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Two-dimensional flow around semi-circular cylinder at zero angle of attack and at Re = 50000 during the self-oscillatory regime has been extensively studied within the URANS method with the use of different-structure grids (multiblock, structured overlapping, unstructured composite), the SST turbulence model and its versions (1993) and (2003) considering the streamline curvature influence modified within the Rodi–Leschziner–Isaev approach and numerical different-approximation methods realized in two codes (VP2/3, Fluent). Experiments have been made on flow around a semi-circular cylinder in the wind tunnel of the Lomonosov Moscow State University, Institute of Mechanics to obtain data for verification of numerical predictions. The double-mode character of a periodic time history of a drag force caused by a periodically forming and disappearing jet flap and acting upon a body is explained. With increasing compressibility at a Mach number ranging from 0 to 0.5, it is observed that periodic flow around the semi-circular cylinder is restructured, and the time history of the drag force acting upon it is described by a dependence close to a sinusoidal one. It is found that, as the Mach number is increased, pressure field distortions in the form of concentric cylindrical waves propagating from the semi-circular cylinder and the vortex street behind it grow over the infrasonic range.

The flow and heat transfer characteristics in slip flow regime at low pressure state are numerically investigated by using CFD solver. The complete boundary conditions of first-order velocity slip are considered by User Defined Functions (UDFs), which include effects of thermal creep and wall curvature, as well as temperature jump. Based on the dimensionless continuity, momentum and energy equations along with these boundary conditions, the numerical simulation for flow and heat transfer behaviors over a circular cylinder is conducted in a wide range of Re = 0.001 to 20 and Kn = 0.01 to 0.1. It is found that the prediction results without considering the slip boundary conditions obviously deviate from the experimental values with the increase of Knudsen number, while the one by the developed model is in good agreement with the experimental results. Meanwhile, the variation of physical fields under different Reynolds and Knudsen numbers is discussed in detail. The empirical correlations in the slip flow regime for predicting average drag coefficient and average Nusselt number are proposed on the basis of simulation data, which makes the traditional method used for continuous flow regime be successfully applied into the prediction of flow and heat transfer performances in the slip flow regime.

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/

The selection of interface boundary conditions between porous-medium and clear-fluid regions is vital for the extensive range of applications in engineering. As such, present paper reports an analytically investigating Stokes flow over Reiner–Rivlin liquid sphere embedded in a porous medium filled with micropolar fluid using Brinkman’s model and assuming uniform flow away from the obstacle. The stream function solution of Brinkman equation is obtained for the flow in porous region, while for the inner flow field the solution is obtained by expanding the stream function in a power series of S . The flow fields are determined explicitly by matching the boundary conditions at the interface of porous region and the liquid sphere. Relevant quantities such as velocity and pressure on the surface of the liquid sphere are determined and exhibited graphically. The mathematical expression of separation parameter SEP is also calculated which shows that no flow separation occurs for the considered flow configuration and also validated by its pictorial presentation. The drag coefficient experienced by a liquid sphere embedded in a porous medium is evaluated. The useful features of the Stokes flow for numerous values of parameters are analyzed and discussed. The dependence of the drag force and stream line pattern on permeability parameter( η 2 ), viscosity ratio( λ ), micropolar parameter( m ), coupling number( N ), and dimensionless parameter S is presented graphically and discussed. The analysis also aims at the explanation of velocity overshoot behavior. Some previous noted results are then also obtained from the ongoing analysis.

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.

The flow around a twisted elliptic cylinder is numerically calculated using large eddy simulation (LES) at the Reynolds number of 2 × 104, along with a circular cylinder for comparison. The mean drag coefficient and the fluctuating lift coefficient of the twisted cylinder are 27.99% and 94.01% lower, respectively, than that of the circular cylinder. The twisted surface leads to more stable wake, longer vortex formation length, higher base pressure and three-dimensional separation. In addition, cluster-based reduced-order modelling (CROM) is performed to analyze phase-dependent variations of the wake flow and evaluate the relationship between derived characteristic flow structures and their impact on forces. It captures the oscillatory behavior of Karman vortex shedding in the wake of the circular cylinder. Meanwhile, two flow regimes, anti-phased and in-phase-dominated vortex shedding, generated by the twisted cylinder are extracted and distinguished. The two regimes prevail alternately with changing probability of occurrence along the spanwise of the twisted cylinder, which can be attributed to the different separation angles. The in-phase-dominated vortex shedding inhibits the effect of Karman vortex shedding and is subjected to a lower force compared with the anti-phased vortex shedding.

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.

