Existence of Weak Solutions Up to Collision for Viscous Fluid‐Solid Systems with Slip

We study in this paper the movement of a rigid solid inside an incompressible Navier‐Stokes flow within a bounded domain. We consider the case where slip is allowed at the fluid/solid interface through a Navier condition. Taking into account slip at the interface is very natural within this model, as classical no‐slip conditions lead to unrealistic collisional behavior between the solid and the domain boundary. We prove for this model existence of weak solutions of Leray type, up to collision, in three dimensions. The key point is that, due to the slip condition, the velocity field is discontinuous across the fluid/solid interface. This prevents obtaining global H¹ bounds on the velocity, which makes many aspects of the theory of weak solutions for Dirichlet conditions inappropriate. © 2014 Wiley Periodicals, Inc.     

Time-dependent Stokes and Navier–Stokes problems with boundary conditions involving pressure, existence and regularity

This paper is concerned with the time-dependent Stokes and Navier–Stokes problems with nonstandard boundary conditions: the pressure is given on some part of the boundary. The stationary case was first studied by Bégue, Conca, Murat and Pironneau and, next, their study were completed by Bernard, mainly about regularity. In this paper, the Stokes problem is studied by a method analogous to that of Temam for the standard problem, combined with regularity results of Bernard for the nonstandard stationary case. We obtain existence, uniqueness and regularity H². In addition, in two dimensions, a regularity W^2,r, r2, is proved. Next, for the nonstandard Navier–Stokes problem, we present some existence, uniqueness and regularity H² results. The proof of existence is based on a fixed point method.     

Vorticity-velocity-pressure formulation for Stokes problem

We propose a three-field formulation for efficiently solving a two-dimensional Stokes problem in the case of nonstandard boundary conditions. More specifically, we consider the case where the pressure and either normal or tangential components of the velocity are prescribed at some given parts of the boundary. The proposed computational methodology consists in reformulating the considered boundary value problem via a mixed-type formulation where the pressure and the vorticity are the principal unknowns while the velocity is the Lagrange multiplier. The obtained formulation is then discretized and a convergence analysis is performed. A priori error estimates are established, and some numerical results are presented to highlight the perfomance of the proposed computational methodology.

     

Long‐time existence of solutions to the Navier–Stokes equations with inflow–outflow and heat convection

Long time existence of regular solutions to the Navier–Stokes equations for velocity and pressure coupled with the heat convection equation for temperature in cylindrical pipe with inflow and outflow is shown. We assume the slip boundary conditions for velocity and the Neumann conditions for temperature. First, an appropriate estimate is shown, and next the existence of solutions is proved by the Leray–Schauder fixed point theorem. The estimate is obtained for a long time, which is possible because L₂ norms of derivatives in the direction along the cylinder of the initial velocity, initial temperature and the external force are sufficiently small. Copyright © 2012 John Wiley & Sons, Ltd.     

A solution for the problem of stokes flow past a porous sphere

The problem of a general non-axisymmetric Stokes flow of a viscous fluid past a porous sphere is considered. The expressions for the velocity and pressure, both inside and outside the sphere are given, when the flow outside satisfies the Stokes equations and the flow inside the sphere is governed by Darcy's law. The expressions for drag and torque are given. It is found that the drag is greater or smaller than the drag in the rigid case, depending on whether the undisturbed velocity is a pure biharmonic or a harmonic respectively. The torque is same as in the rigid case.     

Homogenization of the Navier-Stokes equations with a slip boundary condition

This paper deals with the homogenization of the Stokes or Navier-Stokes equations in a domain containing periodically distributed obstacles, with a slip boundary condition (i.e., the normal component of the velocity is equal to zero, while the tangential velocity is proportional to the tangential component of the normal stress). We generalize our previous results (see [1]) established in the case of a Dirichlet boundary condition; in particular, for a so-called critical size of the obstacles (equal to ε³ in the three-dimensional case, ε being the inter-hole distance), we prove the convergence of the homogenization process to a Brinkman-type law.     

