Contact Velocity Research Articles

Analysing head and trunk motion in the judo osoto-gari technique: relationship to sweeping-leg velocity.

Osoto-gari is a leg throw technique that primarily relies on the hip extension to initiate the sweeping motion of the leg. A high sweep contact velocity is a crucial factor in efficiently executing this technique. While some literature emphasises whole-body coordination in the leg-sweeping action, the roles of trunk and head motion remain unclear. This study investigates head and trunk movements (including the pelvic and upper torso) contributing to higher leg-sweep velocities when executing the judo osoto-gari. Kinematic data were collected from 17 male black-belt judokas using a motion capture system (250 Hz). Pearson product-moment correlation and stepwise linear regression were used to identify kinematic variables linked to the sweeping-leg velocity at sweep contact (SC). Six out of twenty-four variables correlated with sweeping-leg velocity at SC. A stepwise regression model (adjusted R2 = 0.53, p = 0.009) predicted sweeping-leg velocity based on head-tilt angle at maximum sweeping-leg height (MSH) and SC, head-tilt angular velocity at MSH, and trunk-tilt angular velocity at MSH. The findings of this study indicate that (1) increasing the forward angle of the head aids the visual system in rapidly processing spatial information about the target position, thus facilitating the execution of the leg sweep, and (2) a greater forward-tilt rotation of the head, which leads to rapid trunk rotation, is conducive to enhancing sweeping-leg velocity.

Damage evolution simulation and lifetime prediction for composite blades under continuous droplet impacts

Offshore wind power generation is a promising technology in renewable energy applications due to its high reserves of wind energy in sea areas. To improve the energy transformation efficiency, the blade length of the offshore wind turbines has become larger and larger, and it has made rain erosion be one of the most frequent failures during the turbine operation. As the natural rainfall is stochastic in spatial and time domains, it is difficult to depict the damage evolution process caused by rain impact exactly. Therefore, regular and continuous droplet impact simulation and experiments present an alternative methodology for this issue. With the composite structure of the blade and deformability of the liquid droplet in consideration, a fluid solid interaction model will be established to investigate the impact response and subsequent damage evolution. In which, the Smooth Particle Hydrodynamics (SPH) model is utilized to depict the constitutive relationship within the droplet, and Finite Element Method (FEM) is used to construct the Representative Volume Element (RVE) model of the blade leading edge. The impact process is simulated first to obtain the impact pressure distribution at the contact center and velocity field in the droplet. Furthermore, the stress wave propagation in the blade multilayer structure can be analyzed. Owing to the multiaxial fatigue feature of the continuous droplet impact, the continuum damage mechanics is integrated with the fatigue criterion and the Jump-in-Cycle procedure is used to simulate the high-cycle fatigue process. The damage factor distribution on the blade coating surface and its influence on mechanical properties are analyzed. Thereafter, the droplet impact fatigue life can be accumulated based on Miner’s linear rules. The theoretical achievements are validated by experimental data provided by Rain Erosion Testing (RET), which shows a good agreement between each other. As a result, V-N curves and D-N curves, i.e. quantitative relationship between droplet falling conditions and impact fatigue life, are established. The achievements in this study can provide an effective tool for rain erosion mechanism analysis and life prediction in industrial applications.

Association Between Longitudinal Changes in Patellar Tendon Abnormality and Land-Jump Biomechanics in Male Collegiate Basketball Players.

