I'm trying to understand the physical cause of wave drag, beyond the simple statement "the presence of shockwaves increases the drag".
As far as I understand, in the case of a weak BLSWI (so without separation), the drag increase is due to:
- Direct shock losses (momentum deficiency of freestream flow through shockwave)
- A change in the state of the boundary layer due to the compression waves in side the BL
But how exactly does the second point work? If the BL becomes thicker after the SW, wouldn't that mean that the drag decreases because the velocity gradient and therefore the shear stress decrease?
I feel like I'm missing some tiny aspect...
Edit: I've found the following statement in this report, which maybe explains my problem better:
The increase in drag occurs directly because of the wave drag associated with the presence of shock waves. However, the drag also increases because the boundary layer thickness increases due to the sudden pressure rise on the surface due to the shock wave, which leads to increased profile drag. Lynch has estimated that at drag divergence the additional transonic drag is approximately evenly divided between the explicit shock drag and the shock induced additional profile drag
Why does the increase in BL thickness cause more drag? Shouldn't the velocity gradient at the wall reduce, leading in less shear stress? Or should I see it in a way that the already turbulent BL "takes up more energy in its vortical structures" from the flow, due to the increase in thickness?