**3. Visualization of pulsating jet**

The use of visualization plays an important role in the study of behaviour of pulsating water jet. It enables not only the examination of characteristics of the jet such as mean velocity and break-up length of the pulsating jet but also to study the morphology and processes of formation of the pulsating jet and development of pulses in the jet. Furthermore, the visualization can be used to validate results obtained from numerical simulation of the process of generation of pulsating jets using CFD methods.

An original method of visualization of pulsating water jets based on the application of stroboscopic effect was elaborated for the above mentioned purposes. The method enables to obtain visual information not only on instantaneous structure of the pulsating jet but also on the mean structure of the jet. In addition, the stroboscopic effect allows observing process of formation of pulsating water jet by the naked eye. Special stroboscope for the pulsating jet visualization was developed where the frequency of stroboscope flashing is controlled by the frequency of pressure pulsations in the high-pressure system measured upstream from the nozzle exit. An example of the mean structure of pulsating jet with pulsating frequency of 20 kHz can be seen in Fig. 9. Exposure time of the photograph was 1/1000 s and the frequency of stroboscope flashing was about 20 kHz, therefore the figure represents superposition of 20 images of pulsating jet "frozen" by the stroboscope flashing.

Fig. 9. The mean structure of the pulsating water jet generated at 30 MPa (illumination by the stroboscope)

An instantaneous structure of the pulsating water jet with the frequency of 20 kHz was studied using the high-speed camera LaVision VC-HighSpeedStar 5 equipped with image amplifier LaVision HighSpeed IRO. The record rate of the high-speed camera was 35 000 frames per second and the gate was set to 1µs. In addition, visualization of instantaneous structure of the pulsating water jet was also performed using Particle Image Velocimetry (PIV) system consisting of LaVision Imager Intense camera and New Wave Research laser,

Use of Acoustic Waves for Pulsating Water Jet Generation 337

diverse types of jets: single round and fan pulsating jets as well as rotating pulsating jets. The effects of pulsating jets were evaluated in terms of cutting depth, rate of mass-loss or volume removal rate respectively and compared with the effects of continuous water jets

Obtained results show clearly the supremacy of pulsating water jets over continuous ones in terms of their effects on material. Figures 13 to 15 illustrate the effects of various types of both pulsating and continuous jets on metal, rock and concrete samples. Differences in the surface structures created by pulsating and continuous water jets on individual materials are clearly visible in the above mentioned figures. An example of erosion effects of pulsating fan water jets (generated at various pressures) on aluminium samples at variable stand-off

**P C**

**P C**

d)

b)

Fig. 13. Comparison of effects of pulsating (P) and continuous (C) water jets on samples of: a) mild steel (pressure 40 MPa, nozzle dia. 1.98 mm, traverse speed 0.03 m.min-1, standoff distance 140 mm), b) brass (pressure 40 MPa, nozzle dia. 1.98 mm, traverse speed 0.03 m.min-1, standoff distance 140 mm), c) duralumin (pressure 50 MPa, nozzle diameter 1.45 mm, traversing speed 0.05 m.min-1, standoff distance 60 mm) and d) basalt (pressure 50 MPa, nozzle diameter 1.45 mm, traversing speed 1.0 m.min-1, standoff distance 40 mm (P)

under the same operating conditions.

distance is presented in Figure 16.

**P C**

**P C**

c)

a)

and 20 mm (C))

Model Solo 120 with pulse duration 3-5 ns; the optical system was used to produce 1 mm thick sheet of light. An example of the pulsating jet visualized by high-speed camera can be seen in Fig. 10, the same jet visualized by PIV system is presented in Fig. 11.

Fig. 10. The instantaneous structure of the pulsating water jet generated at 30 MPa (highspeed camera)

Fig. 11. The instantaneous structure of the pulsating water jet generated at 30 MPa (PIV system)

The fan (flat) pulsating water jet visualization was performed using the pulsed laser New Wave Research and digital camera Nikon D70s. Figure 12 shows the morphology of fan pulsating water jet generated at a pressure of 20 MPa.

Fig. 12. The instantaneous structure of the fan pulsating water jet generated at 20 MPa (illumination by pulsed laser, camera Nikon D70s)
