• Title/Summary/Keyword: boundary characteristics

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CFD-DEM modeling of snowdrifts on stepped flat roofs

  • Zhao, Lei;Yu, Zhixiang;Zhu, Fu;Qi, Xin;Zhao, Shichun
    • Wind and Structures
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    • v.23 no.6
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    • pp.523-542
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    • 2016
  • Snowdrift formation on roofs should be considered in snowy and windy areas to ensure the safety of buildings. Presently, the prediction of snowdrifts on roofs relies heavily on field measurements, wind tunnel tests and numerical simulations. In this paper, a new snowdrift modeling method by using CFD (Computational Fluid Dynamics) coupled with DEM (Discrete Element Method) is presented, including material parameters and particle size, collision parameters, particle numbers and input modes, boundary conditions of CFD, simulation time and inlet velocity, and coupling calculation process. Not only is the two-way coupling between wind and snow particles which includes the transient changes in snow surface topography, but also the cohesion and collision between snow particles are taken into account. The numerical method is applied to simulate the snowdrift on a typical stepped flat roof. The feasibility of using coupled CFD with DEM to study snowdrift is verified by comparing the simulation results with field measurement results on the snow depth distribution of the lower roof.

Features of the flow over a finite length square prism on a wall at various incidence angles

  • Sohankar, A.;Esfeh, M. Kazemi;Pourjafari, H.;Alam, Md. Mahbub;Wang, Longjun
    • Wind and Structures
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    • v.26 no.5
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    • pp.317-329
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    • 2018
  • Wake characteristics of the flow over a finite square prism at different incidence angles were experimentally investigated using an open-loop wind tunnel. A finite square prism with a width D = 15 mm and a height H = 7D was vertically mounted on a horizontal flat plate. The Reynolds number was varied from $6.5{\times}10^3$ to $28.5{\times}10^3$ and the incidence angle ${\alpha}$ was changed from $0^{\circ}$ to $45^{\circ}$. The ratio of boundary layer thickness to the prism height was about ${\delta}/H=7%$. The time-averaged velocity, turbulence intensity and the vortex shedding frequency were obtained through a single-component hotwire probe. Power spectrum of the streamwise velocity fluctuations revealed that the tip and base vortices shed at the same frequency as that ofspanwise vortices. Furthermore, the results showed that the critical incidence angle corresponding to the maximum Strouhal number and minimum wake width occurs at ${\alpha}_{cr}=15^{\circ}$ which is equal to that reported for an infinite prism. There is a reduction in the size of the wake region along the height of the prism when moving away from the ground plane towards the free end.

Large eddy simulation of blockage effects in the assessment of wind effects on tall buildings

  • Gao, Yang;Gu, Ming;Quan, Yong;Feng, Chengdong
    • Wind and Structures
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    • v.30 no.6
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    • pp.597-616
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    • 2020
  • The blockage effect on the aerodynamic characteristics of tall buildings is a fundamental issue in wind tunnel test but has rarely been addressed. To evaluate the blockage effects on the aerodynamic forces on a square tall building and flow field peripherally, large eddy simulations (LES) were performed on a 3D square cylinder with an aspect ratio of 6:1 under the uniform smooth inflow and turbulent atmospheric boundary layer (ABL) inflow generated by the narrowband synthesis random flow generator (NSRFG). First, a basic case at a blockage ratio (BR) of 0.8% was conducted to validate the adopted numerical methodology. Subsequently, simulations were systematically performed at 6 different BRs. The simulation results were compared in detail to illustrate the differences induced by the blockage, and the mechanism of the blockage effects under turbulent inflow was emphatically analysed. The results reveal that the pressure coefficients, the aerodynamic forces, and the Strouhal number increase monotonically with BRs. Additionally, the increase of BR leads to more coherence of the turbulent structures and the higher intensity of the vortices in the vicinity of the building. Moreover, the blockage effects on the aerodynamic forces and flow field are more significant under smooth inflow than those under turbulent inflow.

