Acoustic and shock loads

Specifying surface-based impedance conditions

You can define the impedance condition on a surface. The impedance is applied to element edges in two dimensions and to element faces in three dimensions. The element-based surface (see Element-based surface definition) contains the element and face information.

SIMPEDANCE, PROPERTY=impedance property table name
surface name

Interaction module: Create Interaction: Acoustic impedance: select surface: Definition: Tabular, Acoustic impedance property: impedance property table name

Specifying element-based impedance conditions

Alternatively, you can define the impedance condition on element faces. The impedance is applied to element edges in two dimensions and to element faces in three dimensions. The edge or face of the element upon which the impedance is placed is identified by an impedance load type and depends on the element type (see About the element library).

IMPEDANCE,  PROPERTY=impedance property table name
element number or set name, impedance load type label

Combining different radiation conditions in the same problem

Since the radiation boundary conditions for the different shapes are spatially local and do not involve discretization in the infinite exterior domain, an exterior boundary can consist of the combination of several shapes. The appropriate boundary condition can then be applied to each part of the boundary. For example, a circular cylinder can be terminated with hemispheres (see Fully and sequentially coupled acoustic-structural analysis of a muffler), or an elliptical cylinder can be terminated with prolate spheroidal halves. This modeling technique is most effective if the boundaries between surfaces are continuous in slope as well as displacement, although this is not essential.

Scattered wave formulation

When the mechanics of a fluid can be described as linear, the observed total acoustic pressure can be decomposed into two components: the known incident wave and the “scattered” wave that is produced by the interaction of the incident wave with structures and/or fluid boundaries. When this superposition is applicable, it is common practice to seek the “scattered” wave field solution directly. When using the scattered wave formulation, the pressures at the acoustic nodes are defined to be only the scattered part of the total pressure. Both acoustic and solid surfaces at the acoustic-structural interface should be loaded in this case.

When using incident wave loads in steady-state dynamic procedures, the scattered wave formulation must be used.

ACOUSTIC WAVE FORMULATION, TYPE=SCATTERED WAVE

Any module: ModelEdit Attributesmodel_name. Toggle on Specify acoustic wave formulation: select Scattered wave

Total wave formulation

The total wave formulation (see Coupled acoustic-structural medium analysis) is particularly applicable when the acoustic medium is capable of cavitation, rendering the fluid mechanical behavior nonlinear. It should also be used if the problem contains either a curved or a finite extent boundary where the pressure history is prescribed. Only the outer acoustic surfaces should be loaded with the incident wave in this case, and the incident wave source must be located exterior to the fluid model. Any impedance or nonreflecting condition that may exist on this outer acoustic boundary applies only on the part of the acoustic solution that does not include the prescribed incident wave field (that is, only the scattered field is subject to the nonreflecting condition). Thus, the applied incident wave loading will travel into the problem domain without being affected by the nonreflecting conditions on the outer acoustic surface.

In the total wave formulation the acoustic pressure degree of freedom stands for the total dynamic acoustic pressure, including contributions from incident and scattered waves and, in Abaqus/Explicit, the dynamic effects of fluid cavitation. The pressure degree of freedom does not include the acoustic static pressure, which can be specified as an initial condition (see Defining initial acoustic static pressure). This acoustic static pressure is used only in determining the cavitation status of the acoustic element nodes and does not apply any static loads to the acoustic or structural mesh at their common wetted interface. It does not apply to analyses using Abaqus/Standard.

ACOUSTIC WAVE FORMULATION, TYPE=TOTAL WAVE

Any module: ModelEdit Attributesmodel_name. Toggle on Specify acoustic wave formulation: select Total wave

Prescribing geometric properties and the speed of the incident wave

You must refer to a property definition for each prescribed incident wave. Incident wave loads in Abaqus may be either planar, spherical, or diffuse. You select a planar incident wave (default), spherical incident wave, or a diffuse field in the incident wave property definition.

Planar incident waves maintain constant amplitude as they travel in space; consequently, the speed and direction of travel are the critical parameters to define. The speed is defined in the incident wave interaction property definition, and the direction is determined by the locations of the source and standoff points you define as part of the incident wave interaction.

For spherical incident wave definitions, the wave reduces in amplitude as a function of space. By default, the amplitude of a spherical wave is inversely proportional to the distance from the source; this behavior is called “acoustic” propagation. For the preferred interface you can modify the default propagation behavior to define spatial decay of the incident wave field. The dimensionless constants $A$, $B$, and $C$ are used to define the spatial decay as a function of the distance $R_j$ between the source point and the loaded point and the distance $R_0$ between the source point and the standoff point:

Refer to Loading due to an incident dilatational wave field for details of the generalized spatial decay formulation.

