1,721,143 research outputs found
Slip velocity of large neutrally buoyant particles in turbulent flows
Full text linkWe discuss possible definitions for a stochastic slip velocity that describes the relative motion between large particles and a turbulent flow. This definition is necessary because the slip velocity used in the standard drag model fails when particle size falls within the inertial subrange of ambient turbulence. We propose two definitions, selected in part due to their simplicity: they do not require filtration of the fluid phase velocity field, nor do they require the construction of conditional averages on particle locations. A key benefit of this simplicity is that the stochastic slip velocity proposed here can be calculated equally well for laboratory, field and numerical experiments. The stochastic slip velocity allows the definition of a Reynolds number that should indicate whether large particles in turbulent flow behave (a) as passive tracers; (b) as a linear filter of the velocity field; or (c) as a nonlinear filter to the velocity field. We calculate the value of stochastic slip for ellipsoidal and spherical particles (the size of the Taylor microscale) measured in laboratory homogeneous isotropic turbulence. The resulting Reynolds number is significantly higher than 1 for both particle shapes, and velocity statistics show that particle motion is a complex nonlinear function of the fluid velocity. We further investigate the nonlinear relationship by comparing the probability distribution of fluctuating velocities for particle and fluid phases
Numerical Assessment of a New Hydro-Pneumatic Suspension System for Motorcycles
Full text linkThis work investigates an innovative hydro-pneumatic suspension system that was recently developed for possibly enhancing the performance of wheeled road vehicles, particularly motorcycles. The device exhibits a highly non-linear behavior, and in particular a strongly regressive spring rate, due to its architecture (featuring a hydropneumatic spring and a coil spring arranged in series). Multibody models are implemented and simulated for a preliminary assessment of its actual benefits and/or drawbacks with respect to common suspension systems. As a starting point of the research, this work focuses on the in-plane dynamics, and in particular on a condition of straight running at constant speed. The load fluctuations on the rear wheel (grip) as well as the velocity of the suspended mass (comfort) under different road inputs at various speeds are analyzed. The numerical results confirm the hydro-pneumatic suspension system as a promising device
Positron emission tomography in ARDS : a new look at an old syndrome
No full textThis is a review of some of the main findings obtained by positron emission tomography (PET) concerning the pathophysiology of acute respiratory distress syndrome (ARDS) and acute lung injury. PET (which is nowadays often combined with computed tomography) is a functional imaging technique based on the detection of a labeled molecule administered to a subject. Based on the molecule used, different lung functions can be imaged. Examples include inhaled, labeled nitrogen, which allows us to visualize regional aeration and ventilation, whereas lung perfusion has been studied by means of labeled water or by injected nitrogen dissolved in saline. With this latter technique, a global assessment of regional gas exchange is possible. Administration of [18F]FDG facilitates the imaging of cellular metabolic activity, reflecting an acute neutrophil-sustained inflammatory process. This technique has been used in experimental ARDS and, more recently, in patients. It showed, for example, that inflammatory activity of the lungs is markedly increased even in "normally aerated" regions at levels that are, in some cases, even higher than in the non-aerated regions
Assessing effort and work of breathing
No full textPurpose of review To discuss the role of measuring functional residual capacity (FRC) during mechanical ventilation to improve patient ventilator settings in order to prevent ventilator-induced lung injury. Recent findings Nowadays, FRC can be measured without the use of tracer gases and without disconnection from the ventilator. It is shown that FRC can provide additional information to optimize the ventilator setting; for example, FRC measurements can differentiate between responders and nonresponders after a recruitment maneuver, and in combination with dynamic compliance one can differentiate between recruitment and overdistention during a positive end-expiratory pressure trial. In addition, FRC measurements enable not only to estimate stress and strain at the bedside, but also to estimate ventilation inhomogeneity. In conclusion, measuring FRC could be extremely valuable during mechanical ventilation, but clinical studies are needed to prove whether this technique will improve outcome
In reply : A High Significance Level after Analysis of Covariance in a Small-group Study?
No full textLung imaging during acute respiratory distress syndrome : CT- and PET-scanning
Full text linkIn this article we review some of the main findings obtained by Computed Tomography (CT) and Positron Emission Tomography (PET) concerning the pathophysiology of Acute Respiratory Distress Syndrome (ARDS). CT is based on the detection of physical densities, allowing measurement of regional aeration. Beginning from the revolutionary concept of the " baby lung" , CT findings later led to a further evolution of the ARDS lung model, showing that the lungs behave as a wet " sponge" , with the dependent regions collapsing under the superimposed weight. More recently, CT scan has been used to quantify the potential for lung recruitment, showing that this is extremely heterogeneous among ARDS patient and tightly linked to mortality. PET (possibly combined with CT) is a functional imaging technique, based on the detection of a labeled molecule administered to a subject. Based on which molecule is used, different functions can be imaged. In the course of experimental ARDS injection of [13N]N2- labeled saline has been used to image regional gas exchange. Administration of [18F]FDG allows the image of cellular metabolic activity, reflecting neutrophils activation during inflammation. This technique has been applied in experimental ARDS and, more recently, in patients showing, for example, that inflammatory activity of the lungs is markedly increased also in " normally aerated" regions and, in some cases, even higher than in the non-aerated ones. We will present here some of the findings obtained by the two techniques in the clinical setting of ARDS, also discussing some of their possible future application
Airway pressure and flow monitoring
No full textPURPOSE OF REVIEW: We report on the evolution of airway pressure and flow monitoring from a pathophysiological tool to the cornerstone of ventilator-induced lung injury (VILI) prevention. RECENT FINDINGS: Protective ventilatory strategies are based on reduction of volume and pressures delivered to the lungs. New evidence, which will need confirmation in further studies, suggests that transpulmonary pressure (alveolar pressure minus pleural pressure), could be used to titrate both the positive end-expiratory pressure (PEEP) level and the inspiratory pressure applied by the ventilator. A limited number of animal studies are strongly supporting a role for inspiratory flow on the development of VILI.Moreover, different airway flow patterns may affect secretion movement, both global, to the alveoli or the glottis, and regional, from lower to higher compliance regions. This intra-lung transfer may be a primary mechanism for the propagation of infections and inflammatory mediators.Alternative monitoring techniques (among others) are the rapid interrupter technique, which can be used to measure airway resistance and patients' inspiratory effort and the forced oscillation technique which could become a bedside technique to estimate recruitment/derecruitment and titrate PEEP. SUMMARY: Airway pressure and flow monitoring is essential for VILI prevention and for an appropriate setting of mechanical ventilation
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