Houdini is HIP - Part 14: Flip Fluids I

Introduction

In this tutorial, we will explore the fundamentals of flip fluids in Houdini, focusing on both small-scale simulations and more complex ocean simulations. By the end of this guide, you will understand the underlying concepts of fluid dynamics using Lagrangian and Eulerian methods, and you will be equipped to create and manipulate flip fluids effectively.

Chapter 1: Understanding Lagrangian and Eulerian Solving

Lagrangian Solving

• Concept: Lagrangian solving involves tracking particles that carry their own data (position, velocity, color).
• Characteristics:
  • Each particle holds attributes.
  • As particles move through frames, they carry their data with them.
  • Can be influenced by external forces (e.g., gravity, air resistance).

Eulerian Solving

• Concept: In Eulerian solving, data (like density or velocity) is represented as fixed voxels in space.
• Characteristics:
  • Voxels do not move; data moves through them.
  • Useful for simulating fluids in a volume where the position is not tracked.

Euler/Lagrange Solving

• Concept: Flip fluids combine both methods, using particles for detail and a grid for overall volume dynamics.
• Workflow:
  • Start with particles that transfer their values to a grid.
  • Use Eulerian methods for calculations on the grid.
  • Feed results back to the particles to update their positions.

Chapter 2: Setting Up Flip Fluids in Houdini

Creating the Base Setup

1. Create a New Node: Name it flipBasics.
2. Add a Sphere: This will act as the source for our fluid.
3. Add a Null Node: Label it as Source.

Sourcing the Fluid

• Using a Scatter Node:
  • Add a scatter node to distribute points over the sphere.
  • Use an ISO Offset to create a shell around the sphere.
  • Set sampling divisions to 100 and total count to 10,000 or more.

Flip Dynamics Network

1. Create a Flip Object: This will hold the fluid properties.
2. Add a Flip Solver: Connect this to the flip object to process the simulation.
3. Adjust Visual Settings:
   • Disable visualizer limits in the flip solver for better focus on particles.

Adjusting Particle Settings

• Set Particle Separation: Lower values increase particle count and detail.
• Add a Ground Plane: Use a merge node to integrate the ground plane into the simulation.
• Add Gravity: Include a gravity node to affect the fluid's behavior.

Fixing Common Issues

• Volume Loss: Increase particle separation to prevent particles from passing through each other.
• Stickiness: Enable stick on collisions in the flip solver to simulate realistic fluid behavior against surfaces.

Chapter 3: Continuous Particle Sourcing

1. Add a POP Source: This will emit particles continuously into the fluid simulation.
2. Adjust Birth Rate: Set a higher constant activation rate to increase particle emission.
3. Monitor Particle Count: Ensure the number of particles corresponds to the defined particle separation.

Chapter 4: Volume Sourcing

• Use a flip source to create a volume-based fluid source.
• Adjust voxel and particle separations to ensure consistency in simulation quality.

Chapter 5: Customizing Fluid Behavior

• Add Velocity: Use velocity attributes to influence fluid movement.
• Scatter and Volume Rasterize: Create a velocity field based on a spiral or other geometry.
• Link Velocity to Flip Dynamics: Ensure that velocity attributes are correctly transferred into the simulation.

Chapter 6: Outputting Flip Fluids

1. Use a DO Input/Output Node: Fetch your dynamics and prepare for export.
2. Apply Fluid Compress: This helps manage file size and efficiency when saving to disk.
3. Use a Particle Fluid Surface: Convert particles into a mesh for rendering.

Visualizing Fluid Attributes

• Create visualizations for velocity and other attributes to analyze simulation performance.

Conclusion

In this tutorial, we covered the fundamentals of flip fluids in Houdini, including Lagrangian and Eulerian methods, setting up fluid simulations, and outputting results. As you experiment with these techniques, consider exploring settings related to fluid viscosity, droplet formation, and external forces to enhance your simulations. Keep practicing to master fluid dynamics in your projects!