
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
[1] | Youngmok Jeon, Dongwook Shin . Immersed hybrid difference methods for elliptic boundary value problems by artificial interface conditions. Electronic Research Archive, 2021, 29(5): 3361-3382. doi: 10.3934/era.2021043 |
[2] | Yijun Chen, Yaning Xie . A kernel-free boundary integral method for reaction-diffusion equations. Electronic Research Archive, 2025, 33(2): 556-581. doi: 10.3934/era.2025026 |
[3] | Derrick Jones, Xu Zhang . A conforming-nonconforming mixed immersed finite element method for unsteady Stokes equations with moving interfaces. Electronic Research Archive, 2021, 29(5): 3171-3191. doi: 10.3934/era.2021032 |
[4] | Yu Lei, Zhi Su, Chao Cheng . Virtual reality in human-robot interaction: Challenges and benefits. Electronic Research Archive, 2023, 31(5): 2374-2408. doi: 10.3934/era.2023121 |
[5] | Yujie Wang, Enxi Zheng, Wenyan Wang . A hybrid method for the interior inverse scattering problem. Electronic Research Archive, 2023, 31(6): 3322-3342. doi: 10.3934/era.2023168 |
[6] | Jon Johnsen . Well-posed final value problems and Duhamel's formula for coercive Lax–Milgram operators. Electronic Research Archive, 2019, 27(0): 20-36. doi: 10.3934/era.2019008 |
[7] | Guanrong Li, Yanping Chen, Yunqing Huang . A hybridized weak Galerkin finite element scheme for general second-order elliptic problems. Electronic Research Archive, 2020, 28(2): 821-836. doi: 10.3934/era.2020042 |
[8] | Hongsong Feng, Shan Zhao . A multigrid based finite difference method for solving parabolic interface problem. Electronic Research Archive, 2021, 29(5): 3141-3170. doi: 10.3934/era.2021031 |
[9] | Abdeljabbar Ghanmi, Hadeel Z. Alzumi, Noureddine Zeddini . A sub-super solution method to continuous weak solutions for a semilinear elliptic boundary value problems on bounded and unbounded domains. Electronic Research Archive, 2024, 32(6): 3742-3757. doi: 10.3934/era.2024170 |
[10] | Yiyuan Qian, Haiming Song, Xiaoshen Wang, Kai Zhang . Primal-dual active-set method for solving the unilateral pricing problem of American better-of options on two assets. Electronic Research Archive, 2022, 30(1): 90-115. doi: 10.3934/era.2022005 |
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+μ∂ν+u=0on Γ, | (1.1b) |
u=0on Γ0. | (1.1c) |
Referring to Fig. 1 let
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
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
The immersed hybrid difference method is consisting of two kinds of finite differences. Let us consider a decomposition of the domain into rectangular cells,
−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+∂ν′u|e=0on e=∂R∩∂R′. | (1.3) |
The continuity of
The paper is organized as follows. In §2 the artificial interface conditions are derived, which will replace the boundary conditions on
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
−Δu−=0in Ω−, | (2.2a) |
u−=u+on Γ, | (2.2b) |
one obtains a solution
u(x)={u+(x),x∈Ω+,u−(x),x∈Ω−. |
By the theory of elliptic operators in [26] we have the following lemmas.
