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 SensorsCranfield University

Defence Academy of the UKShrivenham, 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

Size/Weight: 150-230mm/50–100gEndurance: 20–60min

• Reasons for MAVs:Existing UAVs limited by large sizeNiche exists for MAVs – e.g. indoor flight, low altitude, man-portable

• MAV Essential (Desirable) Attributes:High efficiency High manoeuvrability at low speedsVertical flight & hover capabilitySensor-carrying; autonomous(Stealthy; durable)

Microgyro

Microsensors

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Why insect-like flapping? • Insects are more manoeuvrable• Power requirement:

Insect – 70 W/kg maximumBird – 80 W/kg minimumAeroplane – 150 W/kg

• Speeds:Insects ~ 7mphBirds ~ 15mph

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Wing Kinematics – 1• Flapping Motion

sweepingheavingpitching

• Key PhasesTranslational

downstrokeupstroke

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Wing Kinematics – 1• Flapping Motion

sweepingheavingpitching

• Key PhasesTranslational

downstrokeupstroke

Rotationalstroke reversalhigh angle of attack

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Wing Kinematics – 2

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Mechanical Implementation

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Generic insect wing kinematicsThree important differences when compared

to conventional aircraft:wings stop and start during flightlarge wing-wake interactionshigh angle of attack (45° or more)

Complex kinematics:difficult to determine difficult to understand difficult to reproduce

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Aerodynamics

• Key phenomenaunsteady aerodynamics

apparent massWagner effectreturning wake

leading-edge vortex

[Pho

to: P

rene

let a

l199

7]

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Aerodynamic Modelling – 1• Quasi-3D Model

• 2-D blade elements withattached flowseparated flow

leading-edge vortextrailing-edge wake

• Convert to 3-Dradial chords

+

centre ofrotation

Robofly wing

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Aerodynamic Modelling – 1• Quasi-3D Model

• 2-D blade elements withattached flowseparated flow

leading-edge vortextrailing-edge wake

• Convert to 3-Dradial chordscylindrical cross-planesintegrate along wing span

~

Φ

θ

ξ̂

η

η̂

ξ

η~

ξ

wing

ξ~

~ηη̂

η

ξ̂

ξ

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Aerodynamic Modelling – 2• Model Summary

6 DOF kinematicscirculation-based approachinviscid model with viscosity introduced indirectlynumerical implementation by discrete vortex methodvalidated against experimental data

Wing Geometry

Flow

Moment DataForce and

Aerodynamic Model

Wing Kinematics Visualisation

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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 stallInstead, stable, lift-enhancing (~80%) LEV createdFlapping wing MAVs (FMAVs) need to retain stable LEV for efficiencyWhy 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 (>10000) despite occurrence of Kelvin-Helmholtz instability.

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Questions?