Streamline

Use a plot in 2D (

) or 3D (

) to visualize a vector quantity. A streamline is a curve everywhere tangent to an instantaneous vector field. 3D streamline plot is analogous to the 2D streamlineplot except that there is no height data setting and the starting point selection is different. In 3D, you can also use a Streamline Surface plot to plot streamlines on 3D surfaces. Add Deformation, Color Expression, Export Expressions, Filter, or Selection (Plot Attribute) subnodes as needed. Right-click a or to add these plots.


	Go to Common Results Node Settings for links to information about these sections: Data, Expression, Title, Quality, and Inherit Style.


	To get the length of the streamlines, use an Export>Plot node. In its Settings window, select the Only export starting points and endpoints check box to include one row with the starting point, the endpoint, and the length of the streamline for each streamline.

Streamline Positioning

Select one of these options from the list: (the default), , , or . Then follow one of the methods described:

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Method 1: Specifying the Number of Streamlines and Start Boundaries

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Method 2: Specifying Points by Entering Coordinates

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Method 3: Selecting the Specified Number of Starting Points in the Geometry

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Method 4: Creating Streamlines with Uniform Density

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Method 5: Creating Streamlines with Variable Density and Magnitude Controlled

Selection


	The Selection section is available for some data sets when you select On selected boundaries from the Positioning list under Streamline Positioning.

Select the boundaries from which the streamlines start. By selecting in the window and using the tools in the section, select the boundaries for the starting positions for the streamlines.

Color and Style

In this section you can control the style and color of the streamlines and add arrows to them, if desired.

Line Style

Under , choose , (the default), or from the list to determine the type of streamlines. If you choose , no streamlines appear, but you can plot arrows that are tangential to the streamlines.

Arrow Style

Under , you can add arrows and control their appearance.

From the list, choose (the default) for no arrows, or choose for arrows that are tangential to the streamlines. If you choose , these additional settings are available:

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From the list, choose (the default) to distribute the arrows uniformly over the streamlines’ arc length, to distribute the arrows using the weight function dt/darc, or to distribute the arrows using the weight function darc/dt.

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Select the check box to enter a number for the total number of arrows, on all streamlines, that are plotted. By default, the COMSOL Multiphysics software provides a reasonable number of arrows.

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From the list, choose (the default), , or .

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From the list, choose (the default), , or to make the arrows’ sizes depend on the magnitude of the plotted quantity, if desired. If you choose , the length of the arrows is proportional to the natural logarithm of the magnitude of the quantity they represent. This makes arrows representing small values relatively larger. The value in the field (default: 100) determines the ratio between the smallest and largest values in the range of values for the logarithmic arrow length.

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Use the slider, or select the check box and enter a scale factor in the associated text field if you want to use another scaling than the one used by default.

From the list, choose a color for the streamlines and arrows. Choose to add a custom color using a color palette.

Advanced

Define the following advanced streamline settings as needed.

Advanced Settings for the Streamline Plot

Under , set these general settings. See also Advanced Section Setting Effects.

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The field default is 0.01 for 3D and 0.001 for 2D. Edit to specify how accurately streamlines are computed.

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The field makes it possible to control the length of streamlines. Edit the default (Inf) to control the streamlines’ length. Enter the value as a fraction of the mean bounding box’s size. When the check box is selected (the default), the maximum length refers to the sum of the lengths of the forward and backward parts.

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The field makes sure that the integration does not continue indefinitely. Edit the default (5000) to control when the computation stops.

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The field sets an upper time limit for the integration. The default is infinity (inf).

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The can be adjusted to make sure the integration stops near a stationary point in the field. The default is 0.01.

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The field default is 0.01. This is a fraction of the mean of the lengths of the bounding box of the geometry. If a streamline gets closer to its starting point than this distance, the streamline snaps to its starting point and is plotted as a connected loop. See also Method 5: Creating Streamlines with Variable Density and Magnitude Controlled.

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Select the check box to integrate points from the starting points both in the direction of the vector field and in the opposite direction. This check box is selected by default.

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Select the check box if required. The vector field is normalized pointwise: For each point where the field was evaluated, the vector is replaced by a unit vector in the same direction. If you apply normalization, the speed along the streamline changes. This change means that the other settings in the section (for example, maximum number of integration steps and maximum integration time) are interpreted differently.

Streamline Positioning Section (Continued)

Method 1: Specifying the Number of Streamlines and Start Boundaries

Under , from the list, select .


	The Selection section is made available for some data sets when On selected boundaries is selected from the Positioning list under Streamline Positioning.

Under , select the boundaries from which the streamlines start. By selecting in the window and using the tools in the section, select the boundaries for the starting positions for the streamlines.

Enter the of streamlines (the default is 20). This number is a suggestion for how many streamlines are generated, but there is no guarantee that you get exactly the specified number of streamlines. The reason is that the streamline starting points are placed in a regular grid on the selected boundaries.

