472 research outputs found
On really locking-free mixed finite element methods for the transient incompressible Stokes equations
Inf-sup stable mixed methods for the steady incompressible Stokes
equations that relax the divergence constraint are often claimed to deliver
locking-free discretizations. However, this relaxation leads to a
pressure-dependent contribution in the velocity error, which is proportional
to the inverse of the viscosity, thus giving rise to a (different) locking
phenomenon. However, a recently proposed modification of the right hand side
alone leads to a discretization that is really locking-free, i.e., its
velocity error converges with optimal order and is independent of the
pressure and the smallness of the viscosity. In this contribution, we extend
this approach to the transient incompressible Stokes equations, where besides
the right hand side also the velocity time derivative requires an improved
space discretization. Semi-discrete and fully-discrete a-priori velocity and
pressure error estimates are derived, which show beautiful robustness
properties. Two numerical examples illustrate the superior accuracy of
pressure-robust space discretizations in the case of small viscosities
On really locking-free mixed finite element methods for the transient incompressible Stokes equations
Inf-sup stable mixed methods for the steady incompressible Stokes equations that relax the divergence constraint are often claimed to deliver locking-free discretizations. However, this relaxation leads to a pressure-dependent contribution in the velocity error, which is proportional to the inverse of the viscosity, thus giving rise to a (different) locking phenomenon. However, a recently proposed modification of the right hand side alone leads to a discretization that is really locking-free, i.e., its velocity error converges with optimal order and is independent of the pressure and the smallness of the viscosity. In this contribution, we extend this approach to the transient incompressible Stokes equations, where besides the right hand side also the velocity time derivative requires an improved space discretization. Semi-discrete and fully-discrete a-priori velocity and pressure error estimates are derived, which show beautiful robustness properties. Two numerical examples illustrate the superior accuracy of pressure-robust space discretizations in the case of small viscosities
Towards pressure-robust mixed methods for the incompressible Navier--Stokes equations
In this contribution, classical mixed methods for the incompressible Navier-Stokes equations that relax the divergence constraint and are discretely inf-sup stable, are reviewed. Though the relaxation of the divergence constraint was claimed to be harmless since the beginning of the 1970ies, Poisson locking is just replaced by another more subtle kind of locking phenomenon, which is sometimes called poor mass conservation. Indeed, divergence-free mixed methods and classical mixed methods behave qualitatively in a different way: divergence-free mixed methods are pressure-robust, which means that, e.g., their velocity error is independent of the continuous pressure. The lack of pressure-robustness in classical mixed methods can be traced back to a consistency error of an appropriately defined discrete Helmholtz projector. Numerical analysis and numerical examples reveal that really locking-free mixed methods must be discretely inf-sup stable and pressure-robust, simultaneously. Further, a recent discovery shows that locking-free, pressure-robust mixed methods do not have to be divergence-free. Indeed, relaxing the divergence constraint in the velocity trial functions is harmless, if the relaxation of the divergence constraint in some velocity test functions is repaired, accordingly
On high-order pressure-robust space discretisations, their advantages for incompressible high Reynolds number generalised Beltrami flows and beyond
An improved understanding of the divergence-free constraint for the
incompressible Navier--Stokes equations leads to the observation that a
semi-norm and corresponding equivalence classes of forces are fundamental for
their nonlinear dynamics. The recent concept of {\em pressure-robustness}
allows to distinguish between space discretisations that discretise these
equivalence classes appropriately or not. This contribution compares the
accuracy of pressure-robust and non-pressure-robust space discretisations for
transient high Reynolds number flows, starting from the observation that in
generalised Beltrami flows the nonlinear convection term is balanced by a
strong pressure gradient. Then, pressure-robust methods are shown to outperform
comparable non-pressure-robust space discretisations. Indeed, pressure-robust
methods of formal order are comparably accurate than non-pressure-robust
methods of formal order on coarse meshes. Investigating the material
derivative of incompressible Euler flows, it is conjectured that strong
pressure gradients are typical for non-trivial high Reynolds number flows.
Connections to vortex-dominated flows are established. Thus,
pressure-robustness appears to be a prerequisite for accurate incompressible
flow solvers at high Reynolds numbers. The arguments are supported by numerical
analysis and numerical experiments.Comment: 43 pages, 18 figures, 2 table
Refined a posteriori error estimation for classical and pressure-robust Stokes finite element methods
Recent works showed that pressure-robust modifications of mixed finite
element methods for the Stokes equations outperform their standard versions in
many cases. This is achieved by divergence-free reconstruction operators and
results in pressure independent velocity error estimates which are robust with
respect to small viscosities. In this paper we develop a posteriori error
control which reflects this robustness.
