Exponentially Accelerating Jet in Crossflow

Effects of exponential acceleration on penetration and mixing characteristics of a jet in crosseow have been investigated experimentallyinawatermodel. To imposean exponential acceleration on the eow, both the injection speed and the nozzle width of the jet increased exponentially in the downstream direction of the crosseow. An acceleration parameter ® is dee ned as the ratio of the revolution time of the longitudinal vortex pair to the e-folding time of the acceleration. Theoretically, a signiecant reduction in turbulent entrainment and mixing of the jet with the crosseow 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-eeld 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 eame or reaction length increased up to 50%, revealing a strong effect of the near-e eld forcing on the far-e eld molecular scale mixing. Furthermore, the experiments have shown up to a 50% increase in the penetration of the jet into the crosseow as a result of the acceleration, when compared with a conventional transverse jet. These results demonstrate clearly that, in a free-shear eow, imposing a new timescale by means of an external acceleration ine uences the entrainment and mixing characteristics dramatically, thus providing a new possibility of controlling the eow characteristics.