# calculation of drag

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 March 7, 2006, 01:58 calculation of drag #1 CFD Guest   Posts: n/a For calculation of drag at subsonic, transonic, supersonic and hypersonic regions, what should be the combination of turbulence model and Yplus?

 March 7, 2006, 03:12 Re: calculation of drag #2 Razvan Guest   Posts: n/a The only thing that I am sure of is y+ MUST be around 1 to have better chance of predicting viscous drag, which is much harder to predict than shape drag. This is mostly influential in low subsonic regime. When velocity goes beyond M=1, shock drag appears and viscous drag importance diminishes, but it is still quite high. Anyway, my opinion is that when M>1.5 you have to be much more careful to the shockwave sharpness and wake grid resolution than y+ and turbulence model. About turbulence model, I cannot say that a particular model is clearly better...I would choose S-A in most cases, although RSM would be better, but is too computationally expensive. Razvan

 March 7, 2006, 03:21 Re: calculation of drag #3 CFD Guest   Posts: n/a What should be the wake grid resolution when M>1.5 ? If we r simulating flow over a cylinder at M > 1.5, what should be the wake grid resolution and grid size over the body? How we relate shockwave sharpness with grid resolution? Thanks

 March 7, 2006, 14:58 Re: calculation of drag #4 Freeman Guest   Posts: n/a y+ around 1? But this is when one is using enhanced wall functions, isn't it? While using the standard law of the wall y+ needs to be between 30 and 200... or am I mixing concepts here?

 March 7, 2006, 17:02 Re: calculation of drag #5 Razvan Guest   Posts: n/a Enhanced wall functions do not exist in Fluent. You are talking about EWT (Enhanced Wall Treatment). This is in fact a special implementation of wall functions which is intended to eliminate y+ constraint (it actually minimises the error when y+ is around 11, the worst possible value), but it is not so good, to say the truth. For best results, you need to go to y+=1. Classic wall functions must be used with y+>30 (but also y+<100 for satisfactory results). Most turbulence models are not valid in near-wall region so must use wall functions. S-A and RSM fall into this category. In Fluent, S-A uses EWT only. But you still have to be careful when using it, not to have 4 < y+ < 18!! Good luck, Razvan

 March 8, 2006, 01:26 Re: calculation of drag #6 Freeman Guest   Posts: n/a OK, thanks Razvan: I didn't know that SA only uses EWT! Regards, Freeman

 March 8, 2006, 02:06 Re: calculation of drag #7 CFD Guest   Posts: n/a May you plz inform us about the y+ limits for k-e & k-w models. Please inform me about relavent material from which I can get necessary information and importance about the y+ value limits for different turbulent models. Thanks

 March 8, 2006, 03:05 Re: calculation of drag #8 Razvan Guest   Posts: n/a k-e models are separated in 2 main categories: - high-Re-number models (standard k-e, RNGk-e, Realizable k-e, and other variants), which all need wall functions (standard&non-equilibrium wall functions work well with 30 < y+ < 100, with an upper validity limit of y+=300, and EWT which works best at y+=1); - low-Re-number models (there are many modifications to standard k-e model made by different researchers, in order to make k-e model valid all the way to the wall, but none of it is available through the GUI), accesible by scheme commands in Fluent. k-w models are actually 2: standard k-w and SST k-w. Both are low-Re-number models and to work properly need y+=1, and when you have this y+, you can select "transitional flows" option in GUI. But if you cannot provide a mesh with y+ around 1, you can use the modified versions of these models, which use wall functions, and you may have y+>30 meshes. These are default in Fluent. I hope you got it now, Razvan

 March 8, 2006, 06:35 Re: calculation of drag #9 CFD Guest   Posts: n/a What is the value of Re, below which we use low-Re-model?? In my case mach is 1.2, residuals have converge upto e-5,but Cd is still decreasing. Where should I suppose that solution has converge???

 March 9, 2006, 03:55 Re: calculation of drag #10 Razvan Guest   Posts: n/a Low-Re-number model = valid in B-L, down to the wall (Re number is decreasing in B-L because velocity is decreasing, so at the wall Re=0) High-Re-number model = valid only in bulk flow, not in B-L region, so it needs wall functions. In high velocity flows, Cd is a better convergence criterion, so trust it, not the residuals! Good luck, Razvan

 March 12, 2006, 23:44 Re: calculation of drag #11 CFD Guest   Posts: n/a In the flow (M = 1.4) over a projectile, which turbulent model should be use for the calculation of drag? As reynold is high around the projectile but low near the projectile surface.