Modeling Submerged Structures Loaded by Underwater Explosions with ABAQUS/Explicit

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1 Modeling Submerged Structures Loaded by Underwater Explosions with ABAQUS/Explicit David B. Woyak ABAQUS Solutions Northeast, LLC Abstract: Finite element analysis can be used to predict the transient response of submerged structures that are externally loaded by an acoustic pressure shock wave resulting from an Underwater Explosion (UNDEX). This class of problem is characterized by a strong coupling between the structural motions and acoustic pressures at the fluid-structure wetted interface. The structural behavior is a combination of long time (low frequency) response dominated by an added mass effect, short time (high frequency) response dominated by radiation damping, and intermediate time-frequency response where both added mass and radiation damping behavior are present. For the finite element method to be useful, the analyst must develop modeling techniques and procedures that yield accurate and computationally tractable solutions. Modeling procedures and guidelines were developed for use with an explicit dynamics code that offers advanced features such as: pressure formulated acoustic elements, surface based fluid-structure coupling, surface based absorbing (radiation) boundaries, and automated incident wave loading for the fluid-structure wetted interface. The modeling guidelines address issues such as: location of the fluid acoustic domain outer boundary, meshing of the acoustic domain, representation of the shock wave, and solution efficiency. These modeling procedures and guidelines are demonstrated with an ABAQUS/Explicit analysis of an UNDEX experiment in which a submerged test cylinder was exposed to a 60-pound HBX-1 explosive charge (Kwon & Fox, 1993). General Background ABAQUS/Explicit is an efficient tool for simulating the transient response of structural-acoustic systems, of which the response of submerged structures loaded by acoustic shock waves resulting from an Underwater Explosion (UNDEX) is an important problem class. This paper provides a brief discussion on the general nature of the structural-acoustic interaction and describes modeling studies that address general Finite Element Analysis (FEA) requirements for the accurate simulation of UNDEX problems. The studies described in this report have general application to a wide range of structural-acoustic problems, not just the analysis of submerged structures. An example analysis of a submerged cylinder is used to illustrate an UNDEX problem. UNDEX analyses can be generally characterized as transient simulations of acoustic scattering behavior. However, the objective of an UNDEX analysis is to evaluate the response of the submerged structure and not necessarily the acoustic response. The finite element model for the external acoustic domain must be adequate to represent the influence of the water on the structural response. The discussion herein will be restricted to those cases where the external fluid behaves as a linear acoustic fluid with no cavitation. Therefore, the model of the external acoustic domain need only be tailored to provide an accurate loading on the structure and does not need to accurately represent the acoustic waves that will travel away from the structure. It should be noted 2002 ABAQUS Users Conference 1

2 that procedures for UNDEX analyses which include fluid cavitation will be available in ABAQUS/Explicit with the release of Version 6.3 (Prasad & Cipolla, 2001). The total acoustic pressure in the external fluid that results from an underwater explosion consists of the known incident shock wave (incoming) pressure and the unknown scattered wave (outgoing) pressure. The scattered wave pressure consists of two parts, a reflected part that is associated with the shock wave interacting with an ideal rigid, immovable structure and a vibratory part that results from the motions of the structure at the interface with the fluid. When cavitation is not present, it is desirable to let the external acoustic domain represent only the scattered portion of the total acoustic pressure. The shock wave incident pressure load is applied directly to the structural mesh at the fluid-structure wetted interface. Acoustic loads associated with the reflected part of the scattered pressure are applied to the fluid mesh at the wetted interface. The full scattered pressure (reflected and vibratory) is obtained as the solution for the acoustic element pressure degrees of freedom, and the complete scattered pressure loading on the structure is generated through the fluid-structure coupling equations. The acoustic loads are a characteristic of the incident shock wave, and are obtained from the fluid particle accelerations in a direction normal to the surface that defines the fluid-structure wetted interface. In the discussion that follows the capability of ABAQUS/Explicit to efficiently perform UNDEX analyses is demonstrated. The ABAQUS features utilized in solving this class of problem are: 1. Computationally efficient pressure based acoustic elements. 2. Surface based automated acoustic-structure coupling. 3. Acoustic-structure coupling for mis-matched meshes. 4. Surface based impedance models for representing non-reflecting fluid boundaries. 5. Automated shock wave loading at the fluid-structure interface. Acoustic Domain Outer Boundary UNDEX problems are characterized by a strong coupling between the structural motions and acoustic pressures at the wetted interface. The system response in a strongly coupled structuralacoustic system can be described as being a combination of the following types of response: 1. Late Time - Low Frequency: Characterized by structural wavelengths that are significantly shorter than the associated acoustic wavelengths. The effect of the external fluid on the structure is that of adding an effective mass to the structure on the wetted interface. The scattered energy within the acoustic domain remains near the structure with very little energy radiating away from the structure. 2. Early Time - High Frequency: Characterized by structural wavelengths that are significantly longer than the associated acoustic wavelengths. The effect of the external fluid on the structure is to act as a simple radiation damper at the wetted interface. Most of the scattered energy within the acoustic domain radiates away from the structure. 3. Intermediate Time - Frequency: Characterized by structural wavelengths that are of similar length compared to the associated acoustic wavelengths. The effect of the ABAQUS Users Conference

