
[Sponsors] 
September 13, 2006, 23:11 
The DNS solver requires inputs

#1 
New Member
Andy
Join Date: Mar 2009
Location: Fukuoka, Japan
Posts: 5
Rep Power: 9 
The DNS solver requires inputs for Ea and K0 in "BOXTURBDICT" and for U0sigma, U0alpha, U0kupper and U0lower in "turbulence properties".
I want to simulate a turbulent flow for a known integral length scale and turbulent rms velocity and then apply it to a combustion event. I assume I can estimate these inputs from this and the thermodynamic state of the fluid. How do I go about that? I have found some bits around and could have a guess at their meaning, however its always best to ask especially as it takes me weeks to find a solution. I would like to find the exact definitions of these parameters from literature or a website. Can anybody help? I am happy to update the WIKI once I understand it all. I am sure others having the same problems. 

September 24, 2006, 21:48 
I`d welcome any input from oth

#2 
New Member
Andy
Join Date: Mar 2009
Location: Fukuoka, Japan
Posts: 5
Rep Power: 9 
I`d welcome any input from others who have been through the same process.
Here is my progress to date. The turbulence spectrum appears to be based on the following equation. E(k)= (Ea*(k/k0)**4)exp(2.0*(k/k0)**0.5) That was easy enough. However, the inputs for "turbulenceProperties" appear to be more difficult to define. They appear to be linked to the forcing of turbulence. I have traced the terms U0sigma, U0alpha, U0kupper and U0lower back to UOprocess.C but cannot find the definition of each of these terms or any literature describing them. My best guess at the moment is that U0kupper and U0lower are the maximum and minimum wavelengths used to excite the turbulence. I still cannot work out what U0sigma and U0alpha are exactly. Any help would be greatly appreciated. 

September 25, 2006, 03:22 
UO process uses an UhlenbeckO

#3 
Senior Member
Gavin Tabor
Join Date: Mar 2009
Posts: 181
Rep Power: 9 
UO process uses an UhlenbeckOrnstein stochastic process to generate a stream of random numbers  or rather not quite random as they are stochastic, and so each number is partly related to the one preceeding it. Sigma and alpha are the two parameters in the UO process, controlling the variance and mean (I think) respectively. Have a look at
http://en.wikipedia.org/wiki/OrnsteinUhlenbeck_process for more details. Unfortunately there are various different definitions of the process with different scalings for alpha and sigma, and I'm not clear which reference was used for the original algorithm; I always have to sort through the code to work out the precise definition. Gavin 

November 9, 2006, 22:06 
Thank you for your help.
I

#4 
New Member
Andy
Join Date: Mar 2009
Location: Fukuoka, Japan
Posts: 5
Rep Power: 9 
Thank you for your help.
I am still having problems with the DNS though. My solution always becomes unstable. Below is a summary of my problem. Geometry 5mm x 5mm square 2 "cyclic" patches and an "empty" for the 3rd dimension 256x256 grid size dT= 0.0001s nu= 1.59e5 Ea= 0.00056 K0= 10 UOsigma = 0.1 UOalpha = 0.8 U0Kupper = 300 U0Klower = 7 My K and Epsilon slowly grow until there is no solution. I have tried reducing the timestep and tuning each input to no use. Any ideas? Andy 

November 9, 2006, 22:15 
Are you by any chance doing a

#5 
Senior Member
Hrvoje Jasak
Join Date: Mar 2009
Location: London, England
Posts: 1,781
Rep Power: 22 
Are you by any chance doing a 2D DNS simulation? This will (of course) not work, as the physics is all wrong. If you are doing 3D as you should, you should not be using empty patch type.
Hrv
__________________
Hrvoje Jasak Providing commercial FOAM/OpenFOAM and CFD Consulting: http://wikki.co.uk 

February 21, 2008, 13:15 
boxTurbDict seems to never be

#6 
Senior Member
John Deas
Join Date: Mar 2009
Posts: 160
Rep Power: 9 
boxTurbDict seems to never be accessed. I am a bit confused about the magnitude of the forcing in the current version.


March 10, 2008, 08:23 
Hi, I am trying to create a bo

#7 
Member
Ali Heidari
Join Date: Mar 2009
Location: Surrey, London, United Kingdom
Posts: 39
Rep Power: 9 
Hi, I am trying to create a box with initial turbulent. I have the box dimention and Vrms, I don't know how to relate them to Ea and K0. the formula which is presented here for turbulence energy spectrum [E(k)= (Ea*(k/k0)**4)exp(2.0*(k/k0)**0.5)] is not clear for me, I will be very grateful if some one can clue me in on this problem.


February 13, 2012, 05:01 

#8 
New Member
Terrence Nguyen
Join Date: Jan 2012
Posts: 11
Rep Power: 6 
Hi Andy,
are you still working on this. Since now I just begin with DNS and am facing the same problem as yours here. Would be nice, if you can show me how to overcome this. Thanks in advance, Terrence 

July 30, 2013, 09:51 

#9 
Member
Jack
Join Date: Aug 2012
Posts: 44
Rep Power: 5 
Hi all,
I know this is old, but I would love to know if anyone got anywhere, I am guessing that sigma and alpha are the variance and mean, but I do not know how they relate to the taylor Re. This is what I would LOVE to know! johndeas, boxTurbDict controls the application boxTurb which generates the initial conditions on velocity. This I don't think is too important as the forcing will eventually send the simulation to the correct Re. Andy, The upper limit on K seems very large, as in you are forcing too much of the spectrum and this could well be the simulation is blowing up. Jack 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Point coordinates via orinial point address after decomposing  diana  OpenFOAM Running, Solving & CFD  0  August 6, 2008 10:27 
hex mesh by starting from a group of point  Giulia Valerio  FLUENT  0  November 8, 2007 05:47 
floating point error when starting to iterate  Ralf Schmidt  FLUENT  1  June 2, 2006 06:00 
Granular starting point not at a boundary  Michael  FLUENT  0  February 22, 2006 08:30 
Starting Point to study CFD and code writing  Lynn  Main CFD Forum  3  November 23, 2005 05:31 