Atmospheric Boundary Layer Turbulence
 

Hi All

I am currently dealing with simulating an ABL flow using fluent and I am running into issues with the turbulence viscosity ratio being limited to 10^5. I would like to know what turbulence viscosity ratios can be realistically encountered in simulations of the ABL and whether increasing this limit would still result in a meaningful result.

Regards
A.V.Hariram

Hi, did you find an answer to your question?

I'm facing similar problem, I'm using a velocity profile, a k profile and an epsilon profile for ABL but I get turbulent viscosity limitation no matter what I try.
I'm very interested in your case and how you managed to solve it if you did.

Thanks a lot.

Hi Bollonga.

I've read a paper recently, Appropriate boundary conditions for computational wind engineering models using the k-e turbulence model by Richards and Hoxey, and in it they calculate that at 10m, the turbulence viscosity can be around 2.94, which when divided by the viscosity of air gives you values greater than 10^5. This would mean that you could actually increase the value of the limiter. I've also found a few lines at: http://www.eureka.im/77.html which says that atmospheric turb-viscosity ratios can reach 10^8-10^9. It is not as reliable, but is still something to work with.
Ultimately you have to look at the turbulence model equations you are using and what increasing this limit means. In my case I had a very large length scale for turbulence and was using the k-epsilon class of models thus this increased length would overall increase my turbulence viscosity and hence by increasing the limit I was able to achieve a meaningful result.
With atmospheric flows I believe I've read that turbulence scales can reach 150m, but do some searching in that area to judge if whether or not increasing the limit will give you meaningful results.
I also suggest checking the equations for the turbulence models you are using as I had once made an error in specifying k using FLUENT.

Regards