Effect ff Humpback Whale-Inspired Tubercles on Vertical Axis Wind Turbine Blade

Abstract

Vertical axis wind turbines (VAWT) are gaining popularity for their potential to be used in urban and off-grid areas and in offshore wind farms. A study on the flow control potential of Humpback whale flipper-inspired tubercles on VAWT blade is the key focus of this thesis. This work studies the aerodynamic effect of leading-edge tubercles (LET) on certain flow phenomena such as Laminar Separation Bubble (LSB), stall, and hysteresis of the VAWT blade. The effect of tubercle shape and geometrical parameters on the steady-state and unsteady-state aerodynamic characteristics has also been studied. Initially, a comparative study on unswept and various swept blades is conducted in order to detect the extent of performance enhancement brought about by incorporating LET on both types of blade. The aerodynamic characteristics of NACA 0021 swept and unswept blades have been studied and compared with their tubercle counterparts of tubercle amplitude and wavelength 6% and 21% of the airfoil chord, respectively. Tubercles along the flow direction have been incorporated on blades swept or inclined at 10◦ , 20◦ , and 30◦ and compared to the respective baselines and to unswept blades, in terms of static aerodynamic forces at a Reynolds number of 1 × 105 . It was seen that tubercles are more beneficial on unswept and blades of low sweep than on blades of high sweep angle, in attaining a smooth stall. Further, a detailed study of the effect of LET on the unswept blade has been done by an- alyzing the static force and surface pressure at varied Reynolds numbers ranging from 2.5 × 105 to 6 × 105 . The LSB begins to appear on the suction surface of unmodified blade at an angle of attack (AOA) of 6◦ , which extends between 24% and 35% of the blade chord. With the increase in AOA, the LSB moves forward towards the blade’s leading edge and decreases in size. However, due to the influence of tubercles, LSB is not present on the tubercle blade. Stall occurs earlier in the tubercle blade as com- pared to the baseline blade. The baseline blade exhibits an abrupt and deep stall that occurs in a single step, whereas the tubercle model has a soft gradual stall that occurs iiin multiple stages for all Reynolds numbers studied. Surface pressure measurements for static models further reveal the mechanism of the stall. Stall initially originates at the midsection of the blade, which progresses towards the tips of the baseline blade. However, with the introduction of tubercles, the stall progression towards the tips is inhibited on the tubercle blade. The baseline blade also exhibits static hysteresis for lift, drag, and moment coefficient curves, and its extent increases with an increase in Reynolds number. However, for the tubercle model, hysteresis was completely absent. The effect of tubercle shape and geometrical parameters on steady-state aerodynamics has been studied at a Reynolds number of 5 × 105 using blades incorporated with sinu- soidal and triangular LET of varying amplitude to wavelength ratio (A/W). The static forces on four blades with sinusoidal LET of A/W 0.25, 0.5, 0.75 and 1, and two blades of triangular LET of A/W 0.5 and 1, are analyzed and compared to that of a baseline. For blades of sinusoidal and triangular LET, the lift coefficient is lower than that of the baseline in the pre-stall region and decreases with an increase in A/W. The drag and pitching moment increases with increase in A/W. However the tubercle blades are better performing in the post-stall region, where they have higher lift than the baseline. The tubercle blades have a stall earlier than the baseline. However, the tubercle blades have a better stall characteristic than baseline in terms of smooth stall. The stall gets smoother with increase in A/W of tubercles. The steady-state aerodynamic character- istics of sinusoidal and triangular blades of corresponding A/W are closely identical to one another. Finally, considering the actual blade movement on VAWT, the effect of the LET on dynamic blades is studied. The effect of tubercles on unsteady flow has been studied by pitch-oscillating the above-mentioned blades with sinusoidal and triangular LET at various frequencies and obtaining the forces acting. The baseline blade has the maxi- mum lift coefficient, CLmax but exhibits deep stall. Large hysteresis loops are seen in the stall region for the baseline blade. The triangular and sinusoidal LET help mitigate the intensity of stall and hysteresis of the blade. The tubercle blades (sinusoidal and triangular LET) with the least tubercle amplitude have the highest CLmax after baseline. The blades of the highest tubercle amplitude has the most desirable stall and hysteresis characteristics. The size of the hysteresis loop decreases with an increase in tubercle amplitude. Blades with triangular LET perform similar to the sinusoidal leading edge except for having a higher coefficient of normal force.