Masato Ida

Conservative global gyrokinetic toroidal full-f five-dimensional Vlasov simulation

Abstract A new conservative global gyrokinetic toroidal full-f five-dimensional Vlasov simulation (GT5D) is developed using a novel non-dissipative conservative finite difference scheme. The scheme guarantees numerical stability by satisfying relevant first principles in the modern gyrokinetic theory, and enables robust and accurate simulations of tokamak micro-turbulence. GT5D is verified through comparisons of zonal flow damping tests, linear analyses of ion temperature gradient driven (ITG) modes, and nonlinear ITG turbulence simulations against a global gyrokinetic toroidal δf particle code. In the comparison, global solutions of the ITG turbulence are identified quantitatively by using two gyrokinetic codes based on particle and mesh approaches.

Mitigation Technologies for Damage Induced by Pressure Waves in High-Power Mercury Spallation Neutron Sources (II) : Bubbling Effect to Reduce Pressure Wave

Liquid mercury was suggested to be used as target material for high-power pulsed spallation neutron sources. In order to realize the high-power target, however, the pressure wave is a critical issue, which is caused by the thermal shock in mercury and causes cavitation at the moment when highly intense proton beams bombard mercury. R&D on pressure wave mitigation technologies is carried out for Japan Spallation Neutron Source (JSNS; 1MW/25 Hz). Microbubble injection into the mercury is one of prospective technologies to mitigate the pressure wave. The microbubble effect was experimentally investigated from the viewpoint of pitting damage due to the cavitation in the mercury loop with an electro-magnetic impact testing machine (MIMTM) and numerically examined from the viewpoint of bubble dynamics. In the present study, we confirmed that the microbubble injection is very effective to reduce pitting damage and the amplitude of negative pressure, which causes explosive growth of cavitation bubble.

New conservative gyrokinetic full-f Vlasov code and its comparison to gyrokinetic δf particle-in-cell code

A new conservative gyrokinetic full-f Vlasov code is developed using a finite difference operator which conserves both the L1 and L2 norms. The growth of numerical oscillations is suppressed by conserving the L2 norm, and the code is numerically stable and robust in a long time simulation. In the slab ion temperature gradient driven (ITG) turbulence simulation, the energy conservation and the entropy balance relation are confirmed, and solutions are benchmarked against a conventional delta f particle-in-cell (PIC) code. The results show that the exact particle number conservation and the good energy conservation in the conservative Vlasov simulation are advantageous for a long time micro-turbulence simulation. In the comparison, physical and numerical effects of the nu(parallel to) nonlinearity are clarified for the Vlasov and PIC simulations

Alternative interpretation of the sign reversal of secondary Bjerknes force acting between two pulsating gas bubbles.

It is known that in a certain case, the secondary Bjerknes force (which is a radiation force acting between pulsating bubbles) changes, e.g., from attraction to repulsion, as the bubbles approach each other. In this paper, a theoretical discussion of this phenomenon for two spherical bubbles is described. The present theory based on analysis of the transition frequencies of interacting bubbles [M. Ida, Phys. Lett. A 297, 210 (2002)] provides an interpretation, different from previous ones (e.g., by Doinikov and Zavtrak [Phys. Fluids 7, 1923 (1995)]), of the phenomenon. It is shown, for example, that the reversal that occurs when one bubble is smaller and the other is larger than a resonance size is due to the second-highest transition frequency of the smaller bubble, which cannot be obtained using traditional natural-frequency analysis.

Multibubble cavitation inception

The inception of cavitation in multibubble cases is studied numerically and theoretically to show that it is different from that in single-bubble cases in several aspects. Using a multibubble model based on the Rayleigh–Plesset equation with an acoustic interaction term, we confirmed that the recently reported suppression of cavitation inception due to the interaction of nonidentical bubbles can take place not only in liquid mercury but also in water, and we found that a relatively large bubble can significantly decrease the cavitation threshold pressure of a nearby small bubble. By examining in detail the transition region where the dynamics of the suppressed bubble changes drastically as the interbubble distance changes, we determined that the explosive expansion of a bubble under negative pressure can be interrupted and turn into collapse even though the far-field liquid pressure well exceeds the bubble’s threshold pressure. Numerical results suggest that the interruption of expansion occurs when the b...

Bubble-bubble interaction: a potential source of cavitation noise.

