ACOU WK04 UnderwaterShock

7/23/2019 ACOU WK04 UnderwaterShock

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Workshop 4

Underwater shock analysis

Keywords Version

© Dassault Systèmes, 2013 Structural-Acoustic Analysis with Abaqus

Note: This workshop provides instructions in terms of the AbaqusKeywords interface. If you wish to use the Abaqus GUI interface instead,please see the “Interactive” version of these instructions.

Please complete either the Keywords or Interactive version of thisworkshop.

Goals

When you complete this workshop, you will be able to:

• Set up and analyze underwater shock problems.

• Determine the transient shock response of a submerged cylinder excited by a blastload.

• Understand the concept and the implementation of the Scattered Waveformulation.

• Define acoustic excitation loads via the *INCIDENT WAVE option.

•

Add reflective surfaces and examine the change in results.

Introduction

For this workshop, we will study a classical underwater shock example problem: a

submerged infinitely long elastic cylinder excited by a planar shock wave from a charge

that is relatively close to the structure. The effect of the pressure wave reflecting off the

bottom surface (the seabed) is included in the second part of this workshop. In thisworkshop you will add important modeling details to complete the supplied analysis

input file that already contains basic modeling data such as node, element, and material

definitions, which will help you to leverage the Abaqus and acoustics knowledge that youhave accumulated thus far, and quickly determine the user inputs required in underwater

shock analyses.

Case 1: Infinite cylinder without bottom reflection

The infinite cylinder excited by a plane wave is a two-dimensional problem with a singleplane of symmetry. The half-model for this problem is shown in Figure W4-1. The

infinite cylinder is fully submerged in the fluid so that free surface effects are negligible.

Artificial material properties are used in order to avoid spending an inordinate amount of




W4.2

time, as this is an instructional exercise. The coupling between the structure and the fluid

is enforced using the *TIE option which does not require compatible meshes. The meshdensity of the fluid is chosen so that the shock is captured accurately.

Figure W4-1 Infinite cylinder half-model.

1. Enter the working directory for this workshop:

../acoustics/keywords/workshop4

2. Open the provided incomplete input file inf-cyl-noref.inp in a text editor

and review its contents. This input file uses the Abaqus part/instance/assemblyparadigm. When adding options to this input file, consult the online

documentation to determine where the options go, and to understand the options

further.

The half-cylinder is modeled with 36 S4R shell elements. The fluid is modeledwith AC3D8R elements. The bulk viscosities and the time interval sizes have

been chosen to reduce the run time while capturing the shock accurately, thus

optimizing the efficiency of the solution.

3. After both the part instances have been declared, a number of surfaces have been

defined, primarily for output purposes. Define a node set named LEADING-EDGE

consisting of nodes 37 and 74 of the SHELL instance. Define another node setnamed TRAILING-EDGE consisting of SHELL nodes 1 and 38. These will be

used to generate history output, and to apply boundary conditions. Figure W4-2

shows where the node sets are.

The syntax to be used is:

*NSET, NSET=node_set_name, INSTANCE=instance_name

node1,node2,etc




W4.3

Figure W4-2 Shell model.

4. Define a surface on the shell elements that will be in contact with the acoustic

elements representing the fluid. Use the predefined element set SHELL-FLUID,

and create the surface using the SPOS side of the elements in the set, as shown in

Figure W4-2. Create another surface on the acoustic elements that will be in

contact with the structure. Use the predefined element set FLUID-SHELL, and

create the surface on the S4 side of these elements, as shown in Figure W4-3.

These surfaces will be used to define the loading, and to couple the structuraldisplacements to the acoustic pressures. The syntax to be used is:

*SURFACE, TYPE=ELEMENT, NAME=surface_name

element_set, face_label

5. Define the surface where the radiation boundary condition is active. For this, use

the predefined element set RADIATION, and use face S6, as shown in Figure W4-

3. Name this surface RADIATION.




W4.4

Figure W4-3 Acoustic fluid model.

6. Add the *TIE constraint in order to implicitly couple the response of the structure

to that of the fluid. Use the ADJUST parameter on this option and set it to YES.

This eliminates any separation between the surfaces that might be present due to

round-off errors. Specify the fluid surface to be the slave and the shell surface to

be the master. Name the constraint AC-STR-COUPLING. The syntax is:*TIE, NAME=constraint_name, ADJUST=YES

slave_surface, master_surface

7. Add the boundary conditions. On the node sets LEADING-EDGE and TRAILING-

EDGE, constrain all degrees of freedom except degree of freedom 1. On the node

sets BACK and FRONT constrain degrees of freedom 3 through 5. The syntax is:

*BOUNDARY

node_set, first_dof_constrained, last_dof_constrained

8. Define a nonreflecting boundary impedance on the surface named RADIATION.

Choose a CIRCULAR nonreflecting condition; the radius of the circle is 2.0.

