Jo Aug 9, 2022
Yu Chung Sim, a lecturer at the Faculty of Applied Mathematics, has studied a method to increase the convergent speed of global optimization.
The filled function method is an effective method to get the global optimization solution to multi-dimensional optimization problems.
One of the major problems to be solved in the search for a global optimization solution to multi-dimensional optimization problems is how to escape from the given local minima into a better one.
To solve this problem, a number of methods including Orbit method and Tunneling method have been investigated. Among them the filled function method has been admitted as an effective one and thus, it has been undergoing further study.
The filled function method was first applied to unconstrained nonlinear programming problems, followed by constrained nonlinear programming problems, non-smooth optimization problems and discrete optimization problems.
At present, in order to improve the effectiveness of the global optimization solution search algorithm using a filled function, many scientists have been looking for methods to combine various methods including the filter method and the interior point method with the filled function method.
The main problem of the filled function method is how to construct an auxiliary function called filled function at the given local minima of the objective function obtained from every repetition of the algorithm.
For this, filled functions with one parameter and those with two parameters were proposed, but they are difficult to control when they have more parameters
Recently, a new filled function method that skips the process where they used to get better minima by minimizing an objective function from the local minima of the filled function in the prior algorithms is being applied to continuous optimization problems.
As seen above, you can see that the filled function method can be applied to the global optimization problems arising in practice if we introduce filled functions with few parameters in the continuous optimization problems as well as discrete optimization problems, non-convex optimization problems and non-smooth optimization problems.
Therefore, she has proposed a new filled function that comes from the idea that an objective function and a filled function have the same stationary points and applied it to finding a global minimum solution to constrained nonlinear programming.
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Jo Aug 5, 2022
Ri Yong Ho, a section head at the Faculty of Heat Engineering, has been working hard to make further achievements since he developed a speed control device of a turbo coupling which is widely used in the speed change coupling of a large rotating machine.
The device consists of a turbo coupling, a speed control instrument and a distributed control system.
The fluid coupling, a power transfer device of a rotating machine, can control speed, has a wide control range and has good control characteristics. With a good power transfer characteristic, it can raise energy efficiency.
To use the turbo coupling’s speed control function, we must get a correct understanding of the turbo coupling’s working principle before designing and making a speed control instrument based on it.
The oil system of a turbo coupling consists of a lubricating oil system and a hydraulic oil system. The lubricating oil system functions as a supplier to provide lubricating oil to the coupling’s gears and medals while the hydraulic oil system controls rotating speed. The deeper the coupling is submerged in the oil, the larger the viscosity of oil gets and the faster the output rotating speed gets. The depth of the oil depends on the height of the dipper pipe.
The flow control piston controls oil discharge which passes through the turbo coupling.
The depth of the dipper pipe piston controls the rotating speed; the flow control piston controls the amount of hydraulic oil discharge to allow the turbo coupling to work.
The speed control instrument consists of a rotating motor that converts the control system’s command into the rotation angle, and cam accessories. The rotating motor receives 4~20mA current signals from the control system and in proportion to it, rotates the axis in the range of 0~150°, feeds it back to the control system in proportion to the rotated angle.
Two cams are installed on the axis of the conversion device, and they press on the fluid control piston and the dipper pipe piston individually according to the rotated angle. The main components of the conversion device are the two cams.
Based on the piston’s working principle, the working cycle, the size and installation conditions of the speed control instrument, the cam’s curve radii are decided as follows.
The fluid control cam’s radius: min = 96mm, max =115mm.
The dipper pipe cam’s radius: min = 96mm, max =117mm.
If the above-mentioned turbo coupling is installed between the main feed pump and the motor of the 210MW boiler-turbine block, it controls the rotating speed of the feed pump, which enables to control the output water pressure and the drum level of the boiler.
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Jo Aug 4, 2022
Even though there exist a great number of nations in the world, hardly any of them are like the Korean nation, which built their own state early in ancient times and has developed adorning their history based on one and the same blood and language.
One of the ancient relics that reflect the superiority of the Korean national culture is a pipha-shaped dagger.
Made in the first Korean state founded by King Tangun, pipha-shaped daggers are representative relics that are still unearthed on the Korean Peninsula and all other areas where ancient Koreans used to live. The name pipha-shaped dagger comes from its shape that resembles an ancient musical instrument pipha (a kind of oriental lute). Its birthplace is Pyongyang.
The pipha-shaped dagger representative of the culture in the first half period of Ancient Korea has peculiar features that make a clear distinction from those used by other ethnic groups in the neighbouring countries in those days in respect of structure, quality, shapes of blade and haft, designs engraved on the blade, etc.
