41.5. Boundary Conditions and Loading

Only acoustic boundary conditions and loads are applied, as the model uses only acoustic elements.

A normal surface velocity is applied on the exterior. A transparent port and a radiation boundary are also applied on the same face:

CMSEL,S,source
ESEL,S,ENAME,,220 
ESEL,A,ENAME,,221
SF,ALL,SHLD,-1, 0
SF,ALL,PORT,1
SF,ALL,INF

Another port, used to calculate the absorption coefficient, is defined on the extremities of the tubes:

CMSEL,S,ends 
ESEL,S,ENAME,,220 
ESEL,A,ENAME,,221

SF,ALL,PORT,2

Two models are available for simulating the viscothermal losses in narrow tubes:

A boundary layer impedance (BLI) model is defined on the tube faces in order to account for the viscothermal effects:
```
CMSEL,S,res 
ESEL,S,ENAME,,220 
ESEL,A,ENAME,,221
SF,ALL,BLI
```
A low reduced frequency (LRF) model is defined for each tube via the TB,AFDM,,,,CIRC material model:
```
TB,AFDM,1,,,CIRC
TBDATA,1,Rtube
```

Generally, the LRF model applies to layers and tubes below the cut-off frequency, and the BLI model applies to geometries that are large compared to the viscothermal boundary layer thickness.