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Integrated Computational and Experimental Studies of Flapping-wing Micro Air Vehicle Aerodynamics Kevin Knowles, Peter Wilkins, Salman Ansari, Rafal Zbikowski.

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Presentation on theme: "Integrated Computational and Experimental Studies of Flapping-wing Micro Air Vehicle Aerodynamics Kevin Knowles, Peter Wilkins, Salman Ansari, Rafal Zbikowski."— Presentation transcript:

1 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 3 rd Int Symp on Integrating CFD and Experiments in Aerodynamics, Colorado Springs, 2007

2 Knowles et al. Outline Introduction Flapping-Wing Problem Aerodynamic Model LEV stability Conclusions

3 Knowles et al. Micro Air Vehicles Defined as small flying vehicles with Size/Weight: mm/50–100g Endurance:20–60min Reasons for MAVs: Existing UAVs limited by large size Niche exists for MAVs – e.g. indoor flight, low altitude, man-portable MAV Essential (Desirable) Attributes: High efficiency High manoeuvrability at low speeds Vertical flight & hover capability Sensor-carrying; autonomous (Stealthy; durable) Microgyro Microsensors

4 Knowles et al. 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

5 Knowles et al. Wing Kinematics – 1 Flapping Motion sweeping heaving pitching Key Phases Translational downstroke upstroke

6 Knowles et al. Wing Kinematics – 1 Flapping Motion sweeping heaving pitching Key Phases Translational downstroke upstroke Rotational stroke reversal high angle of attack

7 Knowles et al. Wing Kinematics – 2

8 Knowles et al. Mechanical Implementation

9 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

10 Knowles et al. Aerodynamics Key phenomena unsteady aerodynamics apparent mass Wagner effect returning wake leading-edge vortex [Photo: Prenel et al 1997]

11 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 Robofly wing

12 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

13 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

14 Knowles et al. Flow Visualisation Output

15 Knowles et al. Impulsively-started plate

16 Knowles et al. Validation of Model

17 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?

18 Knowles et al. 2D flows at low Re Re = 5 Re = 10

19 Knowles et al. Influence of Reynolds number α = 45°

20 Knowles et al. 2D flows Re = 500, α = 45°

21 Knowles et al. Influence of Reynolds number α = 45°

22 Knowles et al. Kelvin-Helmholtz instability at Re > 1000 Re 500 Re 5000

23 Knowles et al. Secondary vortices Re = 1000 Re = 5000

24 Knowles et al. 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

25 Knowles et al. Structure of 3D LEV

26 Knowles et al. Stable 3D LEV Re = 120 Re = 500

27 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.

28 Knowles et al. Questions?


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