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2. Numerical Assessment of Flow Around Circular Cylinder

Author

Lekkala, Malakonda Reddy, Latheef, Mohamed, Kim, Do Kyun, Wahab, Mubarak Bin A., di Prisco, Marco, Series Editor, Chen, Sheng-Hong, Series Editor, Vayas, Ioannis, Series Editor, Kumar Shukla, Sanjay, Series Editor, Sharma, Anuj, Series Editor, Kumar, Nagesh, Series Editor, Wang, Chien Ming, Series Editor, Mohammed, Bashar S., editor, Shafiq, Nasir, editor, Rahman M. Kutty, Shamsul, editor, Mohamad, Hisham, editor, and Balogun, Abdul-Lateef, editor

Published
2021
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5. Recent advances in understanding the flow over bluff bodies with different geometries at moderate Reynolds numbers

Author

Lekkala, Malakonda Reddy (author), Latheef, Mohamed (author), Jung, Jae Hwan (author), Coraddu, A. (author), Zhu, Hongjun (author), Srinil, Narakorn (author), Lee, Byung Hyuk (author), Kim, Do Kyun (author), Lekkala, Malakonda Reddy (author), Latheef, Mohamed (author), Jung, Jae Hwan (author), Coraddu, A. (author), Zhu, Hongjun (author), Srinil, Narakorn (author), Lee, Byung Hyuk (author), and Kim, Do Kyun (author)

Abstract

The application of unsteady incompressible flow phenomenon over the bluff bodies has received the attention of many researchers due to the rich and complex physics underpinning these flows, and thus requiring special attention in their modelling and numerical simulations. The wake that forms at the leeside of the bluff body is of particular interest. Reynolds number and geometry are in turn two prominent parameters that govern the formation and subsequent behaviour of this wake. This paper reviews the wake formations for different cylindrical bluff bodies cross-sections such as circular, elliptical, helically twisted elliptical, symmetric wavy, asymmetric wavy, and harbor seal vibrissae cylinders. Alongside the Reynolds number and geometrical shape, the impacts of rotational rate, aspect ratio, angle of attack, and gap ratio between the cylinder and the bottom wall on the hydrodynamic coefficients, Strouhal number, recirculation length and suppression of vortex shedding in the cylinder wake are investigated. In addition, the variation of hydrodynamic coefficients for different cylindrical shapes are compared. Finally, concluding remarks are drawn based on recent advances in understanding the flow features and predictions with CFD methods., Ship Design, Production and Operations

Published
2022
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7. Numerical Simulation of VIV of Risers with Staggered Buoyancy Elements

Author

Lekkala, Malakonda Reddy

Subjects
numerical simulation, riser, buoyancy elements, staggered, offshore teknologi, marin og undervannsteknologi, undervannsteknologi, Technology: 500::Marine technology: 580::Offshore technology: 581 [VDP], VIV
Abstract

Master's thesis in for Offshore technology: Marine and subsea technology Steel Lazy Wave Riser (SLWR) is an attractive deep water riser system design, which allows the platform motion to be decoupled from the touchdown point (TDP) of the riser. When subjected to external flow, both buoyancy element and bare riser section may experience Vortex Induced Vibrations (VIV). Such vibrations are the results of the periodic hydrodynamic forces that are induced by the interaction of slender bodies and external fluid flow. If the vibration period is close to the natural period of the system, it can lead to fast accumulation of fatigue damage to the risers and amplified drag loads. The vortex shedding frequency of the buoyancy element is lower than that of the bare riser section due to its larger diameter. These two shedding process will interact and influence the VIV responses. Such interaction depends on many parameters, e.g. Buoyancy element dimensions and their arrangement etc. Semi-empirical VIV prediction programs, such as VIVANA, SHEAR7 and VIVA are the most commonly used industrial VIV prediction tools. These programs rely on hydrodynamic force coefficient database generated from forced motion test with a rigid cylinder section. However, these database may not be valid for a flexible cylinder with buoyancy elements. Therefore, there are strong needs to obtain force coefficient database, taking into account the influences of the riser and buoyancy element interaction. In present work, the existing riser with staggered buoaycny VIV model test is reviewed. The prediction using VIVANA with default hydrodynamic force coefficient database is evaluated. A new way of obtaining an optimal database directly from VIV tests of a flexible cylinder with staggered buoyancy element is investigated. The hydrodynamic force coefficient database is parameterized and the representative parameters are systematically varied until the predicted frequency, mode and fatigue damage agree with experimental results. The improvements and uncertainties are also discussed.

Published
2016

9. Prediction of Combined Inline and Crossflow Vortex-Induced Vibrations Response of Deepwater Risers.

Author

Jie Wu, Lekkala, Malakonda Reddy, Muk Chen Ong, Passano, Elizabeth, and Voie, Per Erlend

Subjects
*OCEAN engineering, *ENGINES
Abstract

Deepwater risers are susceptible to vortex-induced vibrations (VIV) when subjected to currents. When responding at high modes, fatigue damage in the inline (IL) direction may become equally important as the crossflow (CF) components. Accurate calculation of both IL and CF responses is therefore needed. Empirical VIV prediction programs, such as VIVANA "Passano et al. (2016, "VIVANA--Theory Manual Version 4.8," Trondheim, Norway)," SHEAR7 "(Vandiver, J. K., and Li, L., 2007, "Shear7 v4.5 Program Theoretical Manual," Department of Ocean Engineering, Massachusetts Institute of Technology, Cambridge, MA)," and VIVA "Triantafyllou et al. (1999, "Pragmatic Riser VIV Analysis," Offshore Technology Conference, Houston, TX, May 3-6, Paper No. OTC-10931-MS.)" are the most common tools used by the offshore industry. Progress has been seen in the prediction of CF responses. Efforts have also been made to include an IL load model in VIVANA. A set of excitation coefficient parameters were obtained from rigid cylinder test and adjusted using measured responses of one of the flexible cylinder VIV tests. This set of excitation coefficient parameters is still considered preliminary and further validation is required. Without an accurate IL response prediction, a conservative approach in VIV analysis has to be followed, i.e., all current profiles have to be assumed to be unidirectional or acting in the same direction. The purpose of this paper is to provide a reliable combined IL and CF load model for the empirical VIV prediction programs. VIV prediction using the existing combined IL and CF load model in VIVANA is validated against selected flexible cylinder test data. A case study of a deepwater top tension riser (TTR) has been carried out. The results indicate that VIV fatigue damage using two-dimensional directional current profiles is less conservative compared to the traditional way of using unidirectional current profiles. [ABSTRACT FROM AUTHOR]

Published
2019
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