• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.36 no.9
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• pp.835-840
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• 2008

We investigate the aerodynamic performance of the dragonfly wing which has the cross-sectional corrugation by using the static 2-dimensional unsteady simulation. Computational condition is at Re=150, 1400 and 10,000 with the angles of attack from 0 to 40 degrees. As computational results, the increment of the lift coefficient by corrugation is nearly constant over the critical angle of attack. Also, upper side corrugation of the wing have very little influence on increase of the lift coefficient.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.17 no.1
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• pp.8-15
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• 2005

This study concerns the characteristics of helical flow in a concentric and eccentric annulus with a diameter ratio of 0.52 and 0.9, whose outer cylinders are stationary and inner ones are rotating. Pressure losses and skin friction coefficients have been measured for fully developed flows of water and $0.2\%$ aqueous of sodium carboxymethyl cellulose(CMC), respectively, when the inner cylinder rotates at the speed of $0\~500$ rpm. The effect of rotation on the skin friction coefficient is significantly dependent on the flow regime. In all flow regimes, the skin friction coefficient is increased by the inner cylinder rotation. This study shows the change of skin friction coefficient and wall shear stress corresponding to the variation of rotating speed of the inner cylinder, radius ratio, eccentricity, and working fluids.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.27 no.6
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• pp.766-772
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• 2003

The pseudospectral method for stability analysis was used to find the most influential disturbance mode for transition of plane channel flows and Blasius flow at their critical Reynolds numbers. A number of various oblique disturbance waves were investigated for their pseudospectra and resolvent norm contours in each flow, and an exhaustive search method was employed to find the disturbing waves to which the flows become most unstable. In plane Poiseuille flow an oblique disturbance with a wavelength of 3.59h (where h is the half channel width) at an angle $28.7^{\circ}$ was found to be the most influential for the flow transition to turbulence, and in plane Couette flow it is an oblique wave with a wavelength of 3.49h at an angle of $19.4^{\circ}$. But in Blasius flow it was found that the most influential mode is a normal wave with a wavelength of $3.44{\delta}_{999}$. These results imply that the most influential disturbance mode is closely related to the fundamental acoustic wave with a certain shear sheltering in the respective flow geometry.

• International Journal of Naval Architecture and Ocean Engineering
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• v.7 no.5
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• pp.906-919
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• 2015

The Deep Draft Semi-Submersible (DDS) concepts are known for their favourable vertical motion performance. However, the DDS may experience critical Vortex-Induced Motion (VIM) stemming from the fluctuating forces on the columns. In order to investigate the current-induced excitation forces of VIM, an experimental study of flow characteristics around four square-section cylinders in a square configuration is presented. A number of column spacing ratios and array attack angles were considered to investigate the parametric influences. The results comprise flow patterns, drag and lift forces, as well as Strouhal numbers. It is shown that both the drag and lift forces acting on the cylinders are slightly different between the various L/D values, and the fluctuating forces peak at L/D = 4.14. The lift force of downstream cylinders reaches its maximum at around ${\alpha}=15^{\circ}$. Furthermore, the flow around circular-section-cylinder arrays is also discussed in comparison with that of square cylinders.

• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.8 no.3
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• pp.330-337
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• 1996

The effects of pump and temperature on drag reducing characteristics were investigated with a polymer(PAAM : Polyacrylamide) and three kinds of surfactants(CTAC, STAC, Habon-G) in fully developed turbulent pipe flows with various experimental parameters such as additive concentration(30~500ppm), pipe diameter(4.65mm, 10.85mm), Reynolds number($4{\times}10^4{\sim}10^5$) and working fluid temperature($20{\sim}80^{\circ}C$). The pump effect on PAAM was severe such that the drag reduction rates obtained with pump were decreased upto 30% as compared with those obtained with compressed air in 4.65mm test section. The temperature effect on PAAM was noticeably considerable, that is, the higher temperaute, the less drag reduction rate. On the other hand, no significant pump effect on the surfactants was observed. The drag reducing effectiveness of CTAC was totally lost in the temperature ragne of 60 to $80^{\circ}C$, whereas STAC and Habon-G kept their distinct drag reducing capability at a temperature of $80^{\circ}C$. This study clearly elucidated that for DHC application of drag reducing additives, the pump and temperature effects as well as additive concentration and pipe diameter should be carefully taken into consideration.

