Integrated Computational and Experimental Studies of Flapping-wing Micro Air Vehicle Aerodynamics

Transcription

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 Outline Introduction Flapping-Wing Problem Aerodynamic Model LEV stability Conclusions

3 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 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 Wing Kinematics 1 Flapping Motion sweeping heaving pitching Key Phases Translational downstroke upstroke

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

7 Wing Kinematics 2

8 Mechanical Implementation

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

11 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

12 Aerodynamic Modelling 1 Quasi-3D Model 2-D blade elements with attached flow separated flow leading-edge vortex trailing-edge wake η^ η ξ^ θ Φ η ~ ~ ξ ξ Convert to 3-D radial chords cylindrical cross-planes integrate along wing span ^η η ^ ξ ~ η ~ ξ wing ξ

13 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 Wing Geometry Force and Moment Data Aerodynamic Model Wing Kinematics Flow Visualisation

14 Flow Visualisation Output

15 Impulsively-started plate

16 Validation of Model

17 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 2D flows at low Re Re = 5

19 Influence of Reynolds number α = 45

20 2D flows Re = 500, α = 45

21 Influence of Reynolds number α = 45

22 Kelvin-Helmholtz instability at Re > 1000 Re 500 Re 5000

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

24 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 Structure of 3D LEV

26 Stable 3D LEV Re = 120 Re = 500

27 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 Questions?