PHD POSITION: INERTIAL PARTICLE DYNAMICS IN THE TURBULENT / NON-TURBULENT INTERFACE

In spite of its apparent simplicity, the physics of finite size spheres advected by a fluid hides a whole hierarchy of rich imbricated phenomena, some of which are still shrouded in mystery. This is for instance the case when a particle is in a turbulent environment andor when hydrodynamic couplings emerge between many particles, resulting in subtle collective behaviours. Unveiling the fundamental mechanisms of such phenomena remains crucial to improve our capacity to accurately model and predict particle laden flows. These flows are present in many different processes such as coalescence (rain formation), agglomeration (particles in the atmosphere), settling velocity (sedimentation), chemical transformations (risers), among many others. All such flow configurations present many open fundamental questions and are subject of great interest in current research. Indeed, because of their higher density compared to the carrier fluid, inertial particles interacting with the underlying turbulence tend to form high (clusters) and low (voids) concentration regions. These regions span almost the whole range of turbulence scales, i.e. from the Kolmogorov length scale up to the integral length scale or even larger for superclusters, and the widest regions exhibit selfsimilarity. Such non-trivial spatial organization of particles, a phenomenon referred to as preferential concentration, has strong consequences on the flow dynamics by way of momentum exchanges between phases, of velocity fluctuations generation or of collective dynamics, as well as on particle transport and dispersion. Ultimately, these features control the efficiency of all the applications mentioned above.

This experimental project will study the dynamics of inertial droplets in a turbulent/non-turbulent interface, with and without the presence of shear, and with different orientations with respect to gravity. To conduct this research, we will employ unique facilities and measurement techniques, namely, two wind tunnels equipped with turbulence-producing systems that can be activated differentially to generate a turbulent/nonturbulent interface. With the two experimental setups, this collaboration will be able to cover a wide range of turbulent intensity gradients, shear rates and Reynolds numbers for the study of inertial particle dynamics in Turbulent/non-Turbulent conditions. The joint study will produce data on different droplet sizes that span the range of Stokes numbers, which characterize the inertia of the particles compared to the turbulence microscale, that are of interest in application areas such as fuel injection in energy conversion systems, industrial spray coating, warm rain formation in clouds and sea spray from breaking waves in the surf zone.

Different advances are expected:

• On a first stage, measuring campaigns in UGA-LEGI (Grenoble, France) and in Univ. Washington (Seattle, USA) facilities are envisioned. Measuring techniques such as Phase Doppler interferometry, Particle Image Velocimetry, Phase detection optical probes are already mastered as well as post-processing routines to access drop size, velocity, concentration and combined statistics as done in our former contributions. All the equipment needed is available. The PhD student will spend the first 18 months in Grenoble while the rest of the time he/she will be in Seattle.
• The PhD student will explore how to generate a turbulent/non-turbulent interface in both wind tunnels. This can be achieved by means of an active grid in LEGI or by changing the injection system in University of Washington.
• Finally, a systematic study of clustering of inertial particles on the turbulent/non-turbulent interface  will be performed: the shape and orientation of clusters on such flow, the role of gravity and entrainment, etc…

The expected outputs are the following: (i) to produce insights into the behavior of particle-laden flows in the presence of a Turbulent/non-Turbulent interface, (ii) quantify the properties of clustering on the parameter space (that includes the turbulence parameters, the Stokes number, the orientation of gravity) and (iii) produce some simple models to quantify such phenomenon.