
[Sponsors] 
November 20, 2016, 21:48 

#181  
Senior Member
Elham
Join Date: Oct 2009
Posts: 184
Rep Power: 16 
Quote:
How can I have tauw from mean velocity? 

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

#182 
New Member
Nitin
Join Date: Mar 2012
Location: Bombay
Posts: 16
Rep Power: 14 
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  
Senior Member
Elham
Join Date: Oct 2009
Posts: 184
Rep Power: 16 
Quote:
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 
Senior Member
Ehsan Asgari
Join Date: Apr 2010
Posts: 473
Rep Power: 18 
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  
Senior Member
Timofey Mukha
Join Date: Mar 2012
Location: Stockholm, Sweden
Posts: 119
Rep Power: 14 
Quote:


December 6, 2016, 02:44 

#186  
Senior Member
Elham
Join Date: Oct 2009
Posts: 184
Rep Power: 16 
Quote:
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  
Senior Member
Ehsan Asgari
Join Date: Apr 2010
Posts: 473
Rep Power: 18 
Quote:
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 
New Member
Bo Kong
Join Date: Oct 2016
Location: China
Posts: 22
Rep Power: 9 
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 
Senior Member
Elham
Join Date: Oct 2009
Posts: 184
Rep Power: 16 
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 
New Member
Bo Kong
Join Date: Oct 2016
Location: China
Posts: 22
Rep Power: 9 
Yes，I can see perturbation in x and z directions like the picture shown. Does it right?


December 27, 2016, 22:02 

#191 
New Member
Bo Kong
Join Date: Oct 2016
Location: China
Posts: 22
Rep Power: 9 
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 
Senior Member
Elham
Join Date: Oct 2009
Posts: 184
Rep Power: 16 
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 
New Member
Bo Kong
Join Date: Oct 2016
Location: China
Posts: 22
Rep Power: 9 
Yes，these files are copied from channel395 case in tutorials without any change.


December 30, 2016, 02:59 

#194 
Senior Member
Elham
Join Date: Oct 2009
Posts: 184
Rep Power: 16 

January 2, 2017, 16:03 

#195 
Member
Join Date: Apr 2016
Posts: 34
Rep Power: 10 
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  
New Member
Bo Kong
Join Date: Oct 2016
Location: China
Posts: 22
Rep Power: 9 
Quote:
Here is the contour. 

January 2, 2017, 22:22 

#197  
Senior Member
Elham
Join Date: Oct 2009
Posts: 184
Rep Power: 16 
Quote:


January 2, 2017, 22:25 

#198 
Senior Member
Elham
Join Date: Oct 2009
Posts: 184
Rep Power: 16 
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  
Member
Join Date: Apr 2016
Posts: 34
Rep Power: 10 
Quote:


January 4, 2017, 07:31 

#200  
Senior Member
Elham
Join Date: Oct 2009
Posts: 184
Rep Power: 16 
Quote:


Thread Tools  Search this Thread 
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Pressure inlet boundary conditions for open channel flows  jack2000  OpenFOAM Running, Solving & CFD  5  December 6, 2018 11:00 
LES In Turbulent in channel flow  pankaj saha  Main CFD Forum  18  November 20, 2014 05:49 
LES In Turbulent in channel flow  pankaj saha  Main CFD Forum  8  April 15, 2009 11:34 
Turbulent channel flow  roberthino  OpenFOAM Running, Solving & CFD  5  August 15, 2007 08:35 
Bc for turbulent channel flow  roberthino  OpenFOAM Running, Solving & CFD  0  August 13, 2007 08:12 