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Volume 13, Number 2, June 2023

Through-the-thickness stress distribution in a tubular member has a profound effect on the fatigue behavior of tubular joints commonly found in steel offshore structures. This stress distribution can be characterized by the degree of bending (DoB). Although multi-planar joints are an intrinsic feature of offshore tubular structures and the multi-planarity usually has a considerable effect on the DoB values at the brace-to-chord intersection, few investigations have been reported on the DoB in multi-planar joints due to the complexity of the problem and high cost involved. In the present research, data extracted from the stress analysis of 243 finite element (FE) models, verified based on available parametric equations, was used to study the effects of geometrical parameters on the DoB values in two-planar tubular DYT-joints. Parametric FE study was followed by a set of nonlinear regression analyses to develop six new DoB parametric equations for the fatigue analysis and design of axially loaded two-planar DYT-joints.

Key Words
degree of bending (DoB); fatigue; offshore jacket structure; two-planar tubular DYT-joint

Hamid Ahmadi: Centre for Future Materials, University of Southern Queensland, QLD 4350, Australia
Faculty of Civil Engineering, University of Tabriz, Tabriz 5166616471, Iran;
Center of Excellence in Hydroinformatics, University of Tabriz, Tabriz 5166616471, Iran
Mahdi Ghorbani: Faculty of Civil Engineering, University of Tabriz, Tabriz 5166616471, Iran

In this paper, Moore-Gibson-Thompson theory of thermoelasticity is considered to investigate the fundamental solution and vibration of plane wave in an isotropic photothermoelastic solid. The governing equations are made dimensionless for further investigation. The dimensionless equations are expressed in terms of elementary functions by assuming time harmonic variation of the field variables (displacement, temperature distribution and carrier density distribution). Fundamental solutions are constructed for the system of equations for steady oscillation. Also some preliminary properties of the solution are explored. In the second part, the vibration of plane waves are examined by expressing the governing equation for two dimensional case. It is found that for the non-trivial solution of the equation yield that there exist three longitudinal waves which advance with the distinct speed, and one transverse wave which is free from thermal and carrier density response. The impact of various models (i)Moore-Gibson-Thomson thermoelastic (MGTE)(2019), (ii) Lord and Shulman's (LS)(1967) , (iii) Green and Naghdi type-II(GN-II)(1993) and (iv) Green and Naghdi type-III(GN-III)(1992) on the attributes of waves i.e., phase velocity, attenuation coefficient, specific loss and penetration depth are elaborated by plotting various figures of physical quantities. Various particular cases of interest are also deduced from the present investigations. The results obtained can be used to delineate various semiconductor elements during the coupled thermal, plasma and elastic wave and also find the application in the material and engineering sciences.

Key Words
fundamental solution; Moore-Gibson-Thompson thermoelastic model; photothermoelastic isotropic; plane waves; steady oscillations

Rajneesh Kumar and Anil K. Vashishth: Department of Mathematics, Kurukshetra University, Kurukshetra, Haryana, India
Nidhi Sharma: Department of Mathematics, Maharishi Markandeshwar University Mullana, Ambala, Haryana, India
Supriya Chopra: Department of Mathematics, Maharishi Markandeshwar University Mullana, Ambala, Haryana, India;
Department of Mathematics, Government College for Women, Ambala city, Haryana, India

This study investigates the influence of loading and inflow conditions on tidal turbine performance from a hydrodynamic and hydroacoustic point of view. A boundary element method is utilized for the former to investigate turbine performance at various loading conditions under zero/non-zero yaw inflow. The boundary element method is selected as it has been selected, tested, and validated to be computationally efficient and accurate for marine hydrodynamic problems. Once the hydrodynamic solutions are obtained, such as the time-dependent surface pressures and periodic motion of the turbine blade, they are taken as the known noise sources for the subsequence hydroacoustic analysis based on the Ffowcs Williams-Hawkings formulation given in a form proposed by Farassat. This formulation is coupled with the boundary element method to fully consider the three-dimensional shape of the turbine and the speed of sound in the acoustic analysis. For validations, a model turbine is taken from a reference paper, and the comparison between numerical predictions and experimental data reveals satisfactory agreement in hydrodynamic performance. Importantly, this study shows that the noise patterns and sound pressure levels at both the near- and far-field are affected by different loading conditions and sensitive to the inclination imposed in the incoming flow.

