Fluid Simulation Components

Table of contents

• Fluid Simulation
  • Terrain and Obstacle Input
  • Boundary Conditions
• Fluid Simulation Settings
  • Creation
  • Properties
  • Second Layer
• Flux Fluid Simulation
  • The Pipe (Flux) Model
  • Simulation Characteristics
• Flux Fluid Simulation Settings
  • Creation
  • Specialized Flux Properties
• Flow Fluid Simulation
  • The 2D Height Field Model
  • Performance and Stability
• Flow Fluid Simulation Settings
  • Creation
  • Specialized Flow Properties
• Foam Layer
• Foam Layer Settings
  • Foam Render Settings
  • Foam Decay Settings
  • Foam Pressure Settings
  • Foam Wave Settings
  • Shallow Foam Settings
  • Turbulence Foam Settings
• Fluid Flow Mapping
• Fluid Flow Mapping Settings
• Erosion Layer
  • Multi-Layer Terrain System
  • Terrain Modification Processes
• Erosion Settings
• Terraform Layer
  • Liquefaction (Solid Terrain to Fluid)
  • Fluid Contact Reactions (Fluid and Terrain)
  • Fluid Mixing (Fluid + Fluid to Solid Terrain)
  • Fluid Contact Reaction
  • Terraform Settings
• Terrain Modifier
  • Terrain Modifier Settings
• Fluid Particle Generator
  • Splash Particles
  • Surface Particles
• Fluid RigidBody
  • Prerequisite
  • Fluid Density Calibration
  • Interaction Calculation
  • Fluid Displacement Profile
  • Fluid Interaction Profile
• Fluid RigidBody Lite
• Fluid Simulation Obstacle
  • Settings
• Fluid Event Trigger

This section covers all the simulations and the modification components Fluid Frenzy has to offer.

There are two types of simulations: Flux Fluid Simulation and Flow Fluid Simulation. Both simulations handle the full simulation and the components attached to it using a different method. The user can choose which to simulation to use in their scene depending on which is more suitable for their needs.

Fluid Simulation

Fluid Simulation is the core component of Fluid Frenzy. It handles the full simulation and the components attached to it.

This is the base class for all fluid simulation types (Flux Fluid Simulation and Flow Fluid Simulation). The system uses the Shallow Water Equations to create a physically-based fluid heightfield simulation. This core technology calculates a velocity field that dictates how the fluid moves and flows over a static, underlying ground heightfield, providing a realistic and performant simulation of large water bodies. It defines common properties and systems used by both solvers.

Terrain and Obstacle Input

The simulation’s bottom depth and its interaction with static terrain are managed through a single primary input source. You must select one of the following modes to define the simulation floor, as they are mutually exclusive:

• UnityTerrain (Uses the standard Unity Terrain component)
• Simple/TerraformTerrain (A custom, simplified terrain for faster results)
• Orthographic layer capture (Captures scene geometry from a camera view)
• Custom heightmap (Uses a texture as the terrain height input)
• MeshCollider (Can be used as a simple, static bottom surface if no other base mode is selected)

Additionally, FluidObstacle components can be added to the scene to represent dynamic or static objects that interact with the fluid surface, compatible with any of the primary base modes.

Boundary Conditions

The simulation handles the outer edges of the simulation grid realistically for various scene types:

• Reflective Boundaries The grid boundary forces water to reflect back into the simulation, ideal for enclosed areas like pools or tanks.
• Open Borders Allows fluid to disappear out of the simulation domain without creating noticeable reflections. This is essential for simulating an open ocean or a river with continuous flow.

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Fluid Simulation Settings

This is the abstract base class for all simulation settings, serving as a Scriptable Object asset assigned to a Fluid Simulation. It defines the essential, global properties shared across all fluid simulation types.

By utilizing a Scriptable Object asset, FluidSimulationSettings simplifies the configuration workflow. This approach allows a single settings profile to be reused and instantly modified across multiple Fluid Simulation components within a project, ensuring consistent simulation behavior.

Creation

To create a new simulation settings asset, navigate to: Assets > Create > Fluid Frenzy > Simulation Settings.

Properties

This base class contains all parameters and properties that are universal to the core fluid heightfield simulation system, independent of the specific solver (e.g., Flux, Flow). Any specialized settings for a particular solver will be defined in classes that derive from FluidSimulationSettings.

