Patent Publication Number: US-2022230613-A1

Title: Acoustic attenuation panel for low-frequency waves

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of International Application No. PCT/FR2020/051712, filed on Sep. 30, 2020, which claims priority to and the benefit of FR 19/11130, filed on Oct. 8, 2019. The disclosures of the above applications are incorporated herein by reference in their entireties. 
    
    
     FIELD 
     The present disclosure relates to an acoustic attenuation panel for the treatment of low-frequency waves. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     An aircraft is propelled by one or more propulsion unit(s) each including a turbojet/turboprop engine housed in a tubular nacelle. Each propulsion unit is attached to the aircraft by a mast generally located under a wing or at the level of the fuselage. 
     A nacelle generally has a structure including an upstream section forming an air inlet upstream of the turbojet engine, a middle section configured to surround a fan of the turbojet engine, a downstream section configured to accommodate thrust reversal means and to surround the combustion chamber of the turbojet engine, and generally terminates in an exhaust nozzle the outlet of which is located downstream of the turbojet engine. 
     The air inlet is configured to improve the air intake which supplies the fan of the turbojet engine throughout the flight envelope and channels the air towards the fan. 
     The air inlet includes an air inlet lip forming a leading edge, attached to an annular structure. 
     The annular structure includes an outer fairing providing the outer aerodynamic continuity of the nacelle and an inner fairing providing the inner aerodynamic continuity of the nacelle, in particular with the outer fan casing at the level of the middle section. The air intake lip provides the upstream junction between these two fairings. 
     The inner fairing of the air inlet is exposed to a high flow of air and is located proximate to the blades of the fan. It therefore contributes to the transmission of the noise originating from the turbojet engine to the outside of the aircraft. 
     Also, it is known from the prior art to equip the inner fairing of the air inlet of the nacelle with an acoustic panel in order to attenuate the transmission of the noise generated by the turbojet engine. 
     Typically, the acoustic panel includes a perforated acoustic skin and a honeycomb core which is assembled on the acoustic skin. 
     The honeycomb core includes a plurality of acoustic cells, forming Helmholtz resonators, which are separated from each other by peripheral partitions. 
     The perforated skin is directed towards the noise emission area, so that the acoustic waves can penetrate through the openings of the perforated skin inside the acoustic cells. The acoustic energy is dissipated by visco-thermal effect in the acoustic cells. 
     In particular, acoustic panels are known the honeycomb core of which includes two levels of acoustic cells. These two levels of cells are separated from each other by a micro-perforated septum. 
     The presence of an additional level of acoustic cells allows improving the acoustic performance of the panel. 
     In recent years, the developments of propulsion systems have pursued the reduction of the consumption of aircrafts. To address this need, the overall evolution of aircrafts, in particular commercial aircrafts, tends to provide propulsion units having larger turbojet engine dimensions with larger diameter fans. It is also sought to provide shorter and lighter nacelles so as to reduce the generation of drag in the flight phase. The overall effect resulting in lower fuel consumption. 
     Due to the fan having larger dimensions, its rotational speed decreases and it then generates a lower frequency sonority. However, the acoustic panels of the prior art, which have improved acoustic treatment results in the medium or high frequencies, are not suitable for the acoustic treatment of low frequencies. 
     It is known in the public domain that the height of the cells allows adjusting the frequency at which the acoustic treatment is effective. 
     An increase in the dimensions of the cells also allows acoustic treatment in low frequencies. 
     An adaptation of the panels of the prior art would also consist in an increase in the height of cells of these panels. A target total height of the panel has, moreover, been evaluated to at least 50 mm to allow for an adequate low-frequency acoustic treatment. 
     However, the available space in future propulsion units with short nacelles does not allow integrating acoustic panels with excessive dimensions. 
     We know from the prior art, in particular from the applications WO2015023389 and GB2300081, low-frequency treatments with small bulk which consists in adding in each cell conical obstacles the top of which has an orifice. 
     However, this type of geometry is difficult to achieve on an industrial scale. 
     SUMMARY 
     This section provides a general summary of the disclosure and is not a comprehensive disclosure of its full scope or all of its features. 
     The acoustic attenuation panel according to the present disclosure finds an application in the aeronautics industry, in particular for the use thereof in the aircraft propulsion units. 
