WO2004041638A2

WO2004041638A2 - Device for establishing and sustaining laminar boundary-layer flow

Info

Publication number: WO2004041638A2
Application number: PCT/US2003/035003
Authority: WO
Inventors: Fabio Paolo Bertolotti
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-11-04
Filing date: 2003-11-03
Publication date: 2004-05-21
Anticipated expiration: 2005-05-04
Also published as: AU2003291694A8; AU2003291694A1; WO2004041638A3

Abstract

The outer skin (11) of an aerodynamic body (100) has an elongated surface opening (310) generating a virgin leading-edge (200) in the outer skin, and ingesting the approaching boundary layer flowing over the outer skin to achieve a virgin flow-attachment region on the virgin leading-edge from which laminar flow can develop over a controlled surface region of the outer skin. The ingested air is received by a channel (300) comprising a channel diffuser (330) for decelerating the ingested flow, a channel center-section (340) for transporting the ingested flow, a channel nozzle (350) for re-accelerating the ingested flow, and a channel exit (320) for ejecting the ingested flow at a location outside the controlled surface region. A heating means (401) at the virgin leading edge performs de-icing functions and increases the stability of the laminar flow over the controlled surface region. The aerodynamic body further includes structurally joining the material defining the outer skin portion having the controlled surface region and the material defining the channel into a monolithic laminar-flow unit (150) that can be separated from the remainder of the outer skin to allow access to the interior of the aerodynamic body.

Description

DEVICE FOR ESTABLISHING AND SUSTAINING LAMINAR BOUNDARY-LAYER FLOW

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims the priority, under 35 U.S.C. §119 (e) and PCT Article 8, of U. S. Provisional Application 60/423,587, filed on November 4, 2002, the entire disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

This invention relates to the construction of a structure for the establishment and sustainment of laminar flow over a surface via the ingestion of boundary layer air.

BACKGROUND ART

It is generally known in the art that the laminar-flow condition in the boundary layer of a fluid flowing over the surface of a body can be difficult to establish once the boundary layer is in a turbulent state. It is further known that establishing and sustaining the laminar-flow condition in the boundary layer can reduce the resulting skin friction between the fluid and the body. This is especially pertinent, for example, in the field of aircraft construction, whereby the improvement of the laminar-flow of the boundary layer and the resulting lower skin friction can achieve potential fuel savings in the operation of the aircraft . For these reasons, means for avoiding or clearing turbulence- causing surface contamination, such as insect collisions with the aircraft surface, as well as means for stabilizing the laminar state of the boundary layer flow over the surface, have been widely studied for many decades. In the context of commercial aircraft, known laminar-flow control devices generally must operate with the best efficiency at only one condition, namely the cruise flight condition, and are thus designed primarily for this operating condition.

The boundary-layer flow relevant for the present invention initiates, or is born, at a regular flow-attachment region, also known in the art as a flow stagnation region, such as, but not limited to, the regular flow-attachment region at the leading- edge of the body; at the leading-edge of a member of the body, such as a wing, nacelle, fin, flap, aleron, winglet, or similar appendage; and at the attachment regions occurring in internal flows at points whereat the flow impinges on, and is bifurcated by, a nonporous surface.

If the boundary-layer flow initiates at the regular flow- attachment region in a laminar state, there are several established methods known to those skilled in the art to help maintain the flow in a laminar state. Two of the most widely known methods are the use of an extended region of favorable pressure gradient (i.e. "natural laminar-flow control") and the use of surface suction. Both methods can be combined to form "hybrid laminar-flow control." Localized surface heating has also been shown to stabilize the boundary-layer flow.

Conversely, if the boundary-layer flow is tripped into turbulence near the leading-edge, or at any point along the surface of the object, it is very difficult to re-laminarize the flow at a location further downstream. One of the main technological problems with the establishment and maintanence of the laminar state in a selected boundary-layer flow is the surface roughness created by impact of a foreign object on the surface in the leading-edge region of the body. Such roughness can be created, for example, by the impact of insects on the leading-edge of an aircraft's wing, nacelle, and other appendages, during take-off, landing, and low-altitude flight. The roughness on the object's surface creates turbulent flow, preventing the formation of laminar flow downstream.

Another technological problem is the requirement that the surface of the wing, nacelle, fin, etc, exposed to the boundary-layer flow be smooth enough to support laminar flow, yet be easily displaceable from the object to allow access to the inner machinery on the object. Junctions, rivets, hinges and similar devices found on current designs trip the flow into turbulence, and cannot be used on laminar-flow surfaces described in the prior art .

