Inertial particles in a turbulent/turbulent interface

Turbulent interfaces are ubiquitous in the nature and in many important canonical flows (wakes, jets, mixing layers, boundary layers). In nature, the sharp edges of atmospheric clouds are defined by a turbulent/nonturbulent interface, where the droplet-laden turbulent cloud interior mixes with the unladen outer air. This study presents statistics of sub-Kolmogorov-scale inertial particles, in a parameter range representative of cloud droplets, in a sheared turbulent-turbulent interface. The study focuses on the effect of the inhomogeneous turbulent field on the particles' clustering properties and settling velocity modification. Wind tunnel experiments were carried out in a well-characterised facility where a grid containing 81 independent gas/liquid injectors generates both high intensity turbulence and a field of small inertial droplets. These atomisers, located at the entrance of the test section, contribute significantly to the carrier phase turbulence and are used to create the sheared turbulent/turbulent interface. Selecting which atomisers are on or off enables us to control the mean velocity profile and the gradient of turbulent intensity through the tunnel cross-section. The resulting carrier flow is characterised by a gradient of mean velocity, turbulent velocity rms, and a sharp interface separating two regions with different turbulent scales. Particles' velocities and diameters were measured with a Phase Doppler Particle Analyser, at closely-spaced positions across the turbulent interface. In agreement with previous experimental studies on the topic, particles are shown to be transported from the turbulent side, with higher turbulence intensity, to the low-intensity side by large-scale-energetic eddies. Newly observed in our results is an enhancement of particle preferential concentration and settling velocity in the sheared interface region, caused by the interaction between particle trajectories and turbulent structures of different sizes.