Problem Of Particular Interest Research Articles

DOTAP: Structure, hydration, and the counterion effect

The cationic lipid 1,2-dioleoyl-3-trimethylammonium propane (DOTAP) is one of the original synthetic cationic lipids used for the liposomal transfection of oligonucleotides in gene therapy. The key structural element of DOTAP is its quaternary ammonium headgroup that is responsible for interactions with both nucleic acids and target cell membranes. Because these interactions are fundamental to the design of a major class of transfection lipids, it is important to understand the structure of DOTAP and how it interacts with halide counterions. Here, we use x-ray and neutron diffraction techniques to examine the structure of DOTAP and how chloride (Cl−) and iodide (I−) counterions alter the hydration properties of the DOTAP headgroup. A problem of particular interest is the poor solubility of DOTAP/I− in water solutions. Our results show that the poor solubility results from very tight binding of the I− counterion to the headgroup and the consequent expulsion of water. The structural principles we report here are important for assessing the suitability of DOTAP and its quaternary ammonium derivatives for transfection.

In vitro parasiticidal effect of Plantago barbata (Plantaginaceae) and Empetrum rubrum (Empetraceae) on Leishmania amazonensis

Emerging and re-emerging diseases are a problem of particular interest to the scientific communities.

A Numerical Investigation of the Influence of Geometric Parameters on the Performance of a Multi-Channel Confluent Water Supply

Transportation efficiency is a problem of particular interest in multi-channel confluent water supply engineering. Transportation efficiency depends not only on the system control strategy but also on the pressure loss (pressure difference between the inlet and outlet) and pressure drop (amplitude of outlet pressure fluctuations) of its structure. In this article, sensitivity analyses of the pressure loss and pressure drop to changes in multi-channel confluent water supply geometry are presented. An experimental set-up was established to validate computational fluid dynamic (CFD) predictions and obtain the boundary conditions for two-channel synchronous switching. The influences of the geometric structure varies by the clustered pipe diameter (40 mm < Dc < 80 mm), main pipe diameter (30 mm < Do < 80 mm), channel pitch (60 mm < L < 400 mm) and number of channels (2 ≤ n ≤ 4); those variables were investigated with the help of CFD simulations. The results showed that configuration “C” can be considered a costless method of decreasing pressure loss (βC(2.05) < βA(2.42) < βB(2.64)) and that the different configurations are insensitive to pressure drop. The variations of the influence of channel pitch and clustered pipe diameter on pressure loss have extremes at L/d = 5 and Dc/d = 2.5, respectively, but the effect on pressure drop is not obvious. The main pipe diameter and the inlet velocity have more significant influences on efficiency. The results can be used to choose the proper geometry of multi-channel confluent water supply to enable energy savings.

Cooperator-driven and defector-driven punishments: How do they influence cooperation?

Economic studies have shown that there are two types of regulation schemes which can be considered as a vital part of today's global economy: self-regulation enforced by self-regulation organizations to govern industry practices and government regulation which is considered as another scheme to sustain corporate adherence. An outstanding problem of particular interest is to understand quantitatively the role of these regulation schemes in evolutionary dynamics. Typically, punishment usually occurs for enforcement of regulations. Taking into account both types of punishments to influence the regulations, we develop a game model where six evolutionary situations with corresponding combinations of strategies are considered. Furthermore, a semianalytical method is developed to allow us to give accurate estimations of the boundaries between the phases of full defection and nondefection. We find that, associated with the evolutionary dynamics, for an infinite well-mixed population, the mix of both punishments performs better than one punishment alone in promoting public cooperation, but for a networked population the cooperator-driven punishment turns out to be a better choice. We also reveal the monotonic facilitating effects of the synergy effect, punishment fine, and feedback sensitivity on the public cooperation for an infinite well-mixed population. Conversely, for a networked population an optimal intermediate range of feedback sensitivity is needed to best promote punishers' populations. Overall, a networked structure is overall more favorable for punishers and further for public cooperation, because of both network reciprocity and mutualism between punishers and cooperators who do not punish defectors. We provide physical understandings of the observed phenomena, through a detailed statistical analysis of frequencies of different strategies and spatial pattern formations in different evolution situations. These results provide valuable insights into how to select and enforce suitable regulation measures to let public cooperation remain prevalent, which has potential implications not only for self-regulation, but also for other topics in economics and social science.

