A flow energy harvesting system including a nozzle-diffuser defining a spline-shaped flow channel and a flow energy harvesting device in the spline-shaped flow channel of the nozzle-diffuser. The spline-shaped flow channel includes a converging portion, a diverging portion, and a constriction section between the converging and diverging portions. The flow energy harvesting device includes a flextensional member having a frame and a cantilever extending outward from the frame, and a stack of piezoelectric elements housed in an interior cavity defined in the frame. The cantilever is a non-piezoelectric material. The frame of the flextensional member is in the converging portion and the cantilever is in the constriction section of the spline-shaped flow channel. The frame is configured to deform and elongate the piezoelectric elements to generate a current based on the piezoelectric effect when a fluid flows through the spline-shaped flow channel and generates unbalanced forces on the cantilever due.