Masakazu Tajima

Possible Explanation of the Secondary Flash and Strong Flare on IR Lightcurves upon Impact of Shoemaker-Levy 9

Observations of IR lightcurves of Comet Shoemaker-Levy 9 impacts detected strong thermal emissions at most impact events. During several huge impacts such as C, K, and R events, two precursor flashes were detected before the strong emission (e.g., Graham et al., 1995; Watanabe et al., 1995). Comparison of IR lightcurves with Galileo and HST data suggests that the first flash should correspond to the entry of a comet. At K impact, the secondary flash started rising after 60sec from the first flash and the strong emission started after 350sec from the first flash. To understand the flashes and the strong thermal emission, we have performed direct numerical simulations of break-up and subsequent explosion processes with highly accurate hydrocode. When the energy deposition length is quite long and is 300 km, two groups of materials are ejected and the two peaks corresponding to the secondary flash and strong thermal flare are obtained in the time evolution of density and temperature whose temporal profile agrees quite well with observations.

Simulation on Slamming of a Vessel by CIP Method

Impact of a vessel on water surface is simulated by the CIP (Cubic-Interpolated Pseudoparticle/ Propagation) scheme. The simulation demonstrates that air layer between vessel and water surface plays an important role to determine the pressure profile. In particular at small attack angle (or dead-rise angle), air flow along this layer between vessel and water gains speed sufficient to induce the Kelvin-Helmholtz instability which leads to a water wave of small wavelength. Because of this air layer, the maximum pressure is largely reduced at the small attack angle compared with Wagners theory and becomes much closer to Chuangs experiment.

Study on Flow Phenomenon inside a Nozzle of a Ship Propulsion Equipment Directly Driven by High Pressure Air (Experimental Consideration of Flow inside a Nozzle According to Water-Flow Velocity)

An experimental study by means of pressure measurements and flow visualization was performed to investigate unsteady flows inside a two-dimensional semi-open-type nozzle for a ship propulsion equipment directly driven by high-pressure gas. We found that ejected gas phase and water-flow phase are separated clearly of themselves, and the interface of these phase behave like interfacial waves. It is clarified by flow visualization with a high speed motion camera and a circulating water channel that these interfacial waves change their shapes according to water-flow velocity. The interfacial wavelength becomes longer in response to increasing water-flow velocity, and the mechanism that obtains thrust on the nozzle-wall changes. The thrust and flow patterns for the intermittent gas ejection according to water-flow velocity are also clarified.