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1.
Experiments employing a low-mass-damping cylinder have been conducted to determine the vortex-induced vibration (VIV) response of four suppressors of the flexible-shroud family. The VIV suppressors were inspired in the concept of the Ventilated Trousers (VT), a flexible shroud composed of a flexible net fitted with three-dimensional bobbins. Reynolds number varied between 5 × 103 and 25 × 103, while reduced velocity varied from 2 to 26. The VIV dynamic response showed that the VT suppressed the peak amplitude of vibration down to 40% of that of a bare cylinder. Other flexible shrouds also achieved suppression, but not as efficiently. Drag was reduced during the VIV synchronization range, but remained above the value for a bare static cylinder thereafter. Spectral analysis of displacement and lift revealed that, depending on the geometry and distribution of the bobbins, the flexible shroud can develop an unstable behavior, capturing energy from the wake and sustaining vibrations for higher reduced velocities. PIV measurements of the wake revealed that the entrainment flow through the mesh is necessary to extend the vortex-formation length of the wake; this mechanism only occurs for the VT mesh. 相似文献
2.
A series of flow induced vibration (FIV) experiments for an equilateral triangle prism elastically mounted in a water channel are performed with different system stiffness at constant damping and mass. An amplitude variation coefficient is proposed to describe FIV stationarity in the present study. The FIV of the prism can be divided into three primary regions based on the amplitude and frequency responses, which are the vortex induced vibration (VIV) branch, the transition branch from VIV to galloping, and the galloping branch. The transition branch occurs at the reduced velocity in the range of 7.8 < Ur = U/(fn,air·D) < 10.4, accompanied with a relatively rapid increase in amplitude and a precipitous drop in frequency and vibration stationarity. In addition, the reduced velocity where the transition region is initiated is independent of the system stiffness. The maximum amplitude reaches 3.17 D in the galloping branch. The ratio of the response frequency to the natural frequency of the prism in air remains locked to approximately 0.65 throughout the fully developed galloping branch. Large amplitude responses in an infinite range of flow velocities, excellent vibration stationarity and steady vibration frequencies, which are characteristics of the galloping of the prism, have a positive impact on improving energy conversion. 相似文献
3.
The aim of this paper is to evaluate the accuracy, stability and efficiency of the overset grid approach coupled with the RANS (Reynolds Averaged Navier-Stokes) model via the benchmark computations of flows around a stationary smooth circular cylinder. Two dimensional numerical results are presented within a wide range of Reynolds numbers (6.31 × 104 ∼ 7.57 × 105) including the critical flow regime. All the simulations are carried out using the RANS solver pimpleFoam provided by OpenFOAM, an open source CFD (Computational Fluid Dynamics) toolkit. Firstly, a grid convergence study is performed. The results of the time-averaged drag and lift force coefficients, root-mean square value of lift force coefficient and Strouhal number (St number) are then compared with the experimental data. The velocity, vorticity fields and pressure distribution are also given. One main conclusion is that the numerical solutions in regard to a fixed cylinderare not deteriorated due to the implementation of the overset grid. Furthermore, it can be an appealing approach to facilitate simulations of Vortex Induced Vibrations (VIV), which involves grid deformation. The present study is a good start to implement the overset grid to solve VIV problems in the future. 相似文献
4.
Laminar flow past a circular cylinder with 3 small control rods is investigated by numerical simulation. This study is concerned with the suppression efficacy of vortex induced vibration by small control rods located around a main cylinder. The effects of the attack angle and rod-to-cylinder gap ratio on the hydrodynamics and vibration responses of the main cylinder are investigated. The attack angle of α = 45° is performed as the critical angle for VIV suppression of 3 control rods. The 3 control rods have no effect on VIV suppression when the attack angle is less than the critical angle. The 3 control rods have an excellent VIV suppression efficacy when the attack angle is larger than the critical angle. The transverse vibration frequency of the cylinder with 3 control rods is less than that for an isolated cylinder for all the configurations. The numerical results for the configurations of α = 45° & 60°, G/D = 0.6–1.2 show excellent suppression efficient among the cases investigated in this study. The best suppression efficient is found at α = 45°, G/D = 0.9 for 3 control rods. 2 rods in behind of the main cylinder perform more efficient than that of 1 rod in front for VIV suppression as the gap ratio of G/D less than 1.0. 相似文献
5.
