Interfacial stress analysis of a CFRR-metal adhesively bonded joint with/without defect under hygrothermal environment

The current study focuses on the single-lap adhesively bonded joint of carbon fiber reinforced resin (CFRR), a kind of carbon fiber reinforced polymers (CFRPs), and metal with/without adhesive defect under coupled hygrothermal environment. Theoretical mechanical model, containing key geometric and position parameters of the interfacial defect, is established and solved by differential quadrature method (DQM). Influences of the coupled hygrothermal condition, external mechanical loads, adhesive defect size and position as well as adherend materials to interfacial stresses are explored and discussed. Numerical examples show the significance of consideration of hygrothermal environment for the interfacial stresses, especially the importance of temperature gradient. Taking into account the hygrothermal environment makes the stress fields more complex. The current study may offer theoretical references to investigations of failure mechanisms of adhesively bonded structures of dissimilar materials operating in complex environment conditions.     

High-speed visualization of cavitation evolution around a marine propeller

Journal of Visualization - Nowadays, a lot of types of digital high-speed video camera are available with a wide range of image rates. The new camera technology has made it possible to observe the...     

Effect of the stiffness,inertia and oscillation kinematics on the thrust generation and efficiency of an oscillating-foil propulsion system

This paper presents an experimental study that has investigated the effects of the foil stiffness, inertia and oscillation kinematics on the thrust generation and efficiency of a flexible oscillating-foil propulsion system. A semi-empirical damped-oscillator model, which included a quadratic damping element, was developed and fitted to the experimental results. The model was used to develop explanations for the observed trends in the propulsive performance. For all of the foils constructed for the study, a consistent relationship between the efficiency and frequency ratio was observed. The maximum efficiency occurred at the same frequency ratio that resulted in both a beneficial phasing of the deformation with respect to the driven motion and also the maximum overall amplitude of the motion. For foils of equivalent resonant frequency operating at the same frequency ratio, the stiffer and heavier foils were found to develop greater thrust, likely because the lower effective damping allowed for a greater amplitude of the motion. Increasing the amplitude of the driven motion was found to cause the frequency ratio providing the maximum efficiency to shift towards lower values. The use of combined pitch and heave motions was shown to increase efficiency while reducing thrust compared to the heave-only case.     

Fractional derivative modelling of adhesive cure

This paper considers the modelling of curing adhesive properties using fractional derivatives. A systematic approach is adopted where results can be related to a physical interpretation of the system rather than relying on a purely data-driven approach. The method relies on selecting standard integer order models based on the pre-cure and post-cure behaviour, from which fractional order derivative models are derived. Results from dynamic mechanical testing of two chemistries, a cyanoacrylate adhesive and a methacrylate resin are used to identify the parameter values for their respective fractional models. These results are then used to interpret behaviour of the adhesives during cure such as the onset of solidification.     

舷侧近距离爆炸下舱段模型毁伤试验研究

为探究舷侧近距离爆炸对水面舰船的毁伤效应，设计了大尺度舱段模型，并开展了舷侧近距离水下爆炸试验，试验后测量了舱段模型破坏范围及破坏模式、模型典型部位冲击环境数据、典型部位动态响应。综合对比可发现:（1）舷侧近距离水下爆炸下，在爆心区域可对舱段模型形成严重毁伤破坏，但毁伤范围有限，基本以局部破坏为主；（2）舷侧近距离水下爆炸下可形成较为明显的水射流载荷，主要是由气泡与非完整边界、自由面在耦合过程中形成的，与传统研究的气泡收缩失稳引起的水射流载荷形成机理存在一定的差异；（3）基于板格能量计算方法，建立了舷侧外近距离爆炸下舷侧爆炸破口计算公式，与试验结果吻合较好；（4）近距离爆炸下，舷侧外板的破坏模式与爆距存在较大的关系。本文的研究成果对舰船抗爆防护具有很强的指导意义。     

Aerodynamic external pressure loads on a semi-circular bluff body under wind gusts