One design, known to those skilled in the art, for solving the impact problem on the leading-edge proposes the use of a perforated surface in the leading-edge region of the body, and the ejection of a cleaning fluid through this perforated surface for a brief duration in time. The fluid dissolves the proteins that constitute the insect's body, and the resulting liquid evaporates into the atmosphere. This design has five main shortcomings: first, the manufacturing of the porous surface and the apparatus for dispensing the fluid is expensive, thus counteracting the fuel savings from laminar flow; second, the apparatus adds weight, and this also counteracts the fuel savings; third, the wetted surface picks-up dust, which becomes hardened to the surface when the liquid evaporates; fourth, the apparatus is complex, leading to higher maintenance costs; fifth, the apparatus necessitates re-filling at regular intervals, leading to higher operating costs. Another proposed design for solving the impact problem uses flaps that extend from underneath the leading-edge to cover the leading-edge during take-off and ascent. This device has been successfully applied to objects with rectilinear leading-edges (in plan view) , such as wings, but its application to nacelles, or other bodies with oval or cylindrical shape, is too complex and costly.

Another design for sustaining laminar flow and allowing accessibility to interior machinery in nacelles proposes a single, monolithic, nacelle skin movable along the axis of the nacelle upon rails, or skid pads. However, due to the weight of the rails and added mechanisms, the potential savings in fuel consumption are strongly reduced, making the design not desirable .

A further problem with current laminar-flow devices is their lack of a means for re-establishing laminar flow once turbulent flow is initiated. This shortcoming limits the extent of the laminar- flow region to areas near the flow-attachment line at the leading-edge .

As will be seen from the subsequent description of the preferred embodiments of the present invention, these and other shortcomings of the prior art are overcome.

DISCLOSURE OF THE INVENTION

In view and in consideration of the above, the invention aims to achieve the following objects singly or in combination:

to provide a device for establishing and maintaining boundary-layer flow in the laminar state that:

a) is mechanically simple and light-weight; b) can be located at essentially any region of the object's surface;

c) enables the outer skin sustaining laminar flow to be detachable from the object to allow regular access to the interior of the object;

d) can work in conjunction with established methods for sustaining laminar flow;

e) provides an energetic flow ejection into the boundary- layer to hinder flow separation in regions of adverse pressure gradient;

f) can re-establish laminar boundary-layer flow in the presence of an upstream boundary-layer flow in the turbulent state.

The invention further aims to avoid or overcome the disadvantages of the prior art, and to achieve additional advantages as apparent from the present description, claims, abstract, and drawings .

The above objects have been achieved according to the invention in an outer skin arrangement having an outer surface partitioned into a controlled surface region where laminar flow is expected and an uncontrolled surface region; an elongated surface opening located upstream of the controlled surface region and extending through the outer skin to create a virgin leading-edge in the outer skin, the elongated surface opening ingesting at least the approaching boundary-layer flow and thereby creating a new, virgin flow-attachment region at the virgin leading-edge from which the boundary-layer flows in a laminar state onto the controlled surface region; and a channel for accepting the ingested flow and ejecting the ingested flow at a location on the uncontrolled surface region. The preferred embodiment of the channel comprises a channel diffuser for decelerating the ingested flow and recovering most of the ingested fluid's kinetic energy into pressure, a channel center-section for transporting the ingested flow with minimum skin friction and pressure loss, and a channel nozzle for re- accelerating the ingested flow prior to ejection. The energetic boundary layer can be ejected essentially tangent to the uncontrolled surface region to delay or prevent boundary-layer separation at a location further downstream.

In general, the leading-edge of the body or body appendage separates the oncoming flow into a top flow region and a bottom flow region. The embodiments of the invention can be partitioned in two groups: in Group I, the boundary layer is ingested from one of the top or bottom flow regions and ejected into the other flow region; in Group II, the boundary layer is ingested and ejected in the same flow region.

