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 by either the
MP or
TB,AFDM,,,,MAT command.
Frequency-dependent materials are supported. For more information, see Basic Material Parameters of Acoustic Media.
The FLNS model is only available for the higher order acoustic elements, FLUID220, FLUID221, and FLUID244. To activate the FLNS model, use one of the following KEYOPT(2) settings:
KEYOPT(2) = 5 for the coupled element type:
KEYOPT(2) = 6 for the uncoupled element type:
The solution of an FLNS model supports only full harmonic analysis in viscous-thermal acoustics. This capability does not support mode-superposition harmonic analysis.
The excitation sources for viscous-thermal acoustics are discussed in Excitation Sources in Viscous-Thermal Acoustics.
Defining quiescent pressure and temperature is discussed in Quiescent Pressure and Ambient Temperature.
The pressure that is approximately equal to the normal stress is exerted on the exterior surface of the FLNS model (SF,,PRES) as the acoustic boundary condition.
The non-viscous-thermal acoustic elements are directly coupled to the viscous-thermal acoustic elements via the pressure degree of freedom.
The viscous-thermal acoustic elements are coupled to structural elements with the kinematic condition on the FSI (see Coupling Conditions on the FSI Interface for the FLNS Model in the Theory Reference).
The following table shows boundary conditions most often used with the FLNS model.
Table 11.2: Typical Boundary Conditions (BC) for FLNS Model
| BC Type | Acoustic BC | Viscous BC | Thermal BC |
|---|---|---|---|
| Rigid wall |
|
| T = 0 |
| Normal velocity |
|
| T = 0 |
| Symmetry |
|
|
|
| Pressure |
|
|
|
| Impedence |
|
|
|
where:
 = stress tensor |
 = heat flow vector |
 = outward normal unit vector on the boundary 
|
 = tangential unit vector that is the direction of total
viscous shear force on the boundary |
Example 11.10: FLNS Model Solution
... ! Define viscous-thermal material c0 = 340.6 rho = 1.2256 visc = 17.83e-6 kxx = 0.02534 cp = 1005 cv = 718 bvis=0.6*visc tb,afdm,1,,,mat tbdata,1,rho,c0,visc,kxx,cp,cv tbdata,7,bvis ! ... et,1,220,,6 ! Define viscous-thermal element type ... nsel,s,loc,x,0 sf,all,pres,-1 ! Apply pressure nsel,all bf,all,temp,15 ! Define quiescent temperature bf,all,spre,101325 ! Define quiescent pressure toffst,273 ! Offset from absolute zero to zero nsel,all d,all,vy,0 ! Zero out y component of velocity ! ! Boundary conditions on rigid wall nsel,s,loc,z,0 d,all,temp,0 d,all,vx,0 d,all,vz,0 allsel ! ! Solve the problem /solu ... solve finish ! /post1 set,1,1 prnsol,pres ! Print out pressure prnsol,v,comp ! Print out velocity prnsol,temp ! Print out temperature finish
Additional Recommendations for the FLNS Model
Keep the following points in mind when using the FLNS model:
Even though 6 elements per wavelength are enough for pressure acoustic analysis using higher-order elements, for FLNS usage it is necessary to have 12 elements per wavelength because the low-order pressure shape functions are used.
The boundary conditions must be correctly assigned.
The mean flow boundary condition cannot be used with FLNS.
It is not necessary to constrain the pressure on the FLNS model.
The mesh should be fine enough to resolve the viscous and thermal boundary layer near the wall.
For more information, see The Full Linear Navier-Stokes (FLNS) Model in the Theory Reference.