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 children geometry, an injection zone, an inlet boundary condition, or a background injection:
- A child geometry injection represents the superposition of particles on the fluid field within a sub-volume of the fluid domain. Fluid flow sweeps these particles from the initial positions within the child geometry to other regions in the system. The particle properties, such as initial size distribution, composition, and injection rate, are all inherited from the parent. All children inherit the same properties.
- An injection zone 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 injection zones allow for differentiated particle injection characteristics within a particle family. Children geometry can be added to the injection zone to control where localize where particles associated with the injection zone 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.

When using a child geometry injection, users have three options for adding geometry to the particle parent: (1) via the Add Geometry form, which opens automatically when a new particle parent is created; (2) by importing geometry through the Assembly Explorer; or (3) by importing geometry from a spreadsheet. See the Geometry page for more details.

For an already-existing parent component, you can add children via the Add Geometry command found on the Context Specific Toolbar. Children geometry can also be extracted from an assembly file.

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¶

`General`¶

This option controls the type of particle injections which will be added to the system. Consider both the number and the location of the particles when adding particles to the system. The injection options are listed below and discussed in greater detail in the injection options overview.

Injection Option

This setting contains the different selections for introducing particles into a system. The number of particles added to the system is determined by an injection rate and an injection duration.

Dump: A single impulse addition of particles into the injection geometry. The dump defines the total number of particles added to the system at a single user-defined time.
Feed: A continuous feed of particles into the injection geometry at a user-defined number flow rate over a user-defined period. The feed rate can vary per user-defined expression.
No Injection: The particle feed rate is not directly controlled by the user. Particles are dynamically generated across the system via free-surface Eularian-Lagrangian conversion, immiscible two fluid Eularian-Lagrangian conversion, or volumetric particle generation.

`Volumetric Generation`¶

This functionality is used to dynamically generate particles across the fluid domain according to user-defined generation criteria. The generation criteria are applied on a voxel-by-voxel basis and can be a function of local fluid properties and/or system-level variables. Volumetric generation is typically used to model nucleation processes.

Enabled

This enables volumetric generation based on local fluid conditions.

On

Volumetric generation enabled.

Interval: The time interval at which the generation criteria are tested. Shortening this interval increases the frequency at which the injection criteria are tested.
Code: The user-defined generation criteria which define particle injection.

Off

No volumetric generation.

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.

`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.

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

The following option will launch only with Free Surface and Immiscible Two Fluid models.

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.