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The Active Bodies feature in Phoenix FD allows for two-way interaction between scene geometry and the Phoenix FD simulation.

When an object is selected as an Active Body, the simulation both influences and is influenced by the Active Body's movement. This allows objects such as boats to float over a Phoenix FD ocean simulation and be affected by its waves, or objects placed on the ground to be washed away by flooding waters.

Objects selected as Active Bodies may also be animated in which case they will follow the animation while also being influenced by the Phoenix FD simulation. A speedboat simulation is a great example of this mutual interaction in action - the speedboat is heavily affected by the the water while also able to cut through the waves and produce large amounts of foam. You can balance between the influence of the animation and the fluid from an Active Body's right click properties' Original Animation Influence option.

 A Phoenix FD Liquid Simulation with Active Bodies consists of the following elements:

1. A Liquid Simulator – used for the simulation of water.
2. One or more scene objects – converted to Active Bodies at simulation time.
3. An Active Body Solver – used to specify the scene objects which will be converted to Active Bodies and to set some global options for the Active Body simulation.
4. A Center of Mass gizmo Gizmo – automatically generated for each object when included in the Active Body simulation through the solverSolver.

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To add an Active Bodies Body Solver, go to Command Panel → Create → PhoenixFD → Active Bodies.

Add your scene geometry to the Active Bodies list.

Finally, link the Active Bodies Body Solver to the Phoenix FD Liquid Simulator from the Dynamics roll-out rollout.

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When you start the simulation, the Active Bodies solver Body Solver will create a duplicate of your object and keyframe its Position and Rotation for every single frame.

A couple of things to note here:

1. The ship is sinking - this happens because it's its Density too high. The Density of the Active Body can be tweaked from the original object's Phoenix FD Properties.
2. The Center of Mass Gizmo is above the ship's deck, right at the base of the middle mast - the this is causing the ship to tilt to the side. Instead of relying on the Active Bodies solver Body Solver to guess the correct position for the Center of Mass gizmo, its position can be overriden overridden from the Phoenix FD Properties of the original object and tweaked manually.

To tweak the Density of the active bodyActive Body, right mouse button click on the original object and bring up the Phoenix FD Properties.

You can either manually type an exact value in the Specify Density parameter, or choose one of the provided Density presetsPresets.

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With a lower densityDensity, the object does not now sink entirely underwater.

The erratic spinning of the ship is caused by the incorrectly placed Center of Mass gizmo Gizmo. Placing the Center of Mass somewhere close to the center of an object would make sense for a sphere but not for a ship which is usually bottom-heavy.

To tweak the Center of Mass gizmo, select Select the Center Of Mass Gizmo parented under the original object and enable the Override Center of Mass toggle under its Phoenix FD Properties.For this example, the gizmo is placed place it at the origin (0, 0, 0).

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Now that the Center of Mass is placed at the bottom, the ship is no longer capsizing.

It is clear, however, that the specified density Density is too high, causing the ship to sink too far below the surface.

Select the original object and reduce the Density from the Phoenix FD Properties to make the ship lighter.

In this example, the Density is set to 250.

In this example, the Center of Mass Gizmo of the original geometry is placed at [ (-2.5, 0, 0 ]).

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The ship is now behaving as expected.

You may continue this setup by adding a Phoenix FD Wave Force with an a Phoenix Ocean Texture to generate waves in the simulatorthe Simulator.

The Wave Force produces real simulated waves inside the Simulator. This is an alternative to the render-time displacement you can achieve when plugging a Phoenix Ocean Texture under the Displacement section of the Rendering rollout of a Simulator, which can't interact with submerged objects. The Wave Force is also driven by a Phoenix FD Ocean Texture and allows you to control how strictly the fluid will follow the pure ocean texture or how much freedom the fluid will have to splash and swirl around.