A New Stall-Onset Criterion for Low Speed Dynamic-Stall

Abstract

The Beddoes/Leishman dynamic-stall model has become one of the most popular for the provision of unsteady aerofoil data embedded in much larger codes. The underlying modelling philosophy was that it should be based on the best understanding, or description, of the associated physical phenomena. Even although the model was guided by the flow physics, it requires significant empirical inputs in the formThese conditions are normally associated with pitching and plunging motions and were initially of much interest to helicopter rotor-aerodynamicists, who required an assessment of rotor loads during forward and manoeuvring flight. Today, dynamic stall is also relevant to the performance and durability of wind turbines.Carr [4] gives an illustration for the events of a dynamic stall process, for an aerofoil undergoing a sinusoidal pitch profile, shown in Figure 1. Chronologically, the dynamic stall events start at point (a), where the pitching aerofoil passes the staticstall angle of attack, but without any discernible change in the flow around the airfoil, and the flow remains fully attached.Then the flow reverses near the surface at the trailing edge region starting at point (b), but still with no large separation due to the dynamic effects. This reversal moves up the chord until it covers most of the aerofoil, at which stages ((c) and (d)) the leading edge flow no longer remains attached and a strong vortical flow develops (point (e)). As the vortex enlarges and remains close to the leading edge, there is an obvious increase in the lift-curve slope at (f) and associated vortically induced normal force coefficient, N C , ((g) and (h)). The vortex subsequently convects downstream and finally detaches from the trailing edge, inducing a strong nose-down pitching moment at (i). After that, when the flow over the aerofoil upper surface is fully separated, the lift break (lift stall) occurs (j). As the angle of attack decreases continuously, the reattachment process takes place. According to Niven et al [5], at some angle of attack (close to the static fully-stalled state), the leading edge reattachment occurs. This is characterised by two distinct but related events. First, all the large eddies, due to the stall, are convected over the chord and into the free stream at constant speed and, second, following closely behind is the reestablishment of a fully attached boundary layer. The convective component, i.e. the first, normally takes a few chord lengths of free-stream travel, at which point the lift reaches its lowest value. Then the flow transits to a fully attached state at the end of stage (k). Obviously, the whole process forms a large hysteresis loop, shown in figure 1, which is taken from Carr et al.[6].Leishman [7] explores the flow topology of dynamic stall and summarises that the aerofoil dynamic behaviour is significantly different from the steady case in following three aspects:-Under dynamic conditions, since the circulation is shed into the wake at the trailing edge of the aerofoil, the induced un...

Leading-Edge Flow Structure of a Dynamically Pitching NACA 0012 Airfoil

“…. 12 5 Present experimental conditions overlayed on a lift curve hysteresis loop taken from experiments conducted by Sheng et al [3][4][5]. The experimental conditions are similar to those used in the present study.…”

“…Note that several different types of airfoil are considered. [5][6][7][8][9][10][11][12][13][14][15] The present experimental conditions are indicated in Figure 6 and Table 1, along with a selection of other relevant experiments reported in the literature. It is evident that while realistic conditions are not quite matched, the present experimental conditions are not unusual.…”

“…Note that several different types of airfoil are considered. [5][6][7][8][9][10][11][12][13][14][15] . .…”

Modification of Leishman–Beddoes model incorporating with a new trailing-edge vortex model

“…(pitchingdown:20.43 ) that the Cp of upper surface is obviously increased. The Cp value of upper surface is even larger than the value of lower surface at the point about 0.7 times chord.…”

Study on the Mechanism of the Variable-Speed Rotor Affecting Rotor Aerodynamic Performance

“…In a series of ramp-up dynamic experiments by the University of Glasgow, Sheng, Galbraith and Coton found that the original Leishman-Beddoes algorithm predicts the dynamic stall vortex formation ahead of the actual situation when the Mach number is 0.12 [24,25]. Then, Sheng et al proposed a new dynamic stall criterion in the original Leishman-Beddoes framework in a low Mach number situation (Ma < 0.3).…”