A better understanding of the cavitation-erosion mechanism is important to improve the prediction of the affected region under unsteady cavitation condition. For that, a new model has been proposed to estimate the microjet speed, which is based on a homogeneous mixture assumption and the energy transformation from potential to kinetic energy. A numerical simulation of unsteady cavitating flow around a plane-convex hydrofoil with semi cylindrical obstacle has been carried out, which is based on experimental studies from the cavitation tunnel of the EPFL. Results obtained using the new proposed model, shows that the affected region by the flow has been estimated fairly well and matches experimental results.