Structured panel with integrated skin and sidewalls

A panel includes a core between and connected to a first skin and a second skin. The core includes a corrugated body, a first sidewall and a second sidewall. The corrugated body includes a plurality of corrugations configured from at least a plurality of baffles and a plurality of septums. Each of the corrugations includes a respective one of the baffles and a respective one of the septums. A first cavity extends laterally within the core from a first of the baffles to a first of the septums. The first cavity extends longitudinally within the core from the first sidewall to the second sidewall. The first cavity is fluidly coupled with a passage through a first portion of the first skin. The first portion of the first skin, the first sidewall and the second sidewall are collectively at least partially formed by a ply of material.

BACKGROUND

1. Technical Field

This disclosure relates to structured panels such as, but not limited to, an acoustic panel for attenuating sound generated by an aircraft propulsion system.

2. Background Information

Structured panels such as acoustic panels may be used in various applications to attenuate noise. An acoustic panel, for example, may be configured with a nacelle of an aircraft propulsion system to attenuate noise generated by a gas turbine engine. Such an acoustic panel typically includes a honeycomb core connected between a perforated face skin and a solid, non-perforated back skin. The honeycomb core includes a plurality of resonating chambers. These resonating chambers are tuned by selecting a desired chamber length and, thus, core thickness that corresponds to a specific target frequency of noise to be attenuated. Increasing the core thickness, for example, will typically tune the resonating chambers for attenuating lower frequency noise. Conversely, decreasing the core thickness will typically tune the resonating chambers to attenuate higher frequency noise.

Recent trends in aircraft engine design such as higher bypass ratios, larger fan diameters, slower rotating fans and/or fewer number of fan blades have resulted in those aircraft engines generating relatively low frequency noise. Relatively strict space constraints (e.g., loft envelope) for those engines, however, typically limit or prohibit increasing the thickness of an acoustic panel to tune its resonating chambers for such relatively low frequency noise. There is a need in the art therefore for an acoustic panel operable to attenuate relatively low frequency noise while utilizing the same or less space than previous acoustic panels. There is a further need to provide a panel configuration capable of reducing panel assembly time, complexity and cost.

SUMMARY OF THE DISCLOSURE

According to an aspect of the present disclosure, a panel is provided for attenuating sound. This panel includes a first skin, a second skin and a core between and connected to the first skin and the second skin. The core includes a corrugated body, a first sidewall and a second sidewall. The corrugated body includes a plurality of corrugations configured from at least a plurality of baffles and a plurality of septums. Each of the corrugations includes a respective one of the baffles and a respective one of the septums. A first cavity extends laterally within the core from a first of the baffles to a first of the septums. The first cavity extends longitudinally within the core from the first sidewall to the second sidewall. The first cavity is fluidly coupled with a passage through a first portion of the first skin. The first portion of the first skin, the first sidewall and the second sidewall are collectively at least partially formed by a ply of material.

According to another aspect of the present disclosure, a structured panel is provided that includes a first skin, a second skin and a core between and connected to the first skin and the second skin. The core includes a corrugated body, a first sidewall, a second sidewall and a third sidewall. The corrugated body includes a plurality of corrugations configured from at least a plurality of baffles and a plurality of septums. Each of the corrugations includes a respective one of the baffles and a respective one of the porous septums. A first cavity extends laterally within the core from a first of the baffles to a first of the septums. The first cavity extends longitudinally within the core from the first sidewall to the second sidewall. A second cavity extends laterally within the core from a second of the baffles to a second of the septums. The second cavity extends longitudinally within the core from the second sidewall to the third sidewall. A first portion of the first skin adjacent the first cavity, a second portion of the first skin adjacent the second cavity, the first sidewall, the second sidewall and the third sidewall are collectively at least partially formed by a ply of material.

According to still another aspect of the present disclosure, a method is provided during which a panel is formed for attenuating sound. The panel includes a porous first skin, a second skin and a cellular core connected to and between the porous first skin and the second skin. The cellular core is configured with a first cavity that extends laterally between a baffle and a porous septum and longitudinally between a first sidewall and a second sidewall. A sheet of material is provided with one or more rows of polygonal cutouts. The sheet of material is folded such that a portion of the porous first skin bounding the first cavity, the first sidewall and the second sidewall are collectively at least partially defined by the folded sheet of material.

