Experimental Studies of

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13 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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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 University

Defence Academy of the UK

Shrivenham, England

3
rd

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

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Mechanical Implementation


Knowles et al.

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

[Photo: Prenel
et al

1997]

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

+
c
e
n
t
r
e
o
f
r
o
t
a
t
i
o
n
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

Knowles et al.

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

Re = 10

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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

Knowles et al.

Secondary vortices

Re = 1000 Re = 5000

Knowles et al.

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

Knowles et al.

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?