Wave Simulation

_{* Due to the 2.5D nature of the simulation each cell/pixel represents a world space size: dimension / Number Of Cells. This means scaling the number of cells up can cause the fluid to move slower in world space. The simulation tries to automatically scale as best as possible, but can only move so fast before becoming unstable. Therefore at some point making the Cell Size lower or Acceleration higher will have no effect, if you want your simulation to move faster you will have to reduce the Number Of Cells.}

Rendering

GPU > CPU Readback Synchronization

The simulation fully runs on the GPU but in some cases, interaction with objects that live in the CPU is desired like floating objects. When enabling this the simulation data for height and velocity will be read back to the CPU asynchronously. The reason the simulation data is readback asynchronously is to prevent stalls and improve performance, this does mean that the CPU data is a few frames behind the GPU simulation.

Evaporation

Second Layer

Flux Fluid Simulation

Flux Fluid Simulation is the core component of Fluid Frenzy. It handles the full simulation and the components attached to it.

The FluxFluidSimulation uses a Flux-based algorithm, often referred to as the Pipe Model, to simulate large water bodies. This approach directly calculates the volume exchange (flux) between grid cells, resulting in a highly stable simulation capable of generating very smooth, realistic waves.

The Pipe (Flux) Model

In this model, the fluid domain is discretized into a grid of vertical columns. These columns interact with their four neighbors via virtual pipes. The simulation determines the flow (flux) through these pipes by calculating the difference in pressure (based on fluid level) between adjacent columns. This method of modeling volume exchange guarantees mass conservation and smooth waves.

Simulation Characteristics

The system provides the following trade-offs compared to other simulation types:

Flux Fluid Simulation Settings

Represents the specialized settings for the Flux Fluid Simulation solver. This class extends Fluid Simulation Settings to include specific properties and parameters unique to the Flux simulation algorithm.

As a Scriptable Object asset, FluxFluidSimulationSettings allows for the reuse and quick modification of a specific Flux simulation profile across multiple Flux Fluid Simulation components. This ensures consistency and simplifies rapid iteration on simulation characteristics.

Creation

To create a new Flux-specific simulation settings asset, navigate to: Assets > Create > Fluid Frenzy > Flux > Simulation Settings.

Specialized Flux Properties

This asset contains all settings that are specific to the “Flux” implementation of the fluid solver, inheriting all universal properties from the base Fluid Simulation Settings. These properties are used to control the unique behaviors, stability, and optimizations of the Flux algorithm.

Flow Fluid Simulation

Flow Fluid Simulation is the core component of Fluid Frenzy. It handles the full fluid simulation based on the 2D Shallow Water Equations (SWE) approach for large bodies of water.

The FlowFluidSimulation leverages the Shallow Water Equations (SWE) to simulate large water bodies such as rivers, lakes, and oceans by treating the fluid as a 2D height field. This velocity-based model is computationally efficient, allowing for real-time performance and multiple simulation iterations per frame.

The 2D Height Field Model

The simulation models large bodies of water (like rivers, lakes, and oceans) using a highly efficient 2D Height Field. This model is a streamlined version of full 3D fluid dynamics, designed for fast, real-time performance. It tracks two main components across a grid: the Height Field (h), which manages the overall water level, and the Horizontal Velocity Field (v), which controls the direction and speed of the flow. This approach is key to capturing realistic horizontal phenomena like strong river currents, swirling whirlpools, and how objects are pushed by the flow-features that simpler wave simulations miss.

Performance and Stability

The solver is built for maximum speed and runs on a Fixed Timestep. To maintain accuracy and stability regardless of the rendering frame rate, the simulation employs several techniques:

• Adaptive Stepping The simulation will run multiple times per frame to catch up if the frame rate drops, or skip a frame if the frame rate is higher than the simulation’s fixed timestep.

• Clamping Limits the water’s maximum flow speed and ensures the water level never dips below the terrain (h >= 0). This is vital for stability, especially in dynamic, high-energy water scenes.

• Overshooting Reduction Automatically detects and smooths out unnatural wave artifacts that appear when large waves transition too quickly into shallow areas, making breaking waves look more convincing.

Flow Fluid Simulation Settings

Represents the specialized settings for the Flow Fluid Simulation solver. This class extends Fluid Simulation Settings to include specific properties and parameters unique to the Flow simulation algorithm.

As a Scriptable Object asset, FlowFluidSimulationSettings allows for the reuse and quick modification of a specific Flow simulation profile across multiple Flow Fluid Simulation components. This ensures consistency and simplifies rapid iteration on simulation characteristics.

Creation

To create a new Flow-specific simulation settings asset, navigate to: Assets > Create > Fluid Frenzy > Flow > Simulation Settings.

