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Search Results: 1 - 10 of 756 matches for " vortex shedding. "
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On the Sub-Critical Bifurcation of Anti-Phase and In-Phase Synchronized Vortex Shedding Forms  [PDF]
Yih Ferng Peng
Journal of Modern Physics (JMP) , 2013, DOI: 10.4236/jmp.2013.45B015

Transition of flows past a pair of side-by-side circular cylinders are investigated by numerical simulations and the bifurcation analysis of the numerical results. Various flow patterns behind the cylinder-pair have been identified by the gap ratio (G) and Reynolds number (Re). This study focus on transition of in-phase and anti-phase vortex shedding synchronized forms. A nested Cartesian-grid formulation, in combination with an effective immersed boundary method and a two-step fractional-step procedure, has been adopted to simulate the flows. Numerical results reveal that the in-phase and anti-phase vortex shedding flows at Re = 100 can co-exist at 2.08 ≤G≤ 2.58. Hysteresis loop with increasing/decreasing G at constant Reynolds number Re = 100 is reported.

Turbulence Modeling Applied to Flow over a Hydraulic Ball Check Valve  [PDF]
Leonid Grinis, Vitaly Haslavsky
Engineering (ENG) , 2013, DOI: 10.4236/eng.2013.58081
Abstract: This paper describes an experimental, theoretical model, and a numerical study of concentrated vortex flow past a ball in a hydraulic check valve. The phenomenon of the rotation of the ball around the axis of the device, through which liquid flows, has been found. That is, vibration is caused by the rotation of the ball in the check valve. We observe the rotation of the ball around the longitudinal axis of the check valve. This rotation is induced by vortex shedding from the ball. We will discuss computational simulation and experimental investigations of this strong ball rotation. The frequency of the ball vibration and interaction with the check valve wall has been measured as a function of a wide range of Reynolds numbers. The validity of the computational simulation and of the assumptions on which it is based has been proved experimentally. This study demonstrates the possibility of controlling the vibrations in a hydraulic system and proves to be a very effective suppression of the self-excited vibration.
Multimoment Hydrodynamics in Problem on Flow around a Sphere: Entropy Interpretation of the Appearance and Development of Instability  [PDF]
Igor V. Lebed
Open Journal of Fluid Dynamics (OJFD) , 2014, DOI: 10.4236/ojfd.2014.42015

Multimoment hydrodynamics equations are applied to investigate the phenomena of appearance and development of instability in problem on a flow around a solid sphere at rest. The simplest solution to the multimoment hydrodynamics equations coincides with the Stokes solution to the classic hydrodynamics equations in the limit of small Reynolds number values, . Solution  to the multimoment hydrodynamics equations reproduces recirculating zone in the wake behind the sphere having the form of an axisymmetric toroidal vortex ring. The  solution remains stable while the entropy production in the system exceeds the entropy outflow through the surface confining the system. The passage of the first critical value  is accompanied by the  solution stability loss. The  solution, when loses its stability, reproduces periodic pulsations of the periphery of the recirculating zone in the wake behind the sphere. The  and  solutions to the multimoment hydrodynamics equations interpret a vortex shedding. After the second critical value  is reached, the  solution at the periphery of the recirculating zone and in the far wake is replaced by the  solution. In accordance with the  solution, the periphery of the recirculating zone periodically detached from the core and moves downstream in the form of a vortex ring. After the attainment of the third critical value , the  solution at the periphery of the recirculating zone and in the far wake is replaced by the  solution. In accordance with the  solution, vortex rings penetrate into each other and form the continuous vortex sheet in the wake behind the sphere. The replacement of one unstable flow regime by another unstable regime is governed the tendency of the system to discover the fastest path to depart from the state of statistical equilibrium. Having lost the stability, the system does not reach a new stable position. Such a scenario differs from the ideas of classic hydrodynamics, which interprets the development of instability in terms of bifurcations from one stable state to another stable state. Solutions to the multimoment hydrodynamics equations indicate the direction of instability development, which qualitatively reproduces the experimental data in a wide range of Re values. The problems encountered by classic hydrodynamics when interpreting the observed instability development process are solved on the way toward an increase in the number of principle hydrodynamic values.

Cold-State Investigation on a Flame Holder  [PDF]
Yiqing Du
Journal of Power and Energy Engineering (JPEE) , 2013, DOI: 10.4236/jpee.2013.17008

Slitty bluff body is widely used as a high-performance flame holder in power industry. To understand the flame stability mechanism, the evolution of the near wake over a slitty bluff body in cold state was numerically investigated using the renormalization group (RNG) k-ε model at Reynolds number of 470,000. The coherent structure of the near wake was identified by the vortex shedding simulation. To explain the vortex shedding, a mechanism that single vortex of large size suddenly immerses two shear layers was proposed. To quantitatively compare the near wakes at different gap ratio, a vortex shedding character dimension was first proposed. This character dimension has positive correlation with flame stability. Particle-image velocimetry (PIV) measurements in a close wind tunnel were also carried out to confirm the observation from the numerical study. The evidence shows that the numerical results are of good agreement with the cold-state experiments.

