experimental investigation of the freestream turbulence … · · 2017-12-15swept wing with a...
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Experimental investigation of the freestream turbulence approaching a swept wing with a blunt leading edgeIsabella Fumarola, Mike Gaster, Chris Atkin
DiPaRT – 21st November 2017
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Introduction
• Piercy and Richardson 1928, 1930 – Turbulence in front of the leading edge.
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Sadeh and Brauber, 1981
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Kerr et al., 1994
• Sutera et al.1963 - Vorticity amplification theory
• Kerr, Dold 1994 – Periodic steady vortices in a stagnation-point flow.
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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).
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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
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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∞
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??
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.
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Previous experimentTwo experiments:
• Straight cylinder • Swept wing NACA0050
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Previous investigation
Q∞
Z
XΛ
Z
XU∞
Straight cylinder Swept NACA 0050
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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.
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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.
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Flow direction
Advantages:
- flow at the leading edge: Hiemenz flow
- Boundary layer relatively thick at the attachment line.
- Flat surface, experimentally convenient.
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Model design• Sweep angle 50˚.
• Width of the leading edge to ensure blockage < 20%.
• Swept back due to wind tunnel constraints.
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Flow direction
Wood wing body
Two aluminium inserts
High quality surface
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Attachment-line contamination• 𝑅𝑒 = 240at 18 m/s to avoid attachment-line contamination.
• The flow was turbulent already at low velocity.
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Wind tunnel top view – flow right to left
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Attachment-line contamination
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• 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
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Attachment-line contamination
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New attachment line
laminar
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Model
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Gaster wind tunnel at City University of London Test section 0.91m x 0.91m x 3m Tu < 0.01% at 20m/s.
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y
x
Experiments
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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
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Base flow
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yx
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Base flow
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Base flow – boundary layer
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Spectra
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Base flow - Spectra
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Outside the BL for different speeds Inside the boundary layer
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HW1 Vibration
Traverse vibration test
With and without tape
20Hz and 50Hz disappeared
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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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How to increase turbulence
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Vorticity amplification theory:
“Only the vorticity properly oriented goes under amplification approaching the stagnation point”
STR ING
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How to increase turbulence
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Vorticity amplification theory:
“Only the vorticity properly oriented goes under amplification approaching the stagnation point”
STR ING
GRID
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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
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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
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H2
No disturbance
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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
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H2 – inside the boundary layer
No disturbances
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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
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downstreamH2 – outside the boundary layer
No disturbances
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Vertical string
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V1
740mm
1560mm
Exp. String d U∞ Red x/d Flow
V1 0.23 mm 6-10-15-18 m/s 91-240 1782 Laminarx
y
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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
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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
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External turbulence - Grid
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Grid: parallel rods d=3mm rods, M=20mm, horizontal or vertical orientation, Tu~1%.
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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
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External turbulence – GridSadeh,Sutera, Maeder 1970
Re=250000
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No grid
𝑇𝑢8 =𝑢𝑈100
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□ perpendicular to s.l. ○ parallel to s.l(grid parallel to the chord) (grid perpendicular to the chord)
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Swept experiment Q∞=6-18m/sU∞=4.8-14.5m/s Reu=28096-84289V∞=3.5-10.6m/s Rev=20413-61239
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External turbulence - Hor. vs Vert. Grid
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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.
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Thank [email protected]
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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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