Method 2: Specifying Points by Entering Coordinates

Under , from the list, select .

Select from the list.

Enter and (2D) or , , and (3D) coordinates (SI unit: m). Also use a scalar value to represent a fixed value for some of the coordinates.

Method 3: Selecting the Specified Number of Starting Points in the Geometry

Under , from the list, select .

Select from the list.

Enter the number of (the default is 20).

From the , select . The starting points are then distributed semirandomly but deterministically. You can also choose a or data set, if applicable, to restrict the streamline start positions to a parametric curve or surface.

Method 4: Creating Streamlines with Uniform Density

The algorithm saturates the entire domain with evenly spaced streamlines.

Under , from the list, select .

Enter the between the streamlines (the default is 0.05).

The value for the separating distance is a fraction of the mean of the lengths of the bounding box of the geometry. In this case, a streamline stops whenever it gets too close to another streamline or itself (or if any of the general termination criteria specified in the section is fulfilled).

The list defaults to . If required, select to edit these parameters: , , , , or .

Edit the if streamlines do not behave as expected near boundaries on a coarse mesh — try increasing this number. It is a measurement of the density of points on the boundaries used to set up the structure and is used to measure distances between streamlines. Refining the mesh in the problematic area can also resolve the problem.

Edit the value in the field (a fraction 0–1; default value: 0.5) when a streamline is close to itself, typically for spiraling streamlines. This number controls how big part of the streamline, starting from its starting point, that the streamline itself is allowed to get close to, and it might in some cases be useful in order to get a less cluttered streamline plot.

The is a factor multiplied with the distance specified in the field (as a fraction of the mean of the lengths of the bounding box of the geometry — the default value is 0.05). It sets the minimum distance between streamlines and the starting point for the next streamline.

When the domain is close to be saturated with streamlines, new starting points tend to be positioned where the streamline has nowhere to go before it gets too close other streamlines, resulting in short streamlines. The higher the value of this factor, the more it disqualifies the starting point and thus reduces the number of short streamlines.

The is a factor multiplied with the distance specified in the field. It sets the minimum distance between any pair of streamlines. Thus, this distance is the minimal distance under which the integration of a streamline stops.

By default the list defaults to , and it sets the starting point for the first streamline. It is selected in the element where the highest value of the velocity of the specified vector field occurs. If required, select instead to override the default and enter and coordinates.

Method 5: Creating Streamlines with Variable Density and Magnitude Controlled

To create streamlines with a variable density according to the magnitude of the specified vector field:

Under , from the list, select .

The setting gives proper streamline plots only for incompressible flow fields. In this case, the algorithm places the streamlines so that the flow between each pair of adjacent streamlines is the same throughout the domain, giving streamlines that are more dense where the magnitude of the field is high.

This step depends if it is a 2D or 3D Component.


	For 2D models, enter a Density (the default is 20). This value is roughly the number of streamlines. Prior to streamline generation, the software computes a rough estimate of the total flow of the flow field in the model, divides this value with the specified Density setting, and uses the resulting value as the flow between each pair of adjacent streamlines.


	For 3D models, enter the Min (minimum) distance and Max (maximum) distance between streamlines (the default Min distance is 0.05 and the default Max distance is 0.15). These distances are specified as fractions of the mean of the lengths of the bounding box of the geometry. The minimum velocity in the model is mapped to the minimum distance and the maximum velocity to the maximum distance. Thus every point on a streamline and on the boundary has a separating distance associated with it. Given a set of streamlines, the starting point for the next streamline is selected using these separating distances.

A streamline stops only if it exits the domain or gets too close to its own starting point, using the option in the section (or if any of the general termination criteria specified in the section is fulfilled).

If required, from the list, select to set advanced parameters as described in Method 4: Creating Streamlines with Uniform Density.

Advanced Section Setting Effects

The settings have the following effects:

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When calculating streamlines, the software selects a set of starting points (controlled by the streamline starting points and the number of starting points).

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The algorithm then finds the vectors of the given vector field at these points by interpolation. It normalizes the vector field if that option is selected.

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The algorithm integrates the points along the direction of the vector using the integration tolerance using a second-order Runge-Kutta algorithm.

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At the new positions, the algorithm finds vector values by interpolation and performs another integration.

This process stops if:

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It reaches a predetermined number of integration steps (controlled by the maximum number of integration steps entry).

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The points end up outside the geometry.

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The points reach a “stationary point” where the vector field is zero. Control the meaning of “zero” with the stationary point stop tolerance.

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It has used a predetermined amount of “time” for integrating (control this parameter with the field).

Finally, the software connects the calculated points for each streamline consecutively with straight lines.


	When integrating, the software uses a pseudo-time that has nothing to do with the time in time-dependent problems. Use the massless particle tracing tool to integrate in time-varying fields and to control the real time in stationary fields.