Techniczne nauki / 6. Electrical engineering and electronics

Candidate of Technical Sciences Sukhar'kov O.V.

Odessa National Academy of Telecommunication named after O.S. Popov,Ukraine

Sound generation in the modified hydrodynamic radiator

Currently stream hydrodynamic radiators (HDR) are used to enhance the efficiency of various physical processes [1, 2]. Among the advantages of data converters there are high energy intensity (the ratio of radiated power to mass), low cost of radiators manufacture, small size and simple maintenance. Stream HDRs are conventionally divided into uniflow and counterflow converters [1]. For HDRs of the two types the sound generation mechanism is explained by pulsations of localized two-phase region and transverse bending self-oscillation of axisymmetric jet shell [3]. The author designed and developed a modified HDR with a step obstacle and circular gap nozzle in the form of coaxial discs [4].

In the modified HDR flat submerged axisymmetric jet interacts with a stepped obstacle, which has square niche in front of it [4]. The process of sound generation in the interaction of the incoming flow with a rectangular niche is analyzed [5 – 8]. In the case of small flow rates , when the Reynolds number ( – kinematic liquid viscosity) does not exceed there are stable vortices formed inside the niche (one or two). With this flow mode around the niche no effects associated with the generation of sound arise. Significant increase in Reynolds number leads to oscillation of the vortex at the aft wall of the niche and related vertical oscillation of the shear layer over the niche with a certain period . Frequency of acoustic oscillations excited by the flow is substantially proportional to the flow velocity and inversely proportional to the length of the niche. This confirms the fact that the niche being flown around may be referred as an acoustic generator, in which some part of the flow self-oscillations is excited by aerohydrodynamic feedback [5, 8].

At higher Reynolds numbers (about ) it was found that in addition to the above mentioned mechanism of sound generation, there is another mechanism associated with abrupt periodic release of vortex from niche into the flow [5]. The reason of the phenomenon is the ongoing oscillations of the shear layer that last for some time while the layer is swept away by flow beyond the niche. In English literature the mechanism is known as “wake mode”. Feedback mechanism in the self-oscillatory system has purely hydrodynamic nature [7]. However, in the modified HDR an other kind of oscillation mode feedback is implemented in comparison to the case of the interaction of incoming flow and niche. This is due to the forming of two-phase medium in the rectangular niche of HDR (liquid and steamgas microbubbles).

The proposed physical model of HDR is based on the idea of self-oscillations of submerged circular jet plate in the presence of developed cavitation (fig. 1).

а b

Fig.1. Modified HDR: a – physical model
b – photo of operating radiator

Submerged axisymmetric jet 5 (fig. 1a) outflows perpendicularly to the axis of the radiator from the slit nozzle radiator that is formed by coaxial discs of body 1 and fairing 6. The jet is a kind of an elastic circular jet plate that makes bending oscillations. We can assume that the inner edge of an circular plate is rigidly clamped at the nozzle exit and the outer edge leans freely against the stepped obstacle 3. Among the geometric parameters of the jet plate there are: thickness , width , inner radius and an outer radius . Moreover, the plate thickness is small in comparison to the radius . Hydrodynamic parameters of jet plate are: – density, equivalent module of elasticity of a submerged jet and its velocity at the nozzle exit. A niche with almost square cross-section is made in the body 1 of the radiator. Stepped obstacle 3 (rectangular wedge) helps to ensure that due to the Bernoulli effect a part of the kinetic energy of the jet is spent on the formation of primary toroidal vortex 2 in the niche. In the center of the vortex region 2, the conditions for the cavitation are created. Therefore the niche is filled with two-phase medium (liquid and steamgas microbubbles).

With increase of cavities concentration the pressure inside of the niche increases and reaches critical value. At this point, the deformation of the outer edge of the circular jet plate and release of the contents of the cavitation region 2 to the external environment takes place. This creates a secondary toroidal region 4 with developed cavitation (fig. 1b). Cavitation bubbles collapse in region 4 thus resulting in microshock waves in the surrounding liquid medium. This leads to an increase of the generated acoustic signal. After the cavities ejection the pressure inside the niche becomes smaller than the hydrostatic pressure in the environment and the conditions for the vortex formation are again formed in the niche. Further the described process repeats periodically and pulsations of two-phase region 2 excite bending oscillations in jet plate 5. Frequency of pulsations of cavitation region 2 depends on the jet velocity at the nozzle exit. When the frequency of pulsations of region 2 and the proper frequency of the jet plate 5 are the same the sound signal of maximum level will be generated [4].

Fig. 2 demonstrates a sound record and spectrum of typical sound signals generated by HDR with optimal geometric parameters and the optimum mode of jet discharge. It is clear (fig. 2a), that in this case the radiator generates nonharmonic acoustic signals in the form of short exponential pulses. A significant fraction of the radiated sound energy falls on the fundamental frequency (fig. 2b). It has been found that the radiator can be a source of powerful acoustic waves in the low sound frequencies [4].

а b

Fig.2. Typical sound record (a) and the corresponding spectrum (b)

of the acoustic signal from modified HDR

The experimental investigations of HDR helped to reveal a number of differences of sound generation in comparison with sound generation in the interaction of the incident flow with a rectangular niche. First, in case of the modified HDR fluid flow is formed by slit nozzle as an elastic jet with small thickness [4]. Secondly, the radiator operates in the speed range that results in the formation of cavitation area in the niche. Thirdly, a modified HDR generates sound signal at a very high Reynolds numbers: . Furthermore, with increase of the jet velocity at constant geometrical parameters of HDR the frequency of the main sound tone generated by the HDR decreases. The latter fact is contradicts completely to the classical laws that are specific to a aerohydrodynamic sound radiators.

To answer the question, what makes a flat axisymmetric jet perform strictly periodic oscillations, i.e. operate in a self-oscillation mode, we use the analysis of article [3]. Since the wave size of oscillating circular jet plate is substantially smaller than the wavelength, its outer surface is loaded by an impedance that has mainly mass character. In contrast, the inner surface of the plate is loaded by an impedance having an elastic nature. Moreover, due to the rather high ductility of two-phase medium, the corresponding elasticity may be relatively small. As the result a kind of resonance acoustic system consisting of three series-connected elements: elasticity – jet plate – mass is created. Clear enough, it has its proper frequency, which will be imposed on the transverse oscillations of an circular plate. This is the physical mechanism of self-oscillation feedback of jet plate, the nature of which in this case is purely acoustic.

Literature:

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