The region of outer skin spanning from essentially the elongated surface opening to essentially the channel exit in Group II embodiments can be detached from the body without affecting the ability of the controlled surface region to sustain laminar flow, allowing regular access to the interior of the body. In particular, the junctions between the detachable and fixed portion of the body may trip the boundary layer into the turbulent state, but this boundary layer is ingested or ejected, hence does not affect the laminar boundary-layer state on the controlled surface region. BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be clearly understood, it will now be described in connection with example embodiments, with reference to the accompanying drawings, wherein:

Fig. 1 is a schematic sectional view of an aerodynamic body representing a nacelle cowl;

Fig. 2 is a plot of normalized pressure iso-lines for the aerodynamic body of Fig. 1, moving through air at altitude and speed typical of commercial jet-powered airplanes;

Fig. 3 is a schematic sectional view of the leading-edge region of an aerodynamic body according to the invention;

Fig. 4 is a schematic sectional view of the leading-edge region near the keel of a nacelle at an angle of attack similar to airplane take-off conditions, according to the invention;

Fig. 5 is a schematic sectional view of a portion of the outer skin of an aerodynamic according to one embodiment of the invention;

Fig. 6 is a schematic sectional view showing further detail near the virgin leading edge for the embodiment shown in Fig. 5; Fig. 7 is a view similar to that of Fig. 6, but showing coupling details between an upstream laminar-flow unit and a downstream laminar-flow unit;

Fig. 8 is a schematic sectional view of the leading-edge region of an aerodynamic body according to another embodiment of the invention, showing the flexible membrane fully expanded;

Fig. 9 is the same view as Fig. 8, but with the membrane contracted;

Fig. 10 is a schematic sectioned perspective view of a portion of the outer skin, according to another embodiment of the invention;

Fig. 11 is a schematic perspective view of an engine nacelle incorporating an embodiment of the current invention in a service position allowing access to the space within the nacelle;

Fig. 12 is a schematic perspective view of a commercial aircraft incorporating embodiments of the invention at different locations.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS AND THE BEST MODE FOR CARRYING OUT THE INVENTION

The invention will now be described in connection with an example relating to an aerodynamic body, such as an engine nacelle cowl, which represents a highly effective and preferred application of the invention. As a reference for the following discussion, Fig. 1 schematically shows the basic geometry of a representative aerodynamic body 100 such as a nacelle cowl or the like of an aircraft. The aerodynamic body 100 has an outer skin 11, and a regular flow-attachment region 20 at the leading edge. When this aerodynamic body 100, or especially the nacelle cowl 100, is moving through the air, the free-stream airflow impinges onto the regular flow-attachment region 20 and separates into a top flow region 14 and a bottom flow region 16. Each flow region grazes along the aerodynamic body 100, thereby creating a boundary layer airflow over at least part of the outer surface 12 of outer skin 11.

For reference, Fig. 1 further shows the outer surface 12 of outer skin 11 divided into a controlled surface region 220 adopted to support boundary-layer flow in the laminar state, and an uncontrolled surface region 210, over which the boundary-layer flow may be in a laminar or turbulent state.

Normalized static pressure iso-contours at cruising speed and altitude of a commercial airplane near the outer skin 11 are shown in figure 2. The actual numerical values are only representative, and secondary in importance to the concepts and procedures to be described below. Starting from these basic points and considerations in connection with Figs. 1, 3 and 5, the underlying characteristic features of the invention will first be developed in the following discussion, and then specific details of the applications thereof will be described.

The regular flow-attachment region 20 is subject to frequent impacts from insects and other particles in the air during takeoff, landing and low-altitude flight. The surface roughness created by the accumulation of these impacts creates sufficient disturbance to trip the boundary-layer flow into a turbulent state. In order to provide the conditions for laminar flow over the controlled surface region 220, an elongated surface opening 310 is located downstream of the regular flow-attachment region 20 and immediately upstream of the beginning of the controlled surface region 220. The elongated surface opening penetrates through the outer skin to generate a new virgin leading edge 200 on the outer skin. The elongated surface opening 310 is sufficiently large to ingest at least the entire boundary-layer approaching the elongated surface opening 310, thereby creating a new virgin flow-attachment region at the virgin leading edge 200. The virgin leading edge 200 is exposed to non-turbulent flow, enabling the laminar-flow state to exists in the boundary- layer developing over the controlled surface region 220. The ingested air is accepted by a channel 300, and ejected through channel exit 320 at some location in the uncontrolled surface region 210.

The preferred embodiment of the channel 300 has a channel diffuser 330 for decelerating the ingested flow and converting most of the kinetic energy of the ingested flow into pressure, a channel center-section 340 for transporting the ingested flow, and a channel nozzle 350 for accelerating the ingested flow prior to flow ejection through the channel exit 320. Preferentially, the channel diffuser is located near the elongated surface opening 310. The channel diffuser is designed to provide as rapid a deceleration as possible without appearance of flow separation along its walls. The procedure for establishing such a design is well known to those skilled in the art. Decelerated flow provides the added beneficial effect of low skin friction between the ingested flow and the walls of the channel 300. Additionally, the channel walls in the center-section 340 are made slightly divergent to create an adverse pressure gradient in the flow direction that is strong enough to maintain the skin friction near zero, but not strong enough to cause flow separation. The ingested flow is, thus, transported through the channel center- section 340 with little loss in pressure, and at a small drag penalty.