The most distinctive one is the plane shape of the blade. The blade looks elegant with a sharp end, graceful curves at both cutting edges and a sharp protrusion in the middle or above the middle. A number of daggers of various types and structures have been discovered in the countries and regions that are proud of world-famous ancient civilization, but none of them looks like the original pipha-shaped dagger.
Another feature is its original structure. Those made and used in other countries and regions in ancient times were all cast bodily. Unlike them, pipha-shaped daggers were assembled with individually cast blades, hafts and fitters. In other words, they were built-up daggers while others were monolithic units. This demonstrates that the techniques of making and processing bronze ware owned by ancient Koreans reached a high level.
Pipha-shaped daggers have charming ornamental designs, too. Others found abroad had few ornaments and if any, the ends of hafts were decorated with no more than the shapes of heads of animals and birds. Pipha-shaped daggers, in contrast, were embossed with various sophisticated designs and ornaments on the blades, hafts, and fitters.
That is why the world people say that a pipha-shaped dagger is not a mere weapon but ‘a piece of sophisticated craftwork’.
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Jo Aug 3, 2022
A research group led by Choe Yong Chol, a researcher at the Faculty of Mining Engineering, has been employing his new method of reducing the amount of harmful explosion gas by using vermiculite and acidic deposits of mine water.
Introducing this technology, you can increase the power of explosion much higher than before thanks to the increase in the work ability resulted from the evaporation of combined water in vermiculite as well as the increase in brisance resulted from the increase in explosive density. At the same time, you can reduce the emission of harmful explosion gas such as CO and NOx by adsorption of expanded vermiculite, absorption of water separated from vermiculite and oxidation of metallic oxides in vermiculite at high temperature of explosion.
This method has two stages; manufacturing stage where vermiculite that passed through crushing, separating and distributing processes and acidic deposits of mine water are mixed, and application stage where they are admixed with explosives.
Explosive additives are made with vermiculite of 0.1mm~1.5mm in granulation size and acidic deposits of mine water less than 0.05mm in granulation size in the proportion 3:1. This mixture and Expanded Ammonium Nitrate Explosives are mixed evenly in a ratio of one to five and put into blasting cartridges before use.
Most of the acidic deposits of mine water consist of ferric hydroxide(Fe(OH)3) and basic ferric sulphate(Fe(OH)SO4). In vermiculite, some metallic oxides such as ferric oxide make up about 5.5%, which is less than those contained in acidic deposits of mine water. Trihydric ferric salt and ferric oxides serve as oxidizers that oxidize CO and NO in explosion gas to CO2 and NO2 respectively.
Vermiculite contains about 15% of interformational water which is not changed between micaceous layers at low temperatures but converted into water vapor at over hundreds of degrees. This interformational water removes harmful gases by absorbing soluble gases like NO2, N2O3, N2O4, etc. produced from explosive reactions and from oxidation by trihydric ferric constituents in the additives.
Vermiculite has a strong cation exchange ability. Its cation exchange capacity and base absorption capacity increases by 1.5 times at high temperatures. Thus, at high explosion temperatures vermiculites are expanded and the absorption ability for harmful explosion gas is further increased.
When explosives are mixed with vermiculites and acidic deposits of mine water that are heavier than constituents of explosives, the charge density of explosives increases. Consequently, the detonation velocity and pressure increase and the dynamic explosion power of explosives gets higher.
Vermiculite has a maximum of 18% of interformational and combined waters, which are changed into water vapour at the moment of explosion and expanded thousands of times in volume. It results in a considerable increase in the volume of explosion gas and in the explosion pressure of explosives, which leads to the increase in the static power and blast effects.
If this additive(vermiculit and acidic deposit of mine water) makes up 20% in explosives, removal rate for CO and NOx gases can be more than 93% and 85% respectively. The explosion power of explosives remains similar to or gets higher than before.
With the introduction of this method to all kinds of blasting operations such as driving, you can save a considerable amount of explosives and make a contribution to protecting the environment and improving working conditions by reducing the emission of harmful explosion gas.
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Jo Aug 1, 2022
“Turtle ship” is the very first iron-clad battleship designed by a distinguished patriotic general Ri Sun Sin after being appointed admiral of Left Jolla Province in 1591.
According to the historical book “Richungmugongjonso”, the structure, building method and performance of turtle ships are roughly as follows.
A turtle ship was 35 metres long, 11.8 metres wide and 5.2 metres high, with 10 oars at each side and about 70 muzzles. The sails could be either raised or lowered if necessary.
