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Large Eddy Simulations (LES) with Smagorinsky subgrid scale model have been performed for the flow past two circular cylinders in tandem placed in the vicinity of a horizontal plane wall at very small gap ratios, namely G ∕ D = 0 . 1 , 0 . 3 and 0.5, in three-dimension (3D). The ratio of cylinder center-to-center distance to cylinder diameter, or the pitch ratio, L ∕ D , considered in the simulations is L ∕ D = 2 and 5. This work serves as an extension of Abrahamsen Prsic et al. (2015). In essence, six sets of simulations have been performed in the subcritical Reynolds number regime at R e = 1 . 31 × 1 0 4 . Our major findings can be summarized as follows. (1) At both pitch ratios, the wall proximity has a decreasing effect on the mean drag coefficient of the upstream cylinder. At L ∕ D = 2 , the mean drag coefficient of the downstream cylinder is negative since it is located within the drag inversion separation distance. (2) At L ∕ D = 2 , a squarish cavity-like flow exists between the cylinders and the flow circulates within the cavity. A long lee-wake recirculation zone is found behind the downstream cylinder at G ∕ D = 0 . 1 . However, a much smaller lee-wake recirculation zone is noticed at L ∕ D = 5 with G ∕ D = 0 . 1 . (3) At L ∕ D = 2 , the reattachment is biased to the bottom shear layer due to the deflection from the plane wall, which leads to the formation of the slanted squarish cavity-like flow. At both pitch ratios, as G ∕ D becomes smaller, stronger vortices are found between the two cylinders. Less intense vortices are observed in the near wake of the downstream cylinder due to the vortex shedding suppression of the neighboring wall.

In simulations of flow around a concave curved cylinder, i.e. free-stream aligned with the plane of curvature and directed towards the inner face of the curvature, one cannot avoid interactions between the cylinder and the inlet boundary. To get rid of the effects brought about by this interaction, we consider different lengths of upstream straight extensions at the lower end of the curved cylinder (0D, 5D and 10D, where D is the cylinder diameter), referred to as horizontal extensions. In this study, we directly solve the time-dependent three-dimensional Navier–Stokes equations. Results reveal that the appended horizontal extension allows the boundary layer to develop, so that the velocity profile at the curved cylinder inception is significantly different from the case where no horizontal extension is considered. The laminar boundary layer is thinner than that in the flat plate flow, which is given by Blasius’ solution. The results from 5D and 10D extensions show a clear convergent tendency. We therefore suggest that a horizontal extension is essential for concave curved cylinder flow simulation, and 10D would be a preferred choice. © 2018. This is the authors’ accepted and refereed manuscript to the article. Locked until 14.8.2019 due to copyright restrictions. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

Supersonic flow past a wavy cylinder has been investigated numerically using scale-adaptive simulation technique. The free-stream Reynolds number and Mach number for this supersonic flow are chosen as 2 × 105 and 1.7, respectively. For the sake of comparison and discussion, a corresponding circular cylinder for the same computational parameters also has been calculated. Unlike the subsonic operating condition, drag reduction and fluctuating lift suppression of wavy cylinder in supersonic flow are less dramatic. Results indicate that wavy surface as a flow control technique is invalid for supersonic flow over a circular cylinder. This is because the existence of bow shock-wave and no large-scale vortex shedding in the flow field of circular cylinder. Further, two different flow states have been found in the near wake of wavy cylinder. One is a quasi-steady flow state behind the nodal position with converged wake, and the other is an unsteady flow state behind the saddle position with obvious vortex street. Based on the analyses of flow patterns, pressure power spectral densities, and proper orthogonal decomposition of pressure fields, some novel characteristics of wavy cylinder in supersonic flow are systematically investigated.

A well-developed numerical scheme is used to simulate the steady current over a circular cylinder in proximity with a rigid plane at low Reynolds numbers. The numerical results, including the hydrodynamic forces acting on the cylinder and the flow fields, are obtained and discussed. The reduction of the vorticity generated at the cylinder surface of the gap side is attributed to the decrease of the amplitude of harmonic force components and eventually causes the complete suppression of the vortex shedding process. Particularly, a distinct flow scenario characterized by the agglomeration of two adjacent vortices takes place in the wake when the cylinder is positioned close to the free-slip wall just before the complete cessation of the vortex shedding. This finding might be served to explain the disappearance of regularly arranged vortices observed when the cylinder was towed above the stationary wall in the experiment. In contrast, the flow with a no-slip plane is also considered. The effective gap between the cylinder and the plane is reduced due to the presence of the boundary layer near the plane, and earlier cessation of vortex shedding is observed in comparison with that in the situations of free-slip plane.

The flow dynamics over a vibrissa-shaped cylinder at the sub-critical Reynolds number of 1.8 × 104 are comparatively investigated using the large eddy simulation (LES) approach, with particular emphasis on the wake structure and self-induced force. Three reference configurations with the same hydrodynamic diameters, i.e., a circular cylinder, an elliptical cylinder and a wavy cylinder, are compared with the vibrissa-shaped cylinder configuration. The results demonstrate that the fluctuation of lift force for the vibrissa-shaped cylinder is reduced by approximately 79.2% compared with that for the circular cylinder, and the predicted RMS magnitudes for the four configurations agree reasonably well with the experimental data. For the vibrissa-shaped cylinder, only one sharp peak with relatively low magnitude is observed at the characteristic frequency f D / U 0 = 0.20 , and the wake shedding does not dominate the dynamics, while the vortex structures show significantly three-dimensional features. Meanwhile, the separation line of the recirculation zone shows a remarkable wavy structure that corresponds to the trailing geometrical surface with a spanwise shift of approximately a half-period of undulation. A proper orthogonal decomposition (POD) analysis is performed to explore the three-dimensional structures in the wake behind the vibrissa-shaped cylinder. The POD modes reveal a completely three-dimensional and periodically staggered arrangement along both the spanwise and streamwise directions. The spectra of the first two time coefficients reveal that the wake shedding processes on the nodal and saddle planes are not synchronous. Finally, the phase-averaging results show sequential processes of vortex shedding in the wake behind the vibrissa-shaped cylinder, and demonstrate marked differences between this and the other cylinders. Two symmetrical vortices are attached closely to the backward side of the vibrissa-shaped cylinder, resulting in smaller fluctuations of velocity and wall pressure, which in turn contribute to reducing the vortex-induced force and suppressing its vibrations.

