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March 26, 2010, 08:11 
kw turbulent model and inflow boundary conditions

#1 
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My CFD case concerns a blade section of a marine propeller. The hydrofoil chord is 0.3 m and the Reynolds number is about 1.9E6. I choose the  SST turbulent model and I'm doubtful about the inlet and wall boundary condition of and .
I can evaluate a initial guess from turbulence intensity. I assume a 1% for my case, an open propeller (or 5% is better?). What about ? I read the FLUENT user manual and ESI guidelines. For external flows it's difficult to guess a characteristic turbulent length, so it's better to estimate the turbulent viscosity ratio (typically 110). where is the turbulent viscosity ratio and is the dynamic viscosity. I guess a = 1 for my case, am I right? Last edited by vaina74; March 26, 2010 at 08:29. 

March 27, 2010, 09:28 

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Sandy Lee
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Somebody told me it should be 10 ~100. You can choose any one, because it has just little effect. Right or wrong?


March 28, 2010, 16:25 

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I think it is just the opposite. I read that has a great influence on the turbulence.
Maybe I didn't understand that Last edited by vaina74; March 29, 2010 at 12:32. 

April 10, 2010, 04:28 

#4  
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Sandy Lee
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Quote:


April 12, 2010, 18:20 

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Hi, Sandy.
You're right, but the in the most papers I find a turbulent viscosity ratio (sometimes TVR) of 110 for external flows (maybe do they all use FLUENT and its manual? ). Anyway, I set and obtained very good results for and (less good for a more turbulent flow). I used ESI guidelines to evaluate an initial guess for . I hope a more expert CFD user will answer to this thread. 

April 23, 2010, 12:10 

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Please, I have some questions about implementation of SST turbulent model and wall functions.
1. In most papers or threads of this forum I read that, when a wall function is used, y+ must be greater than 30, if possible closed to 30, so walladjacent first cells centroid is located within the loglaw layer. But someone, with SST, sets y+ above 11. Is it correct? Why? I can't find theoretical support for that. 2. I'm in trouble with inlet boundary conditions for . In FLUENT manual and other papers I read but I find also In other words, my question is: or ? 

April 26, 2010, 06:03 

#7 
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I 'chose' the relation
and In my case, kinematic viscosity is 1.19e6 mē/s (sea water) and inlet velocity is 7.3 m/s. If I guess a turbulence intensity of 1% and a turbulent viscosity ratio of 1, I obtain k=0.008 mē/sē and =6725 1/s! seems to be enormous, compared to values I see in the forum, is anything wrong? Last edited by vaina74; April 26, 2010 at 06:35. 

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