International Marine & OffShore Engineering Conference DEVELOPMENTS of HYDRODYNAMICS MODELING TECHNIQUES for UNDERWATER VEHICLES SERTAC ARSLAN, Asst. Prof. HAYRI ACAR and Prof. MEHMET SERIF KAVSAOGLU International Marine & Offshore Engineering Conference 2014 CONTENTS INTRODUCTION & MOTIVATION DEFINITION OF THE HYDRODYNAMIC COEFFICIENTS METHODOLOGY OF THE CFD ANALYSIS PROCESS VERIFICATION OF HYDRODYNAMIC ANALYSIS MODEL HYDRODYNAMIC ANALYSIS of AUTOSUB UNDERWATER VEHICLE CONCLUSION International Marine & Offshore Engineering Conference 2014 MOTIVATION In this project, new hydrodynamic modeling techniques are developed and all hydrodynamic coefficients of an AUV are calculated in six degree of freedom (6 DOF), by using the developed calculation methods. Firstly, a new hydrodynamic calculation model for underwater vehicles is created and verified. After the verification of CFD solution model, hydrodynamic database of an AUV is constituted and then static stability characteristics are determined. International Marine & Offshore Engineering Conference 2014 INTRODUCTION In recent years, AUVs have become a main tool for surveying below the sea in the scientific, military and commercial applications because of the significant improvement in their performance. In order to design an AUV, it is usually necessary to analyze its hydrodynamics performance, maneuverability and controllability characteristic. International Marine & Offshore Engineering Conference 2014 INTRODUCTION To be examined AUV's performance in the simulation environment; hydrodynamic equations of motion should be established, then hydrodynamic force and moments values should be fed to the equations of motion , and simulation studies should be performed in required maneuvering scenarios. International Marine & Offshore Engineering Conference 2014 INTRODUCTION The hydrodynamic coefficients may be classified into 2 main groups. Static hydrodynamic coefficients Dynamic hydrodynamic coefficients Also, dynamic coefficients may be classified into 3 subgroups such as; Linear damping coefficients (maneuvering coefficients), Linear inertial force coefficients (added mass, inertia coefficients) Nonlinear damping coefficients These coefficients can be obtained by different ways. These are experimental methods, numerical methods (CFD) and empirical methods. International Marine & Offshore Engineering Conference 2014 INTRODUCTION Rotating-Arm Mechanism Planar Motion Mechanism Vertical Planar Motion and Rotating-Arm Mechanism are the most common experimental techniques to measure the hydrodynamic coefficients. Static coefficients, maneuvering coefficients and added mass-inertia coefficients can be calculated in these mechanisms. Using experimental techniques to calculate the hydrodynamic coefficients is a very expensive method. Instead of experimental methods, numerical (computational fluid dynamics) and empirical techniques can be developed and used. International Marine & Offshore Engineering Conference 2014 DEFINITION OF THE HYDRODYNAMIC COEFFICIENTS The coefficients, which mostly affect the maneuverability of an AUV, are static and the linear damping (maneuvering) coefficients. A rectangular Cartesian coordinate system attached to the vehicle. The three components of the hydrodynamic force along the directions x, y, z are denoted by X, Y, Z respectively, and the three components of the hydrodynamic moments by L, M, N. International Marine & Offshore Engineering Conference 2014 DEFINITION OF THE HYDRODYNAMIC COEFFICIENTS The three components of force X, Y, Z and the three components of the moments L, M, N are expanded up to second order terms in the linear and angular velocities u, v, w, p, q, r and the coupled of them. And added mass coefficients should also be taken into consideration. International Marine & Offshore Engineering Conference 2014 DEFINITION OF THE HYDRODYNAMIC COEFFICIENTS Added mass can be modeled as some volume of fluid moving with the object. So, added mass coefficients must be considered and added to the equation of motions. For underwater vehicles added-mass coefficients are constant and independent of maneuvering and wave effects. In the added mass matrix the diagonal terms are always positive, and those terms are dominant to dynamics of vehicle. If the vehicle is symmetrical, the terms except diagonal terms can be neglected. International Marine & Offshore Engineering Conference 2014 METHODOLOGY OF THE CFD ANALYSIS PROCESS In this study, time-independent hydrodynamic coefficients of an AUV will be calculated and hydrodynamic database of the AUV will be created. Previous of the calculation of hydrodynamic coefficients of AUV, hydrodynamic modeling method will be created and verified by a benchmark study. In this study, 3D solid modeling program CATIA, ANSYS GAMBIT and TGRID meshing programs and ANSYS FLUENT commercial program is used. International Marine & Offshore Engineering Conference 2014 VERIFICATION OF HYDRODYNAMIC ANALYSIS MODEL In order to calculate hydrodynamic coefficients correctly by using CFD methods, Benchmark study has to be done. In order to evaluate the capabilities and accuracy of the tools used in this project, DARPA SUBOFF model