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Scale is crucial for the behavior of any simulation. The real-world size of the Simulator in units is important for the simulation dynamics. Large-scale simulations appear to move more slowly, while mid-to-small scale simulations have lots of vigorous movement. When you create your Simulator, you must check the Grid rollout where the real-world extents of the Simulator are shown. If the size of the Simulator in the scene cannot be changed, you can cheat the solver into working as if the scale is larger or smaller by changing the Scene Scale option in the Grid rollout.

The Phoenix solver is not affected by how you choose to view the Display Unit Scale - it is just a matter of convenience.

Go to Customize → Units Setup and set Display Unit Scale to Metric Centimeters.

Also, set the System Units such that 1 Unit equals 1 Centimeter.

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Cloth Preparation

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To represent a balloon, we need geometry with topology suitable for cloth simulations.

Create a Geosphere primitive with 30 segments and set its Radius to 20cm.

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Apply a ProOptimize modifier on the geosphere.

Set the Vertex% value to around 28 to produce a topological pattern suitable for cloth simulations.

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Assign a Cloth modifier to the sphere and head over to the Object Properties menu.

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Select the Geosphere in the list and then set it to Cloth. This sets the Geosphere as a dynamic cloth object in the simulation.

Under the Cloth Properties panel, set the Preset to Silk. The Silk preset allows you to easily produce detailed wrinkles. You can experiment with other presets and settings.

Set Pressure to 20. The Pressure parameter keeps the balloon from deflating and collapsing on itself.

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Under the Cloth modifier, go to the Group sub-object.

Select the top vertex of the balloon (as the arrow indicates) and press the Make Group button.

Then press Preserve so that this vertex won't be affected by the Cloth simulation and the rest of the cloth would hang from it.

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Change your mouse selection mode to Paint Selection. You can control the size of the brush by Right-Mouse-Button clicking on the Paint Selection icon and editing the Paint Selection Brush Size parameter from the General tab.

Optionally, you can set the Viewport shading mode to Hidden Line if you find it hard to see your selection.

In Group sub-object mode, manually select the vertices where you want the balloon to tear apart.

With the vertices selected, press the Make Tear button.

This creates a new Weld Group. The vertices in that group are split and welded together. Once the Constraint strength is not enough to hold them, they will start to break apart.

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Set Constraint Strength to 9999 so that the balloon will hold together at the start of the simulation.

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Set the animation Frame Rate to FPS: 120.

By doing so, we can simulate the cloth in slow motion and speed it up after baking.

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Cloth Simulation

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Reduce the Step parameter to 0.008 or you will get a warning that the Step cannot be greater than frame length. The frame length at 120 FPS is 0.0083(3).

Increase the Scene Scale by setting the cm/unit parameter to 10. This parameter can be used to easily control the "apparent" weight of the balloon.

From the Simulation Parameters rollout, set Subsample to about 10 - 20.

Higher Subsample values prevent the cloth from over-stretching.

Enable Self Collision to prevent the cloth from intersecting with itself.

 

Perform a local simulation to allow the balloon to form an initial resting position, as shown on the image.

Click Simulate Local.

Once the balloon is in the desired shape, we can do a simulation over time.

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Now that we are happy with the shape of the balloon, we can reduce Constraint Strength for the Weld Group to 10.

Press the Simulate button.

The balloon now slowly tears apart as the inflation pressure increases during the simulation.

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Congratulations!

Once the simulation is complete, you can change the Frame Rate back to NTSC.

 

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Note

The following operation cannot be reverted! Consider saving a backup file first!

 

Under the Cloth modifier, click the Create Keys button.

This creates vertex animation keys and essentially bakes the cloth simulation.

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From the Time Configuration window, click the Re-scale Time button.

Set the Length parameter to 50 to speed up the animation playback.

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Phoenix Liquid Simulation

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Since the simulated balloon geometry has no thickness, we can't use it as-is in the Phoenix simulation.

Apply a Shell modifier to the balloon and set Outer Amount to 0.1 cm.

This value can be modified depending on the size of the balloon object and the resolution of the Phoenix simulation.

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As the water in the balloon is supposed to be pressurized, place an emitter inside and use the Outgoing Velocity parameter to mimic the behavior of such a fluid.


Place a Geosphere inside the balloon.

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Create a LiquidSrc helper from the Helperstab of the Command Panel of Phoenix.

Add the Geosphere as an emitter with the Add button.

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Keyframe the Outgoing Velocity parameter of the LiquidSrc helper such that it runs fast in the beginning and then suddenly stops.

The entire point of this simulation is to mimic the effect of water bursting out of a balloon. Find the best value for the LiquidSrc Outgoing Velocity and the overall timing.

Find a good reference image or video and pay attention to the water bursting out of the balloon.


Bonus tip: turn off the LiquidSource's Outgoing Velocity before the balloon is fully broken.

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Create a V-Ray Physical Camera and set the settings based on your needs.


Enable Depth of Field and Motion Blur.
Set the F-Number, Shutter Speed and ISO.

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Simulator Properties

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Create a Liquid Simulator from the Phoenix Toolbar or the Phoenix FD menu of the Create panel.

Adjust its size so that it encompasses the balloon geometry and try to align it to the camera.


The Cell size parameter does not have to be excessively small during the RnD stage - you can set it to 0.3 cm.
Enable Adaptive Grid and configure the range of Maximum Expansion for X, Y and Z as shown in the picture.


Notice that the back of the balloon is partially outside of the simulation grid. This way we can save some resources and reduce the required memory for the simulation. The back side is pretty much irrelevant considering the camera angle.

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Set Gravity to a low value (0.05) so the liquid is almost weightless.

Set the Steps per Frame parameter to 2.

Low values will give you a 'noisy' result which is ideal for explosions. High values (10 or more) tend to smooth out the movement of the liquid. A value of 2 gives a good result, so that's what is used in this tutorial. You can increase it if needed.

The Time Scale is set to 0.7 to slow things down a bit.

Set Surface Tension to 0.05 (as a comparison, the surface tension of water is 0.05 ~0.10).

 

Wetting is enabled, with the Consumed Liquid option set to 0.1, and the Drying Time to 0.01 seconds.

Wetmap particles are born on the surface of objects which come into contact with the liquid. A Wetmap particle group is created which can be used at render time to drive the options of a material using a Phoenix Particle Texture.

Wetmap particles also allow the liquid to stick to the surface of objects when Surface Tension and/or Viscosity is above 0.

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Under Preview, enable Show Mesh. This allows you to preview the liquid mesh during simulation.

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Once happy with the result, increase the grid resolution by reducing the Cell Size.


Set the Cell size to around 0.2 cm.


Run the final simulation.

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Materials and Lighting

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The lighting in the tutorial scene includes a V-Ray Dome light with HDRI assigned.


Optionally, you can place a plane in the background and set its color close to black.

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Balloon Material

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During the Cloth preparation stage, select more vertices to be torn apart to simulate an instantaneous burst.

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On the Phoenix Properties of the balloon geometry, set the Motion Velocity Effect parameter to -1.0.
When the rubber pieces move away from the fluid, the velocities sourced into the simulation point towards the center of the liquid, as opposed to the previous setup, where the balloon geometry is dragging the liquid outward. This causes pressure build-up and random splashing to occur.

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In case there are liquid particles penetrating through the balloon geometry, the easiest solution is to increase the thickness of the balloon geometry.

Increase the inner amount on the Shell modifier to 1.0 cm. Once the simulation is complete, you can set the Inner Amount back to 0 and the Outer Amount to 0.1 cm for final rendering.

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