CFD uses various turbulence models and they differ in their applicability. k -epsilon is widely used for most of the fluid flow problems. But, how standard and realizable k-epsilon differ from the application point of view? Are they suited for different kinds of flow or realizable k-epsilon begin a recent development covers all problems that can be solved by standard model and provides additional capabilities?
The realizable k-ɛ model differs from the standard k−ɛ model in two ways. Firstly it contains a new formulation for the turbulent viscosity: Cμ is not a constant like in the standard model but a variable The second difference is a new transport equation for the dissipation rate, ɛ, that is derived from an exact equation for the transport of the mean-square vorticity fluctuation.
As a result it mainly gives improved predictions for the spreading rate of jets, a superior ability to capture the mean flow of complex structures and for flows involving rotation, boundary layers under strong adverse pressure gradients, separation and re circulation. If you predict such flow in your case you should really think about to use this variation of the k-ɛ model.
But both model variants do use wall functions. So no matter if you use the standard or realizable k−ɛ model you must make sure the y+ -values in your first cell near the wall must not be below 30 and should not excessively exceed values of 100. If your mesh does not fulfill these requirements you need to re mesh your geometry to get reasonable results.
You cannot overcome a poor quality mesh with changing to another turbulence model, especially not if the other equally uses wall functions.
As a guess I would suggest the standard k-ɛ model runs more stable, as the turbulent viscosity is calculated in a less complex way. But if your mesh fulfills my mentioned requirements stability should not be a problem in both cases.
The realizable k-ɛ model differs from the standard k−ɛ model in two ways. Firstly it contains a new formulation for the turbulent viscosity: Cμ is not a constant like in the standard model but a variable The second difference is a new transport equation for the dissipation rate, ɛ, that is derived from an exact equation for the transport of the mean-square vorticity fluctuation.
As a result it mainly gives improved predictions for the spreading rate of jets, a superior ability to capture the mean flow of complex structures and for flows involving rotation, boundary layers under strong adverse pressure gradients, separation and re circulation. If you predict such flow in your case you should really think about to use this variation of the k-ɛ model.
But both model variants do use wall functions. So no matter if you use the standard or realizable k−ɛ model you must make sure the y+ -values in your first cell near the wall must not be below 30 and should not excessively exceed values of 100. If your mesh does not fulfill these requirements you need to re mesh your geometry to get reasonable results.
You cannot overcome a poor quality mesh with changing to another turbulence model, especially not if the other equally uses wall functions.
As a guess I would suggest the standard k-ɛ model runs more stable, as the turbulent viscosity is calculated in a less complex way. But if your mesh fulfills my mentioned requirements stability should not be a problem in both cases.
The difference between two models is the viscosity coeficient Cμ and the dissipaion equation .You can see the define of the two models.
Different scales of motion through changes and viscosity coeficient
Dear Lalmi Djemoui and Manigandan Sekar
I am more concerned about the applicability of the two models. How the different formulation of Cμ and the dissipation equation will impact the applicability?
The most important in the application of this model is that the Cμ is not a constant like in the standard model .The developpement of this model is to eliminate the indetermination at Wall .
Hardik Mistry : It is a not applicable hope and believe, that any 2-equation turbulence model with one integral time scale and one integral length scale can be universally applicable and absolutely accurate for any kind of turbulent flows. Any 2-equation turbulence model is a strong simplification of the real phenomenon of chaotic turbulence. But they are used for industrial flow simulation because they are comparable affordable way to predict the statistical mean properties of turbulent flows and therefore they are found to be an acceptable and affordable compromise for many applications.
If you are going to choose a 2-equation turbulence mdoel for your work, than I would discourage you to select any of the k-epsilon models, but to choose either SST or GEKO model (GEKO - Generalized k-omega model in ANSYS Fluent 2019 R1). Both are by far the better choice over Standard or Realizable k-eps models.And if you are interested in really accurate prediction of turbulent flows you most likely would like to consider hybrid RANS-LES approaches (SAS-SST, SBES) or Wall-modeled LES (WMLES).
Best regards,
Dr. Th. Frank.
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