Vortex dynamics and associated turbulence production in the separated transitional flow and the wake at a moderate Reynolds number

The vortex dynamics in the separated transitional flow and the near-wake over a C4 compressor blade at Re_c = 24,000 and incidence angles of 0°, 2.5°, 5°, 7.5°, and 10° are investigated experimentally. The laminar separation vortex shedding, the trailing edge vortex shedding, and the interaction of the two vortices are captured using the time-resolved particle image velocimetry. To separate the contributions to the overall turbulence production due to various flow dynamics, the orthonormality of proper orthogonal decomposition modes of the velocity components is utilized. When the laminar separation is close to the trailing edge, the laminar separation vortex locks in the wake instability with an approximately subharmonic of the von Karman frequency. Approximately 80 % of the turbulence is produced by the large-scale flow structure governed by the laminar separation vortex and trailing edge vortex. When the shear layer separates near the leading edge, the laminar separation vortex shedding frequency is lower with a smaller streamwise wavelength. The turbulence production rate contributed by the laminar separation vortex shedding increases with incidences. These novel experimental findings offer insight into transition mechanisms and turbulence modeling of the separated flow located near periodic wake flow.