An impeller may include a blade groove region disposed in an outer circumferential portion of at least one end surface of two end surfaces in a rotation axis direction, the blade groove region including a plurality of blades and a plurality of blade grooves. The blade groove may be disposed between adjacent blades, open at one end surface side, and closed at the other end surface side. A housing of the pump includes an opposing groove opposing the blade groove region and extending along the rotation direction of the impeller. A rate of a depth of the opposing groove at an intermediate position excluding both ends of the opposing groove in the rotation direction of the impeller to a depth of each of the blade grooves at the deepest position of the each of the blade grooves may be equal to or less than 0.7.

A fan apparatus is described that comprises a rotational hub that defines a central axis, a first fan blade coupled to the rotational hub and extending radially outward from the central axis, wherein the first fan blade has a first pitch angle; a second fan blade coupled to the rotational hub and extending radially outward from the first central axis, wherein the second fan blade has a second pitch angle opposite the first pitch angle; and a housing that defines cylindrical interior volume with a central axis coaxial with the central axis of the rotational hub, wherein the first and second fan blades reside within the cylindrical interior volume.

The gas propulsion thrust device comprises a cone-shaped propulsion element with a rigid concave internal surface and a second convex external surface. The propulsion element is submerged in gas and aligned with a high-frequency linear actuator, which causes its reciprocal motion along the longitudinal axis, generating thrust. The device includes a thrust chamber that supports the actuator and surrounds the propulsion element, maintaining a consistent gap between the propulsion element and the chamber to direct gas from the second side to the first side. The chamber tapers around the propulsion element, and a gas directing cap is configured to direct gas around the second side towards the first side. Propulsion element is rigid and its shape remains unchanged during the operation. The reciprocal motion creates a gas pressure differential across the propulsion element, generating thrust by propelling gas away from the propulsion element in the opposite direction of propulsion. The device offers efficient and controlled thrust generation.