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Mach number on wall BC

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Old   February 22, 2010, 03:51
Default Mach number on wall BC
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Dear CFD people,

In my simulation in Fluent I am using no slip wall BC. As expected, zero velocity magnitude and velocity components are present on wall BC. However non-zero Mach number values are appearing on the wall surface. Why is that so? Thanks,

ramad
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Old   February 23, 2010, 11:09
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Old   August 5, 2011, 13:58
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Hi,
Actually, I have the same question.

The only reason I expect that can explain this is that the Mach Number isn't calculated directly at the wall, just like the Velocity magnitude is done. Maybe in the middle of the first square instead of the corners.

However, if you try to simulate the same but with wall integration (Y+ smaller then 3), you may find Mach number really close to 0 because the first square is a lot smaller. I tested it and it works pretty well.

But if someone can confirm or correct what I wrote, it would be nice.

Regards,
Alexandre
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Old   June 6, 2014, 15:02
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Quote:
Originally Posted by Alexandre Drouin View Post
Hi,
Actually, I have the same question.

The only reason I expect that can explain this is that the Mach Number isn't calculated directly at the wall, just like the Velocity magnitude is done. Maybe in the middle of the first square instead of the corners.
Quite an old thread, but I'll leave it here for reference in case someone stumbles upon the same problem.

I have made some mesh refinements near the wall in order to analyze if this was only due to mesh size. Indeed the Mach number approximates to zero with increase in the mesh, but there are diminishing returns. It seems to come to a point where increasing the mesh refinement has very little impact on the Mach number at the wall, as shown in the picture for some refinement levels.

It seems it indeed extrapolates the value of the last cell center to the wall, as the variation in the inclination at the last point seems to indicate. What we suspect here is that it does not calculate a wall value for Mach number, since it is a solid, and just extrapolates the value of the adjacent cell center in the flow.

If anyone understands exactly how it works or has more information, please contribute.

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