The relationship between self-reported symptoms and the severity of patellar tendon abnormality (PTA) as seen on magnetic resonance imaging and ultrasound is unclear, but biomechanical testing may resolve this. To (1) compare land-jump limb biomechanics between pre- and postseason timepoints, (2) assess whether seasonal changes in biomechanics are associated with seasonal changes in PTA and symptom severity, and (3) explore models that identify seasonal changes in PTA and symptoms with seasonal changes in biomechanics in collegiate basketball players. Cohort study; Level of evidence, 2. Victorian Institute of Sport Assessment Scale - Patellar Tendon (VISA-P) scores and bilateral measurements from 18 National Collegiate Athletic Association Division I and II male basketball players (n = 36 limbs) at the preseason (visit 1) and postseason (visit 2) timepoints were collected. PTA was graded with ultrasound and magnetic resonance imaging morphology measurements proximally and distally, and 3-dimensional lower extremity sagittal kinematics and kinetics were measured during a land-jump test. Multivariate and chi-square analyses assessed timepoint differences. The association of seasonal (Δ = visit 2 - visit 1) biomechanics with seasonal morphology (ΔPTA: no change/worsened) and symptoms (ΔVISA-P: improved/no change/worsened) was tested with multivariate models. Logistic regressions modeled the accuracy of seasonal biomechanics to classify seasonal PTA and symptoms. Three athletes (6 limbs) at visit 1 and 2 athletes (4 limbs) at visit 2 were symptomatic. VISA-P scores were not significantly different between preseason and postseason. Regarding PTA, multivariate analyses found differences among grouped ground-reaction force variables (P < .05); univariate analyses found that worsened PTA was associated with seasonal decreases in peak vertical jumping force and with seasonal increases in knee flexion velocity at contact and maximum knee flexion velocity (P < .05 for all). Regarding VISA-P scores, multivariate analyses found differences among grouped hip (P < .01) and ankle (P < .05) kinematic variables; univariate analyses found that worsened VISA-P was associated with seasonal increases in hip (P < .01) and knee (P < .01) flexion velocity at contact and seasonal increases in ankle range of motion (P < .05). The findings demonstrated an association between seasonal changes in dynamic lower extremity biomechanics and seasonal changes in patellar tendon imaging signals as well as self-reported symptoms.

Dynamic wetting characteristics of two droplets impacting a spherical dust particle

The removal of dust based on water misting is one of the most effective measures for dust control, and the basis of this method lies in the liquid–solid impact behavior. To further improve the dust removal efficiency, in this work, the removal mechanism was studied at the microscopic level, and the dynamic impact between two water droplets and a spherical dust particle was simulated using the coupled level-set/volume-of-fluid (CLSVOF) method. The optimal parameter ranges for micron-level dust removal were determined through the method of variable control, and the reliability of the obtained results was experimentally verified. When two droplets continuously or simultaneously impact a spherical particle, the liquid film formed undergoes deformation phenomena such as fusion, rupture, stretching, contraction, and fragmentation. The liquid film is more likely to break at higher droplet velocities, leading to the filling of the gas film and hindering the wetting process. The optimum contact angle and velocity range of the liquid droplets to completely encapsulate the dust particles were found to be θ = 2 and V = 10–20 m/s, respectively. The experimental results also showed that for the same dust particle size, the dust removal efficiency initially increased but then decreased with increasing dust concentration. The optimum dust removal efficiency of 98.1 % was achieved for θ = 2 and V = 10 m/s. This study sheds new light on the dust removal mechanism of water mist-based dust removal technologies and can aid in the design and parameter optimization of more effective dust suppression systems.

Analysis and prediction of uneven wear in metro pantograph-rigid catenary system

The non-elastic electrical contact between pantograph strip and wire, via a pantograph-rigid catenary system, results in much wear of metro strip. Every month, hundreds of thousands of strips are consumed in China due to the approximately 9900 kilometers of metro distance. Numerous instances of abnormal strip damage have been observed at the site, likely attributable to the uneven wear of strip. However, the cause of uneven wear on metro strip remains unclear to date. Hence, a wear model was developed based on the thermal equilibrium and wear mechanism, to investigate the uneven wear of strip in metro system. Various parameters, including contact force, electrical current, velocity, and copper content of the strip have been studied to explore their relationship with strip wear. Different laying structures, such as sine, half-wave, and folded line, exhibited varying degrees of uneven wear on the strip surface, with maximum depths observed at 4.20 mm per 104 km, 3.68 mm per 104 km and 1.02 mm per 104 km, respectively. The contact force holds the dominant position in metro wear, followed by current. Additionally, the wear rate also demonstrated sensitivity to the copper content of the strip.

Human-like acceleration and deceleration control of a robot astronaut floating in a space station

The acceleration and deceleration (AD) motions are the basic motion modes of robot astronauts moving in a space station. Controlling the locomotion of the robot astronaut is very challenging due to the strong nonlinearity of its complex multi-body dynamics in a gravity-free environment. However, after training, humans can move well in space stations by pushing the bulkhead, and the motion mechanism of humans is a good reference for robot astronauts. The contribution of this study is modeling the human AD motion in a microgravity environment and proposing a human-like control method for robot astronauts moving in space stations. Specifically, the movement and contact force data of the human body during AD motion were collected on an air-floating platform. Through human AD modeling analysis, the mechanism of human motion is discovered, and semi-sinusoidal primitives of contact forces are proposed for AD motion. Then, a dynamic guidance model of human AD motion is built to complete motion planning under contact constraints, which is used as the expected model for the AD control of robot astronauts. Benefiting from the force primitives, accurate and safe planning of human-like AD motion can be completed. The characteristics and mechanism of human AD motion have been analyzed from the perspective of optimization. Lastly, based on the proposed dynamic guidance model, the AD motion policy is mapped to the robot astronaut system via a system control method based on the equivalent mapping of dynamic responses (force, velocity and pose). Through comparative analysis with real human motion data and simulation results under different conditions, the proposed AD control method can achieve human-like motion efficiently and stably. Even when confronted with errors in the robot’s contact velocities and inertia parameters, the method can significantly reduce the motion errors while ensuring stability.