3 external fluid on the structure is that of adding both effective mass and damping to the structure. Comparable levels of scattered energy remain near the structure and are radiated away from the structure. Due to the high density and bulk modulus of water the finite element model for an UNDEX analysis must be capable of accurately simulating all three ranges of structural-acoustic response. It should be noted that for some other types of structural-acoustic analyses, where the fluid is very light compared to the structure, not all response types may be of equal importance. For example, the added mass effect of air acting upon heavy structures is often of no consequence. The acoustic model for the outer boundary of the external fluid domain must provide adequate non-reflecting behavior over all three time-frequency ranges. Non-reflecting outer boundary models are implemented in ABAQUS/Explicit (Version 6.2) via a surface based boundary impedance. ABAQUS has several imbedded surface impedance models, of which the circular and sphere types were used for analyses described herein. These impedance models are based upon the classical solutions for a 3-dimensional point source (sphere) and a 2-dimensional point source or 3-dimensioanl linear source (circular). The default impedance model corresponds to a simple plane wave radiation condition, which is well suit to simple acoustic tube test simulations. For the late time - low frequency response range, the extent of fluid contributing to the added mass is largest for the longest structural response wavelength. Also, for the early time - high frequency response range, the longest structural response wavelength has the potential to generate efficient radiating acoustic waves with the longest wavelengths. Therefore, the location of the fluid mesh outer boundary can be based upon the structure s longest characteristic response wavelength. A general guideline for locating the acoustic mesh outer boundary was developed by performing a series of analyses representing the harmonic translational motion of a rigid infinite cylinder in an infinite fluid domain. This type of motion is closely related to the transverse motion of a cylinder section for beam bending response modes. A 2-dimensional rigid cylinder cross section (10 radius) was placed within a circular fluid domain, for which the outer boundary was located at 2, 3, and 4 cylinder radii. The fluid was water with a bulk modulus of 345,600 psi and a sound speed of 60,000 inches per second. ABAQUS/Explicit was used to drive the cylinder with a harmonic motion until steady state conditions were achieved. Baseline analyses with very refined acoustic meshes were used to represent the exact solutions. The baseline analyses utilized linear acoustic triangle elements with approximately 42 element divisions per acoustic wavelength. The outer boundary for the baseline analyses was located two full acoustic wavelengths away from the structure, and utilized the circular type impedance boundary. The boundary evaluation models had a minimum of 20 element divisions per acoustic wavelength for the highest driving frequency, and the maximum acoustic element size was set at 1.5 inches for the lowest driving frequencies. Figure 1 shows the baseline results for the complex radiation impedance of the driven cylinder (force/velocity). The impedance values are plotted against the ratio of the structural wavelength, which is the cylinder circumference, to the driving frequency acoustic wavelength. The radiation reactance (imaginary) represents an added-mass effect and the radiation resistance (real) represents the acoustic damping. Figure 1 clearly shows all three time-frequency response ranges, with the radiation impedance transitioning from added mass at low frequencies to radiation damping at high frequencies. Figure 2 shows a plot of the error ratio for the radiation impedance predictions with the evaluation models. The radial thickness of the fluid mesh for the outer boundary at 2, 3 and 4 cylinder radii, corresponds to approximately 1/6, 1/3, and 1/2 of the structural wavelength. Setting the boundary at 2 cylinder radii (1/6 structural wavelength) works 2002 ABAQUS Users Conference 3

