[ANSYS Meshing] Meshing Detail for Ahmed Body Simulation

spggodd · March 27, 2015, 17:54

Hi,

I am trying to simulate the flow over the ahmed body and compare the CFD results to experimental data.

I have followed Scipy's excellent tutorial on this scenario but I am now trying to refine my mesh further as I'm not sure the inflation parameters defined in the video are sufficient.

https://www.youtube.com/watch?v=dZR7Wi70Vec

To find the first cell height I have done some calculations based on boundary layer flow over a flat plate but using 0.7428m as my length based on the top horizontal surface of the ahmed body. However in order to fit 20 cells (as recomended) into the 10%XL boundary layer (1.2 Stretch factor) the first cell height is 4.65E-6m which seems incredibly small and my PC is struggling with the fine mesh requirements.

Should this kind of size be expected?

Also, I have took this value and calculated y+ based on a formula provided by Ansys:

y+=( $\Delta$ y)/(L $\times\sqrt{74}\times Re^{L}_{13/14}$ )

This gives me a y+ of around 1.4 which seems very low for k-Epsilon, literature I've read suggests y+=30 to 200.

Therefore would it be suitable to reduce the 20 cell requirement in the boundary layer calcs in order to reduce the mesh size and improve performance?

edit: I have included my calculation spreadsheet for reference.

Many Thanks
Steve

hwet · March 28, 2015, 09:21

Y+ is actually a measure of the first layer height, if ur y+ is 1.4 it means u can increase the first layer height so the Y+ is upto 30 atleast, 1.4 is wayyy to fine to apply wall functions

read this best explanation ever http://www.computationalfluiddynamic...oundary-layer/

spggodd · March 28, 2015, 10:38

Thanks Hwet, I will have a read through you link now. What I've read so far sounds very useful.

Is it normal practice to calculate the inflation parameters for each edge? For the ahmed body I will have the bottom edge, top edge and the slant. Or is it better to apply the same inflation properties all over?

Do you also need to calculate inflation properties for the non-slip road in the ahmed model? If so what would you use as your characteristic length (L)? (total domain length, body length etc..)

hwet · March 28, 2015, 19:35

Depends on your flow over the different regions, it shouldnt make a very big difference anyways having it the same over the different surfaces, I have only a vague idea of what an ahmed body looks like, it all depends on ur flow physics, just mesh it using the guide i posted earlier, and simulate the flow, if you then think the mesh needs to be refined go back to meshing and do that, it is somewhat a trial and error approach

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March 28, 2015, 09:21		#2
hwet Senior Member Join Date: Mar 2014 Posts: 375 Rep Power: 13	Y+ is actually a measure of the first layer height, if ur y+ is 1.4 it means u can increase the first layer height so the Y+ is upto 30 atleast, 1.4 is wayyy to fine to apply wall functions read this best explanation ever http://www.computationalfluiddynamic...oundary-layer/

March 28, 2015, 10:38		#3
spggodd Member Steven Goddard Join Date: Mar 2015 Posts: 34 Rep Power: 11	Thanks Hwet, I will have a read through you link now. What I've read so far sounds very useful. Is it normal practice to calculate the inflation parameters for each edge? For the ahmed body I will have the bottom edge, top edge and the slant. Or is it better to apply the same inflation properties all over? Do you also need to calculate inflation properties for the non-slip road in the ahmed model? If so what would you use as your characteristic length (L)? (total domain length, body length etc..)

March 28, 2015, 19:35		#4
hwet Senior Member Join Date: Mar 2014 Posts: 375 Rep Power: 13	Depends on your flow over the different regions, it shouldnt make a very big difference anyways having it the same over the different surfaces, I have only a vague idea of what an ahmed body looks like, it all depends on ur flow physics, just mesh it using the guide i posted earlier, and simulate the flow, if you then think the mesh needs to be refined go back to meshing and do that, it is somewhat a trial and error approach