Control of Self-Excited Roll Oscillations of Low-Aspect-Ratio Wings Using Acoustic Excitation

Passive control of roll oscillations of low-aspect-ratio wings using bleed

“…Figure 3 presents the RMS values of the roll angle as a function of angle of attack for AR = 2 flat-plate wing (baseline model). It can be observed that, similar to previous studies reported by Gresham et al (2009Gresham et al ( , 2010b and Hu et al (2011Hu et al ( , 2014, the onset of selfexcited roll oscillations is observed at around α = 12º for the baseline model. At 12º ≤ α < 15º, the roll amplitude gradually increases with increasing angle of attack.…”

“…Phase-averaged PIV measurements of the crossflow vorticity patterns over the rolling wing with tip slot E at α = 17º and Φ = 0º indicate that the tip vortex is located further away from the wing surface for the increasing Φ case and closer to the wing surface for the decreasing Φ case (Figure 7). These hysteresis and time-lag effects are similar to those observed over the free-to-roll baseline model (Gresham et al 2009(Gresham et al , 2010bHu et al 2011Hu et al , 2014), suggesting the presence of the asymmetric flows which lead to the large amplitude self-induced roll oscillations again at 16º ≤ α ≤ 24º (Figure 3).…”

“…It is seen that, for successful slot geometries, roll oscillations are largely attenuated across the whole range of angle of attack. This aspect is different for the previous active flow control methods (Hu et al 2011(Hu et al , 2013(Hu et al , 2014 for which the onset of roll oscillations is delayed up to 4 degrees, but they are ineffective at larger incidences. It is interesting that the present passive control method is far superior to the active control methods tested so far.…”

“…It is also noted that these proposed mechanisms differ from that for the active control of the wing rock of the same low-aspect-ratio wing (Hu et al 2011(Hu et al , 2013(Hu et al , 2014. In these studies, it has been shown that the excitation of the flow results in the formation of the separated shear layer and tip vortices closer to the wing surface, which is more typical of lower wing incidence, thus effectively suppressing roll oscillations.…”

“…In order to suppress such undesired roll oscillations, a number of studies have been conducted using both active (oscillating flaps (Walton and Katz 1992;Katz and Walton 1993), blowing (Wong et al 1994;Sreenatha and Ong 2002), acoustic excitation (Hu et al 2011(Hu et al , 2014, synthetic jets (Hu et al 2013)) and passive (Synolakis et al 1993;Ng et al 1994;Mabey 1997) flow control techniques. In a recent study at the University of Bath, acoustic excitation was applied to attenuate the roll oscillations of an AR = 2 rectangular flat plate wing (Hu et al 2011(Hu et al , 2014. It was found that the onset of the self-excited roll oscillations could be delayed by up to ∆α max ≈ 4º and the magnitude of the oscillations was suppressed by up to ∆Φ rms,max ≈ 30º.…”