Flow interference between four equispaced cylinders when subjected to a cross flow

This paper presents the results of experiments carried out in an open jet wind tunnel on a group of four equispaced cylinders to determine the lift and drag coefficients for a single cylinder in a group and for the group as a whole. The spacing ratio of their centres was in the range 1.1<S/ D<5 while the angle of inclination or stagger angle of the group varied over the range 0° < α < 180°. In all cases the flow Reynolds number was in the subcritical zone at Re=3×10⁴ based on the diameter of a single cylinder. The results are compared with those obtained for flow interference between a group of three cylinders and it is suggested that these results be included with similarly determined data to form a basic code of design.     

Flow around two parallel circular cylinders in a linear shear flow

The flow around two parallel circular cylinders, subjected to a free stream linearly sheared in the spanwise direction was experimentally investigated in the subcritical range of Reynolds numbers. The study describes the effect of mutual interference on pressure distributions, force distribution and vortex shedding frequency along the span of each cylinder. In the tandem arrangement, it was found that there is an abrupt change in these distributions at $\text{L/D = 3.5}$. For $\text{L/D < 3.5}$, there is no vertex shedding from the upstream cylinder and vortex shedding from the downstream one breaks down into spanwise cells. For $\text{L/D > 3.5}$, vortices are shed from the two cylinders such that the Strouhal number, based on local shedding frequency and local free stream velocity, is constant along the span of each cylinder. In the side-by-side arrangement, it was found that for $\text{1.1 < T/D < 1.8}$, the flow around the two cylinders is asymmetrical and bistable. It was also found that at any value of $\text{T/D}$, the Strouhal number based on local vortex shedding frequency and local free stream velocity, is constant along the span.     

The aerodynamic forces acting on groups of two and three circular cylinders when subject to a cross-flow

The fluid drag and lift forces in the in-flow and cross-flow directions, respectively, have been measured on one cylinder in a group of either two or three cylinders, when the group was subject to a uniform cross-flow. This was done for a variety of geometrical patterns, and the effect of static cylinder displacements on the fluid forces investigated. The cross-flow spacing between cylinders was varied from 0.75 to 2.0 cylinder diameters and the in-flow spacing from 1.5 to 5.0 cylinder diameters. In general it was found that the effect of cylinder displacement on the fluid forces for one cylinder in a group of three is very similar to that obtained with one cylinder in a group of two. This led to an attempt to use a superposition principle to determine the fluid forces in the former arrangement from a knowledge of the fluid forces in the latter arrangement. Remarkable success was obtained using this superposition for the above range of cylinder spacings.     

Fatigue cracking in damaged cylinders subjected to cyclic compressive loading

This paper presents the results of analytical and experimental investigations of the occurrence of cracking in thin-walled structures which are damaged and subjected to axial compressive loading with cyclically varying magnitude. The experimental investigation comprised nine test specimens, namely three plain cylinders, three plain ring stiffened and three T-ring stiffened cylinders. The paper describes the method whereby damage was inflicted on the cylinders and describes test results, including strain gauge data.

The analysis was performed using a commercial finite element program which incorporated non-linearities in the strain-displacement and stress-strain relationships. The results of the analysis were found to be in good agreement with the test results, and the occurrence of cracking was noted to be dependent on the combined levels of tensile residual stresses due to the damage and to the levels of compressive stresses due to the imposed loading.     

Experimental study of the interaction between a pair of circular cylinders normal to a uniform shear flow

The flow about two cylinders of diameter D, displaced in a plane normal to a uniformly sheared free stream with a centreline separation H, has been investigated experimentally. The freestream shear parameter (D/U_c)dU/dy was 1.48 × 10^?2, and the Reynolds number based on the freestream central velocity was 4.3 × 10⁴. Three distinct flow regions were observed: for 1.1<H/D<1.8 the flow through the gap between the cylinders is biased and unstable; for 1.8?H/D<2.0 the flow through the gap is biased and stable; and for 2?H/D the gap flow is not biased. The present paper describes the effects of mutual interference on the pressure distributions, the vortex-shedding frequencies and the interaction of the vortex streets for these three regions.     

Flat bar steel ring stiffeners on cylinders subjected to external pressure

It is known that plates are not sensitive to initial imperfections, since, after buckling, their load deformation curve is upwards. Flat rings on circular cylinders are circular ring plates. Therefore, their postbuckling behavior is the same. This is proved theoretically in this paper. Hence, it follows that the slenderness of flat rings is not restricted by stability considerations, in contrast to the ECCS and BS 5500 recommendations.     

Finite Element simulation of dynamic buckling of cylinders subjected to periodic shear

This paper deals with the buckling of cylindrical shells under a dynamic shear load. The aim of our study is to compare static buckling load and buckling load during a sweep frequency excitation. First, we describe the special experimental device and the two Finite Element codes used in this study. In a second part static tests and corresponding Finite Element calculations are presented in order to have a reference buckling load and to understand the effect of initial imperfections. Then, a vibration analysis is performed in order to investigate the effect of geometric imperfection and a preload. In the last part, we discuss dynamic results. When we reach the first eigen frequency, the buckling load drops and the buckling deformations increase due to a parametric resonance. There is a coupling between vibration and buckling modes.     

