Structural and stability characteristics of jets in crossflow

Abstract

This experimental study examines the relationship between transverse jet structural characteristics and the shear layer instabilities forming on the upstream side of the jet column. Jets composed of mixtures of helium and nitrogen were introduced perpendicularly into a low-speed wind tunnel using several alternative injectors: convergent circular nozzles mounted either flush with or elevated above the tunnel floor, and a flush-mounted circular pipe. Both non-intrusive optical diagnostics (planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV)) and intrusive probe-based (hot-wire anemometry) measurements were used to explore a range of jet-to-crossflow momentum flux ratios and density ratios for which previous studies have identified upstream shear layer transition from convective to absolute instability. Remarkable correspondences were identified between formation of the well-known counter-rotating vortex pair (CVP) associated with the jet cross-section and conditions producing strong upstream shear layer vorticity rollup, arising typically from absolute instability in the shear layer. In contrast, asymmetries in the jet mean cross-sectional shape and/or lack of a clear CVP were observed to correspond to weaker, convectively unstable jet shear layers.

Transverse jet mixing characteristics

“…The transitional momentum flux ratio J is much lower for this configuration when compared to injection from a flush nozzle owing to vertical co-flow effects which act to stabilize the elevated jet's upstream shear layer (Megerian et al 2007). The transverse jet upstream shear layer for equidensity (S = 1.00) jet injection from a straight pipe mounted flush with the test section floor shows a transition to absolutely unstable characteristics in the shear layer spectra near J ≈ 10, similar to that for the flush-mounted nozzle (Getsinger et al 2014). Yet differences between the flush nozzle and flush pipe configurations in terms of Strouhal numbers for the fundamental shear layer instabilities and their initiation locations suggest that the jet momentum thickness at injection plays an important role in the growth rate, strength, coherency and frequency of these instabilities.…”

“…Transverse jet mixing characteristics 239 and tilted vortices in the mean, as opposed to the structurally symmetric, single CVP indicated in figure 1 (Kuzo 1995;Smith & Mungal 1998;Shan & Dimotakis 2006;Getsinger et al 2014).…”

“…This transition has important implications for jet structure (Getsinger et al 2014) as well as the type of transverse jet control required to impact jet behaviour (M'Closkey et al 2002;Shapiro et al 2006;Davitian et al 2010b). The evidence for transition in the upstream shear layer instability has been documented for both the equidensity and low-density JICF via alterations in the shear layer's spectral character, reduction in the energy transfer from fundamental to subharmonic frequencies along the shear layer, altered responses to low-level forcing, evidence of a Hopf bifurcation and other relevant observations (Megerian et al 2007;Davitian et al 2010a;Getsinger et al 2012).…”

Turbulent mixing in non-isothermal jet in crossflow

“…Except for TR = 0.5, the CVP has a symmetric pattern for different temperature ratios. As discussed in details by Karagozian [7] and Getsinger et al [70], the non-symmetric CVP in time-averaged flow field can occur for momentum ratios (MR) (approximately) higher than 20 and for the jet Reynolds numbers (approximately) less than 4000. They argue that this condition is related to convectively unstable shear layers.…”

Transient interaction between a reaction control jet and a hypersonic crossflow

“…Smith and Mungal [21] observed that the formation of the jet shear-layer vortices is delayed in a subsonic crossflow with increased velocity ratios R > 5. Recent work in subsonic crossflows shows that this delayed formation is not related to velocity ratio, but rather to shear-layer instabilities [22][23][24]. There are two distinct mechanisms for jet shear-layer roll-up in subsonic crossflow.…”