Smooth Horizontal Surface Research Articles

‘Levitation’ of a falling pencil

If a pencil is held upright on a smooth horizontal surface and allowed to fall, then the bottom end first slides backward and then forward across the surface. If the top end of the pencil is given a firm horizontal push then the bottom end lifts off the surface and the whole pencil becomes airborne.

Calculation of heat transfer coefficients for explosive boiling of a liquid under conditions of low pressure

The paper discusses the problem of determining the temperature head for calculating the heat transfer coefficient during explosive boiling of a liquid on a smooth horizontal surface under conditions of low pressure. The task of the work is obtaining the method for determining the characteristic pressure for calculating the temperature of saturated vapors. As a result, it is found that the difference between the heat transfer coefficients calculated at the pressure specified at the beginning of the experiment and the pressure averaged over time differ on average by 5%.

Two-dimensional collisions of disks and spheres

An oblique collision between two spheres or two disks is usually analysed by ignoring the friction force between the objects. In the present paper, effects of the friction force are examined explicitly to analyse oblique collisions between two circular objects. Two distinct collision modes are described, one where the friction force is due to sliding friction and one where static friction dominates. Experimental results are presented for oblique collisions of two ice hockey pucks. This type of collision could be investigated as a student project, for example by colliding two coins on a smooth horizontal surface or two pucks on an air table.

Power spectra of pressure pulsations in the processes of evaporation/boiling of a liquid at low pressures

This paper presents experimental data on pressure fluctuations during evaporation/boiling of a thin liquid film under conditions of reduced pressure. The experimental data were obtained as a result of studying heat transfer on a smooth horizontal surface in a wide range of changes in the height of the liquid layer. Using the fast Fourier transform, the power spectra of pressure pulsations versus frequency were obtained. It was found that the power spectra of pressure pulsations differ depending on the mode of evaporation/boiling in the system.

Bounce of a biassed ball

If a smooth ball is dropped vertically without spin on a smooth horizontal surface then one might expect the ball to bounce vertically without spin. If it does not then the centre of mass of the ball does not coincide with its geometric centre. An experiment is described where a billiard ball and a superball are deliberately biassed by adding a small steel ball on one side. The results agree well with theoretical calculations.

Oblique bounce of a double sphere on a horizontal surface

The normal coefficient of restitution (COR) for a sphere that bounces obliquely or at right angles on a horizontal surface usually has an approximately constant value between 0 and 1, which are the limits for a completely inelastic or a perfectly elastic collision respectively. Different results are obtained for the bounce of a non-spherical object. Measurements are presented for a double sphere consisting of two golf balls joined together, incident obliquely on a smooth horizontal surface. The COR varied from 0.15 to 1.0 depending on the alignment of the two balls and the incident spin. The results are explained in terms of a simple bounce model where the lower ball slides on the horizontal surface, and in terms of three different values of the COR defined in terms of (a) the bounce height, (b) the vertical speed of the contact point and (c) the ratio of the final to the initial kinetic energy.

Bouncing along a horizontal surface

If a billiard ball bounces along a smooth horizontal surface, the bounce height decreases after every bounce. One might expect that the horizontal speed will also decrease after every bounce. In fact it remains constant. By comparison, the horizontal speed of a superball sometimes increases and sometimes decreases after each bounce.

Evaluation of SPL100 Single Photon Lidar Data

Geiger-mode and single photon lidar sensors have recently emerged on the commercial market, advertising greater collection efficiency than the traditional linear mode lidar (LML) systems. Non-linear photon detection is a new technology for the geospatial community, and its performance characteristics for surveying and mapping are not yet well understood. Therefore, the geospatial quality of the data produced by one of these new sensors, the Leica SPL100, is examined by comparing the achieved lidar point cloud accuracy, precision, digital elevation model (DEM) generation, canopy penetration, and multiple return generation to a LML point cloud. We find the SPL100 has a lower ranging precision than linear mode lidar and that the precision is more negatively affected by surface properties such as low intensity and high incidence angle. The accuracy of the SPL100 point cloud, however, was found to be comparable to LML for smooth horizontal surfaces. A 1 m resolution SPL100 DEM was also comparable to a corresponding LML DEM, but the SPL100 was observed to have a reduced ability to resolve multiple returns through vegetation when compared to a LML sensor. In its current state, the SPL100 is likely best suited for applications in which the need for collection efficiency outweighs the need for maximum precision and canopy penetration and modeling.