has been selected as test case. DARPA model was designed at David Taylor Research Center in order to provide a forum for the CFD community to compare the numerical computations of the flow field over an axisymmetric hull model with and without sail Length: 4 m and appendages. Diameter: 0,508 m International Marine & Offshore Engineering Conference 2014 TEST AND ANALYSIS CONDITIONS The data used in the present study are based on the experiments which were performed in the David Taylor Model Basin on Towing Carriage by David Taylor Research Center . Three configurations of DARPA SUBOFF model are studied and compared with available test results in this report. Bare hull Hull with sail Hull with sail and appendages The static stability experiments were conducted at zero angle of attack and a model speed of 9m/s which correspond to a Reynolds number of about 30 million. In the calculation of Reynolds Number, kinematic viscosity of the water is accepted as 1,307e-06 m2/s. International Marine & Offshore Engineering Conference 2014 INVESTIGATIN OF TEST CASE MODELS 1.Bare Hull Model Bare hull configuration of DARPA model consists of nose, body and aft-body parts. 2. Hull with Sail Model Hull with sail configuration of DARPA model consists of nose, body, aft-body, and sail parts. 3. Hull with Sail and Appendages Model Hull with sail configuration of DARPA model consists of nose, body, aft-body, sail and four appendages parts. International Marine & Offshore Engineering Conference 2014 GRID GENERATION 3D grid generation is used for the all configurations. In order to constitute the grid of the AUV GAMBIT 2.4 software is used. Grids are created in high quality to use in Navier-Stokes equations. To define the boundary layer over the Autosub geometry TGRID 5.0 software is used. The boundary layer is created as two zones. These are inner zone and outer zone. Y+ value is chosen 1 for this study. International Marine & Offshore Engineering Conference 2014 GRID GENERATION At the end of grid generation process, vehicle’s grid and fluid domain’s grid is completed, and the geometry get ready to make hydrodynamic analyses in CFD software. In this study, Navier-Stokes equations are solved with following conditions, Segregated solver model, K-ε Realizable turbulence model, Y+ ~ 1 is chosen. International Marine & Offshore Engineering Conference 2014 RESULTS AND VERIFICATION OF CFD ANALYSES Three different configurations of DARPA model is analyzed in this study. Analyses are done in 9m/s speed and 20 m depth. Analysis Results of Bare Hull Model Experimental results of pressure and friction coefficients are compared with CFD analysis results. International Marine & Offshore Engineering Conference 2014 RESULTS ANS VERIFICATION OF CFD ANALYSES Analysis Results of Hull with Sail Model Experimental results of pressure coefficients and friction coefficients over the hull and sail are compared with CFD results for this configuration. International Marine & Offshore Engineering Conference 2014 RESULTS ANS VERIFICATION OF CFD ANALYSES Analysis Results of Hull with Sail and Appendages Model Pressure coefficients measurement results exist for this configuration, in three stations on the appendages. These stations which presents on the appendages, are located in distance of 10%, 50% and 90% of the total appendages length, respectively. Comparison of pressure coefficients results of appendages stations which is calculated in CFD analyses with experimental results. 25% of the chord is accepted as zero point in the graphics. International Marine & Offshore Engineering Conference 2014 RESULTS ANS VERIFICATION OF CFD ANALYSES International Marine & Offshore Engineering Conference 2014 HYDRODYNAMIC ANALYSIS of AUTOSUB AUTONOMOUS UNDERWATER VEHICLE (AUV) Autosub is a large AUV developed by a team of engineers and oceanographers at the National Oceanography Centre, Southampton, UK. Autosub has been employed in scientific research projects ranging from mapping manganese’s distributions in a sea. Autosub is controlled by four movable control surface mounted at the rear of the vessel in a cruciform arrangement. The control surfaces’ cross sections consist of NACA 0015 hydrofoil. International Marine & Offshore Engineering Conference 2014 ANALYSIS CONDITIONS In this study, a comprehensive study has been made to constitute hydrodynamic database of Autosub AUV in six degree of freedom (6DOF). X, Y, Z forces and K, M, N moments of Autosub is calculated for different maneuvering conditions. Velocity levels: 5, 10, 15, 20 and 25 knots. Angle of attacks: 10, -7, -5, -2, 0, 2, 5, 7, and 10 Sideslip angels: 10, -7, -5, -2, 0, 2, 5, 7, and 10 Elevator deflections: 0, ±5, ±10, ±15, ±20 degrees Rudder deflections: 0, ±5, ±10, ±15, ±20 degrees Turn Radius: 20, 30, 40 Operating Reynolds Number is between: 1,27e+07 - 6,52e+07. Kinematic viscosity of the water is accepted as 1,38e-06. International Marine & Offshore Engineering Conference 2014 GRID GENERATION OF AUTOSUB 3D grid generations are used for all analyses. In order to constitute the grid of the Autosub, GAMBIT 2.4 software is used. Quite quality mesh is used for suitable solution with Navier-Stokes Equations. In