Assessment of approximate models to evaluate transient and cyclical hygrothermoelastic stress in composite plates

The present paper aims at assessing an approximate model to evaluate hygrothermoelastic stress in composite laminated plates. The approximate model is based on straight hypotheses (classical lamination theory of plates) and is able to simulate the effects of complex hygrothermal conditionings, such as those encountered in the life of real structures. For the purpose of the assessment, a 3D analytical solution for hygrothermally stressed plates is employed. The approximate model for plates is also compared to an analytical model for cylinders under transient and cyclical hygrothermal loads [Jacquemin, F., Vautrin, A., 2002a. Modelling of the moisture concentration field due to cyclical hygrothermal conditions in thick laminated pipes. European Journal of Mechanics – A/Solids 21, 845–855; Jacquemin, F., Vautrin, A., 2002b. A closed-form solution for the internal stresses in thick composite cylinders induced by cyclical environmental conditions. Composite Structures 58, 1–9]. This is done in order to assess the capabilities and the limits of a simplified model for plates to represent also the behaviour of cylinders.     

厚壁管道周向导波检测技术实验研究

厚壁管道是火电机组四大管道系统的核心部件,将超声导波技术应用于厚壁管道的无损检测显得十分重要.首先确定厚壁管道检测的激励方式,优化选取适合厚壁管道检测的0.5MHz探头和楔形块角度为60°的斜探头组合.通过改变斜探头与外壁轴向缺陷之间周向距离,在一定范围内仍可检测到缺陷回波,且接收到的周向回波幅值变化不大,表明周向导波...     

厚壁管道周向超声导波缺陷检测研究

厚壁管道在特种承压设备领域中广泛使用,常规无损检测技术难以实现其快速有效地检测。本文采用数值模拟与实验相结合来研究周向导波快速检测厚壁管道的方法。首先,利用有限差分软件研究了不同角度激励下外径269mm、壁厚32mm的厚壁管道中周向导波的传播特性,优化了探头激励角度范围;然后分别制作了55°和45°的斜楔,并搭建了实验系统,研究了周向导波与厚壁管道壁厚方向不同位置缺陷的相互作用规律。研究结果表明,周向导波适用于厚壁管道快速检测。检测时需选择角度适中的探头。激发角度过小时,厚壁管中形成的周向导波模式较多,使得波包宽度较大,影响检测分辨率;而角度过大时,会使得盲区增大,导致靠近内壁区域缺陷漏检。本文的研究结论为厚壁管道缺陷周向导波的实际检测应用提供了指导。     

Experimental study of wall pressure fluctuations in rigid and elastic pipes behind an axisymmetric narrowing

Wall pressure fluctuations, p_t, in rigid and elastic pipes behind a local axisymmetric narrowing are studied. A sharp increase in their rms level in a finite region immediately downstream of the narrowing, leading up to a pronounced maximum upstream of the point of jet reattachment, is found. Approximate estimates both for the distance from the narrowing to the point of maximum rms pressure and for the rms magnitude at this point are obtained. Inspection of the wall pressure power spectrum, P, reveals the presence of low-frequency maxima. The maxima are found to be associated with the large-scale eddies in the regions of separated and reattached flow, and their frequencies are close to the characteristic frequencies of the eddies’ formation. These maxima are the main distinguishing features of the spectrum under investigation compared to the power spectrum of the wall pressure fluctuations in a fully-developed turbulent flow in a pipe without narrowing. A comparative analysis of the data for rigid and elastic pipes shows that changes in the pipe wall bending stiffness cause alterations in the flow structure near the wall and the corresponding redistribution of flow energy among the vortices. This results in an increase in the wall pressure amplitude and the low-frequency level of the wall pressure power spectrum, as well as the appearance of new frequency components in this domain.     

Direct Numerical Simulation of Turbulent Pipe Flow at Moderately High Reynolds Numbers

Fully resolved direct numerical simulations (DNSs) have been performed with a high-order spectral element method to study the flow of an incompressible viscous fluid in a smooth circular pipe of radius R and axial length 25R in the turbulent flow regime at four different friction Reynolds numbers Re _τ ?=?180, 360, 550 and $1\text{,}000$ . The new set of data is put into perspective with other simulation data sets, obtained in pipe, channel and boundary layer geometry. In particular, differences between different pipe DNS are highlighted. It turns out that the pressure is the variable which differs the most between pipes, channels and boundary layers, leading to significantly different mean and pressure fluctuations, potentially linked to a stronger wake region. In the buffer layer, the variation with Reynolds number of the inner peak of axial velocity fluctuation intensity is similar between channel and boundary layer flows, but lower for the pipe, while the inner peak of the pressure fluctuations show negligible differences between pipe and channel flows but is clearly lower than that for the boundary layer, which is the same behaviour as for the fluctuating wall shear stress. Finally, turbulent kinetic energy budgets are almost indistinguishable between the canonical flows close to the wall (up to y ^?+??≈?100), while substantial differences are observed in production and dissipation in the outer layer. A clear Reynolds number dependency is documented for the three flow configurations.     

