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Cavity Thickness on Rotating Propeller Blades Measurements by Two Laser Beams H.D. Stinzing (VWS, Berlin Model Basin, Germany) ABSTRACT A prediction of propeller-induced vibratory hull pressures needs compu- tation of the cavity volume, which de- pends on blade geometry and operating conditions of the propeller. In order to verify or, if necessary, to improve the theoretical model developed by the Hamburg Ship Model Basin (HSVA), meas- urements of thickness and extension of the blade cavitation at two propeller models of different blade geometry were executed by a laser technique, and the results were compared with those following from computations. Extensive preliminary studies for optimizing the laser technique were necessary in order to develop a pre- cise and easily applicable method. The essential feature of the new procedure is the use of two laser beams of constant intensity intersecting at the surfaces of the cavity and of the pro- peller blades, resp. INTRODUCTION As a result of the efforts to in- crease the delivered power of ship propellers, to build lighter hulls and to use propellers with larger diam- eters ship hull vibrations are an increasing problem. These vibrations are mainly caused by forces acting on the propeller. The non-uniform wake field of the ship results in period- ically changing load of the propeller blades which affects the hull through the propeller shaft. In addition, the propeller produces pressure fluctu- ations which are transferred to the hull through the water. An essential reason for the pro- peller-induced pressure fluctuations is the nonsteady propeller cavitation. The extension and the thickness of the cavity on a propeller blade vary ac- cording to the blade position and the Hanns-Dieter Stinzing. Versuchsanstalt Mueller-Breslau-Str. (Schleuseninsel)' operating conditions of the propeller. These volume variations cause pressure fluctuations and thereby vibrations at the afterbody. The prediction of such propeller- induced pressure fluctuations thus re- quires the calculation of the cavity volume. For this the Hamburg Ship Model Basin (HSVA) has developed a theoretical model that had to be con- firmed and, if necessary, improved by measurements executed by the Berlin Model Basin (YWS). The aim of the investigations discussed below was to provide a meth- od to measure the cavity thickness on rotating propeller blades by using the advantages of laser light and to apply this technique to two different pro- peller models operated in the small cavitation tunnel of the VWS. The re- sults had to be compared with calcu- lations made by the HSVA. MEASURING TECHNIQUES Basic Methods The simplest methods to "measure" the cavity thickness are visual ob- servation or simple photographs taken at stroboscopic illumination of the propeller. Both provide useful infor- mation about the shape of the cavity, its thickness, however, can only be roughly estimated. Slightly more accurate results are achieved by stereo-photogrammetry which gives the three-dimensional shape of the cavity. This technique, however, still may produce errors as high as 100 percent (1). Another dis- advantage is the troublesome eval- uation of stereo-photographs. The pin-gauge method uses stream- lined, scaled pins normally fixed to the blade surface (2). They allow an easy estimation of the cavity thick- ness. But great disadvantages of fuer Wasserbau und Schiffbau D-1000 Berlin 12, FR Germany 319

