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

摘要

Statistical and kinematic characteristics of turbulent boundary layer flwos subjected to adverse pressure gradients are found to differ significnatly from those of zero-pressure-gradient ordinary boundary layers. The turbulent energy and shear stress transport vu~2 and vuv occurs in the directio ntoward the wall from the regions away fro mthe wall, as the pressure gradient paramete P~+ increases. This event is completely opposite to that in zero-pressure-gradient cases. The instataneous waveforms of third-order moments are markedly altered in the near-wall region. The quadrant splitting and trajectory analyses reveal the obvious changes do occur in large-amplitude sweep motions (Q4) and outward itneractiosn (Q1). On the other hand, the contributions from other coherent motions, especially the ejectio nmotions (Q2), significantly decrease and their duratiosn become logner, i.e., these motions are dull and less active. Moreover, multi-point simultaneous measuremtns with five X-probes are made to depict the kinematic pictures of hte effects of the adverse pressure gradient on the eddy strucutres.