Random Sweep Research Articles

Space–time structure of weak magnetohydrodynamic turbulence

The two-time energy spectrum of weak magnetohydrodynamic turbulence is found by applying a wave-turbulence closure to the cumulant hierarchy constructed from the dynamical equations. Solutions are facilitated via asymptotic expansions in terms of the small parameter $\varepsilon$ , describing the ratio of time scales corresponding to Alfvénic propagation and nonlinear interactions between counter-propagating Alfvén waves. The strength of nonlinearity at a given spatial scale is further quantified by an integration over all possible delta-correlated modes compliant in a given set of three-wave interactions that are associated with energy flux through the said scale. The wave-turbulence closure for the two-time spectrum uncovers a secularity occurring on a time scale of order $\varepsilon ^{-2}$ , and the asymptotic expansion for the spectrum is reordered in a manner comparable to the one-time case. It is shown that for the regime of stationary turbulence, the two-time energy spectrum exponentially decays on a lagged time scale $(\varepsilon ^2 \gamma _k^s)^{-1}$ in proportion to the strength of the associated three-wave interactions, characterized by nonlinear decorrelation frequency $\gamma _k^s$ . The scaling of the form $k_{\perp } v_0 \chi _0$ exhibited by this frequency is reminiscent of random sweeping by the outer scale with characteristic fluctuation velocity $v_0$ that is modified due to competition with Alfvénic propagation (characterized by $\chi _0$ ) at the said scale. A brief calculation of frequency broadening of the power spectrum due to nonlinear interactions is also presented.

The production of uncertainty in three-dimensional Navier–Stokes turbulence

We derive the evolution equation of the average uncertainty energy for periodic/homogeneous incompressible Navier–Stokes turbulence and show that uncertainty is increased by strain rate compression and decreased by strain rate stretching. We use three different direct numerical simulations (DNS) of non-decaying periodic turbulence and identify a similarity regime where (a) the production and dissipation rates of uncertainty grow together in time, (b) the parts of the uncertainty production rate accountable to average strain rate properties on the one hand and fluctuating strain rate properties on the other also grow together in time, (c) the average uncertainty energies along the three different strain rate principal axes remain constant as a ratio of the total average uncertainty energy, (d) the uncertainty energy spectrum's evolution is self-similar if normalised by the uncertainty's average uncertainty energy and characteristic length and (e) the uncertainty production rate is extremely intermittent and skewed towards extreme compression events even though the most likely uncertainty production rate is zero. Properties (a), (b) and (c) imply that the average uncertainty energy grows exponentially in this similarity time range. The Lyapunov exponent depends on both the Kolmogorov time scale and the smallest Eulerian time scale, indicating a dependence on random large-scale sweeping of dissipative eddies. In the two DNS cases of statistically stationary turbulence, this exponential growth is followed by an exponential of exponential growth, which is, in turn, followed by a linear growth in the one DNS case where the Navier–Stokes forcing also produces uncertainty.

Level of Preparedness of the School Security Personnel and Their Qualifications Towards Institutional Security

School security is one of the prime concerns. It involves students, staff, and every other individual who is present at the premises. Hence, the duty of a school security guard is full of tough challenges. The guard must always stay alert and on his toes. This study determined the level of preparedness of the school security personnel and their qualifications towards institutional security. The respondents of the study include all thirteen (13) security guards in Misamis University Ozamiz City, Misamis Occidental, Philippines that serve as the respondents. The respondents are purposively chosen by the researchers because they are fit to answer the questionnaires provided by the researchers in determining the level of preparedness of the school security personnel and their qualifications towards institutional security. The security guard possessed very good qualifications in relation to their skills, knowledge, and work style, which resulted in a relatively high mean value. The security guard's level of preparedness in terms of operating school practices and programs garnered a moderate level of mean value, indicating that it still needs to be improved. It is evident that the relationship between the qualifications of school security personnel and their level of preparedness is statistically significant. Having a good qualification may positively contribute to a high-level preparedness among the school's security personnel. Most security guards were very efficient in fulfilling their duties due largely to their qualifications, which encompassed their skills, knowledge, and work style. They were able to maintain effective control over the conditions and individuals in their immediate vicinity. On the other hand, security guards thrived in the sector of level of preparedness. This is because they were highly prepared to respond with countermeasures and preventative actions in the case of a crime. It is essential that the performance of school security guards be constantly monitored and evaluated to determine the various strengths and weaknesses of everyone for future training and improvement. In dealing with the school's various problems, security guards are encouraged to maintain their high level of preparedness and qualification. It's also crucial to keep students from bringing weapons onto campus that may threaten other students or instructors, and to conduct random sweeps of students to look for contraband like as drugs and firearms.

A theoretical and empirical assessment of authoritarianism’s effects on behavior, attendance, and performance in urban school systems

ABSTRACT Authoritarian school safety measures have become commonplace in efforts to promote safety in today’s schools. This paper attempts to examine the effects of student engagement with authoritarianism in schools using self-report survey data from students (N= 359) across eight high schools in one large, urban school district in the greater New York City area. With consideration of various frameworks for conceptualizing authority in schools, authoritarianism in schools is operationalized by student self-report of engagement between two time points in (1) random sweeps for contraband; (2) surveillance cameras; (3) metal detector searches; and (4) interaction with authority (i.e., school police). Findings suggest those who have increased engagement with authoritarianism in schools throughout the year report increased occurrence of engagement in maladaptive behavior. Concurrently, exposure to authoritarianism over the course of one school year is associated with student behavior, academic performance, and attendance in ways that vary for different racial and ethnic groups. Findings call for educators and administrators to consider and continually assess the effects of authoritarian practices on student performance when developing and implementing safety protocol in their schools.

