Gemi yapılarının vibro-akustik yaklaşımı ile titreşim ve akustik açısından optimum hale getirilmesi
Acquiring vibrationally and acoustically optimum ship structure through the vibroacoustic methodology
Yazar: MEHMET AVCU
Danışman: PROF. DR. İSMAİL AHMET GÜNEY
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Makine Mühendisliği Ana Bilim Dalı / Makine Mühendisliği Bilim Dalı
Derece: Doktora, 2018
Konu: Deniz Bilimleri = Marine Science ; Makine Mühendisliği = Mechanical Engineering

Özet:
Deniz taşımacılığında artan talebi karşılamak için ihtiyaç duyulan ticari gemi boyut ve hızlarındaki artış gemide yapısal titreşim problemlerine neden olmaktadır. Bunun yanında, ticari gemilerle yapılan deniz taşımacılığı deniz gürültüsünün büyük oranda artmasına sebep olmaktadır. Gemilerdeki gürültü ve titreşimlerin doğurduğu sonuçlar çeşitlidir. Gemide görev yapan ve/veya seyahat eden personel/yolcular üzerinde olduğu gibi limanda bulunduğu ortama da konforu azaltıcı etki yapmaktadır. Ayrıca, artan gemi trafiği nedeniyle artan sualtı akustik gürültüsü deniz canlı yaşamının ekolojik dengesini bozabilmektedir. Benzer konfor azaltıcı etkiler savaş gemileri için de önemini korumaktadır. Bugün savaş gemisi ve denizaltıların başka gemilerin konumlarını saptaması ve onlara hücum edebilmeleri için güçlü sonar sistemlerine sahip olmaları gerekirken diğer gemiler tarafından fark edilmemek ve kendi sonar sistemlerinin performansını etkilememeleri için de mümkün olduğunca düşük akustik ize sahip olmaları gerekmektedir. Bu sebeple bir savaş gemisinin inşa edilmeden daha tasarım aşamasında sualtı akustik izinin hesaplanabilmesi ve tasarımın bu veriler ışığında düzenlenmesi son derece önem arz etmektedir. Gemilerde titreşim ve gürültü problemlerinin çözüm kaynağının erken safhadaki tasarım aşamasında belirlenmesinin önemi ve sonradan yapılacak olan düzeltmelerin çok ağır maliyetler gerektirdiği bilinmektedir. 
Hem ticari gemilerin hem de savaş gemilerinin tasarım aşamasında gürültü ve titreşim açısından uyması zorunlu ulusal veya uluslararası standartlar ile ilgili ülkenin Loyd kuralları, bir sistem/ekipmanın minimum gürültü/titreşim sağlayacak şekilde gemi bünyesine entegrasyonuna yönelik detaylı uygulama yöntemleri belirtmesine rağmen, geminin vibro-akustik yapısal konstrüksiyonunun nasıl olması gerektiğine ilişkin çok fazla bilgi vermemektedir. Bu tezde yapılan kapsamlı çalışma ile, bir geminin yapısal konstrüksiyonunun yapısal titreşim ve sualtı akustiğinin azaltımını sağlayacak şekilde iyileştirilmesi/optimum hale getirilmesine yönelik olarak, gemilerin tasarımı aşamasında kullanılabilecek alternatif bir vibro-akustik hesaplama yöntem ve metodoloji yaklaşımı sunulmuştur. Aynı yaklaşımın gemilere benzer şekilde tasarlanan diğer platformlar ile yapısal konstrüksiyonu önem arz eden mekanik sistemlerin titreşim ve gürültü açısından iyileştirilmesi amacıyla kullanılabilmesi de mümkündür. 
Tez çalışmasına vibro-akustik hesaplama yöntem ve teorilerinin irdelenmesi ile başlanacaktır. Takip eden bölümde, gemi konstrüksiyonunu temsilen seçilen bir araştırma gemi yapısı, gürültü/titreşim kaynak yükleri ile modellenerek tüm frekans bandında gemiye ait yapısal titreşim ve sualtı akustik gürültü değerlerinin İstatiksel Enerji Analizi (SEA) ve Sonlu Elemanlar Metodu (SEM) yöntemi ile (test/ölçüm değerleri ile doğrulanmış olarak) tespit edilmesi sağlanacaktır. Ardından, sualtı akustik izin azaltılması maksadıyla literatürde yapısal titreşim iletim yollarının belirlenmesi için yeni kullanım alanı bulan graf teorisi gemi yapılarına bu anlamda ilk defa uygulanacaktır. Bu amaçla, sualtı akustik gürültünün azaltılmasına yönelik olarak optimum gemi yapısının tespit edilmesi için, dominant transfer iletim yolları (SEA graf yaklaşımı ile) graf teorisi metodu (MPS algoritması) referans çalışmalar baz alınarak yeniden geliştirilecektir. Son olarak, sualtı akustik izin azaltılmasını sağlayacak ve mukavim tekne özelliklerini de kaybetmeyecek optimum (pasif kontrol) yapısal konstrüksiyon belirlenecektir. Bu maksatla, graf teorisi MPS algoritması ile tespit edilen dominant enerji transfer iletim hatları üzerinde belirlenen yapısal konstrüksiyon elemanlarını kullanacak şekilde bir parametrik yaklaşım geliştirilerek, optimum akustik gemi yapısı tasarım çözümleri belirlenecektir. Bu yapı çözümleri, tez kapsamında daha önceden oluşturulmuş olan global gemi SEM ve SEA modellerine uygulanarak optimum akustik yapısal konstrüksiyonun sualtı akustik gürültüde azalım sağladığı gösterilecektir.