A numerical analysis is carried out to investigate the wake flow control behind a circular cylinder with corona discharge. The electrohydrodynamic flow is utilized to suppress the boundary layer separation and modify the vortical structures of the flow past a circular cylinder. Numerical simulations consist of the interaction between the electric and flow fields. In this article, the finite volume approach is employed to simulate the flow affected by EHD actuator. The effects of applied voltage, shape of the grounded electrode, location and radius of the discharge electrodes on the swirling flow patterns and drag reduction have been studied. The Reynolds numbers are considered from 4000 to 16000. The results shown that, the modification in EHD actuator design can be used to enhance the flow control effects. The present achievements indicate that, EHD-induced flow can significantly reduce the wake flow behind a circular cylinder. Moreover, the study suggested that this actuator can be applied to practical separation suppression and drag reduction. Furthermore, the predicted numerical results are in excellent agreement with the experimental data.

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.

Trapped mode in three-dimensional waveguide with parallel vertical walls a distance 2d apart, when there is a circular cylinder of radius placed symmetrically between them, is investigated. The pot...

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.

In this paper, a hybrid algorithm of the lattice Boltzmann method(LBM) and the finite difference lattice Boltzmann method(FDLBM) is proposed. The conventional LBM is known for its high computational efficiency, while the finite difference method can guarantee both accuracy of the description of the complex geometry and the high resolution of the flow field by a local refined anisotropic body-fitted mesh. The hybrid algorithm of the LBM and FDLBM takes advantage of the merits of both the methods. Four benchmark flow problems are simulated to verify the hybrid algorithm, namely, the lid driven cavity flow, the flow past a static circular cylinder, the flow past an impulsively started cylinder and the flow past two cylinders in tandem configuration. Results from these simulations show that the hybrid method proposed in the present work works very well for viscous flows.

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.

Theoretical characteristics of the flow in scrolls of hydraulic turbines whose cross-sectional dimensions and shapes are determined using the equations of the potential flow induced by a vortex sink cylinder of finite length located on an impermeable cylinder of infinite length in an ideal fluid are calculated. The theoretical distributions of the components of the velocity and the directions of its horizontal and vertical projections (vertical turbine) are determined and plotted. They are favorable for hydraulic turbines and are in good agreement with the values set at the stator inlet, which confirms the effectiveness of the new approach to studying potential flows whose velocity potential cannot be expressed analytically.

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.

International audience; Flow-induced vibration in square cylinder arrangement under viscous fluid incompressible cross-flow is investigated in the present work. The purpose is to contribute to better mod-eling and understanding external fluid loads exerted on long thin cylinders inducing flow perturbations. Due to high flow confinement, thin cylinders may be subjected to strong vibrations, which may lead to dynamic instability development. A theoretical approach is developed to determine a stability criterion of the dynamical system. The influence of geometric, mechanical and flow parameters such as reduced velocity and pitch ratio is investigated. The proposed model is derived from the potential flow theory and enhanced through an algebraic phase lag model in order to predict the critical limit of the reduced velocity for a square cylinder arrangement submitted to an external in-line cross flow. A theoretical formulation of the total damping, including added damping in still fluid, the damping due to fluid flow and the damping derived from the phase shift between the fluid load and the tube displacement, is expressed. A function depending on fluid and structure parameters, such as reduced velocity, pitch ratio and Scruton number is thus obtained. It is shown that this function provides a prediction of the dynamic stability limit of the system for several ranges of the major parameters to be considered. The results are compared to experimental reference solutions and to those provided by other theoretical models. This work proposes a consistent original model based on a potential flow theory enriched by using an algebraic formulation based on standard physical assumptions from literature. The major advantage of this model is due to the fact that it is in the same time robust and very user-friendly from a computational point of view thanks to the potential framework. In order to describe fluid and solid dynamics in the domain, terms coming from the potential flow theory are estimated by using a finite element method and complementary terms acting on damping are obtained through an algebraic formulation. Therefore this is a convenient way to propose a hybrid numerical / algebraic model for predicting dynamic instability limit in cylinder arrangements.

A large-eddy simulation method of calculating the flow past an elastically-supported body, has been constructed based on the finite difference method formulated in the coordinates fixed on the moving body. The boundary conditions on the body can be applied in the same way as on a fixed body. Reconstruction of computational mesh or interpolation of the computational points is not needed as the body moves. The coding of the method is simple and the computation is efficient, but the equations of motion must be written in the non-inertial coordinates and the boundary conditions on the outer boundaries of the flow domain that also move must be adjusted as the orientation of the body changes. The method has been tested in the computation of the flow past a rectangular cylinder supported by a spring and dashpot and the motion of the cylinder for which experimental data are available. Comparison with experimental data in the case of a rectangular cylinder of depth to breadth ratio of D/B=2 indicates that the characteristics of the translational oscillation corresponding to the heaving of long beams and the rotational oscillation corresponding to the torsional deformation are reproduced well.

