1. Introduction
In this paper, we shall construct piecewise divergencefree nonconforming virtual elements for Stokes problem in any dimensions. Assume that is a bounded polytope. The Stokes problem is governed by
(1.1) 
where is the velocity field, is the pressure, is the symmetric gradient of , external force field , and constant is the viscosity. The incompressibility constraint in (1.1) describes the conservation of mass for the incompressible fluid.
Since the nonconforming  element is a stable pair for the Stokes problem [21], as the generalization of the nonconforming element, it is spontaneous that the nonconforming virtual element in [5] is adopted to discretize the Stokes problem in [13, 25]. While the incompressibility constraint is not satisfied exactly in general at the discrete level for the discrete methods in [13, 25], which is very important for the Brinkman problem [28] and the NavierStokes problem [24]. To design the discrete method with the exact divergencefree discrete velocity, one idea is to combine the discontinuous Galerkin technique and the conforming finite elements or virtual elements, such as the discontinuous Galerkin conforming method [18] and the divergencefree weak virtual element method [16]. The more compact idea in [7, 6, 2] is to construct divergencefree conforming virtual elements in two and three dimensions by defining the space of shape functions through the local Stokes problem with Dirichlet boundary condition. By enriching an conforming virtual element with some divergencefree functions, a divergencefree nonconforming virtual element in two dimensions is advanced in [29], in which each element in the partition is required to be convex. We refer to [12] for a virtual element method based on the pseudostressvelocity formulation.
Following the ideas in [15, 22], we shall devise piecewise divergencefree nonconforming virtual elements in any dimensions based on the generalized Green’s identity for Stokes problem, which are also
nonconforming. The degrees of freedom of the proposed virtual elements for the velocity are same as those in
[13], i.e. copies of the degrees of freedom of the nonconforming virtual elements in [5]. And the space of shape functions for the velocity is defined from the local Stokes problem with Neumann boundary condition, which is different from that in [7] due to the constraint on the boundary. Our virtual elements are locally divergencefree since .A novelty of this paper is to introduce a local energy projector based on the Stokes problem:
while the local projector is adopted in all the previous papers. The local Stokesbased projector commutes with the divergence operator, i.e.
(1.2) 
Then we define a stabilization involving all the degrees of freedom of the virtual elements for the velocity except those corresponding to . With the help of the local projector and the stabilization, we propose a piecewise divergencefree nonconforming virtual element method for Stokes problem, where the velocity is discretized by the virtual elements and the pressure is discretized by the piecewise polynomials. Furthermore, applying the technique in [7, 17, 26], we remove the degrees of freedom corresponding to for the velocity, reduce the space of shape functions to , and then derive the reduced virtual element method, in which the pressure is discretized by the piecewise constant. Hence we can first acquire the discrete velocity by solving the reduced discrete method, and then recover the discrete pressure elementwisely.
A detailed and rigorous error analysis is presented for the piecewise divergencefree nonconforming virtual element method. We first prove the inverse inequality
(1.3) 
by using the fact that is a norm on the finitedimensional space , where satisfies . Then we derive the norm equivalence of the stabilization on the kernel of the local projector from (1.2) and (1.3). The interpolation error estimate is acquired after setting up the Galerkin orthogonality of the interpolation operator. With the norm equivalence of the stabilization and the interpolation error estimate, we build up the discrete infsup condition, and thus the piecewise divergencefree nonconforming virtual element method is wellposed. Finally the optimal error estimate comes from the discrete infsup condition and the interpolation error estimate in a standard way.
The rest of this paper is organized as follows. In Section 2, we present some notation and inequalities. The divergencefree nonconforming virtual elements, local energy projector, stabilization and interpolation operator are constructed in Section 3. We show the divergencefree nonconforming virtual element method for the Stokes problem and its error analysis in Section 4. A reduced virtual element method is given in Section 5. In Section 6, numerical results are provided to verify the theoretical convergence.
2. Preliminaries
2.1. Notation
Denote by the space of all tensors, the space of all symmetric tensors, and
the space of all skewsymmetric
tensors. Denote the deviatoric part and the trace of the tensor by and accordingly, then we haveGiven a bounded domain and a nonnegative integer , let be the usual Sobolev space of functions on , and
be the usual Sobolev space of functions taking values in the finitedimensional vector space
for being , , or . The corresponding norm and seminorm are denoted respectively by and . Let be the standard inner product on or . If is , we abbreviate , and by , and , respectively. Let be the closure of with respect to the norm . For integer , notation stands for the set of all polynomials over with the total degree no more than . Set . And denote by the vectorial or tensorial version space of . Let () be the orthogonal projector onto ().Let be a family of partitions of into nonoverlapping simple polytopal elements with and . Let be the set of all dimensional faces of the partition for . Moreover, we set for each
Similarly, for , we define
For any , denote by its diameter and fix a unit normal vector . For any , denote by the unit outward normal to . Without causing any confusion, we will abbreviate as for simplicity.
For nonnegative integer , let
Define
and the usual broken type norm and seminorm
Let and be the piecewise counterparts of and with respect to .
