Investigation of shock-induced flow separation over a transonic compressor blade by conditionally averaged PIV and high-speed shadowgraphs

Kurzfassung

The impact of separation control has been investigated in a highly loaded transonic compressor cascade at an inlet Mach number of 1.21 and a chord based Reynolds number of 1.4 x 10^6. Applied control devices are air jet vortex generators (AJVG) and a surface roughness patch. Comparative flows without transition control imply a variation of the upstream turbulence level from 0.5% to 2.5%. Above the suction side, velocities of the unsteady separation region have been captured by particle image velocimetry (PIV). The aerodynamic load alternation due to shock motion results in flexure of the blade surface which have been measured and compensated prior to PIV processing. Single PIV shots indicate shape variations of both the lambda shock system and the associated separation region while the shock foot position is fluctuating within a range of up to 23% of chord. Large sets of statistically independent PIV samples are conditionally averaged upon instantaneous passage shock positions at a resolution of 1% of chord length to quantify the size of flow separation. Large bubble separation occurs if the turbulence of the incoming flow is low. The separation region becomes smaller when AJVGs are applied but still exhibits bubble separation at rear shock positions. The size of the separation region is significantly reduced either if a roughness patch is applied or if the turbulence level of the incoming flow is high. The frequency range of shock motion is analyzed by shock tracking on the basis of high speed shadowgraphs. A Fourier analysis of shock motion in the low frequency range (<1khz) indicates the highest spectral densities for the turbulent case and the lowest densities if roughness patch are applied. Joint probability density distributions of blade displacements and amplitudes of shock movement revealed that upward transverse blade deflections are more frequent at shock positions downstream of the mean shock position while downward deflections are more frequent at frontal shock positions.