10:30
Computational Fluid Dynamics IV
Chair: Arthur Veldman
10:30
30 mins
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NUMERICAL ANALYSIS OF DRAG AND HYDRODYNAMIC DERIVATIVES OF NON-SYMMETRIC AUTONOMOUS UNDERWATER VEHICLE USING CFD
Jeonghoon Park, Sunghoon Kim, Moonhwan Kim, Jaeyeop Choi, Yunho Jeon, Myungsub Shin, Younghun Shin, Dongseok Kang, Jiho Choi
Abstract: Generally, hydrodynamic derivatives of underwater vehicle is obtained by basin model test. However, experiments in basin need to much cost and time, nowadays CFD(Computational Fluid Dynamics) is frequently adopted. In this paper, numerical analysis by CFD was performed to calculate drag and hydrodynamic derivatives of autonomous underwater vehicle(AUV), which was developed by LIG Nex1.
The AUV has external parts installed to hull. RF/GPS antenna and communication modem are attached to upper surface, and side scan sonar to both side surface. These appendages can cause additional drag and non-symmetric phenomena of flow around AUV. In this study, we performed resistance, static elevator/rudder and static angle of attack/drift simulations in order to get hydrodynamic coefficients of AUV and verify non-symmetric effects. The results of this paper show that our predictions are correct. Appendages made resistance increase, and elevator, upper rudder and lower rudder had different control force for same fin angle, respectively. The tendencies of static angle of attack simulations were different for directions of angle of attack. The drag as a function of flight velocity and hydrodynamic derivatives were calculated on the basis of the results in this paper and used in review of configuration design, controller design and analysis of flight chracteristics.
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11:00
30 mins
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INVESTIGATION OF NUMERICAL SCHEMES IN AIR CAVITY COMPUTATIONS
Sankar Menon, Rickard Bensow, Claes Eskilsson
Abstract: Air cavity and air chamber concepts have been proven to be an efficient way for drag reduction in low-speed ships. Series of experiments were conducted in the SSPA cavitation tunnel to simulate the working conditions of an air filled cavity under the hull of a ship. In this paper, study is extended with a numerical validation using a CFD Open Source solver, OpenFOAM \textsuperscript{\textregistered} (OF). Volume of fluid (VOF) approach, which uses phase volume fraction ($\alpha$) is used to compute the incompressible two-phase viscous flow. The influence of different numerical methodologies on the advection of $\alpha$ is studied. Different schemes from diffusive first-order to higher order TVD (Total Variation Dimensioning) schemes like SUPERBEE are tested. Results are also drawn from counter-gradient convective flux implementation in OF VOF approch. Conclusions are drawn from the wave profile, wave sloshing pressure force and viscous force. It was observed that, as more compressive interface capturing methods were used, the aft force was better predicted but distorts the wave profile and under predicts the beach plate force.
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11:30
30 mins
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Free Surface Flow and Wave Impact at Complex Solid Structures
Robert Mayon, Zoheir Sabeur, Tan Mingyi, Kamal Djidjeli
Abstract: There is great scientific interest in further understanding the underlying wave impact dynamics on solid and/or permeable structures for coastal defences. The accurate and validated simulation of the dynamics of the flow at microsecond temporal scale prior to, at and after impact is an outstanding and challenging numerical problem in CFD. When more advanced numerical modelling of free surface flow including air entrapment processes is achieved, more insight into the trends of pulse-like forces involved at impact with solid and/or porous material will enable the understanding of the mechanical stability and integrity of defence structures [1]. A comprehensive analysis of wave impact pressure signals at a solid interface was undertaken. The numerical model setup to generate these signals consisted of the classical dam-break test case simulation with the collapsing flow front impacting the interface. Compressible (and incompressible) multiphase simulations were performed using finite volume method to control the advection of the fluid within the domain. The Volume of Fluid method was used to capture the free surface evolution. Multi-modal oscillatory pressure trends were observed during the compressible simulations. The manifestation of these high frequency oscillations coincided with the entrapment of an air bubble as the plunging breaker jet re-entered the free surface of the fluid. These oscillatory signals were subsequently analysed in the frequency domain. Comparison of the frequency spectra results with both analytic methods [2] and experimental data [3] gave good agreement. The observed air bubble formation in the compressible numerical model and associated oscillatory influence on the pressure signal supports previous experimental observations and theories [4] on entrained air bubbles at impact to be the source of such pressure oscillations.
References
[1] Z.A.Sabeur. 1996. A Parallel Computation of the Navier-Stokes Equations for the Simulation of Free Surface Flows with the Volume of Fluid Method, In ‘Applied parallel computing’. Lecture Notes in Computer Science No 1041, Springer-Verlag, pp. 483-492, Edited by J.Dongara and J.Wasniewski.
[2] Minnaert, M. 1933. XVI. On musical air-bubbles and the sounds of running water. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 16, 235-248.
[3] Hattori, M., Arami, A. & Yui, T. 1994. Wave Impact Pressure on Vertical Walls under Breaking Waves of Various Types. Coastal Engineering, 22, 79-114.
[4] Peregrine, D. H. 2003. Water-wave impact on walls. Ann. Rev. Fluid Mech., 35 (2003), pp. 23–43.
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