The transition between the Stokes equations and the Reynolds equation: A mathematical proof

The Reynolds equation is used to calculate the pressure distribution in a thin layer of lubricant film between two surfaces. Using the asymptotic expansion in the Stokes equations, we show the existence of singular perturbation phenomena whenever the two surfaces are in relative motion. We prove that the Reynolds equation is an approximation of the Stokes equations and that the kind of convergence is strongly related with the boundary conditions on the velocity field.     

Boundary Controllability of a Stationary Stokes System with Linear Convection Observed on an Interior Curve

We study the approximate controllability of a stationary Stokes system with linearized convection in a bounded domain of ^N. The control acts on a part of the boundary and the velocity field is observed on an interior curve (N=2) or surface (N=3). We establish the L ²-approximate controllability under certain compatibility conditions and suitable geometrical assumptions on the curve or surface. We build controls of minimal L ²-norm by duality. To compute the control, we propose a numerical method, based on duality techniques, consisting in the minimization of a nonquadratic functional coupled to a Stokes system. It is tested in several situations leading to interesting numerical results.     

Convergence analysis and computational testing of the finite element discretization of the Navier–Stokes alpha model

This report performs a complete analysis of convergence and rates of convergence of finite element approximations of the Navier–Stokes‐α (NS‐α) regularization of the NSE, under a zero‐divergence constraint on the velocity, to the true solution of the NSE. Convergence of the discrete NS‐α approximate velocity to the true Navier–Stokes velocity is proved and rates of convergence derived, under no‐slip boundary conditions. Generalization of the results herein to periodic boundary conditions is evident. Two‐dimensional experiments are performed, verifying convergence and predicted rates of convergence. It is shown that the NS‐α‐FE solutions converge at the theoretical limit of O(h²) when choosing α = h, in the H¹ norm. Furthermore, in the case of flow over a step the NS‐α model is shown to resolve vortex separation in the recirculation zone. © 2009 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2010     

Error analysis of a weighted least-squares finite element method for 2-D incompressible flows in velocity–stress–pressure formulation

In this paper we are concerned with a weighted least-squares finite element method for approximating the solution of boundary value problems for 2-D viscous incompressible flows. We consider the generalized Stokes equations with velocity boundary conditions. Introducing the auxiliary variables (stresses) of the velocity gradients and combining the divergence free condition with some compatibility conditions, we can recast the original second-order problem as a Petrovski-type first-order elliptic system (called velocity–stress–pressure formulation) in six equations and six unknowns together with Riemann–Hilbert-type boundary conditions. A weighted least-squares finite element method is proposed for solving this extended first-order problem. The finite element approximations are defined to be the minimizers of a weighted least-squares functional over the finite element subspaces of the H¹ product space. With many advantageous features, the analysis also shows that, under suitable assumptions, the method achieves optimal order of convergence both in the L²-norm and in the H¹-norm. © 1998 B. G. Teubner Stuttgart—John Wiley & Sons, Ltd.     

Non‐standard Stokes and Navier–Stokes problems: existence and regularity in stationary case

This paper is devoted to Stokes and Navier–Stokes problems with non‐standard boundary conditions: we consider, in particular, the case where the pressure is given on a part of the boundary. These problems were studied by Bégue, Conca, Murat and Pironneau. They proved the existence of variational solutions, indicating that these were solutions of the initial non‐standard problems, if they are regular enough, but without specifying the conditions on the data which would imply this regularity. In this paper, first we show that the variational solutions, on supposing pressure on the boundary Γ₂ of regularity H^1/2 instead of H^?1/2, have their Laplacians in L² and, therefore, are solutions of non‐standard Stokes problem. Next, we give a result of regularity H², which we generalize, obtaining regularities W^{m, r}, m∈?, m?2, r?2. Finally, by a fixed‐point argument, we prove analogous results for the Navier–Stokes problem, in the case where the viscosity νis large compared to the data. Copyright © 2002 John Wiley & Sons, Ltd.     

