Abstract: A fluid resonator is disclosed in which a fluid flow through and around a cylindrical member positioned parallel to the direction of the fluid flow causes vibration or vibratory waves in the fluid flow. The fluid resonator may take on a number of different forms. Preferably, a plurality of cylindrical members may be concentrically positioned in the fluid flow stream. Variations may also be provided in the structure of the cylindrical element to produce desired effects in specific cases. The fluid resonator may be used for many purposes, including emulsification apparatus and the secondary recovery of oil from oil formations.