Fluid flow straightening techniques

Details Fluid flow straightening techniques Fluid flow straightening techniques

2001-07-09

2003-04-08

Barefoot, Galen L. (Department: 3644)

Aeronautics and astronautics

Aircraft sustentation

Sustaining airfoils

C244S017230, C416S128000

Reexamination Certificate

active

06543726

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention relates initially, and thus generally, to flow straightening techniques and apparatus. More specifically, the present invention relates to techniques and apparatus for preventing the formation of cylindrical vortices in applications where they are detrimental to performance. The applications include, but are in no way limited to, vortex attractors (as disclosed in inventor's co-pending application Ser. No. 09/316,318 entitled “Vortex Attractor”, which is herein incorporated by reference), VTOL aircraft and lifting platforms (an example of which is disclosed in inventor's co-pending Ser. No. 09/728,602, entitled “Lifting Platform”, which is herein incorporated by reference), toroidal vortex attractors (as disclosed in inventor's co-pending Ser. No. 09/829,416, entitled “Toroidal and Compound Vortex Attractor”, which is herein incorporated by reference), and toroidal vortex vacuum cleaners (as disclosed in inventor's co-pending application Ser. No. 09/835,084, entitled “Toroidal Vortex Bagless Vacuum Cleaner”, which is herein incorporated by reference).
BACKGROUND OF THE INVENTION
The need for flow straightening techniques and apparatus has been well established in the prior art, as well as what has been disclosed in prior applications of the inventor. For example, the prior art has prescribed the use of flow straightening in numerous applications such as gas scrubbers, helicopter rotors and fluid flow control. The inventor has specifically prescribed their use in applications such as toroidal vortex attractors, lifting platforms and vacuum cleaners. For the purposes of understanding the fundamental flow characteristics that make flow straightening useful, the flows of very basic systems will first be examined.
FIG. 1
shows a top view of a common prior art horizontal propeller. The construction consists of a motor
102
coupled to a plurality of blades
100
(in this example, four blades are shown). The blades rotate in accordance with direction
101
.
FIG. 2
shows a side view of the same system consisting of blades
100
and motor
102
. In this view, however, the vertical airflow components
200
are shown, with air drawn downwards through the blades
100
and outwards at the ends of the blades
100
. Air drawn downwards through the blades has a pressure, initially lower than ambient, that can be determined using Bernoulli. The outward curvature, however, leads to pressure higher than that above the blades
100
. The pressure difference across a curved stream tube is defined by the equation p=(&rgr;*V
2
)/R, where “p” is the pressure difference, “&rgr;” is the air density, “V” is the airspeed and “R” is the radius of curvature of the stream tube. The increased pressure beneath the blades adds to the efficiency of the propeller and lift. As seen, the curved airflow
200
forms a partial toroidal vortex
201
and the tips of blades
100
.
Of particular concern, however, is the deflection of downward airflow through the blades in the direction of blade movement. This effect is illustrated in FIG.
3
. As blades
100
rotate (the direction of rotation is shown by arrow
300
), momentum is imparted to the incoming airflow
301
such that it is deflected as shown.
FIG. 4
shows a top view of the air flow
301
of FIG.
3
. Again, motor
102
coupled to propeller blades
100
. Specifically, the outward radial airflow
400
combines with tangential flow
401
due to the blade movement to form a resultant spiraling air flow
402
. The effect is more pronounced when the blade
100
pitch is high.
The radial air movement
400
produces a cylindrical vortex. Such vortices have been exhaustively described by the inventor in prior applications. The cylindrical vortex results in a lowering of the air pressure beneath the propeller that is approximately p=(&rgr;*V
t
2
)/R
t
, where “p” is the air pressure, “V
t
” is the tangential air velocity at the tip of propeller
100
produced by its rotation, “R
t
” is the propeller tip radius and “&rgr;” is the air density. Consequently, there are two conflicting pressure generation systems. The first (the partial toroidal vortex), provides increased pressure and lift, while the second (the cylindrical vortex) leads to decreased pressure and decreased lift.
When very close to the ground and when operating with a high propeller pitch, the cylindrical vortex may predominate leading to a critical loss of lift. Thus, a helicopter, or VTOL (vertical take-off and landing) aircraft when very close to the ground may encounter conditions in which the cylindrical vortex predominates and lift is lost. An attempt to counter this by increasing the propeller pitch will increase the cylindrical vortex and further reduce the pressure beneath the blades and thereby lead to a crash. The inventor theorizes that this effect could solve unexplained crashes of helicopters and VTOL aircraft such as the Osprey.
FIG. 5
shows the addition of flow straightening vanes
500
beneath propeller
100
such that the air that enters
502
the propeller-vane system exits
503
with a vertical motion. However, such a system is cumbersome and could not be practically added to a helicopter. Alternatively, such flow straightening vanes have been integrated into prior systems of the inventor's that utilize an outer shroud or a duct.
Thus, it is clear that in order to properly remove rotational components from a flow, flow straightening techniques are necessary and useful. However, while flow straightening vanes have been useful in prior systems (such systems have been the subject of several of the inventor's co-pending patent applications, several of which have already been incorporated by reference), such systems could benefit from flow straightening techniques in accordance with contra-rotating blades, propellers, or impellers.
SURVEY OF THE PRIOR DISCLOSURES
While the prior art does disclose the use of contra-rotating propellers, no where has their use been disclosed as a means to eliminate cylindrical vortices to increase performance of devices employing toroidal vortices. However, the following represent references that the inventor feels are the most relevant, and still, do not approach the scope of the present invention.
Smith et al U.S. Pat. No. 4,422,342 is directed to a method for flow straightening and sampling gas in the stack of a gas scrubber. In normal operation, the gas has a substantially non-axial flow so as to retain, within the stack, droplets carried by the gas. A flow straightener having flow straightening panels is placed in the chamber of the gas scrubber and lifted into the stack. Gas having an axial flow is then sampled. The flow straightener is then removed from the scrubber, and normal operation is resumed. Smith et al simply provides for removable flow straightening vanes to allow for more accurate sampling of an effluent. Nowhere does Smith et al suggest the use of contra-rotating propellers or the elimination of cylindrical vortices.
Marze et al U.S. Pat. No. 5,634,611 discloses an arrangement for a helicopter rotor in which a flow-straightening stator is fixed into the duct downstream of the rotor. It is mounted so that it rotates in the duct, and includes vanes with an aerodynamic profile that straightens out the airflow downstream of the rotor substantially along the axis of the duct, and are each inclined to the radial direction, from the axis of the duct towards its periphery, and in the opposite direction from the direction of rotation of the rotor, and/or inclined at a slant, from the center of the duct to its periphery and from upstream to downstream of the duct. This arrangement is said by the inventors to increase safety, increase performance and decrease noise. However, Marze et al do not disclose any new method or apparatus for straightening a flow, just a particular arrangement therefor. Moreover, since Marze et al are not concerned in this reference with the lift generation of a helicopter, they cannot appreciate the detrimental effects of a cyl

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