*SIMPEDANCE, NONREFLECTING=type surface name, radius

9. Add the incident wave property. Use the name PLANE for this property. Set the

speed of sound in the fluid and the fluid density each equal to 1:

*INCIDENT WAVE INTERACTION PROPERTY, NAME= property_name

c f , ρ f




W4.5

10. Review the input file that you have made changes to again. Make sure that you

understand all the options used in the file. Some of them have been created foryou in advance. Use the documentation to clarify anything that is unclear.

Remember that the *AMPLITUDE definition used in the input deck refers to the

amplitude function of the shock at the standoff point, not at the source point.

Notice that direct user control has been used for the explicit dynamics step. Thisis sometimes necessary in shock simulations, since the stable time increment

automatically calculated by the code may still be too large for an accurate

solution.

11. Now add the incident wave load in the dynamic explicit step. Specify that the

amplitude given is a pressure amplitude, defined as a step function, and that the

wave property is planar. Specify the load both on the solid and the fluidinterfaces. The option to invoke this feature is given below. Enter it in the input

file exactly as it appears:

*INCIDENT WAVE INTERACTION, PRESSURE AMPLITUDE=STEP, PROPERTY=PLANE shell-fluid, source, standoff, -1.0E-4

fluid-shell, source, standoff, -1.0E-4

12. Add history output requests. For node sets, leading-edge, and trailing-

edge, request output of variables U, V , and A . They are displacement, velocity,

and acceleration respectively. The default frequency will be used, which is to save

output at 200 equally spaced intervals during the analysis. The syntax is:

*OUTPUT, HISTORY

*NODE OUTPUT, NSET=node_set

variables

13. The modifications are now complete; save the modified input file. Use thefollowing command to run the Abaqus/Explicit analysis with double precision:

abaqus job=inf-cyl-noref doubleLook at the status file (.sta) while the analysis is running to monitor its

progress. You can also monitor the stable time increment size, which for this step

remains constant at the fixed value specified. This analysis requires small time

increments, in order to retain accuracy and stability. This is why most underwater

shock analyses are better suited for Abaqus/Explicit rather than Abaqus/Standard.

14. Once the analysis is complete, open the ODB file in Abaqus/Viewer.

15. Display a contour plot of the model. Use the Results Tree to display the SHELL

part instance alone (expand the Instances container underneath the output

database named inf-cyl-noref.odb, and click mouse button 3; from the menu

that appears, select Replace).Animate the Mises stress history in the shell (Animate→Time History). The

shock front can be seen to affect the leading edge first and pass gradually over the

whole structure until it reaches the trailing edge. The wave nature of the front can

be seen from the alternating high and low stress regions passing over thestructure. Figure W4-4 shows the Mises stress in the shell 2.6 seconds after the

shock encounters the leading edge.




W4.6

Figure W4-4 Mises stress in the shell after 2.6 seconds.

16. Display the FLUID part instance alone. Change the current field output variable to

POR (Result→Field Output) and animate the result contours. The dynamics of

the shock front can be easily seen in the sequence of plots shown in Figures W4-

5, W4-6 and W4-7. As the shock wave moves over the structure and thesurrounding fluid, it loses energy into the fluid due to non-reflective boundary

condition. The contours show how the POR maximum decreases with time.

Notice how the wave reaching the radiating boundary is not reflected back intothe mesh. The smoothness of the pressure contours in the vicinity of the boundary

is an indication of the effectiveness of the absorbing condition. Jagged or rough

contours indicate that the boundary is too close to the structure, and should bemoved further away, implying that a larger fluid mesh should be used to analyzethe problem. In general, enough fluid should be used to capture the ‘added mass’

effect of infinite surrounding fluid in the physical problem.




W4.7

Figure W4-5 Acoustic pressure in the fluid after 0.6 seconds.

Figure W4-6 Acoustic pressure in the fluid after 1.4 seconds.




W4.8

Figure W4-7 Acoustic pressure in the fluid at the end of the analysis.

17. Another important output variable in shock analyses is the velocity history atcritical points in the structure. These indicate the severity with which the shockhas influenced the structure and perhaps compromised its integrity. Create an X–Y

plot using the saved history data (Result→History Output). Plot the V1 velocity

component for one node from the set TRAILING-EDGE and one node from the

set LEADING-EDGE. This plot is shown in shown in Figure W4-8. Note that the

plot has been optimized for display purposes, and that the frequency of plot

markings is 8 times less than the frequency at which results were extracted. Thisplot clearly illustrates the time delay in the response of the trailing edge due to the

finite shock speed. Notice that the shock tends to be its severest immediately upon

impingement on the structure, and then begins to dissipate energy mitigating its

effect on the shell. The choice of time increment size and damping will have aneffect on the nature of these histories. If you have time after completing this

workshop, investigate the effect of changing the time increment size and the bulk

viscosity. Remember to run all shock analyses using double precision to furtherimprove the accuracy and suppress numerical oscillations.




