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[ANSYS Meshing] Very high aspect ratio

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Old   August 15, 2011, 03:53
Default Very high aspect ratio
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zhao xin
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Hi,

I used Ansys meshing to create a mesh with 10E-6 m first boundary layer height. There are few elements with crazy aspect ratio, around 2000000.....What can I do with this? and unbelieveable I did't get any warning. I used this mesh to run the simulation with double precision and no divergence happened, the result oscillated because of large seperation of the flow. Can i trust this? What can happen with such high aspect ratio?

Thank you
Kind Regards,
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Old   August 16, 2011, 04:14
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10^-6 m thickness of first Bundary layer? What do you simulate actually?

Let's say the number of Prism Layer is usually between 10 - 15 and the total height of BL depends on the flow behaviour and your expectation. But usually is between cm and m.

Check the physics and think about whether you really need such tiny thin BL.

In your case, if the thickness is 10^-6 (Height )and cell size (Length) is in cm, of course you will get a cray aspect ratio.
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Old   August 16, 2011, 05:26
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Quote:
Originally Posted by swiss_zhang View Post
10^-6 m thickness of first Bundary layer? What do you simulate actually?

Let's say the number of Prism Layer is usually between 10 - 15 and the total height of BL depends on the flow behaviour and your expectation. But usually is between cm and m.

Check the physics and think about whether you really need such tiny thin BL.

In your case, if the thickness is 10^-6 (Height )and cell size (Length) is in cm, of course you will get a cray aspect ratio.
Thanks for the reply.
I do need the 10e-6 thickness to get y+ around 1, since I have really high pressure and mach number. Actually I am using mm as the cell size, but there is still some crazy elements. I could not get even smaller cell size due to the enormous number of elements already.

And I have 30 layers to get a relative smooth transition from the BL to the normal flow.
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Old   August 16, 2011, 05:49
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Do you simulate supersonic flow?
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Old   August 16, 2011, 06:17
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Originally Posted by swiss_zhang View Post
Do you simulate supersonic flow?
No, but the Re is 10^6 with high temperature, small ref.length and high pressure. I used Grid spacing calculator http://geolab.larc.nasa.gov/APPS/YPlus/, to calculate my first layer height. And I also calculate the y+ in the result. I'm sure i need 10e-6.
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Old   August 16, 2011, 06:25
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Can you just use wall functions?

Stu

Last edited by stuart23; August 16, 2011 at 06:51.
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Old   August 16, 2011, 08:55
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Quote:
Originally Posted by stuart23 View Post
Can you just use wall functions?

Stu
Absolutely right, Why don't you use the wall function?

Instead of creating a very fine mesh in BL, you can use the wall function. Because the Velocity Profile in BL is predictable and therefore is approximated using Wall Function. Since you want to capture more details in the BL, you can use the SST Turbulence Model and Automatic Wall Function.

Last edited by swiss_zhang; August 16, 2011 at 09:16.
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Old   August 16, 2011, 09:13
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Actually there are 2 methods to solve the flow behaviour near the wall:

1: Wall Funtion Methode

For wall function: y+ < 300 is ok

delta_y = L*y+ * (74)^2*Re^(-13/14), using this formular to estimate the first node spacing delta_y, if the flow passes over a plate

2: Low-Reynold-Number Methode

For Low-Reynold-Number Methode: y+ < 2
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Old   August 16, 2011, 09:17
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Also, aspect ratio isn't everything... You can still have a high aspect ratio (though yours is very very high) and have a happy solver if your angles, skewness, etc. are all still good. This is usually fine with boundary elements...
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Old   August 16, 2011, 09:23
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Slightly off topic, but can someone advise if multigridding speed up the rate of convergence for very fine prism layers? Because the layer is so thin, the velocity gradient normal to the surface would be very small between cells (i.e. low frequency), and therefore would take a lot of iterations without multigridding. I was just wondering out of curiosity...
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Old   August 16, 2011, 09:43
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Quote:
Originally Posted by stuart23 View Post
Can you just use wall functions?

Stu
Yes, I have tried that. Maybe I should stick on the wall function.
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Old   August 16, 2011, 09:46
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Quote:
Originally Posted by swiss_zhang View Post
Absolutely right, Why don't you use the wall function?

Instead of creating a very fine mesh in BL, you can use the wall function. Because the Velocity Profile in BL is predictable and therefore is approximated using Wall Function. Since you want to capture more details in the BL, you can use the SST Turbulence Model and Automatic Wall Function.
Thanks for your effort. I was too aggressive to get the accurate BL. I will estimate them both.
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Old   August 16, 2011, 09:49
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Quote:
Originally Posted by PSYMN View Post
Also, aspect ratio isn't everything... You can still have a high aspect ratio (though yours is very very high) and have a happy solver if your angles, skewness, etc. are all still good. This is usually fine with boundary elements...
It's really high....yeah...but the most amazing thing is i didn't get any warning and it ran very well. Mybe it's not that critical at the boundary.
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