Comparative study on heat pipe performance using aqueous solutions of alcohols

This paper deals with the performance characterization of heat pipes using an aqueous solution of long chain alcohols like n-Butanol, n-Pentanol, n-Hexanol and n-Heptanol as working mediums. These solutions are called as self-rewetting fluids, since these fluid mixtures possess a non-linear dependence of the surface tension with temperature. A cylindrical heat pipe made up of copper with two layers of wrapped screen is used as a wick material and partially filled with the self-rewetting fluid water mixture and tested for its heat transport capability like thermal efficiency and thermal resistance at different inclinations and input power levels. A number of tests have been performed with heat pipes, filled with various aqueous solutions of alcohols with a concentration of 2?ml/l in de-ionized water (DI water) on volume basis. The results obtained for heat pipes using self rewetting fluids show improved performances, when compared to DI water heat pipes.     

Analytical solution for conjugated heat transfer in pipes and ducts

 The conjugated transient forced convection heat transfer for laminar, thermally developing, steady slug flow of a Newtonian fluid of constant thermal properties in semi-infinite pipes and ducts is considered. An analytical solution for three cases, namely (1) steady state, (2) transient with negligible axial conduction in the wall and (3) early stages of time, is obtained. Received on 10 July 2000 / Published online: 29 November 2001     

Transient solution for flow of evaporating fluid in parallel pipes using analysis based on flow patterns

Flow with evaporation in parallel lines with common inlet and outlet headers may result in an uneven flow distribution among the parallel pipes. The prediction of the flow rate distribution in steady state as well as under transient conditions was based on simplified models. In this paper a more accurate time dependent model based on the temporal-local flow pattern in the pipe is presented. The pipe is subdivided into numerical sections and the calculation of the pressure drop in each cell is based on mechanistic models that are specific for the flow pattern in the cell.     

垂直提升管道输送过程流固耦合分析

以垂直提升管道的下管道支撑点为研究对象,用有限元方法对固液两相流输送过程中的流固耦合作用进行应力与位移计算。建立了管道和流体有限元模型,利用ADINA-FSI流固耦合模块进行求解;针对不同启动时间、不同约束条件、不同输送浓度、不同输送速度工况下管道铰接点的应力和位移进行分析。结果表明:随着两相流输送浓度和速度的增加,管道应力和位移也相应地增加;启动时间越短,管道应力和位移也越大;柔性连接时管道铰接点应力和位移远大于固接时的应力和位移。     

Transient effects of orthogonal pipe oscillations on laminar developing incompressible flow

This paper presents a numerical study of the transient developing laminar flow of a Newtonian incompressible fluid in a straight horizontal pipe oscillating around the vertical diameter at its entrance. The flow field is influenced by the tangential and Coriolis forces, which depend on the through‐flow Reynolds number, the oscillation Reynolds number and the angular amplitude of the pipe oscillation. The impulsive start of the latter generates a transient pulsating flow, whose duration increases with axial distance. In any cross‐section, this flow consists of a pair of symmetrical counter‐rotating vortices, which are alternatively clockwise and anti‐clockwise. The circumferentially averaged friction factor and the axial pressure gradient fluctuate with time and are always larger than the corresponding values for a stationary pipe. On the other hand, local axial velocities and local wall shear stress can be smaller than the corresponding stationary pipe values during some part of the pipe oscillation. The fluctuation amplitude of these local variables increases with axial distance and can be as high as 50% of the corresponding stationary pipe value, even at short distances from the pipe entrance. Eventually, the flow field reaches a periodic regime that depends only on the axial position. The results show that the transient flow field depends on the pipe oscillation pattern (initial position and/or direction of initial movement). Copyright © 2000 John Wiley & Sons, Ltd.     