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.

Nonisothermal cross flow of a viscous incompressible fluid around a porous cylinder with a square cross section is considered. Main attention is paid when an impermeable core of the cylinder is surrounded with a porous layer. The full system of Navier–Stokes and energy equations is integrated numerically by the finite-volume method. The hydrodynamic interaction between the flow and the matrix of the porous layer is described by Darcy’s law. At moderate Reynolds numbers, the influence of the permeability of the porous layer on the nature of the flow and the heat exchange between the cylinder and the flow is studied. It is shown that, with increasing permeability, heat transfer from the cylinder increases mainly on its front side. From the analysis of the data obtained, an approximate formula for the mean Nusselt number as a function of the Reynolds and Darcy numbers is derived. The results of the calculation of hydrodynamic and thermal characteristics of the cross-flow around an impermeable and a fully permeable cylinder are also presented.

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.

The present work analyzed the hydromagnetic effect on three-dimensional Couette flow of viscous incompressible, electrically conducting and Newtonian fluid through a porous medium bounded by two horizontal parallel porous flat plates with transverse sinusoidal injection of the fluid at the stationary plate and its corresponding removal by periodic suction through the plate in uniform motion. The flow becomes three dimensional due to this injection/suction velocity. Approximate solutions are obtained for the flow field, the pressure, the skin-friction, the temperature field, and the rate of heat transfer. It is found that the cross velocity w is reduced considerably with a rise in the magnetic body parameter ( M ), permeability parameter ( K ) and injection/suction ( λ ) in the forward flow, while a reverse effect is observed in the backward flow. An increase in K or λ is found to escalate the main flow velocity whereas an increase in the magnetic body parameter ( M ) is shown to exert the opposite effect. Similarly, the shear stress due to main flow is considerably increased with an increase in M and K . The results show that both methods, perturbation and electrical network schemes provides excellent approximations to the solution of this nonlinear system with high accuracy. The acquired knowledge in our study can be used by designers to control Magnetohydrodynamic (MHD) flow as suitable for a certain application. Other possible applications include materials processing, MHD propulsion thermo-fluid dynamics, boundary layer in aerodynamics, chemical engineering etc.

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 effect of a cylindrical bluff body on the interface characteristics of stratified two-phase, oil-water, pipe flows is experimentally investigated with high speed Particle Image Velocimetry (PIV). The motivation was to study the feasibility of flow pattern map actuation by using a transverse cylinder immersed in water in the stratified pattern, and particularly the transition from separated to dispersed flows. The cylinder has a diameter of 5 mm and is located at 6.75 mm from the bottom of the pipe in a 37 mm ID acrylic test section. Velocity profiles were obtained in the middle plane of the pipe. For reference, single phase flows were also investigated for Reynolds numbers from 1550 to 3488. It was found that the flow behind the cylinder was similar to the two dimensional cases, while the presence of the lower pipe wall diverted the vorticity layers towards the top. In two-phase flows, the Froude number (from 1.4 to 1.8) and the depth of the cylinder submergence below the interface affected the generation of waves. For high Froude numbers and low depths of submergence the counter rotating von Karman vortices generated by the cylinder interacted with the interface. In this case, the vorticity clusters from the top of the cylinder were seen to attach at the wave crests. At high depths of submergence, a jet like flow appeared between the top of the cylinder and the interface. High speed imaging revealed that the presence of the cylinder reduced to lower mixture velocities the transition from separated to dual continuous flows where drops of one phase appear into the other.

Computer simulations of steady plus low-amplitude oscillatory flow about a circular cylinder are reported at a fixed Reynolds number of 150 based on the steady component. The conventional Keleugan–Carpenter number based on the oscillatory component is fixed at π/5. The oscillation frequency is varied so as to study a wide spectrum of flows where inertial forces dominate at one end and viscous drag forces at the other as a function of the modified Keleugan–Carpenter number. The hydrodynamic force on the cylinder in-line with the flow direction is represented by Morison's equation and an extended version with three terms. The drag and inertia coefficients in Morison's equation are determined by least-squares fits to data directly computed from integration of skin friction and pressure distributions around the periphery of the cylinder. The root-mean-square value of the residue of reconstructed minus directly-computed forces varies between 2 and 41% depending on the flow parameters. Comparable results can be obtained with a semi-theoretical approach using inviscid inertia and quasi-steady viscous drag terms. Physical explanations for the variation of the force coefficients are provided and implications for pertinent flow–structure interactions are discussed.