In Abaqus incident wave interactions can be used to simulate diffuse incident fields. Diffuse fields are characteristic of reverberant spaces or other situations in which waves from many directions strike a surface. For example, reverberant chambers are constructed intentionally in acoustic test facilities for sound transmission loss measurements. The diffuse field model used in Abaqus, as shown in Figure 2, allows you to specify a seed number $N$; $N^2$ deterministic incident plane waves travel along vectors distributed over a hemisphere so that the incident power per solid angle approximates a diffuse incident field.

Figure 2. Diffuse loading model.

The fluid and the solid surfaces where the incident loading acts are specified in the incident wave loading definition. The incoming wave load is further described by the locations of its source point and of a reference (“standoff”) point where the wave amplitude is specified. For information on how to specify these surfaces and the standoff point, see Identifying the fluid and the solid surfaces for incident wave loading and The standoff point below. For a planar wave the specified locations of the source and the standoff points are used to define the direction of wave propagation.

The speed of the incident wave is prescribed by giving the properties for the incident wave-bearing acoustic medium. These specified properties should be consistent with the properties specified for the fluid discretized using acoustic elements.

For the preferred interface you must define nodes corresponding to the source and standoff points for the incident wave; the node numbers or set names must be specified for each incident wave definition. The node set names, if used, must contain only a single node. Neither the source node nor the standoff node should be connected to any elements in the model.

INCIDENT WAVE INTERACTION PROPERTY, 
NAME=wave property name, TYPE=PLANE or SPHERE
speed of sound, fluid mass density, A, B, C
INCIDENT WAVE INTERACTION, PROPERTY=wave property name
fluid surface name, source node, standoff node, reference magnitude

INCIDENT WAVE INTERACTION PROPERTY, 
NAME=wave property name, TYPE=DIFFUSE
speed of sound, fluid mass density
INCIDENT WAVE INTERACTION, PROPERTY=wave property name
fluid surface name, source node, standoff node, reference magnitude, N

Interaction module: Create Interaction Property: Name: wave property name and Incident wave, Speed of sound in fluid: speed of sound, Fluid density: fluid mass density

Definition: Planar
Definition: Spherical, Propagation model: Acoustic
Definition: Spherical, Propagation model: Generalized decay, 
enter values for A, B, and C
Definition: Diffuse, Seed number: N

Create Interaction: Incident wave: select the source point, select the standoff point, select the region: Wave property: wave property name, Reference magnitude: reference magnitude

Identifying the fluid and the solid surfaces for incident wave loading

In the scattered wave formulation the incident wave loading must be specified on all fluid and solid surfaces that reflect the incident wave with two exceptions:

• those fluid surfaces that have the pressure values directly prescribed using boundary conditions; and

• those fluid surfaces that have symmetry conditions (the symmetry must hold for both the loading and the geometry).

In problems with a fluid-solid interface both surfaces must be specified in the incident wave loading definition for the scattered formulation. See Example: submarine close to the free surface, shown in Figure 4.

When the total pressure-based formulation is specified, the incident wave loading must be specified only on the fluid surfaces that border the infinite region that is excluded from the model. Typically, these surfaces have a nonreflecting radiation condition specified on them, and the implementation ensures that the radiation condition is enforced only on the scattered response of the modeled domain and not on the incident wave itself. See Example: submarine close to the free surface and Example: surface ship, shown in Figure 4 and Figure 5, respectively.

In certain problems, such as blast loads in air, you may decide that the blast wave loads on a structure need to be modeled, but the surrounding fluid medium itself does not. In these problems the incident wave loading is specified only on the solid surfaces since the fluid medium is not modeled. The distinction between the scattered wave formulation and the total wave formulation for handling the incident wave loading is not relevant in these problems since the wave propagation in the fluid medium is of no interest.

The standoff point

In transient analyses the standoff point is a reference point used to specify the pulse loading time history: it is the point at which the user-defined pulse history is assumed to apply with no time delay, phase shift, or spreading loss. In steady-state analyses using discrete planar or spherical sources, the standoff point is the point at which the incident field has zero phase.

