In visualization results of highly-submerged cavitating water jet obtained by a digital camera, the influence of parameters such as: injection pressure, nozzle diameter and geometry, nozzle mounting (for convergent / divergent flow), cavitation number and exit jet velocity, were investigated. In addition, the influence of visualization system position was studied. All the parameters have been found to be of strong influence on the jet appearance and performance

Hutli, E. A.

Ilic, Vojislav

Nedeljkovic, M.

Hutli, Ezddin

Nedeljković, Miloš

Ilić, V.

machinery

Visualization of submerged cavitating jet: Part two - Influences of hydrodynamic conditions, nozzle geometry and visualization system arrangement

University of Queensland eSpace

16th Australasian Fluid Mechanics Conference 
Crown Plaza, Gold Coast, Australia 
2-7 December 2007 
 
Visualization of Submerged Cavitating Jet: Part Two – Influences of Hydrodynamic 
Conditions, Nozzle Geometry and Visualization System Arrangement 
 
E.A. Hutli1 (MSc), M. Nedeljkovic2 (Professor) and Vojislav Ilic3(PhD) 
1,2 Department of Hydraulic Machinery and Energy Systems 
               Belgrade University, Faculty of Mechanical Engineering, Belgrade, Serbia 
 
3 School of Engineering, University of Western Sydney, Australia 
 
Abstract 
In visualization results of highly-submerged cavitating water jet 
obtained by a digital camera, the influence of parameters such as: 
injection pressure, nozzle diameter and geometry, nozzle 
mounting (for convergent / divergent flow), cavitation number 
and exit jet velocity, were investigated. In addition, the influence 
of visualization system position was studied. All the parameters 
have been found to be of strong influence on the jet appearance 
and performance. 
 
Introduction 
Flow visualization has been an important tool in fluid dynamics 
research; it has been used extensively in engineering, physics, 
medical since, meteorology, oceanography and aerodynamics etc. 
As can be determined from the published literature, not  much is 
known about the unsteady behaviours of the cavitating jet and the 
development as well as the collapse of the cavitation clouds on 
the impinging surface, If the unsteady behaviour and the jet 
structure are clarified in detail, it is expected that, the jet working 
capacity can be drastically improved [2, 8]. The flow 
visualization is used here to achieve these goals. In this paper, 
NIKON COOLPIX 990 digital camera was used for visualization 
and investigation of the influences of hydrodynamic conditions, 
nozzle geometry, and position of the visualization system. The 
nozzles used are mounted to be suddenly convergent or 
divergent.. The visualization investigations were done using a 
stroboscope for time 30 µs to illuminate the cavitating jet. The 
flash frequency was 50 Hz while the shutter camera frequency 
was 1/30 or 1/60 s-1. The jet images were taken as a movie at 
around 600 frames in 40s (15 frames/s). The jet behaviour has 
then been followed by extracting appropriate images. 
 
 
 
Figure 1. Nozzle geometry, nozzle holder, and the manner of 
nozzle mounting 
 
Influence of Nozzle Conicity 
Investigation of the influence of nozzle conicity was done by 
mounting the nozzle in two ways, once to have a nozzle as 
suddenly divergent and once to have it suddenly convergent 
(Fig.1). The cavitation number was calculated as (σ=(P2-
Pv)/(0.5ρVj2)). The installation of the visualization system is 
shown in Fig.2 (left). 
 
Analysis for the Divergent Conical Nozzle  
Divergent Nozzle, with inlet diameter of Din=0.45 mm and outlet 
of  Dout=1 mm, (divergent) 
 
 
Figure 2.  Arrangement of the apparatus. 
 
 
 
Figure 3. Appearance of the cavitating jet. Conditios: P1=8bar, 
P2=2.05 bar, σ=17.85, VJ=4.9m/s, T=16oC, Divergent Nozzle. 
 