Passive Turbulence Control (PTC) in the form of selectively distributed surface roughness is used to alter Flow Induced Motion (FIM) of a circular cylinder in a steady flow. The objective is to enhance FIM's synchronization range and amplitude, thus maximizing conversion of hydrokinetic energy into mechanical energy by oscillator in vortex-induced vibration and/or galloping. Through additional viscous damping, mechanical energy is converted to electrical harnessing clean and renewable energy from ocean/river currents. High Reynolds numbers (Re) are required to reach the high-lift TrSL3 (Transition-Shear-Layer-3) flow regime. PTC trips flow separation and energizes the boundary layer, thus inducing higher vorticity and consequently lift. Roughness location, surface coverage, and size are studied using systematic model tests with broad-field laser visualization at 3.0×104<Re<1.2×105 in the low-turbulence free-surface water-channel of the Marine Renewable Energy Laboratory of the University of Michigan. Test results show that 16° roughness coverage is effective in the range (10°-80°) inducing reduced vortex-induced vibration (VIV), enhanced VIV, or galloping. Range of synchronization may increase or decrease, galloping amplitude of oscillation reaches three diameters; wake structures change dramatically reaching up to ten vortices per cycle. Conversion of hydrokinetic energy to mechanical is enhanced strongly with proper PTC. 相似文献
6.
Experimental results are presented in the paper of two elastically supported rigid circular cylinders subjected to steady flows in a flume. The two cylinders were initially placed at various tandem and staggered positions with one in the wake of the other when subject to the steady flows. The in-line centre-to-centre distance varied from 2 to 5 diameters whilst the cross-flow distance from 0 to 2 diameters. The nominal Reynolds numbers were in the sub-critical regime and ranged from 1.12 × 104 to 5.52 × 104, and the nominal reduced velocities from 1.78 to 8.77. The damping ratio of the test set-up is low at 0.003 which gives a combined mass-damping parameter of 0.0046. Both the cylinders were free to respond in both the in-line and the cross-flow directions. The cylinder motion was measured simultaneously with the hydrodynamic loading in the two directions. It was found that the motion trajectories of the downstream cylinder show qualitative difference depending upon whether it is in tandem with the upstream cylinder or in the wake with a transverse offset. The VIV response of the downstream cylinder is dependent upon the reduced velocity of the upstream cylinder and its own reduced velocity based upon the actual mean wake velocity. The drag amplification of the downstream cylinder in the wake appears to be fundamentally different from that of a single VIV cylinder in isolation. Furthermore, unlike the two fixed cylinders in cross flow, the downstream cylinder undergoing VIV no longer experiences a marked non-zero mean lift. The upstream cylinder is largely unaffected by the downstream cylinder when the initial spacing is greater than 3 diameters. On the other hand, the motion response of and the fluid loading on the downstream cylinder are strongly influenced by the upstream cylinder in the spacing range tested. 相似文献
7.
The main purpose of this study is to establish a better understanding of the relationship between drag reduction and surface roughness. Experiments were conducted to measure the force and flow characteristics of a circular cylinder with different types of artificial surface roughness over the range 6 × 103 < Re < 8 × 104 (Re is based on the cylinder diameter D). The roughness cylinder was formed by covering the exterior surface of the cylinder with uniformly distributed (1) sandpaper, (2) netting, and (3) dimples. The roughness coefficient ranged from k/D = 0.0028 to 0.025 (k is the roughness height). A detailed quantitative measurement of the flow field around the cylinder using Particle Imaging Velocimetry (PIV) was carried out. The hydrodynamic force coefficients (drag and lift) of the rough cylinders are compared against those of a smooth cylinder measured under the same flow conditions. It is found that certain configuration of surface roughness significantly reduces the mean drag coefficient of the cylinder, particularly at large Reynolds numbers. In addition, the root-mean-square (r.m.s.) lift coefficient of the rough cylinders is considerably lower than that of a smooth cylinder. 相似文献
8.
Experimental studies were carried out to investigate the response features of an inclined flexible bare cylinder as well as a straked cylinder in a towing tank, with the main purpose of further improving the understanding of the effect of yaw angle on vortex-induced vibration (VIV) suppression. Four yaw angles (a = 0°, 15°, 30°, 45°), which is defined as the angle between the cylinder axis and the plane orthogonal to the oncoming fluid flow, were tested. The cylinder model was towed along the tank to generate a uniform fluid flow. The towing velocity was in the range of 0.05–1.0 m/s with an interval of 0.05 m/s. The corresponding Reynolds number ranged from 800 to 16000. The strakes selected for the experiments had a pitch of 17.5D and a height of 0.25D, which is generally considered as the most effective configuration for VIV suppression of a flexible cylinder in water. The experimental results indicate that VIV suppression effectiveness of the inclined flexible straked cylinder is closely related to the yaw angle. The displacement amplitudes are significantly suppressed in both cross-flow (CF) and in-line (IL) directions at a = 0°. However, with increasing yaw angle, the suppression efficiencies of the CF and IL displacement amplitudes gradually decrease. In addition, the CF dominant frequencies of the straked cylinder obviously deviate from those of the bare cylinder at a = 0° and 15°. This deviation is substantially alleviated with increasing yaw angle. The IL dominant frequencies show less dependency on the yaw angle. Similar trends are also observed on the dominant modes of vibration and the mean drag coefficients. 相似文献
9.
