Updated November 2024
General open positions for MSc, PhD students and Postdocs
At the Environmental Multi-Phase Flow Laboratory we perform mainly experimental research in the broad field of fluid dynamics and heat transfer. Curious students that have a strong affinity to experiments are invited to send their applications.
Specific fields of interest are:
1. Particle-Flow interactions
Particle-Flow interactions can be found in many industrial and environmental processes such as erosion and desertification processes, air conditioning systems, spray-based drug delivery, sedimentation/resuspension processes in waterways and oceans.
New (November 2024): Open positions PostDocs, PhD and MSc
Inertial effects on dispersion of polydisperse beads in a turbulent coaxial jet in the two-way coupling regime
Description of the research: Jet flows can be found all around us both in industrial applications (e.g. jet propulsion, impinging jet cooling), human health (e.g. drug delivery devices, virus spread through coughing), and environmental settings (e.g. volcano eruptions). In many cases, these jets are laden with particles having different sizes whose dynamics govern the downstream development and spatial distribution of these two-phase jets. Although particle-laden jets have been studied extensively during the last decades, still little is known about polydisperse particle-laden jets in the two-way coupling regime. While numerical studies struggle with high computational demands in resolving the relevant spatial and temporal scales, experiments are scarce since spatially and temporally resolved measurements are equally hard to obtain. However, experimental “ground truth” results are essential to improve the modeling of the unresolved scales in the numerical simulations.
This research will leverage combined inline digital holographic cinematography with high-speed stereoscopic particle image velocimetry (stereo-PIV) to resolve the spatio-temporal distributions of both the beads and the surrounding flow field. The focus of the research will be on polydisperse spherical beads in the two-way coupling regime for which hardly any data exists. Furthermore, it is here proposed to use a coaxial jet configuration that has the advantage of being able to control the near-field vortex structure by changing the velocity ratio, between outer and inner jet. By changing the velocity ratio, the beads’ Stokes numbers are changed, thereby controlling their inertial response. In polydisperse particle-laden coaxial jets, this may hold the promise of possible inertia based, bead segregation.
Combined stereoscopic PIV and inline holography
The proposed research will lead to a better, basic understanding of inertial effects on polydisperse particle-laden, turbulent coaxial jets in the two-way coupling regime which is crucial to further improve the current modeling of these flows. Furthermore, as a result of the basic nature of this investigation, this research is expected to have a broad impact on turbulent, polydisperse particle-laden flows in general.
Funding: This research is funded by the Israel Science Foundation under grant no. 2944/24
New (November 2024): Open positions PhD and MSc
Fiber dispersion in coaxial jets
Description of the research: The study of fluid-solid multiphase flows, characterized by the suspension of small solid particles within turbulent carrier fluids, is of significant relevance in both industrial and environmental contexts. Such flows are commonly encountered in processes such as papermaking, fuel combustion, and cyclonic separations, as well as in natural phenomena like sandstorms and the dispersion of soot particles, to name only a few. The inherent complexity of these dispersed two-phase flows arises from the intricate interactions between the suspended particles and the surrounding fluid, making them substantially more complicated to understand and predict than single-phase flows.
Over the past decades, fluid-solid multiphase flows have been extensively studied through experimental and numerical methods. However, much of this research has been limited to flows involving spherical particles. In addition, mostly “idealized” flow conditions, such as homogeneous isotropic turbulence (HIT) or fully developed channel flows, have been investigated, whereas particle behaviour in spatially developing flows has hardly been studied. However, in real-world applications, flows are often spatially evolving and particle shapes deviate from perfect spheres, with irregular and non-spherical shapes being common. This introduces additional complexity, as the interactions between non-spherical particles and turbulent flows are highly dependent on particle shape and orientation. Understanding these interactions is crucial for improving predictions and optimizing industrial processes involving fluid-solid multiphase flows.
Despite the growing recognition of the importance of non-spherical particles in flows, there is a relatively limited body of literature addressing their dynamics in realistic flow conditions. In particular, fibre-like particles represent a significant challenge due to their elongated shape and orientation-dependent interactions with the fluid. This research proposal aims to address this gap by focusing on the behaviour of fibre-like particles in a spatially developing turbulent flow, i.e., a coaxial jet flow, specifically investigating the effects of fibre inertia and length on the dispersion of fibres in coaxial jet flows.
Stereo-PIV and holography setups
The aim of the measurements is to obtain high spatial and temporal resolution information on the instantaneous coupling between the fibre motion and the flow field. This requirement calls for full field measurements that can be achieved by combined particle image velocimetry for the flow field, and particle tracking for the fibres. We will use high spatial and temporal resolution stereoscopic PIV (stereo-PIV) measurements that enable to track the instantaneous 3D flow field as well as the fibre’s translational and rotational motion in a plane combined with high-speed, two-view digital inline holography that enables to track the 3D motion of the fibres.
