Turbulence modeling

 

*    It is crucial to obtain low numerical and convergence errors in turbulent flow simulations. This is because the relevance of turbulence modelling only becomes significant in CFD simulations when other sources of error, especially the numerical and convergence errors, have been eliminated or properly controlled.

*    It is clear that no proper evaluation of the merits of different turbulence models can be made unless the discretisation error of the numerical algorithm is known, and grid sensitivity studies become crucial for all turbulent flow computations.

*    Unfortunately, there is no universally valid general model of turbulence that is accurate for all classes of flows. Validation and calibration of the turbulence model with test data are necessary for all applications*.

*    (* Calibration in this sentence refers to testing the ability of a CFD code to predict global quantities of interest for specific geometries of engineering design interest (see definition in the section about errors and uncertainties) and not to tuning the coefficients of a turbulence model, which cannot be recommended for a novice user).

*    If possible, to select the right turbulence model, examine the effect and sensitivity of results to the turbulence model by changing the turbulence model being used.

*     When using a particular turbulence model, check the published literature to find out if there are any known weaknesses of the model concerned.

*     The weaknesses of the standard k-    model (Launder and Spalding [1974]), which is the most commonly used model in industrial applications, are listed below together with some indications of possible palliative actions which might be fruitfully considered.

*    Choose a suitable near-wall model. Decide whether to use a wall function method, in which the near-wall region is bridged with wall functions, or a low Reynolds number model, in which the flow structure in the viscous sub-layer is resolved.

*    What model to choose depends on the available resources and the requirements for resolution of the boundary layer.

*    The validity of the wall function approach or the use of a low Reynolds number model should be examined for the flow configuration under study.

*    Wall function methods are not valid in the presence of  separated regions and/or strong three-dimensional flows.