Date August 2008
Author W.M. van Rees, MX. van Rijsbergen, G. Kuiper and
TiC. van Terwisga
Address Ship Hydromechanics Laboratory
Mekelweg 2, 26282 CD Deift
TUDeift
Deift University of Technology
An exploratory investigation of cavitation
inception on the pressure side of propellers
by
Wim M. van Rees, Martijn X. van Rijsbergen,
Gert Kuiper and Tom IC. van Terwisga
Report No. 1614-p
2008Proceedings of the FEDSM2008 - 2008 ASME Fluids
Engineering Conference 43rd Forum on Cavitation and
Multiphase flows, August 10-14, 2008, Jacksonville,
Fl, USA, Paper FEDSM2008-55337
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ASME 2009 Fluids Engineering Division Summer Meeting
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Deadlines and other details will be posted on the web page.
Pumping Machinery
Non-Invasive Measurements in Single and
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Fundamental Issues and Perspectives in Fluid
Mechanics
Cavitation Inception Liquid-Solid Flows Gas-Particle Flows
Flow Manipulation and Active Control Flow Applications in Aerospace Fluid Power
Transport Phenomena in Mixing
Transport Phenomena in Manufacturing
Processes
Transport Phenomena in Energy Conversion
from Clean and Sustainable Resources
For additional information, please contact: Dr. Joel T. Park
Naval Surface Warfare Center joel.park@navy.mil
Conference Chair
Fluid Structure Interaction and Flow-Induced
Noise
Numerical Modeling in Aerodynamics and
1-lydrodynamics in Turbomachinery
Applications in Computational Fluid Dynamics (CFD)
Algorithmic Developments in CFD
DNS, LES, and Hybrid LES/RANS Methods Numerical Methods in Multiphase Flows
Micro-Fluidics
Fluid Measurements and Instrumentation Cavitation and Multiphase Flows
Advances in Fluids Engineering Education Automotive Flows Fluid Machinery Biological Flows 56 Ms. Erin Dolan ASME Headquarters DolanE@asme.org
55221 Effect of Dissipative Terms on the Quality of Two and Three I)irrieiisional Euler Flow Solutions Seyed Saied Bahrainian
3:30 - 4:00 Break
59
MONDAY August 11,
2008
PARALLEL SESSONS / 2:00 - 3:30 pm
Grand Ballroom 7 Grand Ballroom 8
City Terrace / / City Terrace 12 Daytona
2nd International Symposium on Flow Applications iii Aerospace 20th Forum on Fluid Machinery
Session 3-2: Cavitation and Session 21-3: Forum no
Session 14-3: Session 17-3:
Fluid-Session 9-1: Multiphase Floss' -2 Flu Id Macli inc ry - 3
Applications in
Computational Fluid Structure Interactionarid Flow-Indnnced Noise AerospaceApplications
-Dynamics -3 3
Chair: William Straka, Penn Chair: Yu-Tai Lee, Naval
Chair: l-Iisashi Chair: Raymond
Chair. Javid State University; Co-Chair Surface Warfare Center;
Ninokata, Tokyo Inst. Gordnier, Air Forces
Bayandor, Royal Mohamed Farhat, EPFL Co-Chair: Jinkook Lee,
Eaton of Tech.; Co-ChairYassin Hassan, Texas Research Lab; Co-Chair,
Subrata Roy, University Melbourne Inst. OfTech.; Co-Chair: A&M University of Florida
David Davis, Aerojet 55135 Rotordynamic Forces 55258 Numerical Study 55059 A Condensed
55350 Controlling arid 55284 Laniinrar-on Tliree-Bladed Inducer of the Pulsating Flow at
Phase Computational
Harnessing Flow- l'urrhni le rut
tinder Super-Synchronous / the Tongue Region of a
Fluid Dynamics Model
Acoustic Resonance in Transition Synchronous Rotating Centrifugal Pump for
for Polymer Melt Flow High-Speed Flows
Modeling Strategies
Cavitation Several Flow-Rates
Qing Tang and
Farukh S Alvi for Thermally
Yoshiki Yoshida, Masato Rail Barrio, Jorge
Michael Bockelie
Protected Airfoils Eguchi, Taiichi Motornura,
Masaharu Uchiurni, Hirotaka
Parrondo, and
Eduardo Blanco Lunz T'obaldini Neto,
Guilherme Araujo Kure and Yoshiyuki Maruta
Lima da Silva and Marcos de Mattos Pimenta
F
55337 Art Exploratory 55303 Transient
55174 CFD Solution 55209 Analysis of 55178 Investigation of Cavitation
Inception on tire Pressure Calculations of PrimpPerformance Parameters of a Two-Canopy
Parachute in a Top- 'recliniques Using TimePassive Wake Mixing Experimental
Investigations of the U Side of Propellers Mikhail P. Strongin
To-Top Fornnation Resolved Digital Particle
Onset of Rotating Wim M. van Rees,Martijn X.
