ABSTRACT: A summary of our recent research is presented. The flow past an end-to-end wing becomes unsteady beyond Re=600 approximately via the primary instability of the wake leading to vortex shedding. Three-dimensionality sets in at Re~1280.9 via mode C instability and hairpin vortices. Interaction of the shear layer vortices with the separated boundary layer leads to formation of a Laminar Separation Bubble (LSB) at Re~20,000, causing a delay in flow separation. Simulations for flow past a finite rectangular wing with NACA 0012 section at Re=1000 for various semi-aspect ratios (0.25≤sAR≤7.5) over a range of angle of attack (0°≤α≤14°) reveal streamwise vortices, which increase in strength and number to occupy an increasing spanwise extent with increase in α. Unlike the prediction from the Lifting Line Theory (LLT) by Prandtl, they result in non-monotonic spanwise variation of local force coefficients for α>8°. Vortex shedding, for α=14°, is single cell and parallel for sAR<3. Shedding is in two cells with oblique angle that varies with time, leading to large spanwise variation in rms of local force coefficients for higher sAR.

Lock-in in vortex-induced vibration (VIV) of a rigid body mounted on an elastic support is described. The response of the body and its effect on the modes of vortex shedding in the flow are discussed for low vs  high Reynolds number flows. Hysteresis and intermittency, with respect to increasing- and decreasing-flow speed, in the response of the body is investigated. FIV of a flexible filament/plate attached behind a bluff body placed in a uniform flow is studied. A plate has infinite natural frequencies; multiple lock-ins are observed. The effect of ratio of the density of structure to the fluid is profound. Galloping response is observed in certain parameter range. The response of the system from the point of view of effective placement of sensors for energy harvesting is discussed. A strategy is proposed to choose appropriate flexibility of the splitter plate to minimize FIV of a body on elastic supports. The inflation and deflation of a balloon is modeled via a membrane. Ovalling oscillations are observed during inflation. A simple model for failure of balloon is proposed.

The aerodynamics of projectiles in various sports is investigated via experiments in a low speed wind tunnel and computational fluid dynamics (CFD).  The role of the seam in the ‘swing’ of a cricket ball is investigated via unsteady force and surface-pressure measurements and oil-flow visualization.  A new cricket ball experiences three flow states with increase in Re: no swing (NS), conventional swing (CS) and reverse swing (RS). A laminar separation bubble (LSB) forms on the seam side in CS and on the non-seam side during RS. CFD studies show that the boundary layer on the seam side is laminar in the initial regime of CS. However, the seam delays the flow separation in the polar zone.  The aerodynamics of a badminton shuttlecock is investigated. It is shown that the deformation of the shuttle impacts the flow and drag very significantly. Preliminary results for the flow past a frisbee are presented. A laminar separation bubble forms on the upper surface.

BIOSKETCH: Sanjay Mittal received his undergraduate education in Aeronautical Engineering at the Indian Institute of Technology Kanpur, India. He went to the University of Minnesota, Minneapolis, USA for his graduate education.  Presently, he is a Professor in the Department of Aerospace Engineering at Indian Institute of Technology Kanpur, India. His research interests include fluid-structure interactions, flow-induced vibration, bluff body flows, aerodynamic shape optimization, shock-boundary layer interactions, high performance computing, modeling of traffic flow, sports aerodynamics and low speed wind-tunnel testing. He is an active badminton player and his love for sports has found its way in his research. He is passionate about teaching, and  recently received the Excellence-in-Teaching Award and the Gopal Das Bhandari Distinguished Teacher Award from his Institute. He is a Fellow of the Indian National Academy of Engineering, Indian National Science Academy, Indian Academy of Sciences and National Academy of Sciences. He received the prestigious Shanti Swarup Bhatnagar Prize in Engineering Sciences, in 2006.

Sanjay Mittal Flyer