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Editorial

Debating New Theory in Neuroscience

  • Citation: Robert A Moss, Joseph V Martin. Debating New Theory in Neuroscience[J]. AIMS Neuroscience, 2014, 1(1): 1-3. doi: 10.3934/Neuroscience.2014.1.1

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  • Magnetohydrodynamics(MHD) is a subject that studies the motion of conductive fluids in magnetic fields. It is mainly used in astrophysics, controlled thermonuclear reactions and industry. The system of MHD equations is a coupled equation system of Navier-Stokes equations of fluid mechanics and Maxwell's equations of electrodynamics (see[1,2]). In this paper, we consider the unsteady incompressible MHD equations as follows:

    $ {ut+(u)uνu+μB×curlB+p=f,xΩ,u=0,xΩ,Bt+Rm1curlcurlBcurl(u×B)=g,xΩ,B=0,xΩ, $ (1.1)

    where $ u $ is velocity, $ \nu $ kinematic viscosity, $ \mu $ magnetic permeability, $ p $ hydrodynamic pressure, $ B $ magnetic field, $ f $ body force, and $ g $ applied current. The coefficients are the magnetic Reynolds number $ Rm $. As in [3], we assume that $ \Omega $ is a convex domain in $ R^d $, $ d = 2, 3 $ and $ t\in [0, T] $. Here, $ u_t = \frac{\partial u}{\partial t} $, $ B_t = \frac{\partial B}{\partial t} $. The term $ u\times B $ explains the effect of hydrodynamic flow on current. The magnetic Reynolds number $ Rm $ is moderate or low (from $ 10^{-2} $ to 1) (see[4]). For the purposes of simplicity, we choose $ Rm = 1 $ in our numerical analysis.

    System (1.1) is considered in conjunction with the following initial boundary conditions

    $ u(x,0)=u0(x),B(x,0)=B0(x),xΩ,u=0,Bn=0,n×curlB=0,xΩ,t>0, $

    where $ {\bf{n}} $ denotes the outer unit normal of $ \partial \Omega $.

    Over that last few decades, there has been a lot of works devoted on the numerical solution of MHD flows. In [5], some mathematical equations related to the MHD equations have been studied. In [6], a weighted regularization approach was applied to incompressible MHD problems. Reference [7,8] decoupled the linear MHD problem involving conducting and insulating regions. A mixed finite element method for stationary incompressible MHD equations was given in [9]. In order to avoid in-sup condition, Gerbeau [10] presented a stabilized finite element method and Salah et al. [11] proposed a Galerkin least-square method. In [12], a two-level finite element method with a stabilizing sub-grid for the incompressible MHD equations has been shown.

    Recently, a convergence analysis of three finite element iterative methods for 2D/3D stationary incompressible MHD equations has been given by Dong and He (see[13]). In order to continue the in-depth explore of three-dimensional incompressible MHD system, an unconditional convergence of the Euler semi-implicit scheme for the three-dimensional incompressible MHD equations was studied by He [14]. In [15], a two-level Newton iteration method for 2D/3D steady incompressible MHD equation was proposed by Dong and He. In addition, Yuksel and Ingram have discussed the full discretization of Grank-Nicolson scheme at small magnetic Reynolds numbers (see[16]). The defect correction finite element method for the MHD equations was shown by Si et. al. [17,18,19]. In [20], a decoupling penalty finite element method for the stationary incompressible MHD equation was presented. To further study this method, we can refer to [21,22,23].

    In this paper, a modified characteristics projection finite element method for the unsteady incompressible magnetohydrodynamics(MHD) equations will be given. In this method, modified characteristics finite element method and the projection method will be combined for solving the incompressible MHD equations. Both the stability and the optimal error estimates both in $ L^2 $ and $ H^1 $ norms for the modified characteristics projection finite element method will be derived. In order to demonstrate the effectiveness of our method, we will present some numerical results at the end of the manuscript. The contents of this paper are divided into sections as follows. To obtain the unconditional stability and optimal error estimates, in section 2, we introduce some notations and present a modified characteristics projection algorithm for the MHD equations. Then, in section 3, we give stability and error analysis and show that this method has optimal convergence order by mathematical induction. In order to demonstrate the effectiveness of our method, some numerical results are presented in the last section.

    In this section, we introduce some notations. We employ the standard Sobolev spaces $ H^{k}(\Omega) = W^{k, 2} $, for nonnegative $ k $ with norm $ \|v\|_{k} = (\sum_{|\beta| = 0}^{k}\|D^{\beta}v\|_{0}^{2})^{1/2} $. For vector-value function, we use the Sobolev spaces $ \mbox{H}^{k}(\Omega) = (H^{k}(\Omega))^{d} $ with norm $ \|\mbox{v}\|_{k} = (\sum_{i = 1}^{d}\|v_{i}\|_{k}^{2})^{1/2} $(see [24,25]). For simplicity, we set:

    $ X=H10(Ω)2,X0={vX:v=0},M=L20(Ω)={qL2(Ω)d:Ωq(x)dx=0},H={vL2(Ω)d:v=0},W={ψ(H1(Ω))d:ψn|Ω=0},W0={ψW:ψ=0}, $

    which are equipped with the norms

    $ vX=v0,qM=q0,ψW=ψ1. $

    In this paper, we will use the following equality

    $ curl(ψ×B)=BψψB,ψ,BW0. $

    Then, we recall the following Poincaré-Friedrichs inequality in $ W $: there holds

    $ c0Ccurlc0,cW, $

    with a constant $ C > 0 $ solely depending on the domain $ \Omega $; (see[26]). Here, we define the following trilinear form $ a_1(\cdot, \cdot, \cdot) $ as follows, for all $ u\in V $, $ w, v\in X $,

    $ a1(u,v,w)=(uw+12(u)w,v)=12(uw,v)12(uv,w) $

    and the properties of the trilinear form $ a_1(\cdot, \cdot, \cdot) $ [27] are helpful in our analysis

    $ a1(u,v,w)=a1(u,w,v),  uX0,v,wX,|a1(u,v,w)|N2u0(v0wL+vL6wL3),  uL2(Ω)2, vX,wL(Ω)2X,|a1(u,v,w)|N2(uLv0+uL3vL6)w0,  uL(Ω)2X, vX,wL2(Ω)2,v0γ0v0,vL6Cv0,wL4C0w0,  wX, $

    where $ N > 0 $ is a constant, $ \gamma_0 $ (only dependent on $ \Omega $) is a positive constant, and $ C_0 $ (only dependent on $ \Omega $) is an embedding constant of $ H^{1}(\Omega)\hookrightarrow L^{4}(\Omega) $ ($ \hookrightarrow $ denotes the continuous embedding).

    Furthermore, we define the Stokes operator $ \mathcal{A}:D(\mathcal{A})\longrightarrow \tilde{H} $ such that $ \mathcal{A} = -P\triangle $, where $ D(\mathcal{A}) = H^{2}(\Omega)^{d}\cap X_{0} $ and $ P $ is an $ L^2 $-projection from $ L^2(\Omega)^d $ to $ \tilde{H} = \{v\in L^2(\Omega)^d; \nabla\cdot v = 0, v\cdot n|_{\partial\Omega} = 0\} $.

    Next, we recall a discrete version of Gronwall's inequality established in [14].

    Lemma 2.1 Let $ C_{0}, a_n, b_n $ and $ d_n $, for integers $ n\geq0 $, be the non-negative numbers such that

    $ am+τmn=1bnτm1n=1dnan+C0   m1. $ (2.1)

    Then

    $ am+τmn=1bnC0exp(τm1n=1dn)   m1. $ (2.2)

    Throughout this paper, we make the following assumptions on the prescribed date for the problem (1.1), which satisfies the regularity of the data needed for our main results.

