海基公司培訓(xùn)講義文件fluent.06.solversettings

1、Solver SettingsOutlineUsing the SolverSetting Solver ParametersConvergenceDefinitionMonitoringStabilityAccelerating ConvergenceAccuracyGrid IndependenceAdaptionAppendix: BackgroundFinite Volume MethodExplicit vs. ImplicitSegregated vs. CoupledTransient SolutionsModify solution parameters or gridNoYe

2、sNoSet the solution parametersInitialize the solutionEnable the solution monitors of interestCalculate a solutionCheck for convergenceCheck for accuracyStopYesSolution Procedure OverviewSolution ParametersChoosing the SolverDiscretization SchemesInitializationConvergenceMonitoring ConvergenceStabili

3、tySetting Under-relaxationSetting Courant numberAccelerating ConvergenceAccuracyGrid IndependenceAdaptionChoosing a SolverChoices are Coupled-Implicit, Coupled-Explicit, or Segregated (Implicit)The Coupled solvers are mended if a strong inter-dependence exists between density, energy, momentum, and/

4、or species.e.g., high speed compressible flow or finite-rate reaction modeled flows.In general, the Coupled-Implicit solver is mended over the coupled-explicit solver.Time required: Implicit solver runs roughly twice as fast.Memory required: Implicit solver requires roughly twice as much memory as c

5、oupled-explicit or segregated-implicit solvers!The Coupled-Explicit solver should only be used for unsteady flows when the characteristic time scale of problem is on same order as that of the acoustics.e.g., tracking transient shock waveThe Segregated (implicit) solver is preferred in all other case

6、s.Lower memory requirements than coupled-implicit solver.Segregated approach provides flexibility in solution procedure.Discretization (Interpolation Methods)Field variables (stored at cell centers) must be interpolated to the faces of the control volumes in the FVM: FLUENT offers a number of interp

7、olation schemes:First-Order Upwind Schemeeasiest to converge, only first order accurate.Power Law Schememore accurate than first-order for flows when Recell 20 Large differences in thermal conductivityfine (original) meshcoarse meshsolution transferAccuracyA converged solution is not necessarily an

8、accurate one.Solve using 2nd order discretization.Ensure that solution is grid-independent.Use adaption to modify grid.If flow features do not seem reasonable:Reconsider physical models and boundary conditions.Examine grid and re-mesh.Mesh Quality and Solution AccuracyNumerical errors are associated

9、 with calculation of cell gradients and cell face interpolations.These errors can be contained:Use higher order discretization schemes.Attempt to align grid with flow.Refine the mesh.Sufficient mesh density is necessary to resolve salient features of flow.Interpolation errors decrease with decreasin

10、g cell size.Minimize variations in cell size.Truncation error is minimized in a uniform mesh.Fluent provides capability to adapt mesh based on cell size variation.Minimize cell skewness and aspect ratio.In general, avoid aspect ratios higher than 5:1 (higher ratios allowed in b.l.).Optimal quad/hex

11、cells have bounded angles of 90 degreesOptimal tri/tet cells are equilateral.Determining Grid IndependenceWhen solution no longer changes with further grid refinement, you have a “grid-independent” solution.Procedure:Obtain new grid:AdaptSave original mesh before adapting.If you know where large gra

12、dients are expected, concentrate the original grid in that region, e.g., boundary layer. Adapt grid.Data from original grid is automatically interpolated to finer grid.file write-bc and file read-bc facilitates set up of new problemfile reread-grid and File Interpolate.Continue calculation to conver

13、gence.Compare results obtained w/different grids.Repeat procedure if necessary.Unsteady Flow Problems Transient solutions are possible with both segregated and coupled solvers.Solver iterates to convergence at each time level, then advances automatically. Solution Initialization defines initial cond

14、ition and must be realistic.For segregated solver:Time step size, t, is input in Iterate panel.t must be small enough to resolve time dependent features and to ensure convergence within 20 iterations.May need to start solution with small t.Number of time steps, N, is also required.N*t = total simula

