US2252256A - Sound attenuator for air impellers - Google Patents

Description

Aug. 12, 1941. E. H. HARRIS 2,252,256

SOUND ATTENUATOR FOR AIR IMPELLERS Filed .Jan. 11, 1939 2 Sheets-Sheet l INVENTOR, I

1941- H. HARRIS 2,252,256

SOUND ATTENUATOR FOR AIR IMPELLERS Patented Aug. 12, 1941 UNITED STATES PATENT OFFICE SOUND ATTENUATOR FOR AIR IMPELLERS Eliot Huntington Harris, New York, N. Y.

Application January 11, 1939, Serial No. 250,365

3 Claims.

My invention relates to a device for attenuating the sounds incident to the rapid rotation of impellers in air.

The device constituting the subject matter of my invention although designed primarily for use in connection with the standard types of impellers or propellers utilized in aeronautics is adaptable for Ventilating fans or for special types of impellers or propellers such for instance as those of the enclosed blade and slotted disc types.

My invention, briefly stated, resides in the disposition about the radial blades of an air or gas impeller or propeller, in circumferential relationship, of an annuloid enclosure, the inner boundaries of which will be so disposed with respect to the rotating impeller blades as to prescribe or define a zone or port for intake and discharge of the air, partaking of the nature of a nozzle, and which for the sake of brevity will be hereinafter referred to as a nozzle.

The said annuloid enclosure comprises two or more chambers with access to the space circumscribed by the nozzle through an aperture or series of apertures through which may pass the gas or air, as well as the sound Waves created by the action of the impeller. These chambers will have such construction and relation one to the other as will serve to attenuate those sounds whose amplitude it is desirable to reduce.

The primary object of my invention is to reduce the volume of those dominant sounds created by the action of a rotating impeller in a field of gas or air, thereby reducing the facility with which the presence of an airplane may be detected, and also to diminish the noise created as stated, within an airplane.

By placing a nozzle around an air impeller it is obvious that both suction and discharge coefficients will be improved. .I have discovered that by use of a properly designed nozzle, with all other conditions maintained, the quantity of air displaced will be increased, or, as a corollary, the thrust of the impeller will be substantially increased to an extent sullicient to compensate for the additional weight when the nozzle is applied to an aircraft. While the employment merely of a nozzle will eliminate certain sounds resulting from the sudden and impulsive change of direction in the air sucked into the impeller stream,

the diminution thus obtained in the volume of twice the magnitude of one of these sounds, it will be seen that the elimination of a sound of minor magnitude will result in only a slight reduction of the total volume of sound. It should be further explained that studies of the sound produced by a fast moving propeller in air show that the sound consists of a series of sounds of independent frequencies as well as many harmonies of the fundamentals. Thus frequencies from approximately to 12,000 per second have been located when examining a propeller from a near-by location. Examined from a distance where no reflecting objects, such as ground contour, may interfere, there will be found certain dominant frequencies, F, that, in most instances, are a function of the number of blades in the propeller and of the speed of its rotation, the flexure vibrations which may be computed from the physical design of the blades and shaft, as well as those indefinite frequencies associated with the shedding of eddies from the blades. My device will attenuate these dominant sounds.

In the accompanying drawings, illustrating different embodiments of my invention:

Fig. 1 shows, in front elevation, a nozzle circumscribed about a rotating impeller with part of the inner, directing surface removed to better show the inner chambers and particularly the radial walls, embodying my invention;

Fig. 2 is a cross section taken on the line 2-2 of Fig. 1 and shows the circumferential annuloid enclosure as being divided at any cross section into one impedance chamber and one resonator chamber, the direction of air flow being indicated by the series of arrows, the air approaching the device at its leading edge and passing beyond the device at its trailing edge;

Fig. 3 shows an embodiment of my invention wherein at any cross section are employed two impedance chambers and two resonating chambers within a nozzle placed circumferentially about a rotating impeller;

Fig. 4 shows a propeller with a circumferential nozzle, and at the cross section illustrated, one impedance and two resonating chambers, the nozzle face forming one enclosing wall.

