Publications

The two-sensor depth from defocus technique for the measurement of drop sizes in a spray is further developed to achieve higher spatial and temporal resolution, to improve estimates of spatial size distribution and number concentration, and to provide additional guidelines for the calibration and design of the optical system for a specific application. The technique and these improvements are demonstrated using the case of secondary atomization when a shock wave interacts with a single drop. This is an application in which both high spatially and temporally resolved number density and size distributions of secondary droplets generated in the wake of the original drop are necessary. 

Particle size measurement is crucial in various applications, be it sizing droplets in inkjet printing or respiratory events, tracking particulate ejection in hypersonic impacts, or detecting floating target markers in free surface flows. Such systems are characterised by extracting quantitative information like size, position, velocity and number density of the dispersed particles, which is typically non-trivial. The existing methods like phase Doppler or digital holography offer precise estimates at the expense of complicated systems, demanding significant expertise. We present a novel volumetric measurement approach for estimating the size and position of dispersed spherical particles that utilises a unique 'Depth from Defocus' (DFD) technique with a single camera. The calibration free sizing enables in-situ examination of hard to measure systems, including naturally occurring phenomena like pathogenic aerosols, pollen dispersion or raindrops. The efficacy of the technique is demonstrated for diverse sparse dispersions, including dots, glass beads, spray droplets, and pollen grains. The simple optical configuration and semi-autonomous calibration procedure make the method readily deployable and accessible, with a scope of applicability across vast research horizons.

Bubbles have always captivated our curiosity with their aesthetics and complexities alike. While the act of blowing bubbles is familiar to everyone, the underlying physics of these fleeting spheres often eludes reasoning. In this letter, we discuss the dynamics of inflating a soap bubble using controlled airflow through a film-coated nozzle. We assess and predict the rate of inflation by varying the source pressure. Visualising the previously unexplored internal flow reveals that air enters the bubble as a round jet, emerging from the nozzle opening and impinges on the expanding concave bubble interface to form a toroidal vortex. Several scaling laws of the associated vortical flow spanning the entire bubble and the vortex core are reported. The observed dynamics of this bubble-confined vortex ring formation indicate universality in certain aspects when compared to the free laminar vortex rings.

Experiments are performed to investigate the interaction of a vortex ring (Reynolds number based on circulation (Re_gamma = 11500) with perforated surface (open area ratio, phi1 = 0.24 and phi2 = 0.44) with different included angles (theta = 60deg - 180deg ). The phenomenon is characterized using techniques like Planer Laser-Induced Fluorescence (PLIF) imaging and Particle Image Velocimetry (PIV). Lagrangian analysis using finite-time Lyapunov exponents (FTLE) and Gamma_2 vortex identification methods are utilised to understand the flow physics. Early observations reveal the growth of induced mushroom structures through the holes as a consequence of placing the perforated surface in the path of the vortex ring. These structures along with Kelvin-Helmholtz (K-H) instability imparts the initial instability to the emerging jets. We discern a sequential emergence of the vortex ring in the form of jets at lower theta value that diminishes at higher values. Except for theta = 150deg cases, where the flow from the two halves starts to talk resulting in a divergence in the circulation ratio, a reformed vortex ring is formed for all cases in the far downstream. A detailed discussion on the downstream vorticity dynamics has been been provided using vorticity contours, time-series variation of circulation and the FTLE fields. By varying the value of theta, we present a more generalised study of vortex ring interacting with perforated surfaces that finds application in multiple domains including flow control, manipulation and vortical cleaning.

The study comprehensively investigates the response of a combusting droplet during its interaction with a high-speed transient flow that is imposed by a coaxially propagating blast wave. The blast wave is generated using a specially designed unique miniature shock generation apparatus that generates blast waves using the wire-explosion technique which facilitates a wide range of shock Mach number (1.03 < Ms < 1.8). The experiments are performed in two configurations: Open field blast wave and focused blast wave. The charging voltage and the configuration determine the shock Mach number (Ms) and flow characteristics. The flame is found to exhibit two major response patterns: partial extinction followed by re-ignition and full extinction. Simultaneously, the droplet also interacts with the flow imposed by the blast wave exhibiting different modes of response ranging from pure deformation, Rayleigh-Taylor piercing bag breakup, and shear-induced stripping. The KH instability is exhibited along the windward side interface of the droplet during the interaction with the blast wave decay profile which gets aggravated when the induced flow interaction ensues. Increasing the Mach number (Ms > 1.1) makes the droplet flame more vulnerable to extinction. However, the flame exhibits stretching and shedding, followed by re-ignition at lower Mach numbers (Ms < 1.06). In all cases, the flame base lifts off in response to the imposed flow, and the advection of the flame base interacting with the flame tip results in flame extinction. The entire interaction occurs in two stages: 1) interaction with the blast wave and the decaying velocity profile associated with it, and 2) interaction with the induced flow behind the blast wave as a result of the entrainment (delayed response). The criteria for partial and complete extinction of flame have been postulated which is in good agreement with the experiments.

An imaging-based optical method and apparatus for measuring geometric characteristics of particles using a single camera. This technique measure the size and depth information from blurred images of spherical particles.

A two-nozzle vortex ring based device for segregation and mobilisation of entities attached or deposited over a substrate. Application include vortex assisted cleaning.