A COMPARISON OF SINGLE PHOTON AND FULL WAVEFORM LIDAR

Abstract. Single photon sensitive LiDAR sensors are currently competing with conventional multi-photon laser scanning systems. The advantage of the prior is the potentially higher area coverage performance, which comes at the price of an increased outlier rate and a lower ranging accuracy. In this contribution, the principles of both technologies are reviewed with special emphasis on their respective properties. In addition, a comparison of Single Photon LiDAR (SPL) and FullWaveform LiDAR data acquired in July and September 2018 in the City of Vienna are presented. From data analysis we concluded that (i) less flight strips are needed to cover the same area with comparable point density with SPL, (ii) the sharpness of the resulting 3D point cloud is higher for the waveform LiDAR dataset, (iii) SPL exhibits moderate vegetation penetration under leaf-on conditions, and (iv) the dispersion of the SPL point cloud assessed in smooth horizontal surface parts competes with waveform LiDAR but is higher by a factor of 2–3 for inclined and grassy surfaces, respectively. Still, SPL yielded satisfactory precision measures mostly below 10 cm.

Electrically Operated Multipurpose Trolley

Present paper deals with design and fabrication of industrial trolley which can be used to transfer luggage or items from one place to another place. A trolley nowadays are important for transferring different items from place to place in our daily life or even in working life as per the requirement. We can see trollies in shopping mall, airport and industries for handling the goods. In the airport, passengers use trollies to transfer their luggage till the check in counters. Sometimes they face higher weight issues at the check in, making them pay the excess baggage charge or dispose of few important items thereof only. This creates an uncomfortable and awkward situation at the check in. And if the passenger is an old age or senior citizen it’s really a very panic situation. Additionally, a survey based on a prepared questionnaire carried out at Muscat airport revealed the need for substantial improvements in the present trollies in terms of comfort in luggage handling, loading and unloading of the luggage and the need for weight of the luggage at the time of loading itself. Furthermore, many food and kitchen industries use trollies to receive and transfer the goods items to the store after weighing them. The Arduino based electronically operated steerable trolley developed here aims to address few of these key issues by measuring the weight of the items directly from the trolley with easy movement by electrical power from battery. The trolley has been designed using AUTOCAD and stress analysis of the structure has also been performed using CREO V3. The fabrication is completed as per the design and tested for its performance. The testing performed on the trolley shown that for the maximum load of 90 kg including the passenger weight, the time taken by the trolley to travel on a smooth horizontal airport surface for covering a distance of 10 m with full and half rated speed of motor is 7 seconds and 9.6 seconds only respectively. The trolley was also successfully tested for elevated slope at the Muscat airport requiring additional 25 seconds more at the full rated speed at maximum loading condition of 90 kg. Additionally the trolley provides few added benefits such as charging of smart devices by using photovoltaic system technology and sliding platform for easy loading and unloading of the goods. The future work includes use of lighter aluminum alloy material along with use of solar panels for operation of the trolley.

