• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.6
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• pp.786-798
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• 2001

This paper describes the phenomena of wake-induced transition of the boundary layers on a NACA0012 airfoil using measured phase-averaged data. Especially, the phase-averaged wall shear stresses are reasonably evaluated using the principle of Computational Preston Tube Method. Due to the passing wake, the turbulent patch is generated in the laminar boundary layer on the airfoil and the boundary layer becomes temporarily transitional. The patches propagate downstream with less speed than free-stream velocity and merge with each other at further down stream station, and the boundary layer becomes more transitional. The generation of turbulent patch at the leading edge of the airfoil mainly depends on velocity defects and turbulent intensity profiles of passing wakes. However, the growth and merging of turbulent patches depend on local streamwise pressure gradients as well as characteristics of turbulent patches. In this transition process, the present experimental data show very similar features to the previous numerical and experimental studies. It is confirmed that the two phase-averaged mean velocity dips appear in the outer region of transitional boundary layer for each passing cycle. Relatively high values of the phase-averaged turbulent fluctuations in the outer region indicate the possibility that breakdown occurs in the outer layer not near the wall.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.30 no.9 s.252
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• pp.921-928
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• 2006

Wake-induced boundary-layer transition on a NACA0012 airfoil with zero angle of attack is experimentally investigated in periodically passing wakes under the moderate level of free-stream turbulence. The periodic wakes are generated by rotating circular cylinders clockwise or counterclockwise around the airfoil. The free-stream turbulence is produced by a grid upstream of the rotating cylinder, and its intensities $(Tu_{\infty})$ at the leading edge of the airfoil are 0.5 and 3.5%, respectively. The Reynolds number (Rec) based on chord length (C) of the airfoil is $2.0{\times}10^5$, and Strouhal number (Stc) of the passing wake is about 1.4. Time- and phase-averaged streamwise mean velocities and turbulence fluctuations are measured with a single hot-wire probe, and especially, the corresponding wall skin friction is evaluated using a computational Preston tube method. The patch under the high free-stream turbulence $(Tu_{\infty}=3.5%)$ grows more greatly in laminar-like regions compared with that under the low turbulence $(Tu_{\infty}=0.5%)$ in laminar regions. The former, however, does not greatly change the turbulence level in very near-wall region while the latter does it. At further downstream, the former interacts vigorously with high environmental turbulence inside the pre-existing transitional boundary layer and gradually loses its identification, whereas the latter keeps growing in the laminar boundary layer. The calmed region is more clearly observed under the lower free-stream turbulence level and with the receding wakes.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.25 no.12
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• pp.1821-1830
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• 2001

The flow around free end of a finite circular cylinder (FC) embedded in an atmospheric boundary layer has been investigated experimentally. The experiments were carried out in a closed-return type subsonic wind tunnel with varying aspect ratio of the finite cylinder mounted vertically on a flat plate. The wakes behind a 2-D cylinder and a finite cylinder located in a uniform flow were measured for comparison. Reynolds number based on the cylinder diameter was about Re=20,000. A hot-wire anemometer was employed to measure the wake velocity and the mean pressure distributions on the cylinder surface were also measured. The flow past the FC free end shows a complicated three-dimensional wake structure and flow phenomenon is quite different from that of 2-D cylinder. The three-dimensional flow structure was attributed to the downwashing counter rotating vortices separated from the FC free end. As the FC aspect ratio decreases, the vortex shedding frequency decreases and the vortex formation length increases compared to that of 2-D cylinder. Due to the descending counter-rotating twin-vortex, near the FC free end, regular vortex shedding from the cylinder is suppressed and the vortex formation region is hardly distinguished. Around the center of the wake, the mean velocity for the FC located in atmospheric boundary layer has large velocity deficit compared to that of uniform flow.