Due to the low skin friction within the channel 300, the ingested flow approaches the channel exit 320 having lost essentially minimal total pressure since the ingestion event. Consequently, the ingested flow can be re-accelerated in the channel nozzle 350 to a velocity approaching the value of the free-stream velocity present over the uncontrolled surface region 210 at the location of the channel exit 320. Since the boundary-layer flow over the outer skin 11 does not benefit from the low skin-friction condition inside the channel 300, the ejection of the ingested flow in a direction tangent to the uncontrolled surface region 210 will energize the overall boundary-layer flow downstream of the channel exit 320, thereby endowing the overall boundary-layer to penetrate into a region of stronger adverse pressure gradients and remain attached longer to the outer skin 11 than possible in the absence of flow ejection from the channel exit 320. A region of stronger adverse pressure gradient is created when the controlled surface region 220, having thereupon a favorable pressure gradient for laminar-flow stabilization, is extended towards the trailing edge of the aerodynamic body 100. Thus, a beneficial consequence of the energetic flow ejection from the channel exit 320 is the possibility of extending the controlled surface region 220 further back towards the trailing edge of the aerodynamic body 100. To improve the display of specific details of the application, we partition the embodiments of this invention into two groups :

Group I : the boundary layer is ingested from the top flow region 14 and ejected into the bottom flow region 16, or vice versa;

Group II: the boundary layer is ingested and ejected in the same flow region.

Group I :

Locations labeled A, A', B, C, and D in figure 2 will be used in the following description of one embodiment of the invention.

In reference to figure 3, a first embodiment of the invention has the virgin leading-edge 200 in the vicinity of the regular flow- attachment region 20 of the nacelle cowl 100. The channel 300 separates the nacelle cowl 100 into a forward nacelle part 114 and a rearward nacelle part 116. The forward nacelle part 114 contains the regular flow-attachment region 20, and is held fixed relative to the rearward nacelle part 116 by azimuthally spaced- apart struts (not shown) spanning the width of the channel 300. The forward nacelle part 114 preferably contains conventional means to heat the surface to prevent ice build-up, such as heating tubes 400. Additionally, the core of the forward nacelle part 114 can be filled with a metal matrix material 410 to add structural strength and aid in the diffusion of heat.

The virgin leading-edge 200 is located downstream from the regular flow-attachment region 20 and is separated from the surface of the forward nacelle part 114 by more than the height of the turbulent boundary-layer approaching the virgin leading- edge 200, and preferably by more than 2 times the boundary-layer height (height is defined by the conventional 99% rule, i.e. the point whereat the stream-wise velocity reaches 99% of the free- stream velocity) so as to guarantee that the virgin leading-edge is exposed to a smooth, turbulence-free, undisturbed flow. The flow ensuing from the new stagnation-region on the virgin leading edge 200 is laminar, and is kept laminar over the controlled surface region 220 by conventional means, such as natural laminar-flow control, or such as surface suction applied over part, or all, of the controlled surface region 220.

In the absence of the channel diffuser 330, channel center- section 340, and channel nozzle 350, the channel exit 320 must be located in a region of lower static pressure than that at the elongated surface opening 310 to establish the desired flow through the channel 300. In reference to figure 2, the elongated surface opening can be located at point labeled "A" and exit at the point labeled "B" . More freedom in the selection of the location of the elongated surface opening and channel exit is obtained with the presence of pressure recovery functionality provided by the channel diffuser 330 and channel nozzle 350, as discussed above. In particular, the elongated surface opening 310 can be moved into a region of lower static pressure, the exit can be moved into a region of higher static pressure, or both, and still obtain the desired flow through the channel.

In the preferred embodiment, a heating means, such as the heater- pipe 401, is located near the virgin leading-edge 200 to avoid ice build-up there. An added advantage of heating the virgin leading-edge 200 and near-by outer skin material is the stabilization of the boundary-layer over the controlled surface region 220 that further enhances the boundary-layer flow to remain in the laminar state. Heat to the virgin leading-edge 200 can be produced by cooling engine-bearing oil, for example, or by extracting heat from the engine's exhaust, so as not to subtract power from the engine.

