Faculty Publications
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Item Experimental and numerical study of laminar separation bubble formation on low Reynolds number airfoil with leading-edge tubercles(Springer, 2020) Sreejith, B.K.; Sathyabhama, A.The present work reports the effect of leading-edge tubercles on aerodynamic performance and flow features of a cambered airfoil E216 at a Reynolds number of 100,000 and at various angles of attack in the pre-stall regime. Amplitude values of 2 mm, 4 mm and 8 mm and wavelength values of 15.5 mm, 31 mm and 62 mm are used for both experimental and simulation studies. The Transition-SST RANS model is used to simulate transition phenomenon (laminar separation bubble) and three-dimensional flow features over the airfoil. Wind tunnel experimental results are used for the performance analysis and the validation of the simulation methodology. The experimental values of Cl and Cd are 1.37 and 0.081, respectively, at a stall angle of 12 ? for the plain airfoil. The experimental results show that the lift generated by tubercled airfoils is higher than that produced by the plain airfoil in the pre-stall region but lower at the stall angle. A maximum benefit of 4.51% in Cl is obtained for the tubercled airfoil with the highest amplitude (8 mm) and wavelength (64 mm) at 6 ? angle of attack. A higher Cd is observed for all the tubercled airfoils than for the plain one. The simulation is mainly carried out to study the flow structure. Simulation results indicate the presence of laminar separation bubbles on the plain airfoil with a straight separation and reattachment line parallel to the trailing edge. The tubercles considerably altered the laminar separation bubble formation and the flow structure. A sinusoidal laminar separation bubble is formed on the tubercled airfoils with reduced bubble length. The laminar separation bubble along the trough is formed ahead of that at peak. Two pairs of counter-rotating vortices are formed on the airfoil surface along the trough at two different chord-wise locations which strongly alter the flow pattern over it. Prandtl’s secondary flow of the first kind is the key reason for the vortex formation. © 2020, The Brazilian Society of Mechanical Sciences and Engineering.Item Leading edge tubercle on wind turbine blade to mitigate problems of stall, hysteresis, and laminar separation bubble(Elsevier Ltd, 2022) Joseph, J.; Sathyabhama, A.Low Reynolds number flows may encounter some phenomena like stall, hysteresis, laminar separation bubble. etc., which can deteriorate the performance or efficiency of aerodynamic devices, including wind turbines. This study investigates the effect of the Humpback whale inspired leading edge tubercle on these characteristics, especially stall and hysteresis by comparing a tubercle blade with its baseline counterpart. Surface pressure measurements on various span wise locations of the blades as well as force measurement are done at Reynolds number ranging from 2.5×105 to 6.5×105. For all Reynolds numbers studied, there is a deterioration in stall angle when tubercles are introduced on the blade. The baseline blade experiences abrupt single step deep stall whereas tubercle blade experiences soft stall in multiple steps. From the surface pressure analysis, it is seen that the baseline blade stalls initially at the middle section which further progresses to the tips with increase in angle of attack. However, the stall progression to the tips is drastically inhibited for tubercle blade. It is also seen that there exists significant hysteresis loop for baseline model at all the studied Reynolds numbers. The extent or size of the hysteresis loop is dependent on the Reynolds number. This phenomenon of hysteresis is absent for tubercle blade. Surface pressure over baseline blade for a specific post stall angle of attack shows two distinct pressure distributions- one for increasing branch of angle of attack and one for the decreasing branch. Identical pressure distribution is obtained for tubercle blade irrespective of the direction of movement of blade. Clear regions of laminar separation bubble can be seen for baseline, whereas it is absent for tubercle blade. © 2022Item Implementation of tubercles on Vertical Axis Wind Turbines (VAWTs): An Aerodynamic Perspective(Elsevier Ltd, 2022) Sridhar, S.; Joseph, J.; Radhakrishnan, J.In recent days, enhancement of Vertical Axis Wind Turbines (VAWTs) by mitigating flow deteriorating effects like dynamic stalling, unsteady wake is given great importance. The following article focuses on implementing four different tubercles on the blades’ leading edge and studying its performance and flow characteristics using CFD techniques. Results indicate that the addition of tubercles generated counter-rotating vortices and delayed flow separation and helped control dynamic stalling. Between azimuth angles 70°–160°, the flow was seen to separate only along the trough regions of the blade and remained attached along the peak regions, thus providing more torque and power. In addition to the enhancements in the flow characteristics, a 28% increase in power coefficient was observed for the optimal configuration at the optimal tip speed ratio. Additionally, a 14% increase in maximum lift generated by the blade was observed. Preliminary aeroacoustics analysis revealed a 12% and 20% decrease in the noise emissions along the blade tip and mid-plane of the turbine, respectively. Hence, it can be shown that tubercles effectively control dynamic stall, reduce noise emissions, and increase the power output of VAWTs. © 2022 Elsevier LtdItem Analyzing dynamic stall on tubercle mounted VAWT blades: A simplistic experimental approach using an oscillating rig(Elsevier Ltd, 2024) Joseph, J.; Sridhar, S.; A, S.; Radhakrishnan, J.Leading-edge tubercles, inspired by the flippers of humpback whales, are widely adopted passive flow control devices to enhance the aerodynamic performance of various lifting surfaces. This experimental study investigates the implementation of sinusoidal and triangular tubercles on H-type Vertical Axis Wind Turbine blades to analyze their effects on dynamic stall characteristics. Experimental tests were conducted using a specially designed oscillating rig to replicate blade motion at different reduced frequencies. The results reveal that tubercle blades exhibit a lower stall angle and maximum normal force compared to the baseline configuration. Moreover, the dynamic stall characteristics of tubercle blades are notably smoother, leading to reduced hysteresis losses. A variation in the tubercle amplitude-wavelength ratio further decreases hysteresis, albeit at the cost of reduced normal force generation. At the highest tested reduced frequency of 0.065, tubercles reduce hysteresis by up to 38%. Despite the reduction in normal force, tubercles effectively mitigate the effects of dynamic stall vortices, resulting in smoother stall behavior. The observed reduction in hysteresis can contribute to enhancing the turbine's lifespan and increasing power production efficiency. This experimental approach provides a cost-effective alternative to more expensive methods for studying dynamic stall characteristics. © 2024 The Authors