with a Vent of Air Image Velocinretry Stall van Rijsbergen, Gert Kuiper
and Tom J.C. van Terwisg from One of Its
Canopy
Mohainmad Javad Izadi
Christopher Weiland, Christopher Michie,
Scott Bressers and
Pavios Vlachos P. B. V. Johansson and M. Henriksson D S E 55217 Art Iniprovenierit ofa
Cavitation Model arid the 55291 Vihra lion-BasedFault Diagnosis of Pump 551)57 Effects of
Endplates on 55006
Experimental N
Application to LES Using WPT Secondary Streaming
Investigation on
G
Shin-ichi Tsuda, Naoki Tani. Nobuhiro Yamanishi and
Pan I-long, and
Zheng Yuan of Oscillating FlowsPast a Circular Quenching Distance
for Aluniiinuni Dust R Chisachi Kato
Cylinder
Flames G
CT. 1-Isu and Yan Sn
MR. Habibzadeh and MI-I. Keyhani 55002 Cavitation Intensity 55257 Numerical
55103 CFD Modeling Measrrrenient by Analysis of
Pump Structure Oscillation- Investigation onlmpeller-Volrrte O\of Slurry Flows iii Horizontal Pipes A New Parametric Method
Approach Interaction in theCe nt rifnrga I Pit till) with Franz H. l-Iernandez, Arniando J. Blanco and 1-ladi Arjmandi Tash, Morteza Radial Gap and Tongue Luis Rojas-SolOrzano Sadeghi, Mohainmad T. Profile Variation
Shervani and M. Mohammad
ABSTRACT
Delayed slicer cavitation inception has occasionally been
observed in the MARIN Depressurized Towing Tank (DTf). The
problems are specifically related to the pressure sideofmodel
ship propellers, and occur despite the application of leading-edge roughness. As a consequence, no cavitation at all or cavita-tion on parts of the propeller blades is observed, in cases where cavitation in the cavitation tunnel or at full scale is present.
In an exploratory investigation, the effect of several
parame-ters that may influence cavitation inception is studied in the DT'L
In particular, the influences of Reynolds number, free-stream
tur-bulence and additional gas nuclei are investigated. It is con-cluded that the presenceof sufficient gas nuclei is crucial for
sheet cavitation inception, even
if
leading-edge roughness isap-plied. With additional nuclei in the propeller inflow, sheet cavi-tation inception in the DTJ' is no longer delayed with respect to the cavitation tunnel.
NOMENCLATURE
D propeller diameter
I
total current through electrolysis grid wiresProceedings of FEDSM2008 2008 ASME Fluids Engineering Conference 43rd Forum on Cavitation and Multiphase flows August 10-14, 2008, Jacksonville, Fl, USA
FEDSM2008-55337
AN EXPLORATORY INVESTIGATION OF CAVITATION INCEPTION ON THE
PRESSURE SIDE OF PROPELLERS
Wim M. van Rees 1,2
Martijn X. van Rijsbergen
Gert Kuiper'
Tom J.C. van Terwisga 1,2
1. Maritime Research Institute Netherlands (MARIN) Wageningen, the Netherlands
2. Delft University of Technology Delft, the Netherlands
k roughness height
p free-stream pressure at the location of the propeller shaft
ml propeller rotation rate
T thrust delivered by propeller
U local velocity
Va advance velocity
X length scaling parameter
POPv
,, propeller cavitation number,
- 0.5pn2D2
J
propeller advance ratio, J =KT thrust coefficient, KT =
pn2D4
T
Rek roughness Reynolds number, Rek =
Uk
Re,, propeller Reynolds iiumber, Re,, - nD2
V
pV,,R
We Weber number, We
Figure 1: Small cavitation spots on the pressure side of the container ship propeller in the DTT are present in a region where cavitation is expected. based on lull-scale reference tests.
