Flow Past Cylinders Research Articles

Application of event-driven molecular dynamics approach to rarefied gas dynamics problems

The numerical method based on event-driven molecular dynamics (EDMD) algorithm was developed for investigation of free-molecular and transitional gas flows in regions with arbitrary boundaries. Method was applied to two classical problems of rarefied gas dynamics: (1) transitional gas flow through plane channel of finite length between two reservoirs with small pressure difference, (2) supersonic flow past cylinder. These very different problems were chosen in order to study efficiency and applicability limits of the EDMD method for rarefied gas dynamics. The importance of the EDMD approach for rarefied gas dynamics is that EDMD is fully deterministic meshless implementation of chosen collision model without stochastic collision pair selection (unlike direct simulation Monte-Carlo method) or simplification of the collision integral (unlike model equations methods). So it gives additional information for the analysis of mechanical model itself with no regard to the features of the algorithm and can be used for verification of other numerical methods.

2-D eddy resolving simulations of flow past a circular array of cylindrical plant stems

In the present study, 2-D large eddy simulations (LES) are conducted for flow past a porous circular array with a solid volume fraction (SVF) of 8.8%, 15.4% and 21.5%. Such simulations are relevant to understanding flow in natural streams and channels containing patches of emerged vegetation. In the simulations discussed in the paper, the porous cylinder of diameter D contains a variable number of identical solid circular cylinders (rigid plant stems) of diameter d = 0.048D. Most of the simulations are conducted at a Reynolds number of 2 100 based on the diameter D and the velocity of the steady uniform incoming flow. Though in all cases wake billows are shed in the regions where the separated shear layers (SSLs) forming on the sides of the porous cylinder interact, the effect of these wake billows on the mean drag is different. While in the high SVF case (21.5%), the total drag force oscillates quasi-regularly in time, similar to the canonical case of a large solid cylinder, in the cases with a lower SVF the shedding of the wake billows takes place sufficiently far from the cylinder such that the unsteady component of the total drag force is negligible. The mean amplitude of the oscillations of the drag force on the individual cylinders is the largest in a streamwise band centered around the center of the porous cylinder, where the wake to wake interactions are the strongest. In all cases the maximum drag force on the individual cylinders is the largest for the cylinders directly exposed to the flow, but this force is always smaller than the one induced on a small isolated cylinder and the average magnitude of the force on the cylinders directly exposed to the flow decreases monotonically with the increase in the SVF. Predictions of the global drag coefficients, Strouhal numbers associated with the wake vortex shedding and individual forces on the cylinders in the array from the present LES are in very good agreement with those of 2-D direct numerical simulations conducted on finer meshes, which suggests LES is a better option to numerically investigate flow in channels containing canopy patches, given that LES is computationally much less expensive than DNS at high Reynolds number. To prove this point, the paper also discusses results of 2-D LES conducted at a much higher Reynolds number, where the near-wake flow is strongly turbulent. For the higher Reynolds number cases, where the influence of the turbulence model is important, the effect of the sub-grid scale model and the predictive capabilities of the unsteady Reynolds averaged Navier-Stokes (RANS) approach to predict flow past porous cylinders are discussed.

Wall proximity effects on the flow past cylinder with flexible filament

The response of thin, elastic filament attached to a circular cylinder is numerically investigated under uniform flow at different wall proximities, GD (0.5, 0.6, 0.8, 1, 1.5, 2 and 2.5). The Reynolds number based on the diameter of the cylinder is 200. Numerical simulations were conducted for a range of reduced velocities Ur (3,4,5 and 6), where Ur = U∞fnD which appropriately captures the synchronization range of vortex-induced vibration(VIV) of the structure. Ur is varied by changing the density of the material. The force coefficients, the frequency of shedding/flapping are observed to be the function of GD and Ur. The positive vortices diffuses faster as GD decreases. The skewness in the amplitude attained by the flapping filament on either side of the cylinder causes unsteady shedding of vortices. Filament flaps in frequency that is closer to its natural frequency at higher GD and at lower GD, the frequency of flaps for all Ur is similar. For lower gap ratios, 0.5 and 0.6, the modulations in flapping amplitude is observed. The modulation is predominant as the Ur increases. During the growth and damping phase of the amplitudes in each modulation cycle, the vortex shedding is observed to be oblique towards the wall. The filament flaps in single mode shape. The frequency of the flapping depends on the Ur and GD. The frequency of the flapping is in resonance with shedding frequency.

