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K and Omega Profiles in Fluent

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Old   September 14, 2015, 15:21
Default K and Omega Profiles in Fluent
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hob
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Hi all,

I am having a small issue with matching experimental data in Fluent 15.0 using standard k-\omega model. From the experimental data obtained I have an x-velocity profile and corresponding RMS profile for the inlet conditions.

In Fluent there are various way to specify the turbulence (via hydraulic diamter etc), however, I want to use the actual RMS profile.

The method I have used thus far is to assume isotropic turbulence and determine the turbulent kinetic energy profile from:

k = \frac{3}{2}\overline{u_{x}^{'}},

where \overline{u_{x}^{'}} is the RMS profile in the x-direction (length).

\omega specified in the Fluent manual is given by

\omega = \frac{\epsilon}{k \beta^{*}},

where \beta^{*} = 0.09, and k is specified above.

I have found an approximation to \epsilon is various papers, listed as

\epsilon \approx 15 \nu   \overline{\frac{\left ( u_{x}^{'} \right )^{2}}{U_{x}}}.

Based on a wind tunnel velocity of approx. 4 ms^{-1} and turbulence intensity I of approx. 1.1%, this equates to average values of \epsilon = 2.0 \times 10^{-9} and \omega = 0.0001.

Is this the correct approach/equations given the experimental data?

Many thanks for any input,

Hob

Last edited by hob; September 15, 2015 at 01:49.
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Old   November 13, 2015, 08:06
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Quote:
Originally Posted by hob View Post
Hi all,

I am having a small issue with matching experimental data in Fluent 15.0 using standard k-\omega model. From the experimental data obtained I have an x-velocity profile and corresponding RMS profile for the inlet conditions.

In Fluent there are various way to specify the turbulence (via hydraulic diamter etc), however, I want to use the actual RMS profile.

The method I have used thus far is to assume isotropic turbulence and determine the turbulent kinetic energy profile from:

k = \frac{3}{2}\overline{u_{x}^{'}},

where \overline{u_{x}^{'}} is the RMS profile in the x-direction (length).

\omega specified in the Fluent manual is given by

\omega = \frac{\epsilon}{k \beta^{*}},

where \beta^{*} = 0.09, and k is specified above.

I have found an approximation to \epsilon is various papers, listed as

\epsilon \approx 15 \nu   \overline{\frac{\left ( u_{x}^{'} \right )^{2}}{U_{x}}}.

Based on a wind tunnel velocity of approx. 4 ms^{-1} and turbulence intensity I of approx. 1.1%, this equates to average values of \epsilon = 2.0 \times 10^{-9} and \omega = 0.0001.

Is this the correct approach/equations given the experimental data?

Many thanks for any input,

Hob
Hi hob,

Did you ever come to a conclusion about this?

Cheers,
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Old   November 13, 2015, 11:14
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hob
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Originally Posted by kingjewel1 View Post
Hi hob,

Did you ever come to a conclusion about this?

Cheers,
In the end I switched to the transitional sst model, the low turbulence value meant the BL growth was very thick for the k-w model, the trans-sst is better for low turbulence/ transitioning models.

I also used hydraulic diameter, although varying the dissipation rate seemed to make little difference compared to variance between turbulence models,

Regards,

Hob

Last edited by hob; November 13, 2015 at 23:46.
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