Alec Petersen defends PhD thesis: Laboratory Investigation of Disperse Multiphase-Turbulent Flows

Ph.D candidate Alec Petersen successfully defended his Ph.D in Aerospace Engineering and Mechanics on May 15th, 2020. He is advised by Filippo Coletti of the St. Anthony Falls Laboratory and Department of Aerospace Engineering and Mechanics at the University of Minnesota. 

Congratulations Dr. Petersen!

Laboratory Investigation of Disperse Multiphase-Turbulent Flows:  Dilute & Dense Distributions of Inertial Particles Settling in Air
Alec Petersen, PhD Candidate in Aerospace Engineering and Mechanics
Advisor: Filippo Coletti, Department of Aerospace Engineering and Mechanics

Turbulent multiphase flows are found throughout our universe, all over Earth and in many man-made systems. Despite surrounding us, their dynamics are still in many ways obscure and require further study. One of the most basic questions is how fast particles settle under gravity but even that can only be predicted for the most simplified case: an isolated particle falling through still air. Multiphase flows are almost always more complicated, and in this thesis, we present our experimental investigation of two particle-laden turbulent flows in air. 

We first focus on the dynamics of dilute distributions of inertial particles falling through turbulent air using particle image velocimetry (PIV) and particle tracking velocimetry (PTV) to simultaneously measure the air and particle velocities.  We find that the settling velocity can be several times larger than the still-air terminal velocity, due to fluid fluctuations preferentially sweeping the particles downward. This preferential sweeping behavior significantly influences the formation of dense particles clusters, which themselves fall faster than isolated particles. Our simultaneous measurements also allow us to calculate particle-fluid slip velocities. These show large variance, and both the instantaneous particle Reynolds number and drag coefficient can greatly differ from their nominal values with implications for how drag is modeled in simulations.

We then consider the case of denser suspensions of inertial particles falling into initially still air, forming particle-laden plumes driven by the downward buoyancy of the particles. Characterizing particle dispersion from these plumes requires understanding relatively basic measures, like the spread rate and settling speed. These in turn depend on the air-entrainment into the plume. We conduct two experiments to measure these quantities: one to capture the particle-phase behavior and another to measure the ambient air velocity. We show that our particle-laden plumes are self-similar, and that the entrainment rate is stable throughout the plume height, allowing us to formulate a 1-D multiphase plume model. Using the measured entrainment rates, the predictions from our model show good agreement with the particle-phase properties of our experiments.