Acoustic propagation velocity v in a fluid depends on flow-velocity vf vectorially and temperature T. Measuring v in two suitable directions provides a good method for determining vf and T simultaneously. This is particularly valuable in gas flows, where the other noncontact flow determination method by light-scattering technique (laser Doppler velocimetry) is difficult because of the negligible Rayleigh scattering efficiencies of gases in the absence of particles (e.g., smoke particles or oil droplets are usually added to the gas to observe light scattering). Furthermore, our noncontact acoustic technique is also applicable to hostile environments (flames, discharges, etc.), where background luminescence may pose severe limitations on light-scattering measurements.
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