• Wind and Structures
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• 제20권2호
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• pp.327-347
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• 2015

This paper investigates the effects of Reynolds number (Re) on the aerodynamic characteristics of a twin-deck bridge. A 1:36 scale sectional model of a twin girder bridge was tested using the Wall of Wind (WOW) open jet wind tunnel facility at Florida International University (FIU). Static tests were performed on the model, instrumented with pressure taps and load cells, at high wind speeds with Re ranging from $1.3{\times}10^6$ to $6.1{\times}10^6$ based on the section width. Results show that the section was almost insensitive to Re when pitched to negative angles of attack. However, mean and fluctuating pressure distributions changed noticeably for zero and positive wind angles of attack while testing at different Re regimes. The pressure results suggested that with the Re increase, a larger separation bubble formed on the bottom surface of the upstream girder accompanied with a narrower wake region. As a result, drag coefficient decreased mildly and negative lift coefficient increased. Flow modification due to the Re increase also helped in distributing forces more equally between the two girders. The bare deck section was found to be prone to vortex shedding with limited dependence on the Re. Based on the observations, vortex mitigation devices attached to the bottom surface were effective in inhibiting vortex shedding, particularly at lower Re regime.

• Wind and Structures
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• 제26권5호
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• pp.305-315
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• 2018

A numerical study based on a delayed detached eddy simulation (DDES) is conducted to investigate the aerodynamic mechanism behind the suppression of vortex-induced vibrations (VIVs) of twin box girders by central grids, which have an inhibition effect on VIVs, as evidenced by the results of section model wind tunnel tests. The mean aerodynamic force coefficients with different attack angles are compared with experimental results to validate the numerical method. Next, the flow structures around the deck and the aerodynamic forces on the deck are analyzed to enhance the understanding of the occurrence of VIVs and the suppression of VIVs by the application of central grids. The results show that shear layers are separated from the upper railings and lower overhaul track of the upstream girder and induce large-scale vortices in the gap that cause periodical lift forces of large amplitude acting on the downstream girder, resulting in VIVs of the bridge deck. However, the VIVs are apparently suppressed by the central grids because the vortices in the central gap are reduced into smaller vortices and become weaker, causing slightly fluctuating lift forces on the deck. In addition, the mean lift force on the deck is mainly caused by the upstream girder, whereas the fluctuating lift force is mainly caused by the downstream girder.

• 한국방재학회 논문집
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• 제8권6호
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• pp.21-30
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• 2008

이 연구에서는 현수교 PWS 및 CFRC 행어로프의 바람에 의한 내풍안정성 평가을 위한 풍동실험을 수행하였다. 풍동실험결과, PE관으로 피복된 PWS 케이블은 단일 케이블에서 와류진동이 발생하고 복수 케이블에서는 중심간격이 $3\sim6D$일때 웨이크갤로핑이 발생하였다. 진동이 발생하지 않는 복수 케이블에서도 풍향각이 달라지면 웨이크갤로핑이나 웨이크플러터가 발생함을 알 수 있었다. CFRC 케이블은 케이블 중심간격 및 풍향각에 관계없이 PWS 케이블에 비해 안정된 내풍성능을 보였다.

• International Journal of Naval Architecture and Ocean Engineering
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• 제13권1호
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• pp.628-640
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• 2021

To investigate the supercavity geometry and gas flow structure for the supercavities with two closure types under the different flow conditions, an inhomogeneous multiphase model with the SST turbulence model was established, and validated by experimental results. The results show that two distinct regions exist inside the supercavity, which include the downstream flow region along the gas-water interface and the reverse flow region. For the twin-vortex supercavity, the internal gas leaks from the supercavity boundary by two paths: the supercavity surface and the two-vortex tubes. Increasing Froude number leads to more internal gas stripped from the supercavity surface. Two types of gas loss exist for the re-entrant jet supercavity with high Froude number, one type is the steady process of gas loss, and the major gas-leaking path is the supercavity surface rather than supercavity closure region. The other type is the unsteady periodic ejection, and the gas cluster of periodic ejection is merely a small part of the gas stored inside the supercavity.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2002년도 학술대회지
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• pp.151-154
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• 2002

The twin impulse wave leads to very complicated flow fields, such as Mach stem, spherical waves, and vortex ring. The twin impulse wave discharged from the exits of the two tubes placed in parallel is investigated to understand detailed flow physics associated with the twin impulse wave, compared with those in a single impulse wave. In the current study, the merging phenomena and propagation characteristics of the impulse waves are investigated using a shock tube experiment and by numerical computations. The Harten-Yee's total variation diminishing (TVD) scheme is used to solve the unsteady, two-dimensional, compressible, Euler equations. The Mach number $M_{s}$, of incident shock wave is changed below 1.5 and the distance between two-parallel tubes, L/d, is changed from 1.2 to 4.0. In the shock tube experiment, the twin impulse waves are visualized by a Schlieren optical system for the purpose of validation of computational work. The results obtained show that on the symmetric axis between two parallel tubes, the peak pressure produced by the twin-impulse waves and its location strongly depend upon the distance between two parallel tubes, L/d and the incident shock Mach number, $M_{s}$. The predicted Schlieren images represent the measured twin-impulse wave with a good accuracy.