In Fig.3 two images reveal that the jet is comprised only of very 
tiny individual bubbles (no clouds), the spreading angle of jet is 
bigger than the corresponding one in other images on the next 
figures for the same case. The existence of tiny bubbles is 
attributed to the inception of cavitation inside the nozzle (not at 
the nozzle outlet). Images in Fig. 4 show that the cavitation 
started to go out from the nozzle as dense clouds and then 
immediately transformed to lighter clouds, which appear as 
smoky clouds after a maximum distance of 20% of the distance 
between the nozzle exit and the target surface (which is denoted 
by X, X=25.67mm) (first 6 images). The analyses of the 
remaining groups of images in Fig.4 show that the jet 
penetration, jet breaking point position, jet-spreading angle, and 
jet density (individual bubbles and bubble clusters) are increased 
as upstream pressure (P1) increases.  at the end of the jet, When 
the jet strikes the target wall at high P1  , it spreads radially and 
covers all the area with  tiny bubbles. These bubbles collapse at 
different radial positions on the target surface or near it. The 
position of bubble collapse depends on the radial pressure 
distribution while the local stress produced as a result is a 
function of two parameters: the bubble content, and the bubble 
size. 
Analysis for the Convergent Nozzle 
Figs.5, for  convergent nozzle (Din=1 mm and outlet of Dout=0.45 
mm), reveals that the jet behaviour with up-stream pressure is the 
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 same as in the case of Divergent Nozzle. However, the position 
of the breaking point, jet penetration spreading angle and the jet 
density in case of nozzle convergence are much bigger than for 
the divergent case. This is attributed to the big difference in the 
velocities for the two nozzle directions. In the case of convergent 
nozzle at P1 = 8 bar, no cavitation was observed. However, at 
P1=25 bar the phenomenon appeared at nozzle exit with irregular 
and rare frequency: it was unstable, and this can be assumed as 
the inception of cavitation (no image was obtained for this case). 
In the groups of images for P1=145 and 195 bar (Fig.5) many 
bubbles distributed throughout the whole area may be observed. 
This feature is related to the intensive vortex action in the jet, 
which leads to the liberation of bubble growth and their 
spreading and floating in the chamber. Finally, these bubbles 
collapse when they meet the point of sufficient pressure in their 
path. However, these gas bubbles have longer lifetime as 
compared to cavitation (vapour)  bubbles.   
 
 
P1=45 bar, P2=2.07bar, σ =2.35, Vj=13.3m/s 
 
P1=95bar, P2=2.08bar, σ =1.12, Vj=19.5m/s 
 
P1=145bar, P2=2.09bar, σ =0.72, Vj=24.1m/s 
 
P1=195 bar, P2=2.1bar, σ =0.53, Vj=28m/s 
 
Figure 4. Cavitating jet started outside at the nozzle exit 
(divergent nozzle din=45mm, dout=1mm),T=16oC1   
                                                          
1 NOTE: The given exit velocities (Vj) during cavitation 
are based on the single phase flow. For the same mass flow 
rate, the actual flow velocity will be higher than given. 
The Influence of Nozzle Dimensions on the Cavitating 
Jet Behaviour 
Visualizations of jets created by two different nozzles (both are 
Divergent Nozzles but of different dimensions) were done. 
Figures 6 and 7 represent the results. Upstream pressure (P1) was 
kept constant, as well as working fluid temperature and standoff 
distance, while jet velocity (VJ) and downstream pressure (P2) 
changed with the nozzle geometry (thus the cavitation number  
also changed). The comparison of obtained results reveals that 
the cavitating jet characteristics are strongly dependent on 
geometry and diameter of the nozzle - jet penetration, jet width, 
jet spreading angle and cavitation cloud density are the 
parameters that significantly change with the nozzle geometry 
change.  
 
