The Measurement of Boundary Layers on a Compressor Blade in Cascade: Part 4 — Flow Fields for Incidence Angles of −1.5 and −8.5 Degrees
Measurements, made with laser Doppler velocimetry, about a double-circular-arc compressor blade in cascade are presented for −1.5 and −8.5 degree incidence angles and a chord Reynolds number near 500,000. Comparisons between the results of the current study and those of our earlier work at a 5.0 degree incidence are made. It is found that in spite of the relative sophistication of the measurement techniques, transition on the pressure surface at the −1.5 degree incidence is dominated by a separation “bubble” too small to be detected by the laser Doppler velocimeter. The development of the boundary layers at −1.5 and 5.0 degrees are found to be similar. In contrast to the flow at these two incidence angles, the leading edge separation “bubble” is on the pressure surface for the −8.5 degree incidence. Here, all of the measured boundary layers on the pressure surface are turbulent — but extremely thin — while on the suction surface, a laminar separation/turbulent reattachment “bubble” lies between roughly 35% and 60% chord. This “bubble” is quite thin, and some problems in interpreting backflow data.