In transient analyses the standoff point should be defined so that it is closer to the source than any point on the surfaces in the model that would reflect the incident wave. Doing so ensures that all the points on these surfaces will be loaded with the specified time history of the source and that the analysis begins before the wave overtakes any portion of these surfaces. To save analysis time, the standoff point is typically on or near the solid surface where the incoming incident wave would be first deflected (see Example: submarine close to the free surface, shown in Figure 4). However, the standoff point is a fixed point in the analysis: if the loaded surfaces move before the incident wave loading begins, due to previous analysis steps or geometric adjustments, the surfaces may envelop the specified standoff point. Care should be taken to define a standoff point such that it remains closer to the incident wave source point than any point on the loaded surfaces at the onset of the loading.

When the total wave formulation is used and the incident wave loading is specified in the first step of the analysis in terms of pressure history, Abaqus automatically initializes the pressure and the pressure rate at the acoustic nodes to values based on the incident wave loading. This allows the acoustic analysis to start with the incident waves partially propagated into the problem domain at time zero and assumes that this propagation had taken place with negligible effect of any volumetric dissipative sources such as the fluid drag. When the incident wave loading is specified in terms of the pressure values, the recommendations given above for selecting a standoff point are valid with the total wave formulation as well. However, when the incident wave loading is specified in terms of acceleration values, the automatic initialization is not done and the standoff point should be located near the exterior fluid boundary of the model such that the standoff point is closer to the source than any point on the exterior boundary. See Example: submarine close to the free surface and Example: surface ship, shown in Figure 4 and Figure 5, respectively.

In steady-state analyses the role of the standoff point is somewhat different. When the incident wave interaction property is of planar or spherical type, you define the real and imaginary parts of the magnitude at the standoff point. Separately, the specified real and imaginary incident waves are taken to have zero phase at the standoff point (combined, these two waves could be equivalent to a single wave with nonzero phase at the standoff). Every location on the loaded surface has a phase shift in the applied pressure or acoustic traction, corresponding to the difference in propagation time between the loaded point and the standoff. This means that an incident wave defined, for example, with a pure real value at the standoff point generates both real and imaginary tractions at all the other points on the loaded surface.

When the incident wave is of diffuse type, the role of the standoff and source points is primarily to orient the loaded surface with respect to the incoming reverberant field. The model used for diffuse incident wave loading applies a set of deterministically defined plane waves, whose directions are defined as vectors connecting the standoff point and an array of points on a hemisphere. This hemisphere is centered at the standoff point, and its apex is the source point. The array of points is set according to the specified seed, $N$, and a deterministic algorithm that arranges $N^2$ points on the hemisphere. The algorithm concentrates the points so that the incident waves in the diffuse field model are concentrated at normal incidence, with fewer waves at oblique angles. The specified amplitude value and reference magnitude are divided equally among the $N^2$ incident waves. The orientation of the hemisphere containing the incident waves in the diffuse model is the same for all of the points on the loaded surface—it does not vary with the local normal vector on the surface.

Defining the amplitude of the source pulse

For transient analyses the time history to be specified by the user is that observed at the standoff point: histories at a point on the loaded surface are computed from the wave type and the location of that point relative to the standoff point. The time history of the acoustic source pulse can be defined either in terms of the fluid pressure values or the fluid particle acceleration values. Pressure time histories can be used for any type of element, such as acoustic, structural, or solid elements; acceleration time histories are applicable only for acoustic elements. In either case a reference magnitude is specified for any given incident-wave-loaded surface, and a reference to a time-history data table defined by an amplitude curve is specified. The reference magnitude varies with time according to the amplitude definition.

For steady-state dynamic analyses the amplitude definition specified as part of the incident wave interaction definition is interpreted as the frequency dependence of the wave at the standoff point.

Currently the source pulse description in terms of fluid particle acceleration history is limited to planar incident waves acting on fluid surfaces in transient analyses. Further, if an impedance condition is specified on the same fluid surface along with incident wave loading, the source pulse is restricted to the pressure history type even for planar incident waves. The source pulse in terms of pressure history can be used without these limitations; i.e., pressure-history-based incident wave loading can be used with fluid or solid surfaces, with or without impedance, and for both planar and spherical incident waves.

