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CONTENTS
Volume 23, Number 2, August 2016
 

Abstract
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Key Words
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Address
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Abstract
Flow-excited acoustic resonance in ducted cavities can produce high levels of acoustic pressure that may lead to severe damage. This occurs when the flow instability over the cavity mouth, which is created by the free shear layer separation at the upstream edge, is coupled with one of the acoustic modes in the accommodating enclosure. Acoustic resonance can cause high amplitude fluctuating acoustic loads in and near the cavity. Such acoustic loads could cause damage in sensitive applications such as aircraft weapon bays. Therefore, the suppression and mitigation of these resonances are very important. Much of the work done in the past focused on the fluid-dynamic oscillation mechanism or suppressing the resonance by altering the edge condition at the shear layer separation. However, the effect of the downstream edge has received much less attention. This paper considers the effect of the impingement edge geometry on the acoustic resonance excitation and Strouhal number values of the flow instabilities in a ducted shallow cavity with an aspect ratio of 1.0. Several edges, including chamfered edges with different angles and round edges with different radii, were investigated. In addition, some downstream edges that have never been studied before, such as saw-tooth edges, spanwise cylinders, higher and lower steps, and straight and delta spoilers, are investigated. The experiments are conducted in an open-loop wind tunnel that can generate flows with a Mach number up to 0.45. The study shows that when some edge geometries, such as lower steps, chamfered, round, and saw-tooth edges, are installed downstream, they demonstrate a promising reduction in the acoustic resonance. On the other hand, higher steps and straight spoilers resulted in intensifying the acoustic resonance. In addition, the effect of edge geometry on the Strouhal number is presented.

Key Words
acoustic resonance; flow impingement; rectangular cavity; downstream edge; strouhal number

Address
Ahmed Omer and Atef Mohany: Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, Oshawa, Ontario L1H 7K4, Canada
Marwan Hassan: School of Engineering, University of Guelph, Guelph, Ontario N1G 2W1, Canada

Abstract
Flow interference is investigated between two tripped cylinders of identical diameter D at stagger angle = 0 ~ 180 and gap spacing ratio P* (= P/D) = 0.1 ~ 5, where a is the angle between the freestream velocity and the line connecting the cylinder centers, and P is the gap width between the cylinders. Two tripwires, each of diameter 0.1D, were attached on each cylinder at azimuthal angle b= +-30, respectively. Time-mean drag coefficient (CD) and fluctuating drag (CDf) and lift (CLf) coefficients on the two tripped cylinders were measured and compared with those on plain cylinders. We also conducted surface pressure measurements to assimilate the fluid dynamics around the cylinders. CD, CDf and CLf all for the plain cylinders are strong function of a and P* due to strong mutual interference between the cylinders, connected to six interactions (Alam and Meyer 2011), namely boundary layer and cylinder, shear-layer/wake and cylinder, shear layer and shear layer, vortex and cylinder, vortex and shear layer, and vortex and vortex interactions. CD, CDf and CLf are very large for vortex and cylinder, vortex and shear layer, and vortex and vortex interactions, i.e., the interactions where vortex is involved. On the other hand, the interference as well as the strong interactions involving vortices is suppressed for the tripped cylinders, resulting in insignificant variations in CD, CDf and CLf with a and P*. In most of the (a, P* ) region, the suppressions in CD, CDf and CLf are about 58%, 65% and 85%, respectively, with maximum suppressions 60%, 80% and 90%.

Key Words
interactions; aerodynamics; two cylinders; tripped cylinders; trip wires; wake; forces; vortex; shear layer; staggered arrangement

Address
Md. Mahbub Alam: Institute for Turbulence-Noise-Vibration Interaction and Control, Shenzhen Graduate School,
Harbin Institute of Technology, Shenzhen 518055, China;
Digital Engineering Laboratory of Offshore Equipment, Shenzhen, China
Sangil Kim : Department of Mechanical Engineering, Kangwon National University, 346 Jungang-ro, Samcheok 25913, Republic of Korea
Kumar Maiti: Department of Applied Mathematics with Oceanology and Computer Programming, Vidyasagar University, Midnapur WB, India


