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-   -   how to calculate y+ in cfx? (http://www.cfd-online.com/Forums/cfx/21846-how-calculate-y-cfx.html)

micaheal November 20, 2005 01:15

how to calculate y+ in cfx?
 
Hi to all.I want to capture boundary layer in cfx .I have studied about solver y plus and y plus.But I could not get it properly.I want to know that how we can give y plus for BL.i.e. how to calculate that these much no of nodes will be require for capturing B.L.Which equation I should use?as far as fluent i know that that no of y+ sholud not be greater than 30 but how to do it cgf?can anybody know Please reply me thanks

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adma November 20, 2005 12:07

Re: how to calculate y+ in cfx?
 
Hi, if you use turbulence model like k-w or k-e, check your res file in CFX-post, you will find a variable named y plus. that is what you want. easy. :) BTW: Y+ is less than 30...I don't think so. The well-accepted value is Y+ < 1.0, at least < 2.0 or 3.0 something. 30 is too high for any case. I don't know why Fluent says so.


guilin November 20, 2005 12:27

Re: how to calculate y+ in cfx?
 
I don't know,sorry

matej November 21, 2005 15:45

Re: how to calculate y+ in cfx?
 
Hi,

about the value of this magical y+ ...

first its value can go from 0 to anything. around 1 is required for near wall models, resolving flow in a BL with many mesh points.

classical wall function model for BL treatment requires y+ = 30 or more (but in some parts of a domain where BL is not important for the features one solves can run into hunderets).

CFX has its own adaptive near wall treatment, which should not be much sensitive to y+ value. have a look into the theory part of the manual searching wall BC.

and the last hint - do not stuck to fluent manual. I know its nicely written, but you'd better look for some CFD book like

An Introduction to Computational Fluid Dynamics: The Finite Volume Method, H. K. Versteeg and W. Malalasekera, Addison Wesley Longman Limited, 1995, reprinted 1996, ISBN 0-582-21884-5

or

Computational Methods for fluid dynamics Joel H. Ferziger and Milovan Peric springer verlag, 1997 corrected 2nd printing isbn 3-540-59434-5

matej



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