Particle Tracing

Use a plot to visualize the trajectory of a massless particle subject to a flow field in 2D (

) or 3D (

). Visualize pathlines (that is, trajectories of particles released in a flow field), which can be time-dependent or static. For time-dependent flows, also use a snapshot in time of the flow field as a static field. The motion of the particles does not affect the flow field. Add a Color Expression, Deformation, Translation, or Transparency (3D only) subnode as needed. Right-click a or to add these plot types from the submenu.


	Go to Common Results Node Settings for links to information about these sections: Data, Expression, Title, Coloring and Style, Quality (Resolution and Recover only), and Inherit Style. See below for sections specific to this plot: Particle Positioning, Release, Quality (ODE solver settings), and Advanced. For Particle Tracing plots, only Solution datasets are allowed as inputs.


	There is an additional setting under Coloring and Style for this plot. The Type of Point Style available includes Comet tail. Comet tail plots provide a convenient way to indicate the direction of travel of particles at a given point in time. The tail of the comet typically points in the opposite direction to the particle velocity — so visually, it is the same as the tail of a comet approaching the sun. See Defining the Coloring and Style for the Comet tail settings.


	To use the Particle Tracing plot type, first select the All Plot Types check box in the Show More Options dialog box. This plot type is intended for visualizing a small number of particles on simple geometries. The Particle Tracing Module has vastly superior particle tracing capabilities and should be used for all but the simplest of models.

Particle Positioning

For 3D models, enter the initial position of particles in the , ,and fields. For 2D models, enter the details as described below.

Select a : or . Boundary coordinates are useful, for example, for flow models with one or more inflow boundaries.

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If is selected, enter the initial position of particles in the and fields. The initial position can be defined as numbers but can also include parameters and variables.

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If is selected, select an item from the list and select an : or .

If is selected, enter the number of grid (the default is 10).

If is selected, enter the .

Release

Under , specify when to : (the default), , or . Select:

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to release particles once at the first available time, typically at time 0 (zero). To delay the release, select the check box and enter a time.

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to release particles at regular intervals starting at the first available time, typically at time 0 (zero). To delay the release, select the check box and enter a time. Enter a . The default is 1.

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to release particles at an arbitrary time point; enter multiple to release particles.

Quality (ODE Solver Settings)

Under , also define the as needed. Go to ODE Solver Settings — Relative Tolerance, ODE Solver Settings — Absolute Tolerance, and ODE Solver Settings — Step Size for details.

Advanced


	The Advanced section contains settings that do not normally need to be adjusted.

Under , also define these settings as needed. Go to Advanced — Termination and Advanced — Instantaneous Flow Field for details.

In the section, edit the and In the section, edit the check box, the default, and the .

ODE Solver Settings — Relative Tolerance

Follow these supplementary instructions for the section found under the section.

Enter a for the ODE solver. The default is 0.001.

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When solving the second-order ODE

for x, the solver first rewrites it as two coupled first-order ODEs: one for the position x and one for the velocity

, each with two components in 2D and three components in 3D.

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The value is the relative error tolerance that the ODE solver uses. It applies to all components of the particle’s position and velocity. The solver controls the step size so that the estimated error e in each integration step satisfies

where rtol is the relative tolerance specified, atolpos is the absolute tolerance for the particle’s position components, and atolvel equals the absolute tolerance for the particle’s velocity components.

ODE Solver Settings — Absolute Tolerance

Follow these supplementary instructions for the section found under the section. Specify the solver’s absolute tolerance. The default is . To enter different values, select from the list and enter a . The field can contain a single value — it applies to all components of the position and is the absolute tolerance.

ODE Solver Settings — Step Size

Follow these supplementary instructions for the section found under the section. Specify the solver The default is — the COMSOL Multiphysics software uses the initial value of the acceleration (force divided by mass) and the relative and absolute tolerances to determine the initial time step.

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The automatic maximum step size is 10% of the total simulation time for time-dependent flows as well as for static flow fields where the end time is manually specified in the section (in the field). For static flow fields where the end time is not set manually, there is no upper limit of the step size. However, in this case, the initial time step is less than or equal to 0.1.

To edit the settings, select from the list and enter values in the and fields.

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The is the longest time step the solver takes. It has higher priority than the ; that is, if an initial step size is set larger than the maximum step size, the solver lowers the initial step size to the maximum step size.


	The initial step size, whether entered manually or computed automatically, is not necessarily the first step the solver takes but is a first try. If this step leads to an error such that the tolerances are not met, the COMSOL software lowers it.

Advanced — Termination

Follow these supplementary instructions for the section found under the section. The section contains settings that determine when to end the particle tracing simulation.

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To specify an upper limit of the number of time steps, click to select the check box and edit the default (1000). The particle simulation ends after this number of steps.

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To specify how close to the geometry boundary the pathlines are cut when they exit the geometry, edit the default (0.001). This is a relative tolerance controlling how close to the geometry boundary the pathlines are cut when they exit the geometry. A lower value cuts the line closer to the geometry boundary.

Advanced — Instantaneous Flow Field

Follow these supplementary instructions for the section found under the section. To specify if you want to plot an instantaneous flow field, even if the solution is time dependent, select the check box. This freezes the time selected previously — for example, from a page in the list — to the value specified and considers this a static flow field.

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Edit the default (partt) if required. Normally it is not necessary to change the default name but the name can be used in expressions as well as for the color when coloring the pathlines according to an expression.

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If required, select the check box and enter a value.