Post-stall flow control on aerofoils by leading-edge flags

Abstract

Self-excited oscillations of flags attached at the leading edge of aerofoils have been investigated at post-stall angles of attack at a chord Reynolds number of 100 000. Significant increases in the time-averaged lift coefficient and stall angle have been observed for three aerofoils: one symmetric, one cambered and one with a sharp leading edge. The aerodynamic improvement is due to the periodic formation of vortices caused by the flag oscillations. When the flag is near the aerofoil surface, it is lifted upwards by the induced velocity of the growing vortex. As the flag moves up, the vortex grows in strength and reaches maximum circulation when the flag is furthest from the aerofoil surface and subsequently sheds. Flags with large stiffness exhibit better spatial and temporal coherence of flag oscillations than the compliant flags, resulting in a larger maximum lift coefficient and higher stall angle. For all aerofoils tested, the best lift enhancement with respect to the clean aerofoils is found when the angle of attack is 6° to 10° above the stall angle of the clean aerofoil. High lift is observed when the flags are locked in with the wake instability in a narrow frequency band, depending on the flag mass ratio and length.

Passive Poststall Flow Control on Nonslender Delta Wings Using Flags

“…The effect of the three-dimensional flow is expected to be more significant for delta wings. For the delta wing with the sweep angle of 50° in this paper, the initial experiments by varying the span and the chordwise location of the flags revealed that optimal flags covered the 50% of the length of the entire leading-edge of the wing and had the optimal location when placed between the mid-chord and the trailing-edge (aft of the mid-chord, see previous study [10] suggest that nearly-rigid flags are more effective in producing high lift due to better spanwise coherence of the flag oscillations.…”

“…While the lift enhancement increases in the post-stall angle of attack and has a local maximum at α = 30°, the dimensionless flag frequency remains roughly constant. This is fundamentally different from the flags on airfoils, for which flag frequency depends on the airfoil angle of attack as the frequency of the wake instability varies (since the effective wake width varies) [10]. For the leading-edge separation on the nonslender delta wing, there is no evidence of locking-in to the wake, which has an absolute instability.…”

“…This is a non-intrusive fullfield measurement technique by tracking speckling patterns on the surface of an object in timeaccurate consecutive image pairs through a correlation method [14,15]. It has been previously used in our laboratory for membrane wings [16], flags on airfoils [9,10], on finite wings [11], and in freestream [17]. A highspeed DIC system was used to provide time-accurate three-dimensional flag deformation.…”

“…Recently it was shown that self-excited oscillations of a small flag attached at the leading-edge of a two-dimensional airfoil at post-stall angles of attack could excite the separated shear layer, induce periodic shedding of leading-edge vortices and lead to increased time-averaged lift force [9]. The frequency of the flag oscillations could be tuned to the frequency of the flow instabilities by properly choosing the properties of the flag [10,11]. For the case of nominally two-dimensional post-stall flows over airfoils and high-aspect-ratio unswept wings, the main flow instability that flags lock-in is the wake instability.…”