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this method result from the dis- turbance of the flow around the blades and from cavitation that may be pro- duced by the pins themselves. The final breakthrough in cavity thickness measurement was achieved by the use of lasers utilizing the strong beaming and the high intensity of their light. The basic idea of this technique was to measure the distance between the light spots that appear where the laser beam hits the surfaces of the blade and the cavity, resp. To make those spots as small as possible the beam has to be pulsated like a stroboscope. These pulsations are gen- erated by means of an acousto-optic or electro-optic modulator, triggered by the propeller shaft. In case of an acousto-optic modulator, which has the same effect as an optical grating, for the selection of the 1st order beam a pinhole has to be attached in front. By using an electro-optic modulator, which rotates the plane of polar- ization of the laser light, a polariz- er is needed additionally. If a single laser beam is used to measure the cavity thickness, a sight device is necessary to define the measuring direction (3). By means of this device a vertical virtual plane containing the measuring direction is selected, while the laser beam enters the tunnel horizontally. In order to hit the cavity where the measuring di- rection penetrates the cavity surface, the laser beam, which initially is po- sitioned on the blade surface, has to be shifted parallel with the propeller shaft. The cavity thickness then fol- lows from the shift with regard to the measuring direction and the blade ge- ometry (Fig.l). In the beginning of a measurement the laser beam, entering the tunnel horizontally and normally to the pro- peller shaft, is adjusted to the meas- uring point on the blade surface while the tunnel pressure is high, i.e. while the propeller is not cavitating. Then the measuring direction is de- fined by means of the sight device and the tunnel pressure is reduced accord- ing to the desired cavitation number. Finally the laser is shifted as far as , that is visible on the lies in the measuring the light spot cavity surface, direction. In the past useful results were obtained by this method. That's why it served as a basis for the investiga- tions within the scope of our proj- ect. Another technique to measure the cavity thickness by means of laser light is to use two convergent and synchronously pulsated laser beams in- tersecting in a point. The beam inter- section first is positioned in the 9 al SIGHT DEVICE ~\~ ACOUSTO - OPTIC -T = _~ t~? ret ~ ( J `~^ t/~1 TRAVERSE SYSTEM fOR LASER DOPPLER VELCCIMETER LIGH T ~STROBO MODU LATCR S l G NAL SCOP_ 5 ~ GNAt _ CoNDITION~ _ mL5 ?ROC SSOR I GENERATOR IPROPELLCR I I DYNAMOME TER I z i> ~ a LASER y N ~ jig-AVERSE N Fig.1 Cavity thickness measurement according to Ukon,Y. and Kurobe,Y.(3) measuring point on the blade surface when the propeller is not cavitating. The cavity thickness there is then calculated from the distance of the two light spots observable on the cav- ity surface, when the propeller is cavitating. This method has been suc- cessfully applied on large ships (4). Method Applied By The VWS Due to the good results obtained with a single pulsated laser beam in Japan, that technique initially was selected to be used at VWS, too. To re- examine the measuring principle, the beam of a He-Ne laser of 5 mW power was electro-optically modulated using a Pockels-cell and a Glan-Taylor po- larizer attached in front of it. All the components were mounted on an op- tical bench that was fixed to a tube stand. So the laser beam could be freely directed towards the propeller in the cavitation tunnel. The 250 V pulses required to control the Pockels-cell, were supplied by a spe- cial video amplifier that on his part was controlled by a pulse generator triggered by the propeller shaft. The 320

