Abstract:

An experimental study was undertaken to explore the coupling that occurs when a rectangular jet is exposed to an oscillating ambient field.  The rectangular nozzle has an aspect ratio of 3:1, and Reynolds numbers of 1100 and 6500.  The jet was placed in the middle of a rectangular acoustic chamber which is driven by a speaker to establish an acoustic resonance.  The location of the jet was at a pressure node which results in a local transverse flow oscillation which can couple to the jet flow.  The relative frequency, characterized by the Strouhal number, was varied from very low to moderately high and three different behaviors were observed.  At the lowest Strouhal number, the jet couples with the acoustics and experiences low frequency oscillations while the preferred mode instability is still present.  The second behavior, at moderate Strouhal numbers, is a strong linear instability coupling where the jet forms instabilities at the forcing frequency near the nozzle exit and mixing is dramatically enhanced.  The last behavior at moderately high Strouhal numbers indicates not linear instability coupling at low amplitude, while high acoustic levels resulted in a weak nonlinear coupling likely caused by dynamics near or even upstream of the nozzle exit.