Yumrubaş dizaynı
The Design of bulbous bows
- Tez No: 39604
- Danışmanlar: PROF.DR. A. YÜCEL ODABAŞI
- Tez Türü: Yüksek Lisans
- Konular: Gemi Mühendisliği, Marine Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1994
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 236
Özet
Yumrubaşın geminin bir çok özelliğini geliştirdiğine şüphe yoktur. Uygun olarak şekillendirilen bir ytmafeaş*- bir geminin yaklaşık olarak tüm özelliklerini etkiler, özellikle hızlı gemiler için, yumrubaş kullanımı, daha iyi bir sualb formu için, şimdiye kadar kabul edilen dizayn prensiplerinden değişiklik yapmaya müsade eder. Yapım maliyetinin yüksek olması sadece dezavantajıdır. Teoriler dizayneri amprîk yolla yumrubaş formu geliştirmeye zorladığı zaman, dizayner kendi deneyimleri ve yayınlanmış meterialler olarak ük iki kaynağa bağımlı olacaktır. Ancak, modern teorik yöntemler göreceli bir değerlendirmeyi mümkün kılar. Nicel değerlendirmede en güvenilir yol, Bunula birlikte, bir gemi için doğru yumrubaş dizaym halen zordur. Yumrubaş dizaynı için, çeşitli yöntemler mevcut olamsma rağmen, yöntemlerin yeterli olduğu söylenemez. Bu nedenle, bu çalışmada yumrubaş dizaynı için bir yöntem önerilir. Önerilen yöntem, Kracbt tarafindan önerilen ve yumrubaş dizaym için yeterli olacağı öne sürülen altı yumrubaş parametresi temel alarak ve hiperelliptik fonksiyonlar kullanarak, bîr ilk yaklaşan olarak yumrubaş formunun beliıienmsi şeklindedir. Bu amaç içm, bu çalışmada bir bilgisayar programı hazırlanmıştır. Yumrubaş formunun ofeettaplosu bu programdan kolayca elde edilebilir. Daha sonra, Neuman-Kelvin problemi çerçevesinde, Dawson yöntemi kutlanılarak, yumrubaşsız gemi ile, ana tekneye ilk yaklaşımdan bulunan yumrubaş formu ugulanan geminin direnç karekteristikleri ve dalga formları vs. karşılaştırılır. Bu verilere bakılarak, yumrubaş formunda ve ana tekneye entegrasyonunda gerekli düzehemeler yapılır. Önerilen yöntem seri 60 4ekne-?ormuna uygulandı ve yöntemin yumrubaş dizaym için yararlı olabileceği görüldü. Bu uygulamada, yumrubaşm dalga direncini azalttığı bir kez daha kanıtlanır. Dalga direncinin cismin geometrisine duyarlı olduğu görülür. Fakat hemen belirtelim ki, bu yöntemde elde edilecek yumrubaş formu mevcut yumrubaş formların içinde en iyisidir. Yani, optimal bir sonuç değildir. Seri 60 içm yapılan uygulamada, direnç yönünden mevcut en iyi yumrubaş, dalga direncini yaklaşık %37 azaltır. Dalga direncindeki bu azalmanın ekonomik önemim belirtmeye gerek yoktur. vi
Özet (Çeviri)
. Taylor was the first who recognised the bulbous bow as an elementary device to reduce the wave making resistance. In 1907 be fitted tbe battleship Delaware with a bulbous bow to increase the speed at constant power. In spite of great activities in the experimental field to explore its potential, 70 years had to pass before tbe bulb finally asserted itself as an elementary device in practical shipbuilding. A suitably rated and shaped bulb affects nearly all the properties of a ship. Especially for fast ship, the use of a bulb allows a departure from hitherto accepted design principles for the benefit of a better underwater form. The higher building costs are the only disadvantage.. The protruding bulb form affects hydrodynamteally a variation of the velocity field inme vicinity of the bow, that is, in the region of the rising ship waves. Primarily the bulb attenuates the bow wave system, which usually is accompanied by a reduction of wave resistance. By smoothing the flow around the fbrebody, there is good reason to believe mat the bulb tends to reduce the viscous resistance too. Therefore, the beneficial action of a protruding bulb depends on the size, the position, and the form of the bulb body. The problem of the bulbous bow is a very clear example of the difficulties of transferring theoretical results into practical applications. This is particularly well shown by a historical survey of the treatment of the bulbous bow problem, which brings to light bom serious misunderstandings and also some instances of very fruitful cooperation between theory and practice, and hence between the analytical and empirical points of view. The difficulty for the practising ship designer is mat the easting theories cannot provide any useful quantitative information on the design of a hull form and of a suitable bulbous bow, so mat the qualitative information that is provided has to be checked and verified by empirical means. Nevertheless, in view of the physical relationships mat exist, it is possible to derive some indications from this basic statement which can serve as a guide in the design of bow bulbs; vii1. The size of the bow bulb and its distance from the wafer surface are among the primary factors that affect the amplitude of the wave systems that it 2. The position of the bow bulb relative to the length of the ship affects the