Ramanujan Faculty Fellow
Indian Institute of Technology Madras (IIT Madras)
Abstract
The fundamental relationship between vortical structures, wake dynamics, and force generation in fluid flows span a wide range of applications, including biological locomotion, the aerodynamics of airfoils and wings, fluid-structure interaction, and gust response of free-falling bodies. The talk will first cover the hydrodynamics of fish-schooling. The oscillation of the leader foil produces excess momentum in its wake, along with increased streamwise and transverse velocity fluctuations. The impingement of vortices from the oscillating foil enables thrust production by the stationary follower foil through the Knoller-Betz effect. The pressure force drives the thrust generation of the follower foil through leading edge suction, making the leading edge geometry an important parameter in energy extraction. Despite the high momentum produced by the oscillating leader, the oscillatory wake modifies the boundary layer and lowers the viscous drag experienced by the follower foil. The effect of follower's mass ratio on its forward locomotion will also be discussed. Next, the wake transition and formation of laminar separation bubble on a nominally two-dimensional airfoil will be presented. The three-dimensional wake, characterized by hairpin vortex structures, is associated with mode C instability and exhibits a period-doubling mechanism and neither odd nor even RT-symmetry. The boundary layer transitions to a turbulent state due to the interaction of shear layer vortices with the separated flow. The reattachment of turbulent boundary layer leads to the formation of a laminar separation bubble (LSB). The airfoil experiences abrupt increase in lift and decrease in drag at the onset of LSB. The aerodynamics of finite wings at low Reynolds number will be discussed with a brief overview of Prandtl's classical lifting line theory (LLT). Unlike LLT predictions, the drag increases with an increase in aspect ratio. The formation of streamwise vortices due to the vortex linkages result in non-monotonic spanwise force distribution with a local peak near the wing-tip. A discrete vortex model is proposed to explain this mechanism. Vortex shedding occurs in distinct cells due to the interaction between the wing-tip vortex and the primary vortices. The Lamb-Oseen vortex model is employed to predict the radius of the forced vortex. Understanding how freely falling bodies interact with unsteady flow disturbances is fundamental to fluid mechanics, with implications spanning atmospheric science, biology, and engineering. The transverse gusts systematically enhance the airborne endurance of free-falling plates by reducing their descent velocity. A horizontal gust induces a relative horizontal motion between the plate and the surrounding fluid, generating circulation and uplifting the plate. In addition, the temporary horizontal displacement of the plate from its own wake further augments this uplift. The altitude gain depends non-monotonically on the governing parameters due to the interplay between aerodynamic forces and body rotation.Overall, these studies establish connections between flow instabilities, vortex interactions, and force generation, advancing the fundamental understanding of fluid-structure interactions in unsteady environments across a range of aerospace and biological applications. They also provide design insights into passive flyers capable of harvesting energy from gusty atmospheric conditions.
About Dr. Jawahar Sivabharathy Samuthira Pandi
Dr. Jawahar Sivabharathy Samuthira Pandi is a Ramanujan Faculty Fellow in the Department of Applied Mechanics and Biomedical Engineering at the Indian Institute of Technology Madras (IIT Madras), India, and a Collaborating Academic Visitor in the Department of Mechanical Engineering and Mechanics at Lehigh University, USA. He received his PhD in Aerospace Engineering from the Indian Institute of Technology Kanpur, where he was awarded the Outstanding PhD Thesis Award. He completed his master's program in Space Engineering and Rocketry from Birla Institute of Technology, Mesra, and his bachelor's degree in Aeronautical Engineering from Rajalakshmi Engineering College, Chennai, where he received the Anna University Gold Medal for securing the first rank in the state. Jawahar's research focuses on computational fluid dynamics, aerodynamics, fluid-structure interaction, vortex-dominated flows, and flow instabilities, with applications to finite wings, bluff bodies, biological locomotion, and the flight dynamics of freely falling bodies. Before joining IIT Madras, he was a Research Associate at the University of Edinburgh, UK, where he worked on the ERC-funded Dandelion Seed Flight project. The investigations were supported by an EPSRC HPC award which provided computational resources for simulations. He is currently the principal investigator of an ANRF-funded research project on the optimization of free-falling plate and wing shapes.