Penetration and mixing of fully modulated turbulent jets in crossflow

Fully modulated, incompressible, turbulent transverse jets were studied experimentally in a water tunnel over a range of pulsing frequencies and duty cycles and at two jet-to;crossflow velocity ratios. The jet flow was completely modulated by operating an in-line solenoid valve resulting in the shutoff of jet supply during a portion of the cycle. The planar laser-induced fluorescence technique was used to determine the penetration, dilution, and structural features of the pulsed jets. The molecular mixing rate was quantified using a chemical reaction between the jet and crossflow fluids, Short. injection times resulted in creation of vortex ring structures, whereas long injection times produced axially elongated turbulent puffs, similar to a segment of a steady jet The latter case resulted in only modest enhancement of the jet penetration depth and dilution. Pulsed Jets dominated by vortex rings had penetration depths significantly greater than a steady jet with the same velocity ratio, up to a factor of 5 at 50 Jet diameters downstream of the exit. For short injection times, duty cycle had a significant effect on the behavior of pulsed jets. Increasing the duty cycle for a fixed injection time reduced the jet penetration, The dilution and mixing of pulsed jets: with short Injection time was also improved over the steady jet for duty cycles as high as 0.5. The greatest increase in the mixing rate was approximately 50% for well-separated pulses with short injection times.