This example problem determines the first flexural mode (egg mode) frequency of an annular ring submerged in water which extends to infinity. A harmonic analysis is performed using a frequency sweep between 34 and 38 Hz.

The distance from the center of the ring to the infinite elements is at least equal to (D/2) + 0.2λ, where D is the outer diameter of the ring, and λ = c/f is the dominant wavelength of the pressure waves.

Using 0.26035 for the outer radius of the ring, 1460 for the speed of sound (c), and 36 for the estimated dominant frequency (f), gives (D/2) + 0.2λ = 0.26035 + (0.2)(1460)/36 = 8.37146. This distance is much greater than that required when using the acoustic infinite element, however, and so a distance of 2x the outer radius of the ring is used ( 2 x .26035 = .5207).

/BATCH,LIST
/VERIFY,EV129-1S
/PREP7
/TITLE,AMA,EV129-1S,FLUID129,HARMONIC ANALYSIS
ET,1,PLANE182,2                                 ! structural element
ET,2,FLUID29                                    ! acoustic fluid element with ux & uy
et,3,129                                        ! acoustic infinite line element
r,3,0.5207,0,0
ET,4,FLUID29,,1,0                               ! acoustic fluid element without ux & uy
                                                ! material properties
MP,EX,1,2.068e11
MP,DENS,1,7929
MP,NUXY,1,0
MP,DENS,2,1030
MP,SONC,2,1460
                                                ! create inner and outer quarter circles
CYL4,0,0,0.254,0,0.26035,90    
CYL4,0,0,0.26035,0,0.5207,90
                                                ! select, assign attribute to and mesh area 1
ASEL,S,AREA,,1
AATT,1,1,1,0
LESIZE,1,,,16,1
LESIZE,3,,,16,1
LESIZE,2,,,1,1
LESIZE,4,,,1,1
MSHKEY,1
MSHAPE,0,2D                                     ! mapped quad mesh
AMESH,1
                                                ! select, assign attribute to and mesh area 2
ASEL,S,AREA,,2
AATT,2,1,2,0
LESIZE,5,,,16,1
LESIZE,7,,,16,1
LESIZE,6,,,5
LESIZE,8,,,5
MSHKEY,0
MSHAPE,0,2D                                     ! mapped quad mesh
AMESH,2
                                                ! reflect quarter circle into semicircle about x-axis
nsym,x,1000,all                                 ! offset node number by 1000
esym,,1000,all
                                                ! reflect semicircle into full circle about y-axis
nsym,y,2000,all                                 ! offset node number by 2000
esym,,2000,all
NUMMRG,ALL                                      ! merge all quantities
                                                ! modify outer 2 layers of el29 into type 4
esel,s,type,,1
nsle,s
esln,s,0
nsle,s
esel,inve
nsle,s
emodif,all,type,4
esel,all
nsel,all
                                                ! define el129 line element
csys,1
nsel,s,loc,x,0.5207
type,3
real,3
mat,2
esurf
esel,all
nsel,all
                                                ! flag interface as fluid-structure interface
nsel,s,loc,x,0.26035
esel,s,type,,2
sf,all,fsi,1
nsel,all
esel,all
FINISH
                                                ! enter solution module
/SOLU
ANTYPE,harmic                                   ! select harmonic analysis
hropt,full
f,19,fx,1000.
f,1019,fx,-1000.
harfrq,34.,38.
nsubst,100
kbc,1
SOLVE
FINISH
                                                ! postprocess
/post26
plcplx,0
nsol,2,1,u,x,d1ux
store
conjug,3,2
prod,4,2,3
sqrt,5,4
*get,uxmx,vari,5,extrem,tmax
/COM -------------------------------------------------------------
/COM                     Expected Result:
/COM
/COM The following "uxmx" should equal 34.52 Hz.
/COM -------------------------------------------------------------
*status,uxmx
finish