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Implicit/Multigrid Algorithms for Incompressible TurbulentFlows on Unstructured Grids
W. Kyle Anderson,* Russ D. Rausch, Daryl L. BonhausNASA Langley Research Center, Hampton, Virginia 23681
An implicit code for computing inviscid and viscous incompressible flows on unstructured grids is described. The foundation of thecode is a backward Euler time discretization for which the linear system is approximately solved at each time step with either a pointimplicit method or a preconditioned Generalized Minimal Residual (GMRES) technique. For the GMRES calculations, several tech-niques are investigated for forming the matrix-vector product. Convergence acceleration is achieved through a multigrid scheme thatuses non-nested coarse grids that are generated using a technique described in the present paper. Convergence characteristics areinvestigated and results are compared with an exact solution for the inviscid flow over a four-element airfoil. Viscous results, whichare compared with experimental data, include the turbulent flow over a NACA 4412 airfoil, a three-element airfoil for which Machnumber effects are investigated, and three-dimensional flow over a wing with a partial-span flap.
*Senior Research Scientist, Aerodynamic and Acoustic MethodsBranch, Fluid Mechanics and Acoustics Division, Member AIAA.
Research performed while a Resident Research Associate,National Research Council. Currently Senior Engineer, LockheedEngineering and Sciences Company, Member AIAA.
Research Scientist, Aerodynamic and Acoustic MethodsBranch, Fluid Mechanics and Acoustics Division, Member AIAA
Copyright 1995 by the American Institute of Aeronautics andAstronautics, Inc. No copyright is asserted in the United Statesunder Title 17, U.S. Code. The U.S. Government has a royalty-freelicense to exercise all rights under the copyright claimed herein forGovernmental purposes. All other rights are reserved by the copy-right owner.
IntroductionIn the past decade, much progress has been made in devel-
oping computational techniques for predicting flow fieldsabout complex configurations. These techniques include bothstructured- and unstructured-grid algorithms. The accuracyand efficiency of these codes is now such that computationalfluid dynamics (CFD) is routinely used in the analysis andimprovement process of existing designs and is a valuabletool in experimental programs and in the design of new con-figurations.
Many existing codes referred to above have been devel-oped in support of the aircraft industry and, therefore, solvethe compressible flow equations because of the need to han-dle the important effects associated with transonic Machnumbers. However, many important problems, such as thosein the automobile industry and in biomechanics, are inher-ently incompressible and must be treated appropriately.
With the success and wide availability in recent years ofthe compressible codes, these codes have naturally been con-sidered for use with incompressible flows by simply loweringthe Mach number to minimize compressibility effects. Unfor-tunately, as the Mach number is successively decreased to-ward zero, the performance of the compressible codes interms of both convergence rate and accuracy suffers greatly.In Ref. 52, Volpe demonstrated the poor performance of com-pressible flow codes under these conditions, particularly forMach numbers below approximately 0.1.
To overcome the difficulties associated with use of com-pressible codes, excellent progress has been made in the useof local preconditioners to extend the applicability of thesecodes to low Mach numbers. Several examples of this tech-nique, as well as the necessary theory, can be found inRefs. 13, 18, 24, 47, 48, 49, and 54. Preconditioning is indeeda viable means of extending the applicability of compressible
flow codes to the low-Mach-number range and continues tobe an area of active research. A computer code that utilizesthis technique has the added benefit of being able to handleboth compressible and incompressible flows. This techniquehas been applied to both steady-state and time-dependentflows; for time-dependent flows, however, a subiterative pro-cess is required to maintain time accuracy.
