CFD Best Practice Recommendations

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CFD Best Practice Recommendations

RECOMMENDATIONS ON GRIDS

The key recommendation is to ensure smooth grids, avoiding abrupt changesin grid size or shape, as this can lead to a significant loss of accuracy.Hence take good care to:

• Define the computational domain, in order to minimize theinfluence and interactions between the flow and the far-field conditions. Inparticular,

– Place inlet and outlet boundaries as far away as possible from theregion of interest. In particular, if uniform far-field conditions are imposed,you should ensure that the boundary is not in a region where the flow may stillvary significantly.

– Avoid inlet or outlet boundaries in regions of strong geometricalchanges or in regions of recirculation.

• Avoid jumps in grid density or in grid size.

• Avoid highly distorted cells or small grid angles.

• Ensure that the grid stretching is continuous.

• Avoid unstructured tetrahedral meshes in boundary layerregions.

• Refine the grids in regions with high gradients, such asboundary layers, leading edges of airfoils and any region where large changesin flow properties might occur.

• Make sure that the number of points in the boundarylayers is sufficient for the expected accuracy. Avoid less than 10 points overthe inner part of the boundary layer thickness.

• Monitor the grid quality by adequate mesh parameters,available in most of the grid generators, such as aspect ratio, internal angle,concavity, skewness, negative volume.

RECOMMENDATIONS ON SOLUTION ASSESSMENT

Once you run your code, the following recommendations will be useful toenhance your confidence in the results obtained:

• Check very carefully the selected boundary conditions forcorrectness and compatibility with the physics of the flow you are modeling.

• Verify all the numerical settings and parameters, beforelaunching the CFD run.

• Verify that your initial solution is acceptable for theproblem to be solved.

• Monitor the convergence to ensure that you reach machineaccuracy. It is recommended to monitor, in addition to the residuals, theconvergence of representative quantities of your problem, such as a drag forceor coefficient, a velocity, temperature or pressure at selected points in theflow domain.

• Look carefully at the behavior of the residualconvergence curve in function of number of iterations. If the behavior isoscillatory, or if the residual does not converge to machine accuracy byshowing a limit cycle at a certain level of residual reduction, it tells youthat some inaccuracy affects your solution process.

• Apply internal consistency and accuracycriteria, by verifying:

– Conservation of global quantities such as totalenthalpy and mass flow in steady flow calculations.

– The entropy production and drag coefficients withinviscid flows, which are strong indicators of the influence of numericaldissipation, as they should be zero.

• Check, whenever possible, the griddependence of the solution by comparing the results obtained on different gridsizes.

• Some quantities are more sensitive thanothers to error sources. Pressure curves are less sensitive than shearstresses, which in turn are less sensitive than temperature gradients or heatfluxes, which require finer grids for a given accuracy level.

• If your calculation appears difficult toconverge, you can

– Look at the residual distribution and associated flowfield for possible hints, e.g. regions with large residuals or unrealisticlevels of the relevant flow parameters.

– Reduce the values of parameters controllingconvergence, such as the CFL number or some under-relaxation parameter, whenavailable.

– Consider the effects of different initial flowconditions.

– Check the effect of the grid quality on the convergencerate.

– Use a more robust numerical scheme, such as a firstorder scheme, during the initial steps of the convergence and switch to moreaccurate numerical schemes as the convergence improves.

RECOMMENDATIONS ON EVALUATION OFUNCERTAINTIES

This is a very difficult issue, as the application uncertainties aregenerally not well defined and require a sound judgment about the physics ofthe considered flow problem. Some recommendations can be offered:

• Attempt to list the most important uncertainties, such as

– Geometrical simplifications and manufacturing tolerances around the CADdefinition.

– Operational conditions, such as inlet velocity or inlet flow angle.

– Physical approximations, such as handling an incompressible flow as alow Mach number compressible flow. This type of uncertainty is manageable, asit can more easily be quantified.

– Uncertainties related to turbulence or other physical models.

• Perform a sensitivity analysis of the relevantuncertainty to investigate its influence.