Using high-speed X-radiography, it is possible to see through granular media while they are flowing. This is crucial when one want to study the dynamics far from any walls, because wall effects can be very important in granular media.
First of all, one can add one or a few denser particles that can be directly followed. Here is an example with a steel ball in a rotating tumbler half-full of plastic beads. Only the first image is displayed, the green curve is the trajectory of the large particle during the recording.
Since granular media are inhomogeneous, it is actually possible to simply track the density fluctuations in the flow. Here is an example of grains flowing down a rectangular hopper. Using Particle Image Velocimetry (i.e. tracking correlations between images), it is possible to get the velocity field inside the hopper.