# FWH_SURFACE_OUTPUT

Specifies a set of surfaces for integrated-surface output parameters for the Ffowcs-Williams Hawkings (FWH) method that is used by the AcuFwh post processing program to propagate acoustic sources to far field locations.

## Type

AcuSolve Command

## Syntax

FWH_SURFACE_OUTPUT("name") {parameters...}

## Qualifier

User-given name.

## Parameters

- coordinates or coord (array) [no default]
- An array of sample points. Each row contains a unique identification number and the x, y and z coordinates of a point in the global xyz coordinate system.
- gap (real) >=0 [=0]
- Tolerance for sample points falling outside of all elements.
- shape (enumerated) [no default]
- Shape of the surfaces in this set.
- three_node_triangle or tri3
- Three-node triangle.
- four_node_quad or quad4
- Four-node quadrilaterial.

- partial_surfaces (array) [no default]
- List of surfaces with no references to parent elements or element sets.
- quadrature (enumerated) [=full]
- Quadrature rule used for surface integration.
- full
- Full quadrature.
- reduced
- Reduced quadrature.

## Description

```
FWH_SURFACE_OUTPUT( "sample surfaces" ) {
coordinates = Read( "sample.crd" )
partial_surfaces = Read( "sample.srf" )
shape = four_node_quad
quadrature = full
}
```

defines an acoustic surface output for use with AcuFwh.

A set of sample points is specified by the coordinates parameter, which is a four-column array corresponding to an identification number and the x, y and z coordinates of each sample point, one row per point. The identification number may be any arbitrary but unique (within the set) integer. Negative numbers are acceptable. The coordinates are in the global xyz coordinate system. This is the same format as the coordinates parameter of the COORDINATE command, but the sample points do not have to be a subset of the nodal coordinates nor related to them in any other way. The coordinates parameter is mandatory. Within AcuPrep, an element is found for each sample point and interpolation shape functions are computed. If a sample point falls outside all fluid elements, then the closest element face is found, up to a distance specified by gap. An error is issued if no element is found.

The partial_surfaces parameter contains the connectivity of the surfaces. This parameter is a multi-column array, but it does not have the same format as the corresponding parameter in other surface commands, for example SURFACE_OUTPUT. The number of columns depends on the shape of the surface. For three_node_triangle, this parameter has four columns, corresponding to a unique (within this set) surface number and the three nodes of the surface. For four_node_quad, partial_surfaces have five columns, corresponding to a surface number and the four nodes of the surface. One row per surface must be given. Since there is no parent element definition the three or four nodes of the surface must be ordered around the periphery of the surface. Also, unlike other surface commands this command does not support higher-order surfaces, that is, six-node triangles, or nodal quadrature. The nodes contained in the surface connectivity must all be defined in the coordinates.