This article addresses the growing need for comprehensive tools to investigate the hemodynamics of ventricular assist devices (VADs) in general and sac-type VADs in particular. Numerical simulations can be very helpful in these efforts. However, full simulation of flow inside sac-type VADs poses several key problems, among them simulation of the mechanical heart valves and calculation of the motion of flexible walls. We present a simplified three-dimensional (3D) numerical model of a sac-VAD chamber. The walls in the simplified model are defined to move according to experimental measurements, and the valves are modeled in fully open or fully closed positions. The model is validated by comparison to a fully coupled fluid-structure interaction numerical simulation and to experimental measurements using continuous digital particle image velocimetry. Our results demonstrate that the flexible wall motion is sensitive to changes in pressure distribution inside the chamber. However, small variations in wall motion do not significantly affect the global features of flow inside the chamber. Therefore, the simplified model can be used to predict the 3D time-dependent flow field in the VAD.