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CONTENTS
Volume 29, Number 1, July 2019
 

Abstract
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Key Words
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Address
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Abstract
A numerical investigation on forces and flow around three square cylinders in side-by-side arrangement is conducted at a Reynolds number Re = 150 with the cylinder center-to-center spacing ratio L/W = 1.1 ~ 9.0, where W is the cylinder side width. The flow at this Re is assumed to be two-dimensional, incompressible, and Newtonian. The flow simulation is conducted by using ANSYS-Fluent. The flow around the three side-by-side cylinders entails some novel flow physics, involving the interaction between the gap and free-stream side flows as well as that between the two gap flows. An increase in L/W from 1.1 to 9.0 leads to five distinct flow regimes, viz., base-bleed flow (L/W < 1.4), flip-flopping flow (1.4 < L/W < 2.1), symmetrically biased beat flow (2.1 < L/W < 2.6), non-biased beat flow (2.6 < L/W < 7.25) and weak interaction flow (7.25 < L/W < 9.0). The gap flow behaviors, time-averaged and fluctuating fluid forces, time-averaged pressure, recirculation bubble, formation length, and wake width in each flow regime are discussed in detail.

Key Words
flow; forces; wake; three cylinders; gap flow

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

Abstract
Much advancement has been made in wind power due to modern technological developments. The wind energy technology is the world\'s fastest-growing energy option. More power can be generated from wind energy by the use of new design and techniques of wind energy machines. The geographical areas with suitable wind speed are more favorable and preferred for wind power deployment over other sources of energy generation. Today\' s wind turbines are mainly the horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs). HAWTs are commercially available in various sizes starting from a few kilowatts to multi-megawatts and are suitable for almost all applications, including both onshore and offshore deployment. On the other hand, VAWTs finds their places in small and residential wind applications. The objective of the present work is to review the technological development, available sizes, efficiencies, structural types, and structural stability of VAWTs. Structural stability and efficiencies of the VAWTS are found to be dependent on the structural shape and size.

Key Words
vertical axis wind turbines; structural stability; advantages; disadvantages; optimization tools; blade failures

Address
Shafiqur Rehman and Luai M. Alhems: Center for Engineering Research, Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
Muhammad M. Rafique: Faculty of Mechanical Engineering, Technische Universität Bergakademie, Freiberg 09599, Germany
Md. Mahbub Alam: Institute for Turbulence-Noise-Vibration Interaction and Control, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China

Abstract
The interaction of head waves with an infinite row of identical, equally spaced, rectangular breakwaters is investigated in the presence of uneven bottom topography. Using linear water wave theory and matched eigenfunction expansion method, the boundary value problem is transformed into a system of linear algebraic equations which are numerically solved to know the velocity potentials completely. Utilizing this method, reflected and transmitted wave energy are computed for different physical parameters along with the wave field in the vicinity of breakwaters. It is observed that the wave field becomes more complicated when the incoming wavelength becomes smaller than the channel width. A critical ratio of the gap width to the channel width, corresponding to the inflection point of the transmitted energy variation, is identified for which 1/3 of the total energy is transmitted. Similarly, depending on the incident wavelength, there is a critical breakwater width for which a minimum energy is transmitted. Further, the accuracy of the computed results is verified by using the derived energy relation.

Key Words
uneven bottom; breakwaters; eigenfunction expansion

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



Abstract
Vortex-induced vibration of three circular cylinders (each of diameter D) in an equilateral triangular arrangement is investigated using the immersed boundary method. The cylinders, with one placed upstream and the other two side-by-side downstream, are free to vibrate in the cross-flow direction. The cylinder center-to-center spacing L is adopted as L/D = 2.0. Other parameters include the Reynolds number Re = 100, mass ratio m* = 2.0, reduced velocity Ur = 2 ~ 15 and damping ratio  = 0. Cylinder vibration responses are dependent on Ur and classified into five regimes, i.e. Regime I (Ur < 3.2), Regime II (3.2 < Ur < 5.0), Regime III (5.0 < Ur < 6.4), Regime IV (6.4 < Ur < 9.2) and Regime V (Ur > 9.2). Different facets of vibration amplitude, hydrodynamic forces, wake patterns and displacement spectra are extracted and presented in detail for each regime.

