A laser doppler method for measuring oscillating flow in prismatic tanks

The flow field inside a partially filled prismatic tank with a rectangular section which oscillates laterally was analyzed using a laser doppler velocimeter. The natural frequencies of the system can be determined with greater accuracy through velocity measurements than through acceleration measurements or flow visualization. Considering the effective value of the horizontal velocity along a vertical line, it is possible to show an out of phase of velocity at different points. This phenomenon, particularly noticeable for the vibration modes of higher order, causes in some points lower RMS values of velocity than those measured near the bottom. The distribution of vertical velocity near the free surface, when the frequency of the acting force is equal to one of the natural frequencies of the liquid tank system, shows that the maxima and the nodes are shifted toward the center of the tank in comparison with the positions computed using the linear theory with the hypothesis of inviscid fluid.