Aerodynamic bodies subjected to pitching motions or oscillations exhibit a stalling behavior different from that observed when the flow over a wing at a fixed angle of attack separates. The latter phenomenon is referred to as static stall, since the angle of attack is fixed. In the case of a dynamically pitching body, such as an airfoil, the shear layer near the leading edge rolls up to form a leading-edge vortex (LEV) which provides additional suction over the upper airfoil surface as it convects downstream. This increased suction leads to performance gains in lift and stall delay, but the LEV quickly becomes unstable and detaches from the airfoil. The LEV detachment is accompanied by a dramatic decrease in lift and sudden increase in pitching moment. Dynamic stall is not a well-understood phenomenon despite its importance to the performance and operational limits of helicopters, flapping wings, and wind turbines. In fact, dynamic stall can lead to violent vibrations and dangerously high loads in these aerodynamic applications, leading to fatigue and structural failure.
Where it concerns helicopters specifically, many experimental, computational, and theoretical investigations have been undertaken in order to grasp the fluid physics at play when an airfoil executes pitching oscillations in a steady low-speed airstream. However, the scenario is complicated when considering a helicopter travelling at especially high forward-flight speeds. In this case, the impact of the time-varying relative velocity seen by the rotor plays a significant role in the dynamics stall process. At sufficiently high speeds, compressibility effects are encountered and shock waves can form on the advancing rotor and they can induce stall. The image at left shows one instantaneous Particle Image Velocimetry (PIV) measurement acquired at the onset of shock-induced stall. This picture was acquired in the 6” × 22” Transonic Wind Tunnel at Ohio State University. This is a very unique facility which allows experimentalists to model and understand the unsteady fluid dynamics associated with compressible dynamic stall with enhanced fidelity. In addition to investigating the effects of airfoil pitching motions, the tunnel was upgraded in 2008 to produce a time-varying compressibility condition by oscillating the free stream Mach number. This is accomplished by rotating a set of oblong vanes at the wind tunnel choke point downstream of the test section. As they rotate, the area ratio (between the choke point and the test section) varies harmonically, as does the test-section Mach number. Dynamic airstream oscillations can be generated over a range of Mach number and frequency relevant to advanced rotorcraft. These features make this facility suitable for experimental modeling of compressible dynamic stall as well as exploration of novel flow control strategies to expand the operating envelope of rotorcraft.
In addition to conventional measurement techniques, the wind tunnel is equipped with optical access. This makes it possible to acquired data using PIV and the fast-responding pressure-sensitive paint (PSP) formulations developed and applied by our research group. These advanced diagnostics may be utilized in dynamic stall investigations to better understand the time-accurate flow topology both on- and off-body. Experimental data gleaned from these investigations are all important for validation of computational models for dynamic stall, which could lead to improved performance gains and more defined operating envelopes as a matter of safety for helicopters.
- Gompertz, K., Kumar, P., Jensen, C.D., Peng, D., Gregory, J.W., and Bons, J.P., 2011, "Modification of a Transonic Blowdown Wind Tunnel to Produce Oscillating Freestream Mach Number," AIAA Journal, vol. 49, no. 11. doi: 10.2514/1.J051090
- Juliano, T.J., Peng, D., Jensen, C.D., Gregory, J.W., Liu, T., Montefort, J., Palluconi, S., Crafton, J., and Fonov, S., 2011, "PSP Measurements on an Oscillating NACA 0012 Airfoil in Compressible Flow," Proceedings of the 41st AIAA Fluid Dynamics Conference and Exhibit, Honolulu, HI (AIAA 2011-3728).