Slotted hydrofoil design optimization to minimize cavitation in amphibious aircraft application: A numerical simulation approach

  • Conesa, Fernando Roca;Liem, Rhea Patricia
    • Advances in aircraft and spacecraft science
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    • v.7 no.4
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    • pp.309-333
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    • 2020
  • The proposed study aims to numerically investigate the performance of hydrofoils in the context of amphibious aircraft application. In particular, we also study the effectiveness of a slotted hydrofoil in minimizing the cavitation phenomenon, to improve the overall water take-off performance of an amphibious aircraft. We use the ICON A5 as a base model for this study. First, we propose an approach to estimate the required hydrofoil surface area and to select the most suitable airfoil shape that can minimize cavitation, thus improving the hydrodynamic efficiency. Once the hydrofoil is selected, we perform 2D numerical studies of the hydrodynamic and cavitating characteristics of a non-slotted hydrofoil on ANSYS Fluent. In this work, we also propose to use a slotted hydrofoil to be a passive method to control the cavitation performance through the boundary layer control. Numerical results of several slotted configurations demonstrate notable improvement on the cavitation performance. We then perform a multiobjective optimization with a response surface model to simultaneously minimize the cavitation and maximize the hydrodynamic efficiency of the hydrofoil. The optimization takes the slot geometry, including the slot angle and lengths, as the design variables. In addition, a global sensitivity study has been carried and it shows that the slot widths are the more dominant factors.

Flutter characteristics of axially functional graded composite wing system

  • Prabhu, L.;Srinivas, J.
    • Advances in aircraft and spacecraft science
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    • v.7 no.4
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    • pp.353-369
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    • 2020
  • This paper presents the flutter analysis and optimum design of axially functionally graded box beam cantilever wing section by considering various geometric and material parameters. The coupled dynamic equations of the continuous model of wing system in terms of material and cross-sectional properties are formulated based on extended Hamilton's principle. By expressing the lift and pitching moment in terms of plunge and pitch displacements, the resultant two continuous equations are simplified using Galerkin's reduced order model. The flutter velocity is predicted from the solution of resultant damped eigenvalue problem. Parametric studies are conducted to know the effects of geometric factors such as taper ratio, thickness, sweep angle as well as material volume fractions and functional grading index on the flutter velocity. A generalized surrogate model is constructed by training the radial basis function network with the parametric data. The optimized material and geometric parameters of the section are predicted by solving the constrained optimal problem using firefly metaheuristics algorithm that employs the developed surrogate model for the function evaluations. The trapezoidal hollow box beam section design with axial functional grading concept is illustrated with combination of aluminium alloy and aluminium with silicon carbide particulates. A good improvement in flutter velocity is noticed by the optimization.

Distance Detection between Vehicles Using Stereo Vision (스테레오 비젼을 이용한 차량간 거리정보 검출)

  • Yang, Seok-Joo;Baek, Joong-Hwan
    • Journal of Advanced Navigation Technology
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    • v.6 no.1
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    • pp.27-36
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    • 2002
  • As the number of autonomous vehicles is increased, drivers are trending toward constant interests in detecting distance between vehicles in close-range and maintaining the distance between forward and backward vehicles for drivers' safety. In this paper, we propose a method of detecting the distance between two vehicles by computing the disparity of the close-rang vehicle using stereo vision. The boundary of the vehicle is obtained by using the modified wavelet transform which has multi-resolution characteristics. Then the disparity between left and right images is computed using coarse-to-fine method and histogram matching. Here we transform the left-right stereo images through 3-steps using the modified wavelet for maintaining the original resolution. An experimental result showed that the proposed method had 4.65% in total error rate.

