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Integrated Computational and Experimental Studies of Flapping-wing Micro Air Vehicle Aerodynamics Kevin Knowles , Peter Wilkins, Salman Ansari, Rafal Zbikowski Department of Aerospace, Power and Sensors Cranfield University Defence Academy of the UK Shrivenham, England 3rd Int Symp on Integrating CFD and Experiments in Aerodynamics, Colorado Springs, 2007 Outline • Introduction • Flapping-Wing Problem • Aerodynamic Model • LEV stability • Conclusions Knowles et al. Micro Air Vehicles • • • Defined as small flying vehicles with Size/Weight: Endurance: 150-230mm/50–100g 20–60min Reasons for MAVs: Existing UAVs limited by large size Niche exists for MAVs – e.g. indoor flight, low altitude, man-portable Microgyro MAV Essential (Desirable) Attributes: High efficiency High manoeuvrability at low speeds Vertical flight & hover capability Sensor-carrying; autonomous (Stealthy; durable) Knowles et al. Microsensors Why insect-like flapping? • • • Insects are more manoeuvrable Power requirement: Insect – 70 W/kg maximum Bird – 80 W/kg minimum Aeroplane – 150 W/kg Speeds: Insects ~ 7mph Birds ~ 15mph Knowles et al. Wing Kinematics – 1 • Flapping Motion sweeping heaving pitching • Key Phases Translational Knowles et al. downstroke upstroke Wing Kinematics – 1 • Flapping Motion sweeping heaving pitching • Key Phases Translational downstroke upstroke Rotational Knowles et al. stroke reversal high angle of attack Wing Kinematics – 2 Knowles et al. Mechanical Implementation Knowles et al. Generic insect wing kinematics Three important differences when compared to conventional aircraft: wings stop and start during flight large wing-wake interactions high angle of attack (45° or more) Complex kinematics: difficult to determine difficult to understand difficult to reproduce Knowles et al. • Key phenomena unsteady aerodynamics apparent mass Wagner effect returning wake leading-edge vortex Knowles et al. [Photo: Prenel et al 1997] Aerodynamics Aerodynamic Modelling – 1 • Quasi-3D Model • 2-D blade elements with attached flow separated flow leading-edge vortex trailing-edge wake + • Convert to 3-D radial chords centre of rotation Robofly wing Knowles et al. Aerodynamic Modelling – 1 • Quasi-3D Model • 2-D blade elements with attached flow separated flow ^ ~ leading-edge vortex trailing-edge wake ~ ^ • Convert to 3-D radial chords cylindrical cross-planes integrate along wing span ~ ^ ^ wing Knowles et al. ~ Aerodynamic Modelling – 2 • Model Summary 6 DOF kinematics circulation-based approach inviscid model with viscosity introduced indirectly numerical implementation by discrete vortex method validated against experimental data Knowles et al. Flow Visualisation Output Knowles et al. Impulsively-started plate Knowles et al. Validation of Model Knowles et al. The leading-edge vortex (LEV) Insect wings operate at high angles of attack (>45°), but no catastrophic stall Instead, stable, lift-enhancing (~80%) LEV created Flapping wing MAVs (FMAVs) need to retain stable LEV for efficiency Why is the LEV stable? Is it due to a 3D effect? Knowles et al. 2D flows at low Re Re = 5 Knowles et al. Influence of Reynolds number α = 45° Knowles et al. 2D flows Re = 500, α = 45° Knowles et al. Influence of Reynolds number α = 45° Knowles et al. Kelvin-Helmholtz instability at Re > 1000 Re 500 Knowles et al. Re 5000 Secondary vortices Re = 1000 Knowles et al. Re = 5000 2D LEV Stability • • • For Re<25, vorticity is dissipated quickly and generated slowly – the LEV cannot grow large enough to become unstable For Re>25, vorticity is generated quickly and dissipated slowly – the LEV grows beyond a stable size In order to stabilise the LEV, vorticity must be extracted – spanwise flow is required for stability Knowles et al. Structure of 3D LEV Knowles et al. Stable 3D LEV Re = 120 Re = 500 Knowles et al. Conclusions • • • LEV is unstable for 2D flows except at very low Reynolds numbers Sweeping motion of 3D wing leads to conical LEV; leads to spanwise flow which extracts vorticity from LEV core and stabilises LEV. 3D LEV stable & lift-enhancing at high Reynolds numbers (>10 000) despite occurrence of Kelvin-Helmholtz instability. Knowles et al. Questions? Knowles et al.