Input files specification

Rhoxyz, version 2020.3

File format

JavaScript Object Notation (JSON) open–standard file format, that uses human readable text in form of attribute–value pairs and array data types, is used as an input for the solver. Graphical setting up of simulations may be implemented, but JSON format is intuitive enough to make as rapid changes as graphical interface could.

The table describing data types used for parameters is given below:

Note: When a default value is available for some parameter, user does not need to specify the parameter (unless they want to override it).

Note: The document is an object, i.e. it must start with curly brackets ’{’ and ’}’. Also, the last element in a list or object must not have comma after its definition.

General parameters

Global parameters of the simulation are listed as follows:

Note: The modern GPU can execute simulations > 20 times faster than the modern CPU. This holds for typical GPUs optimized for 32-bit precision operations (GTX, RTX, etc.). If double precision is needed, GPUs like Quadro, Tesla, etc. will perform better.

Note: 32-bit precision is usually enough if fast approximated/engineering results are needed. For extra large simulations, the implementation of accurate computation independent of precision is a work-in-process.

Example: JSON file with some general parameters.

   {
       "description": "Sway sloshing experiment blah blah",
       "dimensions": 3,
       "precision": "single",
​
   ... other parameters go here ...
​
   }

Relaxation

Relaxation is tightly connected to the scheme and pressure parameters. It says how strong factors dictate the simulation convergence within the time step.

“relaxation”: {

}

Note: The scheme takes minimum value of space– and time–based relaxation. Space–based relaxation is preferable, since it’s less prone to induce oscillations.

Note: Diagonal scaling may help solvability of the system of equations, but yields a bit lower or smoothed out results than they should be.

Example:

   "relaxation": {
       "space": 0.9,
       "walls": 1.0
   }

Pressure

Smooth and accurate pressure field is obtained by solving a Poisson equation. The equation is transformed to a system of linear equations, which is solved by using an iterative solver.

“pressure”: {

}

Example:

   "pressure": {
       "solver": "bicgstab",
       "max_iterations": 300,
       "max_error": 1e-4,
       "gradient_limiter": 1.0
   }

Velocity

Velocity is calculated from the momentum equation. For convection-dominated flows, it may be calculated directly, by explicitly approximating the diffusion. For diffusion-dominated flows, the velocity field should be solved implicitly using an iterative solver.

“velocity”: {

}

Example:

   "velocity": {
       "solver": "jacobi",
       "max_iterations": 300,
       "max_error": 1e-4
   }

Resolution

This category specifies the spacing between nearest neighbors, and the interaction radius. Each point can be connected to only first ring of its neighbours (fast, less diffusive), but better stability may be achieved by enlarging the radius to reach second ring of its neighbours (slower, more diffusive).

“resolution”: {

}

Note: Currently the point cloud resolution is constant in space and time, adaptive refinement is work in progress.

Example:

   "resolution": {
       "spacing": 0.02,
       "truncation": 2.1
   }

Reordering

Each time step points perform Lagrangian advection, i.e. they move along solved streamlines. It can lead to distortion of the point distribution, and therefore mass conservation errors (imagine a rotating disc where points move in the tangential direction and increase the volume of the disc). On the Lagrangian movement, points are trying to keep equidistant neighbours to optimally conserve mass at all time.

“reordering”: {

}

External acceleration

Here we use term gravity as constant acceleration imposed to whole fluid in the domain, at all time. On the other hand, user may impose variable domain movement, which yields variable domain acceleration in time. This may be achieved by setting movement keyframes, or by analytically defining harmonic oscillator.

Note: During the simulation, gravity constant, keyframe motion, and oscillation motion are superposed.

The harmonic oscillator object is created as follows:

“impose_oscillation”: {

}

Note: Setting domain rotation through keyframes is work-in-progress.

Domain

The domain is an axis-aligned minimum bounding box (or AABB), defined by the starting and ending coordinates of its diagonal. It specifies the space where fluid flow is allowed. Currently, the domain size is static, dynamic adaptation of the domain size will be implemented.

“domain”: {

}

Note: Points leaving the domain boundaries are removed from the simulation.

Note: Performance will suffer if user specifies too large domain compared to the space of the fluid body.

Example:

   "domain": {
       "min": [0, 0, 1],
       "max": [5, 1, 3.2]
   }

Patches

The simulation input file does not incorporate any volumetric mesh, but only bounding surfaces (patches) of the domain and objects in the scene. These surfaces can be defined using primitives (list of connected line–segments, quadrilaterals) or imported from the commonly used mesh file formats. Patches can be moveable, i.e. their motions can be imposed similarly as domain motions.

Note: The input is an object patches containing any number of named boundary surfaces.

“patches:” {

“name_of_patch:” {

},

“another_patch: { … }”

}

Note: Generally, it’s not wise to use STL meshes (anywhere!) as they do not incorporate triangle connectivity information. The solver automatically connects triangles in such case.

Example:

   "patches": {
       "tank": {
           "type": "solid",
           "geometry": "polyline",
           "points": [
               [-0.3, 0.3], [-0.3, 0.0], [ 0.3, 0.0], [ 0.3, 0.3], [-0.3, 0.3]
           ],
           "oscillation": {
               "period": 1.5,
               "direction": [0.05, 0]
           },
           "probes": [
               [-0.3, 0.1],
               [0.3, 0.1]
           ]
       },
​
       "water": {
           "type": "free_surface",
           "geometry": "polyline",
           "points": [ [-1, 0.12], [1, 0.12] ]
       }
   }

Fluid

Currently a single fluid phase is supported, but soon multi-phase and variable-phase calculation will be implemented. The specified single fluid moves in the domain specified above. When the boundary surfaces are defined, user may fill the domain space with fluid.

Note: The fluid set-up follows the same as idea as bucket fill or flood fill tools in painting programs. Fluid is filled from some source point until it reaches wall and free-surface boundaries, defined above.

   "fluid": {
       "density": 998.6,
       "viscosity": 1.14e-6,
       "flood_point": [0.0, 0.05]
   }

Output

Besides real-time viewing of the results, the results may be written to hard drive to postprocess them in another software. Currently the results may be written to VTK legacy ASCII file–format, which can be read by ParaView.

Note: The folder for outputting is created where the input file is located.

Note: Along with output fields, the solver writes: log, forces & torques, pressure probes.

Example:

   "output": {
       "time_step": 0.2,
       "times": [0.1666, 1.53],
       "fields": ["velocity", "vorticity"]
   }