• Transactions of the Korean Society of Mechanical Engineers B
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• v.26 no.6
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• pp.823-832
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• 2002

A modified $textsc{k}$-$\varepsilon$model is proposed for calculation of transitional boundary-layer flows with changing pressure gradient. In order to develop the model for this problem, the flow is divided into three regions; pre-transition region, transition region and fully turbulent region. The effect of pressure gradient is taken into account in stream-wise intermittency factor, which bridges the eddy-viscosity models in the pre-transition region and the fully turbulent region. From intermittency data in various flows, Narashima＇s intermittency function, F(${\gamma}$), has been found to be proportional to $\chi$$^{n}$ according to the extent of pressure gradient. Three empirical correlations of intermittency factor being analyzed, the best one was chosen to calculate three benchmark cases of bypass transition flows with different free-stream turbulence intensity under arbitrary pressure gradient. It was found that the variations of skin friction and shape factor as well as the profiles of mean velocity in the transition region were very satisfactorily predicted.

• 유체기계공업학회:학술대회논문집
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• 2000.12a
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• pp.337-344
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• 2000

A modified k-$\epsilon$ model is proposed for calculation of transitional boundary layer flows. In order to develop the eddy viscosity model for the problem, the flow is divided into three regions; namely, pre-transition region, transition region and fully turbulent region. The pre-transition eddy-viscosity is formulated by extending the mixing Length concept. In the transition region, the eddy-viscosity model employs two length scales, i.e., pre-transition length scale and turbulent length scale pertaining to the regions upstream and the downstream, respectively, and a university model of stream-wise intermittency variation is used as a function bridging the pre-transition region and the fully turbulent region. The proposed model is applied to calculate three benchmark cases of the transitional boundary layer flows with different free-stream turbulent intensity ( $1\%{\~}6\%$ ) under zero-pressure gradient. It was found that the profiles of mom velocity and turbulent intensity, local maximum of velocity fluctuations, their locations as well as the stream-wise variation of integral properties such as skin friction, shape factor and maximum velocity fluctuations are very satisfactorily Predicted throughout the flow regions.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.40 no.7
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• pp.435-440
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• 2016

Effects of the increase rate of Reynold number on near-wall turbulent structures are investigated by performing direct numerical simulations of transient turbulent channel flows. The simulations were started with the fully-developed turbulent channel flow at $Re_{\tau}=180$, then temporal accelerations were applied. During the acceleration, the Reynolds number, based on the channel width and the bulk mean velocity, increased almost linearly from 5600 to 13600. To elucidate the effects of flow acceleration rates on near-wall turbulence, a wide range of durations for acceleration were selected. Various turbulent statistics and instantaneous flow fields revealed that the rapid increase of flow rate invoked bypass-transition like phenomena in the transient flow. By contrast, the flow evolved progressively and the bypass transition did not clearly occur during mild flow acceleration. The present study suggests that the transition to the new turbulent regime in transient channel flow is mainly affected by the flow acceleration rate, not by the ratio of the final and initial Reynolds numbers.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.3
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• pp.305-314
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• 2001

A modified model is proposed for calculation of transitional boundary layer flows. In order to develop the eddy viscosity model for the problem, the flow is divided into three regions; namely, pre-transition region, transition region and fully turbulent region. The pre-transition eddy-viscosity is formulated by extending the mixing length concept. In the transition region, the eddy-viscosity model employs two length scales, i.e., pre-transition length scale and turbulent length scale pertaining to the regions upstream and the downstream, respectively, and a universal model of stream-wise intermittency variation is used as a function bridging the pre-transition region and the fully turbulent region. The proposed model is applied to calculate three benchmark cases of the transitional boundary layer flows with different free-stream turbulent intensity (1%∼6%) under zero-pressure gradient. It was found that the profiles of mean velocity and turbulent intensity, local maximum of velocity fluctuations, their locations as well as the stream-wise variation of integral properties such as skin friction, shape factor and maximum velocity fluctuations are very satisfactorily predicted throughout the flow regions.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2004.03a
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• pp.426-432
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• 2004

Large-Eddy Simulation (LES) is applied for the simulation of compressible flat plate boundary with Reynolds number up to 5 X 10$^{5}$ . Numerical examples include shock/boundary layer interaction and boundary layer transition, aiming future application to the analysis of transonic fan/compressor cascades. The present LES code uses hybrid com-pact/WENO scheme for the spatial discretization and compact diagonalized implicit scheme for the time integration. The present code successfully predicted the bypass transition of subsonic boundary layer. As for supersonic turbulent boundary layer, mean and fluctuation velocity of the attached boundary, as well as the evolution of the friction coefficient and the displacement thickness both upstream and downstream of the separation region are all in good agreement with experiment. The separation point also agreed with the experiment. In the simulation of the shock/laminar boundary layer interaction, the dependence of the transition upon the shock strength is reproduced qualitatively, but the extent of the separation region is overpredicted. These numerical examples show that LES can predict the behavior of boundary layer including transition and shock interaction, which are hardly managed by the conventional Reynolds-averaged Navier-Stokes approach, although there needs to be more effort before achieving quantitative agreement.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.3
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• pp.319-325
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• 2003

We investigate the spectra and the pseudospectra in plane Poiseuille flow, plane Couette flow and Blasius flow. At subcritical Reynolds number, the spectra are lied strictly inside the stable complex half-plane, but the pseudospectra are lied in the unstable half-plane, reflecting the large linear transient growth that certain perturbations may excite. It means that the smooth flows may become to turbulent even though all the eigenmodes decay monotonically. We found that pseudospectra is one reason that causes subcritical transition in plane Poiseuille flow and plane Couette flow and bypass transition in Blasius flow.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2004.03a
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• pp.513-520
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• 2004

