Fluids

Fluids

Can’t get correct values for Cl and Cd on a airfoil

    • Riccardo Bianchini
      Subscriber

      Hello, a few days ago I asked help with a mesh, now I have troubles for the validation of a flow around a NACA 4412. I'm trying at 6° but Cl is too low and Cd too high, tried different models and it's the same. Also tried a NACA 0012 with a different type of mesh and it's the same. I'm leaving a few pictures of the mesh and setup. 

      Probably I'm doing something wrong. Thanks.

    • SRP
      Subscriber

      Hi,

      I recommend checking the AIC course for more information regarding the airfoil simulation: Flow over an Airfoil - ANSYS Innovation Courses

      Thanks

      • Riccardo Bianchini
        Subscriber

        Already tried but had no luck. It seems that for 0° AoA I get the correct values, but as soon as I change that I get bad data.

    • SRP
      Subscriber

      Hi,

      Which turbulence simulation model did you use?

      Thanks

      • Riccardo Bianchini
        Subscriber

        Hi. I intended to use k-w SST, but have also tried k-e and S-A and I get somewhat similar results. Which would be good if they weren't wrong, there has to be something simple I'm missing but can't figure out what.

    • SRP
      Subscriber

      Hi,

      Could you please send the screenshot of the following plot and contours:

      1)Residual plot for zero AOA and 6 degree AOA

      2)Cl and Cd plot for both AOA

      3)Velocity/Pressure contour for both AOA

      Thank you

      • Riccardo Bianchini
        Subscriber

        Contour at 0°:

        Residuals at 0°:

        Cl at 0°:

        Cd at 0°:

        Contour at 6°:

        Residuals at 6°:

        Cl at 6°:

        Cd at 6°:

        Changed something in the model and now even the values at 0° are wrong.

    • SRP
      Subscriber

      Hi,

      Can you please uncheck pseudo transient to reduce error and re run simulation once?

      Thank you

      • Riccardo Bianchini
        Subscriber

        Looks about the same even though convergence is slower.

    • SRP
      Subscriber

      Hi,

      The issue might be with turbulence model and corresponding y+ use to create the inflation layer.

      When using low-Re models or any models with enhanced wall treatment, the average Y+ value should be on the order of ~1 to ensure we are capturing the laminar sub-layer. When using wall function models, the Y+ value should ideally be above 15 to avoid erroneous modelling in the buffer layer and the laminar sub-layer. High quality numerical results for the boundary layer will only be obtained if the overall resolution of the boundary layer is sufficient. 

      I suggest to please check the y+ corresponding to the turbulence model used.

      you can also create an additional variable for the eddy viscosity ratio. Then by plotting this variable , and superimposing mesh in the near-wall region, we can visualize the boundary layer resolution.

      Thank you

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