     The present disclosure addresses the above-referenced concerns and provides an acoustic attenuation panel including: 
     a perforated acoustic wall, 
     a first honeycomb structure connected to the perforated acoustic wall, having a plurality of acoustic cells delimited by peripheral partitions, 
     a second honeycomb structure having a plurality of acoustic cells delimited by peripheral partitions, 
     a septum having a plurality of macro-perforations, interposed between said first honeycomb structure and said second honeycomb structure, and 
     each acoustic cell of the first honeycomb structure and each acoustic cell of said at least one second honeycomb structure being disposed opposite a unique perforation of the septum. 
     By macro-perforation, it should be understood perforations the diameter of which is greater than or equal to 1 mm. 
     According to variations of the present disclosure, the acoustic attenuation panel includes one or more of the following optional features considered alone or in all possible combinations: 
     Each perforation of the septum has a diameter between 1 mm and 2 mm. 
     Each acoustic cell of the first honeycomb structure has a height between 5 mm and 10 mm. 
     Each acoustic cell of the second honeycomb structure has a height between 10 mm and 20 mm. 
     The height of the acoustic cells of the first honeycomb structure is lower than the height of the acoustic cells of the second honeycomb structure. 
     The height of the acoustic cells of the first honeycomb structure is equal to the height of the acoustic cells of the second honeycomb structure. 
     The total height of the acoustic panel is less than 30 mm. 
     The macro-perforations are evenly distributed in the septum such that three adjacent macro-perforations form an equilateral triangle, one side of which is equal to the diameter of the acoustic cells of the honeycomb structures and one height of which is equal to 0.86 times the diameter of the acoustic cells of the honeycomb structures+1-20%. 
     The acoustic cells of the second honeycomb structure have a diameter larger than the diameter of the acoustic cells of the first honeycomb structure. 
     According to this variant, the macro-perforations are evenly distributed in the septum so that three adjacent macro-perforations form an equilateral triangle, one side of which is equal to the diameter of the acoustic cells of the second honeycomb structure and one height of which is equal to 0.86 times the diameter of the acoustic cells of the second honeycomb structure+/−20%. 
     The first honeycomb structure is superimposed on the second honeycomb structure such that the peripheral partitions of the cells of the first honeycomb structure are arranged in the geometric continuity of the peripheral partitions of the cells of the second honeycomb structure. 
     Each cell of the first honeycomb structure and each cell of the second honeycomb structure is centered with respect to the single perforation of the septum. 
     Another form of the present disclosure provides a nacelle in which a fan is disposed, the nacelle including an air inlet including an inner face directed opposite the fan, said inner face receiving at least one acoustic attenuation panel as described previously. 
     In an aspect, the nacelle includes an air inlet, a thrust reverser, and an exhaust nozzle, wherein at least one of the air inlet, the thrust reverser, and the exhaust nozzle receives an acoustic attenuation panel as described previously. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings, in which: 
         FIG. 1  is a schematic representation of an acoustic attenuation panel according to the prior art; 
         FIG. 2  is a schematic cross-sectional view of an acoustic panel according to one form of the present disclosure; 
         FIG. 3  is an enlarged view of superimposed cells of an acoustic panel according to a form of the present disclosure; 
         FIG. 4  is a top view of an acoustic panel according to a form of the present disclosure; 
         FIG. 5  is a graphical representation illustrating the difference in the treated frequency ranges between the acoustic panel according to the present disclosure and the acoustic panels of the prior art; 
         FIG. 6  is a graphical representation illustrating the equivalence of the acoustic attenuation results obtained with an acoustic panel according to the present disclosure and an acoustic panel with a total height of 50 mm; 
         FIG. 7  is a table listing the results obtained by varying various parameters of the acoustic panel according to the present disclosure; and 
         FIG. 8  is an illustration of a propulsion unit including a nacelle the air inlet of which receives an acoustic attenuation panel according to the present disclosure. 
     
    
    
     The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features. 
       FIG. 1  is a partial view of an acoustic attenuation panel known from the prior art, in particular from the application FR 2841031. 
