• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.47 no.6
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• pp.429-437
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• 2019

This paper deals with a ZEM (Zero-Effort-Miss) based guidance law for the interception of moving targets and characteristics of the guidance law according to time-to-go estimation methods. To derive the ZEM vector feedback guidance command, we introduce a polynomial function with unknown coefficient, and then we determine the coefficient to satisfy initial and terminal constraints. Since the directions of the guidance command and ZEM vectors are adjusted by the time-to-go, general time-to-go estimation methods are proposed, which can generate the vertical and horizontal guidance commands with respect to an arbitrary reference frame. By performing various numerical simulations, the performance and characteristics of the proposed methods are investigated.

• International Journal of Naval Architecture and Ocean Engineering
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• v.12 no.1
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• pp.832-842
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• 2020

The coordination control of multiple Underactuated Underwater Vehicles (UUVs) moving in three dimensional space is investigated in this paper. The coordinated path following control task is decomposed into two sub tasks, that is, path following control and coordination control. In the spatial curve path following control task, path following error dynamics is build in the Serret-Frenet coordinate frame. The virtual reference object can be chosen freely on the desired spatial path. Considering the speed of the UUV, the line-of-sight navigation is introduced to help the path following errors quickly converge to zero. In the coordination control sub task, the communication topology of multiple UUVs is described by the graph theory. The speed of each UUV is adjusted to achieve the coordination. The path following system and the coordination control system are viewed as the feedback connection system. Input-to-state stable of the coordinated path following system can be proved by small gain theorem. The simulation experiments can further demonstrate the good performance of the control method.

• Journal of Aerospace System Engineering
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• v.15 no.2
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• pp.26-35
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• 2021

In this study, CFD analysis was conducted to compare the aerodynamic performance of the isolated propeller and pusher propeller, which is affected by the wake of wide fuselage. The moving reference frame (MRF) method was used for isolated propeller analysis, while the MRF and sliding mesh method were used sequentially for the pusher propeller to analyze the change in the aerodynamic characteristics based on the azimuth angle. Under the same torque condition, the thrust of the pusher propeller was greater than that of the isolated propeller. Thrust increment of the pusher propeller was mainly generated near the root of the blade where the fuselage wake was concentrated. The net efficiency of the pusher propeller was greater than or equal to that of the isolated propeller. Because of the flat fuselage shape, thrust and torque of the pusher propeller periodically changed with the rotation of the propeller.

• Journal of the Korea Institute of Military Science and Technology
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• v.24 no.1
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• pp.84-98
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• 2021

The hydrodynamic characteristics of amphibious assault vehicles are investigated using commercial CFD code, STAR-CCM+. Resistance performances of a displacement-type vehicle and a semi-planing type vehicle are analyzed in calm water. The self-propelled model is also computed for the semi-planing type vehicle. All computations are performed using an overset mesh system and a RANS based flow-solver coupled with a two-degree of freedom equations of motion. A moving reference frame is applied to simulate revolutions of impeller blades for a waterjet propulsion system. Grid dependency tests are performed to evaluate discretization errors for the mesh systems. The numerical analysis results are compared with the experimental results obtained from model tests. It is shown that RANS is capable of investigating the resistance and self-propulsion characteristics of high-speed amphibious assault vehicles. It is also found that a fully covered side skirt, which is covering tracks, reduces resistance and stern trim, besides increasing propulsive efficiency.

• Journal of Korea Multimedia Society
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• v.16 no.1
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• pp.42-49
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• 2013

The camera motion is accompanied with the translation and/or the rotation of objects in frames of a video sequence. An unnecessary rotation of objects declines the quality of the moving pictures and in addition is a primary cause of the viewers' fatigue. In this paper, a novel method for correcting rotated frames in video sequences is presented, where the modified Mojette transform is applied to the motion-compensated area in each frame. The Mojette transform is one of discrete Radon transforms, and is modified for correcting the rotated frames as follows. First, the bin values in the Mojette transform are determined by using pixels on the projection line and the interpolation of pixels adjacent to the line. Second, the bin values are calculated only at some area determined by the motion estimation between current and reference frames. Finally, only one bin at each projection is computed for reducing the amount of the calculation in the Mojette transform. Through the simulation carried out on various test video sequences, it is shown that the proposed scheme has good performance for correcting the rotation of frames in moving pictures.