Abstract:
The increase of the commercial ship size and speed which is needed to meet the increasing demand in the sea transportation causes structural vibration problems onboard. In addition, the sea transportation with the commercial ships contributes significantly to ambient noise in the ocean. The consequencies that are resulted from the noise and vibration onboard vary. The increasing underwater noise due to the increasing ship traffic may disrupt the ecological balance of sea life as well as adversely effect on ship crew habitability and/or passenger comfort and the ambient noise at the port. Same discomforting influences are significant also for warships.  Nowadays, to detect the positions of other enemy ships and attack them, warships and submarines need powerful sonar systems. Besides that, not to be detected by other ships and to avoid the performance decrease of their own sonar systems, warships and submarines require as low acoustic signature as possible. Thus, to be able to determine the underwater noise signature of a warship at the design stage before shipbuilding is one of the most significant facts of today's warship design. The importance of the solutions of noise and vibration problems which are addressed at the earliest design stage and the great cost increase in later correction efforts are obvious. 
Both commercial ships and warships shall obey the national or international standards and national Loyd rules regarding the noise and vibration at the design stage. Although they contain the detailed practical methods which define the ship-structure-to-systems interface solutions ensuring the minimum noise/vibration transmission through the ship, standards and rules do not give adequate information about the ship vibroacoustic structural configuration. In this thesis, an alternative vibroacoustic predicting methodology approach to be used during the ship design phase has been proposed, which allows one to determine the vibrationally and acoustically optimum ship structure. It is considered that this approach may be applicable for other platforms and mechanical systems, whose structural construction is significant, to improve their vibration and noise vibroacoustic behavior.
The vibroacoustic behavior of a physical system can be modelled using several numerical methods. One of the most common measures used to categorize them is the frequency range, i.e., low-frequency, mid-frequency and high-frequency. Since each frequency range has its own characteristics and has a limited scope of validity, modelling strategies to resemble the vibroacoustic response of a system depends on the frequency range used. In the low-frequency range, deterministic approaches like Finite Element Method (FEM) and Boundary Element Method (BEM) can be used to determine the vibration modes; this is likely because there are few modes and they are well-separated. Nevertheless, in the high-frequency range, modal density and modal overlap are high and the physical system's vibroacoustic behavior is more sensitive to slight changes in the physical parameters due to the range's short wavelengths. Therefore, since these variations must be considered, predicting the response of a single system via deterministic techniques becomes meaningless. Instead, calculating the average responses of a physical system is more rational. Consequently, statistical approaches like the Statistical Energy Analysis (SEA) are the commonly used techniques for this frequency range.
Because there is a large frequency gap between low- and high-frequency ranges, the mid-frequency range is too diverse for an exact description of the system's vibroacoustic response. The vibroacoustic characteristics of the system may fall in between low- and high-frequency ranges, or the system may not present uniform dynamic behavior. To tackle this mid-frequency range problem, several numerical methods and strategies like the Wave Based Methods (WBM), the Asymptotical Scaled Modal Analysis (ASMA), the Energy Distribution Methods, the Statistical modal Energy distribution Analysis (SmEdA), the MODal Energy Analysis (MODENA) approach, the radiative energy transfer methods, the dynamic energy analysis and the FEM-SEA hybrid methods have been developed.
Numerical methods can also be used to solve noise and vibration problems seen in the automotive and marine industries. Usually, these problems consist of a vibroacoustic source that generates an extreme energy level in the other part of the system, normally named as target or receiver. For instance, in a ship's structure, the diesel engine may interfere with the comfort levels of the crew's living and working spaces on the upper level decks. To resolve these quality issues, some parts of the system will need to be modified to decrease the energy level of the target to an adequate level. Analysis methods like FEM, BEM and SEA are used to predict the vibroacoustic response of a system at a given frequency band. Depending on the method used to model the vibroacoustic response, quantities such as acoustic pressure or, structural vibration velocity can be determined. The results yield information about the vibroacoustic behavior of the system, but does not directly provide a solution for the part of the system that needs to be modified. By investigating the system response during postprocessing, a general overview (of the modifications) is provided, reviewed and applied. This procedure does not guarantee the best modification to reduce energy levels of the targeted system component. 
