Coherent structures in rectangular jets

The mixing process of a triple free jet emerging from three rectangular nozzles with a length ratio of between 30 and 15 has been investigated. The center jet was a CO2 jet, and the two outer jets were air jets. The laser Raman scattering technique was used for number-density measurements, Laser-Doppler anemometry for velocity measurement, and a pulsed schlieren technique for qualitative analysis of the mixing process. In the range from the nozzle exit to approximately 10 nozzle widths downstream, a strong influence of the free shear layers on the mixing process has been observed. This process was governed by the rolling up of the shear layer into vortex bands oriented approximately parallel to the nozzle exitplane. A second system with its vortex lines parallel to the main flow direction could also be observed. A qualitative explanation for the observed phenomena is given.