The following topics related to viscous-thermal materials in an acoustic analysis are available:
An acoustic propagating wave in a viscous media is dampened due to the viscosity of the fluid. The interaction between the acoustic pressure wave in a viscous fluid and a rigid wall is not taken into account.
Define the viscosity of a fluid via the MP,VISC command.
Example 5.7: Defining a Viscous Material
mp,dens,1,1.21 ! mass density mp,sonc,1,343 ! sound speed mp,visc,1,1.827e-5 ! dynamic viscosity
For more information, see Acoustic Fundamentals in the Mechanical APDL Theory Reference.
The interaction between an acoustic pressure wave in a viscous fluid and a rigid wall is taken into account.
Specify a rigid wall as a boundary layer via the
SF,Nlist,BLI command.
BLI models are supported in full harmonic acoustic analyses only.
Example 5.8: Defining a BLI Model
mp,dens,1,1.21 ! mass density mp,sonc,1,343 ! sound speed mp,visc,1,1.827e-5 ! dynamic viscosity mp,bvis,1,1.096e-5 ! bulk viscosity mp,kxx,1,0.0257 ! thermal conductivity mp,cvh,1,718 ! heat coefficient at a constant volume per mass mp,c,1,1005 ! heat coefficient at a constant pressure per mass nsel,s,ext ! select exterior nodes sf,all,bli ! flag boundary layer
For more information, see Boundary Layer Impedance (BLI) Model in the Mechanical APDL Theory Reference.
The interaction between an acoustic pressure wave in a viscous fluid and a rigid wall is taken into account for specific structures according to low reduced frequency (LRF) approximation.
Define the LRF model via the
TB,AFDM,,,,TBOPT command.
The following table shows the valid TBOPT values and
input parameters for the LRF model in a viscous-thermal fluid:
Table 5.2: Low Reduced Frequency Models
TBOPT
| Comments | Input Parameters |
|---|---|---|
| THIN | Thin layer between two rigid plates | Thickness of the layer |
| RECT | A tube with a rectangular cross section | Width and height of the rectangle |
| CIRC | A tube with a circular cross section | Radius of the circle |
LRF models are supported in full harmonic acoustic analyses only.
Example 5.9: Defining an LRF Model
The following example input defines a low reduced frequency model with a thin layer:
mp,dens,1,1.21 ! mass density mp,sonc,1,343 ! sound speed mp,visc,1,1.827e-5 ! dynamic viscosity mp,kxx,1,0.0257 ! thermal conductivity mp,cvh,1,718 ! heat coefficient at a constant volume per mass mp,c,1,1005 ! heat coefficient at a constant pressure per mass tb,afdm,1,,,thin ! basic acoustic materials tbfield,freq,f1 ! table at frequency f1 tbdata,1,thick1 ! material parameters table tbfield,freq,f2 ! table frequency f2 tbdata,1, thick2 ! material parameters table
For more information, see Low Reduced Frequency (LRF) Model in the Mechanical APDL Theory Reference.
The FLNS model solves the full linear Navier-Stokes equations with velocity, temperature and pressure variables. The FLNS model has more accurate numerical results than either the BLI or LRF model, especially at higher frequencies, while the viscous-thermal effects play a significant role on the acoustic phenomena in devices with narrow or thin acoustic paths.
The viscous-thermal material properties are defined in Basic Material Parameters of Acoustic Media.