STRUCTURE OF TURBULENT BOUNDARY-LAYER SUBJECTED TO ADVERSE PRESSURE-GRADIENT

A turbulent boundary layer subjected to a sustained adverse pressure g radient is experimentally investigated. Waveforms of fluctuating veloc ity components in the boundary layer, especially in the near-wall regi on, are remarkably elongated in time in comparison with those in zero- pressure-gradient flows, and thus time scales increase with an increas ing pressure gradient parameter P+. The increase in time scales is not in proportion to the corresponding increase in the conventional visco us lime scale nu/u(tau)(2). It is found that the Taylor time scale is most appropriate to describe the essential characteristics of non-equi librium adverse pressure gradient flows. Even the near wall-limiting b ehavior of streamwise velocity fluctuations for different P+ is well c orrelated in the coordinates based on the Taylor time scale. Moreover, in the boundary layer with an adverse pressure gradient, the contribu tion of sweep motions becomes equivalent to that of ejections, and out ward and wallward interactions relatively increase near the wall, whic h evidently indicates a change in coherent structures. (C) 1998 Elsevi er Science Inc. All rights reserved.