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vaina74 March 26, 2010 09:11

k-w turbulent model and inflow boundary conditions
 
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 \kappa-\omega SST turbulent model and I'm doubtful about the inlet and wall boundary condition of k and w.
I can evaluate a \kappa initial guess from turbulence intensity. I assume a 1% for my case, an open propeller (or 5% is better?).
What about \omega? 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 1-10).

\omega = \frac{0.09*k}{\beta*\nu}

where \beta is the turbulent viscosity ratio and \nu is the dynamic viscosity. I guess a \beta = 1 for my case, am I right?

sandy March 27, 2010 10:28

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

vaina74 March 28, 2010 17:25

I think it is just the opposite. I read that \omega has a great influence on the turbulence.
Maybe I didn't understand that :(

sandy April 10, 2010 05:28

Quote:

Originally Posted by vaina74 (Post 251807)
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 \kappa-\omega SST turbulent model and I'm doubtful about the inlet and wall boundary condition of k and w.
I can evaluate a \kappa initial guess from turbulence intensity. I assume a 1% for my case, an open propeller (or 5% is better?).
What about \omega? 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 1-10).

\omega = \frac{0.09*k}{\beta*\nu}

where \beta is the turbulent viscosity ratio and \nu is the dynamic viscosity. I guess a \beta = 1 for my case, am I right?

Hi vaina74, in the ESI guidelines, it is "For external flows the freestream turbulent viscosity will be on the order of laminar viscosity so small values of b are appropriate, say b = 0.1 – 0.2. ", however, in the FLUENT user manual, it is about 1~10. You think, which one is the value we should choose? Thanks.

vaina74 April 12, 2010 19:20

Hi, Sandy.
You're right, but the in the most papers I find a turbulent viscosity ratio (sometimes TVR) of 1-10 for external flows (maybe do they all use FLUENT and its manual? ;)). Anyway, I set \beta=1 and obtained very good results for C_L and C_D (less good for a more turbulent flow). I used ESI guidelines to evaluate an initial guess for \omega.
I hope a more expert CFD user will answer to this thread.

vaina74 April 23, 2010 13:10

Please, I have some questions about implementation of \kappa-\omega 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 wall-adjacent first cells centroid is located within the log-law layer. But someone, with \kappa-\omega 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 \omega. In FLUENT manual and other papers I read
\omega=\frac{\kappa}{\nu_t}
but I find also
\omega=\frac{0.09\cdot\kappa}{\nu_t}
In other words, my question is:
\omega=\frac{\epsilon}{0.09\cdot\kappa}
or
\omega=\frac{\epsilon}{\kappa}?

vaina74 April 26, 2010 07:03

I 'chose' the relation
\omega=\frac{\kappa}{\nu_t}
and
\omega=\frac{\epsilon}{0.09\cdot\kappa}

In my case, kinematic viscosity is 1.19e-6 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 \omega=6725 1/s! \omega seems to be enormous, compared to values I see in the forum, is anything wrong? :(


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