experimental investigation of the freestream turbulence … · · 2017-12-15swept wing with a...

Experimental investigation of the freestream turbulence approaching a swept wing with a blunt leading edgeIsabella Fumarola, Mike Gaster, Chris Atkin

DiPaRT – 21st November 2017

Introduction

• Piercy and Richardson 1928, 1930 – Turbulence in front of the leading edge.

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Sadeh and Brauber, 1981

DiPaRT- 20th-22ndNovember2017,CFMSBristol&BathSciencePark

Kerr et al., 1994

• Sutera et al.1963 - Vorticity amplification theory

• Kerr, Dold 1994 – Periodic steady vortices in a stagnation-point flow.

Vorticity Amplification Theory• The vorticity amplification theory considers a case similar to the Hiemenz

flow.

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• Superimposing a disturbance in the form of a sinusoidal wave ofwavelength λ.

𝜆" = 2𝜋𝜈𝑎

)/+

𝑎=Hiemenz constant𝜈=kinematic viscosity

Sutera et al.1963

• The theory predicts that if thewavelength of the oncomingperturbation is greater than a certainnatural wavelength (𝜆"), the vorticity willbe amplified at the edge of the boundarylayer (δ).


Vorticity Amplification TheoryThe theory has been validated in many experimental investigations:

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• Sadeh et al. , 1970 à stagnation flat plate.

• Bearman , 1972 à wing with a blunt leading edge.

• Sadeh and Sullivan, 1980 à NACA65-010 aerofoil.

• Sadeh et Brauber, 1981 à cylinder.

• and more…

Sadeh and Brauber1981

𝑢-./

𝑥/𝑅


Aim & Motivation

Investigate the behaviour of the vortices when a sweep angleis added to the model.

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U∞=Q∞

Vorticity amplification theory

?V

Λ

Q∞

V∞U∞


??

Aim & Motivation

• The region in front of the leading edge has been studied bothexperimentally and theoretically in un-swept cases. No informationregarding swept wings seems to be available in the literature.

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• The swept case may be of interest to better understand the receptivityprocess due to freestream turbulence in the cross-flow instability, one ofthe main mechanisms of boundary-layer transition from laminar toturbulent.


Previous experimentTwo experiments:

• Straight cylinder • Swept wing NACA0050

DiPaRT- 20th-22ndNovember2017,CFMSBristol&BathSciencePark 7

Previous investigation

Q∞

Z

XΛ

Z

XU∞

Straight cylinder Swept NACA 0050


Outline of the presentation

Outline of the presentation:

• Design of a new model suitable for studying attachment line flows.

• Analysis of swept Hiemenz flow in low turbulence wind tunnel.

• How to vary the incoming turbulence.

• Comparison of low and high turbulence.

• Conclusions and further works.

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Swept model in lower turbulence wind tunnel.


Model designAim: studying the flow at the leading edge of a swept wing.

Model: vertical flat plate fitted in a faring shape, similar to the wing used by Bearman 1971 but swept.


Flow direction

Advantages:

- flow at the leading edge: Hiemenz flow

- Boundary layer relatively thick at the attachment line.

- Flat surface, experimentally convenient.

Model design• Sweep angle 50˚.

• Width of the leading edge to ensure blockage < 20%.

• Swept back due to wind tunnel constraints.

11DiPaRT- 20th-22ndNovember2017,CFMSBristol&BathSciencePark

Flow direction

Wood wing body

Two aluminium inserts

High quality surface

Attachment-line contamination• 𝑅𝑒 = 240at 18 m/s to avoid attachment-line contamination.

• The flow was turbulent already at low velocity.


Wind tunnel top view – flow right to left

Attachment-line contamination


• 𝑅𝑒 = 240at 18 m/s to avoid attachment-line contamination

• Gaster’s device (Gaster,1965)

• Flow laminar up to 25 m/s

Wind tunnel top view – flow right to left

Attachment-line contamination

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New attachment line

laminar


Model


Gaster wind tunnel at City University of London Test section 0.91m x 0.91m x 3m Tu < 0.01% at 20m/s.

y

x

Experiments


Single hot-wire anemometer, boundary layer probe.Band pass filtered between 2Hz-10kHz

The sensor is traversed parallel to the leading edge along the attachment line.

