OBSERVATION OF TRANSITION TO TURBULENCE IN AN AIR-JET BY COLLECTIVE LIGHT-SCATTERING

We report measurements by a heterodyne-detection technique of density fluctuations in a free air jet by collective scattering of a 10-W cw C O2 laser beam. Above a certain threshold of the flow velocity we obser ve a well-defined signal in the lower-frequency side of the spectrum, which can be simply interpreted as resulting from negative flow veloci ties inside the jet (eventually associated with the formation of vorti ces). For increasing values of the flow velocities the negative freque ncy peak. breaks into a sequence of equally spaced peaks that are char acteristic of a quasi-periodic state before developing into a broadban d spectrum (turbulent nonperiodic regime). A measured bifurcation diag ram is presented and discussed in detail. (C) 1996 Optical Society of America