Morphological evolution of microscopic dewetting droplets with slip

We investigate the dewetting of a droplet on a smooth horizontal solid surface for different slip lengths and equilibrium contact angles. Specifically, we solve for the axisymmetric Stokes flow using the boundary element method with (i) the Navier-slip boundary condition at the solid/liquid boundary and (ii) a time-independent equilibrium contact angle at the contact line. When decreasing the rescaled slip length $\tilde{b}$ with respect to the initial central height of the droplet, the typical non-sphericity of a droplet first increases, reaches a maximum at a characteristic rescaled slip length $\tilde{b}_{m}\approx O(0.1{-}1)$ and then decreases. Regarding different equilibrium contact angles, two universal rescalings are proposed to describe the behaviour of the non-sphericity for rescaled slip lengths larger or smaller than $\tilde{b}_{m}$. Around $\tilde{b}_{m}$, the early time evolution of the profiles at the rim can be described by similarity solutions. The results are explained in terms of the structure of the flow field governed by different dissipation channels: elongational flows for $\tilde{b}\gg \tilde{b}_{m}$, friction at the substrate for $\tilde{b}\approx \tilde{b}_{m}$ and shear flows for $\tilde{b}\ll \tilde{b}_{m}$. Following the changes between these dominant dissipation mechanisms, our study indicates a crossover to the quasistatic regime when $\tilde{b}$ is many orders of magnitude smaller than $\tilde{b}_{m}$.

Lattice Boltzmann Simulations of Macro/Microscale Effects on Saturated Pool Boiling Curves for Heated Horizontal Surfaces

Results of lattice Boltzmann (LB) simulations of macroscale effects (heating modes, heater size, and saturation temperature) as well as microscale effects (wettability and roughness) on saturated pool boiling from superheated horizontal surfaces are summarized in this paper. These effects on pool boiling curves from natural convection through nucleate boiling to critical heat flux (CHF) and from transition boiling to film boiling are illustrated. It is found that macroscale effects have negligible influence on nucleate boiling heat transfer, and Rohsenow's correlation equation fits well with the simulated nucleate boiling heat transfer on smooth hydrophilic and hydrophobic horizontal surfaces. Both macroscale and microscale effects have important influence on critical heat flux and transition boiling heat transfer.

Mesoscale simulations of boiling curves and boiling hysteresis under constant wall temperature and constant heat flux conditions

Mesoscale simulations for pool boiling curves and boiling hysteresis on hydrophilic/hydrophobic surfaces, under constant wall temperature/constant wall heat flux conditions, are presented in this paper. It is found that simulated boiling curves in dimensionless form under these two different heating modes are identical in nucleate boiling and film boiling regimes, and they differ only in the transition boiling regime. Saturated temperatures have relatively small effects on boiling curves up to the fully-developed nucleate boiling regime, but have pronounced effects on critical heat flux and on film boiling. Boiling hysteresis between increasing heating and decreasing heating are also simulated numerically. It is confirmed numerically that boiling hysteresis exists in transition boiling regime for both hydrophilic and hydrophobic surfaces under controlled wall heat flux conditions. Under controlled wall temperature conditions, however, boiling hysteresis exists only on a hydrophobic surface during decreasing heat flux but no boiling hysteresis exists on a hydrophilic surface. Rohsenow’s classical correlation equation for nucleate boiling heat transfer matches well with simulated nucleate boiling heat transfer results for smooth horizontal superheated surfaces. Simulated critical heat fluxes are in qualitative agreement with those predicted by Zuber’s hydrodynamic model and by Kandlikar’s analytical model.

Cooperative transport by the ant Novomessor cockerelli

While most ant species carry burdens solitarily, a few have evolved impressive skills at cooperative transport that are important to their ecological success. We examined one of these species, Novomessor cockerelli, with the goals of better understanding how transport groups are coordinated and how cooperation influences the efficiency of load transport. Ants were induced to carry standard artificial loads over a smooth horizontal surface, both in groups and as solitary individuals. Porters in groups were non-randomly distributed around the load, with most pulling from the front while walking backward. Porters were more persistent at the leading edge than at the rear, where high rates of leaving and joining were observed. Solitary porters also pulled the load, but they usually first attempted to walk forward and push, until their interactions with the unwieldy load rotated them into a pulling position. Cooperative transport entailed a loss of efficiency, in that solitary porters moved faster and on straighter paths than groups, even though carrying the same per capita weight. This led to a higher prey delivery rate for individuals than for groups. These findings contradict earlier claims of ‘superefficient’ transport for this species, but are consistent with the overall pattern across the ants, in which only a few swarm-raiding species show higher transport efficiency by groups than individuals. We interpret cooperative transport by N. cockerelli and many other species as the product of selection for competitor avoidance, in contrast to swarm raiders, where the special need for efficient prey retrieval has shaped distinctive transport tactics.