Lemma 2.1. Suppose
‖∂ν−u−‖Hs−1/2(Γ)≤C‖gD‖Hs+1/2(Γ) |
with
Lemma 2.2. Suppose
‖∂ν+u+‖L2(Γ)≤C‖u+‖H1(Γ)+C‖u+‖H1(Ω+)+C‖f‖L2(Ω+), |
and
‖u+‖H1(Γ)≤C‖∂ν+u+‖L2(Γ)+C‖u+‖H1(Ω+)+C‖f‖L2(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),x∈Ω+,u−ϵ(x),x∈Ω−, |
be the solution of the coupled problem;
−Δu+ϵ=fin Ω+, | (2.3a) |
−Δu−ϵ=0in Ω−, | (2.3b) |
with the exterior boundary condition
u+ϵ=u−ϵ,ϵ∂ν+u+ϵ+ϵ∂ν−u−ϵ=−u+ϵ+gDon Γ. | (2.4) |
Here,
Let us introduce function spaces for the extended solution
Hk(Ω−∪Ω+)={u∈C(Ω):u±=u|Ω±∈Hk(Ω±)},Hk0(Ω−∪Ω+)={u∈Hk(Ω−∪Ω+):u|Γ0=0}. |
The extended solution
(∇u−,∇v−)Ω−+(∇u+,∇v+)Ω+−⟨∂ν+u++∂ν−u−,v⟩Γ=(f,v+)Ω+. | (2.5) |
Moreover, the solution
(∇u−ϵ,∇v−)Ω−+(∇u+ϵ,∇v+)Ω+−⟨∂ν+u+ϵ+∂ν−u−ϵ,v⟩Γ,=(∇u−ϵ,∇v−)Ω−+(∇u+ϵ,∇v+)Ω++⟨1ϵ(uϵ−gD),v⟩Γ=(f,v+)Ω+. | (2.6) |
Subtracting (2.5) from (2.6) we have
(∇(uϵ−u),∇v)Ω+⟨1ϵ(uϵ−gD),v⟩Γ=−⟨∂ν+u++∂ν−u−,v⟩Γ. |
Taking
‖uϵ−gD‖L2(Γ)≲ϵ‖∂ν+u++∂ν−u−‖L2(Γ)≲ϵ(‖gD‖H1(Γ)+‖u+‖H1(Ω+)+‖f‖L2(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
∇⋅(ϵ∇u−)=0in Ω−, | (2.9a) |
u−=u+on Γ. | (2.9b) |
Then, the total solution
(ϵ∇u−,∇v−)Ω−+(∇u+,∇v+)Ω+−⟨gN+ϵ∂ν−u−,v⟩Γ=(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) |
−∇⋅(ϵ∇u−ϵ)=0in Ω−, | (2.11b) |
u+ϵ=0on Γ0, | (2.11c) |
with the interface conditions
u+ϵ=u−ϵ,∂ν+u+ϵ+ϵ∂ν−u−ϵ=gNon Γ. | (2.12) |
Then,
(ϵ∇u−ϵ,∇v−)Ω−+(∇u+ϵ,∇v+)Ω+−⟨gN,v⟩Γ=(f,v+)Ω+. | (2.13) |
Subtracting (2.13) from (2.10) one obtains that
(ϵ∇(u–u−ϵ),∇v−)Ω−+(∇(u+−u+ϵ),∇v+)Ω+=(ϵ∂ν−u−,v)Γ. | (2.14) |
Take
‖∇(u+−u+ϵ)‖2L2(Ω+)≲ϵ‖∂ν−u−‖H−1/2(Γ)‖v‖H1/2(Γ)≲ϵ‖u−‖H1(Ω−)‖u+−u+ϵ‖H1(Ω+). |
Notice that
‖∇(u+−u+ϵ)‖L2(Ω+)≲ϵ‖u−‖H1(Ω−). | (2.15) |
Let us consider the the problem with the Robin boundary condition on the interior boundary (we consider only the case
−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
∇⋅(ϵ∇u−)=0in Ω−, | (2.17a) |
u−=u+on Γ. | (2.17b) |
Then,
(ϵ∇u−,∇v−)Ω−+(∇u+,∇v+)Ω+−⟨gR−λu+ϵ∂ν−u−,v⟩Γ=(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+,−∇⋅(ϵ∇u−ϵ)=0in Ω−,u+ϵ=0on Γ0, |
with the interface conditions
u+ϵ=u−ϵ,∂ν+u+ϵ+λu+ϵ+ϵ∂ν−u−ϵ=gRon Γ. |
Then,
(ϵ∇u−ϵ,∇v−)Ω−+(∇u+ϵ,∇v+)Ω+−⟨gR−λu+ϵ,v⟩Γ=(f,v+)Ω+. | (2.19) |
Subtracting (2.19) from (2.18) one obtains that
(ϵ∇(u–u−ϵ),∇v−)Ω−+(∇(u+−u+ϵ),∇v+)Ω++⟨λ(u−uϵ),v⟩Γ=⟨ϵ∂ν−u−,v⟩Γ. |
By a similar argument as in the Neumann condition
‖∇(u+−u+ϵ)‖L2(Ω+)+λ‖u−uϵ‖2L2(Γ)≲ϵ‖u−‖H1(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