INTRODUCTION
Over the last years. significant knowledge about scale ef-fects of marine propeller cavitation has been obtained. In [I
a summary of recent gains in scaling tip vortex cavitation, cav-ity dynamics and ship wakes has been presented. An unresolved problem MARIN occasionally experiences in the Depressurized Towing Tank (DTT) is the delay of sheet cavitation inception on the pressure side of propellers. This delay occurs on all blades, on some blades or on parts of some blades. An example of the latter is given in figure 1. The existence of this problem is ver-ilied by comparing cavitation inception results obtained in the Dri' with reference results from lull scale tests, cavitation tun-nel (CT) tests, and results from numerical computations.
In this paper the problem of sheet cavitation inception on
the pressure side of propellers is investigated. Use is being made of the cavitation tunnel as reference facility. The differences be-tween experiments in the CT and experiments in the DTT give a first indication of the possible causes of the problem. and are therefore briefly discussed.
In experiments in the Dli' the Froude number is maintained.
At a constant ship model speed, this means that pressure-side
cavitation inception is found at relatively low values of the
ro-tation rate, typically in the range 300-500 rpm. The propeller
Reynolds number in these experiments is thus very low. In the CT, pressure-side inception is found by an increase of tunnel ve-locity at a constant rotation rate. The rotation rate of these exper-iments is typically in the range 1000-2000 rpm, corresponding to much higher propeller Reynolds numbers than iii the DYE In both facilities. 60 pm carborundum grains are glued to the First 2-4 mm of the leading edge. both on pressure and suction side, iii order to trip the boundary layer to turbulence.
In the cavitation tunnel the inflow to the propeller disk con-tains sonic level of free-stream turhtileiice because of the recir-culation of the water in this facility. In a previous study the mag-nitude of RMS streaniwise velocity fluctuations in the MARIN C'!' has been measured to be about 0.6-0.8% of the free-stream velocity. The water in the DYF has a turbulence intensity of ap-proximately zero.
2
The nuclei population in the DTT has not been measured re-ceimlly. Measurements conducted twenty-five years ago showed that the nuclei content in the DTT was approximately one order of magnitude lower than in the CT [2; 3]. Since theii, additional
nuclei are generated in every propulsion test by attachiiig a metal
strip directly to the model Ship hull, at a location about a quarter of the model length upstream of the propeller. A current is
ap-plied on this metal strip, which results in the productionof
hydro-gen and oxyhydro-gen bubbles by electrolysisofthe tank water. In the
CT, the nuclei population is kept at a high level by keeping the total air content of the tunnel above an empirically determined threshold.
LITERATURE REVIEW
The possible influence of each of the previously mentioned parameters Reynolds number, free-stream turbulence and nuclei population on sheet cavitation inception will now he discussed based on literature.
A low Reynolds number can have an influence on the
effec-tiveness of roughness in tripping the boundary layer. For
dis-tribttted roughness elements oil a flat plate, a critical roughness Reynolds number exists above which transition occurs directly at the roughness elements [4]. Ii is possible that, at the low pro-peller Reynolds numbers used in pressure side cavitation experi-ments, patches of laminar flow persist along the leading edge. in
[3]. it is shown that laminar attached boundary-layer flow inhibits
sheet cavitation inception. In this case, an increase of propeller Reynolds number would bring the roughness Reynolds number well above the critical value, ensuring a turbulent boundary layer and thus inception.
The influence of free-stream turbulence on cavitation
incep-tion is twofold. First, it enforces early boundary-layer
transi-lion [4], and as such is able to take over the role of leading-edge roughness on model ship propellers [5]. Free-stream turbulence
might therefore also help in case roughness has become
inef-fective due to low roughness Reynolds numbers. Second, free-stream turbulence is stated to have an influence on inception by increasing the amplitude of boundary-layer pressure fluctuations in a lurhulent boundary layer [61. In the CT. free-stream turbu-lence might Iherefore contribute to sheet cavitation inception.