 
P1=45bar, P2=2.07bar, σ =0.097, Vj=65m/s 
 
P1=95 bar, P2=2.15bar, σ =0.047, Vj=96m/s 
 
P1=145bar, P2=2.22bar, σ =0.032, Vj=119m/s 
 
 
P1=195bar, P2=2.28bar, σ =0.024, Vj=138m/s 
 
Figure 5. Cavitating jet started outside at the nozzle exit 
(Convergent nozzle din=1mm, dout=0.45mm), T=16oC 
 
They decreased as the inlet nozzle diameter decreased. Both 
gaseous and vaporous types of cavitation were observed in the 
present study. Gaseous type of cavitation occurred when the 
dissolved gases in high-pressure came out of solution with liquid 
in the low-pressure test section (test chamber), while vapor type 
cavitation, for a given liquid temperature, occurred when 
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 reduction of pressure due to dynamic effects decreased to 
saturation pressure for that temperature, giving rise to 
evaporation (local boiling/cavitation). These two types of 
cavitation were quite distinct in appearance. The gaseous type 
seemed to be formed within the entire jet with bubbles of nearly 
spherical shape. Bubbles did not form clusters.  
 
This type of cavitation is mainly dependent on upstream pressure 
P1. The vapor type cavities strongly depend on the cavitation 
number. Relatively large coalescing vapor cavities may form 
away from, nearby or possibly inside the nozzle itself. Another 
interesting difference between the two types of cavitation is that 
the vapor cavities disappear after a certain distance from the 
nozzle, while the gaseous bubbles tend to persist indefinitely 
(only an increase of P2 destroys them). Gaseous cavities do not 
contribute to the erosion of material [6].  
 
 
P1=45bar, P2=1.92bar, σ =1.93, Vj=14m/s 
 
P1=95bar, P2=1.99bar, σ =0.95, Vj=21m/s 
 
P1=145bar, P2=2.1bar, σ =0.64, Vj=25m/s 
 
P1=195bar, P2=2.2bar, σ =0.5, Vj=29m/s 
 
Figure 6. Cavitating jet started outside at the nozzle (Divegent 
nozzle din=0.55mm, dout=1.1mm), T=16oC 
The formation of gaseous cavities in the present experiments 
could be suppressed by slight pressurization of the test section, 
since it was not possible to control the dissolved gases with 
degassing of water in the test rig system (the water is  assumed to 
be saturated). The existence of these two types of cavitation at 
high upstream pressures (high vortices) may be clearly 
distinguished in the group of images in Figures 9-11 in the next 
paragraph (effect of cavitation , velocity and system 
arrangement) the gas bubbles appears as spots distributed 
randomly in the test chamber. 
 
 
P1=45bar, P2=1.79bar, σ =3.63, Vj=10m/s 
 
P1=95bar, P2=1.8bar, σ =1.78, Vj=14m/s 
 
P1=145bar, P2=1.81bar, σ =1.15, Vj=17.7m/s 
 
P1=195bar, P2=1.84bar, σ =0.87, Vj=20.5m/s 
 
Figure 7. Cavitating jet started outside at the nozzle exit 
(Divergent nozzle din=0.4mm, dout=1mm), T=16oC 
 
Influence of Cavitation Number (σ) and Exit Jet Velocity 
(Vj) on Characteristics of Cavitating Jets 
In order to get better distribution of light to improve the quality 
of images, the flashlight and the camera eye were installed on the 
same side (same window). Since the window area is not large 
883
 enough to accept the camera and the flash together in parallel, the 
flash had to be inclined to the camera direction with an included 
angle θ  (between 30o and 40o). The experimental installation is 
shown in Fig.2 (left). Visualizations were done using convergent 
nozzle (din=1m and dout=0.45mm) first for different cavitation 
numbers σ, which was achieved by changing the downstream 
pressure (P2) and second for different exit jet velocity (VJ) which 
was achieved by changing upstream pressure (P1). Figs. 8 and 9 
show the results, respectively.  The visual analysis reveals that 
the jet appears as a complete solid unit in white colour.. The jet 
penetration is increased as σ or P2 is decreased and when jet 
strikes the target it starts spreading over the target surface and 
formation of rings appears. In the first row in Fig.9 the jet did not 
arrive to the target wall – it disappeared in the region of the 
second third of the full distance between the nozzle exit and the 
target wall (2/3 of X). The jet is more stable as VJ or P1 increase. 
Jet width and jet penetration increased as VJ or P1 increase. 
 