     In this example, the acoustic panel  40 ′ comprises a double resonator, that is to say it comprises two thicknesses of acoustic cells and has a laminated structure including successively, along the direction of the Y axis, a perforated acoustic wall  42 ′, a first honeycomb structure  44 ′, a micro-perforated septum  46 ′, a second honeycomb structure  48 ′ and a solid skin  49 ′ arranged on the second honeycomb structure  48 ′. 
     The perforated acoustic wall  42 ′ is configured to be in contact with the circulating air and is crossed by a plurality of macro-perforations (not shown) through which the acoustic waves can penetrate. 
     Each of the first honeycomb structure  44 ′ and of the second honeycomb structure  48 ′ includes acoustic cells separated from each other by peripheral partitions. 
     The septum  46 ′ is disposed between the first  44 ′ and second  48 ′ honeycomb structures. The micro-perforated septum of the current panels is crossed by a plurality of micro-perforations the diameter of which is conventionally in the range of 0.3 mm with a perforation density in the range of 400,000 to 800,000 holes/m2. 
     The acoustic panels with double resonators and with micro-perforated septum provide for attenuation of acoustic waves at medium and high frequencies, that is to say higher than 1,500 Hz. 
       FIG. 2  is a schematic representation of a cross-section of an acoustic panel according to a first form of the present disclosure. 
     The acoustic attenuation panel  40  is an acoustic panel of the double resonator type including successively, along the direction of the Y axis, a perforated acoustic wall  42 , a first honeycomb structure  44 , a macro-perforated septum  46 , a second honeycomb structure  48 , and a solid wall  49  called reflective wall, devoid of perforation. 
     Indeed, the use of an acoustic panel with a unique honeycomb structure does not allow obtaining a sufficient open surface ratio for a desired acoustic treatment. 
     The perforated acoustic wall  42  comprises a plurality of macro-perforations  420  evenly formed in the acoustic wall  42 . Each perforation  420  has a diameter in the range of 1.5 mm with a perforation density in the range from 40,000 to 100,000 holes/m2. 
     The perforated acoustic wall comprises an opening ratio between 8% and 20% relative to the total surface of the wall. 
     The first honeycomb structure  44  comprises a plurality of acoustic cells  440  delimited by peripheral partitions  445 . The second honeycomb structure  48  comprises a plurality of acoustic cells  480  delimited by peripheral partitions  485 . 
     The first honeycomb structure  44  is superimposed on the second honeycomb structure  48 . The first honeycomb structure  44  is misaligned with respect to the second honeycomb structure  48 , that is to say that the peripheral partitions  445  of the first honeycomb structure  44  are not in geometric continuity with the peripheral partitions  485  of the second honeycomb structure  48 . 
     The first honeycomb structure is directly coupled to the perforated acoustic wall  42  by gluing, for example. 
     The second honeycomb structure is coupled to the solid wall  49  by gluing, for example. 
     The acoustic cells  440  of the first honeycomb structure  44  extend in the Y axis and have a height H 1  between 5 and 10 mm. The height of the acoustic cells  440  of the first honeycomb structure  44  must be at least equal to 5 mm to inhibit coupling phenomena. 
     By height, it should be understood the side that separates the faces of a honeycomb structure. 
     The acoustic cells  480  of the second honeycomb structure  48  extend in the Y axis and have a height H 2  between 10 and 20 mm. 
     The height H 1  of the acoustic cells  440  of the first honeycomb structure  44  in direct contact with the perforated acoustic wall  42  is smaller than the height H 2  of the acoustic cells  480  of the second honeycomb structure  48 . The aim is to make the volume of the acoustic cells  480  of the second honeycomb structure  48  resonate. 
     In a form of the present disclosure not shown, the height of the acoustic cells  440  of the first honeycomb structure  44  is equal to the height of the acoustic cells  480  of the second honeycomb structure  48 . 
     To reduce bulk, the total height HT of the acoustic attenuation panel  40  is less than or equal to 30 mm. In an aspect, the total height HT is less than 25 mm. 
     The total height of the panel takes into account all of the elements constituting said acoustic panel. 
     The first honeycomb structure  44  and the second honeycomb structure  48  are separated from each other by the septum  46 . The septum  46  is interposed between said first honeycomb structure  44  and said second honeycomb structure  48 . The septum  46  extends over the entire surface between the two honeycomb structures, in a direction perpendicular to the Y axis. 