A series of experiments was carried out to study the flow behaviour behind a rotationally oscillating cylinder at a low Reynolds number (Re=300) placed in a recirculation water channel. A stepper motor was used to rotate the cylinder clockwise- and- counterclockwise about its longitudinal axis at selected frequencies. The particle image velocimetry (PIV) technique was used to capture the flow field behind a rotationally oscillating cylinder. Instantaneous and timeaveraged flow fields such as the vorticity contours, streamline topologies and velocity distributions were analyzed. The effects of four rotation angle and frequency ratios F r (F r=f n/f v, the ratio of the forcing frequency f n to the natural vortex shedding frequency f v) on the wake in the lee of a rotationally oscillating cylinder were also examined. The significant wake modification was observed when the cylinder undergoes clockwise-and-counterclockwise motion with amplitude of π, especially in the range of 0.6≤F r≤1.0.

The three-dimensional unsteady problem of impulsive stagnation-point flow on a rotating vertical circular cylinder along with mixed convection heat transfer has been solved, numerically for the first time. This is because similarity solution techniques are not always possible to be accomplished. Initially, the fluid is considered to be at rest and with a uniform temperature T∞ and at t = 0 it starts flowing toward a vertical cylinder at the strength rate of k ¯ and the cylinder surface's temperature rises to Tw, simultaneously. The cylinder possesses a rotational movement, with a constant angular velocity ω. The Navier-Stokes and energy equations in cylindrical coordinate system have been descritized and solved in a 2-D domain by using a SIMPLE based algorithm. The buoyancy effects are also included in the computations, so the problem is investigated as mixed convection heat transfer. Considering a sample case of incompressible flow with Re = 1,5,10,15,20 and Pr = 0.7 , the results of Nusselt number, wall shear-stress and dimensionless velocity and temperature have been obtained at different states of cylinder's wall temperature and for some selected values of Grashof numbers.

Laminar flow past a rotating cylinder near a plane wall is studied numerically using a second-order accurate immersed boundary method. The wake-wall interactions are investigated in detail, for different wall heights and varying rotational rates. Flow patterns are classified based on the wake structure and the effect of rotation on critical wall heights is discussed for both clockwise and counterclockwise rotation. For the configuration studied i.e. rotating cylinder above a bottom plane wall, counterclockwise rotation of cylinder favors the wake-wall interactions, while clockwise rotation influences in an adverse manner. In addition to conventional analysis, the evolution of the vortical structures in the wake region are examined using Lagrangian analysis of individual vortical structures. The diffusion of the positive vortex shed from the wall facing side of the cylinder is accelerated due to the influence of boundary layer with net negative vorticity. The wall augmented diffusion of positive cylinder vortices is compensated by the creation of secondary wall vortices from the bottom wall boundary layer. Counterclockwise rotation of cylinder increases the shear inside boundary layer resulting in more pronounced diffusion.

The existence of a generalized weak solution is proved for the nonstationary problem of motion of a rigid body in the flow of a viscous incompressible fluid filling a cylindrical pipe of arbitrary cross-section. The fluid flow conforms to the Navier–Stokes equations and tends to the Poiseuille flow at infinity. The body moves in accordance with the laws of classical mechanics under the influence of the surrounding fluid and the gravity force directed along the cylinder. Collisions of the body with the boundary of the flow domain are not admitted and, by this reason, the problem is considered until the body approaches the boundary.

A numerical simulation of transverse flow-induced vibrations of a rotating circular cylinder is presented. Rotation rate and direction is imposed to be proportional either to the cylinder’s position or cylinder’s velocity. A 2D Lattice Boltzmann method is used to solve the flow dynamics and the cylinder is spring-mounted and free to undergo transverse vibrations. In all cases, non-dimensional mass is 12.7, Reynolds number 100, and zero damping is considered, so the analysis is focused on the role of the rotation law and the reduced velocity. Results are presented in terms of steady-state oscillations characterization (say amplitude and frequency), fluid forces on the cylinder, and wake-pattern topology. It is shown how the imposed rotation changes the cylinder’s response significantly, fluid forces on it and the flow field in the wake. Depending on the rotation law imposed flow-induced vibrations can be increased or diminished. The former can be of interest for energy harvesting purposes; whereas the later can be useful to avoid unwanted oscillations. When rotatory law is proportional to the cylinder’s position, a galloping-type response has been found. In this case, the cylinder’s amplitude can be reasonably well predicted with a quasi-steady theoretical model. For rotation proportional to the cylinder’s velocity only vortex-induced-type responses have been found.

In this study a new two degrees-of-freedom wake oscillator model is proposed to describe vortex-induced vibrations of elastically supported cylinders capable of moving in cross-flow and in-line directions. The total hydrodynamic force acting on the cylinder is obtained here as a sum of lift and drag forces, which are defined as being proportional to the square of the magnitude of the relative flow velocity around the cylinder. The two van der Pol type oscillators are then used to model fluctuating drag and lift coefficients. As the relative velocity around the cylinder depends both on the fluid flow velocity and the velocity of the cylinder, the equations of motions of the cylinder in cross-flow and in-line directions become coupled through the fluid forces. It is shown that such approximation of the fluid forces allows to obtain the well known low dimensional models in the limit case, and the model proposed by Facchinetti et al. [1] to describe the cross-flow vibrations is used as an example. Existing experimental data and Computational Fluid Dynamics (CFD) results are used to calibrate the proposed model and to verify the obtained predictions of complex fluid-structure interactions for different mass ratios. The "super upper" branch phenomenon, exclusive for a two degrees-of-freedom motion at low mass ratios, has been observed. The influence of the empirical parameters of the wake oscillators and fluid forces coefficients on the dynamic responses is also discussed.

