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CONTENTS
Volume 26, Number 5, May 2018
 

Abstract
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Key Words
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Address
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Abstract
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 6th in the CF direction and the 11th in the IL direction are observed. Both the CF and IL vibrations feature a multi-mode mixed pattern. Mode competition is observed. The 2nd 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.

Key Words
numerical simulation; long flexible cylinder; vortex-induced vibration; mode; fluid force coefficient; phase difference; vortex structure

Address
Chunning Ji, Ziteng Peng, Weilin Chen and 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, Shenzhen Graduate School,
Harbin Institute of Technology, Shenzhen, 518055, China


Abstract
Inspired by the energy harvesting eel, a flexible flag behind a D-shape cylinder in a uniform viscous flow was simulated by using the immersed boundary method (IBM) along with low-speed wind tunnel experimentation. The flag in the wake of the cylinder was strongly influenced by the vortices shed from the upstream cylinder under the vortex-vortex and vortex-body interactions. Geometric and flow parameters were optimized for the flexible flag subjected to passive flapping. The influence of length and bending coefficient of the flexible flag, the diameters (D) of the cylinder and the streamwise spacing between the cylinder and the flag, on the energy generation was examined. Constructive and destructive vortex interaction modes, unidirectional and bidirectional bending and the different flapping frequency were found which explained the variations in the energy of the downstream flag. Voltage output and flapping behavior of the flag were also observed experimentally to find a more direct relationship between the bending of the flag and its power generation.

Key Words
immersed boundary method; fluid-structure interaction; energy harvesting eel; piezoelectric flag

Address
Usman Latif, Chaudary Abdullah, Emad Uddin M., Muhamad Sajid and Samiur Rehman Shah: Department of Mechanical Engineering, SMME, National University of Sciences and Technology (NUST),H-12, Islamabad, 46000, Pakistan
M. Yamin Younis: Department of Mechanical Engineering, Mirpur University of Science and Technology (MUST), Mirpur 10250 (AJK), Pakistan
Aamir Mubasha: Mechanical Engineering Program, Middle East Technical University Northern Cyprus Campus, Mersin, Turkey

Abstract
A numerical study is conducted on the flow characteristics of a rectangular cylinder (chord-to-width ratio C/W = 2 - 10) mounted close to a rigid wall at gap-to-width ratios G/W = 0.25 - 6.25. The effects of G/W and C/W on the Strouhal number, vortex structure, and time-mean drag and lift forces are examined. The results reveal that both G/W and C/W have strong influences on vortex structure, which significantly affects the forces on the cylinder. An increase in G/W leads to four different flow regimes, namely no vortex street flow (G/W < 0.75), single-row vortex street flow (0.75< G/W < 1.25), inverted two-row vortex street flow (1.25 < G/W < 2.5), and two-row vortex street flow (G/W > 2.5). Both Strouhal number and time-mean drag are more sensitive to C/W than to G/W. For a given G/W, Strouhal number grows with C/W while time-mean drag decays with C/W, the growth and decay being large between C/W = 2 and 4. The time-mean drag is largest in the single-row vortex street regime, contributed by a large pressure on the front surface, regardless of C/W. A higher C/W, in general, leads to a higher time-mean lift. The maximum time-mean lift occurs for C/W = 10 at G/W = 0.75, while the minimum time-mean lift appears for C/W = 2 at the same G/W. The impact of C/W on the time-mean lift is more substantial in single-row vortex regime. The effect of G/W on the time-mean lift is larger at a larger C/W.

Key Words
boundary layers; rectangular cylinder; Strouhal number; vortex shedding frequency

Address
J.F. Derakhshandeh: School of Mechanical Engineering of University of Adelaide, Australia
Md. Mahbub Alam: Institute for Turbulence-Noise-Vibration Interaction and Control, Shenzhen Graduate School,
Harbin Institute of Technology, Shenzhen 518055, China


Abstract
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.

Key Words
twin box girders; vortex-induced vibrations; central grids; CFD simulation

Address
Zhiguo Li, Qiang Zhou, Haili Liao and Cunming Ma: Research Center for Wind Engineering, Southwest Jiaotong University, Chengdu, China

Abstract
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 6.5X103 to 28.5X103 and the incidence angle a was changed from 0 to 45. The ratio of boundary layer thickness to the prism height was about s/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 of spanwise vortices. Furthermore, the results showed that the critical incidence angle corresponding to the maximum Strouhal number and minimum wake width occurs at acr = 15o 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.

Key Words
experimental study; finite square prism; incidence angle; low-speed wind tunnel; hotwire; Strouhal number

Address
A. Sohankar and M. Kazemi Esfeh: Department of Mechanical Engineering, Isfahan University of Technology, Isfahan, Iran
H. Pourjafari: Department of Mechanical Engineering, Yazd University, Yazd, Iran
Md. Mahbub Alam and Longjun Wang: Institute for Turbulence-Noise-Vibration Interaction and Control, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China


Abstract
A numerical study on the flow over a square cylinder in the vicinity of a wall is conducted for different Couette-Poiseuille-based non-uniform flow with the non-dimensional pressure gradient P varying from 0 to 5. The non-dimensional gap ratio L (=H/a) is changed from 0.1 to 2, where H is gap height between the cylinder and wall, and a is the cylinder width. The governing equations are solved numerically through finite volume method based on SIMPLE algorithm on a staggered grid system. Both P and L have a substantial influence on the flow structure, time-mean drag coefficient , fluctuating (rms) lift coefficient (CL), and Strouhal number St. The changes in P and L leads to four distinct flow regimes (I, II, III and IV). Following the flow structure change, the , CL, and St all vary greatly with the change in L and/or P. The and CL both grow with increasing P and/or L. The St increases with P for a given L, being less sensitive to L for a smaller P (< 2) and more sensitive to L for a larger P (>2). A strong relationship is observed between the flow regimes and the values of , CL and St. An increase in P affects the pressure distribution more on the top surface than on bottom surface while an increase in L does the opposite.

Key Words
square cylinder; couette-poiseuille flow; aerodynamic characteristics; gap flow

Address
Rajesh Bhatt and Md. Mahbub Alam: nstitute for Turbulence-Noise-Vibration Interaction and Control, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen, China
Dilip K. Maiti: Department of Applied Mathematics with Oceanology and Computer Programming,
Vidyasagar University, Midnapur 721102, India
S. Rehman: Center for Engineering Research, Research Institute, King Fahd University of Petroleum and Minerals,
Dhahran-31261, Saudi Arabia




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