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Old   June 2, 2000, 01:45
Default inlet boundary condition in k-e model
Abhijit Tilak
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I am little preplexed about inlet boundary condition on epsilon. I am solving incompressible turbulent flow between two parallel plates. Currently i am specifying the inlet value of e as equal to generation of k at the nearest neighbouring grid point. Is this correct? This is valid provided the Equilibrium assumption (high Re form) holds. The assumption is not valid in some flows. how should the value of e be specified then? Is there any generalised way of evaluating e value at inlet? thnx
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Old   June 2, 2000, 09:42
Default Re: inlet boundary condition in k-e model
John C. Chien
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(1). In the k-epsilon model, the eddy viscosity is a function of the tke(k) and the tke dissipation(epsilon). This can also be used to derive the epsilon, if you move the eddy viscosity to the right-hand side of the definition. (2). The turbulent eddy viscosity is just some unknown parameters which will give you the proper Reynolds stresses through the eddy viscosity definition. (3). If the inlet flow is uniform, the velocity gradient will be zero, and the level of eddy viscosity value is not going to affect the shear stress calculation. In this case, you are free to set the eddy viscosity level, and derive the epsilon from the eddy viscosity definition. (4). If the inlet flow is a fully-developed channel flow, then you can not set your eddy viscosity distribution. You can use the derived test data for the eddy viscosity or the computed result of a separate channel flow calculation (form uniform flow to the fully-developed flow). (5). If you have a finite thickness boundary layer at the inlet, then you have to be consistent in the boundary layer region so that the flow variables will not change suddenly at the first interior cell or grid. (6). The best way to do is to run a parametric study on the effect of the eddy viscosity level or distribution ( or the epsilon distribution). For the uniform flow, it may not be a big issue. For fully-developed inlet flow, or flow with finite thickness boundary layer, you will have to specify it consistently. (otherwise, the flow will re-adjust it for you right after the inlet station and produce sudden changes in flow variables)
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