Serbest su yüzeyine yakın derinlikteki denizaltının manevra sorunlarının incelenmesi
Investigation of maneuver problems of submarines close to the free surface
Yazar: KAĞAN YÜCE
Danışman: DOÇ. DR. DEVRİM BÜLENT DANIŞMAN
İstanbul Teknik Üniversitesi / Lisansüstü Eğitim Enstitüsü / Gemi İnşaatı ve Gemi Makineleri Mühendisliği Ana Bilim Dalı / Gemi İnşaatı ve Gemi Makineleri Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2023
Konu: Gemi Mühendisliği = Marine Engineering

Özet:
Denizaltılar ve diğer sualtı araçları sahip oldukları sınırlı hacim sebebiyle üretilen enerjinin en verimli halde kullanılması gereken araçlardır. Bu konuda araştırmacıları yönlendirmek maksadıyla 1980'li yıllarda Defence Advanced Research Projects Agency (DARPA) tarafından SUBOFF programı başlatılmıştır. Programda 8 farklı konfigürasyonda Anechoic Flow Facility (AFF) modeli oluşturulmuş olup; bu modeller kullanılarak deneysel ve sayısal birçok araştırma yapılmıştır. Son yıllarda insansız sualtı araçlarındaki gelişmeler nedeniyle araştırmacılar sualtı araçlarının form optimizasyonu, manevra iyileştirmesi, serbest su yüzeyi ve deniz dibinin manevraya etkisi gibi konulara yönelmiştir. Bu çalışmada ise periskop derinliğinde keşif ve şnorkel faaliyeti icra eden denizaltıların umkunu sabit tutarken ortaya çıkan manevra sorunu incelenmiştir. Open Field Operation and Manipulation (OpenFOAM) açık kaynak kodlu yazılım ile AFF-8 modeli kullanılarak Hesaplamalı Akışkanlar Dinamiği (HAD) analizleri yapılmıştır. 'SnappyHexMesh' ağ üreticisi ile oluşturulan ağ ile doğrulama çalışması yapılarak uygun ağ modeli seçilmiştir. Çalışmada modelin oluşturulan akış hacminden bağımsız hareket etmesini sağlayan 'overset' ağ metodu kullanılmıştır. Analizlerde serbest su yüzeyinden çapının 1,1 katı derinliğe yerleştirilen modele ait ufki dümenlerin açısının manevraya etkisi incelenmiştir. İlk olarak sıfır dümen açısı ile yapılan analizde denizaltının başlı yönde dalma hareketini yaptığı belirlenmiştir. Denizaltı personelleri ile yapılan görüşmelerde periskop derinliğindeki bu manevra sorununa karşın ufki dümene yukarı yönde açı verildiği ve baş taraftan balast boşaltma yapıldığı öğrenilmiştir. Sayısal analizlerde OpenFOAM yazılımı ile 'Sonlu Hacimler Yöntemi' kullanan 'interFoam' çözücüsünün 'overset' ağ metodunun kullanıldığı 'overInterDyMFoam' çözücüsü kullanılmıştır. Bu çözücü 'Reynolds Averaged Navier Stokes (RANS)' denklemleri ile izotermal ve birbirine karışmayan sıkıştırılamaz iki akışkan için nümerik olarak çözümü gerçekleştirmektedir. 0 dereceden 30 dereceye kadar 7 farklı dümen açısı ile yapılan analizlerde kıç tarafta bulunan ufki dümenin açısı arttıkça kaldırma kuvveti arttığı, artan kuvvet değeri ağırlık merkezine olan mesafeyle birlikte oluşan moment dalma hareketini engelleme yönünde fayda sağladığı ortaya çıkarılmıştır. AFF-8 modelinde ufki dümenlerin sadece kıç tarafta olması, yelken ya da baş tarafta ufki dümen bulunmaması manevra yönünden dezavantaj sağlamaktadır. Ayrıca balast operasyonu ile ağırlık merkezinin yer değişiminin periskop derinliğindeki denizaltıların manevrasına etkisi ayrı bir inceleme konusunu oluşturmaktadır.

Abstract:
Stealth is one of the most effective features of submarines. It must perform its duties at periscope depth without losing this feature. Submarines and other underwater vehicles are the vehicles where the energy produced should be used in the most efficient way due to the limited volume they have. Although the information about these platforms, which are generally used for military purposes, is difficult to access due to confidentiality, many generic submarine forms have been created to date. In addition to experimental studies using these generic submarine forms, numerical studies are increasing rapidly with the development of computer technology.
The SUBOFF program was initiated by DARPA in the 1980s. AFF models in 8 different configurations were created in the program. Many experimental and numerical studies have been carried out using these models. The AFF model used in this study is AFF-8, and it consists of an axisymmetric hull, two horizontal rudders, two vertical rudders and sail. The model created in 1/24 scale has a length of 4,356 meters. In this study, the model was created using the Rhinoceros® program. Computational Fluid Dynamics (CFD) analyzes were performed using the AFF-8 model with OpenFOAM open source software.