We introduce jumps on ()dimensional faces. Consider two adjacent elements and sharing an interior ()dimensional face . Denote by and the unit outward normals to the common face of the elements and , respectively. For a scalarvalued or tensorvalued function , write and . Then define the jump on as follows:
On a face lying on the boundary , the above term is defined by
Denote the space of rigid motions by
where . For any , is defined by
For positive integer , set . Take being any subspace of such that
(2.1) 
where is the direct sum. One choice of is given by (3.11) in [3, 4]
(2.2) 
where and is the exterior product. We can also take
(2.3) 
where is the centroid of . Let be the orthogonal projector onto .
2.2. Mesh conditions and some inequalities
We impose the following conditions on the mesh in this paper:

Each element is starshaped with respect to a ball with radius , where the chunkiness parameter is uniformly bounded;

There exists a quasiuniform simplicial mesh such that each is a union of some simplexes in .
Throughout this paper, we use “” to mean that “”, where is a generic positive constant independent of the mesh size and the viscosity , but may depend on the chunkiness parameter of the polytope, the degree of polynomials , the dimension of space , and the shape regularity and quasiuniform constants of the virtual triangulation , which may take different values at different appearances. And means and .
Under the mesh condition (A1), we have the trace inequality of [11, (2.18)]
(2.4) 
the PoincaréFriedrichs inequality [11, (2.15)]
(2.5) 
and the Korn’s second inequality [23, 1]
(2.6) 
Recall the BabuškaAziz inequality [19, 8, 20]: for any , there exists such that
(2.7) 
When , we can choose . Combined with the proof of Proposition 9.1.1 in [9], it holds for any satisfying that [14, Lemma 3.4]
(2.8) 
Let be the regular inscribed simplex of , where all the edges of share the common length. It holds for any nonnegative integers and that [22, Lemma 4.3 and Lemma 4.4]
(2.9) 
(2.10) 
Lemma 2.1.
For any nonnegative integers , and , we have
(2.11) 
Proof.
Recall the error estimates of the projection. For each and nonnegative integer , we have
(2.13)  
(2.14) 
Lemma 2.2.
We have for any that
(2.15) 
Proof.
Due to (2.7), there exists such that
Let be the BrezziDouglasMarini interpolation [9, 4], then
It follows from the inverse inequality (2.10) and (2.13) that
Noting that can be spontaneously extended to the domain , let such that . Thus
Again due to being a polynomial,
And it follows from (2.9)
Therefore we arrive at (2.15). ∎
3. DivergenceFree Nonconforming Virtual Elements
We will construct the divergencefree nonconforming virtual elements for Stokes problem in this section.
3.1. Virtual elements
For any , and satisfying , and for each , it follows from the integration by parts
(3.1) 
Following the ideas in [15, 22], suppose and temporarily. Inspired by the Green’s identity (3.1), we propose the following local degrees of freedom of the divergencefree nonconforming virtual elements for Stokes problem
(3.2)  
(3.3) 
Define the space of shape functions as
For any , since , by the direct sum decomposition (2.1) there exists such that . And then for each , thus
Proof.
Consider the local Stokes problem with the Neumann boundary condition
(3.4) 
where , , and . Employing the Green’s identity (3.1), we acquire
(3.5) 
If taking in (3.5), we have the compatibility condition
(3.6) 
Given , , and satisfying the compatibility condition (3.6), due to (3.5), the weak formulation of the local problem (3.4) is to find and such that
(3.7) 
for all and . According to the BabuškaBrezzi theory [9], the mixed formulation (3.7) is uniquely solvable.
On the other hand, if and there exist such that and , then , which implies , and thus is a constant. Hence the function in the definition of forms a onedimensional vector space. As a result, we have
Furthermore, if all the data , and vanish, then the solution of the local Stokes problem (3.4) belongs to . Therefore the dimension of is . ∎
Proof.
Let and suppose all the degrees of freedom (3.2)(3.3) vanish. We get from the integration by parts
Thus . Due to the definition of , there exists some such that , and for each . Then it follows from (3.1) that
which together with indicates . Applying the integration by parts,
Therefore is constant, and thus , as required. ∎
Remark 3.3.
We have for , thus the virtual element is exactly the nonconforming element in [21].
3.2. Local projection
With the degrees of freedom (3.2)(3.3), define a local operator as follows: given , let and be the solution of the local Stokes problem
(3.8)  
(3.9)  
(3.10)  
(3.11) 
Similarly as (3.2) in [11], an equivalent formulation of the local Stokes problem (3.8)(3.11) is
where
with symbols and being the inner products of the tensors and vectors respectively.
The infsup condition (2.15) indicates is a stable pair for Stokes problem, thus the local Stokes problem (3.8)(3.11) is uniquely solvable. To simplify the notation, we will rewrite as . Apparently the projector can be computed using only the degrees of freedom (3.2)(3.3). The unique solvability of the local Stokes problem (3.8)(3.11) implies the operator is a projector, i.e.
(3.12) 
It follows from (3.10)(3.11), (2.13)(2.14) and the Korn’s second inequality (2.6) that
(3.13) 
where . Due to (3.9), the local Stokesbased projector commutes with the divergence operator, i.e.
(3.14) 
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