Variable Time-Step Physics Engine with Continuous Compliance Contact Model for Optimal Robotic Grinding Trajectory Planning.

In the transition from virtual environments to real-world applications, the role of physics engines is crucial for accurately emulating and representing systems. To address the prevalent issue of inaccurate simulations, this paper introduces a novel physics engine uniquely designed with a compliant contact model designed for robotic grinding. It features continuous and variable time-step simulations, emphasizing accurate contact force calculations during object collision. Firstly, the engine derives dynamic equations considering spring stiffness, damping coefficients, coefficients of restitution, and external forces. This facilitates the effective determination of dynamic parameters such as contact force, acceleration, velocity, and position throughout penetration processes continuously. Secondly, the approach utilizes effective inertia in developing the contact model, which is designed for multi-jointed robots through pose transformation. The proposed physics engine effectively captures energy conversion in scenarios with convex contact surface shapes through the application of spring dampers during collisions. Finally, the reliability of the contact solver in the simulation was verified through bouncing ball experiments and robotic grinding experiments under different coefficients of restitution. These experiments effectively recorded the continuous variations in parameters, such as contact force, verifying the integral stability of the system. In summary, this article advances physics engine technology beyond current geometrically constrained contact solutions, enhancing the accuracy of simulations and modeling in virtual environments. This is particularly significant in scenarios wherein there are constant changes in the outside world, such as robotic grinding tasks.

Assessment of Foot Strike Angle and Forward Propulsion with Wearable Sensors in People with Stroke.

Effective retraining of foot elevation and forward propulsion is a critical aspect of gait rehabilitation therapy after stroke, but valuable feedback to enhance these functions is often absent during home-based training. To enable feedback at home, this study assesses the validity of an inertial measurement unit (IMU) to measure the foot strike angle (FSA), and explores eight different kinematic parameters as potential indicators for forward propulsion. Twelve people with stroke performed walking trials while equipped with five IMUs and markers for optical motion analysis (the gold standard). The validity of the IMU-based FSA was assessed via Bland-Altman analysis, ICC, and the repeatability coefficient. Eight different kinematic parameters were compared to the forward propulsion via Pearson correlation. Analyses were performed on a stride-by-stride level and within-subject level. On a stride-by-stride level, the mean difference between the IMU-based FSA and OMCS-based FSA was 1.4 (95% confidence: -3.0; 5.9) degrees, with ICC = 0.97, and a repeatability coefficient of 5.3 degrees. The mean difference for the within-subject analysis was 1.5 (95% confidence: -1.0; 3.9) degrees, with a mean repeatability coefficient of 3.1 (SD: 2.0) degrees. Pearson's r value for all the studied parameters with forward propulsion were below 0.75 for the within-subject analysis, while on a stride-by-stride level the foot angle upon terminal contact and maximum foot angular velocity could be indicative for the peak forward propulsion. In conclusion, the FSA can accurately be assessed with an IMU on the foot in people with stroke during regular walking. However, no suitable kinematic indicator for forward propulsion was identified based on foot and shank movement that could be used for feedback in people with stroke.

Study on the thermal and mass evolution behavior of methane air premixed gas under rigid and flexible obstacles

There has been a lack of public reports on the combustion and explosion risks under the coupling effect of different structural materials in coal mine tunnels. Therefore, this article uses a square pipeline with a cross-section of 0.01m2and a length of 1m to study the methane combustion and explosion process under different blockage rates and rigid and flexible obstacle arrangements, in order to fully reveal the impact of tunnel construction on explosions. The results indicate that when a rigid obstacle is in the forward position, the blockage rate of a flexible obstacle is positively correlated with the flame contact velocity, maximum velocity, and maximum explosion pressure inside the pipeline. When placing a flexible obstacle in the front, as the blockage rate of the flexible obstacle increases, the contact speed and maximum speed first increase and then decrease. As the blockage rate of flexible obstacles increases, the maximum upstream explosion pressure first decreases and then increases, while the total pressure inside the pipeline first increases and then decreases. When flexible and rigid obstacles are combined and placed, they both increase heat transfer, convection, and radiation inside the tube, indirectly reducing the risk of hot air caused by explosions. Under the premise of a flexible obstacle blockage rate of 0.4, the maximum downstream overpressure can reach 2.96 times that of the upstream area, providing data support and theoretical reference for the safe layout of explosion-proof structures and equipment.