An added advantage of this embodiment occurs on the lower part of the nacelle cowl 100 during take-off. This part of the nacelle is known as the keel, and is exposed to a high angle of attack during take-off. In a conventional nacelle design, the boundary-layer tends to separate from the uncontrolled surface region 13 of the nacelle facing the engine fan as the flow accelerates rapidly away from the regular flow-attachment region 20. When boundary-layer separation occurs, the fan blades are subjected to unequal azimuthal loading that can harm the blade's structural integrity, as well as promote the dangerous situation of engine stall. In reference to figure 4, the regular flow-attachment region 20 on the keel-side of the nacelle moves outwardly towards the outer surface of the nacelle due to, in part, the nacelle's angle of attack during take-off and climb. The static pressure at the elongated surface opening 310 is near maximum since the inlet is situated near the regular flow-attachment region, while the channel exit 320 is located within, or near the onset, of the adverse pressure gradient responsible for promoting separation over the uncontrolled surface region 13 of the nacelle facing the engine fan. The combination of the channel diffuser 330, the channel center-section 340 and the channel nozzle 350 works to ingest the flow near the regular flow-attachment region 20 and transport the ingested flow to the channel exit 320 with a minimum loss of total pressure, hence allowing the ingested flow to be ejected from the channel exit 320 with high momentum and, consequently, help the boundary-layer downstream on the channel exit 320 overcome the adverse pressure gradient over the uncontrolled surface region 210 and remain attached this surface.

In a further embodiment of the invention, the forward nacelle part 114 is slidably attached to the rearward nacelle part 116 to allow the elongated surface opening 310 to close during the flight portion in which insect impact is likely. By closing the elongated surface opening 310, the virgin leading-edge 200 is further protected against insect contamination.

Group II :

Embodiments in this group ingested and ejected the boundary layer in the same flow region. In reference to figure 5, an embodiment of the invention comprises a monolithic laminar-flow unit 150 comprising the outer skin 11 between the elongated surface opening 310 and the channel exit 320 functioning as an upper structural element 221, and a lower structural element 222. The upper structural element 221 contains the virgin leading-edge 200 as well as the controlled surface region 220. The lower structural element 222 mechanically joins with the outer skin 11 and provides a smooth continuation of the outer surface 12. The channel 300 is formed by the space between the upper structural element 221 and the lower structural element 222, and comprises the elongated surface opening 310, the channel diffuser 330, the channel center-section 340, the channel nozzle 350 and the channel exit 320. The upper and lower structural elements are fixedly connected via webbing 260 that is azimuthally spaced- apart, as visible in figures 6 and 7. The webbing 260 extends the length of the channel center-section 340, and extends only partially into the channel diffuser 330 and the channel nozzle 350. At the two lateral extremities of the monolithic laminar- flow unit 150, the webbing extends the entire length of the monolithic laminar-flow unit 150 so as to seal the channel 300 from the lateral influx of fluid. This construction leads to a structural shell having a beneficial high stiffness and strength due to the high moment of inertia created by the upper structural element 221 and lower structural element 222.

The combination of laminar flow on the controlled surface region 220 and of low skin-friction drag on the walls of the channel 300 generates a lower total skin friction drag compared to the drag of a fully turbulent boundary-layer over the outer skin 11. The following rough and simple estimate of skin-friction levels exemplifies this fact:

Consider the simplified case where the controlled surface region 220 is flat, and subjected to a zero pressure gradient. For reasons of example, we fix the height of the channel center- section 340 to be 12 times the height of the elongated surface opening 310. If A denotes the surface area of the controlled surface region 220, then the total wetted area of the monolithic laminar-flow unit 150 is approximately three times A, on account of the wetted surface area of the channel 300. The flow on the controlled surface region 220 is laminar, and an approximate value of the local skin friction at a given location x on this surface, measured in the stream-wise direction from the virgin leading-edge 200, is t_lam= 0.332 p U_e ² ( μ / p U_e x) ^1/2 where p is density, μ is viscosity, and U_e is the velocity of the flow-stream outside the boundary-layer (for example, the airplane's velocity) . Assuming that the flow inside the channel is turbulent, the skin friction on the walls of the channel is similarly approximately given by t_trb= 0.0135 p U_c ² ( μ / p U_c x) ^ιn where U_c is the velocity at the edge of the boundary-layer, which we take to be the average velocity inside the channel center- section 340. Both equations are well known in the art and are easily found in the literature. Next, we consider flight at Mach 0.78 at altitude of 31,000 feet. Then U_e = 240 m/s, p = 0.418 kg/m³, μ = 0.0000145 Pa-s, and we assume, as way of example, that the stream-wise extent of the controlled surface region 220 is 2.5 meters. Under these conditions the total skin drag force on the controlled surface region 220, obtained by integrating equation for t_lara in x, is approximately 8.3 Newtons per meter width of surface (i.e. lateral width).

