“…Optical techniques are commonly used in the analysis of velocity profiles near the transparent silo walls. The X-ray technique was frequently applied to obtain information from deeper flow layers [36,37]. Also, other non-invasive measurement techniques were applied to register granular flow, density and velocity fields in flowing zones, among others spy-holes, radio transmitters, positron emission [38], magnetic resonance imaging, radioactive tracers, and ultrasonic speckle velocimetry.…”