The front part of the ship looked like the head of a turtle, where sulphur and nitre were burnt to produce smoke which was supposed to be emitted through the mouth to prevent enemy from noticing it.
It had two muzzles at the top of the turtle head and one muzzle on each side of the door. There were 12 muzzles on the right and left siding walls respectively on which some colours were fixed.
The roof of the ship was made up of large planks to look like a turtleback. There was a narrow way for our warriors only on the turtleback the rest part of which sharp objects like spears and daggers were fixed all over to keep enemies away. In the centre of the ship were 26 chambers, two of which were filled with ironware, five of which stored weapons like guns, arrows and bows, dynamite, swords, etc. and 19 of which were used as bedrooms for warriors. One of the remaining 2 chambers was captain’s cabin and the other one was a bedroom for commanding officers.
As warriors and combat equipment were protected inside the ship and they could look out from inside but nobody could look in from outside, it was possible for warriors in the ship to fire guns on all sides at hundreds of enemy ships at the same time. Some parts of the ship were iron-clad and the rest parts were covered with thick planks. Therefore, contemporary gun fire could never break it down nor burn it down.
Stout and stable in structure and incredible in speed, turtle ships equipped with various kinds of weapons and guns (12 at the right and left, 2 at the front, total 14) and loaded with as many warriors as 160 was an incomparably powerful warship in those days that was capable of destroying any enemy ship. In addition, the reasonable size and shape of the ship ensured good navigability.
Turtle ship, built in an original way by the Korean people, who had exploited and defended the sea from the early times, made a great contribution to defending the country by inflicting disastrous defeat on the Japanese invaders in several sea battles including the battle on the sea off Sachon during the Japanese invasion of Korea in 1592.
Outside Korea, it was not until the Crimean War (1853~1856), which broke out 440 years after the turtle ship, that three iron-clad ships were built. However, they were hardly used because they were too slow and cumbersome.
In conclusion, the first ever iron-clad ship turtle ship is a priceless national heritage that demonstrates the resourcefulness and creative talents of the Korean nation and at the same time it is a common asset of mankind that holds a prominent position in the world history of armament.
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Jo Jul 29, 2022
O Un Suk, a lecturer at the Faculty of Applied Mathematics, has studied the relations of core, Weber set and Shapley value to one another under the condition of strong convexity / weak concavity.
A cooperative game with transferable utility, a TU-game for short, is defined as a pair (N, v) , where N is a finite set of players and v is a mapping that assigns a real number v(S) to each coalition S⊆N. v(φ) is set 0.
One of the important problems in the cooperative game theory is to select an efficient payoff vector, i.e., a way of sharing among players the total worth of the game when they all join a grand coalition. For this aim, various solution concepts have been developed.
The solutions playing an important role in selecting efficient payoff vectors include Core, Weber set, and Shapley value.
In many real situations, however, not all subsets of N can be realized as coalitions or they are not feasible. The interest in subcollections of the power set of N is inspired by the fact that in case of restricted communication or cooperation structure, the standard model of a cooperative game is not applicable. A game on concept lattices is a good example.
A game on concept lattices is a cooperative TU-game defined on concepts, where a concept is a pair (S, S') of S, a subset of players or objects, and S' a subset of attributes.
Such games induce a game on extents and a game on intents.
The inclusion of the Weber set in core plays an important role in the studies of the stability of the Shapley value (inclusion of the Shapley value into the core) because the Shapley value is defined as a mean value of all vertices of the Weber set.
The Weber set does not become the subset of the core under convexity in games on concept lattices different from classical cooperative games. Thus, the concept of strong convexity is introduced in games on extents.
Strong convexity means that the larger coalition each player joins in, the greater the average creation value of them gets.
The following theorem shows that the extent Weber set becomes a subset of the extent core in strongly convex games on extents.
Theorem 1. The extent Weber set is included in the extent core if a game on extents is strongly convex.
Corollary 1. The extent Shapley value assigns an element of the extent core if a game on extents is strongly convex.
The condition of strong convexity is a sufficient condition that guarantees the extent Shapley value to be in the extent core in the games on extents. Therefore, in convex games, not strongly convex, the extent Shapley value may belong to the extent core.
Similarly, it becomes possible to define weak concavity in games on intents as they are in dual relation with games on extents. Then, it can be proved that the intent Weber set is the subset of the intent core under weak convexity.
It is concluded that the extent/intent Shapley value can be used as a rational solution to this game if a game on extents/intents is convex/concave.
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