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309 lines
14 KiB
Markdown
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# 34.2.2 INITIAL CONDITIONS IN Abaqus/CFD
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Products: Abaqus/CFD Abaqus/CAE
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# References
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• “Prescribed conditions: overview,” Section 34.1.1
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• \*INITIAL CONDITIONS
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• “Using the predefined field editors,” Section 16.11 of the Abaqus/CAE User’s Guide, in the HTML version of this guide
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# Overview
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In Abaqus/CFD initial conditions for fluid flow simulation are specified using element sets.
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# Defining initial velocities
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You can define the initial fluid flow velocity in elements; however, if such conditions are omitted, a default value of zero is assumed. Initial velocities must be defined in global directions, regardless of the use of local transformations (see “Transformed coordinate systems,” Section 2.1.5).
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For incompressible flow Abaqus/CFD automatically uses the user-defined boundary conditions and tests the specified initial velocity to be sure that the initial velocity field is divergence-free and that the velocity boundary conditions are compatible with the initial velocity field. If they are not, the initial velocity is projected onto a divergence-free subspace, yielding initial conditions that define a well-posed incompressible Navier-Stokes problem. Therefore, in some circumstances, the user-specified initial velocity may be overridden with a velocity that is divergence-free and matches the velocity boundary conditions.
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Input File Usage: \*INITIAL CONDITIONS, TYPE=VELOCITY, ELEMENT AVERAGE
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Abaqus/CAE Usage: Load module: Create Predefined Field: Step: Initial: Category: Fluid: Fluid velocity
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# Defining initial density
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You can define the initial fluid density in elements. However, if the initial condition is omitted, the material density definition is assumed as default (see “Density,” Section 21.2.1). Similarly, if the initial density is specified on an element set that does not include all fluid elements, the material density is assumed as the default for those elements not contained in the element set.
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Input File Usage: \*INITIAL CONDITIONS, TYPE=DENSITY, ELEMENT AVERAGE
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Abaqus/CAE Usage: Load module: Create Predefined Field: Step: Initial: Category: Fluid: Fluid density
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<!-- source-page: 82 -->
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# Initial pressure for incompressible fluid flow
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For incompressible flows it is not necessary to prescribe the initial pressure condition since the initial pressure field is computed automatically from the initial velocity field and boundary conditions. This is done to ensure proper starting conditions for incompressible flows.
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# Defining initial temperature
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If the energy equation is solved, the initial fluid temperature in elements must be defined.
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Input File Usage: \*INITIAL CONDITIONS, TYPE=TEMPERATURE, ELEMENT AVERAGE
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Abaqus/CAE Usage: Load module: Create Predefined Field: Step: Initial: Category: Fluid: Fluid thermal energy
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# Defining initial Spalart-Allmaras turbulent eddy viscosity directly
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You can define the initial turbulent eddy viscosity, , directly for use with the Spalart-Allmaras turbulence model. It is recommended that you use a value that is three to five times the kinematic viscosity. The kinematic viscosity is the ratio of the molecular viscosity and density $( \nu = \mu / \rho )$ . For more information, see “Viscosity,” Section 26.1.4
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Input File Usage: \*INITIAL CONDITIONS, TYPE=TURBNU, ELEMENT AVERAGE
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Abaqus/CAE Usage: Load module: Create Predefined Field: Step: Initial: Category: Fluid: Fluid turbulence; Eddy viscosity:
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# Defining initial k directly
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You can define the initial turbulent kinetic energy, k, directly for use with the k– and k– turbulence models.
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Input File Usage: \*INITIAL CONDITIONS, TYPE=TURBKE, ELEMENT AVERAGE
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Abaqus/CAE Usage: Use the following option to specify the initial turbulent kinetic energy for the k– RNG turbulence model:
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Load module: Create Predefined Field: Step: Initial: Category:
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Fluid: Fluid turbulence; Turbulent kinetic energy: k
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The k– realizable and k– turbulence models are not supported in Abaqus/CAE.
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# Defining initial epsilon directly
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You can define the initial turbulent kinetic energy dissipation rate, , directly for use with the k– turbulence models.
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Input File Usage: \*INITIAL CONDITIONS, TYPE=TURBEPS, ELEMENT AVERAGE
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<!-- source-page: 83 -->
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Abaqus/CAE Usage: Use the following option to specify the initial turbulent kinetic energy dissipation rate for the k– RNG turbulence model:
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Load module: Create Predefined Field: Step: Initial: Category:
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Fluid: Fluid turbulence; Dissipation rate:
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The k– realizable turbulence model is not supported in Abaqus/CAE.
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Defining initial omega directly
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You can define the initial specific energy dissipation rate, , directly for use with the k– turbulence model.
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Input File Usage: \*INITIAL CONDITIONS, TYPE=TURBOMEGA, ELEMENT AVERAGE
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Abaqus/CAE Usage: The k– turbulence model is not supported in Abaqus/CAE.
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Defining initial turbulence intensity
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You can define the initial turbulence intensity, , for use with the k– , k– , and Spalart-Allmaras turbulence models.