Similarly, by approximating U_c = U_e /12 (i.e. neglecting the small change in flow density through the channel diffuser) and integrating the equation for t_trb in x , the drag produced by the turbulent, slow moving, flow inside the channel 300 is approximately 1.67 Newtons per meter width. This number includes the factor of 2 in wetted area of the channel 300 in comparison with that of the controlled surface region 220. Accordingly, the total drag of the monolithic laminar-flow unit 150, namely 8.3 + 1.67 Newtons per meter width, is only 20 percent higher than that of laminar flow on the controlled surface region 220 per se. In comparison, when the flow over the controlled surface region 220 is turbulent, the drag force is approximately 87 Newtons per meter width. Thus, the monolithic laminar-flow unit 150 yields essentially the same low-drag benefits as that of laminar flow over the controlled surface region 220 per se. A more accurate estimate of skin friction, based on the solution of the governing differential equations of fluid motion applied to a configuration wherein the controlled surface region 220 extends for 85% of the nacelle chord, yields a total external skin friction reduction of about 50%, with the main difference between the rough estimate and the more accurate estimate being the contribution to skin friction by the diffuser and nozzle sections of the channel 300.

In reference to figure 6, the monolithic laminar-flow unit 150 comprises a forward heating tube 402 and a rearward heating tube 403 running parallel to each other and parallel to the virgin leading-edge 200. The forward heating tube 402 and the rearward heating tube 403 run the length of the virgin leading-edge 200 and then connect to each other form a circuit for the heating fluid in which hot fluid flows first through the rearward heating tube and then through the forward heating tube. This arrangement allows the inlet and outlet to the circuit to be conveniently located at one lateral end of the monolithic laminar-flow unit.

In reference to figures 5 and 6, the monolithic laminar-flow unit 150 can be pulled away from the outer skin 11 to allow access to the machinery below. An embodiment of the invention for removably attaching the monolithic laminar-flow unit 150 to the nacelle is shown in figure 11, where the monolithic laminar-flow unit 150 hinges in a conventional fashion at the crown of the nacelle and has conventional means for fastening the monolithic laminar-flow unit 150 shut at the keel of the nacelle. Flexible tubes 404 connect the forward heating tube 402 and the rearward heating tube 403 to the non-movable part of the nacelle and close the overall heating circuitry.

In reference to figure 7, an upstream monolithic laminar-flow unit 152 and a downstream monolithic laminar-flow unit 151, are be joined to each other to provide an extended region of laminar flow. The two units are adopted to provide a continuous passage of channeled air from the upstream monolithic laminar-flow unit 152 to the downstream monolithic laminar-flow unit 151. In particular, the upstream monolithic laminar-flow unit 152 has a weakly converging passage 352 replacing the usual channel nozzle and channel exit. The downstream monolithic laminar-flow unit 151 has a receiving channel 360 matching with, and receiving fluid from, the weakly converging passage 352 of the upstream monolithic laminar-flow unit 152. The fluid passing through the receiving channel 360 mixes with the flow from the channel diffuser 330 of the downstream monolithic laminar-flow unit 151. Preferentially, a lobed mixer 362 aids this mixing process. In order to avoid a loss in pressure in the flow within the channels of the upstream monolithic laminar-flow unit 152 and the downstream monolithic laminar-flow unit 151 seal against each other in hermetic fashion, using, preferentially, 0-rings 380. More than two monolithic laminar-flow units can be chain connected to create an extended controlled surface region sustaining laminar flow.