In case leading-edge roughness is applied, only few nuclei
are necessary for sheet cavitation inception [3: 7]. In Is],a
sim-ilar result was obtained on a propeller without roughness at a
relative high Reynolds number, using a cavitation tunnel with controllable nuclei population. No delay of sheet cavitation
in-ception was found on this propeller, evenif Ilie largest nuclei size
in the water was as small as 1 pm.
pressure-r,hl I: Details 01 the propellers used in the experiments
enlarged view
1600
Figure 2: Details of setup with electrolysis grid (dimensions in nun)
side inception in the DYE' and to compare this with CT results.
SET-UP
All experiments were conducted with an open water test setup. Two propellers were used: a propeller designed for a tanker and one designed for a container ship (table 1). Both were roughened using distributed 60 pus carborundum elements. The tests consisted of cavitation observations of each propeller at four or five thrust coefficients at a constant cavitation num-ber. In every run the thrust and torque delivered by the propeller were measured. The pressure of the dissolved gases in the wa-ter in the DTT was 320 rnbar, measured using an In-Situ T300E tensionometer.
The influence of propeller Reynolds number on sheet cavi-tation inception was tested by conducting a series of experiments at the same KT and a,,. but at different l)ropeller Reynolds nuni-hers. This variation was realized by adjusting the propeller to-tation rate, advance velocity and tank pressure simultaneously. The rotation rate was varied between 500 and 1100 rpm. It is noted that in this manner, leaving the nuclei population in the tank unchanged. the Weber nuniher increases as well, as will be discussed later.
To generate a higher intensity of turbulence in the inflow to the propeller, a square-mesh turbulence grid was constructed out of steel cylindrical rods. The total grid width and height are both 850 tutu, and a mesh width of 38 mm was used with a rod
diame-3
-r
(a) DTr. no turbulence grid (b) DTf. with turbulence grid (ci CTFigure 3: Comparison container ship propeller at highest propeller Reynolds itumber. K = -0.05 (J = 1.11, J = 1.19 and J = 1.05 ), a = 1000 rpm. ,, = 3.0
ter of 2 mm. The grid was placed I in in front of the propeller. in a similar open-water setup as the electrolysis grid described be-low and shown in figure 2. According to [9], this turbulence grid should generate a turbulence intensity of 1.0% at the location of the propeller. The spectrum of the turbulence generated by this grid is comparable to previous measurements in the MARIN cav-itation tunnel. In the experiments with the turbulence grid, the advance velocity was adjusted to correct for the velocity drop caused by the presence of the grid.
Additional nuclei in the inflow to the propeller were gener-ated using an electrolysis grid in front of the propeller, shown iii detail in figure 2. The grid consists of nine horizontal pairs of flat steel bars with a width of 515 nim each. Ihat act as cathode and anode once a current is applied. Electrolysis of the water results in the production of hydrogen and oxygen gas bubbles tltat travel downstream with the flow. In the zero-current reference tests, the electrolysis grid was not removed front the set-up. The tests were conducted with different current strengths. ranging from a total of 0 to 3 ampere.
RESULTS
Effect of propeller Reynolds number
At the highest Reynolds number, cavitation inception in the DTT was still delayed or partially delayed with respect to the cavitation tunnel on both propellers. Figures 3(a) and 3(c) give a comparison between the DIT and CT for the container ship pro-peller. Although cavitation inception was established in the DYE', the radial extent varied between the blades and was still smaller than in the CT, at identical values for K7. a,, and Re,,. On the fanker propeller, cavitation inception was also significantly de-layed with respect to the CT, also at equal rotation rates. In a single case higher Reynolds numbers were used, up to the maxi-mum attainable rotation rate in tile DTT (1550 rpm). In this case, a similar delay in inception with respect 10 the CT was found in
the D1T
Within the tested range of propeller Reynolds numbers no tatiker propeller contaiiici ship propeller
0.9 08 0,7 06 05 04 0.3 02
- uUFmul lJrbUFerrce gud
ruth turbulence grid
520ren 680rpm 840rpm 1000mm
rotation rate
Figure 4: Bar diagram showing average radial extent of cavitation per
blade, and (lie standard deviation taken over all blades, for the container ship propeller with and without grid. K, = 0.05 (J = 1.11 without grid and J = 1.19 with grid) and , = 3.0
consistent effect of this I)aittflieter on sheet cavitation inception was found on either propeller. This is visualized in figure 4 (10-cusing on the results without turbulence grid), where the average radial cavitation exient and standard deviation over the blades at different propeller Reynolds numbers is plotted. This figure also shows that the radial cavitation extent varied greatly between the blades. An example is given in figure 3(a), where blades 3 and 4 both cavitate, but at different radli. These variations in cavitation extent between the blades were present on both propellers. No consistent trend was found that would indicate an influence of
small surface geometry differences between the blades.