Proceeding now to a more detailed description of my invention and referring to Fig. 1 and Fig. 2 of the drawings, the numeral l denotes a nozzle provided with an interior directing surface shown in cross section at 2, shown as partly cut away in Fig. 1, which is placed about the periphery of the circle described by the propeller 3 in rotation on the shaft 4. In the cross sectional view of the nozzle Fig. 2, there is shown an outer enclosing imperforate wall of the nozzle which is indicated by the numeral 5. The annuloid created by the enclosing surfaces 2 and has its axis coincident with the axis of the propeller shaft 3. Within the annuloid there is shown a dividing wall 15 which separates the annuloid enclosure into the impedance chamber 6 and resonator chamber "a. In the directing surface, indicated by the numeral 2, there are provided a series of orifices 8, giving access to the chamber 5. In the dividing wall Ill there are orifices 9 giving access from the impedance chamber 5 to the resonance chamber '1. The annuloid chambers 6 and 'I' may be increased in frequency by placing one or more limiting walls, indicated by the reference numeral l2 as shown, said walls l2 being approximately radial to the axis of the shaft 4 and inside the annuloid space enclosed by the walls 2 and 5.

In Figure 3 there is illustrated a nozzle which is designated by the reference numeral l which departs in the shape of the directing surface from that of the nozzle illustrated by Figs. 1 and 2, and in that the annuloid space between the walls 2 and 5' has been further divided by a solid wall H and there are provided two sets of impedance chambers 6 and two sets of resonator chambers I. The orifices 8 provided in the wall 2' of the nozzle l are here arranged in two groups, one on the suction side of the plane of propeller rotation and one on the discharge side thereof. There are also two sets of orifices 9 interconnecting the impedance chambers 6' and resonating chambers I.

Fig. 4 is similar to the construction of Fig. 3, except that the single impedance chamber 6 has access through two sets of orifices 9 to the two resonator chambers l" and only one series of apertures give communication from impedance chamber 5" to the source of noise.

The proper design of my impedance and resonator chambers is dependent upon a large number of variables. The calculus employed may easily become too involved for ready solution and there are also certain constants required which must be empirically determined. 1 have therefore developed a series of ratios for the design which may be employed with good results.

It must be understood that for each change of impeller design, diameter and critical speed, a different design of my device will be required. There is, however, a considerable tolerance available in the design of the chambers due to the fact that impedance and resonance will be effective over a fairly wide range of frequencies in any given orifice or chamber, and furthermore the nonharmonic relation will maintain between impedance and resonator chambers over any small range of volume or shape change in such chambers. Furthermore the attenuating effect resulting from my proposed arrangement of chambers and their associated orifices is active over the full range of harmonics and to a slightly lesser extent to the 5/4, 2/3 series of frequencies.

The arrangement of chamber for my invention as illustrated by Fig. 3 of the drawings will be described below. The methods for calculation of other types will be obvious to one skilled in the art.

Impedance of orifice leading from a reservoir is given by E. J. Irons in the familiar formula:

where Z=impedance; azvelocity of sound in medium; =density of medium; V=volume of chamber; F=frequency; c=conductance.

From this it will be seen that the impedance varies inversely with the volume and directly with the conductance. Therefore the impedance chamber should be smaller than the resonator chamber and by calculation it may be estimated that the best results are obtained with ratios from 3.6:1 to 7.6:1. Although attenuators have been made with ratios of 1.621 there were special features which would not necessarily hold for this design. The actual size and shape must be on an approximate ratio of tuned frequencies as will be later explained.

An approximation may now be made of the natural frequency of the resonator chamber by the standard formula:

(1 C 21r V where the symbols are the same as above. Multiply the resulting frequency f by 1.15 to get an approximation of the actual frequency for the chambers indicated by the drawings. The result of this computation may be compared with the dominant frequency F to be attenuated. If the estimated frequency is too low, baflies may be designed to be placed approximately radially to the axis of the impeller and the volume of the chamber reduced, then another estimate made.