Traverse

Base flow


yx

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Base flow


Base flow – boundary layer


Spectra

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Base flow - Spectra

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Outside the BL for different speeds Inside the boundary layer


HW1 Vibration

Traverse vibration test

With and without tape

20Hz and 50Hz disappeared


How to increase turbulence

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Vorticity amplification theory:

“Only the vorticity properly oriented goes under amplification approaching the stagnation point”



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STR ING



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STR ING

GRID


0 0.5 10

0.5

1

1.5

2

2.5

3

V/Ve

X [m

m]

0 0.05 0.1 0.15 0.20

0.5

1

1.5

2

2.5

3

VRMS

/Ve

X [m

m]

H1 0.23mmH2 0.23mm

Horizontal string

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Exp. String d U∞ Red x/d Flow

H1 0.23 mm 18 m/s 274 3217 – 6782 Turbulent

H2 0.23 mm 18 m/s <274 613 – 3939 Laminar

H2 H1

740mm141mm

1560mm906 mm

x

y


Horizontal string

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H1 0.23 mm 18 m/s 274 3217 – 6782 Turbulent

H2 0.23 mm 18 m/s <274 613 – 3939 Laminar

H2 H1

740mm141mm

1560mm906 mm

x

y


H2

No disturbance

Horizontal string

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H1 0.23 mm 18 m/s 274 3217 – 6782 Turbulent

H2 0.23 mm 18 m/s <274 613 – 3939 Laminar

H2 H1

740mm141mm

1560mm906 mm

x

y


H2 – inside the boundary layer

No disturbances

Horizontal string

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H1 0.23 mm 18 m/s 274 3217 – 6782 Turbulent

H2 0.23 mm 18 m/s <274 613 – 3939 Laminar

H2 H1

740mm141mm

1560mm906 mm

x

y


downstreamH2 – outside the boundary layer

No disturbances

Vertical string

30

V1

740mm

1560mm


V1 0.23 mm 6-10-15-18 m/s 91-240 1782 Laminarx

y


Vertical string

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V1

740mm141mm

1560mm906 mm

V2V3Exp. String D U∞ Red x/d Conclusion

V1 0.23 mm 6-10-15-18 m/s 91 to 240 1782 LaminarV2 0.23 mm 18 m/s <274 1663 LaminarV3 0.15 mm 18 m/s 109 >40 Laminar

x

y


Vertical string

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V1

740mm141mm

1560mm906 mm

V2V3Exp. String D U∞ Red x/d Conclusion

V1 0.23 mm 6-10-15-18 m/s 91 to 240 1782 LaminarV2 0.23 mm 18 m/s <274 1663 LaminarV3 0.15 mm 18 m/s 109 >40 Laminar

x

y

Flow directionDiPaRT- 20th-22ndNovember2017,CFMSBristol&BathSciencePark

External turbulence - Grid

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Grid: parallel rods d=3mm rods, M=20mm, horizontal or vertical orientation, Tu~1%.


External turbulence - Grid

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Horizontal grid à cylinder parallel to x (parallel to the chord)Vertical grid à cylinders parallel to z (perpendicular to the chord)

horizontal

2D Vertical Flat Plate - Sadeh,Sutera, Maeder 1970

Horizontalgrid


External turbulence – GridSadeh,Sutera, Maeder 1970

Re=250000

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No grid

𝑇𝑢8 =𝑢𝑈100


□ perpendicular to s.l. ○ parallel to s.l(grid parallel to the chord) (grid perpendicular to the chord)


Swept experiment Q∞=6-18m/sU∞=4.8-14.5m/s Reu=28096-84289V∞=3.5-10.6m/s Rev=20413-61239

External turbulence - Hor. vs Vert. Grid


Conclusion• A model suitable to study the flow at the attachment line has

been presented.

• The flow does not present an increase in RMS goingtowards the model in a low turbulence environment.

• Two different techniques to vary the turbulence in the windtunnel have been investigated.

• Further work will be to insert a grid with a lower turbulenceintensity.


Thank [email protected]


The authors would like to acknowledge the financial support of the Engineering and Physical Sciences Research Council

under grant ref. EP/L024888/1 UK National Wind Tunnel Facility,co-ordinated by Imperial College.

And the support of InnovateUK and Enhanced Fidelity Transonic Wing, led by Airbus.

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experimental investigation of the freestream turbulence … · · 2017-12-15swept wing with a...

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