Another available method of extending a compressibleflow code for use at zero Mach number is the method of arti-ficial compressibility first introduced by Chorin.14 In this ap-proach, a pseudo-time derivative of pressure is added to thecontinuity equation, which allows the continuity equation tobe advanced in a time-marching manner, much the same asthe momentum equations. Artificial compressibility has beensuccessfully applied by several researchers for both steady-state and time-dependent flows (e.g., Refs. 12, 21, 32, 35, 36,43, and 44). When the temperature field is not required, thistechnique offers an advantage over preconditioning methodsin that the energy equation is not solved; therefore, the effi-ciency of the algorithm is enhanced both in terms of computertime and reduced memory. The reduction in memory is par-ticularly significant for implicit codes on unstructured gridsbecause the storage associated with the time linearization ofthe fluxes is reduced by the square of the local block size, orroughly 40 percent. As with preconditioning, the use of thismethod for time-dependent flows requires a subiterative pro-cedure to obtain a divergence-free velocity field at each timestep.
Recently, unstructured grids have been explored for CFDproblems (e.g., Refs. 1, 5, 6, 10, 26, and 28). This approachhas several advantages over structured grids for problems thatinvolve complex geometries and flows. The biggest advan-tage is the reduction in time needed to generate grids within acomplex computational domain. Another advantage is thatunstructured grids lend themselves to adaptive-grid methodsbecause new nodes can be added to a localized region of themesh by modifying a small subset of the overall grid datastructure. Although the unstructured-grid approach enjoysthese advantages over structured grids, flow solvers that uti-lize it suffer from several disadvantages. These primarily in-clude a factor of 2-3 increase in memory requirements andcomputer run times on a per grid point basis.
The purpose of the current work is to extend the unstruc-tured-grid compressible flow code described in Refs. 1, 2,and 10 to incompressible flows. The extension provides acode that can be used in the design of airplanes, ships, auto-mobiles, pumps, ducts, and turbomachinery. The extensionalso provides a tool for studying Mach-number effects on
high-lift airfoils because of the existence of both incompress-ible and compressible flow codes with similar levels of nu-merical accuracy that can be run on identical grids. The cur-rent code is a node-based upwind implicit code that usesmultigrid acceleration (in two dimensions) to reduce the com-puter time required for steady-state computations. In this ex-tension, the artificial compressibility approach is used. Thechoice of this technique over preconditioning is based prima-rily on the desire to reduce memory requirements and com-puter time by reducing the number of equations.
In the remainder of the paper, the governing equations aregiven, and the basic solution algorithm is described. Althoughboth two- and three-dimensional results are shown in the pa-per, the description of the equations, algorithms, and bound-ary conditions are limited to two-dimensional flow to con-serve space. Results are presented to demonstrate theincompressible code. Inviscid flow results for a four-elementairfoil are compared with an exact solution and are used forexamining the effects of various parameters on the conver-gence behavior. Viscous, turbulent flow results for the NACA4412 airfoil are compared with experimental data, as well aswith results from a well-known structured-grid compressiblecode run at a low Mach number. In addition, results are pre-sented for a three-element airfoil to study the effects of com-pressibility. These results are compared with results from anunstructured-grid compressible code and with experimentaldata. Finally, three-dimensional turbulent computations areshown for a wing with a partial-span flap.
Governing EquationsThe governing equations are the incompressible Navier-
Stokes equations augmented with artificial compressibility.These equations represent a system of conservation laws for acontrol volume that relates the rate of change of a vector ofaverage state variables to the flux through the volume sur-face. The equations are written in integral form as
where is the outward-pointing unit normal to the controlvolume . The vector of dependent state variables and theinviscid and viscous fluxes normal to the control volumeand are given as
where is the artificial compressibility parameter; andare the Cartesian velocity components in the and direc-tions, respectively; is the velocity normal to the surface ofthe control volume, where
and is the pressure. The shear stresses in Eq. (4) are givenas
where and are the laminar and turbulent viscosities, re-spectively, and is the Reynolds number.
Solution AlgorithmThe baseline flow solver is an implicit upwind algorithm in
which the inviscid fluxes are obtained on the faces of eachcontrol volume with a flux-difference-splitting scheme. Forthe current algorithm, a node-based scheme is used in whichthe variables are stored at the vertices of the grid and theequations are solved on nonoverlapping control volumes thatsurround each node. The viscous terms are evaluated with afinite-volume formulation that is equivalent to a Galerkin typeof approximation for these terms. The solution at each timestep is updated with the linearized backw