4 well at high frequencies but can introduce significant error in the added mass at low frequencies. The error oscillations within the 5% error range at the higher driving frequencies appear to be due to the outer boundary being placed near integer multiples of half the acoustic wavelength. Placing the outer boundary so that the fluid domain thickness is between 1/3 and 1/2 the largest structural wavelength provides for reasonable accuracy when using a sound source based outer boundary surface impedance model. The performance when using a classical plane wave boundary condition is significantly diminished, and would require at least doubling the extent of the fluid domain. Fluid Mesh Requirements for Representing the Shock Wave The classical representation of a spherical shock wave associated with UNDEX loading is characterized by an instantaneous pressure rise to a peak value followed by an exponential decay. An UNDEX analysis in which the external fluid is modeled with finite elements cannot accurately represent a shock wave having an instantaneous pressure rise because the infinite pressure gradient at the shock front implies infinite fluid particle acceleration, so that the acoustic loads associated with the reflected part of the scattered pressure become indeterminate. Therefore, the shock front must be modeled such that the pressure rise occurs over a period of time, designated the rise time. A reasonable value for the rise time can usually be obtained from experimental or anecdotal data. The pressure vs. time history of the shock wave at a known distance form the source can be used to evaluate element size requirements for the acoustic mesh. This is accomplished by means of a simple acoustic tube evaluation model. A simple acoustic tube model was constructed with the linear tetrahedron acoustic elements that will be used in the subsequent UNDEX example analyses. Acoustic loads representative of a planar shock wave are applied at one end of the model with the ABAQUS incident wave loading capability. The end at which the loads are applied represents a rigid immovable wall. The resulting reflected wave travels down the acoustic tube. A simple plane wave absorbing boundary can be applied to the opposite end of the tube, or the tube can be made of sufficient length so that the test analysis finishes before the reflected wave reaches the opposite end. Figure 3 shows the results for a tube analysis in which the nominal element size is equal to 1.5 times the wave propagation distance corresponding to the rise time of the shock front. The only output quantity of concern in these acoustic tube analyses is the pressure at the rigid wall (scattered pressure). An ideal solution for this problem would have the reflected wave being an exact copy of the shock wave. This is clearly not the case for the Figure 3 model. Figure 4 is another analysis with the element size set at 1/4 the rise time propagation distance. The scattered pressure matches the shock pulse very well. However, there is still a fair degree of oscillation in the early time solution. It should be noted that the range in the pressure oscillations can become noticeably less when using brick type acoustic elements. Figure 5 shows some additional solutions with the element size set at 1.5 times the rise time distance, for which the time increment was varied via direct user control. As the time increment is reduced the mean response approaches that of the incident shock wave. This illustrates how simple acoustic tube models can also be used to evaluate the time increment requirements for accurately representing the reflected wave loading. Figure 5 also suggests that a relatively coarse fluid mesh may provide sufficient solution accuracy for the structural response as long as the structure being analyzed responds at low frequencies relative to the reflected wave oscillations. For these cases, the pressure impulse (time integral of pressure) associated with the reflected and ABAQUS Users Conference

