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Improvements to the General Moving Objects Model

This article highlights developments to be released in FLOW-3D version 10.1.

In a FLOW-3D simulation, a general moving object (GMO) is a rigid body with any kind of motion that is either user-prescribed or dynamically coupled with fluid flow. The GMO model allows multiple rigid bodies under independent motion types as well as rigid body interactions including collisions and continuous contact. The GMO model has been successfully applied to many engineering problems. In the upcoming release of FLOW-3D v10.1, certain model limitations have been addressed, such as preventing interpenetration between objects in continuous contact and allowing for porosity in moving objects.

Objects in Continuous Contact

In the currently released GMO model, continuous contact between two objects is simulated using a series of low-strength collisions, called micro-collisions. Examples of continuous contact include sitting, rolling and sliding of one object over another. When these collisions are frictional or inelastic, mechanical energy of the object(s) may gradually decrease, causing interpenetration between the objects. For example, a moving object sitting on a fixed object would gradually sink into the supporting object. In addition, a high fluid force acting on the moving object may also cause or accelerate the sinking process.

The collision model includes collision detection and collision response calculation. In the collision detection, collision is confirmed if 1) overlap between a pair of objects is found, and 2) at the contact point the two objects are moving toward each other. Once collision is detected, the code calculates a collision response. In this work, a two-step approach is used and implemented to prevent interpenetrations, which is applied immediately after the collision response calculation. The first step is to translate one of the colliding objects over a short distance to reduce the overlap depth. The second step is to adjust the object's velocity to conserve its mechanical energy if gravity is present. The second step is necessary because the translation in the first step is artificial and changes the object's mechanical energy more or less through the change of its gravitational potential energy. With the second step the accumulation of such changes over a series of micro-collisions that may introduce significant error to the object's motion behavior is eliminated. The improved GMO model now simulates continuous contact successfully.

In the example below, a filter captures spherical particles as fluid moves through it. Animation 1 shows the simulation result using the existing GMO model. It is seen that the filter cannot stop the particles from going through because the original model fails continuous contact. The result of the improved collision model is presented in Animation 2, where the particles are correctly stopped at the filter.


Animation 1. Simulation of filter using the existing GMO model.
Animation 2. Simulation of filter using the modified GMO model.

Porous Moving Objects

The GMO model has been extended to allow the moving objects to be porous. At each time step, the volume and area fractions are updated in all the mesh cells occupied by the porous objects. The drag force of the porous media on the fluid is computed using the relative velocity between the object and the fluid. Porosity effects are also considered in the calculations of hydraulic forces and torques exerted on the moving objects.

Animation 3. Water fills pore space in a stationary object.
Animation 4. Water fills pore space in a downward moving object.
Animation 5. Water fills pore space in a upward moving object.

Animation 6. Water fills pore space in
an upward moving object

The simulation examples above model a porous cube-shaped object moving in water. Animations 3, 4 and 5 present the computational results when the object is stagnant, moves downward and upward, respectively. It can be seen that water enters the object and gradually fills the open space inside (the void pressure inside is constant). Figure 1 compares the evolution of the fluid volume inside the porous medium for all the three cases. The filling rate is the fastest for the object going downward because the surrounding fluid pressure increases with time. The filling rate is the slowest for the object moving upward due to a decrease in the average fluid pressure. Animation 6 shows the simulation result of the porous moving object emerging from water. While the object is still submerged, water enters the pore space due to the high surrounding pressure. After it emerges out of the water, the fluid inside moves with it and at same time drains due to gravity. The filling and draining processes can be seen more clearly in Figure 2 where evolution of water volume inside the object is shown.

Time variation of water volume in a porous object
Figure 1. Time variation of water volume in a porous object.

Time variation of water volume in a porous object moving in upward direction
Figure 2. Time variation of water volume in a porous object moving in upward direction.


The GMO model has been improved for continuous contact and porous media flow, which extends the application breadth of FLOW-3D. For example, the continuous contact capability can be used to simulate debris flow, landslide, and particle filtration.

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