27.1.4 Displaying Pathlines

Pathlines are used to visualize the flow of massless particles in the problem domain. The particles are released from one or more surfaces that you have created with the tools in the Surface menu (see Chapter  26). A line or rake surface (see Section  26.5) is most commonly used. Figure  27.1.24 shows a sample plot of pathlines.

Figure 27.1.24: Pathline Plot

Note that the display of discrete-phase particle trajectories is discussed in Section  21.13.1.

Steps for Generating Pathlines

You can plot pathlines using the Path Lines panel (Figure  27.1.25).

Display $\rightarrow$ Path Lines...

Figure 27.1.25: The Path Lines Panel

The basic steps for generating pathlines are as follows:

1.

Select the surface(s) from which to release the particles in the Release From Surfaces list.

2.

Set the step size and the maximum number of steps. The Step Size sets the length interval used for computing the next position of a particle. (Note that particle positions are always computed when particles enter/leave a cell; even if you specify a very large step size, the particle positions at the entry/exit of each cell will still be computed and displayed.) The value of Steps sets the maximum number of steps a particle can advance. A particle will stop when it has traveled this number of steps or when it leaves the domain. One simple rule of thumb to follow when setting these two parameters is that if you want the particles to advance through a domain of length $L$, the Step Size times the number of Steps should be approximately equal to $L$.

3.

Set any of the options described below.

4.

Click the Display button to draw the pathlines, or click the Pulse button to animate the particle positions. The Pulse button will become the Stop ! button during the animation, and you must click Stop ! to stop the pulsing.

Options for Pathline Plots

The options mentioned in the procedure above include the following. You can include the grid in the pathline display, control the style of the pathlines (including the twisting of ribbon-style pathlines), and color them by different scalar fields and control the color scale. You can also ``thin'' the pathline display, trace the particle positions in reverse, and draw ``oil-flow'' pathlines. If you are ``pulsing'' the pathlines, you can control the pulse mode. In addition to the regular pathline display, you can also generate an XY plot of a specified quantity along the pathline trajectories. Finally, you can choose node or cell values for display (or plotting).

Including the Grid in the Pathline Display

For some problems, especially complex 3D geometries, you may want to include portions of the grid in your pathline display as spatial reference points. For example, you may want to show the location of an inlet and an outlet along with the pathlines (as in Figure  27.1.24). This is accomplished by turning on the Draw Grid option in the Path Lines panel. The Grid Display panel will appear automatically when you turn on the Draw Grid option, and you can set the grid display parameters there. When you click Display in the Path Lines panel, the grid display, as defined in the Grid Display panel, will be included in the plot of pathlines.

Controlling the Pathline Style

Pathlines can be displayed as lines (with or without arrows), ribbons, cylinders (coarse, medium, or fine), triangles, spheres, or a set of points. You can choose line, line-arrows, point, sphere, ribbon, triangle, coarse-cylinder, medium-cylinder, or fine-cylinder in the Style drop-down list in the Path Lines panel. (Note that pulsing can be done only on point, sphere, or line styles.)

Once you have selected the pathline style, click the Style Attributes... button to set the pathline thickness and other parameters related to the selected Style:

• If you are using the line or line-arrows style, set the Line Width in the Path Style Attributes panel that appears when you click the Style Attributes... button. For line-arrows you will also set the Arrow Scale, which controls the size of the arrow heads.

• If you are using the point style, you will set the Marker Width in the Path Style Attributes panel. The thickness of the pathline will be the thickness of the marker.

• If you are using the sphere style, you will set the Diameter and the Detail in the Path Style Attributes panel.

  The best diameter to use will depend on the dimensions of the domain, the view, and the particle density. However, an adequate starting point would be a diameter on the order of 1/4 of the average cell size or 1/4 step size. Units for the Diameter field correspond to the mesh dimensional units.

  The level of detail applied to the graphical rendering of the spheres can be controlled using the Detail field in the Path Style Attributes panel. The level of detail uses integer values ranging from 4 to 50. Note that the performance of the graphical rendering will be better when using a small level of detail, i.e., very coarse spheres, such as 6 or 8. The rendering performance significantly decreases with higher levels of detail. You should gradually increase the detail to determine the best-case scenario between performance and quality.

  Also note that to take full advantage of spherical rendering, lighting should be turned on in the view. The Gouraud setting provides much smoother looking spheres than the Flat setting and better performance than the Phong setting. For more information on lighting, see Section  27.2.6.

• If you are using the triangle or any of the cylinder styles, you will set the Width in the Path Style Attributes panel. For triangles, the specified value will be half the width of the triangle's base, and for cylinders, the value will be the cylinder's radius.