• Journal of Institute of Control, Robotics and Systems
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• v.13 no.4
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• pp.320-328
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• 2007

This paper presents a stiffness modeling of a low-DOF parallel robot, which takes into account of elastic deformations of joints and links, A low-DOF parallel robot is defined as a spatial parallel robot which has less than six degrees of freedom. Differently from serial chains in a full 6-DOF parallel robot, some of those in a low-DOF parallel robot may be subject to constraint forces as well as actuation forces. The reaction forces due to actuations and constraints in each serial chain can be determined by making use of the theory of reciprocal screws. It is shown that the stiffness of an F-DOF parallel robot can be modeled such that the moving platform is supported by 6 springs related to the reciprocal screws of actuations (F) and constraints (6-F). A general $6{\times}6$ stiffness matrix is derived, which is the sum of the stiffness matrices of actuations and constraints, The compliance of each spring can be precisely determined by modeling the compliance of joints and links in a serial chain as follows; a link is modeled as an Euler beam and the compliance matrix of rotational or prismatic joint is modeled as a $6{\times}6$ diagonal matrix, where one diagonal element about the rotation axis or along the sliding direction is infinite. By summing joint and link compliance matrices with respect to a reference frame and applying unit reciprocal screw to the resulting compliance matrix of a serial chain, the compliance of a spring is determined by the resulting infinitesimal displacement. In order to illustrate this methodology, the stiffness of a Tricept parallel robot has been analyzed. Finally, a numerical example of the optimal design to maximize stiffness in a specified box-shape workspace is presented.

• Journal of Mechanical Science and Technology
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• v.20 no.5
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• pp.710-716
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• 2006

Micro-PIV system with a high speed CCD camera is used to measure the flow field near the advancing meniscus of a water slug in microchannels. Image shifting technique combined with meniscus detecting technique is proposed to measure the relative velocity of the liquid near the meniscus in a moving reference frame. The proposed method is applied to an advancing front of a slug in microchannels with rectangular cross section. In the case of hydrophilic channel, strong flow from the center to the side wall along the meniscus occurs, while in the case of the hydrophobic channel, the fluid flows in the opposite direction. Further, the velocity near the side wall is higher than the center region velocity, exhibiting the characteristics of a strong shear-driven flow. This phenomenon is explained to be due to the existence of small gaps between the slug and the channel wall at each capillary corner so that the gas flows through the gaps inducing high shear on the slug surface. Simulation of the shape of a static droplet inside a cubic cell obtained by using the Surface Evolver program is supportive of the existence of the gap at the rectangular capillary corners. The flow fields in the circular capillary, in which no such gap exists, are also measured. The results show that a similar flow pattern to that of the hydrophilic rectangular capillary (i.e., center-to-wall flow) is always exhibited regardless of the wettability of the channel wall, which is also indicative of the validity of the above-mentioned assertion.

• Fire Science and Engineering
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• v.13 no.3
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• pp.3-17
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• 1999

Smoldering is a non-flaming combustion mode, characterized by thermal degradation and c charring of the virgin material, evolution of smoke and emission of visible glow. A big fire may @ occur even in a confined environment having a limited amount of oxygen, due to smoldering c combustion through a porous solid material. This paper presents a theoretical analysis on the effect of smoldering combustion on fire occurrence based on a report about fire investigation of a real f fire accident. It is assumed that the propagation of the smolder wave is one-dimensional, d downward, opposing an upward forced flow and steady in a frame of reference moving with the s smolder wave. Smoldering combustion is modeled by a one-step reaction mechanism, without c considering pyrolysis. It is found that dominant parameters controlling smoldering combustion i include mass flux of oxidizer entering the reaction zone and void fraction of solid fuel. It is also found that the mechanism of transition to flaming is critically influenced by these two parameters.

• Nuclear Engineering and Technology
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• v.27 no.1
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• pp.45-52
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• 1995

In influence of the vapor phase turbulent stress (i.e., the too-phase Reynolds stress) to the characteristics of two-phase system in a horizontal pipe has been theoretically investigated. The average two-fluid model has been constituted with closure relations for stratified flow in a horizontal pipe. A vapor phase turbulent stress model for the regular interface geometry has been included. It is found that the second order waves propagate in opposite direction with almost the same speed in the moving frame of reference of the liquid phase velocity. Using the well-posedness limit of the two-phase system, the dispersed-stratified How regime boundary has been modeled. Two-phase Froude number has been found to be a convenient parameter in quantifying the onset of slugging as a function of the global void fraction. The influence of the taper phase turbulent stress was found to stabilize the flow stratification.

• Journal of the Korea Institute of Information and Communication Engineering
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• v.13 no.12
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• pp.2693-2700
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• 2009

This paper proposes an effective compression method to utilize the 3D integral image with large amount of data obtained by a lens array in various applications. The conventional compression methods for still images exhibit low performance in terms of coding efficiency and visual quality, since they cannot remove the correlation between elemental images. In the moving picture compression methods, 1D scanning techniques that produce a sequence of elemental images are not enough to remove the directional correlation between elemental images. The proposed method effectively sequences the elemental images from an integral image by the 2D referencing technique and compresses them using the multi-frame referencing of H.264/AVC. The proposed 2D referencing technique selects the optimal reference image according to vertical, horizontal, and diagonal correlation between elemental images. Experimental results show that compression with the sequence of elemental images presents better coding efficiency than that of still image compression. Moreover, the proposed 2D referencing technique is superior to the 1D scanning methods in terms of the objective performance and visual quality.