Vivek N. Prakash

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.

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

Wake-Driven Dynamics of Finite-Sized Buoyant Spheres in Turbulence

Particles suspended in turbulent flows are affected by the turbulence and at the same time act back on the flow. The resulting coupling can give rise to rich variability in their dynamics. Here we report experimental results from an investigation of finite-sized buoyant spheres in turbulence. We find that even a marginal reduction in the particles density from that of the fluid can result in strong modification of its dynamics. In contrast to classical spatial filtering arguments and predictions of particle models, we find that the particle acceleration variance increases with size. We trace this reversed trend back to the growing contribution from wake-induced forces, unaccounted for in current particle models in turbulence. Our findings highlight the need for improved multiphysics based models that account for particle wake effects for a faithful representation of buoyant-sphere dynamics in turbulence.

Energy spectra in turbulent bubbly flows

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

Vortex arrays and ciliary tangles underlie the feeding–swimming trade-off in starfish larvae

-5/3

Flowtrace: simple visualization of coherent structures in biological fluid flows

−5/3 energy spectrum scaling for a single-phase turbulent flow and the

The role of viscosity contrast on plume structure in laboratory modeling of mantle convection

-3

Vortex arrays and ciliary tangles underlie the feeding-swimming tradeoff in starfish larvae

−3 scaling for a swarm of bubbles rising in a quiescent liquid and of bubble-dominated turbulence. The bubblance parameter (Lance & Bataille J. Fluid Mech., vol. 222, 1991, pp. 95–118; Rensen et al., J. Fluid Mech., vol. 538, 2005, pp. 153–187), which measures the ratio of the bubble-induced kinetic energy to the kinetic energy induced by the turbulent liquid fluctuations before bubble injection, is often used to characterise bubbly flow. We vary the bubblance parameter from

Rapid behavioral transitions produce chaotic mixing by a planktonic microswimmer

b=\infty

Tissue Fracture Dynamics governs Mechanics of Morphogenesis in a Simple, Early Divergent Metazoan

b=∞ (pseudoturbulence) to