Patent Description:
Exhaust systems for aircraft or other vehicles may have features that mix hot exhaust air and cooler ambient air to limit temperatures of the surrounding vehicle structure or to limit the heat signature of the vehicle. One method of achieving cooler exhaust temperatures is to attach a flow mixer to the outlet of the exhaust system. At least some known flow mixers include a plurality of lobes, or "flutes," that promote mixing of the exhaust air and cooler ambient air.

A potential drawback is that the stiffness of the flow mixer may be reduced because of the lobed or fluted designs. Consequently, at engine or rotor frequencies that align with the flow mixer's natural frequency, and such alignment of natural frequencies (resonance) may lead to increased deflection and stresses that may limit or prevent use of a fluted design, or may use thicker, heavier duct walls. Further, static and/or dynamic pressures on the walls of the flutes may cause large deflections in the area of the flutes and may impact flow mixing.

Duct size and configuration of an exhaust system may also contribute to natural frequency. At least some known exhaust systems use ducts having relatively small flow mixers in an attempt to avoid certain frequencies without additional stiffening features. However, such relatively low-flow mixers may not provide optimal efficient mixing of hot exhaust air and cooler ambient air. Furthermore, at least some exhaust systems may use multiple small, relatively low-flow mixers to increase the mixing efficiency. However, multiple flow mixers may increase manufacturing costs, maintenance costs, and/or weight associated with the exhaust system.

<CIT>, in accordance with its abstract, relates to a mixing nozzle for an aircraft turbine engine having an improved aerodynamic stiffening ring secured to the lobes of the mixer. Aircraft turbine engines having a rearwardly discharging nozzle are often provided with a mixer having a generally circular forward end adapted to be attached to an engine and axially deepening corrugations leading to a multi-lobed aft end surrounding the nozzle. This mixer mixes ambient cool air with the hot gases exiting the engine thus suppressing engine sound. The lobes are generally unsupported and subject to vibration and excessive deflections in use. A circumferential stiffening ring is secured to the aft end of the mixer to reduce or prevent the vibration while simultaneously enhancing the mixing of cool ambient air with the hot engine exhaust. The stiffening ring preferably has an airfoil cross-section for lowest drag combined with the ability to direct flow past the ring in a selected optimum direction.

<CIT>, in accordance with its abstract, relates to a lobe type mixer that comprises a pair of an upper and a lower symmetric type variable lobes opened from each other. Each lobe comprises plural recessed parts tapered off, and protruded parts adjacent to the recessed parts to be continuous with each other, and side surface parts are provided to compose side surfaces of the lobes so swirl to turn along flow is formed between a high speed gas flow on the inside of the lobes and an introduced air flow on the outside, where the corresponding recessed parts of the upper and lower changeable lobes are coupled with each other by a place member parallel to the flow direction.

In one aspect, and according to claim <NUM>, the system described herein includes a duct for directing a flow of exhaust, the duct including a wall portion defining a passageway having an inlet portion adapted to receive the flow of exhaust and an outlet portion adapted to discharge the flow of exhaust. A plurality of flutes is defined at the outlet portion, and the plurality of flutes include a first flute and a second flute spaced from the first flute. The first flute has a peak, a trough, a first height dimension and a first width dimension generally perpendicular to the first height dimension, and the second flute has a peak, a trough, a second height dimension and a second width dimension generally perpendicular to the second height dimension. Two flute ties are respectively coupled to the first trough and to the second trough.

At least one retainer is couplec to the trough of the first flute and the trough of the second flute in addition to or instead of the at least two flute ties and extends generally parallel to at least one of the first width dimension and the second width dimension, wherein the retainer is configured to restrain relative movement between the first flute and the second flute and relative movement among the first flute, the second flute, and at least one of the inlet portion and the outlet portion during the flow of exhaust through the duct.

In another aspect, not part of the subject-matter of any of the attached claims, a stiffener apparatus for an exhaust duct is described anc includes a first row of flutes and a second row of flutes generally opposite to the first row of flutes, and each flute is elongated along a respective axis, and the stiffener apparatus includes at least two flute ties coupling together at least two flutes from the first row of flutes to at least two flutes from the second row of flutes. At least one retainer is coupled to the at least two flute ties and extends generally perpendicularly to the axis of each of the at least two flutes from the first row of flutes and the at least two flutes from the second row of flutes, wherein the retainer is configured to generally restrain relative movement between the at least two flutes from the first row of flutes and the at least two flutes from the second row of flutes.

In a further aspect, and according to claim <NUM>, a method is described for increasing the resonant frequency of an exhaust system having at least one duct, the duct including a plurality of flutes. Each flute includes a trough and an elongated portion with a respective axis and a lateral portion extending generally perpendicular to the respective axis, wherein the length of the elongated portion is greater than the width of the lateral portion. The method includes selecting a first flute from the plurality of flutes, the first flute having a first axis and selecting a second flute from the plurality of flutes, the second flute having a second axis generally parallel to the first axis. The retainer is oriented to extend generally perpendicular to the first and second axes, and a first portion of the retainer is coupled to the trough of the first flute and a second portion of the retainer is coupled to the trough of the second flute such that the retainer generally restrains movement between the first flute and the second flute.

In one implementation, not part of the subject-matter of any of the attached claims, a method is described for increasing the resonant frequency of an exhaust system having at least one duct, the duct including at least one row of flutes with a first side of the row having flutes oriented in a first direction and a second side of the row having flutes oriented in a second direction generally opposite to the first direction. Each flute defines an elongated portion extending along a respective axis and a lateral portion extending generally perpendicular to the respective axis, wherein the length of the elongated portion is greater than the width of the lateral portion. The method includes selecting a first flute and a second flute from the first side of the row of flutes and selecting a third flute and a fourth flute from the second side of the row of flutes. A first flute tie is coupled to the first flute and the third flute such that the first flute tie is generally parallel to the axis of each of the first flute and the third flute, and a second flute tie is coupled to the second flute and the fourth flute such that the second flute tie is generally parallel to the axis of each of the second flute and the fourth flute. A retainer is oriented generally perpendicular to a length of at least one of the first flute tie and the second flute tie. A first portion of the retainer is coupled to the first flute, and a second portion of the retainer is coupled to the second flute such that the retainer restrains movement between the first flute and the second flute, wherein the retainer restrains movement among the first flute, second flute, third flute, and fourth flute.

