• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.21 no.11
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• pp.1013-1019
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• 2011

Sound attenuation of periodically arranged cylindrical rods is studied numerically and experimentally. Cross section of the cylinder is circular and arrays are in a square lattice. Cylinders are made of steel, and consist of five groups with different diameters from 27.2 mm to 48 mm. Each group has 5 rows, while number of cylinders in a row varies from 17 to 31. The area filling fraction is about 60~61 %, which leads to the stop bandgap(2.9 kHz ~ 8.4 kHz). Sound attenuation is computed using two-dimensional BEM, and measurement is done by using a speaker and microphones in a semi-anechoic room. Comparison of the results by BEM and experiment shows that attenuation spectra are qualitatively in agreement, although experiment gives higher attenuations than BEM. After results by BEM are scaled up in accordance with cylinder diameter, it is observed that attenuation curves are in good agreement, which confirms that analysis by BEM is done correctly. It is also found that the measured bandgaps are shifted toward lower frequency by 0.5 kHz ~ 1.2 kHz, when compared to the predictions obtained from infinitely repeated two-dimensional cylinder arrays.

• International Journal of Precision Engineering and Manufacturing
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• v.9 no.4
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• pp.32-38
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• 2008

This paper describes a fast tool control (FTC)-based diamond turning process for fabricating large-area high-quality micro-lens arrays. The developed FTC unit has a stroke of $48{\mu}m$ and a resonance frequency of 4.9 kHz. Micro-lens arrays were fabricated using a micro-cutting tool with a nose radius of $50{\mu}m$. The FTC unit was integrated with a force sensor so that the initial position of the micro-cutting tool with respect to the workpiece surface could be detected through monitoring the contacting force. The length and depth of the designed parabolic micro-lens profile were $190{\mu}m$ and $20{\mu}m$, respectively. A micro-lens array was fabricated on a cylinder surface over an area of ${\phi}55 mm{\times}40 mm$.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.22 no.11
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• pp.1603-1609
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• 1998

Numerical solutions of Stokes flow through periodic arrays of cylinders were sought using Darcy's law and homogenization theory. Drag and lift forces of each cylinder were computed for various attack angles and pitch-to-diameter ratios. It was found that drag force decreased as principal pressure gradient direction deviated from array direction and that drag force increased exponentially as pitch-to-diameter ratio approached unity. Similar tendency was found in lift force except that lift force increased and then decreased in quadratic manner as attack angle varied.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2011.04a
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• pp.508-509
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• 2011

• Journal of Ocean Engineering and Technology
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• v.36 no.2
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• pp.132-142
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• 2022

We investigate the performance of multi-body wave energy converters (WECs). This investigation considers multiple scattering of water waves by the buoys of a WEC under the generalized mode approach. Predominantly, the effect of a WEC's configuration on its energy extraction is studied in this research. First, single-row terminator and single-column attenuator arrays of vertical cylinders have been studied. The performance of these attenuator arrays shows that the wall effect induced by the periodic buoys influences the wave propagation and energy extraction in these WECs. Further studies show that a single-row terminator array of vertical cylinders performs better than the corresponding single-column attenuator array. Subsequently, multi-row terminator arrays of vertical cylinders are investigated by conducting a parametric study. This parametric study shows that the hydrodynamic property of three resonance phenomena makes energy extraction efficiency drop down, and the magnitude of energy extracted oscillates between the resonance points in these WECs. Finally, a 4×8 terminator array of vertical cylinders is studied to determine the effect of various d_x (x-directional distance between adjacent rows) within this WEC on its performance. In particular, this study enforces at least two equal d_x values within the 4×8 terminator array of vertical cylinders. It shows that a small value of this d_x leads to better energy extraction efficiency in some of these various d_x arrays than that of a corresponding regular array with the same d_x.

