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

Abstract

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 m diameter neutral density particles. Experiments are performed in a 50ϫ 50ϫ 70 mm 3 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 scale. To compare the effects of turbulence and buoyancy, the turbulence intensity ͑u i Ј͒ is scaled by the droplet quiescent rise velocity ͑U q ͒. The droplet diffusion coefficients in horizontal and vertical directions are lower than those of the fluid at low normalized turbulence intensity, but exceed it with increasing normalized turbulence intensity. For most of the present conditions the droplet horizontal diffusion coefficient is higher than the vertical diffusion coefficient, consistent with trends of the droplet velocity fluctuations and in contrast to the trends of the diffusion timescales. The droplet diffusion coefficients scaled by the product of turbulence intensity and an integral length scale are a monotonically increasing function of u i Ј/ U q .

Testing an in-line digital holography ‘inverse method’ for the Lagrangian tracking of evaporating droplets in homogeneous nearly isotropic turbulence

“…A detailed survey can be found in the review paper by Toschi and Bodenschatz [4]. They are based on various principles such as 3D particle tracking velocimetry (3D PTV) with four cameras [5,6], optical imaging with silicon strip detectors [7][8][9][10], extended laser Doppler anemometry (extended LDA) [11], ultrasonic Doppler tracking [12] and digital holography [13][14][15]. Compared to the other techniques mentioned, the main advantage of digital holography is that it provides information on size, which is obviously crucial for evaporation studies and caused us to select it.…”

“…Moreover, it is insensitive to vibrations and easy to implement, two important points for our operating conditions. The in-line digital holography method has already been proved to be successful in studying the turbulent dispersion of slightly buoyant droplets in homogeneous, nearly isotropic turbulence with high-speed cameras [13,18]. However, the rms velocity fluctuations in this case are low, of the order of 10 cm s −1 , and no phase change takes place.…”

A dinoflagellate exploits toxins to immobilize prey prior to ingestion

“…18. Using extensions to Taylor's theory for scalar and particle diffusion (21)(22)(23)26), swimming-induced dispersion coefficients were determined from an ensemble average of the Lagrangian autocorrelation function of particle velocity:…”

Lagrangian measurements of the fast evaporation of falling diethyl ether droplets using in-line digital holography and a high-speed camera

“…They are based on various principles such as 3D particle tracking velocimetry (PTV) with four cameras (Bourgoin et al 2006), optical imaging with silicon strip detectors (Voth et al 2002), extended laser Doppler anemometry (LDA) (Volk et al 2008), ultrasonic Doppler tracking (Mordant et al 2004) and digital holography (Gopalan et al 2008;Lu et al 2008;Katz and Sheng 2010;Nguyen et al 2011). Compared to the other techniques mentioned, the main advantage of digital holography is that it provides information on size, which is obviously crucial for phase-change studies.…”