Aeronautics and astronautics – Aircraft sustentation – Sustaining airfoils
Reexamination Certificate
1999-02-25
2002-05-21
Jordon, Charles T (Department: 3644)
Aeronautics and astronautics
Aircraft sustentation
Sustaining airfoils
C244S130000, C244S207000, C244S208000, C244S00100R
Reexamination Certificate
active
06390418
ABSTRACT:
TECHNICAL FIELD
This invention relates to an acoustic jet having a nozzle directed essentially tangentially downstream into a flow of gas along a surface to control the boundary layer thereof.
BACKGROUND ART
Boundary layer separation is a fundamentally limiting mechanism which constrains the design of gas flow systems. As an example, it is known in the helicopter art that retreating blade stall (boundary layer separation from the leading edge of the rotor blade) establishes limits on rotor load and flight speed. In addition to the loss of capability to generate lift, unsteady blade stall transmits very large impulsive blade pitching moments to the flight control system. In order to prevent excess control loads, stall boundaries are set as a function of rotor load and flight speed. Stall boundaries define the maximum blade loads, which impact maneuverability and agility as well as speed and payload. Similar boundary layer separation problems affect diffusers, fans in air moving equipment and jet engines, airplane wings, other airfoils, fuselages, flow ducts, and other structures having surfaces with aerodynamic profiles.
Gas flow in the boundary layer adjacent to a surface exhibits a reduction in velocity due to friction of the molecular viscosity interacting with the surface, which results in a strong velocity gradient as a function of perpendicular distance from the wall: from zero at the surface, raising to mainstream velocity at the outer edge of the boundary layer. The reduced velocity results in a lower momentum flux, which is the product of the density of the gas times the square of its velocity. This near-wall, low-momentum fluid can be problematic for the case where the static pressure rises along the direction of the flow. For example, along a diverging surface (that is, a surface that tails away from the mean flow direction), as is the case in a diffuser and on the suction side of an airfoil such as a fan blade or an airplane wing, the flow along the surface is accompanied by a pressure rise, which is accomplished only by conversion of momentum flux. If the pressure rise is sufficiently large, the momentum and energy of the gas along the surface is consumed in overcoming this pressure rise, so that the gas particles are finally brought to rest and then flow begins to break away from the wall, resulting in boundary layer separation (FIG.
1
A). Boundary layer separation typically results in the termination of pressure rise (recovery) and hence loss in performance (e.g., airfoil lift) and dramatic decrease in system efficiency, due to conversion of flow energy into turbulence, and eventually into heat. It is known that boundary layer separation can be deterred by increasing the momentum flux of the gas particles flowing near the surface. In the art, the deterrence or reduction of boundary layer separation is typically referred to as “delaying the onset of boundary layer separation”.
The simplest and most common method for reducing boundary layer separation includes small vortex generators, which may typically be tabs extending outwardly from the surface (such as the upper surface of an airplane wing), which shed an array of streamwise vortices along the surface. The vortices transport the low momentum particles near the surface away from the surface, and transport the higher momentum particles flowing at a distance from the surface toward the surface, thereby improving the momentum flux of particles flowing near the surface in the boundary layer downstream of the tabs. This has the effect of deterring boundary layer separation at any given velocity and over a range of angle of attack (where the uncontrolled separation is downstream of the vortex generators). However, as is known, tab-type vortex generators create parasitic drag which limits the degree of boundary layer separation that can be efficiently/practically suppressed.
Another known approach employs continuous flow into or out of the boundary layer. A wall suction upstream of the boundary separation line (that is, the line at which the onset of full boundary layer separation occurs across the surface of an airfoil or a diffuser) simply removes low momentum flux gas particles from the flow adjacent to the surface, the void created thereby being filled by higher momentum flux gas particles drawn in from the flow further out from the surface. A similar approach is simply blowing high energy gas tangentially in the downstream direction through a slot to directly energize the flow adjacent to the surface. Both of these flow techniques, however, require a source of vacuum or a source of pressure and internal piping from the source to the orifices at the surface, which greatly increases the cost, weight and complexity of any such system. These techniques have not as yet been found to be sufficiently effective to justify use over a wide range of applications.
A relatively recent approach, so-called “dynamic separation control” uses perturbations oscillating near the surface, just ahead of the separation point, as are illustrated in U.S. Pat. No. 5,209,438. These include: pivotal flaps which oscillate from being flush with the surface to having a downstream edge thereof extending out from the surface, ribbons parallel to the surface, the mean position of which is oscillated between being coextensive with the surface and extending outwardly into the flow, perpendicular obstructions that oscillate in and out of the flow, and rotating vanes (microturbines) that provide periodic obstruction to the flow, and oscillatory blowing. These devices introduce a periodic disturbance in vorticity to the flow, the vortices being amplified in the unstable separating shear layer into large, spanwise vortical structures (see
FIG. 1B
) which convect high momentum flow toward the surface, thereby enabling some pressure recovery. It is consistently reported in the relevant literature that at least two large coherent vortical structures must be present over the otherwise separated region for the control to be effective. Such a flow is neither attached nor separated, under traditional definitions. However, such perturbations must be actively controlled as a function of all of the flow and geometric parameters, dynamically, requiring expensive modeling of complex unsteady flow structures and/or significant testing to provide information for adapting to flow changes either through open loop scheduling or in response to feedback from sensors in the flow.
A recent variation on the dynamic separation control is the utilization of a so-called “synthetic” jet (also referred to as “acoustic jet” or “streaming”) directed perpendicular to the surface upstream of the boundary separation line of the surface. This approach has been reported as being highly parameter dependent, thus also requiring dynamic control; and, the results achieved to date have not been sufficient to merit the cost and complexity thereof in any product or practical application. In Redinotis et al, “Synthetic Jets, Their Reduced Order Modeling and Applications to Flow Control”, AIAA 99-1000, presented at 37th Aerospace Sciences Meeting & Exhibit, Reno, Nev., Jan. 28, 1999, a laminar flow of water (Reynolds number=6600) flowing around a half-cylinder used a tangential synthetic jet which induced natural instability of the shear layer, leading to large vortical coherent structures of the type referred to with respect to
FIG. 1B
, hereinbefore, which promoted mixing and momentum flux exchange between the inner and outer parts of the boundary layer. As stated therein, the process takes advantage of the Coanda effect. That requires significant local surface curvature in the vicinity, and particularly downstream, of the point of injection of the synthetic jet. Although flow separation was delayed somewhat, it was not eliminated, as shown in FIG.
17
(C) therein.
DISCLOSURE OF INVENTION
Objects of the invention include: absolute adherence of a boundary layer of laminar or turbulent gaseous flow to an adjacent surface; improved boundary layer characteristics in turbulent flow; red
Gysling Daniel L.
McCormick Duane C.
Dinh Tien
Jordon Charles T
United Technologies Corporation
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