Flow-induced vibration of two circular cylinders in tandem arrangement. Part 2: Suppression of vibrations

This paper presents an experimental investigation on suppression of cross-flow vibrations of two circular cylinders in tandem arrangement, conducted at the fluid mechanics laboratory of Kitami Institute of Technology, Japan. To suppress the vibrations of the cylinders, tripping wires were deployed, attached symmetrically about the leading stagnation lines of the cylinders. Five spacing ratios were examined, i.e., L/D=0.1, 0.3, 0.8, 2.0 and 3.2 (L is the gap spacing between the two cylinders; D is the diameter of cylinder), which are representative for five Regimes I (0.1≤L/D<0.2), II (0.2≤L/D<0.6), III (0.6≤L/D<2), IV (2≤L/D<2.7) and V (L/D≥2.7), respectively, as classified in Part 1 [Kim et al., 2009. Flow-induced vibrations of two circular cylinders in tandem arrangement (part 1: characteristics of vibration). Journal of Wind Engineering and Industrial Aerodynamics, submitted together for publication]. Tripping wire position θ measured from the leading stagnation lines of the cylinders was changed from 20° to 60° to determine the optimum range of θ for suppressing structural vibrations. The shear layers separated from the two cylinders were investigated based on flow visualization. The main findings are: (i) the flow-induced vibration on the two cylinders depends strongly on θ, (ii) at θ=20–30° the vibrations on both cylinders are almost completely suppressed for all regimes except V, and (iii) for θ≥40° the vibration amplitudes of both cylinders are considerably larger than those of the plain cylinders, particularly at θ=40°, where the vibration of the upstream cylinder becomes divergent.     

Effect of a T-shaped plate on reduction in fluid forces on two tandem cylinders in a cross-flow

The effect of a T-shaped plate on reduction in fluid forces on two tandem circular cylinders in a uniform cross-flow was examined at a Reynolds number of 6.5×10⁴. The T-shaped plate, with a head-width of 5 mm (one-tenth of cylinder diameter), is used as the control object to control the approaching flow onto the upstream cylinder, resulting in reduction in fluid forces on the cylinders. The trail length of the T-shaped plate was varied in order to bring the head of the T-shaped plate in action at different positions so that an optimum position of the head of the T-shaped plate for reducing fluid forces could be identified. From the result, it was found that, when the T-shaped plate with a trail length of 0.70-1.00 times the cylinder diameter is used, an optimum reduction in fluid forces occurs, and the interference effect of the downstream cylinder on the upstream cylinder is completely suppressed.     

串列双方柱气动干扰效应试验研究

为了进一步澄清串列方柱的气动干扰效应,以串列双方柱为研究对象,在雷诺数Re=8.0×104、间距比P/B为1.25~5(其中P为方柱中心间距、B为方柱边长)条件下,通过风洞试验同步测得上下游方柱沿周向和展向的表面风压,研究了上下游方柱的风压和气动力特性、两方柱之间气动力的柱间相关性、各方柱气动力沿展向的柱内相关性等随方柱间距的变化情况,分析并给出了干扰条件下非高斯风压沿方柱周向的分布区域。研究结果表明：串列双方柱的临界间距比P/B处在3和3.5之间,在临界间距前后,两个方柱的气动性能均会发生突变;气动力的柱间相关性和柱内相关性随方柱间距变化剧烈,间距比P/B=1.25的串列方柱的气动力柱内相关性远强于单方柱;串列双方柱的风压非高斯区域随间距比的变化较大,下游方柱表面风压的非高斯特性较单方柱更为显著。     

Solution for large amplitude vibrations of circular plates via modified Berger's approximation

Complex mathematical formulations for the geometrically nonlinear analysis of structural members have been presented by many researchers using the von-Karman strain-displacement relations. Berger proposed a simplifying approximation to take care of the geometric nonlinearity by neglecting the second strain invariant. Although it is a reasonable approximation, it poses problems when applied to circular plates. This is mainly due to the coupling between the radial and circumferential inplane strains, by the radial displacement. In this article, a modification to the Berger's approximation, applicable to the circular plates, is proposed. The main motivation of the present study is to examine how the modified Berger's approximation works in the case of the large amplitude vibrations of circular plates. It is shown that the modified Berger's approximation gives consistently accurate results, for both the simply supported and clamped circular plates, when compared to the classical results.     