The first cavity may be fluidly coupled with a first passage through the first portion of the first skin. The second cavity may be fluidly coupled with a second passage through the second portion of the first skin.

The first skin may be a non-perforated skin.

The ply of material may be configured from or otherwise include thermoplastic material.

The ply of material may be configured from or otherwise include thermoset material.

The ply of material may be configured from or otherwise include metal.

The ply of material may be configured from or otherwise include fiber-reinforcement within a matrix.

The first portion of the first skin may be further partially formed by a second ply of material that overlaps and is bonded to the ply of material.

The ply of material may be configured with a first through hole. The second ply of material may be configured with a second through hole having a width that is greater than a width of the first through hole. The passage may be formed by at least the first through hole and the second through hole.

The core may further include a third sidewall. A second cavity may extend laterally within the core from a second of the baffles to a second of the septums. The second cavity may extend longitudinally within the core from the first sidewall to the third sidewall. The second cavity may be fluidly coupled with a second passage through a second portion of the first skin. The first portion of the first skin, the second portion of the first skin, the first sidewall, the second sidewall and the third sidewall may be collectively at least partially formed by the ply of material.

A second cavity may extend laterally within the core from a second of the baffles to a second of the septums. The second cavity may extend longitudinally within the core from the first sidewall to the second sidewall. The second cavity may be fluidly coupled with a second passage through a second portion of the first skin. The first portion of the first skin, the second portion of the first skin, the first sidewall and the second sidewall may collectively at least partially be formed by the ply of material.

The core may further include a third sidewall and a fourth sidewall. The corrugated body may be between the first sidewall and the third sidewall and between the second sidewall and the fourth sidewall. A second cavity may extend laterally within the core from the first of the septums to a second of the baffles. The second cavity may extend longitudinally within the core from the third sidewall to the fourth sidewall. The second cavity may be fluidly coupled with the first cavity through one or more perforations in the first of the septums.

A first portion of the second skin bounding the second cavity, the third sidewall and the fourth sidewall may collectively at least partially be formed by a second ply of material.

The third sidewall may be longitudinally aligned with the first sidewall. The fourth sidewall may be longitudinally aligned with the second sidewall.

The first sidewall and the second sidewall may each extend from the first skin to the first of the baffles and the first of the septums.

A first portion of the ply of material may overlap and may be bonded to a second portion of the ply of material to form at least a portion of the first sidewall.

A third portion of the ply of material may overlap and may be bonded to a fourth portion of the ply of material to form at least a portion of the second sidewall.

Prior to formation of the panel, the ply of material may be configured as or otherwise include a sheet of material with one or more rows of polygonal-shaped cutouts.

The ply of material may be configured as or otherwise include a plurality of segments. Each of the segments may include a rectangular base, a plurality of first triangular projections and a plurality of second triangular projections. The first triangular projections may be on a first side of the rectangular base. The second triangular projections may be on a second side of the rectangular base. Peaks of the first triangular projections of a first of the segments may be respectively connected to peaks of the second triangular projections of a second of the segments.

DETAILED DESCRIPTION

FIG. 1is a cutaway perspective illustration of a portion of a structured panel20; e.g., a structural acoustic panel. This structured panel20may be configured to attenuate sound (e.g., noise) generated by an aircraft propulsion system such as, for example, a turbofan propulsion system or a turbojet propulsion system. With such a configuration, the structured panel20may be configured with a nacelle of the propulsion system. The structured panel20, for example, may be configured as or with an inner or outer barrel, a translating sleeve of a thrust reverser, a blocker door, etc. Alternatively, the structured panel20may be configured with another component/structure of the aircraft such as its fuselage or a wing. Furthermore, the structured panel20may be configured to also or alternatively attenuate aircraft related sound other than that generated by the propulsion system. The structured panel20of the present disclosure, however, may alternatively be configured for non-aircraft applications. Furthermore, the structured panel20may also be configured for non-sound attenuation applications.

The structured panel20ofFIG. 1extends longitudinally along a y-axis. The structured panel20extends laterally along an x-axis. The structured panel20extends vertically along a z-axis. The term “vertical” is used herein to describe a depthwise panel direction and is not limited to a gravitational up/down direction. Furthermore, for ease of illustration, the x-y plane is shown as a generally flat plane. However, in other embodiments, the x-y plane and, thus, the structured panel20may be curved and/or follow an undulating geometry. For example, the x-y plane and, thus, the structured panel20may be arcuate, cylindrical or conical with or without radial undulations. Thus, the vertical direction may change at different locations along the x-y plane; e.g., the vertical direction may be a radial direction for a cylindrical, conical or spherical structured panel.

