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Research Papers: Fundamental Issues and Canonical Flows

Effects of Free-Stream Turbulence on Transition of a Separated Boundary Layer Over the Leading-Edge of a Constant Thickness Airfoil

[+] Author and Article Information
A. Samson

Indian Institute of Technology Kanpur,
Kanpur, Uttar Pradesh 208016, India
e-mail: samson@iitk.ac.in

S. Sarkar

Professor
Mem. ASME
Indian Institute of Technology Kanpur, Kanpur,
Uttar Pradesh 208016, India
e-mail: subra@iitk.ac.in

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received April 23, 2014; final manuscript received June 29, 2015; published online September 3, 2015. Assoc. Editor: Feng Liu.

J. Fluids Eng 138(2), 021202 (Sep 03, 2015) (19 pages) Paper No: FE-14-1220; doi: 10.1115/1.4031249 History: Received April 23, 2014; Revised June 29, 2015

This paper describes the change in the transition mechanism of a separated boundary layer formed from the semicircular leading-edge of a constant thickness airfoil as the free-stream turbulence (fst) increases. Experiments are carried out in a low-speed wind tunnel for three levels of fst (Tu = 0.65%, 4.6%, and 7.7%) at two Reynolds numbers (Re) 25,000 and 55,000 (based on the leading-edge diameter). Measurements of velocity and surface pressure along with flow field visualizations are carried out using a planar particle image velocimetry (PIV). The flow undergoes separation in the vicinity of leading-edge and reattaches in the downstream forming a separation bubble. The shear layer is laminar up to 20% of separation length, and then, the perturbations are amplified in the second-half attributing to breakdown and reattachment. The bubble length is highly susceptible to change in Tu. At low fst, the primary mode of instability of the shear layer is Kelvin–Helmholtz (K-H), although the local viscous effect may not be neglected. At high fst, the mechanism of shear layer rollup is bypassed with transient growth of perturbations along with evidence of spot formation. The predominant shedding frequency when normalized with respect to the momentum thickness at separation is almost constant and shows a good agreement with the previous studies. After reattachment, the flow takes longer length to approach a canonical boundary layer.

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Figures

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Fig. 7

Mean streamwise velocity profiles from hotwire measurements at different locations for Re = 25,000: (a) x/D = 0.0875, (b) x/D = 0.2625, (c) x/D = 0.4375, (d) x/D = 0.6125, (e) x/D = 0.7875, (f) x/D = 1.05, (g) x/D = 1.3125, and (h) x/D = 1.75

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Fig. 8

Mean streamwise velocity profiles from hotwire measurements at different locations for Re = 55,000: (a) x/D = 0.06625, (b) x/D = 0.19875, (c) x/D = 0.33125, (d) x/D = 0.46375, (e) x/D = 0.59625, (f) x/D = 0.795, (g) x/D = 0.99375, and (h) x/D = 1.325

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Fig. 9

RMS of streamwise velocity profiles from hotwire measurements at different locations for Re = 25,000 (see Fig. 7 for legend and locations)

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Fig. 6

Time-averaged flow field illustrating mean bubble length for Re = 55,000

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Fig. 5

Time-averaged flow field illustrating mean bubble length for Re = 25,000

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Fig. 4

Pressure recovery coefficient on the top surface of the model from the theoretical stagnation point

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Fig. 3

Percentage uncertainty in U and urms

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Fig. 2

Streamwise turbulence decay for grids A and B

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Fig. 1

Schematic of test setup and wind tunnel along with turbulence generating grids (not to scale)

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Fig. 10

RMS of streamwise velocity profiles from hotwire measurements at different locations for Re = 55,000 (see Fig. 8 for legend and locations)

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Fig. 11

Contours of instantaneous streamwise velocity for Re = 25,000: (a) and (b) Tu = 0.65%, (c) and (d) Tu = 4.6%, and (e) Tu = 7.7%

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Fig. 12

Contours of instantaneous streamwise velocity for Re = 55,000: (a) and (b) Tu = 0.65%, (c) Tu = 4.6%, and (d) Tu = 7.7%

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Fig. 13

Instantaneous velocity spectra for Re = 25,000: (a) x/D = 0.2625, (b) x/D = 0.35, (c) x/D = 0.4375, (d) x/D = 0.525, (e) x/D = 0.7, and (f) x/D = 0.875

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Fig. 14

Instantaneous velocity spectra for Re = 55,000: (a) x/D = 0.19875, (b) x/D = 0.265, (c) x/D = 0.33125, (d) x/D = 0.3975, (e) x/D = 0.53, and (f) x/D = 0.6625

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Fig. 15

Variation of intermittency factor for varying Tu: (a)–(c) Re = 25,000 and (d)–(f) Re = 55,000

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Fig. 16

Spot production rate for different Re and Tu comparing with the previous studies and Mayle's [43] correlation

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Fig. 17

Amplification of disturbances for different Re and Tu

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Fig. 20

Variation of normalized Reynolds shear stress for Re = 55,000, indicating the onset of transition

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Fig. 21

Variation of displacement and momentum thickness compared with Blasius curves: □ δ* from experiments, ○ θ from experiments, ------ δ* from Blasius profile, and - - - - - - θ from Blasius profile

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Fig. 22

Variation of shape factor for different Re and Tu

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Fig. 23

Similarity of shape factor through the transition region for different Re and Tu

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Fig. 24

Variation of Clauser parameter illustrating boundary layer relaxation

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Fig. 18

Comparison of normalized urms profiles indicating the shift of turbulence activity from outer layer toward the wall for varying Tu: (a)–(c) Re = 25,000 and (d)–(f) Re = 55,000

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Fig. 19

Variation of normalized Reynolds shear stress for Re = 25,000, indicating the onset of transition

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