Fluids

Fluids

plot the rms of turbulent velocity

    • abd fs
      Subscriber

      Hi to all,


      As it is known that the root mean square of the turbulent velocity fluctuation can be calculated in RANS as follows (with the turbulent kinetic energy k is known );


      u'=(2k/3)^0.5


      I am doing simulation in Fluent using one equation RANS and i want to plot  the rms of turbulent velocity for this RANS model.


      I tried to plot, but I got a wrong curve




      How can I plot  the rms of turbulent velocity in RANS?


       


      Thank you.


       


       


      EDIT - moving to Fluids. Please post in the correct topics. 


       

    • DrAmine
      Ansys Employee

      Why do you think it is wrong? That is the only way to get root mean square in Boussinesq apporach based turbulence models. 

    • abd fs
      Subscriber

      Hello Amine,


      Because  the real curve is :



      Best regards,


      SERRARE

    • DrAmine
      Ansys Employee

      That is what we call validation: so the plot is correct but you are underestimating the fluctuation.

    • abd fs
      Subscriber

      Thank you for sharing these information here.


      In your opinion how can I fix this problem?


      Best  regards?


      SERRARE

    • Rob
      Ansys Employee

      How much mesh have you got in the near wall region? 

    • abd fs
      Subscriber

      Hello,


      this is my mesh



      I carried out a mesh by adaptation region



      Best  regards,


      SERRARE

    • DrAmine
      Ansys Employee

      1/Use Low-Reynolds mesh with yplus 1 to 5 and at least 15 cells in boundary layers


      2/Make a grid sensitivity analysis and work with high order schemes


      3/Comapre with other RANS turbulence models (SST, Realizable with EWT or Menter Lechner) and especially the one which might account for anisotropy: RSM


      3/Are you plotting the value directly after the expansion region / step  (weak recirculation zone)


      4/Judge your results describe them and document them


       


      there is no bad correct or wrong results: only bad or good comments on results. 

    • abd fs
      Subscriber

      Ok, I will try this and post here the results


      Best regards,


      SERRARE

    • abd fs
      Subscriber

      hello Amine,


      I'm plotting  in this section, designated by the yellow line in the picture. 



      velocity profile




      Best regards,


      SERRARE


       

    • abd fs
      Subscriber

      Hello Sir Amine,


      (1/Use Low-Reynolds mesh with yplus 1 to 5 and at least 15 cells in boundary layers)


      I tried implementing it , but I can't do it if you can help me with pictures that carry the steps to do it.


      Best regards,


      SERRARE

    • DrAmine
      Ansys Employee

      Just make rough estimation for yplus and use conservative cell growth rate for the first cells near the walls. Run once. Have a look into the turbulent viscosity ratio. The Max should of will indicate approximately the middle if boundary layer. If the resolution not enough remesh and rerun...

    • abd fs
      Subscriber

      Thank you so much Amine.


      Ok, I will try this and post here the results


      Best regards,


      SERRARE

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