Jo May 4, 2023
Fort on Mt. Taesong is the largest and oldest of all the fortresses on mountains of Koguryo.
Located in Taesong-dong, Taesong District, Pyongyang City, it was built to defend Anhak Palace, the royal palace of Koguryo from the 3rd to 5th century.
The fort is surrounded by wide and narrow rivers on three sides ― east, west and south ― and the north is linked to a mountain range, so it is very advantageous in terms of military affairs, economy and transportation.
It is 7 076 metres round and the total length of the walls runs to 9 284 metres. It could hold a huge number of people, combat equipment and supplies as it had two vast deep valleys between the six mountain peaks linked to one another. It had food storages, armouries, quarters, general’s terraces, gates, etc.
Pyramid-shaped stones were laid in single-sided piles on the peaks and in double-sided piles across the valleys. Double and treble walls were built on important spots. On the top of the walls were parapets, low zigzag walls, and on important points were bastions to strengthen the defense of the fort.
Abundant in water resource, the fort had as many as 170 ponds including a carp pond and Jangsu Pond. It had two gates ― Somunbong Gate and South Gate, which was 5 kan × 2 kan ( 1 kan = 5.965 feet) in size and 19.5 metres in height.
In the fort, a lot of relics and remains like Koguryo roofing tiles, gilt-bronze images of Buddha, stone boxes for keeping Buddhist scriptures, etc. were discovered, and many legends including the story of a Rok tribe lady associated with the fort have been handed down.
Many relics including some parts of the wall, Jangsu Pavilion, South Gate and Jangsu Pond are now restored to the original state to play an important part in the study of Koguryo’s fortress system.
Fort on Mt. Taesong with strong defences, which served as a reliable one in the fights against foreign invaders, was an impregnable fortress that had hardly been found in the world history of fortress construction.
Ko Ju Il, researcher at the Academy of Social Sciences
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Jo May 2, 2023
U Yong Nam, a section head at the Robotics Institute, has made a study of the mobile properties of an omnidirectional AGV with Mecanum wheels.
A Mecanum wheel, which is designed to move a vehicle in any direction, consists of a series of rollers attached to its circumference.
ADAMS was used for the simulation, where he analyzed the motion in X and Y directions, turning round on the position and oblique.
The simulation result showed that the velocity error in the X-direction movement and turning round on the position of the AGV is small and stabilization is fast, and vice-versa in the Y-direction movement and oblique.
The results verified that an AGV with four Mecanum wheels can operate in a static state.
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Jo Apr 30, 2023
“Koryosajolyo” is a book where the history of Koryo, the first unified state of our nation, is recorded in order of year, month and date.
“Koryosa” (History of Koryo), which was written in 1451, touches a huge amount of materials divided into categories, so it was inconvenient to learn the contemporary history chronologically at a glance.
That is why after the completion of “Koryosa”, a more concise one “Koryosajolyo” was compiled.
“Koryosajolyo” was completed in 1452 by Kim Jong So, Jong Rin Ji et al. who had worked at “Koryosa”, and published in the following year 1453.
The title “Koryosajolyo” means “the main parts of Koryo history” and it consists of 35 volumes, which is about a quarter of “Koryosa” in 139 volumes.
In the book is recorded the history of the feudal Koryo dynasty in order of reign of the 34 kings from the founder king to King Kongyang, and the events and facts during their reigns are arranged in yearly, monthly and daily order.
It gives a large amount of information about the feudal Koryo State ― politics including the state machinery, ranks of government posts, punitive system, anti-aggression and anti-feudal struggle; economy including the land system and tax system; culture including education and art; military affairs including the military system, national defense, etc.; nature including astronomy, geography, natural disaster, etc.; information about individuals that made contributions to the development of science and culture.
Although it reveals some shortcomings due to the class restrictions of the compilers, “Koryosajolyo” serves as an important national classic legacy that shows the history of the Koryo since “Koryosajolyo” and “Koryosa” constitute key documents to the research into the history of the Koryo dynasty by compensating each other’s defects.
