• Wind and Structures
• /
• 제11권5호
• /
• pp.377-389
• /
• 2008

An experimental investigation has been conducted to determine the effectiveness of base shield plates in reducing the drag of a rough circular cylinder in a cross flow at Reynolds numbers in the range $3{\times}10^4{\leq}Re{\leq}10.5{\times}10^4$. Three model configurations were investigated and compared: a plane cylinder (PC), a cylinder with a splitter plate (MC1) and a cylinder fitted with base shield plates (MC2). Each configuration was studied in the sub and supercritical flow regimes. The chord of the plates, L, ranged from 0.22 to 1.50D and the cavity width, G, between the plates was in the range from 0 to 0.93D. It is recognized that base shield plates can be employed more effectively than splitter plates to reduce the aerodynamic drag of circular cylinders in both the sub- and supercritical flow regimes. For subcritical flow regime, one can get 53% and 24% drag reductions for the MC2 and MC1 models with L/D=1.0, respectively, compared with the PC model. For supercritical flow regime however, the corresponding drag reductions are 38% and 7%.

• 설비공학논문집
• /
• 제11권4호
• /
• pp.448-456
• /
• 1999

Variations of thickness and effective thermal conductivity of frost forming on the horizontal] cylinder with respect to time were measured under cross flow. The local heat flux around the cylinder was determined by measuring the radial temperature distribution in the cylinder having small holes drilled axially in which T-type thermocouples were inserted, then by using one dimensional cylindrical heat conduction equation. The thickness and the surface temperature of the frost layer around the cylinder were measured periodically while developing the frost. Each experiment was peformed by varying the Reynolds number, the temperature, and the humidity condition. Specially the dew point temperature of the most cases was below the freezing point. Experimental data showed that the frost layers on the front and the rear surface were thicker than those on the top and the bottom one which was near the separation point. The thickness and effective thermal conductivity of the frost layer were affected by inlet air velocity, temperature, and humidity. Moreover, the effective thermal conductivity and the effective thermal resistance increase with respect to time.

• Journal of Mechanical Science and Technology
• /
• 제19권2호
• /
• pp.704-715
• /
• 2005

In this study, to investigate in-cylinder tumble or swirl intake flow of a gasoline engine, the flow characteristics were examined with opening control valve (OCV) and several swirl control valves (SCV) which intensify intake flow through steady flow experiment, and also turbulent characteristics of in-cylinder flow field were investigated by 2-frame cross-correlation particle image velocimetry (PIV) method. In the investigation of intake turbulent characteristics using PIV method, the different flow characteristics were showed according to OCV or SCV figures. The OCV or SCV installed engine had higher vorticity and turbulent kinetic energy than a baseline engine, especially around the wall and lower part of the cylinder. Above all, SCV B type was superior to the others. About energy dissipation and reynolds shear stress distribution, a baseline engine had larger loss than OCV or SCV installed one because flow impinged on the cylinder wall. It should be concluded, from what has been said above, as swirl component was added to existing tumble flow adequately, it was confirmed that turbulent intensity was enlarged, flow energy was conserved effectively through the experiment. In other words, there is a suggestion that flow characteristics as these affected to in-cylinder combustion positively.

• 대한조선학회논문집
• /
• 제47권6호
• /
• pp.782-786
• /
• 2010

Thrust deduction related to the prediction of power performance of a ship is rather resistance increase, and as a preliminary study for it forces upon a circular cylinder in a uniform flow of ideal fluid due to singularities located behind it are investigated. The circle theorem is used to get the complex velocity potential for the flow field under consideration, and the Blasius theorem is applied to obtain forces upon the circular cylinder. As singularities sinks, point vortices and dipoles and their combinations are treated. $\varepsilon$, standing for the strength of a singularity, and $\delta$, representing the distance between the cylinder and the singularity, are important small parameters for the resistance and lateral forces. For sinks or point vortices it is shown that the dimensionless forces upon the cylinder is O($\epsilon$) if $\epsilon$= O($\delta$) is assumed, and the same holds for dipoles if $\epsilon$= O(${\delta}^3$) is supposed. Forces upon the cylinder by a symmetric pair of sinks are greater than a single sink located at the central plane since there is an additional term due to cross effects, and the same is also valid for the case of dipole. Combination of dipole and a point vortex is also considered and a few new aspects are clarified.

• 한국해양공학회지
• /
• 제15권3호
• /
• pp.1-8
• /
• 2001

With uniform heat generation within the wall of the cylinder placed in a cross flow, heat flows by conduction in the circumferential direction due to the asymmetric nature of the fluid flow around the perimeter of the cylinder. The circumferential heat flow affects the wall temperature distribution to such an extent that in some cases significantly different results may be obtained for geometrically similar surfaces. In the present investigation, the effects of circumferential wall heat conduction on local convective heat transfer is investigated for the case of forced convection around horizontal cylinder in cross flow of air. Two-dimensional temperature distribution $T_w$/(${\gamma}$,${\theta}$) is presented through the numerical analysis. The one-dimensional and two-dimensional solutions are in good agreement with experimental results of local heat transfer coefficients.