Key Words
boundary element method; Ffowcs Williams-Hawkings (FW-H) formulation; hydroacoustics; marine current turbine; renewable energy

Seungnam Kim and Spyros A. Kinnasa: Ocean Engineering Group,
Department of Civil, Architectural and Environmental Engineering, The University of Texas at Austin,
Austin, TX, 78712, U.S.A.

Reliable prediction of the motion of FOWT (floating offshore wind turbine) and associated mooring line tension is important in both design and operation/monitoring processes. In the present study, a 5MW OC4 semisubmersible wind turbine is numerically modeled, simulated, and analyzed by the open-source numerical tool, OpenFAST and in-house numerical tool, Charm3D-FAST. Another commercial-level program FASTv8-OrcaFlex is also introduced for comparison for selected cases. The three simulation programs solve the same turbine-floater-mooring coupled dynamics in time domain while there exist minor differences in the details of the program. Both the motions and mooring-line tensions are calculated and compared with the DeepCWind 1/50 scale model-testing results. The system identification between the numerical and physical models is checked through the static-offset test and free-decay test. Then the system motions and mooring tensions are systematically compared among the simulated results and measured values. Reasonably good agreements between the simulation and measurement are demonstrated for (i) white-noise random waves, (ii) typical random waves, and (iii) typical random waves with steady wind. Based on the comparison between numerical results and experimental data, the relative importance and role of the differences in the numerical methodologies of those three programs can be observed and interpreted. These comparative-study results may provide a certain confidence level and some insight of potential variability in motion and tension predictions for future FOWT designs and applications.

Key Words
MW OC4 semisubmersible; free-decay tests; random waves; responses and tensions; second-order wave forces; simulation vs experiment; turbine-hull-mooring coupled dynamics


Jieyan Chen and Moo-Hyun Kim: Department of Ocean Engineering, Texas A&M University, College Station, TX 77084, USA
Chungkuk Jin: Department of Ocean Engineering and Marine Science, Florida Institute of Technology,
Melbourne, FL 32901, USA

This paper proposes a method for the acoustic imaging wherein the traditional requirement of the relative movement between the transmitter and target is overcome. This is facilitated through the beamforming acoustic array in the transmitter, in which the target is illuminated by the array at various azimuth and elevation angles without the physical movement of the acoustic array. The concept of beam steering of the acoustic array facilitates the formation of the beam at desired angular positions of azimuth and elevation angles. This paper substantiates that the combination of illumination of the target from different azimuth and elevation angles with respect to the transmitter (through the beam steering of beam forming acoustic array) and the beam steering at multiple frequencies (through SF) results in enhanced reconstruction of images of the target in the underwater scenario. This paper also demonstrates the possibility of reconstruction of the image of a target in underwater without invoking the traditional algorithms of Digital Image Processing (DIP). This paper comprehensively and succinctly presents all the empirical formulae required for modelling the acoustic medium and the target to facilitate the reader with a comprehensive summary document incorporating the various parameters of multi-disciplinary nature.

Key Words
acoustic impedance; Inverse Synthetic Aperture Radar (ISAR); Inverse Synthetic Aperture SONAR (ISAS); Near-Field Beam Forming (NF-BF); Synthetic Aperture Radar (SAR); Synthetic Aperture SONAR (SAS); Underwater Acoustic Imaging (UAI); underwater environment

L.S. Praveen, Govind R. Kadambi and S. Malathi: Ramaiah University of Applied Sciences, Bangalore, India
Preetham Shankpal: GE Health science, Bangalore, India

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