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Research Papers: Fundamental Issues and Canonical Flows

Sweeping Jets Issuing From the Face of a Backward-Facing Step

[+] Author and Article Information
Brian T. Bohan, Marc D. Polanka, James L. Rutledge

Air Force Institute of Technology,
WPAFB, OH 45433

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received November 9, 2018; final manuscript received April 17, 2019; published online June 3, 2019. Assoc. Editor: Timothy Lee.This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.

J. Fluids Eng 141(12), 121201 (Jun 03, 2019) (17 pages) Paper No: FE-18-1755; doi: 10.1115/1.4043576 History: Received November 09, 2018; Revised April 17, 2019

This study quantified the performance and fluid disbursal capabilities of several fluidic oscillator variations injecting from the face of a backward-facing step. These devices were designed as a replacement for a pair of nonoscillating fuel injector jets in an ultra-compact combustor. However, these results have relevance whenever fluid is injected from the face of a backward-facing step making the oscillator performance widely applicable. The oscillators were tested with and without coflow and at varying coflow velocities, which controlled the strength of the recirculation behind the backward-facing step. The fluidic oscillators investigated included single as well as paired oscillators that produced in-phase and out-of-phase synchronized jets. The injected fluid disbursal was found to be dependent on the velocity ratio of the freestream air and the injecting jet velocity. Additionally, the oscillation angle was found to be a function of Reynolds number due to the interaction of the oscillating jet with the walls of the models used in the present study. Finally, the oscillation frequency was found to be independent of Reynolds number, throat aspect ratio, working gas, and model scale, which resulted in a Strouhal number of 0.017. This result was supported by nondimensionalizing the published data from several other studies.

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Figures

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Fig. 1

Three-dimensional printed 12-step ring hardware

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Fig. 2

Fluidic oscillator schematic

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Fig. 3

Fluidic oscillator sequence of switching. Adapted from Ref. [4].

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Fig. 4

Linked fluidic oscillators for (a) in-phase jets and (b) out-of-phase jets. Adapted from Ref. [18].

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Fig. 5

Comparison of the oscillator model to the 12-step ring: (a) 12-step ring of the UCC and (b) oscillator model

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Fig. 6

Representative model assembly shown for the single oscillator

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Fig. 7

Oscillators printed into the model base

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Fig. 8

Z-type schlieren setup in the AFRL Discovery wind tunnel facility with light ray paths shown

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Fig. 9

Baseline nonoscillating parallel jets, time-averaged composite images at the lowest and highest CO2 flow rates at two wind tunnel configurations: (a) Vjet = 3.0 m/s, V = 0 m/s, (b) Vjet = 10.9 m/s, V = 0 m/s, (c) Vjet = 3.0 m/s, V = 23 m/s, fuel only model upper configuration, and (d) Vjet = 10.9 m/s, V = 32 m/s, fuel only model upper configuration

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Fig. 10

Secondary circulation behavior observed with the baseline nonoscillating parallel jets using a time-averaged composite image with the model in its original orientation and flipped over: (a) Vjet = 7.6 m/s, V = 28 m/s, model right side up and (b) Vjet = 7.6 m/s, V = 28 m/s, model up side down

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Fig. 11

Secondary circulation pattern observed behind backward-facing steps with injection

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Fig. 12

Single oscillator, time-averaged composite images at the lowest and highest CO2 flow rates at two wind tunnel configurations: (a) Vjet = 7.8 m/s, V = 0 m/s, (b) Vjet = 27.8 m/s, V = 0 m/s, (c) Vjet = 7.8 m/s, V = 23 m/s, fuel only model upper configuration, and (d) Vjet = 27.8 m/s, V = 32 m/s, fuel only model upper configuration

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Fig. 13

Computational velocity vectors showing the interaction of jet induced vortices (highlighted) and the walls for a low and high speed jet

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Fig. 14

Montage image of instantaneous frames spaced 1.8 ms apart showing the in-phase oscillation motion of the in-phase oscillator. The jets are highlighted by colored lines.

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Fig. 15

In-phase linked oscillator, time-averaged composite images at the lowest and highest CO2 flow rates at two wind tunnel configurations: (a) Vjet = 7.8 m/s, V = 0 m/s, (b) Vjet = 27.8 m/s, V = 0 m/s, (c) Vjet = 7.8 m/s, V = 23 m/s, fuel only model upper configuration, and (d) Vjet = 27.8 m/s, V = 32 m/s, fuel only model upper configuration

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Fig. 16

Montage image of instantaneous frames spaced 2.2 ms apart showing the out-of-phase oscillation motion of the out-of-phase oscillator. The jets are highlighted by colored lines.

