1,721,000 research outputs found
Capillary instability on a hydrophilic stripe
No full textA recent experiment showed that cylindrical segments of water filling a hydrophilic stripe on an otherwise hydrophobic surface display a capillary instability when their volume is increased beyond the critical volume at which their apparent contact angle on the surface reaches 90° (Gau et al 1999 Science 283 46–9). Surprisingly, the fluid segments did not break up into droplets—as would be expected for a classical Rayleigh–Plateau instability—but instead displayed a long-wavelength instability where all excess fluid gathered in a single bulge along each stripe. We consider here the dynamics of the flow instability associated with this setup. We perform a linear stability analysis of the capillary flow problem in the inviscid limit. We first confirm previous work showing that all cylindrical segments are linearly unstable if (and only if) their apparent contact angle is larger than 90°. We then demonstrate that the most unstable wavenumber for the surface perturbation decreases to zero as the apparent contact angle of the fluid on the surface approaches 90°, allowing us to re-interpret the creation of bulges in the experiment as a zero-wavenumber capillary instability. A variation of the stability calculation is also considered for the case of a hydrophilic stripe located on a wedge-like geometry.National Science Foundation (U.S.) (grant CTS-0624830)National Science Foundation (U.S.) (grant CBET-0746285
Balanced Splitting and Rebalanced Splitting
No full textMany systems of equations fit naturally in the form . We may separate convection from diffusion, -derivatives from -derivatives, and (especially) linear from nonlinear. We alternate between integrating operators for and . Noncommutativity (in the simplest case, of and ) introduces a splitting error which persists even in the steady state. Second-order accuracy can be obtained by placing the step for between two half-steps of . This splitting method is popular, and we suggest a possible improvement, especially for problems that converge to a steady state. Our idea is to adjust the splitting at each timestep to . We introduce two methods, balanced splitting and rebalanced splitting, for choosing the constant . The execution of these methods is straightforward, but the stability analysis becomes more difficult than for . Experiments with the proposed rebalanced splitting method indicate that it is much more accurate than conventional splitting methods as systems approach steady state. This should be useful in large-scale simulations (e.g., reacting flows). Further exploration may suggest other choices for which work well for different problems.United States. Dept. of Energy. Office of Basic Energy Sciences (Division of Chemical Sciences, Geosciences, and Biosciences Contract DE-FG02-98ER14914)Fulbright ProgramMIT Energy Initiative (Fellowship)National Science Foundation (U.S.) (Grant 1023152
Impact of biofuels on contrail warming
No full textContrails and contrail-cirrus may be the largest source of radiative forcing (RF) attributable to aviation. Biomass-derived alternative jet fuels are a potentially major way to mitigate the climate impacts of aviation by reducing lifecycle CO2 emissions. Given the up to 90% reduction in soot emissions from paraffinic biofuels, the potential for a significant impact on contrail RF due to the reduction in contrail-forming ice nuclei (IN) remains an open question. We simulate contrail formation and evolution to quantify RF over the United States under different emissions scenarios. Replacing conventional jet fuels with paraffinic biofuels generates two competing effects. First, the higher water emissions index results in an increase in contrail occurrence (~ +8%). On the other hand, these contrails are composed of larger diameter crystals (~ +58%) at lower number concentrations (~ −75%), reducing both contrail optical depth (~ −29%) and albedo (~ −32%). The net changes in contrail RF induced by switching to biofuels range from −4% to +18% among a range of assumed ice crystal habits (shapes). In comparison, cleaner burning engines (with no increase in water emissions index) result in changes to net contrail RF ranging between −13% and +5% depending on habit. Thus, we find that even 67% to 75% reductions in aircraft soot emissions are insufficient to substantially reduce warming from contrails, and that the use of biofuels may either increase or decrease contrail warming—contrary to previous expectations of a significant decrease in warming.United States. National Aeronautics and Space Administration (cooperative agreement NNX14AT22A
Examining flow-flame interaction and the characteristic stretch rate in vortex-driven combustion dynamics using PIV and numerical simulation
Full text linkIn this paper, we experimentally investigate the combustion dynamics in lean premixed flames in a laboratory scale backward-facing step combustor in which flame-vortex driven dynamics are observed. A series of tests was conducted using propane/hydrogen/air mixtures for various mixture compositions at the inlet temperature ranging from 300 K to 500 K and at atmospheric pressure. Pressure measurements and high speed particle image velocimetry (PIV) are used to generate pressure response curves and phase-averaged vorticity and streamlines as well as the instantaneous flame front, respectively, which describe unsteady flame and flow dynamics in each operating regime. This work was motivated in part by our earlier study where we showed that the strained flame consumption speed S[subscript c] can be used to collapse the pressure response curves over a wide range of operating conditions. In previous studies, the stretch rate at which S[subscript c] was computed was determined by trial and error. In this study, flame stretch is estimated using the instantaneous flame front and velocity field from the PIV measurement. Independently, we also use computed strained flame speed and the experimental data to determine the characteristic values of stretch rate near the mode transition points at which the flame configuration changes. We show that a common value of the characteristic stretch rate exists across all the flame configurations. The consumption speed computed at the characteristic stretch rate captures the impact of different operating parameters on the combustor dynamics. These results suggest that the unsteady interactions between the turbulent flow and the flame dynamics can be encapsulated in the characteristic stretch rate, which governs the critical flame speed at the mode transitions and thereby plays an important role in determining the stability characteristics of the combustor.King Abdullah University of Science and Technology (Grant KUS-110-010-01
