Noise from small air jets and a quiet valve

Journal of Sound and Vibration (1978) 57(l) 35-40

NOISE

FROM

SMALL AIR JETS

AND A QUIET

VALVE

T. W. LANCEY Faculty ofMechanical Engineering, California State University, Fullerton, California 92634, U.S.A. (Received 26 July 1976, and in revised form 22 July 1977)

Noise measurements of air jets of from 0.0794 to 0.635 cm diameter, with jet exit velocity varying from 54 to 244 m/s, to frequencies of 100 kHz are presented. Results are compared to those previously obtained for larger nozzles; acoustical power spectral density curves are found to be similar to those for the larger nozzles at like velocities. Results of a noise survey conducted near a 0.127 m line size quiet vent valve having approximately 20 000 square jets, 0.127 cm on a side are presented and found to agree with the laboratory nozzle noise data. Noise above a jet velocity of 120 m/s was found to be quadrupole in nature, while below this velocity dipole surface sound was observed; this surface noise is the noise of quiet valves, which operate at low velocities. It is estimated that a quiet valve jet of O-025 cm diameter, with a velocity near 60 m/s will exhibit a peak acoustical power spectral density at frequencies beyond the range of human audibility. 1.

INTRODUCTION

Measurements of noise fields near small jets of various diameters were made and compared to noise measurements made near a quiet vent valve. The aim of this study was to investigate some of the basic noise characteristics of very small jets, so that a comparison might be made with experimental work previously performed on larger jets [l , 21 but at higher jet velocities than normally associated with quiet valve jets. Jet velocities were then reduced to the range of the quiet valve jet velocities in an attempt to determine whether noise from a group of quiet valve jets is quadrupole in nature [3-51 or dipole [6] indicating that the effects of the local solid boundaries predominate. The experimental indication of a dipole source could be reasonably interpreted as a noise source comprised of the turbulent air stream and the solid internal walls of the nozzle through which the air flows, forming the jet as the air exits from the nozzle. The nozzle wall near the exit would then constrain the turbulent fluctuations creating Nose

suppressor \

r-l,

Quiet valve deslqn conflgurotlon Figure 1. Partial section flow radially outward.

quiet valve schematic

with P, shown

at noise suppresser

element

inlet and with

35 0022-460X/78/0308X)035

SOl~OO/O

0

1978 Academic

Press Inc. (London)

Limited

36

T. W. LANCEY

the acoustical dipole source field discussed by Curie [6]. It seems reasonable to expect that as the velocity is decreased, the volume quadrupole noise characterized by intensity Z N Us would give way to surface dipole noise characterized by Z N U6 [7]. Quiet valves contain an element called a noise suppressor; the noise suppressor provides resistance to fluid flow of sufficient magnitude that a large plug stroke is required to achieve appreciable changes in flowrate. Air exits from the noise suppressor through a large group of small jets, on the order of 6 x lop4 m on a side, rectangular in shape. The jet exit velocity is something less than 75 m/s. A schematic of the quiet valve is shown in Figure 1, with the inlet pressure P, shown at the noise suppressor inside surface, downstream of the seat and with air flowing to atmospheric pressure Pa. This indicates that the valve pressure drop is taken along flow channels running between the inner and outer radii. Nozzles of 0.635 cm, 0.3 18 cm, 0.238 cm, 0.159 cm and 0.0794 cm diameter were machined and their noise fields investigated in the laboratory. A O-127m line size quiet vent valve with a noise suppressor having0.127 cm square jets was surveyed in a large outdoors test installation. 2. EXPERIMENTAL SET-UP AND METHODS 2.1. LABORATORY NOZZLE EXPERIMENTS The general design of the nozzle is shown in Figure 2. Dry air was reduced in pressure at a regulator located in a room separate from the laboratory carried by a l/2 inch I.D. heavy wall plastic tube. The nozzle was clamped within the downstream end of the tube. Jet velocity was established by using Fischer and Porter rotameters. All noise measurements were made with a l/4 inch Brtiel and Kjaer (B & K) microphone, Type 4135, located 90” from the jet axis, within 0.1 cm from the nozzle exit plane. The protective grid of the microphone was removed during noise measurements providing flat response (within 1 dB) to 100 kHz.

Figure 2. Nozzle design-dimensions

typical except nozzle diameter d and length of converging section; all

, dimensions are given in inches.