Numerical study of gravity-driven droplet displacement on a surface using the pseudopotential multiphase lattice Boltzmann model with high density ratio

Gravity-driven displacement of a droplet on a grooved surface is studied using the Shan and Chen’s pseudopotential multiphase lattice Boltzmann (LB) model allowing a high density ratio between the gas and liquid phases. To verify and validate the multiphase LB model, we find good agreement of the LB simulations with the pressure difference over a droplet described by Laplace’s law, as well as with the dynamic capillary intrusion process obtained by Washburn’s law. The equilibrium contact angle of a droplet on a smooth horizontal surface is studied as a function of the wettability, finding good agreement with an empirical scheme obtained with Young’s equation. The dynamic behavior of a droplet moving down a vertical surface under different gravitational forces is studied. On a vertical wall, the liquid droplet reaches a terminal velocity, which value depends on the wettability of the surface and strength of the gravitational force. When a hydrophilic groove is introduced along the surface, the droplet shows a complex behavior and, depending on the height of the groove, different patterns and mechanisms of the liquid filling the groove are observed. For small groove heights, the droplet totally fills in the groove. At certain groove height, a liquid bridge is formed between top and bottom surfaces dragging most liquid onto the bottom surface. At increasing height, this liquid bridge is broken, and liquid can be dragged into the groove by adhesion force. At high groove heights, the droplet breaks up in smaller droplets dripping from the top surface onto the bottom surface, and only a small amount of liquid remains in the groove. When the wettability of the groove or surface is changed, the liquid filling behavior changes notably. For a hydrophobic surface, but hydrophilic groove, the groove is filled partly by the liquid, while, for the opposite condition, a hydrophobic groove in a hydrophilic wall, the droplet runs into the groove, but the liquid is again dragged out and no filling of the groove occurs.

Lubricated viscous gravity currents

AbstractWe present a theoretical and experimental study of viscous gravity currents lubricated by another viscous fluid from below. We use lubrication theory to model both layers as Newtonian fluids spreading under their own weight in two-dimensional and axisymmetric settings over a smooth rigid horizontal surface and consider the limit in which vertical shear provides the dominant resistance to the flow in both layers. There are contributions from Poiseuille-like flow driven by buoyancy and Couette-like flow driven by viscous coupling between the layers. The flow is self-similar if both fluids are released simultaneously, and exhibits initial transient behaviour when there is a delay between the initiation of flow in the two layers. We solve for both situations and show that the latter converges towards self-similarity at late times. The flow depends on three key dimensionless parameters relating the relative dynamic viscosities, input fluxes and density differences between the two layers. Provided the density difference between the two layers is bounded away from zero, we find an asymptotic solution in which the front of the lubricant is driven by its own gravitational spreading. There is a singular limit of equal densities in which the lubricant no longer spreads under its own weight in the vicinity of its nose and ends abruptly with a non-zero thickness there. We explore various regimes, from thin lubricating layers underneath a more viscous current to thin surface films coating an underlying more viscous current and find that although a thin film does not greatly influence the more viscous current if it forms a surface coating, it begins to cause interesting dynamics if it lubricates the more viscous current from below. We find experimentally that a lubricated gravity current is prone to a fingering instability.

