[MMatt]

Hello guys,

I have a question regarding flow separation and total pressure. I was told that looking at an iso-surface representation of the total pressure, I would see where the flow separates. But I found that this is not true, or that I do not understand it properly.

What you see below is the iso surface representation (at 0Pa) of a front wing showing the total pressure:



Now, let's take a look at a scalar representation of the velocity on the same wing at the symmetry plane:



We clearly see a large flow separation under the wing a bit after the leading edge. Finally, let's take a look at the pressure representation under that wing:



I have only displayed here negative pressures, so we clearly see that we get low pressure around the side of the wing and not at the symmetry plane, indicating again flow separation.

This is where I get confused because the first representation showed me "flow separation" at the side of the wing and not at the symmetry plane, but velocity and pressure scalar representations showed me this is happening at the symmetry plane!



P.S.: Ignore the mesh quality, this is just to try few things before going to more refined mesh!

[lovecraft22]

Looking at the three pictures you posted (which barely resemble some CFD result honestly) I would conclude that you have flow separation in the three of them close to the endplates. On the first one it looks like your isosurface is thick on the side only, not at the center (the blue stuff you see might be somehow related to your coarse mesh there) where you have low pressure (i.e no separation).

[MMatt]

Well, I did not want to present any result but to show a trend. I don't think knowing the values will help in that case, but maybe I'm wrong?

The picture below has been taken close to the end plate (as opposed to the one posted above that was taken at the symmetry plane):



As you can see there is indeed separation but in a much smaller extend than at the symmetry plane (which makes sense looking at what is behind the wing).

But this is where I get confused, I see people using the total pressure iso-surface representation to detect separation/turbulence. In my case I would imagine this region would be close to the symmetry plane, not at the end plate.

I don't understand.

[duri]

Total pressure would not reveal anything related to separation. It varies along the boundary layer and it is function of v2 and not v. It doesn't give you direction of velocity and the iso-surface can lift off well before separation based on boundary layer velocity. Better way is to plot shear stress and surface streamlines.