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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 UniversityDefence Academy of the UK
Shrivenham, England
3rd Int Symp on Integrating CFD and Experiments in Aerodynamics,
Colorado Springs, 2007
Knowles et al.
Outline
• Introduction
• Flapping-Wing Problem
• Aerodynamic Model
• LEV stability
• Conclusions
Knowles et al.
Micro Air Vehicles • Defined as small flying vehicles with
Size/Weight: 150-230mm/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
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
Knowles et al.
Wing Kinematics – 1
• Flapping Motion sweeping heaving pitching
• Key Phases Translational
downstroke upstroke
Knowles et al.
Wing Kinematics – 1
• Flapping Motion sweeping heaving pitching
• Key Phases Translational
downstroke upstroke
Rotational stroke reversal high angle of attack
Knowles et al.
Wing Kinematics – 2
Knowles et al.
Mechanical Implementation
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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.
Aerodynamics
• Key phenomena unsteady
aerodynamics apparent mass Wagner effect returning wake
leading-edge vortex
[Pho
to: P
rene
l et a
l 199
7]
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
+
centre ofrotation
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
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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
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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 Re 5000
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Secondary vortices
Re = 1000 Re = 5000
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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
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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?
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