Equation 1 shows how the lift force FL produced by a foil depends on parameters such as the fluid density ρ, the flow velocity ν, the lift coefficient CL of the foil, and the active area A of the foil. The fluid velocity at the foil, which is a vector sum of the blade velocity and the free stream fluid flow ν, depends on free stream fluid flow ν, rotation angle θ, and an angular velocity ω of base 415. However, if the free stream fluid velocity ν is small when compared to the rotational velocity of a blade 410, the attack angle of a blade 410 is approximately equal to angle α, and the pitching schedule for extracting energy from a constant free stream fluid flow or for momentum transfer in a specific can be a function rotation angle θ.
7. Propeller 400 in this use will experience a net force in the upstream direction, but a motor or engine of some sort needs to provide shaft power to maintain rotation of propeller 400. While a typical application of this operating mode is thrust generation in order to propel land, air or watercraft, propeller 400 can also be used as a fan device in order to deliver an fluid current for heating, cooling or pneumatic transport of particles, as well as providing a pressure rise like in any air or gas compressor. However, mechanism 340, which controls the orientation of blades 310, can only be optimized for a very limited set of operating parameters, e.g., fluid velocity ν and fluid density ρ.
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In general, the pitching schedule required to produce a von Karman Vortex Street pattern depends on various operating parameters such as the fluid flow speed and rotational speed of the propeller, but the pitching schedule should always be oscillatory. Accordingly, for a low fluid velocity, mechanical linkages, gears or an active servo system with a fixed pitching schedule can vary blade angle α as a function of rotation angle θ, which has a defined relation to the background fluid flow vector ν. While most advantageous at low Reynolds numbers, the propeller device can be employed at all flow speeds. 2. FIG. 2 shows that for a positive (clockwise) angle of attack of the foil 210 in a moving fluid, a clockwise rotating vortex 220 forms on the upper side of foil 220. This vortex 220 will then be swept downstream with the mean fluid flow towards the trailing edge of foil 210, entering the wake of foil 210. Similarly, a negative angle of attack will produce a counter clockwise rotating vortex on the bottom side of the foil.
In the case of energy extraction from the fluid, the fluid flow preferably rotates the propeller, while the pitch changes during propeller rotation may exceed the static stall angle and are of sufficient magnitude to shed vortices and create dynamic lift. Dolor muy fuerte de estomago y espalda . In another aspect of the invention, a propeller device employs unsteady aerodynamic effects, in order to transfer power efficiently to or from a fluid.
Propeller system 400 as described above has at least one blade that is mounted eccentrically to a main shaft 420. A propeller having a single blade 410 is easier to analytically analyze since the wake field of one blade in a multi-blade propeller can affect the flow at other blades. For the example, when system 400 extracts energy from a moving fluid, device 440 may be a generator, pump, or other device receiving drive power resulting from the action of the moving fluid on blades 410 during a pitching process. Alternatively, device 440 can be a motor that drives main shaft 420 to turn base 415, so that blades 410 act on the fluid, for example, during a pitching process to create thrust for propulsion or to create a flow in the fluid. In the case of momentum transfer to the fluid, the dynamic pitch changes during driven rotation may exceed the static stall angle and are of sufficient magnitude to shed vortices.
DETAILED DESCRIPTION In accordance with an aspect of the invention, a propeller used to transfer energy to or from a fluid employs pitching blades in order to use the large dynamic lift coefficients that result from shedding of vortices during dynamic pitching. Base 415 is attached to a main shaft 420 and provides a linkage to blades 410, so that base 415 and main shaft 420 conduct the energy transfer between blades 410 and a device 440, which may be, for example, a generator or motor. The pitching can provide efficient energy transfers even at low fluid flow rates and can be adapted to changing conditions. The foils can produce a force commonly referred to as lift that enables the desired energy transfers. In one specific embodiment, the shedding of vortices creates a flow pattern that is the inverse to the von Karman vortex street. 7 illustrates uniform flow field incident on propeller system 400. Propeller system 400 interacting with the flow field creates the von Karman Vortex street wake pattern and causes a net transfer of flow momentum to propeller system 400, such that the time averaged flow field is that of a wake flow, shown on the right side of FIG.
Propeller 400 can efficiently extract energy from the fluid when the pattern of shed vortices forms a pattern known as the von Karman Vortex Street. Further, the stall angle generally decreases with decreasing fluid velocity, so that the maximum lift that a foil can produce generally drops with the fluid velocity. Further, the self starting ability works even for a single blade located in any angular position of rotation. The attack angle of blade 410 generally depends on blade angle α, rotation angle θ, an angular velocity ω of base 420, and the direction and velocity of free stream fluid flow ν.
Accordingly, a propeller system 400 that causes blades 410 to oscillate between extremes that are greater than the stall angle of attack while extracting energy from a fluid flow will generate a vortex pattern such as shown in FIG. FIG. 3 shows a known propeller including a mechanism that cyclically changes the attack angles of blades mounted transverse to a fluid flow.
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In particular, a programmable control system such as servo control system 400 in FIG. In general, an adaptable pitching schedule is easiest to implement using an electronic servo control system such as illustrated in FIG. Such small changes are sufficient to perform energy transfers when the Reynolds number is large and the desired flow pattern is that of attached flow, but do not produce dynamic stall vortices and the high efficiency provided by dynamic lift. Dolor columna vertebral lumbar . In general, any type of foil can be employed for blades 410, but the type of foil may influence the particular pitch variation process employed in system 400 as described further below. The flow field for example would preferably be oriented in any direction normal to main shaft 420 and can be measured using an anemometer of any kind and/or a weather vane type device. Sensors 456 can measure any desired characteristic of the fluid including but not limited to measuring the fluid flow direction and magnitude.