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pulse generator could produce square- wave pulses of any duty factor. Preliminary tests showed that not nearly all positions on the back of the propeller can be reached by a laser beam directed normally to the tunnel window. That is because of the limited height of the windows as well as the geometry of the propeller. That's why an inclined laser beam must be used inspite of a number of disadvantages: Since the tunnel window is not a coated optical glass but simple perspex, the reflection losses at the outer surface may be quite high. Moreover, the inclined beam is refracted in the window as well as in the tunnel water so that the direction in which it strikes the propeller is unknown. This means that the cavity thickness can no longer be calculated from the laser shift as described above, but has to be measured as the direct distance of the light spots, using a glass scale for instance. This technique is more complicated. A further result of the prelim- inary tests was that, even at 90 angle of incidence, a laser power of only 5 me is too low. Especially in the case of a smooth cavity surface it is difficult and over and above it often impossible to recognize the light spot generated by such a weak laser. In addition, since long light pulses are perceived as lines on the blade or the cavity surface the de- sired punctiform illumination requires very short pulses (approx. 50 ps) which complicates the visual obser- vation. Finally, the intensity losses in the tunnel water, mainly caused by air bubbles, have to be taken into ac- count, too. The results of the preliminary tests called for another measuring principle and in addition for the in- stallation of a more powerful laser. Because of the considerably larger di- mensions of the 35 mW He-Ne laser presently used in the VWS and the very restricted space on the portside of the VWS cavitation tunnel it became necessary to install the laser away and to transport its light to the measuring point through an optical fiber. That means that now, instead of the laser, only the much smaller and lighter fiber positioner had to be shifted. However, substantial losses in light intensity, mainly caused by the coupling into the optical fiber (monomode with a 3 am core), are disadvantageous. The preliminary tests have shown that the laser beam always can be ob- served clearly and sharp-edged where it is crossed by the propeller blades and thus strikes the solid blade surface - provided that adequately long light pulses are used. This led to the idea to use two permanent laser beams intersecting in one point, in- stead of a single pulsated beam, and to utilize the beam intersection, which in the interesting propeller regions always can be clearly seen, as a pointer tip. If this pointer tip is moved parallel with the propeller shaft from a position on the propeller blade to the cavity surface, the cav- ity thickness in this direction is ob- tained directly from that shift. The decisive advantage of this method is the existence of a defined measuring direction independent on the direction of the laser beams. A precondition for the posi- tioning of the beam intersection point is the visual fixation of the pro- peller by additional stroboscopic il- lumination. One can always adjust the intensity of the stroboscope flashes so that the laser light is not out- shined, simply by adequate covering of the reflector. Using a stroboscope has the fur- ther advantage that the complete three-dimensional shape of the cavity can be observed while it is measured. This is important especially when measurements have to be made in crit- ical regions, namely near to the leading edge of the blade or under the bulge of the cavity close to the trailing edge or when bubbles com- plicate the observation of the cavity surface. MEASUREMENTS AND RESULTS Measurements The measurements were performed with models of the six-bladed "Hongkong- Express.' propeller (HS9A No. 2076, 211.11 mm diameter) and the five- bladed "Sydney-Express" propeller (HSVA No. 2054, 200.00 mm diameter). The axial velocities adjusted by means of sieves are shown in Figs.2 and 3. To define the measuring points the suction sides of the two measuring hades (in both cases blade No. 1) have been provided with rasters by polar coordinates consisting of circular arcs spaced in equidistances of 0.02 R (where R designates the propeller ra- dius) and with radii every 2 (Fig.4). This raster was definitely fixed by the point given by half the chord length of the 0.7 R arc. The radius going through this point was also used to define the blade position. In order to obtain realistic measuring conditions for the models data of the ships served as a basis. These are the ship velocity vs=22.5 kn, the rate of revolutions n=92 mind and the torque Q=2,750 kNm for the 321

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[cm] I I I ~/~/ ~ n ';~ -1 ho. _. CD a 3 \ \\\ pot \` ~ I'd 7~ ~ / _ ~ / St b. F i 9 .2 Ax i a 1 i Of 1 ow of prope 1 1 er No . HSVA 2076 "Hongkong Express" and vs=22.0 kn, n=110 min-t and Q=2,000 kNm for the "Sydney Express". These values give the torque coefficients KQ =0 . 045 and KQ -O. 035, resp., and from the open water diagrams result the thrust coefficients KT =0. 250 and KT =0.215, resp. To define the caviation numbers a depth of the propeller shafts of 7.0 m for the "Hongkong Express" and 6.7 m for the "Sydney Express" were assumed. Thus the propeller diameters of 7.6 m and 7.0 m, resp., and 1.0 m of height of the stern wave give the cavitation numbers co. B =0 . 340 and co.~=0.280, resp., with regard to the highest po- sition on 0.8 R. The test setup is shown in Fig.5: The beam of the He-Ne laser (1) is coupled into a 10 m long optical fiber (3) with the aid of a fiber coupler (2). On the other end it is fed into simple LDA-optics (5) using a fiber positioner (4). The optical system, consisting of a beam splitter, a beam displacer and a front lens (focal length 600 mm), divides the entering laser beam into two convergent beams intersecting in one point. For posi . hi CD - _ o 4 ~/ tat Prop. No. HSVA 2076 \\\ _ ma/ ~ ~ ~ \1 F i 9 e 3 Ax i a 1 i Of 1 ow of prone 1 1 er No . HSVA 2054 / t .~\ Prop. No. ItSVA 2054 Fig.4 Polar-coordinate raster for measuring-point def inition tioning this intersection, fiber po- sitioner and optics are fixed to two translation stages (7) with a rotary stage (8) between them. These stages 322

