Air entrapment and bubble formation during droplet impact onto a single cubic pillar

We study the vertical impact of a droplet onto a cubic pillar of comparable size placed on a flat surface, by means of numerical simulations and experiments. Strikingly, during the impact a large volume of air is trapped around the pillar side faces. Impingement upon different positions of the pillar top surface strongly influences the size and the position of the entrapped air. By comparing the droplet morphological changes during the impact from both computations and experiments, we show that the direct numerical simulations, based on the Volume of Fluid method, provide additional and new insight into the droplet dynamics. We elucidate, with the computational results, the three-dimensional air entrapment process as well as the evolution of the entrapped air into bubbles.

Simulation of Delamination Evolution of Slab Ballastless Track under Vertical Impact

During the running of a high-speed train, the wheel may bounce on the rail due to the track irregularity. The wheel bounce could generate a vertical impact, leading to the initiation and expansion of delamination between layers of the track structure. In this paper, the evolution of the interfacial damage and delamination subjected to the vertical impact is simulated using finite element analysis (FEA). In the FEA, a bilinear cohesive zone model (CZM) is adopted to simulate the interface between the track slab and the CA mortar layer. For different levels of impact energy, the contact force, vertical deformation, absorbed energy, area of interfacial damage, and area of delamination are calculated and compared. The effects of the tangential and normal stiffness of the interface on the distribution of interfacial damage and delamination are investigated. The results show that the contact force, vertical deformation, absorbed energy, area of interfacial damage, and area of delamination increase with the increase of the impact energy. The area of interfacial damage in the compression stage is closely related to the tangential stiffness, whereas the area of delamination depends on the normal stiffness. The normal stiffness that gives the largest area of delamination is recommended to be taken as the lower bound of the normal stiffness for both controlling the delamination and preventing an exceedance of the track irregularity limit.

Study of Horizontal Impact Forces Arising from Terrain on Off-Road Vehicles and Minimizing Their Effects on Ride Quality

Vehicles with off-road capabilities in the present times have begun to focus more on ride comfort. One of the most common uses of such vehicles is to help commuters travel on rough terrain, away from paved roads. Vertical suspensions carry out the work of minimizing the impact from objects like rocks and stones that comprise the terrain. However, such undulations in the terrain are not just vertically bulged. The geometry of the object, i.e., the rock/stone and the wheel coming in contact with the object gives rise to the familiar vertical impact forces for which vertical suspensions are provided. The other component of the impact force arising from the same irregular geometry of the undulation, i.e., the horizontal component of impact force which acts parallel to the axle of the wheels remains neglected. This might lead to passengers experiencing sideways swaying while inside the vehicle, even if there are independent vertical suspensions. In this paper, a study of the effects of horizontal component of impact forces on off-road vehicles was done and after that, spring-shock absorber arrangements to counter these forces were analyzed with springs of different spring-stiffness values.

Underdeck Wave Slamming Model Tests for a Drilling Semi-Submersible Unit

Negative air gap and wave slamming load on the deck box of drilling semi-submersible units in severe storm have received a great deal of attention, due to the COSL Innovator accident in 2015. Equally important is vertical slamming load on the MODU underdeck, which is less reported in the literature. The present paper attempts to derive characteristic vertical slamming pressure on the deck bottom, based on an extensive model test program for a drilling semi-submersible unit, CM-SD1000. A total of 96 3-hour wave impact tests were conducted including 4 sea states selected along the DNV steepness criterion curve in 3 wave headings. Two critical sea states were identified and each was tested with 16 random realizations in both the head and the beam waves. 8 force panels were installed on the under-deck to capture vertical wave impact events. It is found that the peak slamming pressures obtained can be fitted well with both Weibull and Gumbel probability function. The extreme vertical impact pressure predicted are of the same order of magnitude as the extreme horizontal impact pressure. The present study also shows that rise velocities of the wave surface relative to the deck bottom have a remarkable correlation with the wave slamming pressure in terms of probability distribution. The relative rise velocities can be properly derived from wave probe measurements. This offers an alternative approach to estimate the vertical impact pressure without resort to force panels. In contrast to horizontal wave slamming, the magnitude and frequency of vertical ones simply increases with significant wave height and wave steepness has much less effect. It is found that the extreme vertical impact pressure can be approximated well by a linear function of the significant wave height. The linear relationship, if validated by more tests, may help evaluate structural strength of the deck bottom before wave basin model testing.

