Dynamic Stall Experiment

ABSTRACT
In this research effort, the Mechanism of dynamic stall has been investigated, to gain more insight in to the events that lead to its occurrence. Numerical flow simulations based on the solution of RANS equations are carried out for this study, using a solver based on the Beam-Warming approximate factorization technique. Dynamic stall occurrence on constant-rate pitching NACA 0012 airfoil was studied in detail. The force, moment coefficients and detailed flow structures were studied in conjunction with the existing vortical theory to provide insights into these events. It was convincingly established that the sudden changes in the behavior of the force and moment coefficients are caused due to the shedding of the clockwise (negative)

When an airfoil is pitching up, the flow separation and hence the stall, is delayed resulting in a higher maximum lift coefficient. However, once the airfoil nears the end of its pitching-up movement and starts the retraction (pitch-down) cycle, a separation region is rapidly formed near the leading edge of the airfoil. This separation region quickly grows till it bursts, causing a massive drop in lift. The effect of dynamic stall continues nearly throughout the retraction cycle causing hysteresis loop behaviour of the force and moment coefficients. Dynamic stall not only reduces the airfoil lift, but also causes large unsteady pitching moments and increased drag resulting in stronger vibrations, high noise level and high control loads. The sudden excessive pitching moment leads to increased torsion which can cause structural fatigue, and even flutter which is likely to lead to catastrophic failure. This severely restricts the performance of rotorcrafts, wind turbines and high performance military aircraft. In today’s environment, such restrictions on the manoeuvrability, speed and performance envelope of modern vehicles and devices are considered unacceptable. Work was done on Dynamic Stall in past 3 decades[6-27] to establish its causes and mechanism, however, the last decade has seen much more emphasis and more detailed studies have been carried to

The same events, as calculated by Numerical Flow Solver used in this study, are depicted in fig:1a and the related vortical structures illustrated. For airfoils oscillating in pitch mode due to the unsteady motion effects, the onset of stall is delayed well beyond the static stall angle (a). Initially the formation of separation bubble is inhibited, but after the vortex is formed (b), it does not shed quickly. In fact, the vortex rides on the airfoil upper surface for extended periods and stall occurs at a much higher angle of attack (h), resulting in a larger maximum lift. In case of airfoils oscillating to moderately high angles of attack, the stall is delayed until the end of the pitch-up cycle. At the beginning of retraction (pitch-down) cycle, the vortex size enlarges rapidly, until it bursts (j). This deep stall, which occurs close to the start of retraction cycle for oscillating airfoils, and at post-stall angles of attack for pitching airfoils, is generally known as dynamic stall. When dynamic stall occurs the lift deteriorates rapidly (j), drag surges and the airfoil is subjected to sudden large changes in the pitching moment