The structured panel20includes a (e.g., acoustic) porous top skin22(e.g., a perforated face skin), a solid, non-perforated bottom skin24(e.g., a back skin) and a cellular core26. Note, the terms “top” and “bottom” are used in this disclosure to describe the relative position of an element as viewed in the figures. The present disclosure, however, is not limited to such an orientation. Furthermore, it is worth noting, the top skin22may be replaced with a solid, non-perforated skin where, for example, the structured panel20is not being used for sound attenuation.

Briefly, the cellular core26is disposed and extends vertically between the top skin22and the bottom skin24. The cellular core26is also connected to the top skin22and the bottom skin24. The cellular core26, for example, may be welded, brazed, fused, adhered and/or otherwise bonded to or integral with the top skin22and/or the bottom skin24as discussed below in further detail.

The top skin22ofFIG. 1is configured as a relatively thin sheet or layer of material that extends longitudinally and laterally along the x-y plane. This top skin material may include, but is not limited to, a metal (e.g., sheet metal), a polymer (e.g., thermoplastic or thermoset material), a fiber reinforced composite (e.g., fiberglass composite, carbon fiber composite, aramid fiber composite, etc.), or a combination thereof. Referring now toFIG. 2, the top skin22has a vertical thickness28, which extends vertically between opposing first skin top and bottom side surfaces. The top skin22includes a plurality of passages30; e.g., perforations such as through-holes. Each of these passages30extends generally vertically through the top skin22between its side surfaces. While the passages30are described above and illustrated inFIG. 2as through-holes for ease of description, one or more of the passages may also or alternatively be formed by one or more interconnected pores in the top skin material in alternative embodiments.

The bottom skin24ofFIG. 1is configured as a relatively thin sheet or layer of (e.g., solid, continuous and/or uninterrupted) material that extends longitudinally and laterally along the x-y plane. This bottom skin material may include, but is not limited to, a metal (e.g., sheet metal), a polymer (e.g., thermoplastic or thermoset material), a fiber reinforced composite (e.g., fiberglass composite, carbon fiber composite, aramid fiber composite, etc.), or a combination thereof. The bottom skin material may be the same as or different than the top skin material. Referring toFIG. 2, the bottom skin24has a vertical thickness32, which extends vertically between opposing second skin top and bottom side surfaces. This vertical thickness32may be substantially equal to or different (e.g., greater or less) than the vertical thickness28of the top skin22.

Referring toFIG. 1(see alsoFIGS. 2-4), the cellular core26extends longitudinally and laterally along the x-y plane. Referring again toFIG. 2, the cellular core26has a vertical thickness34, which extends vertically between opposing core sides respectively abutted against the top skin22and the bottom skin24. The vertical thickness34may be substantially greater than the vertical thickness28,32of the top skin22and/or the bottom skin24. The vertical thickness34of the core26, for example, may be at least ten to forty times (10-40×), or more, greater than the vertical thickness28,32of the skin22,24; however, the structured panel20of the present disclosure is not limited to such an exemplary embodiment.

Referring toFIG. 1, the cellular core26includes a corrugated body36, one or more top sidewalls38and one or more bottom sidewalls40, where the top sidewalls38and the bottom sidewalls40are arranged on opposing sides of the corrugated body36. More particularly, referring toFIG. 3, the top sidewalls38are disposed vertically between the corrugated body36and the top skin22. The bottom sidewalls40are disposed vertically between the corrugated body36and the bottom skin24. Each of the bottom sidewalls40is also aligned longitudinally with a respective one of the top sidewalls38as seen inFIG. 4; however, in other embodiments, one or more of the top sidewalls38may be longitudinally offset from a respective closest bottom sidewall40. Furthermore, while structured panel20is illustrated with a 1:1 ratio of the top sidewalls38to the bottom sidewalls40, the number of top sidewalls38may be different (e.g., greater or less) than the number of bottom sidewalls40in other embodiments. In still other embodiments, the structured panel20may be configured without any top sidewalls38or without any bottom sidewalls40.