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,1 11 it, / ~ I/ \ \ Fig.5 Test setup 1 ~ He-Ne- 1 aser ~ 6 ~ stroboscope 1 amp 2 ~ f i her coup 1 er ~ 7 ~ bans 1 at i on stage ~ 3 ~ opt i ca 1 f i ber ~ 8 ~ rotary stage (4) fiber positioner (9) tube stand (5 ~ LDA-optics are mounted on a manifoldly adjustable tube stand (9). The cavity thickness is measured with the lower translation stage which is aligned parallel with the propeller shaft. With the strobo- scope lamp (6), triggered by the pro- peller shaft, the propeller is flashed in proper phase relation and thus visually fixed. The tests were performed as shown in Fig.6: At first thrust and torque of the propeller are adjusted accord- ing to the coefficients KT and KQ, resp., by adjusting the flow velocity at a given propeller speed. Then the desired blade position is fixed by changing the stroboscope trigger ade- quately. After that the tunnel pres- sure is reduced according to the cav- itation number, to examine the ex- tension of the cavity. Subsequently the pressure is increased until the cavitation disappears, this being nec- essary for positioning the beam inter- section in the measuring point on the blade surface using the rotary and the translation stages. For the same rea- son it is necessary to reduce the in ,3 , , adjust thrust and torque ~ b~;~: i; ~ ~ ~ ~;~f ~ i) , J __ _ , adj ust stroboscope trigger l , , reduce tunne 1 pressure . 1 _ , sketch cavity extens ion ~ . Crimea sur i ng po i nt: I. YeS 1 increase tunnel pressure ~- pos it ion beam i nter- section and read stage adj ustment 1 ~ reduce tunnel pressure , , ~ _ sh if t beam i nter- ~ect i on and read stage adjustment 1 . Fig.6 Schematic of test procedure tensity of the laser light which ex- pediently can be achieved by means of two rotating polarizing filters, in- serted between fiber positioner and LDA-optics. Following the positioning, the light intensity is increased again and the adjustment of the lower trans- lation stage is noted down. Finally the tunnel pressure is lowered again down to the required value to get cavitation and the beam intersection is shifted into the cavity surface with the aid of the lower stage. The adjustment of which is noted down again. The cavity thickness then fol- lows directly from the beam shift. According to this method. points in 16 blade positions measured on propeller HSVA No. 323 330 were 2076

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/\\ N~A ~ =320 . , . (p =33oo ~ =34Oo \, ,,~ ~ =10 (,t'4, ,/,\) ~ ~ =20 Fig.7 Measured cavity thicknesses of propeller No. USVA 2076 (isometric representation) 324

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~ to ~( At ~ :=50 ~.' ~ i] ,, I,:: To ~ =60 'A \ A.' ~ W Fig.8 Measured cavity thicknesses of propeller No. HSVA `076 (isometric representations 325

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~ JO ~ ~ \~ ~ =10 And ~ =40 TV, \`,;~w ~ = Coo Fig.9 Measured cavity thicknesses of propeller No. HSVA 2054 (isometric representation) 326

