty parameter ϵ, however, this ill-conditioning in the VR-T does not affect on the condition number of the global stiffness system.

    Figure 7.  Numerical experiments for the 1-d Dirichlet condition on the P3-grid. The first two figures represent error and convergence rate, respectively, and the right represents the scaled condition number ((Condition Number)×h3).
    Figure 8.  Numerical results for the 1-d Neumann condition on the P3-grid. The scaled condition numbers are almost coincident.

    We provide two-dimensional numerical examples for the Poisson equation on three multiply connected domains with a curved inner boundary. Numerical experiments were performed only on the Q3-grids with the uniform mesh grids. The errors are measured in the discrete L2-norm,

    u2Lh2=RThηijR|u(ηij)|2

    for a mesh Th of Ω=Ω+Ω. Fig. 9 represents three different immersed boundaries and corresponding mesh grids that are used in our computation. Since the inner boundary is treated as an interface in our approach we call it frequently as interface. A mesh grid is called interface coherent if all interface cells are coherent and it is called interface incoherent if there is at least one incoherent interface cell. The incoherent interface cells are treated as a regular cell in our computation.

    Figure 9.  The three different problem domains with the Q3-grid. Grid 1 (left, circle), Grid 2 (middle, rotated ellipse) and Grid 3 (right, five-leafed flower).

    In the immersed methods one wishes to use a fixed mesh (usually, the uniform mesh), independently of the shape of interface or immersed boundary. Then, it is inevitable to have incoherent interface cells for curved interfaces. Let us introduce a measure that indicates the portion of coherent interface cells among all interface cells, which is called the coherence ratio:

    coherence ratio=#(coherent interface cells)#(all interface cells).

    In the Fig. 9 the immersed boundaries satisfy

    x2+y2=0.32,(xy)20.92+(x+y)20.12=1,r=0.5+0.1cos(θ),

    respectively. The the five-leafed flower is expressed in the polar coordinates.

    Fig. 10 represents the coherence ratio for each immersed boundary for the mesh configurations in Fig. 9. The rotated ellipse has the least coherence ratio among them. The five-leafed flower has the less oscillatory coherence ratio.

    Figure 10.  Coherence ratio for the uniform Q3-grids with the circular interface(red), elliptic interface (blue) and flower-like interface (black). The horizontal axis represents the number of subdivisions for each axis of the computational domai.

    Ex. 2. We consider the Poisson equation on the multiply connected domains as shown in Fig. 9.

    Δu=2onΩ+

    where the exact solution is given as u(x,y)=excos(y)+x2 on the square [1,1]×[1,1]. The Dirichlet and Neumann condition are considered. As shown in Section 2 we solve the extended problem

    Δu+ϵ=2in   Ω+,Δuϵ=0in   Ω,

    with the interface conditions

    u+ϵ=uϵ,u+ϵ+ϵν+u+ϵ=ϵνuϵ+gDon   Γ (Dirichlet),u+ϵ=uϵ,ν+u+ϵ=ϵνuϵ+gNon   Γ (Neumann),

    with gD=u|Γ and gN=ν+u|Γ by varying ϵ=hα, α=1,...,4. The computation is performed on the (quasi-) uniform Q3-mesh grid.

    Fig. 11 represents numerical results for the Dirichlet problem on the domain [1,1]×[1,1]D, where D is a disk with radius 0.3, and Fig. 12 represents that for the Neumann condition on the same domain. Both show the optimally expected convergence, O(hα+h4). Unlikely to the one dimensional case the Neumann condition also achieves the optimal convergence.

    Figure 11.  The L2 errors and convergence rates for the Dirichlet condition on the circle of radius 0.3 with the uniform Q3-grid(Grid 1).
    Figure 12.  The L2 errors and convergence rates for the Neumann condition on the circle of radius 0.3 with the uniform Q3-grid(Grid 1).

    Fig. 13 represents the convergence history for the Dirichlet problem on the domain with the rotated-ellipse inner boundary. Even though the coherence rate is relatively lower than those of others, convergence did not seem to be influenced much by the lower coherence ratio. Fig. 14 represents the convergence history for the five-leafed flower. The boundary is non-convex and a poorer or oscillatory convergence is observed with the uniform mesh. Therefore, we adopt a quasi-uniform mesh, which is obtained by modifying the uniform mesh slightly on the places that the interface becomes horizontally or vertically flat. By doing that we can reduce the number of interface cells with reentrant interface boundary, otherwise cells with reentrant interface boundary are recognized as interface incoherent cells in our programing. The cell with reentrant interface is meant by that interface comes in and goes out of a cell through the same edge of a cell. For convex interfaces as in the first two figures in Fig. 9 the four interface collocation points as in the middle of Fig. 6 are used to imposed artificial interface conditions. For the five-leaped flower case we picked two sets of collocation points, each of which are consisting of four collocation points. One set is used to impose [[U]]=0, and the other set is used for the remaining interface condition. This seems to enhance the stability of the VR transformation. The convergences are optimal, however it is much lower than the theoretically expected one when α=4. The bad conditioning by a very small penalty parameter in the VR transformation and non-convexity of the domain seems to have some negative influence in convergence.

    Figure 13.  The L2 errors and convergence rates for the Dirichlet condition on the rotated ellipse with the uniform Q3-grid(Grid 2).
    Figure 14.  The L2 errors and convergence rates for the Dirichlet condition on the five-leafed flower with the quasi-uniform Q3-grid(Grid 3).

    The condition number of the VR transformation increases in the order of O(hα) as h decreases, however the (scaled) condition number for the global stiffness system is independent of the penalty parameter hα as mentioned in the one-dimensional case.

    According to our numerical experiments non-convex interface boundaries induce some difficulties in mesh generation if one wish to reduce the number of cells with reentrant interface, which is an important part for a more regular convergence of numerical solutions. It is intuitively trivial to see that the higher degree methods yields higher coherence ratios, hence it is desired to use at least the Q3-IHD method. Moreover, complexity of the boundary shapes seems to have more influence on convergence than the coherence ratio.

    In this paper the immersed hybrid difference method for elliptic boundary value problems on multiply connected domains is presented. The artificial interface conditions combined with the virtual to real transformation introduced in [14] enable us to solve the boundary value problems on general rectangular meshes which are constructed independently of the geometric shape of the boundaries. This approach dramatically reduces mesh generation efforts when a problem domain has a boundary of complicated shapes. Although we need additional degrees of freedom due to the approximation in Ω, the additional computational cost can be negligible when the size of Ω is relatively small, compared with that of Ω+. The numerical results indicate that the IHD method can be promising in handling problems on domains with a complicatedly shaped boundary. The optimal rate of convergence can be achieved when the parameter α is chosen properly.

    We would like to express sincere thanks to anonymous referees, whose invaluable comments improved the quality of the paper a lot.



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