Flow past finite cylinders of constant curvature

Wake visualization experiments were conducted on a finite curved cylinder whose plane of curvature is aligned with the free stream. The stagnation face of the cylinder is oriented concave or convex to the flow at $230\leqslant Re_{D}\leqslant 916$, where $Re_{D}$ is the cylinder Reynolds number and the curvature is constant and ranges from a straight cylinder to a quarter-ring. While the magnitude of the local angle of incidence to the flow is the same for both orientations, the contrast in their wakes demonstrates a violation of a common approximation known as the ‘independence principle’ for curved cylinders. Vortex shedding always occurred for the convex-oriented cylinder for the Reynolds-number range investigated, along most of the cylinder span, at a constant vortex shedding angle. In contrast, a concave-oriented cylinder could exhibit multiple concurrent wake regimes along its span: two shedding regimes (oblique, normal) and two non-shedding regimes. The occurrence of these wake regimes depended on the curvature, aspect ratio and Reynolds number. In some cases, vortex shedding was entirely suppressed, particularly at higher curvatures. In the laminar wake regime, increasing the curvature or decreasing the aspect ratio restricts vortex shedding to smaller regions along the span of the cylinder. Furthermore, the local angle of incidence where vortex shedding occurs is self-similar across cylinders of the same aspect ratio and varying curvature. After the wake transitions to turbulence, the vortex shedding extends along most of the cylinder span. The difference in the wakes between the concave and convex orientations is attributed to the spanwise flow induced by the finite end conditions, which reduces the generation of spanwise vorticity and increases the incidence of non-shedding and obliquely shedding wakes for the concave cylinder.

Numerical Investigation of Unsteady Wakes Transition and Aerodynamic Characteristics of Flow Past Cylinders

Numerical Investigation of Unsteady Wakes Transition and Aerodynamic Characteristics of Flow Past Cylinders

Investigation of Incompressible Flow Past Two Circular Cylinders of Different Diameters

<p>A two - dimensional Navier-Stokes solver based on finite volume approach using a boundary-fitted curvilinear structured O-grid has been developed to obtain details of unconfined flow past cylinders at low Reynolds number of 100 and 200 based on diameter. Computations made on a single cylinder with smaller domain adopting the convective boundary conditions captured most of the flow features. This concept of a smaller domain, when used to capture the highly complex flow field around two cylinders of the same diameter placed in tandem at a Reynolds number of 200 showed reasonable results. The details of the flow field around two cylinders of different diameters placed at a typical distance of 3L and Reynolds number of 100 could be well captured adopting smaller domain concept. It is observed that the change in diameter of upstream cylinder strongly influences the overall flow field and the drag of the downstream cylinder.</p>

Numerical simulation of flows past multiple cylinders using the hybrid Local Domain Free Discretization and Immersed Boundary Method

Study of the flow around a group of cylinders (such as risers of the offshore platforms) has been the challenging subject because of its inherent complexity. The research on this problem is important because flow interference between cylinders is responsible for some significant changes in the characteristics of fluid loads. In this paper, the hybrid Local Domain-Free Discretization-Immersed Boundary Method (LDFD-IBM), which combines the strengths of LDFD method and Immersed Boundary Method (IBM), is adopted to solve this problem. The advantages of the LDFD-IBM over conventional IBM are its truly second-order accuracy and simple implementation using a regular fixed Cartesian mesh. In the present investigation, the solver has been applied to simulate the unsteady viscous flow past two cylinders and three cylinders at different configurations in a uniform cross flow. The streamlines and vorticity contours are plotted for the flow visualization. The lift and drag coefficients time histories are presented.

Analysis of Flow Past Oscillatory Cylinders Using a Finite Element Fixed Mesh Formulation

In this work, we propose a fixed mesh finite element formulation to solve the fluid dynamic on an Eulerian mesh dealing with immersed bodies in motion. The study is focused on the computation of the fluid dynamic forces acting on immersed bodies which strongly depend on the evolution of the vortex shedding. The frequency of vortex detachment for flow past cylinder problems can be modified when the cylinder moves, promoting the modification of the wake of vortices. Synchronization phenomena appear when the frequencies of the resulting flow pattern coincide with the frequency of the imposed body motion. To study this problem, we propose to describe the immersed body surface by a collection of markers that moves according to the imposed body motion. The markers are updated using a Lagrangian scheme. In this framework, a distinct aspect of the present work is the imposition of the body velocity as an internal immersed boundary condition for the fluid dynamic analysis. To transfer the body velocity to the fluid along the fluid–solid interface, a restriction on the flow velocity is added into the weak form of the Navier–Stokes equations by means of a penalty technique. This work encompasses the study of flows past a crossflow, streamwise, and rotational oscillating cylinders. The results are satisfactorily compared with numerical data reported in the literature, showing a proper behavior for the analysis of long-term vibrating systems at low Reynolds numbers.

