Speakers

Invited speakers:

Mona Rahmani (University of British Columbia, Canada) - Webpage

Effects of gravitational settling on aggregation of microparticles in turbulent flows

Aggregation of microparticles is an important mechanism for the vertical transport of microplastics, mineral particles, and organic matter in natural aqueous environments. In particular, aggregation of particles of different sizes and densities changes the effective settling velocity of the original constituent particles, contributing to large fluxes of microplastics, diatoms, and carbon in the oceans. In these flow settings, particles have low inertia but relatively high settling velocities, making the interplay between gravitational effects and turbulence highly nonlinear. In this talk, we will focus on how the combination of turbulent shear and differential settling can contribute to geometric collision of microparticles in aquatic environments. We use direct numerical simulations of homogeneous and isotropic turbulence (HIT) coupled to the Lagrangian tracking of point particles. Binary collisions between pair particles are detected and upon collision particles merge into an aggregate with a statistical probability that represents their efficiency of aggregation. Our results reveal that for monodispersed particles, gravitational settling reduces the collision rate as the settling particles have shorter residence times within an eddy. Conversely, differential settling markedly enhances the collision rate for bidisperse particles with differing densities. Finally, we will discuss some aspects of the effects of aggregation efficiency or “stickiness” and the growth of biofilm on microplastics on their total aggregation rate.

Marco Edoardo Rosti (Okinawa Institute of Technology, Japan) - webpage

Hydrodynamics of flexible aquatic plants

Submerged vegetation plays a crucial role in aquatic ecosystems by promoting sediment retention, enhancing the transport of suspended species, and attenuating incoming waves. Flexible stems, which bend and sway with water currents, exhibit complex dynamics that, in turn, influence fluid motion. Using fully resolved and coupled numerical simulations with our in-house solver Fujin, we first examine the response of a single flexible stem to a monochromatic surface wave and to broadband turbulent flow. We then extend our analysis to flexible canopies — dense arrays of slender stems anchored to a substrate — exploring both their collective behaviour, such as monami waves, and the individual motion of stems and their interaction with the incoming flow. Our findings provide new insights into the hydrodynamics of submerged vegetation, shedding light on key natural processes while also laying the groundwork for innovative engineering applications.

Alfredo Soldati (TU Wien, Austria)

Martin Sommerfeld (Otto von Guericke University, Germany) - webpage

Strategies for Modelling Non-Spherical Particle Transport in the frame of an Euler/Lagrange Approach

The importance of numerical calculations for supporting optimisation and lay-out of industrial processes involving dispersed multiphase flows is continuously increasing. In predicting dispersed gas-solid flows the general assumption made is that the particles are spherical. However, in most practical situations the particle shape deviates from spherical, either being irregular in shape or having a certain geometry, such as granulates or fibres. In such cases the fluid dynamic transport characteristics (i.e., forces acting on the particles) differ from that of spherical particles and using correlations derived for spheres may yield incorrect results. Additionally, in confined particle-laden flows wall collisions remarkably govern the particle transport. There are in principle three modelling strategies to consider non-spherical particles listed below with increasing modelling effort: (i) The simplest approach is using only averaged drag coefficients which are depending on the particle Reynolds number and the sphericity of the particle considering a fixed orientation with respect to the relative flow; (ii) The second approach applies for irregular shaped particles with sphericities of larger than about 0.7, where the instantaneous forces and moments on the particles are randomly generated from previously generated PDFs obtained by particle-resolved simulations; (iii) The most rigorous approach is valid for regular shaped particles with a defined major axis, like fibres and ellipsoids. Here the particles are tracked by additionally solving for the orientation, requiring resistance coefficients based on orientation and Reynolds number previously generated by particle-resolved simulations. These approaches will be validated through experimental studies obtained for different experimental flow configurations.

Gautier Verhille (IRPHE Marseille, France) - Webpage

Trapping of flexible discs by a vortex

In turbulent flow, particle clustering depends on how particles are either expelled or trapped by vortices. A first step in investigating the influence of flexibility on particle transport in turbulent flows is understanding their dynamics in vortical flows. In my presentation, I will present our latest results on the dynamics of flexible discs, which are heavier than the fluid, in the vicinity of a vortex.

Anthony Wachs (University of British Columbia, Canada) - webpage

On the dynamics of flows laden with angular rigid bodies

We discuss recent results on the dynamics of a single or multiple angular rigid bodies of aspect ratio 1 immersed in the flow of a Newtonian fluid. We analyze various data sets generated by particle resolved simulations that pertained to four specific flow configurations: (i) the flow past a single stationary rigid body, (ii) a single rigid body settling in an otherwise quiescent fluid, (iii) the transverse hydrodynamic force and torque exerted on a transversely rotating and moving rigid body, and (iv) multiple cubic rigid bodies settling in an otherwise quiescent fluid. In (i)-(iii), we use octree adaptive mesh refinement to properly capture the vorticity generation in the boundary layer around the angular rigid body that is key to the wake dynamics. In all cases, the Reynolds number computed with the rigid body characteristic length lies in the interval [10,400] where inertia is dominant. We examine these flows in terms of hydrodynamic force and torque, wake structure and suspension microstructure. Our work contributes to clarify the role of particle angularity in rigid particle-laden flows.