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Research Papers: Fundamental Issues and Canonical Flows

Transition of a Steady to a Periodically Unsteady Flow for Various Jet Widths of a Combined Wall Jet and Offset Jet

[+] Author and Article Information
Tanmoy Mondal

Department of Mechanical Engineering,
Indian Institute of Technology Kharagpur,
West Bengal 721302, India

Manab Kumar Das

Professor
Department of Mechanical Engineering,
Indian Institute of Technology Kharagpur,
West Bengal 721302, India
e-mail: manab@mech.iitkgp.ernet.in

Abhijit Guha

Professor
Department of Mechanical Engineering,
Indian Institute of Technology Kharagpur,
West Bengal 721302, India

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received February 10, 2015; final manuscript received February 3, 2016; published online April 22, 2016. Assoc. Editor: Sharath S. Girimaji.

J. Fluids Eng 138(7), 070907 (Apr 22, 2016) (11 pages) Paper No: FE-15-1099; doi: 10.1115/1.4032750 History: Received February 10, 2015; Revised February 03, 2016

In the present paper, a dual jet consisting of a wall jet and an offset jet has been numerically simulated using two-dimensional unsteady Reynolds-Averaged Navier–Stokes (RANS) equations to examine the effects of jet width (w) variation on the near flow field region. The Reynolds number based on the separation distance between the two jets (d) has been considered to be Re = 10,000. According to the computational results, three distinct flow regimes have been identified as a function of w/d. For w/d ≤ 0.5, the flow field remains to be always steady with two counter-rotating stable vortices in between the two jets. On the contrary, within the range of 0.6 ≤ w/d < 1.6, the flow field reveals a periodic vortex shedding phenomenon similar to what would be observed in the wake of a two-dimensional bluff body. In this flow regime, the Strouhal number of vortex shedding frequency decreases monotonically with the progressive increase in the jet width. For w/d ≥ 1.6, the periodic vortex shedding is still evident, but the Strouhal number becomes insensitive to the variation of jet width.

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Figures

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Fig. 7

Mean velocity vector of a dual jet flow consisting of a wall jet and an offset jet for w/d = 1.5

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Fig. 6

Variation of maximum mean streamwise velocity (U¯m) with the downstream distance X for an offset jet

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Fig. 5

Variation of maximum mean streamwise velocity (U¯m) with the downstream distance X for a wall jet

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Fig. 4

Variation of maximum mean streamwise velocity (U¯m) with the downstream distance X for a dual jet with d/w = 1

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Fig. 3

Grid layout of the computational domain for dual jet flow with w/d = 1

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Fig. 2

Streamwise velocity profiles of dual jet flow with w/d = 1 for three different grid densities at downstream distances X = 1 (converging region), X = 10 (merging region), and X = 30 (wall jet region)

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Fig. 1

Schematic diagram of a typical dual jet flow consisting of a wall jet and an offset jet, where the jet width (w) is larger than the separation distance between the two jets (d)

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Fig. 8

Instantaneous streamline patterns of dual jet flow showing the transition of flow from a purely steady-state to a periodically unsteady-state (a) two steady stable counter-rotating stable vortices for w/d = 0.5 and (b) periodic vortex shedding for w/d = 0.6

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Fig. 9

Instantaneous streamline patterns of dual jet flow for w/d = 2 showing the periodic vortex shedding phenomenon within a time period T at various time instants: (a) T/4, (b) T/2, (c) 3T/4, and (d) T

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Fig. 10

Instantaneous vorticity contours of dual jet flow for (a) w/d = 0.5, (b) w/d = 0.6, (c) w/d = 1, and (d) w/d = 2 (solid and dashed lines indicate positive and negative vorticity, respectively)

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Fig. 11

Instantaneous vorticity contours of dual jet flow for w/d = 1.5 showing the generation of von Kármán like vortex street within a time period T at various time instants: (a) T/4, (b) T/2, (c) 3T/4, and (d) T

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Fig. 16

Variation of Strouhal number (St) with the jet width w/d

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Fig. 12

Time series of streamwise (U) and transverse (V) velocity components depicting (a) steady flow for w/d = 0.5 and (b)–(d) self-sustained periodic flow for w/d = 0.6, 1, and 2, respectively

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Fig. 13

FFT of transverse velocity (V) signals in the range w/d = 0.6–2 for which periodic vortex shedding occurs

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Fig. 14

Phase diagram of streamwise (U) and transverse (V) velocity signals for (a) w/d = 0.5 (a single point) and (b) w/d = 1.4 (a closed-loop)

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Fig. 15

PDF of transverse velocity signal (V) for the cases of w/d at which regular vortex shedding occurs displaying bimodal distribution (two distinct peaks)

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