Performance and accuracy investigations of two Doppler global velocimetry systems applied in parallel

Abstract

Two Doppler global velocimetry systems were applied in parallel to assess their performance in wind tunnel environments. Both DGV systems were mounted on a common traverse surrounding the glass-walled 1.4 × 1.8 m2 test section of the wind tunnel. The traverse normally supports a three-component forward-scatter laser Doppler velocimetry system. The reproducible tip-vortex flow field generated by the blunt tip of an airfoil was chosen for this investigation and was precisely surveyed by LDA just prior to the DGV measurements. Both DGV systems shared the same continuous wave laser light source, laser frequency monitoring and fibre optic light sheet delivery system. The principal differences between the DGV implementations are with regard to the imaging configuration. One configuration relied on a single camera view that observed three successively operated light sheets. In the second configuration, three camera views simultaneously observed a single light sheet using a four-branch fibre imaging bundle. The imaging bundle system had all three viewpoints in a forward scattering arrangement which increased the scattering efficiency but reduced the frequency shift sensitivity. Since all three light sheet observation components were acquired onto the same image frame, acquisition times could be reduced to a minimum. On the other hand, the triple light sheet–single camera system observed two light sheets in forward scatter and one light sheet in backscatter. Although three separate images had to be recorded in succession, the image quality, spatial resolution and signal-to-noise ratio were superior to the imaging bundle system. Comparison of the DGV data with LDV measurements shows very good agreement to within 1–2 m s−1. The remaining discrepancy has a variety of causes, some are related to the reduced resolving power of the fibre imaging bundle system (graininess, smoothing), exact localization of the receiver head with respect to the scene, laser frequency drift or background influences. The extensive data base available allows a detailed study of the influence of each of these factors.