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Fig.6 shows the completed open jet wind tunnel inside the
ISVR’s anechoic chamber (control valve and primary silencer are
in the roof space of the chamber and are not shown in the figure).
Also shown is the new coordinate system (x,y,z) employed for the
cross section of the nozzle exit plane.We now present the overall
facility background noise characteristics for the entire rig as a function
of exit jet velocity.The flow uniformity and turbulence intensity
variation over the jet nozzle of the jet were also measured and
are also presented below.Note that both the acoustic and aerodynamic
measurement results are plotted using the new coordinate
system (x,y,z) as defined in Fig.6.
4.1.Analysis of background noise levels
A microphone was placed at (x,y,z) = (0,0.5,0),i.e.0.5 m vertically
above the centre of the cross-sectional nozzle exit plane to
measure the background noise level inside the anechoic chamber
at different exit jet velocities.This corresponds to a polar angle,
h = 90\2,where h is the angle from the jet axis,as shown in Fig.1.
In addition,another microphone was placed at h = 45\2 (0.35,0.35,
0) to assess the noise directivity of the exit jet.Fig.7a and b show
the narrowband (spectral density) sound pressure level at h = 45\2
and 90\2,respectively pertaining to the open jet wind tunnel over
a range of jet velocities between 33.1 and 99.6 ms\41.These figures
are plotted in the form of power spectral density with a 1 Hz bandwidth
and a frequency resolution,Df of 6.25 Hz.The spectra are
smoothly varying and decay slowly with frequency.
It is also insightful to examine how the sound pressure level
varies with jet velocity as the function of frequency.Fig.8 shows
the dependence of sound pressure level on jet velocity,p2 / VN
for h = 45\2 and 90\2.For h = 45\2,the sound pressure level is observed
to scale as V7.5–V8 in the frequency range between 400 Hz and
10 kHz.This power law is classically associated with quadrupole
jet mixing noise.For h = 90\2,a power law of V6.5 in the frequency
range 100 Hz–2 kHz is observed.This velocity dependence implies
that dipole aerodynamic noise sources are dominant at this measurement
angle.One possible dipole noise source is due to the
boundary layer being scattered at the nozzle lip.Another possible
dipole noise contributor at this frequency range could be due to the
noise breakout from inside of the rig.From 2 kHz and above,the
Fig.6 shows the completed open jet wind tunnel inside the
ISVR’s anechoic chamber (control valve and primary silencer are
in the roof space of the chamber and are not shown in the figure).
Also shown is the new coordinate system (x,y,z) employed for the
cross section of the nozzle exit plane.We now present the overall
facility background noise characteristics for the entire rig as a function
of exit jet velocity.The flow uniformity and turbulence intensity
variation over the jet nozzle of the jet were also measured and
are also presented below.Note that both the acoustic and aerodynamic
measurement results are plotted using the new coordinate
system (x,y,z) as defined in Fig.6.
4.1.Analysis of background noise levels
A microphone was placed at (x,y,z) = (0,0.5,0),i.e.0.5 m vertically
above the centre of the cross-sectional nozzle exit plane to
measure the background noise level inside the anechoic chamber
at different exit jet velocities.This corresponds to a polar angle,
h = 90\2,where h is the angle from the jet axis,as shown in Fig.1.
In addition,another microphone was placed at h = 45\2 (0.35,0.35,
0) to assess the noise directivity of the exit jet.Fig.7a and b show
the narrowband (spectral density) sound pressure level at h = 45\2
and 90\2,respectively pertaining to the open jet wind tunnel over
a range of jet velocities between 33.1 and 99.6 ms\41.These figures
are plotted in the form of power spectral density with a 1 Hz bandwidth
and a frequency resolution,Df of 6.25 Hz.The spectra are
smoothly varying and decay slowly with frequency.
It is also insightful to examine how the sound pressure level
varies with jet velocity as the function of frequency.Fig.8 shows
the dependence of sound pressure level on jet velocity,p2 / VN
for h = 45\2 and 90\2.For h = 45\2,the sound pressure level is observed
to scale as V7.5–V8 in the frequency range between 400 Hz and
10 kHz.This power law is classically associated with quadrupole
jet mixing noise.For h = 90\2,a power law of V6.5 in the frequency
range 100 Hz–2 kHz is observed.This velocity dependence implies
that dipole aerodynamic noise sources are dominant at this measurement
angle.One possible dipole noise source is due to the
boundary layer being scattered at the nozzle lip.Another possible
dipole noise contributor at this frequency range could be due to the
noise breakout from inside of the rig.From 2 kHz and above,the
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