# Documentation

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## Displaying Curl with Stream Ribbons

### What Stream Ribbons Can Show

Stream ribbons illustrate direction of flow, similar to stream lines, but can also show rotation about the flow axis by twisting the ribbon-shaped flow line. The `streamribbon` function enables you to specify a twist angle (in radians) for each vertex in the stream ribbons.

When used in conjunction with the `curl` function, `streamribbon` is useful for displaying the curl angular velocity of a vector field. The following example illustrates this technique.

### 1. Select a Subset of Data to Plot

Load and select a region of interest in the `wind` data set using `subvolume`. Plotting the full data set first can help you select a region of interest.

```load wind lims = [100.64 116.67 17.25 28.75 -0.02 6.86]; [x,y,z,u,v,w] = subvolume(x,y,z,u,v,w,lims); ```

### 2. Calculate Curl Angular Velocity and Wind Speed

Calculate the curl angular velocity and the wind speed.

```cav = curl(x,y,z,u,v,w); wind_speed = sqrt(u.^2 + v.^2 + w.^2); ```

### 3. Create the Stream Ribbons

• Use `meshgrid` to create arrays of starting points for the stream ribbons. See Specifying Starting Points for Stream Plotsfor information on specifying the arrays of starting points.

• `stream3` calculates the stream line vertices with a step size of `.5`.

• `streamribbon` scales the width of the ribbon by a factor of `2` to enhance the visibility of the twisting (which indicates curl angular velocity).

• `streamribbon` returns the handles of the surface objects it creates, which are then used to set the color to red (`FaceColor`), the color of the surface edges to light gray (`EdgeColor`), and slightly increase the brightness of the ambient light reflected when lighting is applied (`AmbientStrength`).

```[sx sy sz] = meshgrid(110,20:5:30,1:5); verts = stream3(x,y,z,u,v,w,sx,sy,sz,.5); h = streamribbon(verts,x,y,z,cav,wind_speed,2); set(h,'FaceColor','r',... 'EdgeColor',[.7 .7 .7],... 'AmbientStrength',.6) ```