The purpose of this study is to examine the maneuvering problem that arises while keeping the depth of the submarines performing reconnaissance and snorkeling activities at periscope depth. First, the appropriate mesh model was selected by verifying the mesh created with the 'snappyHexMesh' mesh generator. This mesh generator is a command for creating a three-dimensional mesh by removing the surface mesh in STL format from the background mesh created with the blockmesh command and adding a surface layer. When this command is run, the 'snappyHexMeshDict' library in the OpenFOAM file directory is used. For the correct solution of the flow around the model, it is necessary to perform the correct mesh casting. The iterative meshing process; it continues according to the quality criteria determined in the 'snappyHexMeshDict' library of the created mesh structure. As a result, the percentage of covering the model of the mesh layer created around the model and the quality of the mesh structure as a result of the check with the 'checkMesh' command are learned. The layer/model coverage ratio in the mesh structures used in the analysis is at least 99%. The flow volume with a flow rate of 5,144 m/s has been sized according to the documents created by International Towing Tank Conference (ITTC). With appropriate initial and boundary conditions, the 'overset' mesh method is used, which allows the model to move independently from the generated flow volume. In the analyzes, the effect of the angle of the horizon rudders on the maneuver of the model, which was placed at a depth of 1.1 times its diameter from the free water surface, was examined. First of all, in the analysis made with zero rudder angle, it was determined that the submarine made the diving motion in the head direction. In the numerical analysis, the 'overInterDyMFoam' solver using the 'overset' mesh method of the 'interFoam' solver using the 'Finite Volume Method' with the OpenFOAM software was used. This solver performs the numerical solution for two isothermal and immiscible incompressible fluids with the 'Reynolds Averaged Navier Stokes (RANS)' equations.
The underwater movements of the submarine with 6 degrees of freedom were investigated with the 'sixDoFRigidBodyMotion' solver. Since we are examining the movements of the submarine at periscope depth, the model movement in the 'dynamicMeshDict' directory is linear only in the longitudinal (-x-axis) and vertical (- z-axis) directions. It is rotationally limited to make fore and aft rotational movement (-y-axis). In this way, the computational cost is reduced. The 'kOmegaSST' turbulence model was used in the calculations.
A validation study was carried out in order to determine that the initial, boundary conditions, mesh structure and solvers that make up the numerical analyzes are correct and to use the most appropriate mesh structure in the parametric study. First, the 'Grid Convergence Index (GCI)' study was carried out, and then the results of the numerical analysis made with the selected mesh structure were compared with the results of the experimental studies. As a result of the verification study, it has been determined that the mesh convergence index of the fine mesh structure with 4 million cells is 1.45%. In order to make comparisons with the experimental data, CFD analyzes were performed at 6 different speeds using the mesh structure obtained by the 'Grid Convergence Index' validation study. In order to minimize the free water surface effect, the model is positioned at a depth of H/D=5.4. As a result of the validation study, the resistance was calculated with an error rate of 3.12% by experimental studies at a flow rate of 5,144 m/s.
In this study, maneuvering problems that occur in submarines that need to cruise continuously at periscope depth were investigated by CFD analysis using the DARPA SUBOFF AFF-8 model. However, it is very difficult to navigate at periscope depth due to the effects of the free water surface effect and the constantly changing weight of the submarines. This situation was determined in the analyzes made with the model positioned at a depth of H/D = 1.1. The number of CFL was kept below 0.7 in time- dependent analyzes. Although submarines make the diving and surfacing movements mainly with ballast operation, they make the pitch movement with the horizon rudders. In the AFF-8 model, the horizontal rudders are at the stern and their distance from the center of gravity determines the direction and magnitude of the force generated on the control surface. Horizon rudders are used to maintain the depth by keeping pitching motion at a minimum when the periscope is at depth. In the analysis made, since the submarines at periscope depth tend to dive upside down, the rudders are controlled in the upward direction in order to create a counter moment to this movement, and the lifting force formed in the downward direction creates a moment in the stern down direction in the center of gravity. In order to parametrically examine the effect of the rudder angle on this movement, CFD analyzes were carried out without changing the center of gravity, simulation time interval and any other parameters.
In the analyzes made with 7 different rudder angles from 0 degrees to 30 degrees, the lift force increases as the angle of the rudder at the stern is increased. It has been revealed in CFD analyzes that the increased force value, together with the distance from the center of gravity, is beneficial in preventing the moment pitch movement. In the interviews with the submarine personnel, it was learned that despite this maneuvering problem at periscope depth, the rudder at the horizon was angled upwards and ballast was discharged from the bow. As a result of the study, it has been determined that the periscope depth model is beneficial in maintaining the umk depth with the moment created by the upward angle given to the stern horizon rudders. The model's horizontal rudders are at the stern and there is no horizon rudder in the bow or sail, which creates a disadvantage in maintaining the depth of the horizon. Considering that the surface area of the rudders also has an effect on the lifting force, the rudder surface area constitutes a different research topic on this subject.
In this study, the maneuvering problem that arises during the exploration and snorkeling activities of submarines at periscope depth close to the free water surface has been investigated. During this maneuver, it has been numerically determined that submarines cannot keep their umk and tend to dive upside down due to their form. In order to eliminate this maneuvering problem, submarines use their horizon rudders to take measures to create a buoyant force on the nose. However, it has been determined in the analyzes that the measure taken creates an additional resistance force on the submarine; this will increase fuel consumption in the first place.