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 kim April 3, 2002 12:28

grid for laminar flow

What is the recommendation for building grid in laminar flow ? In turbulent flow there is the well known restriction for the distance of the first cell expressed by y+ value. But how careful do I have to be when the flow is laminar ? Is it only a problem of numerical accuracy or are there any other limitations ? Thanks for your comments.

 Chetan Kadakia April 3, 2002 15:57

Re: grid for laminar flow

In answer to your question, such a restriction for the laminar case does not exist. There will only be a problem of numerical inaccuracy if you have much too many cells, but for most laminar flows you should not need to have such a high density mesh to obtain grid independence.

The reason there is a recommended minimum distance of y+ when using turbulence models is that the near wall functions are only accurate in certain areas.

Remember when flow is turbulent, the boundary layer has 3 layers. Each layer differs in evaluation.

The near wall functions, depending on which one is selected has equations that work well for certain layers of the boundary layer but not for the others.

In the laminar case, the boundary layer is evaluated the same throughout. Therefore the Navier-Stokes Equation, as applied throughout your domain, will be accurate for the laminar boundary layer.

You should have enough cells in the boundary layer to observe the velocity gradient/profile of the layer. That means at least 10 cells perpindicular to the wall . If you increase to a 100 cells perpindicular to the wall you should get even more accurate results. As you increase the number of cells, accuracy obviously increases. But if you have too many cells, numerical error can be a factor, but that is probably not your worry. Just adapt the boundary and check to see if the change in the grid has caused a change in your solution. Hence start with less cells and iterate and adapt until you have grid independence.

Need more help, or further clarification, just ask.

 kim April 4, 2002 02:44

Re: grid for laminar flow

Thanks Chetan

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