Products: Abaqus/CFD Abaqus/CAE
Boundary conditions:
are used to prescribe the values of all primitive variables involved in a fluid dynamics calculation (e.g., velocities, temperatures, turbulence variables, wall-normal distance, etc.);
can be given as “history” input data (within an analysis step) to add, modify, or remove zero-valued or nonzero boundary conditions; and
can be prescribed through the use of a co-simulation region for multiphysics problems.
Computational fluid dynamics problems typically require the prescription of multiple variables such as pressure, temperature, and velocity for boundary conditions. In practice, boundary conditions tend to appear together to collectively define a physical behavior; e.g., no-slip/no-penetration conditions at a wall. In contrast, Neumann conditions (e.g., prescribed heat flux) are specified as loads (see “Specifying surface-based distributed heat fluxes” in “Thermal loads,” Section 30.4.4). In the absence of a prescribed boundary condition or load, the default behavior for Abaqus/CFD is to enforce a homogeneous (zero) Neumann condition. For example, if the temperature is not specified at a wall, the default behavior is to automatically specify a perfectly insulated boundary; i.e., zero normal heat flux. Similarly, if the velocity is not prescribed, the normal derivative of the velocity is set to zero.
In Abaqus/CFD combinations of boundary conditions that represent an inflow, outflow, or wall behavior are grouped collectively for ease of use. These types of boundary conditions are prescribed using Abaqus/CAE (for more information, see “Using the boundary condition editors,” Section 16.10 of the Abaqus/CAE User's Manual).
An inflow boundary condition is used to describe the flow behavior at a surface where fluid enters the analysis domain. For incompressible flows, inflow conditions can be prescribed for velocity or pressure, temperature, and turbulence variables. If boundary conditions are not specified explicitly for a variable, a homogeneous Neumann condition is automatically assumed. This corresponds to permitting the variable (e.g., temperature) to vary at the inflow and the incoming fluid to correspond to that local variable. Similarly, if pressure is not specified, its normal derivative at the inflow surface is automatically set to zero. The velocity components can be prescribed independently.
| Input File Usage: | You can generate the input file using Abaqus/CAE. |
| Abaqus/CAE Usage: | Load module: Create Boundary Condition: Step: flow_step: Category: Fluid: Fluid inlet/outlet: select inlet regions; and specify momentum (pressure or velocity), thermal energy (temperature), and turbulence conditions at the inlet |
An outflow boundary corresponds to a surface where the fluid flow leaves the analysis domain. In Abaqus/CFD outflow conditions are most frequently associated with a specified pressure. However, all other flow variables can be prescribed at an outflow boundary as well. Similar to an inflow boundary, when a variable is not specified, its normal derivative is assumed to be zero. As such, convective outflows carry their quantities out of the domain at a fixed level, resulting in essentially nonreflecting boundaries.
| Input File Usage: | You can generate the input file using Abaqus/CAE. |
| Abaqus/CAE Usage: | Load module: Create Boundary Condition: Step: flow_step: Category: Fluid: Fluid inlet/outlet: select outlet regions; and specify momentum (pressure or velocity), thermal energy (temperature), and turbulence conditions at the outlet |
Wall boundary conditions are typically associated with the no-slip/no-penetration behavior at a solid surface. However, the behavior at a solid wall may also require the prescription of temperature and, optionally, turbulence variables depending on the flow conditions. In situations where a wall heat flux is required, a heat flux loading must be prescribed in addition to the wall boundary conditions.
Depending on the physical properties of the wall, the wall boundary conditions can be modified to achieve a variety of physical behaviors that include slip, no-slip, infiltration, symmetry, etc.
A no-slip (and no-penetration) wall is a surface where the fluid adheres to the wall without penetrating it. No-slip/no-penetration conditions are prescribed by setting all velocity components equal to the wall velocity (zero if the wall is not moving). If a turbulence model is specified, the wall-normal distance boundary condition must be set to zero at the wall. The boundary conditions for the different turbulence variables depend on the model selected. For the Spalart-Allmaras model, the turbulent eddy viscosity, 
, is set to zero at the wall. For the RNG k–
model, the wall boundary conditions are automatically implemented by the solver using the wall-function approach; no user settings for k or are required because they are prescribed automatically.
| Input File Usage: | You can generate the input file using Abaqus/CAE. |
| Abaqus/CAE Usage: | Load module: Create Boundary Condition: Step: flow_step: Category: Fluid: Fluid wall condition: select regions: Condition: No slip |
A slip wall is a surface where the fluid does not adhere to the wall but cannot penetrate it. This wall condition is modeled by specifying the wall-normal fluid velocity equal to the wall velocity (zero if the wall is not moving). This situation also represents a symmetry condition for fluid flow since the in-plane velocities can vary, but the out-of-plane velocity is zero. In cases where a moving boundary is being considered, an associated set of mesh displacement boundary conditions must be prescribed in conjunction with the surface fluid velocity to achieve the proper behavior.
| Input File Usage: | You can generate the input file using Abaqus/CAE. |
| Abaqus/CAE Usage: | Load module: Create Boundary Condition: Step: flow_step: Category: Fluid: Fluid wall condition: select regions: Condition: Shear |
Infiltration at a surface permits the fluid to penetrate the surface while maintaining the no-slip condition. This wall condition is modeled by specifying the wall-normal velocity equal to the velocity representing the infiltration velocity, while the wall-tangent fluid velocity is equal to the wall velocity (zero if the wall is not moving). In the special case when a turbulence model is implemented, the wall-normal distance boundary condition must be set to zero at the wall. If the Spalart-Allmaras turbulence model is enabled, you can specify the value of the Spalart-Allmaras turbulent eddy viscosity, , that is allowed at the wall due to infiltration. If the RNG k– model is implemented, you can prescribe values at the wall for the turbulent kinetic energy, k, and the dissipation rate, .
| Input File Usage: | You can generate the input file using Abaqus/CAE. |
| Abaqus/CAE Usage: | Load module: Create Boundary Condition: Step: flow_step: Category: Fluid: Fluid wall condition: select regions: Condition: Infiltration |
Temperatures can be prescribed at a wall. By default, if no temperature is prescribed at a wall, a perfectly insulated boundary is specified automatically. For multiphysics applications such as conjugate heat transfer, a variable temperature condition is imposed automatically using a co-simulation region (for more information, see “Preparing an Abaqus/CFD analysis for co-simulation,” Section 14.1.3).
| Input File Usage: | You can generate the input file using Abaqus/CAE. |
| Abaqus/CAE Usage: | Load module: Create Boundary Condition: Step: flow_step: Category: Fluid: Fluid wall condition: Thermal Energy: Specify: Temperature |
Abaqus/CFD provides the capability to perform both deforming-mesh and fluid-structure interaction (FSI) simulations using an arbitrary Lagrangian Eulerian (ALE) methodology for the fluid flow. For FSI and deforming-mesh problems, typically some portion of the fluid domain is deformed consistent with a boundary motion. To manage the mesh motion, you must prescribe displacement boundary conditions on the mesh. For FSI problems, displacement boundary conditions are not permitted at the co-simulation region because these conditions are prescribed automatically.
| Input File Usage: | You can generate the input file using Abaqus/CAE. |
| Abaqus/CAE Usage: | Load module: Create Boundary Condition: Step: flow_step: Displacement/Rotation: select regions and toggle on the degree or degrees of freedom |