This experimental study explores the effect of temporal forcing on gaseous transverse jets, quant... more This experimental study explores the effect of temporal forcing on gaseous transverse jets, quantifying jet response via optical diagnostics. Low level sinusoidal jet excitation is observed to significantly affect structural and mixing characteristics for transverse jets with convectively unstable jet shear layers under unforced conditions. Such excitation, especially in a frequency range close to the fundamental, is found to attenuate the cross-sectional asymmetry naturally occurring in such unforced jets. There is also improved molecular mixing for excited jets with more symmetric cross sections. Yet when the transverse jet's upstream shear layer is absolutely unstable in the absence of forcing, sinusoidal jet excitation barely affects jet structure or mixing, even with a very high forcing amplitude and wider range of forcing frequencies. Different forcing methods, e.g., creating alternative temporal waveforms for jet excitation, are clearly required to control the structural and mixing characteristics of transverse jets based on these differing instabilities.
This experimental study explores and quantifies mixing characteristics associated with a gaseous ... more 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 J 41), density ratios (0.35 S 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 Getsinger et 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.
This experimental study explores the effect of temporal forcing on gaseous transverse jets, quant... more This experimental study explores the effect of temporal forcing on gaseous transverse jets, quantifying jet response via optical diagnostics. Low level sinusoidal jet excitation is observed to significantly affect structural and mixing characteristics for transverse jets with convectively unstable jet shear layers under unforced conditions. Such excitation, especially in a frequency range close to the fundamental, is found to attenuate the cross-sectional asymmetry naturally occurring in such unforced jets. There is also improved molecular mixing for excited jets with more symmetric cross sections. Yet when the transverse jet's upstream shear layer is absolutely unstable in the absence of forcing, sinusoidal jet excitation barely affects jet structure or mixing, even with a very high forcing amplitude and wider range of forcing frequencies. Different forcing methods, e.g., creating alternative temporal waveforms for jet excitation, are clearly required to control the structural and mixing characteristics of transverse jets based on these differing instabilities.
This experimental study explores and quantifies mixing characteristics associated with a gaseous ... more 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 J 41), density ratios (0.35 S 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 Getsinger et 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.
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Papers by TAKESHI SHOJI