Abstract. Hydrodynamic processes due to spherically focusing acoustic shock waves in liquid are discussed. The luminescence registration has been combined with the pressure measurement and high-speed photo recording. The electromagnetic generator of shock wave as a sphere segment is used in the experiments. Complex non-linear hydrodynamic processes are caused by the cavitation in the shock wave (~1mks) focus in liquid both in boundless volume and near a free surfaces. It is shown experimentally, that: 1) Along a focal axis the rarefaction wave is transformed into a compression cavitation shock wave (CSW). The observed effect can be named the effect of "non-linear amplification of sound", 2) Advanced cavitation in the focal area near a free surface results in the quasi-collective collapse of cavities with the "sultan" hydrodynamic effect, 3) The first luminescence pulse correlates with the cavitation breakdown in a liquid. It is suggested, that this luminescence pulse is caused by electrokinetic processes in forming cavities.

INTRODUCTION

It is known, that when focusing the powerful laser radiation into firm and liquid media the optical breakdown is observed, which is followed by destruction of a firm bodies and by cavitation of a liquid [1,2]. The similar effects are observed in focusing short ultrasonic and shock-acoustic beams into condensed media [3]. Under the shock acoustic influence in the focus there arises pulsing cavity by the breaking a liquid in a rarefaction wave as the result of merging the smaller-sized bubbles.

The research of radiation spectra and the hydrodynamic calculations have shown a possibility to attain plasma temperatures inside a collapsing cavity within the range 10⁵-10⁷ K [4,5]. However, as noted in the review [6], the observed cavitation luminescence in the periodic acoustic field can be caused not only by the bubble collapse, but also by attendant electrokinetic processes in splitting bubbles. The interest to the liquid luminescence in acoustic shock waves is due to the possibility of luminescence registration through cavitation time beginning with the early stage where the expanding and coalescence of bubbles take place.


EXPERIMENT

To investigate the luminescence nature complex researches of the luminescence along with the cavitation processes in liquids in a field of spherically focusing acoustic shock wave were performed by the methods of the high-speed photo-recording and the pressure field measurements in water, glycerine and their solutions.

The cavitation processes kinetics in a focal area was recorded by the shadow method with the help of the high-speed photo recorder (СФР-1м) in the slot-hole and frame modes. In the experiments the electromagnetic generator [3] of shock waves as a sphere segment having the focal distance 170 mm, the aperture 220 mm was used. The shock wave generator was installed on the bottom of the bath having dimensions 300 х 300 х 480 mm.

The luminescence signal was recorded from the photo-multiplier tube ФЭУ-35 (a spectral range is from 300 nm to 600 nm). In this case the electromagnetic shock wave generator with a radiating surface as a sphere segment having the focal distance F = 55 mm, the aperture 55 mm was used. The cuvette sizes are 106 х 106 x 130 mm.

The pressure was controlled with a needle pressure having the time resolution 0.05 mks and the space resolution 0.7 mm. The shock wave had a bipolar profile containing the compression wave having the duration t₊=2 mks, and rarefaction wave having the duration t_-=2 mks in the half-height. The experiments were performed for spherically focusing shock wave at pressures P₊ = 2-40 MPa. In the experiments the two-channel oscilloscopes C8-14 and C9-27 were used to record signals.


RESULTS

Figure 1 illustrates a wave picture ahead of the focus.

FIGURE 1. The scheme of wave picture.

The shock wave (1) from electromagnetic generator initiates the shock acoustic breakdown. The emitted stimulated acoustic scattering waves (SAS waves) (2) represent a complex wave field, consisting of a discreet spherical waves radiated by a driven source. The source is a point K which is the crossing of a toroidal surface of marginal rarefaction wave (MRW) (3) on the Z axis. A point K moves along the Z axis with speed u. The cavitation zone front is the radiator of SAS waves. As a result, the transformation of the rarefaction wave into an amplified compression wave will be realised in the direction of shock wave distribution because of the SAS wave interference. The speed of the point K for the given system is easily found from the relation: u = v/cos(Q /2), where v is the MRW speed, Q /2 is the angle between MRW normal v/|v| and the Z axis at the point K. In our case, the formation of CSW will be realised for the angles Q /2 from 35° to 45°, which corresponds to the speed u within the range from 2.1 to 1.8 mm/mks. As evident from the recorded angular characteristic of the shock waves the point K speed exceeds that of shock wave (1.5 mm/mks).

The pressure field measurements have shown that with no liquid breakdown one can observe the condition |P_-|/|P₊|@ 1, where P₊ is the compression wave pressure, P_- is the rarefaction wave pressure. For technical water this condition is fulfilled up to the pressure at the focus P₊ = 4-6 MPa. The further increase of the pressure in the focal area results in the liquid cavitation at the focus. The cavitation limits the increase of the pressure P_-. At the focus there arises pulsing cavity as the result of smaller-sized bubble coalescence (Fig. 2), which gives the “sultan” hydrodynamic effect [7].