Numerical Study on the Effect of Distribution Plates in the Manifolds on the Flow Distribution and Thermal Performance of a Flat Plate Solar Collector

Flow maldistribution represents a problem of particular interest in the engineering field for several thermal systems. In flat plate solar collectors, the thermal efficiency strongly depends on the flow distribution through the riser tubes, where a uniform distribution causes a uniform temperature distribution and therefore a higher efficiency. In this work, a Computational Fluid Dynamics (CFD) numerical analysis has been carried out using the commercial software FLUENT®, in order to determine the flow distribution, pressure drop and hence the thermal efficiency of a solar collector with distribution flow plates inside the manifolds. The obtained numerical solution for this type of thermal systems has been validated with experimental results available in literature for laminar and turbulent flow. Four distribution plate configurations were analyzed. Results show that using two distribution plates in each of both manifolds improves the flow uniformity up to 40% under the same operating conditions when distribution plates are not used. Besides, it is shown that there exists an increase in the overall pressure drop which is practically negligible for the tilt angles commonly employed in the installation of flat plate solar collectors in Mexico. The use of closed end distribution plates on the dividing and combining manifolds allows the thermal efficiency to become close to the ideal thermal efficiency which is obtained with a uniform flow distribution.

The fault ride through technologies for doubly fed induction generator wind turbines

The penetration of large-scale wind power generators to the grid has resulted in an increasing focus on the study on their fault ride through issue that is becoming imminent. The doubly fed induction generator-based wind turbine is still one of the most frequently adopted wind generation units in the market of the installed wind power worldwide due to its good behavior during normal grid conditions. However, due to the sensitivity of doubly fed induction generator to power disturbances, especially to voltage sags, the transient characteristic improvement of doubly fed induction generator during fault, thus meeting the fault ride through requirements, has become a problem of particular interest to wind turbine manufacturers. So, in this article, a comprehensive comparative study on the fault ride through techniques for doubly fed induction generator wind turbine is provided based to three broad subcategories: control algorithms, additional hardware circuit, and combined approaches. Furthermore, a discussion on the different characteristics of the fault ride through capability of these methods has been made, and a case study has been carried out to show the performance of different schemes. Finally, the article suggests a likely future outlook for the fault ride through technique for doubly fed induction generator wind turbines.

The geometry of fusion inspired channel design

This paper is motivated by the problem of integrating multiple sources of measurements. We consider two multiple-input–multiple-output (MIMO) channels, a primary channel and a secondary channel, with dependent input signals. The primary channel carries the signal of interest, and the secondary channel carries a signal that shares a joint distribution with the primary signal. The problem of particular interest is designing the secondary channel matrix, when the primary channel matrix is fixed. We formulate the problem as an optimization problem, in which the optimal secondary channel matrix maximizes an information-based criterion. An analytical solution is provided in a special case. Two fast-to-compute algorithms, one extrinsic and the other intrinsic, are proposed to approximate the optimal solutions in general cases. In particular, the intrinsic algorithm exploits the geometry of the unit sphere, a manifold embedded in Euclidean space. The performances of the proposed algorithms are examined through a simulation study. A discussion of the choice of dimension for the secondary channel is given.

ChIPnorm: A Statistical Method for Normalizing and Identifying Differential Regions in Histone Modification ChIP-seq Libraries

The advent of high-throughput technologies such as ChIP-seq has made possible the study of histone modifications. A problem of particular interest is the identification of regions of the genome where different cell types from the same organism exhibit different patterns of histone enrichment. This problem turns out to be surprisingly difficult, even in simple pairwise comparisons, because of the significant level of noise in ChIP-seq data. In this paper we propose a two-stage statistical method, called ChIPnorm, to normalize ChIP-seq data, and to find differential regions in the genome, given two libraries of histone modifications of different cell types. We show that the ChIPnorm method removes most of the noise and bias in the data and outperforms other normalization methods. We correlate the histone marks with gene expression data and confirm that histone modifications H3K27me3 and H3K4me3 act as respectively a repressor and an activator of genes. Compared to what was previously reported in the literature, we find that a substantially higher fraction of bivalent marks in ES cells for H3K27me3 and H3K4me3 move into a K27-only state. We find that most of the promoter regions in protein-coding genes have differential histone-modification sites. The software for this work can be downloaded from http://lcbb.epfl.ch/software.html.

Calculating a confidence interval on the sum of binned leakage

Calculating the expected number of misclassified outcomes is a standard problem of particular interest for rare-event searches. The Clopper–Pearson method allows calculation of classical confidence intervals on the amount of misclassification if data are all drawn from the same binomial probability distribution. However, data is often better described by breaking it up into several bins, each represented by a different binomial distribution. We describe and provide an algorithm for calculating a classical confidence interval on the expected total number of misclassified events from several bins, based on calibration data with the same probability of misclassification on a bin-by-bin basis. Our method avoids a computationally intensive multidimensional search by introducing a Lagrange multiplier and performing standard root finding. This method has only quadratic time complexity as the number of bins, and produces confidence intervals that are only slightly conservative.