• Wind and Structures
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• 제30권4호
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• pp.367-378
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• 2020

Two-dimensional Delayed Detached Eddy Simulation (DDES) was carried out to investigate the uniform flow over a twin-box bridge deck (TBBD) with various gap ratios of L/C=5.1%, 12.8%, 25.6%, 38.5%, 73.3% and 108.2% (L: the gap-width between two girders, C: the chord length of a single girder) at Reynolds number, Re=4×104. The aerodynamic coefficients of the prototype deck with gap ratio of 73.3% obtained from the present simulation were compared with the previous experimental and numerical data for different attack angles to validate the present numerical method. Particular attention is devoted to the fluctuating pressure distribution and forces, shear layer reattachment position, wake velocity and flow pattern in order to understand the effects of gap ratio on dynamic flow interaction with the twin-box bridge deck. The flow structure is sensitive to the gap, thus a change in L/C thus leads to single-side shedding regime at L/C≤25.6%, and co-shedding regime at L/C≥35.8% distinguished by drastic changes in flow structure and vortex shedding. The gap-ratio-dependent Strouhal number gradually increases from 0.12 to 0.27, though the domain frequencies of vortices shedding from two girders are identical. The mean and fluctuating pressure distributions is significantly influenced by the flow pattern, and thus the fluctuating lift force on two girders increases or decreases with increasing of L/C in the single-side shedding and co-shedding regime, respectively. In addition, the flow mechanisms for the variation in aerodynamic performance with respect to gap ratios are discussed in detail.

• 한국소음진동공학회논문집
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• 제24권12호
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• pp.962-968
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• 2014

The lattice Boltzmann method(LBM) has attracted attention as an alternative numerical algorithm for solving fluid mechanics since the end of the 90's. In these days, its intrinsic unsteadiness and rapid increase in computing power make the LBM be more applicable for computing flow-induced noise as well as fluid dynamics. The lattice Boltzmann method is a weakly compressible scheme, so we can get information about both aerodynamics and aeroacoustics from single simulation. In this paper, numerical analysis on Aeolian tone noise generated by tandem-twin square cylinders in duct is performed using the LBM. For simplicity, laminar two-dimensional fluid models are used. To verify the validity and accuracy of the current numerical techniques, numerical results for the laminar duct and the cylinder flows are compared with the analytical solution and the measurement, respectively. Then, aerodynamic noise of the twin tandem square cylinders is investigated. It is shown that the aerodynamic noise from the twin tandem square cylinders can be reduced by controlling the distance between the cylinders.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집E
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• pp.247-252
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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 wake structures behind a 2-D cylinder and a finite cylinder located in a uniform flow were also 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 is decreased and the vortex formation length is increased compared to that of 2-D cylinder. Due to the descending counter-rotating twin-vortex, in the region near the FC free end, regular vortex shedding from the cylinder is suppressed and the vortex formation region is hardly established. In the wake center region, the mean velocity for the FC located in atmospheric boundary layer has large velocity deficit, compared to that of uniform flow.

• 한국항공우주학회지
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• 제31권7호
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• pp.69-74
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• 2003

Vane tip 개념을 도입하여 소음 특성을 개선하도록 설계된 차세대 로터 블레이드(NRSB-I)의 공력 성능을 기존의 BERP 블레이드와 비교하기 위하여, 제자리 비행에 대한 점성 압축성 계산을 수행하였다. 깃끝 영역의 면적감소에 의하여 설계된 로터 블레이드의 절대 추력이 6-7% 정도 감소하지만, 동력계수의 감소폭이 더 크기 때문에 제자리 비행 성능상의 손실은 거의 없는 것으로 파악되었다. 또한, 깃끝 절단부에서 발생한 와류는 블레이드 표면을 지날 때 확산에 의하여 그 강도가 현저하게 약화되므로, 설계요구조건을 만족시키기 위해서는 절단부 위치에 펜스가 필요함을 확인할 수 있었다.

• 한국추진공학회:학술대회논문집
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• 한국추진공학회 2004년도 제22회 춘계학술대회논문집
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• pp.117-124
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• 2004

The mixing enhancement is one of the most important problems for the development of scramjet engines. The influence of the streamwise vortices produced by a ramp in a unheated supersonic flow on the mixing of twin jets injected from its base was experimentally investigated. Nominal Mach number of the main airstream and of the twin jets at the nozzle exits were 2.35 and 2.0, respectively. Three dimensional velocity distributions near the ramp with and without injection were measured by Particle Image Velocimetry (PIV). A pair of counter rotating streamwise vortices could be seen behind the injector without injection. On the other hand, two pairs of streamwise vortices could be seen with injection. The outer one had the same direction as the vortex pair produced by the ramp, but they were stronger than those produced by the ramp. The inner ones had the opposite directions to the outer ones. It is considered that these vortices enhance the mixing near the injector.