Numerical simulations of granular dam break: Comparison between discrete element, Navier-Stokes, and thin-layer models.

Granular flows occur in various contexts, including laboratory experiments, industrial processes, and natural geophysical flows. To investigate their dynamics, different kinds of physically based models have been developed. These models can be characterized by the length scale at which dynamic processes are described. Discrete models use a microscopic scale to individually model each grain, Navier-Stokes models use a mesoscopic scale to consider elementary volumes of grains, and thin-layer models use a macroscopic scale to model the dynamics of elementary columns of fluids. In each case, the derivation of the associated equations is well-known. However, few studies focus on the extent to which these modeling solutions yield mutually coherent results. In this article, we compare the simulations of a granular dam break on a horizontal or inclined planes for the discrete model convex optimization contact dynamics (COCD), the Navier-Stokes model Basilisk, and the thin-layer depth-averaged model SHALTOP. We show that, although all three models allow reproducing the temporal evolution of the free surface in the horizontal case (except for SHALTOP at the initiation), the modeled flow dynamics are significantly different, and, in particular, during the stopping phase. The stresses measured at the flow's bottom, reflecting the flow dynamics, are in relatively good agreement, but significant variations are obtained with the COCD model due to complex and fast-varying granular lattices. Similar conclusions are drawn using the same rheological parameters to model a granular dam break on an inclined plane. This comparison exercise is essential for assessing the limits and uncertainties of granular flow modeling.

Lung tissue biomechanics imaged with synchrotron phase contrast microtomography in live rats.

The magnitude and distribution of strain imposed on the peripheral airspaces by mechanical ventilation at the microscopic level and the consequent deformations are unknown despite their importance for understanding the mechanisms occurring at the onset of ventilator-induced lung injury. Here a 4-Dimensional (3D + time) image acquisition and processing technique is developed to assess pulmonary acinar biomechanics at microscopic resolution. Synchrotron radiation phase contrast CT with an isotropic voxel size of 6 µm3 is applied in live anesthetized rats under controlled mechanical ventilation. Video animations of regional acinar and vascular strain are acquired in vivo. Maps of strain distribution due to positive-pressure breaths and cardiovascular activity in lung acini and blood vessels are derived based on CT images. Regional strain within the lung peripheral airspaces takes average values of 0.09 ± 0.02. Fitting the expression S = kVn, to the changes in peripheral airspace area (S) and volume (V) during a positive pressure breath yields an exponent n = 0.82 ± 0.03, suggesting predominant alveolar expansion rather than ductal expansion or alveolar recruitment. We conclude that this methodology can be used to assess acinar conformational changes during positive pressure breaths in intact peripheral lung airspaces.

A 1D model of leukocyte adhesion coupling bond dynamics with blood velocity.

Cell adhesion on the vascular wall is a highly coupled process where blood flow and adhesion dynamics are closely linked. Cell dynamics in the vicinity of the vascular wall is driven mechanically by the competition between the drag force of the blood flow and the force exerted by the bonds created between the cell and the wall. Bonds exert a friction force. Here, we propose a mathematical model of such a competitive system, namely leukocytes whose capacity to create bonds with the vascular wall and transmigratory ability are coupled by integrins and chemokines. The model predicts that this coupling gives rise to a dichotomic cell dynamic, whereby cells switch from sliding to firm arrest, through non linear effects. Cells can then transmigrate through the wall. These predicted dynamic regimes are compared to in-vitro trajectories of leukocytes. We expect that competition between friction and drag force in particle dynamics (such as shear stress-controlled nanoparticle capture) can lead to similar dichotomic mode.

In vivo estimates of NO and CO conductance for haemoglobin and for lung transfer in humans.

Membrane conductance (Dm) and capillary lung volume (Vc) derived from NO and CO lung transfer measurements in humans depend on the blood conductance (θ) values of both gases. Many θ values have been proposed in the literature. In the present study, measurements of CO and NO transfer while breathing 15% or 21% O2 allowed the estimation of θNO and the calculation of the optimal equation relating 1/θCO to pulmonary capillary oxygen pressure (PcapO2). In 10 healthy subjects, the mean calculated θNO value was similar to the θNO value previously reported in the literature (4.5mmHgmin(-1)) provided that one among three θCO equations from the literature was chosen. Setting 1/θCO=a·PcapO2+b, optimal values of a and b could be chosen using two methods: 1) by minimizing the difference between Dm/Vc ratios for any PcapO2, 2) by establishing a linear equation relating a and b. Using these methods, we are proposing the equation 1/θCO=0.0062·PcapO2+1.16, which is similar to two equations previously reported in the literature. With this set of θ values, DmCO reached the morphometric range.

