Turbulent flow with adverse pressure gradient (APG)

There has been comprehensive research available in literature discussing the near-wall turbulence on flat plates, where flow does not experience any (zero) pressure gradient (ZPG). However, in many applications, the flow path is not straight, and the slope change of the wall profile applies a pressure gradient to the near-wall flow. If the pressure gradient is negative at a point on the wall, it adversely affects the boundary layer and might cause separation. Boundary layer separation (BLS) due to adverse pressure gradient (APG) is a well-known phenomenon on hydrofoil or nozzle surfaces. It is primarily responsible for generating a re-entrant jet and recirculating flow downstream of a nozzle at the onset of cavitation.

I conducted a series of experiments to understand this phenomenon in a wall-bounded mesoscale water flow passing a converging-diverging nozzle. I used particle image velocimetry to investigate the near-wall turbulent flow at a Reynolds number at which the onset of cavitation happens. The next step is to explore the effect of polymer additives on the turbulent flow with APG.

The following figure shows a sample instantaneous velocity field on the diverging section of the nozzle downstream of the throat, where BLS occurs.