Pipe flow measurements of turbulence and ambiguity using laser-Doppler velocimetry

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Abstract
The laser-Doppler ambiguities predicted by George & Lumley have been verified experimentally for turbulent pipe flows. Experiments were performed at Reynolds numbers from 5000 to 15000 at the centre-line and near the wall. Ambiguity levels were measured from power spectral densities of FM demodulated laser signals and compared with calculations based on the theory. The turbulent spectra for these water flows after accounting for the ambiguity were equivalent to hot-film measurements at similar Reynolds numbers. The feasibility of laser-Doppler measurements very close to the wall in shear flows is demonstrated.