Problem in Turbulent modeling of impinging slot jet

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 October 5, 2009, 08:40 Problem in Turbulent modeling of impinging slot jet #1 New Member   Join Date: Oct 2009 Posts: 7 Rep Power: 7 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 K-e 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 k-e 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 Member   MAZI Join Date: Oct 2009 Posts: 79 Rep Power: 7 Hi Amir. it's better that you first write code with Standard k-e model then you can change it to Low-Re 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 Low-Re model.maybe it be usefull. Writing Turbulence Solver, k-e model

October 21, 2009, 06:52
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Dear Mazdak,

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.

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 Low-Reynolds number model for K-e we don't need any care about the first grid point distance to the wall.

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.

Amir

Quote:
 Originally Posted by mazdak Hi Amir. it's better that you first write code with Standard k-e model then you can change it to Low-Re 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 Low-Re model.maybe it be usefull. Writing Turbulence Solver, k-e model

 October 21, 2009, 07:38 #4 Member   MAZI Join Date: Oct 2009 Posts: 79 Rep Power: 7 Good question Amir. i read one paper, wrote that when you use Low-Re K-E 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 29, 2009, 22:00 #6 Senior Member   Julien de Charentenay Join Date: Jun 2009 Location: Australia Posts: 229 Rep Power: 9 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 high-Reynolds 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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