• Steel and Composite Structures
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• v.51 no.5
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• pp.563-573
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• 2024

This research experimentally evaluated the local stress distribution along the cross-section of composite beams under both positive and negative moments. The experiment utilized a large-scale, two-story, two-by-three bay steel gravity frame with a concrete on metal deck floor system. The composite shear connections, which are nominally assumed to be pinned under gravity loading, can develop non-negligible moment-resisting capacity when subjected to lateral loads. This paper discusses the local stress distribution, orshear lag effects, observed near the beam-to-column connections when subjected to combined gravity and lateral loading. Strain gauges were used for measurements along the beam depth at varying distances from the connection. The experimental data showed amplified shear lag effects near the unconnected region of the beam web and bottom flange under the applied loading conditions. These results indicate that strain does not vary linearly across the beam cross-section adjacent to the connection components. This insight has implications for the use of experimental strain gauge data in estimating beam demands near the connections. These findings can be beneficial in informing instrumentation plans for future experimental studies on composite beams.

• Bulletin of the Society of Naval Architects of Korea
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• v.26 no.4
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• pp.3-13
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• 1989

In recent towing tank experiments, it has been observed that a ship moving near the critical speed $\sqrt{gh}$(g=gravitational acceleration, h=water depth) radiates solitons upstream in an almost periodic manner. As a ,consequence, the ship experiences considerable changes in resistance, trim and sinkage, or better known as squat. Mei and Choi(1987) developed a nonlinear theory for a slender ship by using the method of matched asymptotic expansions. For a certain class of channel width and ship slenderness, they found that the waves generated can be described by an inhomogeneous Korteweg-de Vries(KdV) equation. The leading-order solution properly predicts solitons propagating upstream, but it fails to render three-dimensional waves in the wake. In this paper a new approach has been made by choosing a different class of channel width and ship slenderness. The wave equation in the farfield turns out to be a homogeneous Kadomtsev-Petviashvili(KP) equation, which predicts solitons upstream and three-dimensional waves in the wake. Numerical results for the wave resistance, sinkage and trim reflect the experimentally identified phenomena.

• Wind and Structures
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• v.26 no.5
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• pp.267-277
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• 2018

Numerical simulations are performed of a long flexible cylinder undergoing vortex-induced vibration at a Reynolds number of 500. The cylinder is pinned at both ends, having an aspect ratio of 100 (cylinder length to cylinder diameter) and a mass ratio of 4.2 (structural mass to displaced fluid mass). Temporal and spatial information on the cross-flow (CF) and in-line (IL) vibrations is extracted. High modal vibrations up to the $6^{th}$ in the CF direction and the $11^{th}$ in the IL direction are observed. Both the CF and IL vibrations feature a multi-mode mixed pattern. Mode competition is observed. The $2^{nd}$ mode with a low frequency dominates the IL vibration and its existence is attributed to a wave group propagating back and forth along the span. Distributions of fluid force coefficients are correlated to those of the CF and IL vibrations along the span. Histograms of the x'-y motion phase difference are evaluated from the total simulation time and a complete vibration cycle representing the standing or travelling wave pattern. Correlations between the phase difference and the vibrations are discussed. Vortex structures behind the cylinder show an interwoven near-wake pattern when the standing wave pattern dominates, but an oblique near-wake pattern when the travelling wave pattern prevails.

• Asian Journal of Atmospheric Environment
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• v.8 no.2
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• pp.81-88
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• 2014

Noise-barriers on both sides of the roadway (hereafter referred to as double noise-barriers), are a common feature along roads in Korea, and these are expected to have important effects on the near-road air pollution dispersion of vehicle emissions. This study evaluated the double noise-barrier impact on near-road air pollution dispersion, using a FLUENT computational fluid dynamics (CFD) model. The realizable k-${\varepsilon}$ model in FLUENT CFD code was used to simulate vehicle air pollutant dispersion, in around 11 cases of double noise-barriers. The simulated concentration profiles and surface concentrations under no barrier cases were compared with the experimental results. The results of the simulated flows show the following three regimes in this study: isolated roughness (H/W=0.05), wake interface (H/W=0.1), and skimming flow (H/W>0.15). The results also show that the normalized average concentrations at surface (z=1 m) between the barriers increase with increasing double noise-barrier height; however, normalized average concentrations at the top position between the barriers decrease with increasing barrier height. It was found that the double noise-barrier decreases normalized average concentrations of leeward positions, ranging from 0.8 (H/W=0.1, wake interface) to 0.1 (H/W=0.5, skimming flow) times lower than that of the no barrier case, at 10 x/h downwind position; and ranging from 1.0 (H/W=0.1) to 0.4 (H/W=0.5) times lower than that of the no barrier case, at 60 x/h downwind position.