These results are in good agreement with Soyama [4,5]. In the 2nd  
and 3rd  row of images in Fig.8 and 3rd ,4th and 5th  row of images 
in Fig.9, the gaseous bubbles appear as fog distributed in the 
chamber. 
 
 
 
 
Figure 8.  Influence of cavitation number on the jet behaviour. 
 
Influence of Visualization System Arrangement on the 
Information Content In Cavitating Jet Images 
In the first case the visualization was made with a sheet of white 
paper between the flash and the window. The reason for this was 
to reduce the intensity of light illuminating the jet before entering 
the camera. In addition, the paper enhances the distribution of the 
light in the test chamber Fig.10 (right). In the second case, the 
visualization was made by mounting the flash in such a way to be 
in parallel with the jet direction (in the same direction of the 
flow, Fig.10 (Left (no paper)). The light was from the other side 
of the test chamber with the angle ϕ = 90o to the camera eye i.e. 
the flash was at right angles to the camera. Fig.11, 12 and13 
shows the results (right column in each figure for 1st case while 
Left column for 2nd case). The comparison between the two cases 
reveals that, the position of light and its distribution is very 
important factor in the information provided by the visualization 
process for the cavitating jets. In first case the cavitating jet 
appears as a dark grey shadow. It seems less dense and is 
discontinuous in some locations. 
The difference in the jet features compared with the second case 
may be attributed to the light passing through the jet here, such 
that the jet appears discontinuous.. Also, there exists a reflection 
process of light by the spherical bubbles in or around the jet. 
However, this reflection is not in the direction of the camera eye, 
so the camera does not sense the reflected light. All these factors, 
including the limitation of camera resolution, contribute to the 
fact that bubbles will not appear in the images and the jet appears 
as a dark grey shadow. 
   
 
 
Figure 9. Influence of exit jet velocity (Vj) on the jet behaviour. 
 
 
 
Figure 10. Visualization system  
 
In the second case, and in the two cases when flash was mounted 
as in Fig.2 (both left and right), the light was reflected by the 
spherical bubbles, thus the distances between the cavitation 
clouds (or clusters of bubbles), as a result of the break off, were 
allowed to be seen. The rest of the jet appears as a white portion 
and it contains many tiny bubbles that may reflect some light to 
the direction of the camera.  In Fig.10 at P1=145 bar, the gas 
bubbles appear in the images for the second case. However,  this 
was not observed in the firs case. At P1=195 bar, the gas bubbles 
appear in both cases, which may be explained by the appearance 
of greater number of bubbles at pressure P1=195 bar than at 
P1=145 bar. However, the bubble density (number of bubbles) 
which appears in the images obtained in first case is much less 
than that which appears in the images obtained by the second 
case. Thus we can notice how the information in the image 
depends on the arrangement of the visualization system.  
The interesting point worth mentioning in the end is that the 
definition of cavitation number (σ =P2/P1 as proposed by ASTM 
) is not enough to discribe the phenomenon  or is incorrect since 
the geometry is not included. The normal definition (σ = (P2-
Pv)/(0.5ρVj2)) is more convenient way to describe the 
phenomenon, even if it is not descriptive enough of the whole 
phenomenon. However this definition provides values of σ lower  
 
884
  
 
Figure 11. Influence of visualization system arrangement on the 
visualization data.(.P1=95bar, P2=2.08bar, 
σ=1.12,VЈ=19.5m/s,T=160C) 
 
 
 
Figure 12. Influence of visualization system arrangement on the 
visualization data..( P1=145bar, P2=2.09 bar, σ =0.72, 
VЈ=24.1m/s,T=160C) 
 
when cavitation is more intense. which appears reasonable. This 
may be seen by comparing Figures 4-7. 
 
