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Old   April 1, 2005, 10:48
Default ask a simple question.
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I feel very puzzled about the turbulent K-e models , ..and the SIMPLE method. If SIMPLE method can solve the flow problem, why we need that different turbulence models. Could I just use the SIMPLE method to solve the turbulence flow?
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Old   April 1, 2005, 13:45
Default Re: ask a simple question.
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The problem with solving turbulence method using simple algorithm is the difficulty in resolving all the time scales in your problem. The method used is often referred as DNS when you do a direct simulation of turbulence.The process is time consuming and needs memory.So people resolve only the larger time scales and use a model for the smaller time scale.
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Old   April 2, 2005, 07:56
Default Re: ask a simple question.
John G.
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If you want to capture the flow physics accurately you need to capture all the scales of motion in time and space. This is a DNS. A RANS set of equations tries to surmount this by treating the fluctuations as a seperate entity and in doing so you end up needing a turbulence closure. This is what the K-e model tries to do. In short, what you see in these books igonores the reality of the flow scales.

The problem lies in the unclosed terms, the Reynolds stress, which arises from the RANS averaging of the Navier Stokes equations. The SIMPLE algoithm as you see applied in many books (Versteeg and Malasekera for example) on unsteady flows is applied to instantaneous equations and does not accouunt for all the scales. In otherwords, it is of no use for real turbulent flows unless you do a DNS and since this is not practically possible for most engineering flows, we use RANS of LES techniques.
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