Vortex shedding and heat transfer around a cylinder in pulsating cross-flow have been studied experimentally. Analysis of flow visualization results enabled to classify flow patterns around the cylinder into four groups. New dimensionless number has been submitted, which is a ratio of the inertial force due to the oncoming flow acceleration in its global unsteady motion to the centrifugal inertial force due to curved streamlines around the cylinder. A map of flow patterns around the cylinder has been plotted in terms of the newly submitted dimensionless number and the relative amplitude of oncoming flow pulsations. Distributions of the local heat transfer coefficient over the cylinder surface depending on the flow pattern and the amplitude of forced flow pulsations have been obtained. It has been shown that forced pulsations of oncoming flow can enhance average heat transfer from the cylinder in the cross-flow.

The numerical simulation of the laminar viscous flow past a cylinder performing rotary oscillations around its axis is carried out. The Navier–Stokes equations are solved by finite volume method using the program package OpenFOAM. The values of the amplitude and frequency of forced oscillations are found, at which the maximum reduction of the drag coefficient of the cylinder is achieved.

An example of three-dimensional potential flow of an inviscid incompressible fluid is presented, in which the point of a strict non-zero local minimum of velocity is an internal point of the flow. This proves the impossibility to extend to a three-dimensional flow the two-dimensional velocity minimum principle, according to which in plane potential flows of incompressible fluid the flow velocity cannot have a local non-zero minimum at an internal point.

A first attempt is made for identifying the wake characteristics of circular cylinder confined by a wavy wall at laminar flow regime. Numerical study of flow characteristics past circular cylinder with wavy-wall confinement perpendicular to cylinder axis has been carried out in the range of Reynolds number 20–100. The finite volume-based CFD solver Ansys Fluent (Version 15.0) is used for computations. The results are presented in the form of streamline plots, mean drag co-efficient, flow separation angle and recirculation length. Wavy-wall confinement leads to highly significant changes in the cylinder wake such as the evolution of strong x-plane vortices, enhanced fluid mixing, wake suppression near the crest region and vortex stretching near the trough region on the downstream of the cylinder has been observed. Flow separation angle varies significantly along the axis of the cylinder. Increased wall shear stress on rear surface of the cylinder has also been observed. The part of vorticity magnitude as compared to strain rate has been distinguished and identified using vortex identification methods such as Q-criterion and Lambda-2 criterion.

In the present study, flow control mechanism of single groove on a circular cylinder surface is presented experimentally using Particle image velocimetry (PIV). A square shaped groove is patterned longitudinally on the surface of the cylinder with a diameter of 50 mm. The flow characteristics are studied as a function of angular position of the groove from the forward stagnation point of the cylinder within 0° ≤ θ ≤ 150°. In the current work, instantaneous and time-averaged flow data such as vorticity, ω streamline, Ψ streamwise, u / U o and transverse, v / U o velocity components, turbulent kinetic energy, TKE and RMS of streamwise, u rms and transverse, v rms velocity components are utilized in order to present the results of quantitative analyses. Furthermore, Strouhal numbers are calculated using Karman vortex shedding frequency, f k obtained from single point spectral analysis. It is concluded that a critical angular position of the groove, θ = 80° is observed. The flow separation is controlled within 0° ≤ θ θ = 80°, the flow separation starts to occur in the upstream direction. The instability within the shear layer is also induced on grooved side of the cylinder with frequencies different than Karman vortex shedding frequency, f k .

The turbulent structures formed in a Taylor–Couette (TC) flow established between two concentric counter-rotating cylinders were explored numerically. The shear Reynolds number was set to Reshear = 8000 and the radius ratio was set to ri/ro = 0.5. An optimal flow corresponding to the maximal angular velocity transport between the cylinders was selected for the TC flow. The mean velocity profile reached its steepest value near the cylinders in the optimal TC flow. The streamwise velocity correlations at the outer cylinder in the gap exceeded those at the inner cylinder. The large convective transport of angular velocity in the gap generated a maximal angular velocity flux to achieve the optimal flow. The angular velocity flux generated by the momentum source exceeded that generated by the momentum sink. The vorticity dispersion was larger near the inner cylinder than near the outer cylinder, but vorticity stretching near the outer cylinder exceeded than that near the inner cylinder. The skin fricti...

Finite container sizes in experiments and computer simulations impose artificial boundaries which do not exist when they are meant to mimic ambient fluid of infinite extent. We show here that this is the case with flows past an infinite cylinder placed in an infinite ambient fluid (Stokes’ paradox). Using a highly efficient and stable numerical method that is capable of handling computational domains several orders of magnitude larger than in previous studies, we provide a criterion for the minimum necessary extent around an object in order to provide accurate velocity and pressure fields, which are prerequisites for correct calculation of secondary quantities such as drag coefficient. The careful and extensive simulations performed suggest an improved relation for the drag coefficient as a function of Reynolds number, and identify the most suitable experimental data available in the literature.

Turbulent flow in the axial direction along a cylinder representing a monofilament yarn was reproduced for a relatively wide range of radius Reynolds numbers using direct numerical simulations. In the simulation of the thinnest cylinder, the friction coefficient agreed with a previously published formula. A pair of high- and low-speed streaks was detected around even the thinnest cylinder, whereby it was confirmed that turbulence was sustained according to the analysed turbulence statistics. Even when only a single pair of streaks around the cylinder was detected, the characteristics of the turbulence structures, such as the mean streak spacing based on the viscous length scale, were the same as those in flows over a flat plate. It was found that the friction coefficient changes in a way that maintains the structural characteristics of the flow, consistent with the view that universal characteristics of turbulence structures exist.