Theoretical investigation of the flow in a stenosis

The theoretical investigation of the flow in stenosis is presented. We use a new formulation of the incompressible Navier–Stokes equation in terms of an auxiliary field that differs from the velocity by a gauge transformation [1]. The gauge freedom allows us to formulate simple boundary conditions for the auxiliary field and the gauge field as well. The gauge field eliminates the pressure distribution in the Navier–Stokes equation. The numerical investigation of the creeping flow, depending on the geometrical parameters of the system, is performed. The influence of the pressure drop has been taken into account. An excellent agreement with the analytical results in frames of the film theory was observed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Slip at the surface of a sphere translating perpendicular to a plane wall in micropolar fluid

The Stokes axisymmetrical flow caused by a sphere translating in a micropolar fluid perpendicular to a plane wall at an arbitrary position from the wall is presented using a combined analytical-numerical method. A linear slip, Basset type, boundary condition on the surface of the sphere has been used. To solve the Stokes equations for the fluid velocity field and the microrotation vector, a general solution is constructed from fundamental solutions in both cylindrical, and spherical coordinate systems. Boundary conditions are satisfied first at the plane wall by the Fourier transforms and then on the sphere surface by the collocation method. The drag acting on the sphere is evaluated with good convergence. Numerical results for the hydrodynamic drag force and wall effect with respect to the micropolarity, slip parameters and the separation distance parameter between the sphere and the wall are presented both in tabular and graphical forms. Comparisons are made between the classical fluid and micropolar fluid.      

On compressible Navier–Stokes equations with density dependent viscosities in bounded domains

The present note extends to smooth enough bounded domains recent results about barotropic compressible Navier–Stokes systems with density dependent viscosity coefficients. We show how to get the existence of global weak solutions for both classical Dirichlet and Navier boundary conditions on the velocity, under appropriate constraints on the initial density profile and domain curvature. An additional turbulent drag term in the momentum equation is used to handle the construction of approximate solutions.     

A study of an arbitrary Stokes flow past a fluid coated sphere in a fluid of a different viscosity

A general method to discuss the problem of an arbitrary Stokes flow (both axisymmetric and non-axisymmetric flows) of a viscous, incompressible fluid past a sphere with a thin coating of a fluid of a different viscosity is considered. We derive the expressions for the drag and torque experienced by the fluid coated sphere and also discuss the conditions for the reduction of the drag on the fluid coated sphere. In fact, we show that the drag reduces compared to the drag on a rigid sphere of the same radius when the unperturbed velocity is either harmonic or purely biharmonic, i.e., of the form ${r^2\vec{\textbf{v}}}$ , where ${\vec{\textbf{v}}}$ is a harmonic function. Previously Johnson (J Fluid Mech 110:217–238, 1981), who considered a uniform flow showed that the drag on the fluid coated sphere reduces compared to the drag on the uncoated sphere when the ratio of the surrounding fluid viscosity to the fluid-film viscosity is greater than 4. We show that this result is true when the undisturbed velocity is harmonic or purely biharmonic, uniform flow being a special case of the former. However, we illustrate by an example that the drag may increase in a general Stokes flow even if this ratio is greater than 4. Moreover, when the unperturbed velocity is harmonic or purely biharmonic, and the ratio of the surrounding fluid viscosity to the fluid-film viscosity is greater than 4 for a fixed value of the viscosity of the ambient fluid, we determine the thickness of the coating for which the drag is minimum.     