    Assumption (A1)(see[27,28]): Assume that the boundary of $ \Omega $ is smooth so that the unique solution $ (v, q) $ of the steady Stokes problem

    $ Δv+q=f,v=0inΩ,v|Ω=0, $

    for prescribed $ f\in L^{2}(\Omega)^{3} $ satisfies

    $ v2+q1cf0, $

    and Maxwell's equations

    $ curlcurlB=g,B=0inΩ,n×curlB=0,BnonΩ, $

    for the prescribed $ g\in L^{2}(\Omega)^{3} $ admits a unique solution $ B\in W_{0} $ which satisfies

    $ B2cg0. $

    Assumption (A2): The initial data $ u_0, B_0, f $ satisfy

    $ Au00+B02+sup0tTf0+ft0C. $ (2.3)

    Assumption (A3): We assume that the MHD problem admits a unique local strong solution $ (u, p, B) $ on $ (0, T) $ such that

    $ sup0tTAut0+Bt2+p1+ut0+Bt0+T0(ut21+Bt21+pt21+utt20+Btt20)dtC. $ (2.4)

    Let $ 0 = t_{0} < t_{1} < \cdots < t_{N} = T $ be a uniform partition of the time interval $ [0, T] $ with $ t_{n} = n\tau $, and $ N $ being a positive integer. Set $ u^{n} = (\cdot, t_{n}) $, For any sequence of functions $ \{f^{n}\}_{n = 0}^{N} $, we define

    $ Dτfn=fnfn1τ. $

    At the initial time step, we start with $ U^{0} = \tilde{U}^{0} = u_{0}, B^{0} = \tilde{B}^{0} = b_{0} $ and an arbitrary $ P^{0}\in M\cap H^{1}(\Omega) $. Then, we can give

    $ P0=f0u0u0+νΔu0μB0×curlB0. $

    Then, we give the algorithm as follows

    Algorithm 2.1 (Time-discrete modified characteristics projection finite element method)

    Step 1. Solve $ B^{n+1} $, for instance

    $ Bn+1ˆBnτ+curlcurlBn+1(Bn)Un=gn+1, $ (2.5)
    $ Bn+1n=0,n×curlBn+1=0, on Ω, $ (2.6)

    where

    $ ˆln={ln(ˆx),   ˆx=xUnτΩ,0,   othrewise. $

    Step 2. Find $ \tilde{U}^{n+1} $, such that

    $ ˜Un+1ˆUnτνΔ˜Un+1+Pn+μBn×curlBn+1=fn+1, $ (2.7)
    $ ˜Un+1=0, on Ω. $ (2.8)

    Step 3. Calculate $ (U^{n+1}, P^{n+1}) $, for example

    $ Un+1˜Un+1τ+(Pn+1Pn)=0, $ (2.9)
    $ Un+1=0, $ (2.10)
    $ Un+1=0 on Ω,. $ (2.11)

    The corresponding weak form is

    Step 1. Solve $ B^{n+1} $, for instance

    $ (Bn+1ˆBnτ,ψ)+(curlBn+1,curlψ)((Bn)Un,ψ)=(gn+1,ψ),   ψW. $ (2.12)

    Step 2. Find $ \tilde{U}^{n+1} $, such that

    $ (˜Un+1ˆUnτ,v)+ν(˜Un+1,v)+(Pn,v)+μ(Bnv,Bn+1)μ(vBn,Bn)=(fn+1,v),   vX, $ (2.13)

    Step 3. Calculate $ (U^{n+1}, P^{n+1}) $, for example

    $ (Un+1˜Un+1τ,v)+((Pn+1Pn),v)=0,   vX, $ (2.14)
    $ (Un+1,ϕ)=0,   ϕM. $ (2.15)

    Remark 2.1 As we all know $ (B^{n}\times \mbox{curl}B^{n+1}, v) = (B^{n}\cdot\nabla v, B^{n+1})-(v\cdot\nabla B^{n}, B^{n+1}) $. In order to improve our algorithm, we change it as $ (B^{n}\cdot\nabla v, B^{n+1})-(v\cdot\nabla B^{n}, B^{n}) $.

    We denote $ T_{h} $ be a regular and quasi-uniform partition of the domain $ \Omega $ into the triangles for $ d = 2 $ or tetrahedra for $ d = 3 $ with diameters by a real positive parameter $ h(h\rightarrow 0) $. The finite element pair $ (X_{h}, M_{h}, W_{h}) $ is constructed based on $ T_{h} $.

    Let $ W_{0n}^h = X_h\times W_h $. There exits a constant $ \beta_0 > 0 $ (only dependent on $ \Omega $) such that

    $ sup0(v,ψ)Wh0nd(v,q)v0β0q0,  qMh. $ (2.16)

    There exits a mapping $ r_h\in \mathcal{L}(H^2(\Omega)^2\cap V, X_h) $ satisfying

    $ ((vrhv),q)=0,(vrhv)0Chv2,  vH2(Ω)2V,  qMh, $

    and a $ L^2 $-projection operator $ \rho_h:M\rightarrow M_h $ satisfying

    $ qρhq0Chq1,  qH1(Ω)M, $

    and a mapping $ R_h\in \mathcal{L}(H^2(\Omega)^2\cap V_n, W_h) $ satisfying

    $ (×RhΦ,×Ψ)+(RhΦ,Ψ)=(×Φ,×Ψ)+(Φ,Ψ)=(×Φ,×Ψ),  ΨWh,ΦRhΦ0+hΦRhΦ1Ch2Φ2,  H2(Ω)Vh. $

    Here, we define the discrete Stokes operator $ A_{h} = -P_h\triangle_h $ and $ \triangle_h $ (see [15] and the references therein) defined by

    $ (huh,vh)=(uh,vh),uh,vhVh. $

    Meanwhile, we define the discrete operator $ A_{1h} B_h = R_{1h}(\nabla_h\times \nabla\times B_h+\nabla_h\nabla\cdot B_h)\in W_h $ (see [15]) as follows

    $ (A1hBh,Ψ)=(A121hBh,A121hBh)=(×Bh,×Ψ)+(Bh,Ψ),Bh,ΨWh. $

    We define the Stokes projection $ (R_{h}(u, p), Q_{h}(u, P)):(X, M)\rightarrow (X_{h}, M_{h}) $ by

    $ ν(Rh(u,p)u,vh)(Qh(u,p)p,vh)=0, $ (2.17)
    $ ((Rh(u,p)u),ϕh)=0. $ (2.18)

    By classical FEM theory ([25,29,30]), we have the following results.

    Lemma 2.2 Assume that $ u \in H_{0}^{1}(\Omega)^{d}\cap H^{r+1}(\Omega)^{d} $ and $ p \in L_{0}^{2}(\Omega)\cap H^{r}(\Omega) $. Then,

    $ Rh(u,p)u0+h((Rh(u,p)u)0+Qh(u,p)p0)Chr+1(ur+1+pr), $ (2.19)

    and

    $ Rh(u,p)LC(u2+p1). $ (2.20)

    Lemma 2.3 If $ (u, p)\in W^{2, k}(\Omega)^{d}\times W^{1, k}(\Omega) $ for $ k > d $,

    $ Rh(u,p)LC(uW1,+pL). $ (2.21)

    Algorithm 2.2 (The fully discrete modified characteristics projection finite element method)

    Step 1. Solve $ B_{h}^{n+1} $, for instance

    $ (Bn+1hˆBnhτ,ψh)+(curlBn+1h,curlψh)((Bnh)unh,ψh)=(gn+1,ψh),   ψhWh. $ (2.22)

    where

    $ ˆlnh={lnh(ˆx),   ˆx=xunhτΩ,0,   othrewise. $

    Step 2. Find $ \tilde{U}^{n+1} $, such that

    $ (˜un+1hˆunhτ,vh)+ν(˜un+1h,vh)+(pnh,vh)+μ(Bnhvh,Bn+1h)μ(vhBnh,Bnh)=(fn+1,vh),   vhXh. $ (2.23)

    Step 3. Calculate $ (u_{h}^{n+1}, p_{h}^{n+1}) $, for example

    $ (un+1h˜un+1hτ,vh)+((pn+1hpnh),vh)=0,   vhXh, $ (2.24)
    $ (un+1h,ϕh)=0,   ϕhMh. $ (2.25)

    Here, we make use of the following notation:

    $ en=unUn,˜e=un˜Un,n=0,1,...,N,εn=bnBn,ˆεn=bnˆBn,n=0,1,...,N,ξn=pnPn,n=0,1,...,N. $

    Lemma 3.1 (see[31]). Assume that $ g_{1}, g_{2}, \rho $ are three functions defined in $ \Omega $ and $ \rho|_{\partial\Omega} = 0 $. If

    $ τ(g1W1,+g2W1,)12, $

    then

    $ ρ(xg1(x)τ)ρ(xg2(x)τ)LqCτρW1,q2g1g2Lq1,ρ(xg1(x)τ)ρ(xg2(x)τ)1Cτρ0g1g2W1,, $

    where $ 1/q_{1}+1/q_{2} = 1/q, 1 < q < \infty $.

    The following theorem gives insight on the error estimate between the time-discrete solution and the solution of the time-dependent MHD system (1.1).

    Theorem 3.1 Suppose that assumption A2–A3 are satisfied, there exists a positive $ C > 0 $ such that

    $ max0nm(em+120+μεm+120)+mn=0(en+1en20+μεn+1εn20),+τmn=0(ν˜en+120+μcurlεn+120)Cτ2,max0nm(νem+121+μcurlεm+120)+τmn=1(νen+1en21+μcurlεn+1curlεn20)+τmn=1(Dτen+120+μDτεn+120)Cτ2,τmn=0(en+122+˜en+122+εn+122)Cτ2,τmn=0(DτUn+122+Dτ˜Un+122+DτPn+121+DτBn+122+Un+12W2,d+Bn+12W2,d)+max0nm(Un+12+˜Un+12+Pn+11+Bn+12)C. $

    Proof. Firstly, we prove the following inequality

    $ em2+τ3/4UmW2,d1, $ (3.1)
    $ εm2+τ3/4BmW2,d1, $ (3.2)

    by mathematical induction for $ m = 0, 1, ..., N $.