• International Journal of Naval Architecture and Ocean Engineering
• /
• 제8권4호
• /
• pp.330-343
• /
• 2016

This paper presents results of an experimental investigation of vortex-induced vibration (VIV) of a flexibly mounted and rigid cylinder with two-degrees-of-freedom with respect to varying ratio of in-line natural frequency to cross-flow natural frequency, $f^*$, at a fixed low mass ratio. Combined in-line and cross-flow motion was observed in a sub-critical Reynolds number range. Three-dimensional displacement meter and tension meter were used to measure dynamic responses of the model. To validate the results and the experiment system, x and y response amplitudes and ratio of oscillation frequency to cross-flow natural frequency were compared with other experimental results. It has been found that the higher harmonics, such as third and more vibration components, can occur on a certain part of steel catenary riser under a condition of dual resonance mode. In the present work, however, due to the limitation of a size of circulating water channel, the whole test of a whole configuration of the riser at an adequate scale for VIV phenomenon was not able to be conducted. Instead, we have modeled a rigid cylinder and assumed that the cylinder is a part of steel catenary riser where the higher harmonic motions could occur. Through the experiment, we have found that even though the cylinder was assumed to be rigid, the occurrence of the higher harmonic motions was observed in a small reduced velocity ($V_r$) range, where the influence of the in-line response is relatively large. The transition of the vortex shedding mode from one to another was examined by using time history of x and y directional displacement over all experimental cases. We also observed the influence of in-line restoring force power spectral density with $f^*$.

• Wind and Structures
• /
• 제30권5호
• /
• pp.525-532
• /
• 2020

This work numerically investigates the effects of Reynolds number Re_D (= 100 - 150), cross-sectional aspect ratio AR = ( 0.25 -1.0), and attack angle α (= 0° - 90°) on the forces, Strouhal number, and wake of an elliptical cylinder, where Re_D is based on the freestream velocity and cylinder cross-section height normal to the freestream flow, AR is the ratio of the minor axis to the major axis of the elliptical cylinder, and α is the angle between the cylinder major axis and the incoming flow. At Re_D = 100, two distinct wake structures are identified, namely 'Steady wake' (pattern I) and 'Karman wake followed by a steady wake (pattern II)' when AR and α are varied in the ranges specified. When Re_D is increased to 150, an additional wake pattern, 'Karman wake followed by secondary wake (pattern III)' materializes. Pattern I is characterized by two steady bubbles forming behind the cylinder. Pattern II features Karman vortex street immediately behind the cylinder, with the vortex street transmuting to two steady shear layers downstream. Inflection angle α_i = 32°, 37.5° and 45° are identified for AR = 0.25, 0.5 and 0.75, respectively, where the wake asymmetry is the greatest. The α_i effectively distinguishes the dependence on α and AR of force and vortex shedding frequency at either Re_D. In Pattern III, the Karman street forming behind the cylinder is modified to a secondary vortex street. At a given AR and α, Re_D = 150 renders higher fluctuating lift and Strouhal number than Re_D = 100.

• 한국전산유체공학회:학술대회논문집
• /
• 한국전산유체공학회 2008년도 학술대회
• /
• pp.131-136
• /
• 2008

Three-dimensional characteristics of fluid flow and heat transfer around a wavy circular cylinder having sinusoidal variation in cross sectional area along the spanwise direction are numerically investigated using the immersed boundary method. The three different wavelengths of ${\pi}/4$, ${\pi}/3$ and ${\pi}/2$ and at the fixed wavy amplitude of 0.1 have been considered to investigate the effects of waviness on especially the forced convection heat transfer around a wavy cylinder when the Reynolds and Prandtl numbers are 300 and 0.71, respectively. The present computational results for a wavy cylinder are compared with those for a smooth cylinder. The time- and total surface-averaged Nusselt number for a wavy cylinder with is larger than that for a smooth cylinder, whereas that with ${\lambda}={\pi}/4$ and ${\pi}/3$ is smaller than that for a smooth cylinder. However, because the surface area exposed to heat transfer for a wavy cylinder is larger than that for a smooth cylinder, the total heat transfer rate for a wavy cylinder with different wavelengths of ${\lambda}={\pi}/4$, ${\pi}/3$ and ${\pi}/2$ is larger than that for a smooth cylinder.

• 대한기계학회논문집B
• /
• 제33권2호
• /
• pp.124-132
• /
• 2009

Experiments have been performed to investigate fluid-elastic instability of tube bundles, subjected to twophase cross flow. Fluid-elastic is the most important vibration excitation mechanism for heat exchanger tube bundles subjected to the cross flow. The test section consists of cantilevered flexible cylinder(s) and rigid cylinders of normal square array. From a practical design point of view, fluid-elastic instability may be expressed simply in terms of dimensionless flow velocity and dimensionless mass-damping parameter. For dynamic instability of cylinder rows, added mass, damping and the threshold flow velocity are evaluated. The Fluid-elastic instability coefficient is calculated and then compared to existing results given for tube bundles in normal square array.

• 대한전자공학회논문지
• /
• 제14권3호
• /
• pp.1-5
• /
• 1977

압축성 plasma내에서 sheath로 둘러쌓인 원주형도체에 의한 전자파의 산란에 관하여 해석적인 방법으로 조사하였다. 전자파가 입사하는 경우에 대하여 total scattering cross section과 back scattering cross section을 구하였으며 computer로 계산한 수치결과를 graph로 제시하였다.