Prediction of Turbulent Flow Around a Square Cylinder With Rounded Corners

Predictions are reported of the two-dimensional turbulent flow around a square cylinder with rounded corners at high Reynolds numbers. The effects of rounded corners have proved difficult to predict with conventional turbulence closures, and hence, the adoption in this study of a two-equation closure that has been specifically adapted to account for the interactions between the organized mean-flow motions due to vortex shedding and the random motions due to turbulence. The computations were performed using openfoam and were validated against the data from flows past cylinders with sharp corners. For the case of rounded corners, only the modified turbulence closure succeeded in capturing the consequences of the delayed flow separation manifested mainly in the reduction of the magnitude of the lift and drag forces relative to the sharp-edged case. These and other results presented here argue in favor of the use of the computationally more efficient unsteady Reynolds-averaged Navier-Stokes approach to this important class of flows provided that the effects of vortex shedding are properly accounted for in the turbulence closure.

A computational study of flow past three unequal sized square cylinders at different positions

The flow past three unequal sized side-by-side square cylinders placed in different vertical configurations is investigated numerically using the lattice Boltzmann method for the Reynolds number Re = 160 and different values of the gap spacing between the cylinders, g, (ranging between 0.5 and 5). The present study is devoted to systematic investigation of the effects of cylinders position on the flow patterns. The reported results reveal that the flow patterns change significantly by the variation of cylinders configuration. Depending on the cylinders positions we observed; chaotic, base bleed, binary vortex street, modulated synchronized, inphase vortex shedding, antiphase vortex shedding, and in-antiphase vortex shedding flow patterns. The characteristics of the flow patterns are discussed with the aid of time history analysis of drag and lift coefficients, power spectra analysis of lift coefficients and vorticity contours visualization. The study also includes a detailed discussion on the aerodynamic forces, such as mean drag coefficient, Strouhal number and root-mean-square values of drag and lift coefficients. Our results show that the flow patterns behind three unequal cylinders are distinctly different compared to the flow past equisized square cylinders placed side-by-side. In chaotic flow pattern the secondary cylinder interaction frequency plays an important role especially at the second, third and fourth configurations for all gap spacings. At larger gap spacings for the first and sixth configurations, the primary vortex shedding frequency plays a dominant role and the jet effect almost diminishes between the cylinders.

Lattice Boltzmann Method for Two-Dimensional Unsteady Incompressible Flow

Abstract A Lattice Boltzmann method is used to analyse incompressible fluid flow in a two-dimensional cavity and flow in the channel past cylindrical obstacle. The method solves the Boltzmann’s transport equation using simple computational grid - lattice. With the proper choice of the collision operator, the Boltzmann’s equation can be converted into incompressible Navier-Stokes equation. Lid-driven cavity benchmark case for various Reynolds numbers and flow past cylinder is presented in the article. The method produces stable solutions with results comparable to those in literature and is very easy to implement.

Features of flow past square cylinder with a perforated plate

A numerical investigation was performed on the reduction of the fluid forces acting on the square cylinder in the laminar flow regime with a perforated plate. The effects of geometric parameters such as the distance between the square cylinder and the perforated plate on the wake of the square cylinder were discussed. Furthermore, the flow characteristics such as the drag coefficient, lift coefficient, Strouhal number and flow pattern were obtained. It can be concluded that the drag force of the square cylinder reduces to some extent due to the addition of the perforated plate. The flow structure varies when the perforated plate is located behind the square cylinder. Moreover, the recirculation zone augments with the increase of L/D, and the vortex trace on the upper and lower surface of the square cylinder moves gradually backwards until a stable recirculation zone formed between the square cylinder and the perforated plate.

Application of a coupled Lattice Boltzmann and Immersed Boundary Method for solving conjugated heat transfer problems

The paper presents development of a coupled Lattice Boltzmann and Immersed Boundary Method for solving conjugated heat and fluid flow problems in a domain with complex internal boundaries. The Lattice Boltzmann Method was applied to efficiently solve both Navier-Stokes and energy equations, while the Immersed Boundary Method accounted for presence of objects of complex shape inside the computational domain. The presented approach allowed to use fixed Cartesian grid on which complex internal boundaries were imposed. Several conjugated heat transfer and fluid flow problems in a domain with complex internal boundaries, i.e., laminar natural convection in a square cavity without and with internal cylinder and isothermal flow past stationary cylinder were analyzed. The results obtained were compared with the results presented in the literature successfully verifying the developed numerical simulator.

