Exponentially Accelerating Jet in Crossflow

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Abstract
Effects of exponential acceleration on penetration and mixing characteristics of a jet in crossèow have been investigated experimentallyinawatermodel. To imposean exponential acceleration on the èow, both the injection speed and the nozzle width of the jet increased exponentially in the downstream direction of the crossèow. An acceleration parameter ® is deé ned as the ratio of the revolution time of the longitudinal vortex pair to the e-folding time of the acceleration. Theoretically, a signiécant reduction in turbulent entrainment and mixing of the jet with the crossèow is expected as ® nears unity or as the revolution time of the longitudinal vortex pair in the jet becomes comparable to the characteristic time of acceleration. It was found that the diameter of each vortex in the near-éeld jet cross section is reduced more than a factor of three as ® is increased from 0 to 2.5. In the same ® range, the jet èame or reaction length increased up to 50%, revealing a strong effect of the near-é eld forcing on the far-é eld molecular scale mixing. Furthermore, the experiments have shown up to a 50% increase in the penetration of the jet into the crossèow as a result of the acceleration, when compared with a conventional transverse jet. These results demonstrate clearly that, in a free-shear èow, imposing a new timescale by means of an external acceleration inè uences the entrainment and mixing characteristics dramatically, thus providing a new possibility of controlling the èow characteristics.

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