Monte Carlo numerical experiments may be employed to find components that need modifications. The experiment defines thousands of unique scenarios with random and slightly modified parameters; each parameter is kept within acceptable values and the best parameters that yield the best results are kept. This technique has a high computational cost without guaranteeing the best or optimal solution for the system. One of the other approaches is to combine known optimization algorithms with sensitivity analyses. Then the optimization is achieved in two-stages; the objective function is minimized or maximized in the first stage and the parameters which are the most influential in the variation of the results are determined in the second stage. 
These optimization approaches look for solutions that provide the best results, but they do not attempt to discover the origin of the problem. One of the key issues when modelling the vibroacoustic behavior of a system is specifying how energy is transmitted from a source, where the external energy is known, to a target, where the resulting energy is determined and needs to be reduced. The experimental methods traditionally used to solve these types of vibroacoustic energy minimization problems are known as Transmission Path Analysis (TPA) and Operational Transfer Path Analysis (OTPA) techniques. In the automobile industry, this experimental technique is used extensively; but for the marine sector, the relatively large number of parameters and complexity of the ship structure result in many measurement difficulties.
Additionally, the energy transmission paths can be found numerically, and the SEA method is at the core of the path determination strategy. A graph is a set of elements that share pairwise connections. Furthermore, a SEA model consists of subsystems and the interactions of power flow between them. Therefore, the theoretical similarities between a graph and a SEA system can be found in the premise of each method. The problem of reducing the energy of a set of target subsystems in a SEA model could be solved in the general framework of graph theory. A SEA model can obtain more information by establishing the connection between graph theory and the SEA method via a SEA graph for noise and vibration control purposes.
While determining the vibroacoustic behavior and energy transmission paths for a system, another issue related to noise and vibration control arises when ranking source-to-target energy transmission paths. Ranking the energy transmission paths, i.e., determining the relative importance of these paths, is significant because a small set of dominant paths can carry a high percentage of the overall energy. Ranking energy transmission paths requires a systematic path search in a vibroacoustic system model and a systematic dominant-energy path-ranking approach. 
The graph theory-based systematic procedure for ranking the dominant energy paths developed before will be combined with a SEA graph approach using the path-detection algorithm. Adapting this procedure for optimizing vibroacoustic behavior will be one of the main goals of this dissertation. Another goal is to extend the use and performance of this approach for investigating the vibroacoustic behavior of a ship's structure. 
The acoustic performance of noise and vibration sources can be improved by using adequate and resilient mounting that has passive, semi-passive or active control damping mechanisms. To mitigate the vibration and noise problems related to underwater noise, and to improve the comfort of onboard personnel, the ship's structural design can be optimized to minimize structural vibration. Since this passive control mechanism can be carried out at the structural design phase of a ship, the ship can be improved acoustically without the need for expensive and costly remedial work on a completed vessel. Thus, Maximum Path Search (MPS) algorithm adapted for structural ship design in this dissertation can be regarded as a complementary method to optimization techniques like Monte Carlo or parametric methods.
This dissertation starts focusing on vibroacoustic predicting methods and theories.  In the next section, a research ship structure selected to represent the typical ship structure is numerically modelled with all noise/vibration sources and then, its structural vibration and underwater noise are provided by using SEA and FEM methods (validated with the test/measurement values) in the whole frequency range. Later, the usage of the graph theory, which is newly started to be used for finding the structure-borne energy transmission paths, is extended for the ship structures at the first time to reduce the underwater noise signature. For this purpose, to acquire the optimum ship structure providing the reduced underwater noise signature, the systematic procedure (MPS algorithm) for ranking the dominant energy transmission paths developed before by using graph theory and SEA graph approach are reconstituted again by using the same algorithm and validating it. Finally, the optimum (passive control damping mechanism) structural configuration is determined, which provides the reduced underwater noise signature and keeps the ship structural strength capabilities. For this purpose, a parametric approach, which modifies the ship structural elements belonging to the dominant energy transmission paths found with MPS algorithm, is proposed and, the optimum acoustical ship structural design solutions are determined using this parametric approach. Then, it is shown that the optimum acoustical ship structure ensures the reduced underwater noise by applying these structural solutions to the global ship FEM and SEA models developed before in the thesis.