Flow around a cylinder surrounded by a permeable cylinder in shallow water

The change in flow characteristics downstream of a circular cylinder (inner cylinder) surrounded by an outer permeable cylinder was investigated in shallow water using particle image velocimetry technique. The diameter of the inner cylinder and the water height were kept constant during the experiments as d?=?50?mm and h _w ?=?25?mm, respectively. The depth-averaged free-stream velocity was also kept constant as U?=?170?mm/s which corresponded to a Reynolds number of Re_d?=?8,500 based on the inner cylinder diameter. In order to examine the effect of diameter and porosity of the outer cylinder on flow characteristics of the inner cylinder, five different outer cylinder diameters (D?=?60, 70, 80, 90 and 100?mm) and four different porosities (???=?0.4, 0.5, 0.6 and 0.7) were used. It was shown that both porosity and outer cylinder diameter had a substantial effect on the flow characteristics downstream of the circular cylinder. Turbulent statistics clearly demonstrated that in comparison with the bare cylinder (natural case), turbulent kinetic energy and Reynolds stresses decreased remarkably when an outer cylinder was placed around the inner cylinder. Thereby, the interaction of shear layers of the inner cylinder has been successfully prevented by the presence of outer cylinder. It was suggested by referring to the results that the outer cylinder having 1.6????D/d????2.0 and 0.4????D/d????0.6 should be preferred to have a better flow control in the near wake since the peak magnitude of turbulent kinetic energy was considerably low in comparison with the natural case and it was nearly constant for these mentioned porosities ??, and outer cylinder to inner cylinder diameter ratios D/d.     

Flow around a circular cylinder—structure of the near wake shear layer

The separated shear layer in the near wake of a circular cylinder was investigated using a single hot wire probe, with special attention given to the shear layer instability characteristics. Without end plates to force parallel vortex shedding, the critical Reynolds number for the onset of the instability was 740. The present data, together with all previously published data, show that the ratio of the instability frequency f_sl to the vortex shedding frequency f_v varies as Re^0.65, which is in agreement with the Re^0.67 dependence obtained by Prasad and Williamson [1997, J Fluid Mech 333:375–402]. However, the distribution of f_sl/f_v and the spectra of the longitudinal velocity fluctuation (u) suggest that, on either side of Re=5,000, the shear layer exhibits lower and upper subcritical regimes, in support of the observations by Norberg [1987, publication no. 87/2, Chalmers University of Technology, Sweden] and Prasad and Williamson [1997, J Fluid Mech 343:235–265]. The spectra of u provide strong evidence for the occurrence of vortex pairing in wake shear layers, suggesting that the near wake develops in a similar manner to a mixing layer.     

Flow around a porous cylinder subject to continuous suction or blowing

In the present experimental investigation the surface pressure distribution, vortex shedding frequency, and the wake flow behind a porous circular cylinder are studied when continuous suction or blowing is applied through the cylinder walls. It is found that even moderate levels of suction/blowing (5% of the oncoming streamwise velocity) have a large impact on the flow around the cylinder. Suction delays separation contributing to a narrower wake width, and a corresponding reduction of drag, whereas blowing shows the opposite behaviour. Both uniform suction and blowing display unexpected flow features which are analysed in detail. Suction shows a decrease of the turbulence intensity throughout the whole wake when compared with the natural case, whilst blowing only shows an effect up to five diameters downstream of the cylinder. The drag on the cylinder is shown to increase linearly with the blowing rate, whereas for suction there is a drastic decrease at a specific suction rate. This is shown to be an effect of the separation point moving towards the rear part of the cylinder, similar to what happens when transition to turbulence occurs in the boundary layer on a solid cylinder. The suction/blowing rate can empirically be represented by an effective Reynolds number for the solid cylinder, and an analytical expression for this Reynolds number representation is proposed and verified. Flow visualizations expose the complexity of the flow field in the near wake of the cylinder, and image averaging enables the retrieval of quantitative information, such as the vortex formation length.     

Flow characteristics around a circular cylinder subjected to vortex-induced vibration near a plane boundary

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 V_r; 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.     

Flow past a square cylinder at low Reynolds numbers

Results are presented for the flow past a stationary square cylinder at zero incidence for Reynolds number, Re ? 150. A stabilized finite‐element formulation is employed to discretize the equations of incompressible fluid flow in two‐dimensions. For the first time, values of the laminar separation Reynolds number, Re_s, and separation angle, θ_s, at Re_s are predicted. Also, the variation of θ_s with Re is presented. It is found that the steady separation initiates at Re = 1.15. Contrary to the popular belief that separation originates at the rear sharp corners, it is found to originate from the base point, i.e. θ_s=180^° at Re = Re_s. For Re > 5, θ_s approaches the limit of 135 ^°. The length of the separation bubble increases approximately linearly with increasing Re. The drag coefficient varies as Re^?0.66. Flow characteristics at Re ? 40 are also presented for elliptical cylinders of aspect ratios 0.2, 0.5, 0.8 and 1 (circle) having the same characteristic dimension as the square and major axis oriented normal to the free‐stream. Compared with a circular cylinder, the flow separates at a much lower Re from a square cylinder leading to the formation of a bigger wake (larger bubble length and width). Consequently, at a given Re, the drag on a square cylinder is more than the drag of a circular cylinder. This suggests that a cylinder with square section is more bluff than the one with circular section. Among all the cylinder shapes studied, the square cylinder with sharp corners generates the largest amount of drag. Copyright © 2010 John Wiley & Sons, Ltd.     

