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Unsteady Dynamics Of Wind Turbine Wake, Oscillating Bubble And Falling Card

Helical tip vortices in the wake of a wind turbine, dynamics of a rising and oscillating bubble and trajectories of cards falling under gravity have been investigated here. The near wake flow field of the wind turbine in the Reynolds number range 10 3 ≤ Re ≤ 5 × 10 3 has been explored using qualitative dye flow visualization and quantitative digital particle image velocimetry (DPIV) techniques. Flow visualization studies showed the dye getting trapped in the shape of spirals surrounding the helical vortex cores. It was found that the helical vortex core size was increasing with downstream distance. It was also found that the normalized stream-wise component of the wake velocity decreased with increasing tip-speed ratios. The pitch of the tip vortex monotonically decreased with increasing tip-speed ratios. The evolution of the global wake vorticity field with time has been studied in detail, for the first time. The results indicated that vorticity peaks at the center of the core, as one would expect. The vorticity at the center of the core was also found to decay as the vortex moved downstream, implying that the viscous dissipation was active even at length scales of few diameters. Power spectrum of the stream-wise velocity field at a point showed a distinctive peak at a frequency of 0.5 Hz. In another study, numerical simulations on dynamic motions of bubbles undergoing radial oscillations and rising against to gravity in an ambient quiescent viscous fluid were performed. Bubbles of two different geometric shapes, a spherical cap bubble and a perfectly spherical bubble were considered. Numerical simulations of the nonlinear differential equations showed that radial oscillations of the bubble dramatically modified the rising motion of the bubble. It was found that the rise velocity of an oscillating bubble was much larger than that of a non- oscillating bubble and was strongly oscillatory, with a maximum value attained when the bubble was at its minimum radius. Moreover, the elevation of the oscillating bubble changed suddenly and very steeply, and was much larger than that of a non-oscillating bubble. Finally, aerodynamics of freely falling cards of various geometries was investigated. In this investigation, parallelogram-shape cards were released in still air with their long axis horizontal and acute axis vertical. In the past studies, tumbling and fluttering motions have been observed. Here we observed a new structural instability in which the card moved vertically downwards in the shape of a helix while at the same time undergoing tumbling motions along long axis. In addition, the card was inclined with the vertical and was sinusoidally varying about a mean angle.
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