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TECHNICAL PAPERS

Characteristics of Small Vortices in a Turbulent Axisymmetric Jet

[+] Author and Article Information
Sudhaker Chhabra

Department of Mechanical Engineering, University of Delaware, Newark, DE 19716-3140

Pablo Huq1

Department of Mechanical Engineering, University of Delaware, Newark, DE 19716-3140

Ajay K. Prasad2

Department of Mechanical Engineering, University of Delaware, Newark, DE 19716-3140prasad@me.udel.edu

Clockwise eddies are denoted by negative radius values in the legend in Fig. 4.

It should be noted that slopes of the Re=140 and 600 lines are close to 2 if vortices of larger radii are ignored.

1

College of Marine Studies, University of Delaware.

2

Corresponding author.

J. Fluids Eng 128(3), 439-445 (Oct 25, 2005) (7 pages) doi:10.1115/1.2173292 History: Received September 10, 2004; Revised October 25, 2005

Characteristics of small vortices were studied in axisymmetric jets wherein the Kolmogorov scale was approached by progressively decreasing the Reynolds number while still maintaining turbulent flow. A periodic forcing introduced far upstream of the jet nozzle ensured that the jet was turbulent. A vortex eduction tool was developed and applied to the high-pass filtered 2D velocity field in the axial plane of a turbulent jet while varying Re between 140 and 2600. Vortex population, energy, vorticity, and rms (root-mean-square velocity fluctuations) of the high-pass filtered field were measured to elucidate vortex characteristics. The observed population of vortices decreases dramatically at the Kolmogorov scale. The observed increase in vortex population with decreasing vortex size appears to be in accord with the space-filling argument, in that the vortex population in a two-dimensional domain should grow as R2. The energy density curve obtained from vortex statistics reproduces the 53 slope for the inertial subrange, and the high-pass filtered field accounts for approximately two-thirds of the total rms.

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Copyright © 2006 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Schematic of experimental setup

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Figure 2

Time-averaged streamwise velocity profiles for Re=250: (a) perturbation turned on (turbulent jet) and (b) perturbation turned off (laminar jet)

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Figure 3

Flow chart for identifying a vortex center and determining its radius

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Figure 4

Variation of number of vortices (per frame) with Reynolds number. Negative radius values in the legend correspond to clockwise eddies.

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Figure 5

Linear fits for ratio of vortex population and radius

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Figure 6

Variation in vortex energy with vortex size (energy averaged per frame)

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Figure 7

Variation of nondimensional vortex energy with vortex size

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Figure 8

Variation of centerline rms (u′) of high-pass filtered velocity field with Reynolds number

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Figure 10

Variation of nondimensional vorticity with vortex size

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Figure 9

Variation in average vorticity with vortex size

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