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Research Papers: Fundamental Issues and Canonical Flows

Aerodynamic Characteristics of Asymmetric Bluff Bodies

[+] Author and Article Information
J. C. Hu

Department of Mechanical Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China

Y. Zhou1

Department of Mechanical Engineering, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, Chinammyzhou@polyu.edu.hk

1

Corresponding author.

J. Fluids Eng 131(1), 011206 (Dec 11, 2008) (9 pages) doi:10.1115/1.2979229 History: Received October 09, 2007; Revised July 12, 2008; Published December 11, 2008

The wake of asymmetric bluff bodies was experimentally measured using particle imaging velocimetry, laser Doppler anemometry, load cell, hotwire, and flow visualization techniques at Re=26008500 based on the freestream velocity and the characteristic height of the bluff bodies. Asymmetry is produced by rounding some corners of a square cylinder and leaving others unrounded. It is found that, with increasing corner radius, the flow reversal region is expanded, and the vortex formation length is prolonged. Accordingly, the vortex shedding frequency increases and the base pressure rises, resulting in a reduction in the mean drag as well as the fluctuating drag and lift. It is further found that, while the asymmetric cross section of the cylinder causes the wake centerline to shift toward the sharp corner side of the bluff body, the wake remains globally symmetric about the shifted centerline. The near wake of asymmetric bluff bodies is characterized in detail, including the Reynolds stresses, characteristic velocity, and length scale, and is further compared with that of the symmetric ones.

Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Experimental details. (a) Cross-sectional geometry of cylinders. Asymmetric cylinders: (1) r/d=0.157, (2) 0.236, (3) 0.472. Symmetric cylinders: (4) r/d=0, (5) 0.157, (6) 0.236, (7) 0.472, (8) 0.5. (b) Installation of cylinder and load cell (top view). (c) Coordinate system (x,y) and definitions of θ, U1, and L0.

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Figure 4

Dependence of St (◻,◼) and Cd St (○,●) on corner radius: (a) Re=6000 and (b) 8500. Open symbols stand for the asymmetric cylinders and solid symbols represent the symmetric cylinders. The dashed lines denote the best fit curves to the data.

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Figure 5

LDA-measured cross-flow distributions at x/d=5 and Re=2600. Asymmetric cylinders: (a) 1−U¯∗ and (b) V¯∗. Symmetric cylinders: (c) 1−U¯∗ and (d) V¯∗. The dashed lines denote y/d=0.

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Figure 6

LDA-measured cross-flow distributions at x/d=5 and Re=2600. Asymmetric cylinders: (a) u2¯, (b) v2¯, and (c) uv¯∗. Symmetric cylinders: (d) u2¯, (e) v2¯, and (f) uv¯∗. The dashed lines denote y/d=0.

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Figure 7

Dependence of (a) maximum mean velocity deficit, U1∗ and (b) mean velocity wake half-width, L0∗, on corner radius. Re=2600 and x/d=5.

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Figure 8

Comparison between LDA and PIV measurements at x/d=5 in the wake of the asymmetric cylinder (r/d=0.157, Re=2600): (a) U¯∗, (b) urms∗, and (c) vrms∗

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Figure 9

PIV-measured isocontours of U¯∗ at Re=2600. Asymmetric cylinders: (b) r/d=0.157, (c) 0.236, (d) 0.472. Symmetric cylinders: (a) r/d=0 (square cylinder), (e) 0.5 (circular cylinder), (f) 0.157, (g) 0.236, (h) 0.472. Cutoff value U¯∗=0, contour increment=0.4. The dashed lines denote the wake centerline.

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Figure 10

Dependence of the wake recirculation region length, lc∗, on corner radius at Re=2600

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Figure 11

PIV-measured isocontours of urms∗ at Re=2600. Asymmetric cylinders: (b) r/d=0.157, (c) 0.236, (d) 0.472. Symmetric cylinders: (a) r/d=0 (square cylinder), (e) 0.5 (circular cylinder), (f) 0.157, (g) 0.236, (h) 0.472. Cutoff value urms∗=0.1, contour increment=0.05. The dashed lines denote the wake centerline.

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Figure 12

PIV-measured isocontours of vrms∗ at Re=2600. Asymmetric cylinders: (b) r/d=0.157, (c) 0.236, (d) 0.472. Symmetric cylinders: (a) r/d=0 (square cylinder), (e) 0.5 (circular cylinder), (f) 0.157, (g) 0.236, (h) 0.472. Cutoff value vrms∗=0.1, contour increment=0.1. The dashed lines denote the wake centerline.

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Figure 13

Dependence of the vortex formation length, lf∗, on corner radius at Re=2600

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Figure 14

Dependence of the lateral spacing, luy∗, between the two peaks of urms∗ on corner radius at Re=2600

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Figure 15

Typical photographs of flow visualization behind (a) r/d=0.472 (symmetric) and (b) r/d=0.5 (circular cylinder); Re=500

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