phase difference between the wave systems production by the bulb and by the hull. 3. The extent of the wave systems and their interference depends on the Two main bulbs effects which are very important for bulb design are defined as the interference and breaking effects. Tne interference effect expresses the resistance change due to the interfering free wave systems of main hull and bulb. For slender, fast ships, it gives the main proportion to the total bulb effect Its amount depends on bulb volume and the longitudinal position of the bulb center. The wave-breaking effect includes the energy loss by breaking of too steep bow waves and gives the main contribution to the total bulb effect of foil, slow ships. Its amount depends on wen-distributed bulb volume in the longitudinal direction. Bom bulb effects are Froude number dependent Today the bulbous bow has asserted itself as an elementary device in practical shipbuilding. But the existing design methods are not sufficient for modem bulb design. A well-dimensioned bulb improves most of the properties of ship. Therefore, Qualitative and quantitative guiding rules are necessary for its beneficial application. The linearized theory of wave resistance has provided the main contribution to the understanding of the bulb action. But it is itfn^gfeat use for project engineer. In the preliminary stage of his project, he needs fundamental information on which to base concrete decisions. Later, in the realisation stage, the quantitative as well as qualitative guidelines are important, because the hydrodynamic phenomena are not describaWe by a few geometric form parameters alone. For this reason, the mode of action of a bulb and the influence of bulb parameters on resistance or power reduction, respectively, are described hi a qualitative manner; guidelines tor bulb design are also intruducesed. For an adequate presentation of the hydrodynamic properties of bulbs, it is necessary to systematise the different existing bulb forms by means of geometric parameters. Obviously a definitive description of a bulb shape, just as for a slop form, with a finite number of geometric parameters, is impossiblar But a rough classification is possible using only few parameters. With the shape of the cross section A^ of the bulbous bow at the forward perpendicular (FP) as the main criterion, one can differentiate three main bulb vnitypes. &) A-Type: The drop-shaped sectional area A^ of delta-type with the center of the bulb volume near the base. The Taylor bulb and pear- shaped bulbs belong to Oris type. b) O-Type: This type, with an oval sectional area A,, and a center of area in the middle, has a central volumetric concentration. All the circular, elliptical, and lens-shaped bulbs as well as the cylindrical bulbs belong to this type. c) V-Type: The nabla-type also has a drop-sMped sectional area A^, but its center of area is situated in the upper-half part, indicating a volume concentration near the free surface. Because of its favourable seakeeping properties, this type is the most common bulb. With respect to the lateral contour of the bulbous bow, two typical classes are distinguishable: a) The stem outline remains unchanged as with the Taylor bulb. These bulbous bows do not have favorable properties and are no longer built today. b) The stem outline is changed by the protruding bulb as with all modern bulbous bows. In additional to these classification criteria, quantitative bulb parameters are necessaj$J^&iineation of me bulb form. Kracht is of the opinion that six parameters are sufficient for all practical purposes. The three linear and three nonlinear geometric bulb quantities are reduced the bulb parameters, that is, normalized by the main dimensions of the strips, as described in the followings. The three linear bulb parameters are 1) The breadth parameter, that is, the maximum breadth B» of bulb area Abt at the FP divided by the beam B^ of the ship 2) The length parameter, mat is, the protruding length L^ normalised by L^p of the ship. 3) The depth parameter, mat is, the height Z^ of the foremost point of the bulb over the base divided by the draft Tff at the FP The three nonlinear bulb parameters are 1) The cross-section parameter, that is, the cross-sectional area As, of the bulbous bow at the FP divided by the mid-ship -section area A^ of the IXship. 