Drag Reduction of Ships by Microbubbles

Transcription

1 rag Reduction of his by Microbubbles Yoshiaki Kodama 1, kira Kakugawa 1, Takahito Takahashi 1, higeki Nagaya 1 and Takafumi Kawamura 1 National Maritime Research Institute of Jaan kodama@srimot.go.j The niversity of Tokyo BTRT Microbubbles, a skin friction reduction device, has significant effect, and is esecially suited to shis. The authors constructed a circulating water tunnel for microbubble exeriment and found out strong correlation between skin friction reduction and local void ratio close to the wall. The authors then carried out exeriment by towing a 5m-long flat late shi at 7m/sec in a towing tank, and obtained scale effect data of microbubbles. Based on the data, net drag reduction effect of microbubbles when it is alied to a 3m-long tanker was estimated. It was found out that at fully loaded condition net drag reduction is almost zero, that at ballast condition % net reduction, and that, if the reduction efficiency is doubled, net drag reduction of 5% is obtained at fully loaded condition. Further study to imrove drag reduction efficiency and to solve related roblems is needed. frictional drag. The wave-making drag comonent of such a shi is very small because they move very slowly. nother reason that they are suited is in their shae. Their shae is like a box, excet for bow and stern regions. They have a wide flat bottom, and the bubbles injected at the bottom near the bow stay close to the hull bottom by buoyancy while they are carried by flow all the way to the stern. Thus the injected bubbles can cover the whole hull bottom efficiently. In Jaan, microbubbles has been studied intensively in the ast few years toward its alication to full-scale shis. This aer reorts a review of such studies, focusing on those carried out by our grou in NMRIJ. HY MIROBBBLE? Microbubbles is a drag reduction device that reduces skin friction of a solid body moving in water by injecting small bubbles into the turbulent boundary layer develoing on the solid body. Microbubbles has been studied by many researchers since the ioneering work of Mcormick and Bhattacharyya [1]. Fig.1 shows an examle [] of its skin friction reduction effect. The data was taken in a circulating water tunnel, the bubbles were injected at the to flat wall and skin friction was measured by a skin friction sensor laced downstream of the injection oint. The horizontal axis shows the amount of injected air and the vertical axis shows the ratio of reduced skin friction to that at non-bubble condition. This figure shows that, as the amount of injected air increases, skin friction reduction effect by microbubbles increases u to 8%. his such as tankers lay a major role in marine transortation. They are very large and move very slowly. They are esecially suited to microbubbles. Fig. shows an image of the alication of microbubbles to such a shi. One reason that they are suited is that their skin friction drag comonent occuies about 8% of the total drag. The drag of a shi that moves on the water consists of two comonents, i.e., wave-making drag and skin Fig.1 Measured skin friction reduction by microbubbles [1] Fig. n image of microbubbles alied to a fullscale shi

2 KIN FRITION RETION EFFET OF MIROBBBLE N IT MEHNIM Flow direction ir injection chamber (Position1) P P3 P4 kin Friction Reduction Effect Microbubbles has significant skin friction reduction effect. The authors carried out microbubble exeriment using a circulating water tunnel shown in Fig.3. In the test section, air is injected through a orous late set on the to wall at Position 1. Measurements are carried out in the downstream locations P, P3, and P4, each 5mm aart. t the downstream of the test section, there is a dam tank, injected bubbles are removed by buoyancy, so that continuous microbubble exeriment is ossible. hotograh of microbubbles is shown in Fig.4. The average flow seed in the test section was 7m/sec, or 14 kts, which corresonds to tyical cruising seed of a tanker. The average void ratio, i.e. the ratio of air volume to the total volume of airwater mixture, was 5.3%. The actual longitudinal length of the hoto is 1mm, and the to side corresonds to the to wall of the test section. The bubbles are clustered near the to wall. The bubble diameter was.5mm to 1.mm. Fig.5 shows the measured skin friction reduction. The horizontal axis shows α, the average void ratio in the test section, and the vertical axis shows the ratio of the wall skin friction with and without bubbles. s the amount of injected air increases, skin friction reduction increases, and the maximum reduction of 3% is obtained. The solid line shows the measured result by Merkle et al. []. The authors' results agree with Merkle's at higher void ratio. Fig.6 shows the distribution of local void ratio at α 8%, at three streamwise locations. The local void ratio is defined as the ratio of the air volume to that of air-water mixture at a given oint. This figure shows that the bubbles are clustered near the to wall and tend to diffuse as they move downstream ire meshes 3mm Test section 15mm x 1mm Electro-magnetic flow meter Pum Fig.3 irculating water tunnel for microbubble exeriment Fig.4 Bubble hotograh. 7m/sec, Position, α 5.3%[4] Fig. 5 kin friction reduction by microbubbles [5]. 7m/sec. um tank kin Friction Reduction Mechanism lthough there is no well-established theory for skin friction reduction mechanism of microbubbles, one ossible mechanism is the density effect, which means that, since the density of air is about 1/1 of that of water, the layer of clustered bubbles near solid wall cuts off shear stress un water and reduces skin friction. The results shown in Figs. 5 and 6, although the difference is small, shows that there is correlation between the magnitude of the skin friction reduction and the local void ratio near the solid wall at ositions P, P3, and P4. Guin et al. [3] also show similar results. nother ossible reduction mechanism is the turbulence suression effect, i.e. bubbles suress Fig. 6 Local void ratio distribution. 7m/sec, α 8% [5]

