## Particle Tracing Module Updates

For users of the Particle Tracing Module, COMSOL Multiphysics^{®} version 5.5 brings new virtual mass and pressure gradient forces for the *Particle Tracing for Fluid Flow* interface, improved acoustophoretic force functionality, and more ways to release particles with a size distribution. Learn more about these new features in the Particle Tracing Module below.

### Faster Particle Tracing with Coupled Fields

Some particle tracing models will run significantly faster in version 5.5 than in previous versions. The reduction in computation time is most apparent if any forces exerted on the particles are based on a field that is solved for in a previous study. Typical examples include an electric force modeled with the *Electrostatics* interface or a fluid velocity field modeled with the *Laminar Flow* interface. The speedup is especially noticeable when coupling to a field that was computed using a very fine mesh.

Here are some Application Library examples that show a significant speedup in solving for the particle trajectories, compared to the previous version:

- dielectrophoretic_separation: 69% reduction in time.
- einzel_lens: 74% reduction in time.
- quadrupole_mass_spectrometer: 45% reduction in time.

### Virtual Mass and Pressure Gradient Forces

When you include a *Drag Force* in the *Particle Tracing for Fluid Flow* interface, you now have the option to add two new force contributions, the virtual mass force and the pressure gradient force. The virtual mass term (sometimes called the added mass term) represents the acceleration of the fluid as it occupies the empty space that a moving particle leaves behind, which can make the particle's inertial mass seem greater than its actual mass. These two forces are most significant when the density of the particles is comparable to or less than the density of the surrounding fluid.

*Drag Force settings*

*Settings*window for the

*Drag Force*, with the virtual mass and pressure gradient forces included via a check box.

Application Library examples that use this new functionality:

### Improved Acoustophoretic Force

The *Acoustophoretic Force* feature has been renamed *Acoustophoretic Radiation Force*. This feature has new force expressions that are more accurate, because they account for the viscous and thermal boundary layers that form around particles in an acoustic pressure field. You can now specify whether the particles are solid or liquid. Then you can choose a *Thermodynamic loss model*: *Ideal*, *Viscous*, or *Thermoviscous*.

### Particle Size Distributions

In COMSOL Multiphysics^{®} version 5.5, there is a new option to specify the particle size distribution (PSD) in the *Particle Tracing for Fluid Flow* interface. You can release particles with a diameter distribution, just as you could previously release them with a mass distribution. In addition, when releasing according to a lognormal distribution, you can choose what type of mean and standard deviation you enter.

*Particle Tracing for Fluid Flow interface settings*Particles can have uniform size, or you can sample the mass or diameter from a distribution.

### Preview Grid Release Positions

When you release particles from a grid of points using the *Release from Grid* feature, you can now preview the initial particle positions in the *Graphics* window. In the *Initial Coordinates* section of the *Settings* window, click the *Preview Initial Coordinates* button to view the initial particle coordinates as a grid of points. Click the *Preview Initial Extents* button to view the spatial extents of the initial coordinates as a bounding box. These buttons allow you to check the initial particle positions before running a study.

In addition, when you right-click a *Study* node and click *Get Initial Value*, you can preview the initial particle positions and velocities for all release types.

*Initial particle coordinates viewed as a point grid*

*Graphics*window after clicking the

*Preview Initial Coordinates*button.

*Initial particle coordinates' spatial extents*

*Graphics*window after clicking the

*Preview Initial Extents*button.

### Isotropic Scattering Wall Condition

You can now select *Isotropic scattering* as the wall condition when particles hit boundaries in the geometry. Like the *Diffuse scattering* condition, the *Isotropic scattering* condition causes particles to be reflected with randomly sampled velocity directions around the surface normal. However, whereas the *Diffuse scattering* condition uses a probability distribution based on the cosine law, the *Isotropic scattering* condition follows a probability distribution that gives equal flux across any differential solid angle in the hemisphere.

*Scattering wall condition comparison*Comparison of the diffuse (left) and isotropic (right) scattering wall conditions. Each side shows a distribution of 1000 particles.

### New Options for Secondary Particle Emission at Walls

When secondary particles are released during a particle–wall interaction, you now have additional options for controlling the initial speed of the released secondary particles. In addition to selecting an initial velocity, which lets you sample the particle velocity direction isotropically or according to the cosine law, you can also choose to initialize the particle speed so that it is equal to the incident particle speed, proportional to the incident particle speed, or user defined.

### Built-In Species for Charged Particle Tracing

In the *Charged Particle Tracing* interface, you can now select from a number of built-in species when specifying particle properties. The mass and charge number are then automatically assigned based on the species you choose. Alternatively, you can select *User Defined* and enter values for the mass and charge number directly.

*Selecting particle species in the Charged Particle Tracing interface.*Screenshot of the

*Particle Properties*node for the

*Charged Particle Tracing*interface. As the equation display indicates, this screenshot is taken from an example with relativistic particle tracing.

Application Library examples that use this new functionality:

- childs_law_benchmark
- electron_beam_divergence
- electron_beam_divergence_relativistic
- ion_range_benchmark
- magnetic_lens
- planar_diode
- trapped_protons

### Particle Charging for Fluid Flow

In the *Particle Tracing for Fluid Flow* interface, you can can now add a *Charge Accumulation* feature that models the charging of the particles due to a space charge density in the surrounding fluid. Several different charging models are available to cover the entire range of expected Knudsen numbers: *Lawless*, *Classical diffusion*, *Classical field*, *Classical diffusion and field*, and *White*.

### New Tools for Modeling Electrostatic Precipitators

A new physics interface, *Corona Discharge*, uses an approximate method to compute the charge distribution, making it easier to solve than a fully self-consistent plasma model. A new *Charge Accumulation* feature is available in the Particle Tracing Module that computes the accumulation of charge on particles as they travel through a fluid with nonzero space charge density. These tools can be used to efficiently model electrostatic precipitators. Note that this functionality requires both the Particle Tracing Module and the Plasma Module. You can see these new features in the Electrostatic Precipitator and Positive and Negative Corona Discharges models.