Measurements and calculations of the phase speed of disturbances observed in turbulent jets of three diameters are made at speeds between 80 m/s and 300 m/s, via crosscorrelation of signals from a double focused laser differential interferometer (D-FLDI). A consistent trend in correlation-derived disturbance propagation speeds is observed with varying jet velocity, and the strength of the correlation decreases as the jet is moved laterally through the interferometer away from the focus point. The correlation curves also collapse across varying jet velocities when normalized. The dual FLDI beams are imaged and separation distances characterized using a beam profiler. Spectral density curves from the D-FLDI and hotwire anemometer at similar locations are compared. The effect of orienting FLDI bundles parallel with and at a 45°angle to the mean flow direction is assessed and shown to be minimal in terms of the disturbance velocity computed via cross-correlation.
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