Abstract
Noise propagation within ducts is of practical concern in many areas of
industrial processes where a fluid has to be transported in piping
systems. The paper presents experimental data and visualization of flow in
the vicinity of an abrupt change in cross-section of a circular duct and
on obstacles inside where the acoustic wave generates nonlinear separated
flow and vortex fields.
For noise produced by flow wave of low Mach number, laminar and turbulent
flows are studied us- ing experimental sound intensity (SI) and laser
particle image velocimetry (PIV) technique adopted to acoustics (A-PIV).
The emphasis is put on the development and application of these methods
for better understanding of noise generation inside the acoustic ducts
with different cross-sections. The intensity distribution inside duct is
produced by the action of the sum of modal pressures on the sum of modal
particle velocities. However, acoustic field is extremely complicated
because pressures in non-propagating (cut-off) modes cooperate with
particle velocities in propagating modes, and vice versa. The discrete
frequency sound is strongly influenced by the transmission of higher order
modes in the duct. By under- standing the mechanism of energy in the sound
channels and pipes we can find the best solution to noise abatement
technology.
In the paper, numerous methods of visualization illustrate the vortex flow as
an acoustic velocity or sound intensity stream which can be presented
graphically. Diffraction and scattering phenomena occurring inside and
around the open-end of the acoustic duct are shown.
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