Turbulent jet interaction with a long rise-time pressure signature

A sonic boom signature with a long rise time has the ability to reduce the sonic boom, but it does not necessarily minimize the sonic boom at the ground level because of the real atmospheric turbulence. In this study, an effect of the turbulence on a long rise-time pressure signature was experimentally investigated in a ballistic range facility. To compare the effects of the turbulence on the long and short rise-time pressure signatures, a cone-cylinder projectile that simultaneously produces these pressure signatures was designed. The pressure waves interacted with a turbulent field generated by a circular nozzle. The turbulence effects were evaluated using flow diagnostic techniques: high-speed schlieren photography, a point-diffraction interferometer, and a pressure measurement. In spite of the fact that the long and short rise-time pressure signatures simultaneously travel through the turbulent field, the turbulence effects do not give the same contribution to these overpressures. Regarding the long rise-time pressure signature, the overpressure fluctuation due to the turbulence interaction is almost uniform, and a standard deviation 1.5 times greater than that of the no-turbulence case is observed. By contrast, a short rise-time pressure signature which passed through the same turbulent field is strongly affected by the turbulence. A standard deviation increases by a factor of 14 because of the turbulence interaction. Additionally, there is a non-correlation between the overpressure fluctuations of the long and short rise-time pressure signatures. These results deduce that the length of the rise time is important to the turbulence effects such as the shock focusing/diffracting

Observation of the Formation of Multiple Shock Waves at the Collapse of Cavitation Bubbles for Improvement of Energy Convergence

The collapse of a cavitation bubble is always associated with the radiation of intense shock waves, which are highly relevant in a variety of applications. To radiate a strong shock wave, it is necessary to converge energy at the collapse, and understanding generation processes of multiple shock waves at the collapse is a key issue. In the present study, we investigated the formation of multiple shock waves generated by the collapse of a laser-induced bubble. We used a high-speed imaging system with unprecedented spatiotemporal resolution. We developed a triggering procedure of high precision and reproducibility based on the deflection of a laser beam by the shockwave passage. The high-speed videos clearly show that: (A) a first shockwave is emitted as the micro-jet hits the bottom of the bubble interface, followed by a second shock wave due to the collapse of the remaining toroidal bubble; (B) a sequential collapse of elongated bubbles, where the top part of the bubble collapses slightly before the bottom of the bubble; and (C) the formation of compression shock waves from multiple sites on a toroidal bubble

Jetting from cavitation bubbles due to multiple shockwaves

We present experimental observations of microjets formed by cavitation microbubbles. Anunderwater electric discharge, applied beneath a flat free surface, produces a primary compressionwave, which undergoes several phase inversions upon reflections from the free surface and spark-bubble interface. The first reflection yields a tension wave, which produces a cloud of secondarycavitation bubbles in the liquid, some of which form microjets upon collapse. The tuning of thesereflections enables an effective control of the microjet direction in the bubble cloud. All of the jetsof the microbubbles between the spark bubble and free surface are directed radially away from thespark bubble. The mechanical response of an alumina plate placed between the electrodes and freesurface generates a quasi-planar compression wave, which, following its multiple reflections fromthe free surface and plate, orients the microjets in the same direction toward the plate. These obser-vations imply that the jet direction is determined mainly by the secondary compression wave,which is the first and thus most energetic compression wave acting on a sufficiently grown cavita-tion bubble.ISSN:0003-6951ISSN:1077-311