• Korean Journal of Air-Conditioning and Refrigeration Engineering
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• v.14 no.10
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• pp.833-843
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• 2002

The present experimental and numerical investigations are performed for the characteristics of transitional flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one rotating. The pressure losses and skin- friction coefficients have been measured for the fully developed flow of water and glycerine-water solution (44%) with the inner cylinder rotating at speed of 0∼600 nm, respectively. The transitional flow has been examined by the measurement of pressure losses to reveal the relation of the Reynolds and Rossby numbers with the skin-friction coefficients. The occurrence of transition has been checked by the gradient changes of pressure losses and skin-friction coefficients with respect to the Reynolds numbers. The increasing rate of skin-friction coefficient due to the rotation is uniform for laminar flow regime, whereas it is suddenly reduced for transitional flow regime and, then, it is gradually declined for turbulent flow regime.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.31 no.7
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• pp.15-25
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• 2003

An experimental. study is carried out to investigate the near-wake characteristics of an airfoil oscillating in pitch. An NACA 4412 airfoil is sinusoidally pitched about the quarter chord point between the angle of attack -6$^{\circ}$ and +6$^{\circ}$. A hot-wire anemometer is used to measure the phase-averaged mean velocities in the near-wake region of an oscillating airfoil. The freestream velocities of present work are 3.4, 12.4, 26.2 m/s, and the corresponding Reynolds numbers are 5.3${\times}10^4$, 1.9${\times}10^5$, 4.l${\times}10^5$, and the reduced frequency is 0.1. Streamwise velocity profiles are presented to show the Reynolds number effects on the near-wake region behind an airfoil oscillating in pitch. All the cases in these measurements show that the velocity defects by the change of the Reynolds number are very large at the lowest Reynolds number $R_N$=5.3${\times}10^4$: and are small at the other Reynolds numbers ($R_N$=1.9${\times}10^5$ and 4.l${\times}10^5$) in the near-wake region. A significant difference of phase-averaged mean velocity between 5.3${\times}10^4$, and 1.9${\times}10^5$ is observed. The present study shows that a critical value of Reynolds number in the near-wake of an oscillating airfoil exists in the range between 5.3${\times}10^4$, and 1.9${\times}10^5$.

• Journal of the Korean Society of Industry Convergence
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• v.9 no.4
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• pp.247-253
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• 2006

Power consumption for double-stage paddle impeller in spherical and cylindrical agitated vessel was measured over a wide range of Reynolds number from laminar to turbulent flow regions. The power correlation was obtained which was applied to both spherical and cylindrical vessel, when the apparent diameter of the spherical vessel was equal to the diameter of the cylindrical vessel which had a height equal to its diameter and had the same volume as the spherical vessel. The power consumption for the double-stage impeller was dependent upon the distance of among the impeller in the agitated vessels, as follows: $$f/2={\frac{C_L}{Re_G}}+{\frac{Ct}{2}}({\frac{C_tr}{Re_g}}+Re_g)^{-m}$$

• Proceedings of the KSME Conference
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• 2002.08a
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• pp.521-524
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• 2002

In the present study, kinetic energy of laminar steady flow 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 kinetic energy distributions by using the Particle Image Velocimetry(PIV) system with the data acquisition and processing system of Cactus 2000 software. The results obtained from experimental studies are summarized as follows : (1) The critical Reynolds number for a change from laminar steady flow to transitional steadt flow was about 1910, in the 50 region of dimensionless axial position (x/Dh) whirh was considered as a fully developed flow region. (2) Maximum kinetic energy of laminar steady flow was gradually increased as the Reynolds number increased.

• Proceedings of the KSME Conference
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• 2001.11b
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• pp.545-550
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• 2001

A two-dimensional laminar flow through a channel, on which a couple of symmetric vertical fins are attached, is investigated. The stokes flow for this channel flow is investigated analytically and laminar flow numerically. For analytic solution, the method of eigen function expansion and collocation method are employed. For numerical solution, finite difference method(FDM) is used to obtain vorticity and stream function. From the results, streamline patterns are shown and the pressure drop due to the attached fins is calculated, which depends on the length of fins and Reynolds number. While $Re, streamline pattern is symmetric, a pair of additional asymmetric solutions appear for $Re>Re_c$, where the critical Reynolds number $Re_c$ depends on the length of the fin.

