Dangerous Mode Research Articles

Assessing multimodal school travel safety in North Carolina

School transportation has been the subject of numerous federal and state policies since the early twentieth century–the Safe Routes to School program is the most recent example. However, few recent studies have thoroughly analyzed the risks and costs associated with different modes of transportation to school. Our descriptive study assessed the injury and fatality rates and related safety costs of different modes of school transportation using crash and exposure data from North Carolina, USA from 2005 to 2012. We found that riding with a teen driver is the most dangerous mode on a per trip basis with injury rates 20 times higher and fatality rates 90 times higher than school buses, which had the lowest injury rates. Non-motorized modes had per trip injury rates equivalent to school buses but per trip fatality rates were 15 times higher than for school buses. The economic costs of school travel-related injuries and fatalities for walking, biking, and teen drivers were substantially higher than other modes. This research has important policy implications because it quantified the risks of different school travel modes which allows policymakers to consider how safety investments can reduce risks. Decades of effort by schools, communities, and the government have made school buses a very safe mode and endeavored to reduce risks to teen drivers. This study highlighted the need for these same actors to reduce the risks of injury for walking and bicycling. As more improvements are made to infrastructure around schools, repeated studies of this type will allow practitioners to examine whether the improvements help mitigate the risks.

Linear instability, nonlinear instability and ligament dynamics in three-dimensional laminar two-layer liquid–liquid flows

AbstractWe consider the linear and nonlinear stability of two-phase density-matched but viscosity-contrasted fluids subject to laminar Poiseuille flow in a channel, paying particular attention to the formation of three-dimensional waves. A combination of Orr–Sommerfeld–Squire analysis (both modal and non-modal) with direct numerical simulation of the three-dimensional two-phase Navier–Stokes equations is used. For the parameter regimes under consideration, under linear theory, the most unstable waves are two-dimensional. Nevertheless, we demonstrate several mechanisms whereby three-dimensional waves enter the system, and dominate at late time. There exists a direct route, whereby three-dimensional waves are amplified by the standard linear mechanism; for certain parameter classes, such waves grow at a rate less than but comparable to that of the most dangerous two-dimensional mode. Additionally, there is a weakly nonlinear route, whereby a purely spanwise wave grows according to transient linear theory and subsequently couples to a streamwise mode in weakly nonlinear fashion. Consideration is also given to the ultimate state of these waves: persistent three-dimensional nonlinear waves are stretched and distorted by the base flow, thereby producing regimes of ligaments, ‘sheets’ or ‘interfacial turbulence’. Depending on the parameter regime, these regimes are observed either in isolation, or acting together.

A fracture condition incorporating the most unfavourable orientation of the crack

A fracture condition incorporating the most unfavourable orientation of the crack has been derived to improve the safety of loaded brittle components with complex shape, whose loading results in a three-dimensional stress state. With a single calculation, an answer is provided to the important question whether a randomly oriented crack at a particular location in the stressed component will cause fracture. Brittle fracture is a dangerous failure mode and requires a conservative design calculation. The presented experimental results show that the locus of stress intensity factors which result in mixed-mode fracture is associated with significant uncertainty. Consequently, a new approach to design of safety–critical components has been proposed, based on a conservative safe zone, located away from the scatter band defining fracture states. A postprocessor based on the proposed fracture condition and conservative safe zone can be easily developed, for testing loaded safety–critical components with complex shape. For each finite element, only a single computation is made, which guarantees a high computational speed. This makes the proposed approach particularly useful for incorporation in a design optimisation loop.

First measurements of hydrodynamic instability growth in indirectly driven implosions at ignition-relevant conditions on the National Ignition Facility.

Ignition experiments have shown an anomalous susceptibility to hydrodynamic instability growth. To help understand these results, the first hydrodynamic instability growth measurements in indirectly driven implosions on the National Ignition Facility were performed at ignition conditions with peak radiation temperatures up to ∼300 eV. Plastic capsules with two-dimensional preimposed, sinusoidal outer surface modulations of initial wavelengths of 240 (corresponding to a Legendre mode number of 30), 120 (mode 60), and 80 μm (mode 90) were imploded by using actual low-adiabat ignition laser pulses. The measured growth was in excellent agreement, validating 2D hydra simulations for the most dangerous modes in the acceleration phase. These results reinforce confidence in the predictive capability of calculations that are paramount to illuminating the path toward ignition.

