[Moritz87]

Dear users,

I've come to this forum since I'm new to Ansys and have some problems with an acoustic FSI calculation.

In the scope of my master's thesis, I want to calculate the eigenfrequencies of an acoustic cavity. The cavity consists of a steel-walled box, which surrounds a volume of gas (air). The box was modeled with SOLID95-elements, the fluid was modeled with FLUID220-elements. The interface between fluid and solid was defined as an FSI-flag. Now, I want to investigate the influence of the box on the eigenfrequencies of the gas and vice versa.

Unfortunately, the results of my calculation differ with every calculation up to a factor of 5. I have posted my APDL-code, does anybody know why this mistake happens?

I'm very thankfull for every help.!


My code:


!!! preamble ================================================== =====

FINISH
/CLEAR

!/TITLE, Eigenmoden FSI-Volumen

!!! preprocessor ================================================== =

/PREP7


! definition of parameters:

Komp = 1420000 ! Kompressionsmodul bei Athmosphärendruck; Einheit: Pa
rho0 = 12
C = SQRT(Komp/rho0)
Ref = 0.00002 ! Referenzschalldruck

E_x = 210000000000
E_y = 210000000000
E_z = 210000000000

nu_x = 0.3
nu_y = 0.3
nu_z = 0.3

rho = 7850

! definition of element types:

ET,1,FLUID220,,1 ! with FSI
ET,2,FLUID220,,0 ! without FSI
ET,3,SOLID95

! definition of materials:

!Material 1:
MP,DENS,1,rho0
MP,SONC,1,C
MP,MU,1,0

!Material 2:
MP,DENS,2,rho0
MP,SONC,2,C
MP,MU,2,0

!Material 3:
MP,EX,3,E_x
MP,EY,3,E_y
MP,EZ,3,E_z

MP,PRXY,3,nu_x
MP,PRXZ,3,nu_z
MP,PRYZ,3,nu_y

MP,DENS,3,rho



! definition of geometry

!lengths:
L_x = 8
L_y = 3
L_z = 4

!lengths of fluid- and solid elements
L_Ele_Fluid = 0.5
L_Ele_Solid = 0.5

! definition of keypoints:

S_X = L_x/2
S_Y = L_y/2
S_Z = L_z/2

! keypoints of inner fluid body:
K,1,S_X,-S_Y,S_Z
K,2,-S_X,-S_Y,S_Z
K,3,-S_X,-S_Y,-S_Z
K,4,S_X,-S_Y,-S_Z

K,5,S_X,S_Y,S_Z
K,6,-S_X,S_Y,S_Z
K,7,-S_X,S_Y,-S_Z
K,8,S_X,S_Y,-S_Z


! keypoints of right outer fluid body:
K,9,S_X+L_Ele_Solid,-S_Y,S_Z
K,10,S_X,-S_Y,S_Z
K,11,S_X,-S_Y,-S_Z
K,12,S_X+L_Ele_Solid,-S_Y,-S_Z

K,13,S_X+L_Ele_Solid,S_Y,S_Z
K,14,S_X,S_Y,S_Z
K,15,S_X,S_Y,-S_Z
K,16,S_X+L_Ele_Solid,S_Y,-S_Z


! keypoints of left outer fluid body:
K,17,-S_X,-S_Y,S_Z
K,18,-(S_X+L_Ele_Solid),-S_Y,S_Z
K,19,-(S_X+L_Ele_Solid),-S_Y,-S_Z
K,20,-S_X,-S_Y,-S_Z

K,21,-S_X,S_Y,S_Z
K,22,-(S_X+L_Ele_Solid),S_Y,S_Z
K,23,-(S_X+L_Ele_Solid),S_Y,-S_Z
K,24,-S_X,S_Y,-S_Z


! keypoints of frontal outer fluid body:
K,25,(S_X+L_Ele_Solid),-S_Y,(S_Z+L_Ele_Solid)
K,26,-(S_X+L_Ele_Solid),-S_Y,(S_Z+L_Ele_Solid)
K,27,-(S_X+L_Ele_Solid),-S_Y,S_Z
K,28,(S_X+L_Ele_Solid),-S_Y,S_Z