On the phase between pressure and heat release fluctuations for propane/hydrogen flames and its role in mode transitions
Full text linkThis paper presents an experimental investigation into mode-transitions observed in a 50-kW, atmospheric pressure, backward-facing step combustor burning lean premixed C[subscript 3]H[subscript 8]/H[subscript 2] fuel mixtures over a range of equivalence ratios, fuel compositions and preheat temperatures. The combustor exhibits distinct acoustic response and dynamic flame shape (collectively referred to as “dynamic modes”) depending on the operating conditions. We simultaneously measure the dynamic pressure and flame chemiluminescence to examine the phase between pressure (p′) and heat release fluctuations (q′) in the observed dynamic modes. Results show that the heat release is in phase with the pressure oscillations (θ[subscript qp] ≈ 0) at the onset of a dynamic mode, while as the operating conditions change within the mode, the phase grows until it reaches a critical value θ[subscript qp] = θ[subscript c], at which the combustor switches to another dynamic mode. According to the classical Rayleigh criterion, this critical phase (θ[subscript c]) should be π/2, whereas our data show that the transition occurs well below this value. A linear acoustic energy balance shows that this critical phase marks the point where acoustic losses across the system boundaries equal the energy addition from the combustion process to the acoustic field. Based on the extended Rayleigh criterion in which the acoustic energy fluxes through the system boundaries as well as the typical Rayleigh source term (p′q′) are included, we derive an extended Rayleigh index defined as R[subscript e] = θ[subscript qp]/θ[subscript c], which varies between 0 and 1. This index, plotted against a density-weighted strained consumption speed, indicates that the impact of the operating parameters on the dynamic mode selection of the combustor collapses onto a family of curves, which quantify the state of the combustor within a dynamic mode. At R[subscript e] = 0, the combustor enters a mode, and switches to another as R[subscript e] approaches 1. The results provide a metric for quantifying the instability margins of fuel-flexible combustors operating at a wide range of conditions.King Abdullah University of Science and Technology (Grant KUS-110-010-01
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Ember: a quasi-one-dimensional, unsteady flame solver
No full text<p>Ember is a quasi-one-dimensional, unsteady reacting flow solver. It can be used to simulate a number of fundamental flame configurations, including premixed laminar flames, opposed flow strained flames (premixed or diffusion), axisymmetric (tubular) flames with positive or negative curvature, and steady 2D flames in a prescribed velocity field (using the method of lines).</p>
<p>Ember integrates the governing equations using a variant on the standard Strang splitting method called <em>rebalanced splitting</em>, which eliminates steady-state errors.</p>
<p>Ember’s solver is implemented primarily in C++, which is made accessible to the user as a Python module to enable script-driven input files and provide extensibility through user-supplied functions. Ember is parallelized using the Intel TBB library to take advantage of modern multi-core processors.</p>
Public health impacts of excess NOx emissions from Volkswagen diesel passenger vehicles in Germany
No full textIn September 2015, the Volkswagen Group (VW) admitted the use of 'defeat devices' designed to lower emissions measured during VW vehicle testing for regulatory purposes. Globally, 11 million cars sold between 2008 and 2015 are affected, including about 2.6 million in Germany. On-road emissions tests have yielded mean on-road NO[subscript x] emissions for these cars of 0.85 g km[superscript −1], over four times the applicable European limit of 0.18 g km[superscript −1]. This study estimates the human health impacts and costs associated with excess emissions from VW cars driven in Germany. A distribution of on-road emissions factors is derived from existing measurements and combined with sales data and a vehicle fleet model to estimate total excess NO[subscript x] emissions. These emissions are distributed on a 25 by 28 km grid covering Europe, using the German Federal Environmental Protection Agency's (UBA) estimate of the spatial distribution of NO[subscript x] emissions from passenger cars in Germany. We use the GEOS-Chem chemistry-transport model to predict the corresponding increase in population exposure to fine particulate matter and ozone in the European Union, Switzerland, and Norway, and a set of concentration-response functions to estimate mortality outcomes in terms of early deaths and of life-years lost. Integrated over the sales period (2008–2015), we estimate median mortality impacts from VW excess emissions in Germany to be 1200 premature deaths in Europe, corresponding to 13 000 life-years lost and 1.9 billion EUR in costs associated with life-years lost. Approximately 60% of mortality costs occur outside Germany. For the current fleet, we estimate that if on-road emissions for all affected VW vehicles in Germany are reduced to the applicable European emission standard by the end of 2017, this would avert 29 000 life-years lost and 4.1 billion 2015 EUR in health costs (median estimates) relative to a counterfactual case with no recall.Germany. Umweltbundesamt (UBA
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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