Bug: Drone can’t stay in a closed tunnel shape like a doughnut

[AI4IS]

Hi there,
I created a tunnel model (.stl) that is hollow inside. I import it into urdf. I set the collision attribute same as vision attribute (both that stl file) but the drone just can’t stay inside the tunnel and bounce away. If I comment collision attribute of tunnel and set mass to zero, the drone can stay inside. So I guess pybullet can’t deal with hollow objects’ collision. Is there anyone figure it out? I need your help, plz

Tunnel urdf here:

<?xml version="1.0"?>
<robot name="tubehigh">
    <link name="tube">
        <visual>
            <geometry>
                <mesh filename="tunnel3.stl" scale="1 1 1"/>
            </geometry>
            <material name="red">
                <color rgba="1.0 0.0 0.0 1.0"/>
            </material>
        </visual>
        <collision>
            <geometry>
                <mesh filename="tunnel3.stl" scale="1 1 1"/>
            </geometry>
        </collision>
        <inertial>
            <mass value="1.0"/>
            <inertia ixx="0 " ixy="0" ixz="0" iyy="0" iyz="0" izz="0"/>
        </inertial>
    </link>
</robot>

Drone urdf here:

<?xml version="1.0" ?>

<robot name="cf2">

  <properties arm="0.0397" kf="3.16e-10" km="7.94e-12" thrust2weight="2.25" max_speed_kmh="30" gnd_eff_coeff="11.36859" prop_radius="2.31348e-2" drag_coeff_xy="9.1785e-7" drag_coeff_z="10.311e-7" dw_coeff_1="2267.18" dw_coeff_2=".16" dw_coeff_3="-.11" />

  <link name="base_link"> 

    <inertial>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <mass value="0.027"/>
      <inertia ixx="1.4e-5" ixy="0.0" ixz="0.0" iyy="1.4e-5" iyz="0.0" izz="2.17e-5"/>
    </inertial>

    <!-- links>
      <carlos url="https://arxiv.org/pdf/1608.05786.pdf" />
      <julian url="https://www.research-collection.ethz.ch/handle/20.500.11850/214143" />
      <mit url="http://groups.csail.mit.edu/robotics-center/public_papers/Landry15.pdf" />
    </links -->

    <visual>
      <origin rpy="0 0 55" xyz="0 0 0"/>
      <geometry>
        <mesh filename="./cf2.dae" scale=" 1 1 1"/>
      </geometry> 
      <material name="grey">
        <color rgba=".5 .5 .5 1"/>
      </material> 
    </visual>

    <collision>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <geometry>
        <cylinder radius=".06" length=".025"/>
      </geometry>
    </collision>  

  </link>

  <link name="prop0_link">
    <inertial>
      <origin rpy="0 0 0" xyz="0.028 -0.028 0"/>
      <mass value="0"/>
      <inertia ixx="0" ixy="0" ixz="0" iyy="0" iyz="0" izz="0"/>
    </inertial>
  </link>
  <joint name="prop0_joint" type="fixed">
    <parent link="base_link"/>
    <child link="prop0_link"/>
  </joint>

  <link name="prop1_link">
    <inertial>
      <origin rpy="0 0 0" xyz="-0.028 -0.028 0"/>
      <mass value="0"/>
      <inertia ixx="0" ixy="0" ixz="0" iyy="0" iyz="0" izz="0"/>
    </inertial>
  </link>
  <joint name="prop1_joint" type="fixed">
    <parent link="base_link"/>
    <child link="prop1_link"/>
  </joint>

  <link name="prop2_link">
    <inertial>
      <origin rpy="0 0 0" xyz="-0.028 0.028 0"/>
      <mass value="0"/>
      <inertia ixx="0" ixy="0" ixz="0" iyy="0" iyz="0" izz="0"/>
    </inertial>
  </link>
  <joint name="prop2_joint" type="fixed">
    <parent link="base_link"/>
    <child link="prop2_link"/>
  </joint>

  <link name="prop3_link">
    <inertial>
      <origin rpy="0 0 0" xyz="0.028 0.028 0"/>
      <mass value="0"/>
      <inertia ixx="0" ixy="0" ixz="0" iyy="0" iyz="0" izz="0"/>
    </inertial>
  </link>
  <joint name="prop3_joint" type="fixed">
    <parent link="base_link"/>
    <child link="prop3_link"/>
  </joint>

  <link name="center_of_mass_link">
    <inertial>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <mass value="0"/>
      <inertia ixx="0" ixy="0" ixz="0" iyy="0" iyz="0" izz="0"/>
    </inertial>
  </link>
  <joint name="center_of_mass_joint" type="fixed">
    <parent link="base_link"/>
    <child link="center_of_mass_link"/>
  </joint>

</robot>

[AI4IS]

I use vhacd in pybullet, but it seems that vhacd can't decompose the object correctly for my doughnut, some part of it is not hollow.

[AI4IS]

assets.zip
These are my .obj and _vhacd.obj files.