integrated computational and experimental studies of flapping-wing micro air vehicle aerodynamics...

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

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

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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

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Wing Kinematics – 1

• Flapping Motion sweeping heaving pitching

• Key Phases Translational

downstroke upstroke

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Wing Kinematics – 1

• Flapping Motion sweeping heaving pitching

• Key Phases Translational

downstroke upstroke

Rotational stroke reversal high angle of attack

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Wing Kinematics – 2

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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

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Aerodynamics

• Key phenomena unsteady

aerodynamics apparent mass Wagner effect returning wake

leading-edge vortex

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to: P

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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.

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Questions?