FIGURE 2. Cavitation near free surface.

The photo-registrogram (Fig. 3) illustrates the cavitation bubble formations as point K passages through the focal area. Then one can observe the bubble pulsation and emitted shock waves at the bubble collapse. The time of the first and the second bubble pulsation T₁, T₂ depend on the shock wave pressure as well as the liquid properties. For example, T₁ = 130-320 mks, T₂/T₁ = 0.4-0.5 for technical water at pressure P₊ = 15-25 MPa.

FIGURE 3. Photo-registrogram (a) and luminescence (b) near free surface.

The registration of light flashes has been made in the setting-up where the focus is close to the free surface. This setting-up is chosen after the preliminary experiments, that when focusing shock wave on the free surface the luminescence amplitude (1) in Figure 3 increases by one order of magnitude as against the results of the focusing into the liquid volume given the same shock wave parameters [8]. This is caused by increasing the cavitation zone through amplification of rarefaction wave by reflecting compression wave on the free surface [3].

In additional series of experiments the luminescence registration has been combined with the pressure measurement at a depth of 0.5-1 mm under the free surface. The space of light radiation has been chosen using the special lens system with a diaphragm. The radiation has been recorded from the area 5 mm in diameter adjacent to the pressure gauge. In Figure 4 are shown the oscillograms of simultaneous registration of the pressure at the focus measured by the gauge submerged at a depth of 1 mm (a), and the luminescence at the focal zone (b) at spherically focusing on the free surface of the 10 percent glycerine solution in water.

FIGURE 4. Simultaneous measurement of pressure (a) and luminescence (b) in the focus.

The similar results have been also observed in pure water and glycerine and their mixtures. Notice that when changing water to glycerine the luminescence amplitude increases almost in factor of forty, all other experimental conditions are the same.


DISCUSSION

Shown in diagram is the formation of the cavitation zone in the focal area. This process as a whole will be called the acoustic shock wave breakdown, because the general patterns of hydrodynamic processes in this case are similar with the laser breakdown of a liquid.

It follows from analysis of the high-speed photo recording and pressure measurements that the SAS wave source are expansible cavitation bubbles in the point K trace, which form complex structure of the wave field. Observed in the range of angles from Q /2 to p is the SAS wave scattering. Conversely, in the range of angles from 0 to Q /2 the conditions of amplification when adding the discreet SAS waves are provided by virtue of a super sonic movement of the cavitation source (Fig. 1). As a consequence, at this solid angle from discreet SAS waves there arises a wave pattern of the explosion shock wave type which can be named the cavitation shock wave (CSW).

So, the shock wave focusing into a liquid leads to the explosion-breakdown at the focus, which is caused by the non-linear absorption of the focused shock wave energy. The shock wave breakdown, as well as the laser breakdown, is followed by the radiation of SAS waves whose superposition can form CSW.

It follows from the given results, that the onset of liquid luminescence correlates with the moment of coming rarefaction waves and, hence, with the cavity expanding and coalescence. The obtained experimental results testify that there exist several mechanisms of a cavitation luminescence in a liquid, in addition to the already known model of collapsing a cavity. These mechanisms are concerned with the liquid breakdown in rarefaction waves.

In [9] the electrical pulses in the liquid breakdown zone have been registered experimentally. Based on this study, it is believed that the electrical discharge followed by the light radiation due to surface region with small radius of curvature formations during bubble coalescence may occur.

REFERENCES

1. Askarian, G.A., Prokhorov, A.M., Chanturia, G.F. et al. JETP 44 2180-2181 (1963) (in Russian)
2. Teslenko, V.S. IEEE Transaction on Electrical Insulation 26 1195-1200 (1991)
3. Teslenko, V.S. Technical Physics Letters 20 199-201 (1994)
4. Crum, L.A. J. Acoust. Soc. Amer 95 559-562 (1994)
5. Moss, W.C., Clarke, D.B., White, J.W. et al. “Sonoluminescence, shock waves, and micro-thermonuclear fusion”. in Proceedings of the Conference on Shock Compression of Condensed Matter, Seattle, Washington, August 13-18, 1995, pp. 453-458.
6. Margulis, M.A. Russ. J. Phys. Chem 69 2217-2222 (1995) (in Russian)
7. Kedrinskii, V.K. Archives of Mechanics 26 535-540 (1974)
8. Teslenko, V.S., Sankin, G.N. and Drozhzhin, A.P. Combustion, Explosion and Shock Waves 35 N6 (1999) (in printing)
9. Stebnovsky, S.V. J. Appl. Mech. and Tech. Phys N2 285-291 (1989)

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