Contact-line instability of liquids spreading on top of rotating substrates

A (film or) droplet of viscous liquid spreads isothermally on a smooth horizontal solid surface. The lubrication approximation is used to study the linear stability of thin (films or) droplets, subject to capillary, gravitational, and centrifugal forces, and a variety of contact-angle-versus-speed conditions. All equations are derived for plane spreading films and rotationally-symmetric spreading droplets, while the discussion of the results is carried out for the droplets. It is found that in general two types of two-dimensional base states develop. Early on there is a simple convex contour and later a contour with a pronounced capillary ridge near the contact line. While the convex contour remains stable, the capillary-ridge contour becomes unstable with regard to disturbances, which are periodic in the lateral direction. As the contact line advances in time, this instability involves a transition from two-dimensional to three-dimensional spreading, whereas modes with increasing wave numbers become successively unstable. The onset of the instability is controlled by gravitational, centrifugal, and capillary forces, whereas gravitational and capillary forces tend to stabilize and centrifugal forces tend to destabilize the system. For partially-wetting systems, the neutral stability appears to be not affected by the static advancing contact angle, though the growth rates are modified.

Sensory Ecology of Water Detection by Bats: A Field Experiment

Bats face a great risk of dehydration, so sensory mechanisms for water recognition are crucial for their survival. In the laboratory, bats recognized any smooth horizontal surface as water because these provide analogous reflections of echolocation calls. We tested whether bats also approach smooth horizontal surfaces other than water to drink in nature by partly covering watering troughs used by hundreds of bats with a Perspex layer mimicking water. We aimed 1) to confirm that under natural conditions too bats mistake any horizontal smooth surface for water by testing this on large numbers of individuals from a range of species and 2) to assess the occurrence of learning effects. Eleven bat species mistook Perspex for water relying chiefly on echoacoustic information. Using black instead of transparent Perspex did not deter bats from attempting to drink. In Barbastella barbastellus no echolocation differences occurred between bats approaching the water and the Perspex surfaces respectively, confirming that bats perceive water and Perspex to be acoustically similar. The drinking attempt rates at the fake surface were often lower than those recorded in the laboratory: bats then either left the site or moved to the control water surface. This suggests that bats modified their behaviour as soon as the lack of drinking reward had overridden the influence of echoacoustic information. Regardless of which of two adjoining surfaces was covered, bats preferentially approached and attempted to drink from the first surface encountered, probably because they followed a common route, involving spatial memory and perhaps social coordination. Overall, although acoustic recognition itself is stereotyped and its importance in the drinking process overwhelming, our findings point at the role of experience in increasing behavioural flexibility under natural conditions.

Pool Boiling CHF for Pentane, Hexane, Methanol, FC-72, FC-87, and R113 on a Smooth Horizontal Surface

Pool boiling critical heat flux (CHF) has been measured for pentane, hexane, methanol, FC-72, FC-87, and R113 on a 25.4 mm diameter smooth brass horizontal surface at five different reduced pressures ranging from 0.01to 0.24. The CHF data are compared with various established CHF models as well as the new mechanistic CHF lift-off model recently proposed by the authors. The dependence of CHF on pressure is examined, and it is found that the lift-off model gives a reasonably good prediction of changes in CHF with step changes in the reduced pressure. The R113 and FC-72 boiling curves suggest that a partially wetting transition boiling regime is encountered just prior to CHF.

Self-induced jumping of a rigid body of revolution on a smooth horizontal surface

The article is devoted to the study of the motion of a rigid body of revolution on a rigid and perfectly smooth horizontal surface under the influence of the uniform gravitational field. Basic equations are listed and their solutions are given. The unilateral contact between the body and the plane at non-steady motion is investigated and the procedure of calculation of threshold values of the body energy above which the contact is broken is given. In contrast to Shimomura et al. [Dynamics of an axisymmetric body spinning on a horizontal surface. II. Self-induced jumping. Proc. R. Soc. A 461 (2005) 1775–1809], who assumed sliding friction in their analysis, it is found that the self-induced jumping can also occur in the absence of friction at the very beginning of the motion. The free motion after the contact is lost and impact of the body when it again makes contact with the plane is discussed. The motion of a spheroid and a disk which illustrate the results of the general theory are discussed in some detail.