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Fig. 17

Out-of-phase linked oscillator, time-averaged composite images at the lowest and highest CO2 flow rates at two wind tunnel configurations: (a) Vjet = 7.8 m/s, V = 0 m/s and (b) Vjet = 27.8 m/s, V = 0 m/s

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Fig. 18

Baseline, nonoscillating jets, time-averaged composite images at a CO2 jet velocity of 8.1 m/s (1.5 × 10−4 kg/s) with the fuel only step model: (a) V = 0 m/s, velocity ratio = 0, (b) V = 4 m/s, velocity ratio = 0.50, (c) V = 6 m/s, velocity ratio = 0.74, (d) V = 8 m/s, velocity ratio = 0.99, (e) V = 10 m/s, velocity ratio = 1.24, (f) V = 14 m/s, velocity ratio = 1.74, (g) V = 18 m/s, velocity ratio = 2.23, and (h) V = 28 m/s, velocity ratio = 3.47

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Fig. 19

Baseline, nonoscillating jets, time-averaged composite images at a CO2 jet velocity of 13.5 m/s (2.7 × 10−4 kg/s) with the fuel only step model: (a) V = 0 m/s, velocity ratio = 0, (b) V = 4 m/s, velocity ratio = 0.30, (c) V = 8 m/s, velocity ratio = 0.59, (d) V = 12 m/s, velocity ratio = 0.89, (e) V = 16 m/s, velocity ratio = 1.18, (f) V = 20 m/s, velocity ratio = 1.48, and (g) tunnel = 24 m/s, velocity ratio = 1.78

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Fig. 20

Single oscillator, time-averaged composite images at a CO2 jet velocity of 19.1 m/s (1.5 × 10−4 kg/s) with the fuel only step model: (a) V = 0 m/s, velocity ratio = 0, (b) V = 4 m/s, velocity ratio = 0.21, (c) V = 8 m/s, velocity ratio = 0.42, (d) V = 12 m/s, velocity ratio = 0.63, (e) V = 16 m/s, velocity ratio = 0.84, (f) V = 19 m/s, velocity ratio = 0.99, (g) V = 20 m/s, velocity ratio = 1.04, (h) V = 24 m/s, velocity ratio = 1.25, and (i) V = 28 m/s, velocity ratio = 1.46

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Fig. 21

Single oscillator, time-averaged composite images at a CO2 jet velocity of 8.4 m/s (6.4 × 10−5 kg/s) with the fuel only step model: (a) V = 0 m/s, velocity ratio = 0, (b) V = 4 m/s, velocity ratio = 0.48, (c) V = 8 m/s, velocity ratio = 0.95, (d) V = 12 m/s, velocity ratio = 1.43, and (e) V = 16 m/s, velocity ratio = 1.91

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Fig. 22

In-phase linked oscillator, time-averaged composite images at a CO2 jet velocity of 19.1 m/s (1.5 × 10−4 kg/s) with the fuel only step model: (a) V = 0 m/s, velocity ratio = 0, (b) V = 4 m/s, velocity ratio = 0.21, (c) V = 8 m/s, velocity ratio = 0.42, (d) V = 12 m/s, velocity ratio = 0.63, (e) V = 16 m/s, velocity ratio = 0.84, (f) V = 19 m/s, velocity ratio = 1.00, (g) V = 20 m/s, velocity ratio = 1.05, (h) V = 24 m/s, velocity ratio = 1.26, and (i) V = 28 m/s, velocity ratio = 1.47

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Fig. 23

In-phase linked oscillator, time-averaged composite images at a CO2 jet velocity of 8.4 m/s (6.4 × 10−5 kg/s) with the fuel only step model: (a) V = 0 m/s, velocity ratio = 0, (b) V = 4 m/s, velocity ratio = 0.47, (c) V = 8 m/s, velocity ratio = 0.94, (d) V = 12 m/s, velocity ratio = 1.42, and (e) V = 16 m/s, velocity ratio = 1.89

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Fig. 24

Correlation of jet penetration depth into the freestream normalized by backward-facing step height relative to velocity ratio

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Fig. 25

Experimental oscillator frequency measurements taken from high-speed video files and pressure transducers

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Fig. 26

Nondimensional correlation of oscillator frequency using the throat width as the reference length

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Fig. 27

Correlation of oscillator angle to a throat width scaled Reynold number (ReW)

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