The study of ultimate strength in notched cylinders subjected to axial compression

Inclusion of various defects, e.g. imperfection or notches, in cylinders during their service life is expected and identifying of the load bearing capacity of the structure in the presence of those imperfection and damages are necessary. In the present paper, the plastic buckling of notched cylinders is studied experimentally and numerically for various groups of the cylinder dimensions. The study focuses on influences of various characteristics of notches, such as orientation, position and notch length on the buckling capacity of the cylindrical shells. In this direction, the amount of absorbed internal strain energy affected by inserting notch is calculated and compared with intact ones.     

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.     

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。     

Flow past a blunt flat plate subjected to the disturbance of incident vortex street

An experimental investigation was conducted to study the flow past a square leading-edge flat plate subjected to the disturbance of a Karman vortex street. The experiments were carried out in a wind tunnel using flow visualization together with surface pressure measurements, at a Reynolds number of 1000 based on the plate thickness. The incident vortex street was generated by employing a symmetrical airfoil located at a certain distance upstream of the test plate. The vertical offset between the incident vortex street and the plate was varied in the range 0–5 of plate thicknesses. It was found that at small vertical offsets, the incident vortices were distorted or split by the presence of the plate and secondary vortex shedding from the leading edge was induced, resulting in a sharp rise in mean pressure distribution immediately downstream of the leading edge. At large vertical offsets, the incident vortices preserved their coherence when passing over the plate and the separated shear layer was observed reattaching to the plate surface at a more downstream position as the vertical offset was increased. The pressure measurements also reflected the variation of the reattachment length with the vertical offset.     

Hydraulic-mechanical properties of loess and its behavior when subjected to infiltration-induced wetting

Bulletin of Engineering Geology and the Environment - Water retention, unsaturated permeability, and deformation-failure characteristics induced by infiltration were studied by analyzing...     

Shear strength of a fibre-reinforced clay at large shear displacement when subjected to different stress histories

The topic of fibre-reinforced soil has been introduced and studied increasingly in the past few decades. However, the shear strength response of fibre-reinforced clay soils with different initial void ratio values when subjected to large shear displacement has not been explored in the literature. The purpose of this study is to evaluate the shear strength responses of fibre-reinforced clay soils when remoulded with relatively small and large initial void ratio and subjected to large shear displacement. In order to exclude the composition variability of the fibre-reinforced samples when subjected to various normal effective stresses, a series of multi-stage reverse drained direct shear test was undertaken with four reverse cycles of ±7 mm, ±7 mm, ±7 mm and +14 mm to achieve an accumulative horizontal shear displacement up to 56 mm that is 93% of the sample dimension. The first stage of the testing programme was carried out on soil samples consolidated at normal effective stress of 600 kPa and unloaded to 50 kPa, followed by 4 shear cycles at normal effective stresses of 50, 100 and 200 kPa, respectively. The results of these tests confirmed significant effective stress ratio improvement with fibre reinforcement, even at large shear displacement to the fourth cycle. However, the rate of improvement decreased with normal effective stress and initial void ratio. Based on the experiments carried out in this study, the optimum fibre content to increase the shear strength of the clay soil with initial void ratio of 0.64 was found to be 0.25% with 140%, 81% and 23% increase in the stress ratio over that of the unreinforced soil at normal effective stresses of 50, 100 and 200 kPa, respectively. The second stage of the testing programme was conducted on a set of samples consolidated and sheared at normal effective stresses of 50, 100 and 200 kPa, respectively. The optimum fibre content was found to be related only to the initial void ratio of the soil, irrespective of the stress history of the soil and the applied normal effective stress. The shear stress ratio of the fibre-reinforced clay soils at large shear displacement was found to be relatively independent of the stress history of the soil. For all soil samples tested in this study, the stress ratio at 200 kPa normal effective stress was found to remain between 0.45 and 0.60.     

Wind-tunnel tests on a model antenna rotating in a cross flow

Aerodynamic loads acting on a rotating antenna depend essentially on its speed of rotation. Due to the complexity of wind-tunnel tests on a rotating model, the evaluation of these loads is usually obtained through tests carried out on a non-rotating model, suitably modifying the results with theoretical or semi-empirical methods to account for rotation. In order to investigate the accuracy of this methodology, wind-tunnel tests on a rotating antenna placed in a cross flow were carried out and the experimental results were compared with those obtained from tests on a static model with subsequent application of the quasi-steady theory to determine the loads whilst in rotation. Differences emerged from the comparison between the two procedures, some of which were quite significant; an attempt has been made to determine the reasons leading to these differences.