On interference between two circular cylinders in staggered arrangement at high subcritical Reynolds numbers

The results of a wind-tunnel investigation on the interference between two identical parallel circular cylinders arranged in staggered configurations, carried out in a uniform smooth flow at high subcritical Reynolds number, are presented. On the basis of the pressure measurements and the flow visualization, three main flow patterns are classified and discussed. Close attention is paid to the switching phenomenon of two different pressure patterns at critical angles. This switching creates discontinuity of large lift forces on cylinders. With measurements of velocity profiles and power spectral analysis in the gap between the two cylinders, the mechanism of the appearance of this extreme aerodynamic force is discussed.     

Wind tunnel testing of yawed and inclined circular cylinders in the context of field observations of stay-cable vibrations

To assist the understanding of wind- and rain–wind-induced stay-cable vibrations observed in a field, sectional cable models were tested in the wind tunnel. Circular cylinders with different spatial orientations were tested to investigate the effects of cable inclination and wind direction on the excitation mechanism. The dynamic responses of the cylinders over different reduced velocity ranges were compared. In conjunction with the response measurement, the fluctuation of the pressure in the near wake of the cylinders was measured to provide an indication of the flow structure that is responsible for the excitation mechanism. This paper also compares wind tunnel observations with the corresponding characteristics of field prototype stay-cable vibrations.     

Vortex-induced vibration tests of circular cylinders connected with typical joints in transmission towers

Vortex-induced vibration (VIV) tests on full-scale cylinders are undertaken to study the vibration performance of steel tubes connected with typical joints in transmission towers, including [-shaped gusset plate connection, U-shaped gusset plate connection, cross-gusset connection and the flange. Due to the asymmetric flexural stiffness for the cross section of the [-shaped or U-shaped gusset plate, VIV is only generated about the minor axis. The paper presents the relationship between the slenderness ratio and the occurrence wind speed of VIV about the minor axis. Moreover, it has been shown that VIV can occur not only in laminar flows, but also in turbulent flows, and the amplitude decreases as the turbulence intensity rises. The amplitude is affected by the wind attack angle, and drops as the wind attack angle decreases. It is revealed that for the cylinders connected with [-shaped gusset plates with the slenderness ratio of 100∼200, the value of Strouhal number ranges from 0.20 to 0.21and the reduced velocity from 5.0 to 5.5.     

冷凝器流程布置的数值模拟研究(2):计算结果与分析

运用分布参数模型对2~4排管冷凝器的流程布置方式进行了模拟计算,并对其换热和流动特性进行了分析和研究。研究结果表明,单回路冷凝器中,逆流布置换热效果最好,叉流其次,顺流最差;多排管冷凝器应尽可能采用分流方式,以减小压降,增强换热;布置冷凝器流程时,重力的影响不可忽略;当风量不变时,必然存在一最佳制冷剂流量使冷凝器性能达到最佳;当制冷剂流量保持不变时,也相应存在一个最佳风量使冷凝器性能最佳。     

钻孔灌注桩反循环工艺在渭河三号桥中的应用

分析了钻孔灌注桩反循环工艺的原理,探讨了钻孔桩反循环成孔工艺的优点,通过具体工程实例,进一步说明了钻孔桩反循环成孔工艺的采用能大大提高桩基础的成孔和灌注速度,节约混凝土用量,保证成桩质量。     

Study of ground vibrations induced by railway traffic in a 3D FEM model formulated in the time domain: experimental validation

In this paper, a three-dimensional numerical model is applied for studying railway vibrations. The numerical model is based on finite element method formulated in the time domain with an implicit scheme of integration. In the first part of the paper the numerical approach is briefly described. An uncoupled scheme is applied: (i) firstly the train loads are calculated taking into account dynamic excitation with a 2D vehicle-track model; (ii) the computed loads on each sleeper are introduced into a 3D numerical model, developed in the commercial code Plaxis, which is used to simulate the wave propagation in the ground. In the second part of the paper a real case study, located in Portugal, is presented, being used for the experimental validation of the proposed model. The numerical results of ground vibrations show an acceptable agreement with real measurements. Therefore, the proposed approach can be used as a reliable prediction tool based on PLAXIS software, which allows simulating railway vibrations in some specific cases such as soft soils, inhomogeneous geometries in the longitudinal direction of the track and transition zones.     