Chaining multiple monolithic laminar-flow units together provides at least three advantages. First, any undesired tripping of the laminar boundary-layer over one monolithic laminar-flow unit generates turbulent flow only up to the beginning of the following monolithic laminar-flow unit. Thus, the propagation of turbulent flow is blocked, yielding a robust system for laminar-flow control. Second, laminar flow needs to be maintained only for at least part of the stream-wise extent of each monolithic laminar-flow unit, and since each monolithic laminar- flow unit provides its own virgin leading-edge 200, the extent is limited. This property allows, amongst other things, more flexibility in selecting a surface pressure distribution in natural laminar-flow control, or reduced suction strength in active laminar-flow control. Third, access to the machinery under each monolithic laminar-flow unit can be provided when the embodiments are displaced in consecutive order. For example, in figure 7, the downstream monolithic laminar-flow unit 151 needs to be lifted away, or displaced, prior to the upstream monolithic laminar-flow unit 152. The reverse order is also possible, provided the junction between the embodiments is appropriately changed in orientation.

The small frontal area of the virgin leading-edge 200 greatly reduces the probability of insect impact on the virgin leading- edge 200. In reference to figures 8 and 9, an inflatable surface patch 450 can be placed upstream of the virgin leading-edge 200 to guarantee zero change of insect impact on the virgin leading- edge. The inflatable surface patch 450 rests upon a perforated surface 460 when not inflated. The perforated surface 460 provides exposure of the inflatable surface patch 450 to a pressurizable channel 470. This channel is pressurized when the inflation of the patch 450 is desired, and is slightly de- pressurized at other times to make the patch 450 press against the perforated surface 460, thereby producing a smooth continuation of the surface neighboring the patch 450.

In reference to figure 10, in another embodiment of the invention, a liquid-spray mechanism is located upstream, and in the vicinity of, the virgin leading-edge 200. The liquid-spray mechanism comprises a supply manifold 480 that feeds and ejects cleaning fluid through flush mounted surface orifices 482 to clean the virgin leading-edge. Preferably, the cleaning fluid contains additives for dissolving the largely protein makeup of insects .

The embodiments of the invention can be applied at many locations on a commercial jet 10. In reference to figure 12, a partial list of the locations that can receive the embodiments of this invention comprises: the nacelle 160, the upper wing surface 162, the lower wing surface (not shown) , the winglets (not shown) , the horizontal stabilizer 164, the vertical stabilizer (fin) 166, and the fuselage 168, especially in regions near a change in surface pressure, such as near the end of the well known fuselage bulge in the Boeing 747.

INDUSTRIAL APPLICABILITY

The present inventive structure for influencing or controlling the boundary-layer flow of a fluid along a surface is especially applicable to the outer skin of nacelles, and airfoil members such as the wings, stabilizers, fins and control surfaces of aircraft. The inventive structure is further applicable to any other situation involving a relatively high speed boundary-layer flow of a fluid along a surface, such as fan blades and turbine blades within a jet engine, helicopter rotor blades, ship propellers, ship rudders, blades of wind turbines, as well as surfaces of missiles, torpedoes, and submarines.

Claims

1. A laminar flow apparatus comprising: an outer skin having an outer surface adapted to be exposed to a fluid flow including a boundary-layer flow stream flowing along said outer surface, the outer skin further having an elongated surface opening communicating therethrough to create a new virgin leading-edge on said outer skin, the elongated surface opening ingesting at least said boundary-layer flow stream arriving at the elongated surface opening, the process of ingestion creating a virgin flow-attachment region on the virgin leading-edge, the outer surface further having a controlled surface region adopted to promote laminar flow, said controlled surface region extending downstream of said virgin leading-edge, and; a channel communicating with the elongated surface opening and receiving said ingested boundary-layer flow stream, the channel having a channel diffuser for decelerating the ingested flow stream, a channel center- section for transporting the ingested flow stream, a channel nozzle section for accelerating the ingested flow stream, and a channel exit for ejecting the ingested flow stream; characterized in that said channel exit ejects the ingested flow stream at a location outside the controlled surface region.

2. The laminar flow apparatus according to claim 1, further comprising a heating means for heating the virtual leading edge.

3. The laminar flow apparatus according to claim 1, further comprising an inflatable surface patch upstream of the elongated surface opening.

4. The laminar flow apparatus according to claim 1, further comprising flush mounted surface orifices on the outer skin located upstream of elongated surface opening, the surface orifices ejecting cleaning fluid onto the virtual leading edge.