3 Fig.7 5m-long flat late shi turbulence in the boundary layer, resulting in skin friction reduction. Kato et al. [6] measured turbulence intensity very close to the solid wall in the bubbly channel flow and showed that turbulence intensity decreases as the skin friction increases. The authors think that, in reality, both mechanisms work to get skin friction reduction. LE EFFET IN KIN FRITION RETION BY MIROBBBLE f/f distance from the leading edge of the bow x (m) (a) V5m/s q. q.4 In order to be able to estimate skin friction reduction effect of microbubbles when it is alied to a full-scale shi, it is necessary to carry out largescale exeriments and find out how far in the downstream direction the skin friction reduction effect of microbubbles ersists after injection. atanabe et al.[7] measured the reduction effect by using a flat late shi of 4m long and 6cm wide in a towing tank. The authors carried out a similar exeriment using a flat late shi of 5m long and 1m wide in their 4m-long towing tank. Fig.7 shows the flat late shi that they used, and Fig.8 shows the distribution of skin friction reduction in the streamwise direction. It is seen that the skin friction reduction effect ersists to the downstream end at both seeds of 5m/sec and 7m/sec, which is a good sign for its alication to a large shi. The skin friction reduction effect is smaller at the higher seed of 7m/sec, whose reason may be that injected bubbles diffuse from the solid wall faster at higher seeds. Further study is needed. atanabe et al. [6] obtained higher skin friction reduction at 7m/sec than the authors. One of the differences between the two is that they used a orous late for bubble injection and the authors used an array-of-holes late, which has many 1mm diameter holes arranged in arrays. Further study on this oint may lead to imrovement in skin friction efficiency. Figs. 9(a), (b) show reduction of the total drag of the 5m-long shi by microbubbles [8]. The horizontal axis shows the flow rate of injected air nondimensionalized by the shi seed V and the injection area (5cm 1cm). The vertical axis of Fig.9(a) shows the ratio of the total drag with bubbles to that without bubbles. The vertical axis of Fig.9(b) shows the ratio of the estimated skin friction of the art downstream of the air injection late with and without bubbles. The former was estimated, f/f diatance from the leading edge of the bow x (m) (b) V7m/s q. q.4 Fig.8 Local skin friction (ir injection at bow) Rt/Rt Rf/Rf V7m/s V5m/s qq/v V7m/s V5m/s q Q/V (a)total drag Fig.9 rag reduction (ir injection at bow) (b)integrated skin friction downstream of injection late

4 based on the assumtion that the skin friction reduction occurs only in the area downstream of the air injection late, and the latter was estimated, based on the well-established choenherr skin friction formula for a flat late. These two figures show the same data with different denominators. Fig.9(b) shows that the maximum skin friction reduction of 3% is obtained at V7m/sec. NET RG RETION BY MIROBBBLE In order to assess the alicability of microbubbles to shis, it is necessary to discuss its net drag reduction effect by taking into account the energy needed for injecting air bubbles into water. In this section, net drag reduction of microbubbles is estimated following the line of argument by Merkle [] with slight modification. Net ork Ratio The net work ratio r, i.e., the ratio of net work rate needed to roel a shi while injecting bubbles to that with no bubbles, is r net o + + : work rate to roel a shi in non-bubble condition net : net work rate to roel a shi in bubble condition : shi's drag in non-bubble condition : shi's drag in bubble condition : shi's seed. ssumed to be unchanged by bubble injection. : work rate for bubble injection r 1. when net drag reduction is zero, and r < 1. when there is net drag reduction effect. is exressed by taking into account the energy loss due to head ressure at injection oint and the local ressure there. (1) 1 Q ( ρ gd + ρ ) () Q : air flow rate for bubble injection ρ : water density g : gravity acceleration d : water deth at injection oint : local ressure coefficient at injection oint hi's drag is exressed in a conventional nondimensional form. 1 ρ T 1 ρ ( F + ) 1 [(1 + K) F + ] ρ (3) : wetted surface area of a shi T : total drag coefficient F : frictional drag coefficient : wave drag coefficient F : frictional drag coefficient of equivalent flat late (having the same area and length as the shi) K : form factor of a shi hull choenherr's emirical formula is used to estimate frictional drag of the equivalent flat late..4 log1 log( R ef ), (4) R e is the Reynolds number., the drag coefficient of a shi in bubble condition, is exressed as 1 ρ T 1 F ρ ( F + ) 1 [(1 + K) F + ] and K do not change it is assumed that ρ, (5) with bubble injection. Thus, the net work ratio r is exressed as r + F + r + F Q Fd + 1+ r (1 + K) F (1 + r ) (6)