The microphone signal was filtered by a Krohn-Hite Model 3100 filter. Calibration of the filter was performed by using a function generator and dual beam oscilloscope to determine contiguous bands from 1 kHz to 100 kHz for which the output to input signal ratio of the filter was unity across the band and decreasing beyond the band limits. It was found that the band widths Af were O-3 of the center frequencies fc;the filter output rolled off 3 dB, as defined by the square of the voltage signal ratio equal to l/2, at frequencies within 10% of the band limit frequencies. A B & K Type 2209 impulse precision sound level meter received the filtered signal and indicated the sound pressure level. Laboratory dimensions were 17 m x 9 m x 5 m height, with the jet 1.5 m from the floor and the nearest object on which the jet impinged, 12 m from the nozzle exit. The laboratory was not anechoic, and the noise signal was undoubtedly augmented somewhat by reflection from

NOISE FROM SMALL JETS AND A QUIET VALVE

37

the surroundings. However, the effects of reflection are considered minimal, because the jet noise occurs at high frequencies, well beyond many normal modes of the laboratory and readily absorbed at the boundaries. Also, results show a linear inverse relation between the rms acoustic pressure and the distance Yfrom the jet axis, which indicates that direct noise from the jet predominates over extraneous sources. Experimental velocities varied from 54 m/s to 244 m/s. Measurements were taken at r = 0.152 m and r = 0.305 m. Most measurements were at least 10 dB over the measured background SPL. 2.2. QUIET VALVE TEST A noise survey of the quiet valve was conducted outdoors in the test rig shown in Figure 3. Nitrogen was passed through the valve at 3.0 kg/s, flowing to atmosphere with an upstream pressure of 4.8 x lo5 Pa. Jet exit velocity was 62 m/s. One third octave band measurements were made, by using a l/2 inch B & K microphone with a windscreen, and a B & K Type 3347 Nitrogen SO”%B

Quietvalve

@:!2

Eel IOm Figure

3.

Quiet valve test rig.

real time analyzer. All measurements were more than 10 dB over the background SPL. The valve contained slightly over 20 000 exit jets, but only l/2 of the jets were considered audible. It is well known that noise levels behind a jet are much lower than those forward. In a quiet valve, the jet exits from a solid cylindrical wall which serves as a baffle, blocking rearward propagation of sound. Therefore, only the 10 000 jets visible to the observer were considered audible. 3. RESULTS

AND

DISCUSSION

3.1. DATA REDUCTION Data were reduced for presentation in the form of power spectral density curves. The mean squared acoustical pressure P,' is related to the acoustical power spectral density @“cf>by P: = ,=@(f.)d$ I

(1)

0

If the dimensionless Strouhal number S is used, where S=fd/U,

(2)

and G(S) is defined as a dimensionless acoustical power spectral density, then rc

Pi =

s

(fpa’)‘;@(S)d(fd/U). 0

Combining expressions (1) and (3) gives

Q(f) r2U

G(S) = --(3p U’)’ d2 d ’

following Mollo-Christensen,

Kolpin and Martuccelli [l].

(3)

38

T. W. LANCEY

Frequency

(w/Zrr)

Figure 4. Acoustical power spectral density vs. frequency. diameter nozzle; n, 0.127 cm square quiet valve jet.

For each band, P,’ is calculated (APSD) is calculated as

0,

0.0794 cm diameter nozzle; ? ,?0.159 cm

from the data, and the acoustical

power spectral

density

with the dynamic pressure term omitted and the square root taken, in the form presented by Mollo-Christensen [2]. The APSD’s for the O-0794 and 0.159 cm nozzles at U = 183 m/s and for the quiet valve at 62 m/s are plotted versus frequency on Figure 4 and versus Strouhal number on Figure 5.

Strouhal number

(/‘d/U1

Figure 5. Acoustical power spectral density vs. Strouhal number. Symbols same as Figure 4.

3.2.

NOISE SPECTRA

Figure 4 not only illustrates the large increase in spectral density with increased velocity but also shows the shift in the location of the spectral density peak. Because the maximum frequency of human audibility is less than 20 kHz, it is evident that by suitable selection of jet size and exit velocity, the spectral density peak can be moved beyond 20 kHz. Acoustical power spectral density peaks occur near S = O-2 for these small jets, as expected, and as found experimentally for large jets. However, near the jet axis Ahuja and Lush [8, 91

