MEASUREMENTS OF TURBULENCE IN COMPRESSIBLE LOW-DENSITY FLOWS AT THE INLET OF A TRANSONIC LINEAR CASCADE WITH AND WITHOUT UNSTEADY WAKES

In the present work, hot-wire anemometry was employed for the characterization of the turbulent field at the inlet of a high-speed low-pressure turbine cascade, in terms of turbulence intensity and integral length scales. This work addresses two major topics relevant to the turbomachinery field: the application of hot-wire anemometry in transonic and rarefied flow regimes and the decoupling of the deterministic and the stochastic fluctuations when measuring unsteady phenomena. In compressible and rarefied flows, a hot-wire is strongly sensitive to both density and velocity fluctuations, and the commonly used Nusselt-Reynolds correlations are not valid. In this paper, a non-dimensional calibration methodology, based on Nusselt, Reynolds and Knudsen numbers, was coupled with a sensitivity analysis and employed to post-process the experimental dataset, allowing to decouple the fluctuations of density and velocity and compute the turbulence parameters. In the presence of unsteady wakes generated upstream of the cascade, two different phase-locked averaging techniques were employed to distinguish the wake deterministic fluctuations from the background turbulence intensity.