When the source pulse is specified using pressure values and is applied on a fluid surface, the pressure gradient is computed and applied as a pressure-conjugate load on these surfaces. Hence, it is desirable to define the pulse amplitude to begin with a zero value, particularly when the cavitation in the fluid is a concern. If the structural response is of primary concern and the scattered formulation is being used, any initial jump in the pressure amplitude can be addressed by applying additional concentrated loads on the structural nodes that are tied to the acoustic mesh, corresponding to the initial jump in the incident wave pressure amplitude. Clearly, the additional load on any given structural node should be active from the instance the incident wave first arrives at that structural node. However, the scattered wave solution in the fluid still needs careful interpretation taking the initial jump into account.

INCIDENT WAVE INTERACTION, PRESSURE AMPLITUDE=amplitude data table name
solid or fluid surface name, source node, standoff node, reference magnitude

INCIDENT WAVE INTERACTION, ACCELERATION AMPLITUDE=amplitude data table name
fluid surface name, source node, standoff node, reference magnitude

INCIDENT WAVE INTERACTION, REAL 

INCIDENT WAVE INTERACTION, IMAGINARY

Interaction module: Create Interaction: Incident wave: select the source point, select the standoff point, select the region: Reference magnitude: reference magnitude

Definition: Pressure or Acceleration, Pressure amplitude or 
Acceleration amplitude: amplitude data table name

Toggle on Real amplitude and/or Imaginary amplitude: amplitude data table name

Defining bubble loading for spherical incident wave loading

An underwater explosion forms a highly compressed gas bubble that interacts with the surrounding water, generating an outward-propagating shock wave. The gas bubble floats upward as it generates these waves changing the relative positions of the source and the loaded surfaces. The loading effects due to bubble formation can be defined for spherical incident wave loading by using a bubble definition in conjunction with the incident wave loading definition.

The bubble dynamics can be described using a model internal to Abaqus or by using tabulated data. Abaqus has a built-in mechanical model of the bubble interacting with the surrounding fluid, which is simulated numerically to generate a set of data prior to running the finite element analysis. You can specify the explosive material parameters, ending time, and other parameters that affect the computation of the bubble amplitude curve used, as shown in Table 2.

All of the parameters specified affect only the bubble amplitude; other physical parameters in the problem are independent. You can suppress the effects of wave loss in the bubble dynamics and introduce empirical flow drag, if desired. Detailed information about the bubble mechanical model is given in Loading due to an incident dilatational wave field.

In an underwater explosion event a bubble migrates upward toward, and possibly reaches, the free water surface. If the bubble migration reaches the free water surface during the specified analysis time, Abaqus applies loads of zero magnitude after this point.

Model data about the bubble simulation are written to the data (.dat) file. During an Abaqus/Standard analysis history data are written each increment to the output database (.odb) file. The history data include the radius of the bubble and the bubble depth below the free water surface. For reference, the pressure and acoustic load quantities at the standoff point are also written to the data file; these load terms include the direct plane-wave term and the spherical spreading (“afterflow”) effect (see Loading due to an incident dilatational wave field).

For the preferred interface the loading effects due to bubble formation can be defined for spherical incident wave loading using the UNDEX charge property definition. Because the bubble simulation uses spherical symmetry, the incident wave interaction property must define a spherical wave.

INCIDENT WAVE INTERACTION PROPERTY, 
NAME=wave property name, TYPE=SPHERE
UNDEX CHARGE PROPERTY
data defining the UNDEX charge
INCIDENT WAVE INTERACTION, PROPERTY=wave property name,
 UNDEX
fluid surface name, source node, standoff node, reference magnitude

Interaction module: Create Interaction Property: Name: wave property name and Incident wave: Definition: Spherical, Propagation model: UNDEX charge, enter data defining the UNDEX charge
Create Interaction: Incident wave: Definition: UNDEX, Wave property: wave property name, enter data defining the UNDEX charge

Load or Interaction module: Create Amplitude: Name: pressure 
and select Tabular
Interaction module: Create Interaction: Incident wave: select the standoff point: Definition: Pressure, Pressure amplitude: pressure

Load or Interaction module: Create Amplitude: Name: name 
and select Tabular
Load module: Create Boundary Condition: select step: Displacement/Rotation or Velocity/Angular velocity: select the source node as the region and toggle on the degree or degrees of freedom, Amplitude: name

Modeling incident wave loading on a moving structure

To model the effect of relative motion between a structure (such as a ship) and the wave source during the analysis using the preferred interface, the source node may be assigned a velocity. It is assumed that the entire fluid-solid model is moving at a velocity with respect to the source node during the loading and that the speed of the model's motion is low compared to the speed of propagation of the incident wave. That is, the effect of the speed of the source is neglected in the computation of the loads, but the change in position of the source is included. This is equivalent to assuming that the relative motion between the source and the model is at a low Mach number. Relative motion can be specified only for transient analyses.

