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New results for fractional ordinary differential equations in fuzzy metric space

  • In this paper, we primarily focused on the existence and uniqueness of the initial value problem for fractional order fuzzy ordinary differential equations in a fuzzy metric space. First, definitions and relevant properties of the Gamma function and Beta function within a fuzzy metric space were provided. Second, by employing the principle of fuzzy compression mapping and Choquet integral of fuzzy numerical functions, we established the existence and uniqueness of solutions to initial value problems for fuzzy ordinary differential equations. Finally, several examples were presented to demonstrate the validity of our obtained results.

    Citation: Li Chen, Suyun Wang, Yongjun Li, Jinying Wei. New results for fractional ordinary differential equations in fuzzy metric space[J]. AIMS Mathematics, 2024, 9(6): 13861-13873. doi: 10.3934/math.2024674

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  • In this paper, we primarily focused on the existence and uniqueness of the initial value problem for fractional order fuzzy ordinary differential equations in a fuzzy metric space. First, definitions and relevant properties of the Gamma function and Beta function within a fuzzy metric space were provided. Second, by employing the principle of fuzzy compression mapping and Choquet integral of fuzzy numerical functions, we established the existence and uniqueness of solutions to initial value problems for fuzzy ordinary differential equations. Finally, several examples were presented to demonstrate the validity of our obtained results.



    The Cahn-Hilliard-Hele-Shaw system is a very important mathematical model which describes the motion of a viscous incompressible fluid between two closely spaced parallel plates and can be viewed as the simplification of the Cahn-Hilliard-Navier-Stokes system [1,2,3]. The model are widely applied in different fields, such as simulations of nonlinear tumor growth and neovascularization [4,5,6,7], spinodal decomposition in a Hele-Shaw cell [8], and two-phase flow in porous medium [9,10], etc.

    The Cahn-Hilliard-Hele-Shaw system is a gradient system coupled with fluid motion, which is difficult to solve because of its complex form. For this model, purely explicit methods are limited by strict time step constraints for stability, and completely implicit numerical methods must contend with potentially large systems of nonlinear algebraic equations [11]. There have been many effective numerical schemes for the Cahn-Hilliard-Hele-Shaw system. Guo et al. proposed a semi-implicit time integration scheme based on convex splitting technique, and proved the unconditional stability of the fully discrete scheme of the Cahn-Hilliard-Hele-Shaw system [12]. S.M. Wise put forward an unconditionally stable finite difference scheme for the Cahn-Hilliard-Hele-Shaw [13]. Chen et al. established a finite difference simulation of Gagliardo-Nirenberg-type inequalities to analyze stability and convergence [14]. Liu et al. developed a mixed finite element numerical scheme for the Cahn-Hilliard-Hele-Shaw system and proved its unconditional stability [15]. Guo carried out a numerical analysis for the Cahn-Hilliard-Hele-Shaw system with variable mobility and logarithmic Flory-Huggins potential [16]. The above mentioned works are numerical methods to solve the Cahn-Hilliard-Hele-Shaw system. However, there are few researches on the modified Cahn-Hilliard-Hele-Shaw system.

    The modified Cahn-Hilliard equation (also named Cahn-Hilliard-Oono equation) used to suppress phase coarsening in [17] is as follows

    ϕt+Δ(εΔϕ1εf(ϕ))+θ(ϕˉϕ0)=0,xΩ,0<tT, (1.1)
    ϕn=(νΔϕf(ϕ))n,xΩ, (1.2)
    ϕ(x,0)=ϕ0(x),xΩ, (1.3)

    where ¯ϕ0:=1|Ω|Ωϕ0(x)dx. More works on the modified Cahn-Hilliard equation can be found in [18,19,20,21]. For the modified Cahn-Hilliard equation, when θ = 0, the equation becomes the classical Cahn-Hilliard equation[22,23,24]. When the modified Cahn-Hilliard is coupled with the Darcy equation, the modified Cahn-Hilliard-Hele-Shaw equation can be obtained. Jia et al. introduced a novel finite element method for the modified Cahn-Hilliard-Hele-Shaw system [25], in which the time discretization was based on the convex splitting of the energy functional in the modified Cahn-Hilliard equation. Of course, the above numerical methods are directly solved based on the coupling equation, and the solving process is complicated. To solve this kind of problem, many decoupled methods have been proposed to solve the Cahn-Hilliard-Hele-Shaw system in recent years. Han [26] presented a decoupled unconditionally stable numerical scheme for the Cahn-Hilliard-Hele-Shaw system with variable viscosity, in which the operator-splitting strategy and the pressure-stabilization technique were used to completely decouple the nonlinear Cahn-Hilliard equation from pressure. Similar strategies were also adopted in [27]. Then, Gao [28] studied the fully decoupled numerical scheme of the Cahn-Hilliard-Hele-Shaw model, in which the scalar auxiliary variable method was used to deal with the nonlinear term in the free energy. Similarly, decoupled schemes are also effectively used in other systems and models recently. Zhao et al. [29] developed an energy-stable scheme for a binary hydrodynamic phase field model of mixtures of nematic liquid crystals and viscous fluids. A second-order decoupled energy-stable schemes for Cahn-Hilliard-Navier-Stokes equations was suggested in [30]. For thermodynamically consistent models, Zhao [31] investigated a general numerical framework for designing linear, energy stable, and decoupled numerical algorithms. However, to the best of our knowledge, there are few researches on decoupling methods of the modified Cahn-Hilliard-Hele-Shaw system, it will be the purpose of our paper.

    Based on Eqs (1.1)-(1.3), the modified Cahn-Hilliard-Hele-Shaw system with double well potential is given by

    tϕ+(ϕu)=Δμ,inΩ×(0,T), (1.4)
    μ=f(ϕ)ε2Δϕ+ξ,inΩ×(0,T), (1.5)
    Δξ=θ(ϕ¯ϕ0),inΩ×(0,T), (1.6)
    u=(p+γϕμ),inΩ×(0,T), (1.7)
    u=0,inΩ×(0,T), (1.8)
    ϕ|t=0=ϕ0,inΩ, (1.9)
    nϕ=nμ=0,un=0,onΩ×(0,T). (1.10)

    where ΩRd(d=2,3). ϕ is the concentration field, u is the advective velocity, ε>0 is the constant to measure the thickness of the transition layer between the two phases, and μ is the chemical potential. f(ϕ) is the derivative of the double well potential F(ϕ), ξ is an auxiliary variable. p and γ represent the pressure and the dimensionless surface tension parameter, respectively. n is the unit outer normal of the boundary Ω. when θ = 0, the equation becomes the classical Cahn-Hilliard-Hele-Shaws equation. With regard to the double well potential corresponding to f(ϕ) in Eq (1.2), the following ˇF(ϕ) can be taken[32,33,34]

    ˇF(ϕ)=ˇF1(ϕ)+ˇF2(ϕ):=(ϕ2+14)+{2ϕ+34,ϕ1,32ϕ2+14ϕ4,ϕ[1,1],2ϕ+34,ϕ1. (1.11)

    Correspondingly, the derivatives of ˇF(ϕ) can be split as follows

    ˇf(ϕ)=ˇF(ϕ)=ˇf1(ϕ)+ˇf2(ϕ)=2ϕ+{2,ϕ1,3ϕ+ϕ3,ϕ[1,1],2,ϕ1. (1.12)

    F(ϕ) and f(ϕ) are replaced by ˇF(ϕ) and its derivative ˇf(ϕ), which are still recorded as F and f for simplicity. Typically, the free energy functional of a modified Cahn-Hilliard-Hele-Shaw system with double well potential is given by

    E(ϕ)=Ω(ε22|ϕ|2+F(ϕ))dx. (1.13)

    In this paper, a decoupled finite element scheme for a modified Cahn-Hilliard-Hele-Shaw system with double well potential is proposed. The temporal discretization is based on the convex splitting of the energy functional in the modified Cahn-Hilliard equation, and the spacial discretization is carried out by the mixed finite element method. The computation of the velocity u is separated from the computation of the pressure p by using an operator-splitting strategy. A Possion equation is solved to update the pressure at each time step. We prove that the the proposed scheme is unconditionally stable in energy, and the error analyses are obtained. Finally, the numerical results verify the theoretical analysis. The rest of this article is structured as follows.The finite element discrete scheme of the Cahn-Hilliard-Hele-Shaw system combing with the convex splitting is given in Section 2; The theoretical preparations and stability of the proposed numerical scheme are proved in Section 3; The error analyses of the proposed scheme are addressed in Section 4; Some numerical examples are given to verify the previous theory in Section 5, and the conclusion is given in Section 6.