We consider the one dimensional elliptic Dirichlet condition on two disjoint intervals
−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
−u−xx=0onΩ−=(Γ1,Γ2),u−=g1 on Γ1,u−=g2 on Γ2. | (3.2) |
From here on we call
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
−Dxxu+ϵ=fin Ω+, | (3.3a) |
−Dxxu−ϵ=0in Ω−, | (3.3b) |
with the exterior boundary condition
u+ϵ=u−ϵ,ϵ∂ν+u+ϵ+ϵ∂ν−u−ϵ=−u+ϵ+g1on Γ1, | (3.4a) |
u+ϵ=u−ϵ,ϵ∂ν−u+ϵ+ϵ∂ν+u−ϵ=−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
For the sake of simplicity we use
u(x)={u+(x),x∈Ω+u−(x),x∈Ω−. |
We are going to use the finite difference method. Then, the approximation
Dhxxu2k−1=u2k−2u2k−1+u2k−2(hk−1/2)2 |
with
Now, we consider an extension
˜u(x)={˜u+(x),x∈[0,η2k],˜u−(x),x∈[η2k−2,η2l+2],˜u+(x),x∈[η2l,1], |
with reference to Fig. 2. Here,
Dhxx˜u±2k−1=˜u±2k−2˜u±2k−1+˜u±2k−2(hk−1/2)2,Dhx˜u±2k−1=˜u±2k−˜u±2k−2hk−1, | (3.5a) |
∂hν+˜u±2k−2=3˜u±2k−2−4˜u±2k−1+˜u±2khk−1,∂hν−˜u±2k=3˜u±2k−4˜u±2k−1+˜u±2k−2hk−1. | (3.5b) |
on the interface cell
Now, we consider how to obtain
˜u+(Γ1)=˜u−(Γ1), | (3.6a) |
ϵ˜u+x(Γ1)+˜u+(Γ1)=ϵ˜u−x(Γ1)+g1, | (3.6b) |
−˜u+xx(η2k−1)=−˜u−xx(η2k−1)+f2k−1. | (3.6c) |
To impose the conditions in (3.6) the Lagrange interpolations,
[˜u−2k−2,˜u+2k−1,˜u+2k]T=M[˜u+2k−2,˜u−2k−1,˜u−2k]T+L[0,g1,f2k−1]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˜u−2k−1=−˜u−2k−2˜u−2k−1+˜u−2k−2(hk−1/2)2=−(1+m13)˜u−2k−(2−m12)˜u−2k−1+m11˜u+2k−2(hk−1/2)2−l12g1+l13f2k−1(hk−1/2)2,∂hν+˜u+2k−2=3˜u+2k−2−4˜u+2k−1+˜u+2khk−1=(3−4m21+m31)˜u+2k−2+(−4m22+m32)˜u−2k−1+(−4m23+m33)˜u−2khk−1+(−4l22+l32)g1+(−4l23+l33)f2k−1hk−1∂hν−˜u−2k=3˜u−2k−4˜u−2k−1+˜u−2k−2hk−1=(3+m13)˜u−2k+(−4+m12)˜u−2k−1+m11˜u+2k−2hk−1+l12g1+l13f2k−1hk−1, |
where
Theorem 3.1. Let
Proof. The matrix
[[u]]Γ1=0,ϵ[[ux]]Γ1=−u+(Γ1),[[uxx]]η2k−1=0. | (3.7) |
Consider the function
To show that
u+2k−2=K(η2k−2−Γ1), u−2k−1=−K(η2k−1−Γ1), u−2k=−K(η2k−Γ1). |
The Taylor expansion yields that
u+(Γ1)=u+2k−1+δhk−12Dxu+2k−1+δ2h2k−18Dxxu+2k−1=−δhk−12K(η2k−2−Γ1)hk−1+δ2h2k−18K(η2k−2−Γ1)(hk−1/2)2=δ2−δ2K(η2k−2−Γ1)=δ(1−δ)(1+δ)4Khk−1 |
where
Now, let us consider the other case so that the virtual values are
u+2k−2=K(η2k−2−Γ1), u+2k−1=K(η2k−1−Γ1), u−2k=−K(η2k−Γ1). |
The Taylor expansion yields that
u−(Γ1)=u−2k−1+δhk−12Dxu−2k−1+δ2h2k−18Dxxu−2k−1=−δhk−12K(η2k−Γ1)hk−1−δ2h2k−18K(η2k−Γ1)(hk−1/2)2=−K2(δ+δ2)(η2k−Γ1)=−Khk−14(δ−δ3), |