In addition to prescribing boundary conditions at the source node, a small mass element must be defined at the source node.

BOUNDARY, TYPE=DISPLACEMENT or VELOCITY,  AMPLITUDE=name
source node, degrees of freedom

Load module: Create Boundary Condition: select step: Velocity/Angular velocity or Displacement/Rotation: select regions and toggle on the degree or degrees of freedom, Amplitude: name

Specifying the reflection effects

The waves emanating from the source may reflect off plane surfaces, such as seabeds or sea surfaces, before reaching the specified standoff point. Thus, the incident wave loading consists of the waves arriving from a direct path from the source, as well as those arriving from reflections off the planes. In Abaqus an arbitrary number of these planes can be defined, each with its own location, orientation, and reflection coefficient.

If no reflection coefficient is specified, the plane is assumed to be nonreflective; a zero reflected pressure is applied. If a reflection coefficient is specified, the magnitude of the reflected waves are modified by the reflection coefficient $Q$ according to the formula:

Only real values for $Q$ are used.

The reflection planes are allowed only for incident waves that are defined in terms of fluid pressure values. Only one reflection off each plane is considered. If the effect of many successive reflections is important, these surfaces should be part of the finite element model. Reflection planes should not be used at a boundary of the finite element model if the total wave formulation is used, since in that case the incident wave will be reflected automatically by that boundary.

INCIDENT WAVE REFLECTION

Boundary with prescribed pressure

The acoustic pressure degree of freedom at nodes of acoustic elements can be prescribed using a boundary condition. However, since you can use the nodal acoustic pressure in an Abaqus analysis to refer to the total pressure at that point or to only the scattered component, care must be exercised in some circumstances.

When the total wave formulation is used, a boundary condition alone is sufficient to specify a prescribed total dynamic pressure on a boundary.

In an analysis without incident wave loading, the nodal degree of freedom is generally equal to the total acoustic pressure at that point. Therefore, its value can be prescribed using a boundary condition in a manner consistent with other boundary conditions in Abaqus. For example, you may set the acoustic pressure at all of the nodes at a duct inlet to a prescribed amplitude to analyze the propagation of waves along the duct. The free surface of a body of water can be modeled by setting the acoustic pressure to zero at the surface.

When incident wave loading is used, the scattered wave formulation defines the nodal acoustic degree of freedom to be equal to the scattered pressure. Consequently, a boundary condition definition for this degree of freedom affects the scattered pressure only. The total acoustic pressure at a node is not directly accessible in this formulation. Specification of the total pressure in a scattered formulation analysis is nevertheless required in some instances (for example, when modeling a free surface of a body of water). In this case, one of the following methods should be used.

If the fluid surface with prescribed total pressure is planar, unbroken, and of infinite extent, an incident wave reflection plane and a boundary condition can be used together to model the fact that the total pressure is zero on the free surface. A “soft” incident wave reflection plane coincident with the free surface will make sure that the structure is subjected to the incident wave load reflected off the free surface. A boundary condition setting the acoustic pressure in the surface equal to zero will make sure that any scattered waves emitted by the structure are reflected properly. The scattered wave solution in the fluid must be interpreted taking into consideration the fact that the incident field now includes a reflection of the source as well. If the fluid surface with prescribed total pressure is planar but broken by an object, such as a floating ship, this modeling technique may still be applied. However, the reflected loads due to the incident wave are computed as if the reflection plane passes through the hull of the ship; this approximation neglects some diffraction effects and may or may not be applicable in all situations of interest.

Alternatively, the free surface condition of the fluid can be eliminated by modeling the top layer of the fluid using structural elements, such as membrane elements, instead of acoustic elements. The “structural fluid” surface and the “acoustic fluid” surface are then coupled using either a surface-based mesh tie constraint (Mesh tie constraints) or, in Abaqus/Standard, acoustic-structural interface elements; and the incident wave loading must be applied on both the “structural fluid” and the “acoustic fluid” surfaces. The material properties of the “structural fluid” elements should be similar to those of the adjacent acoustic fluid. In Abaqus/Explicit the thickness of the “structural fluid” elements must be such that the masses at nodes on either side of the coupling constraint are nearly equal. This modeling technique allows the geometry of the surface on which total pressure is to be prescribed to depart from an unbroken, infinite plane. As a secondary benefit of this technique, you can obtain the velocity profile on the free surface since the displacement degrees of freedom are now activated at the “structural fluid” nodes. If a nonzero pressure boundary condition is desired, it can be applied as a distributed loading on the other side of the “structural fluid” elements.

