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Research Papers: Fluid-Structure Interaction

# Flow-Acoustic Coupling in $T$-Junctions: Effect of $T$-Junction Geometry

[+] Author and Article Information

Department of Mechanical Engineering, McMaster University, 1280 Main Street West, Hamilton, ON, L8S 4L7, Canadaziadas@mcmaster.ca

K. W. McLaren

Department of Mechanical Engineering, McMaster University, 1280 Main Street West, Hamilton, ON, L8S 4L7, Canada

Y. Li

Department of Applied Physics, Eindhoven University of Technology, Eindhoven, 5612 AZ, The Netherlands

J. Pressure Vessel Technol 131(4), 041302 (Jul 14, 2009) (14 pages) doi:10.1115/1.3148188 History: Received February 15, 2008; Revised October 25, 2008; Published July 14, 2009

## Abstract

The flow-acoustic coupling mechanism in a $T$-junction, which combines flows from two branches, forming the “cross-bar” of the $T$-junction, into one pipe, forming the “stem” of the $T$-junction, is investigated experimentally. The $T$-junction has a step pipe expansion at its inlets. The shear layer separating from this step expansion is found to excite intense acoustic resonances over multiple ranges of flow velocity. The excited acoustic mode is confined to the branch pipes and has an acoustic pressure node at the centerline of the $T$-junction. The length of the expansion section of the $T$-junction is found to control the frequency of the shear layer oscillation and therefore determines the ranges of flow velocity over which acoustic resonances are excited. Introducing asymmetry in the $T$-junction expansion length has shown little influence on the excitation of acoustic resonance. An additional $T$-junction arrangement made of rectangular cross-sectional ducts is also investigated to facilitate a flow visualization study of unsteady flow structures in the $T$-junction during acoustic resonance, and thereby improve understanding of the acoustic resonance mechanism and the nature of the aero-acoustic sources in the $T$-junction.

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Copyright © 2009 by American Society of Mechanical Engineers
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## Figures

Figure 1

Schematic of a turbine by-pass piping system. (1) steam turbine isolation valve; (2) bypass isolation valve; (3) bypass control valve; and (4) T-junction being investigated.

Figure 2

Schematic of various T-junction geometries. (a) T-junction with transition zone, (b) T-junction without transition zone, and (c) T-junction with asymmetric transition zone.

Figure 3

Primary branch acoustic mode

Figure 4

Typical spectra of T-junction without transition zone. (a) Spectra in branch pipe and (b) spectra in main pipe (15).

Figure 5

Cylindrical pipe test setup. ● denotes microphone locations.

Figure 6

Rectangular duct T-junction test setup

Figure 7

Schematic of various rectangular duct T-junction geometries

Figure 8

Fog injection nozzle arrangement and geometry

Figure 9

Experimental arrangement for flow visualization

Figure 10

Typical waterfall plot of pressure spectra, L/D=2.5

Figure 11

Comparison of typical responses for T-junctions with and without a transition zone. ×, T-junction with transition zone, L/D=2; ▲, T-junction without transition zone.

Figure 12

(a) Normalized acoustic pressure of the primary mode as a function of reduced velocity, based on the branch pipe diameter d. ◆, L/D=1.0; ◼, 1.25; △, 1.5; ×, 1.75; and -×-, 2.0. (b) Normalized acoustic pressure of the primary mode as a function of reduced velocity, based on the branch pipe diameter d. -×- L/D=2.0; ◼, 2.5; ●, 3.75; and ▲, 5.0.

Figure 13

(a) Normalized acoustic pressure of the primary mode as a function of reduced velocity, based on half the transition zone length, L. ◆, L/D=1.0; ◼, 1.25; △, 1. 5; ×, 1.75; and -×-, 2.0. (b) Normalized acoustic pressure of the primary mode as a function of reduced velocity, based on half the transition zone length, L. -×-, L/D=2.0; ◼, 2.5; ● 3.75; and ▲ 5.0.

Figure 14

Normalized acoustic pressure of the primary mode as a function of the reduced velocity for the four tested rectangular duct cases

Figure 15

Comparison of normalized acoustic pressure of Case 1 to linear superposition of Cases 2 and Case 3. -×- Case 1; △, Case 2+Case 3.

Figure 16

Instantaneous flow structure images and sketches for a complete cycle of Mode A oscillation. V=1.65 and f=118 Hz.

Figure 17

Instantaneous flow sketches for the complete cycle of two vortices formed at the left outer and inner shear layers. V=1.65 and f=118 Hz.

Figure 18

Instantaneous flow structure images and sketches for a complete cycle of Mode B oscillation. V=0.95 and f=120 Hz.

Figure 19

(a) Normalized acoustic pressure of the primary mode as a function of reduced velocity, based on the larger of L1 and L2. L1/D=1.75. See Fig. 2 for definition of L1 and L2. ◆ L2/D=1.0; ◼ 1.25; △, 1.5; ×, 1.75; and -×-, 2.0. (b) Normalized acoustic pressure of the primary mode as a function of reduced velocity, based on the larger of L1 and L2. L1/D=1.75. See Fig. 2 for definition of L1 and L2. -×-, L2/D=2.0; ◼, 2.5; ●, 3.75; and ▲ 5.0.

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