The validity of the independence principle applied to the vortex-induced vibration (VIV) of an inclined cylinder in steady flow is investigated by conducting numerical simulations. In order to create a perfect end-effect-free condition, periodic boundary condition is applied on the two end boundaries that are perpendicular to the cylinder. It is found that the response amplitude and frequency for an inclination angle of α = 45° agree well with their counterparts for α = 0°. The numerical results demonstrated the validity of the independence principle in the case of vortex-induced vibration, which has not been demonstrated by laboratory tests due to the difficulty in avoiding the end effects. 相似文献
10.
Large eddy simulations of the flow around a circular cylinder at high Reynolds numbers are reported. Five Reynolds numbers were chosen, such that the drag crisis was captured. A total of 18 cases were computed to investigate the effect of gridding strategy, turbulence modelling, numerical schemes and domain width on the results. It was found that unstructured grids provide better resolution of key flow features, when a ‘reasonable’ grid size is to be maintained.When using coarse grids for large eddy simulation, the effect of turbulence models and numerical schemes becomes more pronounced. The dynamic mixed Smagorinsky model was found to be superior to the Smagorinsky model, since the model coefficient is allowed to dynamically adjust based on the local flow and grid size. A blended upwind-central convection scheme was also found to provide the best accuracy, since a fully central scheme exhibits artificial wiggles, due to dispersion errors, which pollute the solution.Mean drag, fluctuating lift Strouhal number and base pressure are compared to experiments and empirical estimates for Reynolds numbers ranging from 6.31 × 104 to 5.06 × 105. In terms of the drag coefficient, the drag crisis is well captured by the present simulations, although the other integral quantities (rms lift and Strouhal number) show larger discrepancies. For the lowest Reynolds number, the drag is seen to be more sensitive to the domain width than the spanwise grid spacing, while at the higher Reynolds numbers the grid resolution plays a more important role, due to the larger extent of the turbulent boundary layer. 相似文献
11.
Flow past a circular cylinder with multiple small control rods is studied by numerical simulation for ReD ranging from 1161.3 to 6387.1. The Reynolds-Averaged-Navier–Stokes (RANS) equations and shear stress transport (SST) k − ω turbulence model are used to calculate the vortex field, while a fourth-order Runge–Kutta method is employed for evaluating the structure dynamics of the cylinder group. Comparisons with experimental results demonstrate the validation of this method. This study is concerned with the vortex induced vibration (VIV) suppression efficacy of small control rods placed around a main cylinder. The effects of control rod number, diameter ratio, spacing ratio and Reynolds number on the hydrodynamics and vibration responses of the main cylinder are investigated. The reduced percents of in-line and cross-flow amplitudes and the increased percents of the whole cross-sectional area of cylinders and the drag coefficient are used to give a comprehensive evaluation. Results of simulation indicate that placing small rods with appropriate number at appropriate locations can achieve good suppression effectiveness at a wide range of Reynolds number. The numerical result for the case with nine control rods, diameter ratio of 0.15 and spacing ratio of 0.6 shows the best suppression effect among the cases investigated in this study. 相似文献
12.
The purpose of the present work is <!–<query>The highlights are in an incorrect format. Hence they have been deleted. Please refer the online instructions: http://www.elsevier.com/highlights and provide 3-5 bullet points.</query>–>to study the effect of the Reynolds number on the near-wake structure and separating shear layers behind a circular cylinder. Three-dimensional unsteady large-eddy simulation is carried out and two different subgrid scale models are applied in order to evaluate the turbulent wake reasonably. The Reynolds number based on the free-stream velocity and the cylinder diameter is ranging from Re = 5500–41,300, corresponding to the full development of the shear-layer instability in the intermediate subcritical flow regime. For a complete validation of this numerical study, hydrodynamic bulk coefficients are computed and compared to experimental measurements and numerical studies in the literature. Special focus is made on the variations of both the large-scale near-wake structure and the small-scale shear-layer instability with increasing Reynolds numbers. The present numerical study clearly shows the broadband nature of the shear-layer instability as well as the dependence of the shear-layer frequency especially on the high Reynolds numbers. 相似文献
13.
