Question about transitional flow
 

Hi

Is the transitional boundary layer thicker than the turbulent boundary layer? Could transitional flow cause instability like strong vortex?

No. But if there is separation, it can be .

How do you define BL separation? If the BL is separated from the wall due to the discontinuity of the wall geometry, would you also call it 'BL separation'?

If the BL is separated from the wall due to the discontinuity of the wall geometry, I think this statement is wrong, Flow Separation depends on fluid property, Geomety, surface roughness

Refer Boundary layer theory by schlichting

I meant: like there is a 90 degree corner, then due to this discontinuity of the geometry, the BL fly away. Could we also call this as BL separation?

both are different dont confuse wall separation and flow separation

Use LES model to capture exact flow separation