    Since $ U^{0} = u^{0}, B^{0} = b^{0} $ the above inequality hold for $ m = 0 $.

    We assume that (3.1) and (3.2) hold for $ m\leq n $ for some integer $ n\geq0 $. By Sobolev embedding theory, we get

    $ UmLumL+Cem2C, $ (3.3)
    $ BmLbmL+Cεm2C, $ (3.4)

    and

    $ τUmW1,1/4, $ (3.5)
    $ τBmW1,1/4, $ (3.6)

    when $ \tau\leq\tau_{1} $ for some positive constant $ \tau_{1} $.

    To prove (3.1) and (3.2) for $ m = n+1 $, we rewrite (1.1) by

    $ Dτun+1νΔun+1+pn+1=fn+1unˉunτ+Rn+1tr1μbn+1×curlbn+1, $ (3.7)
    $ un+1=0, $ (3.8)
    $ Dτbn+1+curlcurlbn+1=gn+1bnˉbnτ+Rn+1tr2+bn+1un+1. $ (3.9)

    where $ u^{n+1} = U(t_{n+1}), b^{n+1} = B(t_{n+1}) $, and

    $ ˉln={ln(ˉx),ˉx=xunτΩ,0,otherwise. $

    and

    $ Rn+1tr1=un+1ˉunτun+1t(un+1)un+1,Rn+1tr2=bn+1ˉbnτbn+1t(un+1)bn+1, $

    define the truncation error. We can refer to ([31]), there holds that

    $ τNm=1Rmtr120Cτ2,τNm=1Rmtr220Cτ2. $ (3.10)

    The corresponding weak form is

    $ (Dτun+1,v)+ν(un+1,v)(pn+1,v)=(fn+1,v)(unˉunτ,v)+(Rn+1tr1,v)μ(bn+1×curlbn+1,v), $ (3.11)
    $ (un+1,ϕ)=0, $ (3.12)
    $ (Dτbn+1,ψ)+(curlbn+1,curlψ)=(gn+1,ψ)(bnˉbnτ,ψ)+(Rn+1tr2,ψ)+(bn+1(un+1un),ψ)+(bn+1un,ψ). $ (3.13)

    Combining (2.13) and (2.14), we obtain

    $ (Un+1ˆUnτ,v)+ν(˜Un+1,v)(Pn+1,v)+μ(Bnv,Bn+1)μ(vBn,Bn)=(fn+1,v). $ (3.14)

    Then, we rewrite (3.14) and (2.12) as

    $ (DτUn+1,v)+ν(˜Un+1,v)(Pn+1,v)=(fn+1,v)(UnˆUnτ,v)μ(Bnv,Bn+1)+μ(vBn,Bn), $ (3.15)

    and

    $ (DτBn+1,ψ)+(curlBn+1,curlψ)=(gn+1,ψ)+(BnUn,ψ)(BnˆBnτ,ψ). $ (3.16)

    Subtracting (3.15) and (3.16) from (3.11) and (3.13), respectively, leads to

    $ (Dτen+1,v)+ν(˜en+1,v)(ξn+1,v)=(unˉun(UnˆUn)τ,v)+(Rn+1tr1,v)μ((bn+1bn)vv(bn+1bn),bn+1)+μ(vbn,bn+1bn)+μ(vεn,bn)+μ(vBn,εn)μ(εnv,bn+1)μ(Bnv,εn+1), $ (3.17)

    and

    $ (Dτεn+1,ψ)+(curlεn+1,curlψ)=(bnˉbn(BnˆBn)τ,ψ)+(Rn+1tr2,ψ)+(bn+1(un+1un)+(bn+1bn)un+εnun+Bnen,ψ). $ (3.18)

    Testing (2.9) by $ \tau e^{n+1} $, since $ \nabla\cdot e^{n+1} = 0 $, we arrive at

    $ (en+1,˜en+1)=(en+1,en+1). $ (3.19)

    Testing (2.9) by $ \tau e^{n} $, we get

    $ (en,˜en+1)=(en,en+1). $ (3.20)

    Subtracting (3.20) from (3.19), we can obtain

    $ (en+1en,˜en+1)=(en+1en,en+1). $ (3.21)

    Let $ v = 2\tau \tilde{e}^{n+1} $ in (3.17), we obtain

    $ en+120en20+en+1en20+2ντ˜en+1202τ(ξn+1,˜en+1)=2((en(ˉunˆUn),˜en+1)+2τ(Rn+1tr1,˜en+1)2μτ((bn+1bn)˜en+1˜en+1(bn+1bn),bn)+2μτ(˜en+1bn,bn+1bn)+2μτ(˜en+1εn,bn)+2μτ(˜en+1Bn,εn)2μτ(εn˜en+1,bn+1)2μτ(Bn˜en+1,εn+1). $ (3.22)

    Let $ \psi = 2\mu\tau \varepsilon^{n+1} $ in (3.18), we get

    $ μεn+120μεn20+μεn+1εn20+2μτcurlεn+120=2μ(εn(ˉbnˆBn),εn+1)+2μτ(Rn+1tr2,εn+1)+2μτ(bn+1(un+1un)+(bn+1bn)un+εnun+Bnen,εn+1). $ (3.23)

    Taking sum of (3.22) and (3.23) yields

    $ en+120en20+μεn+120μεn20+en+1en20+μεn+1εn20+2ντ˜en+120+2μτcurlεn+1202τ(ξn+1,˜en+1)=2(enˆen,˜en+1)2(ˆunˉun,˜en+1)+2τ(Rn+1tr1,˜en+1)2μτ((bn+1bn)˜en+1,bn)+2μτ(˜en+1(bn+1bn),bn)+2μτ(˜en+1bn,bn+1bn)+2μτ(˜en+1εn,bn)+2μτ(˜en+1Bn,εn)2μτ(εn˜en+1,bn+1)2μτ(Bn˜en+1,εn+1)2μ(εnˆεn,εn+1)2μ(ˆbnˉbn,εn+1)+2μτ(Rn+1tr2,εn+1)+2μτ(bn+1(un+1un),εn+1)+2μτ((bn+1bn)un,εn+1)+2μτ(εnunεn+1)+2μτ(Bnen,εn+1). $ (3.24)

    From (2.9), we have

    $ ˜en+1=en+1+τ(ξn+1ξn)τ(pn+1pn). $

    Then, we can obtain

    $ 2τ(ξn+1,˜en+1)=2τ(ξn+1,en+1)+2τ2(ξn+1,(ξn+1ξn))4τ2(ξn+1,(pn+1pn))=τ2(ξn+120ξn20+ξn+1ξn20)2τ2(ξn+1,(pn+1pn))τ2(ξn+120ξn20+(ξn+1ξn)20)Cτ2ξn+10(pn+1pn)0=τ2(ξn+120ξn20+(ξn+1ξn)20)τ(Cτξn+10(pn+1pn)0)τ2(ξn+120ξn20+(ξn+1ξn)20)τ(Cτ2ξn+120+14(pn+1pn)20)=τ2(ξn+120ξn20+(ξn+1ξn)20)Cτ3ξn+120+14(pn+1pn)20). $

    On the other hand, using Lemma 3.1, we deduce that

    $ 2|(enˆen,˜en+1)|2enˆen1˜en+10Cτen0UnW1,˜en+10ντ12˜en+120+Cτen20,2|(ˆunˉun,˜en+1)|CˆunˉunL6/5˜en+1L6CτunL3en0˜en+10ντ12˜en+120+Cτen20,2τ|(Rn+1tr1,˜en+1)|Rn+1tr10˜en+10ντ12˜en+120+CτRn+1tr120,2μτ|((bn+1bn)˜en+1,bn+1)|Cτbn+1bn0˜en+10bn+12ντ12˜en+120+Cτ3bn+1t20bn+122,2μτ|(˜en+1(bn+1bn),bn+1)|Cτ˜en+1L6bn+1L3bn+1bn0ντ12˜en+120+Cτ3bn+1t20bn+122,2μτ|(˜en+1bn,bn+1bn)|Cτ˜en+1L6bn+1L3bn+1bn0ντ12˜en+120+Cτ3bn+1t20bn+122,2μτ|(˜en+1εn,bn)|Cτ˜en+10bn2εn0ντ12˜en+120+Cτεn20bn22,2μτ|(˜en+1Bn,εn)|Cτ˜en+10BnLεn0ντ12˜en+120+Cτεn20Bn2L,2μτ|(εn˜en+1,bn+1)|Cτεn0˜en+10bn+1Lντ12˜en+120+Cτεn20bn+122,2μτ|(Bn˜en+1,εn+1)|CτBnL˜en+10εn+10ντ12˜en+120+Cτεn20,2μτ|(εnˆεn,εn+1)|Cτεn0BnW1,εn+10μτ8curlεn+120+Cτεn20,2μτ|(ˆbnˉbn,εn+1)|CτˆbnˉbnL6/5εn+1L6CτbnL3εn0εn+10μτ8curlεn+120+Cτεn20,2μτ|(Rn+1tr2,εn+1)|CτRn+1tr20εn+10μτ8curlεn+120+CτRn+1tr220,2μτ|(bn+1(un+1un),εn+1)|Cτbn+12un+1un0curlεn+10μτ8curlεn+120+Cτun+1un20,2μτ|((bn+1bn)un,εn+1)|Cτbn+1bn0unLεn+10μτ8curlεn+120+Cτ3bn+1t20un2L,2μτ|(εnun,εn+1)|Cτεn0unL3curlεn+10μτ8curlεn+120+Cτεn20un2L3,2μτ|(Bnen,εn+1)|CτBnLen0curlεn+10μτ8curlεn+120+Cτen20. $

