Help! How to hold turbulence in a 2d channel flow RANS simulation

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 July 27, 2012, 10:58 Help! How to hold turbulence in a 2d channel flow RANS simulation #1 Member   dw Join Date: Jul 2012 Posts: 32 Rep Power: 7 Newbie here. I am doing a 2D rans simulation to investigate some turbulence models. I am wondering how to make the flow turbulent? For example in a simple Re_tau=180 channel flow, or a little bit more than that. Pressure gradient driven, using some low Re turbulence model, and keep the mesh having y+<1. Then I set the initial nut=100*nu, k/e>100*delta_t, mass is kept constant on the solver's side (I am using an unsteady solver (URANS)). But I found the flow is always laminar or laminarized. This is very annoying. But I read so many papers, they all have a perfect velocity profile, How did they do that? Any secrets? Or what did I do wrong? Any ideas? -------- 1/153

July 27, 2012, 13:44
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Filippo Maria Denaro
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Quote:
 Originally Posted by 1/153 Newbie here. I am doing a 2D rans simulation to investigate some turbulence models. I am wondering how to make the flow turbulent? For example in a simple Re_tau=180 channel flow, or a little bit more than that. Pressure gradient driven, using some low Re turbulence model, and keep the mesh having y+<1. Then I set the initial nut=100*nu, k/e>100*delta_t, mass is kept constant on the solver's side (I am using an unsteady solver (URANS)). But I found the flow is always laminar or laminarized. This is very annoying. But I read so many papers, they all have a perfect velocity profile, How did they do that? Any secrets? Or what did I do wrong? Any ideas? -------- 1/153
Could you post your velocity profile compared to the DNS at Re=180?

 July 27, 2012, 13:49 #3 Senior Member     Daniel WEI (老魏) Join Date: Mar 2009 Location: Beijing, China Posts: 689 Blog Entries: 9 Rep Power: 15 __________________ ~ Daniel WEI ------------- Boeing Research & Technology - China Beijing, China Email

 July 27, 2012, 13:57 #4 Member   dw Join Date: Jul 2012 Posts: 32 Rep Power: 7 Thanks for helping. Yes the velocity profile is here: [shared just for 24 hours] http://f.imgtmp.com/Dcdfq.jpg

 July 27, 2012, 14:59 #5 Member   dw Join Date: Jul 2012 Posts: 32 Rep Power: 7 Is that because 2D cant sustain the turbulence structures, so to damp out all the turbulence? Or is that I should use RANS not URANS?

July 27, 2012, 15:09
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Filippo Maria Denaro
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Quote:
 Originally Posted by 1/153 Is that because 2D cant sustain the turbulence structures, so to damp out all the turbulence? Or is that I should use RANS not URANS?

No ... you are solving a 2D statistically steady solution, you never see in RANS any other variable that zero-order statistics....
But your problem is that you are solving a smaller Re number, your profile arrives at y+ about 10 at the half-heigh!

 July 27, 2012, 15:16 #7 Member   dw Join Date: Jul 2012 Posts: 32 Rep Power: 7 I will test with other LRN models to see if my new implemented model is wrong somewhere. But have you ever use steady RANS to get a correct profile before? Which turb model did you use? Thanks Last edited by 1/153; July 27, 2012 at 15:32.

 July 27, 2012, 15:39 #8 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 3,862 Rep Power: 42 RANS is perfect in turbulent channel flow, you have some wrong in your setting/input

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