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October 5, 2009, 08:40 
Problem in Turbulent modeling of impinging slot jet

#1 
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Dear Friends,
I wrote a code with Fortran for laminar impinging slot jet and I verify it with experimental and numerical references. Now when I add the low reynolds models of Ke turbulent model to my code, unfortunately I couldn't get the converge from code and if I could the results is not the same with available numerical results. Recently, I found some articles which it says we should relaxed the Eddy viscosity and Generation term as well as other variables.(eg. u,v,p,T,k,e) but I am not sure this additional relaxation factor help me to get converge from code. If anybody has a same experience in such a Low Reynolds ke Modeling could help me and tell me what should I do with this problem, I really appreciate that. Because I am at the last semester in my master program and need to get the result in order to be graduate. Best Regard, Amir 

October 21, 2009, 02:05 

#2 
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Hi Amir.
it's better that you first write code with Standard ke model then you can change it to LowRe model.to know that is your codeis correct or no. Ret and f_mi_u maybe produce some error in code because of their value. what's the type of your grid? staggered or collocated? in blow link two persons discused about LowRe model.maybe it be usefull. http://www.cfdonline.com/Forums/mai...kemodel.html 

October 21, 2009, 06:52 

#3  
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Dear Mazdak,
Thanks for your reply. As I wrote in the last post, I wrote it for collocated grid and in the laminar I got aggreement result with experimental and available numerical result. As you suggested to read this link http://www.cfdonline.com/Forums/main/63353writingturbulencesolverkemodel.html I wondered about the wall treatment and the first grid point subject which they discusted about that. Before it I thought when we use the LowReynolds number model for Ke we don't need any care about the first grid point distance to the wall. Anyway I really confused about this subject. If I need to put any special distance to the wall for the first grid, how I can define it? I can't underestand if I wanna put the Y+ term in my equations,should I generate the grid depends on it? how I can define the Y+? If you have any experience in this subject, I wondered If you can share with me. Thanks in advanced. Amir Quote:


October 21, 2009, 07:38 

#4 
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Good question Amir.
i read one paper, wrote that when you use LowRe KE model you must capture y+<5. Amir Y+ is a solution dependent quantity that it's definition is: Y+= density*friction velocity*y/viscosity where y is distance between first grid nodes and the wall. y: faseleye avalin grid ta divare.yani faseleye nazdiktarin noghte be divare For example, when you double the mesh (thereby halving the wall distance), the new y+ does not necessarily become half of the y+ for the original mesh. pleas send me your email address. Mazdak_parsi2000@yahoo.com 

October 27, 2009, 17:11 
Please helpppppppp

#5 
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Dear Mazdak, Thanks for your comments.
Dear all, Please Please help me ;( As I describe my problem before , I am doing 2D numerical of turbulent impinging slot jet. Actually it doesn't have any different with fullydeveloped channel. there are jus some difference in boundary conditions. Anyway let me give u more about my numerical method, Finite volume based SIMPLE, SIMPLER algorithm are followed and also considered herein are some numerical spatial discretization schemes, e.g., hybrid, power law, secondorder upwind, and QUICK schemes, to test their performance in predicting this complex flow. Collocated grid is used and nonuniform grids are used along the walls and at the entrance of jet. I used different node number in Y direction, from 100 to 240. But u know I have no idea, If I need to put any special distance to the wall for the first grid, how I can define it? I can't understand if I wanna put the Y+ term in my equations,should I generate the grid depends on it? how I can define the Y+? If Y+ is a solution dependent quantity that it's definition is: Y+= density*friction velocity*y/viscosity where y is distance between first grid nodes and the wall. so How I can put my first grid near the wall in Y+<5 for low reynolds number models when i don't have the value for friction velocity?!! before generation the grid base on Y+ how i can run the code and got the value of y+?!!! I am really confused. Waiting for your suggestions and comments. Thanks in advanced. Amir. 

October 29, 2009, 22:00 

#6 
Senior Member

Hi Amir,
You may want to distinguish the convergence problem from the physical model problem (although both might be linked). Re the convergence, are you experiencing divergence or erratic convergence? Normally, I would expect that adding turbulence would help convergence due to turbulent viscosity. I would suggest using a simplified boundary condition (maybe highReynolds one) to check this aspect. Re the wall treatment: y is given by your mesh. Y+ is obtained after (during) the calculation: the friction velocity is derived from the velocity field. The normal process is: (a) fix a grid, (b) run a simulation to convergence, (c) extract y+ from the results, (d) modify the grid to have y+ < 5 and redo step b and c. Note that the velocity field may differ for the refined grid, thereby having an impact of y+, i.e. you may have to go through the process a few times. I am not sure what references/sources you are using, but you may want to have a look at the wiki section on turbulence modeling, a fluent user manual (if you can access one). There was a technical paper from CFX on their adaptive wall function for turbulence and heat transfer (used to be accessible online, but I can't get hold of the link now). Good luck. Julien 

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