15、ted time.To iterate without advancing time step, use 0 time steps.PISO may aid in accelerating convergence for each time step.Unsteady Modeling Options Adaptive Time SteppingControls how time step size changes.User-Defined inputs also available.Time averaged data may be acquired.Particularly useful

16、for LES turbulence modeling.If desirable, animations should be set up before iterating (flow visualization).For Coupled Solver, Courant number defines in practice:global time step size for coupled explicit solver.pseudo-time step size for coupled implicit solver.Real time step size must still be def

17、ined.SummarySolution procedure for the segregated and coupled solvers is the same:Calculate until you get a converged solution.Obtain second-order solution ( mended).Refine grid and recalculate until grid-independent solution is obtained.All solvers provide tools for judging and improving convergenc

18、e and ensuring stability.All solvers provide tools for checking and improving accuracy.Solution accuracy will depend on the appropriateness of the physical models that you choose and the boundary conditions that you specify.AppendixBackgroundFinite Volume MethodExplicit vs. ImplicitSegregated vs. Co

19、upledTransient SolutionsBackground: Finite Volume Method - 1FLUENT solvers are based on the finite volume method.Domain is discretized into a finite set of control volumes or cells.General transport equation for mass, momentum, energy, etc. is applied to each cell and discretized. For cell p,unstead

20、yconvectiondiffusiongenerationFluid region of pipe flow discretized into finite set of control volumes (mesh). control volumeAll equations are solved to render flow field.Background: Finite Volume Method - 2Each transport equation is discretized into algebraic form. For cell p, face fadjacent cells,

21、 nbcell pDiscretized equations require information at cell centers and faces.Field data (material properties, velocities, etc.) are stored at cell centers.Face values can be expressed in terms of local and adjacent cell values.Discretization accuracy depends upon stencil size.The discretized equatio

22、n can be expressed simply as:Equation is written out for every control volume in domain resulting in an equation set.Equation sets are solved iteratively.Coefficients ap and anb are typically functions of solution variables (nonlinear and coupled).Coefficients are written to use values of solution v

23、ariables from previous iteration.Linearization: removing coefficients dependencies on .De-coupling: removing coefficients dependencies on other solution variables.Coefficients are updated with each iteration.For a given iteration, coefficients are constant.p can either be solved explicitly or implic

24、itly.Background: LinearizationAssumptions are made about the knowledge of nb:Explicit linearization - unknown value in each cell computed from relations that include only existing values (nb assumed known from previous iteration).p solved explicitly using Runge-Kutta scheme.Implicit linearization -

25、p and nb are assumed unknown and are solved using linear equation techniques.Equations that are implicitly linearized tend to have less restrictive stability requirements.The equation set is solved simultaneously using a second iterative loop (e.g., point Gauss-Seidel).Background: Explicit vs. Impli

26、cit Background: Coupled vs. SegregatedSegregated SolverIf the only unknowns in a given equation are assumed to be for a single variable, then the equation set can be solved without regard for the solution of other variables.coefficients ap and anb are scalars.Coupled SolverIf more than one variable

27、is unknown in each equation, and each variable is defined by its own transport equation, then the equation set is coupled together.coefficients ap and anb are Neqx Neq matrices is a vector of the dependent variables, p, u, v, w, T, YTBackground: Segregated SolverIn the segregated solver, each equati

28、on is solved separately.The continuity equation takes the form of a pressure correction equation as part of SIMPLE algorithm.Under-relaxation factors are included in the discretized equations.Included to improve stability of iterative process.Under-relaxation factor, , in effect, limits change in va

29、riable from one iteration to next:Update properties.Solve momentum equations (u, v, w velocity).Solve pressure-correction (continuity) equation.Update pressure, face mass flow rate.Solve energy, species, turbulence, and other scalar equations.Converged?StopNoYesBackground: Coupled SolverContinuity, momentum, energy, and species are solved simultaneously in the coupled solver.Equations are modified to resolve compressible and pressible flow.Transient term is always included.Steady-state solution is formed as time increases and transients tend