Referring toFIG. 5, the corrugated body36includes a plurality of corrugations42. These corrugations42along with the top sidewalls38and the bottom sidewalls40are arranged together to configure the cellular core26as an open cavity (e.g., open cell) structure as shown inFIG. 1. Referring toFIGS. 2 and 4, this open cavity structure forms a plurality of cavities44and46. The top cavities44are vertically between the corrugated body36and the top skin22. Each of these top cavities44may be fluidly coupled with one or more respective passages30in the top skin22. The bottom cavities46are vertically between the corrugated body36and the bottom skin24. Each of these bottom cavities46may be fluidly coupled with a respective one of the top cavities44through one or more passages48(e.g., perforations) in the corrugated body36.

Referring toFIG. 5, the corrugations42are arranged in a laterally extending array. This arrangement provides the corrugated body36with an accordion wall structure. More particularly, the corrugations42are configured from at least a plurality of solid (e.g., non-perforated) baffles50and a plurality of porous septums52(e.g., perforated septums; passaged shown inFIG. 2). However, in other (e.g., non-sound attenuating) embodiments, one or more or each of the septums52may be replaced with a solid panel similar to the baffles50.

Referring toFIG. 2, each of the baffles50may be configured as a solid, continuous and/or uninterrupted panel of core material. Each of the septums52may be configured as a panel of core material with one or more passages48; e.g., perforation such as through-holes. While these passages48are described and illustrated inFIG. 2as through-holes for ease of description, one or more of the passages may also or alternatively be formed by one or more interconnected pores in the septum material.

Each corrugation42includes a respective one of the baffles50and a respective one of the septums52. Each of these corrugation portions50,52may extend longitudinally along an entire longitudinal length of the respective corrugation42as shown inFIG. 5; however, the present disclosure is not limited to such a configuration.

Referring toFIG. 2, each baffle50extends laterally and/or vertically (e.g., diagonally) from its top end54to its bottom end56. The top end54of the baffle50is connected to and adjacent the top skin22. The bottom end56of the baffle50is connected to and adjacent the bottom skin24. The bottom end56of the baffle50is also connected to and contiguous with a bottom end58of a respective septum54in the same corrugation42. This septum52extends from the bottom end58to its top end60. The bottom end58of the septum52is connected to and adjacent the bottom skin24. The top end60of the septum52is connected to and adjacent the top skin22. The top end60of the septum52may also be connected to and contiguous with the top end54of a baffle50in a laterally adjacent one of the corrugations42.

Referring toFIG. 5, each corrugation42forms a top channel62within the corrugated body36. This top channel62extends laterally between the baffle50and the septum52of the corrugation42. The top channel62extends vertically into the corrugated body36to the interface/connection between the baffle50and the septum52. The top channel62may also extend longitudinally along the entire longitudinal length of the corrugation42.

Each laterally adjacent pair of the corrugations42form a bottom channel64within the corrugated body36. The bottom channel64extends laterally between the septum52of a first of the adjacent corrugations42to the baffle50of a second of the adjacent corrugations42. The bottom channel64extends vertically into the corrugated body36to the interface/connection between the respective baffle50and septum52. The bottom channel64may also extend longitudinally along the entire longitudinal lengths of the laterally adjacent corrugations42. The top channels62and the bottom channels64are positioned on opposing sides of the corrugated body36.

Referring toFIGS. 1 and 4, the top sidewalls38are discretely spaced longitudinally along the top channels62and the corrugations42. Referring now toFIGS. 1 and 3, each top sidewall38extends laterally across one or more of the top channels62and the corrugations42. For example, each top sidewall38includes one or more top sidewall elements66(e.g., triangular panels) arranged in a laterally extending array.

Each top sidewall element66is configured with a shape that substantially matches a cross-sectional shape of a respective one of the top channels62. Each top sidewall element66is disposed within a respective one of the top channels62and configured to substantially fluidly isolate longitudinally adjacent portions (i.e., cavities44) of that top channel62from one another. More particularly, the top sidewall element66extends laterally across the top channel62between the respective baffle50and the respective septum52. The top sidewall element66extends vertically into the top channel62from the top skin22to the interface/connection between the respective baffle50and septum52. The top sidewall element66is connected to (e.g., formed integral with) the top skin22. The top sidewall element66is also connected to (e.g., adhered and/or otherwise bonded) to the respective baffle50and septum52.

