Yoshiyuki Tagawa

Three-dimensional Lagrangian Voronoï analysis for clustering of particles and bubbles in turbulence

Three-dimensional Voronoi analysis is used to quantify the clustering of inertial particles in homogeneous isotropic turbulence using data sets from numerics in the point particle limit and one experimental data set. We study the clustering behaviour at different density ratios, particle response times (i.e. Stokes numbers ) and two Taylor–Reynolds numbers ( and 180). The probability density functions (p.d.f.s) of the Voronoi cell volumes of light and heavy particles show different behaviour from that of randomly distributed particles, i.e. fluid tracers, implying that clustering is present. The standard deviation of the p.d.f. normalized by that of randomly distributed particles is used to quantify the clustering. The clustering for both light and heavy particles is stronger for higher . Light particles show maximum clustering for around 1–2 for both Taylor–Reynolds numbers. The experimental data set shows reasonable agreement with the numerical results. The results are consistent with previous investigations employing other approaches to quantify the clustering. We also present the joint p.d.f.s of enstrophy and Voronoi volumes and their Lagrangian autocorrelations. The small Voronoi volumes of light particles correspond to regions of higher enstrophy than those of heavy particles, indicating that light particles cluster in higher vorticity regions. The Lagrangian temporal autocorrelation function of Voronoi volumes shows that the clustering of light particles lasts much longer than that of heavy or neutrally buoyant particles. Due to inertial effects arising from the density contrast with the surrounding liquid, light and heavy particles remain clustered for much longer times than the flow structures which cause the clustering

Highly focused supersonic microjets: numerical simulations

By focusing a laser pulse inside a capillary partially filled with liquid, a vapour bubble is created that emits a pressure wave. This pressure wave travels through the liquid and creates a fast, focused axisymmetric microjet when it is reflected at the meniscus. We numerically investigate the formation of this microjet using axisymmetric boundary integral simulations, where we model the pressure wave as a pressure pulse applied on the bubble. We find a good agreement between the simulations and experimental results in terms of the time evolution of the jet and on all parameters that can be compared directly. We present a simple analytical model that accurately predicts the velocity of the jet after the pressure pulse and its maximum velocity

Lagrangian statistics of light particles in turbulence

We study the Lagrangian velocity and acceleration statistics of light particles (micro-bubbles in water) in homogeneous isotropic turbulence. Micro-bubbles with a diameter db = 340 ?m and Stokes number from 0.02 to 0.09 are dispersed in a turbulent water tunnel operated at Taylor-Reynolds numbers (Re?) ranging from 160 to 265. We reconstruct the bubble trajectories by employing three-dimensional particle tracking velocimetry. It is found that the probability density functions (PDFs) of the micro-bubble acceleration show a highly non-Gaussian behavior with flatness values in the range 23 to 30. The acceleration flatness values show an increasing trend with Re?, consistent with previous experiments [G. Voth, A. La Porta, A. M. Crawford, J. Alexander, and E. Bodenschatz, ?Measurement of particle accelerations in fully developed turbulence,? J. Fluid Mech. 469, 121 (2002)]10.1017/S0022112002001842 and numerics [T. Ishihara, Y. Kaneda, M. Yokokawa, K. Itakura, and A. Uno, ?Small-scale statistics in highresolution direct numerical simulation of turbulence: Reynolds number dependence of one-point velocity gradient statistics,? J. Fluid Mech. 592, 335 (2007)]10.1017/S0022112007008531 . These acceleration PDFs show a higher intermittency compared to tracers [S. Ayyalasomayajula, Z. Warhaft, and L. R. Collins, ?Modeling inertial particle acceleration statistics in isotropic turbulence,? Phys. Fluids. 20, 095104 (2008)]10.1063/1.2976174 and heavy particles [S. Ayyalasomayajula, A. Gylfason, L. R. Collins, E. Bodenschatz, and Z. Warhaft, ?Lagrangian measurements of inertial particle accelerations in grid generated wind tunnel turbulence,? Phys. Rev. Lett. 97, 144507 (2006)]10.1103/PhysRevLett.97.144507 in wind tunnel experiments. In addition, the micro-bubble acceleration autocorrelation function decorrelates slower with increasing Re?. We also compare our results with experiments in von Karman flows and point-particle direct numerical simulations with periodic boundary conditions.

Probability of spontaneously resolvable conglomerates for racemic acid/racemic amine salts predicted on the basis of the results of diastereomeric resolutions

The salts of racemic acids with racemic amines, of which the enantiopure component is a good resolving agent for the corresponding counterpart in diastereomeric resolution, have an extremely high tendency to be conglomerates: the results of diastereomeric resolutions are highly informative for the prediction of conglomerates, which can be enantioseparated by preferential crystallization developed independently from diastereomeric resolution over one and a half centuries.

How gravity and size affect the acceleration statistics of bubbles in turbulence

We report the results of the first systematic Lagrangian experimental investigation in a previously unexplored regime of very light (air bubbles in water) and large (D/ 1) particles in turbulence. Using a traversing camera setup and particle tracking, we study the Lagrangian acceleration statistics of 3mm diameter (D) bubbles in a water tunnel with nearly homogeneous and isotropic turbulence generated by an active grid. The Reynolds number (Re ) is varied from 145 to 230, resulting in size ratios, D/ , in the range of 7.3-12.5, where is the Kolmogorov length scale. The experiments reveal that gravity increases the acceleration variance and reduces the intermittency of the probability density function (PDF) in the vertical direction. Once the gravity

Wall forces on a sphere in a rotating liquid-filled cylinder

We experimentally study the behavior of a particle slightly denser than the surrounding liquid in solid body rotating flow. Earlier work revealed that a heavy particle has an unstable equilibrium point in unbounded rotating flows[G. O. Roberts, D. M Kornfeld, and W. W Fowlis, J. Fluid Mech.229, 555–567 (Year: 1991)10.1017/S0022112091003166]. In the confinement of the rotational flow by a cylindrical wall a heavy sphere with density 1.05 g/cm3 describes an orbital motion in our experiments. This is due to the effect of the wall near the sphere, i.e., a repulsive force (F W ). We model F W on the sphere as a function of the distance from the wall (L): F W ∝L −4 as proposed by Takemura et al. [J. Fluid Mech.495, 235–253 (Year: 2003)10.1017/S0022112003006232]. Remarkably, the path evaluated from the model including F W reproduces the experimentally measured trajectory. In addition during an orbital motion the particle does not spin around its axis, and we provide a possible explanation for this phenomenon.

Energy spectra in turbulent bubbly flows

We conduct experiments in a turbulent bubbly flow to study the nature of the transition between the classical

The clustering morphology of freely rising deformable bubbles

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Cavitation onset caused by acceleration

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On pressure impulse of a laser-induced underwater shock wave

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