2) The lateral parameter, that is, the area of ram bow A^ in me longitudinal plane normalised by A^ 3) The volumetric parameter, mat is, the volume Vra of the protrudiiig part of the bulb divided by the volume of displacement Vw of the ship. Krach has provided a large number of curves and diagrams to be used as a basis for bulb design and these may be utilized where the block coefficien is betwen 0.56 and 0.82. The curves have been costructedby reanalysing a large number of old model tests results, jp|.this material has been complemented where necessary with new model tests. There are a number of methods available for the design of bulbous bows. But it cannot be said that those methods are sufficient for the design of bulbous bow. Therefore, a new method to design bulbous bow is presented in mis study. The methods consist of two stages: In the first stage, computer aided bulbous bow design is achieved. For mis aim, a computer program, which is based on six parameters define above and uses hiper elliptical functions to determine bulbous bow form, is prepared and given in mis study. The program uses some other parameters, such as, radius of the end of the waterline at the foremost point of the bulb, type of bulb, the apsis of the point that bulbous bow is tangent on the baseline, the ordinate of point of heRgas bulb at the load waterline. Tie form of bulbous bow is determined with mis program, so mat all of the input parameters are entered by designer. Moreover, figures which are usual trend of the parameter which is proposed by Kracht is given in this study. The output of this program is table of ofisets of bulb form. It is clear mat the form of the bulbous bulb determined in the first stage is a usual trend form, and so, this form should be improved. Moreover, mis form has to be checked whether it reduce wave resistances or not For this aim, Dawson's method is proposed. There has been a lot of contribution to the low-Proude number linearization schemes in the wave-making resistance problem, after Dawson (1977) has paved a new way by introducing his free surface condition and numerical algorithm which is also based on Hess and Smith's (1966) method. Wave resistance, wave pattern, pressures and velocities on the body, heave force and trim moment can be calculated with a computer program which is prepared according to Dawson's method. Preparation of input data needs a largeamount of manual work which should be devoted mostly to distribute panels on the hull surface as well as on a portion of the free surface. In the second stage, firstly, wave pattern and wave resistance of the main hall are compared with that of hull with bulb which is determined in the first stage. And then, according to this conciliation, the form of bulb may be altered to improve its wave resistance. The same procedure is carried out for altered forms, and the procedure is to be carried out until sufficient result is obtained This method-applied on Series 60 hull form and it is found that this method is useful for bulbous bow design. From the results, following consolations can be given:. Dawson's method gives acceptable results, and since the results are used relatively, the method is more confidence.. ft is once again proofed that well-shaped bulb can reduce wave resistance.. Wave resistance is sensitive to geometry of the hull. It is found that acceptable results can be obtained, with the proposed method.. The method does not give optimal result But, in order to obtained efficient xssuhY vaiious bulb forms can be searched, and the form mat gives well result can be selected in the existing forms. The form of bulb mat determined by this method reduce 37 percentage of wave resistande. This is a good result Too much emphasis on the percentage gains possible by fitting the bulb could mislead the designer into producing a poor parent form in order to show that large reductions tn power are possible by fitting it with a bulb, instead of refining the form without the bulb first and then fitting it with a bulb which, while it may produce a smaller percentage gain, still leads to a better overall result If results from several investigations are compared care must be taken to ensure mat the basis ofcomparison is the same. Various percentage figures for the gains produced by a bulb can be obtained, depeııdSsg^ whether:. the design speed is equal to or higher then the critical speed,. the tests with and without the bulb ware carried out at the same displacement or at the same draught, ». whether tbe favourable effects produced by bulb ware taken advantage of by trimming the model,. whether the same ship length was used for calculating the fiictional resistance,. whether the same correlation factors were employed when transferring the model results to me full-scale ship,. whether the values that are compared were obtained from resistance or from propulsion tests,. whether the parent form had good or bad resistance properties. m the long run decisive factor for fitting a bulb ais the question of economics. Several authors have stressed this point and have given impressive examples which show quickly the cost of fitting a bulb can be recovered and that is economical to perform careful model tests with the object of producing an optimum bulb design. Ml
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