    Substituting these above inequality into (3.24), we obtain

    $ en+120en20+μεn+120μεn20+τ2ξn+120τ2ξn20+en+1en20+μεn+1εn20+τ2(ξn+1ξn)20+ντ˜en+120+μτcurlεn+120Cτ(en20+εn20+εn+120)+Cτ(Rn+1tr120+Rn+1tr220)+Cτ3(ξn+120+ξn20)+Cτ3. $ (3.25)

    Taking sum of the (3.25) for $ n $ from $ 0 $ to $ m\leq N $ and using discrete Gronwall's inequality Lemma 2.1, we get

    $ max0nm(em+120+μεm+120+τ2ξm+120)+mn=0(en+1en20+μεn+1εn20)+τmn=0(ν˜en+120+μcurlεn+120)Cτ2. $ (3.26)

    Owing to (3.21), we have

    $ ((en+1en),˜en+1)=((en+1en),en+1). $ (3.27)

    To acquire the $ H^{1} $ estimates, we take $ v = 2\tau D_{\tau}e^{n+1} $ in (3.17) to get

    $ ν(en+121en21+en+1en21)+2τDτen+120=2((en(ˉunˆUn),Dτen+1)+2τ(Rn+1tr1,Dτen+1)2μτ((bn+1bn)Dτen+1Dτen+1(bn+1bn),bn+1)+2μτ(Dτen+1bn,bn+1bn)+2μτ(Dτen+1εn,bn)+2μτ(Dτen+1Bn,εn)2μτ(εnDτen+1,bn+1)2μτ(BnDτen+1,εn+1). $ (3.28)

    Then, we take $ \psi = 2\mu\tau D_{\tau}\varepsilon^{n+1} $ in (3.18) to get

    $ μ(curlεn+120curlεn20+curlεn+1curlεn20)+2μτDτεn+120=2μ(εn(ˉbnˆBn),Dτεn+1)+2μτ(Rn+1tr2,Dτεn+1)+2μτ(bn+1(un+1un)+(bn+1bn)un+εnun+Bnen,Dτεn+1). $ (3.29)

    Combining (3.28) and (3.29), we obtain

    $ ν(en+121en21+en+1en21)+μ(curlεn+120curlεn20+curlεn+1curlεn20)+2τDτen+120+2ντDτεn+120=2(enˆen,Dτen+1)2(ˆunˉun,Dτen+1)+2τ(Rn+1tr1,Dτen+1)2μτ((bn+1bn)Dτen+1,bn+1)+2μτ(Dτen+1(bn+1bn),bn+1)+2μτ(Dτen+1bn,bn+1bn)+2μτ(Dτen+1εn,bn)+2μτ(Dτen+1Bn,εn)2μτ(εnDτen+1,bn+1)2μτ(BnDτen+1,εn+1)2μ(εnˆεn,εn+1)2μ(ˆbnˉbn,Dτεn+1)+2μτ(Rn+1tr2,Dτεn+1)+2μτ(bn+1(un+1un),Dτεn+1)+2μτ((bn+1bn)un,Dτεn+1)+2μτ(εnun,Dτεn+1)+2μτ(Bnen,Dτεn+1). $ (3.30)

    Similarly, by Lemma 3.1, we have

    $ 2|(enˆen,Dτen+1)|2enW1,2UnLDτen+10τ16Dτen+120+Cτen21,2|(ˆunˉun,Dτen+1)|CunW1,4enL4Dτen+10CunW1,4en1Dτen+10τ16Dτen+120+Cτen21,2τ|(Rn+1tr1,Dτen+1)|Rn+1tr10Dτen+10τ16Dτen+120+CτRn+1tr120,2μτ|((bn+1bn)Dτen+1,bn+1)|Cτ(bn+1bn)0Dτen+10bn+12τ16Dτen+120+Cτ3bn+1t20bn+122,2μτ|(Dτen+1(bn+1bn),bn+1)|CτDτen+10(bn+1bn)0bn+12τ16Dτen+120+Cτ3bn+1t20bn+122,2μτ|(Dτen+1bn,bn+1bn)|CτDτen+10bnLbn+1bn0τ16Dτen+120+Cτ3bn+1t20bn2L,2μτ|(Dτen+1εn,bn)|CτDτen+10εn0bnLτ16Dτen+120+Cτcurlεn20bn2L,2μτ|(Dτen+1Bn,εn)|CτDτen+10BnL3εnL6τ16Dτen+120+Cτεn20Bn2L3,τ16Dτen+120+Cτcurlεn20Bn2L3,2μτ|(εnDτen+1,bn+1)|Cτεn0Dτen+10bn+12τ16Dτen+120+Cτcurlεn20bn+122,2μτ|(BnDτen+1,εn+1)|CτBnLεn+10Dτen+10τ16Dτen+120+Cτcurlεn+120Bn2L,2μτ|(εnˆεn,Dτεn+1)|CτεnW1,2BnLDτεn+10μτ8Dτεn+120+Cτcurlεn20,2μτ|(ˆbnˉbn,Dτεn+1)|CτbnL4εnL4Dτεn+10μτ8Dτεn+120+Cτcurlεn20,2μτ|(Rn+1tr2,Dτεn+1)|CτRn+1tr20Dτεn+10μτ8Dτεn+120+CτRn+1tr220,2μτ|(bn+1(un+1un),Dτεn+1)|Cτbn+12(un+1un)0Dτεn+10μτ8Dτεn+120+Cτ(un+1un)20,2μτ|((bn+1bn)un,Dτεn+1)|Cτbn+1bn0unLDτεn+10μτ8Dτεn+120+Cτ3bn+1t20un2L,2μτ|(εnun,Dτεn+1)|Cτεn0unLDτεn+10μτ8Dτεn+120+Cτcurlεn20un2L,2μτ|(Bnen,Dτεn+1)|CτBnLen0Dτεn+10μτ8Dτεn+120+Cτen20Bn2L. $

    Substituting these above inequality into (3.30), we obtain

    $ ν(en+121en21+en+1en21)+μ(curlεn+120curlεn20+curlεn+1curlεn20)+τDτen+120+μτDτεn+120Cτ(en21+curlεn+120+curlεn20)+Cτ(Rn+1tr120+Rn+1tr220)+Cτ3. $ (3.31)

    Taking sum of the (3.31) for $ n $ from $ 0 $ to $ m\leq N $ and using discrete Gronwall's inequality Lemma 2.1, we arrive at

    $ max0nm(νem+121+μcurlεm+120)+τmn=1(νen+1en21+μcurlεn+1curlεn20)+τmn=1(Dτen+120+μDτεn+120)Cτ2. $ (3.32)

    Furthermore, the standard result for (3.17) and (3.18) with $ p = 2 $, respectively, leads to

    $ ˜en+12CDτen+10+Cτenˆen0+CτˆUnˉUn0+CRn+1tr10+Cbn+1bn0bn+1L+Cbn+1bn0bn+1L+CbnLbn+1bn0+Cεn0bnL+CBnW1,εn0+Cεn0bn+1L+CBnLcurlεn+10+ξn+11CDτen+10+CRn+1tr10+ξn+11+Cτ, $ (3.33)
    $ εn+12CDτεn+10+Cτεnˆεn0+Cτˆbnˉbn0+CRn+1tr20+Cbn+1Lun+1un0+Cbn+1bn0unL+Cεn0unL+CBnLen0CDτen+10+CRn+1tr0+Cτ. $ (3.34)

    Thanks to (3.19), we can obtain

    $ (Δen+1,Δen+1)=(Δ˜en+1,Δen+1),en+12˜en+12. $

    Then, we have

    $ τmn=0(en+122+˜en+122+εn+122)Cτ2. $ (3.35)

