Seeding-free inlet flow distortion measurement by filtered Rayleigh scattering: diagnostic approach and verification

The expected close coupling between engine and fuselage of future aero-engine architectures will lead to highly distorted inflows at the engine face, presenting a major design risk for efficient and reliable engine operation. In particular, the increase in flow unsteadiness is perceived as a significant challenge. In this context, the Cranfield Complex Intake Test Facility (CCITF) is currently being installed at Cranfield University to reproduce the anticipated level of total pressure and swirl distortion arising from novel, closely coupled airframe-engine configurations. To address the expected demand for much more comprehensive flow field data, it is intended to establish the filtered Rayleigh scattering (FRS) technique for non-intrusive testing of aero-engine intake flows. Unlike the previously used particle image velocimetry (PIV) or Doppler global velocimetry (DGV), which are limited to the measurement of a single flow quantity, FRS can be used for the combined planar measurement of velocity and scalar fields without the need to add a flow tracer. In this study, an FRS concept with the ability to simultaneously measure high-accuracy time-averaged and time-resolved three-component velocity, static pressure and temperature fields is verified on a simplified mock-up of the CCITF facility. Time-averaged results show excellent agreement with benchmark laser Doppler anemometry (LDA) velocities, static pressure probe measurements and analytical temperature calculations. Moreover, it is shown that the developed concept can be used to determine multiple flow variables from a single-frequency measurement, opening the path towards time-resolved multi-parameter measurements by FRS.