# FINITE_MASS_BOUNDARY_CONDITION

Specifies the finite mass boundary conditions and interaction parameters for a particle surface.

## Type

AcuTrace Command

## Syntax

FINITE_MASS_BOUNDARY_CONDITION ("name") {parameters}

## Qualifier

User-given name.

## Parameters

- particle_surface or surface (string) [no default]
- Name of the particle surface.
- wall_type or type (enumerated) [=reflect]
- Wall type.
- reflect
- Particles reflect at surface.
- stop or trap
- Particles stop at surface but remain active.
- terminate or escape
- Particles stop at surface and become inactive.

- wall_en_type or en_type (enumerated) [=constant]
- Type of normal coefficient of restitution. Applies only if wall_type is
reflect.
- constant
- Coefficient is constant.
- piecewise_linear or linear
- Coefficient is a piecewise linear function of the normal component of the impact velocity.
- cubic_spline or spline
- Coefficient is a cubic spline function of the normal component of the impact velocity.

- constant_wall_en or wall_en [=1.0]
- Normal coefficient of restitution. Applies only if wall_type is reflect and wall_en_type is constant.
- wall_en_curve_fit_values or en_values [no default]
- A two-column array of normal-velocity/normal-coefficient-of-restitution data values. Used when wall_en_type is piecewise_linear or cubic_spline. Applies only if wall_type is reflect.
- wall_et_type or et_type (enumerated) [=constant]
- Type of tangential coefficient of restitution. Applies only if wall_type is
reflect.
- constant
- Coefficient is constant.
- piecewise_linear or linear
- Coefficient is a piecewise linear function of the normal component of the impact velocity.
- cubic_spline or spline
- Coefficient is a cubic spline function of the normal component of the impact velocity.

- constant_wall_et or wall_et [=1.0]
- Tangential coefficient of restitution. Applies only if wall_type is reflect and wall_en_type is constant.
- wall_et_curve_fit_values or et_values [no default]
- A two-column array of normal-velocity/tangential-coefficient-of-restitution data values. Used when wall_et_type is piecewise_linear or cubic_spline. Applies if wall_type is reflect.

## Description

The FINITE_MASS_BOUNDARY_CONDITION command specifies the particle/wall interaction at a given AcuSolve particle surface of type wall, slip, or symmetry. An AcuSolve particle surface is a surface that is named either in a PARTICLE_SURFACE command or in a SIMPLE_BOUNDARY_CONDITION command.

```
PARTICLE_SURFACE( "upper_wall" ) {
type = wall
...
}
```

```
SIMPLE_BOUNDARY_CONDITION( "upper_wall" ) {
type = wall
...
}
```

```
FINITE_MASS_BOUNDARY_CONDITION( "upper_wall" ) {
particle_surface = "upper_wall"
...
}
```

```
FINITE_MASS_BOUNDARY_CONDITION( "finite mass upper wall parameters" ) {
particle_surface = upper_wall
...
}
```

- the particle reflects off the wall (wall_type = reflect)
- the particle stops but continues to be actively involved in the particle trace (wall_type = stop)
- the particle trace terminates (wall_type = terminate)

When a particle reflects off a wall, the normal and tangential components of its velocity, ${\overrightarrow{u}}_{n}$ and ${\overrightarrow{u}}_{t}$ (in the wall frame of reference) are given by

where ${e}_{n}$ and ${e}_{t}$ are the normal and tangential coefficients of restitution, and ${\overrightarrow{u}}_{n,i}$ and ${\overrightarrow{u}}_{t,i}$ are the incident values of the normal and tangential components of the particle velocity. ${e}_{n}$ and ${e}_{t}$ always lie between 0 and 1.

Each coefficient can be specified as a constant, a piecewise linear function of the magnitude of the incident normal velocity, or a cubic spline function of the magnitude of the incident normal velocity. Note that AcuTrace clips the values of the coefficients so that they lie between 0 and 1.

```
wall_en_type = constant
wall_et_type = constant
wall_en = 1.0
wall_et = 1.0
```

```
wall_en_type = piecewise_linear
wall_et_type = piecewise_linear
en_values = { 1.0, 0.1 ;
10.0, 0.5 ;
100.0, 1.0 }
et_values = { 1.0, 0.1 ;
10.0, 0.5 ;
100.0, 1.0 }
```

```
wall_en_type = cubic_spline
wall_et_type = cubic_spline
```

cubic spline interpolants are used for velocity magnitudes between 1 and 100.