Large-scale three-dimensional cavitating structures can be found in the wake of two-dimensional hydrofoils, as a result of sheet/cloud cavitation on the suction side. This type of cavitation produces unsteady lift on most hydrofoils, including the NACA 0015 hydrofoil studied here, but is periodic and therefore offers the potential for control. In addition to hydrofoils on marine vehicles complex cavitation characteristics are observed in many types of fluid machinery. Examples range from the high-pressure fuel pumps in the Space Shuttle Main Engine to a variety of hydroturbines. Associated with the deleterious effects of performance breakdown, noise, and vibration, there is a possibility of erosion. The purpose of this research is to investigate the twophase flow structure in the wake of a hydrofoil undergoing unsteady partial cavitation using an integrated experimental/numerical approach. This topic provides both numerical and experimental challenges. A two-dimensional NACA 0015 hydrofoil was selected for study, because of its previous use by several investigators around the world. The simulation methodology is based on a Large Eddy Simulation (LES), using a barotropic phase model to couple the continuity and momentum equations. The complementary experiments were carried out at two different scales in two different water tunnels. Tests at the St. Anthony Falls Laboratory (SAFL) were carried out in a 0.19×0.19m2 water tunnel and a geometrically scaled up series of tests was carried out in the 0.3×0.3 m2 water tunnel at the Versuchsanstalt fu¨r Wasserbau (VAO) in Obernach, Germany. The tests were designed to complement each other and to capitalize on the special features of each facility. Time-resolved Particle Image Velocimetry (TR-PIV) was used at SAFL to confirm the existence of the large-scale flow structure observed with the LES.