    From (3.35), we can arrive at

    $ max0nmUn+12max0nm(un+12+en+12)C, $ (3.36)
    $ max0nm˜Un+12max0nm(un+12+˜en+12)C, $ (3.37)
    $ max0nmPn+11max0nm(pn+11+ξn+11)C, $ (3.38)
    $ max0nmBn+12max0nm(bn+12+εn+12)C, $ (3.39)
    $ τmn=0DτUn+1222τmn=0(Dτun+122+Dτen+122)C, $ (3.40)
    $ τmn=0Dτ˜Un+1222τmn=0(Dτun+122+Dτ˜en+122)C, $ (3.41)
    $ τmn=0DτPn+1212τmn=0(Dτpn+121+Dτξn+121)C, $ (3.42)
    $ τmn=0DτBn+1222τmn=0(Dτbn+122+Dτεn+122)C. $ (3.43)

    From (2.7) and (2.5), we have

    $ DτUn+1νΔ˜Un+1+Pn+1=fn+1UnˆUnτμBn×curlBn+1, $ (3.44)

    and

    $ DτBn+1+curlcurlBn+1=gn+1+(Bn)UnBn+1ˆBnτ. $ (3.45)

    By (2.9), we have

    $ ΔUn+1=×טUn+1. $ (3.46)

    Considering the identity $ \nabla^2\tilde{U}^{n+1} = \nabla\nabla\cdot\tilde{U}^{n+1}-\nabla\times\nabla\times\tilde{U}^{n+1} $ and (3.46), we can obtain

    $ Δ˜Un+1=˜Un+1ΔUn+1. $ (3.47)

    Now, the standard result for the Stokes system (3.17) and (3.18) with $ p = d^{*} > 2 $, respectively, leads to

    $ Un+1W2,d+Pn+1˜Un+1W1,dCDτUn+1Ld+CτˆUnUnLd+Cfn+1Ld+CBn×curlBn+1LdC(DτUn+1Ld+UnLdUnL+fn+1Ld+BnLdcurlBn+1Ld), $ (3.48)
    $ Bn+1W2,dCDτBn+1Ld+Cgn+1Ld+CτˆBnBnLd+CBnUnLdC(DτBn+1Ld+gn+1Ld+BnLdBnL+BnLdUnLd). $ (3.49)

    By (3.48) and (3.49), we get

    $ τmn=0(Un+12W2,d+Bn+12W2,d)C. $ (3.50)

    Thus, we can obtain

    $ en+12+τ3/4Un+1W2,d1, $ (3.51)
    $ εn+12+τ3/4Bn+1W2,d1. $ (3.52)

    The proof is complete.

    For the sake of simplicity, we denote $ R_{h}^{n} = R_{h}(U^{n}, P^{n}), Q_{h}^{n} = Q_{h}(U^{n}, P^{n}), R_{0h} = R_{0h}B^{n}, n = 1, 2, ..., N $, and $ e_{h}^{n} = u_{h}^{n}-R_{h}^{n}, \tilde{e}_{h}^{n} = \tilde{u}_{h}^{n}-R_{h}^{n}, \varepsilon_{h}^{n} = B_{h}^{n}-R_{0h}^{n}, n = 1, 2, ..., N $, where $ R_{0h}:L^{2}(\Omega)^{3} \rightarrow W_{h} $ is the $ L^{2} $-orthogonal projection, where

    $ R0hww0+h(R0hww)0Chr+1wr+1, $ (3.53)
    $ R0hwLCw2, $ (3.54)
    $ R0hwLCwW1,. $ (3.55)

    Theorem 3.2 Suppose that assumption A2–A3 are satisfied, there exists a positive $ C > 0 $ such that

    $ um+1h20+μBm+1h20+mn=0(un+1hunh20+μBn+1hBnh20)+τmn=0(ν˜un+1h20+μcurlBn+1h20)C,um+1h21+μcurlBm+1h20+mn=0(un+1hunh21+μcurlBn+1hcurlBnh20)+τmn=0(νAun+1h20+μcurlBn+1h20)C,em+1h20+μεm+1h20+mn=0(en+1henh20+μεn+1hεnh20)+τmn=0(ν˜en+1h20+μcurlεn+1h20)Ch4,un+1h2L+μBn+1h2L+τmn=0(un+1h2W1,+μBn+1h2W1,)C. $

    Proof. We prove following estimates

    $ emh20+μεmh20+τnm=0(νemh20+μcurlεmh20)Ch4, $ (3.56)

    by mathematical induction for $ m = 0, 1, ..., N $.

    When $ m = 0, u_{h}^{0} = u_{0}, B_{h}^{0} = B_{0} $. It is obvious (3.56) holds at the initial time step. We assume that (3.56) holds for $ 0\leq m\leq n $ for some integer $ n\geq0 $. By (2.20), (2.21), (3.54), (3.55), Theorem 3.1 and inverse inequality, we infer

    $ τumhW1,(emhW1,+RmhW1,)C(hd2emh0+UmW1,+PmL)14, $ (3.57)
    $ τBmhW1,(εmhW1,+Rm0hW1,)C(hd2εmh0+BmW1,)14, $ (3.58)

    and

    $ τumhL(emhL+RmhL)C(hd2emh0+Um2+Pm1)C, $ (3.59)
    $ τBmhL(εmhL+Rm0hL)C(hd2εmh0+Bm2)C. $ (3.60)

    Combining (2.23) and (2.24), we obtain

    $ (un+1hˆunhτ,vh)+ν(˜un+1h,vh)(pn+1h,vh)+μ(Bnhvh,Bn+1h)μ(vhBnh,Bnh)=(fn+1,vh). $ (3.61)

    When $ m = n+1 $, letting $ v_{h} = 2\tau \tilde{u}_{h}^{n+1} $ in (3.61), we can obtain

    $ 2(un+1hunh+unhˆunh,˜un+1h)+2ντ(˜un+1h,˜un+1h)2τ(pn+1h,˜un+1h)+2μτ(Bnh˜un+1h,Bn+1h)2μτ(˜un+1hBnh,Bnh)=2τ(fn+1,˜un+1h). $ (3.62)

    Letting $ v_{h} = \tau u_{h}^{n+1} $ in (2.24), since $ \nabla\cdot u_{h}^{n+1} = 0 $, it follows that

    $ (un+1h,˜un+1h)=(un+1h,un+1h). $ (3.63)

    Taking $ v_{h} = \tau u_{h}^{n} $ in (2.24), we get

    $ (unh,˜un+1h)=(unh,un+1h). $ (3.64)

    Subtracting (3.64) from (3.63), we can obtain

    $ (un+1hunh,˜un+1h)=(un+1hunh,un+1h). $ (3.65)

    Using $ 2(a-b, a) = \|a\|_{0}^{2}-\|b\|_{0}^{2}+\|a-b\|_{0}^{2} $, leads to

    $ un+1h20unh20+un+1hunh20+2ντ˜un+1h202τ(pn+1h,˜un+1h)=2τ(fn+1,˜un+1h)+2(ˆunhunh,˜un+1h)2μτ(Bnh˜un+1h,Bn+1h). $ (3.66)

    Taking $ v_{h} = \nabla p_{h}^{n+1} $ in (2.24), we deduce

    $ 2τ(pn+1h,˜un+1)=2τ2pn+1h202τ2pnh20+2τ2pn+1hpnh20. $ (3.67)

    When $ m = n+1 $, letting $ \psi_{h} = 2\mu\tau B_{h}^{n+1} $ in (2.22), we can obtain

    $ 2μ(Bn+1hBnh+BnhˆBnh,Bn+1h)+2μτ(curlBn+1h,curlBn+1h)2μτ(Bnhunh,Bn+1h)=2μτ(gn+1,Bn+1h). $ (3.68)

    Using $ 2(a-b, a) = \|a\|_{0}^{2}-\|b\|_{0}^{2}+\|a-b\|_{0}^{2} $, we have

    $ μBn+1h20μBnh20+μBn+1hBnh20+2μτcurlBn+1h20=2μτ(gn+1,Bn+1h)+2μτ(Bnhunh,Bn+1h)+2μ(ˆBnhBnh,Bn+1h). $ (3.69)

    Taking sum of (3.66) and (3.69) yields

    $ un+1h20unh20+μBn+1h20μBnh20+2τ2pn+1h202τ2pnh20+un+1hunh20+μBn+1hBnh20+2τ2pn+1hpnh20+2ντ˜un+1h20+2μτcurlBn+1h20=2τ(fn+1,˜un+1h)+2μτ(gn+1,Bn+1h)+2(ˆunhunh,˜un+1h)2μτ(Bn˜un+1h,Bn+1h)+2μ(ˆBnhBnh,Bn+1h)+2μτ(Bnhunh,Bn+1h). $ (3.70)

    Then, we can obtain

    $ 2τ|(fn+1,˜un+1h)|Cτfn+10˜un+1h0ντ4˜un+1h20+Cτfn+120,2μτ|(gn+1,Bn+1h)|Cτgn+10Bn+1h0μτ4curlBn+1h20+Cτgn+120,2|(ˆunhunh,˜un+1h)|Cτˆunhunh6/5˜un+1hL6CτunhL3unh0˜un+1h0ντ4˜un+1h20+Cτunh20,2μτ|(Bn˜un+1h,Bn+1h)|CτBnL˜un+1h0Bn+1h0ντ4˜un+1h20+CτBn+1h20,2μ|(ˆBnhBnh,Bn+1h)|CτˆBnhBnh1Bn+1h1CτBnh0BnhW1,curlBn+1h0μτ4curlBn+1h20+CτBnh20Bnh2W1,,2μτ|(Bnunh,Bn+1h)|CτBnLunh0curlBn+1h0μτ4curlBn+1h20+Cτunh20. $

