• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.37 no.4
• /
• pp.336-343
• /
• 2009

Secondary flow losses can be as high as 30~50% of the total aerodynamic losses generated in the cascade of a turbine. Therefore, these are important part for improving a turbine efficiency. As well, many studies have been performed to decrease the secondary flow losses. The present study deals with the leading edge fences on a wing-body to decrease a horseshoe vortex, one of the factors to generate the secondary flow losses, and investigates the characteristics of the generated horseshoe vortex as the shape factors, such as the installed height, and length of the fence. The study was investigated using $FLUENT^{TM}$. Total pressure loss coefficient was improved about 4.0 % at the best case than the baseline.

• The Bulletin of The Korean Astronomical Society
• /
• v.40 no.1
• /
• pp.63.1-63.1
• /
• 2015

In this study, we determine coronal electron density distributions by analyzing DH type II radio observations based on the assumption: a DH type II radio burst is generated by the shock formed at a CME leading edge. For this, we consider 11 Wind/WAVES DH type II radio bursts (from 2000 to 2003 and from 2010 to 2012) associated with SOHO/LASCO limb CMEs using the following criteria: (1) the fundamental and second harmonic emission lanes are well identified; (2) its associated CME is clearly identified in the LASCO-C2 or C3 field of view at the time of type II observation. For these events, we determine the lowest frequencies of their fundamental emission lanes and the heights of their leading edges. Coronal electron density distributions are obtained by minimizing the root mean square error between the observed heights of CME leading edges and the heights of DH type II radio bursts from assumed electron density distributions. We find that the estimated coronal electron density distribution ranges from 2.5 to 10.2-fold Saito's coronal electron density models.

• Journal of Power System Engineering
• /
• v.17 no.3
• /
• pp.35-43
• /
• 2013

Generally, the chassis of an indoor RAC is composed of a rear-guider and a stabilizer. The aerodynamic performance of a cross-flow fan is strongly influenced by the various design factors of the chassis of an indoor RAC. The purpose of this paper is to select the optimum design factors through the aerodynamic performance of a cross-flow fan. The design factors are the leading angle of a rear-guider (${\theta}_1$), a stabilizer setup angle(${\theta}_2$), a rear-guider clearance(${\epsilon}_1$), and a stabilizer clearance(${\epsilon}_2$), respectively. As a result, the optimum design factors of an indoor RAC can be presented as a combination of ${\theta}_1=33^{\circ}$, ${\theta}_2=55^{\circ}$, ${\epsilon}_1=6{\sim}8mm$, and ${\epsilon}_2=7mm$ through the analysis of a static pressure coefficient and a static pressure efficiency.

• Journal of Contemporary Eastern Asia
• /
• v.13 no.1
• /
• pp.69-81
• /
• 2014

While much in the field of public management has emphasized the importance of nonprofit advocacy activities in policy and decision-making procedures, few have considered the relevance and impact of leading actors on structuring diverse patterns of information sharing and communication through social media. Building nonprofit advocacy is a complicated process for a single organization to undertake, but social media applications such as Facebook and Twitter have facilitated nonprofit organizations and stakeholders to effectively share information and communicate with each other for identifying their mission as it relates to environmental issues. By analyzing the Australian Marine Conservation Society's (AMCS) Twitter network data from the period 1 April to 20 April, 2013, this research discovered diverse patterns in nonprofit advocacy by leading actors in building advocacy. Based on the webometrics approach, analysis results show that nonprofit advocacy through social media is structured by dynamic information flows and intercommunications among participants and followers of the AMCS. Also, the findings indicate that the news media and international and domestic nonprofit organizations have a leading role in building nonprofit advocacy by clustering with their followers.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.23 no.3
• /
• pp.419-430
• /
• 1999

Measurements of local blowing ratio and ammonia-diazo flow visualizations have been conducted for a shower-head film cooling on a first-stage turbine stator. In this study, six rows of normal holes are drilled symmetrically on the semicircular leading edge of a simulated blunt body. The measurements show that for an average blowing ratio based on freestream velocity, M, of 0.5, local average mass flow rate through the first two rows of the holes is less than those through the second and third two rows of the holes, and the fraction of mass flow rate through the first two rows to total mass flow rate has a tendency to increase with the increment of M. The flow visualizations reveal that the injection through the first two row results in inferior film coverage even In the case of M = 0.5, meanwhile the row of holes situated at farther downstream location provides higher film-cooling performances for all tested M. This is because film-cooling effectiveness depends on local mainflow velocity at the hole location as well as the mass flow rate through each row.