    Let L2(Ω) is a space of square integrable function and Hk(Ω),Hk0(Ω) denote the usual Sobolev spaces. L2(Ω) inner product and its norm are denoted by (u,v)=Ωu(x)v(x)dx, ϕ=ϕL2(Ω)=(ϕ,ϕ). The weak formulation of the modified Cahn-Hilliard-Hele-Shaw system with double well potential can be written as

    {(tϕ,v)+((ϕu),v)+(μ,v)=0,vH1(Ω),(μ,w)(f1(ϕ)+f2(ϕ),w)ε2(ϕ,w)(ξ,w)=0,wH1(Ω),(ξ,ψ)θ(ϕ¯ϕ0,ψ)=0,ψH1(Ω),(p+γϕμ,q)=0.qH1(Ω). (2.1)

    where,

    f1(ϕ)=2ϕ,f1(ϕ)=2.f2(ϕ)={2,ϕ13ϕ+ϕ3,ϕ[1,1]2,ϕ1,f2(ϕ)=3(ϕ21)0.

    Let N be a positive integer and 0=t0<t1<<tN=T be a uniform partition of [0,T], where ti=iτ, i=0,1,,N1, τ=TN.

    The semi-discrete scheme of the modified Cahn-Hilliard-Hele-Shaw system with double well potential is as follows. For n0, find {ϕn+1,μn+1,ξn+1,pn+1} such that

    (ϕn+1ϕnτ,v)(ϕnun+1,v)+(μn+1,v)=0, (2.2)
    (μn+1,w)(f1(ϕn+1)+f2(ϕn),w)ε2(ϕn+1,w)(ξn+1,w)=0, (2.3)
    (ξn+1,ψ)θ(ϕn+1¯ϕ0,ψ)=0, (2.4)
    ((pn+1pn),q)=(un+1,q), (2.5)

    where the velocity is given by

    un+1=(pn+γϕnμn+1). (2.6)

    Combing with the idea of the literatures [26,35], the computation of the modified Cahn-Hilliard equations (2.2)-(2.4) are decoupled from Eq (2.5) after substituting un+1 into Eq (2.2), since the pressure is explicit in Eq (2.6). The velocity un+1 is regarded as an intermediate velocity by using the incremental projection method similar to the Navier-Stokes equation. The real velocity ˜un+1 is obtained from the intermediate velocity and satisfies

    ˜un+1un+1=(pn+1pn),˜un+1=0. (2.7)

    Then Eq (2.6) and Eq (2.7) are added together to obtain the original Eq (1.7). If the divergence operator is applied to both side of Eq (2.7), the real velocity ˜un+1 will vanished. We have

    (pn+1pn)=un+1. (2.8)

    Let Th={K} be a regular partition of the domain Ω that is divided into triangles with the size h=max0iNhi. Sh is a piecewise polynomial space, which is defined as

    Sh={υhC0(Ω)|υh|KPk(x,y),KTh}H1(Ω),

    where Pk(x,y) is a polynomial of degree at most r.

    Let us denote

    L20:={uL2(Ω)|(u,1)=0},ˆSh:=ShL20,
    ˆH1:=H1(Ω)L20,ˆH1:={vH1(Ω)|(v,1)=0}.

    The corresponding fully discrete scheme have the following expression, find {ϕn+1h,μn+1h,ξn+1h,pn+1h}Sh×Sh׈Sh׈Sh, such that

    (ϕn+1hϕnhτ,vh)(ϕnhun+1h,vh)+(μn+1h,vh)=0, (2.9)
    (μn+1h,wh)(f1(ϕn+1h)+f2(ϕnh),wh)ε2(ϕn+1h,wh)(ξn+1h,wh)=0, (2.10)
    (ξn+1h,ψh)θ(ϕn+1h¯ϕ0,ψh)=0, (2.11)
    ((pn+1hpnh),qh)=(un+1h,qh), (2.12)

    where the velocity is given by

    un+1h=(pnh+γϕnhμn+1h). (2.13)

    Definition 3.1. [36] The Ritz projection operator Rh(Ω): ϕH1(Ω)Sh satisfies

    ((Rhϕϕ),χ)=0,χSh,(Rhϕϕ,1)=0. (3.1)

    and have the following estimates,

    RhϕH1(Ω)CϕH1,ϕH1(Ω), (3.2)
    ϕRhϕ+hϕRhϕH1(Ω)Chq+1ϕHq+1,ϕHq+1(Ω). (3.3)

    Definition 3.2. [36] Define the operator Th:ˆH1ˆH1 through the following variational problems, given ζˆH1, find Th(ζ)ˆH1 such that

    (Th(ζ),χ)=(ζ,χ),χˆH1. (3.4)

    Lemma 3.1. [12,15] Let ζ,φˆH1 and set

    (ζ,φ)1,h:=(Th(ζ),Th(φ))=(ζ,Th(φ))=(Th(ζ),φ), (3.5)

    where (,)1,h defines an inner product on the ˆH1 and its corresponding H1 norm is written as

    ζ1,h=(ζ,ζ)1,h=sup0χˆH1(ζ,χ)χ. (3.6)

    Consequently, for χˆH1,ζˆH1,

    |(ζ,χ)|ζ1,hχ. (3.7)

    Furthermore, the following Poincareˊ inequalities holds,

    \begin{eqnarray} &\|\zeta\|_{-1, h}\leq C\|\zeta\|, \; \; \; \; \; \; \; \; \; \; \; \; \forall\zeta\in L^{2}_{0}.& \end{eqnarray} (3.8)

    Definition 3.3. [12,15] Define \mathbf{W}: = \{\mathbf{u}\in\mathbf{L}^{2}(\Omega)|(\mathbf{u}, \nabla q), \forall q\in H^{1}(\Omega)\} . The projection operator \mathcal{P}:\mathbf{w}\in \mathbf{L}^{2}(\Omega)\rightarrow \mathbf{W} is defined as

    \begin{eqnarray} &\mathcal{P}(\mathbf{w}) = \nabla p+\mathbf{w}, & \end{eqnarray} (3.9)

    where p\in\dot{H}^{1}: = \{\phi\in H^{1}(\Omega)|(\phi, 1) = 0\} is the unique solution to

    \begin{eqnarray} &(\nabla p+\mathbf{w}, \nabla q) = 0, \; \; \; \; \; \; \forall q\in{H}^{1}(\Omega).& \end{eqnarray} (3.10)

    Lemma 3.2. [12,15] Projection operator \mathcal{P} is linear and satisfies the following properties

    \begin{eqnarray} &(\mathcal{P}(\mathbf{w})-\mathbf{w}, \mathbf{v}) = 0, \; \; \; \; \; \forall \mathbf{v}\in\mathbf{W}, & \end{eqnarray} (3.11)

    and

    \begin{eqnarray} &\|\mathcal{P}(\mathbf{w})\|\leq\|\mathbf{w}\|.& \end{eqnarray} (3.12)

    Definition 3.4. [12,15] Define \mathbf{W}_{h}: = \{\mathbf{u}_{h}\in\mathbf{L}^{2}(\Omega)|(\mathbf{u}_{h}, \nabla q_{h}) = 0, \forall q_{h}\in S_{h}\} . The projection operator \mathcal{P}_{h}:\mathbf{w}\in\mathbf{L}^{2}(\Omega)\rightarrow \mathbf{W}_{h} is defined as

    \begin{eqnarray} &\mathcal{P}_{h}(\mathbf{w}) = \nabla p_{h}+\mathbf{w}, & \end{eqnarray} (3.13)

    where p_{h}\in\hat{S_{h}} is the unique solution to

    \begin{eqnarray} &(\nabla p_{h}+\mathbf{w}, \nabla q_{h}) = 0, \; \; \; \; \; \; \forall q_{h}\in\hat{S_{h}}(\Omega).& \end{eqnarray} (3.14)