In many cases, cavital ion inception occurred independently on different blades and at a time well beyond the initial start-lip time. Once inception occurred, the cavitation persisted during the test run.
Effect of free-stream turbulence
The increase of turbulence intensity in the inliow to the
tanker ship propeller did not give an improvetnent of inception behavior. There was still scatter between the blades, and cavita-tion incepcavita-tion was still mostly established at some time well into the test run.
For the container ship propeller, on the other hand, the test runs with the turbulence grid did show a significant improvement of cavitation inception behavior. The delay of inception did not completely disappear, but it was smaller with the addition of the turbulence grid. Figure 3(b) gives an example of this: with turbu-lence grid the radial extent corresponds better to the CT results, and there is more consistency between the blades. In the bar diagram shown in figure 4, the results with turbulence grid illus-trate Ibis mprovemenl as well. The difference in sensitivity to the presence of the turbulence grid will he discussed later ill this paper.
4
Figure 5: View on the leading edge of the container ship propeller in the
DTT, without electrolysis (left) and with electrolysis (right, I = 3.0 A). Kr = 0.06 (J = 0.973), a = 680 rpm and ri = 3.0
Effect of nuclei
The experiments with the electrolysis grid showed that cavi-tation inception on both propellers occurs earlier when additional gas nuclei are present in the flow. Cavitation was either present in cases where it was absent in the zero-current condition, or the radial extent of the cavity was increased with respect to the zero-current condition. An example of the first case is given in
figure 5.
The radial extent of sheet cavitation was quite sensitive to the strength of the current, as illustrated in figure 6. The middle part of this figure gives the radial distribution of the minimum pressure coefficient for the container ship propeller. Since the minimum pressure coefficient increases for higher radii, (lie ra-dial extent of the cavity on this propeller is a good indicator for the actual inception pressure. The figure shows that an increase of the current through the electrolysis wires improved the
agree-nietit between the DTT and the CT experiments, and decreased the difference between inception pressure and vapor pressure in the DTF.
In some additional experiments with the container ship pro-peller, the water was shown to have a memory' of previous runs with electrolysis. First the propeller was tested at zero current strength (no additional nuclei), then at a current strength of 1.8A and Ihen again at zero current strength. The second time the pro-peller was tested without a current, a signiticant improvement was found with respect to the first time (see first and third bars in figure 6). The results imply that the nuclei generated during one run do persist in the water and influence subsequent runs.
DISCUSSION
Effect of propeller Reynolds number
influ-08 07 .4 06 4 06 03 02
,adai canlshanenitni ,ad,at minimum preasuin cuothcienl disiritution
86 06 3Cuu, r
Figure 6: The average radial extent of cavitation inception, and the
Stan-dard deviation over all blades, observed in experiments (left), the nu-merically calculated radial distribution of niinimutn pressure coefficient values (middle), and a video frame of blade 2(right, I= 3.OA).
Con-tainer ship propeller.Kr = 0.06(J= 0.973). a = 680 rpm and a = 3.0
ence of Reynolds number might be suppressed as well. However, since inception is delayed even at rotation rates identical to those used in the CT, the propeller Reynolds number can hot he causing this delay.
An increase of propeller rotation rate, keeping cavitation number and advance ratio constant, simultaneously alters the propeller Reynolds number and the Weber number, assuming that the nuclei content remains unchanged during experiments. As a result, the critical bubble radius required for cavitation in-ception decreases and a larger nticlei population is effective for cavitation inception (figure 7, natural nuclei size/number rela-tionship from [101 assumed). Nevertheless, no effect of an in-crease of Propeller Reynolds number on cavitation inception was found in the DTT. This implies that most nuclei in this facility are very small, with corresponding critical pressures much lower than the vapot pressure. and large nuclei are extremely scarce.