A finite element method for the axisymmetric three‐field Stokes system with a discontinuous pressure

A quadrilateral based velocity‐pressure‐extrastress tensor mixed finite element method for solving the three‐field Stokes system in the axisymmetric case is studied. The method derived from Fortin's Q₂ − P₁ velocity‐pressure element is to be used in connection with the standard Galerkin formulation. This makes it particularly suitable for the numerical simulation of viscoelastic flow. It is proven to be second‐order convergent in the natural weighted Sobolev norms, for the system under consideration. The crucial result that the method is uniformly stable is proven for the case of rectangular meshes. © 1999 John Wiley & Sons, Inc. Numer Methods Partial Differential Eq 15: 739–763, 1999     

Junction between a 3-D Stokes flow and a 2-D thin film with pressure boundary conditions

A 3-D Stokes equation modelling the flow in the inlet region of a lubricated device is studied. Boundary conditions on both the pressure and the velocity are considered. The geometry induces that the flow region is divided into two parts: the first one is a thin gap with height ?h, where a change of scale is performed, and the second one is a cavity whose dimensions are large with respect to ?. Two equivalent variational formulations are proposed and lead to accurate a priori estimates. The resulting asymptotic equation for the pressure in the thin gap is the currently used Reynolds equation, whose boundary conditions are obtained via the junction with the 3-D cavity. Both hydrostatic and hydrodynamic cases are considered.     

Numerical simulations of thermal convection under the influence of an inclined magnetic field by using solenoidal bases

The effect of an inclined homogeneous magnetic field on thermal convection between rigid plates heated from below under the influence of gravity is numerically simulated in a computational domain with periodic horizontal extent. The numerical technique is based on solenoidal (divergence‐free) basis functions satisfying the boundary conditions for both the velocity and the induced magnetic field. Thus, the divergence‐free conditions for both velocity and magnetic field are satisfied exactly. The expansion bases for the thermal field are also constructed to satisfy the boundary conditions. The governing partial differential equations are reduced to a system of ordinary differential equations under Galerkin projection and subsequently integrated in time numerically. The projection is performed by using a dual solenoidal bases set such that the pressure term is eliminated in the process. The quasi‐steady relationship between the velocity and the induced magnetic field corresponding to the liquid metals or melts is used to generate the solenoidal bases for the magnetic field from those for the velocity field. The technique is validated in the linear case for both oblique and vertical case by reproducing the marginal stability curves for varying Chandrasekhar number. Some numerical simulations are performed for either case in the nonlinear regime for Prandtl numbers Pr = 0.05 and Pr = 0.1. Copyright © 2014 John Wiley & Sons, Ltd.     

On the accuracy of the viscous form in simulations of incompressible flow problems

We present a numerical study of drag/lift and flux estimates using two forms of Navier‐Stokes equations (NSE) that are equivalent in the continuum formulation but not in the discrete finite element formulation. The two investigated forms of the NSE differ in the viscous term that is represented in one form by νΔ u with ν being the viscosity and 2ν?·?^S u in the other form where ?^S represents the deformation tensor. The study consists of numerical analysis of the two forms and computations of drag/lift, pressure drop on the cylinder problem and computations of flux for the Poiseuille flow. The main objective is to provide a clear comparison of the reference values for the maximal drag and lift coefficient at the cylinder and for the pressure difference between the front and the back of the cylinder at the final time for the two forms of NSEs. Our computational results of the reference values do not differ significantly between the two forms, but the differences are there. For the Poiseuille flow, the differences in the flux computations were much smaller, and this agreed with the computationally obtained results of the divergence of the velocity field. © 2011 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 28: 523–541, 2012     

On generalized Stokes’ and Brinkman’s equations with a pressure- and shear-dependent viscosity and drag coefficient

We study generalizations of the Darcy, Forchheimer, Brinkman and Stokes problem in which the viscosity and the drag coefficient depend on the shear rate and the pressure. We focus on existence of weak solutions to the problem, with the chief aim to capture as wide a group of viscosities and drag coefficients as mathematically feasible and to provide a theory that holds under minimal, not very restrictive conditions. Even in the case of generalized Stokes system, the established result answers a question on existence of weak solutions that has been open so far.