The restless mind while driving: drivers' thoughts behind the wheel.

UNLABELLED: Recent research has clearly shown that inattention when driving has an indisputable impact on road safety. "Mind wandering" (MW), an inattentional state caused by a shift in attention from the ongoing task to inner thoughts, is not only frequent in everyday activities but also known to impact performance. There is a growing body of research investigating the concept of MW, suggesting potential causes that could foster such a phenomenon. Only one epidemiological study has focused on this issue in a critical driving context (Galéra et al., 2012), and it revealed the harmful effects of MW in increasing the risk of a car crash. Experimental studies rather consider that driver would adduce in MW (Lemercier et al., 2014). When the driving context is too hard or the thought too difficult to proceed, driver reduced their MW. The aim of this paper is to examine this issue using the most recent trip of ordinary drivers whose MW state did not lead to a road accident. Using a questionnaire, information was collected about the participants' most recent trip as a driver, including: (1) personal characteristics, (2) context in which MW occurs, (3) awareness of MW episodes and finally (4) characteristics of the thoughts. RESULTS: revealed that MW affected 85.2% of the drivers, who spent on average 34.74% of their trip in a MW state. Moreover, we found that the contexts which favor MW are situations in which less of the driver's attention is needed to drive, such as familiar commutes, monotonous motorways or by-passes, or when drivers were alone in their cars. In these MW situations, the drivers quickly became aware of their MW episodes. Thoughts tend to involve neutral private concerns, related to present- or future-oriented content. Our findings suggest that MW is a functional state aiming to solve current problems. Future investigations should focus on this critical concept of MW when driving, both to identify safety issues and to provide suitable solutions for drivers subject to a wandering mind.

Distraction and driving: results from a case-control responsibility study of traffic crash injured drivers interviewed at the emergency room.

BACKGROUND: Use of cellular phones has been shown to be associated with crashes but many external distractions remain to be studied. OBJECTIVE: To assess the risk associated with diversion of attention due to unexpected events or secondary tasks at the wheel. DESIGN: Responsibility case-control study. SETTING: Adult emergency department of the Bordeaux University Hospital (France) from April 2010 to August 2011. PARTICIPANTS: 955 injured drivers presenting as a result of motor vehicle crash. MAIN OUTCOME MEASURES: The main outcome variable was responsibility for the crash. Exposures were external distraction, alcohol use, psychotropic medicine use, and sleep deprivation. Potential confounders were sociodemographic and crash characteristics. RESULTS: Beyond classical risk factor found to be associated with responsibility, results showed that distracting events inside the vehicle (picking up an object), distraction due to driver activity (smoking) and distracting events occurring outside were associated with an increased probability of being at fault. These distraction-related factors accounted for 8% of injurious road crashes. LIMITATIONS: Retrospective responsibility self-assessment. CONCLUSIONS: Diverted attention may carry more risk than expected. Our results are supporting recent research efforts to detect periods of driving vulnerability related to inattention.

Mind wandering and driving: responsibility case-control study.

OBJECTIVE: To assess the association between mind wandering (thinking unrelated to the task at hand) and the risk of being responsible for a motor vehicle crash. DESIGN: Responsibility case-control study. SETTING: Adult emergency department of a university hospital in France, April 2010 to August 2011. PARTICIPANTS: 955 drivers injured in a motor vehicle crash. MAIN OUTCOME MEASURES: Responsibility for the crash, mind wandering, external distraction, negative affect, alcohol use, psychotropic drug use, and sleep deprivation. Potential confounders were sociodemographic and crash characteristics. RESULTS: Intense mind wandering (highly disrupting/distracting content) was associated with responsibility for a traffic crash (17% (78 of 453 crashes in which the driver was thought to be responsible) v 9% (43 of 502 crashes in which the driver was not thought to be responsible); adjusted odds ratio 2.12, 95% confidence interval 1.37 to 3.28). CONCLUSIONS: Mind wandering while driving, by decoupling attention from visual and auditory perceptions, can jeopardise the ability of the driver to incorporate information from the environment, thereby threatening safety on the roads.