In order to develop a diaphragm-type ventricular assist device (VAD), we studied the flow field change following structural modifications. We devised a center flow-type pump by putting a small projection on the center of the housing and/or diaphragm to provide a center in the flow field, and examined the following four types of VADs: N type without a projection, D type with a projection on the diaphragm, H type with a projection on the housing, and DH type with projections on both the diaphragm and housing. Computational fluid dynamics (CFD) was used for flow simulation. Particle image velocimetry (PIV) was also used to verify the reliability of the CFD method and to determine how the flow field changes in the presence of a projection. The results of the PIV and CFD analyses were comparable. The placement of a projection on the housing was most effective in rectifying the flow field.