• Wind and Structures
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• v.14 no.1
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• pp.35-53
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• 2011

Spherical object in wind is a common scenario in daily life and engineering practice. The main challenge in understanding the aerodynamics in turbulent wind lies in the multi-aspect of turbulence. This paper presents a wind tunnel study, which focuses on the role of turbulence integral length scale ${\Lambda}$ on the drag of a sphere. Particular turbulent flow conditions were achieved via the proper combination of wind speed, orifice perforated plate, sphere diameter (D) and distance downstream from the plate. The drag was measured in turbulent flow with $2.2{\times}10^4{\leq}Re{\leq}8{\times}10^4$, $0.043{\leq}{\Lambda}/D{\leq}3.24$, and turbulence intensity Tu up to 6.3%. Our results confirmed the general trends of decreasing drag coefficient and critical Reynolds number with increasing turbulence intensity. More interestingly, the unique role of the relative integral length scale has been revealed. Over the range of conditions studied, an integral length of approximately 65% the sphere diameter is most effective in reducing the drag.

• Journal of Advanced Marine Engineering and Technology
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• v.25 no.4
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• pp.817-824
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• 2001

In the present study, flow characteristics of turbulent oscillatory flows in the exit region connected to the square-sectional $180^{\circ}$curved duct was investigated experimentally. The experimental study for air flows was conducted to measure velocity profiles, shear stress distributions by using the Laser Doppler Velocimetry(L.D.V) system with the data acquisition and processing system of Rotating Machinery Resolver(R.M.R) and PHASE software. The results obtained from the experimentation were summarized as follows : The critical Reynolds number for a change from transitional oscillatory flow to turbulent oscillatory flow was about 75,000 in the 90 region of dimensionless axial position (x/Dh) which was considered as a fully developed flow region. In the turbulent oscillatory flow, velocity profiles of the inflow period in the entrance region were gradually developed, but those of the outflow period were not changed nearly. Shear stress distributions of turbulent oscillatory flow was gradually increased as the flow proceeds to downstream.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.33 no.1
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• pp.17-27
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• 2009

Water jet impingement cooling is used to remove heat from high-temperature surfaces such as hot steel plates in the steel manufacturing process (thermo-mechanical cooling process; TMCP). In those processes, uniform cooling is the most critical factor to ensure high strength steel and good quality. In this study, experiments are performed to measure the heat transfer coefficient together with the inverse heat conduction problem (IHCP) analysis for a plate cooled by planar water jet. In the inverse heat transfer analysis, spatial and temporal variations of heat transfer coefficient, with no information regarding its functional form, are determined by employing the conjugate gradient method with an adjoint problem. To estimate the two dimensional distribution of heat transfer coefficient and heat flux for planar waterjet cooling, eight thermo-couple are installed inside the plate. The results show that heat transfer coefficient is approximately uniform in the span-wise direction in the early stage of cooling. In the later stage where the forced-convection effect is important, the heat transfer coefficient becomes larger in the edge region. The surface temperature vs. heat flux characteristics are also investigated for the entire boiling regimes. In addition, the heat transfer rate for the two different plate geometries are compared at the same Reynolds number.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2017.05a
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• pp.1077-1079
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• 2017

The cavitation in the turbopump inducer progresses from the inception to the critical point, and finally develops to a breakdown which sharply declined in head. In this paper, we evaluated characteristics and predicted empirical equations about the cavitation inception of a turbopump inducer. The empirical equation of the cavitation inception for the elliptical plate was relatively well predicted to the turbopump inducer. However, in case of the marine propeller, it showed a big difference due to Reynolds number under the operating point.

• Korean Chemical Engineering Research
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• v.56 no.2
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• pp.252-260
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• 2018

Energy, a critical input, is to be efficiently managed via waste heat recovery and energy reuse for the economic viability of a process industry. In particular, cement manufacture demands a huge quantum of energy, for the necessary reactions. Huge amounts of hot effluent gases are generated. Energy recovery from these waste gases is an area that is of contemporary research interest. Now, about 75% of total heat recovery takes place in the riser of the suspension pre-heater system. This article deals with the hydrodynamic and heat transfer aspects of riser typically used in the cement industry. An experimental apparatus was designed and fabricated with provision for the measurement of gas pressure and solid temperatures at different heights of the riser. The system studied was air - chalcopyrite taken in different particle sizes. Acceleration length ($L_A$) determined at different parametric levels was fitted to an empirical correlation: $L_A/d_t=4.91902(d_p/d_t)^{0.10058}(w_s/w_g)^{-0.11691}(u_g{\mu}_g/d_t^2g{\rho}_g)^{0.28574}({\rho}_p/{\rho}_g)^{0.42484}$. An empirical model was developed for Nusselt number as a function of Reynolds and Prandtl numbers using regression analysis: $Nu=0.40969(Re_p)^{0.99953}(Pr)^{0.03569}$.