In the present study, a traveling wave wall (TWW) was employed to manipulate the vortex shedding behind a fixed circular cylinder based on a 2-D CFD numerical simulation method at a low Reynolds number of 200. The study mainly focused on four types of TWW propagation directions combined with nine different wave velocities to eliminate vortex shedding, and the lift and drag coefficients and vortex shedding modes at different propagation directions or velocities were analyzed in detail to access the effectiveness of the TWW flow control method. The control mechanism of eliminating wake stemming from the flow around a TWW-controlled circular cylinder was revealed by the boundary vorticity flux (BVF) and relative flow fields. The results show that the type of downstream propagating TWW can effectively eliminate the alternating wake. When the ratio of the wave velocity to the oncoming velocity is 2.0, the fluctuations of the lift coefficients descended to the lowest point. The averages of the drag coefficients decreased with increasing wave velocity and descended to the lowest point when the ratio of the wave velocity to the oncoming velocity was 5.0. Owing to the capture of small-scale vortices, there was a relatively larger level of vorticity in the wave valley of the TWW cylinder. The relative flow fields showed that the vortex shedding from the cylinder was completely eliminated and that the goal of eliminating the oscillating wake and suppressing the potential vortex-induced vibration (VIV) of flow around a cylinder was achieved.

A velocity decomposition method is developed for the solution of three-dimensional, unsteady flows. The velocity vector is decomposed into an irrotational component (viscous-potential velocity) and a vortical component (vortical velocity). The vortical velocity is selected so that it is zero outside of the rotational region of the flow field and the flow in the irrotational region can thus be solely described by the viscous-potential velocity. The formulation is devised to employ both the velocity potential and the Navier–Stokes-based numerical methods such that the field discretization required by the Navier–Stokes solver can be reduced to only encompass the rotational region of the flow field and the number of unknowns that are to be solved by the Navier–Stokes solver is greatly reduced. A higher-order boundary-element method is used to solve for the viscous potential by applying a viscous boundary condition to the body surface. The finite-volume method is used to solve for the total velocity on a reduced domain, using the viscous-potential velocity as the boundary condition on the extent of the domain. The two solution procedures are tightly coupled in time. The viscous-potential velocity and the total velocity are time dependent due to the unsteadiness in the boundary layer and the wake. The solver is applied to solve three-dimensional, laminar and turbulent unsteady flows. For turbulent flows, the solver is applied for both Unsteady-Reynolds-Averaging-Navier–Stokes and Large-Eddy-Simulation computations.

The dynamics of vortical structures in flow over a circular cylinder in the vicinity of a flat plate is investigated using particle image velocimetry (PIV). The cylinder is placed above the flat plate with its axis parallel to the wall and normal to the flow direction. The Reynolds number $Re_{D}$ based on the cylinder diameter $D$ is 1072 and the gap $G$ between the cylinder and the flat plate is varied from gap-to-diameter ratio $G/D=0$ to $G/D=3.0$. The flow statistics and vortex dynamics are strongly dependent on the gap ratio $G/D$. Statistics show that as the cylinder comes close to the wall ($G/D\leqslant 2.0$), the cylinder wake becomes more and more asymmetric and a boundary layer separation is induced on the flat plate downstream of the cylinder. The wake vortex shedding frequency increases with decreasing $G/D$ until a critical gap ratio (about $G/D=0.25$) below which the vortex shedding is irregular. The deflection of the gap flow away from the wall and its following interaction with the upper shear layer may be the cause of the higher shedding frequency. The vortex dynamics is investigated based on the phase-averaged flow field and virtual dye visualization in the instantaneous PIV velocity field. It is revealed that when the cylinder is close to the wall ($G/D=2.0$), the cylinder wake vortices can periodically induce secondary spanwise vortices near the wall. As the cylinder approaches the wall ($G/D=1.0$) the secondary vortex can directly interact with the lower wake vortex, and a further approaching of the cylinder ($G/D=0.5$) can result in more complex interactions among the secondary vortex, the lower wake vortex and the upper wake vortex. The breakdown of vortices into filamentary debris during vortex interactions is clearly revealed by the coloured virtual dye visualizations. For $G/D, the lower shear layer is strongly inhibited and only the upper shear layer can shed vortices. Investigation of the vortex formation, evolution and interaction in the flow promotes the understanding of the flow physics for different gap ratios.

The present paper deals with the numerical simulation of unsteady, two-dimensional laminar fluid flow past a circular cylinder. An unstructured grid finite volume method has been used, and the commercial CFD code ANSYS Fluent was employed to perform the simulations. Effects of injection of a second dissimilar fluid from two diametrically opposite peripheral slots on the cylinder are of prime interest in the study. Two distinct injection arrangements have been investigated–a co-counter flow arrangement, in which the injected fluid is aligned with the free stream flow, and a cross flow arrangement, in which the second fluid is injected perpendicular to the free stream flow direction. A parametric variation of the velocity of the injected stream in the downstream wake region of the cylinder have been studied qualitatively by observing the velocity, vorticity, and mass fraction contours. Effects of injection on the vortex shedding have also been studied by observing the variations of the Strouhal number. The two injection arrangements affected the vortex shedding in distinctly different manners. Effect of lateral walls on the flow dynamics were also investigated. The presence of lateral walls was found to delay the onset of aperiodic wake structures and to promote better mixing in the downstream wake of the cylinder. Proper orthogonal decomposition of the vorticity field was also performed in order to identify the dominant modes and their respective enstrophy contents. Significant differences in the modal structures were observed with increase in the value of the control parameter. At lower values, large scale POD modes dominate, while at higher values, the wake comprised of small scale structures, for both the flow arrangements. A gradual dominance of the injected stream on the free stream was also observed.

