Investigation of unsteady separation over pitching airfoils at high Reynolds numbers

Surface shear-stress measurements were obtained on an NACA 0012 airfoil model, undergoing a pitch-up motion from 0° to 45° angle of attack at a constant rate, using an array of surface-mounted hot-film sensors. Dominant features in these data and in the standard deviations computed from these data were examined and related to events in the development and evolution of the unsteady separation over the suction surface. Trends in the behavior of these features are presented for a range of non-dimensional pitch rates (0.010 ensemble-averaged value Introduction and Background The unsteady flow over a pitching or oscillating airfoil is dominated by the development of a large vortical structure known as the dynamic-stall vortex (DSV). The formation and subsequent separation of this vortex results in the phenomenon known as dynamic stall. Interest in the study of this process has been largely motivated by the need for a better understanding of helicopter blade aerodynamics. More recently, motivation has come from interest in improving the maneuverability of aircraft. There has been a great deal of work devoted to the investigation of this complicated unsteady flow phenomenon. The reviews by McCroskey, Gad-el-Hak, Ericsson and Reding, and Doligalski et al. summarize a large portion of this work. Although some of this work examined the mechanisms responsible for the formation of the DSV, most of the results pertain to the evolution of the flow field once the vortex has formed and the detachment of the vortex, which leads to dynamic stall. The origin of the DSV has been recently studied in detail at the Illinois Institute of Technology (IIT). Work by Acharya and Metwally, and Karim and Acharya, have investigated in great detail the evolution of the unsteady separation and formation of the DSV over a two-dimensional pitching airfoil. The techniques used consisted of smoke-wire flow visualization, surface pressure measurements, and vorticity production calculations. Computational work by Visbal provided additional insight into this process. This work shows that the vorticity present in the DSV originates from a concentrated source near the leading edge on the pressure surface of the airfoil. This vorticity is transported to the suction surface in a thin shear layer. A thin region of reverse-flowing fluid in the leading edge region accumulates beneath this shear layer, causing it to lift away from the surface. This reverse flow near the surface is produced by the increasing adverse pressure gradient over the suction surface as the angle of attack increases. The initial flow reversal occurs just downstream of the leading edge region on the suction surface, and is independent of the reverse flow initiated at the trailing edge. As the airfoil " Graduate Research Assistant, Fluid Dynamics Research Center. Student Member, AIAA. " Professor, Fluid Dynamics Research Center. Associate Fellow, AIAA, presently with AlliedSignal Engines, Phoenix, AZ. Copyright © by John W. Kiedaisch and Mukund Acharya. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. 1 American Institute of Aeronautics and Astronautics Copyright© 1998, American Institute of Aeronautics and Astronautics, Inc. continues to pitch up, accumulation of vorticity in the shear layer downchord of the reverse-flow region interacts with the outer flow, causing the shear layer to roll up and form the DSV. The DSV remains attached to the suction surface of the airfoil over a range of angles of attack, enhancing the unsteady lift force. The abrupt detachment of the DSV, due to a secondary flow eruption, causes this lift enhancement to be lost, resulting in dynamic stall. The previous work shows that the processes involved in the formation and evolution of the DSV depend strongly on pitch rate. The dimensionless pitch rate (a) is the key parameter that affects the behavior of this unsteady flow. This parameter is the ratio of the tip velocity of the pitching airfoil and the free-stream velocity. At higher pitch rates, the unsteady effects are stronger, and the processes of DSV formation and detachment are delayed to higher angles of attack. As the pitch rate is decreased, the unsteady effects become weaker, and shear-layer instabilities begin to occur farther upchord on the airfoil surface. The mechanisms leading to the DSV formation occur at earlier angles of attack, the shear-layer roll up becomes more gradual, and the hold time of the DSV above the suction surface is greatly reduced. After a certain pitch rate, the hold time is no longer distinguishable, and the DSV appears to propagate downchord as it forms. It has also been shown that as the pitch rate is reduced, the adverse pressure gradient over the suction surface increases, which allows transition to turbulence to occur at earlier angles of attack. The influence of Reynolds number on the DSV formation process is not well understood. As the chord Reynolds number (Rec) increases, the boundary layer becomes thinner and the inertial effects are greater. Earlier transition to turbulence in the boundary layer, especially at lower pitch rates, becomes an important factor and adds complexity to the unsteady flow. However, at higher pitch rates, it has been shown that certain critical flow mechanisms in the unsteady flow evolution are insensitive to changes in Reynolds number for Reynolds numbers ranging in order from 10 to 10. The work by Acharya, Karim, and Metwally shows very similar flow development over this range of Reynolds number for higher pitch rates (ct>0.1). Also, they show that the occurrence of the local leading-edge flow reversal and shear-layer lift-up are not affected by changing Reynolds number over this range. There is great interest in the development of systems for controlling DSV formation and detachment on helicopter rotor blades and on wings of highly maneuverable aircraft. In helicopter applications, complete suppression of the DSV is desired, while in aircraft applications, it is desirable to allow the DSV to form and delay its detachment from the suction surface to take advantage of the lift enhancement it provides. The capability to accomplish both of these goals has been demonstrated in the work by Karim and Acharya and by Alrefai and Acharya through the use of leading-edge suction. However, to improve and optimize these techniques and develop new control schemes, a better understanding of the unsteady flow mechanisms that play a part in the very early stages of DSV formation, as well as the mechanisms responsible for DSV detachment, is essential. Most practical applications involve low pitch rates (a* = 0.01-0.2) and high Reynolds numbers (Rec > 2.5xl0). The flow mechanisms which lead to the formation of the DSV described in the previous section are only well understood for pitch rates greater than a* = 0.1. Also, there is very little or no information about the nature of the unsteady flow field for Reynolds numbers greater than 500,000. Figure 1 shows a summary of the Rec and cc values at which past experimental and computational investigations were carried out. This figure only shows investigations where a pitch-up motion profile was used. The solid circles represent the present research, while the open symbols represent past experiments and the small solid symbols represent past computations. Clearly, a detailed investigation of the unsteady flow development at low pitch rates and higher Reynolds numbers is needed to close the gap between research and applications and to answer a number of fundamental questions about the nature of this process. First, it is unclear if the same mechanisms and events that lead to the formation of the DSV at higher pitch rates contribute in the same way or behave in a similar manner at lower pitch rates. Also, it is not known if the flow reversal and shear-layer roll-up events remain independent of chord Reynolds number at higher Reynolds numbers. Finally, it is not known if transition in the shear layer is an important factor. Rec 1.5E+6 Applications