I am simulating 3D turbulent flow around a circular cylinder using k-omega SST at a Re number of 300000. I have read a lot about turbulent models however I don’t understand how to calculate the k and omega values for the initial conditions and boundary conditions. How are the values for the initia…

Hi @vyvo ! Before we talk about the boundary conditions may I ask why you decided for a RANS type simulation in this case? You have to keep in mind that statistics has been applied a priori to the equations and you will receive a time-averaged flow field corresponding to a long-time exposure. For a…

Hello @jousefm , thank you for your response. I chose RANS simulation because I was lead to believe that LES would make resolving near wall regions difficult. Why would LES be a better choice? When you say ‘not use the whole length of the cylinder’, do you mean make the cylinder a shorter length …

Hi @vyvo ! It really depends on what you want to achieve. If you are fine with smoothed fields as I previously mentioned you can use RANS and a corresponding turbulence model. If you are interested in “instantaneous snapshots” of your flow LES is the way to go. With “length” I mean the longest dire…

Hi @jousefm I have changed my simulation to LES as I am more interested in the instantaneous solutions rather than averaged. Since wall functions would not be appropriate for my simulation, how do I know that they have not been used? Do certain models use them and others do not? I would also li…

Hi @vyvo , you would have to explicitly define the wall functions to be used. You do not have to do that if your y^+ value is at approximately 1 with progressive coarsening towards the core of the flow. [image] vyvo: Since wall functions would not be appropriate for my simulation, how do I know…

I have ran my simulation but I can’t get it converge? I think it is something wrong with my BC but I don’t really know what the problem is.

Hi @vyvo , will have a look at it later on :slight_smile: Best, Jousef

Hi @vyvo , Your boundary conditions are fine as you’ve previously successfully ran a simulation with them. I would probably change the top bottom BC to a slip wall so as to reduce the chances that the domain is interacting and influencing your simulation. The issue, I believe, is your Courant numbe…

Hi @Get_Barried and @vyvo [image] Get_Barried: As shown here in a discussion, the PIMPLE algorithm allows a Courant number of more than 1. Of course, how much more is hard to tell, but I think 459 is much too large. As for why this is possible, I haven’t read through how the PIMPLE equations wo…

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Turbulent k and omega values for external flows

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jousefm September 27, 2018, 6:10pm 8

Hi @vyvo,

will have a look at it later on

Best,

Jousef