Flow past a circular cylinder in a uniform flow is investigated for 1 × 10 4 ≤ Re ≤ 4 × 10 5 . A stabilized finite element method is used to solve the incompressible flow equations in primitive variables in three dimensions. The computations capture the phenomenon of drag crisis: a significant reduction in drag with increase in Re in the critical regime. The mechanism for this decrease in drag during the drag crisis is explored. It is found that the transition of the boundary layer from a laminar to turbulent state, as well as the formation of the laminar separation bubble (LSB), is intermittent. The LSB does not exist in the sub-critical regime while it appears at all times beyond the critical regime. The frequency of its appearance as well as the duration of its stay, in the critical Re regime, increases with increase in Re . This is established by studying the rms of the high pass filtered fluctuations in the surface pressure and flow close to the cylinder. A procedure to capture these fluctuations due to the shear layer activity responsible for the formation of LSB is proposed and implemented. It is utilized to estimate the intermittency factor of the LSB at various Re . It is found that the intermittency can be utilized to explain the variation of mean drag with Re . Weakening of vortex shedding is observed in critical regime and beyond.

In this paper, complicated hydrodynamics of a horizontal circular cylinder entering water is investigated numerically for low Froude numbers. A numerical approach is used to model the solid-liquid interactions in presence of a free-surface. The governing equations for the 3D incompressible fluid flow are continuity and Navier-Stokes equations along with an equation for the free surface advection. To track the free-surface motion, the fast-fictitious-domain method is integrated into the volume-of-fluid (VOF) technique. The governing equations are solved everywhere in the computational domain including the horizontal cylinder. A rigid body motion is applied to the region occupied by the circular cylinder. The no-slip boundary condition on the solid-liquid interface is exerted implicitly via increasing the viscosity of the region occupied by the solid. To validate the numerical scheme, the results are compared with those of the experiments available in the literature. The effects of cylinder diameter, length, impact velocity, and cylinder-water density ratio on the non-dimensional depth are also investigated.

Vortex-induced vibration (VIV) has been studied extensively and much attention has been devoted to the “lock-in” phenomenon, which is primarily responsible for fatigue damage to structures. Nevertheless, the behavior of the surrounding flow can also be important. This paper investigates the flow characteristics of the wake in the initial branch for a vibrating cylinder using a new PIV technique capable of a high sampling rate of 200 Hz. The experiments were carried out over a wide range of reduced velocities to better understand the important trends. The mean velocities and turbulence characteristics were obtained by ensemble averaging repeated velocity measurements. This study reveals new insights on the flow characteristics of the wake in the initial branch. In particular, the cylinder vibration is found to lead to the formation of oblique jets, which have a profound influence on the mean flow velocity, turbulence intensity, formation and convection of the vortices, and variations to the stagnation and separation points of the cylinder during vibration. Accepted version

Numerical solutions are presented for fluid flow past a rotating elliptic cylinder for different non-dimensional rotational rates (0.5, 1.0 and 1.5) and various axis ratios (0.1, 0.3, 0.5, 0.7 and 1.0). The two-dimensional, incompressible Navier–Stokes equations are solved using Immersed Boundary method on a non-body conforming Cartesian grid. Reynolds number based on the perimeter of the elliptic cylinder is 100. Periodic flow after an impulsive start of the elliptic cylinder is studied. Flow patterns and variation of aerodynamic force coefficients are reported for different set of parameters and it is observed to be very different from that of rotating circular cylinder. As the cylinder rotates, the trailing edge of the cylinder aids the flow while the leading edge deters the flow. This has significant effect on the wake region and vortex shedding pattern. Complex interactions between cylinder and shed vortices result in many interesting phenomenon, such as formation of hovering vortex. Parameters affecting the formation of hovering vortex and its underlying mechanism are analyzed. Aerodynamic force coefficients and vortex shedding frequencies are presented and it is observed that force coefficients have multiple amplitudes and frequencies.

The flow around an isolated cylinder with a Boger fluid was studied experimentally by partially immersing a small vertical cylinder in a test fluid contained in an annular tank slowly rotating on a turntable. To approximate isolation, the ratio of the tank width to cylinder diameter varied from 40/1 to 300/1, and Reynolds numbers ranged from 10−5 to 0.5. Drag was measured by a custom-made mechanical/optical system and the flow field was mapped using particle image velocimetry (PIV). The experimental liquids were two Newtonian fluids and two polyisobutylene/polybutene Boger fluids, and the cylinders had various diameters. The Newtonian drag data, corrected for end and wall effects, agree with Kaplun's low-Re asymptotic formula. Elasticity increased the drag, starting at a Deborah number of 0.6, by 50% at a De of 2.5, and by more at higher Deborah numbers. PIV measurements were made around a cylinder—upstream, downstream, and transversely—and at 180° from it. The velocity measurements showed that the wake increased with De, among other findings. To relate the increased drag to fluid elasticity, normal stresses due to elasticity were found from deformation rates derived from the PIV data, but these stresses appear to be insufficient to account for the effect.

Basing upon the ideal inviscid perfect gas model the aerodynamic characteristics and the gasdynamic function distributions over the surface of a cylinder in a steady supersonic three-dimensional flow are studied in the absence and the presence of combined intense air injection from the flat-ended nose and the lateral surface of the body into the shock layer. The purpose of the study is to investigate the influence of gas injection from different regions of the cylinder surface on supersonic flow past the cylinder at different angles of attack.