Effects of water mist addition on kerosene pool fire

The use of water mist to extinguish fire is a problem of particular interest since the banning of halogen-based agents for environmental reasons. This interest is reflected in the large number of researches performed on the main fire-extinguishing mechanisms of water mist: heat extraction, oxygen displacement and attenuation of heat fluxes. In contrast, there are still little known about the chemical and some other aspects of water mist addition on the pool fire. In this paper, a phenomenological study was conducted of the effect of water mist addition on the kerosene pool fire through the measurement of the heat release rate, CO, CO2 and O2 species concentration in combustion. The experimental results show that there is a significant enhancement effect at the beginning stage of water mist addition. Then, the flame size was decreased abruptly. By physical suppression effect combined with chemical effect, the experiments’ results are explained especially. The study of effects of water mist on pool fire will be useful for optimizing designation of water mist fire-suppression system, improving the fire suppression efficiency and extending their application field.

2004 M6.0 Parkfield earthquake characterization using Time Reversal

Time reversal has proved to be a robust source location method in acoustics and is now being developed for a number of seismic applications. One problem of particular interest is locating sources where the signal-to-noise ratio is small. These include small earthquakes (<M5.5) or atypical seismic sources with a small seismic energy radiation (e.g., tremor, slow earthquakes). Time reversal has been shown to be very robust and work in the presence of poor data, low signal to noise ratio, etc. We present a prototype study showing the power of time reversal, using seismic data from the 2004 M6.0 Parkfield earthquake, which is the world's best recorded event to date and thus one of the most studied. The back-propagation of recorded seismic data in a 3D Earth velocity model is numerically carried out. We show that the reconstructed reverse wave-field exhibits clear focusing at the source point but also displays a four-lobe radiation pattern for each type of rebroadcast waves (body, surface), which is consistent with the known source mechanism: a right-lateral strike slip along the almost-vertical San Andrea fault.

The Sum of Distances Between Vertices of a Convex Polygon with Unit Perimeter

In the recent paper [1] the authors gave a very nice overview of many solved as well as unsolved extremal problems in plane Euclidean geometry. Among these problems we found the following isoperimetric problem of particular interest. Let n > 3, let xi, ... , xn be the (pairwise different) vertices of a convex polygon with unit perimeter in the Euclidean plane, and let Sn be the sum of the distances between these points, i.e.,

Chemical and physical effects of water vapor addition on diffusion flames

The use of water mist to extinguish fires is a problem of particular interest since the banning of halogen-based agents for environmental reasons. This interest is reflected in the large number of studies performed on the main mechanisms of extinguishment: heat extraction, oxygen displacement and attenuation of radiant heat fluxes. In contrast, little is still known about the chemical and some other aspects of water vapor addition in hydrocarbon diffusion flames. In this paper, a phenomenological study is conducted of the effect of water vapor addition, through the base of a small-scale heptane pool fire. Heptane, supported on a pool of liquid water, is burned as a pool flame while the water underneath is heated to boiling. This is a unique and original method in that boiling the water under the fuel is used for adding water vapor at the base of a diffusion flame where chemical reactions and air entrainment take place. This region is the most interesting regarding flame reactivity and soot formation and, in spite of the perturbing influence of the injected water vapor, the major characteristics of the flame are conserved. The procedure permits measurements of exhaust gas composition by means of a flow through system. Measurements of temperature, CO and CO 2 species concentrations, and monochromatic absorption coefficient which can be associated with soot concentration, are performed. It is seen that adding water vapor in such a way affects both physical phenomena and chemical reactions. That water vapor inhibits the soot formation and shifts CO to CO 2 is further confirmed by injection of an inert gas instead of water vapor. However, the resulting heat release is not efficient enough to counteract the cooling effect of water vapor and the temperature is significantly decreased.

