Visualization of Results

Visualization of Results

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Results of an FEBio analysis can be visualized and analyzed in FEBioStudio. In this article, we’ll discuss some commonly used tools for looking at your results.

Opening the Analysis Results

The results of an FEBio analysis are saved in a plot file, which includes the mesh data as well as the plot variables selected by the user in the model file. Select File\Open Model File and select the desired xplt file (making sure that FEBio plot files or *.xplt is the desired file format). The user is then prompted with a dialog box that allows the user to select what they wish to import. Users can choose to load all states, load the last state, or list the specific states to load. The latter two choices may be preferred when computer memory is limited. More information can be found in tutorial “Performing Your First Analysis in FEBioStudio.”

Viewing Model Colormap and Animation

Once the plot file is loaded, plot variables may be visualized on the mesh using a colormap. Once a plot variable has been selected, the colormap can be turned on by selecting the Toggle Colormap button , located in the main toolbar. Different scalar plot variables can be visualized, such as the magnitude or components of the displacement and velocity vectors, or principal normal stresses. An example is shown in Figure 1 for an elastic solid subjected to a uniaxial stretch, where the displacement magnitude is displayed. The evolution through the different times steps can be animated by pressing the Play button . More information can be found in tutorial “Performing Your First Analysis in FEBioStudio.”

Figure 1: Visualizing displacement colormap in FEBioStudio and PostView.

Visualizing Graphs and Exporting Data

Scalar plot variables can also be visualized in a graph. An element, face, edge, or node can be selected after first pressing the Select elements button, Select faces button, Select edges button, and Select nodes button, which can be found beneath the Graphics View. Multiple elements/surfaces/nodes can be selected by checking the Select connected button, or holding the Shift key and boxing the selection, or Shift clicking. The graph can be viewed by clicking Post->New Graph from the menu. Figure 2 shows the graph of the displacement magnitude at one selected element, versus time, in the same uniaxial stretch model. It is also possible to create scatter plots, displaying one variable versus another.

Figure 2: Graph showing total displacement in an element of uniaxial stretch problem

This data can be exported by clicking the Copy to clipboard button and pasting in a spreadsheet. More information can be found in tutorial “Performing Your First Analysis in FEBioStudio.”

How Data is Interpreted from FEBio

Plot variables represent data fields; each data field has a Type, such as scalar (float), vector (vec3f), symmetric tensor (mat3fs), etc. Each data field also has a Class, which indicates if it is associated with a node, face, or element. The Type and Class of each Data field may be viewed in the Data tab of the Post panel. In particular, the degrees of freedom for any of the finite element analysis types in FEBio are saved as nodal variables. For example, for structural mechanics, the displacement vector is a nodal variable, while for fluid mechanics, the (scalar) fluid dilatation and the fluid velocity vector are nodal variables (the latter referred to as nodal fluid velocity).

Data fields associated with elements represent quantities that are typically evaluated at certain sampling points within the elements, usually the integration points. For example, the solid stress in structural mechanics, the fluid pressure in fluid mechanics, or the solid velocity and acceleration in a dynamic structural mechanics analysis. For each element variable, the plot file includes a single measure representing its average value over the sampling points within the element, at each time step. Similarly, surface variables may represent (averaged) quantities such as the contact pressure on a contact surface in a structural mechanics analysis, or the fluid mass flow rate across that surface in a fluid mechanics analysis.

When displaying a Colormap for a variable, by default the color is interpolated based on nodal values. For nodal variables, the interpolation is based on the values imported from the plot file. For element variables, when the Nodal Smoothing property in the Colormap is set to Yes (default), their Colormap nodal values are calculated by averaging the element values from all elements connected to that node. The same applies to face variables. This averaging procedure produces a smoothing of the displayed data. Setting the Nodal Smoothing property in the Colormap to No, displays element variables with a solid color over the entire element.

For example, in fluid mechanics, fluid velocity is an element variable representing the fluid velocity evaluated at integration points inside the element, using the nodal fluid velocity values. Similarly, in biphasic analyses, fluid pressure is an element variable evaluated from the effective fluid pressure nodal values. In general, element data is smoother than nodal data because of this averaging process.

When plotting a Graph, selecting nodes for plotting nodal variables, elements for plotting element variables, and faces for plotting face variables, ensures that the graph is displaying the exact values saved in the plot file.

Mirror and Cut Planes

The user can also cut the model along a plane, or alternatively, mirror the model along a plane in the case of a symmetric model. This can be done by using the Plane cut and Mirror plane buttons respectively. An example of a plane cut is shown in Figure 3. More information can be found in the tutorial “Visualizing Results of Your First CFD Model in FEBioStudio.”

Figure 3: Using the plane cut feature

Visualizing Vector Data

Vector data may be displayed as glyphs (e.g., arrows, spheres, boxes), or as streamlines, particle flow, or volume flow. These features can be accessed from the Vector plot , Stream lines plot , Particle flow plot , and Volume flow plot buttons. An example of a vector arrow plot is shown in Figure 4 showing the displacement vector for the uniaxial stretch of a block. An example using the stream lines plot is shown for a carotid bifurcated artery fluid mechanics problem in Figure 5. When these features are used to display element data, the material may need to be set to transparent to allow their visualization. This can be done in the Materials tab by changing the Transparency property and setting it to a value less than 1 (0.2-0.5 recommended). More information can be found in the tutorial “Visualizing Results of Your First CFD Model in FEBioStudio.”

Figure 4: Using the vector plot feature, representing displacement as arrows.
Figure 5: Using the streamline plot feature to visualize fluid velocity.

Visualizing Tensor Data

Second order tensors can also be visualized with glyphs representing their eigenvectors (for symmetric tensors), column vectors, or row vectors. This feature can be accessed from the Tensor plot button. The glyphs include arrows, boxes, lines, or ellipsoids. An example is shown in Figure 6 which visualizes the stress ellipsoids (superposed with the color map of the displacement magnitude). More information can be found in the tutorial “Visualizing Results of Your First CFD Model in FEBioStudio.”

Figure 6: Using the tensor plot feature, representing stress spherically.

Exporting Images and Videos

FEBioStudio also provides features that allow users to take picture snapshots and record videos.

Before you make a snapshot or a recording, you need to select the region of the Graphics View that you want to capture. The region is referred to as the Capture Frame and can be shown view the menu View/Show Capture Frame. (Or press the numeric keypad 0 button.) By clicking and dragging the boundary of the capture frame, you can position it anywhere on the view. Click-and-drag the lower-right corner to resize the capture frame. You can also double-click the border of the capture frame to bring up its properties. In the properties dialog you can specify the position and size precisely.

A picture can be taken with the Snapshot button , which allows the user to save a snapshot of the current Capture Frame (menu View->Show Capture Frame). A window is shown with a preview of the capture image. From this window, the image can be saved to file, or copied to the clipboard.

A video of the content of the Capture Frame can be recorded through the menu option Record->New. A dialog appears that allows the user to select a video format. Depending on the selected format, additional dialogs may ask the user to configure the video stream. To start the recording, select the menu Record\Start. From this point, every change to the Graphics View will be recorded as a frame into the video stream. Select the menu Record\Pause to pause the recording and Record\Stop to stop the video.

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