The main difficulty lies in the volume contribution of the standard
residual-based approach that includes the -norm of the right-hand side.
However, the velocity is only steered by the divergence-free part of this
source term. An efficient error estimator must approximate this divergence-free
part in a proper manner, otherwise it can be dominated by the pressure error.
To overcome this difficulty a novel approach is suggested that uses arguments
from the stream function and vorticity formulation of the Navier--Stokes
equations. The novel error estimators only take the of the
right-hand side into account and so lead to provably reliable, efficient and
pressure-independent upper bounds in case of a pressure-robust method in
particular in pressure-dominant situations. This is also confirmed by some
numerical examples with the novel pressure-robust modifications of the
Taylor--Hood and mini finite element methods
Locking free and gradient robust H(div)-conforming HDG methods for linear elasticity
Robust discretization methods for (nearly-incompressible) linear elasticity
are free of volume-locking and gradient-robust. While volume-locking is a
well-known problem that can be dealt with in many different discretization
approaches, the concept of gradient-robustness for linear elasticity is new. We
discuss both aspects and propose novel Hybrid Discontinuous Galerkin (HDG)
methods for linear elasticity. The starting point for these methods is a
divergence-conforming discretization. As a consequence of its well-behaved
Stokes limit the method is gradient-robust and free of volume-locking. To
improve computational efficiency, we additionally consider discretizations with
relaxed divergence-conformity and a modification which re-enables
gradient-robustness, yielding a robust and quasi-optimal discretization also in
the sense of HDG superconvergence
Locking free and gradient robust H(div)-conforming HDG methods for linear elasticity
Robust discretization methods for (nearly-incompressible) linear elasticity are free of volume-locking and gradient-robust. While volume-locking is a well-known problem that can be dealt with in many different discretization approaches, the concept of gradient-robustness for linear elasticity is new. We discuss both aspects and propose novel Hybrid Discontinuous Galerkin (HDG) methods for linear elasticity. The starting point for these methods is a divergence-conforming discretization. As a consequence of its well-behaved Stokes limit the method is gradient-robust and free of volume-locking. To improve computational efficiency, we additionally consider discretizations with relaxed divergence-conformity and a modification which re-enables gradient-robustness, yielding a robust and quasi-optimal discretization also in the sense of HDG superconvergence
Divergence-preserving reconstructions on polygons and a really pressure-robust virtual element method for the Stokes problem
Non divergence-free discretisations for the incompressible Stokes problem may suffer from a lack of pressure-robustness characterised by large discretisations errors due to irrotational forces in the momentum balance. This paper argues that also divergence-free virtual element methods (VEM) on polygonal meshes are not really pressure-robust as long as the right-hand side is not discretised in a careful manner. To be able to evaluate the right-hand side for the testfunctions, some explicit interpolation of the virtual testfunctions is needed that can be evaluated pointwise everywhere. The standard discretisation via an L2 -bestapproximation does not preserve the divergence and so destroys the orthogonality between divergence-free testfunctions and possibly eminent gradient forces in the right-hand side. To repair this orthogonality and restore pressure-robustness another divergence-preserving reconstruction is suggested based on Raviart--Thomas approximations on local subtriangulations of the polygons. All findings are proven theoretically and are demonstrated numerically in two dimensions. The construction is also interesting for hybrid high-order methods on polygonal or polyhedral meshes
Refined a posteriori error estimation for classical and pressure-robust Stokes finite element methods
Recent works showed that pressure-robust modifications of mixed finite
element methods for the Stokes equations outperform their standard versions
in many cases. This is achieved by divergence-free reconstruction operators
and results in pressure-independent velocity error estimates which are robust
with respect to small viscosities. In this paper we develop a posteriori
error control which reflects this robustness. The main difficulty lies in the
volume contribution of the standard residual-based approach that includes the
L2-norm of the right-hand side. However, the velocity is only steered by the
divergence-free part of this source term. An efficient error estimator must
approximate this divergence-free part in a proper manner, otherwise it can be
dominated by the pressure error. To overcome this difficulty a novel approach
is suggested that uses arguments from the stream function and vorticity
formulation of the NavierStokes equations. The novel error estimators only
take the curl of the righthand side into account and so lead to provably
reliable, efficient and pressure-independent upper bounds in case of a
pressure-robust method in particular in pressure-dominant situations. This is
also confirmed by some numerical examples with the novel pressure-robust
modifications of the TaylorHood and mini finite element methods
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