Quasi-Coherent Structures in Adverse-Pressure-Gradient Turbulent Boundary Layer.

Abstract

Statistical characteristics of turbulent boundary layer flows subjected to adverse pressure gradients are known to differ significantly from those of zero-pressure -gradient ordinary boundary layers. In this study, the dynamical characteristics of the quasi-coherent structures in adverse pressure gradient flow are investigated experimetally. As the pressure gradient paramenter P+ increases, the turbulent energy and shear stress transport <νu2> - and <υuυ> -, in the direction toward the wall from the region away from the wall, become significant in contrast to those in zero-pressure-gradient cases. The quadrant splitting and trajectory analyses reveal the obvious changes do occur in large-amplitude sweep motions(Q4)and outward interactions(Q1). On the other hand, the contribution 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.