     The septum  46  differs from that of the prior art in that it is not micro-perforated but macro-perforated, that is to say that it has a plurality of perforations  460  having a diameter greater than or equal to 1 mm. In an aspect, the diameter of the macro-perforations  460  of the septum  46  is between 1 and 2 mm. 
     Each acoustic cell  440  of the first honeycomb structure and each acoustic cell  480  of said second honeycomb structure  48  is disposed opposite a unique perforation  460  of the septum  46 . That is to say each acoustic cell is disposed opposite one single macro-perforation of the septum  46 . 
     The principle is to define the drilling pattern of the septum, that is to say the distance between the macro-perforations so that in general, most of the surface of the septum has a unique perforation per acoustic cell in each honeycomb structure ( FIG. 4 ). It should be noted that, during industrial production, edge effects may occur marginally according to which a peripheral partition of a honeycomb structure may be positioned at the level of a perforation. In these marginal cases, there will potentially be a half perforation per cell or a full perforation and a half perforation at one end of the cell. This does not call into question the effectiveness of the panel conferred thereon by the fact that most of the macro-perforations are arranged in a unique manner opposite each of the acoustic cells of the first honeycomb structure and of the second honeycomb structure. 
     As an unintended consequence of the present disclosure, the macro-perforations  460  of the septum  46  allow obtaining an acoustic attenuation behavior in low frequencies without an increase in the height of the acoustic cells. The macro-perforations  460  of the septum  46  allow forcing the entire volume of the second honeycomb structure  48  to resonate over its height H 2 , thus allowing for an improved acoustic treatment of low-frequency waves. 
     The septum  46  may, for example, be made of an organic composite comprising one to three layer(s) of fiberglass fabrics embedded in an epoxy resin hardened by polymerization. 
       FIG. 3  is an enlarged view of a partial section of the acoustic panel according to another form of the present disclosure. Each acoustic cell has a hexagonal shape. 
     In this form, the first honeycomb structure  44  is superimposed on the second honeycomb structure  48  such that the peripheral partitions  445  of the cells of the first honeycomb structure are arranged in the geometric continuity of the peripheral partitions  445  of the cells of the second honeycomb structure  48 . 
     Each acoustic cell  440  of the first honeycomb structure  44  is arranged opposite an acoustic cell  480  of the second honeycomb structure  48 . 
     The septum  46  is provided between the acoustic cell  440  of the first honeycomb structure  44  and the acoustic cell  480  of the second honeycomb structure  48 . 
     The septum comprises a unique circular macro-perforation  460  disposed opposite the acoustic cell  440  of the first honeycomb structure  44  and the acoustic cell  480  of the second honeycomb structure  48 . Thus, each acoustic cell  440  of the first honeycomb structure  44  and each cell  480  of the second honeycomb structure are centered with respect to the unique perforation  460  of the septum  46 . 
     In the present example, the height H 1  of the acoustic cell  440  of the first honeycomb structure  44  is smaller than the height H 2  of the acoustic cell  480  of the second honeycomb structure  48 . 
       FIG. 4  is a partial view of a longitudinal section of the acoustic attenuation panel according to a further form of the present disclosure. 
       FIG. 4  illustrates the definition of the drilling pattern of the septum  46 , that is to say the definition of the distance between each of the macro-perforations  460  in the septum  46 . 
     The definition of the drilling pattern is defined so as to obtain a unique macro-perforation  460  opposite each acoustic cell of each honeycomb structure. 
     This figure illustrates the first honeycomb structure  44  comprising the plurality of acoustic cells  440  in the foreground, the second honeycomb structure  48  comprising the plurality of acoustic cells  480  in the background, as well as the macro-perforations  460  of the septum disposed between the first honeycomb structure  44  and the second honeycomb structure  48 . 
     In the present example, the diameter D 1  of the acoustic cells  440  of the first honeycomb structure  44  is equal to the diameter D 2  of the acoustic cells  480  of the second honeycomb structure  48 . 
     To obtain a unique macro-perforation  460  opposite each of the acoustic cells  440 ,  480 , the drilling step in the septum  46  must be in the range of the diameter D 1 , D 2  of the acoustic cells  440 ,  480  while taking into account the tolerances. The diameter D 1 , D 2  is defined as being the diameter of the circle inscribed in the hexagon of an acoustic cell. 
