Boundary conditions
A boundary condition prescribes the value of a solution variable at part of the model — a displacement held at zero, a pore pressure fixed at a water table. These are Dirichlet conditions, as distinct from the loads, which prescribe forces and fluxes instead.
Boundary conditions belong to a step. A condition defined in one step does not carry into the next unless it is defined there too, which is what lets a boundary be released partway through an analysis.
The branch is divided into two groups.
Direct boundary conditions
The value is prescribed directly. Six conditions are available.
| Condition | Assigned to |
|---|---|
| Solid displacement | Points, lines, surfaces, volumes |
| Solid rotation | Points, lines, surfaces |
| Pore water pressure | Points, lines, surfaces, volumes |
| Pore air pressure | Points, lines, surfaces, volumes |
| Water displacement | Points, lines, surfaces, volumes |
| User Boundary | Points, lines, surfaces |
Per-direction settings
Solid displacement, Solid rotation and Water displacement are given
per direction. Each of x, y and z takes one of three states:
| State | Meaning |
|---|---|
| free | The direction is unconstrained |
| fixed | The direction is held at zero |
| prescribed | The direction is held at the Value entered beside it |
A direction set to prescribed with an amplitude follows the amplitude over
the step, which is how a displacement-controlled test is applied.
Pore water pressure and Pore air pressure take a single value rather than three directions.
Type
Each condition carries a Type, which decides how the prescribed value is distributed over the assigned entities. Beyond a plain constant, a value can be given as a gradient — a Direction, a Gradient and a Zero level — which is how a hydrostatic pore pressure distribution is prescribed from a single water table elevation rather than node by node.
Amplitude
Every condition can reference an Amplitude, which scales the prescribed value over the step. An Edit amplitude command opens the amplitude manager directly from the tree.
Without an amplitude the value is applied instantly at the start of the step. Applying a large displacement or pressure instantly is a common cause of convergence trouble; a ramp amplitude usually fixes it.
Flexible boundary conditions
Flexible conditions change during the step in a way that depends on the solution, rather than following a fixed value.
| Condition | Assigned to |
|---|---|
| Solid displacement | Points, lines, surfaces, volumes |
| Solid rotation | Points, lines, surfaces |
| Pore water pressure | Points, lines, surfaces, volumes |
| Pore air pressure | Points, lines, surfaces, volumes |
| Water displacement | Points, lines, surfaces, volumes |
Type is None or Moving hydrostatic. The moving hydrostatic type
describes a water table that rises or falls during the step, given by a
Direction of application, a Slope, and two zero levels with the times at
which they apply:
| Field | Meaning |
|---|---|
| Direction of application | x, y or z |
| Slope | Gradient of the pressure distribution |
| Zero-level 0 | Elevation of the water table at t0 |
| Initial time (t0) | Time at which the first level applies |
| Zero-level 1 | Elevation of the water table at t1 |
| Final time (t1) | Time at which the second level applies |
| Constant | Value held outside the interval |
This is what a rising or falling water table needs: the level moves with time, and the pressure at every node follows from it.
Boundary conditions are described under Boundary, Boundary-Flexible and UBoundary in the numgeo reference manual.