Balaji Gopalan

Experimental investigation of turbulent diffusion of slightly buoyant droplets in locally isotropic turbulence

High-speed inline digital holographic cinematography is used for studying turbulent diffusion of slightly buoyant 0.5–1.2 mm diameter diesel droplets and 50 μm diameter neutral density particles. Experiments are performed in a 50×50×70 mm3 sample volume in a controlled, nearly isotropic turbulence facility, which is characterized by two dimensional particle image velocimetry. An automated tracking program has been used for measuring velocity time history of more than 17 000 droplets and 15 000 particles. For most of the present conditions, rms values of horizontal droplet velocity exceed those of the fluid. The rms values of droplet vertical velocity are higher than those of the fluid only for the highest turbulence level. The turbulent diffusion coefficient is calculated by integration of the ensemble-averaged Lagrangian velocity autocovariance. Trends of the asymptotic droplet diffusion coefficient are examined by noting that it can be viewed as a product of a mean square velocity and a diffusion time s...

Stretching of turbulent eddies generates cavitation near a stagnation point

Cavitation inherently occurs in regions with low pressure. However, in recent experiments involving a high-speed bubbly jet impinging on a blunt body, we have observed rapid growth and stretching of bubbles near the stagnation point. The analysis of holographic particle image velocimetry data shows that turbulent eddies which originated from the jet are being stretched by the strain field in the vicinity of the stagnation point to form powerful vortices. Their strength and size along with the local strain rate are sufficient for decreasing the pressure in the vortex core below the vapor pressure, explaining the occurrence of cavitation in this region.

Diffusion of Slightly Buoyant Droplets in Isotropic Turbulence

We study the diffusion of slightly buoyant droplets in isotropic turbulence using High Speed Digital Holographic PIV. Droplets (Specific Gravity 0.85) are injected in the central portion of an isotropic turbulence facility with weak mean flow. Perpendicular digital inline holograms are recorded in a 37 × 37 × 37 mm3 region of interest using two high speed cameras. Data are recorded at 250 frames per second (2000 frames per second is the maximum possible frame rate). An automated program is developed to obtain two dimensional tracks of the droplets from two orthogonal images and match them to get three dimensional tracks. Cross correlation of droplet images are used for measuring their velocities. The time series are low pass filtered to obtain accurate time history of droplet velocities. Data analysis determines the PDF of velocity and acceleration in three dimensions. The time history also enables us to calculate the three dimensional Lagrangian velocity autocorrelation function for different droplet radii. Integration of these functions gives us the diffusion coefficients. For shorter time scales, when the diffusion need not be Fickian we can use the three dimensional trajectories to calculate the generalized dispersion tensor and measure the time elapsed for diffusion to become Fickian.Copyright

DIESEL DROPLET DIFFUSION IN ISOTROPIC TURBULENCE WITH DIGITAL HOLOGRAPHIC CINEMATOGRAPHY

High-speed in-line digital holographic cinematography was used to investigate the diffusion of droplets in locally isotropic turbulence. Droplets of diesel fuel (0.3–0.9mm diameter, specific gravity of 0.85) were injected into a 37×37×37mm3 sample volume located in the center of a 160-liter tank. The turbulence was generated by 4 spinning grids, located symmetrically in the corners of the tank, and was characterized prior to the experiments. The sample volume was back illuminated with two perpendicular collimated beams of coherent laser light and time series of in-line holograms were recorded with two high-speed digital cameras at 500 frames/sec. Numerical reconstruction generated a time series of high-resolution images of the droplets throughout the sample volume. We developed an algorithm for automatically detecting the droplet trajectories from each view, for matching the two views to obtain the three-dimensional tracks, and for calculating the time history of velocity. We also measured the mean fluid motion using 2-D PIV. The data enabled us to calculate the Lagrangian velocity autocorrelation function.Copyright

Lagrangian Statistics of Droplet Velocity and Acceleration in Isotropic Turbulence

The addition of dispersants, water and oil soluble surfactants that lower the interfacial tension of the crude oil, along with oceanic turbulence can breakdown oil spills into droplets. Knowledge of the dispersion rate of these droplets by oceanic turbulence is essential for the development of better models to assess the environmental impact of spills. The objective of this research is to study, experimentally, the dispersion of oil droplets in turbulent flows. The measurements are performed in a specialized laboratory facility that enables generation of carefully controlled, isotropic, homogeneous turbulence at a wide range of fully characterized intensities and length scales. The oil dispersion is visualized using high-speed inline digital holographic cinematography. Holographic data has been analyzed and Lagrangian statistics of droplet velocity, dispersion and acceleration has been calculated. As the relative size of the droplet diameter to the Kolmogorov length scale and its Stokes number increases, the acceleration autocorrelation shifts from dropping to zero faster than the fluid particles to slower.© 2009 ASME

Break Up of Viscous Crude Oil Droplets Mixed With Dispersants in Locally Isotropic Turbulence

Our objective is to visualize the breakup of crude oil droplets, mixed with dispersants, in a turbulent flow. The current measurements are performed for a sample crude oil from Alaska National Slope mixed with dispersant COREXIT 9527. The droplet breakup measurements are conducted in a nearly homogeneous and isotropic turbulence facility, the central portion of which is characterized using 2-D PIV. The droplet break up is visualized at high spatial and temporal resolution using one view high speed, in-line digital holography. Subsequent analysis reveals relevant length scales and timescales involved in the break up process.Copyright

Lagrangian Motion of Slightly Buoyant Droplets and Fluid Particles in Isotropic Turbulence

The diffusion of slightly buoyant diesel oil droplets in isotropic turbulence is studied using high speed in-line digital holographic cinematography. Diesel fuel droplets with specific gravity 0.85 are injected into a 50×50×70 mm3 sample volume located at the central portion of a nearly isotropic turbulence facility. The turbulence in the sample volume is fully characterized using 2D PIV. Probability density functions of the Lagrangian droplet velocity are very close to a Gaussian distribution, which justifies the use of Taylor’s [1] model to calculate diffusion parameters. Similar to Friedman & Katz [2] data, our current results confirm that the mean rise velocity of diesel droplets becomes higher than the quiescent rise velocity at high turbulence levels. For most of the present droplet sizes and turbulence level, the rms of the horizontal droplet velocity fluctuations exceeds that of the horizontal fluid velocity fluctuations. The rms values of the vertical droplet velocity fluctuations are higher than those of the fluid only for the highest turbulence level. The droplet to fluid velocity rms ratio in both directions increases with turbulence level, but decreases with increasing droplet size. Assuming Fickian diffusion, Lagrangian auto-correlation functions of 22,000 droplet tracks are used for calculating the diffusion coefficient as functions of droplet size and turbulence level. Using all the data, we show that the diffusion coefficient scaled by quiescent rise velocity and the turbulence integral length scale is a monotonically increasing function of the turbulence level normalized by the droplet quiescent rise velocity.Copyright