order to obtain significant result and to be time efficient node numbers in the grids are optimized. International Marine & Offshore Engineering Conference 2014 ANALYSIS RESULTS OF AUTOSUB In this study, hydrodynamic database of Autosub AUV is constituted in 6DOF by using verified CFD calculation model for underwater vehicles. Around the critical Reynolds number, the stability characteristic of fluids is unstable. It is shown that, the flow regime where Autosub cruise in is far away from the critical Reynolds number and stable. Increment of drag force is compatible with velocities Variation of the axial forces with angle of attacks is too much in high speeds and very little in low speeds. International Marine & Offshore Engineering Conference 2014 ANALYSIS RESULTS OF AUTOSUB Yaw force decrease with sideslip angle according to velocity levels. Variation of pitching moment is investigated in 10 knots cruise condition. Pitching moment increase with angle of attack for different elevator deflections. Trim conditions of Autosub in pitch plane can also be checked. International Marine & Offshore Engineering Conference 2014 ANALYSIS RESULTS OF AUTOSUB Longitudinal static stability can be investigated according to CM-α graphics. 𝑆𝑡𝑎𝑡𝑖𝑐 𝑆𝑡𝑎𝑏𝑖𝑙𝑖𝑡𝑦 𝐶𝑟𝑖𝑡𝑒𝑟𝑖𝑎 => 𝜕𝐶𝑀 <0 𝜕𝛼 Autosub has statically unstable geometry. This is because of the center of gravity point is the behind of center of pressure point of Autosub. Center of gravity point distance to the nose of the geometry is 3.159 meter but, the resultant hydrodynamic force point is in front of the center of gravity. In the Autosub geometry, aft-body part produces a negative lift; in this situation the center of the pitch moment moves to front of the vehicle and this situation pushes forwards the center of pressure point in front of the center of the gravity point, so that the geometry becomes unstable. International Marine & Offshore Engineering Conference 2014 CONCLUSION In the scope of this study, dynamic stability analyses and performance analyses of Autosub are also performed. After the stability analyses, it can be said that, these underwater geometries such as Autosub are usually statically unstable but dynamically stable. Because of the specific damping effects of underwater worlds such as cross coupled terms, added mass terms and munk moments, underwater vehicles stability characteristics get stable. In the hydrodynamic analyses, it can be said that; Reynolds Averaged Navier Stokes (RANS) equations are solved without any problem and the verified CFD calculation models can be used to calculate any of hydrodynamic coefficients in 6 degree of freedom. In this project; a new numerical and empirical hydrodynamic calculation method is developed to calculate hydrodynamics coefficients of Autosub AUV in 6DOF. Calculated numerical results are compared with the experimental results and it is seen that, calculated results are close to experimental results adequately, to rely on performing hydrodynamic analyses with CFD methods. In this project, a new hydrodynamic analysis method is developed and verified for underwater vehicles. By using the verified analysis models, hydrodynamic performance and characteristic of any underwater vehicles can be investigated and determined. International Marine & Offshore Engineering Conference 2014 REFERENCES (1) Cindy, C. W., “Steady and unsteady force and moment data on a DARPA submarine,” Master of Science in Aerospace Engineering, Department of Aerospace and Ocean Engineering, Virginia Polytechnic Institute and State University. (2) Data Sheet, Overview of Model Testing Facilities, NTNU. (3) Fossen, T., I., 1994: “Guidance and Control of Ocean Vehicles, Wiley, H., Sons, 5-90. (4) Gertler, M., Grant, R. H., 1967: Standard Equations of Motion for Submarine Simulations. (5) Huang, T., Liu, H-L, Groves, N., Forlini, T., Blanton, J., Growing, S., 1994: Measurement of Flows over an Axisymmetric Body with Various appendages in a Wind Tunnel: the DARPA Suboff Experimental Program, David Taylor Model Basin, Washington, USA. (6) Jones, D. A., Clarke, D. B., Brayshaw, I. B., Barillon, J. L, and Anderson B., 2002: The Calculation of Hydrodynamic Coefficients for Underwater Vehicles, Maritime Platforms Division/Platform Sciences Laboratory, Australia. (7) Jun, B. H., Park, J. Y., Lee, F. Y., Lee, P. M., Lee, C. M., Kim, K., Lim, Y. Y. and Oh, J. H. (2009). Development of the AUV ISIMI and a free running test in an Ocean Engineering Basin, Ocean Engineering Research Department, Republic of Korea. (8) Nancy, C. G., Thomas, T. H., Ming, S. C,. 1989: Geometric Characteristic of Darpa Suboff Model, Ship Hydromechanics Department, David Taylor Research Center, USA. (9) Phillips, A., Furlong, M., Turnock, S. R., The Use of Computational Fluid Dynamics to Determine the Dynamic Stability of an Autonomous Underwater Vehicle, National Oceanography Center, Southampton, England. (10) http://www.km.kongsberg.com (11) http://www.noc.soton.ac.uk (12) http://web.mit.edu/towtank/www/ International Marine & Offshore Engineering Conference 2014 THANKS… ANY QUESTIONS? International Marine & Offshore Engineering Conference 2014
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