Session G45: Particle-Laden Flows: Particle-Turbulence Interactions I

Jamming of growing hydrogel particles in highly turbulent Taylor-Couette flow

To investigate dense suspensions and jamming in turbulent flow, we add superabsorbent hydrogel particles into a turbulent Taylor-Couette flow (Reynolds numbers O(Re) = 10^6). Starting from a small volume fraction of 1.3% the particles slowly grow, altering the volume fraction until a volume fraction of approximately 64% is finally reached (50 times their original volume). We monitor the size of the particles using high speed imaging while simultaneously measuring the torque on the inner cylinder. Comparing this torque to the response of the system for a single-phase fluid we calculate an effective viscosity of the turbulent suspension. The effective viscosity is found to increase when the volume fraction increases. Finally, at high volume fractions, the axial mixing of the system is inhibited, and the system exhibits jamming. Remarkably, the increase of the effective viscosity is non-monotonic in time, which we attribute to shear-banding in the jammed state.   

Experimental investigation of turbulence modification by inertial particles in a horizontal turbulent pipe flow

Particle-laden pipe flow is widely encountered in various industries where efficient and reliable prediction on the effect of particles on turbulence is required. Recently, numerical simulations have outpaced experimental investigations with little cross-validation, and most experiments are in vertical pipes to minimise the gravity effect. As such, relatively little is known about the changes to the dynamics of turbulence owing to inertial particles under the influence of gravity. In this study, the effects of volume fraction, Stokes number and solid-fluid density ratio on turbulent pipe flow are experimentally investigated. The experiments were conducted in a smooth-wall horizontal pipe with diameter D=20.5mm, using two inertial particles with diameters Dp=250μm and 437μm in the range of friction Stokes number St=1.09-3.87 at volume fractions (0-1%), Reτ≈195, and at particle-fluid density ratios of 1 (not gravity effect) and 1.05 (influenced by gravity). Particle image velocimetry technique is employed to record the two-phase behaviours and investigate the intricate turbulent modification. Results reveal that the mean streamwise velocity, fluctuating intensities and Reynolds shear stress decrease more at higher Stokes number and volume fraction than in a neutrally-buoyant scenario. The presence of inertial particles induces an apparent turbulent attenuation, which is also verified by the pre-multiplied energy spectra of streamwise fluctuating velocity and instantaneous vertical fluctuating velocity field. Due to the deposition of solid phase at density ratio 1.05, the bottom-half of pipe displays a more significant turbulent attenuation than the top-half.

  

Electrostatic effects on deposition pattern and kinematic statistics of inertial particles in turbulent wall-bounded flow

Particles in wall-bounded turbulence is a canonical problem that captures the essential physics in many applications, such as spacecraft landings, dust storms, and sand deposition in jet engines. These phenomena involve particle-turbulence interactions that are further complicated through long range electrostatic interactions between particles and the wall. This could modify the mechanisms leading to particle deposition which become of crucial concern for many applications. To better understand this problem, experiments were conducted to study the near-wall motion of charged particles in a turbulent boundary layer. In addition, the deposition pattern and its evolution in time with Lagrangian particle trajectories were obtained to elucidate what leads the particles towards deposition or collision. The charge of the particles is systematically changed using an ionization wire. These experimental results will provide valuable data that will lead to a better understanding of particle-turbulence interactions and how electrostatics influences those interactions.   

Inertial particle dynamics in a turbulent/non-turbulent interface

Turbulent/non-turbulent interfaces are ubiquitous in nature and represent the physics of many important canonical flows (wakes, jets, mixing layers). Two separate flow regions with different turbulent intensities share a sharp interface, where we study the dynamics of inertial particles. The transport of inertial particles across T/NT interfaces plays a role in the description of cloud boundaries, where the droplet-laden turbulent cloud interior mixes with the unladen laminar outer flow. This experimental study focuses on the entrainment of inertial particles across a turbulent/non-turbulent interface.

Spanwise profiles of particle vertical and horizontal velocities were measured by laser interferometry in a wind tunnel. Turbulence is generated by a passive grid combined with gas/liquid atomizers that introduce the inertial particles. These atomizers, at the entrance of the test section, contribute significantly to the carrier phase turbulence and sheared velocity profile. Selecting the atomizer injection profile enables us to control the gradient of turbulent intensity through the tunnel cross-sectional area. Particle velocity distributions are not Gaussian across the interface, as opposed to on the turbulent side where the statistics are canonically normal. Conditional statistics on particle velocities confirm that the transport of droplets from the turbulent to the non-turbulent side is dominated by the integral-scale turbulent eddies.   

Clustering of charged particles in a vertical turbulence channel

Particles in many familiar multiphase flows, including desert sand in atmospheric flows, biomass particles in fluidized bed reactors, planetary dust in spacecraft landings, and ash in volcanic eruptions, naturally accumulate large electrostatic charges either triboelectrically or through exposure to an external ionization field. To investigate how particle charges modulate particle-turbulence interactions, experiments in a unique vertical turbulence channel equipped with modifications to study electrostatics were conducted. Of particular interest is how electrostatic interactions alter particle clustering and streaking dynamics in the turbulent boundary layer. To study this problem, two high-speed cameras were used to capture particle trajectories in the wall-normal and wall-parallel planes, which allows particle clustering dynamics to be directly correlated to particle-wall interactions. After individual trajectories are captured using a Lagrangian particle tracking algorithm, clusters are identified using a Voronoi tessellation analysis and tracked in time. By measuring variations in both pair-wise clustering statistics and the dynamics of the identified clusters, the critical range of the electrostatic Stokes number where electrostatic interactions are comparable to particle-turbulence interactions can be identified. These findings help unveil the role of electrostatic forces on particle clustering and deposition dynamics in turbulent boundary layers.