Exponentially Accelerating Jet in Crossflow

Effects of exponential acceleration on penetration and mixing characteristics of a jet in crossflow were investigated experimentally in a water model. To impose an exponential acceleration on the flow, both the injection speed and the nozzle width of the jet increased exponentially in the downstream direction of the crossflow. Theoretically, a significant reduction in turbulent entrainment and mixing of the jet with the crossflow is expected as alpha, an acceleration parameter, 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-field jet cross section is reduced more than a factor of three as alpha is increased from 0 to 2.5. In the same alpha range, the jet flame or reaction length increased up to 50 percent, revealing a strong effect of the near-field forcing on the far-field molecular scale mixing. The experiments showed up to a 50 percent increase in the penetration of the jet into the crossflow as a result of the acceleration, when compared with a conventional transverse jet. In a free-shear flow, imposing a new timescale by means of an external acceleration influences the entrainment and mixing characteristics dramatically, thus providing a new possibility of controlling the flow characteristics. (Author)