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The Department of Mechanical, Materials, and Aerospace Engineering presents the 2024–2025 seminar series featuring Cameron Tropea, a Henry Ford Visiting Professor at Indian Institute of Technology Madras, who will present “Quantitative Hydrodynamics from Trajectory Imaging.” This seminar will take place on Wednesday, November 20, 2024, from 12:45–1:45 p.m. in room 104 of the Rettaliata Engineering Center.

Abstract 

In this study, we experimentally examine the behavior of a free-falling rigid sphere impacting normally onto, and penetrating into a quiescent liquid pool. Parameters, which are varied, include the impact velocity, the density, and the diameter of the sphere. Observations of the sphere trajectory in time are made using two orthogonally placed high-speed cameras, yielding the velocity and acceleration vector through repeated differentiation of the time resolved trajectories. 

The novelty of this study is twofold. On the one hand, a methodology is introduced by which the instantaneous forces acting on the sphere can be derived by tracking the sphere trajectory. To do this, we work in a natural coordinate system aligned with the path line of the sphere. In particular, the instantaneous lift and drag forces can be separately estimated. This methodology applies to any free body motion. 

On the other hand, using this approach, the forces acting on the sphere can be quantitatively evaluated over the investigated parameter range. These results reveal that when decelerating, the sphere experiences a very high drag force compared with steady flow. This is attributed to an upstream shift of the mean boundary-layer separation. In this context an instability is identified, whereby the instantaneous drag coefficient is positively correlated with the rate of deceleration. On the other hand, the sphere also experiences significant lift force fluctuations, attributed to unsteady and asymmetric wake fluctuations. Moreover, the trajectories can be reduced to three stages, common for all impact Reynolds numbers and density ratios when expressed in dimensionless form. In addition, the sphere trajectory, velocity, and deceleration for different impact parameters exhibit an extremely high degree of uniformity when cast in dimensionless form. This offers valuable predictions of how far a sphere penetrates in time and the forces acting on it. 

Biography

Cameron Tropea graduated from the University of Toronto in engineering sciences, followed by a master’s degree in mechanical engineering (1977). He completed his Ph.D. in Civil Engineering at the Technical University of Karlsruhe (1982) and Ph.D. in Habilitation in Fluid Mechanics at the University of Erlangen-Nürnberg (1991) where he was appointed as professor of fluid mechanics until 1997. This was followed by an appointment as head of the Institute of Fluid Mechanics and Aerodynamics at the Technische Universität Darmstadt. Since 2003 he has been editor-in-chief of the Springer Nature journal Experiments in Fluids and past director of the Center of Smart Interfaces (CSI) from 2007–2014. His research interests include optical measurement techniques in fluid mechanics, interfacial transport and wetting phenomena, atomization and spray processes, and unsteady aerodynamics. He was a member of the Scientific Commission of the Council of Science and Humanities in Germany (Wissenschaftsrat) from 2016–2022. Tropea retired from the TU Darmstadt in April 2020 and presently holds the Henry Ford Visiting Professorship Chair at Indian Institute of Technology Madras and a VAJRA faculty position at Indian Institute of Science Bangalore.