A relevant question that comes up is the question whether and what the minimum Reynolds number is below which cav-itation inception is no longer possible. This question is posed here specifically in the context of bubble screening, which is the deflection of the bubble path so that it avoids the low pressure region just behind the stagnation point on the foil [11]. High speed video observations of previous experiments showed that a strong deflection of the larger nuclei did indeed occur in the flow about the leading edge of the propeller blade. Based on the force equilibrium of pressure and viscous drag forces on a bubble in a Stokes flow, a first approximation of the deflection of the bubble
path can be niade. For a given body the deflection of the bub-ble path normal to the flow streamlines is proportional to
r2U.
where r is Ilic bubble radius and U is the free-stream velocity. It follows that a lower flow velocity, or a lower Reynolds num-ber, increases the bubble path dehiection. A larger bubble size, as needed to keep the Weber number constant at a lower flowveloc-5
nuclei population in Dli, with electrolysis
- nuclei population in OTt, without electrolyois
sueceptible inception ouciw low Ru, without electrolysis high Re. without electrolysis low Re, with electrolysis
P51 Pmin p log(critical nuclei pressure)
Ibobbie size)
Figure 7: Conceptual illustration of increasing Reynolds number while leaving the nuclei tlopulatiott unaffected: the minimum pressure in the t1ow decreases and more nuclei to the flow are effective for cavitatioti inception.
ity, will also increase the deflection of the bubble path. It is there-fore conceivable that below a certain Reynolds number value, all hubbies with critical pressures higher than the minimum flow pressure are deflected so that they avoid the low-pressure region. In the context of Reynolds number inlltienccs on cavitation inception, it. is noteworthy that [121 states that the rupture of the water is not only governed by normal stresses or pressures. but that tangential stresses also contribute to it. In I 13] it is argued that the effect of shear stress on fluid strain can not he neglected
with respect to the effect of normal stresses (or pressures). Future research is needed to find a general answer for the critical Reynolds number, below which no inception of sheet cav-itation on a foil occurs.
Effect of free-stream turbulence
The experinienLs with added free-stream turbulence give am-bivalent results: a delay of cavitation inception persists on the tanker propeller, but it is significantly diminished on the con-tamer ship propeller. To explain this ambivalence, the grid cav-itation ntttnher in these two experiments is further examined. In the first case, the cavitation number based on inflow velocity is approximately 9. The container ship propeller experiments, on the other hand, are conducted at higher flow velocities, resulting in a value of the cavitation number of approximately 1 .5. This value is very close to the minimum pressure coefficient of the grid wires. It is thus likely that the flow behind the turbulence grid did cavitate in the experiments with the container ship pro-peller. As a consequence, nuclei were generated that removed inception delay. This explanation is in line with the fact that ad-ditional gas nuclei in the inflow eliminated the delay in cavitation inception.
current results, it can only be said that a free-stream turbulence intensity of 1% has had no influence on cavitation inception on roughened propellers in the DTT.
Effect of nuclei
The results with the electrolysis grid indicate that even with
leading-edge roughness applied to the propeller blades,
free-stream nuclei are an important parameter for sheet cavitation in-ception. With additional nuclei, the cavitation inception pressure in the DYF increases to values much closer to the vapor pressure. Furthermore, the large variations between the blades observed in the runs without electrolysis are drastically reduced, which indi-cates that the encounter frequency of large nuclei has increased as well. The addition of electrolysis to the inflow apparently has an effect as illustrated schematically in figure 7: with electroly-sis the critical pressure of the largest nuclei is closer to the vapor pressure and there are more nuclei effective in the inception pro-cess. It is left for future research to investigate whether the small difference between the CT and the 3.0 ampere case in the D1T
(see figure 6) can be solved by increasing the current strength
even further. Alternatively, this difference might also be caused by uncertainties in radial extent and/or thrust coefficient.
The effect of additional nuclei on sheet cavitation inception contradicts with an earlier conclusion in [3], pg. 174: 'the appli-cation of roughness at the leading edge in a condition with low nuclei content will generate sheet cavitation in all conditions'.
This statement is based on the existence of microscopic
low-pressure regions in the roughness strip. Once cavitation occurs in these regions, this takes over the role of free-stream nuclei.
