Extension of the fringe projection method to measure shape and position of the centre of mass of granular flow deposit

This paper illustrates the application of a new optical measuring technique, the fringe projection method, to obtain the final deposit thickness of unconstrained granular flow experiments. This allows retrieving the position of the centre of mass, important information for the study of rock avalanche propagation. The fringe projection method consists in projecting alternate lines of dark and light (fringes) on the deposit surface. When fringes are projected on a planar surface, they are straight and equally spaced, whereas on a rough surface they are distorted and this distortion is related to the thickness of the object. This information is contained in the object optical print (phase map) which can be computed using different algorithms; in the present study a formula which implies the use of three different images has been chosen. The obtained phase is directly proportional to the height of the object by a factor which has been computed as the ratio of the maximum height of the deposit and the maximum value of the phase. The validation on the extensive experimental campaign has finally allowed adapting and developing the fringe projection method using the tools at disposition at the laboratory with very limited expenses and a good accuracy.