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

Statistical and kinematic characteristics of turbulent boundary layer flows subjected to adverse pressure gradients are found to differ significantly from those of zero-pressure-gradient ordinary boundary layers. The turbulent energy and shear stress transport <(vu(2))over bar> and (vuv) over bar occurs in the direction toward the wall from the regions away from the wall, as the pressure gradient parameter P(+) increases. This event is completely opposite to that in zero-pressure-gradient cases. The instantaneous waveforms of third-order moments are markedly altered in the near-wall region. The quadrant splitting and trajectory analyses reveal that obvious changes do occur in large-amplitude sweep motions (Q4) and outward interactions (Q1). On the other hand, the contributions from other coherent motions, especially the ejection motions (Q2), significantly decrease and their durations become longer, i.e., these motions are dull and less active. Moreover, multi-point simultaneous measurements with five X-probes are made to depict the kinematic pictures of the effects of the adverse pressure gradient on the eddy structures. (C) 2000 Elsevier Science Inc. All rights reserved.