Flow boundary conditions
A flow boundary condition supplies conditions for several solver fields. A single API object can therefore prescribe velocity, pressure, temperature, and, when \(k\)-\(\omega\) SST is enabled, the turbulence fields.
Fan boundaries follow the same field-level pattern but add a pressure rise from a fan curve. They are described in Fan and linked-fan boundaries.
velocity_inlet
| Field | Condition |
|---|---|
| Velocity | Dirichlet: uniform inward-normal speed |
| Pressure | Neumann: zero normal gradient |
| Temperature | Dirichlet: temperature |
| Turbulence | Dirichlet: tke and sdr derived from turbulent_intensity and viscosity_ratio |
velocity_inlet applies speed normal to the selected patch and directed into the domain. It does not represent a developed profile or preserve tangential velocity. The requested speed therefore fixes the volumetric flow through the meshed opening area.
ambient_pressure_inlet
| Field | Condition |
|---|---|
| Velocity | Dirichlet: inward-normal value updated from the current face flux |
| Pressure | Dirichlet: ambient_pressure minus inflow dynamic pressure |
| Temperature | Dirichlet: temperature |
| Turbulence | Dirichlet: tke and sdr updated from the current inflow speed |
Only pressure differences affect the incompressible flow solution, so pressure values are normally supplied relative to a consistent datum.
At an ambient_pressure_inlet, Vanellus treats ambient_pressure as the pressure available upstream of the opening. On inflow it sets the boundary static pressure to
\[ p_b = p_{\mathrm{ambient}} - \frac{1}{2}\rho U_n^2. \]
The boundary velocity is normal to the opening and follows the flow produced by the pressure field. Vanellus deliberately omits gravitational potential from this condition. Equal ambient pressures at openings of different heights therefore do not create a pressure difference solely from their elevation when Boussinesq buoyancy is enabled.
static_pressure_outlet
| Field | Condition |
|---|---|
| Velocity | Neumann: zero normal gradient |
| Pressure | Dirichlet: pressure |
| Temperature | Neumann: zero normal gradient |
| Turbulence | Neumann: zero normal gradient for tke and sdr |
This condition is intended for outflow. If recirculation reaches it, the zero-gradient treatment does not provide temperature or turbulence data for incoming fluid.
flow_rate_outlet
| Field | Condition |
|---|---|
| Velocity | Neumann: zero normal gradient |
| Pressure | Dirichlet: updated to reach target_flow_rate |
| Temperature | Neumann: zero normal gradient |
| Turbulence | Neumann: zero normal gradient for tke and sdr |
Vanellus integrates the current volumetric flow over the complete selected patch and adjusts the outlet pressure toward target_flow_rate. It does not impose a uniform outlet velocity.
Its relaxation_factor damps the pressure-controller update. It is separate from the pressure relaxation in numerics: one controls the boundary target and the other controls the SIMPLE pressure field.
ambient_pressure_inlet_outlet
| Field | Condition |
|---|---|
| Velocity | Inflow — Dirichlet: inward-normal value from the face flux. Outflow — Neumann: zero normal gradient. |
| Pressure | Inflow — Dirichlet: ambient minus dynamic pressure. Outflow — Dirichlet: ambient_pressure. |
| Temperature | Inflow — Dirichlet: temperature.Outflow — Neumann: zero normal gradient. |
| Turbulence | Inflow — Dirichlet: tke and sdr from the inlet inputs.Outflow — Neumann: zero normal gradient. |
The treatment is chosen independently on every meshed face and can change as the solution develops, so one patch can contain inlet and outlet faces at the same time. This is more suitable than static_pressure_outlet where backflow is physically plausible, but an unintended flow direction can become self-sustaining. Inspect the final velocity direction near the boundary rather than relying on its name.
See also
- Turbulence inlet conditions and wall treatment explains how the inlet turbulence fields are derived.
- Thermal boundary conditions explains the selectable conditions applied to walls.
- Boussinesq buoyancy and reference temperature explains the pressure and body-force treatment used for buoyant flow.
- Boundary placement and defaults explains placement and default surface behavior.
- API reference lists exact request fields and limits.