1. The document describes conducting acoustic analyses in ANSYS to analyze circular ducts with different boundary conditions and excitations.
2. It examines natural frequencies of ducts with rigid walls, pressure distributions in ducts with harmonic excitations, and impedance of a duct radiating into free space.
3. The analyses are performed with acoustic elements like FLUID30 in ANSYS and examine properties like natural frequencies, pressure and velocity distributions, and mechanical impedance of a piston-duct system.
2. Objective
1. Calculate the natural frequencies of a 3D duct with rigid walls with various end conditions.
2. Sound pressure distribution along the duct for a harmonic volume velocity excitation at one end of a duct
with finite length.
3. Pressure distribution along an infinitely long duct.
4. Pressure distribution along a duct with a finite length that has a frequency varying impedance at one end of
the duct radiating into free space.
๐ข1
๐ข2
z
x
y
L
a
3. Element types available for Acoustic Analyses in
ANSYS based on pressure formulation
Name 2D/3D Nodes Description
FLUID29 2D 4 Planar element
FLUID129 2D 2 Line element for simulating
an infinite boundary
FLUID30 3D 8 Brick element
FLUID130 3D 4,8 Planer element for
simulating an infinite
boundary
FLUID220 3D 20 Brick element
FLUID221 3D 10 Tetrahedral element
4. Natural frequencies
Description parameter Value
Diameter 2a 0.1m
Length L 3m
Speed of sound ๐0 343m/s
Density ฯ0 1.21 kg/m3
Velocity of piston ๐ข2 0.0
Velocity at rigid end ๐ข1 0.0
configuration schematic Mode index n= Natural frequencies Mode shape
Rigid-rigid 0,1,2 n๐0/2L Cos(nฯx/L)
Open-rigid 1,3,5 n๐0/4L Cos(nฯx/2L)
Open-open 1,2,3 n๐0/2L sin(nฯx/L)
10. Pressure and velocity distribution along the duct
Description parameter Value
Diameter 2a 0.1m
Length L 3m
Speed of sound ๐0 343m/s
Density ฯ0 1.21 kg/m3
Velocity of piston ๐ข2 1.0 m/s
Velocity at rigid end ๐ข1 0.0
Excitation frequency f 200Hz
Parameters used in the analysis of a circular duct with forced excitation at one end
11. -1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Particle velocity along a Piston-Rigid Duct
1 2 3
100
110
120
130
140
150
-500
-400
-300
-200
-100
0
100
200
300
400
500
Mic Position along Duct [m]
Mic Position along Duct [m]
Mic Position along Duct [m]
Soundpressurelevel[dB]ParticleVelocity[m/s]
ImaginaryPressure[Pa]
13. Semi-infinite duct
โข Applied an absorbing boundary to the outlet of the duct to simulate a
semi-infinite duct
โข The upstream inlet end provides an acoustic excitation as a surface
velocity of 1 m/s.
โข Outlet end has a radiation boundary applied which is one method of
specifying an absorbing boundary.
18. Commands which will couple all the nodes associated with inlet axis
which will create a rigid piston face and motion in the other direction
will be zero
22. Imaginary part of mechanical impedance of piston attached to a Duct
Frequency [Hz]
Imaginaryimpedance[Ns/m]