    Lemma 3.3. [14,15] Projection operator \mathcal{P}_{h} is linear and satisfies the following properties

    \begin{eqnarray} &(\mathcal{P}_{h}(\mathbf{w})-\mathbf{w}, \mathbf{v}_{h}) = 0, \; \; \; \; \; \forall \mathbf{v}_{h}\in\mathbf{W}_{h}, & \end{eqnarray} (3.15)

    and

    \begin{eqnarray} &\|\mathcal{P}_{h}(\mathbf{w})\|\leq\|\mathbf{w}\|.& \end{eqnarray} (3.16)

    Lemma 3.4. [12,15] Suppose that \mathbf{w}\in\mathbf{H}^{q}(\Omega) with the compatible boundary condition \mathbf{w}\cdot\mathbf{n} = 0 on \partial\Omega and q\in\mathbf{H}^{q+1}(\Omega) , then

    \begin{eqnarray} &\|\mathcal{P}_{h}(\mathbf w)-\mathcal{P}(\mathbf{w})\| = \|\nabla(p-p_{h})\|\leq Ch^{q}|p|_{H^{q+1}}.& \end{eqnarray} (3.17)

    Theorem 3.1. Let \{\phi^{n+1}_{h}, \mu^{n+1}_{h}, p^{n+1}_{h}, \xi^{n+1}_{h}\} be the unique solution of Eqs (2.9-2.12). Define

    \begin{eqnarray} &\Xi(\phi^{n+1}_{h}): = E(\phi^{n+1}_{h})+\|\phi^{n+1}_{h}\|^{2}+\dfrac{\theta}{2}\|\phi^{n+1}_{h}-\overline{\phi}_{0}\|^{2}_{-1, h} +\dfrac{\tau}{2\gamma}\|\nabla p^{n+1}_{h}\|^{2}. \end{eqnarray} (3.18)

    Then for any h, \tau, \varepsilon > 0, n\geq0 , scheme (2.9)-(2.12) satisfies the following property,

    \begin{eqnarray} \begin{split} &\Xi(\phi^{n+1}_{h})+\tau\|\nabla\mu^{n+1}_{h}\|^{2}+\|\phi^{n+1}_{h}-\phi^{n}_{h}\|^{2} +\dfrac{\varepsilon^{2}}{2}\|\nabla\phi^{n+1}_{h}-\nabla\phi^{n}_{h}\|^{2}\\ &+\dfrac{\theta}{2}\|\phi^{n+1}_{h}-\phi^{n}_{h}\|^{2}_{-1, h}+\dfrac{\tau}{2\gamma}\|\mathbf{u}^{n+1}_{h}\|^{2}\leq\Xi(\phi^{n}_{h}). \end{split} \end{eqnarray} (3.19)

    Proof. Taking v_{h} = \tau\mu^{n+1}_{h} in Eq (2.9), one has

    \begin{eqnarray} &(\phi^{n+1}_{h}-\phi^{n}_{h}, \mu^{n+1}_{h})-\tau(\phi^{n}_{h}\mathbf{u}^{n+1}_{h}, \mu^{n+1}_{h})+\tau\|\nabla\mu^{n+1}_{h}\|^{2} = 0.& \end{eqnarray} (3.20)

    In Eq (2.10), f_{1}(\phi^{n+1}_{h}) = 2\phi^{n+1}_{h} , f_{2}(\phi^{n}_{h}) = (\phi^{n}_{h})^{3}-3\phi^{n}_{h} . For f_{2}(\phi^{n}_{h}) , through Taylor expansion

    F_{2}(\phi^{n+1}_{h}) = F_{2}(\phi^{n}_{h})+f_{2}(\phi^{n}_{h})(\phi^{n+1}_{h}-\phi^{n}_{h})+\dfrac{f_{2}'({\eta})}{2}(\phi^{n+1}_{h}-\phi^{n}_{h})^{2}.

    where \eta is a number between \phi^{n}_{h} and \phi^{n+1}_{h} , we have

    f_{2}(\phi^{n}_{h})(\phi^{n+1}_{h}-\phi^{n}_{h}) = (F_{2}(\phi^{n+1}_{h})-F_{2}(\phi^{n}_{h}), 1)-\dfrac{f_{2}'(\eta)}{2}(\phi^{n+1}_{h}-\phi^{n}_{h})^{2}.

    Then, choosing w_{h} = -(\phi^{n+1}_{h}-\phi^{n}_{h}) and using the fact that (a, a-b) = \dfrac{1}{2}[a^{2}-b^{2}+(a-b)^{2}] give

    \begin{eqnarray} \begin{split} &-(\mu^{n+1}_{h}, \phi^{n+1}_{h}-\phi^{n}_{h})+(\|\phi^{n+1}_{h}\|^{2}-\|\phi^{n}_{h}\|^{2}+\|\phi^{n+1}_{h}-\phi^{n}_{h}\|^{2}) +(F_{2}(\phi^{n+1}_{h})-F_{2}(\phi^{n}_{h}), 1)\\ &-\dfrac{f_{2}'(\eta)}{2}\|\phi^{n+1}_{h}-\phi^{n}_{h}\|^{2}+\dfrac{\varepsilon^{2}}{2}(\|\nabla\phi^{n+1}_{h}\|^{2}-\|\nabla\phi^{n}_{h}\|^{2}+\|\nabla\phi^{n+1}_{h} -\nabla\phi^{n}_{h}\|^{2})\\&+(\xi^{n+1}_{h}, \phi^{n+1}_{h}-\phi^{n}_{h}) = 0. \end{split} \end{eqnarray} (3.21)

    Replacing \psi_{h} by -T_{h}(\phi^{n+1}_{h}-\phi^{n}_{h}) in Eq (2.11). By Eq (3.1) in definition 3.1, Eq (3.5) in lemma 3.1 and (a, a-b) = \dfrac{1}{2}[a^{2}-b^{2}+(a-b)^{2}] , one obtains

    \begin{eqnarray} &-(\xi^{n+1}_{h}, \phi^{n+1}_{h}-\phi^{n}_{h})+\dfrac{\theta}{2}(\|\phi^{n+1}_{h}-\overline{\phi}_{0}\|^{2}_{-1, h}-\|\phi^{n}_{h}-\overline{\phi}_{0}\|^{2}_{-1, h} +\|\phi^{n+1}_{h}-\phi^{n}_{h}\|^{2}_{-1, h}) = 0.\; \; \; \; \; \; \; & \end{eqnarray} (3.22)

    Next, we take inner product of Eq (2.13) with \dfrac{\tau}{\gamma}\mathbf{u}^{n+1}_{h} to get

    \begin{eqnarray} &\dfrac{\tau}{\gamma}\|\mathbf{u}^{n+1}_{h}\|^{2}+\dfrac{\tau}{\gamma}(\nabla p^{n}_{h}, \mathbf{u}^{n+1}_{h}) = -\tau(\phi^{n}_{h}\mu^{n+1}_{h}, \mathbf{u}^{n+1}_{h}).& \end{eqnarray} (3.23)

    Now, taking q_{h} = \dfrac{\tau}{\gamma}p^{n}_{h} and using the fact that (a-b, 2b) = a^{2}-b^{2}-(a-b)^{2} in Eq (2.12), we arrived at

    \begin{eqnarray} &\dfrac{\tau}{2\gamma}(\|\nabla p^{n+1}_{h}\|^{2}-\|\nabla p^{n}_{h}\|^{2}-\|\nabla p^{n+1}_{h}-\nabla p^{n}_{h}\|^{2}) = \dfrac{\tau}{\gamma}(\mathbf{u}^{n+1}_{h}, \nabla p^{n}_{h}).& \end{eqnarray} (3.24)

    To deal with the \dfrac{\tau}{2\gamma}\|\nabla p^{n+1}_{h}-\nabla p^{n}_{h}\|^{2} in Eq (3.24), replacing q_{h} with (p^{n+1}_{h}-p^{n}_{h}) in Eq (2.12) and using Cauchy-Schwarz inequalities, the following estimation can be obtained

    \begin{eqnarray} &\|\nabla p^{n+1}_{h}-\nabla p^{n}_{h}\|^{2}\leq\|\mathbf{u}^{n+1}_{h}\|^{2}.& \end{eqnarray} (3.25)

    Combining Eqs (3.23)-(3.25), it can be written as

    \begin{eqnarray} &\dfrac{\tau}{2\gamma}\|\mathbf{u}^{n+1}_{h}\|^{2}+\dfrac{\tau}{2\gamma}(\|\nabla p^{n+1}_{h}\|^{2}-\|\nabla p^{n}_{h}\|^{2}) = -\tau(\phi^{n}_{h}\mu^{n+1}_{h}, \mathbf{u}^{n+1}_{h}).& \end{eqnarray} (3.26)