Referring toFIGS. 1 and 4, the bottom sidewalls40are discretely spaced longitudinally along the bottom channels64and the corrugations42. Referring now toFIGS. 1 and 3, each bottom sidewall40extends laterally across one or more of the bottom channels64and the corrugations42. For example, each bottom sidewall40includes one or more bottom sidewall elements68(e.g., triangular panels) arranged in a laterally extending array.

Each bottom sidewall element68is configured with a shape that substantially matches a cross-sectional shape of a respective one of the bottom channels64. Each bottom sidewall element68is disposed within a respective one of the bottom channels64and configured to substantially fluidly isolate longitudinally adjacent portions (i.e., cavities46) of that bottom channel64from one another. More particularly, the bottom sidewall element68extends laterally across the bottom channel64between the respective baffle50and the respective septum52. The bottom sidewall element68extends vertically into the bottom channel64from the bottom skin24to the interface/connection between the respective baffle50and septum52. The bottom sidewall element68is connected to (e.g., formed integral with) the bottom skin24. The bottom sidewall element68is also connected to (e.g., adhered and/or otherwise bonded) to the respective baffle50and septum52.

Referring toFIGS. 6 and 7, the structured panel20includes a plurality of resonance chambers70. Each resonance chamber70includes a respective one of the top cavities44and a respective one of the bottom cavities46. Each resonance chamber70extends laterally between and is formed by a laterally adjacent pair of the baffles50. Each of the resonance chambers70extends vertically between the top skin22and the bottom skin24. Each of the resonance chambers70extends longitudinally between a laterally adjacent pair of the top sidewalls38and a laterally adjacent pair of the bottom sidewalls40. Each septum52is disposed within and divides a respective one of the resonance chambers70into fluidly coupled sub-chambers. More particularly, the passages48(e.g., perforations) in the septum52fluidly couple the sub-chambers (i.e., the top and bottom cavities44and46) together.

A length of each resonance chamber70extends diagonally between the top skin22and the bottom skin24and through a respective one of the septums52. The length72of the resonance chamber70therefore is longer than the vertical thickness34of the cellular core26. This enables noise attenuation of relatively low frequency noise without increasing the vertical thickness34of the core26and, thus, a vertical thickness of the structured panel20. For example, each resonance chamber70may receive acoustic waves through the passages30in the top skin22. The resonance chamber70may reverse the phase of one or more frequencies of those sound waves using known acoustic reflection principles and subsequently direct the reverse phase sound waves out of the structured panel20through the passages30to destructively interfere with other incoming acoustic waves.

The corrugated body36may be constructed from any suitable material or materials. The corrugated body36, for example, may be constructed from a metal (e.g., sheet metal such as aluminum or titanium sheet metal), a polymer (e.g., thermoplastic or thermoset material), a fiber reinforced composite (e.g., fiberglass composite, carbon fiber composite, aramid fiber composite, etc.), or a combination thereof. The corrugated body36may be constructed from the same material(s) as the top skin22and/or the bottom skin24, or a different material or materials.

The top and the bottom sidewalls38and40may be constructed from any suitable material or materials. Each sidewall38,40, for example, may be constructed from a metal (e.g., sheet metal such as aluminum or titanium sheet metal, or woven metallic wire), a polymer (e.g., thermoplastic or thermoset material), a fiber reinforced composite (e.g., fiberglass composite, carbon fiber composite, aramid fiber composite, etc.), or a combination thereof. Each sidewall38,40is constructed from the same material(s) as a respective skin22,24. In the embodiments ofFIGS. 8 and 9, for example, the top sidewalls38are formed integral with the top skin22. Similarly, in the embodiments ofFIGS. 10 and 11, the bottom sidewalls40are formed integral with the bottom skin24. Each sidewall38,40may be constructed from the same material(s) as the corrugated body36; however, in other embodiments, the sidewalls38and/or40may be constructed from different material(s) than the corrugated body36.

Referring toFIG. 8, at least a portion (or an entirety) of the top skin22and one or more (or each) of the top sidewalls38are collectively at least partially (or completely) formed by a single ply74(e.g., sheet, layer) of material such as, but not limited to, a single piece of sheet metal, a single layer of fibrous material in resin (e.g., thermoplastic or thermoset) matrix, a single sheet of polymer (e.g., thermoplastic or thermoset material), etc. In the embodiment ofFIG. 8, the top skin22has a single layer construction whereas each of the sidewalls38has a double layer construction due to overlapping portions of the ply74of material. However, in other embodiments for example as shown inFIG. 9, the top skin22may have a multi-layer construction where, for example, the top skin22includes at least one additional ply76of material.