In another implementation, an aircraft is described which includes an implementation of a duct as described herein.

Further, the disclosure also comprises duct for directing a flow of exhaust gases, not part of the subject-matter of any of the attached claims, the duct comprising: a wall portion defining a passageway having an inlet portion adapted to receive the flow of exhaust gases and an outlet portion adapted to discharge the flow of exhaust gases; a plurality of flutes defined at the outlet portion, the plurality of flutes including a first flute and a second flute spaced from the first flute, the first flute having a first peak, a first trough, a first height dimension, and a first width dimension generally perpendicular to the first height dimension, the second flute having a second peak, a second trough, a second height dimension, and a second width dimension generally perpendicular to the second height dimension; and at least one retainer coupled to the first trough and the second trough and extending generally parallel to at least one of the first width dimension and the second width dimension, wherein the at least one retainer is configured to restrain relative movement between the first flute and the second flute and relative movement among the first flute, the second flute, and at least one of the inlet portion and the outlet portion during the flow of exhaust gases through the duct.

The duct can comprise a third flute generally opposed to the first flute; a fourth flute generally opposed to the second flute; a first flute tie coupled to the third flute and the first flute; a second flute tie coupled to the fourth flute and the second flute, and wherein the at least one retainer is coupled to the first flute tie and the second flute tie.

A first axis may correspond to the first height dimension, the third flute may have a third height dimension, and a third axis may correspond to the third height dimension, and wherein the first axis and the third axis are generally collinear with respect to one another.

The third flute may have a third height dimension and a third axis may correspond to the third height dimension, the fourth flute can have a fourth height dimension and a fourth axis may correspond to the fourth height dimension, and wherein the third axis and the fourth axis can be generally parallel to one another.

The first flute can be adjacent the second flute; the third flute can be adjacent the fourth flute; and the first flute tie can be generally parallel to the second flute tie.

The duct may further comprise: a third flute generally opposed to the first flute; a first flute tie coupled to the third flute and the first flute; a fourth flute generally opposed to the second flute; a second flute tie coupled to the fourth flute and the second flute; a fifth flute between the first flute and the second flute; and a sixth flute generally opposed to the fifth flute.

The retainer can be coupled to the first flute tie and the second flute tie, and the third flute can be configured to have freedom of movement with respect to at least one of the at least one retainer, the first flute, the second flute, and the fourth flute.

The duct can further comprise a third flute tie coupled to the fifth flute tie and the sixth flute tie.

The at least one retainer can be coupled to the first flute tie, the second flute tie, and the third flute tie.

The duct can comprise a third flute generally opposed to the first flute; a first flute tie coupled to the third flute and the first flute; a fourth flute generally opposed to the second flute; a second flute tie coupled to the fourth flute and the second flute; a fifth flute; a sixth flute generally opposed to the fifth flute; a seventh flute; and an eighth flute generally opposed to the seventh flute.

The duct may further comprise a third flute tie coupled to the fifth flute and the sixth flute; and a fourth flute tie coupled to the seventh flute and the eighth flute.

At least one retainer can be coupled to the first flute tie, the second flute tie, the third flute tie, and the fourth flute tie.

The at least one retainer may comprise a first retainer and a second retainer.

The first retainer can be coupled to at least the first flute tie and the second flute tie, and the second retainer can be coupled to at least the third flute tie and the fourth flute tie.

At least one of the plurality of flutes can be configured to have freedom of movement with respect to at least one other of the plurality of flutes.

A first axis may correspond to the first height dimension and a second axis may correspond to the second height dimension, and wherein the first axis and the second axis can be generally parallel to one another.

The outlet portion has can have centerline, a first axis corresponding to the first height dimension, and a second axis corresponding to the second height dimension, and wherein the first axis and the second axis may extend generally radially with respect to the centerline.

The first height dimension can be greater than the first width dimension and the second height dimension can be greater than the second width dimension.

An aircraft may comprise an exhaust system comprising this duct.

The disclosure also relates to a stiffener mechanism for an exhaust duct, such as descried above, not part of the subject-matter of any of the attached claims, the exhaust duct including a first row of flutes and a second row of flutes generally opposite to the first row of flutes, each flute being elongated along a respective axis, the stiffener mechanism comprising: at least two flute ties coupling together at least two flutes from the first row of flutes to at least two flutes from the second row of flutes; and at least one retainer coupled to the at least two flute ties and extending generally perpendicularly to the axis of each of the at least two flutes from the first row of flutes and the at least two flutes from the second row of flutes, wherein the at least one retainer is configured to generally restrain relative movement between the at least two flutes from the first row of flutes and the at least two flutes from the second row of flutes.

The disclosure also relates to a method of increasing a resonant frequency of an exhaust system, not part of the subject-matter of any of the attached claims and having at least one duct, the duct including a plurality of flutes, each flute including a trough and an elongated portion with a respective axis and a lateral portion extending generally perpendicular to the respective axis, wherein a length of the elongated portion is greater than a width of the lateral portion, the method comprising: selecting a first flute from the plurality of flutes, the first flute having a first axis; selecting a second flute from the plurality of flutes, the second flute having a second axis generally parallel to the first axis; orienting a retainer to extend generally perpendicular to the first and second axes; and coupling a first portion of the retainer to the trough of the first flute and a second portion of the retainer to the trough of the second flute such that the retainer generally restrains movement between the first flute and the second flute.

Having thus described exemplary aspects of the disclosure in general terms, various features and attendant advantages of the disclosed concepts will become more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, which are not necessarily drawn to scale, in which like reference characters designate the same or similar parts throughout the several views, and wherein:.

Examples of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all examples of the disclosure are shown. Indeed, various exemplary aspects of the disclosure may be embodied in many different forms and should not be construed as limited to the examples set forth herein. Rather, these examples are provided so that this disclosure will be thorough and complete and will fully convey the scope of the disclosure to those skilled in the art.