Deterministic surface roughness effects on elastic material contact with shear thinning fluid media

Abstract The formation of lubrication films is described using the hydrodynamic lubrication theory, which is based on the Reynolds equation that includes shear thinning behaviors of lubricant. Contacting surfaces are considered to undergo elastic deformation owing to concentrated contact pressures that exceed 1.0 GPa in most engineering applications. Under the contact condition of a high load on a relatively small contact area, elastic deformation of contacting bodies directly influences the formation of the lubricated film. Elastohydrodynamic lubrication (EHL) analysis is applied to correctly analyze the lubricated contact. Under an EHL contact, the scale of the lubrication film thickness is frequently less than that of the surface roughness that results from either the manufacturing or running-in processes. In this work, surface roughness is considered in detail, and two-dimensional surface roughness is measured as that characterizing general engineering surface roughness. The deterministic method regarding the surface roughness is considered for computing EHL film formation under several contact conditions such as load, contact velocity, and elasticity of contacting materials.

Impact-aware task-space quadratic-programming control

Robots usually establish contacts at rigid surfaces with near-zero relative velocities. Otherwise, impact-induced energy propagates in the robot’s linkage and may cause irreversible damage to the hardware. Moreover, abrupt changes in task-space contact velocity and peak impact forces also result in abrupt changes in robot joint velocities and torques; which can compromise controllers’ stability, especially for those based on smooth models. In reality, several tasks would require establishing contact with moderately high velocity. We propose to enhance task-space multi-objective controllers formulated as a quadratic program to be resilient to frictional impacts in three dimensions. We devise new constraints and reformulate the usual ones to be robust to the abrupt joint state changes mentioned earlier. The impact event becomes a controlled process once the optimal control search space is aware of: (1) the hardware-affordable impact bounds and (2) analytically computed feasible set (polyhedra) that constrain post-impact critical states. Prior to and nearby the targeted contact spot, we assume, at each control cycle, that the impact will occur at the next iteration. This somewhat one-step preview makes our controller robust to impact time and location. To assess our approach, we experimented its resilience to moderate impacts with the Panda manipulator and achieved swift grabbing tasks with the HRP-4 humanoid robot.

Effects of Medicine Wall Ball Throws With Whole-Body Vibration on Bat Swing Performance in Collegiate Baseball Players.

Buso, D, Willardson, JM, and Shafer, AB. Research note: Effects of medicine wall ball throws with whole-body vibration on bat swing performance in collegiate baseball players. J Strength Cond Res 37(10): 2071-2075, 2023-The purpose of this study was to examine the effect of medicine ball wall throws performed with whole-body vibration on bat swing performance in 13 highly trained collegiate baseball players. The experimental session involved the athletes' usual warm-up followed by baseline bat swings. The baseline bat swings were followed by 3 sets of 5 medicine ball wall throws while standing on a whole-body vibration platform set at 40 Hz and with 30 seconds rest between sets. Subjects then rested passively for 5 minutes before taking post-test bat swings. Performance was evaluated with bat speed (m·second-1), time to contact (secs), rotational acceleration (G's), and exit velocity (m·second-1). Paired t tests were used to compare baseline and post-test values (p ≤ 0.05). Effect size (d) statistics were also calculated to determine the magnitude of treatment effects. It was found that bat speed (p = 0.02; d = 0.22) and exit velocity (p = 0.03; d = 0.48) were significantly faster versus baseline, whereas rotational acceleration (p = 0.10; d = 0.23) and time to contact (p = 0.44; d = -0.12) were not significantly different from baseline, respectively. These results suggest that the addition of medicine ball wall throws with whole-body vibration can significantly increase key parameters of bat swing performance. Practitioners and coaches may consider including these combined exercise stimuli as part of workouts or during games for potentially greater success at the plate.

