Fluids

Fluids

fluent : turbulent flow u+ vs y+

    • abd fs
      Subscriber

      Hi everyone,


      As you may have seen from my last post (https://forum.ansys.com/forums/topic/separartion-flow/) I have been modeling turbulent flow around a ramp 25° . I do not have much experience so please bear with me even if my questions are simple.


      I have been wanting to check the U+ vs Y+ values in order to determine if the mesh around at the wall level is fine enough to properly model the different boundary layer regions.


      I would like to plot u+ against y+ to have the velocity profile near the walls.


      Did anybody already try that?  


       


       

    • DrAmine
      Ansys Employee

      Yes you need to write the functions for u plus and yplus to plot them at a certain position normal to the wall

    • abd fs
      Subscriber

      Thank you very much for your kind help sir


      I want to plot the famous TBL curve U+ vs y+, to detect the 3 zones :



      Best regards,


       


      SERRARE

    • abd fs
      Subscriber

      Someone can help me? With some tips, comments?


       


      Thanks.


      SERRAR

    • Rob
      Ansys Employee

      The velocity profile can be plotted using a line and an xy plot: you're plotting velocity against position (perpendicular to the wall).  Assuming you have a fine enough mesh you'll see the expected profile. 


      y+ is a pre-set value (we decide what we want) and is used to calculate theoretical height of the first cell: I think it's the mid point, but look it up in the manuals. This value can then be plotted from Fluent, but only really exists as data on the wall, to check if your mesh is fine enough. 


      u+ is also another derived term, and you've put the curves in your earlier post.  


       


      Edit: moved to Fluids


       


      I'm not sure why you'd need to use CFD to plot the u+ against y+  curve? 


       

    • abd fs
      Subscriber

       Dear all,


      I am having extreme difficulty getting this to work. Likely because I have no clue what I am doing.


      Iam required to compare the law-of-the wall profile to the Uplus v Yplus computed in fluent for a K-omega SST turbulent flow.Fluent gives Yplus but not Uplus.


       


      I tried defining Uplus as a Custom Field Function, my  yplus<1. For uplus =ylus i used 


      I got



       


      For  logarithmic u+ 



      I got


      My question is: how can plot the famous TBL curve U+ vs y+, to detect the 3 zones ?



      Best regards,


       


       


      SERRARE


       

    • DrAmine
      Ansys Employee
      You need to define y+ and u+. Then create a line at a certain position normal to a wall then you can xy plot to plot the profile you want. U+ is the velocity in the cell / square root of wall shear stress divided by the density.
    • abd fs
      Subscriber

      Thank you so much AMINE I forgot this, 


      I will try this and post here the results


      Best regards,


      SERRARE

    • mutlaq404
      Subscriber

      Mr. Abd How you calculate Yplus?

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