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Research Papers: Fundamental Issues and Canonical Flows

Features of a Laminar Separated Boundary Layer Near the Leading-Edge of a Model Airfoil for Different Angles of Attack: An Experimental Study

[+] Author and Article Information
K. Anand

Department of Mechanical Engineering,
Indian Institute of Technology Kanpur,
Kanpur, Uttar Pradesh 208016, India

S. Sarkar

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

1Present address: Department of Mechanical Engineering, SASTRA University, Thanjavur, Tamilnadu 613 401, India.

2Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received October 7, 2015; final manuscript received August 26, 2016; published online November 3, 2016. Editor: Malcolm J. Andrews.

J. Fluids Eng 139(2), 021201 (Nov 03, 2016) (14 pages) Paper No: FE-15-1723; doi: 10.1115/1.4034606 History: Received October 07, 2015; Revised August 26, 2016

The evolution of a separated boundary layer over a model airfoil with semicircular leading-edge has been illustrated for angles of attack (α) varying from −3 deg to 10 deg, where the Reynolds number (Rec) based on chord is 1.6 × 105 and the inlet freestream turbulence (fst) being 1.2%. The features of boundary layer are described through measurements of velocity and surface pressure besides the flow visualization using a planar particle image velocimetry (PIV). Freestream perturbations are amplified because of enhanced receptivity of the separated boundary layer resulting in pockets of disturbances, which then propagate downstream attributing to random fluctuations near the reattachment. The separation and reattachment locations including the onset and end of transition are identified for changing α. The reattachment point changes from 18.8% to 47.7% of chord with the onset of separation at almost 7%, whereas the onset of transition moves upstream from 13.2% to 9% with increasing α. The bubble bursting occurs at α = 10 deg. The transition in the separated boundary layer occurs through Kelvin–Helmholtz (K–H) instability for α = 0 deg and 3 deg, whereas the K–H mechanism is bypassed for higher α with significant viscous effect.

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Figures

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Fig. 1

Schematic of the tunnel and the model

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Fig. 2

Sketch illustrating a separated boundary layer with onset of separation, transition, and reattachment point on the model airfoil. The origin is the point where the flat portion is blended tangentially with the semicircular leading-edge.

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Fig. 3

Contours of (a) mean and ((b) and (c)) instantaneous streamwise velocity superimposed with vectors for α = 0 deg

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Fig. 4

Contours of (a) mean and ((b) and (c)) instantaneous streamwise velocity superimposed with vectors for α = 5 deg

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Fig. 5

Contours of (a) mean and (b) instantaneous streamwise velocity superimposed with vectors for α = 10 deg depicting bubble bursting

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Fig. 6

Instantaneous contours of ωz and−λ2 superimposed with velocity fluctuation vectors for α = 0 deg ((a) and (b)) and α = 5 deg ((c) and (d))

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Fig. 7

(a) Variations of surface pressure on the model for various α: Zoomed view indicates that the maximum pressure moves upstream of theoretical stagnation point for high α. (b) Identification of onset of separation, transition, and reattachment for α = 0 deg.

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Fig. 8

Comparison of Cp with data of Arena and Mueller [8] on a similar model configuration

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Fig. 9

Identification of onset of separation, transition, and reattachment from the spatial growth rate of Cp for (a) α = 0 deg and (b) α = 3 deg

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Fig. 10

Variation of laminar shear layer length ll/lb as a function of α and is compared with data of Arena and Mueller [8]

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Fig. 11

Time-averaged streamwise velocity profiles obtained from hotwire measurements for (a) α = 0 deg, (b) α = 5 deg, and (c) α = 10 deg. Inflection layer, where ∂2u¯/∂y2=0, is represented by dashed line.

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Fig. 12

urms profiles along the model for (a) α = 0 deg, (b) α = 5 deg, and (c) α = 10 deg

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Fig. 13

vrms profiles along the model for (a) α = 0 deg, (b) α = 5 deg, and (c) α = 10 deg

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Fig. 14

Contours of turbulent kinetic energy in the separated region from PIV measurements for α = 0 deg and 5 deg

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Fig. 15

Normalized shear stress distribution in the separated region from PIV measurements, illustrating the onset of transition for α = 0 deg and 5 deg

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Fig. 16

Time traces of velocity fluctuations u′ along the inflection line of the separated boundary layer for α = 0 deg, 3 deg, 5 deg, and 10 deg

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Fig. 17

Spectra of streamwise velocity fluctuations along the inflection line of the separated boundary layer for α = 0 deg, 3 deg, 5 deg, and 10 deg

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Fig. 18

Amplification of maximum (a) urms and (b) vrms in the streamwise direction illustrating development of the separated boundary layer for α = 0 deg, 3 deg, 5 deg, and 10 deg

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Fig. 19

Streamwise velocity superimposed with its gradient toillustrate the critical layer for α = 0 deg. Triangle denotes normalized velocity gradient ϕ = (∂u/∂y)/(∂u/∂y)max and square indicates velocity u/U (values within the flow reversal region are deleted).

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Fig. 20

Variation of intermittency along the inflection line of the shear layer for α = 0 deg and 5 deg

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Fig. 21

Variations of boundary layer integral parameters for α = 0 deg, 3 deg, 5 deg, and 10 deg (solid symbols represent δ*)

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