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November 20, 2016, 21:48 

#181  
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Elham
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How can I have tauw from mean velocity? 

November 21, 2016, 15:58 
Hairpin Vortices in turbulent channel flow using DNS

#182 
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Nitin
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Hello Foamers
I am performing DNS of turbulent channel flow to understand the relationship b/w hairpin vortices and heat transfer associated with it. In the literature, the formation of hairpin vortices is explained as follows: First of all, vortices formation in spanwise (z) direction due to gradient of streamwise velocity (du/dy) in wall normal (y) direction. Stretching and lifting of spanwise (z) vortices results in two counterrotating vortices in streamwise (x) direction. These vortices elongate further in x direction and eventually connect at y+=15 and results in formation of hairpin. I am not able to understand the driving force behind stretching and lifting? Please share your experience and understanding. 

November 21, 2016, 20:27 

#183  
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Elham
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Dear all, I am wondering in yPlus calculation if it is correct to have the gradient of U, mag(snGrad(U))? or it must be something like mag(snGrad(flowDirection & U)))? Although I have tested it and the solver stopped. Since we need the gradient of velocity in streamwise direction not in the whole directions. Cheers, Elham 

November 25, 2016, 13:11 
Implicit and Explicit SGS Stress

#184 
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Syavash Asgari
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Hi,
I think this is a quite important subject. I have tried to simulate channel395 through both dynamic Smagorinsky (local) and an explicit SGS stress tensor adding to momentum equations. The first approach is a wellknown eddyviscosity method which calculates nuSgs and adds it to nu to constitute nuEff and finally calculates divDevReff from something like: fvm::laplacian(nuEff, U) The second approach does not calculate nuSgs directly, but instead calculates SGS stress tensor B from the following relation: B = 2 * nu_t * S_ij where nu_t is equal to nuSgs and S_ij is the resolved strain rate tensor. The only difference from the first approach is that divDevReff is now an explicit source term which is added to momentum equation as the following term: fvc:: div(B)  fvm::laplacian(nu, U) Well, I would expect to get the same results in term of viscous stress (u_tau) in the case of channel395 with default grid spacing, but it is not as thought. The first approach (currently used in OF) yields a reasonable Re_tau=370 but the second approach strongly overpredicts viscous stress, yielding a Re_tau=500!! Does anyone have some sort of experience with this type of SGS stress modeling?! I would highly appreciate if someone could give me insight. I wonder if this big difference should be expected when modelling SGS stress explicitly ??! Syavash 

November 26, 2016, 06:22 

#185  
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Timofey Mukha
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December 6, 2016, 02:44 

#186  
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Elham
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I finally could get rid of the scale difference from my results and DNS ones in wall coordinate by calculating u_tau as following: u_tau=sqrt(wallShearStressLES) and wallShearStressLES is a utility. Then I calculate y+. 

December 6, 2016, 04:09 

#187  
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Syavash Asgari
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For whoever that might be interested, to calculate friction velocity and y+ one must use wallShearStressLES instead of yPlusLES. The latter only accounts for eddyviscosity models while the former suits both approaches. Syavash 

December 25, 2016, 00:22 
problem with perturbU

#188 
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Bo Kong
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Hi,
I am trying to run the channel395 case and using perturbU to initial. But when I run for 100 flow cross time, it is still in laminar flow. can anybody give me some advice? Thanks! Bo Kong 

December 27, 2016, 06:44 

#189 
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Elham
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Can you see any perturbation after using perturbU utility? Any small perturbation is enough. Then you need to have enough time to get turbulent.


December 27, 2016, 22:00 

#190 
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Bo Kong
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Yes，I can see perturbation in x and z directions like the picture shown. Does it right?


December 27, 2016, 22:02 

#191 
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Bo Kong
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Yes，I can see perturbation in x and z directions, and keep run for 200 flow cross time, I think it is long enough.


December 28, 2016, 21:39 

#192 
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Elham
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Is the fvOption dictionary in your system directory and active? It forces the pressure difference through the channel. All of the boundary conditions should be cyclic as well.


December 30, 2016, 02:33 

#193 
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Bo Kong
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Yes，these files are copied from channel395 case in tutorials without any change.


December 30, 2016, 02:59 

#194 
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Elham
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January 2, 2017, 16:03 

#195 
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Hello everybody,
I am trying to generate turbulent inlet boundary condition for my LES simulation using mapped boundary condition. So, I start with a uniform inlet for my velocity and I map the velocity profile further downstream back to the inlet plane. The problem is that the turbulence decays over time and I end up with a laminar velocity profile with no disturbances. Can anyone please suggest what the problem could be? Many thanks in advance. 

January 2, 2017, 20:22 

#196  
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Bo Kong
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Quote:
Here is the contour. 

January 2, 2017, 22:22 

#197  
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Elham
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January 2, 2017, 22:25 

#198 
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Elham
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Happy new year too.
It is quite laminar. The problem is not the number of flow throughs. You may missed something in the BCs definition. All of BCs except the walls has to be cyclic and fvOptionDict has to be on. Check them all with channel395 tutorial please. 

January 3, 2017, 00:57 

#199  
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January 4, 2017, 07:31 

#200  
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Elham
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