Session PQ: Particle Laden Flows IV

Direct Visualization of Rapid Convective Deposition of Microsphere Monolayers

Micron-sized microspheres were deposited into thin films via rapid convective deposition using a similar method to that studied by Prevo and Velev, Langmuir, 2003. By varying deposition rate and blade angle, the optimal operating ranges in which 2D close-packed arrays of microspheres existed were obtained. Previous models do not consider the effect of blade angle and blade surface energy on the deposition rate. Using a confocal laser scanning microscope, dynamic self-assembly of colloidal particles under capillary force during solvent evaporation was revealed. The resulting microstructure controlled by varying the macroscale parameters and interaction between substrate and colloidal particles played an important role in formation of ordered crystalline arrays. These interactions were explored through a model comparing the residence time of a particle in the thin film and the characteristic time of capillary-driven crystallization to describe the morphology and microstructure of deposited particles.   

Lagrangian statistics of inertial particles in turbulent flow

Being able to accurately model and predict the dynamics of dispersed inclusions transported by a turbulent flow, remains a challenge with important scientific, environmental and economical stakes. One critical and difficult point is to correctly describe the turbulent dynamics of particles over a wide range of sizes and densities. We present high resolution acoustical Lagrangian measurements of inertial particles transported in a grid generated turbulent flow. The size of the particles and their density have been systematically varied. Our measurements show that Lagrangian statistics of the dispersed particles do exhibit non tirvial, and so far unpredicted, size and density effects. This has important consequences in terms of modelling of the turbulent transport of dispersed inclusions.   

Transport of Finite-Sized Particles in Chaotic Flow

By extending traditional particle tracking techniques, we study the dynamics of neutrally buoyant finite-sized particles in a quasi-2D spatiotemporally chaotic flow. We simultaneously measure the flow field and the trajectories of millimeter-scale particles so that the two can be directly compared. While the single-point statistics of the particles are indistinguishable from the flow statistics, the particles often move in directions that are systematically different from the underlying flow. These differences are especially evident when Lagrangian statistics are considered.   

Towards quantifying the collision kernel of inertial particles in homogeneous isotropic turbulence

We present digital holographic particle image velocimetry (DHPIV) measurements of the radial distribution function (RDF) and the relative velocity probability density function (PDF) of inertial particle pairs in homogeneous isotropic turbulence generated by fans in an enclosed box. The RDF and relative velocity PDF are the essential statistical inputs to the particle-pair collision kernel (Sundaram \& Collins 1997). The measurements are compared to direct numerical simulations (DNS) at a similar Reynolds number. Results show qualitative agreement of the relative velocity PDF from experiments and DNS. Measurements of the temporal development of the RDF demonstrate the existence of an extended quasi-steady-state regime, over which comparisons of the measured two-particle statistics to DNS can be made, justifying a previous RDF comparison by Salazar et al (2008). Further considerations of finite-volume effects on the RDF are considered.   

Particle suspension by a vortex ring convecting parallel to a planar surface

Experiments to qualitatively characterize the flow disturbance due to a vortex ring at the surface of a plane horizontal solid wall are described. The ultimate goal of this study is to characterize the role of turbulent boundary layer coherent structures in suspending particles into the flow. Experiments were performed by introducing vortex rings into a stationary fluid. The motion of the rings, tagged with dye, was recorded with digital video. The current study focuses on the effect of the planar wall on the vortex ring trajectory and topology, as well as the velocity and pressure disturbances induced at the wall by the ring. These observations are reported for a range of vortex ring strengths and distances between the vortex ring and the wall.   

Influence of gravity on inertial particle clustering in turbulence

We report results from experiments aimed at studying inertial particles in homogeneous, isotropic turbulence, under the influence of gravitational settling. Conditions are selected to investigate the transition from negligible role of gravity to gravitationally dominated, as is expected to occur in atmospheric clouds. We measure droplet clustering, relative velocities, and the distribution of collision angles in this range. The experiments are carried out in a laboratory chamber with nearly homogeneous, isotropic turbulence. The turbulence is characterized using LDV and 2-frame holographic particle tracking velocimetry. We seed the flow with particles of various Stokes and Froude numbers and use digital holography to obtain 3D particle positions and velocities. From particle positions, we investigate the impact of gravity on inertial clustering through the calculation of the radial distribution function and we compare to computational results and other recent experiments.