## Research overview:

My research focuses on understanding the physical processes in industrial and natural flows using advanced numerical simulations (e.g. direct numerical simulations or particle resolved simulations) and analytical tools. My research interests are in laminar and turbulent flows that are complex due to the presence of either density stratification or solid particles.

*Particle-laden flows:*

Many natural and industrial flows are laden with solid particles. The applications may range from dense suspensions to turbulent flows carrying particles significantly smaller than the Kolmogorov length scale of the flow. We have studied several applications of such flows in the past.

*Dewatering of suspensions: *We are currently investigating the dewatering of rigid and flexible particle suspensions through compression using particle resolved numerical simulations. The results of this study will help us understand better the constitutive laws used in the theoretical studies of the same processes.

Collaborators: Mark Martinez (UBC), Luca Brandt (KTH), Arash Alizad-Banaei (KTH)

*Settling of polydisperse and non-spherical particles in quiescent and turbulent flows: *While the particle settling problem is a well-studied field of research, the effects of particle polydispersity, non-sphericity and interactions with turbulence are still not well understood. Each of these questions however pose a challenge for the transport of particle and fluid mixtures in industrial settings. We are using numerical simulations to gain a better insight into each of these effects.

Settling of non-spherical and bidisperse particles in a quiescent flow

Collaborators: Anthony Wachs (UBC), Mark Martinez (UBC), Nicholas McIntosh (UBC), Cornelia Hoehr (TRIUMF)

*Rheological properties of suspensions under shear: *In this study, we examine the effective viscosity and diffusion coefficient of a suspension of spherical particles subject to a constant shear rate. Particles with higher inertia, that represent many real-world examples, exhibit a distinctively higher effective viscosity and diffusion coefficients.

Microstructure of shear stresses in the suspension of spherical particles (Rahmani et al. 2018, PoF)

*Collaborators:* Anthony Wachs (UBC), Abdelkader Hammouti (IFP Energies nouvelles)

*Heat transfer in particle-laden turbulent flows:*

*Solid particle solar receivers: *The solar energy can efficiently be harvested by concentrating the solar radiation on a mixture of gas and fine solid particles with high absorptivity and thereby extracting energy from the heated mixture. However, a profound understanding of the couplings between the particle concentration, the turbulent flow and the heat transfer from particles to the gas phase is required prior to designing the solid particle solar receivers at large scales. Here, we examine the effects of particle polydispersity on the thermal performance of the system.

Radiative heat transfer in turbulent flows laden with polydisperse particles (Rahmani et al. 2018, IJMF)

*Collaborators*: Ali Mani (Stanford), Gianluca Geraci (Stanford), Gianluca Iaccarino (Stanford)

**Density-stratified flows:**

An important mechanism for mixing in oceanic and atmospheric flows and in sheared mixing layers is the development and turbulent breakdown of shear instabilities, or Kelvin-Helmholtz instabilities, the more commonly observed type of shear instability. We have been interested in investigating mixing properties in density-stratified shear flows.

*Collaborators:* Greg Lawrence (UBC), Brian Seymour (UBC), Anirban Guha (IIT, Kanpur), Ted Tedford (UBC), Wenjing Dong (NYU)

*Mixing transition: *As the Reynolds number increases, the mixing behaviour transitions from diffusion-limited to entrainment-limited. At sufficiently high Reynolds numbers the amount of mixing induced by shear instabilities should reach a plateau. However this trend can be altered by the pairing of two adjacent vortices and its significant implications for entrainment and mixing.

Mixing transition in turbulent Kelvin-Helmholtz billows.

(Rahmani et al. 2014, JFM)

*Prandtl number effects: *The Prandtl number, the ratio of kinematic viscosity to molecular diffusivity of the stratifying agent (typically salt or heat), can vary over three decades in natural and industrial flows. High Prandtl numbers result in very sharp density interfaces and pose an extreme challenge for numerical simulations. In high Prandtl number flows mixing occurs at a slower rate, but the flow remains energetic for a longer period of time. We are interested in understanding how these two factors interplay to determine the amount and efficiency of mixing in various environmental and industrial flows.

The effect of Prandtl number on mixing at different stages of the KH billow life cycle.

(Rahmani et al. 2016, PoF)