• Journal of the Korean Society of Visualization
• /
• v.15 no.3
• /
• pp.52-56
• /
• 2017

In this study, we investigate the flow characteristics of lift-type disk which behaves the up-down motion using the large eddy simulation (LES) and immersed boundary method (IBM). Also, we perform the noise analysis using pressure field at 1.35 m distance and reveal the cause of noise to observe the vortical structure analysis of flow result. It is observed that vortical structure and wind shear were generated at leading edge and tower with high velocity deficit and flow separation. High magnitude of flow noise was observed in low frequency range which is from 30 Hz to 60 Hz. It was observed that vortical structure at leading edge was generated in frequency range from 33.3 Hz to 41.6 Hz. Temporal characteristic in vortical structure at leading edge was similar to noise characteristics, having the similar frequency ranges.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.24 no.2
• /
• pp.177-187
• /
• 2000

Measurements of temperature fields and film-cooling effectiveness have been conducted for a shower-head film cooling on the leading edge of a blunt body, which simulates a first-stage turbine stator. In this study, three injection cases are employed for an average blowing ratio based on freestream velocity, M, of 0.5, 1.0 and 1.5. Two (Case 1), four (Case 2) and six (Case 3) rows of normal holes are symmetrically drilled on the three tested circular-cylinder leading edges. The measurements show that regardless of M, the film-cooling effectiveness increases as the injection row is situated at farther downstream location. In Case 1, the film-cooling effectiveness is highest for M = 0.5 and lowest for M = 1.5. On the contrary, in Case 3, the film-cooling effectiveness is highest for M = 1.0 and lowest for M = 0.5. When M = 0.5, the film coverage by the first row of the injection holes deteriorates as the number of the injection row increases. In particular, the film-cooling effectiveness due to the injection through the first row of the holes in Case 3, has a nearly zero value.

• Transactions of the Korean Society of Mechanical Engineers B
• /
• v.28 no.6
• /
• pp.672-681
• /
• 2004

Direct numerical simulation was carried out to study the vortical structures of the flow around a wall-mounted cube in a channel at Re=1,000 and Re=3,500 based on cubic height and bulk mean velocity. The cubic obstacle is situated in the entrance region of the channel flow where the boundary layers are developing. Upstream of the obstacle, steady and unsteady laminar horseshoe vortex systems are observed at Re=1,000 and Re=3,500, respectively; the near-wake flow is turbulent in both cases. The flow separates at each leading sharp edge of the cube, and subsequent vortex roll-up is noticed in the corresponding free-shear layer. The vortex shedding from the upper leading edge (upper vortices) and that from the two lateral leading edges (lateral vortices) are both quasi-periodic and their frequencies are computed. The upper and lateral vortices further develop into hairpin and Λ vortices, respectively. A series of instantaneous contours of the second invariant of velocity gradient tensor helps us identify spatial and temporal behaviors of the vortices in detail. The results indicate that the length and time scales of the vortical structures at Re=3,500 are much shorter than those at Re:1,000. Correlations between the upper and lateral vortices are also reported.

• Transactions of the Korean Society of Mechanical Engineers
• /
• v.19 no.4
• /
• pp.1134-1147
• /
• 1995

POD (proper orthogonal decomposition) is applied to turbulent leading-edge separation bubble to extract coherent structures. A two-dimensional leading-edge separation bubble is simulated by discrete-vortex method, where a time-dependent source forcing is incorporated. Based on the wealth of numerical data, POD is applied in a range of the forcing amplitude ( $A_{o}$ = 0, 0.5, 1.0 and 1.5) and forcing frequency (0 .leq. $f_{F}$H/ $U_{\infty}$.leq. 0.3). It is demonstrated that the structures of POD have noticeable changes with local forcings. In an effort to investigate the mechanism of decreasing reattachment length, dynamic behaviors of the expansion coefficients and contributions of the eigenfunctions of POD are scrutinized. As the forcing amplitude increases, the large-scale vortex structures are formed near the forcing amplitude increases, the large-scale vortex structures are formed near the separation point and the flow structures become more organized and more regular, accompanying with the reduction of reattachment length. By further inverstigation of POD global entropy, it is seen that the reattachment length is closely linked to the degree of organization of the flow structures.es.s.

• 한국전산유체공학회:학술대회논문집
• /
• 2009.04a
• /
• pp.43-50
• /
• 2009

In order to reduce the download around Smart UAV(SUAV) at hovering and transition mode, flow control using synthetic jet has been performed. Many of the complex tilt rotor flow features are captured including wing leading and trailing edge separation, and the large region of separated flow beneath the wing. First, in order to control the trailing edge separation, synthetic jet is located at 30, 95% of flap chord length. The flow control using synthetic jet on flap shows that stall characteristics depending on several mode can be improved through separation vortices resizing. Also, a flap jet and a 0.01c jet which control the separation efficiently are applied at the same time at each test case because controlling the leading edge separation is essential for download reduction. As a result, time averaged download is reduced about 18% comparing with no control case at hovering mode and 48% at transition mode. These research results show that if flow control using leading edge jet and trailing edge jet is used effectively to the SUAV in overall flight mode, flight performance and stability can be improved.