ABAQUS AC3D8R Element
Quality Assessment
By Michael Hubenthal
ABAQUS AC3D8R Element Quality Assessment
Background
 AC3D8R is an acoustic element used for
modeling wave propagation in Abaqus Explicit
 It can be used to model fluid-structure interaction
behavior
 The accuracy of the structural response is
dependent on the accuracy of the fluid mesh
 The AC3D8R accuracy can vary with element
shape
Relevant Geometric Characteristics




Element Size
Aspect Ratio
Skew
Orientation
ABAQUS BENCHMARKS MANUAL
Element Verification Concept




Abaqus verification tests look at the first node in a
mesh of ideally shaped elements
This project will perform verification of the AC3D8R
element for geometry that is less than ideal
This project will develop a baseline comparison
using ideal geometry and a known wave
This project will compare the geometric
characteristics to the baseline results
Methodology

Calculate theoretical wave speed in water
Set up the baseline model and develop a known wave to
propagate through the model (triangle wave)

Build and test a fluid mesh with varying aspect ratio

…Methodology

Build and test a fluid mesh with varying mesh density

Build and test a fluid mesh with varying element skew

Build and test a fluid mesh with varying element orientation relative
to wave

Compare results from all the cases
Element Size



1 meter length of fluid
Cube shaped elements
Vary element size only
Element Size Results
Element Size (t=0.00068165sec)
500.00
theoretical
element size 1/4 of baseline
400.00
element size 1/2 of baseline
baseline
element size 2x baseline
Pressure (N/m^2)
300.00
element size 3x baseline
element size 4x baseline
element size 10x baseline
200.00
100.00
0.00
0
0.1
0.2
0.3
0.4
0.5
-100.00
distance (m)
0.6
0.7
0.8
0.9
1
Element Density




1 meter length of fluid
Cube shaped elements
Constant volume
Compare to element size test to consider if multiple elements
affect the results
Element Density Results
(0.0025in x 0.0025in x 0.0025in) elements
(0.005in x 0.005in x 0.005in) elements
5.00E+02
500
density 2x2
4.00E+02
element size 1/2 of
baseline
300
pressure (N/m^2)
pressure (N/m^2)
400
density 4x4
element size 1/4 of baseline
200
100
0
3.00E+02
2.00E+02
1.00E+02
0.00E+00
0.5
0.6
0.7
0.8
-100
0.9
1
0.5
0.6
0.7
0.8
-1.00E+02
distance (in)
distance (in)
0.9
1
Aspect Ratio



1 meter length of fluid
Constant volume
Vary element aspect ratio
Aspect Ratio Results
Aspect Ratio
500
400
Pressure (N/m^2)
300
aspect 1 to 4
aspect 1 to 3
aspect 1 to 2
aspect 2 to 1
aspect 4 to 1
baseline
Theoretical
200
100
0
0
0.1
0.2
0.3
0.4
0.5
-100
distance (m)
0.6
0.7
0.8
0.9
1
Element Skew



1 meter length of fluid
Same number of elements as baseline
Compare to baseline to consider if element skew affects the
results
Element Skew Results
Element Skew (t=0.00068165sec)
500
400
Pressure (N/m^2)
300
5 deg. Skew
15 deg. Skew
30 deg. Skew
45 deg. Skew
Theoretical
baseline
200
100
0
0
0.1
0.2
0.3
0.4
0.5
-100
distance (in)
0.6
0.7
0.8
0.9
1
Element Orientation



1 meter length of fluid
Same number of cube elements as baseline
Compare to baseline to consider if element orientation affects
the results
Element Orientation Results
Element Orientation (t=0.00068165sec)
500.00
theoretical
baseline
orientation
400.00
Pressure (N/m^2)
300.00
200.00
100.00
0.00
0
0.1
0.2
0.3
0.4
0.5
-100.00
distance (m)
0.6
0.7
0.8
0.9
1
Propagation Length



10 meter length of fluid
Same element edge lengths as element size test models
Compare to element size test to consider if element
propagation length affects the results
1 meter
10 meter
Propagation Length Results
Extended Length Propagation (t=0.0068165sec)
500.00
Theoretical
1/2 size baseline-long
1/4 size baseline-long
baseline_long
400.00
Pressure (N/m^2)
300.00
200.00
100.00
0.00
9
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
9.9
10
-100.00
Distance (m)
1 meter length
10 meter length
Model
Maximum
Pressure
Accuracy
Maximum
Pressure
Accuracy
Baseline
480.80
96.2%
456.02
91.2%
½ baseline element size
488.30
97.7%
475.46
95.1%
¼ baseline element size
492.67
98.5%
484.52
96.9%
Conclusions


Element length in the direction of wave propagation is the
most important geometric element characteristic
The modeled fluid extent should be minimized to prevent
losses