The systems and/or methods described herein may restrict or restrain movement of an exhaust system by, at least in part, stiffening the exhaust system or a portion thereof to increase the natural frequency of the exhaust system while facilitating efficient mixing of hot exhaust air and cooler ambient air. More specifically, the herein-described systems and methods may restrain and/or stiffen an exhaust duct. In one aspect, at least one retainer is coupled to at least two flutes of a fluted duct to prevent relative movement between the flutes, thus facilitating reduction in stresses in the flutes.

Referring now to <FIG>, an aircraft <NUM> is illustrated. In a particular implementation, aircraft <NUM> is a rotorcraft. In other suitable implementations, the aircraft <NUM> may be vehicle that travels through airspace, such as, but not limited to, airplanes, unmanned aerial vehicles (UAVs), gliders, helicopters, spacecraft, and reusable launch vehicles, and/or any other object that travels through airspace. Furthermore, although the embodiments described herein are described as related to an aircraft, it is contemplated that systems and methods described herein can be implemented on any ground vehicle or waterborne vessel.

In the exemplary implementation, aircraft <NUM> is a rotorcraft including a nose <NUM>, a body <NUM>, a boom <NUM>, and a tail <NUM>. A rotorshaft <NUM> extends outward from body <NUM> and is coupled to at least one rotor <NUM> that rotates about rotorshaft <NUM> to provide aircraft <NUM> with lift and thrust. Aircraft <NUM> also includes an exhaust system <NUM> that includes an engine <NUM>, an exhaust duct <NUM>, and a flow mixer <NUM>. Engine <NUM> is generally located in body <NUM> and is coupled to rotorshaft <NUM> such that engine <NUM> provides the power necessary to rotate rotorshaft <NUM>. During operation, engine <NUM> generates hot exhaust gases <NUM> that are channeled through exhaust duct <NUM> and discharged from aircraft <NUM> through flow mixer <NUM> to the relatively cooler ambient air <NUM>. Flow mixer <NUM> is configured to mix the flows of exhaust gases <NUM> and ambient air <NUM> to produce a gas mixture <NUM> that reduces the effect of hot exhaust gases <NUM> impinging on downstream components of aircraft <NUM>, such as boom <NUM> and tail <NUM>. Mixing of exhaust gases <NUM> and ambient air <NUM> also facilitates reducing the heat signature of aircraft <NUM> to potentially conceal aircraft <NUM> from detection and heat seeking weapons.

<FIG> is a perspective view of a portion of exhaust system <NUM> illustrating exhaust duct <NUM>, which includes an inlet end, or portion, <NUM>, and an outlet end, or portion, <NUM> with an exemplary flow mixer <NUM>. An intermediate portion <NUM> between the inlet portion <NUM> and the outlet portion <NUM> is configured to channel the flow of exhaust between the inlet portion <NUM> and the outlet portion <NUM>. Flow mixer <NUM> is one implementation of flow mixer <NUM> that may be used with exhaust system <NUM>. In the implementation shown in <FIG>, flow mixer <NUM> is fabricated from a ceramic matrix composite (CMC) material that is able to repeatedly withstand exposure to hot exhaust gases <NUM>. In another suitable implementation, flow mixer <NUM> is fabricated from a metallic or metal alloy material. Generally, flow mixer <NUM> may be fabricated from any material that facilitates operation of exhaust system <NUM> as described herein.

Inlet end <NUM> of exhaust duct <NUM> is coupled to an outlet end of exhaust system <NUM>. In the exemplary embodiment, outlet end <NUM> of exhaust duct <NUM> transitions gradually from the inlet end <NUM> to a lobed or fluted shape that facilitates mixing flow of hot exhaust gases <NUM> from exhaust duct <NUM> with cooler ambient air. Outlet end <NUM> includes a plurality of flutes, generally F, that are spaced circumferentially about outlet end <NUM> to form flow mixer <NUM>. More specifically, in the exemplary implementation, plurality of flutes F at outlet end <NUM> includes a plurality of adjacent upper flutes <NUM> and a plurality of adjacent lower flutes <NUM> that are spaced from one another in two horizontal rows perpendicular to the plane of outlet end <NUM>. Upper flutes <NUM> are spaced from lower flutes <NUM> by a predetermined distance D<NUM> such that a gap <NUM> is defined between upper flutes <NUM> and lower flutes <NUM>. In another suitable implementation, flow mixer <NUM> may be oriented at <NUM> degrees or some other angle from the orientation shown in <FIG> such that outlet end <NUM> includes two substantially vertically-oriented or otherwise oriented rows of flutes. Generally, flow mixer <NUM> may be oriented in any manner that facilitates operation of flow mixer as described herein.

Outlet end <NUM> is formed by continuous an inner surface <NUM> and an outer surface <NUM>, respectively, of a wall portion, generally <NUM>. Wall portion <NUM> includes elongated portions <NUM> forms a plurality of vertically-oriented, alternating flute peaks and flute troughs. More specifically, upper flutes <NUM> each include an upper peak <NUM>, and lower flutes <NUM> each include a lower peak <NUM>. Similarly, upper flutes <NUM> each include an upper trough <NUM>, and lower flutes <NUM> each include a lower trough <NUM>. Wall portion <NUM> defines a plurality of sidewalls <NUM> and a plurality of sidewalls <NUM>. Each sidewall <NUM> and <NUM> has a height dimension, and a width dimension is defined between adjacent sidewalls <NUM> or <NUM>. The height dimensions are oriented substantially vertically and are parallel each adjacent sidewall <NUM> and/or <NUM>. In another suitable implementation, sidewalls <NUM> and/or <NUM> may have any orientation and may not be parallel to an adjacent sidewall <NUM> and/or <NUM>.

Upper flutes <NUM> may be defined by plurality of sidewalls <NUM>. Each upper peak <NUM> extends between a pair of adjacent sidewalls <NUM>, and, similarly, each upper trough <NUM> extends between an adjacent pair of sidewalls <NUM> such that one upper peak <NUM> and an adjacent upper trough <NUM> share a common sidewall <NUM>. Similarly, with respect to lower flutes <NUM>, each lower peak <NUM> extends between a pair of adjacent sidewalls <NUM>, and, similarly, each lower trough <NUM> extends between an adjacent pair of sidewalls <NUM> such that one lower peak <NUM> and an adjacent lower trough <NUM> share a common sidewall <NUM>.

