Investigation on vibro-impacts of driveline system based on a nonlinear clearance element with time-varying stiffness and oil-squeeze damping

The transient vibro-impacts induced by clearance between the connected rotors in driveline system easily causes serious transient noise and vibration, especially between the gear teeth with backlash. To analyze the transient vibro-impacts of the driveline system excited by a step-down engine torque, a new piecewise nonlinear clearance element with time-varying stiffness and oil squeeze damping is proposed, and an 8 degree-of-freedom lumped parameters model with the new piecewise nonlinear clearance elements is established. The transient vibro-impact phenomena of the vehicle driveline during fast disengagement of the clutch are numerically simulated. Colormaps of angular acceleration and vibro-impact force shows the difference of frequency components from transient impact to stable tooth-meshing. The phase plane reveals the phenomenon of multiple impacts and rebounds in each transient impact, and shows the relationship between the relative contact displacement and velocity. The frequency responses of the angular velocity, angular acceleration and vibro-impact forces with time-varying stiffness and linear stiffness are compared respectively. Compared with the widely used clearance element with piecewise linear stiffness, the new nonlinear clearance element with the piecewise nonlinear time-varying stiffness can better reveal the transient vibro-impact responses between the driving and driven gears. Lastly, the transient vibro-impact results of driveline system are verified by the vehicle experiments.

Network-based forecasting of climate phenomena

Network theory, as emerging from complex systems science, can provide critical predictive power for mitigating the global warming crisis and other societal challenges. Here we discuss the main differences of this approach to classical numerical modeling and highlight several cases where the network approach substantially improved the prediction of high-impact phenomena: 1) El Niño events, 2) droughts in the central Amazon, 3) extreme rainfall in the eastern Central Andes, 4) the Indian summer monsoon, and 5) extreme stratospheric polar vortex states that influence the occurrence of wintertime cold spells in northern Eurasia. In this perspective, we argue that network-based approaches can gainfully complement numerical modeling.

Experimental Study on Droplet Impact Phenomena on Burning Liquid Pool Surfaces

In this study, the phenomena of water droplet impact on burning methanol and n-heptane pool surfaces were experimentally investigated under various size and velocity conditions of impact droplet. In the n-heptane pool, the temperature increased to the deeper location of the pool and the droplet impact velocity was slower, as compared with those in the methanol pool. These results were caused by the higher heat release rate of the n-heptane pool. However, the impact droplet sizes on the burning methanol and n-heptane pool surfaces appeared to be similar. By visualizing the droplet impact phenomena, the impact pattern maps of burning methanol and n-heptane pool surfaces were constructed and compared with the previous impact pattern maps of their unburned pool surfaces. In the burning and unburned methanol and unburned n-heptane pools, patterns of single jet and splash with secondary jet were observed. On the contrary, in the burning n-heptane pool, patterns of single jet and canopy were observed.

Flowering in bursting bubbles with viscoelastic interfaces

The lifetime of bubbles, from formation to rupture, attracts attention because bubbles are often present in natural and industrial processes, and their geometry, drainage, coarsening, and rupture strongly affect those operations. Bubble rupture happens rapidly, and it may generate a cascade of small droplets or bubbles. Once a hole is nucleated within a bubble, it opens up with a variety of shapes and velocities depending on the liquid properties. A range of bubble rupture modes are reported in literature in which the reduction of a surface energy drives the rupture against inertial and viscous forces. The role of surface viscoelasticity of the liquid film in this colorful scenario is, however, still unknown. We found that the presence of interfacial viscoelasticity has a profound effect in the bubble bursting dynamics. Indeed, we observed different bubble bursting mechanisms upon the transition from viscous-controlled to surface viscoelasticity-controlled rupture. When this transition occurs, a bursting bubble resembling the blooming of a flower is observed. A simple modeling argument is proposed, leading to the prediction of the characteristic length scales and the number and shape of the bubble flower petals, thus paving the way for the control of liquid formulations with surface viscoelasticity as a key ingredient. These findings can have important implications in the study of bubble dynamics, with consequences for the numerous processes involving bubble rupture. Bubble flowering can indeed impact phenomena such as the spreading of nutrients in nature or the life of cells in bioreactors.