Flow and flow control modeling for a drilling riser system with auxiliary lines

In this numerical study, it was mainly intended to investigate the flow and flow control for a practical drilling riser system with auxiliary lines at various incidence angles. The flow past the riser system was simulated through solving the Reynolds averaged Navier–Stokes equations and k−ω turbulence model. The cases about fluid flow past single cylinder, two staggered cylinders and eight even-distributed auxiliary lines around main line were simulated first, and the results match well with previous researches. The Reynolds number (Re) used here was 35,000, which belongs to the subcritical Reynolds region, and the flow in this Reynolds region can shows the typical flow characteristics in practical drilling operator. The effects of the incidence angle, which was in the range of 0–360° and at 30° intervals, on the drag and lift coefficients, the pressure distribution around the main line, the vortex shedding frequencies and the flow structure were investigated. The mean and RMS values of the drag and lift coefficients of the main line vary with the incidence angle irregularly due to the complexity of the geometry of the riser system. The flow patterns can be classified into six types based on the time-averaged streamlines. The effects of the auxiliary lines on mean force coefficients of the main line in different flow patterns were discussed in detail based on the distributions of the pressure on the main line. The auxiliary lines can suppress the vortex shedding on the main line at all incidence angles. Especially, in the CVP (clamped vortices pair, α=210° and α=330°) pattern, the vortex shedding behind the main line is suppressed most effectively by multiple downstream auxiliary lines. The mean value of the drag coefficient is just about 45.5% of that of the single cylinder, and the RMS value of the lift coefficient is about 1.5% of that of the single cylinder. This is the favorable condition for drilling operation. The worst effect of suppression occurs in the SLA (shear layers afflux, α=90°) pattern, and the RMS value of the lift coefficient is about 76.8% of that of the single cylinder. Due to the interaction between the vortices shed from different lines in riser system, the frequencies of the lift coefficients of the lines in riser system change largely, and some higher frequencies appear in the lift coefficients of the main line. As α=60° and α=120°, the Strouhal number of the main line is about 0.75.

CFD simulations and experimental measurements of flow past free-surface piercing, finite length cylinders with varying aspect ratios

In this work, the flow past surface-piercing cylinders with free lower ends is studied numerically with computational fluid dynamics (CFD) at varying aspect ratios (AR = H/D) from around 1 to 19, and compared against experimental measurement across the same range. Numerical and experimental work is carried out at Re = 2900 and Fr = 0.65. The aspect ratio of the cylinder is varied by raising and lowering the cylinder into the water. The resulting flow field and loads on the cylinder as a result of changing the cylinder’s depth of submergence are investigated in terms of drag, lift and frequency content. First, the effects of changing the aspect ratio are examined. CFD simulations demonstrate a reduced drag coefficient, consistent with the experimental measurements, as compared to a fully-submerged, infinite cylinder at the same Reynolds number. A significant reduction in the drag coefficient is observed at an aspect ratio of 2. For aspect ratios 3 and below, Karman vortex shedding is completely suppressed due to the dominance of free surface effects. With increasing aspect ratio the Strouhal number is found to approach 0.2. Second, the flow regime is investigated as a function of cylinder length for a cylinder with aspect ratio of 12. Different regions of vortex shedding are observed along the length of the cylinder, as demonstrated by lift forces, Strouhal numbers and CFD flow visualizations. Strouhal numbers below 0.2 were observed for regions near the free surface. Additionally, the drag coefficient was found to decrease near the free surface. This study over a wide range of aspect ratios exhibits the effects of a free surface, free end and also the length of the cylinder on both the flow regime and loading.