Fine Flow Structure at the Miscible Fluids Contact Domain Boundary in the Impact Mode of Free-Falling Drop Coalescence

Registration of the flow pattern and the matter distribution of a free falling liquid drop in a target fluid at rest in the impact mode of coalescence when the kinetic energy (KEn) of the drop exceeds its available surface potential energy (ASPe) was carried out by photo and video recording. We studied the evolution of the fine flow structure at the initial stage of the cavity formation. To carry out color registration, the observation field was illuminated by several matrix LED and fiber-optic sources of constant light. The planning of experiments and interpretation of the results were based on the properties of the complete solutions of the fundamental equations of a fluid mechanics system, including the transfer and conversion of energy processes. Complete solutions of the system of equations describe large-scale flow components that are waves or vortices as well as thin jets (ligaments, filaments, fibers, trickles). In experiments, the jets are accelerated by the converted available surface potential energy (ASPe) when the free surfaces of merging fluids were eliminated. The experiments were performed with the coalescence of water, solutions of alizarin ink, potassium permanganate, and copper sulfate or iron sulfate drops in deep water. In all cases, at the initial contact, the drop begins to lose its continuity and breaks up into a thin veil and jets, the velocity of which exceeds the drop contact velocity. Small droplets, the size of which grows with time, are thrown into the air from spikes at the jet tops. On the surface of the liquid, the fine jets leave colored traces that form linear and reticular structures. Part of the jets penetrating through the bottom and wall of the cavity forms an intermediate covering layer. The jets forming the inside layer are separated by interfaces of the target fluid. The processes of molecular diffusion equalize the density differences and form an intermediate layer with sharp boundaries in the target fluid. All noted structural features of the flow are also visualized when a fresh water drop isothermally spreads in the same tap water. Molecular diffusion processes gradually smooth out the fast-changing boundary of merging fluids, which at the initial stage has a complex and irregular shape. Similar flow patterns were observed in all performed experiments; however, the geometric features of the flow depend on the individual thermodynamic and kinetic parameters of the contacting fluids.

Kinetic friction of structurally superlubric 2D material interfaces

The ultra-low kinetic friction Fk of 2D structurally superlubric interfaces, connected with the fast motion of the incommensurate moiré pattern, is often invoked for its linear increase with velocity v0 and area AS, but never seriously addressed and calculated so far. Here we do that, exemplifying with a twisted graphene layer sliding on top of bulk graphite – a demonstration case that could easily be generalized to other systems. Neglecting quantum effects and assuming a classical Langevin dynamics, we derive friction expressions valid in two temperature regimes. At low temperatures the nonzero sliding friction velocity derivative dFk/dv0 is shown by Adelman–Doll–Kantorovich type approximations to be equivalent to that of a bilayer whose substrate is affected by an analytically derived effective damping parameter, replacing the semi-infinite substrate. At high temperatures, friction grows proportional to temperature as analytically required by fluctuation–dissipation. The theory is validated by non-equilibrium molecular dynamics simulations with different contact areas, velocities, twist angles and temperatures. Using 6°-twisted graphene on Bernal graphite as a prototype we find a shear stress of measurable magnitude, from 25kPa at low temperature to 260kPa at room temperature, yet only at high sliding velocities such as 100m/s. However, it will linearly drop many orders of magnitude below measurable values at common experimental velocities such as 1μm/s, a factor 10−8 lower. The low but not ultra-low “engineering superlubric” friction measured in existing experiments should therefore be attributed to defects and/or edges, whose contribution surpasses by far the negligible moiré contribution.

Analysis of direct shear test of incineration slag reinforced with waste tire strips by discrete element method

The mechanical properties of incineration slag and waste tire reinforced incineration slag (RIS) were investigated by a series of large direct shear tests. The results show that the addition of waste tire strips can improve the shear mechanical properties of incineration slag. The mechanical behavior and evolution law of RIS were mainly studied by macroscopic laboratory tests, discrete element method (DEM) simulation and microscopic theory analysis. At the same time, the microscopic mechanical properties are analyzed from the microscopic perspectives of contact force chain, porosity, velocity field, stress deflection, fabric analysis and coordination number. At the microscopic level, the contact force chain, the principal stress field and the contact fabric all show similar evolution laws. It reflects the consistency of the microscopic response of particles under the effect of macroscopic external force. The porosity and coordination number reflect the macroscopic dilatancy of the sample from the microscopic change, and explain the reinforcement mechanism from the microscopic point of view.