BOUNDARY
INCIDENT WAVE REFLECTION

TIE
INCIDENT WAVE INTERACTION

BOUNDARY

Load module: Create BC: choose Other for the Category and Acoustic pressure for the Types for Selected Step

Defining air blast loading for incident shock waves using the CONWEP model in Abaqus/Explicit

An explosion in air forms a highly compressed gas mass that interacts with the surrounding air, generating an outward-propagating shock wave. The loading effects due to an explosion in air can be defined, for spherical incident waves (air blast) or hemispherical incident waves (surface blast), by empirical data provided by the CONWEP model in conjunction with the incident wave loading definition.

Unlike an acoustic wave, a blast wave corresponds to a shock wave with discontinuities in pressure, density, etc. across the wave front. Figure 3 shows a typical pressure history of a blast wave.

Figure 3. Pressure history of a blast wave.

The CONWEP model uses a scaled distance based on the distance of the loading surface from the source of the explosion and the amount of explosive detonated. For a given scaled distance, the model provides the following empirical data: the maximum overpressure (above atmospheric), the arrival time, the positive phase duration, and the exponential decay coefficient for both the incident pressure and the reflected pressure. Using these parameters, the entire time history of both the incident pressure and reflected pressure as shown in Figure 3 can be constructed. Use of a standoff point is not required.

The total pressure, $P\left(t\right)$, on a surface due to the blast wave is a function of the incident pressure, $P_{incident}\left(t\right)$, the reflected pressure, $P_{reflect}\left(t\right)$, and the angle of incidence, $\theta$, which is defined as the angle between the normal of the loading surface and the vector that points from the surface to the explosion source. The total pressure is defined as

The air blast loading due to the total pressure can be scaled using a magnitude scale factor.

A detonation time can be specified if the explosion does not occur at the start of the analysis. The detonation time needs to be given in total time; see Conventions for a description of the time convention. The arrival time at a location is defined as the elapsed time for the wave to arrive at that location after detonation.

The CONWEP empirical data are given in a specific set of units, which must be converted to the units used in the analysis. You will need to specify multiplying factors for conversion of these units to SI units. For the specification of the mass of the explosive in TNT equivalence, you can choose any convenient mass unit, which can be different from the mass unit used in the analysis. For computation of the pressure loading, you will need to specify multiplying factors for conversion of length, time, and pressure units used in the analysis to SI units. Some typical conversion multiplier values are given in Table 3.

For any given amount of explosive, the CONWEP empirical data are valid only within a range of distances from the source. The minimum distance at which the data are valid corresponds to the charge radius. Thus, the analysis terminates if the distance of any part of the loading surface from the source is less than the charge radius. For distances that are larger than the maximum valid range, linear extrapolation is used up to an extended maximum range where the reflected pressure decreases to zero. No loading is applied beyond the extended maximum range.

The CONWEP empirical data do not account for shadowing by intervening objects or for any effects due to confinement. In the definition of incident wave interaction using the CONWEP model, you cannot use incident wave reflection.

The CONWEP pressure load can be requested as element face variable output to the output database file (see Abaqus/Explicit output variable identifiers).

INCIDENT WAVE INTERACTION PROPERTY,
NAME=wave property name, TYPE=AIR BLAST or SURFACE BLAST
CONWEP CHARGE PROPERTY
data defining the CONWEP charge
INCIDENT WAVE INTERACTION, PROPERTY=wave property name,
CONWEP
loading surface name, source node,  detonation time, magnitude scale factor

Interaction module: Create Interaction Property: Name: wave property name and Incident wave: Definition: Air blast or Surface blast: enter data defining the CONWEP charge

Interaction module: Create Interaction: Name: incident wave name and Incident wave: select the source point: CONWEP (Air/Surface blast): select the region: CONWEP Data: enter data defining the time of detonation and magnitude scale factor

Prescribing the geometric properties and the speed of the incident wave (alternative interface)

Conceptually, the alternative interface is the same as the preferred interface; however, the usages are different. For conceptual information, see Prescribing geometric properties and the speed of the incident wave.

