Flatback Airfoils: An Experimental Evaluation of Aerodynamic Performance and Vortex Shedding Behavior

A two-dimensional experimental examination of the effect of splitter plate length on both aerodynamic performance and vortex shedding behavior as well as an initial evaluation of the shedding coherence for a flatback airfoil is presented. The FB-3500-1750 is a thick airfoil with a blunt trailing edge (also called a flatback airfoil) designed for the inboard region of wind turbine blades. Its maximum thickness is 35% of the airfoil chord, with a trailing edge thickness (t[subscript TE]) of 17.5%. In the present study, the FB-3500-1750 airfoil was tested in the University of California, Davis aeronautical wind tunnel at a chord Reynolds number of 666,000. Drag reductions of at least 27% from the baseline were observed with the inclusion of a splitter plate length of 50% t[subscript TE]. Additionally, increasing splitter plate length was shown to continue to decrease the base drag. Maximum lift reductions and earlier stall angles were also observed with increasing splitter plate length though with the 50% t[subscript TE] configuration, the loss in maximum lift coefficient was limited to 5%. Examining vortex shedding behavior, increases in the nondimensional shedding frequency, Strouhal number, were seen with increasing splitter plate length for the measured range. Lastly, for the baseline FB-3500-1750 airfoil, the spanwise correlation length of the vortex shedding was determined as 4-5.5 t[subscript TE] for all the angle of attack and transition (fixed and free) configurations evaluated.