Viscoacoustic squeeze-film force on a rigid disk undergoing small axial oscillations

This paper investigates the air flow induced by a rigid circular disk or piston vibrating harmonically along its axis of symmetry in the immediate vicinity of a parallel surface. Previous attempts to characterize these so-called ‘squeeze-film’ systems largely relied on simplifications afforded by neglecting either fluid acceleration or viscous forces inside the thin enclosed gas layer. The present viscoacoustic analysis employs the asymptotic limit of small vibration amplitudes to investigate the flow by systematic reduction of the Navier–Stokes equations in two distinct flow regions, namely, the inner gaseous film where streamlines are nearly parallel to the confining walls and the near-edge region of non-slender flow that features gas exchange with the surrounding stagnant atmosphere. The flow in the gaseous film depends on the relevant Stokes number, defined as the ratio of the characteristic viscous time across the film to the characteristic oscillation time, and on a compressibility parameter, defined as the square of the ratio of the acoustic time for radial pressure equilibration to the oscillation time. A Strouhal number based on the local residence time emerges as an additional governing parameter for the near-edge region, which is incompressible at leading order. The method of matched asymptotic expansions is used to describe the solution in both regions, across which the time-averaged pressure exhibits comparable variations that give opposing contributions to the resulting time-averaged force experienced by the disk or piston. A diagram structured with the Stokes number and compressibility parameter as coordinates reveals that this steady squeeze-film force, typically repulsive for small values of the Stokes number, alternates to attraction across a critical separation contour in the parametric domain that exists for all Strouhal numbers. This analysis provides, for the first time, a unifying viscoacoustic theory of axisymmetric squeeze films, which yields a reduced parametric description for the time-averaged repulsion/attraction force that is potentially useful in applications including non-contact fluid bearings and robot locomotion.

Revisiting Gas to Solids Ratio for Activated Sludge Clarification by Electrolytic Hydrogen Bubbles: Theoretical and Experimental Evaluations

Molecular hydrogen (H2), as the green energy carrier from water electrolysis, can be utilized for separation of suspended micro-particles as electroflotation (EF). This study provides practical guidelines for the gas to solids (G/S) ratio as the governing parameter in EF, based on theoretical estimations and experiments for clarification of activated sludge. The G/S ratio in EF was controlled linearly by current density (j), under quasi-consistent current efficiency (at j > 8 mA/cm2) for H2 (~1) and O2 (~0.4) bubble generations on Ti cathode and IrTaOx anode, respectively. Based on the measured sizes of bubbles (approximated to 35 µm) and biological flocs (discretized to mean sizes of 22.5, 40, 60, 135, and 150 µm), batch flotation experiments estimated the maximum collision-attachment efficiency of 0.057. The rise velocities of floc-bubble aggregate were computed to derive the limiting G/S ratio to overcome the given influent hydraulic loading. Consequently, the estimates (5.23 × 10−4 and 5.92 × 10−4 at hydraulic loading of 0.87 and 1.73 cm/min, respectively) were compatible with the continuous EF experiments.

Performance Evaluation of Geometric Modification on the Stability of Road Cut Slope Using FE Based Plaxis Software

Slope failures are among the common geo-environmental natural hazards in the hilly and mountainous terrain of the world. Specially it is the major difficulty for the development of construction as it causes considerable damage on the infrastructure, human life and property. Different causes of slope failure and stabilization methods are proposed by different scholars. In this study the performance of geometric modification in slope stability was investigated using numerical method. The study uses slope height, slope angle and slope profile i.e. single slope, multi slope and bench slope as a governing parameter in the performance evaluation of geometric modification on the slope stability. The evaluation was conducted on a newly constructed road cut slope using a finite element based plaxis software. The result from performance evaluation of slope profiles show that geometric modification provides better and economical slope stability. The stability of slope decreases with increase in slope height and slope angle leading to an uneconomical design of high slopes in a single slope profile. However, the use of benching improves the stability of cut slope (i.e. the use of 2 m and 3 m bench improves the factor of safety by 7.5% and 12% from single slope profile). The method is more effective in steep slopes. Similarly, the use of a multi slope profile improves the stability of slope in stratified soil with varied strength. The performance is more significant when it is used in combination with benches. The study also provides comparison of slope profiles based on different criteria’s and recommend the selection profile based on site-specific considerations.

Triggering avalanches by transverse perturbations in a rotating drum

We study the role of small-scale perturbations in the onset of avalanches in a rotating drum in the stick-slip regime. By vibrating the system along the axis of rotation with an amplitude orders of magnitude smaller than the particles’ diameter, we found that the order parameter that properly describes the system is the kinetic energy. We also show that, for high enough frequencies, the onset of the avalanche is determined by the amplitude of the oscillation, contrary to previous studies that showed that either acceleration or velocity was the governing parameter. Finally, we present a theoretical model that explains the transition between the continuous and discrete avalanche regimes as a supercritical Hopf bifurcation.

Magneto-active elastic shells with tunable buckling strength

Shell buckling is central in many biological structures and advanced functional materials, even if, traditionally, this elastic instability has been regarded as a catastrophic phenomenon to be avoided for engineering structures. Either way, predicting critical buckling conditions remains a long-standing challenge. The subcritical nature of shell buckling imparts extreme sensitivity to material and geometric imperfections. Consequently, measured critical loads are inevitably lower than classic theoretical predictions. Here, we present a robust mechanism to dynamically tune the buckling strength of shells, exploiting the coupling between mechanics and magnetism. Our experiments on pressurized spherical shells made of a hard-magnetic elastomer demonstrate the tunability of their buckling pressure via magnetic actuation. We develop a theoretical model for thin magnetic elastic shells, which rationalizes the underlying mechanism, in excellent agreement with experiments. A dimensionless magneto-elastic buckling number is recognized as the key governing parameter, combining the geometric, mechanical, and magnetic properties of the system.