INCIDENT WAVE PROPERTY, NAME=wave property name,
 TYPE=PLANE or SPHERE
data lines to specify the location of the acoustic source and the standoff point
INCIDENT WAVE FLUID PROPERTY
bulk modulus, mass density
INCIDENT WAVE, PROPERTY=wave property name

Defining the time history of the source pulse (alternative interface)

Conceptually, the alternative interface is the same as the preferred interface; however, the usages are different. For conceptual information, see Defining the amplitude of the source pulse.

INCIDENT WAVE, PRESSURE AMPLITUDE=amplitude data table name
solid or fluid surface name, reference magnitude

INCIDENT WAVE, ACCELERATION AMPLITUDE=amplitude data table 
name
fluid surface name, reference magnitude

Defining bubble loading for spherical incident wave loading (alternative interface)

Conceptually, the alternative interface is the same as the preferred interface; however, the usages are different. For conceptual information, see Defining bubble loading for spherical incident wave loading.

To define the bubble dynamics using a model internal to Abaqus, you can specify a bubble amplitude. Use of the bubble loading amplitude is generally similar to the use of any other amplitude in Abaqus.

AMPLITUDE, DEFINITION=BUBBLE, NAME=name
INCIDENT WAVE PROPERTY, TYPE=SPHERE, 
NAME=wave property name
INCIDENT WAVE, PRESSURE AMPLITUDE=name
solid or fluid surface name, reference magnitude

Specifying the reflection effects (alternative interface)

Conceptually, the alternative interface is the same as the preferred interface; however, the usages are different. For conceptual information, see Specifying the reflection effects.

INCIDENT WAVE REFLECTION

Modeling incident wave loading on a moving structure (alternative interface)

To model the effect of rigid motion of a structure such as a ship during the incident wave loading history, the standoff point can have a specified velocity. It is assumed that the entire fluid-solid model is moving at this velocity with respect to the source point during the loading and that the speed of the model's motion is low compared to the speed of propagation of the incident wave.

INCIDENT WAVE PROPERTY, NAME=wave property name
data line to specify the velocity of the standoff point

Scattered wave solution

Here the scattered wave response in the acoustic medium is of interest along with that of the structure to the incident wave loading. Cavitation in the fluid is not considered in a scattered wave formulation. Similarly, the initial hydrostatic pressure in the fluid is not modeled.

The zero dynamic acoustic pressure boundary condition on the free surface requires both a “soft” reflection plane coinciding with the free surface $A_0$ and a zero scattered pressure boundary condition at the nodes on this free surface. The incident wave loading is applied on the fluid surface, $A_{fw}$, and on the wet solid surface, $A_{sw}$. The incident wave loading can be only of pressure amplitude type since the loading includes a solid surface.

A good location for the standoff node is marked as A in Figure 4. This node is in the fluid, close to the structure, and closer to incident wave source S than any portion of the seabed or the free surface. The standoff node's offset from the loaded surfaces is exaggerated for emphasis in the figure.

The radiation condition is specified on the acoustic surface $A_{inf}$ such that the scattered wave impinging on this boundary with the infinite medium does not reflect back into the computational domain. The seabed is modeled with an incident wave reflection plane on surface $A_{sb}$. The reflection loss at this seabed surface is modeled using an impedance property.

If the response of the structure in the nonlinear regime is of interest, the initial stress state in the structure should be established using Abaqus/Standard in a static analysis. The stress state in the structure is then imported into Abaqus/Explicit, and the loading on the solid surfaces causing the initial stress state is respecified in the acoustic analysis.

The following template schematically shows some of the Abaqus input file options that are used to solve this problem using the scattered wave formulation:

HEADING
…
SURFACE, NAME=$A_{fw}$ 
Data lines to define the acoustic surface that is wetting the solid
SURFACE, NAME=$A_{sw}$ 
Data lines to define the solid surface that is wetted by the fluid
SURFACE, NAME=$A_{inf}$ 
Data lines to define the acoustic surface separating the modeled region from the infinite medium
INCIDENT WAVE INTERACTION PROPERTY, NAME=IWPROP
AMPLITUDE, DEFINITION=TABULAR, NAME=PRESSUREVTIME
TIE, NAME=COUPLING
$A_{fw}$, $A_{sw}$ 
STEP
** For an Abaqus/Standard analysis:
DYNAMIC
** For an Abaqus/Explicit analysis:
DYNAMIC, EXPLICIT
** Load the acoustic surface
INCIDENT WAVE INTERACTION, PRESSURE AMPLITUDE=PRESSUREVTIME,
PROPERTY=IWPROP
$A_{fw}$, source node, standoff node, reference magnitude
INCIDENT WAVE REFLECTION
Data lines for the reflection plane over the seabed $A_{sb}$, seabed_Q 
INCIDENT WAVE REFLECTION
Data lines for a "soft" reflection plane over the free surface $A_0$.
** Load the solid surface
INCIDENT WAVE INTERACTION, PRESSURE AMPLITUDE=PRESSUREVTIME,
PROPERTY=IWPROP
$A_{sw}$, source node, standoff node, reference magnitude
INCIDENT WAVE REFLECTION
Data lines for the reflection plane over the seabed $A_{sb}$,  seabed_Q 
INCIDENT WAVE REFLECTION
Data lines for a "soft" reflection plane over the free surface $A_0$.
BOUNDARY
** zero pressure boundary condition on the free surface
Set of nodes on the free surface $A_0$, 8, 8, 0.0
SIMPEDANCE
$A_{inf}$, 
END STEP

Total wave solution

Here the total wave response in the acoustic medium is of interest along with that of the structure to the incident wave loading. Cavitation in the fluid may be included. Similarly, a linearly varying initial hydrostatic pressure in the fluid can be specified.

The zero dynamic acoustic pressure boundary condition on the free surfaces requires only a zero pressure boundary condition at the nodes on this free surface. A reflection plane should not be included along the free surface. The incident wave loading is applied only on the fluid surface, $A_{inf}$, that separates the modeled region from the surrounding infinite acoustic medium. No incident wave should be applied directly on the structure surfaces. If the incident wave is considered planar, an acceleration-type amplitude can be used with the incident wave loading. Otherwise, a pressure-type amplitude must be used with the incident wave loading.

An ideal location for the standoff node depends on the type of amplitude used for the time history of the incident wave loading. The location A shown in Figure 4 can be used if the incident wave loading time history is of pressure amplitude type. Otherwise, the location B that is just on the boundary $A_{inf}$ and closer to the source S than any part of either the seabed or the free surface can be used.

The nonreflecting impedance condition is specified on the acoustic surface, $A_{inf}$, such that the scattered part of the total wave impinging on this boundary with the infinite medium does not reflect back into the computational domain. The seabed is modeled with an incident wave reflection plane on the surface $A_{sb}$.

If the response of the structure in the nonlinear regime is of interest, the initial stress state in the structure should be established using Abaqus/Standard in a static analysis. The stress state in the structure is then imported into Abaqus/Explicit, and the loading on the solid surfaces causing the initial stress state is respecified in the acoustic analysis.

The following template schematically shows some of the input file options that are used to solve this problem using the total wave formulation:

HEADING
…
ACOUSTIC WAVE FORMULATION, TYPE=TOTAL WAVE
MATERIAL, NAME=CAVITATING_FLUID
ACOUSTIC MEDIUM, BULK MODULUS
Data lines to define the fluid bulk modulus
ACOUSTIC MEDIUM, CAVITATION LIMIT
Data lines to define the fluid cavitation limit
…
SURFACE, NAME=$A_{fw}$ 
Data lines to define the acoustic surface that is wetting the solid
SURFACE, NAME=$A_{sw}$ 
Data lines to define the solid surface that is wetted by the fluid
SURFACE, NAME=$A_{inf}$ 
Data lines to define the acoustic surface separating the modeled region from the infinite medium
INCIDENT WAVE INTERACTION PROPERTY, NAME=IWPROP
AMPLITUDE, DEFINITION=TABULAR, NAME=PRESSUREVTIME
Data lines to define the pressure-time history at the standoff point
TIE, NAME=COUPLING
$A_{fw}$, $A_{sw}$ 
INITIAL CONDITIONS, TYPE=ACOUSTIC STATIC PRESSURE
Data lines to define the initial linear hydrostatic pressure in the fluid
STEP
DYNAMIC, EXPLICIT
** Load the acoustic surface
INCIDENT WAVE INTERACTION, PRESSURE AMPLITUDE=PRESSUREVTIME, 
PROPERTY=IWPROP
$A_{inf}$, source node, standoff node, reference magnitude
INCIDENT WAVE REFLECTION
Data lines for the reflection plane over the seabed $A_{sb}$, seabed_Q 
BOUNDARY
** zero pressure boundary condition on the free surface
Set of nodes on the free surface $A_0$, 8, 8, 0.0
SIMPEDANCE
$A_{inf}$, 
END STEP