子母管结构尾迹涡脱落的近壁面效应

利用底部激光照射方法对水流下近壁面处的子母管结构尾流场进行PIV粒子图像速度场测量。取旋流强度作为流场旋涡强弱的表征参量,研究了无量纲子母管距离比(G/D)、母管与壁面的间距比(e/D)以及子母管直径比(d/D)等因素对结构尾涡脱落的影响。在亚临界流动条件下,当G/D>0.25时,子管对母管的涡脱落影响较小,二者尾迹相互独立,形成两列独立涡街;当G/D<0.15时,子管对母管的涡脱落抑制作用明显,同时母管对子管的涡脱落也有抑制作用。根据涡旋的运动特征可将结构后方的尾流场分为四种不同的涡脱落模式:子管涡脱落模式、双管涡脱落模式、涡脱落抑制模式以及整体涡脱落模式,并确定了各种模式发生的条件范围。在子母管间距一定时,子管涡脱落强度随着直径比d/D的增大而增大;而母管涡脱落强度随着d/D的增大而减小

Vortex-induced transverse vibration of piggyback pipelines in steady current

基于量纲分析,设计模型试验研究稳态海流作用下子母管结构的横向涡激振动。通过对结构横向振动位移、水动力载荷和流场速度的同步测量分析,研究子母管结构横向涡激振动幅值和频率随约减速度的变化规律以及母管的水动力特性。实验结果表明,子母管间距比和质量比对管道横向振动和水动力有较大影响。在管间距比为0.1~0.5范围内,子母管结构涡激振动存在明显的不对称性。随着子母管间距比的减小,结构横向最大振幅增大,涡激振动的约减速度范围变宽;随着质量比的增大,涡激振动的约减速度范围变窄。母管的平均阻力系数随子母管间距比的增大单调递减,而平均升力系数则呈现非单调变化的特征

Behavior of vortex-induced vibration of a circular cylinder near a deformable wall with two degrees of freedom in steady flow

The behavior of vortex-induced vibration of a two-degree-of-freedom cylinder near a deformable wall in steady flow is investigated experimentally. The typical phenomenon of the two-degree-of-freedom cylinder's VIV is discussed. The influences of initial gap between the cylinder and the wall on the dynamic responses of the cylinder are analyzed. The comparison is made about dynamic responses of the cylinder with one and two degrees of freedom. Experimental results show that the vibration of the cylinder near a deformable wall with a small value of initial gap-to-diameter ratios can generally be divided into two phases. The initial gap-to-diameter ratios have a noticeable influence on the occurrence of transverse vibration. The transverse maximum amplitude of the cylinder with two degrees of freedom is larger than that of the cylinder with one degree of freedom under the condition with the same values of other parameters. However, the vibration frequency of the cylinder for the two degrees of freedom case is smaller than that for the one degree of freedom case at the same value of Vr number

The experimental study on local scour around a circular pipe undergoing vortex-induced vibration in steady flow

The local scour around a circular pipe undergoing vortex-induced vibration is investigated experimentally in a water flume. The typical characteristic of local scour is analyzed in detail. The influences of initial gap-to-diameter ratio (e(0)/D), the mode of pipe vibration and the sand type on the local scour are discussed. Experimental results indicate that there exists strong intensity of scour when the pipe moves close to the surface of the sandy bed. The vibration of the pipe results in the rapid development of sandy bed's scouring. With the increase of gap-to-diameter ratio the distance between the center of the pipe and the position of the maximum scour depth gets larger. The maximum scour depth does not vary monotonically with the gap-to-diameter ratio, but reaches the largest one at certain gap-to-diameter ratio between the values of e(0)/D = 0 and 2. There is not much difference of the scour profiles around the vibrating pipe between one degree and two degrees of freedom involved in this study

Submarine pipeline lateral instability on a sloping sandy seabed

Ocean-current induced pipeline on-bottom stability on a sloping sandy seabed involves a complex interaction between the hydrodynamic loading, the untrenched pipeline and the neighboring soil. In this study, a newly-designed pipe-soil interaction facility and a flow-structure-soil interaction flume have been utilized for full-scale physical modeling of the pipeline instability on a sloping sand-bed, including the downslope instability and the upslope instability. Unlike the pipeline lateral stability on the horizontal seabed, an initial lateral-soil-resistance is developed and the static-instability might be triggered for the sloping seabed. According to dimensionless analyses, an ultimate lateral-soil-resistance coefficient is proposed to describe the interaction of the pipe with the sloping sand-bed. Experimental results indicate that sand-bed slope angle, pipe submerged weight and end-constraints have much influence on pipe on-bottom stability. No matter for the upslope instability or the downslope instability, the corresponding lateral-soil-resistance coefficient for a sloping sand-bed is larger than that for a horizontal sand-bed

Physical modeling and swirling strength analysis of vortex shedding from near-bed piggyback pipelines

The vortex shedding from near-bed piggyback pipelines in a steady flow has been investigated experimentally in a large water flume. A specially arranged PIV system with upward-illumination of pulsed laser arrays from the flume bottom was employed for the flow visualization and quantitative measurement of the lee-wake flow in a sub-critical regime around the piggyback pipelines in the proximity of a plane boundary. Based on dimensional analyses, a dimensionless maximum swirling strength (W-m) is used for analyzing the vortex shedding intensity and its frequency. Time-averaged swirling strength analyses indicate that the lee-wake patterns for the near-bed piggyback pipelines are dependent on the configuration factors, including the gap-to-diameter ratio (e/D), the spacing-to-diameter ratio (G/D), and the diameter ratio of two pipes (d/D), etc. The swirling strength in the lee-wake is obviously asymmetric for piggyback pipelines with bed proximity. For the fixed values of G/D and d/D, the maximum swirling strength decreases with the decrease of e/D. Moreover, for the examined G/D range (0 &lt;= G/D &lt;= 0.5), minimum values of W-m and corresponding VIV amplitude for the piggyback pipelines are evidently within the same range of spacing-to-diameter ratio G/D approximate to 0.05-0.20