The Linearized Euler, Transient (let) interface (), found under the Acoustics > Aeroacoustics branch () when adding a physics interface, is used to compute the acoustic variations in density, velocity, and pressure in the presence of a stationary background mean-flow that is well approximated by an ideal gas flow. The physics interface is used for aeroacoustic simulations that can be described by the linearized Euler equations.

The equations defined by the Linearized Euler, Transient interface are the linearized continuity, momentum (Euler), and energy equations. The physics interface solves for the acoustic variations in the density ρ, velocity field u, and pressure p. The equations are formulated in the time domain. The background mean flow can be any stationary gas flow that is well approximated by an ideal gas. The coupling between the acoustic field and the background flow does not include any predefined flow induced noise. As the equations do not include any loss mechanisms, nonacoustic modes and instabilities can exist in the time domain.

The equations are defined using a scattered-field formulation just as in The Linearized Euler, Frequency Domain Interface and allows the use of Background Acoustic Fields. Open nonreflecting conditions are set up with the Absorbing Layers for the Linearized Euler, Transient Interface.

Coupling between a background mean flow, computed from a Fluid Flow model, and the Linearized Euler model is handled by the Background Fluid Flow Coupling multiphysics coupling and the dedicated Mapping study. Details are also found in the Mapping Between Fluid Flow and Acoustics Mesh section.

When this physics interface is added, these default nodes are also added to the Model Builder — Linearized Euler Model, Rigid Wall, and Initial Values. For axisymmetric components an Axial Symmetry node is also added.

Then, from the Physics toolbar, add other nodes that implement, for example, boundary conditions and sources. You can also right-click Linearized Euler 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 let.

Select the Linearized Euler formulation as either Full linearized Euler (the default) or the Linearized perturbed compressible equations (LPCE). The first formulation is valid for all Mach numbers (as long as there is not shock formation) while the second is only valid for low Mach numbers (Ma < 0.3). The second formulation has some additional stability properties as vorticity waves cannot propagate (they are not supported in this limit), see Ref. 27 and Linearized Euler Model.

See Stabilization in the frequency domain interface for details about the Stabilization method options.

In the time domain an additional Limit small time steps effect on stabilization time scale option can be selected (not selected per default). This option can be turned on in models where very small time steps are taken by the solver, for example, if a very high Maximum frequency to resolve is set. This can in some situations lead to unphysical oscillations in the solution. This option can be turned on to remedy this. If selected for a model that does not exhibit these numerical challenges, the option can lead to an overly diffusive stabilization.

To display this section, click the Show More Options button () and select Advanced Physics Options in the Show More Options dialog. In the Damping Parameters for Absorbing Layers section you can change and control the values of the artificial damping added in the Absorbing Layers for the Linearized Euler, Transient Interface. Enter a value for the Numerical viscosity parameter μnum (default value is 100 Pa·s) and the Numerical viscosity curvature n (default value is 2).

Enter the Maximum frequency to resolve in the model. The default frequency is set to 1000[Hz] but should be changed to reflect the frequency content of the sources used in the model. Select the Time stepping (method) as Fixed (preferred) the default and recommended or Free. The Free option is in general not recommended for wave problems. The generated solver will be adequate in most situations if the computational mesh also resolves the frequency content in the model. Note that any changes made to these settings (after the model is solved the first time) will only be reflected in the solver if Show Default Solver or Reset Solver to Defaults is selected in the study.

See Discretization in the frequency domain interface for details.

This physics interface defines these dependent variables (fields): the Acoustic Density rho, the Acoustic Velocity u and its components, and the Acoustic Pressure p. The name can be changed but the names of fields and dependent variables must be unique within a model.