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o ~ 2 ._ _ ~ ~i I _ _ _ 0 900 E ~ 4 a) E \ >, 2- . ._ ~ _ 6 rat ~\ total propel ler / \ / ~ single blade Prop. No. HSVA 2076 ~I_ _ - 1 - - 1 80 270 360 b 1 ade pos it ion So . Prop. No. HSVA 2054 A \~4 Angle blade \~/ 4/ 4/ y 0 90 1 80 270 360 b 1 ade pos i tion go _ Fig.10 Cavity volume variation and 139 points in 7 positions on propeller HSVA No. 2054. Results The measured cavity thicknesses are shown in Figs.7 to 9 in an isometric representation, which gives a good survey of the cavity geometry and its changes according to the blade posi- tion. To facilitate the calculation of the cavity volume as a step function, the cavity thickness measured in di- rection of the propeller shaft has been converted to the direction normal to the blade surface. The required angles were determined experimentally by reflecting a laser beam at the blade surface. The cavity volumes _ appearing on a single blade and on the complete pro- peller during one revolution are shown in Fig.10. It is conspicuous that the "Hongkong-Express" propeller produces a considerably greater cavity volume than the "Sydney-Express" propeller despite the higher cavitation number. But the resultant volume fluctuations are much smaller on the "Hongkong in the course of one revolution Express" than on the "Sydney-Express propeller, which may be explained by the larger number of blades, the dif- ferent blade geometry and especially the larger skew. The calculations made by the HSVA differ considerably from the results of the VWS measurements as Fig.11 shows. This applies to the cavitation inception, the extension of the cavity and the cavity thickness likewise. According to the calculations, on the "Hongkong-Express" propeller the cavitation incepts later and closer to the blade root and it disappears much quicker than is revealed by the meas- urements. Correspondingly, the differ- ences in cavity extension may be quite large. At blade positions between ~=350 and ~=0, measurements and cal- culations agree fairly well, but out- side this range the calculated exten- sions are considerably smaller than the measured values. Such differences are inevitably connected with dif- ferences in cavity thickness. Similar values are only obtained for the blade position of ~=350. At the following positions the measured cavity thick- nesses may exceed the results obtained 327

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art ~ lo it' ~ ~: Prop. No. HSVA 2076 >7.~' I'm ) .~ ~J~ /: Prop. No. HSVI I54 \ Fig.ll Measured and calculated (hatched) cavity thicknesses 328

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by linear calculation by a factor of 5. The greatest differences in each case appear at the trailing edge of The propeller blade. On the "Sydney-Express" propeller the calculated cavitation inception appears much earlier than the measured one, which is in contrast to the "Hongkong-Express" propeller. Accord- ingly, up to blade positions of ~-0 and ~-10 the calculated cavity exten- sions are larger than the measured ones. For subsequent positions the situation is vice versa. Furthermore it can be stated that the measured cavities are located closer to the tip region than the calculated ones. Concerning the cavity thickness on the "Sydney-Express" propeller the smallest differences belong to blade positions near to ~=350. After that the measured values become consid- erably larger and again the greatest differences can be observed at the trailing edge. One possible reason for the dis- crepancy between measurement and calculation results might have been the poor condition of the propellers. Especially the "Hongkong-Express" pro- peller, that is made of aluminum, has a lot of corrosion pits at the leading edge. Another reason may be that the cavity thickness depends on the con- centration of the cavitation nuclei and macroscopic gas bubbles within the tunnel water, but these parameters have not been taken into account in the calculations. However, to be able at least to estimate the effect of the gas bubbles on the measured cavity thicknesses, the number and size of the bubbles (relative values) were determined by a light scatter method at nearly all measurements and shown as histograms. In addition, at all tests the oxygen content of the tunnel water was meas- ured by means of an electrode. The histograms didn't give any significant differences in bubble-size spectra. According to visual observa- tion one may also suppose that the propeller, working in a closed cir- cuit, produces his own bubble spec- trum. After a relatively short period of about one minute the same situation can be observed again and again. How- ever, this does not exclude that dif- ferent bubble spectra in fact result in different cavity thicknesses. The oxygen measurements proved that the content of dissolved gas does not affect the cavity thickness. A test run over a whole day, constantly revealed the same thickness for a given position, while the oxygen con- tent was greatly reduced by debasing. The accuracy of these thickness meas- urements was 0.1 to 0.2 mm. The same can be assumed for most of the meas- urements performed. Only if the cavity is hard to be observed, or if meas- urements must be taken under the bulge of the cavity or if heavy fluctuations or bubbles complicate positioning on the cavity surface, the accuracy of measurement is smaller. In such cases it is estimated to be 0.5 mm. CONCLUSIONS The striking advantage of the new method is its well-defined measuring direction. Its try-out with two pro- peller models has shown that it meas- ures cavities fast and exactly. A Japanese report (5) received after closing the investigations described, seems to confirm the practicability of this technique. The following task is to improve the theoretical model. In addition ef- forts should be made to modify the measuring technique so that the cavity outside the propeller blades, i.e. within the range of the tip vortex, can be measured, too. Finally a method has to be developed to vary and record size and number of the macroscopic gas bubbles of the tunnel water in order to clarify their effect on the pro- peller cavitation. REFERENCES 1. Sontvedt, T. and Frivold, H., "Low Frequency Variation of the Sur- face Shape of Tip Region Cavitation of Marine Propeller Blades and Corre- sponding Disturbances on Nearby Solid Bound-aries", Proceedinas of the llth SYmoo-sium on Naval Hydrodynamics, London 1976, pp. 717-729 e 2. Chiba, N., Sasajima, T., and Hoshino, T., "Prediction of Propeller- Induced Fluctuating Pressures and Cor- relation with Full Scale Data", Pro- ceedinas of the 13th Symposium on Naval Hydrodynamics Tokyo 1980 pp. t r 89-103. 3. Ukon r Y. and Kurobe, Y. r ''Meas- urements of Cavity Thickness Distribu- tion on Marine Propellers by Laser Scattering Technique", Report of Shin Research Institute, Vol. 19, No. 3, 1982, pp. 1-12. 4. Rodama, Y., Takei, Y., and A. Rakugawa, "Measurements of Cavity Thickness on a Full Scale Ship Using Lasers and a TV Camera", Pacers of Shin Research Institute, No. 73, Dec. 1983. 5. N.N., "Twenty Years of Ship Re- search at IHI with the Ship Model Basin 1966-1986", Research Institute Ishikawajima-Harima Heavy Industries Co., Ltd., Anniversary Publication, Tokyo 1987. 329

