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-   -   ystar and yplus (http://www.cfd-online.com/Forums/fluent/35295-ystar-yplus.html)

sarah_ron December 16, 2004 10:33

ystar and yplus
 
Posted By: Abhijeet Vaidya <vabhi@hotmail.com> Date: Tue, 29 Oct 2002, 10:25 a.m.

hi friends,

You must be aware of ustar & yplus as fluent defines it ! Going by their definitions , I was thinking that they should be exactly same if ystar is > 11.25. But when I solved turbulent flow over an expanding channel (pl note that this is a benchmark problem for laminar flow so the geometry of this channel must be familier to the readers), then yplus goes to zero at the reattachment poing which is obvious but even in the regions where ystar is > 11.25 , there is difference in the value of ystar and yplus which means that even when ystar > 11.25, still u_tau = (cmu^0.25)*(tke^0.5) does not hold ? But theory says otherwise ! Can somebody help ?


me December 16, 2004 12:19

Re: ystar and yplus
 
They should be close to each other in equilibrium boundary layers where u_tau =(Cmu^0.25)*tke^0.5 is a good approximation. In non-equilibrium flows (e.g., diffusers you're considering), however, they can be quite different. A quintessential example is reattchment or stagnation point where u_tau and therefore y+ vanishes (tau_w = 0), yet TKE has a finite, usually quite large value there making ystar finite.


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