In this work, the characteristics of flow over a circular cylinder with two filaments are numerically investigated. The circular cylinder is fixed, and the filaments with the same properties are symmetrically attached to the rear surface of the cylinder. Moreover, the leading ends of the filaments are fixed, and their trailing ends are free to flap in the wake of the cylinder. To implement numerical simulations, an improved immersed boundary-lattice Boltzmann, which is efficient for moving boundary problems, is employed. By fixing the length of two filaments, their influences on the flow control of the cylinder are examined systematically through varying the filament mass ( M r ) , the filament flexibility ( w r ) , and the attachment angle between two filaments (α). Moreover, the effect of flow viscosity is also examined. Based on the numerical results established, it is found that the fluctuation of lift force together with the mean drag force can be suppressed effectively. In the meanwhile, the flow control of two filaments is independent of Reynolds number. Particularly, the filaments with relatively low flexibility and large attachment angle can achieve high performance, which is caused by the vortex interaction between the cylinder and the filaments as well as the pressure redistribution along the surface of the cylinder.

Potential flow is considered in a cylindrical coordinate system (r, z, θ), caused by a vortex filament located on the z-axis and a finite length vortex inflow-cylinder of uniform intensity distribution located on an infinite impermeable cylinder, the centerline of which coincides with the z axis, in infinite space filled with an ideal liquid. Equipotential surface (line) equations and flow surfaces (lines) are given, along with relations for determining velocity components. It is shown that the flow lines caused by a vortex inflow-cylinder lie on the flow surfaces created generated by the inflow-cylinder. Equations are given that allow lines of total flow to be projected onto planes normal to the z axis using the finite difference method. It is noted that the shape of the flow lines on the respective flow surfaces depend only on the ratio of the vortex filament intensity to inflow-cylinder flow rate.

This work proposes a numerical investigation of the main flow field characteristics and pressure field analysis around a freely oscillating rigid circular cylinder immersed in a high Reynolds number flow in the subcritical range. The cylinder is characterized by high value of mass ratio and mass damping, while the damping itself is quite low. Here the numerical results are compared with experimental data obtained in the Politecnico di Milano wind tunnel under the same fluid dynamic and mechanical conditions. The preliminary check of this numerical setup was provided by considering the case of a fixed cylinder in the subcritical region. Based on this benchmark, the full setup was checked by considering fluid dynamic conditions outside the lock-in region. Finally, a number of points were investigated on the cylinder's steady state response curve in the lock-in region. The numerical model yielded good results in terms of cylinder amplitude response, aerodynamic forces and pressure field analysis, in agreement with the results of the experiment. Analysis of the numerical reconstruction of the flow field evolution is therefore considered to yield further information on the vortex shedding mode, especially in the transition region between 2S and 2P mode.

The question of possible analytical forms for the mean velocity profile in a near-wall turbulent flow is addressed. An approach based on the use of dispersion relations for the flow velocity is developed in the context of a two-dimensional channel flow. It is shown that for an incompressible flow conserving vorticity, there exists a decomposition of the velocity field into rotational and potential components, such that the restriction of the former to an arbitrary cross section of the channel is a functional of the vorticity and velocity distributions over that cross section, while the latter is divergence-free and bounded downstream thereof. By eliminating the unknown potential component with the help of a dispersion relation, a nonlinear integro-differential equation for the flow velocity is obtained. It is then analysed within an asymptotic expansion in the small ratio v*/U of the friction velocity to the mean flow velocity. Upon statistical averaging in the lowest nontrivial order, this equati...

Although vortex-induced vibration (VIV) has been extensively studied, much of existing literature deals with uniform flow in the absence of a boundary. The VIV flow field of a structure close to a boundary generally remains unexplored, but it can have important engineering implications, such as pipeline scour if the boundary is an erodible seabed. In this paper, laboratory experiments are performed to investigate the flow characteristics of an elastically mounted circular cylinder undergoing VIV, and a rigid plane boundary is considered to simplify the problem. The initial gap-to-diameter ratio is fixed at 0.8, and six different reduced velocities are considered. The velocity field is measured using a high resolution particle image velocimetry (PIV) system, which has several advantages over traditional PIV systems, including high sampling rate and the ability to mitigate scatter of laser light near the boundary, allowing accurate measurements at the viscous sublayer. This paper presents the vibration amplitude and oscillation frequency for different Vr; in addition, the mean velocity field, turbulence characteristics, vortex behavior, gap flow velocity, and normal/shear stresses on the boundary were measured/calculated, leading to new insights on the flow field behavior.

The topology and evolution of three-dimensional flow past a cylinder of slenderness ratio SR=1 mounted in a wind tunnel is examined for 0.1≤Re≤325 (based on the diameter of the cylinder) where steady-state solutions have been obtained. Direct numerical simulations were computed using an in-house parallel finite element code. The three-dimensional theory of separation is used to analyze and interpret the flow phenomena. Results indicate that symmetry breaking occurs at Re=1, while the first prominent structure is a horseshoe vortex downstream from the cylinder. At Re=150, two foci are observed, indicating the formation of two tornadolike vortices downstream. Concurrently, another horseshoe vortex is formed upstream from the cylinder. For higher Reynolds numbers, the flow downstream is segmented to upper and lower parts, whereas the topology of the flow on the solid boundaries remains unaltered. Pressure distributions show that pressure, the key physical parameter in the flow, decreases everywhere except im...

Flow around a circular cylinder is a fundamental fluid mechanics problem of practical importance. The dynamic characteristics of the pressure and velocity fields of unsteady incompressible laminar and turbulent wakes behind a circular cylinder are investigated numerically and analyzed simulation. The governing equations, written in the velocity pressure formulation are solved using 2-D finite volume method. In the present work is based on the cylinder diameter 10 2 Reynolds number for flow analysis slow and also turbulent based on the diameter of the cylindrical Reynolds number 10 5 as well as for the analysis of the flow around a cylindrical has been considered. The aim of this research is to investigate the influence of Reynolds number on flow parameters and verify the correctness of the calculations carried out by the governing equations of fluid mechanicsflow. Research on the hydraulic parameters of the flow around a cylinder help of ANSYS software. The frequencies of the drag and lift oscillations obtained theoretically agree well with the experimental results. The pressure and drag coefficients for different Reynolds numbers were also computed and compared with experimental and other numerical results. Due to faster convergence, 2-D finite volume method is found very much prospective for turbulent flow as well as laminar flow.

