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The
basic design goal for a combustion engine silencer
is to achieve a certain noise attenuation within
a maximum allowable pressure loss and within a
defined envelope. Silentor A/S has specialized
in excellent low frequency attenuation at low-pressure
loss in exhaust systems for the automotive, marine
and industrial market. This is achieved by optimizing
the design of diffusers in our silencers.
CFD is well suited to calculate
the pressure loss through the silencer, and using
STAR-CD, the pressure loss can be calculated with
an accuracy of 10-20% of measurements. At Silentor
A/S, we use the same geometrical model for both
the pressure loss calculation and the noise attenuation
prediction.
The noise attenuation performance
of a silencer can be measured by sending loudspeaker-generated
noise through the silencer, and measuring the transmitted
noise. The same thing can be done in CFD calculations
where pressure waves are set as the input transient
boundary condition, and the signal emitted from
the silencer is calculated to predict the noise
attenuation.
The secondary silencer in a
two-silencer system for a low noise diesel engine
is shown in Figure 1. The left-hand side shows
the velocity field used in the pressure loss calculation
and the right hand side shows the local sound pressure
for a 200 Hz tone. Normally, a broad sound source
is used, but for illustration a pure tone is chosen
here.
Figure 2 shows a comparison
between calculated and measured attenuation in
the 0-3000 Hz range. There are detailed discrepancies
between calculations and measurement, but the overall
shape is accurately captured. The start of attenuation
and the first deep resonance at 600 Hz, which originate
from the inlet pipe, is well captured. Even though
not shown here, we have found that the effect of
using absorption material in the silencer can also
be calculated, thereby enabling the amount and
position of absorbent to be optimized.
In conclusion, we found that
the calculation of pressure loss through a silencer
is accurately predictive, and that CFD is also
effective in noise attenuation prediction and can
be used for calculating the frequency where attenuation
starts where resonances occur in the damping spectrum
and the overall level of attenuation. Thus, we
have a tool for both design optimization, as well
as for understanding silencer performance. The
calculations can also be used for more fundamental
questions such as the trade offs between different
diffuser designs, which can be difficult to do
in any other way.
For further information, please contact Mads Røgild, ca@silentor.com
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Figure
1: Axis symmetric velocity field through the
silencer (left) and sound pressure propagation
of a 200 Hz tone (right)
Figure 2: Comparison
between measured and calculated noise attenuation through
a Silentor low frequency attenuation silencer
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