     The drilling step P 1  of the septum  46  in the length of the panel is defined by the general rule: D+/−20%. With D corresponding to the diameter D 1 , D 2  of the acoustic cells  440 ,  480 . 
     The drilling step P 2  of the septum  46  in the width of the panel is defined by the general rule: (D×0.86)+/−20%. With D corresponding to the diameter D 1 , D 2  of the acoustic cells  440 ,  480 . 
     Thus, the macro-perforations  460  are evenly distributed in the septum such that three adjacent macro-perforations  460  form an equilateral triangle  5 , one side P 1  of which is equal to the diameter D 1 , D 2  of the acoustic cells  440 ,  480  of the first honeycomb structure  44  and of the second honeycomb structure  48 , and one height P 2  of which is equal to 0.86 times the diameter D 1 , D 2  of the acoustic cells  440 ,  480  of the first honeycomb structure  44  and of the second honeycomb structure  48 +/−20%. 
     Thus, for example, by using honeycomb structures with ⅜″ dimensions known to those skilled in the art, for which the acoustic cells have a diameter D 1 , D 2  equal to 9.52 mm, the drilling step P 1  of the septum  46  in the length is set to 9.5 mm and the drilling step P 2  of the septum in the width of the acoustic panel is set to 8.3 mm. 
     This allows obtaining a unique perforation  460  of the septum  46  opposite each acoustic cell  440 ,  480  of the honeycomb structures such that the acoustic attenuation is improved. 
     The present disclosure is not limited to these types of honeycomb structures, and thus honeycomb structures with larger or smaller dimensions may be employed while remaining within the teachings herein. 
     In yet another form of the present disclosure not shown, the diameter D 2  of the acoustic cells  480  of the second honeycomb structure  48  is larger than the diameter D 1  of the acoustic cells  440  of the first honeycomb structure  44 . This form provides for an improved volume effect and obtaining equivalent acoustic attenuation results by reducing the height H 2  of the acoustic cells of the second honeycomb structure  48  and therefore a greater gain in size. 
     In this form, the drilling step is defined according to the diameter D 2  of the acoustic cells  480  of the second honeycomb structure  48 , that is to say according to the largest diameter of the cells. 
     Thus, the macro-perforations  460  are evenly distributed in the septum such that three adjacent macro-perforations  460  form an equilateral triangle  5 , one side P 1  of which is equal to the diameter of the acoustic cells  480  of the second honeycomb structure  48 , and one height P 2  of which is equal to 0.86 times the diameter of the acoustic cells  480  of the second honeycomb structure  48 +/−20%. 
       FIG. 5  is a comparative graph of the acoustic attenuation results obtained between the acoustic panel according to the present disclosure and the acoustic panels of the prior art. 
     The curve A illustrates the acoustic attenuation result obtained with the acoustic attenuation panel according to the present disclosure in which the height H 1  of the acoustic cells  440  of the first honeycomb structure  44  is equal to 5 mm, the height H 2  of the acoustic cells  480  of the second honeycomb structure  48  is equal to 15 mm, and the diameter of the macro-perforations  460  of the septum  46  is equal to 1 mm. 
     The curve B illustrates the acoustic attenuation result obtained with an acoustic panel of the prior art having a unique honeycomb structure called “Single Degree of Freedom” and having a total height of 20 mm. 
     The curve C illustrates the acoustic attenuation result obtained with an acoustic panel of the prior art having two honeycomb structures separated by a micro-perforated septum called “Double Degree of Freedom” and having a total height of 20 mm. 
     As the graph illustrates, the acoustic panels of the prior art provide satisfactory acoustic treatments in the medium at high frequencies. In addition, the so-called “Double Degree of Freedom” panels with micro-perforated septum allow widening of the acoustic attenuation to high frequencies compared to a so-called “Single Degree of Freedom” panel of the same total height. 
     In turn, the acoustic panel according to the present disclosure allows achieving an acoustic attenuation of the characteristic low-frequency waves of new propulsion units, for the same bulk. 
       FIG. 6  is a graphical representation that illustrates the acoustic attenuation results obtained with an acoustic panel according to the present disclosure and an acoustic panel having a total height of 50 mm. 
