Laser-Doppler velocity measurements in an under-expanded free jet

Velocity measurements in an under-expanded supersonic free jet have been made using the Fabry-Perot laser-Doppler technique. The jet was produced by a nozzle of exit diameter 2·7 mm operating at a pressure ratio P 0 / P infinity =6·6. Measurement of the complex structure, characteristic of this type of jet, on such a small scale proved to be an excellent test of the capability of the Fabry-Perot laser-Doppler technique for making air velocity measurements.Sufficient scattering particles were present in the flow to make artificial seeding unnecessary. The particles were found to respond to the velocity change across the Mach disc (a normal shock) in a distance of similar 0·1 mm. Laser-Doppler velocity measurements were compared with velocity measurements deduced from Pitot pressure measurements. The difference in results between the two methods was < 1%. Sufficient laser-Doppler velocity measurements were obtained to construct the axial and four transverse velocity profiles which clearly showed the positions and magnitudes of the major features characteristic of an under-expanded free jet.