5 r F d ( 1+ K) F gd : ratio of wave drag to viscous drag : Froude number based on water deth How to Increase rag Reduction Effect sing eq.(6), we can say that following five oints are imortant to increase skin friction reduction effect and thus decrease r. (1)Reduce r, i.e., choose a hull form that has small wave drag. ()Reduce F F, i.e., increase skin friction reduction effect by microbubbles. (3)Reduce rate. (4)Increase Q F d, i.e., reduce injected air flow gd, i.e., seed u and/or make draft shallower. This is because the loss(work needed for air injection) is static(seed-indeendent), and the gain (skin friction reduction) is dynamic(roortional to seed squared). (5)Reduce, i.e., inject air at a location that has low ressure. rag Reduction Estimation in ase of a Tanker The net work ratio r is estimated for a large tanker with 3m length and the seed of 14kts. Tyical arameter values for this case are, hi length L 3m, d m.4l, 7m / sec,. 35 F d.5 r. 5,, 9 Re.1 1 F. 14 (full load), K, (7) The relation between injected air flow rate Q and skin friction reduction effect is roughly / F F estimated, using the data of a 5m-long flat late shi shown in Fig.9(b). First, the skin friction reduction effect of a full-scale shi is / F F assumed to be. 77, the same value / F F Q as that of the 5m-long flat late shi. Then ( means "shi"), the air flow rate in full scale is estimated by multilying Q ( M means M 3 "model") m / s, the air injection rate at 5m-long flat late shi, with the area ratio 864 to be M Q / 3 1.1m s. In this case, using eq.(4.6), the r net work ratio becomes 1. 78, slightly greater than 1, which means that net drag reduction is not obtained. On the other hand, in the ballast condition (emty condition), the draft becomes d 1m, then r. 979, aroximately % net reduction being obtained. This means that, using the resent level of technology, a tanker that runs between Jaan and the Middle East can get net drag reduction one way. If the amount of air can be economized to half by imroving techniques, the net work ratio becomes r. 95 even at full load condition of d m. In general, in order for an energy-saving effect not to be embedded into measurement errors, it has to be 5% at least, and therefore, this much of net drag reduction is needed. This rough estimation suggests that, in order to ut microbubbles into ractical use, it is necessary to imrove drag reduction efficiency at least twice as much and/or to combine the technique with other efforts such as develoing a new hull form suited fro microbubbles like one with very shallow draft and very wide flat bottom. lso it is necessary to take measures to revent fouling at air injection, and to estimate influence on noise and vibration roblems, and to revent them if necessary. The research done by the resent authors and resented in this aer was carried out under research rograms shown below. "tudy on Reduction of Fluid-dynamic rag sing Next Generation F", Ministry of Transort, "R39: tudy on kin Friction Reduction of his", hibuilding Research ssociation of Jaan, "mart ontrol of Turbulence: Millennium hallenge for Innovative Thermal and Fluids ystem", MEXT, - 4. Lastly, a full-scale microbubble exeriment using a 15m-long shi will be carried out by the R39 roject.

6 REFERENE [1]Mcormick, M.E. and Bhattacharyya, R., 1973, rag Reduction of a ubmersible Hull by Electrolysis, Naval Engineers Journal, Vol.85, No., []. Merkle,. and. eutsch:"rag Reduction in Liquid Boundary Layers by Gas Injection", Progress in stronautics and eronautics vol.13, I. [3] Guin M.M., Kato, H., Maeda, M., and Miyanaga, M., 1996, Reduction of kin Friction by Microbubbles and Its Relation with Near-all Bubble oncentration in a hannel, Journal of Marine cience and Technology, Vol. 1, No. 5, [4]Takahashi,T. et al.,1997,"treamwise istribution of the kin Friction Reduction by Microbubbles", J. of the ociety of Naval rchitects of Jaan, vol.18,.1-8. [5]Y. Kodama,et al.,: Exerimental study on microbubbles and their alicability to shis for skin friction reduction., International Journal of Heat and Fluid Flow1,. [6]Kato, H., Iwashita, T., Miyanaga, M., Yamaguchi, H, 1998, Effect of Microbubble luster on Turbulent Flow tructure, ITM ym. on Mechanics of Passive and ctive Flow ontrol, Gottingen, Germany,.1-6. [7]atanabe, O. et al.,1998,"measurements of rag Reduction by Microbubbles sing Very Long hi Models", J. of oc. Naval rchitects, Jaan, vol.183, [8]T. Takahashi et al.:"exerimental tudy on rag Reduction by Microbubbles sing a 5m-long Flat Plate hi", nd International ymosium on Turbulence and hear Flow Phenomena (TFP-), June, 1.