On the simulation of the flow around a circular cylinder at Re=140,000

The flow around a circular cylinder at Reynolds number of 1.4 × 10⁵ is examined with Reynolds-Averaged Navier–Stokes equations (RANS) and Scale-Resolving Simulation (SRS) methods. Such problem is in the upper limit of the flow regime where turbulent transition occurs in the free shear-layers and so the flow dynamics is dominated by the spatial development of vortex-shedding structure, and in particular by the Kelvin–Helmholtz rollers and turbulence onset. The objectives of this investigation are threefold: (i) determine the aptitude of distinct RANS and SRS models to simulate the correct flow regime; (ii) compare the predictions of selected methods with available experimental measurements; and (iii) examine key modelling and flow features that contribute to the observed results. The evaluated models range from RANS supplemented with linear, transition, and non-linear turbulent viscosity closures, to hybrid and bridging SRS methods. Bridging computations are conducted at various constant degrees of physical resolution (range of resolved scales). The results illustrate the complexity of predicting the present flow problem. It is shown that RANS and SRS formulations modelling turbulence in boundary-layers with the selected linear turbulent viscosity closures lead to a premature onset of turbulence which alters the flow regime of the simulations. Although the transition and non-linear RANS closures can predict the correct flow regime, the outcome of this study indicates that solely the bridging model at constant physical resolution is able to achieve an accurate and physics-based prediction of the flow dynamics. Nonetheless, the necessary degree of physical resolution makes the numerical requisites of such computations demanding.     

Numerical study on the vortex motion patterns around a rotating circular cylinder and their critical characters

A hybrid finite difference and vortex method (HFDV), based on the domain decomposition method (DDM), is used for calculating the flow around a rotating circular cylinder at Reynolds number Re=1000, 200 and the angular‐to‐rectilinear speed ratio α∈(0.5, 3.25) respectively. A fully implicit third‐order eccentric finite difference scheme is adopted in the finite difference method, and the deduced large broad band sparse matrix equations are solved by a highly efficient modified incomplete LU decomposition conjugate gradient method (MILU‐CG). The long‐time, fully developed features about the variations of the vortex patterns in the wake, as well as the drag and lift forces on the cylinder, are given. The calculated streamline contours are in good agreement with the experimentally visualized flow pictures. The existence of the critical state is confirmed again, and the single side shed vortex pattern at the critical state is shown for the first time. Also, the optimized lift‐to‐drag force ratio is obtained near the critical state. Copyright © 1999 John Wiley & Sons, Ltd.     

Structure of stratified flow around a cylinder at low internal froude number

The flow pattern around a horizontal cylinder towed at constant velocity in a continuously stratified fluid is visualized by the shadow method. The velocities in the leading flow disturbance, i. e., in the flow-blocking region ahead of the cylinder, are presented. In the body wake, a new class of small-size structures in the density gradient field is revealed against the background of a smooth velocity profile. The evolution of the flow pattern with variation of the parameters of body motion is studied. Institute of Mechanics Problems, Moscow 117526. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 1, pp. 80–88, January–February, 1999.     

Studies on two-phase cross flow. Part I: Flow characteristics around a cylinder

A two-phase flow around a body has scarcely been studied until now, though the flow is used in many industrial components. The cross flows around a spacer in a fuel assembly of light water reactors (LWR) and tube supports in a steam generator are closely related to the long-term reliability and the safety. The present study has been planned to clarify the two-phase flow and heat transfer characteristics around a body including the unknown complicated flow behavior. In the first report, the flow characteristics near and behind a cylinder which was located in a vertical upward air-water bubbly flow were investigated. From the observation of the flow patterns and the measurements of the distributions of void fraction, liquid velocity and static pressure, it is revealed that the vortex flow and the change of the static pressure and liquid velocity distribution around the cylinder resulted in the large distortion of the void fraction distribution around the cylinder. The most noticeable phenomena in the wake were that the peaks of the local void fraction appeared in the vicinity of the cylinder surface near the separation point and in the wake behind the cylinder.     