Near Wake of a Horizontal Circular Cylinder in Stably Stratified Flows  [PDF]
Yuji Ohya, Takanori Uchida, Tomoyuki Nagai
Open Journal of Fluid Dynamics (OJFD) , 2013, DOI: 10.4236/ojfd.2013.34038
Abstract: The near wake of a circular cylinder in linearly stratified flows of finite depth was experimentally investigated by means of flow visualization and measurements of vortex shedding frequencies, at Reynolds numbers 3.5 × 103-1.2 × 104 and stratification parameters kd 0-2.0. The non-dimensional parameter kd is defined as kd = Nd/U, where N is the Brunt-Vaisala frequency, d, the diameter of the cylinder, and U, the approaching flow velocity. The study demonstrates that as kd increases from zero, the vortex shedding from a circular cylinder progressively strengthens, while the Strouhal number gradually becomes lower than that for homogeneous flow. This phenomenon can be explained by the effect of the increasingly stable stratification which enhances the two-dimensionality of the near-wake flow of the circular cylinder; the enhanced two-dimensionality of the flow strengthens the roll-up of the separated shear layer. Above a certain value of kd, however, vortex formation and shedding are strongly suppressed and the Strouhal number rises sharply. This observation is attributable to the development of stationary lee waves downstream of the circular cylinder because the lee waves strongly suppress vertical fluid motions.
Rotational Oscillation Effect on Flow Characteristics of a Circular Cylinder at Low Reynolds Number  [PDF]
Alice Rosa da Silva, Aristeu da Silveira-Neto, Ant?nio Marcos Gon?alves de Lima
World Journal of Mechanics (WJM) , 2015, DOI: 10.4236/wjm.2015.510019
Abstract: Two dimensional numerical simulations of flow around a rotationally oscillating circular cylinder were performed at Re = 1000. A wide range of forcing frequencies, fr, and three values of oscillation amplitudes, A, are considered. Different vortex shedding modes are observed for a fixed A at several values of fr, as well as for a fixed fr at different values of A. The 2C mode of vortex shedding was obtained in the present study. It is important to point out that this mode has not been observed by other investigators for rotationally oscillating case. Also, it is verified that this mechanism has great influence on the drag coefficient for high frequency values. Furthermore, the lift and pressure coefficients and the power spectra density are also analyzed.
Journal of Engineering Studies and Research , 2012,
Abstract: The purpose of the paper is to visualise the vortex shedding process behind a bluff body. It was considered a plane flow of a viscous fluid (water) around a "T" profile having the geometric sizes modified. It was measured the fluid flow speed in the section of the measurements and the frequency of the vortex detachment from the profile. The vortex development phenomenon depends by the profile’s geometry and the Reynolds number of the flow. The relation between Reynolds number and Strouhal number was obtained for different sizes "T" profiles. The experimental study is necessary to improve the vortex flowmeter performances.
Measurement of Aerodynamic Sound Source around a Circular Cylinder by Particle Image Velocimetry  [PDF]
Yasuyuki Oguma, Takayuki Yamagata, Nobuyuki Fujisawa
Journal of Flow Control, Measurement & Visualization (JFCMV) , 2014, DOI: 10.4236/jfcmv.2014.23012
Abstract: The purpose of this paper is to propose a sound localization method as an alternative of the time-resolved particle image velocimetry (PIV) system for detecting the aerodynamic sound source of a circular cylinder in a uniform flow. The sound source intensity of a circular cylinder in a uniform flow is evaluated by measuring the time-derivative of instantaneous velocity field in the flow field using a pair of planar PIV system. It allows the visualization of the sound source intensity distribution, which is the time-derivative of the vector product of vorticity and velocity. The experimental results indicate that the aerodynamic sound is generated from the separation point and the velocity fluctuation in the separating shear layer from the circular cylinder. These results agree qualitatively with the previous findings from experiment and numerical simulation, which supports the validity of the present experimental method for evaluating the sound source intensity distribution.
Numerical Simulation of Interaction of a Current with a Circular Cylinder near a Rigid Bed  [PDF]
M. Y. Abdollahzadeh Jamalabadi, M. Oveisi
Journal of Applied Mathematics and Physics (JAMP) , 2016, DOI: 10.4236/jamp.2016.42047
Abstract: The numerical modeling of a 2D flow around a horizontal cylinder near a rigid bed with the gap ratios G/D = 0, 0.2, 0.3 at Reynolds numbers 840, 4500, 9000, and 9500 is investigated by using weakly compressible smoothed particle hydrodynamics. The velocity field and the separation angles from the present simulations are compared with those obtained from the experimental measurements and are in a good agreement. The results show that the maximum value of shear stress on the bed increases as the cylinder closes the bed and suddenly decreases when the cylinder contact the wall.
High Reynolds number oscillations of a circular cylinder
Hirata, Miguel H.;Pereira, Luiz Antonio A.;Recicar, Jan N.;Moura, Washington H. de;
Journal of the Brazilian Society of Mechanical Sciences and Engineering , 2008, DOI: 10.1590/S1678-58782008000400006
Abstract: this paper concerns the numerical simulation of the flow around an oscillating circular cylinder, which moves with constant velocity in a quiescent newtonian fluid with constant properties. for each time step of the simulation a number of discrete lamb vortices is placed close to the body surface; the intensity of each of these is determined such as to satisfy the no-slip boundary condition. the aerodynamic loads acting on the surface of cylinder are computed using the integral formulation derived from the pressure poisson equation. the influence of the frequency and amplitude oscillation on the aerodynamic loads and on the strouhal number is presented and discussed.
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