Dynamics of a Simulated Demonstration March: An Efficient Sensitivity Analysis

Protest demonstrations are a manifestation of fundamental rights. Authorities are responsible for guiding protesters safely along predefined routes, typically set in an urban built environment. Microscopic crowd simulations support decision-makers in finding sustainable crowd management strategies. Planning routes usually requires knowledge about the length of the demonstration march. This case study quantifies the impact of two uncertain parameters, the number of protesters and the standard deviation of their free-flow speeds, on the length of a protest march through Kaiserslautern, Germany. Over 1000 participants walking through more than 100,000 m2 lead to a computationally demanding model that cannot be analyzed with a standard Monte Carlo ansatz. We select and apply analysis methods that are efficient for large topographies. This combination constitutes the main novelty of this paper: We compute Sobol’ indices with two different methods, based on polynomial chaos expansions, for a down-scaled version of the original set-up and compare them to Monte Carlo computations. We employ the more accurate of the approaches for the full-scale scenario. The global sensitivity analysis reveals a shift in the governing parameter from the number of protesters to the standard deviation of their free-flow speeds over time, stressing the benefits of a time-dependent analysis. We discuss typical actions, for example floats that reduce the variation of the free-flow speed, and their effectiveness in view of the findings.

Residual compression property and response of honeycomb sandwich structures subjected to single and repeated quasi-static indentation

This paper investigated the response and residual compression properties of honeycomb sandwich structures subjected to single quasi-static indentation (QSI) and repeated QSI (RQSI). The damage depth after repeated low-velocity impact (LVI) was considered as the governing parameter during the QSI experiments. Three-dimensional digital image correlation method was applied to determine deformation of the front panels after LVI and QSI to describe damage to honeycomb structures. For specimens with the same depth, it is found that the residual strength of QSI was less than that of LVI and close to that of RQSI. Results indicated that it is more reasonable to describe the damage by volume than by depth.

NEW RATIONAL MODEL FOR THE CURRENT-INDUCED THREE DIMENSIONAL SCOUR BENEATH SUBMERGED PIPELINES

New experiments involving the three-dimensional current-induced live-bed scour beneath submerged horizontal cylinders are presented, spanning larger Shields parameter and cylinder-to-sediment diameter ratio than previous studies. Specific emphasis is on gaining a better understanding of, and ability to predict, the span migration velocity during the initial and subsequent development of such a scour hole. Consistent with previous experimental observations, both a primary (faster) and secondary (slower) span migration are observed. Process visualization of suspended sediment patterns are in line with prior speculation that this transition coincides with reduced local bed shear stress amplifications as the scour hole both deepens and widens. Dimensional analysis and physical insight are combined, leading to a new rational model for predicting the span migration velocity in both live-bed and clear-water regimes, with predictions naturally coinciding at the limit of far field incipient motion conditions. In both regimes the data cluster as predicted, and fitted closed-form expressions are provided for predicting the span migration velocity. The rational approach likewise includes a new and simple criterion for the transition from primary to secondary migration in the live-bed regime. In the clear-water regime the model incorporates primary dependence on the ratio of the Shields parameter to its critical value, resolving apparent contradictions with a previous study which suggests that the depth-based Froude number is the most important governing parameter. The developed rational model can be used to quantitatively predict all known major features of the span migration velocity in the early stages of the three dimensional (live-bed and clear-water) scour beneath submerged horizontal cylinders induced by perpendicular flow, and can hence be regarded as the first complete model for this evolution.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/7BCXoqafbH4

Pre-chamber ignition systems: A methodological proposal to reproduce a reference case in a simplified experimental facility for fundamental studies

To further understand the processes and phenomena taking place in the pre-chamber (PC) ignition concept, many studies under simplified conditions have been carried out in different experimental facilities (e.g. constant volume chambers and rapid compression machines). However limited information is provided about how the volume, orifice diameter and number of orifices were defined, raising the question whether the results are representative of engine-like conditions or not. This novel study arises from the necessity to determine a methodology to reproduce a reference pre-chamber, preserving as much as possible its jet characteristics. A theoretical development based on the first law of thermodynamics has been performed, and a relationship between the effective flow area, pre-chamber volume and engine speed is proposed as the governing parameter of the mass exchange between chambers. Besides, relaying on the know-how of gas jets, a relationship between the orifice diameter, jet tip penetration and engine speed is suggested as the criterion to preserve the relative jet penetration (respect to the distance from the PC hole to the combustion chamber walls). A numerical validation of these assumptions was carried out using a one-dimensional flow calculator to estimate the thermodynamic properties and mass transfer between chambers, and a one-dimensional spray model to estimate the penetration of the PC combustion products jets. Finally, preserving the ratio between the total area of the PC holes and the product of the PC volume and the engine speed for two pre-chamber geometries, an identical pressure rise rate, in an angular basis, is achieved in both pre-chambers. Furthermore, the same relative jet penetration rate, in an angular basis, can be also achieved, even under different engine speeds, when the ratio between the orifice diameter and the product of the square of the jet free length and the engine speed is preserved.