Massless Tracers¶

Introduction¶

Massless tracer particles have no inertia, follow fluid streamlines, and are one-way coupled to the fluid. They are useful for visualizing flow and predicting residence times. They are also useful for characterizing system blend times.

As the name implies, massless tracers have no mass (e.g., no specified density or diameter). Unlike inertial particles, massless tracers cannot be two-way coupled to the fluid and do not participate in particle-particle interactions. However, they can be assigned particle variables and are used to investigate the system from the perspective of a moving fluid parcel.

Adding Massless Tracers¶

When adding massless tracers to a system, there are two factors to consider: (1) the number of particles entering the system, and (2) the location of the particles entering the system.

The number of particles entering the system is defined by an injection rate and an injection duration. Pay close attention to the appropriate number of particles added to the system as very large particle counts can slow performance. Various injection options are provided for defining these parameters.
The location of the particle injection is typically defined using an injection zone, an inlet boundary condition, or a background injection:
- A particle injection allows users to define custom injection regions with local injection rates, injection size distributions, and initial particle compositions that differ from those defined on the particle parent. Particle injections allow for differentiated particle characteristics within a particle family. Children geometry can be added to the injection to control where particles are added to the system.
- An inlet boundary condition represents the superposition of particles on the fluid field along the boundary condition surface. Boundary condition flow sweeps these particles from the initial positions along the boundary condition to other regions in the system.
- A background injection produces a uniform distribution of particles across the full fluid field. This addition, if applied at the start of a process, represents a system with an initially well-dispersed ensemble of particles.

Massless tracers can also enter the system through Eularian conversion and volumetric generation via a user-defined function. Eularian conversion is used to model fluid-to-fluid conversion processes, as realized during jet breakup, air entrainment, and two-fluid dispersion processes. Volumetric generation is used to model bubble nucleation, particle crystallization, and so forth.

Note

Use caution when adding more than a billion particles. System memory limits the total number of particles that can be tracked. On a single workstation, this limit is 50–100 million particles. On a GPU cluster, this limit is about 1 billion particles.

Property Grid¶

If a static body is present, the following section will launch:

`Static Body Interaction`¶

Static Body Option

This parameter specifies how each particle set interacts with each solid body family.

Bounce: The particles bounce off the solid body family.
Stick: The particles stick to the solid body family.
Pass Thru: The particles pass through the solid body family.

`Moving Body Interaction`¶

Moving Body Option

This parameter specifies how each particle set interacts with each moving body family.

Bounce: The particles bounce off the moving body family.
Pass Thru: The particles pass through the moving body family.

`Advanced`¶

Compute Nearest Neighbor Distribution

Computes the average nearest neighbor separation distance and nearest neighbor separation distance probability distribution function. See additional discussion in Theory: Nearest Neighbor Distribution. The Particle Output Data page describes the output for this option.

On: Particle distribution data are computed.
Off: Particle distribution data are not computed.

Compute Pressure Gradient Force

When this option is active, the pressure gradient force on the particle is calculated. This force acts from regions of higher pressure toward regions of lower pressure, influencing the motion of particles immersed in the fluid.

For a small particle within a fluid, the pressure gradient force can be expressed as:

\[F_p = - V_p \nabla P\]

where \(V_p\) is the volume of the particle and \(\nabla P\) is the pressure gradient and the location of the particle.

Off: Does not compute pressure gradient force.
On: Computes pressure gradient force.

Enable Removal UDF

Enable removal user-defined function.

Off

Removal UDF not enabled.

On

Removal UDF enabled.

Removal UDF: no units | This UDF removes particles based on particle properties or local fluid conditions. One output must defined within the UDF: a Boolean variable named remove. This output value determines if a particle is to be removed from the simulation. If set to true, the particle is removed. This is a local UDF, calculated on a particle-by-particle basis using the local particle/fluid properties.

Download Sample File: Removal

If Free Surface or Immiscible Two-Fluid models are employed, the following option will launch.

Particle Interface Stiffness Factor: This parameter describes how aggressively particles will be pushed away from the free surface or immiscible two fluid interface. It mimics the effects of surface tension on particle dynamics and trapping at the fluid interface. The default value is one. Higher values imply stiffer interfaces. A value of zero implies that the particles can pass through the interface.

Massless Tracer Output Data¶

Particle output data is summarized on the Particle Output Data page. These outputs include individual particle motion and properties, ensemble statistics, spatial fields constructed from the local particle population, as well as particle exit statistics.