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May 5, 2013, 11:59 
Question about conjugate heat transfer mesh for laminar model

#1 
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Meimei Wang
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Hi,
Setting Y+<1 is essential to more accurately predict the conjugate heat transfer behavior. But if the laminar model is used, then there won't be a Y+ value any more. I'm wondering how large is the wall cell width at the conjugate heat transfer interface shall I set up for laminar flow?
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May 5, 2013, 19:05 

#2 
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Why is there no y+? The y+ parameter is still defined for laminar flow.


May 6, 2013, 12:34 

#3  
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Meimei Wang
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Quote:
http://www.cfdonline.com/Forums/mai...inarflow.html How would you decide the first cell width for laminar flow for conjugate heat transfer simulation?
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May 6, 2013, 13:03 

#4 
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What does y+ really represent? Whether turbulence is present or not, y+ is a way of defining a grid resolution sufficient to capture an important gradient at the wall. The best approach to your problem is to run a case, refine your grid and repeat. Continue to do so your heat transfer reaches a limiting value.


May 6, 2013, 13:19 

#5  
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Meimei Wang
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Quote:
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May 6, 2013, 14:22 

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If you know your Re and fluid velocity, assume a parabolic velocity profile. You can estimate how thick the BL is (or if it's fully developed internal flow then you can use the radius of your pipe/duct). Then simply decide how many points you need to reproduce this gradient. If you have any idea of how much heat flux to expect, then you can use that to estimate the thermal gradient and base your grid spacing off that. But the process is similar  you estimate the distance of the overall change (i.e. the boundary layer thickness) and the shape of the change (parabolic for laminar flow) and the pick a spacing to give a good representation (10 points, 20 points across the BL, etc.)


May 7, 2013, 05:22 

#7  
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Meimei Wang
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Quote:
http://en.wikipedia.org/wiki/Blasius_boundary_layer . But how do we estimate the thermal boundary layer thickness (if heat flux is already known)? Is there any theory or estimation method about that?
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May 7, 2013, 08:51 

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Look for information on the Pohlhausen method.


May 7, 2013, 09:40 

#9 
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Meimei Wang
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I look at it. It seems that it is not for thermal boundary layer.
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May 7, 2013, 10:13 

#10 
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Reza
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Use the Prandtl number to calculate the thermal boundary layer thickness:
(viscous boundarylayer thickness) / (thermal boundarylayer thickness) = f(Pr) If I remember correctly, f(Pr) = Pr^0.3 is a good start for most materials. If the pipe that you are looking at is very long and straight, then depending on the boundary conditions, you can find the gradient of temperature at wall analytically (for constant heat flux and constant temperature this value is known, for others, it is a value in between these two). So, you just need to have a first layer thickness that is fine enough to capture that gradient. Coming back to y+, yes, we don't use y+ in laminar flow, but y+ is just a Reynolds number defined by a very special velocity (shearstressbased velocity), so y+ at the thickness of first layer just is a measure of hoe steep the velocity profile is near the wall. If you have a large y+ (no matter if it is laminar or turbulent) the first layer is too thick. So, in my opinion, you can still use the y+~1 criterion, but it might be overkill for laminar flow. 

May 7, 2013, 11:33 

#11  
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Meimei Wang
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Quote:
Thank you very much! By the way, how to estimate the turbulent thermal boundary layer before the simulation? Or we just try to make y+<1? The estimation (viscous boundarylayer thickness) / (thermal boundarylayer thickness) = f(Pr) with f(Pr) = Pr^0.3 is really nice. Can you tell me what is the name of this theory? Where could I find it?
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May 7, 2013, 12:13 

#12 
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Reza
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Unfortunately, I can't remember what the name of the correlation is, but the fact that the ratio of boundary layers is equal to Prandtl number is because of the definition of that number.
Pr = nu / alpha nu is kinematic viscosity, but it also is a measure of how fast viscous information travels in the fluid. Take a look at the solution for a plate in a semiinfinite liquid. When you start moving the plate at time zero, and look at how the edge of boundary layer grows as a function of time, you'll see its growth speed is a function of nu. The same applies for thermal diffusivity (and mass diffusivity or any other measure of diffusivity for that matter). If you look at a semiinfinite domain, and introduce a change in a quantity that can diffuse (like internal energy or temperature) and look at the edge of influenced area over time, you'll find out that the speed that this area is growing is a function of the diffusivity of that quantity (for temperature, it is alpha). Now, edge of a boundary layer, is just the loci of fluid particles that have felt the presence of a wall (in other words, they have received the information that a wall exists). So to compare the thickness of thermal and viscous boundary layer, is to compare the speed at which the thermal and viscous information travel in a liquid. And we saw that these speeds are functions of their corresponding diffusivities (or Prandtl number). Sorry for my loooong answer. I hope it points you in the right direction. 

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