Experiments on Leading-Edge Induced Separated Shear Layer Under Various Imposed Pressure Gradients

Abstract

The behaviour of a separated shear layer past a semi-circular leading edge flat plate, its transition and reattachment downstream to separation are investigated for different imposed pressure gradients. The experiments are carried out in a blowing tunnel for a Reynolds number of 2.44×105 (based on chord and free-stream velocity). The mean flow characteristics and the instantaneous vector field are documented using a two-component LDA and a planar PIV, whereas, surface pressures are measured with Electronically scanned pressure (ESP). The onset of separation occurs near the blend point for all values of β (flap angle deflection), however, a considerable shift is noticed in the point of reattachment. The dimensions of the separation bubble is highly susceptible to β and plays an important role in the activity of the outer shear layer. Instantaneous results from PIV show a significant unsteadiness in the shear layer at about 30% of the bubble length, which is further amplified in the second half of the bubble leading to three-dimensional motions. The reverse flow velocity is higher for a favourable pressure gradient (β = +30°) and is found to be 21% of the free stream velocity. The Reynolds number calculated based on ll (laminar shear layer length), falls in the range of 0.9×104 to 1.4×104. The numerical values concerning the criterion for separation and reattachment agree well with the available literature.

Characteristics of a separated flow past a semicircular leading-edge airfoil model under different imposed pressure gradient

“…Hence, the sensor needs to be calibrated against the voltage signals produced for a range of known pressures. The accuracy of the scanners on calibration is found to be well within 0.05% of the full-scale pressure range of ±2500 Pa. 22 Figure 2 details the instrumentation chain for measuring the surface pressure. The scanners and sensors are powered by ±12 V and ±5 V DC supply.…”

“…A flap of 2 D length is hinged at the trailing end of the model, which aides its movement on either side by 45°. The schematic of the tunnel and the test section housing the model is detailed in Figure 1 of Reference 22.…”

“…The onset of separation and reattachment locations are identified, where Cf becomes zero. 22 Downstream to separation, changes in Cf are almost negligible and positive, attributing to the existence of the “calm region.” The negative values of Cf in pursuit of the calm region indicate the existence of a reverse flow vortex. It is interesting to note that the results harnessed from Cp are found to complement the predictions by Cf.…”

“…However, the trend in Cf over the “dead-air region” and the region past the “transition” remains the same compared to the earlier literature. 22

Figure 9.Comparison of time-averaged bubble length through (a & c) PIV and (b) ESP.

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“…Experiments on the shear layer excitation past the leading edge of a model airfoil by Anand et al 22 using PIV, laser doppler anemometer, and electronically scanned pressure (ESP) were made for different flap angles at an elevated stream turbulence for a Re of 2.44 × 10 5 . In this study, the time-averaged bubble parameters were shown to be highly susceptible to the variation of flap angles.…”