Mel’niсk V. N., Karachun V.V.

National Technical University of Ukraine «KPI»

tHE MAIN FEATURES OF THE AERODYNAMIC FLOW

The main features of the aerodynamic flow, surrounding MM in supersonic flight, viewed from the circuit shown in fig. 1. Chief among them is the extraordinary influence of strong shock waves at the head aerodynamic fairing (HAF) and in places of sharp change in profile of fuselage. The turbulent boundary layer can be separated from the body and interact with the shock wave. In this case, in the vicinity of the boundary layer the intense pressure pulsations appear, which are then in the form of sound waves propagate downstream. The resulting turbulent layer has no certain speed of convection. Its correlation distance is of the same order as the thickness of the boundary layer. On the other hand, the waves move through the flow with velocity of sound on the flow, so the wave front, moving downward, will have on the hull missile Mach number equal to , and the front of waves moving upward, .

The characteristic size of these waves is determined by the scale of turbulence, as well as the Mach numberof the rocket and is equal to the product of scale of turbulence on the value , or on , depending on the direction of wave motion on the flow (upward or downward). Most of the flow near the wall moves at a speed below the speed of sound, so that the pressure field induced by them on the wall is alike to a field of pressures in the subsonic boundary layer (fig. 1).

The external area of the boundary layer move relative to the body of missile at supersonic speeds, so the turbulence will be the cause of the Mach wave, which upon reaching the shell launcher will create a new source of noise. These waves are especially dangerous, because they can be quite intense. With the increase in flight speed the large part of the boundary flow becomes supersonic and, thus, Mach waves will be radiated by sources that previously were moving slowly. This, in turn, lead to an increase of the dimensionless level of pressures and a decrease in the relative velocity of convection.

This effect was first detected at low supersonic speeds. He was unstable, but with increasing speed gradually stabilized, since at high gradients it demanded major changes of flow velocity for any significant increase in the turbulent boundary layer. These observations are consistent with experimental data.

Let’s analyze another important case – the rocket flight at subsonic speed and distant from other objects. Between the experimental works presented here the most significant two are – Willmarth's WW, Wooldridge's CE and Hodgson's T.H. In the first work it is shown that the mean-square pressure is 2,15 bigger the average shear stress and 0,0047 times – the dynamic pressure. In the second - these numbers are 2,2 and 0,005, respectively. It is claimed that the spatial scale of turbulence is of the degree of the boundary layer thickness, and the pressure field is moving downward with a velocity of convection, which lies within 0,56 .. 0,83 of free flow velocity. Another, sometimes more important source of pressure pulsations on the surface of the MM at subsonic speed, is due to the noise of the jet of rocket engine.

Fig. 1 schematically shows the structure of the jet and features of the generated sound. When considering problem of the sound pressure at the surface of the fuselage a more detailed examination of some properties of this field is necessary. Theoretical studies suggest the existence of the more intensive Mach waves in the direction where there is equality . Obviously, for the surface of the shell, this equality never holds, because here the angle is close to π.

Satisfactory theoretical interpretation must be based on the equation with the Doppler effect , which in this case will be equal to , where the Mach number for the motion of vortices on the air. Sound pressure must satisfy the equation:

(1)

The scale of the pressure field in this case is relatively large, to be exact - of the order of the diameter of the nozzle. This is characteristically to the sound radiation of high-speed flows of rocket engines. Convection of vortices in the direction of the rocket increases this scale, changing the frequency in accordance with the Doppler factor . Movement of the mother missile on the air environment can significantly affect on the level of noise. The change will be proportional to the value , which is consistent with the above. The scale of the pressure field is not changed, but the frequency is adjusted by the amount , since this factor determines the change in velocity of the waves on the hull of the missile.

As for liftoff from the surface of the Earth (or from the mine), as well as mobile missile launch bases, the sound field here is very complex. There is not only the direct acoustic radiation, but the reflected sound field. The latter fact is explained with the reverb effect.

There is no doubt that the main source of noise is highly directive Mach waves, the appearance of which is predetermined by the condition . The theory explains that in this case the sound intensity is proportional to the third degree of escape velocity and the square of flux density. Both of these statements are in satisfactory agreement with experiment. However, only when you run out of the mine missile can be subjected to the sound radiation of this type, because only in this case we have the surface reflecting the sound in the direction of the missile.

The main source of vibration of rocket body during launch is a sound, created by the strong turbulent flow, falling on the ground. The sound emitted from equivalent dipoles, exceed the one generated by quadrupoles, and has a maximum thrust of the normal to the surface of the launch pad. As the distance – the intensity of the sound is significantly reduced.

Mean-square value of pressure in this case is proportional to the sixth degree of velocity, because here the scale of turbulence varies from very high values associated with the slow movement of the flow from the reflecting surface to low values, about the thickness of the supersonic boundary layer. Spectrum of induced acoustic pressures has sufficiently wide range of frequencies.

On the rocket there are two major changes in the sound field. Near the nozzle exit sound is most intense and decreases with distance towards the bow. The second effect is less obvious. Near the nozzle exit has a small-scale turbulent flow, which creates a high-frequency radiation, while the main sources of low frequency sound are relatively far away.

A few words about some less-studied, aspects of the noise of rockets. The law of the third degree of speed - the square of the density, a pronounced orientation of the Mach angle, as well as the possibility of using the Strouhal number as the scale of frequency – all of these features, predicted theoretically, have now a reliable practical confirmation. There is enough convincing live-justification of the fact that the main cause of noise of MM is radiation emitted by moving with supersonic speed quadrupoles. It is found that 0,5% of the power of modern media is emitted in the form of sound.

Consideration of noise would be incomplete if we do not raise the least clear aspects of the problem. These include the influence of strong shock waves in the zone of mixing of the jet, the temperature heterogeneity as a result of poor combustion, turbulence damping sound waves and a few others.

Thus, it is logical to assume that local overheating of surface adds to their emitted sound. However, the experiment proves the opposite.

Incomplete combustion of fuel increases the noise by 5 dB compared to the nominal regime.

The presence of shock waves, in essence, reduces the emitted sound.

Generally speaking, the sound generated by the temperature inhomogeneities, and strong shock waves is negligible compared with the sound, created by other turbulent sources. This is confirmed by experiment. However, these factors may impact directly on the flow and change the nature of the turbulent sources, which will lead eventually to a change in their effectiveness.