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TECHNICAL PAPERS

# Experimental Investigation of Cavity-Induced Acoustic Oscillations in Confined Supersonic Flow

[+] Author and Article Information
B. Umesh Chandra1

Department of Aerospace Engineering, Indian Institute of Technology - Madras, Chennai - 600 036, IndiaUmesh.Bhayaraju@dlr.de

S. R. Chakravarthy

Department of Aerospace Engineering, Indian Institute of Technology - Madras, Chennai - 600 036, India

1

Present Address: DLR- German Aerospace Centre, Institute of Propulsion Technology, Linder Höhe, 51147, Cologne, Germany.

J. Fluids Eng 127(4), 761-769 (Apr 08, 2005) (9 pages) doi:10.1115/1.1949642 History: Received August 26, 2003; Revised March 28, 2005; Accepted April 08, 2005

## Abstract

The focus of the present work is the acoustic oscillations exhibited by confined supersonic flow past a rectangular cavity of varying length-to-depth $(L∕D)$ ratio, with a view to identify optimal dimensions for application in scramjet combustors. Experiments were conducted to study the acoustic oscillations induced by supersonic flow at a Mach number of 1.5 past a rectangular cavity of variable dimensions mounted on one wall of a rectangular duct. The effect of $L∕D$ ratio of the cavity on the dominant acoustic modes registered on the wall of the duct opposite to the cavity is investigated. The range of $L∕D$ ratio varied is 0.25–6.25. The dominant acoustic modes and the amplitudes are observed to be quite sensitive to $L∕D$ ratio in the above range. Shifts in the dominant acoustic modes are observed predominantly for $L∕D≈0.94$ and $L∕D≈1.5$. The variation of the Strouhal number with $L∕D$ ratio indicates a transition in the modal content in the $0.94 range. Further shifts in the dominant frequencies are observed in the $1.5 range. Peak amplitudes occur at $L∕D$ ratios of around 0.75 and 2.25, with over twice the magnitude at the former than at the latter condition. Time-averaged schlieren visualization indicates the presence of quasi-steady shocks at about 0.75 the length of the cavity for $L∕D⩽1$ as opposed to being nearly at the trailing edge for higher $L∕D$ ratio. Instantaneous phase-locked schlieren images show the quasi-steady shocks are due to the movement of vortices and compression waves along the length of the cavity. It is also observed that the number of vortices in the shear layer roll up along the length of the cavity increases corresponding to the mode shifts for cavities with $L∕D>1$. Such distinct streamwise oscillations are also observed for cavities with $L∕D<1$, when the length is appreciable. The presence of higher modes in the acoustic oscillations is correlated with shocks produced at the lip of the cavity at a different frequency than the compression waves inside the cavity.

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## Figures

Figure 5

Effect of cavity depth on frequency and amplitude for cavities with L=40mm

Figure 6

Effect of L∕D ratio on Strouhal number based on (a) length and (b) depth of the cavity

Figure 7

Effect of the cavity L∕D ratio on the Strouhal number based on the cavity length and the various discrete frequencies excited for cavities of 50 mm length. Data from Heller (7) taken for L∕D=4.0, 5.7, and 7.0 at free-stream Mach number 1.5.

Figure 8

Real-time acoustic signals along with their respective acoustic spectra for typical shallow and deep cavities

Figure 9

Effect of L∕D ratio on acoustic amplitude at the dominant frequency

Figure 10

Time-averaged schlieren images for a cavity of 40 mm length and different depths

Figure 11

Instantaneous schlieren images of flow over cavities that do not excite discrete oscillations

Figure 12

Phase-locked images and corresponding acoustic spectrum for a cavity with L=40mm and D=15mm(L∕D=2.67)

Figure 13

Instantaneous images at different time instants and corresponding acoustic spectrum for a cavity with L=40mm and D=27mm(L∕D=1.48)

Figure 14

Phase-locked images and the corresponding acoustic spectrum for a cavity with L=40mm and D=70mm(L∕D=0.57)

Figure 4

Effect of cavity depth on dominant frequency for various lengths

Figure 3

Spectra of acoustic signals measured (a) in the cavity and (b) at the wall of the test section opposite the cavity, for a cavity with L=30mm and D=15mm(L∕D=2.0)

Figure 2

Typical spectra of acoustic oscillations for different cavity dimensions

Figure 1

Schematic of the test section

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