Ri Won Chol, researcher at the Academy of Social Sciences
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Jo Apr 28, 2023
A research team led by Kim Sung Ri, a section head at the Faculty of Mechanical Science and Technology, has carried out a CFD simulation on a vertical turbine flowmeter with air and hydrogen and compared the meter factors. Then, they have conducted research on improving its starting characteristic in a low flowrate domain.
Generally, the meter factors of a turbine flowmeter for different densities of gases are different.
To fix this problem, they carried out a CFD simulation on a vertical turbine flowmeter that employs air and hydrogen as working media, and analyzed the effect of the density of gas on meter factors.
First, they created a geometrical model and mesh, and performed a CFD simulation by ANSYS Fluent using an SST κ-ω turbulence model. The result showed that as the density of air is much higher than hydrogen, the zero-load rotational speed of air is much higher than that of hydrogen for the same flowrates, and in contrast, the meter factor of air is smaller than that of hydrogen. It also showed that the meter factor of hydrogen had nonlinearity in a low flowrate domain, while the meter factor of air was almost constant over the whole flowrate domain.
Second, they carried out CFD simulations on vertical turbine flowmeters with 2 and 6 inlet channels, respectively, and analyzed the effect of the number of inlet channels on the starting characteristic. The result showed that in case of 2 channels, the zero-load rotational speed in a low flowrate domain was much higher than in case of 6 channels and the meter factor had linearity.
In conclusion, adjusting the area of an inlet channel is a good method for improving the performance of gas flowmeters, and in particular, this method is the most effective for measuring low-density gas in a low flowrate domain.
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Jo Apr 27, 2023
Many crashing accidents of helicopters and multicopters are attributable to the propellers plunging into a vortex ring state. Therefore, safe operation of a helicopter or a multicopter needs a thorough study on the vortex ring of a propeller and the vortex ring state.
Though many studies have been conducted on a vortex ring and its state, few of them deal with the cause of a vortex ring and determination of the critical descent velocity.
Myong Song Nam, an institute head at the Faculty of Mechanical Science and Technology, has determined the cause of the vortex ring of a descending propeller and the critical descent velocity by CFD method.
He used the MRF(Moving Reference Frame) method of Fluent for adequate prediction of the wake flows of a propeller.
The simulation showed that the origin of the vortex ring is not the vorticity line, but the interaction between the wake stream tube and the surrounding air, and that as the position of the vortex ring core varies with the increase in the descent velocity, the vortex ring approaches the propeller and consequently, the propeller enters into a vortex ring state and the vortex ring sharply moves above the propeller. Then, he predicted the boundary value of the descent velocity at which the propeller enters into a vortex ring state. The vortex ring state relates closely to the position of a vortex ring core or the thrust, which starts to vary suddenly at about 4.2m/s of descent velocity.
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Jo Apr 26, 2023
U Yong Nam, a section head at the Robotics Institute, has developed a tower clock.
The tower clock is composed of a driving ball-worm gearing box (1:84) and a clock gearing box (reduction ratio of the hour hand and the minute hand 1:12). The accuracy of the clock is ensured by the side clearance and the motion smoothness of the driving ball-worm gearing box.
When the worm shaft rotates in the driving ball-worm gearing box, the rolling balls in the circular workspace between the worm and the worm wheel transfer torque to the ball-worm wheel. Then, the balls circulate along the ball recirculation passage through 2~3 teeth. At the input and output of the passage is a piece to guide the balls back smoothly. And there is a ball-bush to stop the balls from dispersing.
The control system is composed of a main clock and terminal clocks calibrated by the national standard time. The control device consists of a digit I/O part, a serial communication part, a display, control buttons, and a power circuit.
The terminal clocks receive time pulses through RS-485 communication every 30s from the main clock and send them to the drive module to drive the motor.
The large-size tower clock with a ball-worm gearing box has several advantages.
First, it is much easier to achieve the dimensional accuracy and the assembling accuracy of the gearing box than a clock with a planetary gearing box.
Second, its service life is semipermanent or permanent.
Third, it ensures a high degree of accuracy and it is easy to operate.
It will find use at the front gates of factories and enterprises, in the waiting rooms of stations, and at public places.
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