Another method of computing the factors involved is to consider one impedance chamber and its connecting resonator chamber as a coupled system. Then the natural frequency of the coupled system where M=the combined vibrating mass, mi -the mass within the impedance chamber and ma: the mass within the resonator chamber. The mass may be estimated by M:=A p Z, where A: area, =density, l=length including end correction. End correction would be approximately 0.29 times the diameter of equivalent cylinder. The density must be chosen for the atmospheric conditions considered. At one atmosphere pressure, 0 C. and no humidity, p may be assumed as 0.00129 in the C. G. S. system. The natural frequency of the resonator chamber 712, and the impedance chamber 121 may be computed by the formula;

l ing m where L, the inertance,

and C, the capacitance,

It may also be noted that for a thin plate opening the value of A1 takes on the character of the radius as the inertance is considered as occurring in the orifice whereas A in the capacitance formula is a function of the volume and should be so computed. Integrals of these formulae may be used when the shape of the chambers is complex, as, for example, when a complete annuloid space is used as a single chamber. It will be found, however, that for all usual conditions the chambers will consist of an annuloid space divided by radially disposed walls.

When the frequency of the resonator chamber is estimated approximately to the frequency F, or 2 F, using the lowest value of the exponent possible, the pitch of one resonator chamber should be empirically tuned to the frequency F, or to a frequency of the series noted above. When the above conditions are satisfied the correct design has been attained.

The frequency of the impedance chamber will then be attained as follows: first, determine the logarithm of the frequency of the resonator chamber; second, find the logarithm of twice that number; then, to the logarithm-of the first frequency add one twelfth of the difference between the two logarithms found. The antilog of this will be the base frequency of the impedance chamber, A frequency of 1/2 times this figure will be used to determine the approximate volume by the formula and tested until the volume of the impedance chamber falls within the ratio figures given previously. This may be done by use of the resonance formula given above to obtain the approximate volume when the exact design will be obtained empirically.

The ratio of the total area of openings, apertures, slots or means of communication from the impedance chamber to the cross sectional area of the nozzle at the vena contracta may be approximately 2:1.

It should be kept in mind that when testing the natural frequency of a chamber that the openings in the chamber wall should be according to their final location as these openings afiect the frequency.

It is to be understood that my invention is not to be regarded as being limited by any or all of the constructions herein illustrated and described, such illustrations and descriptions being merely for the purpose of showing how the principles underlying my invention may be carried into practise.

The particular curvature of the intake and discharge nozzle is no part of my invention. From some point either side of the plane of propeller rotation the inner boundaries of the annuloid are divergent from the propeller axis.

The materials employed in the construction may be any lightweight material lending itself to thin wall construction, such as steel or aluminum sheet or compressed asbestos board or the like. Welded steel sheets have performed satisfactorily in tests.

The support for my device may be a simple frame attached to the structure supporting the prime mover, or my Whole device may be attached to the impeller itself, all as will be understood by those skilled in the art without further disclosure. If the latter construction is employed, the lift of the rotating cylinder must, of course, be balanced and the yaw engendered thereby must be corrected by proper vanes. Sound insulated walls and other construction details are of course applicable but form no part of my invention. Neither does the shape or design of the impeller become any part of this invention.

I claim:

1. A sound attenuator for air impellers comprising an annuloid having an outer imperforate surface and an inner perforated surface, said surfaces joined together at the leading and trailing edges, said inner surface having the greater diameters of its annular form at its axial extremities, the lesser diameter occurring in the central section of its axial length, walls Within said annuloid to divide same into impedance and resonance chambers, means for passage of sound Waves from exterior of said annuloid into said impedance chambers, and orifices between said impedance chambers and said resonator charn bers.

2. An attenuator for the sounds created by the rapid rotation of a propeller on its shaft, said attenuator comprising an annuloid whose axis coincides with the axis of said shaft, said annuloid being formed by an outer imperforate surface and an inner surface containing openings, the axial extremities of said inner surface of the annuloid having greater diameter than the central section of said inner surface, walls between the inner and outer surfaces of said annuloid dividing the enclosure into a multiplicity of chamber groups, each group being composed of one impedance chamber and one associated resonator chamber and apertures in said inner surface for the passage of sound waves from the exterior of said annuloid into said impedance chambers.

3. A sonic attenuator for air impellers compirsing an annuloid formed by an outer imperforate and an inner perforate surface, said inner surface being divergent from its central section to its leading edge, said inner surface being divergent from its central section to its trailing edge, walls within said annuloid from and between said inner and outer surfaces dividing said annuloidal enclosure into a multiplicity of impedance and resonator chambers, means of communication between each impedance chamber and the exterior of the annuloid through said inner perforate surface and orifices between said impedance chambers and their adjoining resonator surfaces.

ELIGT HUNTINGTON HARRIS.

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