Abstract
The dynamics of a transonic buffet flow on the suction side of a highly loaded 2-D compressor blade model is investigated at a chord-based Reynolds number of 1.4 times 10^6 and an inlet Mach number of 1.05. Near stall the detached bow shock exhibits pronounced modal shock oscillations at buffet frequencies of {omega ^*}=2pi f c / U_1=2.27 which are not related to any structural aeroelastic modes. High-speed PIV at several stations along the chord provides chordwise velocity spectra and wave propagation velocities of shock-induced perturbations. For this purpose, a double-pulse laser system with a high-repetition rate was set up consisting of two combined DPSS lasers. This enables time-resolved PIV using frame straddling at up to 100 kfps and pulse separations down to 800 ns. Synchronized high-speed shadowgraph imaging simultaneously locates the position of the bow shock. Based on cross-correlations between velocity time records at two points and between velocity and shock position, the propagation velocity of the modal perturbations is determined upstream and downstream from the shock. The measured data indicate that feedback occurs between a region immediately downstream of the shock foot and a plane upstream of the shock, up to which transverse velocity disturbances are convected. This observation is contrary to Lee’s model Lee (AIAA J 28(5):942–944, 1990) which describes self-sustained shock-buffet as a consequence of shock–trailing edge interactions.Graphic
Highlights
In transonic turbomachinery blading, the correct prediction of shock positions and unsteadiness is essential in order to correctly estimate buffet boundary limits as well as frequency, size and location of shock-induced separations
Lee’s model (Lee 1990, 2001) on self-sustained shock oscillations is based on a feedback loop by pressure waves created near the shock foot which convect downstream toward the trailing edge (TE), where they generate upstream propagating waves which induce the shock motion
The results suggest that for the present flow shock buffet at ∗ = 2.3 and 4.5 is associated with shock-induced pressure fluctuations that originate from the low-momentum region immediately downstream of the shock foot near x = 34 mm and travel upstream in the form of longitudinal velocity oscillations
Summary
The correct prediction of shock positions and unsteadiness is essential in order to correctly estimate buffet boundary limits as well as frequency, size and location of shock-induced separations. Lee’s model (Lee 1990, 2001) on self-sustained shock oscillations is based on a feedback loop by pressure waves created near the shock foot which convect downstream toward the trailing edge (TE), where they generate upstream propagating waves which induce the shock motion. On this basis and drawing on experiments on a supercritical airfoil, Lee suggested that the buffet period is equal to the sum of the propagation times required for the disturbances to travel from shock to TE and back:
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