Abstract
Aerodynamic effects, such as drag force and flow-induced vibration (FIV), on civil engineering structures can be minimized by optimally modifying the structure shape. This work investigates the turbulent wake of a square prism with its faces modified into a sinusoidal wave along the spanwise direction using three-dimensional large eddy simulation (LES) and particle image velocimetry (PIV) techniques at Reynolds number ReDm = 16,500 22,000, based on the nominal width (Dm) of the prism and free-stream velocity (U). Two arrangements are considered: (i) the top and bottom faces of the prism are shaped into the sinusoidal waves (termed as WSP-A), and (ii) the front and rear faces are modified into the sinusoidal waves (WSP-B). The sinusoidal waves have a wavelength of 6Dm and an amplitude of 0.15Dm. It has been found that the wavy faces lead to more three-dimensional free shear layers in the near wake than the flat faces (smooth square prism). As a result, the roll-up of shear layers is postponed. Furthermore, the near-wake vortical structures exhibit dominant periodic variations along the spanwise direction; the minimum (i.e., saddle) and maximum (i.e., node) cross-sections of the modified prisms have narrow and wide wakes, respectively. The wake recirculation bubble of the modified prism is wider and longer, compared with its smooth counterpart, thus resulting in a significant drag reduction and fluctuating lift suppression (up to 8.7% and 78.2%, respectively, for the case of WSP-A). Multiple dominant frequencies of vortex shedding, which are distinct from that of the smooth prism, are detected in the near wake of the wavy prisms. The present study may shed light on the understanding of the underlying physical mechanisms of FIV control, in terms of passive modification of the bluff-body shape.

Key Words
square prism; sinusoidal wavy face; passive control

Address
Y.F. Lin: Shenzhen Key Laboratory of Urban Planning and Decision Making, Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China;
Parsons Brinckerhoff (Asia) Ltd., Hong Kong
H.L. Bai: Department of Civil and Environmental Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
Md. Mahbub Alam: Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, China



Abstract
The interaction between two different shaped structures is very important to be understood. Fluid-structure interactions and aerodynamics of a circular cylinder in the wake of a V-shaped cylinder are examined experimentally, including forces, shedding frequencies, lock-in process, etc., with the V-shaped cylinder width d varying from d/D = 0.6 to 2, where D is the circular cylinder diameter. While the streamwise separation between the circular cylinder and V-shaped cylinder was 10D fixed, the transverse distance T between them was varied from T/D = 0 to 1.5. While fluid force and shedding frequency of the circular cylinder were measured using a load cell installed in the circular cylinder, measurement of shedding frequency of the V-shaped cylinder was done by a hotwire. The major findings are: (i) a larger d begets a larger velocity deficit in the wake; (ii) with increase in d/D, the lock-in between the shedding from the two cylinders is centered at d/D = 1.1, occurring at d/D 0.95-1.35 depending on T/D; (iii) at a given T/D, when d/D is increased, the fluctuating lift grows and reaches a maximum before decaying; the d/D corresponding to the maximum fluctuating lift is dependent on T/D, and the relationship between them is linear, expressed as ; that is, a larger d/D corresponds to a greater T/D for the maximum fluctuating lift.

Key Words
cylinder; lock-in; wake; fluctuating lift force

Address
Sangil Kim: Department of Mechanical Engineering, Kangwon National University, 346 Jungang-ro, Samcheok 25913, Republic of Korea
Md. Mahbub Alam: Institute for Turbulence-Noise-Vibration Interaction and Control, Shenzhen Graduate School,
Harbin Institute of Technology, Shenzhen 518055, China;
Digital Engineering Laboratory of Offshore Equipment, Shenzhen, China
Mohammad Russel: School of Food and Environment, Key laboratory of Industrial Ecology and Environmental Engineering, Ministry of Education, Dalian University of Technology, Panjin 124221, China


Abstract
The structural integrity of tube bundles represents a major concern when dealing with high risk industries, such as nuclear steam generators, where the rupture of a tube or tubes will lead to the undesired mixing of the primary and secondary fluids. Flow-induced vibration is one of the major concerns that could compromise the structural integrity. The vibration is caused by fluid flow excitation. While there are several excitation mechanisms that could contribute to these vibrations, fluidelastic instability is generally regarded as the most severe. When this mechanism prevails, it could cause serious damage to tube arrays in a very short period of time. The tubes are therefore stiffened by means of supports to avoid these vibrations. To accommodate the thermal expansion of the tube, as well as to facilitate the installation of these tube bundles, clearances are allowed between the tubes and their supports. Progressive tube wear and chemical cleaning gradually increases the clearances between the tubes and their supports, which can lead to more frequent and severe tube/support impact and rubbing. These increased impacts can lead to tube damage due to fatigue and/or wear at the support locations. This paper presents simulations of a loosely supported multi-span U- bend tube subjected to turbulence and fluidelastic instability forces. The mathematical model for the loosely-supported tubes and the fluidelastic instability model is presented. The model is then utilized to simulate the nonlinear response of a U-bend tube with flat bar supports subjected to cross-flow. The effect of the support clearance as well as the support offset are investigated. Special attention is given to the tube/support interaction parameters that affect wear, such as impact and normal work rate.

Key Words
flow-induced vibrations; fluidelastic instability; impact; friction; loosely-supported tubes; nuclear steam generators

Address
Marwan Hassan: School of Engineering, University of Guelph, Guelph, Ontario, Canada
Atef Mohany: Department of Mechanical Engineering, University of Ontario Institute of Technology,
Oshawa, Ontario, Canada



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