A wall-bounded turbulent mixing layer flow over an open step: I. Time-mean and spectral characteristics

Abstract

The time-mean and spectral characteristics of a wall-bounded turbulent mixing layer flow over an open step were investigated experimentally. Extensive spatio-temporal measurements of the wall pressure fluctuations and velocity fluctuations were performed using a microphone array and an X-type gold-plated hotwire probe, respectively. Air flow was entrained through the open step, forming a wall-bounded turbulent mixing layer flow. Three flow configurations, with step heights of H/δ = 0.67, 1.00 and 1.67 (δ was defined as the boundary layer thickness 2 mm upstream of the step edge), were chosen to examine the influence of the step height on the evolution of the wall-bounded turbulent mixing layer. As the step height increased from H/δ = 0.67 to 1.67, the velocity fluctuations and the associated wall pressure fluctuations were intensified due to enhancement of the large-scale vortical structures. Moreover, as the step height was increased over this range, the inclination of the large-scale vortical structure varied from backward to forward, causing elevation of the wall-bounded turbulent mixing layer. The contribution of the local flow structures to the wall pressure fluctuations increased with increasing step height.

Keywords:

• entrainment
• turbulent mixing layer
• wall pressure fluctuations
• large-scale vortical structure