Intro-9-aerodynamic-noise.pdf
-
Upload
concord1103 -
Category
Documents
-
view
17 -
download
2
description
Transcript of Intro-9-aerodynamic-noise.pdf
NVH Basics (Intro – 9) Aerodynamic Noise Page 1
Rev. 12/30/2010
9. Aerodynamic Noise (Windnoise)
9.0 Introduction This section deals with aerodynamic noise due to the flow of air around the vehicle. The air flow produces three
major effects: - Noise due to turbulence - Noise due to pressure differences causing air leaks through the dynamic seals (aspiration noise) - Tones and whistles due to Aeolian noise as the air flows past antenna, roof-racks and similar obstacles. 9.1 Variation of Aerodynamic Noise with Vehicle Speed 9.1.1 Turbulence Noise The sound power due to turbulent flow is expected to increase roughly as the 6th power of speed V:
62 VPturb
Or about 18 dB per doubling of speed. This sound must generally pass through a barrier such as window glass.
Then the noise is commonly measured using the A-weighted scale. The result is that in practice the interior aerodynamics noise increases by about 16 dB per speed doubling.
9.1.2 Aspiration Noise This type of noise often increases suddenly at a critical speed. This is because negative pressure differences can
cause doors to move away from the vehicle body to such an extent that the seals begin to leak. 9.1.3 Aeolian Noise The sound level due to tones and whistles is due to turbulence and also increases by about 16 dB per doubling of
speed.
9.2 Aerodynamic Noise Spectrum 9.2.1 Interior Turbulence Noise Figure 9.2 Typical Aerodynamic Noise Spectrum
0
10
20
30
40
50
60
70
80
90
31
.5 40
50
63
80
10
01
25
16
02
00
25
03
15
40
05
00
63
08
00
10
00
12
50
16
00
20
00
25
00
31
50
40
00
50
00
63
00
80
00
Linear 1/3r Octave Spectrum
A-Weighted 1/3 Octave Spectrum
NVH Basics (Intro – 9) Aerodynamic Noise Page 2
Rev. 12/30/2010
The spectrum of aerodynamic noise is usually measured on 1/3rd octave basis. In that case the linear spectrum falls at a rate of between 5 and 8 dB per octave, while the A-weight spectrum peaks in the region of 250 to 500 Hz. This is illustrated in figure 9.2 where the linear spectrum falls at 6.5 dB/Octave.
The exterior turbulent flow has numerous sources such as the vehicle hood, side-mirrors and A-pillars. Each
produces a spectrum with a different maximum frequency so that the overall external pressure spectrum is relatively flat.
9.2.2 Aspiration Noise Spectrum Acoustic resonance phenomena in the aspiration path cause resonance spikes at fixed frequencies. 9.2.3 Interior Aeolian Noise Spectrum The spectrum will consist of “spikes” at one or more frequencies. The frequency of the spikes is proportional to
vehicle speed. 9.3 Reduction of Noise Sources 9.3.1 Turbulent Noise Noise reduction is usually achieved by careful design of surfaces subject to airflow. In particular, “steps” should be
avoided. Considering the A-Pillar arrangement of figure 9.3.1 we should minimize steps at the windshield, A-Pillar and door frame.
Figure 9.3.1: Arrangement at A-Pillar 9.3.2 Aspiration Noise Noise reduction is usually achieved by careful design of surfaces the seals. Both the inner and outer seal must close
of the aspiration path. This must be achieved while keeping door closing effort with the optimum customer range. Two important factors must be dealt with:
- The sealing must be robust against manufacturing variation - The sealing must be robust against door motion due to negative pressure differentials.
Windshield
A-Pillar
Side Window Glass
Inner Seal
Outer Seal
NVH Basics (Intro – 9) Aerodynamic Noise Page 3
Rev. 12/30/2010
9.3.3 Aeolian Tones This are typically cause by an airstream passing over a long slim cylinder such as an antenna. For example, if we
consider an antenna in an airstream with: d: Antenna Diameter u: airstream velocity Vortices are shed at a frequency given by:
d
uSf S=Strouhal number ~ .2
This type of flow occurs for Reynold’s numbers in the range 300 to 100,000
Figure 9.3.3: Plan View of Antenna Vortex Shedding The Reynold.s number R is:
duR : Density ρ=1.3 x kg/m3 ; Viscosity μ=1.78 x 10-5 N.s.m-2
For an antenna with d~ 3 mm and a velocity u ~ 30 m/sec (aprox 60 mph) we have:
6500108.1
)3)(3)(3.1( 3 R Hzf 2000103
302. 3
Wavelength: mf
c17.
Notice that the sound is mostly emitted at right angles to the airstream and to the antenna or obstruction. This
means that the sound in the interior of the vehicle can be reduced by careful positioning of the obstruction (or made worse by poor positioning. Also it can be seen that the transverse elements of a roof rack could easily cause a problem.
The strength of the Aeolian tone can often be reduced by varying the section of the problem obstruction. For
example the antenna diameter may be varied by stepping. This does not totally eliminate the noise but much reduces the noticeable tone.
u
Dipole Radiation
Vortices
φ
NVH Basics (Intro – 9) Aerodynamic Noise Page 4
Rev. 12/30/2010
Figure 9.3.3: Stepped Antenna Concept Another popular solution is to wind a spiral wire around a tapered antenna.