• The Journal of Korean Institute of Electromagnetic Engineering and Science
• /
• v.29 no.1
• /
• pp.28-31
• /
• 2018

This paper investigates the ground reflection effect on the channel capacity of an orbital angular momentum (OAM) multi-mode transmission system that uses uniform circular array (UCA) antennas. The ground-reflected signals cause inter-mode interferences between the OAM modes, and lead to system performance degradation. The OAM multi-mode channel capacity severely degrades owing to the ground reflection as the transmission distance increases. Increasing the UCA height above the ground and using highly directive array elements can mitigate the ground reflection effect and increase the channel capacity.

• Journal of Advanced Marine Engineering and Technology
• /
• v.34 no.5
• /
• pp.703-710
• /
• 2010

Recently, small hydropower attracts attention because of its clean, renewable and abundant energy resources to develop. However, suitable turbine type is not determined yet in the range of small hydropower and it is necessary to study for the effective turbine type. Therefore, a 80kW-Class cross-flow turbine is adopted in this study because of its simple structure and high possibility of applying to small hydropower. The result shows that as effective head increases, tangential and radial flow velocities increase and thus, the increased tangential velocity contributes to the increase of angular momentum and output torque.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.36 no.1
• /
• pp.48-56
• /
• 2008

Nowadays, CMG(Control Moment Gyros) becomes one of the essential actuators for satellite attitude control. The method to define the key requirements of CMG is suggested to avoid CMG's singularity problem for the limited envelope of angular momentum of 2H. Furthermore, the analysis and simulation are carried out to provide a necessary guideline when three CMGs are used for spacecraft control purpose. An improved configuration of redundant four CMG cluster, slightly different from the conventional configuration, is proposed not only to avoid the CMG singularity problem, but to improve agility about roll or pitch-axis.

• Proceedings of the Korean Society of Propulsion Engineers Conference
• /
• 2008.05a
• /
• pp.115-118
• /
• 2008

Many theoretical and experimental studies have been conducted to investigate elements of the hydrodynamic process, such as variations in liquid film thickness or air core diameter. From these studies, some theoretical relationships have been established through an approximated analytical solution of flow hydrodynamics in a swirl nozzle. However, experimental studies on elements such as internal flow have not produced conclusive results. In this study, the variations and stability of the internal flow were examined by visualizing the air core and measuring the liquid film thickness.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.21 no.6
• /
• pp.793-803
• /
• 1997

This experimental study was mainly investigated on the swirl flow characteristics in the cylinder generated by a helical intake port. LDA system was used for the measurement of in-cylinder velocity fields. Tangential and axial velocity profiles, with varying valve lifts, valve eccentricity ratios and axial distance, were measured. When the intake valve was set in the cylinder center, we could find that in-cylinder swirl flow fields were composed of a forced vortex motion and a free vortex motion in the vicinity of the cylinder center and the cylinder wall respectively. In case of valve eccentricity ratio, N$_{y}$ = 0.45, the vortex flow which rotates to the opposite direction of a main rotating flow in the cylinder was found. And the reverse flow toward the cylinder head surface was also found in axial velocity profile and it showed the tendency of the linear decrease in the region of 0.leq.Y/B.leq.1.2.2.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.30 no.1 s.244
• /
• pp.67-73
• /
• 2006

This investigation deals with the spin-up flows in a circular container of aspect ratio, 2.0. Shear front is generated in the transient spin-up process and propagating from the side wall to the central axis in a rotating container. Propagation of the shear front to the axis in a rotating container means the region acquires an angular momentum transfer from the solid walls. Propagating speed of the shear front depends on the apparent viscosity of polymer solution. Two kinds of polymer solutions are considered as a working fluid: one is CMC and the other is CTAB solution. CMC solution has larger apparent viscosity than that of water, and CTAB shows varying apparent viscosities depending on the applied shear rates. Transient and spatial variations of the apparent viscosities of the present polymer solutions (CTAB and CMC) cause different speeds of the propagating shear front. In practice, CMC solution that has larger values of apparent viscosity than that of water always shows rapid approach to the steady state in comparison of the behavior of the flows with water. However, for the CTAB solution, the speed of the propagating of the shear front changes with the local magnitude of its apparent viscosity. Consequently, the prediction of Wedemeyer's model quantitatively agrees with the present experimental results.

