|The effects of free-stream turbulence intensity on the flow over a NACA 0018 airfoil are studied experimentally in a wind tunnel facility. A parametric study is performed over a range of chord Reynolds numbers from 100 000 to 200 000, angles of attack from 0 to 20, and free-stream turbulence intensities from 0.09% to 2.03% in order to unravel the effects of each parameter on suction side laminar separation bubble topology and the resulting changes in airfoil lift. In order to investigate the effects of free-stream turbulence intensity on the streamwise and spanwise flow development within a separation bubble, flow field measurements are made using planar Particle Image Velocimetry for an angle of attack of 4, chord Reynolds numbers of 80 000 and 125 000, and free-stream turbulence intensities between 0.10% and 1.94%.
The results show that increasing the level of free-stream turbulence intensity leads to a reduction in the length of the separation bubble formed over the suction side of the airfoil. The reduction in bubble length is a result of a downstream shift in mean separation as well as an upstream shift in mean transition and, consequently, mean reattachment. At low angles of attack, the reduction in separation bubble length leads to a slight reduction in airfoil lift, while at pre-stall angles of attack the reduction in separation bubble length alleviates the loss of suction at the location of the suction peak, thereby increasing lift, and can delay stall. While the effects of turbulence intensity and chord Reynolds number on the mean flow are shown to be similar, their effects on transition are shown to be notably different. The upstream shift in mean transition with increasing turbulence intensity is shown to be the result of disturbances reaching higher amplitudes earlier upstream as the level of turbulence intensity is increased, despite increased bubble stability. This result suggests that the increased initial perturbation amplitude at elevated turbulence intensity levels is solely responsible for the upstream shift in mean transition. In contrast, the upstream shift in mean transition with increasing Reynolds number is a result of decreased bubble stability.
Wavenumber-frequency spectra of velocity fluctuations in the separated shear layer show that disturbances become more broadband in both time and space with increasing turbulence intensity. In addition, the results show that as the level of free-stream turbulence intensity is increased, the spanwise coherence of shear layer rollers decreases at the location of roll-up, leading to earlier vortex breakdown. At elevated levels of turbulence intensity, streamwise streaks of low speed fluid are observed, and originate in the boundary layer upstream of the separation bubble. These streaks form as a result of the onset of bypass transition, leading to significant changes in bubble dynamics, particularly at the highest level of turbulence intensity investigated. The results suggest that the transition mechanism in the separation bubble at the highest level of turbulence intensity investigated is altered.