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Old   July 14, 2000, 06:15
Default transition zone
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dilip
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Hi, As my earlier message was incomplete, i posted that again. My queries were : (1) Can we predict the heat transfer and pressure drop in the transition region with the help of cfd? (2) In case of standard k-e model with wall function approach, y+ should be between 30-50. In this range, friction factor and heat transfer co-efficient change even for fully developed turbulent flow in a pipe. So how shall we attain the mesh independent solution? Or we have to go on changing the mesh size till we reach to the experimental value.

Please help me to clear my funda. Thanks in advance.

Dilip
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Old   July 15, 2000, 21:07
Default Re: transition zone
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John C. Chien
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(1). The laminar-turbulence transition is a tough research field. It has been simulated since the days of boundary layer/mixing length theory till present time of low Reynolds number models. (2). The wall function treatment replaces the numerical calculation of the near the wall region by a single parameter family of the Law of the Wall, which is also implicitly included in the turbulence model of the outer region. (3). In other words, both regions have the universal Law of the Wall function included in the solutions. The iteration method simply tries to find the proper v* value such that the two Law of the Wall solutions will join properly at the interface, that is the mesh boundary of Y+=30~300. (4). The other way to say is: Plot the velocity solution in semi-log scale, using v* as a parameter (as done in the Law of the wall function). Pick a point in the Y+ scale at a point between Y+=30 and Y+=300, and divide the flow field into two regions. The inner region is represented by the Law of the Wall function, and the outer region is represented by the mesh independent numerical solution of the Law of the Wall function. At the interface point, the sigle parameter is v* from both regions, and the U at the interface, which must be continuous. (5). By the way, the mesh independent numerical solution is for the outer region only. As long as the interface point Y+ is within the range, there is no need the alter the interface location (associated with the Law of the Wall region). And the final Y+ is part of the solution. There is a similar discussion in the previous messages on this subject. Use the search to find it.
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Old   July 17, 2000, 05:54
Default Re: transition zone
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Dilip
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John, Thanks a lot for your nice explanation. But i have still some queries on transition that i'm posting later. I expect your comments on that. dilip
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