• Journal of the Korean Society for Aviation and Aeronautics
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• v.17 no.1
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• pp.29-38
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• 2009

This paper surveys the historical perspective and design considerations for developing the human powered aircraft(HPA). Especially the weight and materials, aerodynamics, flight controls, and power trains are focused. The average power a human can produce and sustain is approximately 200${\sim}$250 W which is a critical design constraint of HPA. The survey shows that the empty weight of HPA was in the 30${\sim}$40 kg range(90${\sim}$110 kg include pilot). Thus, in order to design a successful HPA, the value of power to weight ratio should be 2.0 W/kg or above. The HPA design technique could be applied directly to the development of an unmanned high altitude airplanes used for atmospheric research, where light structures, low Reynolds number aerodynamics and high efficiency propeller design are required as well.

• Journal of ILASS-Korea
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• v.9 no.4
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• pp.83-89
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• 2004

Vaporization characteristics of a liquid heptane droplet in high-pressure and temperature flow field are numerically studied. Variable thermodynamic and transport properties and high-pressure effects are taken into account in order to consider real gas effects. Droplet Vaporization in convective environments was investigated on the basis of droplet vaporization in quiescent and convective environment. In quiescent environments, droplet lifetime is directly proportional to pressure at the subcritical temperature range but it is inversely proportional to pressure at the supercritical temperature range. In convective environment, droplet deformation becomes stronger by increasing Reynolds number due to increase of velocity while droplet deformation is relatively weak at a higher pressure for the same Reynolds number cases.

• Wind and Structures
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• v.16 no.5
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• pp.457-476
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• 2013

This paper presents theoretical background for a semi-empirical, mathematical model of critical vortex excitation of slender structures of compact cross-sections. The model can be applied to slender tower-like structures (chimneys, towers), and to slender elements of structures (masts, pylons, cables). Many empirical formulas describing across-wind load at vortex excitation depending on several flow parameters, Reynolds number range, structure geometry and lock-in phenomenon can be found in literature. The aim of this paper is to demonstrate mathematical background of the vortex excitation model for a theoretical case of the structure section. Extrapolation of the mathematical model for the application to real structures is also presented. Considerations are devoted to various cases of wind flow (steady and unsteady), ranges of Reynolds number and lateral vibrations of structures or their absence. Numerical implementation of the model with application to real structures is also proposed.

• Steel and Composite Structures
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• v.30 no.3
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• pp.281-292
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• 2019

In this paper, analysis of critical fluid velocity and heat transfer in the nanocomposite pipes conveying nanofluid is presented. The pipe is reinforced by carbon nanotubes (CNTs) and the fluid is mixed by $AL_2O_3$ nanoparticles. The material properties of the nanocomposite pipe and nanofluid are considered temperature-dependent and the structure is subjected to magnetic field. The forces of fluid viscosity and turbulent pressure are obtained using momentum equations of fluid. Based on energy balance, the convection of inner and outer fluids, conduction of pipe and heat generation are considered. For mathematical modeling of the nanocomposite pipes, the first order shear deformation theory (FSDT) and energy method are used. Utilizing the Lagrange method, the coupled pipe-nanofluid motion equations are derived. Applying a semi-analytical method, the motion equations are solved for obtaining the critical fluid velocity and critical Reynolds and Nusselt numbers. The effects of CNTs volume percent, $AL_2O_3$ nanoparticles volume percent, length to radius ratio of the pipe and shell surface roughness were shown on the critical fluid velocity, critical Reynolds and Nusselt numbers. The results are validated with other published work which shows the accuracy of obtained results of this work. Numerical results indicate that for heat generation of $Q=10MW/m^3$, adding 6% $AL_2O_3$ nanoparticles to the fluid increases 20% the critical fluid velocity and 15% the Nusselt number which can be useful for heat exchangers.

• 한국전산유체공학회:학술대회논문집
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• 2001.10a
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• pp.84-90
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• 2001

Secondary instability of flow past a circular cylinder is examined using direct numerical simulation at Reynolds number 220 and 250. The higher-order finite difference scheme is employed for the spatial distributions along with the second order Adams-Bashforth and the first order backward-Euler time integration. In x-y plane, the convection term is applied by the 5th order upwind scheme, and the pressure and viscosity terms are applied by the 4th order central difference. In spanwise, Navier-Stokes equation is distributed using Spectral Method. The critical Reynolds number for this instability is found to be about Re=190. The secondary instability leads re three-dimensionality with a spanwise wavelength about 4 cylinder diameters at onset (A-mode). Results of three-dimensional effect in wake of a circular cylinder are represented with spanwise and streamwise vorticity contours as Reynolds numbers.