Instability of an inhomogeneous bacterial suspension subjected to a chemo-attractant gradient

AbstractThe stability of a suspension of chemotactic bacteria confined in an infinitely long channel and subjected to a stationary, linear chemo-attractant gradient is investigated. While swimming, individual bacteria exert force dipoles on the fluid, which at the continuum level lead to a stress depending upon the bacterial orientation and number density fields. The presence of the attractant gradient causes bacteria to tumble less frequently when swimming along the gradient, leading to a mean orientation and a non-zero chemotactic drift velocity$U_0$in that direction. At long length and time scales compared to those associated with the persistence of bacterial swimming, fluxes due to chemotaxis and the random run–tumble motion of bacteria balance to yield an exponentially varying number density profile across the channel in the base state. The associated bacterial stress field is also exponentially varying and is normal. This spatially non-uniform base state is unstable to fluctuations in the bacterial concentration field when the scaled bacterial concentration$\beta = (3 C/8) \langle n_0\rangle L^2 H$exceeds a critical value determined by a Péclet number defined as${\mathit{Pe}} = U_0 H/\kappa $. Here,$C$is a non-dimensional dipole strength, which depends on the geometry of the bacterium,$\langle n_0\rangle $is the bacterial concentration averaged across the channel of depth$H$,$L$is the total length of the bacterium,$\kappa $is the bacterial diffusivity, and$\beta _{\mathit{crit}}$is a monotonically decreasing function of${\mathit{Pe}}$, with$\beta _{\mathit{crit}} \sim 720/{\mathit{Pe}}^3$for${\mathit{Pe}} \ll 1$and$\beta _{\mathit{crit}} \sim 2$for${\mathit{Pe}} \gg 1$. The instability is the result of the coupling between the active stress-driven fluid flow and the bacterial concentration, and manifests as rectangular convection patterns. When$\beta $first exceeds$\beta _{\mathit{crit}}$, the unstable wavelengths are large with$\lambda \gg H$and the mode of instability is stationary. Although oscillatory modes appear when$\lambda \leq O(H)$and$\beta > 247$, the most dangerous mode of instability is found to be always stationary with a wavelength$\lambda _m/H \sim {\mathit{Pe}}^{-1}$. To study the coupling between the previously analysed orientation shear instability mechanism of bacterial suspensions and the new chemotaxis-driven instability, a new set of continuum equations that consistently account for weak chemotaxis, rotation of bacteria by weak fluid shear and weak non-continuum effects along with their coupled effects has been derived. The stability analysis of those equations showed that the orientation shear mechanism has only a negligible influence on the critical concentration for the present chemotaxis-induced instability when the suspension depth is large, and it is the latter that has the lowest critical concentration.

Minimum-mass panels under probabilistic aeroelastic flutter constraints

Aeroelastic flutter is a dangerous failure mode, and aircraft structural components are designed under a deterministic flutter margin. Meeting this safety factor may result in overly-conservative structures, however, an alternative approach incorporates uncertainties into the computational models, and imposes a maximum allowable flutter probability during the optimization process. This technique is demonstrated for the variable-thickness design of an elastic panel subjected to supersonic flow. A performance measure approach based on the first-order reliability method incorporates probabilistic flutter constraints during the search for a minimum-mass panel. Optimization results are given for uncertainties in the panel's boundary conditions, and for non-deterministic thickness design variables.

Instabilities of thermal gravitational convection and heat transfer in the Czochralski model at different Prandtl numbers

The results of the calculations of critical Grash of numbers, at which flowfield and temperature fluctuations originate in the axisymmetric and three-dimensional models of crystal growth by pulling from a melt, are presented. The salient features of the convection and heat transfer structure in the zones of stabilization and changeover of dangerous modes are studied over a wide Prandtl number range under different boundary conditions on the melt surface and compared with the experimental data.

Identifying and correcting communication failures among health professionals working in the Emergency Department

ObjectiveThe aim of this study was to identify effective corrective measures to ensure patient safety in the Paediatric Emergency Department (ED). MethodsIn order to outline a clear picture of these risks, we conducted a Failure Mode and Effects Analysis (FMEA) and a Failure Mode, Effects, and Criticality Analysis (FMECA), at a Emergency Department of a Children’s Teaching Hospital in Northern Italy. The Error Modes were categorised according to Vincent’s Taxonomy of Causal Factors and correlated with the Risk Priority Number (RPN) to determine the priority criteria for the implementation of corrective actions. ResultsThe analysis of the process and outlining the risks allowed to identify 22 possible failures of the process. We came up with a mean RPN of 182, and values >100 were considered to have a high impact and therefore entailed a corrective action. ConclusionsMapping the process allowed to identify risks linked to health professionals’ non-technical skills. In particular, we found that the most dangerous Failure Modes for their frequency and harmfulness were those related to communication among health professionals.