Buckle propagation in pipe-in-pipe systems.: Part I. Experiments

Pipe-in-pipe systems are extensively used in offshore pipeline applications in which thermal insulation of the pipeline is necessary. Typically, the space between the two pipes is either empty or contains insulation material which provide minimal mechanical support to the system. In deepwater applications, the carrier pipe must be designed to resist collapse due to the ambient external pressure while the design of the inner pipe is usually governed by the pressure of the hydrocarbons it carries. The integrity of the two-pipe system in the event of accidental collapse of the carrier pipe is an issue of concern.In Part I of this two-part report, the results of an extensive experimental study of the problem are presented. The experiments were carried out on two-inch diameter carrier tubes with D/t values of 24.1, 21.1 and 16.7 and inner tubes of several diameters and wall thicknesses. In most cases local collapse of the outer tube led to simultaneous collapse of the inner one. Subsequently, the collapse propagated simultaneously collapsing both pipes. The propagation pressure of the two-pipe system (P_P2) has been quantified parametrically. An interesting second mode of collapse propagation was also discovered in which the carrier pipe collapses leaving the inner one intact. Propagation of collapse affecting both pipes is still possible but usually at a higher pressure level. The pressure at which this switch takes place has been found to closely correspond to the propagation pressure of a carrier pipe with a solid rod insert (P_PS) of the same diameter as the inner pipe. The parametric dependence of P_PS has also been established experimentally. Part I finishes with a discussion of how these new critical pressures of pipe-in-pipe systems should influence the design of such pipelines.     

Dynamic response of micro-pillar sensors measuring fluctuating wall-shear-stress

We present in this paper test results of flexible micro-pillars and pillar arrays for wall shear stress measurements in flows with fluctuating wall shear stress such as unsteady separated flows or turbulent flows. Previous papers reported on the sensing principle and fabrication process. Static calibrations have shown this sensor to have a maximum nonlinearity of 1% over two orders of wall-shear-stress. For measurements in flows with fluctuating wall shear stress the dynamic response has been experimentally verified in an oscillating pipe flow and compared to a calculated response based on Stokes’ and Oseen’s solution for unsteady flow around a cylinder. The results demonstrate good agreement under the given boundary conditions of cylindrical micro-pillars and the limit of viscous Stokes-flow around the pillar. Depending on the fluid and pillar geometry, different response curves result ranging from a flat low-pass filtered response to a strong resonant behavior. Two different methods are developed to detect the frequency content and the directional wall shear stress information from image processing of large sensor films with arrays of micro-pillars of different geometry. Design rules are given to achieve the optimal conditions with respect to signal-to-noise ratio, sensitivity and bandwidth for measurements in turbulent flows.     

复杂载荷下双金属复合管的屈曲失效研究

本文采用有限元方法系统地研究了复杂载荷下双金属复合管的屈曲失效，三维有限元数值模型考虑了双金属复合管的准静态复合成型制造过程中产生的残余应力，分析了外基管直径、内衬管壁厚、内外管初始间隙、内衬管屈服强度、内压等因素对双金属复合管屈曲失效的影响。结果表明，加载路径、复合管的几何尺寸及内衬管的屈服强度对双金属复合管的屈曲性能均有较大影响，内充压力可以延迟内衬管的屈曲失效。     

Experimental Investigation on Turbulent Structure of Drag Reducing Channel Flow with Blowing Polymer Solution from the Wall

Turbulent drag reducing flow with blowing polymer solution from the channel wall was investigated experimentally using particle image velocimetry (PIV). Experiments were carried out with varying conditions of blowing polymer solution (e.g. weight concentration of polymer solution). Reynolds number based on the channel height and mean velocity was set to 20000 and 40000. When the polymer solution was blown from the channel wall, streamwise velocity fluctuation little increased, but wall-normal velocity fluctuation, Reynolds shear stress and correlation coefficient decreased significantly only near the blower wall. This behavior corresponds to the decrease of the ejection and sweep in the near-wall region observed by the investigation of instantaneous velocity map. On the contrary, this characteristic behavior was not observed at a position away from the blower wall (y/(H/2) > 0.4) and the scatter plot was almost the same as that of the water flow in this region. These results suggest that there are two regions in the drag reducing flow with blowing polymer solution from the wall; one is a non-Newtonian region which exists near the blower wall, and the other is a Newtonian region at a distance from the wall. The non-Newtonian region plays a key role in the drag reduction by the blowing polymer solution.     

Transient hydrodynamics of in-line valves in viscoelastic pressurized pipes: long-period analysis

The literature contains few reports devoted to the analysis of the effects of a partially closed in-line valve on the characteristics of transients in viscoelastic pressurized pipes. In this paper a contribution to the analysis of the long-period behavior of pressure is offered from both the experimental and numerical modeling point of view. In the first part, laboratory tests and the related results??noticeably extensive with respect to the existing literature??are examined. More precisely, the dependance of the damping of the dimensionless pressure maximum values on the initial conditions and in-line valve local head loss coefficient is shown. In the second part, a 1-D numerical model is developed by determining its parameters within a physically based procedure. Model parameters are obtained by considering transients in a constant-diameter pipe (single pipe) and then exported to the case of pipes with a partially closed in-line valve (in-line valve pipe). Moreover, particular attention is devoted to the modalities of specifying boundary conditions. In particular, the quasi-steady-state approach is followed for determining the transient local head loss due to the partially closed in-line valve and the actual supply conditions and characteristics of the maneuver are taken into account. Finally, the effect of unsteady friction and viscoelasticity is examined in both single and in-line valve pipes.     