A horizontal, circular cylinder fitted with one bilge keel is forced to rotate harmonically around its axis. The bilge keel load and hull pressure distribution are investigated. A fully submerged condition (infinite fluid), and three partly-submerged conditions are considered. A two-dimensional numerical study is performed, and the results are validated against recently published experimental data by van’t Veer et al. [30]. In addition, comparisons for mass and drag coefficients are also made with experimental data for plate in infinite fluid (Keulegan and Carpenter [8]), and wall-mounted plate (Sarpkaya and O’Keefe [9]) in oscillatory flow.A Navier–Stokes solver based on the Finite Volume Method is adopted for solving laminar flow of incompressible water. The free-surface condition is linearized by neglecting the nonlinear free-surface terms and the influence of viscous stresses in the free surface zone, while the body-boundary condition is exact. This simplified modeling of the problem required the mesh to be fine only around the bilge keels, leading to a total number of cells around N ≃ 1 ×104, which reduced computational cost significantly.The influence of draft and amplitude of oscillations on the bilge keel force and hull pressure distribution are considered. The bilge keel force is presented in terms of non-dimensional drag and mass coefficients including higher harmonic components. The numerical results are also compared with the industry standard empirical method for calculation of roll damping proposed by Ikeda et al. [4]. In general, a good agreement between the results of the present numerical method and the experimental data is obtained and the differences with those predicted by the empirical method are addressed. 相似文献
14.
《Deep Sea Research Part I: Oceanographic Research Papers》2007,54(5):687-709
We use hydrographic and buoy data to compare the initial temperature fields and Lagrangian evolution of water parcels in two vortices generated by the southward flowing Canary Current passing around the island of Gran Canaria Island. One vortex is anticyclonic, shed in June 1998 as the result of an incident current of about 0.05 m s−1, and the second one is cyclonic, shed in June 2005 with the impinging current estimated as 0.03 m s−1. The two vortices exhibit contrasting characteristics yet display some important similarities. The isopycnals are depressed in the core of the anticyclonic vortex, at least down to a depth of 700 m, whilst they dome up in the core of the cyclonic vortex but only down to 450 m. In the top 300 m the depression/doming of the isotherms is similar for both vortices, with a maximum vertical displacement of the isotherm of about 80 m, which correspond to temperature anomalies of some 2.5 °C at a given depth. A simple method is developed to obtain the initial orbital velocity field from the temperature data, from which we estimate peak values of 0.7 and 0.5 m s−1 for the anticyclonic and cyclonic vortices, respectively. The buoys, three for the anticyclonic vortex and two for the cyclonic one, were drougued at 100 m depth, below the surface mixed layer, and their initial velocities are consistent with the above values. In both vortices, the buoys revolve either within a central core, where the rotation rate remains stable and large for several weeks, or in an outer ring, where the rotation rate is significantly smaller and displays large radial fluctuations. Within the inner core the anticyclonic vortex has significant inward radial velocity, while the cyclonic vortex has near-zero radial mean motions. The cyclonic vortex rotates more slowly than the anticyclonic, their initial periods being 4.5 and 2.5 days, respectively. A simple axisymmetric model with radial diffusion (coefficient Kh≅25 m2 s−1) and advection reproduces the observations reasonably well, the diffusive effect being more important than that resulting from the observed radial advection. The model also supports the hypothesis that the rotation rate of cyclonic vortices is less than that of anticyclonic vortices, as otherwise they would become inertially unstable. Both the buoys data and sea surface temperature images confirm that the vortices evolve from youth to maturity, as the cores shrink and the outer rings expands, and then to a decay stage, as the core rotation rates decrease, though frequent interactions with other mesoscale structures result in more accelerated aging. Despite these interaction they last many months as coherent structures south of the Canary Islands. 相似文献
15.
We describe experiments with multi-directional focused waves interacted with a vertical circular cylinder in a 3D wave basin. The focus of this study is on the run-up of multi-directional focused waves, wave forces, and wave pressures on the cylinder. Part I, the study on wave run-up, has already been presented by Li et al. (2012). In this paper, the analysis of the wave force on the vertical cylinder is presented.In this experiment, a cylinder with 0.25 m in diameter was adopted and different wave parameters, such as focused wave amplitude, peak frequency, frequency bandwidth and directional spreading index, are considered. The model scale kpa (kp is the wave number corresponding to peak frequency, a is the radium of the cylinder) varies from 0.32 to 0.65. The maximum forces of multi-directional focused wave on cylinder were measured and investigated. The results showed that the wave parameters have a significant influence on the wave force, and that the spatial profile of the surface of multi-directional focused wave can also affect its force on the cylinder, which is different from two-dimensional wave. In addition, the ‘secondary loading cycle’ phenomenon was also observed and discussed. In our experiments, the ‘secondary loading cycles’ occur when kA > 0.36 for all cases. While in some referred small scale experiments, the secondary load cycles are observed even for kA = 0.2, when the waves are longer enough. To larger model scale, the pronounced secondary load cycle occurs with larger wave steepness waves. 相似文献
16.
