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Old   July 3, 2012, 02:08
Default micro scale calculation problem
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Dear all,

Now my simulation model is that 10mm cyclinder with 20Ám little radial clearance. I want to calculate the fluid is through the clearance condition.

-which turbulent model and wall function is suitable to use?
- how can i check mesh size in layer position with y plus in a reasonable range?

hope someone can help me. thank you in advance
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Old   July 3, 2012, 05:56
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Quote:
Originally Posted by Guava Wang View Post
Dear all,

Now my simulation model is that 10mm cyclinder with 20Ám little radial clearance. I want to calculate the fluid is through the clearance condition.

-which turbulent model and wall function is suitable to use?
- how can i check mesh size in layer position with y plus in a reasonable range?

hope someone can help me. thank you in advance
I think that in microfluidic you have more relevant effects by viscosity and capillarity than worry about turbulence. I don't think you have a Re number sufficiently high to substain turbulent structures....
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Old   July 3, 2012, 22:56
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Originally Posted by FMDenaro View Post
I think that in microfluidic you have more relevant effects by viscosity and capillarity than worry about turbulence. I don't think you have a Re number sufficiently high to substain turbulent structures....
hi FMDenaro,

In the radial clearance, the velocity is 30m/s, which is turbulent structure. now my main issue is how to adapt the mesh size in reasonable range to assure the calculation accurate.

have a nice day
guava
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Old   July 4, 2012, 03:44
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Originally Posted by Guava Wang View Post
hi FMDenaro,

In the radial clearance, the velocity is 30m/s, which is turbulent structure. now my main issue is how to adapt the mesh size in reasonable range to assure the calculation accurate.

have a nice day
guava

Hi,
I am not an expert about microfluidic, I just see from some estimation, velocity and lenght of 20Ám turn out to produce a small Re number. What about the axial velocity? I doubt that the energy production can balance the energy dissipation to substain turbulent structures and produce inertial range in the energy spectra... but I could be in error
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Old   July 4, 2012, 04:32
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What kind of fluid are you simulating? How high is the viscosity?

Estimating Re with an approximate value of air (1.5 e-5 m^2 s^-1) yields Re=40.

So your flow is definitely laminar (assuming the fluid you use doesn't have a lower viscosity than air) since the critical Re is in the range of 2300 for a circular pipe.
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Old   July 4, 2012, 21:02
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Originally Posted by flotus1 View Post
What kind of fluid are you simulating? How high is the viscosity?

Estimating Re with an approximate value of air (1.5 e-5 m^2 s^-1) yields Re=40.

So your flow is definitely laminar (assuming the fluid you use doesn't have a lower viscosity than air) since the critical Re is in the range of 2300 for a circular pipe.
hi flotus1,
viscosity is 2.24e-07 m^2 s^-1, and i have check the Re number, which is larger than 2300 in the clearance.
I can't upload the model as a picture, maybe the problem didn't give a clear description. my proposal for this case is to know the leakage with piston through the clearance 20 Ám.

have a nice day
guava
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Old   July 5, 2012, 01:26
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Ok sorry for the detour, now lets get back to your initial question.

Since Re is still pretty low, the computational cost of fully resolving the boundary layer without a wall-function should not be too high.
Try to get the Y+ values below 1 to achieve this with a small growth factor (1.05-1.1) in wall-normal direction.

Thanks to the low Re again, the choice of the turbulence model will not be too important in this simulation. Any of the basic k-epsilon formulations should yield reasonable results.
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Old   July 5, 2012, 02:27
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Ok sorry for the detour, now lets get back to your initial question.

Since Re is still pretty low, the computational cost of fully resolving the boundary layer without a wall-function should not be too high.
Try to get the Y+ values below 1 to achieve this with a small growth factor (1.05-1.1) in wall-normal direction.

Thanks to the low Re again, the choice of the turbulence model will not be too important in this simulation. Any of the basic k-epsilon formulations should yield reasonable results.

I agree...what's more, your Re_tau is quite low such as a DNS is realizable
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Old   July 12, 2012, 23:07
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thanks. finally i adapt K-E model with scalable wall function, that can give good result.
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