where
Theorem 3.2. Let
Proof. The matrix
[[u]]Γ1=0,(u+x−ϵu−x)(Γ1)=0,(u+xx−ϵu−xx)(η2k−1)=0. | (3.8) |
Let us consider
To show that
u+2k−2=K, u−2k−1=−Kϵ, u−2k=−Kϵ. |
The Taylor expansion yields that
u+(Γ1)=u+2k−1+δhk−12Dxu+2k−1+δ2h2k−18Dxxu+2k−1=−δhk−12Khk−1+δ2h2k−18K(hk−1/2)2=K2(δ2−δ) |
Using
Remark 1. In our approach we use the penalty parameter
Now, let us introduce the hybrid difference method. Let
−DhxxU+2j+1=f2j+1,Ij⊂Ω+, | (3.9a) |
(∂hν−+∂hν+)U−2j+2=0, | (3.9b) |
−DhxxU−2j+1=f2j+1,Ij⊂Ω−, | (3.10a) |
(∂hν−+∂hν+)U+2j=0, | (3.10b) |
and on the interface cell
−DhxxU−2k−1=f2k−1,−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
In this case we have the four real and four virtual values in the interface cell. The real values are
U+(Γ)=U−(Γ)U+(Γ)+ϵU+x(Γ)=ϵU−x(Γ)+gD(Γ)−U+xx(η4k+1)=−U−xx(η4k+1)+f4k+1−U+xx(η4k+2)=−U−xx(η4k+2)+f4k+2} |
For the Neumann condition we use the following VR-T,
U+(Γ)=U−(Γ) | (3.12a) |
U+x(Γ)=ϵU−x(Γ)+gN(Γ) | (3.12b) |
−U+xx(η4k+1)=−ϵU−xx(η4k+1)+f4k+1−U+xx(η4k+2)=−ϵU−xx(η4k+2)+f4k+2.} | (3.12c) |
For the Neumann VR-T we can use
U+x(Γ)+λU(Γ)=ϵU−x(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)2−U+(η8)−2U+(η4)+U+(η1)(k1/2)2=f+(η4) | (4.1) |
on the cell
[[∂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
In Fig. 6, two grid types are displayed. The left one is the
The VR-T for the Dirichlet condition (2.3):
U+(τ1)=U−(τ1)U+(τ2)=U−(τ2)} | (4.3a) |
U+(τ1)+ϵ∂ν+U+(τ1)=−ϵ∂ν−U−(τ1)+gD(τ1)U+(τ2)+ϵ∂ν+U+(τ2)=−ϵ∂ν−U−(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)=−ϵ∂ν−U−(τ1)+gN(τ1)∂ν+U+(τ2)=−ϵ∂ν−U−(τ2)+gN(τ2)}−ΔU+(η3)=−∇.(ϵ∇U−)(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
∂ν+U+(τ1)=−ϵ∂ν−U−(τ1)+gN(τ1)∂ν+U+(τ2)=−ϵ∂ν−U−(τ2)+gN(τ2)}−ΔU+(η3)=−∇.(ϵ∇U−)(η3)+f(η3):consistency | (4.4b) |
Here,
In order to define the VR transformation, the number of equations required is
In order to impose the constitutive equations, we need binomial spaces for
(P1): The degree of freedom of
(P2):
(P3):
Here,
The following two binomial spaces are likely to be most commonly used,
●
●
It is easy to see that
Remark 2. Let the extended solutions satisfy
infU±∈Q∗k(R)‖˜u±−U±‖∞,R≤Chk+1‖u±‖k+1,∞,R, |
for
infU±∈Q∗2(R)‖˜u±−U±‖∞,R≤Ch2‖u±‖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
We perform numerical tests for a one-dimensional problem.
