• Journal of Ocean Engineering and Technology
• /
• v.27 no.5
• /
• pp.16-21
• /
• 2013

The flow around a circular cylinder advancing beneath the free surface is numerically investigated using a VOF method. The simulations cover Froude numbers in the range of 0.2~0.6 and gap ratios (h/d) in the range of 0.1~2.0, where h is the distance from the free surface to a cylinder, and d is the diameter of a cylinder at Reynolds number 180. It is observed that the vortex suppression effect and surface deformation increase as the gap ratio decreases or the Froude number increases. The most important results of the present study are as follows. The proximity of the free surface causes an initial increase in the Strouhal number and drag coefficient, and the maximum Strouhal number and drag coefficient occur in the range of 0.5~0.7. However, this trend reverses as the gap ratio becomes small, and the lift coefficient increases downward as the gap ratio decreases.

• Journal of the Society of Naval Architects of Korea
• /
• v.50 no.5
• /
• pp.324-333
• /
• 2013

Recently, rudder erosion due to cavitation has been frequently reported on a semi-spade rudder of a high-speed large ship. This problem raises economic and safety issues when operating ships. The semi-spade rudders have a gap between the horn/pintle and the movable wing part. Due to this gap, a discontinuous surface, cavitation phenomenon arises and results in unresolved problems such as rudder erosion. In this study, we made a rudder model for 2-D experiments using the NACA0020 and also manufactured gap flow blocking devices to insert to the gap of the model. In order to study the gap flow characteristics at various rudder deflection angles($5^{\circ}$, $10^{\circ}$, $35^{\circ}$) and the effect of the gap flow blocking devices, we carried out the velocity measurements using PIV(Particle Image Velocimetry) techniques and cavitation observation using high speed camera in Seoul National University cavitation tunnel. To observe the gap cavitation on a semi-spade rudder, we slowly lowered the inside pressure of the cavitation tunnel until cavitation occurred near the gap and then captured it using high-speed camera with the frame rate of 4300 fps(frame per second). During this procedure, cavitation numbers and the generated location were recorded, and these experimental data were compared with CFD results calculated by commercial code, Fluent. When we use gap flow blocking device to block the gap, it showed a different flow character compared with previous observation without the device. With the device blocking the gap, the flow velocity increases on the suction side, while it decreases on the pressure side. Therefore, we can conclude that the gap flow blocking device results in a high lift-force effect. And we can also observe that the cavitation inception is delayed.

• School Mathematics
• /
• v.10 no.3
• /
• pp.357-374
• /
• 2008

The complex number system is supposed to introduce first chapter in the first grade of high school. When number system is expanded to complex numbers, the main aim is to understand preservation of algebraic structure with regard to the flow of curriculum and textbook. This research reviewed overall alternative articulation and treatment of textbooks from a logical viewpoint. Two research questions are developed below. First, in the structure of the current curriculum, when we consider student's 'level', how are the alternative articulation and treatment of textbooks in complex unit on a logical point of view? Second, What are more logical alternative articulation and treatment? What alternative articulation and treatment are suitable for a running goal? and what are the improvement which is definitive?

• Journal of Korean Society of Forest Science
• /
• v.89 no.5
• /
• pp.543-551
• /
• 2000

This study was conducted to understand Pinus densiflora regeneration characteristics from canopy gaps due to disturbance in natural forests located at Mt. Joongwang in Kangwon-do. The line-transect method was adopted to analyze crown architecture of Pinus densiflora. The saplings of Pinus densiflora in the canopy gap showed high adaptation to growth environment at their early regeneration stage, and showed different characteristics in crown architecture. Variation of branching angle in the main branch of Pinus densiflora was small in canopy gap. Primary branch growth showed was high during 4~5 year-old period and slowly low after that period. Average 5 of primary branch were generated from stem in canopy. Average 4 of secondary branch were generated from primary branch in canopy gap. Primary branches generated from the stem were uniformly distributed at all cardinal directions. When canopy gap size is $100{\sim}120m^2$, secondary branches generated from primary branch had mostly high numbers between $S44^{\circ}E$ and $S90^{\circ}W$.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.23 no.9
• /
• pp.1192-1200
• /
• 1999

