# Drag Reduction and CFD

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 January 20, 2008, 04:21 Drag Reduction and CFD #1 Daniel Guest   Posts: n/a Hi all, I have some questions again, 1. Can CFD predict Drag reduction? How is it? 2. If it can, then by which ways, DNS or LES or DES or RANS? which one is better? Is there anything I should noticed during prediction? For example, cylinder flow. I am using LES, and I am not sure whether the LES method has the capability or not. Any help is appreciated! Daniel

 January 20, 2008, 06:51 Re: Drag Reduction and CFD #2 Usman Guest   Posts: n/a Hi Daniel 1. Yes CFD can predict drag reduction. You calculate the force on your body in streamwise direction and then calculate your Cd using drag formula. 2. I suppose if your case is unsteady then you can use LES. I have not tried to use LES to calculate drag though. I personally think you should use RANS modelling to calculate your drag which will take less computing resources and you can get results as good as LES. For RANS you will have to use very fine grid near the surface to capture the boundary layer. We are talking about Yplus values in the range of 1-5. Usman

 January 20, 2008, 08:29 Re: Drag Reduction and CFD #3 Daniel Guest   Posts: n/a Thank you, Usman! Yplus should be in the range of 1-5, Great! I will try. And If I choose RANS, then which one should I use, 3D or 2D? I guess it must be 3D, for turbulence is 3 dimensional, since turbulence is the very reason for drag reduction, so to capture it well is neccessary, right? But, many cases such as sylinder flow is almost 2 dimensional... Daniel

 January 20, 2008, 11:32 Re: Drag Reduction and CFD #4 Usman Guest   Posts: n/a Yes turbulence is 3D and it will give you more accurate and stable results if you use 3D model. However, in some cases such as airfoils you can simulate 2D flow because there is not much happening in third dimension so it does not affect the stability of simulation. Thats what happening i would guess! Usman

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