Three-stranded rope is widely used in fishing gear and mooring system. Results of numerical simulation are presented for flow around a three-stranded rope in uniform flow. The simulation was carried out to study the hydrodynamic characteristics of pressure and velocity fields of steady incompressible laminar and turbulent wakes behind a three-stranded rope. A three-cylinder configuration and single circular cylinder configuration are used to model the three-stranded rope in the two-dimensional simulation. The governing equations, Navier-Stokes equations, are solved by using two-dimensional finite volume method. The turbulence flow is simulated using Standard κ-e model and Shear-Stress Transport κ-ω (SST) model. The drag of the three-cylinder model and single cylinder model is calculated for different Reynolds numbers by using control volume analysis method. The pressure coefficient is also calculated for the turbulent model and laminar model based on the control surface method. From the comparison of the drag coefficient and the pressure of the single cylinder and three-cylinder models, it is found that the drag coefficients of the three-cylinder model are generally 1.3–1.5 times those of the single circular cylinder for different Reynolds numbers. Comparing the numerical results with water tank test data, the results of the three-cylinder model are closer to the experiment results than the single cylinder model results.

A pressure-based finite volume method on unstructured grids is developed for incompressible viscoelastic fluid flow simulations. To avoid the need for information about the stresses at any singular point within the computational domain, an edge (or face for 3D cases) based formulation with all variables located at the cell centroid is used. To avoid possible decoupling between the stress and velocity fields, and ensure consistencies in discretization under the collocated variable arrangement, the deformation rates and stress tensors at the midpoint of an interface straddled by two adjacent cells are linearly interpolated with the variable gradients expressed in terms of the values at all neighboring cell centroids using a least-squares method. An innovative solution strategy that removes the need for explicitly specifying the flow boundary conditions at an open boundary, originally developed for incompressible Newtonian flow simulations, has been extended for viscoelastic cases. With this method, the traditional requirement for flow conditions at an outflow boundary, i.e. fully developed flow, is removed. The outflow channel can thus be truncated so that the flow outlet can be located arbitrarily as long as there is no inflow (e.g. due to recirculation) across this boundary. This not only greatly reduces computational costs, but avoids numerical instabilities due to inconsistencies between the imposed flow behavior and that calculated at the outflow boundary for cases where the outflow channel is not long enough to realize the flow condition imposed. The proposed methodology is demonstrated/validated by simulating, • developing flow of an Oldroyd-B fluid between two parallel plates, and • slip-stick flow of an upper convected Maxwell fluid; with successively truncated flow domains. These results clearly demonstrate that the predicted flow behavior is unaffected by using even a highly truncated domain, whilst this methodology allows a higher critical Weissenberg number to be reached than previously reported in the literature.

The principle of velocity decomposition is used to efficiently and accurately solve the Navier–Stokes boundary-value problem for lifting flows. The velocity vector is decomposed as the sum of irrotational and vortical components. The irrotational component is represented using a velocity potential which satisfies the Laplace equation with a modified boundary condition that is written in terms of the Navier–Stokes solution. A viscous potential can be found which satisfies the Navier–Stokes problem directly outside of the rotational regions of the fluid such that the fluid domain over which the Navier–Stokes equations must be solved numerically can be greatly reduced. The viscous potential is used as a Dirichlet boundary condition for the total velocity on the boundaries of the reduced fluid domain. The velocity decomposition approach is used to solve for the flow over a 2D NACA0012 foil in both laminar and turbulent regimes.

The flow characteristics and drag force of a square cylinder with a front jet injection were experimentally investigated in a wind tunnel. The evolution process of the characteristic flow patterns was recorded by the laser-assisted smoke flow visualization method. The time-averaged velocity fields measured by particle image velocimetry (PIV) were applied to analyze the velocity vectors, streamline patterns, vorticity contours, velocity distributions, and time histories of instantaneous velocities around the upstream region of the square cylinder. The drag force experienced by the square cylinder was obtained by measuring the surface pressures on the front and rear faces. The results show that the jet emitted from the upstream surface of the square cylinder periodically swings left-and-right in the experimental range of injection ratio ⩽0.9. In the time-averaged velocity field, the jet flow impinges the freestream at a four-way saddle and subsequently bifurcates into two streams; one stream goes toward the left edge of the upstream surface, while the other stream directs toward the right edge of the upstream surface. Two recirculation regions formed above the upstream surface of the square cylinder are enclosed by those two streams. The vorticity contours around the upstream surface of the square cylinder are characterized by two adjacent vorticity-concentrated areas of opposite signs. The time histories of the instantaneous velocities around the four-way saddle, jet exit, and shear layer of the swinging jet represent periodic oscillation. This characteristic oscillation frequency is dominated by the wake instability. The recirculation regions formed above the cylinder’s upstream surface prevent impingement from the freestream. Consequently, the surface pressure coefficients on the upstream surface of the square cylinder are reduced. This reduction in the surface pressure coefficient decreases the drag force acting on the square cylinder.

The electric-static body force model is obtained by solving Maxwell's electromagnetic equations. Based on the electro-static model, numerical modeling of flow around a cylinder with a dielectric barrier discharge (DBD) plasma effect is also presented. The flow streamlines between the numerical simulation and the particle image velocimetry (PIV) experiment are consistent. According to the numerical simulation, DBD plasma can reduce the drag coefficient and change the vortex shedding frequencies of flow around the cylinder.

Wake and heat transfer characteristics around a heated circular cylinder were studied numerically in this paper. Heat transfer from a heated cylinder to the freestream flow was in mixed convection regime, with the free convection-bouyancy driven flow in opposite direction to the forced convection-main flow. Numerical simulations were performed for three Reynolds numbers of 100, 135 and 200, with the Richardson (Ri = Gr/Re2) numbers varied from 0 to 1. Results showed that buoyancy force significantly altered wake formation behind the heated cylinder, further resulted in increasing drag and decreasing Nusselt number.

