Streamline
Use a Streamline 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 streamline plot 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 2D Plot Group or 3D Plot Group 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 Positioning list: On selected boundaries (the default), Starting-point controlled, Uniform density, or Magnitude controlled. Then follow one of the methods described:
Selection
The Selection section is available for some datasets 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 Graphics window and using the tools in the Selection 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 Line style, choose None, Line (the default), Ribbon, or Tube from the Type list to determine the type of streamlines. If you choose None, no streamlines appear, but you can plot arrows that are tangential to the streamlines. If you choose Ribbon, then enter an expression for the ribbon width in the Width expression field (SI unit: m). If desired, you can also adjust the width by selecting the Width scale factor check box and enter a scale factor in the corresponding text field.
Point Style
Under Point style, you can add points or arrows and control their appearance.
From the Type list, choose None (the default) for no points or arrows, Arrow for static arrows that are tangential to the streamlines, Interactive arrow for arrows that are tangential to the streamlines and that can move along the streamlines with the local integration time, or Interactive point for points that can move along the streamlines with the local integration time.
If you choose Arrow, these additional settings are available:
From the Arrow distribution list, choose Equal arc length (the default) to distribute the arrows uniformly over the streamlines’ arc length, Equal time to distribute the arrows using the weight function dt/darc, or Equal inverse time to distribute the arrows using the weight function darc/dt.
Select the Number of arrows 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.
If you choose Interactive arrow, these additional settings are available:
Specify a value in the Local time field (default: 0) for the local integration time along the streamlines for which the arrows are plotted. It is possible for integration times to be negative: For a starting point in the interior of a domain, streamlines are integrated both forward and backward in time unless you clear the Allow backward time integration check box in the Advanced section.
In the Extra release times field, enter any additional times for releasing arrows, or click the Range button () to define the extra release times. By default, there are no extra times.
The following settings are available for both the Arrow and the Interactive arrow types:
From the Arrow type list, choose Arrowhead (the default), Arrow, or Cone.
From the Arrow length list, choose Normalized (the default), Logarithmic, or Proportional to make the arrows’ sizes depend on the magnitude of the plotted quantity, if desired. If you choose Logarithmic, 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 Range quotient field (default: 100) determines the ratio between the smallest and largest values in the range of values for the logarithmic arrow length.
Use the slider, or select the Scale factor 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.
If you choose Interactive point, these following settings are available:
Specify a value in the Local time field (default: 0) for the local integration time along the streamlines for which the points are plotted. It is possible for integration times to be negative: For a starting point in the interior of a domain, streamlines are integrated both forward and backward in time unless you clear the Allow backward time integration check box in the Advanced section.
In the Extra release times field, enter any additional times for releasing points, or click the Range button () to define the extra release times. By default, there are no extra times.
Select or enter an expression for the points’ radii in the Point radius expression field (SI unit: m). By default, the radius is scaled automatically. To enter a scale factor for the radius, select the Radius scale factor check box and enter a scale factor in the corresponding text field.
Select the Fixed size check box if you want to display the points with a fixed onscreen size.
From the Color list, choose a color for the streamlines and arrows. Choose Custom to add a custom color using a color palette. This list is not available if you use a Color Expression subnode to define the colors for the streamlines.
Advanced
Define the following advanced streamline settings as needed.
Advanced Settings for the Streamline Plot
Under Advanced, set these general settings. See also Advanced Section Setting Effects.
The Integration tolerance field default is 0.01 for 3D and 0.001 for 2D. Edit to specify how accurately streamlines are computed.
The Maximum streamline length 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 Allow backward time integration check box is selected (the default), the maximum length refers to the sum of the lengths of the forward and backward parts.
The Maximum number of integration steps field makes sure that the integration does not continue indefinitely. Edit the default (5000) to control when the computation stops.
The Maximum integration time field sets an upper time limit for the integration. The default is infinity (inf).
The Stationary point stop tolerance can be adjusted to make sure the integration stops near a stationary point in the field. The default is 0.01.
The Loop tolerance 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.
Select the Allow backward time integration 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.
Select the Normalize vector field 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 Advanced 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
1
Under Streamline Positioning, from the Positioning list, select On selected boundaries.
The Selection section is made available for some datasets when On selected boundaries is selected from the Positioning list under Streamline Positioning.
2
Under Selection, select the boundaries from which the streamlines start. By selecting in the Graphics window and using the tools in the Selection section, select the boundaries for the starting positions for the streamlines.
3
Enter the Number 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
1
Under Streamline Positioning, from the Positioning list, select Starting-point controlled.
2
Select Coordinates from the Entry method list.
3
Enter x and y (2D) or x, y, and z (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
1
Under Streamline Positioning, from the Positioning list, select Starting-point controlled.
2
Select Number of points from the Entry method list.
3
Enter the number of Points (the default is 20).
4
From the Along curve or surface list, select None. The starting points are then distributed semirandomly but deterministically. You can also choose a Cut Line, Cut Plane, Parameterized Curve or Parameterized Surface dataset, if applicable, to restrict the streamline start positions to a cut line, cut plane, parametric curve, or parametric surface.
Method 4: Creating Streamlines with Uniform Density
The algorithm saturates the entire domain with evenly spaced streamlines.
1
Under Streamline Positioning, from the Positioning list, select Uniform density.
2
Enter the Separating distance 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 Advanced section is fulfilled).
3
The Advanced parameters list defaults to Automatic. If required, select Manual to edit these parameters: Boundary element refinement, Fraction of streamline length to ignore, Starting distance factor, Terminating distance factor, or First starting point.
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Edit the Boundary element refinement 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.
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Edit the value in the Fraction of streamlines to ignore 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.
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The Starting distance factor is a factor multiplied with the distance specified in the Separating distance 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.
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The Terminating distance factor is a factor multiplied with the distance specified in the Separating distance 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.
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By default the First starting point list defaults to Automatic, 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 Manual instead to override the default and enter x and y 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:
1
Under Streamline Positioning, from the Positioning list, select Magnitude controlled.
The Magnitude controlled 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.
2
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 Loop tolerance option in the Advanced section (or if any of the general termination criteria specified in the Advanced section is fulfilled).
3
If required, from the Advanced parameters list, select Manual to set advanced parameters as described in Method 4: Creating Streamlines with Uniform Density.
Advanced Section Setting Effects
The Advanced settings have the following effects:
This process stops if:
Finally, the software connects the calculated points for each streamline consecutively with straight lines.