Immersed tunnel settlements. Part 1: nature of settlements

The prediction of short-term and long-term settlements is crucial in the design of all types of immersed tunnels. Settlement can result in immediate or future problems due to cracking and leakage, which can lead to corrosion of structural reinforcing steel, ice on the roadway, life-long pumping costs and deterioration of tunnel finish. This paper provides an overview of settlement experience that may be of help to future tunnel designers in more fully understanding this phenomenon when developing their projects. This is the first part of a two-part paper. Part 2 describes a series of immersed tunnel settlement case histories.     

An improved formulation for the assessment of the capacity load of circular rings and cylindrical shells under external pressure. Part 1. Analytical derivation

In this two-part set of articles the capacity load of circular rings under external pressure is investigated assuming as a starting point the classic Levy–Timoshenko approach, which is still at the bases of most design codes for cylindrical shells. This first paper presents a perfected analytical formulation of the problem, which accurately accounts for the onset of plasticity and incorporates the effect of various geometrical imperfections. A rigorous non-linear formulation is first derived and, subsequently, an algebraic expression which avoids the recourse to numerical solution strategies is established.     

A comprehensive study of two fire sources in a road tunnel: Considering different arrangement of obstacles

This paper uses Fire Dynamics Simulator (FDS) to study various arrangements of different vehicles at upstream of two fire sources. In order to make a comprehensive study, the effects of two fire sources in both lateral and longitudinal directions are investigated. The results reveal that the behavior of two fire sources, in both perpendicular directions, is directly influenced by distance between them. For small vehicles, variations of the arrangement and distance between the vehicles and fire sources do not affect the calculated Critical Ventilation Velocity (CVV). However, the presence of medium vehicles strengthens the influence of inertia force rather than buoyant force of fire plume in the tunnel. Accordingly, when there is a short distance between fires and medium obstructions, less air ventilation is needed to prevent smoke back-layering. Eventually, far distance between the vehicles and the fires results in vanishing obstruction effects. Consequently, CVV is the same as the case in which there is no vehicle in the tunnel.     

Seismic resistance of composite beam-columns in multi-storey structures. Part 1: Experimental studies

The behaviour of partially encased composite beam-columns under the combined effects of earthquake and axial loads is experimentally investigated in a series of cyclic and pseudo-dynamic tests. The beam-column specimens tested in the experimental programme use an improved system of detailing which offers advantages in terms of efficiency and economy of fabrication and achieves an enhanced seismic performance by delaying the onset of flange buckling, while still retaining the traditional benefits offered by composite columns. Two cyclic and five pseudodynamic tests using a range of axial loads and seismic excitations have been performed and the results presented and examined. The experiments demonstrate the excellent performance of the element, and reaffirm the applicability of composite beam-columns to the earthquake-resistant design of multi-storey structures.     

Wind tunnel investigation of natural ventilation through multiple stacks. Part 1: Mean values

Wind tunnel experiments have been performed on a scale model to study unsteady natural ventilation through multiple stacks. A previously developed hot-wire technique was used to obtain both mean and instantaneous flow characteristics. The work is described in two Parts.     

Postbuckling and imperfection sensitivity analysis of structures in the plastic range. Part 1: Model analysis

In this paper, two simple models which complement one another in mirroring the plastic buckling properties of structures are analysed in detail to introduce a scheme for postbuckling and imperfection sensitivity analysis that is suitable for tackling real structures.

The variation of the size of the unloading area is used as the perturbation parameter and a special perturbation scheme is designed to deal with the moving boundary problems caused by plastic unloading, so that the extension of Koiter's theory of initial elastic postbuckling to the plastic range becomes possible.     

Environmental Impact of Automatic Fire Sprinklers: Part 1. Residential Sprinklers Revisited in the Age of Sustainability

A recent study on the environmental impact of automatic fire sprinklers is documented in a two part series. The current paper provides a background on residential sprinkler technology from its inception until the present, and analyzes the contribution of risk factors, such as fire, on the total lifecycle carbon emissions of one- and two-family dwellings and the reduction to that contribution achieved via the use of automatic fire sprinklers. The analysis shows that the risk of fire increases the carbon emissions of a building over its’ lifecycle. The results illustrate that sound risk management is necessary to achieving sustainability and avoiding unintended consequences. These findings are especially important as society seeks to design and build more energy- and resource-efficient, environmentally sustainable buildings.