    Summing Eqs (3.20)-(3.26), one concludes that

    \begin{eqnarray} \begin{split} &\tau\|\nabla\mu^{n+1}_{h}\|^{2}+(\|\phi^{n+1}_{h}\|^{2}-\|\phi^{n}_{h}\|^{2}+\|\phi^{n+1}_{h}-\phi^{n}_{h}\|^{2}) +(F_{2}(\phi^{n+1}_{h})-F_{2}(\phi^{n}_{h}), 1)\\ &-\dfrac{f_{2}'(\eta)}{2}\|\phi^{n+1}_{h}-\phi^{n}_{h}\|^{2}+\dfrac{\varepsilon^{2}}{2}(\|\nabla\phi^{n+1}_{h}\|^{2}-\|\nabla\phi^{n}_{h}\|^{2}+\|\nabla\phi^{n+1}_{h} -\nabla\phi^{n}_{h}\|^{2})\\ &+\dfrac{\theta}{2}(\|\phi^{n+1}_{h}-\overline{\phi}_{0}\|^{2}_{-1, h}-\|\phi^{n}_{h}-\overline{\phi}_{0}\|^{2}_{-1, h} +\|\phi^{n+1}_{h}-\phi^{n}_{h}\|^{2}_{-1, h})+\dfrac{\tau}{2\gamma}\|\mathbf{u}^{n+1}_{h}\|^{2}\\ &+\dfrac{\tau}{2\gamma}(\|\nabla p^{n+1}_{h}\|^{2}-\|\nabla p^{n}_{h}\|^{2}) = 0. \end{split} \end{eqnarray} (3.27)

    Since f_{2}'(\phi) = 3(\phi^{2}-1)\leq0 , \phi\in[-1, 1] , there is \dfrac{f_{2}'(\eta)}{2}\|\phi^{n+1}_{h}-\phi^{n}_{h}\|^{2}\leq0 by Taylor expansion. Therefore,

    \begin{eqnarray} \begin{split} &\Xi(\phi^{n+1}_{h})-\Xi(\phi^{n}_{h})+\tau\|\nabla\mu^{n+1}_{h}\|^{2}+\|\phi^{n+1}_{h}-\phi^{n}_{h}\|^{2}+\dfrac{\varepsilon^{2}}{2} \|\nabla\phi^{n+1}_{h} -\nabla\phi^{n}_{h}\|^{2}\\ &+\dfrac{\theta}{2}\|\phi^{n+1}_{h}-\phi^{n}_{h}\|^{2}_{-1, h}+\dfrac{\tau}{2\gamma}\|\mathbf{u}^{n+1}_{h}\|^{2}\leq0. \end{split} \end{eqnarray} (3.28)

    The proof is completed.

    Corollary 3.1. Suppose that \Xi({\phi^{0}_{h}})\leq C_{0} , there is a constant C > 0 independent of \tau and h , such that the following estimates hold for any \tau, h > 0 ,

    \begin{eqnarray} &\max\limits_{0\leq n\leq N}(\|\nabla\phi^{n+1}_{h}\|^{2}+\|\phi^{n+1}_{h}\|^{2}+\|\phi^{n+1}_{h}-\overline{\phi}_{0}\|^{2}_{-1, h})\leq C, \end{eqnarray} (3.29)
    \begin{eqnarray} &\max\limits_{0\leq n\leq N}\|\nabla p^{n+1}_{h}\|^{2}\leq C, \end{eqnarray} (3.30)
    \begin{eqnarray} &\sum\limits_{i = 0}^{N}(\|\phi^{i+1}_{h}-\phi^{i}_{h}\|^{2}+\|\nabla\phi^{i+1}_{h}-\nabla\phi^{i}_{h}\|^{2}+\|\phi^{i+1}_{h}-\phi^{i}_{h}\|^{2}_{-1, h})\leq C, \end{eqnarray} (3.31)
    \begin{eqnarray} &\sum\limits_{i = 0}^{N}\tau(\|\nabla\mu^{i+1}_{h}\|^{2}+\|\mathbf{u}^{i+1}_{h}\|^{2})\leq C. \end{eqnarray} (3.32)

    Proof. Summing the Eq (3.19) from i = 0\; \rm{to}\; N , we get

    \begin{eqnarray} \begin{split} &\Xi(\phi^{N}_{h})+\tau\sum\limits_{i = 0}^{N}\|\nabla\mu^{i+1}_{h}\|^{2}+\dfrac{\tau}{2\gamma}\sum\limits_{i = 0}^{N}\|\mathbf{u}^{i+1}_{h}\|^{2} +\sum\limits_{i = 0}^{N}\|\phi^{i+1}_{h}-\phi^{i}_{h}\|^{2}\\ &+\dfrac{\varepsilon^{2}}{2}\sum\limits_{i = 0}^{N}\|\nabla\phi^{i+1}_{h}-\nabla\phi^{i}_{h}\|^{2}+\dfrac{\theta}{2}\sum\limits_{i = 0}^{N}\|\phi^{i+1}_{h}-\phi^{i}_{h}\|^{2}_{-1, h} \leq\Xi(\phi^{0}_{h})\leq C. \end{split} \end{eqnarray} (3.33)

    The proof is completed.

    In this section, we assume that the weak solution \{\phi, \mu, \xi, p\} satisfies the following regularity

    \begin{align*} &\phi\in H^{1}(0, T;H^{q+1}(\Omega))\cap L^{\infty}(0, T;H^{1}(\Omega))\cap L^{\infty}(0, T;H^{q+1}(\Omega)), \\ &\mu\in L^{\infty}(0, T;H^{1}(\Omega))\cap L^{2}(0, T;H^{q+1}(\Omega)), \\ &\xi\in L^{2}((0, T;H^{q+1}(\Omega))), \\ &\mathbf{u}\in L^{\infty}(0, T;\mathbf{H}^{q}(\Omega)), \\ &\phi\nabla\mu\in L^{\infty}(0, T;H^{q}(\Omega)). \end{align*}

    For the convenience of subsequent analysis, we introduce some notations,

    \begin{eqnarray*} \begin{split} &\phi^{n+1} = \phi(t_{n+1}), \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \; \delta_{\tau}\phi^{n+1} = \dfrac{\phi^{n+1}-\phi^{n}}{\tau}, \\ &\tilde{e}^{n+1}_{\phi} = \phi^{n+1}-R_{h}\phi^{n+1}, \; \; \; \; \; \; \; \; \; \; \; \hat{e}^{n+1}_{\phi} = R_{h}\phi^{n+1}-\phi^{n+1}_{h}, \\ &\tilde{e}^{n+1}_{\mu} = \mu^{n+1}-R_{h}\mu^{n+1}, \; \; \; \; \; \; \; \; \; \; \; \hat{e}^{n+1}_{\mu} = R_{h}\mu^{n+1}-\mu^{n+1}_{h}, \\ &\tilde{e}^{n+1}_{\xi} = \xi^{n+1}-R_{h}\xi^{n+1}, \; \; \; \; \; \; \; \; \; \; \; \; \hat{e}^{n+1}_{\xi} = R_{h}\xi^{n+1}-\xi^{n+1}_{h}, \\ &\tilde{e}^{n+1}_{p} = p^{n+1}-R_{h}p^{n+1}, \; \; \; \; \; \; \; \; \; \; \; \hat{e}^{n+1}_{p} = R_{h}p^{n+1}-p^{n+1}_{h}, \\ &\sigma(\phi^{n+1}) = \delta_{\tau}R_{h}\phi^{n+1}-\partial_{t}\phi^{n+1}. \end{split} \end{eqnarray*}

    Lemma 4.1. [36] Suppose the \{\phi, \mu, \xi, p\} is the solution to Eq (2.1), the following estimate holds

    \begin{eqnarray} &\|\sigma(\phi^{n+1})\|^{2}\leq Ch^{2q+2}+C\tau^{2}.& \end{eqnarray} (4.1)