Effect of Characteristic Parameters of Exponential Cohesive Zone Model on Mode I Fracture of Laminated Composites

Delamination is a particularly dangerous damage mode of high performance laminated composites. In order to describe the composites ductile cracking and its progressive evolution accurately, the adjusted exponential cohesive zone model (CZM) is adopted, which correlates the tensile traction with the corresponding interfacial separation along the fracturing interfacial zone. At first the adjusted exponential CZM is used to simulate the mode I delamination of the standard double cantilever beam (DCB). The simulated results are in good agreement with the corrected beam theory and the corresponding experimental results. Then in order to research how the interfacial properties influence the mode I fracture, the interfacial strength and the critical energy release rate are studied. The main results are obtained as follows. The interfacial strength plays a crucial role in the laminated composites delamination onset, and it affects the peak load significantly if there is not a pre-crack. Once the delamination propagation begins to occur in the laminated composites, the responses of the load-displacement plots are relatively insensitive to the interfacial strength, and only the critical energy release rate is of critical importance. Furthermore, the peak load increases with the increase of the critical energy release rate and interfacial strength.

Development of a Selective Friedel–Crafts Alkylation Surrogate: Safe Operating Conditions through Mechanistic Understanding

This article describes a selective one-pot, Friedel–Crafts acylation/ketone reduction protocol, effectively a surrogate for the Friedel–Crafts alkylation reaction with a primary alkyl halide. A potentially dangerous failure mode was identified, resulting in the uncontrolled evolution of hydrogen. A series of mechanistic experiments, including analysis by 27Al NMR, was undertaken, and the reaction mechanism elucidated. Finally, the use of React IR to ensure real-time reaction safety was demonstrated.

Dynamic stability of an aerodynamically efficient motorcycle

Motorcycles exhibit two potentially dangerous oscillatory modes known as ‘wobble’ and ‘weave’. The former is reminiscent of supermarket castor shimmy, while the latter is a low frequency ‘fish-tailing’ motion that involves a combination of rolling, yawing, steering and side-slipping motions. These unwanted dynamic features, which can occur when two-wheeled vehicles are operated at speed, have been studied extensively. The aim of this paper is to use mathematical analysis to identify important stability trends in the on-going design of a novel aerodynamically efficient motorcycle known as the ECOSSE Spirit ES1. A mathematical model of the ES1 is developed using a multi-body dynamics software package called VehicleSim [Anon, VehicleSim Lisp Reference Manual Version 1.0, Mechanical Simulation Corporation, 2008. Available at http://www.carsim.com]. This high-fidelity motorcycle model includes realistic tyre–road contact geometry, a comprehensive tyre model, tyre relaxation and a flexible frame. A parameter set representative of a modern high-performance machine and rider is used. Local stability is investigated via the eigenvalues of the linearised models that are associated with equilibrium points of interest. A comprehensive study of the effects of frame flexibilities, acceleration, aerodynamics and tyre variations is presented, and an optimal passive steering compensator is derived. It is shown that the traditional steering damper cannot be used to stabilise the ES1 over its entire operating speed range. A simple passive compensator, involving an inerter is proposed. Flexibility can be introduced deliberately into various chassis components to change the stability characteristics of the vehicle; the implications of this idea are studied.

Probabilistic High-Cycle Fretting Fatigue Assessment of Gas Turbine Engine Components

High-cycle fatigue (HCF) is arguably one of the costliest sources of in-service damage in military aircraft engines. HCF of turbine blades and disks can pose a significant engine risk because fatigue failure can result from resonant vibratory stresses sustained over a relatively short time. A common approach to mitigate HCF risk is to avoid dangerous resonant vibration modes (first bending and torsion modes, etc.) and instabilities (flutter and rotating stall) in the operating range. However, it might be impossible to avoid all the resonance for all flight conditions. In this paper, a methodology is presented to assess the influences of HCF loading on the fracture risk of gas turbine engine components subjected to fretting fatigue. The methodology is based on an integration of a global finite element analysis of the disk-blade assembly, numerical solution of the singular integral equations using the CAPRI (Contact Analysis for Profiles of Random Indenters) and Worst Case Fret methods, and risk assessment using the DARWIN (Design Assessment of Reliability with Inspection) probabilistic fracture mechanics code. The methodology is illustrated for an actual military engine disk under real life loading conditions.