An approach to grid-to-rod fretting wear modeling using dynamic mortar contact

This article presents the development of a grid-to-rod fretting (GTRF) modeling methodology that utilizes dynamic mortar contact. We leverage a recently developed computational framework for modeling thermomechanical contact in dynamic simulations from the nuclear fuel performance code Bison. Usage of mortar contact ensures a smooth distribution of normal and frictional contact forces, displacements, and velocities on the contact interface, thus facilitating an accurate computation of wear. The integration within an advanced nuclear fuel performance code enables analysis of complex interactions between the fuel, cladding, and spacer grid. Such interactions include swelling, creep, fuel fragmentation, burnup, plenum pressure, and fission gas release. Our methodology is demonstrated via the modeling of two spacer grid geometries and the influence of fuel-cladding mechanical contact on the generation of fretting wear.

Sea Launch Adapter Response in Rough Sea Conditions Considering Strain Rate Effects

The launch vehicle sea launch system is a typical strong nonlinear complex coupled system. There is relative motion between the launch vehicle and the frame-type launcher in the offshore wind and wave environment. The multiple impact loads between the adapters and the guideways are nonlinear and discontinuous. The contact velocities between adapters and guideways are different. The strain rate of polyurethane foam should be considered in the adapter model. Based on the experimental data of polyurethane foam compression, a more accurate modified phenomenology macroscopic model is established. Dynamic models of the sea launch system with and without adapter strain rate effects are established respectively for comparative analysis. The results show that the dynamic effect of adapter strain rate effects on launch vehicle sea launch cannot be ignored.

A robust and contact resolving Riemann solver for the two-dimensional ideal magnetohydrodynamics equations

This paper presents a new cell-centered numerical method for ideal magnetohydrodynamics (MHD) that can be used in Lagrangian or Eulerian discretization. A two-dimensional Riemann solver based on the HLLC-type MHD method is established, which can be viewed as an extension from the HLLC-2D in hydrodynamics. The main feature of the algorithm is to introduce a nodal contact velocity and ensure the compatibility between edge fluxes and the nodal flow intrinsically. It transforms naturally from Lagrangian setting to the Eulerian setting in terms of grid nodal velocity, and gains benefits of the Lagrangian nature of the scheme. In the Lagrangian approach, the finite volume scheme itself can keep the magnetic field divergence-free strictly, while in the Eulerian case, a special constrained transport (CT) algorithm is constructed from the discontinuous fluxes on cell interfaces to ensure solenoidal nature again. Numerical tests are presented to demonstrate the performance of this new solver and compare the difference between the Lagrangian and Eulerian methods.

Intrusive and Impact Modes of a Falling Drop Coalescence with a Target Fluid at Rest

The evolution of the falling drop substance transfer in a target fluid at rest was traced by high-speed video techniques. Two flow modes were studied: slow intrusive flow, when the KE of the drop was comparable or less than the available potential energy (APSE), and a fast impact flow, at a relatively high drop contact velocity. For the substance transfer visualization, a drop of alizarin ink solution at various concentrations was used. The use of transparent partially colored fluid allows tracing the drop matter motion in the bulk and on the fluid free surface. The traditional side and frontal view of flow patterns were registered and analyzed. In both flow modes, the substance of the drop partially remained on the free surface and partially went into the target fluid bulk, where it was distributed non-uniformly. In the intrusive mode, the drop substance partially remained on the surface, while the main mass of the drop flowed into the thickness of the target fluid, forming the lenticular colored domain. The intrusion was gradually transformed into an annular vortex. In the impact mode, the drop broke up into individual fibers during the coalescence, creating linear and reticular structures on the surface of the cavity and the crown. The flow patterns composed of individual fibers were rapidly rebuilt as the flow evolved and the splash emerged and decayed. The sizes of cavities and colored fluid domains were compared in different flow regimes as well. The total energy transfer and transformation impact on the flow structure formation and dynamics was revealed.

The friction effects in the stick-slip phenomena of the human skin

In tribosystems that include contact with a flexible body, friction-induced vibration may be present. Dimensionless analysis coupled with numerical manipulation was employed to model the discontinuity between the static and kinetic behaviour of the friction. The present paper discusses the nature of stick-slip between a human fingertip and standard printing paper, intending to describe the theoretical stability conditions of movement. The amplitude of the stick-slip phenomenon is analised as a function of the human skin's rheological and tribological properties, the system's rigidity, and the sliding contact velocity.