Detection of Bursts in a Favorable Pressure Gradient with the VITA Technique.

Abstract

The variable-interval time averaging (VITA) tchnique is applied to detect bursts in the turbulent boundary layer at favorable pressure gradients generated by a two-dimensional asymmetric contraction. Three averaging periods based on (1) the Eulerian micro-time scale, (2) the inner time scale and (3) the outer time scale are used to obtain conditionally averaged streamwise velocities with the VITA technique. The first of these scales is derived from the auto-correlation of the streamwise-fluctuating velocity component, and the second and third scales are obtained from the inner and the outer variables of boundary layers, respectively. Results of experiments show that when the Eulerian micro-time scale at Y+=15 is used as the averaging period in the VITA technique, the conditional averages at Y+=15 are similar and independent of the pressure gradient. On the other hand when the averaging periods based on the inner or the outer time scales are used, the conditional averages become irregular as the pressure gradient decreases.