Flow mixer <NUM> may be configured such that each upper flute <NUM> is oriented in parallel with an opposing lower flute <NUM>. That is, each upper flute <NUM> of the plurality of upper flutes <NUM> is generally aligned along on an axis Y, such as illustrated in <FIG>, namely, Y<NUM>, Y<NUM>, etc., and a corresponding lower flute <NUM> of the plurality of lower flutes <NUM> is aligned along the same axis Y<NUM>, Y<NUM>, etc.. More specifically, an apex <NUM> (shown in <FIG>) of each upper trough <NUM> is aligned with an apex <NUM> (shown in <FIG>) of a corresponding lower trough <NUM> such that apexes <NUM> and <NUM> are spaced apart by predetermined distance D<NUM> along a respective axis Y. Because upper troughs <NUM> are aligned with a corresponding lower trough <NUM>, it follows that each upper peak <NUM> is aligned with a corresponding lower peak <NUM> and that upper sidewalls <NUM> are aligned with lower sidewalls <NUM>.

Upper peaks <NUM> and lower peaks <NUM> and upper troughs <NUM> and lower troughs <NUM> facilitate mixing cool ambient air <NUM> with hot exhaust gases <NUM> to facilitate producing a steady and spatially uniform flow of gas mixture <NUM> (shown in <FIG>). In operation, the flow of ambient air <NUM> is directed along exhaust duct <NUM> and around peaks <NUM> and <NUM> and through troughs <NUM> and <NUM> where at least a portion of the flow of ambient air <NUM> is directed towards an axis <NUM>. Simultaneously, hot exhaust gases <NUM> is directed through exhaust duct <NUM> and through peaks <NUM> and <NUM> and around troughs <NUM> and <NUM> where at least a portion of hot exhaust gases <NUM> is directed towards axis <NUM>. Peaks <NUM> and <NUM> and troughs <NUM> and <NUM> substantially vertically "slice" each respective flow of ambient air <NUM> and gases <NUM> to facilitate mixing flows of gases <NUM> and ambient air <NUM> into the flow of gas mixture <NUM> that is cooler than the flow of hot exhaust gases <NUM>.

<FIG> is an enlarged view of a portion <NUM> (shown in <FIG>) of flow mixer <NUM> illustrating a stiffener mechanism <NUM> and retainer, generally <NUM>, coupled thereto. Retainer <NUM> is discussed in more detail below. Stiffener mechanism <NUM> is coupled between an upper flute <NUM> and a corresponding lower flute <NUM>, and, more specifically, between an upper trough <NUM> and a corresponding lower trough <NUM>. In such a configuration, stiffener mechanism <NUM> can restrict movement of upper flutes <NUM> and lower flutes <NUM> with respect to each other. Coupling upper flutes <NUM> and lower flutes <NUM> together using stiffener mechanism <NUM> facilitates raising the natural frequency of flow mixer <NUM> sufficiently to prevent or avoid deflections of upper flutes <NUM> and lower flutes <NUM> resulting from resonance with a rotor frequency. Although stiffener mechanism <NUM> is described herein as extending between corresponding upper and lower sets of upper flutes <NUM> and lower flutes <NUM> of flow mixer <NUM>, it is contemplated that stiffener mechanism <NUM> may be used on any fluted flow mixer and is not limited to use with only flow mixer <NUM> as described herein. For example, stiffener mechanism <NUM> may extend between two or more flutes of a substantially radial flow mixer having circumferentially-spaced flutes. In such a configuration, stiffener mechanism <NUM> may extend between any number of flutes and is not restricted to extending between only two such flutes.

Stiffener mechanism <NUM> preferably includes an upper cap <NUM>, a lower cap <NUM>, and a flute tie, or body portion, <NUM> coupled between upper cap <NUM> and lower cap <NUM>. One upper cap <NUM> is positioned within an upper trough <NUM> of each pair of opposing upper troughs <NUM> and lower troughs <NUM> such that an arcuate bottom surface <NUM> of upper cap <NUM> is in contact with a substantially complementary arcuate surface <NUM> of upper trough <NUM>. Similarly, one lower cap <NUM> is positioned within a lower trough <NUM> of each pair of opposing upper trough <NUM> and lower troughs <NUM> such that an arcuate bottom surface <NUM> of lower cap <NUM> is in contact with a substantially complementary arcuate surface <NUM> of lower trough <NUM>.

Furthermore, each upper cap <NUM> includes an opening <NUM> defined therethrough that is configured to receive an upper fastener <NUM> inserted therein. Similarly, each lower cap <NUM> includes an opening <NUM> defined therethrough that is configured to receive a lower fastener <NUM> inserted therein. Each opening <NUM> and <NUM> is counter-bored such that the top of each fastener <NUM> and <NUM> is substantially flush with a top surface of a respective cap <NUM> and <NUM>. As such, upper caps <NUM> and lower caps <NUM> and upper fasteners <NUM> and lower fasteners <NUM> have a substantially thin profile within respective upper troughs <NUM> and lower troughs <NUM>. The thin profile of caps <NUM> and <NUM> facilitates a laminar airflow over the top of caps <NUM> and <NUM> such that caps <NUM> and <NUM> do not interfere with the performance of flow mixer <NUM> and/or cause turbulence in the airflow. Each of upper caps <NUM> and lower caps <NUM> can include at least one flow mixing feature, such as a flute F that facilitates efficient mixing of hot exhaust gases <NUM> with ambient air <NUM>.

In <FIG>, body portion <NUM> of stiffener mechanism <NUM> may include an upper end <NUM> coupled to upper trough <NUM> and a lower end <NUM> coupled to lower trough <NUM>. Upper end <NUM> includes an upper opening <NUM> that is aligned with opening <NUM> in upper cap <NUM> such that upper fastener <NUM> is inserted through opening <NUM>, through upper trough <NUM>, and into opening <NUM>. As such, at least a portion of upper trough <NUM> is coupled between upper cap <NUM> and upper end <NUM> of body portion <NUM>. Similarly, lower end <NUM> includes a lower opening <NUM> that is aligned with opening <NUM> in lower cap <NUM> such that lower fastener <NUM> is inserted through opening <NUM>, through lower trough <NUM>, and into the lower opening <NUM>. As such, at least a portion of lower trough <NUM> is coupled between lower cap <NUM> and lower end <NUM> of body portion <NUM>. In a configuration in which each of fasteners <NUM> and <NUM> are oriented substantially parallel to respective sidewalls <NUM> and <NUM>, fasteners <NUM> and <NUM> are subjected to primarily tension and/or compression loading when upper flutes <NUM> and lower flutes <NUM> are subjected to stresses that would cause deflections if not for stiffener mechanism <NUM>.

