For this example I'll be working on a 150 pixel square canvas. The first step is to use the rectangular selection tool to divide the canvas roughly in half, so that in my example the entire lower half of the canvas is selected. Fill this with your color of choice, red is shown here.
Now from the menu select Filters / Distorts / IWarp, and take a look at the dialog box that pops up. Using the 'Deform Mode' radio buttons, you can choose either a clockwise or counter clockwise spin. The two sliders below titled 'Deform Radius' and 'Deform Amount' control the action of the plugin. Since I want my entire canvas area affected by the swirl, I've set 'Deform Radius' to 100. I've chosen a middle value for 'Deform Amount', but playing with this slider may give other interesting results.
Now with my settings in place, I place my mouse cursor directly over the center of the preview area in the plugin dialog, and begin clicking repeatedly in the same spot. With each click, a bit more swirl effect is introduced in the preview. When you are satisfied, click OK. Use the 'Reset' button to return the preview to it's original state and begin again.
Lastly, if you enable the 'Adaptive Supersample' tickbox, the sliders below will become active. These control the quality of the rendering for the final swirl art. The top slider 'Max Depth' controls the amount of supersampling that will be applied. The lower slider entitled 'Threshold' will vary the supersampling based on the image data. Adjust these to preference, but be aware that high 'Max Depth' values, along with a low 'Threshold' value can lead to much longer processing times (though often with much better smoothness). Here is the swirl at supersampling maximum settings:
Finally, if you wish to simulate a motion blur on a rotating prop hub, you can use the Filter / Blur / Motion Blur filter for this effect. Here you can see the settings I've chosen, as well as the results.
