MUFFLERS AND SILENCERS BY BLOCK DIAGRAMS
Genuine block diagram models of flow ducts, mufflers, silencers and resonators are constructed by the user on the System Model Interface (SMI) by simple mouse actions. SMI has several layers. The complete block diagram model is drawn in layer 1. Figure 1 is a snapshot of layer 1 displaying a block diagram model of the cold end of an exhaust line of a passenger car, a schematic of which is shown in Fig.2.
Block diagram strategy
The network strategy of ADEM is based, conceptually and mathematically, on giving users the freedom of constructing their own acoustic models in 1-D and 3-D, by using the element database of the software, without being involved in any mathematics or programming, simply by drawing block diagrams. The process of generating a mathematical model from a block diagram is carried out by the computer. This involves complex mathematical operations, which are often published as research articles in scientific journals.
For example, in Figure 1, the elements numbered 45, which are generically called UsedMacro, are formulated by the user and represent the green regions of the oval and circular mufflers. A UserMacro can be saved with a special name so that it can be used later as an acoustic two-port element. Thus, however complex the actual system configuration may be, its block diagram model may always be represented in layer 1 of SMI as a cascade of two-port elements, This strategy simplifies the reading of complex block diagrams and it is ideally suited for teamwork and customization for specific projects.
Block diagrams of UserMacros (1-D or 3-D) are constructed in sublayers of SMI. For example, Figure 3 is a block-diagram of the oval muffler in Figure 2. When a UserMacro is linked to a sublayer, the color of its sides turns bright-green. The linked sublayer number can be input as a parameter of a UserMacro. Then several different prototypes can be run in batch.
The acoustic source driving a duct system can be defined in ADEM explicitly or implicitly.
The explicit method: SourceMacro
Any multiple-input-single-output duct system with spectral sources (defined subsequently) is represented by the generic one-port element called SourceMacro. This is connected as the terminal element on the source side of the block diagram in layer 1. For example, Fig. 4a is a schematic of a laboratory setup, where the acoustic field in a duct system is generated by two pressure drivers in tandem, for the measurement of some acoustic parameters of the system. The SourceMacro is used to model the upstream side of the (equivalent) source plane as shown in Fig. 4b. A SourceMacro is similar to a UserMacro, in that, it must be linked to a sub-layer of SMI, where its block diagram model is drawn. Fig 4c is a block diagram of the SourceMacro in Fig. 4b.
Two-port acoustic source are applied by using the element SOURCEL, the characteristics of which may be based on actuator-disk models or measurements. This may co-exist with a SourceMacro or an implicitly defined one-port source, or may be the primary source.
The implicit method
If a duct system is driven by a one-port source, the program assumes that it is applied at the inlet node of the system, which is defined as the unconnected inlet node on the source side of the block diagram in layer 1. For example, in Figure 1 the source is applied by the implicit method at the inlet node of element 2, which corresponds to the inlet of the circular muffler in Figure 2.
The type of an implicit source is selected from the main menu and the source data are input in respective datasheets (see Video 1). The following source types can be applied implicitly:
Rigid piston driven by arbitrary velocity or displacement signal.
One-port source with frequency dependent impedance and pressure strength. Simple tools are provided for calculating crisp spectral source characteristics of loudspeakers and the crisp or fuzzy equivalent source characteristics of engine exhausts and intakes. There is also a tool for moving multiple crisp spectral sources to an equivalent source plane. Steady and non-steady spectral one-port sources, the characteristics of which are determined from the microphone signals in source measurements can be applied implicitly and Campbell diagrams can be produced in case of non-steady sources.
This is a non-steady one-port spectral source. Its datasheet allows input of the equivalent source characteristics corresponding to the selected orders of fluid machinery rotational speeds. Typical applications include prediction of the intake and exhaust noise of automobiles as function of the engine speed.
Applies a given acoustic power spectrum at the source plane, irrespectively of the load.
This option is chosen when calculating an acoustic parameter that do not depend on the source characteristics (e.g., transmission loss of a 1-D system).
Acoustic element database
The following table summarizes the categories of the over 60 acoustic elements contained in the database of the software. Each category includes one-dimensional and three-dimensional elements with mean flow. Using this database, users can create an infinity of new elements.
|Perforated pipe packs||Any number of perforated pipes enclosed in hard- or soft-walled casing in any communication topology; mean cross-flow or grazing flow configuration; sound absorbent material application; discrete or continuous perforate models.|
|Area changes||Open or closed; through flow or flow reversing; multiple inlets or outlets; side-inlet or side-outlet|
|Junctions||With any number of connecting ducts including branch and splitter configurations|
|Pipes and ducts||Uniform or non-uniform; mean temperature and pressure gradients; narrow viscothermal; lined; open or porous walls; distensible walls.|
|Macros (user formulated)||Generic passive two port (based on ADEM database): generic passive two-port (measured or FEM or BEM based); generic active one-port (equivalent of arbitrary miso system)|
|Boundary||Open (flanged or unflanged, intake or exhaust with mean flow); closed; pressure-release, anechoic; measured; multi-mode cut-on|
|Open end radiation||Free-field; fractional free-field; diffuse field.|
|Compounds||Several common mufflers, chambers and resonators|
|Baffles||Perforated uniformly or over regions|
|After treatment||Catalytic converter; Diesel particulate filter|
Element data are entered in element datasheets, which open when you click within the area of a block. Element datasheets have formula input capability and error filters for number input format. A typical input datasheet is shown in Figure 4. Detailed definitions of the input parameters on each datasheet are accessed on-line by clicking on the Help button. The input fields accept algebraic formulas in APL language and are checked for syntax. The mean temperature and the mean pressure in elements are input only for a reference operational point and no input is required for the mean flow velocity.