    Theorem 4.1. Suppose the solutions of the initial problem Eq (2.1) and the fully discrete scheme Eqs (2.9)-(2.12) are \{\phi, \mu, \xi, p\} and \{\phi^{n+1}_{h}, \mu^{n+1}_{h}, \xi^{n+1}_{h}, p^{n+1}_{h}\} , respectively. Then for any h\;, \tau\; > \; 0 , the following estimate holds

    \begin{eqnarray} \begin{split} &\sum\limits_{i = 0}^{n}\tau \|\nabla\hat{e}^{i+1}_{\mu}\|^{2}+\varepsilon^{2}\|\nabla\hat{e}^{n+1}_{\phi}\|^{2}+\theta\|\hat{e}^{n+1}_{\phi}\|^{2}_{-1, h}+\sum\limits_{i = 0}^{n} \tau\varepsilon\gamma\|\nabla\hat{e}^{i+1}_{p}\|^{2}\\ &+\sum\limits_{i = 0}^{n}\tau\gamma\|\mathcal{P}_{h}(\phi^{i}_{h}\nabla\hat{e}^{i+1}_{\mu})\|^{2}\leq C\tau^{2}+Ch^{2q}. \end{split} \end{eqnarray} (4.2)

    Proof. Subtracting Eqs (2.9)-(2.12) from Eq (2.1) at t = n+1 , one has

    \begin{eqnarray} &-(\sigma(\phi^{n+1}), v_{h})+(\delta_{\tau}\hat{e}^{n+1}_{\phi}, v_{h})+(\phi^{n+1}(\nabla p^{n+1}+\gamma\phi^{n+1}\nabla\mu^{n+1}), \nabla v_{h}) \end{eqnarray} (4.3)
    \begin{eqnarray} &-(\phi^{n}_{h}(\nabla p^{n}_{h}+\gamma\phi^{n}_{h}\nabla\mu^{n+1}_{h}), \nabla v_{h})+(\nabla\hat{e}^{n+1}_{\mu}, \nabla v_{h}) = 0, \\ &(\tilde{e}^{n+1}_{\mu}, w_{h})+(\hat{e}^{n+1}_{\mu}, w_{h})+(f_{1}(\phi^{n+1}_{h})+f_{2}(\phi^{n}_{h}), w_{h})-(f(\phi^{n+1}), w_{h}) \end{eqnarray} (4.4)
    \begin{eqnarray} &-\varepsilon^{2}(\nabla\hat{e}^{n+1}_{\phi}, \nabla w_{h})-(\hat{e}^{n+1}_{\xi}, w_{h})-(\tilde{e}^{n+1}_{\xi}, w_{h}) = 0, \\ &(\nabla\hat{e}^{n+1}_{\xi}, \nabla\psi_{h})+(\nabla\tilde{e}^{n+1}_{\xi}, \nabla\psi_{h})-\theta(\hat{e}^{n+1}_{\phi}, \psi_{h})-\theta(\tilde{e}^{n+1}_{\phi}, \psi_{h}) = 0, \end{eqnarray} (4.5)
    \begin{eqnarray} &(\nabla\hat{e}^{n+1}_{p}, \nabla q_{h})+(\gamma\phi^{n+1}\nabla\mu^{n+1}-\gamma\phi^{n}_{h}\nabla\mu^{n+1}_{h}, \nabla q_{h}) = 0. \end{eqnarray} (4.6)

    We choose v_{h} = \hat{e}^{n+1}_{\mu} in Eq (4.3), w_{h} = -\delta_{\tau}\hat{e}^{n+1}_{\phi} in Eq (4.4), \psi_{h} = -T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi}) in Eq (4.5), q_{h} = \varepsilon\hat{e}^{n+1}_{p} in Eq (4.6) and sum them to get

    \begin{eqnarray} \begin{split} &\quad \|\nabla\hat{e}^{n+1}_{\mu}\|^{2}+\frac{\varepsilon^{2}}{2\tau}(\|\nabla\hat{e}^{n+1}_{\phi}\|^{2}-\|\nabla\hat{e}^{n}_{\phi}\|^{2}+\|\nabla\hat{e}^{n+1}_{\phi}-\nabla\hat{e}^{n}_{\phi}\|^{2})\\ &\quad+\frac{\theta}{2\tau}(\|\hat{e}^{n+1}_{\phi}\|^{2}_{-1, h}-\|\hat{e}^{n}_{\phi}\|^{2}_{-1, h}+\|\hat{e}^{n+1}_{\phi}-\hat{e}^{n}_{\phi}\|^{2}_{-1, h}) +\varepsilon\|\nabla\hat{e}^{n+1}_{p}\|^{2}\\ & = (\sigma(\phi^{n+1}), \hat{e}^{n+1}_{\mu})+(\tilde{e}^{n+1}_{\mu}, \delta_{\tau}\hat{e}^{n+1}_{\phi})-\theta(\tilde{e}^{n+1}_{\phi}, T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi}))\\ &\quad+(f_{1}(\phi^{n+1}_{h})+f_{2}(\phi^{n}_{h}), \delta_{\tau}\hat{e}^{n+1}_{\phi})-(f(\phi^{n+1}), \delta_{\tau}\hat{e}^{n+1}_{\phi})\\ &\quad-(\phi^{n+1}(\nabla p^{n+1}+\gamma\phi^{n+1}\nabla\mu^{n+1}), \nabla\hat{e}^{n+1}_{\mu})+(\phi^{n}_{h}(\nabla p^{n}_{h}+\gamma\phi^{n}_{h}\nabla\mu^{n+1}_{h}), \nabla\hat{e}^{n+1}_{\mu})\\ &\quad-\varepsilon\gamma(\phi^{n+1}\nabla\mu^{n+1}-\phi^{n}_{h}\nabla\mu^{n+1}_{h}, \nabla\hat{e}^{n+1}_{p}) = \sum\limits_{i = 1}^{6}M_{i}, \end{split} \end{eqnarray} (4.7)

    where we denote

    \begin{eqnarray} \begin{split} &M_{1} = (\sigma(\phi^{n+1}), \hat{e}^{n+1}_{\mu}), \\ &M_{2} = (\tilde{e}^{n+1}_{\mu}, \delta_{\tau}\hat{e}^{n+1}_{\phi}), \\ &M_{3} = -\theta(\tilde{e}^{n+1}_{\phi}, T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi})), \\ &M_{4} = (f_{1}(\phi^{n+1}_{h})+f_{2}(\phi^{n}_{h}), \delta_{\tau}\hat{e}^{n+1}_{\phi})-(f(\phi^{n+1}), \delta_{\tau}\hat{e}^{n+1}_{\phi}), \\ &M_{5} = -(\phi^{n+1}(\nabla p^{n+1}+\gamma\phi^{n+1}\nabla\mu^{n+1}), \nabla\hat{e}^{n+1}_{\mu})+(\phi^{n}_{h}(\nabla p^{n}_{h}+\gamma\phi^{n}_{h}\nabla\mu^{n+1}_{h}), \nabla\hat{e}^{n+1}_{\mu}), \\ &M_{6} = -\varepsilon\gamma(\phi^{n+1}\nabla\mu^{n+1}-\phi^{n}_{h}\nabla\mu^{n+1}_{h}, \nabla\hat{e}^{n+1}_{p}). \end{split} \end{eqnarray} (4.8)

    Next, we estimate M_{i} . According to the poincar \acute{e} inequality, the Cauchy-Schwarz inequality, the Young inequality and lemma 4.1, one obtains

    \begin{align} \begin{split} M_{1}& = (\sigma(\phi^{n+1}), \hat{e}^{n+1}_{\mu})\\ &\leq|(\sigma(\phi^{n+1}), \hat{e}^{n+1}_{\mu})|\\ &\leq|(\sigma(\phi^{n+1}), \hat{e}^{n+1}_{\mu}-\overline{\hat{e}^{n+1}_{\mu}})|\\ &\leq\|\sigma(\phi^{n+1})\|\|\nabla\hat{e}^{n+1}_{\mu}\|\\ &\leq\frac{1}{M}\|\sigma(\phi^{n+1})\|^{2}+\frac{M}{4}\|\nabla\hat{e}^{n+1}_{\mu}\|^{2}\\ &\leq C\tau^{2}+Ch^{2q+2}+\frac{M}{4}\|\nabla\hat{e}^{n+1}_{\mu}\|^{2}. \end{split} \end{align} (4.9)