Evaluating the ductility and shear behaviour of carbon fibre reinforced polymer and glass fibre reinforced polymer reinforced concrete columns

AbstractBrittle shear failure in short reinforced concrete (RC) columns has been identified as one of the most dangerous failure modes that may cause collapse of bridges, based on research findings and damage reconnaissance of past earthquakes. Using ABAQUS software, the effect of carbon fibre reinforced polymer (CFRP) and glass fibre reinforced polymer (GFRP) on retrofitting of existing circular RC columns, and also enhancing of shear strength have been studied. Five types of concrete columns were studied in the ‘as built’ condition to investigate the shear failure mode of columns. These columns were assessed again after being retrofitted by using CFRP and GFRP. Based on findings of this research, the effect of CFRP jackets on improving the shear behaviour of column is more than that of GFRP jackets, while GFRP jackets are more effective than CFRP jackets, regarding their contribution in increasing the column ductility. Therefore, these FRP composites can be utilized in different conditions, according to column demand for increasing the ductility or shear strength. Copyright © 2010 John Wiley & Sons, Ltd.

Thermal Instability in a Horizontal Porous Channel with Horizontal Through Flow and Symmetric Wall Heat Fluxes

The linear thermoconvective instability of the basic parallel flow in a plane and horizontal porous channel is investigated. The boundary walls are assumed to be impermeable and subject to symmetric and uniform heat fluxes. The wall heat fluxes produce either a net heating or a net cooling of the fluid saturated porous medium. A horizontal mass flow rate is externally impressed leading to a stationary basic state with a temperature gradient inclined to the vertical. A region of possibly unstable thermal stratification exists either in the lower half-channel (boundary heating), or in the upper half-channel (boundary cooling). The convective instability of the basic flow is governed by the Rayleigh number and by the Peclet number. In the case of boundary heating, the thermal instability arises when the Rayleigh number exceeds its critical value, that depends on the Peclet number. The change of the critical Rayleigh number as a function of the Peclet number is determined numerically for arbitrary normal modes oblique to the basic flow direction. The most dangerous modes are the longitudinal rolls, with a wave vector perpendicular to the basic velocity. There exists a minimum value of the Peclet number, 19.1971, below which no linear instability is detected.

Failure Mode and Effect Analysis of a Thermal Power Plant for Enhancing its Reliability

Failure Mode and Effect Analysis (FMEA) has a well deserved reputation for systematic and thorough evaluation of failures at the system, sub-system or component level in all manufacturing and processing sectors. These organizations are looking for the final product to be “safe and reliable”. FMEA helps designers to identify and eliminate/control dangerous failure modes, minimizing damage to the system and its users. This paper, as an extension to the prior research work, introduces an insight into the reasons of failure and its effects in a thermal power plant opted for the case study, based on conceptual designs in context of FMEA. The analysis takes into account preparation of appropriate diagnostic and maintenance procedures with the aim of enhancement of thermal plant reliability. The FMEA technique used may be helpful for the design and maintenance departments to curtail the downtime of the plant.

Effects of wall-heating on the linear instability characteristics of pressure-driven two-layer channel flow

The linear stability analysis of a pressure-driven two-layer channel flow of two immiscible, Newtonian and incompressible fluids is considered. The walls of the channel are maintained at different constant temperatures and Nahme's law is applied to model the temperature dependence of the fluid viscosity. A modified Orr–Sommerfeld equation for the disturbance streamfunction coupled to a linearized energy equation is derived and solved using a spectral collocation method. Our results indicate that increasing the dimensionless top wall temperature has a non-monotonic effect on the linear stability characteristics. We also found that increasing the thermal conductivity and density ratios stabilise the flow for the set of parameter values considered; the viscosity ratio has a non-monotonic effect on the maximal growth rate. An energy ‘budget’ analysis shows that the most dangerous mode is of ‘interfacial’ type.