5 incident shock waves should have relatively good correlation. Figure 6 shows the pressure impulse curves generated from the Figure 5 analyses, and suggests that using a time increment that is less than or equal to 1/20 of the rise time may provide good results with the coarse acoustic mesh for a low frequency structural system. Example Problem Description The UNDEX example problem is based upon an experiment in which a submerged test cylinder was exposed to a shock wave produced by a 60-pound HBX-1 explosive charge (Kwon & Fox, 1993). The test cylinder is made of T6061-T6 aluminum, has an overall length of meters, an outside diameter of meters, a wall thickness of 6.35 millimeters and welded endcaps that are 24.5 millimeters thick. The cylinder was suspended horizontally in a 40-meter deep fresh water test quarry (sound speed =1463 meters/second). The 60-pound HBX-1 explosive charge and the cylinder were both placed at a depth of 3.66 meters, with the charge centered off the side of the cylinder and located 7.62 meters from the cylinder surface. The suspension depths, charge offset and duration of the test were selected such that cavitation of the fluid would not be significant and no bubble pulse would occur. During the UNDEX test, two pressure transducers were positioned 7.62 meters from the charge, away from the cylinder, but at the same depth as the cylinder. These transducers provided an experimental determination for the pressure vs. time history of the incident spherical shock wave as it traveled by the point on the cylinder closest to the charge. Figure 7 is a time history of the recorded shock wave pressure used as input to the ABAQUS/Explicit analyses. Strain gages were placed at several locations on the outer surface of the test cylinder, as shown in Figure 8. The strain gage experimental data was filtered at 2000 Hz, with the experimental data presented herein obtained by digitizing the published Kwon & Fox strain data. ABAQUS/Explicit Model & Results The test cylinder was meshed with 2400 S4R finite strain shell elements and contained 2402 nodes (14412 dof) on 40 circumferential and 53 axial element divisions. The element connectivity is such that each shell normal is directed into the external fluid. The shell element nodes are positioned on the outside surface of the test cylinder. The cylinder body elements directly adjacent to the endcaps have reduced mass & stiffness and are only used to provide a surface that corresponds to the thickness of the endcaps. BEAM type MPCs are used to rigidly tie the endcaps to the main cylinder body. The external fluid is meshed with 4-noded AC3D4 acoustic tetrahedral elements. The outer boundary of the external fluid is represented by cylindrical and spherical surfaces with the appropriate surface impedance absorbing conditions. The characteristic radius of the fluid outer boundary is set at 3 shell radii, thus the thickness of fluid modeled about the cylinder represents approximately 1/3 of the cylinder s outer circumference (rigid body translational wavelength). Based upon the mesh boundary study this location should be sufficient to provide reasonably accurate results. Figure 9 shows the cylinder and first acoustic mesh that was used in the analysis, with the top half of fluid removed for clarity. The shock wave rise time is milliseconds, corresponding to a wave propagation distance of meters. The nominal element size at the wetted interface is also set at meters and increases in size to a nominal meters at the 2002 ABAQUS Users Conference 5

6 outer fluid boundary. An acoustic element size of meters corresponds to approximately 12 element divisions per acoustic wavelength for a 1500 Hz response. Figure 10 provides the results of the acoustic tube validation for this degree of mesh refinement. Acoustic Mesh #1 contains elements and 7947 pressure degrees of freedom. Figure 11 shows the second acoustic mesh that was used in the analysis, with the top half of fluid removed for clarity. The nominal element size at the wetted interface is set at meters and increases in size to a nominal meters at the outer fluid boundary. Figure 12 provides the results of the acoustic tube validation corresponding to acoustic Mesh #2, which contains elements and pressure degrees of freedom. Figure 13 shows the ABAQUS predicted axial strain response at strain gage location B1 when using the coarse (#1) and refined (#2) acoustic meshes. The response curves are very close both in magnitude and phasing. The close correlation between the two analyses was also apparent at the other strain gage locations. This indicates that for the applied UNDEX loading the structural response times are long when compared to the reflected wave oscillations obtained in the acoustic tube validation analyses. This result was predictable when considering an eigenvalue analysis for the cylinder with no external fluid. The modes that have the greatest potential for producing damage have frequencies well below 1500 Hz, and will be further reduced when the cylinder is submerged due to the added mass effect. The cylinder modes have response periods that are significantly longer than the shock wave rise time or reflected wave pressure oscillations. Thus, for this particular example, using an acoustic mesh and solution time increment that reasonably captures the shock wave reflected pulse and can represent the scattered acoustic waves at the structural response frequencies is adequate for obtaining a good solution. The response shown in Figure 13 is dominated by the fundamental beam bending mode of the cylinder, for which the dominant motion is transverse to the cylinder axis. At any point along the cylinder axis the motion is dominated by a translation of the cross section through the fluid, similar to the motion used in the infinite cylinder modeling study. The only damping mechanisms in the analyses were due to acoustic radiation and the /Explicit default values for element bulk viscosity. The acoustic model does not include any losses due to hydrodynamic drag (fluid viscosity) associated with the motions of the cylinder. The effect of hydrodynamic drag on the late time response of the cylinder is clearly shown in Figure 14, where the predicted axial strain response is compared to the experimental data. The experimental data was digitized from a published curve (Kwon & Fox, 1993), and was shifted by 0.2 milliseconds in order to align the experimental and analysis time axes. The solution designated as ALPHA = 0, represents the original analysis, whereas the analysis designated as ALPHA=750 utilized mass proportional damping (10% critical at 600 Hz) applied to the cylinder as an approximation for the effects of hydrodynamic drag. The application of ALPHA damping does not have an adverse effect on the solution critical time increment. ALPHA damping does not significantly affect the early time response (high frequency), but does significantly reduce the late time response (low frequency). This is consistent with what is observed with the experimental data. In any event, ignoring hydrodynamic drag in an UNDEX analysis will produce conservative (high) levels for the structural response, which is often a desirable trait when doing a design evaluation analysis. Figure 15 shows the levels of Accumulated Plastic Strain (PEEQ) on the outer surface of the shell at the end of the analysis with fluid Mesh #1 and ALPHA= 750. No change occurs in the peak plastic strain level of the cylinder wall after the first 0.34 milliseconds. Recalling that the entire shock pulse duration is 2.0 milliseconds, this truly is an early time response. No change occurs in the peak accumulated plastic strain of the endcaps after 2.84 milliseconds, which indicates that the ABAQUS Users Conference