Accelerated aging of unidirectional hybrid composites under the long-term elevated temperature and moisture concentration

Despite their high performances, composites with polymer matrix are very sensible to the increase in temperature and moisture concentration. During long years of services, both phenomena cause a critical transient hygrothermal transverse stresses, particularly at first-ply; i.e. at two edges of the composite plates. Therefore, significant degradation of hygrothermal characteristics and ultimate strengths of materials are occurred. To get an explicit relation between the durability and the damage probability of the composite, quadratic failure criterion in stress space is used. This criterion enables us to find a direct relation between transient hygrothermal stresses produced by the increase in temperature and moisture concentration and the ultimate strengths. It is necessary to calculate the strength ratio R from initial to saturation time for each condition imposed of temperature and moisture concentration. The strength ratio gives a point of view on the damage probability of the composite plates, where the rupture occurs if R = 1. In order to limit the consequences of simultaneous effects of temperature and moisture concentration, unidirectional hybrid composites in graphite epoxy was proposed. To reach this aim, hygrothermal transverse stresses are calculated through the thickness of unidirectional hybrid plate. Finally, the strength ratio was evaluated along of the plate with a gradual increase in temperature and moisture concentration.     

Large-eddy simulation of thermal fatigue in a mixing tee

Temperature fluctuations occur due to thermal mixing of hot and cold streams in the T-junctions of the piping system in nuclear power plants, which may cause thermal fatigue of piping system. In this paper, three-dimensional, unsteady numerical simulations of coolant temperature fluctuations at a mixing T-junction of equal diameter pipes were performed using the large eddy simulation (LES) turbulent model. The experiments used in this paper to benchmark the simulations were performed by Hitachi Ltd. The calculated normalized mean temperatures and fluctuating temperatures are in good agreement with the measurements. The influence of the time-step ranging from 100 Hz to 1000 Hz on the numerical simulation results was explored. The simulation results indicate that all the results with different frequencies agree well with the experimental data. Finally, the attenuation of fluctuation of fluid temperature was also investigated. It is found that, drastic fluctuation occurs within the range of less than L/D = 4.0; the fluctuation of fluid temperature does not always attenuate from the pipe center to the wall due to the continuous generation of vortexes. At the top wall, the position of L/D = 1.5 has a minimum normalized mean temperature and a peak value of root-mean square temperature, whereas at the bottom wall, the position having the same characteristics is L/D = 2.0.     

Damage detection based on the propagation of longitudinal guided wave in a bimetal composite pipe

This paper investigates the damage detection based on the propagation of guided wave in bimetal composite pipes, which can identify damage locations in both axial and circumferential directions. The feasibility of the method is showed by numerical simulations using FEM code ANSYS. Mode analysis is used to evaluate the guided wave mode and its structure, which can provide the basis of the mode selection in measurements scheme. The guided wave propagation in a damaged pipe is computed by transient analysis. 16 nodes around the pipe wall, as probes, are used to record the guided wave signal. When Pseudo Margenau—Hill distribution (PMHD) for each signal is carried out, three types of modes could be found, which are led mode, excited mode and lag mode in sequences. Based on the results, the arrival time of the excited mode could be used to locate damage in axial direction, and the energy distribution around the pipe of lag mode is consistent with the damage in circumferential direction. The simulation illustrated the possibility of detecting damage location in both axial and circumferential directions based on longitudinal ultrasonic guided waves only.     

Reduction of beam refraction in optical pipe flow experiments by use of sheet-fabricated pipe walls

A measurement section is presented that allows for the study of liquid flows in straight pipes with noninvasive optical techniques. The amount of refraction of light rays is minimized by the use of short pipe sections manufactured of transparent film. For the situation of water, it is shown that walls as thin as 85?μm can be used in turbulent flows at high velocities. In our specific case, a flow of water at Reynolds numbers upto Re _D =300?000 in a pipe of Ø 70?mm diameter (average velocity 4?m/s) was realized without unacceptable vibrations of the pipe wall. In this situation laser Doppler measurements can be carried out without the need to correct for the position of the measurement volume or changes in the velocity response of the system.