
CONTENTS 
Volume 26, Number 5, May 2018 
 Preface .
 
Abstract; Full Text (79K)  pages ii.  DOI: 10.12989/was.2018.26.5.00i 
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Key Words
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Address
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 Vortexinduced vibration of a long flexible cylinder in uniform crossflow Chunning Ji, Ziteng Peng, Md. Mahbub Alam, Weilin Chen and Dong Xu:
 
Abstract; Full Text (2031K)  pages 267277.  DOI: 10.12989/was.2018.26.5.267 
Abstract
Numerical simulations are performed of a long flexible cylinder undergoing vortexinduced 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 crossflow (CF) and inline (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 multimode 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 xy 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 nearwake pattern when the standing wave pattern dominates, but an oblique nearwake pattern when the travelling wave pattern prevails.
Key Words
numerical simulation; long flexible cylinder; vortexinduced 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 TurbulenceNoiseVibration Interaction and Control, Shenzhen Graduate School,
Harbin Institute of Technology, Shenzhen, 518055, China
 Experimental and numerical investigation of the energy harvesting flexible flag in the wake of a bluff body Usman Latif, Chaudary Abdullah, Emad Uddin, M. Yamin Younis, Muhamad Sajid, Samiur Rehman Shah and Aamir Mubasha
 
Abstract; Full Text (2462K)  pages 279292.  DOI: 10.12989/was.2018.26.5.279 
Abstract
Inspired by the energy harvesting eel, a flexible flag behind a Dshape cylinder in a uniform viscous flow was simulated by using the immersed boundary method (IBM) along with lowspeed wind tunnel experimentation. The flag in the wake of the cylinder was strongly influenced by the vortices shed from the upstream cylinder under the vortexvortex and vortexbody 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; fluidstructure 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),H12, 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
 Flow structures around rectangular cylinder in the vicinity of a wall J.F. Derakhshandeh and Md. Mahbub Alam
 
Abstract; Full Text (1559K)  pages 293304.  DOI: 10.12989/was.2018.26.5.293 
Abstract
A numerical study is conducted on the flow characteristics of a rectangular cylinder (chordtowidth ratio C/W = 2  10) mounted close to a rigid wall at gaptowidth ratios G/W = 0.25  6.25. The effects of G/W and C/W on the Strouhal number, vortex structure, and timemean 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), singlerow vortex street flow (0.75< G/W < 1.25), inverted tworow vortex street flow (1.25 < G/W < 2.5), and tworow vortex street flow (G/W > 2.5). Both Strouhal number and timemean drag are more sensitive to C/W than to G/W. For a given G/W, Strouhal number grows with C/W while timemean drag decays with C/W, the growth and decay being large between C/W = 2 and 4. The timemean drag is largest in the singlerow 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 timemean lift. The maximum timemean lift occurs for C/W = 10 at G/W = 0.75, while the minimum timemean lift appears for C/W = 2 at the same G/W. The impact of C/W on the timemean lift is more substantial in singlerow vortex regime. The effect of G/W on the timemean 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 TurbulenceNoiseVibration Interaction and Control, Shenzhen Graduate School,
Harbin Institute of Technology, Shenzhen 518055, China
 Numerical studies of the suppression of vortexinduced vibrations of twin box girders by central grids Zhiguo Li, Qiang Zhou, Haili Liao and Cunming Ma
 
Abstract; Full Text (1794K)  pages 305315.  DOI: 10.12989/was.2018.26.5.305 
Abstract
A numerical study based on a delayed detached eddy simulation (DDES) is conducted to investigate the aerodynamic mechanism behind the suppression of vortexinduced 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 largescale 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; vortexinduced 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
 Features of the flow over a finite length square prism on a wall at various incidence angles A. Sohankar, M. Kazemi Esfeh, H. Pourjafari, Md. Mahbub Alam and Longjun Wang
 
Abstract; Full Text (2711K)  pages 317329.  DOI: 10.12989/was.2018.26.5.317 
Abstract
Wake characteristics of the flow over a finite square prism at different incidence angles were experimentally investigated using an openloop 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 timeaveraged velocity, turbulence intensity and the vortex shedding frequency were obtained through a singlecomponent 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; lowspeed 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 TurbulenceNoiseVibration Interaction and Control, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China
 CouettePoiseuille flow based nonlinear flow over a square cylinder near plane wall Rajesh Bhatt, Dilip K. Maiti, Md. Mahbub Alamand S. Rehman
 
Abstract; Full Text (2151K)  pages 331341.  DOI: 10.12989/was.2018.26.5.331 
Abstract
A numerical study on the flow over a square cylinder in the vicinity of a wall is conducted for different CouettePoiseuillebased nonuniform flow with the nondimensional pressure gradient P varying from 0 to 5. The nondimensional 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, timemean 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; couettepoiseuille flow; aerodynamic characteristics; gap flow
Address
Rajesh Bhatt and Md. Mahbub Alam: nstitute for TurbulenceNoiseVibration 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,
Dhahran31261, Saudi Arabia