W4.9

Figure W4-8 Velocity histories at the leading and trailing edges of the shell.

18. If time permits, run the same analysis in Abaqus/Standard, and compare the runtimes and results between the two solvers. An Abaqus/Standard input file is

provided (inf-cyl-noref-standard.inp). The results do not differ

significantly, but the analysis time does.

Case 2: Infinite Cylinder with bottom reflection

In this case, we study the effect of adding a single reflecting surface to the problem. Youcan easily extend this to multiple surfaces. The main difference between this case and theprevious one is that here we will assume that the explosion occurs relatively close to the

structure, and hence we will specify a spherical wave front. This is a modeling decision

that is for the user to decide. The effect of reflecting surfaces is handled in Abaqus byusing imaging techniques where the incident pressure wave is made up of both primary

and image source contributions with the appropriate time delays calculated automatically.

The cylinder geometry and the fluid properties are the same as those defined for theprevious case. While the structure is symmetric, the loading is not on account of thereflecting plane. Therefore we do not use symmetry in this case. The full cylinder model

is shown in Figure W4–9. A single charge is located on the X -axis 10 units away from the

cylinder ( Z-) axis. The bottom surface is located 4.6904 units below the cylinder axis. Itis assumed that 80% of the incoming wave is reflected off the bottom surface. The model

consists of 72 4-node quadrilateral shell elements and 2880 AC3D8R acoustic elements.

The standoff point is placed along the X -axis, at the point where the structure and the

fluid are in contact.




W4.10

Figure W4-9 Full cylinder model.

1. Open the file inf-cyl-ref.inp. This file is incomplete and needs somemodifications. Since you have already created sets, surfaces, etc in the first part of

this workshop, you will not do so again. This section focuses on the differences

between the previous case and this one.

2. First add the structural boundary conditions. The only mechanical constraintsrequired for this case are the ones that constrain degrees of freedom 3 through 5

for the node sets BACK and FRONT.

3. Notice that ‘TYPE=SPHERE’ has been used on the *INCIDENT WAVEINTERACTION PROPERTY option in this model. In the previous case, thewavefront was planar.

4. The reflective properties of the plane must be described. We know that the planereflects 80% of the incoming waves. Add the *INCIDENT WAVE

REFLECTION option as a suboption of *INCIDENT WAVE INTERACTIONwithin the step definition. Remember that the normal direction must point away

from source point. This means that the normal direction for the reflecting plane is

(0, −1, 0). The reflecting plane is located 4.6904 units below the charge. Thesyntax needed is:

*incident wave reflection

distance_from_charge, normal_direction_cosines, reflection coefficient

5. Once you have made the above changes, submit the analysis using:

abaqus job=inf-cyl-ref double

6. Monitor the progress of the analysis by looking at the status (.sta) file. Once the

analysis has completed successfully, open the output database (ODB) in

Abaqus/Viewer and examine the results.

7. In a simple model such as this one, we can compare the results obtained using thereflecting surface to those obtained by replacing the reflecting plane with anothercharge. This charge is called the image charge and is internally generated by

Abaqus whenever a reflecting surface is included. A model with this image




W4.11

charge has been created for you so that you can compare results from the two

types of approaches. Open the file inf-cyl-2ch.inp, and review its contents.

8. Notice that there are two *INCIDENT WAVE INTERACTION PROPERTY

options in this input file. The first is named SPHERE and is exactly the same as

before. In addition, there is another one called IMAGE. The source point for this

image charge is below where the reflecting plane was in the previous case, at thesame distance below the plane as the primary charge is above. The standoff point

is located on the line joining the image source point to the center of the circle

representing the cylinder. In order to get the time delay due to reflection correctlycomputed, the image standoff is positioned such that the distance from the image

source to the image standoff is the same as that between the primary source and

the primary standoff. In addition, we know that the reflecting plane was reflecting80% of the incoming energy. So we scale the magnitude of the image source by

0.8. Once this is done, the results obtained using this modeling approach are

identical to those of the previous analysis.

9. Run this analysis using abaqus job=inf-cyl-2ch double and look at the

results. You should find that the results are exactly the same as those of previous job. Either of the two approaches can be used in a general situation.

Note: Complete input files are available for your convenience. You mayconsult these files if you encounter difficulties following the instructionsoutlined here or if you wish to check your work. The input files are named

inf-cyl-noref-complete.inp

inf-cyl-ref-complete.inp

and are available in the working directory for this workshop.

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ACOU WK04 UnderwaterShock