Effect of Boundary Layer Trip and Tubercles on Aerodynamic Performance of E216 Airfoil

Abstract

The thesis presents experimental and numerical investigation on the aerodynamic performance of airfoil E216 in prestall region at Reynolds number (Re) of 1 × 105. The effect of boundary layer trip (BLT) of different shapes and leading edge tubercles are studied with an aim to improve the aerodynamic performance and to eliminate the laminar separation bubble (LSB) formation on the airfoil. Finally the effect of modification are addressed in terms of performance improvement in the power output of a conceptual turbine which is capable of generating 100W at wind speed of 6.5 m/s. In the study Rectangular (RT), Right angled triangular (RA) and Isosceles triangular (IT) shaped boundary layer trips of different height located at two different positions are investigated. Wind tunnel experiment is conducted with rectangular boundary layer trip and is used for validation of numerical methodology. The amplitude of the tubercles are varied from 2 mm to 8 mm and wavelength from 15.5 mm to 62 mm. The different combination of the amplitudes and wavelengths resulted in total of nine models. Out of these, wind tunnel experiment are conducted on three models and are used to validate the numerical results. Langtry-Menter 4-equation Transitional SST Model or γ − Reθ - SST model is used in the study to model laminar separation bubble and effect of trip and tubercles on it. The investigation revealed that the the airfoil stalls at 120 with lift coefficient (Cl) of 1.37 and drag coefficient (Cd) of 0.063. Maximum value of lift to drag ratio of 42.46 is obtained at AOA of 40. Surface pressure distribution over the airfoil shows the presence of laminar separation bubble. The laminar separation bubble (LSB) formed at a vdistance of x/c = 0.22 from leading edge at AOA of 60. The location of laminar separation bubble moved upstream with increase in AOA. Based on the location of laminar separation bubble at AOA of 60, boundary layer trips are positioned at 0.17c and 0.10c from leading edge of the airfoil. Result showed that boundary layer trip eliminates LSB partially or completely and improves the aerodynamic performance of the airfoil. Highest improvements of 16.7% in Cd and 34.6% in Cl=Cd are obtained for location-2 with the rectangular trip having lowest trip height of 0.3 mm at AOA of 80. In all the cases, improvement in performance is observed only up to trip height of 0.5 mm. There is no observable advantage for isosceles and right-angled triangular trips over rectangular trips. Improvement in Cl is observed for most of the tubercled models and is significant at high angles of attack. But simultaneous increase in drag coefficient resulted in little improvement in Cl=Cd for most of the cases. But tubercled model with amplitude 2 mm and wavelength of 62 mm (A2W62) produced a peak value of 46.91 at AOA 60 which is higher than the baseline by 7.37%. Compared to baseline, there is high suction peak pressure along the trough and lower along the peak. The low amplitude and low wavelength tubercled model exhibited smooth Cp distribution without any sign of strong LSB. The LSB moves upstream with increasing amplitude and wavelength. LSB along the trough is formed ahead of that at peak inducing three-dimensional wavy shaped LSB unlike the straight LSB for the baseline. The tubercles considerably reduced the size of LSB compared to baseline. Two pairs of counter rotating vortices are formed on the airfoil surface between the adjacent peaks at two different chord-wise locations which strongly alter the flow pattern over it. The effect of trips and tubercles is demonstrated using a wind turbine performance analysis using BEM theory and it is seen that average improvement in power coefficient by 1.65% is obtained with the boundary layer trip and by 0.64% is achieved with the tubercles.