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An Analysis on 3-Dimensional Temperature Distribution of Jet Vanes for a Thrust Vector Control (추력방향조종용 제트베인의 3차원 온도분포 해석)

  • Hwang, Ki-Young
    • Proceedings of the Korean Society of Propulsion Engineers Conference
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    • 2011.11a
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    • pp.283-291
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    • 2011
  • A computational investigation has been carried out to study the heat transfer characteristics of jet vane assembly used for the thrust vector control(TVC) of a vertical launch motor. In this study, the coefficients of convective heat transfer on the jet vane are calculated using the solutions of thermal boundary-layer equation and several semi-empirical equations. The calculation of 3-dimensional temperature distribution for the jet vane assembly was performed using the softwares called PATRAN and ABAQUS. The accuracy of the present numerical method is verified by comparing with the measured and calculated temperatures within jet vane shaft. The temporal variation of jet vane temperatures for three deflection angles(0o, 12.5o, 25o) was discussed.

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Thermo-mechanical analysis of road structures used in the on-line electric vehicle system

  • Yang, B.J.;Na, S.;Jang, J.G.;Kim, H.K.;Lee, H.K.
    • Structural Engineering and Mechanics
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    • v.53 no.3
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    • pp.519-536
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    • 2015
  • On-line electric vehicle (OLEV) is a new eco-friendly transportation system that collects electricity from a power cable buried beneath the road surface, allowing the system to resolve various problems associated with batteries in electric vehicles. This paper presents a finite element (FE) based thermo-mechanical analysis of precast concrete structures that are utilized in the OLEV system. An experimental study is also conducted to identify materials used for a joint filler, and the observed experimental results are applied to the FE analysis. Traffic loading and boundary conditions are modeled in accordance with the related standards and environmental characteristics of a road system. A series of structural analyses concerning various test scenarios are conducted to investigate the sensitivity of design parameters and to evaluate the structural performance of the road system.

Investigation of 3-D dynamic wind loads on lattice towers

  • Zou, Lianghao;Liang, Shuguo;Li, Q.S.;Zhao, Lin;Ge, Yaojun
    • Wind and Structures
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    • v.11 no.4
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    • pp.323-340
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    • 2008
  • In this paper, the along-wind, across-wind as well as torsional dynamic wind loads on three kinds of lattice tower models are investigated using the base balance technique in a boundary layer wind tunnel. The models were specially designed, and their fundamental frequencies in the directions of the three principal axes are still in the frequency range of the spectra of wind loads on lattice towers. In order to clear contaminations to the spectra of wind loads induced by model resonance, the generalized force spectra of the first mode of the models in along-wind, across-wind and torsional directions were derived based on measured base moments of the models. The RMS generalized force coefficients are also obtained by removing the contributions of model resonance. Finally, the characteristics of the 3-D dynamic wind loads, especially those of the across-wind dynamic loads, on the three kinds of lattice towers are presented and discussed.

Effect of building volume and opening size on fluctuating internal pressures

  • Ginger, John D.;Holmes, John D.;Kopp, Gregory A.
    • Wind and Structures
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    • v.11 no.5
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    • pp.361-376
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    • 2008
  • This paper considers internal pressure fluctuations for a range of building volumes and dominant wall opening areas. The study recognizes that the air flow in and out of the dominant opening in the envelope generates Helmholtz resonance, which can amplify the internal pressure fluctuations compared to the external pressure, at the opening. Numerical methods were used to estimate fluctuating standard deviation and peak (i.e. design) internal pressures from full-scale measured external pressures. The ratios of standard deviation and peak internal pressures to the external pressures at a dominant windward wall opening of area, AW are presented in terms of the non-dimensional opening size to volume parameter, $S^*=(a_s/\bar{U}_h)^2(A_W^{3/2}/V_{Ie})$ where $a_s$ is the speed of sound, $\bar{U}_h$ is the mean wind speed at the top of the building and $V_{Ie}$ is the effective internal volume. The standard deviation of internal pressure exceeds the external pressures at the opening, for $S^*$ greater than about 0.75, showing increasing amplification with increasing $S^*$. The peak internal pressure can be expected to exceed the peak external pressure at the opening by 10% to 50%, for $S^*$ greater than about 5. A dominant leeward wall opening also produces similar fluctuating internal pressure characteristics.