Gravity

Gravity#

The gravity model applies a gravitational body force to the fluid momentum and energy equations, enabling simulation of buoyancy-driven flows. Gravity is applied globally to all fluid zones in the simulation and is disabled by default.

Available Parameters#

Parameter

Description

Gravity (toggle)

Enable or disable the gravitational body force

Direction

Unit vector defining the direction of gravitational acceleration [X, Y, Z]

Magnitude

Magnitude of the gravitational acceleration

Detailed Descriptions#

Gravity (toggle)#

Enables or disables the gravitational body force. When enabled, gravity is applied globally to all fluid zones by adding a body force to the momentum equations and a work term to the energy equation: \(\mathbf{f}_{\text{mom}} = \rho \, \mathbf{g}, \qquad f_{\text{energy}} = \rho \, (\mathbf{g} \cdot \mathbf{u})\)

  • Default: Disabled (off)

Direction#

The unit vector specifying the direction of the gravitational acceleration. Defines the orientation of the gravity vector in the simulation coordinate system.

  • Default: X: 0, Y: 0, Z: -1 (pointing in the negative Z-direction)

  • Example: (0, 0, -1) for vertical-down in a Z-up coordinate system, (0, -1, 0) for a Y-up coordinate system

Notes:

  • The direction vector will be normalised automatically; you do not need to supply a unit vector.

  • Only available when the Gravity toggle is enabled.

Magnitude#

The magnitude of the gravitational acceleration.

  • Default: 9.81 m/s² (Earth surface gravity)

  • Example: 9.81 m/s², 1.62 m/s² (lunar gravity)

Notes:

  • Must be a positive value.

  • Only available when the Gravity toggle is enabled.

💡 Tips

  • When using a Z-up coordinate system, the default direction (0, 0, -1) corresponds to Earth’s downward gravity and requires no adjustment.

  • For conjugate heat transfer (CHT) simulations with buoyancy-driven convection, enabling gravity is essential for physically accurate results.

🐍 Python Example Usage

See also

Python API:

import flow360 as fl

# Gravity is configured via the `gravity` field on the Fluid model.
# Using fl.Gravity() applies Earth's defaults: direction=(0, 0, -1), magnitude=9.81 m/s^2.
fluid = fl.Fluid(
    gravity=fl.Gravity(),
)

# Custom direction and magnitude (e.g. a Y-up coordinate system, lunar gravity)
fluid_custom = fl.Fluid(
    gravity=fl.Gravity(
        direction=(0, -1, 0),
        magnitude=1.62 * fl.u.m / fl.u.s**2,
    ),
)
Contents