Modelling Shallow Water Correctly

Hi,

I'm a master student trying to use MuJoCo to design an amphibious robot. To simulate the water, my current methodology is to adaptively change the viscosity, density, and wind speed of the simulation based on the position of the robot on a hfield amphibious terrain (see bottom right of video for values).

This somewhat works however is not ideal/accurate as fluid forces are being applied across the entire body whereas in real life in amphibious terrain only half the body would be in water.

https://github.com/google-deepmind/mujoco/assets/125491092/410d9807-f11b-404a-b461-976a430c01dd

Is it possible to apply various wind forces on various points of the hfield to mimic top half being air and bottom half being water. Any other suggestions or approaches would be much appreciated.

def controller(model, data):

    global water_bounds
    global rho_a
    global beta_a
    global rho_w
    global beta_w
    global valley_height
    global fluid_vel
    global wind_vel
    global water_present

    if water_present:
        if valley_height <= water_bounds:
            model.opt.viscosity = beta_w #viscosity of water
            model.opt.density = rho_w #density of water
            model.opt.wind = fluid_vel #speed of water (given it changes viscosity of entire system, wind will increase the speed of the entire denser/more viscous water flow as if its water flowing)
        else:
            model.opt.viscosity = beta_a #viscosity of air
            model.opt.density = rho_a #density of air
            model.opt.wind = wind_vel #no wind
    else:
            model.opt.viscosity = beta_a #viscosity of air
            model.opt.density = rho_a #density of air
            model.opt.wind = wind_vel #no wind

<mujoco>

    <asset>   
      <mesh name = "chassis" file = "C:\Users\Chassis.stl"/>
      <mesh name = "wheel_1" file = "C:\Users\Wheel1_pl.stl"/>
      <mesh name = "wheel_2" file = "C:\Users\Wheel2_pl.stl"/>
      <mesh name = "wheel_3" file = "C:\Users\Wheel3_pl.stl"/>
      <mesh name = "wheel_4" file = "C:\Users\Wheel4_pl.stl"/>

      <texture name="grid" type="2d" builtin="checker" width="512" height="512" rgb2="0 0 0" rgb1="1 1 1"/>
      <material name="grid" texture="grid" texrepeat="2 2" texuniform="true" reflectance=".6"/>
    </asset>
    <default>
        <default class="chassis">
             <geom type="mesh" mesh="chassis" mass="2.279" rgba="01.0 0.549 0.0 1" fluidcoef=".5 .25 1.5 0 0" fluidshape="ellipsoid"/>
        </default>
        <default class="wheel">
            <geom fitscale="1.2" type="mesh" friction="2 0.005 0.0001" contype="1" conaffinity="0" mass="0.285" material="grid" fluidcoef=".5 .25 1.5 0 0" fluidshape="ellipsoid"/>

        </default>
    </default>
    <worldbody>
        <body name="chassis_body" pos="-4.5 0 1" euler="0 0 90">
            <joint type="free"/>
            <geom class="chassis"/>
            <site name="robot_marker" pos="0 0 0" size="0.025" type="sphere" rgba="1 1 1 0"/>  

            <body name="wheel1_body">
                <joint name="wheel1_joint" type="hinge" pos="0.25 0.04 0.02" axis="1 0 0" damping="0.05"/>
                <geom class="wheel" mesh="wheel_1"/>
            </body>

            <body name="wheel2_body">
                <joint name="wheel2_joint" type="hinge" pos="-0.05 0.04 0.02" axis="1 0 0" damping="0.05"/>
                <geom class="wheel" mesh="wheel_2"/>
            </body>

            <body name="wheel3_body">
                <joint name="wheel3_joint" type="hinge" pos="0.25 0.26 0.02" axis="1 0 0" damping="0.05"/>
                <geom class="wheel" mesh="wheel_3"/>
            </body>

            <body name="wheel4_body">
                <joint name="wheel4_joint" type="hinge" pos="-0.05 0.26 0.02"  axis="1 0 0" damping="0.05"/>
                <geom class="wheel" mesh="wheel_4"/>
            </body>
        </body>
    </worldbody>

    <actuator>
    <motor name="rl_wheel" joint="wheel3_joint" gear="1" ctrlrange="-1 1" forcerange="-1 1"/>
    <motor name="rr_wheel" joint="wheel4_joint" gear="1" ctrlrange="-1 1" forcerange="-1 1"/>
    <motor name="fl_wheel" joint="wheel1_joint" gear="1" ctrlrange="-1 1" forcerange="-1 1"/>
    <motor name="fr_wheel" joint="wheel2_joint" gear="1" ctrlrange="-1 1" forcerange="-1 1"/>
    </actuator>

    <sensor>
        <framepos name ="robot position" objtype="site" objname="robot_marker"/>
    </sensor>

</mujoco>

google-deepmind / mujoco

Modelling Shallow Water Correctly #1691