K,29,(S_X+L_Ele_Solid),S_Y,(S_Z+L_Ele_Solid)
K,30,-(S_X+L_Ele_Solid),S_Y,(S_Z+L_Ele_Solid)
K,31,-(S_X+L_Ele_Solid),S_Y,S_Z
K,32,(S_X+L_Ele_Solid),S_Y,S_Z


! keypoints of back outer fluid body:
K,33,(S_X+L_Ele_Solid),-S_Y,-(S_Z+L_Ele_Solid)
K,34,-(S_X+L_Ele_Solid),-S_Y,-(S_Z+L_Ele_Solid)
K,35,-(S_X+L_Ele_Solid),-S_Y,-S_Z
K,36,(S_X+L_Ele_Solid),-S_Y,-S_Z

K,37,(S_X+L_Ele_Solid),S_Y,-(S_Z+L_Ele_Solid)
K,38,-(S_X+L_Ele_Solid),S_Y,-(S_Z+L_Ele_Solid)
K,39,-(S_X+L_Ele_Solid),S_Y,-S_Z
K,40,(S_X+L_Ele_Solid),S_Y,-S_Z


! keypoints of lower outer fluid body:
K,41,(S_X+L_Ele_Solid),-(S_Y+L_Ele_Solid),(S_Z+L_Ele_Solid)
K,42,-(S_X+L_Ele_Solid),-(S_Y+L_Ele_Solid),(S_Z+L_Ele_Solid)
K,43,-(S_X+L_Ele_Solid),-(S_Y+L_Ele_Solid),-(S_Z+L_Ele_Solid)
K,44,(S_X+L_Ele_Solid),-(S_Y+L_Ele_Solid),-(S_Z+L_Ele_Solid)

K,45,(S_X+L_Ele_Solid),-(S_Y),(S_Z+L_Ele_Solid)
K,46,-(S_X+L_Ele_Solid),-(S_Y),(S_Z+L_Ele_Solid)
K,47,-(S_X+L_Ele_Solid),-(S_Y),-(S_Z+L_Ele_Solid)
K,48,(S_X+L_Ele_Solid),-(S_Y),-(S_Z+L_Ele_Solid)


! keypoints of upper outer fluid body:
K,49,(S_X+L_Ele_Solid),(S_Y),(S_Z+L_Ele_Solid)
K,50,-(S_X+L_Ele_Solid),(S_Y),(S_Z+L_Ele_Solid)
K,51,-(S_X+L_Ele_Solid),(S_Y),-(S_Z+L_Ele_Solid)
K,52,(S_X+L_Ele_Solid),(S_Y),-(S_Z+L_Ele_Solid)

K,53,(S_X+L_Ele_Solid),(S_Y+L_Ele_Solid),(S_Z+L_El e_Solid)
K,54,-(S_X+L_Ele_Solid),(S_Y+L_Ele_Solid),(S_Z+L_Ele_Sol id)
K,55,-(S_X+L_Ele_Solid),(S_Y+L_Ele_Solid),-(S_Z+L_Ele_Solid)
K,56,(S_X+L_Ele_Solid),(S_Y+L_Ele_Solid),-(S_Z+L_Ele_Solid)


!generation of volumes:
V,1,2,3,4,5,6,7,8 ! Fluid mitte ! Volumen 1
V,9,10,11,12,13,14,15,16 ! Solid außen rechts ! Volumen 2
V,17,18,19,20,21,22,23,24 ! Solid außen links 3
V,25,26,27,28,29,30,31,32 ! Solid außen vorne 4
V,33,34,35,36,37,38,39,40 ! Solid außen hinten 5
V,41,42,43,44,45,46,47,48 ! Solid außen unten 7
V,49,50,51,52,53,54,55,56 ! Solid außen oben 6


! meshing of volumes 2 to 7; SOLID95
TYPE,3
MAT,3
VSEL,S,VOLU,,2,7
ESIZE,L_Ele_Fluid
VMESH,ALL
VSEL,ALL
ALLSEL