Specialized Flow Properties

This asset contains all settings that are specific to the “Flow” implementation of the fluid solver, building upon the universal properties inherited from the base Fluid Simulation Settings. These properties control the unique behaviors and optimizations of the Flow algorithm.

Foam Layer

Foam Layer is an Fluid Layer that can be attached to the Fluid Simulation. It generates a foam map based on the current state of the Fluid Simulation. There are several inputs from the Fluid Simulation that are used to generate this map (Pressure, Y Velocity, and Slope). The influence of each of these inputs can be controlled by the Foam Layer Settings. Note: This component lives on a GameObject but also needs to be added to the Layers list of the Fluid Simulation.

Foam Layer Settings

A Scriptable Object asset containing configuration parameters for a Foam Layer.

Using a settings asset allows for easy reuse and centralized modification of foam behaviors across multiple Foam Layer instances.

To create a new settings asset, navigate to: Assets > Create > Fluid Frenzy > Foam Layer Settings.

Foam Render Settings

Foam Decay Settings

Foam Pressure Settings

Foam Wave Settings

Shallow Foam Settings

Turbulence Foam Settings

Fluid Flow Mapping

Fluid Flow Mapping is an extension layer that enables and controls flow mapping functionality in the simulation and rendering side of Fluid Frenzy. The layer generates the flow map procedurally using the flow of the fluid simulation. The rendering data is automatically passed to the material assigned to the Fluid Renderer. There are several settings to control the visuals of the flow mapping in the layer which can be set in the Fluid Flow Mapping Settings asset assigned to this layer.

• Settings - a Fluid Flow Mapping Settings asset which holds the settings to be used for this Flow Mapping Layer

Fluid Flow Mapping Settings

A Scriptable Object asset containing configuration parameters for Fluid Flow Mapping.

Using a settings asset allows for easy reuse and centralized modification of flow mapping behaviors across multiple simulation instances.

To create a new settings asset, navigate to: Assets > Create > Fluid Frenzy > Flow Mapping Settings.

Erosion Layer

The Erosion Layer is an extension layer for the Fluid Simulation that simulates physically-based hydraulic erosion, slope-based slippage, and terrain modification based on the fluid’s state and terrain slope.

The Erosion Layer is a texture-based system that analyzes the fluid simulation’s state (fluid height and velocity) and the terrain’s geometry to dynamically modify the ground. Modifications are applied directly to the associated terrain input (e.g., SimpleTerrain or TerraformTerrain) and fed back into the simulation to affect fluid flow.

Multi-Layer Terrain System

The erosion system allows you to define up to four distinct terrain layers, each with its own properties (e.g., hardness, color). The layers are structured from the bottom up:

• Layer Structure The first element (Layer 0) represents the bottom-most layer (e.g., bedrock), and each subsequent element represents the material stacked on top.
• User Clarity For clarity in the editor, you can give each layer a custom name (e.g., “Rock,” “Soil,” or “Snow”) to help keep track of what it represents.

Terrain Modification Processes

1. Hydraulic Erosion (Water-Based)

This process converts terrain material into sediment using two-dimensional textures, driven by the fluid’s velocity field.

• Erosion Higher fluid velocity removes terrain material from the heightmap and transfers it into a Sediment Map.
• Transport The Sediment Map is continuously updated and advected (moved) across the grid according to the fluid’s velocity field.
• Deposition Sediment is redeposited back onto the terrain heightmap in areas where the fluid flow is slower.

2. Slope-Based Slippage

This process simulates gravity-driven effects (like landslides and thermal erosion). Material is removed and shifted to lower areas wherever the terrain’s slope angle exceeds a configurable threshold (the material’s angle of repose), helping to smooth steep cliffs over time.

Erosion Settings

Converts a Terrain Layer Mask into a vector representation for shader operations.

Terraform Layer

The Terraform Layer is an advanced extension of the Erosion Layer that enables dynamic terrain synthesis and multi-fluid interaction (e.g., “God game” mechanics) by simulating complex layer transformations.

The Terraform Layer extends the base erosion system by adding highly customizable rules for material transformation between fluid and terrain layers. This facilitates complex real-time behaviors like fluid mixing, liquefaction, and contact-based reactions that modify both the terrain heightmap and splatmap.

Liquefaction (Solid Terrain to Fluid)

This feature allows a terrain layer (e.g., snow or ice) to dissolve into a selected fluid layer (e.g., water) over time. You can configure the Liquify Rate and the Liquify Amount (conversion ratio of terrain height to fluid depth) independently for each terrain layer.

