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 Cuong NK February 20, 2006 23:57

Modeling of fluctuating wind flow

Hi everybody,

I using CFX to study the effects of wind flow on a bluff body of a building. The turbulence model is DES. The velocity boundary at the inlet is modeled using power law function (velocity increases with height). I arranged some monitor points in the fluid domain and on the surface of the bluff body to monitor the velocity and pressure for transient run.

My problem is: I want to plot a time history of the velocity at one certain point in the fluid domain (far from the bluff body) and I expect that it is fluctuate with the pattern that looks like the full scale measurement. But the result of the velocity we get from CFX is the average value, so it is a constant (do not change with time).

If any body knows how to get the fluctuating wind velocity at a point inside the fluid domain, please give me some guidance or instructions.

Thank you very much in advance!

Cuong NK

 Stuart February 21, 2006 09:15

Re: Modeling of fluctuating wind flow

I take it that the point that you are interested in is upstream of the bluff body. I'm not 100% sure how you would have to set up the problem, but you would need to do is to prescribe a fluctuating inlet profile where both the velocities and turbulence qantities vary with time. The Uk wind engineering society held a conference in Birmingham where there was a presentation on this same issue. Ideally you should look at an empty domain and look to maintain a given profile through the domain. The average profile should match tthat of your data and the spectral densities (I believe as I'm no expert) should match. There will be people on here who are much more expert in this then I am. I know for the conference there was a test case where delegates modelled a 3d cube in an atmospheric Boundary layer. For this test case profiles were presented and it would be a useful source of validation information for you. hope this helps and I hope others can help ???

 Cuong NK February 21, 2006 18:03

Re: Modeling of fluctuating wind flow

Thank you very much Stuart, You are true that the monitor point is upstream of the bluff body. Initially I am also think that I should simulate a fluctuating velocity at the inlet, but after that I discussed with a CFX developer and he recommended me that even if I model the fluctuating velocity at the inlet, it will be averaged when transport into the domain. Then I only model the average velocity and the turbulence intensity profile at the inlet.

I think that I can get the actual velocity at a point inside the domain through the average velocity and turbulence intensity, but I don't know how to get turbulence intensity since the given formula in CFX manual is valid at the inlet only.

I am not sure that my approach is correct or not. I would appreciate if some one can help me on that.

Cuong NK

 Stuart February 22, 2006 06:43

Re: Modeling of fluctuating wind flow

The problem you may have is maintaining the profile if you specify a fluctuating in let velocity. Normally the fluctuations would be maintained by the obsticles on the ground. So in the case of wind tunnels - roughness blocks are introduced to generate the boundary layer. When you model a domain using say a steady state simulation, then you can use a roughness applied to the ground wall boundary condition. This together with your inlet profiles should ensure you have a steady well maintianed profile through your domain. Now you are moving into a transient simulation. In theory fluctuations are occuring all the time. DES simulations assume that the large eddys are simulated but the small eddies are taken care of using a RANS type simulation. What does this then mean to the fluctuating component that you would find in an Atmospheric boundary layer ??? I think this is probably an area of research ?? How to best combine the time saving characteristics of a RANS simulation with the more realistic/accurate? simulations of a DES approach.

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