The Thermoviscous Acoustics, SLNS Approximation (slns) interface (), found under the Thermoviscous Acoustics branch () when adding a physics interface, is used to compute the propagation of acoustic waves using the so-called Sequential Linearized Navier-Stokes or SLNS approximation to include the thermoviscous boundary layer losses in a computationally efficient manner. The interface solves the governing equations in the frequency domain.

The SLNS formulation is based on a Helmholtz decomposition of the full set of governing equations of the The Thermoviscous Acoustics, Frequency Domain Interface (also sometimes referred to as FLNS or full linearized Navier-Stokes). The resulting three scalar Helmholtz equations are solved sequentially. These are for the viscous scaling function Ψv, the thermal scaling function Ψth, and the acoustic pressure p. The method is computationally efficient and will capture most thermoviscous losses correctly. This interface is in particular suited for large system simulations. The main assumption for the decomposition is that the acoustical wavelength is much larger than the thickness of the viscous and the thermal boundary layers.

The formulation does not capture losses fully in regions with so-called redirectional flow. This happens at sudden geometry changes, like a sudden expansion. For many models, these additional losses will be minimal compared to the pure boundary layer losses in the remaining regions of the model. The method solves the thermoviscous problem, by including all the rotational (incompressible) flow components into the boundary layers only, through the scaling functions. In regions with redirectional flow, rotational components exist outside the boundary layers and are not captured. For detailed modeling of, for example, perforates or microperforated plates use the full formulation of The Thermoviscous Acoustics, Frequency Domain Interface.

The physics interface solves the equations in the frequency domain assuming all fields and sources to be harmonic. The harmonic variation of all fields and sources is given by eiωt using the +iω convention. Linear acoustics is assumed.

The Thermoviscous Acoustics, SLNS Approximation interface is formulated in the scattered field formulation where the total field (subscript t) is the sum of the scattered field and a possible background acoustic field (subscript b).

When this physics interface is added, these default nodes are also added to the Model Builder — Thermoviscous Acoustics Model, Wall, and Initial Values. Then, from the Physics toolbar, add other nodes that implement, for example, boundary conditions and sources. You can also right-click Thermoviscous Acoustics, SLNS Approximation to select physics features from the context menu.

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 slns.

See settings for Equation for the Pressure Acoustics, Frequency Domain interface.

If needed select the Adiabatic formulation of the equations.

See the settings for Sound Pressure Level Settings for the Pressure Acoustics, Frequency Domain interface.

See settings for Global Port Settings for the Pressure Acoustics, Frequency Domain interface.

From the list, select the element order for the Pressure, the Viscous scaling function, and the Thermal scaling function. The default uses Quadratic (Lagrange) for all the fields.

This physics interface defines these dependent variables (fields), the Acoustic pressure p (p), the Viscous scaling function Ψv (Psiv), and the Thermal scaling function Ψth (Psith). The names can be changed but the names of fields and dependent variables must be unique within a model.