    Substituting these above estimates into (3.70), we obtain

    $ un+1h20unh20+μBn+1h20μBnh20+τ2pn+1h20τ2pnh20+un+1hunh20+μBn+1hBnh20+2τ2pn+1hpnh20+ντ˜un+1h20+μτcurlBn+1h20Cτfn+120+Cτgn+120+Cτunh20+CτBnh20. $ (3.71)

    Taking sum of the (3.71) for $ n $ from $ 0 $ to $ m\leq N $ and using discrete Gronwall's inequality Lemma 2.1, we get

    $ um+1h20+μBm+1h20+τ2pm+1h20+mn=0(un+1hunh20+μBn+1hBnh20)+τmn=0(ν˜un+1h20+μcurlBn+1h20)C. $

    Letting $ v_{h} = 2\tau\mathcal{A}u_{h}^{n+1} $ in (3.61), we obtain

    $ 2(un+1hunh+unhˆunh,Aun+1h)+2ντ(A˜un+1h,Aun+1h)+2μτ(BnhAun+1h,Bn+1h)2μτ(Aun+1hBnh,Bnh)=2τ(fn+1,Aun+1h). $ (3.72)

    From (3.63), we arrive at

    $ (A˜un+1h,Aun+1h)=(Aun+1h,Aun+1h),(A˜un+1h,Aun+1h)=Aun+1h20. $

    Using $ 2(a-b, a) = \|a\|_{0}^{2}-\|b\|_{0}^{2}+\|a-b\|_{0}^{2} $, leads to

    $ un+1h21unh21+un+1hunh21+2ντAun+1h20=2τ(fn+1,Aun+1h)+2(ˆunhunh,Aun+1h)2μτ(BnhAun+1h,Bn+1h). $ (3.73)

    Letting $ \psi_{h} = 2\mu\tau \mbox{curlcurl}B_{h}^{n+1} $ in (2.22), we can obtain

    $ μcurlBn+1h20μcurlBnh20+μcurlBn+1hcurlBnh20+2μτcurlcurlBn+1h20=2μτ(gn+1,curlcurlBn+1h)+2μτ(Bnhunh,curlcurlBn+1h)+2μ(ˆBnhBnh,curlcurlBn+1h). $ (3.74)

    Taking sum of (3.73) and (3.74) yields

    $ un+1h21unh21+μcurlBn+1h20μcurlBnh20+un+1hunh21+μcurlBn+1hcurlBnh20+2ντAun+1h20+2μτcurlcurlBn+1h20=2τ(fn+1,Aun+1h)+2(ˆunhunh,Aun+1h)+2τ(gn+1,curlcurlBn+1h)2μτ(BnhAun+1h,Bn+1h)+2μτ(Bnhunh,curlcurlBn+1h)+2μ(ˆBnhBnh,curlcurlBn+1h). $ (3.75)

    Then, we get

    $ 2τ|(fn+1,Aun+1h)|Cτfn+10Aun+1h0ντ6Aun+1h20+Cτfn+120,2μτ|(gn+1,curlcurlBn+1h)|Cτgn+10curlcurlBn+1h0μτ4curlcurlBn+1h20+Cτgn+120,2|(ˆunhunh,Aun+1h)|CτunhW1,4unhL4Aun+1h0ντ6Aun+1h20+Cτunh21,2μτ|(BnhAun+1h,Bn+1h)|CτBnhW1,Aun+1h0Bn+1h0ντ6Aun+1h20+CτcurlBn+1h20,2μτ|(Bnhunh,curlcurlBn+1h)|CτBnhLunh0curlcurlBn+1h0μτ4curlcurlBn+1h20+Cτunh20,2μ|(ˆBnhBnh,curlcurlBn+1h)|CτBnhW1,4BnhL4curlcurlBn+1h0μτ4curlcurlBn+1h20+CτBnh2W1,4Bnh2L4μτ4curlcurlBn+1h20+CτcurlBnh20. $

    Substituting these above estimates into (3.75), we obtain

    $ un+1h21unh21+μcurlBn+1h20μcurlBnh20+un+1hunh21+μcurlBn+1hcurlBnh20+ντAun+1h20+μτcurlcurlBn+1h20Cτfn+120+Cτgn+120+Cτunh21+Cτ(curlBn+1h20+curlBnh20). $ (3.76)

    Taking sum of the (3.76) for $ n $ from $ 0 $ to $ m\leq N $ and using discrete Gronwall's inequality Lemma 2.1, we get

    $ um+1h21+μcurlBm+1h20+mn=0(un+1hunh21+μcurlBn+1hcurlBnh20)+τmn=0(νAun+1h20+μcurlcurlBn+1h20)C. $

    Then, we rewrite (3.61) and (2.22) as

    $ (Dτun+1h,vh)+ν(˜un+1h,vh)(Pn+1h,vh)=(fn+1,vh)(unhˆunhτ,vh)μ(Bnhvh,Bn+1h)+μ(vhBnh,Bnh), $ (3.77)

    and

    $ (DτBn+1h,ψh)+(curlBn+1h,curlψh)=(gn+1,ψh)+(Bnhunh,ψh)(BnhˆBnhτ,ψh). $ (3.78)

    Subtracting (3.15) and (3.16) from (3.77) and (3.78), respectively, leads to

    $ (Dτen+1h,vh)+ν(˜en+1h,vh)(pn+1hQn+1h,vh)=(Dτ(Un+1Rn+1h),vh)+(ˆunhunhτ,vh)+(UnˆUnτ,vh)μ(εnhvh,Bn+1h)μ((Rn0hBn)vh,Bn+1h)μ(Bnvh,εn+1h)μ(Bnvh,Rn+10hBn+1)+μ(vhεnh,Bnh)+μ(vhBn,εnh)μ(vhBn,Rn0hBn)+μ(vh(Rn0hBn),Bnh), $ (3.79)

    and

    $ (Dτεn+1h,ψh)+(curlεn+1h,curlψnh)=(Dτ(Bn+1Rn+10h),ψh)+(BnhˆBnhτ,ψh)+(ˆBnBnτ,ψh)+(εnhunh,ψh)+((Rn0hBn)unh,ψh)+(Bn(RnhUn),ψh)+(Bnenh,ψh). $ (3.80)

    Taking $ v_{h} = 2\tau e_{h}^{n+1} $ in (2.24), we can obtain

    $ (en+1h,˜en+1h)=(en+1h,en+1h),(Dτen+1h,˜en+1h)=(Dτen+1h,en+1h),(en+1h,˜en+1h)=(en+1h,en+1h)=en+1h20. $

    Let $ \theta^{n} = U^{n}-R_{h}^{n}, \eta^{n} = B^{n}-R_{0h}^{n} $. By taking $ v_{h} = 2\tau e_{h}^{n+1} $ in (3.79), we deduce

    $ en+1h20enh20+en+1henh20+2ντen+1h20=2τ(DτUn+1Rh(DτUn+1,DτPn+1),en+1h)+2(ˆenhenh,en+1h)+2(θnˆθn,en+1h)2μτ(εnhen+1h,Bn+1h)2μτ(ηnen+1h,Bn+1h)2μτ(Bnen+1h,εn+1h)2μτ(Bnen+1h,ηn+1)+2μτ(en+1hεnh,Bnh)+2μτ(en+1hBn,εnh)+2μτ(en+1hBn,ηn)+2μτ(en+1hηn,Bnh). $ (3.81)

    By taking $ \psi_{h} = 2\mu\tau \varepsilon_{h}^{n+1} $ in (3.80), we have

    $ μεn+1h20μεnh20+μεn+1hεnh20+2μτcurlεn+1h20=2τ(DτBn+1R0hDτBn+1),εn+1h)+2μ(εnhˆεnh,εn+1h)+2μ(ηnˆηn,εn+1h)+2μτ(εnhunh,εn+1h)2μτ(ηnunh,εn+1h)+2μτ(Bn(RnhUn),εn+1h)+2μτ(Bnenh,εn+1h). $ (3.82)

