Experimental and numerical study of laminar separation bubble formation on low Reynolds number airfoil with leading-edge tubercles

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 C-l and C-d are 1.37 and 0.081, respectively, at a stall angle of 12 degrees 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 C-l is obtained for the tubercled airfoil with the highest amplitude (8 mm) and wavelength (64 mm) at 6 degrees angle of attack. A higher C-d 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.