Application of wall treatment models versus near-wall-approach
 

Dear all,

I am simulating a slurry flow in a pipe (3D case) with a diameter of 3,2 cm, velocity of 1 m/s. Based on the estimation of the first layer thickness at y+=1, I have to have something about 1,6e-5 m. When I do meshing with sweep method and inflation layers, I get very high skewness as well as high aspect ratio in the boundary layer region which is obvious. I am not sure what can I do with this problem. My questions are:

1) Can I simulate the flow even with high skewness but with "correct' first layer thickness?

2) Can I use Enhaced wall treatment in this cases and do not make the boundary layers so thin?

3) Will the EWT work properly if my boundary layers are not very fined?

Thank you very much in advanced for your replies.

Why are you aiming for a y+ of 1, this will resolve the boundary layer and will require a very high computational time and effort.
you can use the standard or scaleable wall function and aim for a y+ of 30-100.
EWT i believe is for resolving the boundary layer with no boundary functions.

Also, for inflation layers high aspect ratio is acceptable. if you do want to resolve the boundary layer you can get away with having a high aspect ratio of the inflation layer specially if it is a bounded flow. if you dont have a specific reason for a y+ of 1, use a wall function it is the better approach.

Dear hwet,

Thank you very much for your reply.

I am not sure do I need to resolve it or not. I thought that it will give me a better convergence if I aim for y+=1 in a pipe flow with small diameter (3,2 cm). Do you think it is better to use first layer thickness for y+=30 and use scaleable wall fucntion? why? only computational time?

near the wall the flow velocity increases like a log function. this is what a wall function is. rather than using extremely small mesh to use capture this near wall region, we use wall function since we know how the velocity there will grow, any the wall function takes into account.

yes basically it only saves computational effort, but since near the wall the flow may be laminar, the standard turbulence models may not be applicable.

If you use a Y+ as low as 1 then you must use Enhanced Wall Treatment because any wall functions will give bad accuracy for that sort of a Y+ value. Although if the flow does not have large pressure gradients or any such effects that may cause boundary layer to separate then to use Y+ above 30 and wall functions should be sufficiently good.

Dear aeroujj and hwet,

Thank you for your replies.

I am not sure what do you mean bu separation of boundary layer. You mean if I have such adverse pressure gradient so that I have the separation point?

Just use a wall function man, you will be ok