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Input File Usage: \*INITIAL CONDITIONS, TYPE=TURBINTENSITY, ELEMENT AVERAGE
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Abaqus/CAE Usage: Defining the initial turbulence intensity is not supported in Abaqus/CAE.
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Defining initial turbulent length scale
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You can define the initial turbulent length scale, , for use with the k– , k– , and Spalart-Allmaras turbulence models.
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Input File Usage: \*INITIAL CONDITIONS, TYPE=TURBLENGTHSCALE, ELEMENT AVERAGE
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Abaqus/CAE Usage: Defining the initial turbulent length scale is not supported in Abaqus/CAE.
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Defining the initial characteristic velocity scale
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You can define the initial characteristic velocity scale, , for use with the k– , k– , and Spalart-Allmaras turbulence models.
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Input File Usage: \*INITIAL CONDITIONS, TYPE=TURBVELOCITYSCALE, ELEMENT AVERAGE
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Abaqus/CAE Usage: Defining the initial characteristic velocity scale is not supported in Abaqus/CAE.
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# Defining the initial eddy to molecular viscosity ratio
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You can define the initial eddy to molecular viscosity ratio, , for use with the k– , k– , and Spalart-Allmaras turbulence models. The ratio of the eddy to molecular viscosity is defined by
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<!-- source-page: 84 -->
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$$
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T V R = \frac {\mu_ {T}}{\mu},
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$$
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where $\mu _ { T }$ is the eddy viscosity and $\mu$ is the molecular viscosity. For more information about viscosity see “Viscosity,” Section 26.1.4.
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Input File Usage: \*INITIAL CONDITIONS, TYPE=TURBVISCOSITYRATIO, ELEMENT AVERAGE
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Abaqus/CAE Usage: Defining the initial eddy to molecular viscosity ratio is not supported in Abaqus/CAE.
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# Defining initial Spalart-Allmaras turbulent eddy viscosity from turbulence properties
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You can specify the initial Spalart-Allmaras turbulent eddy viscosity, $\tilde { \nu } ,$ using the turbulence properties described above.
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# Using the turbulence intensity, the turbulent length scale, and a characteristic velocity scale
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The value of is obtained from the specified turbulence intensity, $T I ;$ the turbulent length scale, $l ;$ and the characteristic velocity of the problem, $U _ { \infty }$ ; as
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$$
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\tilde {\nu} = \sqrt {\frac {3}{2}} C _ {\mu} ^ {1 / 4} (T I \cdot U _ {\infty}) l.
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$$
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$C _ { \mu }$ is a model coefficient that is used in the $\pmb { k } { - } \varepsilon$ models to compute the eddy viscosity $( \mu _ { T } ~ =$ $\rho C _ { \mu } \frac { k ^ { 2 } } { \varepsilon } \big ) ;$ ; it does not exist in the Spalart-Allmaras model. However, the standard $k { \mathrm { - } } \varepsilon \ C _ { \mu } = 0 . 0 9$ is included for consistency between turbulence models when the turbulence intensity, velocity scale, and length scale are used to specify initial turbulent conditions. A characteristic velocity scale is required to avoid cases when the initial velocity field is zero.
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# Using the eddy to molecular viscosity ratio
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Abaqus/CFD solves the following equation to obtain the value of $\tilde { \nu }$ using the ratio of the eddy to molecular viscosity:
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$$
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T V R = \chi f _ {v 1} (\chi),
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$$
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$$
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f _ {v 1} (\chi) = \frac {\chi^ {3}}{\chi^ {3} + C _ {v 1} ^ {3}},
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$$
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where $\begin{array} { r } { \chi = \frac { \tilde { \nu } } { \nu } } \end{array}$
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<!-- source-page: 85 -->
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# Defining initial k using the turbulence intensity and a characteristic velocity scale
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The initial turbulent kinetic energy, k, for the k– and k– turbulence models is obtained using the turbulence intensity, , and a characteristic velocity scale, $U _ { \infty }$ . Once these quantities are defined as described above, the turbulent kinetic energy is computed internally as
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$$
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k = \frac {3}{2} (T I \cdot U _ {\infty}) ^ {2}.
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$$
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# Defining initial epsilon from turbulence properties
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You can specify the initial energy dissipation rate for the k– turbulence models using the turbulence properties described above.
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# Using initial k and the eddy to molecular viscosity ratio
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The initial can be specified using the initial k and the eddy to molecular viscosity ratio, . Once these quantities are defined, the energy dissipation rate is computed internally as
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$$
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\varepsilon = C _ {\mu} \frac {k ^ {2}}{(T V R \cdot \nu)},
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$$
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where $C _ { \mu }$ is a turbulent model coefficient and is the fluid kinematic viscosity $( \nu = \mu / \rho )$ .
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# Using initial k and the turbulent length scale
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The initial can be specified using the initial k and the turbulent length scale, . Once these quantities are defined, the energy dissipation rate is computed internally as
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$$
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\varepsilon = C _ {\mu} ^ {\frac {3}{4}} \frac {k ^ {3 / 2}}{l},
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$$
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where $C _ { \mu }$ is a turbulent model coefficient.
