Effects of adverse pressure gradient on quasi-coherent structures in turbulent boundary layer

Abstract

The chapter discusses an experimental investigation made on the quasi-coherent structures in nonequilibrium turbulent boundary layers subjected to adverse pressure gradients. The results can be summarized as follows: In the near-wall region of the adverse-pressure-gradient (APG) flow, the large-amplitude sweep motions (Q4) and outward interactions (Q1) occur frequently, and the duration of Q4-motions is shorter than that of the ejections (Q2). The contributions from the coherent motions, especially the ejection motions, significantly decrease. Thus, the turbulent transport quantities in the log region of the APG flow have net values different from those in the zero-pressure-gradient (ZPG) flow. From the trajectory analysis, the two patterns Q2-Q1-Q4 and Q4-Q1-Q4 are found to be strongly affected in the log region of the APG flow. In the APG flow, marked transfer of high momentum fluid towards the wall (Q4) and the following reflection near the wall without significant loss of the momentum (Q1) occur more frequently during a short duration. On the other hand, the duration of the Q2-Q3-Q2 pattern in the APG flow becomes longer with less active motions than that in the ZPG flow. From observations of the instantaneous velocity vector fields, it is confirmed that in the APG flow, the small-scale vortex motions near the wall, which are usually observed in the ZPG flow and exert a decisive influence on the fluid motions in the outer layer, are greatly weakened, and the fluid motions from the outer to inner layer predominate.