hydrodynamic_params_tutorial

Adjusting the Hydrodynamic Model

• The hydrodynamics models we use for the default USV is implemented in the Simple Hydrodynamics plugin
• The coefficients of this plugin characterize the linear and nonlinear drag on the WAM-V
• The buoyancy model we use for our default USV is implemented via our Surface plugin
• This simulates buoyancy of objects at the surface of a fluid.
• We attach an instance to each of Surface plugin to each hull, and an instance of the Simple Hydrodynamics plugin to the whole boat (wamv_gazebo_dynamics_plugin.xacro).

Steps

Step 1: Modify parameters

For the Surface plugin:

<plugin filename="libSurface.so" name="vrx::Surface">
 <link_name>${namespace}/base_link</link_name>
 <hull_length>4.9</hull_length>
 <hull_radius>0.213</hull_radius>
 <fluid_level>0</fluid_level>
 <points>
    <point>0.6 1.03 0</point>
    <point>-1.4 1.03 0</point>
  </points>
  <wavefield>
    <topic>/vrx/wavefield/parameters</topic>
  </wavefield>
</plugin>

This is the attached plugin for a single hull boat, where the hull is simplified as a cylinder defined by hull_length and hull_radius.

For the WAM-V, try decreasing hull_length by half.

It's worth noting that the plugin assumes a cylindrical hull by default, but it is possible to override this behavior.

Step 2: Rebuild

Recompile your workspace:

cd <VRX_WS>
GZ_VERSION=garden 
colcon build --merge-install

Step 3: Test

Launch the simulation:

ros2 launch vrx_gz competition.launch.py world:=sydney_regatta

Zoom in to your WAM-V. If you only adjusted the hull length for one of the hulls, the WAM-V should appear lopsided. If you adjusted it for both hulls, the WAM-V should be sitting lower down in the water.

Step 4:

Return modified WAM-V Surface plugin variables to their original values.

For the Simple Hydrodynamics plugin

<plugin
  filename="libSimpleHydrodynamics.so"
  name="vrx::SimpleHydrodynamics">
  <link_name>${namespace}/base_link</link_name>
  <!-- Added mass -->
  <xDotU>0.0</xDotU>
  <yDotV>0.0</yDotV>
  <nDotR>0.0</nDotR>
  <!-- Linear and quadratic drag -->
  <xU>100.0</xU>
  <xUU>150.0</xUU>
  <yV>100.0</yV>
  <yVV>100.0</yVV>
  <zW>500.0</zW>
  <kP>300.0</kP>
  <kPP>600.0</kPP>
  <mQ>900.0</mQ>
  <mQQ>900.0</mQQ>
  <nR>800.0</nR>
  <nRR>800.0</nRR>
</plugin>

xU and xUU are the linear and nonlinear drag terms, respectively, in the surge direction. Increase the value of each variable to the point of being able to see a visible change in the simulation (muliplying by two seems like a good start). Then, rebuild following Steps 2 & 3.

The WAM-V's driving characteristics should have changed noticeably. If you increased the values as suggested, the WAM-V should seem slower in the surge direction.

For a deeper dive into the effect each coefficient has, this FAU Publication https://doi.org/10.1016/j.oceaneng.2016.09.037 gives a good overview.