0
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.

Copyright © 2016 by ASME
Your Session has timed out. Please sign back in to continue.

References

Figures

Grahic Jump Location
Fig. 1

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

Grahic Jump Location
Fig. 2

Streamwise turbulence decay for grids A and B

Grahic Jump Location
Fig. 3

Percentage uncertainty in U and urms

Grahic Jump Location
Fig. 4

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

Grahic Jump Location
Fig. 5

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

Grahic Jump Location
Fig. 6

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

Grahic Jump Location
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

Grahic Jump Location
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

Grahic Jump Location
Fig. 9

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

Grahic Jump Location
Fig. 10

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

Grahic Jump Location
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%

Grahic Jump Location
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%

Grahic Jump Location
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

Grahic Jump Location
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

Grahic Jump Location
Fig. 15

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

Grahic Jump Location
Fig. 16

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

Grahic Jump Location
Fig. 17

Amplification of disturbances for different Re and Tu

Grahic Jump Location
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

Grahic Jump Location
Fig. 19

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

Grahic Jump Location
Fig. 20

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

Grahic Jump Location
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

Grahic Jump Location
Fig. 22

Variation of shape factor for different Re and Tu

Grahic Jump Location
Fig. 23

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

Grahic Jump Location
Fig. 24

Variation of Clauser parameter illustrating boundary layer relaxation

Tables

Errata

Discussions

Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging and repositioning the boxes below.

Related Journal Articles
Related eBook Content
Topic Collections

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In