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why first grid point is at log layer in ke model

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Old   May 12, 2007, 00:41
Default why first grid point is at log layer in ke model
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student
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sir, why we cannot go upto the wall when applying std ke model..what will happen if we apply.. how low Re ke model eliminates this defect. ie when applying Low Re ke model we can go up the wall. how? plz expalin thanks
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Old   May 12, 2007, 07:30
Default Re: why first grid point is at log layer in ke mod
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Anantha
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Hello,

In the k-e model, when you write the modeled k and e equations, you have few constants with the production and dissipation terms of the epsilon equation. The values are calculated based on some experiments and are valid only in the log law region. This is one reason. More important is, when you see some terms (consider production term) in the modeled 'k' equation and exact 'k' equation, the behavior will not be same as we go close to the wall in your usual k-e equation. In fact there is one order of magnitude difference. To solve this usually damping functions are used such that even the modeled terms behave as same as the exact terms. The models with these damping functions are referred as Low Re Number models.

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Old   May 12, 2007, 08:20
Default Re: why first grid point is at log layer in ke mod
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Helo Anantha..thank you very much for your brief expalnation. but the models with damping functions termes as Low Re model..what Low Re exactly represents. whether this model is applied only low turbulent reynolds no..
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Old   May 12, 2007, 08:22
Default Re: why first grid point is at log layer in ke mod
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slight correction in previous question. sori for that..

Helo Anantha..thank you very much for your brief expalnation. but why models with damping functions termed as Low Re model..what Low Re exactly represents. whether this model is applied only low turbulent reynolds no..

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Old   May 12, 2007, 08:26
Default Re: why first grid point is at log layer in ke mod
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Anantha
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Hello,

The Low Re number does not mean that it is applicable only for the flows with very low over all Re. It refers to the local Re. As you go close to the wall, due to no slip condition, velocity goes to zero rapidly. As I mentioned earlier the damping functions take care of the behavior of each terms in the transport equations. But far away from the wall when Re is high, the effect of damping functions won't be there (damping functions are designed so). Thus it means that these models are good in the entire domain and is not restricted to low turb Re areas. But the difficulty is you need to resolve your grid with a y+ close to 1.

Regards Anantha
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Old   May 12, 2007, 23:38
Default Re: why first grid point is at log layer in ke mod
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thankyou very much Anantha. can i have your mail id.. if i have further doubts i can clarify with you.

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Old   May 13, 2007, 12:25
Default Re: why first grid point is at log layer in ke mod
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Anantha
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Hello, The above email link will do I suppose... you are welcome to send me emails and what ever help I can offer, i will definitely do. All the best.
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