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Scramjet modelling y+ value

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Old   April 21, 2012, 14:41
Default Scramjet modelling y+ value
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Stephen
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Hi, I am modelling a Scramjet intake and think that I have a fine enough mesh however after reading countless documentation about Y+ values I am more confused than ever.

I have an incoming velocity of 2122m/s (Mach 7 at 30,000 feet) along with the corresponding atmospheric conditions. RE = 3.3*10^7

I am modelling this in CFX and have a boundary layer height of 0.0001m.
I have calculated this using an assumed value for y+ of 220. The thing is I have no idea if that is acceptable or not. Does anyone have any thoughts?

I can always further refine the mesh but don't want to go increase the number of nodes to a ridiculous number for a model that is only 0.5m long.
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Old   April 22, 2012, 07:06
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Brad Fowler
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If you aren't greatly interested with the boundary layer effects, it could be suitable. What turbulence model are you using? The y+ value depends greatly on what turbulence model you're applying, and what kind of results you're after.
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Old   April 22, 2012, 07:49
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In addition to Brad's comment, the answer to the question "is my mesh fine enough?" is always "Do a sensitivity check and find out". Always better to determine for yourself.
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Old   April 22, 2012, 07:57
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Sijal Ahmed Memon (turboenginner@gmail.com)
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At such high Mach numbers, boundary layers effects are minimum. So in my point of view you are good with Y+ around 100.
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Old   April 22, 2012, 08:14
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Probably true. But it is best to prove it by doing a sensitivity study, then you will know how big an effect the boundary layer has on the flow. It may even suggest you can coarsen your grid and maintain accuracy (unlikely, but possible).
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Old   April 22, 2012, 09:20
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Quote:
Probably true. But it is best to prove it by doing a sensitivity study, then you will know how big an effect the boundary layer has on the flow.
Very true. Need to check results at Y+ = 1 , 30, 100 and 200 while maintaining the enough no of nodes in the boundary layer (min = 10-15).


Quote:
It may even suggest you can coarsen your grid and maintain accuracy (unlikely, but possible).
I also think it is unlikely.


PS. @moonser: Y+ and overall no. of nodes in domain should be studied separately. For such high Mach no. flows the main problem is the resolution of shock waves, so you many need to refine the mesh in the regions of expected shock waves.
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Old   April 23, 2012, 00:12
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I agree with #2
The y+ value depends greatly on what turbulence model
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Old   April 23, 2012, 00:20
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i'm interesting in your work. I'm performing studies about scramjet in fluent. Mach is 2.5 in 3d. is your model 2d ?
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