Molecular Dynamics Simulations of Titanium Dioxide as Model System for Size Effects in Aerosol Deposition

Up to now, the role of particle sizes on deformation features of ceramic particles in aerosol deposition (AD), is not fully understood. For distinguishing general features, two-dimensional molecular dynamics (MD) simulations are applied to study associated phenomena. The nanoparticles are assumed to have original sizes of 10-300 nm and to impact the substrate at velocities of 100-800 m/s. The applied Lennard-Jones potential for the modelled nanoparticles were adjusted to mimic the mechanical properties of TiO2-anatase. For small particles, the simulations reveal three different impact behaviors of (i) rebounding, (ii) bonding and (iii) fragmentation based on initial size and impact velocity, whereas, the larger ones do not show the bonding behavior. The upper limit of the bonding regime shrinks with increasing particle sizes, the field then diminishing for the largest ones. The different impact phenomena were analyzed by evolution of the stress, strain and temperature fields. Stress and strain field results showed that “localized inelastic deformation” occurred at particle sites spreading from the interface to the substrate to the particle core. Calculated temperature fields show a local rise of around 1200 Kelvin due to the inelastic deformation, which is probably sufficient for thermal activation of further deformation features.

Design of impact dampers for transporting cargoes by two-link vehicles

This paper considers the influence of the transitional modes of movement (acceleration, braking) of a multi-link vehicle on the vibration protection of transported non-fixed or partially fixed cargoes. The impact phenomenon, in this case, can be strengthened by the existence of coupling mechanisms between the links of a multi-link vehicle. To reduce such horizontal impact loads, it is advisable to use elements with viscoelastic damping in the coupling devices of a multi-link vehicle. To study the actual impact phenomena during the transportation of non-fixed or partially fixed cargoes under the extreme modes of movement of two-link vehicles, it is proposed to use a flat two- and three-mass dynamic model with viscoelastic damping. At the same time, the theory of elastic impact has been applied while the elastic-damping characteristics of vehicles' suspensions were not taken into consideration. It has been shown that the reported research results make it possible to estimate the approximate values of the mechanical parameters for restrictive devices that protect non-fixed or partially fixed cargoes from impact, during the transition modes of transportation, depending on the conditions of motion. This practically makes it possible to select the rational design parameters for the elements of viscoelastic restrictive devices, in particular elastic elements and dampers, in order to reduce impact loads on non-fixed heavy cargoes during transportation under extreme modes of movement. Based on this study, a procedure of vibration protection of non-fixed or partially fixed cargoes in the body of a two-link vehicle during its uneven movement has been proposed, which implies determining the maximum dynamic loads on these cargoes as well as the possibility of choosing the rational design parameters for restrictive devices in order to prevent or reduce the impact of these cargoes hitting the restrictive devices

Numerical Simulation of Green Water on Deck with a Hybrid Eulerian-Lagrangian Method

Green water on the ship deck in rough sea conditions may induce extreme impulsive wave impacts on superstructures and result in severe structural damage. It is of great importance to consider green water loads in ship structure design. However, there are many challenges in predicting green water loads due to the strongly nonlinear wave-ship interactions and the multiphase, multi-scale nature of the wave impact phenomena. In this article, a three-dimensional hybrid Eulerian-Lagrangian approach is proposed for simulating green water loads on the ship deck. It is extended from an efficient and accurate two-dimensional method developed for fluid-structure interaction problems. In this method, the flow field is solved on a fixed regular Cartesian grid system in an Eulerian framework, whereas the solid body motion is tracked with a set of markers immersed in the fluid and solved in a Lagrangian framework. Two benchmark cases, green water on a fixed simplified Floating Production Storage and Offloading (FPSO) model and green water on ship, are simulated. Comparison between experimental data and numerical results shows that our method is a viable choice for predicting green water loads.

Experimental study on two consecutive droplets impacting onto an inclined solid surface

The present study is concerned with the experimental impingement of two consecutive droplets on an inclined solid surface. Attention is mainly paid to the effects of impingement timing with various oblique angles (Φ) of the surface on the impact phenomena, which mainly affect the maximum droplet spreading diameter. The investigation considers four impingement scenarios differentiated by impingement timing, namely Case 1: single-droplet impingement; Case 2 of Δt1: the moment when the leading droplet starts spreading along the oblique surface; Case 3 of Δt2: the moment when the leading droplet reaches its maximum spreading; and Case 4 of Δt3: the moment when the leading droplet starts retracting. It is observed that deformation behavior of two successive droplets impacting on the inclined surface experiences a complex asymmetric morphology evolution due to the enhancement of gravity effect and various conditions of the impingement timing. The merged droplet becomes slender with increasing oblique surface angle in the final steady shape, causing the decrease in the value of front and back contact angles. The impingement timing has a significant influence on the change of the maximum height of the merged droplet. The coalesced droplet spreads to the maximum dimensionless width diameter at Δt = Δt2 and the oblique angle of Φ = 45°, but reaches the maximum dimensionless height for Δt = Δt2 at Φ = 30°. The front contact angles converge to a fixed value eventually for all conditions of impingement timing, and the values become lower with the increasing surface inclination.