    Using Eq (3.7) in lemma 3.1, the Young inequality and Eq (3.3) in definition 3.1, we have

    \begin{eqnarray} \begin{split} M_{2}& = (\tilde{e}^{n+1}_{\mu}, \delta_{\tau}\hat{e}^{n+1}_{\phi})\leq|(\tilde{e}^{n+1}_{\mu}, \delta_{\tau}\hat{e}^{n+1}_{\phi})|\\ &\leq\|\nabla\tilde{e}^{n+1}_{\mu}\|\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|_{-1, h}\leq\frac{1}{\alpha}\|\nabla\tilde{e}^{n+1}_{\mu}\|^{2}+\frac{\alpha}{4}\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|_{-1, h}^{2}\\ &\leq Ch^{2q}+\frac{\alpha}{4}\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|_{-1, h}^{2}. \end{split} \end{eqnarray} (4.10)

    Similarly, according to lemma 3.1, the Schwarz inequality, the Young inequality, and Eq (3.3) in definition 3.1, we can estimate M_{3} as follows,

    \begin{eqnarray} \begin{split} M_{3}& = -\theta(\tilde{e}^{n+1}_{\phi}, T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi}))\\ &\leq\theta|(\tilde{e}^{n+1}_{\phi}, T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi}))|\\ &\leq\theta\|\tilde{e}^{n+1}_{\phi}\|\|T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi})\|\\ &\leq\theta\|\tilde{e}^{n+1}_{\phi}\|\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|_{-1, h}\\ &\leq\frac{\theta^{2}}{2}\|\tilde{e}^{n+1}_{\phi}\|^{2}+\frac{\alpha}{4}\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|^{2}_{-1, h}\\ &\leq Ch^{2q+2}+\frac{\alpha}{4}\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|_{-1, h}. \end{split} \end{eqnarray} (4.11)

    As for M_{4} , there is f_{1}(\phi^{n+1})-f_{1}(\phi^{n+1}_{h}) = 2(\phi^{n+1}-\phi^{n+1}_{h}) for f_{1}(\phi) = 2\phi , and f_{2}(\phi^{n+1})-f_{2}(\phi^{n}_{h})\leq C(\phi^{n+1}-\phi^{n}_{h}) for f_{2}(\phi) = \phi^{3}-3\phi . Then, according to lemma 3.1, the Young inequality, definition 3.1 and Taylor extension \|\nabla\tau\delta_{\tau}\phi(t)\|^{2}\leq C\tau^{2} , the following inequality is established

    \begin{eqnarray} \begin{split} M_{4}&=(f_{1}(\phi^{n+1}_{h})+f_{2}(\phi^{n}_{h}),\delta_{\tau}\hat{e}^{n+1}_{\phi})-(f(\phi^{n+1}),\delta_{\tau}\hat{e}^{n+1}_{\phi})\\ &\leq|(f_{1}(\phi^{n+1})-f_{1}(\phi^{n+1}_{h}),\delta_{\tau}\hat{e}^{n+1}_{\phi})+(f_{2}(\phi^{n+1})-f_{2}(\phi^{n}_{h}),\delta_{\tau}\hat{e}^{n+1}_{\phi})|\\ &\leq|2((\phi^{n+1}-\phi^{n+1}_{h}),\delta_{\tau}\hat{e}^{n+1}_{\phi})+f_{2}'(\eta)((\phi^{n+1}-\phi^{n}_{h}),\delta_{\tau}\hat{e}^{n+1}_{\phi})|\\ &\leq2\|\nabla(\phi^{n+1}-\phi^{n+1}_{h})\|\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|_{-1,h} +L\|\nabla(\phi^{n+1}-\phi^{n}_{h})\|\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|_{-1,h}\\ &\leq\frac{16}{\alpha}\|\nabla\tilde{e}^{n+1}_{\phi}\|^{2}+\frac{4}{\alpha}\|\nabla\hat{e}^{n+1}_{\phi}\|^{2} +\frac{16L}{\alpha}\|\nabla(\phi^{n+1}-\phi^{n})\|^{2}+\frac{16L}{\alpha}\|\nabla\tilde{e}^{n}_{\phi}\|^{2}\\ &\quad+\frac{16L}{\alpha}\|\nabla\hat{e}^{n}_{\phi}\|^{2}+\frac{\alpha}{2}\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|_{-1,h}^{2}\\ &\leq C\tau^{2}+Ch^{2q}+\frac{4}{\alpha}\|\nabla\hat{e}^{n+1}_{\phi}\|^{2}+C\|\nabla\hat{e}^{n}_{\phi}\|^{2}+\frac{\alpha}{2}\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|_{-1,h}^{2}. \end{split} \end{eqnarray} (4.12)

    To deal with M_{5} , we denote b(\phi, \mathbf{u}, v): = (\phi\mathbf{u}, \nabla v) . Referring to the method in [15], M_{5} can be analyzed as

    \begin{eqnarray} \begin{split} M_{5}& = -(\phi^{n+1}(\nabla p^{n+1}+\gamma\phi^{n+1}\nabla\mu^{n+1}), \nabla\hat{e}^{n+1}_{\mu})+(\phi^{n}_{h}(\nabla p^{n}_{h}+\gamma\phi^{n}_{h}\nabla\mu^{n+1}_{h}), \nabla\hat{e}^{n+1}_{\mu})\\ & = -b(\phi^{n+1}, \mathbf{u}^{n+1}, \hat{e}^{n+1}_{\mu})+b(\phi^{n}_{h}, \mathbf{u}^{n+1}_{h}, \hat{e}^{n+1}_{\mu})\\ & = -b(\tilde{e}^{n+1}_{\phi}, \mathbf{u}^{n+1}, \hat{e}^{n+1}_{\mu})-b(\tau\delta_{\tau}R_{h}\phi^{n+1}, \mathbf{u}^{n+1}, \hat{e}^{n+1}_{\mu})\\ &\quad-b(\hat{e}^{n}_{\phi}, \mathbf{u}^{n+1}, \hat{e}^{n+1}_{\mu})-b(\phi^{n}_{h}, \mathbf{u}^{n+1}-\mathbf{u}^{n+1}_{h}, \hat{e}^{n+1}_{\mu})\\ &\leq b(\tilde{e}^{n+1}_{\phi}, \mathbf{u}^{n+1}, \hat{e}^{n+1}_{\mu})+b(\tau\delta_{\tau}R_{h}\phi^{n+1}, \mathbf{u}^{n+1}, \hat{e}^{n+1}_{\mu})\\ &\quad+b(\hat{e}^{n}_{\phi}, \mathbf{u}^{n+1}, \hat{e}^{n+1}_{\mu})+b(\phi^{n}_{h}, \mathbf{u}^{n+1}-\mathbf{u}^{n+1}_{h}, \hat{e}^{n+1}_{\mu})\\ &\leq CD(\tau^{2}+h^{2q})+CD\|\nabla\hat{e}^{n}_{\phi}\|^{2}+\frac{1}{4}\|\nabla\hat{e}^{n+1}_{\mu}\|^{2} -\gamma\|\mathcal{P}_{h}(\phi^{n}_{h}\nabla\hat{e}^{n+1}_{\mu})\|^{2}, \end{split} \end{eqnarray} (4.13)

    where D: = \|\phi^{n}_{h}\|^{4}_{L^{\infty}}+1\leq C . Therefore,

    \begin{eqnarray} &M_{5}\leq C\tau^{2}+Ch^{2q}+C\|\nabla\hat{e}^{n}_{\phi}\|^{2}+\dfrac{1}{4}\|\nabla\hat{e}^{n+1}_{\mu}\|^{2} -\gamma\|\mathcal{P}_{h}(\phi^{n}_{h}\nabla\hat{e}^{n+1}_{\mu})\|^{2}.& \end{eqnarray} (4.14)

