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General Moving Objects Model

Editor’s Note: This is the first in a series of articles highlighting development work underway for Flow Science's next release of FLOW-3D.

Every object in the universe is in motion. And all these moving objects, on Earth at least, move in a fluid (gas, liquid or multiphase mixture).

Users of FLOW-3D (Version 8.2 and prior) can model moving objects in fluid, but there are severe limits. The object movement must be prescribed, and it must be either a rectilinear motion along the Z axis or an axi-symmetric rotation. For some cases, these limitations pose no significant problems. In many other cases, however, there is a need to model a more general object motion, including its full interaction with the surrounding fluid.

Long the source of much head-scratching by Flow Science developers, the ability to simulate coupled general solid body motion is now under development and expected to be released in August 2004. According to the principal developer, Dr. Gengsheng Wei (see Staff Profile), the goal of the new General Moving Object model is to allow users to model general six-degrees-of-freedom and full coupling of fluid and solid body dynamics.

Imagine, for example, a glass of tea. You drop some ice cubes into the tea and stir the tea with a straw. The straw causes the water to move, which in turn moves the ice cubes. Now FLOW-3D can simulate the whole process quite well.

Three categories of solid object motion are under development: objects moving with a prescribed motion, objects moving due to prescribed external forces (other than hydraulic forces and gravity), and objects moving freely in flowing fluid under full object-fluid interaction (termed free object motion in FLOW-3D). Combinations of these three options will also be allowed.

The new model uses the same finite volume, structured grid as always. Changes to volume and area fractions due to object movement are calculated at each time step. Up to 500 moving objects are allowed in one computation.

For free object motion, the dynamics of moving objects and the dynamics of fluid flow are calculated and fully coupled through the boundary conditions that they provide to each other. Hydraulic forces and torques in all three directions (x, y, and z) are calculated at each time step and available for output.

The model is very easy to apply. A user only needs to provide the shape, initial velocity and mass density (or inertia parameters) of the solid body. Everything else is left to FLOW-3D. If the object's motion is prescribed, the dynamics of the object are not calculated but the fluid flow is affected by the moving objects.

An interesting, real-world example is provided in the animation above. Here, in a tank containing two fluids, two beaters with closely-located heads are rotating in opposite directions to cause the fluids to mix. Because the blades of the beaters move through each others domain of motion, without the FAVOR™-based moving object capability, this problem would be very difficult to solve.