Furthermore, the tests on the rotating model were used to verify the efficiency of load reduction of a new fin configuration previously analysed with tests on a static model. Tests showed that significant reductions in torque can be obtained with the new fin configuration, confirming the possibility of its use in practical applications.     

高温下钢筋混凝土异形柱的试验研究

进行了11根钢筋混凝土异形柱和1根钢筋混凝土方形柱在ISO834标准升温过程下的明火试验,分析了轴压比和受火方式对高温下不同截面形状的钢筋混凝土柱的破坏形态、轴向变形,以及耐火极限等的影响。试验结果表明:(1)钢筋混凝土异形柱的耐火性能明显低于方形柱。(2)轴压比对异形柱的耐火极限影响较大。(3)一般而言,十字形柱的耐火性能比T形柱好,T形柱比L形柱好;但当轴压比较大(例如0.55)时,T形柱的耐火性能有可能好于十字形柱。最后,利用三种高温下钢筋混凝土异形柱的数值分析模型对12根试验柱的耐火极限进行了计算,比较了不同模型的模拟效果。     

Interaction between inclined pile groups subjected to harmonic vibrations

Vertical and inclined piles are used in seismic areas where they could be subjected to oblique harmonic vibration loads. The effect of closely spaced battered piles on the pile–soil–pile interaction has not yet been fully recognized. A simple analytical method, based on the elasto-dynamic theory by Novak and his associates, is used in the present study to characterize vertical and inclined isolated cylindrical piles subjected to inclined harmonic vibrations. The free field movement of the ground in the vicinity of the piles is determined using an approximate approach based on the interference of the cylindrical wave field originating along each pile shaft and spreading radially outward. In calculating the interaction factor between two battered piles, an analysis has been carried out to demonstrate the effect of the presence of a neighboring pile (receiver) while the first pile (source) is loaded. In this situation, it has been found that the movement of the source pile head is decreased when a receiver pile is present. Also, the effects of the pile–pile distance, the group geometry, the length of the piles, and the inclined angle for each or all of the piles have been studied and the corresponding results will be presented.     

钢筋混凝土异形柱高温下力学性能的数值模拟

为研究高温下钢筋混凝土异形柱的力学性能,结合结构截面在力和温度作用下的非线性增量有限元分析方法和钢筋混凝土柱耐火极限简化分析方法,编制了钢筋混凝土异形柱高温反应的全过程分析程序RCSSCF。程序考虑了轴力的二阶效应,程序的有效性得到了其他学者试验结果的验证。提出混凝土卸载段的映射温度流动途径,以及应变切线模量、温度切线模量和时间切线模量的计算方法。利用该程序对具有相同横截面积的异形柱和方形柱的高温性能进行比较,包括截面温度大于500℃的损伤严重区域占全截面面积的比例和两端固端的异形柱和方形柱在四周受火和不同轴压比下的耐火极限。分析表明:异形柱的耐火性能明显低于方形柱,耐火极限受轴压比影响大;十字形柱的耐火性能比T形柱好,T形柱则又比L形柱好。     

A computational study of the interactions of three adjacent burning shrubs subjected to wind

A three-dimensional physics-based model was used to investigate the effect of shrub size, shrub separation distance and wind on the burnout times of shrubs. The shrub considered for this study was chamise. Two shrub sizes with different physical dimensions and initial masses with two wind speeds were considered. The study was performed for an array of three shrubs with separation distances ranging from zero to one shrub diameter. The shrubs were situated at the vertices of an equilateral triangle with two shrubs placed upwind (upstream shrubs) of the third shrub (downstream shrub). For a smaller shrub and a higher wind speed, the direction of fire front propagation within the shrub was found to be opposite to the direction of the wind, which resulted in a longer burning time for the shrub. For larger shrubs, a decrease in burning time was observed with an increase in wind speed. The burnout time for upstream shrubs increased with an increase in shrub separation distance for all shrub sizes and wind speeds considered. The burnout time for the downstream shrub was found to decrease with an increase in the separation distance, reach a minimum, and then increase with an increase in separation distance. The trends observed in burnout times for downstream shrub were attributed to the balance between heat feedback into the downstream shrub from the flames in upstream shrubs and availability of sufficient oxygen for combustion to take place.     

A numerical analysis of transient flow past road vehicles subjected to pitching oscillation

Conventionally, the pitching instability of road vehicles has been controlled mechanically through the application of suspension systems. The present study demonstrates how unsteady aerodynamics can be exploited for such control by properly configuring vehicle body shapes. To discern the effect of unsteady aerodynamics on road vehicle stability, large eddy simulation has been conducted to simulate the flow past simplified vehicle models. Forced-sinusoidal-pitching oscillation was imposed on the models during the simulation to probe their dynamic responses. Numerical results were compared with wind tunnel measurements for validation, and good agreement is attained. Unsteady flow structures above the rear section of the vehicles were found to significantly affect their pitching stability. Depending on the vehicle body shape configurations, the induced aerodynamic force tended to either enhance or restrain the vehicles' pitching instability.