Flow behavior around a square cylinder subject to modulation of a planar jet issued from upstream surface

The flow behavior in the up- and downstream regions of a square cylinder subject to the modulation of a planar jet issued from the cylinder׳s front surface was studied using the laser-assisted smoke flow visualization method and hot-wire anemometer measurement. Reynolds numbers were from 1628 to 13 000. The drag force experienced by the square cylinder was obtained by measuring the surface pressures on the up- and downstream faces. The temporally evolving smoke flow patterns in the up- and downstream regions were synchronously revealed through the smoke flow visualization. The frequency characteristics of the instability waves in the up- and downstream regions were synchronously detected by the two hot-wire anemometers. Four characteristic flow modes were observed within the different ranges of the injection ratios. At the low injection ratios (IR<1), the ‘swinging jet’ appeared. The jet swung periodically leftward and rightward and formed a fluid bubble on the front surface. The fluid bubble contained a pair of counter-rotating vortices and presented a periodic variation in its height. At moderately low injection ratios (1<IR<4.3), the ‘deflected oscillating jet’ appeared. The jet was deflected in either the left or the right direction and wrapped around one of the edges of the square cylinder. Both the swinging and oscillating motions of the jet in the swinging jet and deflected oscillating jet modes were induced by the periodic feedback pressure signals generated by the vortex shedding in the wake. At the moderately high (4.3<IR<8.3) and high (IR>8.3) injection ratios, the ‘deflection jet’ and ‘penetrating jet’ appeared. The jet detached from the cylinder׳s front surface and penetrated a long distance into the upstream region due to large jet momentum. Neither periodic jet oscillation in the upstream region nor vortex shedding in the wake was observed. The drag coefficient was found to be decreasing quickly with increasing the injection ratio.     

Flow near a vibrating cylinder

The results are given of experimental investigations into the change in the structure of the secondary steady flows near a circular cylinder vibrating in a fluid at rest in the direction perpendicular to its axis in a wide range of vibration amplitudes at high Reynolds numbers. For vibration amplitudes less than the cylinder diameter, not only viscous waves but also four vortices are generated near the surface of the cylinder and four steady flows at some distance from it. During a period of the vibration, the vortices near the cylinder change their shape and position. At vibration amplitudes comparable with the cylinder diameter, the vortices become separated and form two vortex streets. If the vibration amplitude is increased further, the streets bifurcate.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 190–192, July–August, 1982.     

Flow past a rotating cylinder translating at different gap heights along a wall

The flow past a rotating circular cylinder translating parallel to a wall at different heights is investigated for Reynolds numbers up to 400 for three discrete rotation rates. In particular, the various wake transitions that occur as a function of gap height are quantified for the three cases examined: non-rotation, and forward and reverse rotations. At low gap heights, only a single steady three-dimensional mode is found to become unstable on the steady base flow. As the gap height is increased, several new three-dimensional modes are observed, of which one attains large amplitudes in the near wake and another preferentially in the far wake. At still larger gap heights, the transition sequence resembles that observed in a rotating cylinder wake, for which the wake first undergoes transition to a periodic state, prior to the onset of three-dimensional flow. Parameter space maps showing the neutral stability curves and regions of instability for each mode are presented for each rotation rate, together with a discussion of the spatio-temporal characteristics and spatial distributions of the new modes. Finally, the force coefficients for the steady and periodic two-dimensional base flows are presented.     

Wavy mode of the streaked flow around an oscillating cylinder in a stratified fluid at rest

The structures of the flow induced by a vertical circular cylinder performing transverse oscillations in a linearly stratified fluid at rest are investigated. The density gradient inhibits the onset of the three-dimensional instability. The instability appears as regularly spaced streaked flows along the cylinder axis. Each streaked flow follows a wavy path in the horizontal plane in a certain range of amplitude and frequency of the oscillations.     

Experimental investigation of the wave-induced flow around a surface-touching cylinder

The wave-induced flow around a circular cylinder near both a rigid wall and an erodible bed is experimentally investigated using Particle Tracking Velocimetry (PTV). The aim of this study is to gain quantitative information on the local mean flow, the vorticity dynamics and the evolution of the erodible bed. The flow is characterized in terms of the Keulegan–Carpenter (KC), Reynolds (Re) and Ursell (U_r) numbers. The effects of changing these parameters over the ranges 1<KC<31, 3×10³<Re<2.6×10⁴ and 1.5<U_r<152 are investigated. For KC<1.1 the flow does not separate. When KC increases, the flow becomes unstable and large-scale vortical structures develop. The dimensionless intensity (|Γ^⁎|) depends non-monotonically on KC, with a local maximum at KC=17, and the dimensionless area of the same macrovortex (A^⁎) follows a somewhat similar law. Although the dimensionless boundary layer thickness (δ^⁎) exhibits some discontinuities between KC regimes, it decreases with KC at x/D=0.5, as x/D=1 weakly depends on KC and can be regarded as constant (δ^⁎=0.7) and then, increases with KC when moving away from the cylinder. These findings are used to interpret the physics governing the flow around a cylinder touching a wall and are compared with available results from the literature (Sumer et al., 1991). The evolution of the scour mechanism occurring over an erodible sandy bed is also investigated. The validity of some empirical formulas in the literature is also tested on the basis of the available dataset. The empirical relationships of Cevik and Yuksel (1999) and Sumer and Fredsøe (1990) for the dimensionless scour depth (S/D) agree well with our results. The dimensionless scour width (W_s/D) is predicted well by Sumer and Fredsøe's (2002) empirical equation for KC<23, whereas Catano-Lopera and Garcia's (2007) formula is more accurate for higher values of KC.