Hydropower is an important supplement to intermittent renewables such as wind and solar power. Because of its flexibility, it contributes to grid stability. However, when used in off-design condition, this flexibility presents challenges in turbine operation. Cavitation is one of such issue in reaction turbines, i.e., Francis turbine. Increase in velocity combined with flow separation on the suction surface of runner blade as well as guide vane results in pressure to fall below vapor pressure and thus development of cavitation. Guide vane and runner blades are basically extension and combination of the NACA 65-021 hydrofoils, therefore a single three-dimensional NACA 65-021 hydrofoil is investigated for cavitation to develop a theoretical understanding of cavitation dynamics. ANSYS ICEM is used to create a three-dimensional structural mesh of a single hydrofoil. Unsteady Reynolds Averaged Navier–Stokes simulations are performed for cavitation investigation. RNG k-ε model is used with mixture modeling. Overprediction of turbulent viscosity in the cavity presents a major challenge in cavitation prediction with the current modeling approach. So, the correction in turbulent viscosity is attempted by incorporating density correction in cavity region, in the formulation of turbulent viscosity (DCM). The pressure coefficient over suction surface in cavity region is in good agreement with earlier experimental study. Further, pressure fluctuation over hydrofoil surface is discussed. FFT of drag coefficient reported two dominant frequencies. DCM model also predicted re-entrant jet formation near trailing edge of hydrofoil.