# is it laminar or is it not (links included)...

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 September 5, 2001, 07:14 is it laminar or is it not (links included)... #1 Dean Schrage Guest   Posts: n/a I have implemented a solution to the K-E transport equations to solve for turbulent viscosities. My question involves activation. When do I turn on this model? Or do I leave it on all the time? That is, the K-E solutions yields a turbulent viscosity coefficient. It doesn't really know the state of the flow. It's just a dumb advective diffusive solver with sources and boundary conditions. Well not really dumb but sort of. For lamina flow cases, I can still generate turbulent viscosity coefficients near to the molecular values and generally a fraction smaller. If I am to turn off the turbulence model, I would need to decide if turbulence is present prior to simulating these quantities for more complex flow systems. This seems to be defeating the purpose. If the K-E simulation is correct then I suppose it should yield eddy diffusivities which apply properly. On the other hand if I leave the model running, am I generating correct values. The following links show viscosity ratios for laminar flow cases, backstep and natural convection cavity http://www.geocities.com/ctacourse/K...onExamples.pdf And perhaps that forms the question which is more academic than applied. For example, in supposed laminar flows (2D duct, Re <2300) is there a degree of turbulence which is not registered in engineering calculations but can still exists at a smaller intensity and can be determined with these 2 equation models. Thanks. regards Dean

 September 5, 2001, 12:05 Re: is it laminar or is it not (links included)... #2 John C. Chien Guest   Posts: n/a (1). CFD is Numerical Analysis and Mathematical Modeling in Fluid Mechanics. (2). Turbulence model is the result of turbulence modeling. And whether to "turn it on" or to "keep it running" all the time, is the decision of your numerical analysis. (3). You will have to try different approaches like the inventor Edison, or the British carpenter who invented very accurate clocks. (4). CFD is not the way to discover new physics, it is your activities to model the your fluid mechanics problem. (5). If you are traveling by plane, and the turbine blade failed, then you don't care whether it is academic or applied, whether the model is turned on or off.

 September 6, 2001, 04:19 Re: is it laminar or is it not (links included)... #3 sylvain Guest   Posts: n/a k-eps model(s) do(es) NOT handle laminar to turbulent transition. If you turn on the turbulence model, then the result is turbulent even if the Reynolds Number is lower than the Critic Reynolds Number, which could be "sometimes" possible but most of the time, in this case, the results will be far from reality. Regards, Sylvain

 September 6, 2001, 04:26 Re: is it laminar or is it not (links included)... #4 Peter liang Guest   Posts: n/a So, how about first order upwind scheme? Does it handle both turbulent n' laminar?

 September 6, 2001, 06:05 Re: is it laminar or is it not (links included)... #5 Dean Schrage Guest   Posts: n/a I don't understand your question about upwinding

 September 6, 2001, 07:50 Re: is it laminar or is it not (links included)... #6 sylvain Guest   Posts: n/a I was just talking about k-eps turbulence models used to close Reynolds Average Navier Stokes equations. This doesn't depend on the numerical scheme used, does it ?

 September 7, 2001, 19:57 3 cells in inlet; 6x9 cavity flow #8 John C. Chien Guest   Posts: n/a (1)I was finally able to take a look at the webpage, by moving the address one subdirectory up. (2). There are results of calculations one for the backstep flow, with 3 cells at the inlet, and the other for the cavity flow, with 6x9 cells. (3). In my opinion, the inlet location must be at least several step height away from the step corner, and the inlet mesh size must be or order 30 cells. Using only 3 cells at the inlet is just a JOKE. (4). For the cavity flow (I think it is a cavity flow. I could be wrong), more than twenty years ago, I was using 125x125. And most people are using 51x51 nowadays. Using 6x9 size is just CRAZY.

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