| This study provides a detailed exploration of the nearfield three-dimensional shear layer instabilities associated with a gaseous, isothermal jet injected normally into crossflow, also known as the transverse jet. Jet injection from nozzles which are flush as well as elevated with respect to the tunnel wall are explored experimentally in this study, for jet-to-crossflow velocity ratios R in the range 1 ≲ R ≤ 10 and with jet Reynolds numbers of 2000 and 3000. The results indicate that the nature of the transverse jet instability is significantly different from that of the free jet, and that it changes as the crossflow velocity is increased. Dominant instability modes are observed to be strengthened, to move closer to the jet orifice, and to increase in frequency as crossflow velocity increases for the regime 3.5 < R ≤ 10. The instabilities also exhibit mode shifting downstream along the jet shear layer for either nozzle configuration at these moderately high values of R. When R is reduced below 3.2 for the flush jet, and below 1.25 for the elevated jet, single mode instabilities are dramatically strengthened, forming almost immediately within the shear layer, in addition to higher harmonic modes. Under these conditions the dominant and initial mode frequencies tend to decrease with increasing crossflow.;Low level jet forcing has little influence on the shear layer response when these strong modes are present, suggesting a transition to absolute instability, in contrast to the more significant influence of low level forcing for higher R values, consistent with a convectively unstable flow. The differences in the stability characteristics of the jet in crossflow suggest the necessity of a "two-pronged" approach to the control of transverse jet penetration and spread, depending on the values of R, which is explored via smoke visualization in this study. For the transverse jet where R > 3.2, low and moderate levels of sinusoidal forcing of the jet can be used to effectively control jet penetration and spread. For the case where R is relatively low, below 3.2 for the flush jet and below 1.25 for the elevated jet, strong jet forcing with a prescribed time scale, e.g., via square wave forcing, is required to maximize the impact on jet behavior. It is demonstrated that when the applied jet forcing waveform is a square wave with a temporal pulse width that lies within a specific range associated with vortex ring optimization, excellent jet response can be obtained, even under globally unstable conditions. Separate studies of strong sinusoidal forcing produce "control maps" that are similar to those for other globally unstable flowfields, but optimized square wave forcing nevertheless produces improved transverse jet penetration and spread in contrast to sinusoidal forcing. |