2. Heat Transfer: Impinging jet cooling
Unsteady aero-thermal physics of coaxial synthetic jet impingement cooling
As cooling needs of electronics are soaring, there is a demand for removal of high thermal loads that cannot be handled by passive cooling techniques and require active thermal cooling to ensure the electronics perform as designed. Impinging jet cooling has been widely used due to its high local heat transfer coefficients. However, a disadvantage is that it requires a fan or a pressurized air source which makes the system bulky and noisy. A novel technique that has been investigated for the last decade is the use of zero-net-mass flux impinging jets (termed “synthetic jets” (SJs)). Synthetic jets are compact and do not require outside air supply and have a low power demand. The use of SJs was first investigated for flow control but has recently found an application as a ground breaking technology in impinging synthetic jet cooling. In contrast to single SJs, coaxial synthetic jet (CSJ) configurations have hardly been investigated and only recently sparked the interest of few researchers. Flow field measurements by state-of-the-art, phase-locked (stereoscopic) particle image velocimetry combined with high-speed IR measurements of temperature distributions will provide detailed quantitative information on heat transfer efficiency for different operating conditions of the CSJs as well as elucidate the underlying unsteady physical phenomena leading to heat transfer enhancement.
Schematic layout of the CSJ facility, stereo-PIV setup and IR-camera
Funding: This research is funded by the Pazy Foundation
New (November 2024): Open positions PhD and MSc
3. Liquid jet atomization
Gas-assisted atomization (hereafter termed “gas atomization”) of a liquid jet into a spray of small droplets is commonly used in many industrial applications ranging from combustion processes, agriculture, medical therapies, and powder fabrication, among others. The breakup of the liquid jet is the result of a sequence of instabilities. First, shear-induced Kelvin-Helmholtz (KH) instabilities give rise to liquid undulations, and once their crests destabilize through Raleigh-type instabilities, they are stretched into ligaments by the fast gas flow and fragment into small droplets under the influence of capillary forces and gas Reynolds stresses. The final spray properties that are of interest include the droplet size and velocity distributions, as well as the number of droplets per unit volume and their spatial distribution. The required droplet size produced by an atomization process is dictated by the application and should ideally be small enough and have a relatively narrow size distribution. Spray production involves three main steps, (i) liquid jet deformation, (ii) primary breakup, and (iii) secondary breakup. Most experiments have focused on the spray cone angle, the breakup length, the instability wave lengths, and the mean droplet diameters, whereas primary breakup is experimentally least documented. As a result, there is a lack of understanding of primary breakup mechanisms for gas atomization nozzles, and their current design is based on experience rather than physical understanding.
Do you want to play a role in the sustainable energy transition and manufacturing by developing innovative technologies for metal powder production? The production of metal powders for powder metallurgy and additive manufacturing requires high sphericity, uniform particle distribution, and optimal size characteristics. Molten metal atomization is the key process that transforms molten material into fine, spherical particles using high-velocity gas streams. This project aims to develop an experimentally validated CFD model for this process, advancing the understanding of the physical and chemical mechanisms involved in close-coupled gas atomization (CCGA). The overall objective of this study is the development of the experimentally validated CFD model for molten metal atomization.
Digital inline holography experiments on close-coupled gas atomization (CCGA) will be performed on atomization experiments of glycerol/water mixtures (change in viscosity) and solutions with surfactants (change in surface tension). These liquids with different physical properties are used to mimic the typical properties of molten metal. The experiments will provide critical data on velocity and droplet size distributions, as well as the number of droplets per unit volume and their spatial distribution in the spray.
Example hologram reconstruction of liquid jet atomization
4. Visualization of particle-laden flows using optically active particles (November 2024)
In the last two decades, with the advent of tomographic particle image velocimetry (tomo-PIV), there has been much progress in performing three-dimensional (3D) flow field measurements, and commercially available systems are now widely used. To optimally register the scattered light intensities and utilize the higher intensities of the “forward” Mie scattered light, cameras may be positioned at small angles between the incoming light beam and the camera. However, an in-line configuration of camera and illuminating beam (to fully utilize the strong forward light scattering) is impossible since the cameras’ sensors would saturate by the background illumination. The disadvantage of using side scattering is that expensive, fraught with safety issues high-power lasers need to be used to make tiny particles (small enough to follow changes in the flow) detectable. Forward light scattering is not used to its full potential by the commonly used quantitative 3D flow measurement techniques.
The goal of the present research is to develop the technique of registering the light transmitted by optically active particles (OAPs) and show its potential in the study of particle-laden flows as well as 3D flow measurements. The imaging technique based on OAPs relies on their ability to alter the polarization of the light they transmit while unaltered background light is filtered out by a setup of polarizers. The feasibility of this optical approach has been validated for optically active nylon fibers suspended in a turbulent channel flow . The measured translational velocities of the fibers and their in-plane rotations and rotation rates were in excellent agreement with independent digital inline holography measurements. In the present study, we plan to further extend this approach to the imaging of tracer particles. We illustrate the methodology by imaging optically active cellulose microcrystalline particles, CMPs, (diameter 20$ microns, specific gravity 1.5) used as tracers in a turbulent water channel flow. The extension of this approach to three-dimensional flow field measurements of particle-laden turbulent flows is pursued.
Experimental setup and results of optically active particles