Identification of higher order acoustic modes in distributed pipe flow

Journal ofSound

and Vibration (1989) 129(l),

IDENTIFICATION

166-167

OF HIGHER DISTURBED

ORDER ACOUSTIC PIPE FLOW

MODES

IN

1. INTRODUCTION

High levels of external noise radiation from piping systems occur when the internal turbulent flow, disturbed by a pipe fitting, separates from the pipe wall. The disturbed flow generates an intense internal sound field, comprising plane waves and higher order acoustic modes which propagate throughout the piping system: it excites the pipe wall vibrations, which in turn generate external sound. Therefore, it is necessary to understand the mechanism of generation and propagation of higher order acoustic modes in the piping system. In the pressure spectrum, acoustic modes can generally be identified by the step increase in power spectral density which occurs at their cut-off frequencies. However, at higher frequencies, lower power spectral density and close proximity of higher order mode cut-ons as the frequency increases make acoustic mode identification more difficult by this method. In this note, information about the relative phase of the pressure signals from two pressure transducers mounted in the pipe wall at opposite ends of a diameter has been used for the mode cut-ons identification. 2.

EXPERIMENTAL

RESULTS

The flow rig is shown schematically in Figure 1. Air is drawn in the test-pipe (a = 36.27 mm) from the atmosphere through an inlet bell-mouth and discharges into vacuum tanks. A sonic choke (d, = 62.4 mm), immediately upstream of the vacuum tanks, ensures steady running conditions. Flow separation in the pipe was caused -by an orifice plate (d,, = 55 mm), A fully developed turbulent velocity profile (M, = O-33) was established upstream of the orifice plate before separation. A detailed description of the experimental set-up is given in references [l, 21. The relative phase of the pressure signals from two 6-35 mm Briiel and Kjaer microphones was obtained by using a Hewlett Packard spectrum analyzer (model 3582A). The wall pressure spectrum and phase relationship at X = -2.01 are shown in Figure 2. Frequencies corresponding to cut-ons of the various acoustic modes are also marked on Figure 2. This .shows that the acoustic modes can still be identified from phase changes when the modes are scarcely discernible in the pressure spectrum itself. This is a useful analysis, particularly in view of the close proximity of higher order mode cut-ons at the

Figure

1. Schematic

of the flow rig.

166 0022460X/89/040166+02

$03.00/O

@ 1989 Academic

Press Limited

LETTERS

TO

THE

167

EDITOR

I

I

I

I

5

IO

15

20

;<200

Frequency (kHz)

Figure 2. Wall pressure spectrum and phase relationship between signals from two diametrically microphones

opposite

at X = -2.01.

higher frequencies. However, by using this technique only modes at which a phase change from 0” to 180” or 180” to 0” occurs [i.e., (1,O); (2,0); (3,0); (4,0); (5,O); (2,1); (3,l); etc.] can be identified. Measurements were made under various flow conditions to study the effect of flow rate on the modal cut-off frequencies. These results will be reported in a separate paper. N. K. AGARWAL~

Department of Mechanical Engineering, University of Adelaide, Adelaide, South Australia 5601 (Received

7 July 1988) REFERENCES

1985 Ph.D. Thesis, University ofAdelaide, South Australia. Relationship between internal sound generation and characteristics of flow in a region of tlow separation due to disturbance of fully-developed turbulent flow in a pipe. 2. M. K. BULL and N. K. AGARWAL 1983 Proceedings of the 8th Australian Fluid Mechanics Conference, New Castle, N.S. W., 1,4B.l-4B.4. Characteristics of flow separation due to an orifice plate in fully developed turbulent pipe flow. 1. N. K. AGARWAL

t Present address: Department of Aerospace and Ocean Engineering, University, Blacksburg, Virginia 24061, U.S.A.

APPENDIX:

: i M,

GL 6

Virginia Polytechnic

Institute and State

NOMENCLATURE

pipe radius throat diameter of the choke orifice plate hole diameter centerline Mach number upstream of separation streamwise distance non-dimensional streamwise distance, (=x/2a) power spectral density of wall pressure

(at a reference station, X = -3.8)