This experimental study explores and quantifies mixing characteristics associated with a gaseous round jet injected perpendicularly into cross-flow for a range of flow and injection conditions. The study utilizes acetone planar laser-induced fluorescence imaging to determine mixing metrics in both centreplane and cross-sectional planes of the jet, for a range of jet-to-cross-flow momentum flux ratios ($2\leqslant J\leqslant 41$), density ratios ($0.35\leqslant S\leqslant 1.0$) and injector configurations (flush nozzle, flush pipe and elevated nozzle), all at a fixed jet Reynolds number of 1900. For the majority of conditions explored, there is a direct correspondence between the nature of the jet’s upstream shear layer instabilities and structure, as documented in detail in Getsingeret al.(J. Fluid Mech., vol. 760, 2014, pp. 342–367), and the jet’s mixing characteristics, consistent with diffusion-dominated processes, but with a few notable exceptions. When quantified as a function of distance along the jet trajectory, mixing metrics for jets in cross-flow with an absolutely unstable upstream shear layer and relatively symmetric counter-rotating vortex pair cross-sectional structure tend to show better local molecular mixing than for jets with convectively unstable upstream shear layers and generally asymmetric cross-sectional structures. Yet the spatial evolution of mixing with downstream distance can be greater for a few specific convectively unstable conditions, apparently associated with the initiation and nature of shear layer rollup as a trigger for improved mixing. A notable exception to these trends concerns conditions where the equidensity jet in cross-flow has an upstream shear layer that is already absolutely unstable, and the jet density is then reduced in comparison with that of the cross-flow. Here, density ratios below unity tend to mix less well than for equidensity conditions, demonstrated to result from differences in the nature of higher-density cross-flow entrainment into lower-density shear layer vortices.

A flat plate film cooling experimental facility has enabled a study of the flowfield features and film performance characteristics of a single row of standard axial-shaped holes (ASH). Surface IR thermography and particle image velocimetry (PIV) were used to quantify blowing ratios impact on both flow and heat transfer in the ASH film row configuration. At constant freestream flow conditions, the blowing ratio was adjusted by altering the coolant flow rate. Performance was analyzed via IR measurements at three nominal blowing ratios by contours of adiabatic effectiveness as well as lateral and area-averaged adiabatic effectiveness. 2D PIV measurements were utilized as an additional flow diagnostic to examine the film jet structural variation with blowing ratio, and to provide a link between the behavior of the near-wall flowfield and the surface cooling.

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 W ere 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.

This experimental study explores both the natural and acoustically forced behavior of variable density transverse jets. A recent study by Getsinger et al. determined that transverse jets (of Reynolds number Rej = 1800) likely transition to global instability in response to su cient lowering of the jet-to-cross ow density ratio S (below 0.45-0.40) or momentum ux ratio J (below 10). This transition is characterized by weak shear layer oscillations easily overcome by external forcing for the convectively unstable (high S and J) case, and strong pure-tone oscillations resistant to external forcing for the globally unstable (low S and J) case. Here, simultaneous PLIF and PIV measurements of jet uid concentration and velocity elds are obtained in the jet neareld, as well as standalone PLIF measurements with a larger eld of view. Neareld PLIF/PIV measurements show agreement with hotwire measurements in terms of jet structural features and the character of the shear layer instabilities. The e ect of this apparent transition on jet mixing is also examined here, with implications for control of jet behavior in practical applications.

The near-field shear layer instabilities forming in round transverse jets of variable (reduced) densities relative to the crossflow are investigated through gas-phase experiments. Jets composed of helium and nitrogen mixtures are injected from a converging nozzle mounted flush with an injection wall into air crossflow, allowing the jet-to-crossflow density ratio S to be varied between 1.00, the equidensity case, and 0.14, at constant jet Reynolds number Re j  = 1,800. Jet-to-crossflow momentum flux ratios J are examined in the range \(\infty>J\geq5\) at incremental values of the density ratio S. The results of single-component hotwire measurements in the jet shear layer indicate that a transition to global instability likely occurs as J is brought below approximately 10, and/or as S is brought below approximately 0.45–0.40. This observation appears to link many previous independent studies of both equidensity transverse jets and low-density free jets, which may become globally unstable under alteration of J and S, respectively. However, the dynamical character of the transition to global instability in the low-density transverse jet displays differences under independent variation of J and S, which may indicate the predominance of different modes.