• Wind and Structures
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• 제29권1호
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• pp.33-41
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• 2019

The interaction of head waves with an infinite row of identical, equally spaced, rectangular breakwaters is investigated in the presence of uneven bottom topography. Using linear water wave theory and matched eigenfunction expansion method, the boundary value problem is transformed into a system of linear algebraic equations which are numerically solved to know the velocity potentials completely. Utilizing this method, reflected and transmitted wave energy are computed for different physical parameters along with the wave field in the vicinity of breakwaters. It is observed that the wave field becomes more complicated when the incoming wavelength becomes smaller than the channel width. A critical ratio of the gap width to the channel width, corresponding to the inflection point of the transmitted energy variation, is identified for which 1/3 of the total energy is transmitted. Similarly, depending on the incident wavelength, there is a critical breakwater width for which a minimum energy is transmitted. Further, the accuracy of the computed results is verified by using the derived energy relation.

• Wind and Structures
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• 제14권5호
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• pp.413-434
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• 2011

Without output covariance estimation, one reference-based Stochastic Subspace Technique (SST) for extracting modal parameters and flutter derivatives of bridge deck is developed and programmed. Compared with the covariance-driven SST and the oscillation signals incurred by oncoming or signature turbulence that adopted by previous investigators, the newly-presented identification scheme is less time-consuming in computation and a more desired accuracy should be contributed to high-quality free oscillated signals excited by specific initial displacement. The reliability and identification precision of this technique are confirmed by a numerical example. For the 3-DOF sectional models of Sutong Bridge deck (streamlined) and Suramadu Bridge deck (bluff) in wind tunnel tests, with different wind velocities, the lateral bending, vertical bending, torsional frequencies and damping ratios as well as 18 flutter derivatives are extracted by using SST. The flutter derivatives of two kinds of typical decks are compared with the pseudo-steady theoretical values, and the performance of $H_1{^*}$, $H_3{^*}$, $A_1{^*}$, $A_3{^*}$ is very stable and well-matched with each other, respectively. The lateral direct flutter derivatives $P_5{^*}$, $P_6{^*}$ are comparatively more accurate than other relevant lateral components. Experimental procedure seems to be more critical than identification technique for refining the estimation precision.

• 동력기계공학회지
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• 제21권6호
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• pp.5-12
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• 2017

The steady-state, incompressible and three-dimensional numerical analysis was carried out to investigate the internal flow fields characteristics according to wind tunnel contraction type. The turbulence model used in this study is a realizable $k-{\varepsilon}$ modified from the standard $k-{\varepsilon}$ model. As a results, the distribution of the axial mean velocity components along the central axis of the flow model is very similar to the ASME and BE types, and the cubic and cosine types. When the flow passes through the interior space of the analytical models, the flow resistance at the inlet of the plenum chamber is the largest at BS type contraction, but the smallest at cubic type contraction. The boundary layer thickness is the smallest in the cosine type contraction as the axial distance increases. The maximum turbulent kinetic energy in the test section is the smallest in the order of the contraction of cubic type and cosine type. Comprehensively, cubic type contraction is the best choice for wind tunnel performance, and cosine type contraction can be the next best solution.

• 한국소음진동공학회:학술대회논문집
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• 한국소음진동공학회 2014년도 춘계학술대회 논문집
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• pp.128-134
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• 2014

A Experimental study on frequency characteristics of the microphone array covered with Kevlar in closed test section wind tunnel. Microphones that are flush mounted in a closed test section wall of wind tunnel are subject to very high flow noise resulting from the turbulence in the wall boundary layer. At this time the microphones measure the strong hydrodynamic fluctuations generated by the flow. The phenomena are referred to a microphone self-noise and a method for reducing this has studied. In this paper the array that covered with acoustically transparent Kevlar sheet was designed and made to reduce the flow-induced self-noise. For the validation frequency characteristics of the Kevlar, the microphone array was installed on the wall and test was performed for white noise and sine wave of several frequencies using loudspeaker. In addition, the paper compared the signals as a tension of Kevlar. The results were presented that tend to decrease the sound pressure level at high frequency above 3500Hz according to existence of Kevlar.

• 한국소음진동공학회논문집
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• 제14권5호
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• pp.402-409
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• 2004

Numerical analysis of static and dynamic wind effects on civil engineering structures was performed. Long-span suspension bridges are flexible structures that are highly sensitive to the action of the wind. Aerodynamic effect often becomes a governing factor in the design process of bridges and aeroelastic stability boundary becomes a prime criterion which should be confirmed during the structural design stage of bridges because the long-span suspension bridges are prone to the aerodynamic instabilities caused by wind. If the wind velocity exceeds the critical velocity that the bridge can withstand, then the bridge fails due to the phenomenon of flutter. Buffeting caused by turbulence results in structural fatigue, which could lead to the failure of a bridge. Navier-Stokes equations are used for the aeroelastic analysis of bridge girder section. The aeroelastic simulation is carried out to study the aeroelastic stability of bridges using both Computational Fluid Dynamic (CFD) and Computational Structural Dynamic (CSD) schemes.