NOISE FROM SMALL JETS AND A QUIET VALVE

39

have found spectrum peaks near fd/c = 0.2 (independent of velocity) where c is the ambient velocity of sound. No measurements were made near the jet axis for this study. The peak acoustical power spectral density agrees with that obtained by Mollo-Christensen [2] and serves then to provide a scaling relationship which holds over jet areas varying by a factor of 2500 (nozzle diameters tested vary from 0.032 inch to 1.6 inch [2]). For the quiet valve tested, the peak acoustical spectral density occurred near 10 kHz, at S = 0.2. It is then apparent that a 0.025 cm (0.010 inch) quiet valve jet at U = 62 m/s would move the peak to well beyond the audible range, as are the peaks for the two nozzles shown on Figure 4. Previous experiments performed by Jenvey [lo] indicate an attenuation of the noise produced by small, enclosed jets. The jets tested in this study were not enclosed, and no attenuation was evident. Jenvey represents the predicted sound power level for three orifices by one curve. If a correction of 10 Log,, (area ratio) is applied to his predicted curve and largest orifice, curves 6 dB and 12 dB below the existing curve would be generated, providing would then be reduced to 2 or a WPL prediction for each orifice. The apparent attenuation 3 dB, perhaps within the range of data scatter. 3.3. NOISE SOURCE Peak acoustical power spectral densities (PAPSD) were plotted versus jet exit velocity for all the experiments, as shown on Figure 6. A curve representing quadrupole volume sound is drawn in dashed lines at the higher velocities, given by PAPSD

= 9.5 x lO-8 u4.

(6‘)

This relation is a good approximation down to about U = 120 m/s. Below 120 m/s, a curve representing dipole surface sound is drawn as a solid line, a better fit to the experimental data. This curve is given by PAPSD

= 1.1 x 1O-5 I/‘.

(7)

Exit velocbty(m/s) Figure 6. Peak acoustical power spectral density vs. jet exit velocity. o, d= 0.0794 cm; 0, d = 0.159cm; A, d= 0.238 cm; 0, d= 0.318 cm; ?? , d= 0.635 cm; x, 0.127 cm square quiet valve jet; ----, equation (6); -, equation (7).

40

T. W. LANCEY

The data point for the quiet valve tested falls very near this curve, indicating close agreement with the nozzle experiments and that the noise field of a quiet vent valve is dipole in nature. This has been found in the absence of a solid object obstructing the jet which might provide a source for Aeolian tones or edge tones. It seems the nozzle inside wall near the exit provides the sounding board for the noise detected and the nozzle diameter remains the length scale for Strouhal number scaling.

4. CONCLUSIONS

Measurements of noise from small jets were made at jet velocities from 54 to 244 m/s and from a quiet valve at a jet velocity of 62 m/s. Results are in good agreement with previous experiments with larger subsonic jets, at the higher velocities, showing the noise to be quadrupole in nature with a peak acoustical power spectral density occurring near S = O-2 and magnitudes near those of much larger nozzles, at equal velocities. At the lower speeds applicable to quiet valves, the noise is dipole in nature with peak power spectral densities again near S = 0.2, scaled on the nozzle diameter. For typical quiet valve velocities, a jet diameter of 0.025 cm would cause the peak acoustical power spectral density to occur at a frequency beyond the human audibility range.

ACKNOWLEDGMENTS

The author wishes to express his appreciation to Joe Weatherstone of Briiel and Kjaer Instruments, Inc., for providing the real time analyzer for our use in valve testing and to Alan Keskinen of Vacco Industries, Inc., for providing the quiet valve and test system.

REFERENCES 1. E. MOLO-CHRISTENSEN, M. A. KOLPIN and J. R. MARTUCCELLI1964 Journal of FluidMechanics 18, 285-301. Experiments on jet flows and jet noise far field spectra and directivity patterns. 2. E. MOLLO-CHRISTENSEN 1967 Journal of Applied Mechanics 34, l-7. Jet noise and shear flow instability seen from an experimenter’s viewpoint. 3. M. J. LIGHTHILL 1952 Proceedings of the Royal Society A211, 564587. On sound generated aerodynamically-I. General theory. 4. M. J. LIGHTHILL 1954 Proceedings of the Royal Society A222, l-32. On sound generated aerodynamically-11. Turbulence as a source of sound. 5. M. J. LIGHTHILL1963 American Institute of Aeronautics and Astronautics Journal 1, 1507-l 5 17. Jet noise. 6. N. CURLE195.5Proceedings of the Royal Society A231,505-514.The influence of solid boundaries upon aerodynamic sound. 7. W. C. MEECHAM1965 Journal of the Acoustical Society of America 37, 516-522. Surface and

volume sound from boundary layers. 8. K. K. AHUJA 1973 Journal of Sound and Vibration 29,155-168. Correlation and prediction of jet noise. 9. P. A. LUSH 1971 Journal ofFluid Mechanics 46,477-500. Measurements of subsonic jet noise and comparison with theory. 10. P. L. JENVEY1975 Journal of Sound and Vibration 41,506509. Gas pressure reducing valve noise.