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Documentation

Error: Insufficient Mesh Resolution for Turbulence Modeling

Warning

Mesh resolution might not be sufficient for correct turbulence modeling. Please check your results and consider refining the mesh.

Users should note that this mesh resolution error page is relevant to the LBM solver. If you are using one of the OpenFOAMĀ® solvers, please check our other error-related articles.

What Happened?

With the current mesh resolution, y+ values in the domain exceeded the value of 2000. This will likely result in inaccuracies in turbulence modeling.

What Could Be the Possible Reason?

The LBM solver uses wall functions to model the near-wall profiles. y+ values up to 1000 are acceptable. Optimally, they should remain below 500.

When the y+ values exceed the threshold of 2000, a warning message appears in the Event log. In such cases, the results are likely impacted by poor turbulence modeling.

coarse mesh lbm y+ too high
Figure 1: Warning message displayed when y+ is greater than 2000 in the domain.

What Can I Do Now?

There are two main ways to obtain lower values for y+:

  • Refine the surfaces further: The LBM solver uses cartesian mesh where each mesh element is a cube. By generating finer cube elements closer to the walls, the resulting y+ values will be reduced. Therefore, the wall functions will be able to capture the near-wall profiles more accurately.

    If the mesh at the surface is already reasonably refined, then this option is not optimal. In this case, increasing the mesh resolution on the surfaces can result in much more expensive simulations. With smaller cells, the time step sizes will also be smaller.
  • Applying a Reynolds scaling factor: Another possibility is to apply a Reynolds scaling factor. This parameter is accessible in the Mesh settings tab.
reynolds scaling factor set up lbm
Figure 2: Setting up a Reynolds scaling factor. A factor of 0.01 will reduce the Reynolds number by 100x.

Did you know?

SimScale allows its users to control and visualize the y+ values for OpenFOAMĀ® simulations. Want to learn how? Visit this knowledge base article for a detailed answer.

The actual CAD geometry doesn’t need any adjustments to use a Reynolds scaling factor. The algorithm automatically increases the kinematic viscosity \(\nu\) of the fluid, which will scale the Reynolds number \(Re\) appropriately.

$$Re = \frac{VD}{\nu} \tag{1}$$

In the Reynolds number definition, \(V\) is the fluid velocity and \(D\) is the characteristic length.

Please visit this documentation page for additional recommendations and limitations of using the Reynolds scaling factor.

Important Information

If none of the above suggestions solved your problem, then please post the issue on our forum or contact us.

Last updated: September 11th, 2020

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