    According to the definition 3.3, definition 3.4, lemma 3.4, Taylor expansion \|\nabla\tau\delta_{\tau}p^{n+1}\|^{2}\leq C\tau^{2} , the Cauchy-Schwarz inequality and the Young inequality, the following error estimation formulation holds

    \begin{eqnarray} \begin{split} M_{6}&=-\varepsilon\gamma(\phi^{n+1}\nabla\mu^{n+1}-\phi^{n}_{h}\nabla\mu^{n+1}_{h},\nabla\hat{e}^{n+1}_{p})\\ &=-\varepsilon\gamma(\nabla p^{n+1}+\phi^{n+1}\nabla\mu^{n+1},\nabla\hat{e}^{n+1}_{p})+\varepsilon\gamma(\nabla p^{n+1}-\nabla p^{n},\nabla\hat{e}^{n+1}_{p})\\ &\quad+\varepsilon\gamma(\nabla p^{n}-\nabla p^{n}_{h},\nabla\hat{e}^{n+1}_{p})+\varepsilon\gamma(\nabla p^{n}_{h}+\phi^{n}_{h}\nabla\mu^{n+1}_{h},\nabla\hat{e}^{n+1}_{p})\\ &\leq\varepsilon\gamma\|\nabla( p^{n+1}-p^{n})\|\|\nabla\hat{e}^{n+1}_{p}\|+\varepsilon\gamma\|\nabla(p^{n}-p^{n}_{h})\|\|\nabla\hat{e}^{n+1}_{p}\|\\ &\leq\frac{\varepsilon\gamma}{2}\|\nabla\tau\delta_{\tau}p^{n+1}\|^{2}+\frac{\varepsilon\gamma}{2}\|\nabla(p^{n}-p^{n}_{h})\|^{2}+\varepsilon\gamma\|\nabla\hat{e}^{n+1}_{p}\|^{2}\\ &\leq C\tau^{2}+Ch^{2q}+\varepsilon\gamma\|\nabla\hat{e}^{n+1}_{p}\|^{2}. \end{split} \end{eqnarray} (4.15)

    Combining Eqs (4.7)-(4.15) gives

    \begin{eqnarray} \begin{split} &\frac{1}{2}\|\nabla\hat{e}^{n+1}_{\mu}\|^{2}+\frac{\varepsilon^{2}}{2\tau}(\|\nabla\hat{e}^{n+1}_{\phi}\|^{2}-\|\nabla\hat{e}^{n}_{\phi}\|^{2}+\|\nabla\hat{e}^{n+1}_{\phi}-\nabla\hat{e}^{n}_{\phi}\|^{2})\\ &+\frac{\theta}{2\tau}(\|\hat{e}^{n+1}_{\phi}\|^{2}_{-1,h}-\|\hat{e}^{n}_{\phi}\|^{2}_{-1,h}+\|\hat{e}^{n+1}_{\phi}-\hat{e}^{n}_{\phi}\|^{2}_{-1,h})+\varepsilon\gamma\|\nabla\hat{e}^{n+1}_{p}\|^{2}\\ &+\gamma\|\mathcal{P}_{h}(\phi^{n}_{h}\nabla\hat{e}^{n+1}_{\mu})\|^{2}\\ \leq& C\tau^{2}+Ch^{2q}+C\|\nabla\hat{e}^{n}_{\phi}\|^{2}+\frac{4}{\alpha}\|\nabla\hat{e}^{n+1}_{p}\|^{2}+\varepsilon\gamma\|\nabla\hat{e}^{n+1}_{p}\|^{2}\\ &+\alpha\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|^{2}_{-1,h}. \end{split} \end{eqnarray} (4.16)

    For \|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|^{2}_{-1, h} , taking \alpha v_{h} = T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi}) in Eq (4.3) and using a similar idea as M_{5} , the following inequality can be obtained,

    \begin{eqnarray} \begin{split} \alpha\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|^{2}_{-1, h} = &\alpha(\sigma(\phi^{n+1}), T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi}))-\alpha(\nabla\hat{e}^{n+1}_{\mu}, \nabla T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi}))\\ &-\alpha(\phi^{n+1}(\nabla p^{n+1}+\gamma\phi^{n+1}\nabla\mu^{n+1}), \nabla T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi}))\\ &+\alpha(\phi^{n}_{h}(\nabla p^{n}_{h}+\gamma\phi^{n}_{h}\nabla\mu^{n+1}_{h}), \nabla T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi}))\\ \leq&\alpha\|\sigma(\phi^{n+1})\|\|T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi})\|+\alpha\|\nabla\hat{e}^{n+1}_{\mu}\|\|T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi})\|\\ &+\alpha b(\tilde{e}^{n+1}_{\phi}, \mathbf{u}^{n+1}, T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi}))+\alpha b(\tau\delta_{\tau}R_{h}\phi^{n+1}, \mathbf{u}^{n+1}, T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi}))\\ &+\alpha b(\hat{e}^{n}_{\phi}, \mathbf{u}^{n+1}, T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi}))+\alpha b(\phi^{n}_{h}, \mathbf{u}^{n+1}-\mathbf{u}^{n+1}_{h}, T_{h}(\delta_{\tau}\hat{e}^{n+1}_{\phi}))\\ \leq&2\alpha\|\sigma(\phi^{n+1})\|^{2}+\frac{\alpha}{8}\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|^{2}_{-1, h}+\alpha \|\nabla\hat{e}^{n+1}_{\mu}\|^{2}+\frac{\alpha}{4}\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|^{2}_{-1, h}\\ &+CD(\tau^{2}+h^{2q})-\frac{\gamma}{\alpha}\|\mathcal{P}_{h}(\phi^{n}_{h}\nabla\hat{e}^{n+1}_{\mu})\|^{2}+CD\|\nabla\hat{e}^{n}_{\phi}\|^{2}+\frac{\alpha}{8}\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|^{2}_{-1, h}\\ \leq&C\tau^{2}+Ch^{2q+2}+CD(\tau^{2}+h^{2q})-\frac{\gamma}{\alpha}\|\mathcal{P}_{h}(\phi^{n}_{h}\nabla\hat{e}^{n+1}_{\mu})\|^{2}\\ &+\alpha\|\nabla\hat{e}^{n+1}_{\mu}\|^{2}+CD\|\nabla\hat{e}^{n}_{\phi}\|^{2}+\frac{\alpha}{2}\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|^{2}_{-1, h}. \end{split} \end{eqnarray} (4.17)

    Therefore, it follows that

    \begin{eqnarray} &\alpha\|\delta_{\tau}\hat{e}^{n+1}_{\phi}\|^{2}_{-1, h}\leq C\tau^{2}+Ch^{2q}+2\alpha\|\nabla\hat{e}^{n+1}_{\mu}\|^{2}+C\|\nabla\hat{e}^{n}_{\phi}\|^{2}-\dfrac{\gamma}{\alpha}\|\mathcal{P}_{h}(\phi^{n}_{h}\nabla\hat{e}^{n+1}_{\mu})\|^{2}.& \end{eqnarray} (4.18)

    Then, combining Eq (4.16) with Eq (4.18) and multiplying by 2\tau , one has

    \begin{eqnarray} \begin{split} &\tau\|\nabla\hat{e}^{n+1}_{\mu}\|^{2}+\varepsilon^{2}(\|\nabla\hat{e}^{n+1}_{\phi}\|^{2}-\|\nabla\hat{e}^{n}_{\phi}\|^{2}+\|\nabla\hat{e}^{n+1}_{\phi}-\nabla\hat{e}^{n}_{\phi}\|^{2})\\ &+\theta(\|\hat{e}^{n+1}_{\phi}\|^{2}_{-1,h}-\|\hat{e}^{n}_{\phi}\|^{2}_{-1,h}+\|\hat{e}^{n+1}_{\phi}-\hat{e}^{n}_{\phi}\|^{2}_{-1,h}) +2\tau\varepsilon\|\nabla\hat{e}^{n+1}_{p}\|^{2}\\ &+2\tau\gamma\|\mathcal{P}_{h}(\phi^{n}_{h}\nabla\hat{e}^{n+1}_{\mu})\|^{2}\\ \leq&C\tau\tau^{2}+C\tau h^{2q}+C\tau\|\nabla\hat{e}^{n}_{\phi}\|^{2} +{\dfrac{8\tau}{\alpha}}\|\nabla\hat{e}^{n+1}_{\phi}\|^{2}+2\tau\varepsilon\gamma\|\nabla\hat{e}^{n+1}_{p}\|^{2}\\ &+2\alpha\tau\|\nabla\hat{e}^{n+1}_{\mu}\|^{2}-\frac{2\tau\gamma}{\alpha}\|\mathcal{P}_{h}(\phi^{n}_{h}\nabla\hat{e}^{n+1}_{\mu})\|^{2}. \end{split} \end{eqnarray} (4.19)

    Finally, we take the appropriate \alpha(0 < \alpha\leq \dfrac{1}{2}) and add the above estimates from i=0 to n. When 0 < \tau\leq\dfrac{\alpha\varepsilon^{2}}{8}, according to the discrete Gronwall inequality, one concludes that

    \begin{eqnarray} \begin{split} &\sum\limits_{i = 0}^{n}\tau \|\nabla\hat{e}^{i+1}_{\mu}\|^{2}+\varepsilon^{2}\|\nabla\hat{e}^{n+1}_{\phi}\|^{2}+\theta\|\hat{e}^{n+1}_{\phi}\|^{2}_{-1, h}+\sum\limits_{i = 0}^{n} \tau\varepsilon\gamma\|\nabla\hat{e}^{i+1}_{p}\|^{2}\\ &+\sum\limits_{i = 0}^{n}\tau\gamma\|\mathcal{P}_{h}(\phi^{i}_{h}\nabla\hat{e}^{i+1}_{\mu})\|^{2}\leq C\tau^{2}+Ch^{2q}. \end{split} \end{eqnarray} (4.20)

    The proof is completed.

