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Toilet Flushing Applications

Simulation of a weir-type
flow caused by a toilet flushing.

Ever wonder how toilets work? They're actually quite complicated. When the handle is pushed, water begins to fill the bowl. When the fluid level in the bowl rises above the top of the trap (behind the bowl), a weir-type flow begins. When the flow is fast enough, a bubble forms in the top of the trap creating a siphon. At that point, the siphon pulls the water out of the bowl and the toilet flushes.

In many locales, water conservation is an important issue, and low-flow toilets are required for both home and commercial use. But if a toilet doesn’t get the job done on the first try, the water-conservative objective is defeated. FLOW-3D can be used to model various designs to achieve the optimum results.

Flushing Examples

In the 3D animation (below left), FLOW-3D demonstrates the flushing sequence of a water closet. Two regions of water are initialized along with a ball. The ball is modeled using the General Moving Object Model to simulate fully coupled fluid-solid motion with six degrees of freedom. Gravity forces water from flush tank into the water closet. The analysis demonstrates the flow profile and pressure contours indicating regions of stagnation and duration for the ball to exit the domain. Different mass and shapes can be used instead of the ball. Residual water can also be analyzed during the flushing process.

A cross sectional plot (below right) shows flow recirculation in a water closet and detailed flow profile. The Collision model was used to simulate the ball that predicts the bouncing based on a prescribed coefficient of restitution and friction. The transient sharp interface between water and air is maintained well using FLOW-3D’s TruVOF method.

3D Toilet Flush: transient pressure contours during flushing of a water closet (left);
2D Toilet Flush: cross section of a water closet—transient vector representation of velocity magnitude contours (right).

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