• Journal of the Korean Society of Propulsion Engineers
• /
• v.12 no.6
• /
• pp.30-37
• /
• 2008

Many theoretical and experimental studies have been conducted to investigate elements of swirl injector hydrodynamics, such as variations in liquid film thickness or air core diameter. From these studies, some theoretical relationships have been established through an approximate analytical solution of flow hydrodynamics in a swirl nozzle. However, experimental studies on elements such as the stability of internal flow have not produced conclusive results. In this study, the stability of the internal flow under tangential entry conditions was examined by visualizing the formation of the air core in the swirl chamber and measuring the liquid film thickness in the orifice.

• Korean Journal of Applied Biomechanics
• /
• v.14 no.1
• /
• pp.65-81
• /
• 2004

The purposes of this study were to determine the functions of actions of the limbs during each of the three support phases of the triple jump and their relationships with the performance of the triple jump. Four elite male triple jumpers were participated as subjects. The Pearson product moment correlation coefficient were used to determine and compare the relationships between the change in each component of the normalized angular momentum of the whole body about center of gravity and the actions of the extremities during different support phases. A level of significance at $\alpha$=.05 was set. After analyzing the angular momentum and correlation during support phase of the hop, step, and jump, the following findings are obtained: The actions of the arms created a side-somersaulting angular momentum about the whole body center of gravity toward the side of the free leg during the support phase of the step, and a somersaulting angular momentum about the whole body center of gravity during each support phase. The action of the free leg created a somersaulting angular momentum about the whole body center of gravity during the support phases of the hop and step.

• Journal of Astronomy and Space Sciences
• /
• v.6 no.1
• /
• pp.17-28
• /
• 1989

The classical solar wind theory proposed by Parker(1963) explains well the dynamics of the wind pheonomena such as stellar wind and accretion disk. While the stellar wind system like the solar wind has the spherically symmetric wind structure, there are various jet phenomena which collimate the system into the narrow space. We can find these dynamical structures in SS433, in the optical jet of M87, and around the active galactic nulei. We present the dynamical structure of the jet system in disks, which conserves the angular monetum, with the Keplerian rotation and the strong relation between the geometrical cross section and the physical change of the jet stream on the basis of the hydrodynamic equations.

• Korean Journal of Applied Biomechanics
• /
• v.13 no.1
• /
• pp.185-204
• /
• 2003

The purpose of this study was to obtain the data for stable and accurate techniques of the free throw in basketball. The subjects of this study were seven male basketball player consisted of college students athletes. Free throw motions were taken by video camera. The three-dimensional coordinates was processed by DLT. The variables were the velocity, the angular velocity of the upper extremity segments, degree, and angular momentum. The result of analysis is summarized as follows. 1. The velocity and angular velocity of the upper extremity segment was showed an gradual increase and a smooth velocity transfer, transferring from proximal segment to distal segment at free throw motion in basketball. 2. The local term and remote term angular velocity momentum of the proximal segment showed larger than that of the distal segment in X, Y, Z axis component all. 3. The remote term angular momentum was showed larger than that of the local term angular momentum in X, Y, Z axis component all. 4. The angular motion of the upper trunk and upper arm, upper arm and forearm was showed in opposite direction and symmetrical angular momentum in local term angular momentum of the Y and Z axis component. 5. All the segments of upper extremity segment was showed left rotation in remote term angular momentum of the Y axis component and right rotation in remote term angular momentum of the Z axis component.