The coupled evolution of an eroding cylinder immersed in a fluid within the subcritical Reynolds range is explored with scale resolving simulations. Erosion of the cylinder is driven by fluid shear stress. Karman vortex shedding features in the wake and these oscillations occur on a significantly smaller time scale compared to the slowly eroding cylinder boundary. Temporal and spatial averaging across the cylinder span allows mean wall statistics such as wall shear to be evaluated; with geometry evolving in 2-D and the flow field simulated in 3-D. The cylinder develops into a rounded triangular body with uniform wall shear stress which is in agreement with existing theory and experiments. We introduce a node shuffle algorithm to reposition nodes around the cylinder boundary with a uniform distribution such that the mesh quality is preserved under high boundary deformation. A cylinder is then modelled within an infinite array of other cylinders by simulating a repeating unit cell and their profile evolution is studied. A similar terminal form is discovered for large cylinder spacings with consistent flow conditions and an intermediate profile was found with a closely packed lattice before reaching the common terminal form.

Numerical investigation of heat transfer phenomena of low Reynolds number nano-fluid flow over an isothermal cylinder is presented in this paper. Steady state governing equations (continuity, N–S and energy equations) have been solved using finite volume method. Stationary heat transfer, and flow characteristics over the cylinder have been studied for water based copper nanofluid with different solid fraction values. The effect of volume fraction of nano- particles on the fluid flow and heat transfer were investigated numerically. It was found that at a given Nusselt number, drag coefficient, re-circulation length, and pressure coefficient increase by increasing the volume fraction of nano-particles.

A numerical analysis was carried out to examine the effect of pulsating flows around a semi-circular (heated) cylinder placed in a horizontal confined empty channel. The heat transfer induced as an outcome of non-zero mean sinusoidally varying flow past a semi-circular cylinder was investigated. For this purpose, computations are carried out for the following range of parameters: wall confinement (or blockage ratio, β) = 25%; Prandtl number (Pr) = 7 (water as a working fluid); Reynolds number (Re) = 10–100; Strouhal number (St) = 0–2; and amplitude of oscillation (A) = 0–0.6. The current situation is numerically investigated by solving the continuity, momentum and energy equations using the finite volume method—based solver Ansys Fluent. Results in terms of total drag coefficient and Nusselt number have been presented and discussed. The nature of flow for each considered case is reported. The flow (streamlines) and thermal (isothermal contours) patterns have been analyzed. The maximum augmentations of about 22 and 10% were obtained in drag coefficient and Nusselt number, respectively. It is noteworthy that amongst all the cases studied, an appreciable amount of augmentation is observed when St = 1 and A = 0.6 (with respect to the case of non-pulsating flow i.e. St = 0).

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 coupled viscous-inviscid interaction scheme combining the continuity equation for potential flow with the three-dimensional integral boundary layer equations is presented. The inviscid problem is ...

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 flow past an in-line forced oscillating square cylinder at Reynolds number of 200 is studied using an in-house code, named constrained interpolation profile method developed in Zhejiang University (CIP-ZJU). The model is established in the Cartesian coordinate system using the CIP method to discretise the Navier-Stokes equations. The fluid-structure interaction is treated as a multiphase flow of the liquid and solid phases to be solved simultaneously. An immersed boundary method is used to deal with the boundary of the solid body. The CFD model is first applied to the computation of the flow past a fixed square cylinder for its validation. Computations are then performed for the flow past a square cylinder oscillating in the streamwise direction. Considerable attention is paid to the symmetric and anti-symmetric modes of the vortex shedding in the oscillating square cylinder wake. Various oscillation amplitudes and frequencies are simulated and their effects on the vortex shedding modes are analyzed via Lissajous patterns of the unsteady lift coefficient. The relationship among the lift coefficient, the drag coefficient and the lock-on range is also investigated quantitatively.

Secondary flows and corresponding endwall heat transfer around a fully permeable (porous) short circular cylinder differ from those around an impermeable (solid) cylinder. There exists a through flow which is portion of the mainstream entering the front of the cylinder and exiting at the rear of the cylinder, and the alteration of an adverse pressure gradient along the plane of symmetry that causes three-dimensional boundary layer separation (or forms horseshoe vortices). This study experimentally demonstrates how these distinctive features vary horseshoe vortices, wake patterns, and endwall heat transfer around the porous short circular cylinder. Furthermore, the fluidic effect of the extent of the through-flow that is determined by the permeability of the porous cylinder on downstream heat transfer is discussed.

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.

Within the framework of two-dimensional Euler equations, the stability is investigated of a system consisting of an inner unfastened round cylinder, an incompressible fluid flow around this cylinder with circular streamlines, and an outer fastened cylinder (vessel). An equation is obtained for the natural frequencies with different mean flows between the cylinders. Accurate solutions of this equation are derived and an analysis of these solutions is given. An energetic investigation of the loss of stability in the system is carried out.

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.