Influence of wall suction on the organized motion in a turbulent boundary layer

Abstract

The effect of wall suction on the organized motion of a tubulent boundary layer is examined experimentally both in a wind tunnel and in a water tunnel. In the windtunnel boundary layer, which developed over a slighly heated surface, temperature fluctuations were simultaneously obtained at several points, aligned in either the x (streamwise) or y (normal to the wall) direction. The temperature traces reveal the existence of two spatially coherent events, characterized either by a sudden decrease (cooling) or by a sudden increase (heating) of temperature. Estimates are presented for the average convection velocity, and average frequency of these events. The average convection velocity of ‘coolings’ is about 15% larger than that of ‘heatings’, the velocity of both events exhibiting an important local maximum in the buffer region. Near the wall, the convection velocity of both events is increased slightly by suction while their average frequency is reduced by suction. Away from the wall, the average inclination of ‘coolings’ and ‘heatings’ is about 40° without suction; suction does not alter the inclination of ‘coolings’ but increases that of ‘heatings’ to about 50°. Visualizations in the water tunnel indicate that suction increases the stability and the longitudinal coherence of low-speed streaks. They also show that suction reduces the average frequency of dye ejections into the outer layer.