• 한국군사과학기술학회지
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• 제13권6호
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• pp.1026-1033
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• 2010

The analysis of water-entry impact forces acting on the fin shaft of high speed water-entry body is described. During the entry of high speed body into water, the physical phenomenon and flow properties are analyzed. A proper analysis model is established and the method to estimate the flow force which causes impact torque at the fin shaft is described. It is assumed that the fin shaft is damaged by the force which is induced by contacting with cavity wall. The pressure distribution of fin and the maximum torque are estimated and compared with breaking force. Conclusively, it is hard to resist water-entry impact force in terms of the reinforcement of fin shaft. Additionally safe equipment is essentially required.

• 대한기계학회논문집B
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• 제23권2호
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• pp.234-242
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• 1999

Flow phenomena such as the pressure transients Inside a high-speed railway tunnel and the Impulsive waves at the exit of the tunnel are closely associated with the characteristics of the entry compression wave, which is generated by a train entering the tunnel. Tunnel entrance hood may be an effective means for alleviating the Impulsive waves and pressure transients. The objective of the current work is to explore the effects of the train nose shape and the entrance hood on the characteristics of the entry compression wave. Numerical calculations using the method of characteristics were applied to one-dimensional, unsteady, compressible flow field with respect to high-speed railway/tunnel systems. Two types of the entrance hoods and various train nose shapes were employed to reveal their influences on the entry compression wave for a wide range of train speeds. The results showed that the entry compression wave length increases as the train nose becomes longer and the train speed becomes lower. The entry compression wave length in the tunnel with hood becomes longer than that of no hood. Maximum pressure gradient in the compression wavefront reduces by the entrance hood. The results of the current work provide useful data for the design of tunnel entrance hood.

• 대한물리의학회지
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• 제10권1호
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• pp.23-35
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• 2015

PURPOSE: People with physical disabilities such as cerebral palsy usually experience obstacles when interacting with computer through conventional keyboard because of their motor disabilities. The purpose of this study is empirically compare of text entry(alphabet and word) speed and accuracy using the three different keyboard type on four students(male 2 and female 2) with cerebral palsy. METHODS: This research design used a replicated single-case experimental approach to compare the individual performance. An alternating treatments design was used to examine the effectiveness of standard QWERTY keyboard and alternative keyboard(mini and big keyboard) on computer access for students with cerebral palsy. To avoid changes in posture that influence a keyboard character entry training and evaluation was carried out using his sitting in a wheelchair. Compass software program used in this study as an assessment tool to measure speed and accuracy when performance of text entry(alphabet and word). This was repeated until the stable status of reaction time. RESULTS: As a result, the alternative keyboard seems to be the most effective device for students with cerebral palsy to perform text entry. But various factors such as peculiarity of motor disabilities, experience and preferences of the user are heavily related. CONCLUSION: Thus, we must perform the objective and systematic assessment for computer access and if sustained training is accomplished, it could to improve speed and accuracy of text entry(alphabet and word).

• International Journal of Naval Architecture and Ocean Engineering
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• 제13권1호
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• pp.450-465
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• 2021

In this study, an attempt has been made to investigate the water-entry characteristics of the high-speed parallel projectile numerically. The shear stress transport k-𝜔 turbulence model and the Zwart-Gerber-Belamri cavitation model based on the Reynolds-Averaged Navier-Stokes method were used. The grid independent inspection and grid convergence index is carried out and verified. The influences of the parallel water-entry on flow filed characteristics, trajectory stability and drag reduction performance for different values of initial water-entry speed (𝜈₀ = 280 m/s, 340 m/s, 400 m/s) and clearance between the parallel projectiles (L_p = 0.5D, 1.0D, 2.0D, 3.0D) are presented and analyzed in detail. Under the condition of the parallel water-entry, it can be found that due to the intense interference between the parallel projectiles, the distribution of cavity is non-uniform and part of the projectile is exposed to water, resulting in the destruction of the cavity structure and the decline of trajectory stability. In addition, the parallel projectile suffers more severe lateral force that separates the two projectiles. The drag reduction performance is impacted and the velocity attenuation is accelerated as the clearance between the parallel projectiles reduces.

• 한국철도학회:학술대회논문집
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• 한국철도학회 1998년도 창립기념 춘계학술대회 논문집
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• pp.596-603
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• 1998

The entry compression wave, which is generated at the entrance of the tunnel, is almost always associated with the pressure transients in the tunnel as well as the impulse noise at the exit of the tunnel. It is highly required to design the train nose shape that can minimize such undesirable phenomena. The objective of the current work is to investigate the effects of the train nose shape on the entry compression wave. Numerical computations were applied to one-dimensional unsteady compressible flow in high-speed railway train/tunnel systems. A various shape of train noses were tested for a wide range of train speeds. The results showed that the strength of the entry compression wave is not influenced by the train nose shape, but the time variation of pressure in the entry compression wavefront is strongly related to the train nose shape. The current method of the characteristics was able to represent a desirable nose shape for various train speeds. Optimum nose shape was found to considerably reduce the maximum pressure gradient of the entry compression wave.