A steady-state/transient performance simulation model was newly developed for the propulsion system of the CRW (Canard Rotor Wing) type UAV (Unmanned Aerial Vehicle) during flight mode transition. The CRW type UAV has a new concept RPV (Remotely Piloted Vehicle) which can fly at two flight modes such as the take-off/landing and low speed forward flight mode using the rotary wing driven by engine bypass exhaust gas and the high speed forward flight mode using the stopped wing and main engine thrust. The propulsion system of the CRW type UAV consists of the main engine system and the duct system. The flight vehicle may generally select a proper type and specific engine with acceptable thrust level to meet the flight mission in the propulsion system design phase. In this study, a turbojet engine with one spool was selected by decision of the vehicle system designer, and the duct system is composed of main duct, rotor duct, master valve, rotor tip-jet nozzles, and variable area main nozzle. In order to establish the safe flight mode transition region of the propulsion system, steady-state and transient performance simulation should be needed. Using this simulation model, the optimal fuel flow schedules were obtained to keep the proper surge margin and the turbine inlet temperature limitation through steady-state and transient performance estimation. Furthermore, these analysis results will be used to the control optimization of the propulsion system, later. In the transient performance model, ICV (Inter-Component Volume) model was used. The performance analysis using the developed models was performed at various flight conditions and fuel flow schedules, and these results could set the safe flight mode transition region to satisfy the turbine inlet temperature overshoot limitation as well as the compressor surge margin. Because the engine performance simulation results without the duct system were well agreed with the engine manufacturer's data and the analysis results using a commercial program, it was confirmed that the validity of the proposed performance model was verified. However, the propulsion system performance model including the duct system will be compared with experimental measuring data, later.

• Journal of Chest Surgery
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• v.53 no.1
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• pp.8-15
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• 2020

Background: We aimed to investigate the associations of critical care provided in a cardiac surgical intensive care unit (CSICU) staffed by an attending intensivist with improvements in intensive care unit (ICU) quality and reductions in postoperative complications. Methods: Patients who underwent elective isolated coronary artery bypass grafting (CABG) between January 2007 and December 2012 (the control group) were propensity-matched (1:1) to CABG patients between January 2013 and June 2018 (the intensivist group). Results: Using propensity score matching, 302 patients were extracted from each group. The proportion of patients with at least 1 postoperative complication was significantly lower in the intensivist group than in the control group (17.2% vs. 28.5%, p=0.001). In the intensivist group, the duration of mechanical ventilation (6.4±13.7 hours vs. 13.7±49.3 hours, p=0.013) and length of ICU stay (28.7±33.9 hours vs. 41.7±90.4 hours, p=0.018) were significantly shorter than in the control group. The proportions of patients with prolonged mechanical ventilation (2.3% vs. 7.6%, p=0.006), delirium (1.3% vs. 6.3%, p=0.003) and acute kidney injury (1.3% vs. 5.3%, p=0.012) were significantly lower in the intensivist group than in the control group. Conclusion: A transition from an open ICU model with trainee coverage to a closed ICU model with attending intensivist coverage can be expected to yield improvements in CSICU quality and reductions in postoperative complications.

• Journal of the Korea Society of Computer and Information
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• v.24 no.6
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• pp.99-107
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• 2019

With the development of various types of military Unmanned Aircraft(UA)s, the need for interworking and integration between different platforms gradually increased. In order to ensure interoperability at each military UA System(UAS) level, North Atlantic Treaty Organization(NATO) has established STANAG-4586 "Standard Interfaces of Unmanned Aircraft(UA) Control Systems(UCS) for NATO UA Interoperability-Interface Control Document". This paper looks at the basic design structure of STANAG-4586 and the changes on Edition 4 to enhance joint operational capability through reflecting and updating the interoperability design of the military UAS. In particular, we analyze the enhanced Datalink Transition/Handover Procedure and Autonomous functions, one of the biggest features added to the edition. Through this, we propose a modification of UA data link exclusive control using UA Bypass structure, which was impossible in the one-to-one communication structure between existing UA and Core UCS(CUCS). We also suggest ways to improve UA operational reliability by applying Autonomous Functions that directly decides how to deal with emergency situations, rather than by a remote operator over CUCS.

• Journal of Chest Surgery
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• v.57 no.3
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• pp.312-314
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• 2024

A 3-year-old boy with Glenn physiology exhibited refractory heart failure with reduced ejection fraction. To improve the patient's oxygen saturation, he underwent ventricular assist device (VAD) implantation with concomitant Fontan completion. The extracardiac conduit Fontan operation was performed with a 4-mm fenestration. For VAD implantation, Berlin Heart cannulas were positioned at the left ventricular apex and the neo-aorta. Following weaning from cardiopulmonary bypass, a temporary continuous-flow VAD, equipped with an oxygenator, was utilized for support. After a stabilization period of 1 week, the continuous-flow VAD was replaced with a durable pulsatile-flow device. Following 3 months of support, the patient underwent transplantation without complications. The completion of the Fontan procedure at the time of VAD implantation, along with the use of a temporary continuous-flow device with an oxygenator, may aid in stabilizing postoperative hemodynamics. This approach could contribute to a safe transition to a durable pulsatile VAD in patients with Glenn physiology.