Fence probe measurement of flow reversal in separating turbulent boundary layers

Abstract

A small fence probe was evaluated for measurements in the time-dependent flow reversal region of the transition from boundary layer to separated flow. For moderate and high Reynolds numbers, the fence probe is demonstrated to be a usable tool for the measurement of the reverse flow associated with separation. Although the present probe pressure transducer system was limited to approximately 200 Hz, pulses of positive and negative shear stress were readily detected. At or near the location of zero surface shear stress, the measurements were limited by the signal-to-noise ratio. For the separated flow investigated, a marked reduction in the pressure gradient occurred when the fence probe indicated approximately 20 % reversal for the higher Reynolds numbers. The reversal increased to 24 % for the lower Reynolds numbers. The measurements indicate that flow reversal alone may not be adequate to identify the degree of separation. Upstream of turbulent boundary layer (intermittent) separation, the duration of the reversed shear stress was found to be very short (0.002–0.007 s), suggesting a local, small-scale, impulse-type separation. At and beyond the location of intermittent separation, the shear stress reversal duration was an order of magnitude longer. Estimates of the maximum and minimum surface shear stress in the separation region were also obtained with the fence probe.