Jo Jul 13, 2025
Inverse hyperbolic problems appear in underwater sound search, geophysics, electrodynamics, etc.
In most practical applications, most of the physical phenomena are modeled by multidimensional equations. Therefore, considerable attention has been devoted to the analysis of multidimensional inverse problems. However, to the best of our knowledge, the solvability of the hyperbolic inverse source problems has only been studied for one-dimensional linear equations.
Ryom Su Jong, a post-graduate student of the Faculty of Applied Mathematics, has investigated the unique solvability of an inverse problem of determining a solely time-dependent source in a multidimensional semilinear hyperbolic equation.
This inverse problem models the identification of underwater sound source when its position is known. She addressed a numerical scheme together with the existence and uniqueness of the weak solution by means of Rothe’s time-discretization method. Moreover, she evaluated the error of the semi-discretization scheme.
The time-discretization method she applied is a powerful and efficient tool for solving a wide range of evolution equations.
...
Jo Jul 12, 2025
From childhood, the people of Koguryo possessed of exceptional martial habit were all good at horse riding and they accepted it as an important requirement of life.
The mural paintings of Koguryo tombs including murals of dancing, Yaksuri mural paintings and murals of hunting give a lifelike portrayal of the life of Koguryo people who enjoyed riding with excellent horsemanship. In particular, the delightful scenes depicting some people hunting wild animals with bows and arrows while dashing vigorously on horseback show master horsemanship and archery of the Koguryo people in an intensive way.
Through such hunting, the Koguryo people further refined their riding skills and trained courage and matchless capability to defeat any aggressors at a single blow without fear.
The Koguryo people learned riding skills through their routine martial arts training as well as from hunting wild beasts.
The mural painting of Tokhungri tomb vividly depicts the horse-riding archery training on a fixed target. The wall is painted with 4 competitors, 2 judges and one scorer.
Although the mural shows the scene of horse-riding archery only, the Koguryo people further improved their riding skills through various martial arts training including racing, fencing and spearing on horseback.
As a result, the distinguished horsemanship of the Koguryo people played a decisive role in the war to defend the land and the dignity and sovereignty of the country and nation.
...
Jo Jul 11, 2025
Cold pilger mills include a roll cold-pipe rolling mill and a roller cold-pipe rolling mill. As the roll cold-pipe rolling mill involves difficult roll processing, the roller cold-pipe rolling mill is now in wide use.
Determination of rolling force during periodic rolling in roller cold pilger mills is one of the important problems in determining the design parameters. In particular, the roller cold pilger mill is subject to periodic reciprocating motion so that the deformation zone and rolling force continue to change during one cycle, and thus it is important to determine the maximum rolling force.
Han Myong Jin, a researcher at the Faculty of Materials Science and Technology, has determined the average pressure and contact area between the rolling roller and the workpiece as one of the basic data for determination of the design parameters of two-row roller cold pilger mills, and investigated the rolling force.
First, he derived a formula of the maximum rolling force and performed a comparison using the finite element analysis tool Deform-3D. Then, he installed a force sensor under the inclined support plate and compared it with the measured results to confirm the accuracy of calculation.
When producing a copper tube of ϕ24mm×0.5mm in size from a workpiece of ϕ30mm×3mm in size in a two-row roller cold-pipe rolling mill, the rolling force analysis by the computer-coupled rolling force measuring device and the rolling force calculation confirmed that the rolling force formula is relatively accurate, with 3% in the first row and 0.73% in the second row. In the same way, the comparison of the rolling force by the simulation analysis data with Deform-3D and the rolling force determined by the rolling force formula showed that the rolling force formula is relatively accurate, with 2.8% in the first row and 1.96% in the second row.
...
Jo Jul 10, 2025
A turbogenerator is one of the main equipment for large-capacity power stations such as nuclear power plants. In order to design a turbogenerator of high operational reliability, thermal characteristics should be analyzed exactly.
The direct gas-cooled rotor winding in large power turbogenerators is cooled by gas (hydrogen) that flows along the U-type or radial ventilation duct. Many researchers and experts have published their research results on the temperature characteristics of winding of direct gas-cooled rotor in large turbine generators larger than 1 000MW. However, the detailed research results on the cooling duct dimension at the radial cooling rotor windings are rarely mentioned in the literature.
An Se Gwang, a researcher at the Faculty of Electrical Engineering, has determined the reasonable cross section of an axial ventilation duct that makes the distribution of flow velocity uniform in the radial cooling slot of a 1 000MW 4-pole turbogenerator and the reasonable value of the width coefficient of cooling duct and segment length.
First, he proposed a mathematical model to analyze the temperature feature of radial cooling rotors and confirmed the range of practical value to determine the reasonable number and dimension of radial ventilation ducts.
Then, he proved that the gas velocity distribution has an equal value along the axial length when the input cross section of the axial ventilation duct is increased 1.67 times the output cross section, with the effect of revolution in consideration.
The research result is helpful for the design standard of large 4-pole turbine generators, and it can be applied to the design and building of 1 000 MW 4-pole turbine generators.
...
Jo Jul 9, 2025
Kangso Mineral Water is not only unique in its taste but also efficacious for treating various diseases and protecting health.
Our people already started using Kangso Mineral Water hundreds of years ago.
“Kangsogunji”, published in 1936, says the following.
There was a mineral spring in Jonghwa ri of Kangso myon. Over 250 years ago, a crane which got hurt in the leg flew down to the spring to drink the water and wash its body with it every morning. After a few days, it was fully recovered and flew away. Thus, the mineral water was discovered to be of medicinal value.
It tastes refreshing and stimulates the tip of the tongue, which tastes just like a fizzy drink. In addition, habitual drinking of it helps cure many diseases including gastric disorder, anemia, constipation, inflammation of the bladder, etc.
This tells us that Kangso Mineral Water has hundreds of years of use history and it has been one of our country’s specialties well known from long ago for its unique taste and mysterious and marvelous medicinal effect.
On December 7, 1947, when he visited the Kangso Mineral Water spring, President
Today, the modern Kangso Mineral Water Factory near the spring is producing a huge amount of bottled mineral water to supply to every part of the country including Pyongyang City.
...
Jo Jul 8, 2025
Manganese is an important alloying element indispensable in the metallurgical industry and it is widely deposited in the form of oxides in nature.
The reduction reaction of manganese oxides in the production of metallic manganese or ferromanganese from these manganese raw materials is of great importance, and there have been many thermodynamic and kinetic studies on it. In particular, several kinetic models describing the carbothermic reduction reaction of MnO have been proposed and experimentally examined.
However, there are some errors in the calculation and experimental results in the models proposed by the previous authors, and no studies have yet applied the fractional order to the kinetic modeling of the reduction reaction of MnO by solid carbon.
Kim Un Dok, a researcher at the Faculty of Metal Engineering, has developed a kinetic model for the reduction process by solid carbon of MnO in high-carbon ferromanganese slags using fractional differential equations (FDE), determined the relationship between fractional order, fractional rate constant and temperature, and verified the accuracy of the fractional order model through the comparison with the results of previous studies.
Comparing the preset values calculated from the FDE and previous models with the experimental values, the root-mean-square errors (RMSE) were 0.005 and 0.029, respectively, and the coefficients of determination were 0.999 and 0.980, respectively, which shows that the proposed model is more accurate.
...
