XML guide: definition of attributes of rigid objects

Would you please consider the following questions about the attributes of floating objects given in the XML guide (version April 2019, section 2.6)?

These attributes can be computed by GenCase or be custom-defined in the code. In order to specify the values correctly, I would need to know more about their definitions.

The attribute center is defined as the 'gravity center of the rigid object'.
  • Q1: Is this the 'centre of mass'? I presume so, this is just about matching terminologies.
  • Q2: Which reference system should the coordinates be given in? The global one (the domain, as from inside definition) or the local one (the object, as from inside a setmkbound for example)?
  • Q3: Reformulation of Q2. Say that I draw an object after rotating the global reference system (rotate x=...), and do not revert to the original system (no matrixreset) before moving on to the floatings specifications. Will the coordinates of "center" live in the original or in the modified reference system?
  • Q4: Linked to Q1. Does the code takes into account eccentricity when the centre of mass is not the centroid of the figure?
The attribute inertia is defined as 'momentum of inertia of the rigid object'. There is a slip of the pen since it should be 'moment of inertia'.
  • Q5: With respect to which axes are these moments of inertia?
  • Q6: Is the code informed about the principal moments of inertia of objects with a predefined shape such as drawbox, drawsphere?
  • Q7: Similar to Q3. If the object I drew is no longer aligned with the global reference system, which axes are considered for the moments of inertia?
Although I anticipate that the Project Chrono implementation clarifies these question, coding the old way may still be attractive for simple enough cases. So, in your answers, references to what the basic code does do and to what it does not do are equally valued.

Thanks in advance for dealing with this.

Comments

  • Some responses to your questions here:

    Q1: Is this the 'centre of mass'? I presume so, this is just about matching terminologies.
    YES

    Q2: Which reference system should the coordinates be given in? The global one (the domain, as from inside definition) or the local one (the object, as from inside a setmkbound for example)?
    Global ones of course, however this is easy to test to get a faster response ;)

    Q3: Reformulation of Q2. Say that I draw an object after rotating the global reference system (rotate x=...), and do not revert to the original system (no matrixreset) before moving on to the floatings specifications. Will the coordinates of "center" live in the original or in the modified reference system?
    Response in https://dual.sphysics.org/vanilla/discussion/1631/centre-of-mass-of-eccentric-objects-rotated-with-rotateaxis#latest

    Q4: Linked to Q1. Does the code takes into account eccentricity when the centre of mass is not the centroid of the figure?
    Not sure, let us check...

    The attribute inertia is defined as 'momentum of inertia of the rigid object'. There is a slip of the pen since it should be 'moment of inertia'.
    Yes, we have to correct this. Thanks

    Q5: With respect to which axes are these moments of inertia?
    X-axis, Y-axis and Z-axis

    Q6: Is the code informed about the principal moments of inertia of objects with a predefined shape such as drawbox, drawsphere?
    No need, since we use the general formulation to compute any figure, so that you can check that results for a box, for example, will converge to theoretical value with accuracy.

    Q7: Similar to Q3. If the object I drew is no longer aligned with the global reference system, which axes are considered for the moments of inertia?
    X-axis, Y-axis and Z-axis
  • edited June 13
    Thanks for following this up, first of all.

    Re Q2, you have been fast enough to make it worth waiting :-)

    Re Q5 and Q7, the givens for one object I have are the moments of inertia with respect to the principal axes of inertia, which always have the centre of mass as origin.

    Do I then understand it well that the correct procedure to input this object is
    1. to convert principal moments of inertia into those with respect to the lattice/global reference system;
    2. to give these latter moments as an attributes within ' floating'
    3. to let DualSPHysics calculate the moments of inertia wrt the global axes at each time step, in the line of the answer to Q6.
    This link to moments of inertia on wikipedia is for the sake of a common terminology. Corrections and integrations always welcome.
  • In order to compute the initial moment of inertia:
    a) you can allow GenCase to compute the value according to the moment of inertia of a set of points with mass
    b) you can define the values you know in the XML

    however, after this time=0, the code will compute the rotations using that moment of inertia, not sure what you mean with "DualSPHysics calculate the moments of inertia wrt the global axes at each time step"?
  • edited June 14
    Thanks Alex. I try to explain.

    From Q5/Q7 I understood that we impose the conservation of angular momentum with respect to the lattice/global axes at each time step. Logical.

    If the moments of inertia are referred to the principal axes of inertia, these are properties/attributes of the object and need not to be recomputed at each time step. However, as time goes by, the solver would need to convert these object attributes into the moments with respect of the global axes, based on the motion and rotation of the object, again to enforce the conservation of angular momentum around those global axes.

    If the moments of inertia are given relative to the global reference axes, and the object roams and rotates, the moments of inertia change at each time step. This because the distribution of masses with respect to those global axes has changed. And the code should take care of this. Now I understand that is the way DSPH works.

    This was my (perhaps confused/confusing) thinking behind "DualSPHysics calculate the moments of inertia wrt the global axes at each time step". But this topic comes closer to the other companion post https://dual.sphysics.org/vanilla/discussion/1631/centre-of-mass-of-eccentric-objects-rotated-with-rotateaxis , so I will continue it there.
Sign In or Register to comment.