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DISCUSSION . Jlnzhang Feng Pennsylvania State University, USA (China) Cavitation measuring is difficult. It is more so near the blade tip where the cavity bulb is usually fluctuating. The author, however, has measured the bulb thickness remarkably close to the blade tip as shown on Fig. 7 and Fig. 8. Is this because the author used time average to smooth out the fluctuation or because the propeller the author used in the test has a very little loading at the blade tip? AUTHORS' REPLY The operating conditions of both the propeller models were derived from normal operating conditions of the full scale propellers (see values). So, their loads should have been normal, too. At these loads and the cavitation numbers mentioned, cavity fluctuations did not occur at the blade tip but at the blade root boundary of the cavity where its thickness was small. There, indeed, mean values were measured. But due to the thin cavity these values contribute only very little to the cavity volume, the quantity wanted. DISCUSSION Spyros A. Kinnas Massachusetts Institute of Technology, USA The paper offers valuable experimental information of unsteady cavity shapes, which can be used to validate existing analytical techniques. I would like to address the following two points though: (a) The technique used in the paper measures the cavity thickness only on the propeller blade. As it appears though from Figs. 7-9, the cavity seems to extend beyond the trailing edge of the blade. Is the volume of the cavity extending behind the trailing edge accounted for in the computation of the cavity volume? (b) The analytical method that is used appears to underpredict the cavity extend and volume -substantially. A reference for this method would be nice to have been given. Is it a quasi-steady vs unsteady method and is it a stripwise 2-D method vs a completely 3-D method? AUTHORS' REPLY Figs. 7-9 indicate that the cavities indeed extended beyond the trailing edges of the propeller blades. The technique described does not enable cavities to be measured outside the blades. So the specified cavity volumes only include the measured blade cavities. For the calculation of the cavity thicknesses, a quasi-steady and 2-D method was used. It is described in detail by K.-Y. Chao and H. Streckwall in ~Kavitationsuntersuchungen, Druckschwankungsmessungen und Vibrationsbewertung an schnellen Containerschiffen hoher Leistun,. HSVA-Rep. No. 1569, August 1989. 330