Modeling Flow Control Devices
Is there an easy way to model flow control devices such as valves, trap doors, or other simple mechanisms?
The General Moving Object (GMO) model simulates the motion of solid objects and the interaction of those objects with fluid. The motion of a rigid body can be either dynamically coupled with fluid flow or can be prescribed by the user. This naturally leads to the use of the GMO model in applications where the rigid body moves with respect to the fluid/inertial frame, such as the moving parts in an engine, a moving boat, a fluid mixer or a rigid body in a shaking tank.
Somewhat less intuitive is the use of the GMO model in applications where one needs to control the flow of a fluid. For such applications one may specify a simple GMO object with translational or pivoted motion as a flow control device. As an example, waste water treatment might use air bubbles released periodically from the bottom of the tank to prevent sewage from settling down at the bottom. Figure 1 shows a two-dimensional view of the geometry and fluid configuration for this particular problem at different times.
In Figure 1, results from a simulation using the GMO model to simulate the generation of periodic air bubbles. Timesteps proceed in clockwise direction from top left. This is a one-fluid free surface case. Blue corresponds to a fluid fraction of zero (bubble), while red corresponds to a fluid fraction of one (water).
In Figure 1, the image at top-left corner shows the moving and non-moving component. The boundary condition at the bottom boundary is defined as specified pressure with a fluid fraction of zero. For a one-fluid free surface case, this would mean that the pressure of void (air bubble) is held constant at this boundary. The diameter of the moving GMO component defines the air bubble while the non-moving solid component defines the bubble source by blocking the rest of the bottom boundary.
For the moving component a prescribed translational motion in z-direction is achieved by specifying a time dependent w (z component of) velocity. The air bubble is generated during the upward motion of the moving component; during its downward motion, the air bubble detaches itself from the boundary because of buoyancy.
The above example demonstrates a simple use of the GMO model to generate periodic air bubbles. The same concepts can be extended to model other types of flow control devices.