Low Reynolds number External Flow around Bio-inspired objects: Effect of Geometric Shape and Surface Modification on Drag Reduction

Geometry of the body and surface modifications play an important role in influencing the wake dynamics including the onset of vortex shedding. This work explores these effects on both streamline and bluff bodies using two-dimensional particle image velocimetry (PIV) based experiments. Specifically, we investigate the flow dynamics around (i) a superhydrophobic circular cylinder, (ii) a superhydrophobic hydrofoil, (iii) a Teflon or cello taped hydrofoil and (iv) a corrugated hydrofoil with the primary objective of understanding the effect of geometric shapes and surface modifications on the ensuing flow. In addition, we analyze the effect of slip when the streamline body is at an angle of attack and the effect of corrugations on the flow at ultra-low Reynolds numbers. Various engineering parameters are calculated and thoroughly examined to understand the flow dynamics with experiments conducted in the Reynolds number range of 45 – 30800 and angles of attack -15° – 20°.

The effects of superhydrophobic circular cylinder on the flow observed in our analysis are: a delay in the onset of vortex shedding, a difference in the coherent structures present in the wake in comparison to an Acrylic cylinder, maximum reduction of the drag coefficient value (by 15%) and a formation of P+S vortex shedding pattern at Re = 860. Moving on to the hydrofoils with superhydrophobicity, we observed, a reduction in the Reynolds number range for laminar vortex shedding regime at angle of attack 10°, a reduction in the separation bubble size at angle of attack 15° and a maximum reduction in the drag coefficient values by about 40% compared to other surfaces. The study conceptualizes the formation of a virtual hydrofoil around the corrugated hydrofoil owing to the trapped vortices in the corrugations. The virtual surface acts as a travelling wave with a definite velocity which inducing a slip. A fluid roller bearing phenomenon has been proposed to explain the shedding mechanism around the corrugated hydrofoil. This thesis provides an insight to the corrugated hydrofoil surface dynamics by identifying the important role of partially merged co-rotating vortices. As the generation of partial slip is directly proportional to the lift coefficient and inversely proportional to the drag coefficient, we conclude that the corrugated hydrofoil shows better performance in terms of both surface modification and shape modification at low Reynolds numbers (Re < 10000). The passive flow control in the corrugated hydrofoil is the underlying mechanism which enables higher aerodynamic performance. The corrugated shape is the result of natural evolution and the study suggests that bio-mimicking gives more insight towards better technological advances.