• International Journal of Naval Architecture and Ocean Engineering
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• v.7 no.5
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• pp.906-919
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• 2015

The Deep Draft Semi-Submersible (DDS) concepts are known for their favourable vertical motion performance. However, the DDS may experience critical Vortex-Induced Motion (VIM) stemming from the fluctuating forces on the columns. In order to investigate the current-induced excitation forces of VIM, an experimental study of flow characteristics around four square-section cylinders in a square configuration is presented. A number of column spacing ratios and array attack angles were considered to investigate the parametric influences. The results comprise flow patterns, drag and lift forces, as well as Strouhal numbers. It is shown that both the drag and lift forces acting on the cylinders are slightly different between the various L/D values, and the fluctuating forces peak at L/D = 4.14. The lift force of downstream cylinders reaches its maximum at around ${\alpha}=15^{\circ}$. Furthermore, the flow around circular-section-cylinder arrays is also discussed in comparison with that of square cylinders.

• Journal of Ocean Engineering and Technology
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• v.27 no.5
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• pp.51-62
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• 2013

The water wave interactions on any three-dimensional structure of arbitrary geometry can be calculated numerically through the use of source distribution or Green's function techniques. However, such a method can be computationally expensive. In the present study, the water wave interactions in floating circular cylinder arrays were investigated numerically using the eigenfunction expansion method with the three- dimensional potential theory to reduce the computational expense. The wave excitation force, added mass coefficient, radiation damping coefficient, and wave run-up are presented with the water wave interactions in an array of 5 or 9 cylinders. The effects of the number of cylinders and the spacing between them are examined because the water wave interactions in floating circular cylinder arrays are significantly dependent upon these.

• Journal of Ocean Engineering and Technology
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• v.1 no.2
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• pp.28-38
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• 1987

Experimental studies are conducted for the wave forces acting on the vertically mounted circular piles in the waves. Two-three-cylinder arrays are equally spaced and the spacings(S/D) as well as the incident angles of various waves are changed to study their separate effects on the wave forces. The numerical results based on the diffraction theory are in good agreement with the experimental results, and the diffraction theory well predicts the trend of the wave forces when the spacings and the incident angles are changed.

• Journal of Ocean Engineering and Technology
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• v.19 no.1 s.62
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• pp.8-13
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• 2005

In a multiple array of vertical cylinders, flaw patterns are very complex and very interactive between cylinders. The patterns are turbulent and non-linear depending on various factors. The gap and flow incoming velocity of upstream can affect on the downstream cylinder. In this study, the flaw characteristics around two vertical cylinders are investigated numerically and experimentally. As the gap between cylinders is changed at fixed coming velocity, the pressure distributions around cylinders are observed and compared by experimental and numerical approaches. The F.D.M and multi-block method are applied in the study. The pressures at 12 points around the cylinder are measured in the experiment. The results can be applied in the understanding and design of multiple pile array structures.

• The Proceeding of the Korean Institute of Electromagnetic Engineering and Science
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• v.5 no.2
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• pp.46-54
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• 1994

In this paper, an analysis and design method is proposed, which is to improve the directivity of microstrip array antennas conformed to a cylindrical surface. In the case of forming an arc-array in the circumferential direction on a circula-cylinder surface, the circular-cylinder can be approximated to a polygonal-pillar and on each pillar-planes the sub-arrays, Dolph-Tschebyscheff array and uniform array with a beam steered in the desired direction, would make a sharp directivity for the total cylin- drical array antenna. And the radiation pattern according to the type of its sub arrays is analyzed and compared using the cylindrical-cavity codel. A cylindrical microstrip array antenna, with 12 elements and uniform arra as a sub-array which have an equal distance$\lambda_0$/2between the elements, is manufactured and conformed to a cylinder with radius of 6 The measured data of side lobe level, HPBW and FNBW are - 13dB, $9^{\circ}$, and $15{\circ}$, ,respectively. This result shows a good improvement on the directivity comparing with a linear array.