 
 
Figure 13. Influence of visualization system arrangement on the 
visualization data..( P1=195 bar, P2=2.1 bar, σ =0.53, VЈ=28 m/s, 
T=160C) 
 
Conclusion 
The jet behaviour and its features depend on nozzle mounting 
(convergent or divergent configuration). The hydrodynamic 
conditions for cavitation inception and its position depend on the 
nozzle geometry. The hydrodynamic condition has a big 
influence on the jet behaviour and its features. Both gaseous and 
vapor types of cavitation appear at high upstream pressures. The 
life of gaseous cavitation is longer than of vapour cavitation. 
Vapor types of cavitation depend on cavitation number or on 
downstream pressure P2, while gaseous cavitation depends 
mainly on upstream pressure P1. Both of them depend on the 
nozzle geometry. The visualization arrangement has a big 
influence on the quality of jet images, and hence on the 
information that can be obtained from them The difficulties of 
capturing the instant of the  collapsing bubbles are related to 
inadequate temporal resolution of illuminating and recording 
system, and the  huge number of the bubbles in the cavitating jet. 
 
Nomenclature 
 
σ cavitation number  
2
2
1
ref.
vref.
ρu
pp
σ
−=  
refp  Reference (downstream) pressure (bar) 
vp (T) Saturation (vapor) pressure (bar), 
Lρ (T) Density of the liquid (kg/m3), 
T Fluid temperature [0C] 
885
 refu  Reference velocity - exit jet velocity (m/s)        = Q / A = VJ 
Q = K * √(P1-P2)  - flow rate (m3/s) 
A Nozzle outlet cross-section area (m2) 
P1 Upstream pressure (bar) (absolute) 
P2 Downstream pressure (bar) (absolute) 
X Stand-off distance (mm) 
L Nozzle length  
din, duot Inlet and outlet  nozzle diameter (mm) 
K = 4.78E-09 for divergent ; = 6.17E-09 for 
convergent nozzle (m3/s/Pa1/2) 
 
References 
[1] Keiichi S, Yasuhiro S. Unstable Cavitation Behaviour in a 
Circular-Cylindrical Orifice Flow. Trans JSME, 
International Journal, Ser.B, Vol.45, No.3, pp.638-645, 2002 
[2] Ganippa LC, Bark G, Andersson S, Chomiak J. Comparison 
of Cavitation Phenomena In Transparent Scaled-Up Single-
Hole Diesel Nozzles, CAV2001, session A9.005, 2001. 
[3] Soyama H., High-Speed Observation of a Cavitating Jet in 
Air. Trans ASME, Journal of Fluids Engineering, Vol. 127, 
pp 1095-1101, November 2005. 
[4] Soyama H, Ikohagi T, Oba R. Observation of the Cavitating 
Jet in a Narrow Watercourse, Cavitation and Multiphase 
Flow, ASME, FED-Vol.194, 79-82, 1994. 
[5] Soyama H, Lichtarowicz A, Lampard D. Useful Correlations 
for Cavitating Jets, 3rd  international symposium on 
cavitation, Grenoble, France, 1998. 
[6] Vijay MM, Zou C, Tavoularis S. A Study of the 
Characteristics of Cavitating Water Jets by Photography and 
Erosion Jet Cutting Technology – Proceedings of 10th   
International Conference, 37-67, Elsevier 1991. 
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Visualization of Submerged Cavitating Jet: Part Two – Influences of Hydrodynamic Conditions, Nozzle Geometry and Visualization System Arrangement

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Visualization of Submerged Cavitating Jet: Part Two – Influences of Hydrodynamic Conditions, Nozzle Geometry and Visualization System Arrangement

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