TURBULENT BOUNDARY-LAYER OVER RIBLETS - CONDITIONAL ANALYSIS OF EJECTION-LIKE EVENTS

Experimental data obtained with single-sensor hot-wire anemometry in t he turbulent boundary layer over a flat and a riblet surface are condi tionally analyzed by means of the U-level technique. Attention is focu sed on the ejection events, as detected by the conditional algorithm: their frequency of occurrence, as well as the probability density dist ributions of their duration, time separation, and intensity are determ ined. The ensemble-averaged ejection event is also considered, and the concept of grouping is introduced in order to gain information on the frequency of the overall bursting process. On the basis of the condit ional analysis, the modifications induced by the riblets in the turbul ent boundary layer are evidenced and described. It turns out that the turbulent structures in the near-wail region of the riblet flow are ch aracterized by smaller time and length scales in the streamwise direct ion. The results obtained are also interpreted in the frame of propose d conceptual models for the bursting cycle of near-wall turbulence, in which the inflectional spanwise instability of the low speed streaks is dynamically relevant. (C) 1997 by Elsevier Science Inc.