    Combining (3.81) and (3.82), we obtain

    $ en+1h20enh20+μεn+1h20μεnh20+en+1henh20+Sεn+1hεnh20+2ντen+1h20+2μτcurlεn+1h20=2τ(DτUn+1Rh(DτUn+1,DτPn+1),en+1h)+2(ˆenhenh,en+1h)+2(θnˆθn,en+1h)2μτ(εnhen+1h,Bn+1h)2μτ(ηnen+1h,Bn+1h)2μτ(Bnen+1h,Bn+1h)2μτ(Bnen+1h,ηn+1)+2μτ(en+1hεnh,Bnh)+2μτ(en+1hBn,εnh)+2μτ(en+1hBn,ηn)+2μτ(en+1hηn,Bnh)+2τ(DτBn+1R0hDτBn+1),εn+1h)+2μ(εnhˆεnh,εn+1h)+2μ(ηnˆηn,εn+1h)+2μτ(εnhunh,εn+1h)2μτ(ηnunh,εn+1h)+2μτ(Bn(RnhUn),εn+1h)2μτ(Bnenh,Bn+1h). $ (3.83)

    Due to (2.19) and Theorem 3.1, we can get $ \|\theta^{n}\|_{0}\leq Ch^{2}(\|U^{n}\|_{2}+\|P^{n}\|_{1})\leq Ch^{2} $, $ \|\eta^{n}\|_{0}\leq Ch^{2}\|B^{n}\|_{2} $. By Lemma 3.1, there holds that

    $ 2τ|(DτUn+1Rh(DτUn+1,DτPn+1),en+1h)|Cτh2en+1h0(DτUn+12+DτPn+11)ντ20en+1h20+Cτh4(DτUn+122+DτPn+121),2|(ˆenhenh,en+1h)|Cenhˆenh1en+1h0Cτenh0unhW1,en+1h0ντ20en+1h20+Cτenh20,2|(θnˆθn,en+1h)|Cθnˆθn1en+1h0Cτθn0unhW1,en+1h0ντ20en+1h20+Cτθn20,2μτ|(εnhen+1h,Bn+1h)|Cτεnh0en+1h0Bn+1hLντ20en+1h20+Cτεnh20,2μτ|(ηnen+1h,Bn+1h)|Cτηn0en+1h0Bn+1hLντ20en+1h20+Cτηn20,2μτ|(Bnen+1h,εn+1h)|CτBnLen+1h0εn+1h0ντ20en+1h20+Cτεn+1h20,2μτ|(Bnen+1h,ηn+1)|CτBn2en+1h0ηn+10ντ20en+1h20+Cτηn+120,2μτ|(en+1hεnh,Bnh)|Cτen+1h0εnh0BnhLντ20en+1h20+Cτεnh20,2μτ|(en+1hBn,εn+1h)|Cτen+1h0Bn2εnh0ντ20en+1h20+Cτεnh20,2μτ|(en+1hBn,ηn)|Cτen+1h0Bn2ηn0ντ20en+1h20+Cτηn20,2μτ|(en+1hηn,Bnh)|Cτen+1h0ηn0BnhLντ20en+1h20+Cτηn20,2τ|(DτBn+1R0hDτBn+1,εn+1h)|Cτh2εn+1h0DτBn+12ηn0μτ16curlεn+1h20+Cτh4DτBn+122,2μ|(εnhˆεnh,εn+1h)|Cεnhˆεnh0εn+1h0Cτcurlεnh0BnhLεn+1h0μτ16curlεn+1h20+Cτεn+1h20,2μ|(ηnˆηn,εn+1h)|Cηnˆηn1εn+1h0Cτηn0BnhW1,curlεn+1h0μτ16curlεn+1h20+Cτηn20,2μτ|(εnhunh,εn+1h)|Cτεnh0unhL3εn+1hL6μτ16curlεn+1h20+Cτεn+1h20,2μτ|(ηnunh,εn+1h)|Cτηn0unhL3εn+1hL6μτ16curlεn+1h20+Cτηn20,2μτ|(Bn(RnhUn),εn+1h)|CτBn2θn0εn+1h0μτ16curlεn+1h20+Cτθn20,2μτ|(Bnenh,εn+1h)|CτBnLenh0curlεn+1h0μτ16curlεn+1h20+Cτenh20. $

    Substituting these above estimates into (3.83), we obtain

    $ \begin{align} &\|e_{h}^{n+1}\|_{0}^{2}-\|e_{h}^{n}\|_{0}^{2}+\mu\|\varepsilon_{h}^{n+1}\|_{0}^{2}-\mu\|\varepsilon_{h}^{n}\|_{0}^{2} +\|e_{h}^{n+1}-e_{h}^{n}\|_{0}^{2} +\mu\|\varepsilon_{h}^{n+1}-\varepsilon_{h}^{n}\|_{0}^{2} \\ &+\nu\tau\| \nabla e_{h}^{n+1}\|_{0}^{2}+\mu\tau\|\mbox{curl}\varepsilon_{h}^{n+1}\|_{0}^{2} \\ \leq & C\tau(\|e_{h}^{n}\|_{0}^{2}+\|\varepsilon_{h}^{n}\|_{0}^{2}+\|\varepsilon_{h}^{n+1}\|_{0}^{2}) + C\tau h^{4}. \end{align} $ (3.84)

    Taking the sum of the (3.76) for $ n $ from $ 0 $ to $ m\leq N $ and using the discrete Gronwall's inequality Lemma 2.1, we get

    $ \begin{align*} &\|e_{h}^{m+1}\|_{0}^{2}+\mu\|\varepsilon_{h}^{m+1}\|_{0}^{2} +\sum\limits_{n = 0}^{m}(\|e_{h}^{n+1}-e_{h}^{n}\|_{0}^{2} +\mu\|\varepsilon_{h}^{n+1}-\varepsilon_{h}^{n}\|_{0}^{2}) \nonumber\\ &+\tau \sum\limits_{n = 0}^m\left(\nu\|\nabla e_{h}^{n+1}\|_{0}^{2} +\mu\|\mbox{curl}\varepsilon_{h}^{n+1}\|_{0}^{2}\right)\leq C h^{4}. \end{align*} $

    Then, we can deduce

    $ \begin{align*} \max\limits_{0\leq n\leq m}\|u_{h}^{n+1}\|_{L^{\infty}} &\leq\max\limits_{0\leq n\leq m}(\|R_{h}^{n+1}\|_{L^{\infty}}+\|e_{h}^{n+1}\|_{L^{\infty}}) \nonumber\\ &\leq C\max\limits_{0\leq n\leq m}\|U^{n+1}\|_{2}+C\max\limits_{0\leq n\leq m}\|P^{n+1}\|_{1}+Ch^{-\frac{1}{2}}\max\limits_{0\leq n\leq m}\|e_{h}^{n+1}\|_{0} \leq C, \nonumber\\ \max\limits_{0\leq n\leq m}\|B_{h}^{n+1}\|_{L^{\infty}} &\leq\max\limits_{0\leq n\leq m}(\|R_{0h}^{n+1}\|_{L^{\infty}}+\|\varepsilon_{h}^{n+1}\|_{L^{\infty}}) \nonumber\\ &\leq C\max\limits_{0\leq n\leq m}\|B^{n+1}\|_{2}+Ch^{-\frac{1}{2}}\max\limits_{0\leq n\leq m}\|\varepsilon_{h}^{n+1}\|_{0} \leq C, \nonumber\\ \tau\sum\limits_{n = 0}^m\|u_{h}^{n+1}\|_{W^{1,\infty}}^{2} &\leq 2\tau\sum\limits_{n = 0}^m(\|R_{h}^{n+1}\|_{W^{1,\infty}}^{2}+\|e_{h}^{n+1}\|_{W^{1,\infty}}^{2}) \nonumber\\ &\leq C\tau\sum\limits_{n = 0}^m(\|U^{n+1}\|_{W^{1,\infty}}^{2}+\|P^{n+1}\|_{L^{\infty}}^{2}) +C\tau h^{-2}\sum\limits_{n = 0}^m\|\nabla e_{h}^{n+1}\|_{0}^{2}\leq C, \nonumber\\ \tau\sum\limits_{n = 0}^m\|B_{h}^{n+1}\|_{W^{1,\infty}}^{2} &\leq 2\tau\sum\limits_{n = 0}^m(\|R_{0h}^{n+1}\|_{W^{1,\infty}}^{2}+\|\varepsilon_{h}^{n+1}\|_{W^{1,\infty}}^{2}) \nonumber\\ &\leq C\tau\sum\limits_{n = 0}^m\|B^{n+1}\|_{W^{1,\infty}}^{2} +C\tau h^{-2}\sum\limits_{n = 0}^m\|\nabla \varepsilon_{h}^{n+1}\|_{0}^{2} \leq C. \end{align*} $

    Thus, all the results in Theorem 3.2 have been proved.

    In order to show the effect of our method, we present some numerical results. Here, we consider the

    $ \begin{align*} u_1& = (y+y^4)*cos(t),\\ u_2& = (x+x^2)*cos(t),\\ p& = (2.0*x-1.0)*(2.0*y-1.0)*cos(t),\\ B_1& = (sin(y)+y)*cos(t),\\ B_2& = (sin(x)+x^2)*cos(t). \end{align*} $

    The boundary conditions and the forcing terms are given by the exact solution. The finite element spaces are chosen as P1b-P1-P1b finite element spaces. Here, we choose $ \tau = h^2 $ and $ h = 1/n $, $ n = 8, 16, 24, 32, 40, 48 $. Different Reynolds number $ Re $ and magnetic Reynolds number $ Rm $ are chosen to show the effect of our method, the numerical results were shown in Tables 16. Here, we use the software package FreeFEM++ [32] for our program.