A novel two dimensional pure-Lagrangian vortex method based on instantaneous vorticity conserved boundary conditions (IVCBC) has been applied to investigate the hydrodynamic coefficients and patterns flow around two circular cylinders in side-by-side arrangements with pitch ratios T / D =1.1–7 at a high Reynolds number Re =6×10 4 . The benchmark case of viscous flow around a circular cylinder was first carried out to validate the novel vortex method. Then simulations of the flow around two side by side circular cylinders were performed. A new scheme was proposed to distinguish the wide wake (WW) and narrow wake (NW) by judging the velocity direction of a specific point in the gap flow. The hydrodynamic coefficients and Strouhal numbers were thoroughly studied as a function of the pitch ratios. A total of five kinds of flow patterns were identified. During the investigation of biased flow, an intermediate frequency between frequencies of NW and WW was found and approached to that of a single cylinder. The results show good agreement with the previously published experimental and numerical data.

In this paper we introduce a simplified variant of the well-known Taylor–Couette flow. The aim is to develop and investigate a model problem which is as simple as possible while admitting a wide range of behaviour, and which can be used for further study into stability, transition and ultimately control of flow. As opposed to models based on ordinary differential equations, this model is fully specified by a set of partial differential equations that describe the evolution of the three velocity components over two spatial dimensions, in one meridian plane between the two counter-rotating coaxial cylinders. We assume axisymmetric perturbations of the flow in a narrow gap limit of the governing equations and, considering the evolution of the flow in a narrow strip of fluid between the two cylinders, we assume periodic boundary conditions along the radial and axial directions, with special additional symmetry constraints. In the paper, we present linear stability analysis of the first bifurcation, leading to the well known Taylor vortices, and of the secondary bifurcation, which, depending on the type of symmetries imposed on the solution, can lead to wave-like solutions travelling along the axial direction. In addition, we show results of numerical simulations to highlight the wide range of flow structures that emerge, from simple uni-directional flow to chaotic motion, even with the restriction placed on the flow.

The characteristics of the flow field of a circular cylinder having a small vertical plate at the upstream side are investigated by measuring the fluid force on the circular cylinder and by visualizing the flow field using particle image velocimetry. The experimental parameters are the width ratios (H/B = 0.2–0.6) of the vertical plates to the circular cylinder's diameter, and the gap ratios (G/B = 0–3) between the circular cylinder and the vertical plate. The drag reduction rate and Strouhal number of the circular cylinder increased and then decreased with G/B in the case of the same H/B. The drag reduction rate increased with H/B in the case of the same G/B. In the case of a circular cylinder having a small vertical plate, the stagnation regions are represented on the upstream and downstream sides of the circular cylinder, and the size of that region on the upstream side increased with H/B.

The effect of a uniform through-surface flow (velocity$U_{b}$) on a rigid and stationary cylinder and sphere (radius$a$) fixed in a free stream (velocity$U_{\infty }$) is analysed analytically and numerically. The flow is characterised by a dimensionless blow velocity${\it\Lambda}\,(=U_{b}/U_{\infty })$and Reynolds number$Re\,(=2aU_{\infty }/{\it\nu}$, where${\it\nu}$is the kinematic viscosity). High resolution numerical calculations are compared against theoretical predictions over the range$-3\leqslant {\it\Lambda}\leqslant 3$and$Re=1,10,100$for planar flow past a cylinder and axisymmetric flow past a sphere. For$-{\it\Lambda}\gg 1$, the flow is viscously dominated in a thin boundary layer of thickness${\it\nu}/|U_{b}|$adjacent to the rigid surface which develops in a time${\it\nu}/U_{b}^{2}$; the surface vorticity scales as$Re|{\it\Lambda}|U_{\infty }/a$for a cylinder and sphere. A boundary layer analysis is developed to analyse the unsteady viscous forces. Numerical results show that the surface pressure and vorticity distribution within the boundary layer agrees with a steady state analysis. The flow downstream of the body is irrotational so the wake volume flux,$Q_{w}$, is zero and the drag force is$F_{D}=-{\it\rho}U_{\infty }Q_{b}$, where${\it\rho}$is the density of the fluid and$Q_{b}$is the normal flux through the body surface. The drag coefficient is therefore$-2{\rm\pi}{\it\Lambda}$or$-8{\it\Lambda}$for a cylinder or sphere, respectively. A dissipation argument is applied to analyse the drag force; the rate of working of the drag force is balanced by viscous dissipation, flux of stagnation pressure and rate of work by viscous stresses due to sucking. At large$Re|{\it\Lambda}|$, the drag force is largely determined by viscous dissipation for a cylinder, with a weak contribution by the normal viscous stresses, while for a sphere, only$3/4$of the drag force is determined by viscous dissipation with the remaining$1/4$due to the flux of stagnation pressure through the sphere surface. When${\it\Lambda}\gg 1$, the boundary layer thickness initially grows linearly with time as vorticity is blown away from the rigid surface. The vorticity in the boundary layer is weakly dependent on viscous effects and scales as$U_{\infty }/a{\it\Lambda}$or$U_{\infty }/a{\it\Lambda}^{3/2}$for a cylinder and sphere, respectively. For large blow velocity, the vorticity is swept into two well-separated shear layers and the maximum vorticity decreases due to diffusion. The drag force is related to the vorticity distribution on the body surface and an approximate expression can be derived by considering the first term of a Fourier expansion in the surface vorticity. It is found that the drag coefficient$C_{D}$for a cylinder (corrected for flow boundedness) is weakly dependent on${\it\Lambda}$while for a sphere,$C_{D}$decreases with${\it\Lambda}$.