Turbulence parameters and convergence
I have observed that turbulent parameters played a major role in the convergence of a simulation. Initially when I simulated with the default values of intensity ans viscosity ratio (5 and 10) the convergence is bad and when I switched over to use intensity and hydraulic diameter (= 4A/P) there is an improvement in the convergence.
Could some one please provide a reasoning to this behavior |
Yes, turbulence makes a big difference to fluid flow. I am glad you figured that out :)
In simple 2-eqn turbulence models the turbulence is modelled to increase the local viscosity (and diffusivity for other equations). This means turbulence is assumed to slow a flow down and make it more viscous - this is in effect a reduction in Reynolds number and convergence is easier in low Reynolds number flows compared to high. (Only up to a point, CFX has great troubles converging for very small Re numbers <<1). |
Hi Glenn, Thank you very much for the clarification so when a convergence difficulty is observed with a simulation how is it possible to sense at as an effect of improper or unphysical turbulence settings.
And if a good convergence is acheived by say reducing the viscosity ratio and increasing the intensity of turbulence then is it implied that such turbulence settings represent the real physics |
The decision as to whether to choose a laminar or turbulent model only depends on the Reynolds number and whether the flow is turbulent or not. But the choice of which turbulence model to use for a turbulent flow is complex - all I will say is that the SST turbulence model is the best all-round model and use that one unless you have a very good reason to choose something else.
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