This image shows the calculated forward and backward Finite Time Lyapunov Exponent (FTLE) based on vortex ring experiments. Ridges in the forward and backward FTLE can be thought of physically as repelling and attracting structures, respectively, within the flow. The stronger the color intensity, the stronger the repulsion or attraction. In this image, the repelling structure is shown in red and the attracting structure is superimposed in blue. Caught in between the attracting and repelling structures forms what is known as the "vortex bubble" and can clearly be seen as the ellipse in the center of the image.