Influence of Periodically Unsteady Wake Flow on the Flow Separation in Blade Channels

The influence of periodically unsteady perturbations on the turbulent flow along the suction side of turbine blades is investigated in a test rig. The blade suction side is represented by a flat plate 550 mm in length. The pressure profile typically encountered in a turbine blade channel is generated by a curved wall opposite to the flat plate. The angle of the divergent part of the test channel and hence the pressure can be increased to induce flow separation on the flat plate. For simulation of the wakes from the upstream blade row, the incoming flow is periodically disturbed by a wake generator consisting of five flat profiles arranged in front and parallel to the plate rotating with adjustable speed and phase angle. An LDV with high spatial resolution is used to measure averaged and fluctuating components of the velocity inside the boundary layer flow down to a distance of y = 0.05 mm from the plate surface, determining the boundary layer parameters as well as the wall shear stress. By Fourier analysis of the measured time-related velocity distributions, the stochastic and periodic parts of the overall turbulence are identified. With a periodic wake flow the separation is shifted downstream as compared to the steady flow situation. This is due to the energization of the boundary layer flow associated with the conversion of periodic in stochastic parts of the turbulence. Conclusions resulting from the experimental findings for the theoretical understanding of the flow turbine cascades are discussed in particular with respect to turbulence modeling.