     The curve A represents the acoustic attenuation obtained with an acoustic panel according to the present disclosure wherein the acoustic cells  440  of the first honeycomb structure  44  have a height H 1  equal to 5 mm, the acoustic cells  480  of the second honeycomb structure  48  have a height H 2  equal to 15 mm, and the macro-perforations  460  of the septum  46  have a diameter of 1 mm. 
     The curve D represents the acoustic attenuation obtained with an acoustic panel having a unique honeycomb structure called “Single Degree of Freedom” and the total height of which is 50 mm. 
     The curve D illustrates the theory that the increase in the dimensions of the cells provides for the treatment of the waves in low frequencies. 
     The curve A demonstrates that the acoustic panel according to the present disclosure allows obtaining a desired acoustic attenuation of low-frequency waves with a gain of size of 60% compared to the acoustic panel of total height equal to 50 mm. 
     The acoustic attenuation panel according to the present disclosure allows obtaining an improved acoustic treatment of the low-frequency waves over a more selective frequency range. 
       FIG. 7  is a table that illustrates the acoustic length equivalence obtained with the acoustic panels according to the present disclosure for which different parameters have been varied. 
     The tested parameters include the height H 1  of the acoustic cells  440  of the first honeycomb structure, the height H 2  of the acoustic cells  480  of the second honeycomb structure, and the diameter of the macro-perforations  460  of the septum  46 . 
     The columns present, from left to right: the height H 1  of the acoustic cells  440  of the first honeycomb structure  44 , the height H 2  of the acoustic cells  480  of the second honeycomb structure  48 , the total height of the acoustic attenuation panel according to the present disclosure, the diameter of the macro-perforation of the septum  46 , and the equivalence of the wavelength. 
     The parameters of the height H 1  of the acoustic cells of the first honeycomb structure have been tested between 5 and 10 mm. The parameters of the height H 2  of the acoustic cells of the second honeycomb structure have been tested between 15 and 20 mm such that the total height of the acoustic panels of the present disclosure is always less than 30 mm. The diameter of the macro-perforation of the septum  46  has been tested between 1 and 1.5 mm. 
     The results demonstrate the obtainment of acoustic lengths equivalent to those obtained with acoustic panels having a total height in the range of 50 mm. The acoustic panels according to the present disclosure allow obtaining acoustic lengths significantly larger than the total height thereof. 
     To reduce bulk as much as possible, the height H 2  of the acoustic cells  480  of the second honeycomb structure  48  as well as the diameter of the macro-perforation are chosen according to the targeted frequency then the height H 1  of the acoustic cells  440  of the first honeycomb structure  44  is reduced as much as possible. 
       FIG. 8  is an illustration of a propulsion unit  1  extending according to a longitudinal axis X comprising a short nacelle  2  and a turbojet engine  3 . The nacelle  2  has a structure comprising an upstream section forming an air inlet  200 , a middle section  210  comprising fan cowls configured to surround a fan  30  of the turbojet engine  3 , a downstream section  220  comprising a thrust reverser and configured to surround the combustion chamber of the turbojet engine and an exhaust nozzle  230 . 
     The air inlet  200  comprises an inner face  205  directed opposite the fan  30 , said internal face receives at least one acoustic attenuation panel according to the present disclosure. 
     As illustrated in  FIG. 8 , other components of the nacelle such as the thrust reverser, for example, can receive the acoustic attenuation panel according to the present disclosure. 
     As a result, the acoustic panel according to the present disclosure provides a low-frequency acoustic treatment with acoustic performance equivalent to the acoustic treatment obtained with acoustic panels of total height in the range of 50 mm, with a gain of size in the range of 60% in comparison with these. 
     Unless otherwise expressly indicated herein, all numerical values indicating mechanical/thermal properties, compositional percentages, dimensions and/or tolerances, or other characteristics are to be understood as modified by the word “about” or “approximately” in describing the scope of the present disclosure. This modification is desired for various reasons including industrial practice, material, manufacturing, and assembly tolerances, and testing capability. 
     As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A OR B OR C), using a non-exclusive logical OR, and should not be construed to mean “at least one of A, at least one of B, and at least one of C.” 
     The description of the disclosure is merely exemplary in nature and, thus, variations that do not depart from the substance of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.