• Wind and Structures
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• 제24권1호
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• pp.79-93
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• 2017

Wind-induced fluctuating internal pressures in a building with a dominant opening can be described by a second-order non-linear differential equation. However, the accuracy and efficiency of the governing equation in predicting internal pressure fluctuations depend upon two ill-defined parameters: inertial coefficient $C_I$ and loss coefficient $C_L$, since $C_I$ determines the un-damped oscillation frequency of an air slug at the opening, while $C_L$ controls the decay ratio of the fluctuating internal pressure. This study particularly focused on the value of loss coefficient and its influence factors including: opening configuration and location, internal volumes, as well as wind speed and approaching flow turbulence. A simplified formula was presented to predict loss coefficient, therefore an approximate relationship between the standard deviation of internal and external pressures can be estimated using Vickery's approach. The study shows that the loss coefficient governs the peak response of the internal pressure spectrum which, in turn, will directly influence the standard deviation of the fluctuating internal pressure. The approaching flow characteristic and opening location have a remarkable effect on the parameter $C_L$.

• Wind and Structures
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• 제27권2호
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• pp.123-136
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• 2018

The effects of steep and shallow hills on a stationary tornado-like vortex with a swirl ratio of 0.4 are simulated and quantified as Fractional Speed Up Ratios (FSUR) at three different locations of the vortex with respect to the crests of the hills. Steady state Reynolds Averaged Naiver Stokes (RANS) equations closed using Reynolds Stress Turbulence model are used to simulate stationary tornadoes. The tornado wind field obtained from the numerical simulations is first validated with previous experimental and numerical studies by comparing radial and tangential velocities, and ground static pressure. A modified fractional speed-up ratio (FSUR) evaluation technique, appropriate to the complexity of the tornadic flow, is then developed. The effects of the hill on the radial, tangential and vertical flow components are assessed. It is observed that the effect of the hill on the radial and vertical component of the flow is more pronounced, compared to the tangential component. Besides, the presence of the hill is also seen to relocate the center of tornadic flow. New FSUR values are produced for shallow and steep hills.

• 대기
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• 제21권1호
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• pp.95-108
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• 2011

The effects of an apartment complex on flow and pollutant dispersion in an urban area are numerically investigated using a computational fluid dynamics (CFD) model. The CFD model is based on the Reynolds-averaged Navier-Stokes equations and includes the renormalization group k-${\varepsilon}$ turbulence model. The geographic information system (GIS) data is used as an input data of the CFD model. Eight numerical simulations are carried out for different inflow directions and, for each inflow direction, the effects of an apartment complex are investigated, comparing the characteristics of flow and dispersion before and after construction of the apartment complex in detail. The observation data of automatic weather system (AWS) is analyzed. The windrose analysis shows that the wind speed and direction after the construction of the complex are quite different from those before the construction. The construction of the apartment complex resulted in the decrease in wind speed at the downwind region. It is also shown that the wind speed increased partially inside the apartment complex due to the channeling effect to satisfy the mass continuity. On the whole, the wind speed decreased at the downwind region due to the drag effect by the apartment complex. As a result, the passive pollutant concentration increased (decreased) near the downwind region of (within) the apartment complex compared with that before the construction.

• Wind and Structures
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• 제22권5호
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• pp.573-594
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• 2016

Simultaneous pressure measurements on 2D square prisms with various corner modifications were performed in uniform flow with low turbulence level, and the testing Reynolds numbers varied from $1.0{\times}10^5$ to $4.8{\times}10^5$. Experimental models were a square prism, three chamfered-corner square prisms (B/D=5%, 10%, and 15%, where B is the chamfered corner dimension and D is the cross-sectional dimension), and six rounded-corner square prisms (R/D =5%, 10%, 15%, 20%, 30%, and 40%, where R is the corner radius). Experimental results of drag coefficients, wind pressure distributions, power spectra of aerodynamic force coefficients, and Strouhal numbers are presented. Ten models are divided into various categories according to the variations of mean drag coefficients with Reynolds number. The mean drag coefficients of models with $B/D{\leq}15%$ and $R/D{\leq}15%$ are unaffected by the Reynolds number. On the contrary, the mean drag coefficients of models with R/D=20%, 30%, and 40% are obviously dependent on Reynolds number. Wind pressure distributions around each model are analyzed according to the categorized results.The influence mechanisms of corner modifications on the aerodynamic characteristics of the square prism are revealed from the perspective of flow around the model, which can be obtained by analyzing the local pressures acting on the model surface.

• Wind and Structures
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• 제1권4호
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• pp.337-349
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• 1998

A numerical and experimental study was performed for the wind flow field in one area, comprising a group of several pavilions separated by passageways, of the EXPO '98 - a World Exposition (Lisbon, Portugal). The focus of this study is the characterization of the flow field to assess pedestrian comfort. The predictions were obtained employing the Reynolds averaged Navier-Stokes equations with the turbulence effects dealt with the ${\kappa}-{\varepsilon}$ RNG model. The discretization of the differential equations was accomplished with the control volume formulation in a Cartesian coordinate system, and an advanced segregated procedure was used to achieve the link between continuity and momentum equations. The evaluation of the overall numerical model was performed by comparing its predictions against experimental data for a square cylinder placed in a channel. The predicted values, for the practical geometry studied, are in a good agreement with the experimental data, showing the performance and the reliability of the ${\kappa}-{\varepsilon}$ RNG model and suggesting that the numerical simulation is a reliable methodology to provide the required information.