Turbulent Spot Development Under a Moderate Adverse Pressure Gradient

Abstract

Triggered turbulent spots are under investigation in a wind tunnel. A turbulent spot was initiated in a flat plate boundary layer under a moderate adverse pressure gradient. The spot was traversed at four streamwise locations using conventional hot-wire anemometry techniques. A triggering jet provided a phase reference for data sampling. Phase-averaged velocity traces, boundary layer integral properties and contours of velocity perturbation and disturbance level are presented. The central region resembles a zero pressure gradient spot but much of the span is dominated by the different behavioral stages of a stongly-amplified wave packet. The celerities of the spot leading and trailing edges under an adverse pressure gradient are significantly lower than those associated with zero pressure gradient spots but the lateral spreading rate is much higher. This combination of turbulent spot and wave packet, which spreads at an included angle as high as 60° is quite different from the well-documented zero pressure gradient spot spreading at an included angle of about 20°. Improvements in transition region predictions are therefore dependent on further detailed measurements of spots and wave packets under adverse pressure gradients.