Investigation of incipient dynamic stall over pitching airfoils using hot-film sensors

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 were examined and related to events in the development and evolution of the dynamic-stall vortex through comparison with well-known features in the surface-pressure distributions. 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 study of unsteady flow over pitching airfoils was first motivated by the need for a better understanding of helicopter blade aerodynamics. More recently, the interest has spread to applications involving highly maneuverable aircraft. The central feature of this unsteady flow is the formation of a coherent vortical structure known as the dynamic stall vortex (DSV). This vortex is formed on the suction surface of an airfoil during a pitch-up motion. As the pitch-up motion proceeds, the vortex remains above the suction surface, thereby providing an additional transient lift component. When the vortex eventually detaches and convects downstream, this lift enhancement is suddenly removed, resulting in dynamic stall. A great deal of work has been devoted to the investigation of this complex flowfield. Reviews by McCroskey , Gad-el-Hak , Ericsson and Reding , and Doligalski et al. summarize much of this work. While some of this work studied the mechanisms leading to the formation of the DSV, most of it has concentrated on the flowfield development once the DSV has formed and on the detachment of the DSV, which leads to dynamic stall. Recent experimental studies at the Illinois Institute of Technology' (IIT) have investigated in more detail the evolution of the unsteady flowfield over the surface of a twodimensional pitching airfoil through the use of flow visualization, surface-pressure measurements, and vorticity-production calculations. Along with recent theoretical advances in the understanding of unsteady, viscous flows, this work has given new insight into the physical mechanisms involved in the formation of the DSV. It shows that a thin region of reverse-flow fluid accumulates near the leading edge of the airfoil, causing shear layer lift-up and the formation of the DSV. The DSV remains attached to the suction surface of the airfoil until a secondary-flow eruption abruptly separates it from the shear layer. Computational work by Visbal reveals features similar to those seen in these experimental studies. The unsteady pressure gradient over the pitching airfoil which causes the DSV formation is a strong function of pitch rate. Much of the previous work has studied dynamic stall at relatively high pitch rates, while most applications involve lower pitch rates. At higher pitch rates, the processes that lead to the formation of the DSV are delayed to higher angles of attack. Also, the DSV remains attached to the suction " Graduate Research Assistant, Fluid Dynamics Research Center. Student Member, A1AA Professor, Fluid Dynamics Research Center. Associate Fellow, A1AA Copyright © by John W. Kiedaisch and Mukund Acharya. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. surface of the airfoil to higher angles of attack. The unsteady effects become weaker as the pitch rate is decreased. At lower pitch rates, shear-layer instabilities begin to occur further upchord on the airfoil surface. The mechanisms causing DSV formation occur at lower angles of attack, and transition to turbulence in the shear layer begins to influence the flow development. The time between DSV formation and the secondary eruption causing its detachment decreases considerably. From the past work, it is unclear if the mechanisms believed to be responsible for DSV formation at higher pitch rates are also the key factors in DSV formation at lower pitch rates, or if the increased adverse pressure gradient and the resulting earlier transition to turbulence modifies DSV evolution. There is much interest in the development of systems for controlling DSV formation and detachment. Past work at IIT by Karim and Acharya and Alrefai and Acharya are examples of this trend. However, in order to improve and develop new control schemes, a better understanding of the very early stages of DSV formation at low pitch rates is essential. Objectives The objectives of this work are to investigate the early stages of dynamic-stall vortex formation over a range of pitch rates using surface shear stress and local flow direction data obtained from an array of surfacemounted hot-film sensors. The focus is on understanding the flow mechanisms at lower pitch rates and higher Reynolds Numbers, where shear-layer instability and earlier transition to turbulence may play more significant roles in DSV development. The present paper describes shear-stress measurements over a wide range of pitch rates at chord Reynolds numbers up to 0.5 million. The flow direction results will be presented in a future paper. Experimental Arrangement Wind Tunnel and Model Design The experiments were conducted in the Andrew Fejer Unsteady Wind Tunnel at IIT's Fluid Dynamics Research Center. This facility is a closedcircuit low-speed wind tunnel with a maximum free stream velocity of 40 m/s and a turbulence intensity of 0.03%. The test section measures 0.61x0.61m in crosssection and 3.1 m in length. A more detailed description of this facility can be found in Karim and Acharya. The airfoil model used is the one described by Acharya and Metwally. It has an NACA 0012 profile, a chord length of 30 cm, and a span of 60 cm. The pitch axis is located at quarter-chord. A high-torque brushless servo motor is used to pitch the model. This motor incorporates resolver feedback with softwareprogrammable tuning and control, allowing motion control and position information accurate to within ±0.09 degrees. Instrumentation The model is instrumented with a multi-element array of thin-film sensors, shown in Figure la. This array consists of 50 nickel sensors spaced 2.54 mm apart, each with two 18.4 cm long copper leads to which the electrical connections are made. The sensors and leads are deposited on a 0.05 mm thin flexible polymide substrate. The array is wrapped around the leading edge of the airfoil, as shown in Figure Ib, with the row of sensors oriented parallel to the chord line approximately 5 cm from mid-span. Each sensor is approximately 1.5 mm long, 0.15 mm wide, and has a nominal cold resistance of 8.0 ohms. This array allows sensor 50 sensor 33 (x/c=0) (a) Hot-film sensor array. sensor 1 pressure surface I pitch axis sensor 50 (1/4 chard) (x/c-0.126) i . . . . i . . . . i . . . . i . . . . i . . . . i . . . . i . . . . i . . . . i . . 0 O.I 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 I