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-   -   K omega SST vs Realizable K epsilon (https://www.cfd-online.com/Forums/main/244414-k-omega-sst-vs-realizable-k-epsilon.html)

AhmadHij August 7, 2022 11:26

K omega SST vs Realizable K epsilon
 
Greetings,
I am working on a 2D transient simulation of vertical axis wind turbine. here are some of the Dimensions:
Diameter =2 m
Chord=0.265 m
Number of blades=2
pitch angle=6 degrees
airfoil: NACA 0021
Boundary conditions :
inlet velocity=8 m/s
turbulence intensity=0.5%
Reynolds number is around 10^5.
I tried first the K omega SST turbulence model, and it gave very low results in comparison with the experimental data (by comparing the average Cp value), so I then tried the Realizable K epsilon model, and It gave much better results.
My question is can I use the Realizable K epsilon model for vertical axis wind turbines where there are a lot of flow separation and why it is giving much better results than K omega SST model ?

jola August 7, 2022 14:22

The SST k-omega model often predict separation on convex surfaces too early. If your operating point has significant suction-side separations then the SST k-omega model will probably predict too early separations and thereby incorrect Cp values.

Your Reynolds number is quite low and you will probably have a fairly large laminar part of your suction-side boundary layer. This will make the separation problem even worse.

Using a Relizable k-epsilon model can improve the results. If you have experimental data for a similar case you should use that to decide which turbulence model to use.

Another way to improve the results is to use a transition model, but since you have so low turbulence levels this is a bit tricky since you might have natural transition and not by-pass transition caused by the free-stream turbulence.


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