Insertion of a Viscoelastic Layer to Reduce the Propagation of Energy by Vertical Impacts of Slamming in Planing Hull Vessels

For the design of vessels built by GFRP laminates, an insert with a viscoelastic layer is proposed to reduce the spread of damage produced by the vertical impact of the ship's bottom with the sea or slamming phenomenon. Using vertical drops-weight impact machine that reproduce the energy inferred to the panel during navigation, the propagation of the damage of OoA cured prepreg panels is studied comparing it with modified panels with insertion of viscoelastic layer. The use of acceleration data reading allows the benefits of viscoelastic modification during impact to be quantified through the developed formulation. The force, displacement and energy returned by the panel after impact have also been quantified, which does not become intralaminar and interlaminar damage. It is shown that under 40 joules of impact, the viscoelastic sheet has its best ability to return energy and above 130 joules it loses its capacity.

Milling Studies in an Impact Crusher I: Kinetics Modelling Based on Population Balance Modelling

A number of experiments were conducted on a laboratory batch impact crusher to investigate the effects of particle size and impeller speed on grinding rate and product size distribution. The experiments involved feeding a fixed mass of particles through a funnel into the crusher up to four times, and monitoring the grinding achieved with each pass. The duration of each pass was approximately 20 s; thus, this amounted to a total time of 1 min and 20 s of grinding for four passes. The population balance model (PBM) was then used to describe the breakage process, and its effectiveness as a tool for describing the breakage process in the vertical impact crusher is assessed. It was observed that low impeller speeds require longer crushing time to break the particles significantly whilst for higher speeds, longer crushing time is not desirable as grinding rate sharply decreases as the crushing time increases, hence the process becomes inefficient. Results also showed that larger particle sizes require shorter breakage time whilst smaller feed particles require longer breakage time.

Multi-objective optimization of the suspension parameters in the in-wheel electric vehicle

The hub motor significantly increases the unsprung mass of electric in-wheel vehicles, which deteriorates the ride comfort and safety of vehicles and which can be effectively improved by optimizing the main suspension parameters of vehicles reasonably, so a multi-objective optimization method of main suspension parameters based on adaptive particle swarm algorithm is proposed and the dynamic model of a half in-wheel electric vehicle is established. Taking the stiffness coefficient of the suspension damping spring and damping coefficient of the damper as independent variables, the vertical acceleration of the body, the pitch acceleration and the vertical impact force of the hub motor as optimization variables, and the dynamic deflection of the suspension and the dynamic load of the wheel as constraint variables, the multi-objective optimization function is constructed, and the parameters are simulated and optimized under the compound pavement. The simulation results show that the vertical acceleration and pitch acceleration are reduced by 20.2% and 18.4% respectively, the vertical impact force of the front hub motor is reduced by 3.7%, and the ride comfort and safety are significantly improved.

Out-of-round tram wheels – Multibody simulation study based on measured wheel rim geometry

The article discusses the influence of out-of-roundness ( OOR) type of wheel shape deviations on its interaction with the rail. The analysis was carried out using multibody simulation technique ( MBS). Measurements of tram wheel rims and sample models of flat spots were used as input data. It has been observed that the presence of wheel OOR deviations caused a significant increase in vertical impact loads at the interface between the wheel and the rail, which favours rolling contact fatigue formation. Deviations created at the stage of manufacturing the wheel rim (usually while the profile of the rim itself is turned on a lathe) were amplified by increasing amplitudes and were also visible during the measurement of worn rims, thus creating inequalities with dominant amplitudes. Flat spots present on the rolling surfaces of the wheels cause cyclic jumps of vertical force at the contact point of the wheel and the rail, with maximum values depending on the depth and radius of the rounded edges of the flat spot, and thus subject to change in subsequent stages of the flat spot development. Taking into account the observations made, preventive measures have been proposed against the development of OOR deviations in tram wheels, the origin of which is primarily in the machining process.