Body portion <NUM> may be configured to not unnecessarily impede the flow of enable hot exhaust gases <NUM> (shown in <FIG>) through outlet end <NUM>. As such, body portion <NUM> facilitates maintaining the aerodynamic and performance features of flow mixer <NUM>. , Body portion <NUM> can include any cross-sectional profile shape that enables flow mixer <NUM> to operate as described herein.

In <FIG> and <FIG>, stiffener mechanism <NUM> is preferably fabricated from a metallic or metal alloy material that is able to repeatedly withstand exposure to hot exhaust gases <NUM>. In another suitable implementation, stiffener mechanism <NUM> is fabricated from a CMC material. Generally, stiffener mechanism <NUM> may be fabricated from any material that facilitates operation of exhaust system <NUM> as described herein. Furthermore, body portion <NUM> may be fabricated from a different material than upper caps <NUM> and lower caps <NUM>. The body portion <NUM> is preferably removable from caps <NUM> and <NUM> and from flow mixer <NUM> to enable replacement thereof due to impact events or prolonged exposure. Additionally, body portion <NUM> may be substantially hollow with the exception of where fasteners <NUM> and <NUM> are inserted therein. Such a hollow structure reduces the weight of stiffener mechanism <NUM> and may be fabricated using a <NUM>-dimensional printing process of any material described above.

Although <FIG>, <FIG>, and <FIG> depict a stiffener mechanism <NUM> between each pair or corresponding upper flutes <NUM> and lower flutes <NUM>, it is contemplated that flow mixer <NUM> may include fewer stiffener mechanisms <NUM>, such as at only every other pair of flutes <NUM> and <NUM> or only a stiffener mechanism <NUM> at the ends of each plurality of flutes <NUM> and <NUM>. Generally, flow mixer <NUM> may include any number of stiffener mechanisms <NUM> that enable exhaust system <NUM> to operate as described herein.

Turning to <FIG>, <FIG> and <FIG>, the retainer <NUM> of stiffener mechanism <NUM> will be discussed in more detail. In <FIG>, retainer <NUM> is shown spanning between and interconnecting the body portions <NUM> joining opposing upper troughs <NUM> and lower troughs <NUM> of flutes F, each of the flutes defining a height dimension H and a width dimension W generally perpendicular to the height dimension, wherein the height dimension is greater than the width dimension. The joining together of body portions, or flute ties, <NUM> adds further rigidity and stiffness to flow mixer <NUM>, thereby facilitating the further raising the natural frequency of flow mixer <NUM>.

Retainer, generally <NUM>, could be a single member or could include two or more retainer segments (as shown in <FIG> and <FIG>), can be implemented in a variety of different configurations for stiffening exhaust system <NUM>, and only several of such configurations are shown herein. In the implementation shown in <FIG>, retainer <NUM> extends generally perpendicular to one or more vertical axes, e.g., Y<NUM>-<NUM> (shown in <FIG>) that extend collinear to the respective height dimension H of each flute F. Such vertical axes are generally parallel to one another and, accordingly, are generally perpendicular with respect to and correspond to the width dimensions W that are measured in the direction of horizontal axes X, e.g., X<NUM>, X<NUM>, of each flute F of an upper row <NUM> and a lower row <NUM> of flutes F (shown in <FIG>). Flutes F in upper row <NUM> are considered to be upper flutes <NUM>, and flutes F in lower row <NUM> are considered to be lower flutes <NUM>. Retainer <NUM> is configured to generally restrain relative movement between adjacent flutes F and also between flutes F and remainder of the exhaust system <NUM>, including inlet portion <NUM> and the outlet portion <NUM>.

Retainer <NUM> is preferably, an elongated strip having a top surface <NUM>, a bottom surface <NUM>, and a rear surface, or trailing edge, <NUM>, and could be constructed of a material such as INCONEL® <NUM>, or any other suitable material. ("Inconel" is a registered trademark of Huntington Alloys Corporation of West Virginia, USA. ) Retainer <NUM> can be coupled to troughs <NUM> and/or <NUM> of flutes F via one or more flute ties <NUM> by welding, adhesive, mechanical fasteners (not shown), or other suitable means. The trailing surface of flute ties <NUM> may include a slot or notch <NUM> (<FIG>) configured to receive retainer <NUM>. The notch <NUM> may include an angled or curved surface complimentary to the dog-eared, or faceted, cross-section of a generally aerodynamic leading edge <NUM> of retainer <NUM> formed from angled surfaces 404a, 404b, 404c (<FIG>), configured to reduce drag of the flow of exhaust over the retainer <NUM>. Alternately, retainer <NUM> could be coupled to troughs <NUM> and/or <NUM> of flutes F by being formed integrally with flute ties <NUM> and/or integrally with flutes F and/or flow mixer <NUM>, if desired. While the length, width, and thickness of retainer <NUM> can be different depending on particular applications and configurations, in one non-limiting implementation, retainer <NUM> can be between approximately <NUM> (<NUM> inches) and <NUM> (<NUM> inches) (in length, between approximately <NUM> (<NUM> inches) and <NUM> (<NUM> inches) in width <NUM> and between approximately <NUM> +/- <NUM> (<NUM> +/- <NUM> inches) and <NUM> +/- <NUM> (<NUM> +/- <NUM> inches) in thickness <NUM>.

Retainer <NUM> could also be directly attached to the troughs <NUM> and/or <NUM> of flutes F in in addition or instead of to flute ties <NUM>, either directly to such troughs <NUM> and/or <NUM>, such as by welding, bonding, adhesives, etc., or by mechanical fasteners, such as show in <FIG>. In <FIG>, retainers 400a and 400b are each attached to caps <NUM> and/or <NUM>, respectively by welds <NUM>, adhesive, or some other suitable attachment. Caps <NUM> and <NUM> are received within troughs <NUM> and <NUM> respectively, and into or through openings (not shown) troughs <NUM> and/or <NUM>. A washer <NUM> is interposed between an upper fastener <NUM> and opening <NUM> of upper cap <NUM> and between a lower fastener <NUM> and opening <NUM> of lower cap <NUM>. The bottom surface <NUM> of upper cap <NUM> is curved to nest within upper trough <NUM>, and similarly, the bottom surface <NUM> of lower cap <NUM> is curved to be received by and nest within lower trough <NUM>.