The interaction between microbubbles through pressure pulses has been studied to show that it can be a source of cavitation noise. A recent report demonstrated that the acoustic noise generated by a shrimp originates from the collapse of a cavitation bubble produced when the shrimp closes its snapper claw. The recorded acoustic signal contains a broadband noise that consists of positive and negative pulses, but a theoretical model for single bubbles fails to reproduce the negative ones. Using a nonlinear multibubble model, we have shown here that the negative pulses can be explained by considering the interaction of microbubbles formed after the cavitation bubble has collapsed and fragmented: Positive pulses produced at the collapse of the microbubbles hit and impulsively compress neighboring microbubbles to generate reflected pulses whose amplitudes are negative. Discussing the details of the noise generation process, we have found that no negative pulses are generated if the internal pressure of the reflecting bubble is very high when hit by a positive pulse.

Investigation of Transition Frequencies of Two Acoustically Coupled Bubbles Using a Direct Numerical Simulation Technique

The theoretical results regarding the “transition frequencies” of two acoustically interacting bubbles have been verified numerically. The theory provided by Ida [Phys. Lett. A 297 (2002) 210] predicted the existence of three transition frequencies per bubble, each of which has the phase difference of π/2 between a bubbles pulsation and the external sound field, while previous theories predicted only two natural frequencies which cause such phase shifts. Namely, two of the three transition frequencies correspond to the natural frequencies, while the remaining does not. In a subsequent paper [M. Ida: Phys. Rev. E 67 (2003) 056617], it was shown theoretically that transition frequencies other than the natural frequencies may cause the sign reversal of the secondary Bjerknes force acting between pulsating bubbles. In the present study, we employ a direct numerical simulation technique that uses the compressible Navier–Stokes equations with a surface-tension term as the governing equations to investigate the...

Phase properties and interaction force of acoustically interacting bubbles : A complementary study of the transition frequency

The transition frequencies of multibubble systems in a sound field are reexamined to confirm their existence and further clarify their physical properties. We have recently suggested that a gas bubble interacting with a neighboring bubble in a sound field has up to three transition frequencies which invert the pulsation phase of the bubble, e.g., from in phase to out of phase with the driving sound [M. Ida, Phys. Lett. A 297, 210 (2002)]. The number is larger than that of the resonance frequencies of the double-bubble system, which also invert the pulsation phase of the bubbles. Namely, one of the three transition frequencies does not correspond to the resonance frequencies. We have furthermore suggested that the newly derived characteristic frequency plays a crucial role in the determination of the sign of the interaction force (the secondary Bjerknes force) acting between pulsating bubbles [M. Ida, Phys. Rev. E 67, 056617 (2003)]. More specifically, it has been shown that the height relation between the...

Avoided crossings in three coupled oscillators as a model system of acoustic bubbles.

The resonance frequencies and oscillation phases of three acoustically coupled bubbles are examined to show that avoided crossings can appear in a multibubble system. Via a simple coupled oscillator model, we show that if at least three bubbles exist, it is possible for their resonance frequencies as functions of the separation distances between the bubbles to experience an avoided crossing. Furthermore, by focusing our attention on the oscillation phases and based on analysis of the transition frequencies [M. Ida, Phys. Lett. A 297, 210 (2002); J. Phys. Soc. Jpn. 71, 1214 (2002)] of the coupled bubbles, we show that a distinct state exchange takes place between the bubbles at a point in the avoided crossing region, where a resonance frequency of the triple-bubble system crosses with a transition frequency not corresponding to the resonance frequencies.

Mitigation Technologies for Damage Induced by Pressure Waves in High-Power Mercury Spallation Neutron Sources (III)—Consideration of the Effect of Microbubbles on Pressure Wave Propagation through a Water Test—

The effects of microbubbles dispersed in a liquid on a high-rising-rate pressure wave were experimentally investigated with water. Intense, high-rising-rate pressure waves with a rise time of about 1.5 ms were produced by a spark discharge in water, and gas microbubbles were produced by two different bubble generators. Particular attention was focused on the attenuation effect of microbubbles on propagating pressure waves. The dependence of the attenuation effect on the radius and void fraction of the microbubbles was carefully examined. It was found that when the microbubbles are sufficiently small (e.g., about 50 μm in peak radius), the amplitude of wall vibration induced by the spark-induced pressure wave is dramatically decreased with an increase in void fraction. The present study provides strong experimental evidence that microbubbles can act as a strong absorber for high-rising-rate pressure waves as recently predicted numerically.