When fluid flow passes a cylinder, the drag crisis phenomenon occurs between the sub-critical and the super-critical Reynolds numbers. The focus of the present studies was on the numerical prediction of the drag crisis based on CFD methods. In this work, block structured meshes with refined grids near the cylinder surface and in the downstream were employed. Both 2D and 3D simulations were performed using various turbulence models, including the SST k − ω model, the k − ϵ model, the SST with LCTM, the DES model, and the LES model. In the convergence studies, the effects of the grid size, the time step, the first grid size and the aspect ratio (for 3D simulations) on the solutions were examined. The errors due to spatial and time discretizations were quantified according to a V&V procedure. Validation studies were carried out for various Reynolds numbers between Re = 6.31 × 104 and 7.57 × 105. The averaged drag force, the RMS of lift force and the Strouhal number were compared with experimental data. The studies indicated that standard 2D and 3D RANS methods were inadequate to capture the drag crisis phenomenon. The LES method however has the potential to address the problem. 相似文献
17.
The problem of unsteady, laminar flow past a circular cylinder which starts translating and oscillating impulsively from rest in a viscous fluid is numerically investigated at a Reynolds number of R = 103. The flow is incompressible and two-dimensional, and the cylinder oscillations are harmonic. The transverse oscillations are only allowed when the maximum oscillatory-to-translational velocity ratio is 0.5. The investigation is based on an implicit finite difference scheme for integrating the unsteady Navier-Stokes equations together with the mass-conservation equation in their vorticity stream function formulation. A non-inertial coordinate transformation is used so that the grid mesh remains fixed relative to the accelerating cylinder. Present calculations are performed within the range of sufficiently large oscillation amplitude to induce separation. The time variation of the in-line and transverse force coefficients are presented. The study also focuses on the laminar asymmetric flow structure in the near-wake region. In this flow regime, it is found that there is alternate shedding of vortices from either side of the cylinder over an oscillation cycle (as predicted experimentally); this is the classical mode of vortex shedding leading to formation of the Kármán street. 相似文献
18.
The flow patterns in the near wake of a cylinder (either circular or square in shape, D=25 mm) placed in the proximity of a fully developed turbulent boundary layer (thickness δ=0.4D) are investigated experimentally using particle image velocimetry (PIV). The effects of changing the gap height (S) between the cylinder bottom and the wall surface, over the gap ratio range S/D=0.1–1.0, have been investigated. The results show that both the ensemble-averaged and instantaneous flow fields are strongly dependent on S/D. The flow patterns for the two types of cylinders share many similarities with respect to the change in S/D, such as the reduced recirculation length and increased velocity fluctuation in the near wake with increasing S/D, as well as the trend of suppression of vortex shedding at small S/D and onset of vortex shedding at large S/D. However, developments of the shear layers, in terms of wake width, flow curvature, etc., are considerably different for these two types of cylinders. In general, the wake development and momentum exchange for the square cylinder are slower those for the circular cylinder at the same gap ratio. Correspondingly, it is shown that the periodic vortex shedding is delayed and weakened in the case of square cylinder, as compared to that of the circular cylinder at the same S/D. 相似文献
19.
20.
Stephen R. Turnock Alexander B. Phillips Joe Banks Rachel Nicholls-Lee 《Ocean Engineering》2011,38(11-12):1300-1307
An improved method is developed to couple an inner domain solution of the blade element momentum theory with an outer domain solution of the Reynolds averaged Navier Stokes equations for evaluating performance of tidal current turbines. A mesh sensitivity study shows that a mesh of at least 6 M cells with at least 40% of these within the turbine wake is required to ensure satisfactory convergence of the velocity deficit. In addition to the usually applied axial momentum source terms, angular momentum and turbulence intensity source terms are shown to be required to model the near wake evolution. Three different lateral turbine spacing of 2, 4 and 6 turbine diameters are used to demonstrate the influence of the effective channel blockage on the velocity distribution in the turbine bypass region, the rate of spread of the wake and the recovery of velocity distribution. A final study shows that for a fixed number of turbines minimising the lateral spacing within each row, with a small number of staggered rows spaced as longitudinally as far apart as practical, is the most effective strategy for energy capture. 相似文献