Fluid Contact Reactions (Fluid and Terrain)

This system allows each terrain layer to react specifically when it comes into contact with any of the fluid layers. Reactions can be configured to:

• Dissolve Consume the terrain and/or the fluid over time.
• Convert Change the terrain into a new terrain layer (with a new splat channel) or convert the terrain into a different fluid layer (e.g., snow reacting to Lava to create Water).
• Volume Control Adjust the Terrain Volume and Fluid Volume multipliers to simulate material expansion or compression during the transformation.

Fluid Mixing (Fluid + Fluid to Solid Terrain)

When two different fluid layers occupy the same cell, this system can be triggered to simulate a reaction (e.g., water and lava mixing to cool and solidify into rock).

• Solidification The mixing fluids are consumed and deposited as a new, solid terrain layer onto the heightmap and splatmap.
• Particle Emission The system can emit visual particles (e.g., steam) upon mixing, with customizable settings for Emission Rate, color, and lifetime.

Setup Note: While this component must be attached to a GameObject, it also requires registration in the Extension Layers list to function correctly.

Fluid Contact Reaction

Defines the changes that occur when a specific terrain layer comes into contact with fluid.

This class acts as a “recipe” for interactions, such as Lava turning Grass into Rock (scorching) or Water turning Soil into Mud.

Terraform Settings

An extended configuration profile for the Terraform Layer, including standard erosion settings and advanced contact interactions.

Terrain Modifier

Terrain Modifier is a component used to interactively modify the underlying terrain heightfield within the Fluid Simulation.

This component acts as a “brush” for precise terrain editing, allowing you to raise, lower, or set the height of the solid ground layer. It is ideal for dynamic world sculpting or in-game level editing.

Note: This component requires and works in conjunction with a specialized terrain system on the Fluid Simulation, such as the TerraformLayer or ErosionLayer and the Simple/TerraformTerrain types, to enable terrain height modifications.

The modification effect is defined by the Terrain Modifier Settings and applied within a localized area determined by the selected Terrain Input Mode and Size.

Terrain Modifier Settings

Fluid Particle Generator

_{This functionality is subject to future changes.}

Fluid Particle Generator is an Fluid Layer extension that analyzes the fluid simulation’s dynamics to spawn and manage visual particle effects for foam and spray.

This component generates two distinct types of particles by detecting areas of high turbulence and breaking waves within the fluid simulation:

• Splash Particles (Spray/Droplets) Ballistic particles spawned at high-energy events (e.g., breaking waves, collisions). These particles inherit the fluid’s velocity at the moment of spawn and follow a physical trajectory (like spray or droplets) until their lifetime expires.
• Surface Particles (Foam/Bubbles) Particles spawned on top of the fluid surface, primarily in areas of high turbulence. They are continuously advected (moved) by the simulation’s velocity field, acting as a visual representation of sea foam or churn. These particles can often be rendered to an off-screen buffer for use as a foam mask in the water shader.

Splash Particles

Particle System

| Property | Description | | :— | :— | | Update Mode | Defines the execution strategy for the particle update loop. | | Max Particles | The maximum number of particles that can be active simultaneously in the simulation buffer.

This value determines the size of the GPU Graphics Buffer allocated for the system. Increasing this allows for denser effects but increases VRAM usage and GPU processing cost. | | Material | The material used to render the particle geometry.

Requirement: The assigned material must use a shader capable of procedural instantiation, such as the included ProceduralParticle or ProceduralParticleUnlit shaders. | | Layer | The Unity Layer index assigned to the rendered particles.

This is used to control visibility via Camera Culling Masks, allowing specific cameras (e.g., UI or reflection probes) to ignore these particles. |

Surface Particles

Particle System

| Property | Description | | :— | :— | | Update Mode | Defines the execution strategy for the particle update loop. | | Max Particles | The maximum number of particles that can be active simultaneously in the simulation buffer.

This value determines the size of the GPU Graphics Buffer allocated for the system. Increasing this allows for denser effects but increases VRAM usage and GPU processing cost. | | Material | The material used to render the particle geometry.

Requirement: The assigned material must use a shader capable of procedural instantiation, such as the included ProceduralParticle or ProceduralParticleUnlit shaders. | | Layer | The Unity Layer index assigned to the rendered particles.

This is used to control visibility via Camera Culling Masks, allowing specific cameras (e.g., UI or reflection probes) to ignore these particles. |

Fluid RigidBody

Enables high-fidelity, two-way physics coupling between a Rigidbody and a Fluid Simulation’s heightfield grid, allowing the body to be affected by the fluid and to generate displacement, wakes, and splashes.

