Coherent structures in the near field of round jets

The present paper reports measurements of axial and radial velocities obtained with laser-Doppler anemometry in the transitional region of a round jet. The predominant frequencies of the coherent structures detected in the non-artificially excited jet, correspond to Strouhal numbers, around 0.33 and 0.55 and an interpretation of these observations is made. The use of external excitation can modify some characteristics of the jets disturbing the balance between the different flow structures, namely those corresponding to the two values of St. The jet diameter was 30.0 mm and the velocities ranged from near zero up to 15 m/s, corresponding to Reynolds number of 2.87 × 10⁴.To obtain the velocity probability distributions, power spectral distributions and auto-correlations, together with mean and rms velocities a LDA with double Bragg cell system was used. The Doppler signal was analysed with a counter interfaced to an Apple II microcomputer and a Racal magnetic tape.