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Bump in flat plate boundary layer

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Old   July 13, 2015, 10:17
Default Bump in flat plate boundary layer
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DCK
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Hi Everybody,

I am a beginner to CFD. I tried to simulate the flow over a flat plat, i.e. simulate the LAMINAR boundary layer to compare it with the BLASIUS solution. The velocity at the inlet is 1 m/s. The attached plot shows the velocity profile at 1m downstream of the leading edge.

If you look at the flow well outside of the boundary layer (let's say y>0.15 m), you see that the velocity is at around 1.01 m/s instead of the 1.00 m/s I specified. Does this make sense? I wondered if this is due to the fact that I set up a wind tunnel in CCM+, modeling the floor as a wall, and both side walls as well as the ceiling as a symmetry plane. Hence, to maintain overall mass continuity, the flow outside of the boundary layer needs to be a bit faster than the freestream velocity, to make up for the smaller velocities in the boundary layer. Am I correct?

What I do not understand at all is the bump around 0.02 m < y < 0.1 m. Where does this come from? Is there a physical Explanation? Or is this an error due to some mistake(s) I made setting up the simulation?

Regards
Daniel
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File Type: jpg FlatPlate.jpg (54.6 KB, 34 views)
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Old   July 22, 2015, 10:02
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is this a RANS anlaysis? if so, what turbulence model are you using? what wall treatment? is your y+ appropriate? it looks like your boundary layer isn't resolved all that well, but without seeing your mesh I can't say for sure.

we need more information to determine a cause.
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Old   July 23, 2015, 01:55
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DCK
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Hi,

it's a RANS analysis without turbulence modeling, i.e. the flow is modeled as being laminar. I think I found the reason: I placed the leading edge of the flat plate immediately downstream of the inlet face. Meanwhile I found out that this should not be done. Instead, i placed a slip plate right at the beginning of the flow region, and then I attached the flat plate (no-slip wall of course) downstream of the slip plate. Now it works!
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