Non‐parametric Curve Estimation by Wavelet Thresholding with Locally Stationary Errors

An important aspect in the modelling of biological phenomena in living organisms, whether the measurements are of blood pressure, enzyme levels, biomechanical movements or heartbeats, etc., is time variation in the data. Thus, the recovery of a ‘smooth’ regression or trend function from noisy time‐varying sampled data becomes a problem of particular interest. Here we use non‐linear wavelet thresholding to estimate a regression or a trend function in the presence of additive noise which, in contrast to most existing models, does not need to be stationary. (Here, non‐stationarity means that the spectral behaviour of the noise is allowed to change slowly over time). We develop a procedure to adapt existing threshold rules to such situations, e.g. that of a time‐varying variance in the errors. Moreover, in the model of curve estimation for functions belonging to a Besov class with locally stationary errors, we derive a near‐optimal rate for the ‐risk between the unknown function and our soft or hard threshold estimator, which holds in the general case of an error distribution with bounded cumulants. In the case of Gaussian errors, a lower bound on the asymptotic minimax rate in the wavelet coefficient domain is also obtained. Also it is argued that a stronger adaptivity result is possible by the use of a particular location and level dependent threshold obtained by minimizing Stein's unbiased estimate of the risk. In this respect, our work generalizes previous results, which cover the situation of correlated, but stationary errors. A natural application of our approach is the estimation of the trend function of non‐stationary time series under the model of local stationarity. The method is illustrated on both an interesting simulated example and a biostatistical data‐set, measurements of sheep luteinizing hormone, which exhibits a clear non‐stationarity in its variance.

Effect of the fuel boiling point on the boilover burning of liquid fuels spilled on water

The burnign of a liquid fuel floating on water is a problem of particular interest in fires resulting from accidental liquid fuel spills in open waters. Under certain conditions, the fuel burns in a disruptive fashion, known as boilover, which is caused by the boiling of the water underneath, and results in a sharp increase in burning rate and external radiation, and often in the explosive burning of the fuel. In this work, smallscale experiments were conducted to determine the effect of the fuel boiling point on the onset and characteristics of the boilover burning of a fuel spilled on water. To simplify the nature of the results, the fuel layer thickness and pool diameter were kept constant. Fuels tested ad boiling points ranging from 383 to 560 K, and included a heating oil, a crude oil, and five single-component fuels. Measurements were conducted on the liquid fuel and water temperature histories, with emphasis on the temperature at the fuel/water interface. These measurements were complemented with the simultaneous video recording of the bubble nucleation and dynamics at the fuel/water interface, and of the flame plume above the pool. The results of the study corroborate that boilover is due to the nucleation of the water at the fuel/water interface, and that, consequently, for boilover to occur the boiling point of the fuel must be at least above the saturation temperature of water. The boilover intensity increases as the boiling point of the fuel increases, primarily because the thickness of the fuel and superheated water layers is larger at the onset of boilover. Thicker fuel and superheated water layers result in a stronger and faster ejection of the fuel from the pan toward the flame, and consequently in a more explosive and hazardous boilover event.

Calculating decay heat removal rates via natural circulation along variable heat flux reactor fuel plates

A recently developed integral technique is applied to natural convection cooling along test reactor fuel plates. The technique is demonstrated for water and air flow. In the case of air flow, the process is characterized by a large temperature rise along the fuel channel, thereby rendering the commonly applied Boussinesq approximation invalid. This case is a heat transfer problem of particular interest in accident analyses such as determining the level of decay heat dissipation possible, without exceeding the melting temperature of the fuel, subsequent to a hypothetical loss of primary coolant.

Predicting the Likelihood to Vote in Pre-Election Polls

A problem of particular interest in pre-election polls is to predict the likelihood that a sampled individual, whether respondent or non-respondent, will vote. This can be especially difficult in state and local elections since voter turnout is low compared to national elections. We study this problem by means of validated pre-election polls of registered voters in a US city which were conducted for the 1988 Presidential and 1989 Virginia State Gubernatorial elections. Results indicate that respondents and 'refusals' are significantly more likely to vote than non-respondents who cannot be contacted, and that a respondent's self-described likelihood to vote is the best indicator that he will. A logit model is used to predict the likelihood that a registered voter will vote in the State election based on voting status in the Presidential election, age and sex. The presence of the variable sex in the model is explained by the high profile of the abortion issue in the campaign. 1 Background A problem of particular interest in pre-election polls is to predict the likelihood that a sampled individual will vote. Answers to questions aimed at assessing a respondent's likelihood to vote are used to determine what respondent base to use to predict the election outcome. Perry (1973, 1979) concluded that the answer to the question 'are you registered' is the best predictor followed by the respondent's self-described likelihood to vote. Bolstein (1989) obtained results consistent with this in a study restricted to registered voters. It is also of interest to study the likelihood of non-respondents to vote, both by type of non-respondent and in comparison with respondents. This can be important in projecting election results if the non-respondents have different voting patterns. In his study, Bolstein (1989) found no difference in the likelihood to vote among the respondents and those who refused to be interviewed, and that the likelihood to vote was significantly greater in these 'contact' groups than in the 'call-rule exhausted' group, and significantly greater in the combined 'refusal' and 'call-rule exhausted' groups than in the 'unavailable at time of interview' group. The problem of predicting whether an individual will vote can be especially difficult in state and local elections since voter turnout is low compared with national elections. The work of Traugott & Tucker (1984), in which they concluded that past voting behavior is a strong predictor of whether or not an individual will vote, suggests that modeling current voting status (will or will not vote in upcoming election) in terms of past voting status might be especially useful in predicting whether or not non-respondents will vote. In this paper we use a logit model to represent voting status in the 1989 Virginia Gubernatorial election as a function of voting status in the 1988 Presidential election, sex and age. The latter two variables are available on voter registration lists and hence are known for non- respondents as well as respondents. It is no surprise that age is a factor in voting status since it is well-known that voting is a habitual characteristic that requires time to take hold. Thus, Bolstein (1989) found that registered voters under 30 were significantly less likely to vote than those 30 or older in the 1988 election. In a study of spousal voter turnout in the 1984 election based on Current Population Survey data, Straits (1990) also found that the odds of voting increases with age. On the other hand, sex is a factor in our