Ex. 1. Let us consider the differential equation,
u(x)={ex−1,0≤x≤47,sin(πx),57<x≤1. |
Then, the extended solution satisfies
The immersed boundary value problem solves the extended problem,
{−Dxxu+ϵ=ex,0≤x≤47,−Dxxu+ϵ=π2sin(πx),57≤x≤1,−Dxxu−ϵ=0,47<x<57, |
with
u+ϵ=u−ϵ,u+ϵ+ϵ∂ν+u+ϵ=−ϵ∂ν−u−ϵ+e47−1 at x=47,u+ϵ=u−ϵ,u+ϵ+ϵ∂ν+u+ϵ=−ϵ∂ν−u−ϵ+sin(57π) at x=57, |
for the Dirichlet condition, and the interface conditions for the Neumann condition are
u+ϵ=u−ϵ,∂ν+u+ϵ=−ϵ∂ν−u−ϵ+e47 at x=47,u+ϵ=u−ϵ,∂ν+u+ϵ=−ϵ∂ν−u−ϵ−πcos(57π) at x=57. |
The penalty parameter is taken to be
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
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
‖u‖2Lh2=∑R∈Th∑ηij∈R|u(ηij)|2 |
for a mesh
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,(x−y)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.
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
−Δu+ϵ=−2in Ω+,−Δu−ϵ=0in Ω−, |
with the interface conditions
u+ϵ=u−ϵ,u+ϵ+ϵ∂ν+u+ϵ=−ϵ∂ν−u−ϵ+gDon Γ (Dirichlet),u+ϵ=u−ϵ,∂ν+u+ϵ=−ϵ∂ν−u−ϵ+gNon Γ (Neumann), |
with
Fig. 11 represents numerical results for the Dirichlet problem on the domain
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
The condition number of the VR transformation increases in the order of
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
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
We would like to express sincere thanks to anonymous referees, whose invaluable comments improved the quality of the paper a lot.
[1] |
On the implementation of mixed methods as nonconforming methods for second-order elliptic problems. Math. Comp. (1995) 64: 943-972. ![]() |
[2] |
Unified analysis of discontinuous Galerkin methods for elliptic problems. SIAM J. Numer. Anal. (2002) 39: 1749-1779. ![]() |
[3] |
High-order accurate discontinuous finite element solution of the 2D Euler equations. J. Comput. Phys. (1997) 138: 251-285. ![]() |
[4] | D. Braess, Finite Elements, Theory, Fast Solvers, and Applications in Solid Mechanics, 2nd edition, Cambridge University Press, 2001. |
[5] |
Isoparametric C0 interior penalty methods for plate bending problems on smooth domains. Calcolo (2013) 50: 35-67. ![]() |
[6] |
Two families of mixed finite elements for second order elliptic problems. Numer. Math. (1985) 47: 217-235. ![]() |
[7] |
Discontinuous Galerkin methods for first-order hyperbolic problems. Math. Models Methods Appl. Sci. (2004) 14: 1893-1903. ![]() |
[8] |
Quadratic immersed finite element spaces and their approximation capabilities. Adv. Comput. Math. (2006) 24: 81-112. ![]() |
[9] |
Bridging the hybrid high-order and hybridizable discontinuous Galerkin methods. ESAIM Math. Model. Numer. Anal. (2016) 50: 635-650. ![]() |
[10] |
Discontinuous Galerkin Methods for Friedrichs' systems. I. General theory. SIAM J. Numer. Anal. (2006) 44: 753-778. ![]() |
[11] |
A non-oscillatory Eulerian approach to interfaces in multimaterial flows (the ghost fluid method). J. Comput. Phys. (1999) 152: 457-492. ![]() |
[12] |
The ghost fluid method for deflagration and detonation discontinuities. J. Comput. Phys. (1999) 154: 393-427. ![]() |
[13] |
Partition of unity extension of functions on complex domains. J. Comput. Phys. (2018) 375: 57-79. ![]() |
[14] |
Y. Jeon, An immersed hybrid difference method for the elliptic interface equation, preprint. doi: 10.13140/RG.2.2.27746.58566
