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
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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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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
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Outline
• Introduction
• Flapping-Wing Problem
• Aerodynamic Model
• LEV stability
• Conclusions
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Micro Air Vehicles • Defined as small flying vehicles with
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
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Aerodynamics
• Key phenomena unsteady
aerodynamics apparent mass Wagner effect returning wake
leading-edge vortex
[Pho
to: P
rene
l et a
l 199
7]
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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
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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
~
~
^
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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
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Flow Visualisation Output
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Impulsively-started plate
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Validation of Model
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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?
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2D flows at low Re
Re = 5
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Influence of Reynolds number
α = 45°
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2D flows
Re = 500, α = 45°
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Influence of Reynolds number
α = 45°
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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
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Structure of 3D LEV
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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.