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Old   June 6, 2000, 07:01
Default turbulence in turbulence
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ram
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hi .. i am a novice inCFD.. i am modelling flow using k-e model for turbulence. in the standard k-e model equations( 2D), how can one integrate the rate of production of k or e terms over the control volume. i am integrating FVintegration over 2D space? 2) what can one take the relation for mixing length for flow over variable x-area with many internal obstructions? 3)we have two different k-e model equations for low Re and high Re. how high or low should be this Re value for the use of respective equations?
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Old   June 6, 2000, 09:43
Default Re: turbulence in turbulence
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John C. Chien
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(1). The volume is always 3-D, so, make the thickness equal to unit thickness. (2). If you are using mixing length, you need to know the distribution of it in the flow field. You can specify the mixing length distribution or you can use a model to calculate it. (that is your job) (3). As long as the universal log-law portion of the velocity profile exists, you can always use the high Reynolds number model.(sublayer region excluded) (4). You can always use the low Reynolds number model for the whole flow field, any Reynolds number. So, you can not use the high Reynolds number model for the laminar and the transitional flow range. (it is the flow behavior associated with the Reynolds number, not the value of the Reynolds number itself.)
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Old   June 7, 2000, 03:18
Default Re: turbulence in turbulence
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Jonas Bredberg
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Hi,

low-Re and high-Re turbulence models could be a bit confusing a first. This Reynolds number is not related to the bulk Reynolds number, but rather a local Reynolds number. Generally a low-Re turbulence model needs to resolve the flowfield even in the viscous sublayer, while a high-Re turbulence model is satisfied with placing the first interior calculation node in the logarithmic layer. The different regions are normally distinguished using a normalised wall distance, y^+=yu_{\tau}/\nu, where u_{\tau} is the friction velocity (u_{\tau}=\sqrt(\tau_w/\rho), \tau_w is the wall friction), y is the wall normal distance, and \nu being the kinematic viscosity. The viscous sublayer is below y^+=5, and using a low-Re turbulence model you need a number of nodes in this region, with the first below y^+=1. The logarithmic region starts at around y^+=30.

Regards Jonas
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