Karachun V.V, Mel’niсk V. N., Chebotarjova I.G.

National Technical University of Ukraine «KPI»

GENERATORS OF POWERFUL AERODYNAMIC NOISE

BASED ON THE ROTOR DYNAMIC SIREN

In practice, the study of properties of mechanical structures, including on-board equipment, working in conditions of acoustic effects of high intensity and wide frequency range, of domestic and foreign scientists and engineers made significant progress. A methodology for calculating approximate (for the infinite length) mechanical models and some specified types (limited sizes) models. However, these calculations are approximate in nature, with highly random and cannot show actual picture of the influence of acoustic radiation on material of structures and elements of the on-board equipment. Determinative and final in likelihood, at present, is still experimental study of acoustic endurance of products on specially equipped for the test stands.

When creating these stands, it is especially being solved the issue of choice of acoustic radiation sources - with efficient, easy to establish and operate and, of course, available with the possibility of forming a sound field of the desired composition and close to natural conditions. Most of these requirements, particularly for first and main parameter are satisfied with sirens.

It should be noted that still have not been made sufficient calculations of sirens, which have been brought to engineering solutions. Not conducted a comparative analysis of properties and monorotor and multiplerotor structures, not solved the problem of optimization which are of practical interest. Finally, it is not covered in the literature the issue of influence on working siren of kinematic and force excitation from the placement location.

This part is devoted to the generalization of existing data, systematization of methods of calculation and design of sirens, opening of new features and properties. However, here it is not set the problem of basic solving the issues of creation of sound field using sirens, but only some aspects of theory and practice, which, according to the authors, either need serious adjusting, or still not reflected in studies of creative professionals are contemplated.

It is known that acoustic emission with intensity above 150 dB, can serve as a reason of phenomena of fatigue in the material. The development of cracks, of course, starts from a place of stress concentration – in riveted joints, cutting-out and therefore similar to this occurs even in the center panels.

The structural response to acoustic pressure, as determined, largely depends on the frequency, spatial and temporal characteristics of acoustic pressure. Thus, while experimental studying aircraft ІЛ-18 fuselage it was found that the effect of loads due to propeller operation leads to the emergence of strains in the material at a frequency of 72 Hz of pronounced pulsed character. In response, in the structure having its own rapidly damping fluctuations, with no resonance forms that indicates a significant distance on the right axis of natural-vibration frequency of elements.

The main features of the load of the fuselage in acoustic field of jet engines and turbulent boundary layer is a broadband frequency spectrum, the random nature of changes in value of acoustic pressure in time. This leads to that in the thin-walled elements of structure it is generated lots of fluctuations, which may coincide with their own. Comparing the results of experimental studies, it can be concluded that the most dangerous, in terms of occurrence of acoustic fatigue in the material structure and promoting this spatial-frequency resonance, is a wideband frequency load, that occurs, for example in the area of jet stream in, in case of turbulence because of air flow separation and so on.

Despite improvement in recent years, progress in developing analytical methods for assessing the degree of influence of acoustic radiation on the material of structures, the most reliable and the final face is still experimental research. In this regard, first and foremost issue faced is a choice of source of intense sound radiation.

Currently there are widely used powerful sources of noise such as air spray, wind tunnel, jet flow engines, air screws, speakers, sirens.

Noise spectrum of jet flow (hot and cold) best represents the natural range of engines, and noise level, reproducible by them is 150...170 dB. Essential defect of this method is low efficiency - about 1%.

Wind tunnel contribute the largest approximation to the conditions of excitation of structures from turbulent boundary layer (efficiency about 1…2%).

Speakers (efficiency 5...20%) have a wide frequency range - up to 2000 Hz, while the sound pressure level does not exceed 150 dB in small cells. In revibration cells, by the way, they can break from their generable noise.

The largest spread among other types of sources of noise received siren. Sirens allow to generate sound pressure 160...180 dB with frequency range from 50 Hz to 5000 Hz. Their efficiency is the highest and is about 4 ... 40%.

One of the first sources of broadband noise was proposed in USA by von Hyrke multiplerotor siren. Along with the advantage - the opportunity to generate noise to the overall sound pressure of 170 dB and the acoustic power of 50 kW – multiplerotor siren has several disadvantages. Neither theoretically nor experimentally, it is not ascertained the optimal parameters of rotor to obtain a specified range, there are also significant difficulties in the manufacturing, building and operation of these sirens.

