**Diffusion of Particles in Isotropic Turbulence Using High Speed Digital Holographic Cinematography**

High speed in-line digital holographic cinematography is used for studying turbulent diffusion of several particles: slightly buoyant 0.5-1.2mm diesel droplets (our primary focus), 50microns neutral density particles and the phytoplankton Cosinodiscus Radiatus (60-80microns). Experiments are performed in a 50x50x70mm3 sample volume in a controlled, nearly isotropic turbulence facility. Diffusion rate is calculated by integration of the Lagrangian velocity autocorrelation. Droplet diffusion rate in horizontal (Dx) and vertical direction (Dy) is lower than that of fluid at low turbulence level and exceeds it at higher turbulence level. Dx is greater than Dy for most of the data due to horizontal droplet velocity RMS exceeding the vertical one. The vertical droplet diffusion timescale (Ty) is higher than the horizontal timescale (Tx). The droplet diffusion coefficient scaled by quiescent rise velocity (Uq) times turbulence integral length scale, Uq2Tx or Uq2Ty monotonically increases as a function of fluid RMS scaled by Uq. Experiments with phytoplanktons, in salt water with Kolmogorov scale (k) 1mm as observed in the ocean, show that since phytoplankton size ~ (1/15)k their diffusion is similar to fluid diffusion.

Gopalan, B., E. Malkiel, L, Karp-Boss, J. Sheng, J. Katz, 2008. Diffusion of Particles in Isotropic Turbulence Using High Speed Digital Holographic Cinematography. 2008 Ocean Sciences Meeting, Orlando, FL, March 2-7.