    In this part, some numerical examples are used to verify the correctness and validity of the theoretical analysis. Next, let us take the initial conditions \phi_{0} = 0.24*cos(2\pi x)cos(2\pi y)+0.4*cos(\pi x)cos(3\pi y) , and the domain of the calculation is [0, 1]\times[0, 1] .

    For Tables 1 and 2, the parameters are chosen as follows, \tau = 0.01, T = 0.1 , \varepsilon = 0.14 and mesh steps h = \frac{1}{16}, \frac{1}{32}, \frac{1}{64}, \frac{1}{128} . The spatial convergence orders of relative error \|\hat{e}_{\phi}\|_{H^{1}} are close to 1, which is consistent with the convergence order obtained from theoretical analysis. Moreover, different \theta and \gamma have little effect on the corresponding convergence order.

    Table 1.  The spacial convergence rate of \hat{e}_{\phi} with \theta = 0 .
    \gamma=0.02 h \|\hat{e}_{\phi}\|_{H^{1}} rate \gamma=0.5 \|\hat{e}_{\phi}\|_{H^{1}} rate
    \frac{1}{16} 0.114235 0.112924
    \frac{1}{32} 0.0538164 1.08589 0.0532056 1.08571
    \frac{1}{64} 0.0276215 0.962255 0.0272203 0.966891
    \frac{1}{128} 0.0137438 1.00701 0.0135257 1.00898

     | Show Table
    DownLoad: CSV
    Table 2.  The spacial convergence rate of \hat{e}_{\phi} with \theta = 0.5 .
    \gamma=0.02 h \|\hat{e}_{\phi}\|_{H^{1}} rate \gamma=0.5 \|\hat{e}_{\phi}\|_{H^{1}} rate
    \frac{1}{16} 0.113003 0.113255
    \frac{1}{32} 0.05391 1.06774 0.0529089 1.09799
    \frac{1}{64} 0.0275791 0.966978 0.0272109 0.959326
    \frac{1}{128} 0.0137206 1.00723 0.0134845 1.01288

     | Show Table
    DownLoad: CSV

    For Tables 3 and 4, the parameters are chosen as follows, \varepsilon = 0.01, T = 0.1, h = \tau = 0.0625, 0.03125, 0.015625. The temporal convergence orders of relative error \|\hat{e}_{\phi}\|_{H^{1}} are close to 1, which is consistent with the convergence order obtained from theoretical analysis.

    Table 3.  The temporal convergence rate of \hat{e}_{\phi} with \theta = 0.01 .
    \gamma=0.01\tau \|\hat{e}_{\phi}\|_{H^{1}}rate \gamma=0.08 \|\hat{e}_{\phi}\|_{H^{1}} rate
    0.0625 1.00089 1.14072
    0.03125 0.50614 0.983677 0.998929 0.981451
    0.015625 0.259234 0.965282 0.505928 0.968364

     | Show Table
    DownLoad: CSV
    Table 4.  Let us test the energy dissipation of our proposed scheme. The energy functional Eq (1.13) of the modified Cahn-Hilliard-Hele-Shaw system Eqs (1.4)-(1.10) can be discreteized as.
    \gamma=0.01 \tau \|\hat{e}_{\phi}\|_{H^{1}}rate \gamma=0.08 \|\hat{e}_{\phi}\|_{H^{1}} rate
    0.0625 1.00089 1.14072
    0.03125 0.506139 0.983679 0.998926 0.981446
    0.015625 0.259234 0.96528 0.505928 0.968365

     | Show Table
    DownLoad: CSV

    Let us test the energy dissipation of our proposed scheme. The energy functional Eq (1.13) of the modified Cahn-Hilliard-Hele-Shaw system Eqs (1.4)-(1.10) can be discreteized as

    \begin{eqnarray} E(\phi_{h}^{n+1}) = \int_{\Omega}(\frac{\varepsilon^{2}}{2}|\nabla\phi_{h}^{n+1}|^{2}+F(\phi_{h}^{n+1}))dx. \end{eqnarray} (5.1)

    Correspondingly, the modified energy of the fully discrete scheme Eqs (2.9)-(2.13) is defined as

    \begin{eqnarray} &\Xi(\phi^{n+1}_{h}): = E(\phi^{n+1}_{h})+\|\phi^{n+1}_{h}\|^{2}+\dfrac{\theta}{2}\|\phi^{n+1}_{h}-\overline{\phi}_{0}\|^{2}_{-1, h} +\dfrac{\tau}{2\gamma}\|\nabla p^{n+1}_{h}\|^{2}. \end{eqnarray} (5.2)

    For the test, the parameters are chosen as follows: T = 5 , \tau = 0.001 , h = \dfrac{1}{64} , \varepsilon = 0.4 , \gamma = 0.5 . In Figure 1, we can see that the energy functional is non-increasing for \theta = 0, 0.1, 1.

    Figure 1.  the evolutions of discrete energy.

    In this part, we present the phase separation dynamics that is called spinodal decomposition in the modified Cahn-Hilliard-Hele-Shaw system. In the simulation, the computational domain is chosen as [0, 1]\times[0, 1] , the parameters are chosen as follows: \varepsilon = 0.05 , \gamma = 0.45 , \tau = 0.0001 . Then, let us take the initial condition

    \phi_{0} = 2*rand()-1,

    where rand()\in [0, 1] . The process of coarsening is shown in the following figures. From figures 2-19, we can see that the contours of \phi are gradually coarsened over time. However, the profiles obtained by different \theta are similar at the same time T . From left to right, the coarsening processes of \theta = 50, 200 are not obvious compared with the coarsening processes of \theta = 0 . We know the bigger \theta can suppress the coarsening process.

    Figure 2.  T = 0.0001, \theta = 0 .
    Figure 3.  T = 0.0001, \theta = 50 .
    Figure 4.  T = 0.0001, \theta = 200 .
    Figure 5.  T = 0.0005, \theta = 0 .
    Figure 6.  T = 0.0005, \theta = 50 .
    Figure 7.  T = 0.0005, \theta = 200 .
    Figure 8.  T = 0.001, \theta = 0 .
    Figure 9.  T = 0.001, \theta = 50 .
    Figure 10.  T = 0.001, \theta = 200 .
    Figure 11.  T = 0.005, \theta = 0 .
    Figure 12.  T = 0.005, \theta = 50 .
    Figure 13.  T = 0.005, \theta = 200 .
    Figure 14.  T = 0.01, \theta = 0 .
    Figure 15.  T = 0.01, \theta = 50 .
    Figure 16.  T = 0.01, \theta = 200 .
    Figure 17.  T = 0.02, \theta = 0 .
    Figure 18.  T = 0.02, \theta = 50 .
    Figure 19.  T = 0.02, \theta = 200 .

    In this paper, a decoupled scheme of the modified Cahn-Hilliard-Hele-Shaw system is studied. In our scheme, the velocity and pressure are decoupled, and a Possion equation is solved to update the pressure at each time step. Unconditional stability of the scheme in energy is proved. The convergence analysis are addressed in the frame of finite element method. Furthermore, the theoretical part is verified by several numerical examples. The results show that the numerical examples are consistent with the results of the theoretical part.

    The work is supported by the the Provincial Natural Science Foundation of Shanxi (No. 201901D111123) and Key Research and Development (R & D) Projects of Shanxi Province (No. 201903D121038).

    The authors declare no conflicts of interest in this paper.



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