Abstract
An in-line digital holography technique is tested, the objective being to measure Lagrangian three-dimensional (3D) trajectories and the size evolution of droplets evaporating in high-Reλ strong turbulence. The experiment is performed in homogeneous, nearly isotropic turbulence (50 × 50 × 50 mm3) created by the meeting of six synthetic jets. The holograms of droplets are recorded with a single high-speed camera at frame rates of 1–3 kHz. While hologram time series are generally processed using a classical approach based on the Fresnel transform, we follow an ‘inverse problem’ approach leading to improved size and 3D position accuracy and both in-field and out-of-field detection. The reconstruction method is validated with 60 μm diameter water droplets released from a piezoelectric injector ‘on-demand’ and which do not appreciably evaporate in the sample volume. Lagrangian statistics on 1000 reconstructed tracks are presented. Although improved, uncertainty on the depth positions remains higher, as expected in in-line digital holography. An additional filter is used to reduce the effect of this uncertainty when calculating the droplet velocities and accelerations along this direction. The diameters measured along the trajectories remain constant within ±1.6%, thus indicating that accuracy on size is high enough for evaporation studies. The method is then tested with R114 freon droplets at an early stage of evaporation. The striking feature is the presence on each hologram of a thermal wake image, aligned with the relative velocity fluctuations ‘seen’ by the droplets (visualization of the Lagrangian fluid motion about the droplet). Its orientation compares rather well with that calculated by using a dynamical equation for describing the droplet motion. A decrease of size due to evaporation is measured for the droplet that remains longest in the turbulence domain.
Highlights
Affected by inertia and so differs from that of fluid particles
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 [11], ultrasonic Doppler tracking [12] and digital holography [13,14,15]
The sizing accuracy along the tracks is found to be of the order of ±10% with a simple sizing algorithm, which is still far from the accuracy of other local techniques such as phase Doppler anemometry (PDA)
Summary
Affected by inertia and so differs from that of fluid particles. But as the droplets evaporate, they become smaller and smaller compared to the relevant scales of the turbulence and, at the limit, can approach the behaviour of passive tracers. These authors tested an in-line single high-speed camera method on droplets dispersing in strong turbulence within a box [19] and compared its accuracy to that of a stereo setup with two high-speed cameras They showed that the in-line method is capable of measuring particle size and trajectories in this type of flow, and that it has certain advantages compared to the stereo method: simplified setup and calibration, need of less laser power, no particle-matching between cameras and a deeper sample volume (even for small numerical aperture experiments). From their results, obtained with a standard hologram reconstruction method, a number of limitations of in-line digital holography can be listed. The sizing accuracy along the tracks is found to be of the order of ±10% with a simple sizing algorithm (pixel-counting), which is still far from the accuracy of other local techniques such as phase Doppler anemometry (PDA)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