7 endcap response may be influenced to a greater extent by the late time response. Unfortunately, there were no strain gages attached to the endcaps and the cylinder wall gages located in the regions of high plastic strain failed during the test. Table 1 compares the peak accumulated plastic strains obtained with the two acoustic meshes, with and without ALPHA damping. The results comparison between Mesh #1 and Mesh #2 is very good. The effect of ALPHA damping on the cylinder wall PEEQ is very small, but is significant for the endcap response. Table 1 also provides a comparison of the solution times for the analyses, and illustrates the solution efficiency of the acoustic elements as compares to structural elements. Conclusions ABAQUS/Explicit provides an efficient means to evaluate the transient response of structuralacoustic systems loaded by external acoustic sources. This was illustrated with the analysis of a submerged cylinder acted upon by a shock wave generated by an underwater explosion. The modeling studies presented in this paper indicate that sufficient accuracy for a submerged structure s response can be obtained when positioning the external absorbing boundary of the acoustic domain a distance from the structure of between 1/3 to 1/2 the longest characteristic structural wavelength. Modeling studies also indicated that the degree of refinement in the acoustic domain mesh can be tailored to the characteristics of the shock pulse and the nature of structural response, i.e., short vs. long response times as compared to the shock pulse transient. Table 1. Model statistics and results comparisons. Item Cylinder Only Acoustic Mesh #1 Acoustic Mesh #2 No. Acoustic Elements N/A No. Acoustic DOF N/A No. Shell Elements No. Shell DOF Solution Time Increments CPU Time (Seconds) CPU per Increment PEEQ Cylinder Wall (Alpha = 750) N/A PEEQ Cylinder Wall (Alpha = 0) PEEQ Endcap Center (Alpha = 750) PEEQ Endcap Center (Alpha = 0) N/A N/A N/A ABAQUS Users Conference 7

8 Figure 1. Baseline radiation impedance results for an infinite rigid cylinder. Figure 2. Error ratio for the radiation impedance evaluation models ABAQUS Users Conference

9 Figure 3. Acoustic tube with elements 1-½ the rise time propagation distance. Figure 4. Acoustic tube with elements ¼ the rise time propagation distance ABAQUS Users Conference 9

10 Figure 5. Additional results for 1-½ rise time element mesh. Figure 6. Pressure impulse curves from Figure 5 analyses ABAQUS Users Conference

11 Figure 7. Incident shock wave pressure transient. Figure 8. Test Cylinder Strain Gage Locations ABAQUS Users Conference 11

12 Figure 9. Cylinder and acoustic Mesh #1. Figure 10. Acoustic tube validation results for Mesh # ABAQUS Users Conference

13 Figure 11. Cylinder and acoustic Mesh #2. Figure 12. Acoustic tube validation results for Mesh # ABAQUS Users Conference 13

14 Figure 13. Axial strain response at gage location B1. Figure 14. Comparison plots of response at strain gage B ABAQUS Users Conference

15 Figure 15. Accumulated plastic strain in the test cylinder. References 1. Kwon, Y.W. and P.K. Fox, Underwater Shock Response of a Cylinder Subjected to a Side- On Explosion, Computers and Structures, Vol. 48, No. 4, Prasad, B.R. Nimmagadda and J. Cipolla, A Pressure Based Cavitation Model for Underwater Shock Problems, Shock and Vibration Symposium, Paper U30, November, ABAQUS Users Conference 15