Referring toFIG. 10, at least a portion (or an entirety) of the bottom skin24and one or more (or each) of the bottom sidewalls40are similarly collectively at least partially (or completely) formed by a single ply78(e.g., sheet, layer) of material such as, but not limited to, a single piece of sheet metal, a single layer of fibrous material in resin (e.g., thermoplastic or thermoset) matrix, a single sheet of polymer (e.g., thermoplastic or thermoset material), etc. In the embodiment ofFIG. 10, the bottom skin24has a single layer construction whereas each of the sidewalls40has a double layer construction due to overlapping portions of the ply78of material. However, in other embodiments for example as shown inFIG. 11, the bottom skin24may have a multi-layer construction where, for example, the bottom skin24includes at least one additional ply80of material.

To at least partially form a respective skin22,24and sidewalls38,40from a ply (e.g.,74,78) of material as described above, a full sheet of material (e.g., an unmanipulated, stock sheet of material) may be patterned as shown, for example, inFIG. 12. In particular, a plurality of cutouts82and84are formed in a full sheet of material to provide the patterned sheet of material. In the embodiment ofFIG. 12, the cutouts82are polygonal-shaped (e.g., diamond shaped) and arranged in one or more lateral arrays. The cutouts84are also polygonal-shaped (e.g., triangular shaped) and arranged in one or more lateral arrays. The cutouts82and84provide the patterned sheet of material ofFIG. 12with one or more segments86; e.g., integral portions. Each of these segments86includes a (e.g., rectangular) base88, a plurality of first (e.g., triangular or otherwise polygonal) projections90and a plurality of second (e.g., triangular or otherwise polygonal) projections92. The base88is disposed between the first and the second projections92such that the first projections90are on a first side of the base88and the second projections92are on a second, opposite side of the base88. Distal ends (e.g., peaks) of the first projections90are respectively connected to distal ends (e.g., peaks) of the second projections92of an adjacent segment, and vice versa.

After the patterning, the patterned sheet of material is folded along the fold lines94and96. The fold lines94correspond to folds in a first direction. The fold lines96correspond to folds in a second, opposite direction. After the folding, each base88forms a portion of a respective skin22,24; e.g., seeFIGS. 8-11. Each folded over and overlapping pair of projections90and92forms a respective sidewall element66,68; e.g., seeFIGS. 8-11

After the folding and the provision of at least two of the folded and patterned sheets of material, these folded and patterned sheets of material are attached (e.g., bonded) to opposing sides of the corrugated body36to form the structured panel20. Of course, additional sheets of material may be attached to the structure to provide, for example, multi-layer skins as shown, for example, inFIGS. 9 and 11.

In some embodiments, the base88of the patterned material for the top skin22may be perforated prior to the folding. In other embodiments, the top skin22may be perforated after the folded and patterned sheet of material is attached to the corrugated body36.

Referring toFIG. 9, in some embodiments, the ply74of material includes a plurality of first apertures98(e.g., through-holes) and the ply76of material includes a plurality of second apertures100(e.g., through-holes), which are respectively laterally and longitudinally aligned with the first apertures98. Each first aperture98may have a first aperture width (e.g., diameter) that is substantially (e.g., +/−5%) or exactly equal to a second aperture width (e.g., diameter) of a respective aligned second aperture100. However, in other embodiments, the second aperture width may be sized different (e.g., greater) than the first aperture width as shown, for example, inFIG. 13.

FIG. 14illustrates another embodiment of the corrugated body36for the structured panel20. This corrugated body36is configured with non-linear (e.g., zig-zagging) first and second channels62and64.

In some embodiments, referring toFIG. 5, an upper or lower peak102between a respective pair of baffle50and septum52may be sharp. In other embodiments, referring toFIG. 15, one or more of the peaks102may be blunt; e.g., curved.

In some embodiments, each baffle50and/or each septum52may follow a substantially flat, linear trajectory as illustrated inFIG. 15; see alsoFIG. 5. In other embodiments, each baffle50and/or each septum52may follow a curved (e.g., sinusoidal) trajectory as illustrated inFIG. 16.