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

Abstract

Measurements, made with laser Doppler velocimetry, about a double-circular-arc compressor blade in cascade are presented for −1.5 and −8.5 deg 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 deg 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 deg 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 deg is 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 deg 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 percent and 60 percent chord. This bubble is quite thin, and some problems in interpreting the backflow data are discussed.