• International Journal of Naval Architecture and Ocean Engineering
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• 제11권1호
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• pp.495-509
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• 2019

The objective of this study is to analyze the compressibility effects of multiphase cavitating flow during the water-entry process. For this purpose, the water-entry of a projectile at transonic speed is investigated computationally. A temperature-adjusted Tait equation is used to describe the compressibility effects in water, and air and vapor are treated as ideal gases. First, the computational methodology is validated by comparing the simulation results with the experimental measurements of drag coefficient and the theoretical results of cavity shape. Second, based on the computational methodology, the hydrodynamic characteristics of flow are investigated. After analyzing the cavitating flow in compressible and incompressible fluids, the characteristics under compressible conditions are focused upon. The results show that the compressibility effects play a significant role in the development of cavitation and the pressure inside the cavity. More specifically, the drag coefficient and cavity size tend to be larger in the compressible case than those in the incompressible case. Furthermore, the influence of entry velocities on the hydrodynamic characteristics is investigated to provide an insight into the compressibility effects on cavitating flow. The results show that the drag coefficient and the impact pressure vary with the entry velocity, and the prediction formulas for drag coefficient and impact pressure are established respectively in the present study.

• 한국전산유체공학회지
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• 제4권1호
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• pp.1-7
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• 1999

The numerical methodology for computing tile impact forces and water entry behaviors of high speed water entry bodies was been developed. Since the present method assumed the impact occurs within a very short time interval. the viscous effects do not have enough time to play a significant role in the impact forces, that is, the flow around a water-entry object was assumed as an incompressible potential flow and is solved by the source panel method. The elements fully submerged into the water are routinely treated, but the elements intersected by the effective planar free surface are redefined and reorganized to be amenable to the source panel method. To validate the present code, it was applied to disk, cone and ogive model and compared with experimental data. Good agreement was obtained. The water entry behavior such as the bouncing phenomena from the free surface was also simulated using the impact forces and two degree of freedom dynamic equation. Physically acceptable results were obtained.

• Journal of Advanced Marine Engineering and Technology
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• 제26권5호
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• pp.596-606
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• 2002

The three-dimensional unsteady compressible Euler equation solver with ALE, CFD code, PAM-FLOW based on FEM method has been applied to analyze the flow field around the high speed train which is entering into a channel. From the present study, the pressure and flow transients were calculated and analyzed. The generation of compression wave was observed ahead of train and the high pressure in the gap between the train and the tunnel was also found due to the blockage effects. It was found that abrupt fluctuation in pressure exists in the region from train nose to shoulder of train corresponding to 10％ of total length of train during tunnel entry. Computed time history of aerodynamic forces of train during tunnel entry show that drag coefficient rapidly rises and saturates at about non-dimensional time 0.31. The total increase of drag coefficient before and after tunnel entry is about 1.1%. Transient profile of lift force shows similar pattern to drag coefficient except abrupt drop after saturation and lift force in the tunnel increases 0.08% more than that before tunnel entry.

• 대한인간공학회지
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• 제36권5호
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• pp.355-372
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• 2017

Objective: The aim of this study is to investigate the performance and usability of ten-finger text entry on Tablet PCs. Background: Generally a soft keyboard is used on Tablet PCs. However, the soft keyboard's performance is usually worse than physical keyboard's performance. In this study, we proposed a modified keyboard for tablet PCs to improve the performance of ten-finger text entry and evaluated the performance and subjective ratings of the keyboard. Method: The modified soft keyboard that is suggested in this study was compared with current Google and Samsung soft keyboards on Tablet PCs. Results: The three keyboards were not significantly different in terms of typing speed, error rate, and mental workload and showed bad performance. Also, the subjective ratings were not shown positively. Conclusion: Based on our results, ten-finger text entry using soft keyboards on Tablet PCs seems to be very difficult. However, we need to research the possibility continuously since ten-finger text entry can improve typing speed. Application: Our study can be a starting point of research that explores ten-finger text entry on Tablet PCs. The new soft keyboard design can be one of the soft keyboard alternatives. However, the key factors to improve the performance and usability of the soft keyboard will not be 'key size' or 'convenience to typing special characters or numbers', but other factors (e.g., 'tactile feedback').

• 대한인간공학회지
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• 제26권2호
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• pp.149-155
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• 2007

Recently, various devices requiring text input such as mobile phone IPTV, PDA and UMPC are emerging. The frequency of text entry for them is also increasing. This study was focused on the evaluation of Korean text entry interface. Various models to evaluate text entry interfaces have been proposed. Most of models were based on human cognitive process for text input. The cognitive process was divided into two components; visual scanning process and finger movement process. The time spent for visual scanning process was modeled as Hick-Hyman law, while the time for finger movement was determined as Fitts' law. There are three questions on the model-based evaluation of text entry interface. Firstly, are human cognitive processes (visual scanning and finger movement) during the entry of text sequentially occurring as the models. Secondly, is it possible to predict real text input time by previous models. Thirdly, does the human cognitive process for text input vary according to users' text entry speed. There was time gap between the real measured text input time and predicted time. The time gap was larger in the case of participants with high speed to enter text. The reason was found out investigating Eye-Hand Coordination during text input process. Differently from an assumption that visual scan on the keyboard is followed by a finger movement, the experienced group performed both visual scanning and finger movement simultaneously. Arrival Lead Time was investigated to measure the extent of time overlapping between two processes. 'Arrival Lead Time' is the interval between the eye fixation on the target button and the button click. In addition to the arrival lead time, it was revealed that the experienced group uses the less number of fixations during text entry than the novice group. This result will contribute to the improvement of evaluation model for text entry interface.