• If you are using the ribbon style, clicking on the Style Attributes... button will open the Ribbon Attributes panel, in which you can set the ribbon's Width. You can also specify parameters for twisting the ribbon pathlines. In the Twist By drop-down list, you can select a scalar field on which the pathline twisting is based (e.g., helicity). Select the desired category in the upper list and then select a related quantity in the lower list. Note that the twisting may not be displayed smoothly because the scalar field by which you are twisting the pathline is calculated at cell centers only (and not interpolated to a particle's position). The Twist Scale sets the amount of twist for the selected scalar field. To magnify the twist for a field with very little change, increase this factor; to display less twist for a field with dramatic changes, decrease this factor. (When you click Compute, the Max field will be updated to show the range of the Twist By scalar field.)

Controlling Pathline Colors

By default, the pathlines are colored by the particle ID number. That is, each particle's path will be a different color. You can also choose the color based on the surface from where the pathlines were released from using the surface ID as the particle variable. You can choose to color the pathlines by any of the scalar fields in the Color By drop-down list. (Select the desired category in the upper list and then select a related quantity in the lower list.) If you color the pathlines by velocity magnitude, for example, each particle's path will be colored depending on the speed of the particle at each point in the path.

The range of values of the selected scalar field will, by default, be the upper and lower limits of that field in the entire domain. The color scale will map to these values accordingly. If you prefer to restrict the range of the scalar field, turn off the Auto Range option (under Options) and set the Min and Max values manually beneath the Color By list. If you color the pathlines by velocity, and you limit the range to values between 30 and 60 m/s, for example, the ``lowest'' color will be used when the particle speed falls below 30 m/s and the ``highest'' color will be used when the particle speed exceeds 60 m/s. To show the default range at any time, click the Compute button and the Min and Max fields will be updated.

``Thinning'' Pathlines

If your pathline plot is difficult to understand because there are too many paths displayed, you can ``thin out'' the pathlines by changing the Skip value in the Path Lines panel. By default, Skip is set to 0, indicating that a pathline will be drawn from each face on the selected surface (e.g., $n$ pathlines). If you increase Skip to 1, every other pathline will be displayed, yielding $n/2$ pathlines. If you increase Skip to 2, every third pathline will be displayed, yielding $n/3$, and so on. The order of faces on the selected surface will determine which pathlines are skipped or drawn; thus adaption and reordering will change the appearance of the pathline display when a non-zero Skip value is used.

Reversing the Pathlines

If you are interested in determining the source of a particle for which you know the final destination (e.g., a particle that leaves the domain through an exit boundary), you can reverse the pathlines and follow them from their destination back to their source. To do this, turn on the Reverse option in the Path Lines panel. All other inputs for defining the pathlines will be exactly the same as for forward pathlines; the only difference is that the surface(s) selected in the Release From Surfaces list will be the final destination of the particles instead of their source.

Plotting Oil-Flow Pathlines

If you want to display ``oil-flow'' pathlines (i.e., pathlines that are constrained to lie on a particular boundary), turn on the Oil Flow option in the Path Lines panel. You will then need to select a single boundary zone in the On Zone list. The selected zone is the boundary on which the oil-flow pathlines will lie.

Controlling the Pulse Mode

If you are going to use the Pulse button in the Path Lines panel to animate the pathlines, you can choose one of two pulse modes for the release of particles that follow the pathlines. To release a single wave of particles, select the Single option under Pulse Mode. To release particles continuously from the initial positions, select the Continuous option.

Generating an XY Plot Along Pathline Trajectories

If you want to generate an XY plot along the trajectories of the pathlines you have defined, turn on the XY Plot option in the Path Lines panel. The Color By drop-down list will be replaced by Y Axis Function and X Axis Function lists. Select the variable to be plotted on the $y$ axis in the Y Axis Function list, and specify whether you want to plot this quantity as a function of the Time elapsed along the trajectory, or the Path Length along the trajectory by selecting the appropriate item in the X Axis Function drop-down list. Specify the Step Size, number of Steps, and other parameters as usual for a standard pathline display. Then click Plot to display the XY plot.

Once you have generated an XY plot, you may want to save the plot data to a file. You can read this file into FLUENT at a later time and plot it alone using the File XY Plot panel, as described in Section  27.8.3, or add it to a plot of solution data, as described in Section  27.8.2.

To save the plot data to a file, turn on the Write to File option in the Path Lines panel. The Plot button will change to the Write... button. Clicking on the Write... button will open the Select File dialog box, in which you can specify a name and save a file containing the plot data. The format of this file is described in Section  27.8.5.

Choosing Node or Cell Values

In FLUENT you can determine the scalar field value at a particle location using the computed cell-center values or values that have been interpolated to the nodes. By default, the Node Values option is turned on, and the interpolated values are used. If you prefer to use the cell values, turn the Node Values option off.

If you are plotting pathlines to show the effect of a porous medium or fan, to depict a shock wave, or to show any other discontinuities or jumps in the plotted variable, you should use cell values; if you use node values in such cases, the discontinuity will be smeared by the node averaging for graphics and will not be shown clearly in the plot.