Fluids


Introduction

IMPORTANT: This is experimental and outdated. Use with caution.

The package allows the simulation of fluids in Gazebo. The fluid particle interactions are computed on the GPU using the Fluidix library (if a nvidia GPU is not available the simulation will run in CPU mode).

Prerequisites:

Install

  1. Install CUDA (recommended 6.0)

    sudo apt-get install nvidia-cuda-dev nvidia-cuda-toolkit
    
  2. Install Fluidix

    Use the online form to get links to Fluidix

    mkdir /tmp/fluidix
    unzip ~/Downloads/Fluidix*.zip -d /tmp/fluidix
    cd /tmp/fluidix
    sudo ./install.sh
    
  3. Install gazebo from source using the fluid_sph branch

    [...]
    cd ~; hg clone https://bitbucket.org/osrf/gazebo
    cd ~/gazebo
    hg up fluid_sph
    [...]
    

How the package works

The fluid simulation runs as a separate physics engine which interacts with the rigid body physics engine of Gazebo through an interface (include/FluidEngine.hh).

The interaction includes: * collision detection * forces / torques application on the rigid objects * visualization of the particles

The core of the fluid simulation is written in the src/FluidEngine.cu cuda file and it is built upon the basic SPH example.

The package contains two plugins, one world plugin for updating the fluid and its interactions (FluidWorldPlugin.cc). And one GUI system plugin for visualizing the fluid particles(FluidVisPlugin.cc).

Running the plugin:

  1. Set the gazebo plugin and model paths

    echo "export GAZEBO_PLUGIN_PATH=/<path_to_gazebo_source>/gazebo/build/plugins:${GAZEBO_PLUGIN_PATH}" >> ~/.bashrc
    echo "export GAZEBO_MODEL_PATH=/<path_to_gazebo_source>/gazebo/media/models:${GAZEBO_MODEL_PATH}" >> ~/.bashrc
    source ~/.bashrc
    
  2. Add the plugin to your world file, or use one of the examples from the package.

    gedit ~/fluid.world
    

    Copy the following into the open editor, save, and quit.

    <?xml version="1.0"?>
    <sdf version="1.5">
      <world name="fluid_world">
    
        <!-- A global light source -->
        <include>
          <uri>model://sun</uri>
        </include>
    
        <!-- A box (plane + fluid is not supported) -->
        <model name="box">
          <static>true</static>
          <pose>0 0 0 0 0 0</pose>
          <link name="link">
            <collision name="collision">
              <geometry>
                <box>
                  <size>20 20 0.1</size>
                </box>
              </geometry>
            </collision>
            <visual name="visual">
              <geometry>
                <box>
                  <size>20 20 0.1</size>
                </box>
              </geometry>
            </visual>
          </link>
        </model>
    
        <plugin name="FluidWorldPlugin" filename="libFluidWorldPlugin.so">
          <world_position>0 0 1.01</world_position>
          <world_size>1.5 1 10</world_size>
          <fluid_position>-0.5 0.0 0.8</fluid_position>
          <fluid_volume>0.4 0.95 0.5</fluid_volume>
          <particle_nr>0</particle_nr>
        </plugin>
    
      </world>
    </sdf>
    

    Where:

    • <world_position> and <world_size> set the fluid world center position and its size
    • <fluid_position> and <fluid_volume> set the center position of the fluid and its volume to be filled with particles
    • <particle_nr> if set to 0, the given volume will be filled with fluid particles, otherwise the given particle number will be spawned.
  3. Run gazebo client with the system plugin:

    gazebo ~/fluid.world -g /tmp/gz_fluid/build/libFluidVisPlugin.so
    

To know:

Collisions meshes

In order for the fluid simulation to detect collisions gazebo needs to use .stl files for collision meshes.

Possible issues:

In CMakeLists.txt, the cuda compiler might need graphics card specific flags:

SET(CUDA_NVCC_FLAGS "-arch;sm_30 -use_fast_math -lm -ldl -lrt -Xcompiler \"-fPIC\"")

Fluidix CMake include path

For a default install the fluidix headers are located in /opt/fluidix/include, if the paths differ make sure to change them accordingly in the src/CMakeLists.txt file.

Some code explanation:

  • The world plugin FluidWorldPlugin.cc:

    • in the constructor the fluid engine is initialized
    • in FluidWorldPlugin::Load the sdf parameters are loaded, the fluid world is created, fluid is added, the objects from the environment are recreated in the fluid environment (when possible)
    • in FluidWorldPlugin::Init the publishers of the objects and fluids particles positions are initialized
    • FluidWorldPlugin::OnUpdate is called every world update event, there the fluid engine is updated one timestamp, a msg is sent for the rendering plugin with all the new particles position, and computed forces and torques are applied on the rigid objects.
  • The system plugin FluidVisPlugin.cc, used for rendering the fluid:

    • in FluidVisPlugin::Load the arguments (if given) are loaded for the type of rendering, sphere or default point. When sphere is selected the rendering gets slower if many particles are loaded.
    • in FluidVisPlugin::Init the subscribes for the fluid particle positions are loaded.
    • in FluidVisPlugin::RenderAsPointsUpdate or FluidVisPlugin::RenderAsSpheresUpdate, (depending on the rendering type) if a new message with the particle positions is available, these will be rendered.
  • The fluid simulation engine FluidEngine.cu is similar to the SPH example from Fluidix.

Unfinished parts, TODOs:

If somebody is interested in further contributing to the package, many features still need work:

  • Implementing a newer SPH: PCISPH or IISPH for faster simulation and no compression of the fluid. This can be done in the src/FluidEngine.cu file by changing the algorithm.

  • Currently the simulation only has the box implemented as a standard shape, see FluidEngine::AddMovableBox from src/FluidEngine.cu, it is implemented similarly to this example. Using the same idea the rest of the collision types can be implemented as well: cylinder, sphere, plane. After implementation these need to be added in the FluidWorldPlugin::CreateFluidCollision method, similarly to the box type.

  • The force and torque interaction is done via the particles surface collisions, pressure force from the liquid is not taken into account. A fancier algorithm would greatly increase realism.