This brief communication concerns the unsteady aerodynamic external pressure loads acting on a semi-circular bluff body lying on a floor under wind gusts and describes the theoretical model, experimental setup, and experimental results obtained. The experimental setup is based on an open circuit, closed test section, low speed wind tunnel, which includes a sinusoidal gust generating mechanism, designed and built at the Instituto de Microgravedad “Ignacio Da Riva” of the Universidad Politécnica de Madrid (IDR/UPM). Based on the potential flow theory, a theoretical model has been proposed to analyse the problem, and experimental tests have been performed to study the unsteady aerodynamic loads on a semi-circular bluff body. By fitting the theoretical model predictions with the experimental results, influencing parameters of the unsteady aerodynamic loads are ascertained. The values of these parameters can help in clarifying the phenomenon of the external pressure loads on semi-circular bluff body under various gust frequencies. The theoretical model proposed allows the pressure variation to be split into two contributions, a quasi-steady term and an unsteady term with a simple physical meaning.     

高强度钢手工焊接后过程的声发射实时监测研究

本针对实际工艺中使用的高强度钢，重点对手工焊接的冷却过程利用声发射技术进行实时监测。本次试验是和先前在不同型号的钢板在手工焊接过程中所产生的声发射信号进行比较的基础上进行的，主要目的是寻找高强度钢在手工焊接的冷却中所产生的活性裂纹在冷却阶段的声信号特征。     

Fluid–structure interaction during the impact of a cylindrical shell on a thin layer of water

A two-dimensional unsteady analysis of an elastic circular cylindrical shell that enters a thin layer of an ideal incompressible liquid is considered. The cylinder initially touches the liquid free surface at a single point and then penetrates the liquid layer at a constant vertical velocity. The problem is coupled because the liquid flow, the shape of the elastic shell and the geometry of the contact region between the body and the liquid must be determined simultaneously. The flow region is subdivided into four complementary regions that exhibit different properties: the region beneath the entering body surface, the jet root, the spray jet, and the outer region. A complete solution is obtained by matching the solutions within these four subdomains. The structural analysis is based on the normal-mode method. Strain-time histories of the inner surface of the cylinder are of particular interest. In the case of a very flexible shell three distinct regimes of the impact process were found. For a high impact velocity the lower part of the shell flattens and the shell does not enter the water. For a moderate impact velocity the shell reaches the bottom and an effect of “fluid capture” may occur. For a low impact velocity the shell penetrates the liquid, but the size of the contact region decreases before the shell reaches the bottom. This behaviour corresponds to exit or “reflection” of the shell from the water layer.     

Numerical simulation with a TVD–FVM method for circular cylinder wake control by a fairing

A water drop-shaped fairing is applied to control the wake behind a circular cylinder and to suppress the formation of Karman vortex street in this paper. The results are evaluated using high resolution CFD technique. A finite-volume total variation diminishing (TVD) approach based upon the recently proposed elemental velocity vector transformation (EVVT) method, which aims at solving the incompressible turbulent flow for irregular boundary conditions with renormalization group (RNG) turbulence model, is used to simulate the flow field around circular cylinder systems. The calculations are carried out with cylinder systems with and without fairings, while the fairings have different top shape angles within the range of $30°~90°$. The Reynolds number ranges from 1000 to 50 000. It is shown that the simulation results of present numerical method reaches good agreement with the available experimental and numerical simulation data of typical circular cylinder flow and a fixed fairing cylinder system flow. Compared with bare cylinder, the faired bluff structures can obviously reduce the lift and drag forces and alter the vortex shedding frequency. Overall, the mean drag coefficient can be reduced up to about (10–31)% and the RMS lift coefficient can be reduced up to (30–99)% for all faired systems at given Reynolds numbers. The influence of Reynolds number and attack angles on the flow field characters of bare cylinder and faired cylinders is also discussed. The faired structures with shape angles within $30°~45°$under zero-attack-angle-inflow case are considered as the optimal structures, with which the mean drag coefficient and the RMS lift coefficient can be reduced up to (26–31)% and (98–99)%, respectively. Considering the influence of attack angles on lift and drag coefficients reduction, 75° shaped faired structure may be taken as a proper option.