The original siren design to create random noise is suggested by staff of Southampton University. Modulation of air flow is implemented here by using steel strips with holes that are located under residual cyclic quadratic chain code. Stretching the strip with high speed, the air flow is being broken, thus emerging sound vibrations, the level of which reaches 165 dB. To design deficiencies can be a complex operation of a steel strip ascribed.

Thus, for this time a sufficient number of booths to test products on the acoustic strength is created. In frequency principle they can be divided into two classes - discrete and broadband. And, second, as already noted, the best way reproduce natural conditions.

In domestic literature first and, apparently, quite extensive information on the theory and calculation of sirens is in the works of M.I. Karnovskiy, L.Ye. Matohnyuk. A mathematical model of rotary siren is created, the influence of geometric parameters of windows and the distance between the disk of rotor and stator on the structure of the spectrum and intensity generable sound is researched.

At a later time a number of studies abroad and in Ukraine were also dedicated to creating and describing the work of sirens.

Due to the rapid development of powerful drive units, especially in rocket production, again facing a question of research elemenets of structures and on-board equipment on the acoustic resistance, especially in light intensity of 150 dB and above. Unfortunately, it occurred that it is a small number of power generators of sound, and the theory, calculation and design of pneumatic rotary sirens to generate broadband acoustic effects of high intensity either partially researched, or have significant deficiencies.

Seeing early said, let’s formulate some aspects of the principles of the theory, calculation and design sirens of broadband noise of high intensity. Along with this, let’s give some details of work of sirens that are related to kinematic effects, but not reflected in current research.

Let’s illustrate the work of monorotor siren by example of its known technical implementation. Mechanical model is a two coaxial discs with radial windows, one of which - the rotor - rolling, the other - stator - immobile and rigidly attached to the body of siren. Stream of air under pressure comes in the camera of siren, and then passing through the window of the stator, is broken by suspending rotor and creates sound vibrations of environment.

Taking discs of small size in comparison with wavelength, we consider acoustic process subordinate to linear differential equation.

For the chosen geometry of windows, air flow modulation function f (t) will repeat the geometry of the windows of the rotor and, therefore, can be represented in the form of periodic, quasi-trapezoid function with period T = 2π / ω (fig.1). Here ω - angular speed of rotor (sec^-1). Duration of single pulse (t₀+t₁) is defined by equation (t₀+t₁)=T/m, and m - the number of equal parts which the circle of rotor is broken into. Thus, single pulse duration will determine the minimum window of rotor size.

Generally speaking, a form of modulation function may be slightly different and is determined in each case experimentally, as here, among other things, a great role have parameters of the compressor system. For example, the modulation function may take the form of commuted sinusoid, triangular form, combined parabolic arcs and other more complicated configurations.

To obtain the necessary form of modulation function it is necessary to ensure the pressure in the free stream, at least 2 ... 3 atm., so that further increase of pressure does not decisive influence on the velocity of air in the exhaust stream that detects mostly acoustic power (fig.2).

On the other hand, during siren work it is available simultaneous flow of air through the multiple windows that will be reflected on the size of acoustic radiation. Therefore, we should ensure, for example, double or triple reserve capacity of compressor to achieve the necessary continuity of pressure flow environment that injected.

To ensure the random nature of distribution of windows in a circle of disk of rotor, they are affixed on the remainder of cyclic quadratic chain code, which, in some areas of the window can be close, and on the other missing entirely. In those areas where windows are put, the value of function is defined as the expression of modulation f(t)=λ_kf₀ (0≤λ_k≤1, k - number of area), where windows are missing – function of modulation is zero.

Let’s assume for simplicity that the modulation function f(t) is even and periodic, ie f(t)=f(t+T). In this case it can be represented in the form of expansions into a trigonometric series

with coefficient

where n – harmonic number; f₀ – amplitude; π = 3,14 .

Not considering those areas where the modulation function is zero, ie f(t)=0 and summing on areas with impulses, we get –

where p – number of single pulses during the period T, so 1< р ≤m; sum means that the values of parameter k are not consecutive, but only for areas with impulses, ie, f(t)=λ_kf₀; t₁ - time of the impulse front shaping.