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CONTENTS
Volume 30, Number 5, May 2020
 

Abstract


Key Words


Address
Harbin Institute of Technology (Shenzhen)

Abstract
Two-dimensional numerical simulations are conducted at a low Reynolds number Re = 100 to investigate the near wake of three identical circular cylinders that are arranged in an equilateral triangular configuration. The incident angle of the three-cylinder configuration with respect to incoming flow is varied from θ= 0° to 60°, while the spacing between adjacent cylinders (L) covers a wide range of L/D = 1.25-7.0, where D is diameter of the cylinder. Typical flow structures in the near wake of the three-cylinder configuration are identified, including a single Karman vortex street, bistable flip-flopping near wake, anti-phase and/or in-phase vortex shedding, shear layer reattachment, and vortex impingement, depending on the configuration (L/D, θ). The behavior of Strouhal number (St) is discussed in detail, echoing the distinct structures of near wake. Furthermore, fluid forces on the individual cylinders are examined, which, though highly depending on (L/D, θ), exhibit a close correlation to the near wake behavior.

Key Words
cylinder near wake; fluid structure interaction

Address
Honglei Bai: School of Aeronautics and Astronautics, Sun Yat-sen University (Shenzhen), Shenzhen, China
Yufeng Lin: WSP (Asia) Ltd, Kowloon Bay, Kowloon, Hong Kong SAR, China
Md. Mahbub Alam: Institute for Turbulence-Noise-Vibration Interaction and Control, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China

Abstract
Vortex-induced vibrations of a yawed flexible cylinder near a plane boundary are numerically investigated at a Reynolds number Ren = 500 based on normal component of freestream velocity. Free to oscillate in the in-line and cross-flow directions, the cylinder with an aspect ratio of 25 is pinned-pinned at both ends at a fixed wall-cylinder gap ratio G/D = 0.8, where D is the cylinder diameter. The cylinder yaw angle (

Key Words
yawed flexible cylinder; direct numerical simulation; Independence Principle; vortex-shedding pattern

Address
Zhimeng Zhang, Chunning Ji, Dong Xu: State Key Laboratory of Hydraulic Engineering Simulation & Safety, Tianjin University, Tianjin, 300072, China
Md. Mahbub Alam : Institute for Turbulence-Noise-Vibration Interaction and Control, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China

Abstract
This work investigates Reynolds number Re (= 50 - 200) effects on the flows around a single cylinder and the two tandem (center-to-center spacing 𝐿∗= 𝐿/𝐷 = 4) cylinders, each of a diameter D. Vorticity structures, Strouhal numbers, and time-mean and fluctuating forces are presented and discussed. For the single cylinder, with increasing Re in the range examined, the vorticity magnitude, Strouhal number and fluctuating lift all monotonically rise but time-mean drag, vortex formation length, and lateral distance between the two rows of vortices all shrink. For the two tandem cylinders, the increase in Re leads to the formation of three distinct flows, namely reattachment flow (50 ≤ Re ≤ 75), transition flow (75 ≤ Re ≤100), and coshedding flow (100 ≤ Re ≤ 200). The reattachment flow at Re = 50 is steady. When Re is increased from 75 to 200, the Strouhal number of the two cylinders, jumping from 0.113 to 0.15 in the transition flow regime, swells to 0.188. The two-cylinder flow is more sensitive to Re than the single cylinder flow. Fluctuating lift is greater for the downstream cylinder than the upstream cylinder while time-mean drag is higher for the upstream cylinder than for the other. The time-mean drags of the upstream cylinder and single cylinder behaves similar to each other, both declining with increasing Re.

Key Words
Laminar flow regime; tandem circular cylinders; vortex shedding; wake structure; Strouhal number

Address
Javad Farrokhi Derakhshandeh: College of Engineering and Technology, American University of the Middle East, Kuwait
Md. Mahbub Alam: Institute for Turbulence-Noise-Vibration Interaction and Control, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China

Abstract
A study, using potential water wave theory, is conducted on the oblique water wave motion over two fixed submerged rectangular blocks (breakwaters) placed over a finite step bottom. We have considered infinite and semi-infinite fluid domains. In both domains, the Fourier expansion method is employed to obtain the velocity potentials explicitly in terms of the infinite Fourier series. The unknown coefficients appearing in the velocity potentials are determined by the eigenfunction expansion matching method at the interfaces. The derived velocity potentials are used to compute the hydrodynamic horizontal and vertical forces acting on the submerged blocks for different values of block thickness, gap spacing between the two blocks, and submergence depth of the upper block from the mean free surface. In addition, the wave load on the vertical wall is computed in the case of the semi-infinite fluid domain for different values of blocks width and the incident wave angle. It is observed that the amplitudes of hydrodynamic forces are negligible for larger values of the wavenumber. Furthermore, the upper block experiences a higher hydrodynamic force than the lower block, regardless of the gap spacing, submergence depth, and block thickness.