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# Defining initial omega from turbulence properties
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You can specify the initial specific energy dissipation rate for the k– turbulence model using the turbulence properties described above.
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# Using initial k and the eddy to molecular viscosity ratio
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The initial can be specified using the initial k and the eddy to molecular viscosity ratio, . Once these quantities are defined, the specific energy dissipation rate is computed internally as
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$$
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\omega = \frac {k}{(T V R \cdot \nu)}.
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$$
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<!-- source-page: 86 -->
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# Using initial k and the turbulent length scale
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The initial can be specified using the initial k and the turbulent length scale, . Once these quantities are defined, the specific energy dissipation rate is computed internally as
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$$
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\omega = \frac {k ^ {1 / 2}}{\beta^ {* ^ {1 / 4}} l},
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$$
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where $\beta ^ { * }$ is a turbulent model coefficient.
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<!-- source-page: 87 -->
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# 34.3 Boundary conditions
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• “Boundary conditions in Abaqus/Standard and Abaqus/Explicit,” Section 34.3.1
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• “Boundary conditions in Abaqus/CFD,” Section 34.3.2
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<!-- source-page: 88 -->
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<!-- source-page: 89 -->
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# 34.3.1 BOUNDARY CONDITIONS IN Abaqus/Standard AND Abaqus/Explicit
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Products: Abaqus/Standard Abaqus/Explicit Abaqus/CAE
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# References
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• “Defining a model in Abaqus,” Section 1.3.1
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• “Prescribed conditions: overview,” Section 34.1.1
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• “VDISP,” Section 1.2.2 of the Abaqus User Subroutines Reference Guide
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• “DISP,” Section 1.1.4 of the Abaqus User Subroutines Reference Guide
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• \*BOUNDARY
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• “Using the boundary condition editors,” Section 16.10 of the Abaqus/CAE User’s Guide, in the HTML version of this guide
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# Overview
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Boundary conditions:
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• can be used to specify the values of all basic solution variables (displacements, rotations, warping amplitude, fluid pressures, pore pressures, temperatures, electrical potentials, normalized concentrations, acoustic pressures, or connector material flow) at nodes;
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• can be given as “model” input data (within the initial step in Abaqus/CAE) to define zero-valued boundary conditions;
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• can be given as “history” input data (within an analysis step) to add, modify, or remove zero-valued or nonzero boundary conditions; and
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• can be defined by the user through subroutines DISP for Abaqus/Standard and VDISP for Abaqus/Explicit.
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Relative motions in connector elements can be prescribed similar to boundary conditions. See “Connector actuation,” Section 31.1.3, for more detailed information.
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# Prescribing boundary conditions as model data
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Only zero-valued boundary conditions can be prescribed as model data (i.e., in the initial step in Abaqus/CAE). You can specify the data using either “direct” or “type” format. As described below, the “type” format is a way of conveniently specifying common types of boundary conditions in stress/displacement analyses. “Direct” format must be used in all other analysis types.
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For both “direct” and “type” format you specify the region of the model to which the boundary conditions apply and the degrees of freedom to be restrained. (See “Conventions,” Section 1.2.2, for the degree of freedom numbers used in Abaqus.)
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Boundary conditions prescribed as model data can be modified or removed during analysis steps.
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<!-- source-page: 90 -->
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Input File Usage: \*BOUNDARY
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Any number of data lines can be used to specify boundary conditions, and in stress/displacement analyses both “direct” and “type” format can be specified with a single use of the \*BOUNDARY option.
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Abaqus/CAE Usage: Load module: Create Boundary Condition: Step: Initial
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# Using the direct format
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You can choose to enter the degrees of freedom to be constrained directly.
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Input File Usage: Either a single degree of freedom or the first and last of a range of degrees of freedom can be specified.
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\*BOUNDARY node or node set, degree of freedom \*BOUNDARY node or node set, first degree of freedom, last degree of freedom
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For example,
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\*BOUNDARY
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EDGE, 1
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indicates that all nodes in node set EDGE are constrained in degree of freedom 1 ( ), while the data line
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EDGE, 1, 4
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indicates that all nodes in node set EDGE are constrained in degrees of freedom 1–4 ( , , , ).
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Abaqus/CAE Usage: Load module: Create Boundary Condition: Step: Initial
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Use one of the following options:
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Category: Mechanical; Displacement/Rotation, Velocity/Angular velocity, or Acceleration/Angular acceleration; select regions and toggle on the degree or degrees of freedom
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Category: Electrical/Magnetic; Electric potential; select regions
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Category: Other; Temperature, Pore pressure, Mass concentration, Acoustic pressure, or Connector material flow; select regions
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If you are specifying a temperature boundary condition for a shell region, you can enter multiple degrees of freedom, from 11 to 31, inclusive.
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# Using the “type” format in stress/displacement analyses
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The type of boundary condition can be specified instead of degrees of freedom. The following boundary condition “types” are available in both Abaqus/Standard and Abaqus/Explicit:
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XSYMM Symmetry about a plane (degrees of freedom ).
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