Improvement on lidar data processing for stratospheric aerosol measurements

For lidar measurements of stratospheric aerosols; signal-induced noise (SIN) from a photomultiplier (PMT) has been a problem of particular interest. In this paper, we succeed in simulating lidar signals affected by the PMT, after finding a long tail with a decay time of ~200 micros in the PMT's response to an impulselike light exposure. The PMT studied was an RCA 8852. Computer simulation quantitatively revealed that the SIN caused by the delayed response became greater than the real signal at high altitudes. Based on the results of simulation, a proposal was made to find a practical method for identifying and removing the SIN from the actual lidar signals. In addition, an improved method for the lidar signal calibration was proposed by taking into account the systematic noise component, including background light as well as SIN, in formulating the clean air calibration (the matching method). Validity of the proposed methods was demonstrated by using them both with an actual lidar signal and a simulated lidar signal with SIN.

Graph theory in the study of metal cluster bonding topology: Applications to metal clusters having fused polyhedra

The energy levels in a delocalized two- or three-dimensional chemical structure are related to the eigenvalues of the graph representing the corresponding bonding topology. Such relatively crude but computationally undemanding graph theory-derived models provide a clear demonstration of the close relationship between two-dimensional aromatic systems such as benzene and three-dimensional aromatic systems such as deltahedral boranes, carboranes, and metal clusters. The basic building blocks for the three-dimensional aromatic systems are deltahedra, having no degree 3 vertices. Delocalized bonding in such systems having v vertices requires two electrons for a multicenter core bond as well as 2v electrons for pairwise surface bonding. A problem of particular interest is how metal cluster polyhedra can fuse together, leading ultimately to the infinite structures of the bulk metals. As a model for such processes the fusion of rhodium carbonyl octahedra is examined using graph theory-derived methods. These lead to reasonable electron-precise models for the bonding topologies in the “biphenyl analogue” [Rh12(CO)30]2−, the “naphthalene analogue” [Rh9(CO)19]3−, the “anthracene analogue” H2Rh12(CO)25, and the “perinaphthene analogue” [Rh11(CO)23]3−. Similar models can also be developed for clusters based on centered larger rhodium polyhedra as exemplified by the centered cuboctahedral clusters of the type [Rh13(CO)24H5–q]q− (q = 2, 3, 4) representing a fragment of the hexagonal close-packed metal structure.

Matrix and Network Models in Archaeology

An important part of an archaeologist's work is to assign dates to finds, or deposits. For example, the deposits may be graves in a prehistoric cemetery or strata in a trench dug at the site of an ancient settlement. The deposits may contain artifacts such as pottery, jewelry, or tools. Under very fortuitous circumstances, an inscription or other unambiguous characteristic of an artifact literally labels the deposit with a date. However, such luck is not common, and so archaeologists use mathematical and statistical techniques to help solve the problem of dating deposits. This article describes the fairly recent development of the use of matrices and networks to solve an ordering problem of particular interest to archaeologists called the seriation problem, and illustrates these techniques with two examples from the archaeological literature. These same ideas have been successfully applied in genetics, psychology, and management. Most of the work discussed is due to D. G. Kendall, E. M. Wilkinson, and G. Laporte, but the formulations and proofs supplied here are generally simpler than the original ones. An extensive survey of the problem from an archaeologist's point of view appears in [22], and an overview of the application of mathematics to archaeology can be found in [6], [11], and [3].