**Acoustic Waves as Manipulative Tools**

320 Acoustic Waves – From Microdevices to Helioseismology

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**15** 

Josef Foldyna

*Czech Republic* 

(1)

**Use of Acoustic Waves for** 

*Institute of Geonics of the ASCR, v. v. i., Ostrava* 

**Pulsating Water Jet Generation** 

The technology of a high-speed water jet cutting and disintegration of various materials attained considerable growth during the last decades. Continuous high-speed water jets are currently used in many industrial applications such as cutting of various materials, cleaning and removal of surface layers. However, despite the impressive advances made recently in the field of water jetting, substantial attention of number of research teams throughout the world is still paid to the improvement of the performance of the technology, its adaptation to environmental requirements and making it more beneficial from the economic point of view. An obvious method of the water jetting performance improvement is to generate jets at ultra-high pressures. The feasibility of cutting metals with pure water jets at pressures close to 690 MPa was investigated already in early nineties of the last century (Raghavan & Ting, 1991). Such a high pressure, however, induces extreme overtension of high-pressure parts of

An alternate approach, as shown in this chapter, is to eliminate the need for such high pressures by pulsing the jet. It is well known that the collision of a high-velocity liquid mass with a solid generates short high-pressure transients which can cause serious damage to the surface and interior of the target material. The liquid impact on a solid surface consists of two main stages (see Fig. 1). During the first stage, the liquid behaves in a compressible manner generating the so-called ''water-hammer'' pressures. These high pressures are responsible for most of the damage resulting from liquid impact on the solid surface. The situation shortly after the initial impact of the liquid on the solid surface is illustrated in Fig. 2. After the release of the impact pressure, the second stage of the liquid impact begins. Once incompressible stream line flow is established, the pressure on the central axis falls to

The force distribution on liquid jet impact on the solid surface can be summarized as follows: initially a small central area of the first contact is compressed under a uniform

where *v* is the impact velocity and *ρ1*, *ρ2* and *c1*, *c2* are the densities and the shock velocities in

the much lower Bernoulli stagnation pressure that lasts for relatively long time.

pressure. The magnitude of the impact pressure pi on the central axis is given by

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the cutting system which has adverse effect on their lifetime.

the liquid and the solid, respectively (de Haller, 1933).

**1. Introduction** 