! coupling of degrees of freedom of solid bodies:
ESEL,S,TYPE,,3,3
CPINTF,UX,0.0001
CPINTF,UY,0.0001
CPINTF,UZ,0.0001
ESEL,ALL


! meshing of volume1; FLUID220
TYPE,1
MAT,1
VSEL,S,VOLU,,1
ESIZE,L_Ele_Fluid
VMESH,1,1
VSEL,ALL


ESEL,S,TYPE,,1
NSEL,S,EXT
ESLN,ALL
EMODIF,ALL,TYPE,2
ALLSEL


! define FSI-flag at border fluid-solid
ESEL,S,TYPE,,2 ! select fluid elements
NSEL,S,EXT ! nodes on the exterior surface
SF,ALL,FSI
SFELIST,ALL,FSI
ESEL,ALL
NSEL,ALL

!SFELIST,ALL,FSI


! contstrain displacements of solid volumes at all edges:
NSEL,S,LOC,X,-(L_X/2 + L_Ele_Solid) !1
NSEL,R,LOC,Z,-(L_Z/2 + L_Ele_Solid)
D,ALL,ALL,0
NSEL,ALL

NSEL,S,LOC,X,-(L_X/2 + L_Ele_Solid) !2
NSEL,R,LOC,Z,(L_Z/2 + L_Ele_Solid)
D,ALL,ALL,0
NSEL,ALL

NSEL,S,LOC,X,(L_X/2 + L_Ele_Solid) !3
NSEL,R,LOC,Z,(-L_Z/2 + L_Ele_Solid)
D,ALL,ALL,0
NSEL,ALL

NSEL,S,LOC,X,(L_X/2 + L_Ele_Solid) !4
NSEL,R,LOC,Z,(L_Z/2 + L_Ele_Solid)
D,ALL,ALL,0
NSEL,ALL

NSEL,S,LOC,X,-(L_X/2 + L_Ele_Solid) !5
NSEL,R,LOC,Y,-(L_Y/2 + L_Ele_Solid)
D,ALL,ALL,0
NSEL,ALL

NSEL,S,LOC,X,-(L_X/2 + L_Ele_Solid) !6
NSEL,R,LOC,Y,(L_Y/2 + L_Ele_Solid)
D,ALL,ALL,0
NSEL,ALL

NSEL,S,LOC,X,(L_X/2 + L_Ele_Solid) !7
NSEL,R,LOC,Y,-(L_Y/2 + L_Ele_Solid)
D,ALL,ALL,0
NSEL,ALL

NSEL,S,LOC,X,(L_X/2 + L_Ele_Solid) !8
NSEL,R,LOC,Y,(L_Y/2 + L_Ele_Solid)
D,ALL,ALL,0
NSEL,ALL

NSEL,S,LOC,Z,-(L_Z/2 + L_Ele_Solid) !9
NSEL,R,LOC,Y,(L_Y/2 + L_Ele_Solid)
D,ALL,ALL,0
NSEL,ALL

NSEL,S,LOC,Z,-(L_Z/2 + L_Ele_Solid) !10
NSEL,R,LOC,Y,-(L_Y/2 + L_Ele_Solid)
D,ALL,ALL,0
NSEL,ALL

NSEL,S,LOC,Z,(L_Z/2 + L_Ele_Solid) !11
NSEL,R,LOC,Y,-(L_Y/2 + L_Ele_Solid)
D,ALL,ALL,0
NSEL,ALL

NSEL,S,LOC,Z,(L_Z/2 + L_Ele_Solid) !12
NSEL,R,LOC,Y,(L_Y/2 + L_Ele_Solid)
D,ALL,ALL,0
NSEL,ALL

FINISH ! /PREP7



!!! solution ================================================== ==========================

/SOLU

ANTYPE,2
MODOPT,UNSYM,40,1
MXPAND,40
SOLVE

FINISH ! /SOLU

Thanks in advance,
Moritz Becker

[stumpy]

Don't know the problem, but if you search the "Knowledge Resources > Solutions" section on the ANSYS customer portal for "2026060" you'll find a similar example. Comparing the input files may help.