Fluid machine, wind turbine, and method for increasing velocity of internal flow of fluid machine, utilizing unsteady flow

A fluid machine, a wind turbine, and a method for increasing velocity of an internal flow is provided. A basic shape of the casing has a cycloid curve having a convex portion at a throat section between the front and rear edges. The throat section and a vortex generating face disposed downstream of the throat section to generate the negative pressure region are formed on an inner circumferential face of the casing. A first outflow gradient is formed on an outer circumferential face of the casing and at the rear edge. A second outflow gradient is formed on a boundary of a throat-side adjacent face adjacent to the vortex generating portion to separate the internal flow. The negative pressure region of high vorticity is generated behind the vortex generating face by virtue of the non-streamline shaped casing to increase the velocity of the internal flow.