<FIG> illustrates a flow mixer 200a preferably including five upper troughs <NUM> coupled together directly with retainer 400c, and five lower troughs <NUM> are coupled together with retainer 400d. The five upper troughs are axially opposed to the five lower troughs along axes Y<NUM>, Y<NUM>, Y<NUM>, Y<NUM>, and Y<NUM>, respectively. Retainers 400c and 400d could be coupled to troughs <NUM> and/or <NUM>, respectively, through use of adhesive, welds, integral formation with troughs <NUM> and/or <NUM>, and/or some other suitable fastening manner. In this implementation, flute ties <NUM> may be eliminated, and in such case, the upper flutes <NUM> will be generally fixed with respect to relative movement to one another, and likewise, the lower troughs <NUM> will be generally limited to relative movement with respect to one another; however, the upper row <NUM> of flutes F could experience at least relative lateral movement with respect to the lower row <NUM> of flutes F. Depending on the application, allowing for relative generally lateral movement between upper row <NUM> and lower row <NUM> of flutes F may be desirable.

<FIG> illustrates a flow mixer 200b preferably including flutes F' extending generally radially from a center C. Retainers, or retainer segments, 400e directly attach to troughs between adjacent flutes F', via caps 302a and fasteners 314a, to reduce relative movement between the flutes F' and thereby stiffen flow mixer 200b. Instead of caps 302a and fasteners 314a, retainers 400e could be coupled to the troughs through welding, adhesives, bonding, and/or integral formation with the flutes F' or in some other suitable manner.

<FIG> illustrates a flow mixer 200c preferably including a row of upper flutes 206a and a row of lower flutes 208a, with opposed pairs of upper and lower flutes F each being interconnected via mechanical fasteners, such as upper caps <NUM> and lower caps <NUM> to flute ties <NUM>. However, it is to be understood that flute ties <NUM> could be coupled to the flutes through welding, adhesive, integral formation with the flutes F or in some other suitable manner. A first retainer 400f couples together two pairs of outboard flutes Fa on one side of flow mixer 200c. A second retainer <NUM> couples together two pairs of outboard flutes Fc on the other side of flow mixer <NUM>. The center, or middle, pair of opposed flutes Fb is not laterally restrained by a retainer and is thus freer to experience relative movement with respect to either of the remaining pairs of flutes Fa, Fb. This selective application of retainers to specific pairs of flutes allows for the tailoring of stiffness and, accordingly, natural frequency characteristics for a given situation and/or environment of flow mixer 200c.

<FIG> illustrates a flow mixer 200d preferably including a row of upper flutes 206a and a row of lower flutes 208a, with opposed pairs of upper and lower flutes F each being interconnected via mechanical fasteners, such as upper caps <NUM> and lower caps <NUM> to flute ties <NUM>. As noted above, it is to be understood that flute ties <NUM> could be coupled to the flutes through welding, adhesives, mechanical fasteners, integral formation with the flutes F or in some other suitable manner. Retainer <NUM> joins together three center, or middle, pair of flutes Fd, while each outboard pair of flutes Fe, Ff of flow mixer 200d is not laterally restrained by retainer <NUM>, and each outboard pair of flutes Fe, Ff is thus relatively free to experience relative movement with respect to each other and with respect to the three central pairs of flutes Fd, as well as the other outboard pair of flutes. This is another non-limiting selective application of retainers which may be used to tailor the stiffness and, accordingly, the natural frequency characteristics for a given situation and/or environment of flow mixer 200d.

Given the above, the resonant frequency of an exhaust system may be increased by selecting flutes F to be restrained and orienting the elongated retainer to extend generally perpendicular to the elongated axis of each of the selected flutes, and, while maintaining the elongated retainer generally perpendicular to the elongated axis of each of the flutes, coupling the retainer to the flutes directly and/or to flute ties <NUM> attached to such selected flutes, such that the retainer <NUM> generally restrains movement between the selected flutes.

The examples described herein include systems and apparatuses that are able to raise the natural frequency of a flow mixer in order to avoid the resonant frequencies of cyclical vibrations, such as the operation of one or more rotors, propellers, etc. The examples described herein include flow mixers that include a plurality of flutes for mixing of a hot exhaust air stream and a relatively cooler ambient air stream. The flow mixers include a stiffener mechanism having ties that extend between the upper and lower flutes and one or more horizontally extending retainers that connect, or couple, two or more of the flute ties together and being configured to prevent deflections of the flutes due to vibrational stresses and resonant frequencies. In one implementation, the stiffener mechanism includes caps positioned within corresponding upper and lower flute troughs and a body portion that extends between the troughs and is coupled to each of the caps. In such a configuration, the stiffener mechanism is loaded primarily with tension/compression forces during operation. A laterally-extending retainer strip may be coupled to one or more flute ties to generally fix or restrain relative movement of the flute ties, and correspondingly, the flutes to which the flute ties are coupled, thereby providing a stiffening effect. The stiffener mechanism may be integrally formed between the upper and lower flutes and the adjacent fluted ties of the flow mixer. The stiffener mechanism may be coupled to corresponding sidewalls of the flutes such that the stiffener mechanism is loaded primarily with shear forces during operation.

The disclosure described herein facilitates raising the natural frequency of the flow mixer and preventing deflection of the flow mixer flutes due to aligning resonant frequencies and rotor vibrations. Such stiffening enables the use of larger size mixers that may provide more efficient flow mixing and which may also lengthen the service lifetime of the flow mixer due to reduced material fatigue. As such, the costs associated with manufacturing and maintaining multiple flow mixers may be reduced. Additionally, the stiffening potentially creates a more constant flute area, which could facilitate more efficient flow mixing, and while maintaining the aerodynamic shape and properties of the flow mixer.