The present study investigates flow characteristics in a cavity with one rotating disk and co-rotating disks for application to HDD. The experiments are conducted for rotating Reynolds numbers of $5.5{\times}104$ to $1.10{\times}105$ and for gap ratios of 0.059 to 0.175 in a single rotating and 0.047 to 0.094 in co-rotating disk. Time-resolved velocity components and turbulence intensity on the rotating disks are obtained by using LDA measurements. Detailed Knowledge of the flow characteristics is essential to analyze flow vibration and heat transfer and to design head-arm assembly and hub height in HDD. The results indicate that the velocity field in HDD is changed largely by the rotating Reynolds numbers and hub height of the disk.

• Journal of Korean Society of Industrial and Systems Engineering
• /
• v.27 no.3
• /
• pp.66-74
• /
• 2004

In this paper the authors presents a new method, named FR-SERVQUAL, of evaluating perceived service quality in Public Sectors, using triangle fuzzy numbers and semantic differential scale. By conventional quantification methods, it is not easy to express the notion of a linguistic variables and customers' subjective judgements. In contrast to the conventional PZB methods which express the customers' perception of quality as a function of gap between the expected and perceived service, this paper suggests to use the ratio of the two. Through an application example, this paper shows that the current FR-SERVQUAL approach provides a more realistic way of measuring service quality compared to existing methods.

• The Journal of the Korea Contents Association
• /
• v.6 no.2
• /
• pp.162-168
• /
• 2006

This paper presents a high performance CORDIC circuit based on redundant numbers yielding a minimal number of iteration stages. The minimal number of iteration stages reflects the iteration number yielding a smaller computation error than the truncation error. The minimal number of iterations is found n-4 for $n\geq16$, where n is the number of input angle bits. The CORDIC circuit is based on a redundant number system with a constant scale factor The circuit performs sine and cosine calculations with a delay of {5 (n-4)+ 2[$log_{2}n$]}$\DeltaT$.

• The Transactions of The Korean Institute of Electrical Engineers
• /
• v.58 no.3
• /
• pp.509-515
• /
• 2009

When a squirrel cage induction motor is loaded, the magnetic noise can increase depending on the load current. It is due to the variation of air gap harmonic fluxes from the rotor current induced by loading. This unfavorable noise can be anticipated by analysing the radial force waves in the air gap, the mode shapes of them, and stator core natural frequencies at each mode. With the experimental tests with the different rotor slot numbers, the variation of magnetic noise depending on the load current is studied and the method to reduce the magnetic noise is suggested with the newly developed magnetic noise analysis program.

• The Transactions of The Korean Institute of Electrical Engineers
• /
• v.56 no.4
• /
• pp.773-776
• /
• 2007

A microstructure array has been proposed for micro plasma generation using electroplating and double exposed process. A stable atmospheric plasma has been generated at a low voltage by utilizing the micro electrode gap. Self-aligned microstructure can provide uniform electrode overlap with precisely controlled gap between the electrodes. The proposed structure allows for triode operation, which can expand the generated plasma over a large area by applying a lateral electric field. Electrical characteristics of the micro triode confirm the large numbers of the plasma ions are drifted to the secondary cathode by the lateral electrical field.

• Proceedings of the KIEE Conference
• /
• 2008.05a
• /
• pp.147-148
• /
• 2008

In this paper, we calculated the axial magnetic field at mid-gap plane between upper and lower electrode in vacuum interrupter by means of commercial finite element method Maxwell 3D and compared on the basis of "effective area" criterion. The models used in this paper are coil type(axial magnetic field) vacuum interrupter electrodes which have different numbers of coil segment. We used Dr. Schulmann's experimental equation which indicates minimum critical value of axial magnetic field to diffuse arc.