# CFD_external aero problems

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 April 3, 2015, 06:15 CFD_external aero problems #1 New Member   Argyris Apost Join Date: Nov 2014 Posts: 7 Rep Power: 11 Hi everyone, i am new to ANSA and i am interested in making a CFD study of a simple car. In order to correctly predict separation regions i am using K-w SST model. For this to work i choose y+ 11 so the first height is 0.25mm. But when i create the layers all the cells of the road violate the aspect ratio criterion, even when i make it 100, and non orthogonality. Surface mesh of the car: min 5mm max 20mm Windtunnel: 5mm-300mm Sizebox_around_car: max surf:30 max vol:30 Sizebox_wake:max surf:40 max vol:40 I've tried many things such as making different layers for the road with first height cell 2.5mm but then i had skewed elements between the layers of the car and that of the road. I 'm interested in a 6-8 million cell mesh. Any advice about how to solve these problems and what are the best sizing for the mesh would be helpful. Thanks in advance, Argyris

July 31, 2015, 10:43
#2
Senior Member

Lefteris
Join Date: Oct 2011
Location: UK
Posts: 339
Rep Power: 15
Hi Argyris.

Late reply, you've probably already solved the issue which I think doesn't have to do with ANSA.

I think that the Aspect Ratio of the cells inside the boundary layer could be in the order of 10^3 not just 100. You can live with that I guess. If not, you can increase the number of points in the longitudinal direction but you'll end up with a massive cell count.

Out of curiosity, why y+=11? It seems quite odd. I mean, if you are to use such a small value, why not go down to 1 as Menter suggests for his model? (even though the formulation in Fluent is supposed to be y+ insensitive. And also the underlying assumption is that you use fluent, right?)

Best Regards,
Lefteris

Quote:
 Originally Posted by Argyris Hi everyone, i am new to ANSA and i am interested in making a CFD study of a simple car. In order to correctly predict separation regions i am using K-w SST model. For this to work i choose y+ 11 so the first height is 0.25mm. But when i create the layers all the cells of the road violate the aspect ratio criterion, even when i make it 100, and non orthogonality. Surface mesh of the car: min 5mm max 20mm Windtunnel: 5mm-300mm Sizebox_around_car: max surf:30 max vol:30 Sizebox_wake:max surf:40 max vol:40 I've tried many things such as making different layers for the road with first height cell 2.5mm but then i had skewed elements between the layers of the car and that of the road. I 'm interested in a 6-8 million cell mesh. Any advice about how to solve these problems and what are the best sizing for the mesh would be helpful. Thanks in advance, Argyris
__________________
Lefteris