The Pipe Acoustics, Frequency Domain Interface
The Pipe Acoustics, Frequency Domain (pafd) interface (), found under the Acoustics>Acoustic-Structure Interaction branch () when adding a physics interface, is used to compute the acoustic pressure and velocity variations when modeling the propagation of sound waves in flexible pipe systems. The governing equations are formulated in a general way to include the possibility of a stationary background flow. The physics interface can for example be used to compute the propagation of sound waves in HVAC systems, other large piping systems, or simply in an organ pipe.
In the frequency domain all sources and variations are assumed to be harmonic. The solved equations assume that the propagating waves are plane. The propagation of higher-order modes that exist above their cut-off frequency, dictated by the pipe cross section, is not modeled.
The equations governing the propagation of sound in pipes stem from considering momentum, mass, and energy balances for a control volume of a piece of pipe. The resulting equations are expressed in the cross-sectional averaged variables and reduce the equations to a 1D component with scalar dependent variables. The physics interface is available in 3D on edges and points, and in 2D on boundaries and points.
When this physics interface is added, these default nodes are also added to the Model Builder—Fluid Properties, Pipe Properties, Closed, and Initial Values. Then, from the Physics toolbar, add other nodes that implement, for example, boundary conditions and point conditions. You can also right-click Pipe Acoustics, Frequency Domain to select physics features from the context menu.
Settings
The Label is the default physics interface name.
The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.
The default Name (for the first physics interface in the model) is pafd.
Sound Pressure Level Settings
The zero level on the dB scale varies with the type of fluid. That value is a reference pressure that corresponds to 0 dB. This variable occurs in calculations of the sound pressure level Lp based on the root mean square (rms) pressure prms, such that
where pref is the reference pressure and the star (*) represents the complex conjugate. This is an expression valid for the case of harmonically time-varying acoustic pressure p.
Select a Reference pressure for the sound pressure level based on the fluid type:
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Use reference pressure for air to use a reference pressure of 20 μPa (20·10−6 Pa).
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Use reference pressure for water to use a reference pressure of 1 μPa (1·10−6 Pa).
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User-defined reference pressure to enter a reference pressure pref, SPL (SI unit: Pa). The default value is the same as for air, 20 μPa.
For postprocessing, plot the sound pressure level pafd.Lp, which depends on the selected reference pressure. You can also plot the rms intensity magnitude pafd.I_rms or the instantaneous intensity magnitude pafd.I_inst.
Dependent Variables
This section is used to define the dependent variables (fields) for Pressure p (SI unit: Pa) and Tangential velocity u (SI unit: m/s). If required, edit the name, but dependent variables must be unique within a model.
Discretization
This section controls the element types used in the finite element formulation.
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Edge, Boundary, Point, and Pair Nodes for the Pipe Acoustics Interfaces
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Theory for the Pipe Acoustics Interfaces