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 CO(2) laser beam. Above a certain threshold of the flow velocity we observe a well-defined signal in the lower-frequency side of the spectrum, which can be simply interpreted as resulting from negative flow velocities inside the jet (eventually associated with the formation of vortices). For increasing values of the flow velocities the negative frequency peak breaks into a sequence of equally spaced peaks that are characteristic of a quasi-periodic state before developing into a broadband spectrum (turbulent nonperiodic regime). A measured bifurcation diagram is presented and discussed in detail.
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