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Research Papers

Identifying Turbulent Spots in Transitional Boundary Layers

[+] Author and Article Information
Brendan Rehill

e-mail: brendan.rehill@ul.ie

Ed J. Walsh

Stokes Institute,
University of Limerick,
Limerick, Ireland

Luca Brandt

e-mail: luca@mech.kth.se

Philipp Schlatter

Linné Flow Centre,
KTH Mechanics,
Stockholm, Sweden

Tamer A. Zaki

Department of Mechanical Engineering,
Imperial College London,
London, UK
e-mail: t.zaki@imperial.ac.uk

Donald M. McEligot

Mechanical Engineering Department,
University of Idaho,
Idaho Falls, Idaho 83402
e-mail: donaldm@uidaho.edu

Contributed by the International Gas Turbine Institute (IGTI) of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received July 11, 2011; final manuscript received August 19, 2011; published online October 30, 2012. Editor: David Wisler.

J. Turbomach 135(1), 011019 (Oct 30, 2012) (8 pages) Paper No: TURBO-11-1121; doi: 10.1115/1.4006395 History: Received July 11, 2011; Revised August 19, 2011

An artificial turbulent spot is simulated in a zero free-stream turbulence base flow and a base flow with organized streaks. Six identification methods are used in order to isolate the turbulent spot from the surrounding nonturbulent fluid. These are (i) instantaneous wall-normal velocity v, (ii) instantaneous spanwise velocity w, (iii) instantaneous turbulent dissipation, (iv) λ2 criterion, (v) Q criterion, and (vi) gradient of the finite time Lyapunov exponent. All methods are effective in isolating the turbulent spot from the streaks. The robustness of each technique is determined from the sensitivity of the maximum spot dimensions to changes in threshold level. The Q criterion shows the least sensitivity for the zero free-stream turbulence case and the instantaneous turbulent dissipation technique is least sensitive in the organized streaks case. For both cases the v technique was the most sensitive to changes in threshold level.

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References

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Figures

Grahic Jump Location
Fig. 6

Vortex pair + organized streaks: visualization of wall-parallel plane at y/δ0*=4 showing edge of turbulent spot denoted by the black line for each identification method. (i) v, (ii) w, (iii) turbulent dissipation, (iv) λ2 criterion, (v) Q criterion, and (vi) FTLE gradient. Flow direction is from left to right.

Grahic Jump Location
Fig. 5

Vortex pair + zero FST: visualization of wall-parallel plane at y/δ0*=4 showing edge of turbulent spot denoted by the black line for each identification method. (i) v, (ii) w, (iii) turbulent dissipation, (iv) λ2 criterion, (v) Q criterion, and (vi) FTLE gradient. Flow direction is from left to right.

Grahic Jump Location
Fig. 4

Vortex pair + organized streaks: (a) x-z plane of FTLE field at wall-parallel plane at y/δ0*=4 and (b) streamwise gradient of FTLE field at the same wall-parallel plane. This isolates the spot from the surrounding streaks.

Grahic Jump Location
Fig. 3

Visualization of wall-parallel plane at y/δ0*=4 showing typical identification procedure. (a) Instantaneous wall-normal velocity, (b) threshold applied, black denotes regions above this threshold, (c) filtering and smoothing to remove holes within the spot, and (d) edge of turbulent spot denoted by black line superimposed on original wall-normal velocity image.

Grahic Jump Location
Fig. 2

Threshold levels based on streamwise- and spanwise-averaged instantaneous velocity

Grahic Jump Location
Fig. 1

Streamwise velocity in an x-z plane at a wall-normal position of y/δ0*=4. (a) Zero free-stream turbulence and (b) organized streaks.

Grahic Jump Location
Fig. 7

Vortex pair + zero FST: sensitivity analysis of maximum spot dimensions to ± 20% changes in threshold level. All dimensions are normalized with respect to the dimension at the reference threshold value (L = length, H = height, Wid = width, Vol = volume). (Blue - -×- -) v, (green - -▵- -) w, (light blue solid line) dissipation, (purple - -○- -) λ2, (black - -□- -) Q, (red dashed line) FTLE gradient.

Grahic Jump Location
Fig. 8

Vortex pair + organized streaks: sensitivity analysis of maximum spot dimensions to ±20% changes in threshold level. All dimensions are normalized with respect to the dimension at the reference threshold value (L = length, H = height, Wid = width, Vol = volume). (Blue - -×- -) v, (green - -▵- -) w, (light blue solid line) dissipation, (purple - -○- -) λ2, (black - -□- -) Q, (red dashed line) FTLE gradient.

Grahic Jump Location
Fig. 9

Vortex pair + zero FST: change in spot shape with change in threshold. Left: wall-parallel plane (y/δ0*=4). Right: centerline wall-normal plane (y/δ0*=0). * reference threshold, ○-20% reference threshold, □ +20% reference threshold.

Grahic Jump Location
Fig. 10

Vortex pair + organized streaks: change in spot shape with change in threshold. Left: wall-parallel plane (y/δ0*=4). Right: centerline wall-normal plane (z/δ0*=0). * reference threshold, ○-20% reference threshold, □ +20% reference threshold.

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