    Table 1.  The numerical results for $ Re = 1.0 $ and $ Rm = 1.0 $ for different $ h $ at $ T = 1.0 $.
    $ 1/h$ $ \|{\bf{u}}_h^1-{\bf{u}}\|_0$ $ \|\nabla({\bf{u}}_{h}^1-{\bf{u}}_1)\|_0$ $ \|p_h^1-p\|_0$ $ \|{\bf{B}}_h^1-{\bf{B}}\|_0$ $ \|\nabla({\bf{B}}_h^1-{\bf{B}})\|_0$ $ \|E_h^1-E\|_0$ $ |\int_{\Omega} \nabla\cdot B dx|$ $ |\int_{\Omega} \nabla\cdot {\bf{u}} dx |$
    8 0.00395939 0.104518 0.0633311 0.00146071 0.0443105 0.0402426 4.95209e-17 1.99024e-14
    16 0.00109704 0.0527177 0.0212408 0.000384435 0.0221793 0.0208612 1.41995e-16 4.15508e-15
    24 0.000473611 0.0347495 0.0118957 0.000175071 0.0148256 0.0141051 5.03032e-17 5.95751e-15
    32 0.000256132 0.0258999 0.010131 9.82405e-05 0.0111161 0.010656 3.35182e-16 7.38832e-15
    40 0.000160786 0.0206654 0.00831346 6.2778e-05 0.00889124 0.00856254 7.97451e-16 5.55468e-15
    48 0.000110796 0.0171977 0.00648101 4.35568e-05 0.00740848 0.00715659 1.00015e-16 4.15077e-15

     | Show Table
    DownLoad: CSV
    Table 2.  Convergence order for $ Re = 1.0 $ and $ Rm = 1.0 $ for different $ h $ at $ T = 1.0 $.
    $ 1/h$ $ {\bf{u}}-L^2$ $ {\bf{u}}-H_1$ $ P-L^2$ $ {\bf{B}}-L^2$ $ {\bf{B}}-H_1$
    16 1.85166 0.987389 1.57608 1.92586 0.998435
    24 2.07166 1.02792 1.42982 1.93995 0.99343
    32 2.13671 1.0217 0.5582 2.00837 1.00097
    40 2.08665 1.01181 0.88607 2.00685 1.00084
    48 2.04246 1.00748 1.36571 2.00491 1.00066

     | Show Table
    DownLoad: CSV
    Table 3.  The numerical results for $ Re = 100.0 $ and $ Rm = 100.0 $ for different $ h $ at $ T = 1.0 $.
    $ 1/h$ $ \|{\bf{u}}_h^1-{\bf{u}}\|_0$ $ \|\nabla({\bf{u}}_{h}^1-{\bf{u}}_1)\|_0$ $ \|p_h^1-p\|_0$ $ \|{\bf{B}}_h^1-{\bf{B}}\|_0$ $ \|\nabla({\bf{B}}_h^1-{\bf{B}})\|_0$ $ \|E_h^1-E\|_0$ $ |\int_{\Omega} \nabla\cdot B dx|$ $ |\int_{\Omega} \nabla\cdot {\bf{u}} dx |$
    8 0.0172667 0.279161 0.0354359 0.0127351 0.19731 0.00779509 3.41253e-17 3.17954e-14
    16 0.00233046 0.0697115 0.01409 0.00240034 0.0479792 0.00159673 9.63754e-17 1.3337e-14
    24 0.000831767 0.0398145 0.00892872 0.000977778 0.0269431 0.000711847 8.03834e-18 8.42217e-15
    32 0.000422379 0.0280313 0.00649219 0.000527044 0.0184095 0.000385869 2.36579e-16 5.31439e-15
    40 0.000256947 0.0217553 0.00511173 0.000330314 0.013926 0.000239563 7.44033e-16 2.85026e-15
    48 0.000173343 0.0178297 0.00421705 0.000226765 0.0112143 0.000162916 7.14376e-17 2.41418e-15

     | Show Table
    DownLoad: CSV
    Table 4.  Convergence order for $ Re = 100.0 $ and $ Rm = 100.0 $ for different $ h $ at $ T = 1.0 $.
    $ 1/h$ $ {\bf{u}}-L^2$ $ {\bf{u}}-H_1$ $ P-L^2$ $ {\bf{B}}-L^2$ $ {\bf{B}}-H_1$
    16 2.88931 2.00163 1.33054 2.4075 2.03998
    24 2.54095 1.38147 1.12512 2.21495 1.42315
    32 2.35555 1.21978 1.10773 2.1482 1.32391
    40 2.22742 1.13589 1.07134 2.09391 1.2508
    48 2.1588 1.09144 1.05529 2.063 1.1878

     | Show Table
    DownLoad: CSV
    Table 5.  The numerical results for $ Re = 1000.0 $ and $ Rm = 1000.0 $ for different $ h $ at $ T = 1.0 $.
    $ 1/h$ $ \|{\bf{u}}_h^1-{\bf{u}}\|_0$ $ \|\nabla({\bf{u}}_{h}^1-{\bf{u}}_1)\|_0$ $ \|p_h^1-p\|_0$ $ \|{\bf{B}}_h^1-{\bf{B}}\|_0$ $ \|\nabla({\bf{B}}_h^1-{\bf{B}})\|_0$ $ \|E_h^1-E\|_0$ $ |\int_{\Omega} \nabla\cdot B dx|$ $ |\int_{\Omega} \nabla\cdot {\bf{u}} dx |$
    8 0.0397337 1.68458 0.0330041 0.029124 1.17999 0.0213862 1.41814e-16 1.99053e-14
    16 0.00470743 0.368592 0.0137702 0.00403407 0.219747 0.00228304 7.44847e-17 1.26213e-14
    24 0.00158913 0.177215 0.00901776 0.00143967 0.106852 0.000843484 8.59434e-17 8.328e-15
    32 0.000750714 0.102493 0.00644056 0.000756826 0.0713168 0.000456685 2.75943e-16 5.37049e-15
    40 0.000430867 0.0675245 0.00509083 0.000460115 0.0510193 0.000286991 8.76007e-16 2.73806e-15
    48 0.000276193 0.0481754 0.00421193 0.000306103 0.038122 0.000195792 1.25266e-16 2.4751e-15

     | Show Table
    DownLoad: CSV
    Table 6.  Convergence order for $ Re = 1000.0 $ and $ Rm = 1000.0 $ for different $ h $ at $ T = 1.0 $.
    $ 1/h$ $ {\bf{u}}-L^2$ $ {\bf{u}}-H_1$ $ P-L^2$ $ {\bf{B}}-L^2$ $ {\bf{B}}-H_1$
    16 3.07735 2.19229 1.26109 2.8519 2.42487
    24 2.6783 1.80614 1.04402 2.54119 1.77829
    32 2.60675 1.90339 1.16997 2.23523 1.40541
    40 2.48819 1.8701 1.05392 2.23021 1.50095
    48 2.43909 1.85191 1.03949 2.23536 1.59834

     | Show Table
    DownLoad: CSV

    Tables 1 and 2 give the numerical results for $ Re = 1.0 $ and $ Rm = 1.0 $. In order to show the effect of our method for high Reynolds number, we give the numerical results for $ Re = 100.0, \ 1000.0 $ and $ Rm = 100.0, \ 1000.0 $. Tables 3 and 4 give the numerical results for $ Re = 100.0 $ and $ Rm = 100.0 $. Tables 5 and 6 give the numerical results for $ Re = 1000.0 $ and $ Rm = 1000.0 $. It shows that our method is effect for high Reynolds numbers. It shows that the errors are goes small as the space step goes small and the convergence orders are optimal. We can see that $ |\int_{\Omega} \nabla\cdot B dx| $ and $ |\int_{\Omega} \nabla\cdot {{\bf u}} dx| $ are small, which means that our method can conserve the Gauss's law very well.

    In this paper, we have given a modified characteristics projection finite element method for the unsteady incompressible magnetohydrodynamics(MHD) equations. In this method, the modified characteristics finite element method and the projection method were combined for solving the incompressible MHD equations. Both the stability and the optimal error estimates in $ L^2 $ and $ H^1 $ norms for the modified characteristics projection finite element method have been given. In order to demonstrate the effectiveness of our method, some numerical results were given at the end of the manuscript.

    The authors would like to thank the anonymous referees for their valuable suggestions and comments, which helped to improve the quality of the paper. This work is supported in part by National Natural Science Foundation of China (No. 11971152), China Postdoctoral Science Foundation (No. 2018M630907) and the Key scientific research projects Henan Colleges and Universities (No. 19B110007).

    The authors declare there is no conflict of interest in this paper.

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