Three-dimensional instabilities in oscillatory flow past elliptic cylinders

We investigate the early development of instabilities in the oscillatory viscous flow past cylinders with elliptic cross-sections using three-dimensional direct numerical simulations. This is a classical hydrodynamic problem for circular cylinders, but other configurations have received only marginal attention. Computed results for some different aspect ratios ${\it\Lambda}$ from 1 : 1 to 1 : 3, all with the major axis of the ellipse aligned in the main flow direction, show good qualitative agreement with Hall’s stability theory (J. Fluid Mech., vol. 146, 1984, pp. 347–367), which predicts a cusp-shaped curve for the onset of the primary instability. The three-dimensional flow structures for aspect ratios larger than 2 : 3 resemble those of a circular cylinder, whereas the elliptical cross-section with the lowest aspect ratio of 1 : 3 exhibits oblate rather than tubular three-dimensional flow structures as well as a pair of counter-rotating spanwise vortices which emerges near the tips of the ellipse. Contrary to a circular cylinder, instabilities for an elliptic cylinder with sufficiently high eccentricity emerge from four rather than two different locations in accordance with the Hall theory.

Numerical analysis of confined flow past permeable square cylinder with CE/SE method

The unsteady flow field past a two-dimensional permeable square cylinder was predicted by an updated CE/SE method. The periodic patterns of streamline and vortex shedding were obtained. The evolution of lift and drag coefficients were analysed. Results of 2D computations were compared with the experimental data. It found that the Reynolds number dominated the magnitude and frequency of the vorticity given porosity and Darcy number. It proved that the CE/SE method was an effective tool for predicting viscous flows in porous media.

Numerical analysis of confined flow past permeable square cylinder with CE/SE method

The unsteady flow field past a two-dimensional permeable square cylinder was predicted by an updated CE/SE method. The periodic patterns of streamline and vortex shedding were obtained. The evolution of lift and drag coefficients were analysed. Results of 2D computations were compared with the experimental data. It found that the Reynolds number dominated the magnitude and frequency of the vorticity given porosity and Darcy number. It proved that the CE/SE method was an effective tool for predicting viscous flows in porous media.

Transient free convection flow past vertical cylinder with constant heat flux and mass transfer

Abstract This paper describes a one dimensional unsteady natural convection flow past an infinite vertical cylinder with heat and mass transfer under the effect of constant heat flux at the surface of the cylinder. Closed form solutions of the dimensionless unsteady linear governing boundary layer equations are obtained in terms of Bessel functions and modified Bessel functions by Laplace transform method. The numerical values of velocity, temperature and concentration profiles are obtained for different values of the physical parameters namely, thermal Grashof number, mass Grashof number, Prandtl number, Schmidt number and time and presented in graphs. Also, skin friction and Sherwood number are shown graphically and discussed. It is observed that the velocity and temperature increase unboundedly with time, while the concentration approaches steady state at larger times.

U-shaped fairings suppress vortex-induced vibrations for cylinders in cross-flow

We employ three-dimensional direct and large-eddy numerical simulations of the vibrations and flow past cylinders fitted with free-to-rotate U-shaped fairings placed in a cross-flow at Reynolds number $100\leqslant \mathit{Re}\leqslant 10\,000$. Such fairings are nearly neutrally buoyant devices fitted along the axis of long circular risers to suppress vortex-induced vibrations (VIVs). We consider three different geometric configurations: a homogeneous fairing, and two configurations (denoted A and AB) involving a gap between adjacent segments. For the latter two cases, we investigate the effect of the gap on the hydrodynamic force coefficients and the translational and rotational motions of the system. For all configurations, as the Reynolds number increases beyond 500, both the lift and drag coefficients decrease. Compared to a plain cylinder, a homogeneous fairing system (no gaps) can help reduce the drag force coefficient by 15 % for reduced velocity $U^{\ast }=4.65$, while a type A gap system can reduce the drag force coefficient by almost 50 % for reduced velocity $U^{\ast }=3.5,4.65,6$, and, correspondingly, the vibration response of the combined system, as well as the fairing rotation amplitude, are substantially reduced. For a homogeneous fairing, the cross-flow amplitude is reduced by about 80 %, whereas for fairings with a gap longer than half a cylinder diameter, VIVs are completely eliminated, resulting in additional reduction in the drag coefficient. We have related such VIV suppression or elimination to the features of the wake flow structure. We find that a gap causes the generation of strong streamwise vorticity in the gap region that interferes destructively with the vorticity generated by the fairings, hence disorganizing the formation of coherent spanwise cortical patterns. We provide visualization of the incoherent wake flow that leads to total elimination of the vibration and rotation of the fairing–cylinder system. Finally, we investigate the effect of the friction coefficient between cylinder and fairing. The effect overall is small, even when the friction coefficients of adjacent segments are different. In some cases the equilibrium positions of the fairings are rotated by a small angle on either side of the centreline, in a symmetry-breaking bifurcation, which depends strongly on Reynolds number.