Fan and linked-fan boundaries
Vanellus has two fan boundary models:
- An exterior fan uses
fan_inletto draw from an external environment and discharge into the domain. - A linked internal fan uses a
linked_fan_intake, where fluid leaves the simulated region, and alinked_fan_exhaust, where it returns.
Neither model resolves the blades or fluid volume inside the fan. Both represent its pressure rise across boundary patches.
Fan curve
Both models apply a fan curve at the discharge boundary: fan_inlet for an exterior fan or linked_fan_exhaust for a linked fan.
flow_ratesandstatic_pressuresare paired samples of volumetric flow rate and static-pressure rise. The curve must describe static, not total, pressure rise.- Vanellus interpolates the current flow rate on the curve to obtain the pressure rise.
relaxation_factorunder-relaxes changes in pressure rise between SIMPLE iterations; it does not change the final fan curve.
At both discharge boundaries, velocity is a Dirichlet condition. Its inward-normal value is updated from the current volumetric face flux, with zero tangential components.
For intake static pressure \(p_i\), fan pressure rise \(\Delta p_{\mathrm{fan}}(Q)\), density \(\rho\), and normal discharge speed \(U_n\), the discharge boundary pressure is
\[ p_b = p_i + \Delta p_{\mathrm{fan}}(Q) - \frac{1}{2}\rho U_n^2. \]
The fan curve and the resistance of the simulated flow path determine the operating point.
fan_inlet
| Field | Condition |
|---|---|
| Velocity | Dirichlet: inward-normal value updated from the current volumetric face flux |
| Pressure | Dirichlet: ambient_pressure plus the fan pressure rise, minus discharge dynamic pressure |
| Temperature | Dirichlet: temperature |
| Turbulence | Dirichlet: tke and sdr from turbulent_intensity and viscosity_ratio |
For an exterior fan, the intake pressure \(p_i\) is ambient_pressure. The fan curve therefore raises the pressure from the external environment before applying it at the boundary.
linked_fan_intake and linked_fan_exhaust
A linked fan uses two patches with the same tag:
linked_fan_intakeis where fluid leaves the surrounding simulated region and enters the omitted fan;linked_fan_exhaustis where fluid returns from the fan and enters the simulated region.
The exhaust uses the same discharge treatment as fan_inlet, but takes its input from the linked intake rather than an external environment:
| Field | Condition |
|---|---|
| Velocity | Intake — Neumann: zero normal gradient. Exhaust — Dirichlet: inward-normal value updated from the current volumetric face flux. |
| Pressure | Intake — Dirichlet: adjusted until intake flow matches exhaust flow. Exhaust — Dirichlet: the equation above with \(p_i\) equal to the arithmetic mean intake pressure. |
| Temperature | Intake — Neumann: zero normal gradient. Exhaust — Dirichlet: arithmetic mean intake-cell temperature, without area or mass-flow weighting. |
| Turbulence | Intake — Neumann: zero normal gradient. Exhaust — Dirichlet: tke and sdr from its turbulent_intensity and viscosity_ratio. |
The fan curve and relaxation_factor are supplied on linked_fan_exhaust.
The model adds no heat from fan inefficiency or motor power.
The linked patches can have different shapes and areas, but both must represent the complete flow transported through the same fan. Check their final integrated flow rates and the pressure rise rather than comparing local velocities.
See also
- Drive airflow from a fan curve configures an exterior fan.
- Add an internal fan configures paired linked boundaries.
- Flow boundary conditions explains the underlying inlet, outlet, and pressure behavior.
- API reference lists curve fields and accepted values.