Accordingly, a method is described herein of increasing the resonant frequency of exhaust system <NUM>, which has at least one exhaust duct <NUM> with flutes F. As shown in <FIG>, each flute F includes a trough <NUM> or <NUM> and an elongated portion <NUM> with a respective axis Y<NUM>, Y<NUM>, etc. and a lateral portion <NUM> having width W extending generally perpendicular to the respective axis Y<NUM>, Y<NUM>, etc., such that the length of the elongated portion <NUM> is greater than the width W of such lateral portion <NUM>. The method includes selecting a first flute <NUM> from the plurality of flutes F, the first flute having a first axis Y<NUM>, and also selecting a second flute <NUM>. The second flute <NUM> has a second axis Y<NUM> generally parallel to the first axis Y<NUM>. The method includes orienting a retainer 400c to extend generally perpendicular to the first and second axes Y<NUM>, Y<NUM>. The method also includes coupling a first portion <NUM> of the retainer 400c to the trough <NUM> of the first flute <NUM> and a second portion <NUM> of the retainer 400c to the trough <NUM> of the second flute <NUM> such that the retainer 400c generally restrains movement between the first flute <NUM> and the second flute <NUM>.

Referring <FIG>, the disclosure may be described in the context of an aircraft manufacturing and service method <NUM> and via an aircraft <NUM> (shown in <FIG>). Aircraft <NUM> (shown in <FIG>) is an example of aircraft <NUM>. During method <NUM>, specification and design data of aircraft <NUM> may be used <NUM> during the manufacturing process and other materials associated with the airframe may be procured <NUM>. During production, component and subassembly manufacturing <NUM> and system integration <NUM> of aircraft <NUM> occurs, prior to aircraft <NUM> entering its certification and delivery process <NUM>. Upon successful satisfaction and completion of airframe certification, aircraft <NUM> may be placed in service <NUM>. While in service by a customer, aircraft <NUM> is scheduled for periodic, routine, and scheduled maintenance and service <NUM>, including any modification, reconfiguration, and/or refurbishment, for example. The manufacturing and service method <NUM> may be implemented via vehicles other than an aircraft. The exhaust duct <NUM> and/or stiffener mechanism <NUM> described herein may be procured <NUM>, assembled <NUM>, integrated <NUM>, and/or maintained/serviced <NUM>.

Each portion and process associated with aircraft manufacturing and/or service <NUM> may be performed or completed by a system integrator, a third party, and/or an operator (e.g., a customer). For the purposes of this description, a system integrator may include without limitation any number of aircraft manufacturers and major-system subcontractors; a third party may include without limitation any number of venders, subcontractors, and suppliers; and an operator may be an airline, leasing company, military entity, service organization, and so on.

As shown in <FIG>, aircraft <NUM> produced via method <NUM> may include an airframe <NUM> having a plurality of systems <NUM> and an interior <NUM>. Examples of high-level systems <NUM> include one or more of a propulsion system <NUM>, an electrical system <NUM>, a hydraulic system <NUM>, and/or an environmental system <NUM>. Any number of other systems may be included. The exhaust duct <NUM> and/or stiffener mechanism <NUM> are preferably parts of propulsion system <NUM>.

Apparatus and methods embodied herein may be employed during any one or more of the stages of method <NUM>. For example, components or subassemblies corresponding to production process <NUM> may be fabricated or manufactured in a manner similar to components or subassemblies produced while aircraft <NUM> is in service. Also, one or more apparatus implementations, method implementations, or a combination thereof may be utilized during the production stages <NUM> and <NUM>, for example, by substantially expediting assembly of, and/or reducing the cost of assembly of aircraft <NUM>. Similarly, one or more of apparatus implementations, method implementations, or a combination thereof may be utilized while aircraft <NUM> is being serviced or maintained, for example, during scheduled maintenance and service <NUM>.

Further, the aircraft manufacturing and service method described herein may be used in any manufacturing, modification, repair and/or service operation.

In summary, the disclosure described herein include a duct for directing a flow of exhaust, the duct including a wall portion defining a passageway having an inlet portion adapted to receive the flow of exhaust and an outlet portion adapted to discharge the flow of exhaust. A plurality of flutes is defined at the outlet portion, and the plurality of flutes include a first flute and a second flute spaced from the first flute. The first flute has a peak, a trough, a first height dimension and a first width dimension generally perpendicular to the first height dimension, and the second flute has a peak, a trough, a second height dimension and a second width dimension generally perpendicular to the second height dimension. At least one retainer is coupled to the trough of the first flute and the trough of the second flute and extends generally parallel to at least one of the first width dimension and the second width dimension, wherein the retainer is configured to restrain relative movement between the first flute and the second flute and relative movement among the first flute, the second flute, and at least one of the inlet portion and the outlet portion during the flow of exhaust through the duct.

A first axis preferably corresponds to the first height dimension and a second axis preferably corresponds to the second height dimension, and the first axis and the second axis are preferably generally parallel to one another.

The first flute can be adjacent the second flute, and in another aspect a fourth flute can be generally opposed to the second flute, and a second flute tie is coupled to the fourth flute and the second flute. The third flute may have s a third height dimension and a third axis can correspond to the third height dimension, the fourth flute may have a fourth height dimension and a fourth axis may correspond to the fourth height dimension, and the third axis and the fourth axis can be generally parallel to one another. The retainer may be coupled to the first flute tie and second flute tie.

The retainer can be an elongated strip configured to extend at least between the first flute and the second flute, and in another aspect the retainer may include a generally aerodynamic angled leading edge configured to reduce drag of the flow of exhaust over the retainer.

A third flute may generally be opposed to the first flute, and a first flute tie coupled to the third flute and the first flute, and in another aspect, a first axis corresponds to the first height dimension, the third flute has a third height dimension, and a third axis corresponds to the third height dimension, and wherein the first axis and the third axis are generally collinear with respect to one another.

The retainer can be welded, bonded, adhered, formed integrally with the first flute tie and second flute tie and/or mechanically fastened to the first flute tie and the second flute tie. A first mechanical fastener may be coupled to the first flute, a second mechanical fastener coupled to the second flute, and the retainer coupled to at least one of the first mechanical fastener and the second mechanical fastener. The at least one retainer can include a first retainer and a second retainer, and the duct can further include a first mechanical fastener coupled to the first flute, a second mechanical fastener coupled to the second flute, the first retainer coupled to the first mechanical fastener, and the second retainer coupled to the second mechanical fastener.