Liner-on-plasma system near stagnation: Stabilizing effect of a magnetic cushion

This brief communication is concerned with the adiabatic compression of a high-beta plasma by a heavy liner. Elongated cylindrical and quasi-cylindrical geometries are considered. The magnetic field in a plasma is parallel to the axis, whereas the drive field has azimuthal direction. During the liner acceleration, the most dangerous modes are axisymmetric (m = 0) modes. Near stagnation, these modes are further amplified at the inner surface, as the liner is decelerated by the isotropic pressure of a high-beta plasma. This picture, however, is not complete: due to a heat loss from the plasma core to the relatively cold liner, a zone of a strong axial magnetic field may appear between a hot, high-beta plasma and a cold liner. This magnetic cushion is backed from inside by a very high-beta plasma. The stability of such a system with respect to m = 0 modes is studied and the conclusion is drawn that the stabilizing effect of the magnetic cushion remains strong even for relatively thin cushions and moderate magnetic fields in them.

Influence of higher order modes on the beam stability in the high power superconducting proton linac

Higher order modes (HOMs) can severely limit the operation of superconducting cavities in a linear accelerator with high beam current, high duty factor, and complex pulse structure. The full HOM spectrum has to be analyzed in order to identify potentially dangerous modes already during the design phase and to define their damping requirements. For this purpose a dedicated beam simulation code simulation of higher order mode dynamics (SMD) focused on beam-HOM interaction was developed, taking into account important effects like the HOM frequency spread, beam input jitter, different chopping patterns, as well as klystron and alignment errors. Here, SMD is used to investigate the influence of HOMs in detail in the superconducting proton linac at CERN and their potential to drive beam instabilities in the longitudinal and transverse plane.

Interfacial instability in turbulent flow over a liquid film in a channel

We revisit the stability of a deformable interface that separates a fully-developed turbulent gas flow from a thin layer of laminar liquid. Although this problem has received considerable attention previously, a model that requires no fitting parameters and that uses a base-state profile that has been validated against experiments is, as yet, unavailable. Furthermore, the significance of wave-induced perturbations in turbulent stresses remains unclear. To address these outstanding issues, we investigate this problem and introduce a turbulent base-state velocity that requires specification of a flow rate or a pressure drop only; no adjustable parameters are necessary. This base state is validated extensively against available experimental data as well as the results of direct numerical simulations. In addition, the effect of perturbations in the turbulent stress distributions is investigated, and demonstrated to be small for cases wherein the liquid layer is thin. The detailed modelling of the liquid layer also elicits two unstable modes, ‘interfacial’ and ‘internal’, with the former being the more dominant of the two. We show that it is possible for interfacial roughness to reduce the growth rate of the interfacial mode in relation to that of the internal one, promoting the latter, to the status of most dangerous mode. Additionally, we introduce an approximate measure to distinguish between ‘slow’ and ‘fast’ waves, the latter being the case for ‘critical-layer’-induced instabilities; we demonstrate that for the parameter ranges studied, the large majority of the waves are ‘slow’. Finally, comparisons of our linear stability predictions are made with experimental data in terms of critical parameters for onset of wave-formation, wave speeds and wavelengths; these yield agreement within the bounds of experimental error.

Pedestrian Safety Programs in Centers of Large Cities: Institutional Settings and Identified Barriers

Although pedestrian fatalities in the United States have decreased, walking remains one of the more dangerous travel modes. Concern for pedestrian safety is likely to be felt more strongly in the centers of large cities (defined as cities with populations of greater than 500,000) than in other, smaller cities. The needs and the issues in the centers of large cities need to be understood when planning officials develop and implement pedestrian safety programs. Planning officials from 13 large cities (Chicago, Illinois; Columbus, Ohio; Denver, Colorado; Indianapolis, Indiana; Los Angeles, California; Milwaukee, Wisconsin; New York; Philadelphia, Pennsylvania; Phoenix, Arizona; San Francisco and San Jose, California; Seattle, Washington; and Washington, D.C.) were interviewed to discuss institutional settings, interagency collaboration, high-risk population groups, and institutional barriers. Results revealed that collaboration of multiple stakeholders was typical, but several barriers needed to be addressed to further improve pedestrian safety. The barriers included competing priorities between agencies, lack of resources, auto-dependent culture, and data gaps. More holistic approaches are needed as safety countermeasures alone may not always be sufficient for the enhancement of pedestrian safety. Often, pedestrian safety is related to pedestrians’ socioeconomic status, physical environment, and even adverse local issues. Thus, participation and formalized policy integration among multiple parties are required to create a win–win safety policy that can be built on the basis of frequent dialogue, information sharing, and identification of shared goals and objectives.