• Journal of Advanced Marine Engineering and Technology
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• v.41 no.3
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• pp.209-215
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• 2017

A large floating offshore wind-wave hybrid power generation system with an area of 150 m2 and four 3 MW class wind turbine generators was installed at each column top. In accordance with the wind turbine arrangement, the wake generated from upstream turbines can adversely affect the power performance and load characteristics of downstream turbines. Therefore, an optimal arrangement design, obtained through a detailed flow analysis focusing on wake interference, is necessary. In this study, to determine the power characteristics and annual energy production (AEP) of individual wind turbines, transient computational fluid dynamics, considering wind velocity variation (8 m/s, 11.7 m/s, 19 m/s, and 25 m/s), was conducted under different platform conditions ($0^{\circ}$, $22.5^{\circ}$, and $45^{\circ}$). The AEP was calculated using a Rayleigh distribution, depending on the wind turbine arrangement. In addition, we suggested an optimal arrangement design to minimize wake losses, based on the AEP.

• Transactions of the Korean Society of Mechanical Engineers B
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• v.20 no.8
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• pp.2637-2649
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• 1996

Flow characteristics and aerodynamic forces acting on an elliptic cylinder placed in a plane boundary layer were investigated experimentally. Four cylinder models with axis ratio(major axis to minor axis, AR=A/B) of 1, 2, 3, and 4 having the same equivalent diameter were used in this experiment. The Reynolds number based on the equivalent diameter $D_e$(=20mm) was 13,000. In the case of circular cylinder, regular vortex shedding occurs for the cylinder gaps larger than G/B=0.3 and is not almost related to the boundary layer thickness. But, for the elliptic cylinders, the vortex shedding frequency is increased with increasing the gap ratio (G/B) and the axis ratio (AR) of elliptic cylinders. The maximum drag coefficient acting on a circular cylinder is mainly affected by the boundary layer thickness. But, the elliptic cylinders(AR$\geq$2), except for the smaller gap G/B<0.2, show a nearly constant drag coefficient which is much smaller than that of a circular cylinder. The base pressure on the flat plate decreases with increasing the axis ratio(AR) of the elliptic cylinder. In the case of a circular cylinder, the base pressure has the minimum value at the gap ratio G/B=0.4, but it occurs at G/D=2 for elliptic cylinders. The mean velocity of the cylinder wake is quickly recovered at a small cylinder height ratio(H/$\delta$), but the turbulent intensity is rapidly recovered at a large cylinder height ratio(H/$\delta$). The effective wake region in the plane boundary layer is shrinkaged with increasing the axis ratio(AR) of elliptic cylinder. And the drag coefficient and streamwise turbulent intensity of the elliptic cylinder with AR=4 are less than half of those for the circular cylinder(AR=1).

• Transactions of the Korean Society of Automotive Engineers
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• v.6 no.2
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• pp.12-20
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• 1998

Exter ballistics of a typical high-speed projectile is studied through a flow-visualization experiment and an unstructured grid Navier-Srokes computation. Experiment produced a schlieren photograph that adequately shows the characteristic features of this complex flow, namely two kinds of oblique cone shocks and turbulent wake developing into the downstream. A hybrid scheme of finite volume-element method is used to simulate the compressible Reynolds-Averaged Navier-Stok- es solution on unstructured grids. Osher's approximate Riemann solver is used to discretize the cinvection term. Higher-order spatial accuracy is obtained by MUSCL extension and van Albada ty- pe flux limiter is used to stabilize the numerical oscillation near the solution discontinuity. Accurate Gakerkin method is used to discretize the viscous term. Explict fourth-order Runge-Kutta method is used for the time-stepping, which simplifies the application of MUSCL extension. A two-layer k-$\varepsilon$ turbulence model is used to simulate the turbulent wakes accurately. Axisymmetric folw and two-dimensional flow with an angle of attack have been computed. Grid-dependency is also checked by carrying out the computation with doubled meshes. 2-D calculation shows that effect of angle of attack on the flow field is negligible. Axi-symmetric results of the computation agrees well with the flow visualization. Primary oblique shock is represented within 2-3 meshes in numerical results, and the varicose mode of the vortex shedding is clearly captured in the turbulent wake region.