Optical flow sensors based on moiré effect

This paper continues recent research of the author on the applications of dynamically generated moiré patterns in gaseous and transparent fluid flows. Moiré patterns are special interference patterns that appear when two or several quasi-aligned periodic structures are superposed. Depending on the detector, geometrically unrelated macro patterns emerge in the shape of darker and lighter areas as an effect of optical interference of the overlapping patterns as well as aliasing of the resulting pattern with the sensor’s elements. A series of well-known antialiasing practices such as low pass optical filtering can be used to counter this effect when normally imaging everyday objects but on a careful analysis the moiré pattern itself has merit as a tool of investigating small displacements in transparent fluids without any interference. The system described in this paper is able to detect minute changes in velocity of a fluid without perturbing it -unlike the traditional methods where classic flow sensors are placed within- and is therefore a good addition to the experimental aerodynamicist. It also can be extended to work for larger displacements/ lower sensitivities by adjusting the forming grid patterns as will be shown.