General Moving Objects Model
The interaction between moving rigid bodies, or objects, and flowing fluids is a complex physical phenomenon. On one end of the spectrum, the object drives the fluid flow with little effect on the object’s motion, for example the motion of the plunger during filling of a high pressure die casting process. On the other end, the motion of the fluid and the motion of the rigid object are completely coupled, like with floating debris through a culvert. In between, additional forces and torques applied to the moving object can exert considerable influence on the fluid but can be overcome by the motion of the fluid; for example, the thrust and torque applied to a boat moving upstream in a river. Moving objects don’t just interact with the water, but moving objects can collide or come into continuous contact with other moving or stationary objects.
The FLOW-3D product family offers a diverse set of modeling approaches to simulate the wide range of rigid body motion from fully coupled fluid-object motion to simple rigid body translation. The General Moving Object (GMO) model is one of the most broadly applied capabilities in the product family. Engineers across industries use it to simulate everything from check valves and plungers to wave energy converters and rotating machinery — all without resorting to complex body-fitted meshes or remeshing workflows.
Modeling Capabilities
The GMO model supports a wide range of motion and interaction types:
- Up to 500 moving objects per simulation, each with independent motion type
- Six degrees of freedom, rotation about a fixed axis or point, translation in fixed directions, or other more complicated constrained motion patterns
- Fully coupled fluid-object interaction, or user-prescribed motion
- Objects composed of multiple materials, each with defined density
- No restriction on geometry or motion complexity
- Time-dependent forces and torques applied to objects
- Collision dynamics and continuous contact between moving and stationary objects
- Springs, ropes, and mooring lines tethered to objects
- Porosity allowed within objects
- Heat conduction and convection through objects
- Force and torque contributions from hydraulic, gravitational, non-inertial, spring, and user-defined sources
- Full postprocessing output including forces, torques, velocities, and positions
Prescribed Motion
When the motion of an object is known in advance — such as a gate opening at a defined rate, or a plunger following a specified velocity profile — the GMO model allows that motion to be prescribed directly. The fluid response is then computed as the object moves, without requiring the solver to determine the driving forces dynamically. This is useful for hydraulic control structures, casting shot sleeves, and any application where the motion profile is a design input rather than a simulation output.
Example: Tainter Gate Opening
In this FLOW-3D HYDRO simulation, a tainter gate opens using prescribed rotational velocity about a fixed axis. As the gate lifts, high-velocity flow is released through the opening and over the energy dissipators downstream. The top view shows velocity distribution across the stilling basin; the cross-section below shows the jet profile and hydraulic conditions under the gate.
FLOW-3D HYDRO simulation of a tainter gate opening using prescribed rotational motion, showing velocity distribution in plan view (top) and cross-section (bottom).
Coupled Fluid-Object Motion
When an object’s motion is driven by the fluid rather than prescribed, the GMO model solves the coupled system dynamically. Forces and torques acting on the object — from fluid pressure, gravity, springs, or user-defined sources — are computed at each time step and used to update the object’s position and velocity. This enables simulation of floating bodies, fluid-driven mechanisms, wave energy devices, and any system where the interaction between fluid and structure is bidirectional.
Example: Wave Energy Converter
This FLOW-3D HYDRO simulation models an oscillating water column wave energy converter using a two-fluid VOF model to simulate air flow through a simple turbine. As waves move the water column inside the device, air is driven back and forth through the turbine, generating kinetic energy. The simulation tracks turbine kinetic energy and air flow rate over time, capturing the oscillatory energy extraction that characterizes this class of device.
FLOW-3D HYDRO simulation of a wave energy converter showing the physical device (top left), 2-fluid VOF model cross-section with air velocity (bottom left), and turbine kinetic energy and air flow rate time histories (right).
Example: Rotating Casting Wheel
In this FLOW-3D CAST simulation, aluminum ingots are produced using a rotating casting wheel. The rotation of the wheel and the translation of the molds are both defined as moving geometry, allowing the complete filling sequence to be modeled as the molds pass under the pouring station. Entrained air concentrations are tracked across successive mold positions, giving engineers insight into where oxide and air inclusions are most likely to occur.
FLOW-3D CAST simulation of aluminum ingot production on a rotating casting wheel, showing entrained air distribution across mold positions.
Example: Dosing Simulation of HPDC Process
This FLOW-3D CAST simulation models the process of pouring molten aluminum from a ladle into a shot sleeve. The ladle follows a prescribed pouring motion while the plunger waits at the end of the sleeve. Thermal contours capture the temperature distribution of the melt as it flows through the ladle spout, showing both the thermal loss during dosing and the wave propagation driven by the momentum of the pour. This thermal profile is critical for determining melt conditions in the shot sleeve before the plunger pushes the metal into the die.
FLOW-3D CAST dosing simulation showing molten aluminum poured from a ladle into a shot sleeve, colored by temperature. The ladle pouring motion and plunger position are both modeled as GMOs.
Applications
The General Moving Objects model is used across the FLOW-3D product family for a wide range of engineering problems, including:
- Hydraulic control structures — tainter gates, flap gates, sluice gates
- Check valves and flow-driven valve mechanisms
- Plunger motion in HPDC shot sleeves
- Wave energy converters
- Rotating machinery — casting wheels, mixers, centrifuges
- Automotive gearbox lubrication and fuel sloshing
- Moored and floating bodies in coastal and offshore flows
- Squeeze pins and moving die components in metal casting