5. In combination with a aerodynamic body possessing a regular flow-attachment region when exposed to oncoming fluid flow, a laminar-flow apparatus comprising: an outer skin having an outer surface exposed to a boundary-layer flow stream flowing along said outer surface when said aerodynamic body is exposed to oncoming fluid flow, the outer skin further having an elongated surface opening communicating therethrough to create a virgin leading-edge in the outer skin, the elongated surface opening being located downstream from said regular flow attachment region and ingesting at least said boundary- layer flow stream arriving at the elongated surface opening, the process of ingestion forming a virgin flow- attachment region on said virgin leading-edge, the outer surface further having a controlled surface region adopted to promote laminar flow, said controlled surface region extending downstream of said virgin leading-edge, and; a channel communicating with the elongated surface opening and receiving said ingested boundary-layer flow stream, the channel having a channel diffuser for decelerating the ingested flow stream, a channel center- section for transporting the ingested flow stream, and a channel exit for ejecting the ingested flow stream; characterized in that said channel exit ejects the ingested flow stream at a location outside the controlled surface region.

6. The laminar flow apparatus according to claim 5, wherein the channel further comprises a channel nozzle positioned between the channel center-section and the channel exit, the channel nozzle accelerating the ingested flow prior to ejection through the channel exit.

7. The laminar flow apparatus according to claim 6, characterized in that said elongated surface opening is oriented essentially parallel to said regular flow- attachment region.

8. The laminar flow apparatus according to claim 6, characterized in that said regular flow-attachment region partitions the oncoming flow into a top flow region and a bottom flow region, said elongated surface opening ingests flow from the top flow region and said channel exit ejects flow into the bottom flow region.

9. The laminar flow apparatus according to claim 6, characterized in that said streamlined object is an engine nacelle, said channel divides said nacelle into a forward nacelle part containing said regular flow-attachment region and a rearward nacelle part, and said forward nacelle part is slidably united with the rearward nacelle part so as to allow the width of said channel to be varied.

10. The laminar flow apparatus according to claim 6, characterized in that said regular flow-attachment region partitions the oncoming flow into a top flow region and a bottom flow region, and said channel exit ejects the injected flow stream into the same flow region from which the elongated surface opening ingests said boundary-layer flow.

11. The laminar flow apparatus according to claim 10, characterized in that the channel runs essentially parallel to said outer skin.

12. The laminar flow apparatus of claim 11, characterized in that the channel is defined by: the outer skin portion extending from the virtual leading edge to the channel exit, said outer skin portion functioning as an upper structural element, and a lower structural element, the lower structural element being in structural union and fixedly connected with said upper structural element to form a monolithic laminar-flow unit, and further characterized in that the lower structural element mechanically joins with the outer skin portion upstream of the elongated opening and the outer skin portion downstream of the channel exit.

13. The laminar flow apparatus of claim 12, characterized in that the monolithic laminar-flow unit is separable from the outer skin portion upstream of the elongated opening and from the outer skin portion downstream of the channel exit, thereby allowing access to the space within the aerodynamic body immediately below the monolithic laminar-flow unit.

14. The laminar flow apparatus of claim 6, wherein the aerodynamic body is part of an engine nacelle.

15. The laminar flow apparatus of claim 6, wherein the aerodynamic body is part of an aircraft lifting surface.

16. In combination with an aerodynamic body possessing an outer skin having an outer surface adapted to be exposed to a fluid flow including a boundary-layer flow stream flowing along said outer surface, a laminar-flow apparatus comprising: a multitude of aligned laminar-flow units consecutively arranged in the downstream direction, each laminar-flow unit comprising: an elongated surface opening communicating through the outer skin to create a virgin leading-edge on said outer skin, the elongated surface opening being located downstream from said regular flow attachment region and ingesting at least said boundary-layer flow stream arriving at the elongated surface opening to form a virgin flow-attachment region on the virgin leading-edge, the outer surface further having a controlled surface region adopted to promote laminar flow, said controlled surface region extending downstream of said virgin leading-edge, and; a channel communicating with the elongated surface opening and receiving said ingested boundary- layer flow stream, the channel having a channel diffuser for decelerating the ingested flow stream and a channel center-section for transporting the ingested flow stream, the channel running essentially parallel to the outer surface, the channel defined by the outer skin portion extending from the virtual leading edge to the end of the channel center-section, said outer skin portion functioning as an upper structural element, and defined by a lower structural element, the lower structural element being in structural union and fixedly connected with said upper structural element to form a monolithic structure;

each laminar-flow unit except the last laminar-flow unit transferring the ingested fluid onto the channel of the next downstream laminar-flow unit,

the last laminar-flow unit having a channel nozzle downstream of the channel center-section and accelerating the ingested flow, the last laminar-flow unit having a channel exit for ejecting the ingested flow at a location outside all the controlled surface regions of said laminar- flow apparatus .