![]() |
[15] |
Hybrid difference methods for PDEs. J. Sci. Comput. (2015) 64: 508-521. ![]() |
[16] |
Hybrid spectral difference methods for elliptic equations on exterior domains with the discrete radial absorbing boundary condition. J. Sci. Comput. (2018) 75: 889-905. ![]() |
[17] |
Hybrid spectral difference methods for an elliptic equation. Comput. Methods Appl. Math. (2017) 17: 253-267. ![]() |
[18] | Y. Jeon and D. Sheen, Upwind hybrid spectral difference methods for steady-state Navier–Stokes equations, in Contemporary Computational Mathematics - A Celebration of the 80th Birthday of Ian Sloan, Springer International Publishing (eds. J. Dick, F.Y. Kuo and H. Woźniakowski), (2018), 621–644. |
[19] |
High-order accurate implementation of solid wall boundary conditions in curved geometries. J. Comput. Phys. (2006) 211: 492-512. ![]() |
[20] |
Compact finite difference schemes with spectral-like resolution. J. Comput. Phys. (1992) 103: 16-42. ![]() |
[21] |
The immersed interface method for elliptic equations with discontinuous coefficients and singular sources. SIAM J. Numer. Anal. (1994) 31: 1019-1044. ![]() |
[22] |
Immersed interface methods for Stokes flow with elastic boundaries or surface tension. SIAM J. Sci. Comput. (1997) 18: 709-735. ![]() |
[23] |
High order solution of Poisson problems with piecewise constant coefficients and interface jumps. J. Comput. Phys. (2017) 335: 497-515. ![]() |
[24] |
The fast solution of Poisson's and the biharmonic equations on irregular regions. SIAM J. Numer. Anal. (1984) 21: 285-299. ![]() |
[25] |
Fast high order accurate solution of Laplace's equation on irregular regions. SIAM J. Sci. Statist. Comput. (1985) 6: 144-157. ![]() |
[26] | (2000) Strongly Elliptic Systems and Boundary Integral Equations.Cambridge University Press. |
[27] |
The immersed boundary method. Acta Numer. (2002) 11: 479-517. ![]() |
[28] |
A sharp-interface active penalty method for the incompressible Navier–Stokes equations. J. Sci. Comput. (2015) 62: 53-77. ![]() |
[29] |
Immersed boundary smooth extension: A high-order method for solving PDE on arbitrary smooth domains using Fourier spectral methods. J. Comput. Phys. (2016) 304: 252-274. ![]() |
[30] |
Sixth order compact scheme combined with multigrid method and extrapolation technique for 2D Poisson equation. J. Comput. Phys. (2009) 228: 137-146. ![]() |
[31] |
The explicit-jump immersed interface method: Finite difference methods for PDEs with piecewise smooth solutions. SIAM J. Numer. Anal. (2000) 37: 827-862. ![]() |
[32] |
Y. Xie and W. Ying, A fourth-order kernel-free boundary integral method for implicitly defined surfaces in three space dimensions, J. Comput. Phys., 415 (2020), 109526, 29 pp. doi: 10.1016/j.jcp.2020.109526
![]() |
[33] |
A kernel-free boundary integral method for elliptic boundary value problems. J. Comput. Phys. (2007) 227: 1046-1074. ![]() |
[34] |
A kernel-free boundary integral method for implicitly defined surfaces. J. Comput. Phys. (2013) 252: 606-624. ![]() |
1. | Yoonjeong Choi, Gwanghyun Jo, Do Y. Kwak, Young Ju Lee, Locally conservative discontinuous bubble scheme for Darcy flow and its application to Hele-Shaw equation based on structured grids, 2023, 92, 1017-1398, 1127, 10.1007/s11075-022-01333-8 |