Fluids

Fluids

high continity residual for LES simulation for flow over aerofoil

    • zein.elserfy
      Subscriber

      I am running Iincompressible LES simulation for NACA0012 aerofoil. My simulation domain is 3 c upstream, 7 downstream and 3 c to upper and lower boundaries and 0.1c in spanwise direction. The LES simulation is initialized using RANS steady state simulation.


      RANS steady state velocity field



      i used two boundaries as inlet (velocity inlet) and i define the velocity components ( ucos(10.8),usin(10.8),0) and other two as pressure outlet, while the front and back are periodic.


      The solver used for solving momentum equation is PISO solver ,fixed time step of 1e-6 is usedand max number of iteration/time step is 4


      The schemes used in the simulation second order upwind for momentum,second order for pressure and least square cell based for the gradient


      The relaxation factor is 0.3 for pressure and monetum


      The problem is that the continuity residual does not reach the set residual convergence 1e-6 after 4 iteration/time step


        iter        continuity      x-velocity        y-velocity      z-velocity      time/iter


       


        61163   1.9651e-05   3.8508e-07   3.3605e-07    3.6221e-07   0:001  20


        61164   2.4330e-05   7.6474e-07   6.7809e-07    7.3229e-07   0:001  19


        61165   2.0330e-05   5.4125e-07   4.7720e-07    5.1500e-07   0:00:27  18


        61166   1.9703e-05   3.8512e-07   3.3598e-07    3.6219e-07   0:00:28  17


        step flow-time lift flow-time drag


        9845 9.8450e-03 1.0801e+00 9.8450e-03 3.8341e-02


        Flow time = 0.009844999760389328s, time step = 9845


       


      is there something wrong in my case setting?


       

    • DrAmine
      Ansys Employee

      Have you run precursor RANS simulation where you quantified the gird size and time step size required to resolve a certain amount of turbulence kinetic energy?

    • zein.elserfy
      Subscriber

      Thanks for you reply Abenhadj


      after running the RANS simulation, I used the result as initial solution for LES using (init-turb-vel-fluctuations)


      I use this time step (10e-6) as it was quiet enough to keep convective courant less than 1 over the simulation domain


      The y+ value over the aerofoil is between (1-3) and the first cell height (1e-5).


      and x+ and z+ values are 200 and 150 respectively  


      no of nodes on aerofoil is 312 and 21 nodes in the spanwise direction


      chord length=0.3048m


      free stream velocity=71.3 m/s and Rynold number is 1.5 million


      best regards


      Zein

    • DrAmine
      Ansys Employee
      Still coarse for real LES. Please re-check the grid in precursor run or use SBES.
    • zein.elserfy
      Subscriber

      I changed simulation to SBES but still have the same problem with continuity residuals


       

    • DrAmine
      Ansys Employee

      As I said still coarse your run: you need to quantify in precursor run if you are having enough resolution.


      Moreover you can start from scratch now and not from steady-state result.

    • DrAmine
      Ansys Employee

      Add more information about your model (even the LES one) + pciture of let say 100 time steps where one can easily check the behavior of the residuals.

    • zein.elserfy
      Subscriber

      I have check y+ value and it is between 1-3, while x+=200 and Z+=150,so this means that the grid resolution is not enough for LES but i think it is reasonable for SBES

    • zein.elserfy
      Subscriber

       



      This is the residuals for LES simulation


      The LES subgrid model is WALE.


      The mesh is around 1.7 million control volume


       

    • DrAmine
      Ansys Employee

      SRS methods are not only for wall regions: if you are not paying the appropriate resolution in the bulk you are just doing an expensive URANS.


      Residuals looks okay: better to plot small number of them to see whether you have a monotonic decrease or not.

    • zein.elserfy
      Subscriber

      This is the residual plot (no of iteration/time step=5) for 40 time steps



      is there guidlines for the mesh away from the walls?


    • DrAmine
      Ansys Employee

      You might allow more iteration per time step: Say max 10 and set Report Interval to 3.


      LES or SRS Grid cannot just be judged by just looking into it. But what you are sharing is fine probably very fine in the regions of no interest. For that reason a precursor run. Seems that the wake region is not really well resolved if the wake does deflect to top or bottom.


       


       

    • zein.elserfy
      Subscriber

      i will run the simulation for zero angle of attack on the same mesh and check if the continuity residuals are affected or not

    • DrAmine
      Ansys Employee

      Have a fun!

Viewing 13 reply threads
  • You must be logged in to reply to this topic.