Pizza3 POST Documentation Index
Select a POST example from the left panel to view its documentation. You can have access to the POST tools documentation here.
Version: Pizza3 v.1.006
Maintained by: INRAE\olivier.vitrac@agroparistech.fr
POST_example1
The workshop is organized into two interconnected parts to demonstrate how spatial and temporal information can be reconstructed from the dynamics of multi-particle systems, as utilized in Pizza3. The workshop focuses on managing large data files generated from MD-like simulations that contain atomistic information such as positions, velocities, forces, and more. Through specific post-treatment, these details are converted into meaningful time or spatial averages or fluctuating quantities.
Part 1: Temporal Information Extraction
Objective: To extract temporal information from large dump files (5-100 GB) generated by LAMMPS that are beyond the processing capacity of conventional computers.
Applied Physics: The section emphasizes simple physical concepts, aiming to illustrate how scalar quantities can be retrieved and plotted with low computational cost.
Key Learnings:
- Preprocessing of large files.
- Analysis of temporal evolution for selected particles.
- Study of solid obstacle dynamics (including position and movement, shape deformation, and moment of inertia analysis).
Part 2: Spatial Reconstruction and Field Analysis
Objective: Utilizing the results from Part 1, this section aims to reconstruct various fields (velocity, stresses) through the definitions of underlying physics and kernel interpolation.
Key Learnings:
- First description of fluid-solid interactions.
- Visualization techniques.
- Overview of possible extensions.
Overall, the workshop provides hands-on experience and detailed understanding of translating complex particle dynamics into meaningful physical insights, with an emphasis on efficient computational methodologies.
POST_example2
The second part of the worshop will demonstrate how to interpret simulated data in space by projecting them on stuctured or unstructured grids, and to extract stresses.
The visualization techniques and use of the Verlet list are more sophisticated than in part 1. Computations are more expensive and require understanding of vectorial algebra.
The approaches are illustrated on more realistic data.
POST_example3
This extension of the Part2 discusses advanced shear-stress details beyond those presented in Part2. It uses in particular the last features introduced recently: local vrial stress tensor calculated along with Landshoff and Hertz forces and Cauchy stresses reconstructed from projected forces on a Cartesian grid.
The main purpose of this document is to encourage a common understanding how the Landshoff forces can be equilibrated with Hertz contact forces and lead to a steady state. The physical origin and the mathematical formalism of theses forces are very different in nature and emerge from large simulations due to the transfer of momentum mediated between all these forces.
The main important equation: how Hertz contacts can develop a force which give sense of friction at wall and equates the shear stress in the flow regardless of the rigidity of Hertz contacts? There is a possible direct validation by noting that the The viscous tensor component is associated to the flow (x) can be defined independently from forces:
In MD-like simulations, a similar stress can be derived from the virial stress tensor as:
where V is the volume, 
is the α-component of the distance vector between particles i, j , and 
is the β-component of the force between particles i and j.
is the α-component of the distance vector between particles i, j , and is the β-component of the force between particles i and j.For a validation based on example2 results, the most relevant components are those relating to shear and normal forces, specifically 
and 
.
and . - corresponds to the shear effects and should be closely related to the SPH-based shear stress
in the fluid. This component would be a primary point of comparison. - captures the effects of the Hertzian contacts along the y-direction (i.e., the direction opposite to which the wall is moving). In the Hertz contact model, the "macroscopic" normal force is acting along the y-direction, which contributes to this component.
Therefore, based on the equality of mechanical states between the fluid and the wall, we should expect:
Contact: INRAE\olivier.vitrac@agroparistech.fr
Documentation Created on: 2025-02-20 21:47:30