Geometric scattering removal in cross-correlation Doppler global velocimetry by structured illumination

Abstract

The biases introduced by background light contributions in planar, Doppler-based velocimetry are significantly reduced by the application of structured illumination. The method works by measuring and separating desired flow-scattered signals from unwanted reflections and secondary scattering contributions via a sinusoidal intensity modulation across the laser sheet used to interrogate the flow. In the current work we consider the influence of background light on cross-correlation Doppler global velocimetry (CC-DGV) and present the results for measurements in a turbulent free jet with an exit Mach number of 0.5 in a measurement arrangement with high intensity background contributions. It is shown that if the ratio of the particle-scattering (i.e. Mie scattering) intensity to the background solid surface scattering (i.e. geometric scattering) intensity is greater than 10, then the measured signal displays a significant bias away from the true velocity. With structured illumination, the measured intensity corresponding to geometric scattering is reduced to negligible levels, reminiscent of a measurement arrangement with little or no background light influence. Centreline velocity measurements with this background subtraction method are compared with Kiel-type pressure probe measurements, yielding a root-mean-square (RMS) difference between the two of 1.2 m s-1 (less than 1% of the full scale velocity), well within the published uncertainty bound of the CC-DGV technique, underscoring the robustness of this approach. In addition to the centreline validation, the streamwise vorticity and total pressure are also analysed, showing trends indicative of the S-type shape of the duct and aggressive flow entrainment experienced in this environment.