The component performs geometry-to-fluid interaction using an optimized system that leverages Unity Jobs for multithreaded performance.

Prerequisite

To allow the CPU to access the fluid height for buoyancy calculations, the Read Back Height (CPU Height Read) setting must be enabled on the Fluid Simulation.

Fluid Density Calibration

To ensure objects float correctly, the global fluid density setting, accessed via Fluid Density, must be calibrated to match your project’s physics scale (where mass and size often do not follow real-world proportions).

If Rigidbodies (e.g., a vehicle with a mass of 1500) sink when they should float, the fluid density is likely too low for that mass magnitude. A good starting point is to set the Fluid Density value to be similar to the mass of an average-sized object you expect to float.

Interaction Calculation

• Fluid to Solids Coupling (Forces) The component calculates and applies realistic forces to the object’s Rigidbody. Forces are determined by analyzing the volume and state of the body’s submerged sub-triangulated geometry against the fluid’s state. These include: Buoyancy: Calculated from the weight of the displaced fluid, based on the submerged volume. Drag and Lift: Applied based on the relative velocity of the body to the fluid.
• Solids to Fluid Coupling (Displacement) The object’s movement displaces the fluid, generating wakes and splashes. This is calculated by applying the volume and velocity changes of the submerged sub-triangles to the fluid’s closest height field and velocity grid cells. The effect is decayed exponentially with increasing distance from the surface.
• Requirements and Limitations The component requires a Rigidbody and a supported Collider, such as Mesh Collider, Sphere Collider, Box Collider, or Capsule Collider, on the same GameObject. This component is not compatible with WebGL due to its reliance on compute shaders.

Fluid Displacement Profile

A container for the physics coefficients that define how a solid object displaces and applies forces TO the fluid simulation. This controls the “splash” and “wake” created by the object.

Fluid Interaction Profile

A container for the physics coefficients that define how a Rigidbody is affected by fluid forces (drag, lift).

Fluid RigidBody Lite

Fluid Rigid Body Lite is a lightweight component for simplified interaction between a Rigidbody and the Fluid Simulation.

This component applies physics effects such as buoyancy, drag, and advection (movement by the current), and allows the object to generate visual wave and splash effects on the fluid surface. It is designed for performance with minimal setup.

Requirement: This component requires both a Rigidbody and a Collider to be attached to the Game Object.

Prerequisite: To allow the CPU to access the fluid height for buoyancy calculations, the Read Back Height (CPU Height Read) setting must be enabled on the Fluid Simulation.

Waves

Splashes

Physics

Splash Particle System

Fluid Simulation Obstacle

Fluid Simulation Obstacle is a component that can be added to any object with a Renderer component attached, or configured to use a procedural shape.

When this component is attached, its shape and height are orthographically rendered onto the fluid simulation’s underlying ground heightfield. This tells the Fluid Simulation where the obstacle is, allowing the fluid to correctly flow around or over it. The obstacle itself can be moved dynamically during runtime.

Important Note on Movement: The simulation’s heightfield model means the fluid cannot flow under the obstacle. If the obstacle moves quickly into a location where water is currently present, that water will be instantly forced on top of the obstacle. Rapidly moving a large obstacle can cause extreme visual artifacts (like large, unnatural splashes or wild fluid behavior) due to the sudden volume displacement. It is generally advised to use this component on objects that are mostly rounded (e.g., rocks or islands) or those that do not create highly concave shapes with the surrounding terrain.

Settings

Fluid Event Trigger

FluidEventTrigger is a component that can be used to see if and when a object is in a region with fluid The most dominant/highest fluid will be reported.

• onFluidEnter - the event that will be triggered when fluid enters the trigger.
• onFluidExit - the event that will be triggered when fluid has left the trigger.
• fluidLayer - the current fluid layer at the location of this trigger.
• fluidHeight - the height of the fluid at the location of this trigger.
• isInFluid - is the trigger in fluid or not.

Adding the following functions to a script allows the them to be registered to the OnFluidEnter and OnFluidExit event as shown in the image above.

public void OnFluidEnter(FluidFrenzy.FluidEventTrigger evt)
{
    Debug.LogFormat("Fluid Layer {0} entered trigger {1} ", evt.name, evt.fluidLayer);
}
public void OnFluidExit(FluidFrenzy.FluidEventTrigger evt)
{
    Debug.LogFormat("Fluid Layer {0} exit trigger {1} ", evt.name, evt.fluidLayer);
}

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