From the present results it follows that cavitation in these
mi-croscopic low-pressure regions still depends on the free-stream nuclei content. The present results also show that the actual in-ception pressure depends on the strength of the current applied on the electrolysis grid. This is in disagreement with [8], where small nuclei were sufficient to establish sheet cavitation and an increase in nuclei size did not alter the inception pressure. An cx-planation for this discrepancy requires closer observations of the role of leading-edge roughness, free-stream nuclei and pressure distribution on sheet cavitation inception, and will he the focus of future research.
The low nuclei content in the DIT is thought to be caused by a new deaeration technique, adopted four years ago. Apparently this technique decreases both dissolved and undissolved gas in the water. This explains why the problem occurs more often in recent years than before. Furthermore, computed streamline vi-sualizations revealed that the nuclei generated with an electroly-sis strip on ship models, as mentioned in the introduction, only arrive in the top region of the propeller disk, Since pressure-side cavitation inception occurs typically on the upcoming blade (see figure 1), the nuclei from the electrolysis strips are ineffective for pressure-side cavitation inception. Suction side cavitation, on the
6
other hand, occurs typically on the blade in top position, where
the electrolysis nuclei are abundantly available. It will be
investi-gated whether the general association of inception delay with the pressure side of propellers, is related to this unequal distribution of nuclei over the propeller disk in propulsion tests.
CONCLUSIONS AND RECOMMENDATIONS
'l'he pressure side inception problems in the DTT, as de-scribed in this paper, are related to an extremely low nuclei con-tent in this facility. The generation of additional gas nuclei in the inflow to the propeller can restore inception to pressures compa-rable to those found in the cavitation tunnel. A generalization of the findings is that both size and number of free-stream nuclei are important for sheet cavitation inception, even if leading-edge roughness is applied. This contradicts the present theory that with the application of leading-edge roughness, the free-stream nuclei population in the water is of no importance.
A practical next step will be to improve the method used to increase the nuclei content in the inflow to the propeller. When a propeller operates in a ship wake, a very high encounter
fre-quency is needed because inception occurs in every propeller
revolution. Further research will be focused on the concept of
a lower limit in Reynolds number, and on a more detailed
exam-ination of the sheet cavitation inception process, including the role of roughness, free-stream nuclei and pressure distribution.
REFERENCES
van Terwisga, T. J., van Wijngaarden, E., Bosschers, J., and
Kuiper, G., 2006. "Cavitation research on ship propellers". In Sixth International Symposium on Cavitation, Wagenin-gen, The Netherlands (CAV2006).
[21 Arndt, R. E., and Keller, A. P., 1975. Free gas content ef-fects on cavitation inception and noise in a free shear flow.
Tech. Rep. II 85V, Netherlands Ship Model Basin (NSMB).
[3] Kuiper, 0., 1981. "Cavitation inception on ship propeller models". PhI) thesis, Netherlands Ship Model Basin, Wa-geningen.
[41 Tani, I., 1969. "Boundary-layer transition". Annual Review of Fluid Mechanics, 1(1), pp. 169-196.
[5] Korkut, E., and Atlar, M., 2002. "On the importance of the effect of turbulence in cavitation inception tests of marine propellers". In Proceedings of the Royal Society of Lon-don, Vol. 458, pp. 29-48.
[61 Keller, A. P., and Rott, H. K., 1997. "The effect of flow
turbulence on cavitation inception". In ASME Fluids Engi-neering Division Summer Meeting.
181 Gindroz, B., and Billet, M., 1998. "influence of the nuclei on the cavitation inception for different types of cavitation
on ship propellers". Journal of Fluids Engineering, 120,
pp. 171-178.
[91 Roach, P., 1987. "The generation of nearly isotropic turbu-lence by means of grids". Heat and Fluid Flow, 8(2), June, pp. 82 - 92.
[101 Brennen, C. E., 1995. Cavitation and Bubble Dynamics.
Oxford University Press.
[111 Johnsen, V., and Hsieh, T., 1966. "The influence of trajec-tories of gas nuclei on cavitation inception". In Sixth Sym-posium on Naval Hydrodynamics, Washington D.C. USA. [121 Joseph, D. D., 1998. "Cavitation and the state of stress in a
flowing liquid". Journal of Fluid Mechanics, 366, PP.
367-378.
[13] Bouziad, Y. A., 2006. "Physical modelling of leading edge cavitation: computational methodologies and application to hydiatilic machinery". PhD thesis, École Polytechnique Fédérale de Lausanne.
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