The outlet portion of the duct may have a centerline, a first axis corresponds to the first height dimension, and a second axis corresponds to the second height dimension, and the first axis and the second axis extend generally radially with respect to the centerline.

In the duct, a third flute may generally be opposed to the first flute, and a first flute tie can be coupled to the third flute and the first flute. A fourth flute may be generally opposed to the second flute, and a second flute tie may be coupled to the fourth flute and the second flute. A fifth flute may be between the first flute and the second flute, and a sixth flute may generally be opposed to the fifth flute.

The retainer can be coupled to the first flute tie and the second flute tie, and the third flute can be configured to have freedom of movement with respect to at least one of the retainer, the first flute, the second flute, the third flute, and the fourth flute. A third flute tie can be coupled to the fifth flute tie and the sixth flute tie and/or the retainer can be coupled to the first flute tie, the second flute tie, and the third flute tie.

A third flute can be generally opposed to the first flute, and a first flute tie can be coupled to the third flute and the first flute. A fourth flute can be generally opposed to the second flute, and a second flute tie can be coupled to the fourth flute and the second flute. A fifth flute may be included, and a sixth flute may generally be opposed to the fifth flute. A seventh flute can be included, and an eighth flute may generally be opposed to the seventh flute. A third flute tie can be coupled to the fifth flute and the sixth flute, and a fourth flute tie can be coupled to the seventh flute and the eighth flute. Additional aspects include the at least one retainer having a first retainer and a second retainer and/or the first retainer coupled to at least the first flute tie and the second flute tie, and the second retainer coupled to at least the third flute tie and the fourth flute tie. In other aspects, at least one of the plurality of flutes is configured to have freedom of movement with respect to at least one other of the plurality of flutes and/or the retainer is coupled to the first flute tie, the second flute tie, the third flute tie, and the fourth flute tie.

In the duct, the plurality of flutes can be arranged in opposing rows of flutes and/or the first height dimension is greater than the first width dimension and the second height dimension is greater than the second width dimension. A stiffener apparatus for an exhaust duct can include a first row of flutes and a second row of flutes generally opposite to the first row of flutes, and each flute is elongated along a respective axis, and the stiffener apparatus includes at least two flute ties coupling together at least two flutes from the first row of flutes to at least two flutes from the second row of flutes. At least one retainer is coupled to the at least two flute ties and extends generally perpendicularly to the axis of each of the at least two flutes from the first row of flutes and the at least two flutes from the second row of flutes, wherein the retainer is configured to generally restrain relative movement between the at least two flutes from the first row of flutes and the at least two flutes from the second row of flutes.

A method is described for increasing the resonant frequency of an exhaust system having at least one duct, the duct including a plurality of flutes. Each flute includes a trough and an elongated portion with a respective axis and a lateral portion extending generally perpendicular to the respective axis, wherein the length of the elongated portion is greater than the width of the lateral portion. The method includes selecting a first flute from the plurality of flutes, the first flute having a first axis and selecting a second flute from the plurality of flutes, the second flute having a second axis generally parallel to the first axis. The retainer is oriented to extend generally perpendicular to the first and second axes, and a first portion of the retainer is coupled to the trough of the first flute and a second portion of the retainer is coupled to the trough of the second flute such that the retainer generally restrains movement between the first flute and the second flute.

A method is also described for increasing the resonant frequency of an exhaust system having at least one duct, the duct including at least one row of flutes with a first side of the row having flutes oriented in a first direction and a second side of the row having flutes oriented in a second direction generally opposite to the first direction. Each flute defines an elongated portion extending along a respective axis and a lateral portion extending generally perpendicular to the respective axis, wherein the length of the elongated portion is greater than the width of the lateral portion. The method includes selecting a first flute and a second flute from the first side of the row of flutes and selecting a third flute and a fourth flute from the second side of the row of flutes. A first flute tie is coupled to the first flute and the third flute such that the first flute tie is generally parallel to the axis of each of the first flute and the third flute, and a second flute tie is coupled to the second flute and the fourth flute such that the second flute tie is generally parallel to the axis of each of the second flute and the fourth flute. A retainer is oriented generally perpendicular to a length of at least one of the first flute tie and the second flute tie. A first portion of the retainer is coupled to the first flute, and a second portion of the retainer is coupled to the second flute such that the retainer restrains movement between the first flute and the second flute, wherein the retainer restrains movement among the first flute, second flute, third flute, and fourth flute.

Although specific features of various examples of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing.

This written description uses examples to disclose various examples, which include the best mode, to enable any person skilled in the art to practice those examples, including making and using any devices or systems and performing any incorporated methods. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims.

Claim 1:
A duct (<NUM>) for directing a flow of exhaust gases (<NUM>), the duct comprising:
- a flow mixer (<NUM>) having an inlet portion (<NUM>) adapted to receive the flow of exhaust gases and an outlet portion (<NUM>) adapted to discharge the flow of exhaust gases;
- a plurality of flutes, defined at the outlet portion, the plurality of flutes including a first flute (<NUM>, <NUM>) and a second flute (<NUM>, <NUM>), spaced from the first flute, the first flute having a first peak (<NUM>), a first trough (<NUM>), a first height dimension, and a first width dimension, generally perpendicular to the first height dimension, the second flute having a second peak (<NUM>), a second trough (<NUM>), a second height dimension, and a second width dimension generally perpendicular to the second height dimension; and
- at least two flute ties (<NUM>), wherein a first flute tie is coupled to the first trough (<NUM>) and wherein a second flute tie is coupled to the second trough (<NUM>); and
- at least one retainer (<NUM>), directly attached to the first trough (<NUM>) and the second trough (<NUM>) of the flutes in addition to or instead of the at least two flute ties (<NUM>) and extending generally parallel to at least one of the first width dimension and the second width dimension;
- wherein:
- the at least one retainer is coupled to the flute tie; and
- the at least one retainer is configured to restrain relative movement between the first flute and the second flute and relative movement among the first flute, the second flute, and at least one of the inlet portion and the outlet portion during the flow of exhaust gases through the duct.