Self-excited vibrations of transonic blading in turbomachines are partly due to unsteady viscous flow effects and shock-boundary layer interaction. To investigate the unsteady flow, experiments have been performed in a transonic windtunnel at single blades and in a cascade, where the central blade is either mounted elastically or driven by electromagnetic shakers to torsional vibration.
The unsteady flow is measured by a stroboscopic schlieren-method including high-speed movies and a recently developed laser-density gradient-technique. The vibration of the blade is controlled by strain gauges.
The test results reveal:
- severe shock wave and boundary layer oscillations occur with separation alternating between shock-induced and trailing edge separation,
- the unsteadiness of the flow largely depends on Mach- and Reynolds number,
- with pitching vibration of the blade, forced and self-excited shock wave oscillations interfere with each other.