I am attempting to couple acoustic infinite element matrices from Abaqus with a custom solver. During matrix validation using a minimal example, two critical discrepancies emerged. Here is my test configuration and observed issues:
Model Setup
*Element Type: Single CIN3D8 acoustic infinite element
*Node coordinates: (0.5,0.5,0), (-0.5,0.5,0), (-0.5,-0.5,0), (0.5,-0.5,0)
*Acoustic reference node: (0,0,-100)
*Boundary Condition: Unit acoustic pressure applied at (0.5,0.5,0)
*Expected Result: 40 pressure DOFs with the prescribed DOF = 1.0
1. Null Mass/Damping Matrices in Global Matrix Export:
Only stiffness matrix (STIF) contains non-zero values; mass (MASS) and damping (DMPV) matrices are null.Is this expected for acoustic infinite elements, or is there an error in matrix generation methodology?
2. Residual Mismatch in Equation Validation:
Calculated K·U yields non-zero residuals across all DOFs, contradicting theoretical expectations where residuals should be zero except at the prescribed DOF (as shown below):
K 40×40 ⋅U 40×1=F 40×1 (Expected: F i =0 except at BC node)
Thank you for your expertise.
The inp file is as follow:
************************************************************************************************************************************
*Heading
** Job name: Isoparametric_matrix Model name: Isoparametric_matrix
** Generated by: Abaqus/CAE 6.14-1
*Preprint, echo=NO, model=NO, history=NO, contact=NO
**
** PARTS
**
*Part, name=PART-1
*Node
1, 0.5, 0.5, 0.
2, -0.5, 0.5, 0.
3, -0.5, -0.5, 0.
4, 0.5, -0.5, 0.
5, 0., 0., -100.
*Element, type=ACIN3D4
1, 1, 2, 3, 4
*Nset, nset=PART-1-REFPT_, internal
5,
*Nset, nset=INFINITE, generate
1, 4, 1
*Elset, elset=INFINITE
1,
** Section: Infinite
*Solid Section, elset=Infinite, material=Fluid, ref node=Part-1-RefPt_, order=10
1.,
*End Part
**
**
** ASSEMBLY
**
*Assembly, name=Assembly
**
*Instance, name=PART-1-1, part=PART-1
*End Instance
**
*Nset, nset=UNITACOUSTICPRESSURE, instance=PART-1-1
1,
*End Assembly
**
** MATERIALS
**
*Material, name=FLUID
*Acoustic Medium
2.25e+09,
*Density
1000.,
** ----------------------------------------------------------------
**
** STEP: Step-1
**
*Step, name=Step-1, nlgeom=NO, perturbation
*Steady State Dynamics, direct, frequency scale=LINEAR, friction damping=NO
1., 1., 1, 1.
**
** BOUNDARY CONDITIONS
**
** Name: Aco-BC-1 Type: Acoustic pressure
*Boundary, real
UNITACOUSTICPRESSURE, 8, 8, 1.
**
** OUTPUT REQUESTS
**
**
** FIELD OUTPUT: F-Output-1
**
*Output, field, variable=PRESELECT
**
** HISTORY OUTPUT: H-Output-1
**
*Output, history, variable=PRESELECT
*End Step
*Step
*MATRIX GENERATE, STIFFNESS, MASS, VISCOUS DAMPING, STRUCTURAL DAMPING, LOAD
*MATRIX OUTPUT, STIFFNESS, MASS, VISCOUS DAMPING, STRUCTURAL DAMPING, LOAD, FORMAT=COORDINATE
*End Step
************************************************************************************************************************************