Key Words
submerged breakwaters; uneven bottom; semi-infinite fluid domain

Address
Ramnarayan Mondal: Institute for Turbulence-Noise-Vibration Interaction and Control Harbin Institute of Technology, Shenzhen 518055, China
Md. Mahbub Alam: Digital Engineering Laboratory of Offshore Equipment, Shenzhen, China

Abstract
The aeroelastic stability of a long-span suspension footbridge with a bluff deck (prototype section) was examined through static and dynamic wind tunnel tests using a 1:10 scale sectional model of the main girder, and the corresponding aerodynamic countermeasures were proposed in order to improve the stability. First, dynamic tests of the prototype sectional model in vertical and torsional motions were carried out at three attack angles (α= 3°, 0°, -3°). The results show that the galloping instability of the sectional model occurs at α = 3° and 0°, an observation that has never been made before. Then, the various aerodynamic countermeasures were examined through the dynamic model tests. It was found that the openings set on the vertical web of the prototype section (web-opening section) mitigate the galloping completely for all three attack angles. Finally, static tests of both the prototype and web-opening sectional models were performed to obtain the aerodynamic coefficients, which were further used to investigate the galloping mechanism by applying the Den Hartog criterion. The total damping of the prototype and web-opening models were obtained with consideration of the structural and aerodynamic damping. The total damping of the prototype model was negative for α = 0° to 7°, with the minimum value being -1.07%, suggesting the occurrence of galloping, while that of the web-opening model was positive for all investigated attack angles of α = -12° to 12°.

Key Words
long-span suspension footbridge; galloping instability; web opening; Den Hartog criterion; damping

Address
Ruwei Ma, Qiang Zhou and Mingshui Li: Research Centre for Wind Engineering, Southwest Jiaotong University, Chengdu, China/
Key Laboratory for Wind Engineering of Sichuan Province, Chengdu, China

Abstract
Advancements in materialistic life styles and increasing awareness about adverse climatic changes and its negative effects on human life have been the driving force of finding new and clean sources of energy. Wind power has become technologically mature and commercially acceptable on global scale. However, fossil fuels have been the major sources of energy in most countries, renewable energy (particularly wind) is now booming worldwide. To cope with this wind energy technology, various related aspects have to be understood by the scientific, engineering, utility, and contracting communities. This study is an effort towards the understanding of the (i) wind turbine blade and tower structural stability issues, (ii) turbine blade and tower failures and remedial measures, (iii) weather and seismic effects on turbine blade and tower failures, (iv) gear box failures, and (v) turbine blade and tower failure analysis tools.

Key Words
wind power; wind turbine; turbine blade failure; structural stability; tower failure

Address
Shafiqur Rehman: 1Center for Engineering Research, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
Md. Mahbub Alam: 2Institute for Turbulence-Noise-Vibration Interaction and Control, Harbin Institute of Technology (Shenzhen),
University Town, Xili, Shenzhen 518055, China

Abstract
This work numerically investigates the effects of Reynolds number ReD (= 100 - 150), cross-sectional aspect ratio AR = ( 0.25 -1.0), and attack angle a(0° - 90°) on the forces, Strouhal number, and wake of an elliptical cylinder, where ReD 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 a is the angle between the cylinder major axis and the incoming flow. At ReD = 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 a are varied in the ranges specified. When ReD 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 anglle ai=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 ai effectively distinguishes the dependence on a and AR of force and vortex shedding frequency at either ReD. In Pattern III, the Karman street forming behind the cylinder is modified to a secondary vortex street. At a given AR and a, ReD = 150 renders higher fluctuating lift and Strouhal number than ReD = 100.

Key Words


Address
Xiaoyu Shi, Md. Mahbub Alam: Institute of Turbulence-Noise-Vibration Interaction and Control, Harbin Institute of Technology (Shenzhen), Shenzhen, China
Honglei Bai : School of Aeronautics and Astronautics, Sun Yat-sen University (Shenzhen), China
Hanfeng Wang: School of Civil Engineering, Central South University, Changsha, China

Abstract
Large Eddy Simulation (LES) is used to study the effects of steady slot suction on the aerodynamic forces of and flow around a wall-mounted finite-length square cylinder. The aspect ratio H/d of the tested cylinder is 5, where H and d are the cylinder height and width, respectively. The Reynolds number based on free-stream oncoming flow velocity Us/U∞) is is varied as Q = 0, 1 and 3, where Us is the velocity at the entrance of the suction slot. It is found that the free-end steady slot suction can effectively suppress the aerodynamic forces of the model. The maximum reduction of aerodynamic forces occurs at Q = 1, with the time-mean drag, fluctuating drag, and fluctuating lift reduced by 3.75%, 19.08%, 40.91%, respectively. For Q = 3, all aerodynamic forces are still smaller than those for Q = 0 (uncontrolled case), but obviously higher than those for Q = 1. The involved control mechanism is successfully revealed, based on the comparison of the flow around cylinder free end and the near wake for the three tested Q values.

Key Words


Address
Hanfeng Wang : School of Civil Engineering, Central South University, Changsha, China/
National Engineering Laboratory for High-speed Railway Construction, Central South University, Changsha, China
Lingwei Zeng: School of Civil Engineering, Central South University, Changsha, China
Md. Mahbub Alam : Institute for Turbulence-Noise-Vibration Interaction and Control, Harbin Institute of Technology (Shenzhen), Shenzhen, China
Wei Guo: School of Civil Engineering, Central South University, Changsha, China/National Engineering Laboratory for High-speed Railway Construction, Central South University, Changsha, China



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