k-w turbulent model and inflow boundary conditions

vaina74 · March 26, 2010, 08:11

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

and

.
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, 09:28

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

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

and

.
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

and

(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 m²/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 m²/s² and $\omega$ =6725 1/s! $\omega$ seems to be enormous, compared to values I see in the forum, is anything wrong?

March 26, 2010, 08:11	k-w turbulent model and inflow boundary conditions	#1
vaina74 Senior Member Join Date: Feb 2010 Posts: 213 Rep Power: 17	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 and . 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). $\omega = \frac{0.09k}{\beta\nu}$ where $\beta$ is the turbulent viscosity ratio and $\nu$ is the dynamic viscosity. I guess a $\beta$ = 1 for my case, am I right? Last edited by vaina74; March 26, 2010 at 08:29.

March 28, 2010, 17:25		#3
vaina74 Senior Member Join Date: Feb 2010 Posts: 213 Rep Power: 17	I think it is just the opposite. I read that $\omega$ has a great influence on the turbulence. Maybe I didn't understand that Last edited by vaina74; March 29, 2010 at 13:32.

April 26, 2010, 07:03		#7
vaina74 Senior Member Join Date: Feb 2010 Posts: 213 Rep Power: 17	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 m²/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 m²/s² and $\omega$ =6725 1/s! $\omega$ seems to be enormous, compared to values I see in the forum, is anything wrong? Last edited by vaina74; April 26, 2010 at 07:35.

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March 27, 2010, 09:28		#2
sandy Senior Member Sandy Lee Join Date: Mar 2009 Posts: 213 Rep Power: 18	Somebody told me it should be 10 ~100. You can choose any one, because it has just little effect. Right or wrong?

April 12, 2010, 19:20		#5
vaina74 Senior Member Join Date: Feb 2010 Posts: 213 Rep Power: 17	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 and (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.

April 23, 2010, 13:10		#6
vaina74 Senior Member Join Date: Feb 2010 Posts: 213 Rep Power: 17	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 $\omega=\frac{\epsilon}{\kappa}$ ?