Key Words
vortex-induced vibration; circular cylinder; triangular arrangement; immersed boundary method

Address
Weilin Chen and Dong Xu: State Key Laboratory of Hydraulic Engineering Simulation & Safety, Tianjin University, Tianjin, 300072, China
Chunning Ji:State Key Laboratory of Hydraulic Engineering Simulation & Safety, Tianjin University, Tianjin, 300072, China;
Collaborative Innovation Centre for Advanced Ship and Deep-Sea Exploration, Shanghai 200240, China
Md. Mahbub Alam: Institute for Turbulence-Noise-Vibration Interaction and Control, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China

Abstract
The effect of vortex impingement on the fluid dynamics around a cylinder submerged in the wake of another of different diameters is numerically investigated at a Reynolds number Re = 200. While the diameter (D) of the downstream cylinder is fixed, impinging vortices are produced from the upstream cylinder diameter (d) varied as d/D = 0.24, 0.4, 0.6, 0.8 and 1.0, with a spacing ratio L = 5.5d, where L is the distance between the center of the upstream cylinder to the front stagnation point of the downstream cylinder. Two-dimensional simulations are carried out using the finite volume method. Fluid forces acting on the two cylinders are correlated with impinging vortices, vortex shedding, and wake structure. Different facets of wake formation, wake structure, and flow separation and their connections to fluid forces are discussed.

Key Words
complex terrain; typhoon wind field; CFD simulation; surface roughness length; topography

Address
Farhan Zafar, Md. Mahbub Alam and Zaka Muhammad: Institute for Turbulence-Noise-Vibration Interaction and Control
Harbin Institute of Technology (Shenzhen),Shenzhen 518055, China
Md. Islam: Department of Mechanical Engineering, Khalifa University of Science and Technology,
P.O. Box 2533, Abu Dhabi, United Arab Emirates


Abstract
A heated square cylinder (with height A*) is kept parallel to the cold wall at a fixed gap height 0.5A* from the wall. Another adiabatic rectangular cylinder (of same height A* and width 0.5A*) is placed upstream in an inline tandem arrangement. The spacing between the two cylinders is fixed at 3.0A*. The inlet flow is taken as Couette-Poiseuille flow based non-linear velocity profile. The conventional fluid (also known as base fluid) is chosen as water (W) whereas the nanoparticle material is selected as Al2O3. Numerical simulations are performed by using SIMPLE algorithm based Finite Volume approach with staggered grid arrangement. The dependencies of hydrodynamic and heat transfer characteristics of the cylinder on non-dimensional parameters governing the nanofluids and the fluid flow are explored here. A critical discussion is made on the mechanism of improvement/reduction (due to the presence of the upstream cylinder) of heat transfer and drag coefficient, in comparison to those of an isolated cylinder. It is observed that the heat transfer increases with the increase in the non-linearity in the incident velocity profile at the inlet. For the present range studied, particle concentration has a negligible effect on heat transfer.

Key Words
tandem arrangement; heated cylinder; pressure gradient; Couette-Poiseuille flow; FVM

Address
Swati Sharma and Bhupender K. Sharma: Department of Mathematics, Birla Institute of Technology and Science, Pilani - 333031 RJ, India
Dilip K. Maiti: Department of Applied Mathematics with Oceanology and Computer Programming, Vidyasagar University, Midnapur-721102, WB, India
Md. Mahbub Alam: Institute for Turbulence-Noise-Vibration Interaction and Control, Harbin Institute of Technology (Shenzhen), Shenzhen 518055, China



Abstract
A steady slot suction near the free-end leading edge of a finite-length square cylinder was used to control its aerodynamic forces and vortex-induced vibration (VIV). The freestream oncoming flow velocity (U) was from 3.8 m/s to 12.8 m/s. The width of the tested cylinder d = 40 mm and aspect ratio H/d = 5, where H was the height of the cylinder. The corresponding Reynolds number was from 10,400 to 35,000. The tested suction ratio Q, defined as the ratio of suction velocity (Us) at the slot over the oncoming flow velocity at which the strongest VIV occurs (Uv), ranged from 0 to 3. It was found that the free-end slot suction can effectively attenuate the VIV of a cantilevered square cylinder. In the experiments, the RMS value of the VIV amplitude reduced quickly with Q increasing from 0 to 1, then kept approximately constant for Q > 1. The maximum reduction of the VIV occurs at Q = 1, with the vibration amplitude reduced by 92% , relative to the uncontrolled case. Moreover, the overall fluctuation lift of the finite-length square cylinder was also suppressed with the maximum reduction of 87%, which occurred at Q = 1. It was interesting to discover that the free-end shear flow was sensitive to the slot suction near the leading edge. The turbulent kinetic energy (TKE) of the flow over the free end was the highest at Q = 1, which may result in the strongest mixing between the high momentum free-end shear flow and the near wake.

Key Words
vortex-induced vibration; cantilevered square cylinder; steady suction; flow control

Address
Li Ying, Li Shiqing, Zeng Lingwei and Wang Hanfeng: School of civil engineering, Central South University, Changsha, China


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