Short Term Traffic State Prediction via Hyperparameter Optimization Based Classifiers.

Short-term traffic state prediction has become an integral component of an advanced traveler information system (ATIS) in intelligent transportation systems (ITS). Accurate modeling and short-term traffic prediction are quite challenging due to its intricate characteristics, stochastic, and dynamic traffic processes. Existing works in this area follow different modeling approaches that are focused to fit speed, density, or the volume data. However, the accuracy of such modeling approaches has been frequently questioned, thereby traffic state prediction over the short-term from such methods inflicts an overfitting issue. We address this issue to accurately model short-term future traffic state prediction using state-of-the-art models via hyperparameter optimization. To do so, we focused on different machine learning classifiers such as local deep support vector machine (LD-SVM), decision jungles, multi-layers perceptron (MLP), and CN2 rule induction. Moreover, traffic states are evaluated using traffic attributes such as level of service (LOS) horizons and simple if–then rules at different time intervals. Our findings show that hyperparameter optimization via random sweep yielded superior results. The overall prediction performances obtained an average improvement by over 95%, such that the decision jungle and LD-SVM achieved an accuracy of 0.982 and 0.975, respectively. The experimental results show the robustness and superior performances of decision jungles (DJ) over other methods.

Damping enhancement of softmag material by vacuum annealing in vibration environment

In the present study, the effect of vacuum annealing process on the damping properties of the softmag material which is used aerospace applications was observed. The softmag (mu-metal) material was used in aerospace applications due to its high permeability to shield sensitive electronic equipment against electromagnetic radiations. The equipment that are used in aerospace applications should withstand heavy vibrations which are generally experienced during the flight. Random vibrations are experienced due to the thrust forces from fuel combustion and the turbines impart sinusoidal vibrations. The vibration response of the softmag metal box was studied by imparting both sinusoidal and random vibration sweeps of frequency range 20–2000 Hz at three differentaccelerations for each type of vibration. The vibration response was also studied after vacuum annealing the box at the same vibration conditions. The vibration amplitudes and forces of vacuum annealed box were reduced compared to that of softmag box without vacuum annealing. The damping of the box was increased by around 6%. This improvement in damping will have a significant effect on the life of the electronic equipment mounting structures.

DYNAMICAL FIELD LINE CONNECTIVITY IN MAGNETIC TURBULENCE

Point-to-point magnetic connectivity has a stochastic character whenever magnetic fluctuations cause a field line random walk, but this can also change due to dynamical activity. Comparing the instantaneous magnetic connectivity from the same point at two different times, we provide a nonperturbative analytic theory for the ensemble average perpendicular displacement of the magnetic field line, given the power spectrum of magnetic fluctuations. For simplicity, the theory is developed in the context of transverse turbulence, and is numerically evaluated for the noisy reduced MHD model. Our formalism accounts for the dynamical decorrelation of magnetic fluctuations due to wave propagation, local nonlinear distortion, random sweeping, and convection by a bulk wind flow relative to the observer. The diffusion coefficient DX of the time-differenced displacement becomes twice the usual field line diffusion coefficient Dx at large time displacement t or large distance z along the mean field (corresponding to a pair of uncorrelated random walks), though for a low Kubo number (in the quasilinear regime) it can oscillate at intermediate values of t and z. At high Kubo number the dynamical decorrelation decays mainly from the nonlinear term and DX tends monotonically toward 2Dx with increasing t and z. The formalism and results presented here are relevant to a variety of astrophysical processes, such as electron transport and heating patterns in coronal loops and the solar transition region, changing magnetic connection to particle sources near the Sun or at a planetary bow shock, and thickening of coronal hole boundaries.

Stochastic Quasi-Fejér Block-Coordinate Fixed Point Iterations with Random Sweeping

This work proposes block-coordinate fixed point algorithms with applications to nonlinear analysis and optimization in Hilbert spaces. The asymptotic analysis relies on a notion of stochastic quasi-Fejer monotonicity, which is thoroughly investigated. The iterative methods under consideration feature random sweeping rules to select arbitrarily the blocks of variables that are activated over the course of the iterations and they allow for stochastic errors in the evaluation of the operators. Algorithms using quasi-nonexpansive operators or compositions of averaged nonexpansive operators are constructed, and weak and strong convergence results are established for the sequences they generate. As a by-product, novel block-coordinate operator splitting methods are obtained for solving structured monotone inclusion and convex minimization problems. In particular, the proposed framework leads to random block-coordinate versions of the Douglas--Rachford and forward-backward algorithms and of some of their variant...

Students, Security, and Race

In the wake of the terrible shootings in Newtown, Connecticut, our nation has turned its attention to school security. For example, several states have passed or are considering passing legislation that will provide new funding to schools for security equipment and law enforcement officers. Strict security measures in schools are certainly not new. In response to prior acts of school violence, many public schools for years have relied on metal detectors, random sweeps, locked gates, surveillance cameras, and law enforcement officers to promote school safety. Before policymakers and school officials invest more money in strict security measures, this Article provides additional factors that should be considered. First, drawing on recent, restricted data from the U.S. Department of Education, this Article presents an original empirical analysis revealing that low-income students and minority students are much more likely to experience intense security conditions in their schools than other students, even when taking into account neighborhood crime, school crime, and school disorder. These findings raise concerns that such inequalities may continue or worsen as policymakers provide additional funding for security measures. Second, this Article argues that strict security measures do not support long-term solutions needed to effectively prevent school violence. Indeed, strict security measures exacerbate the underlying problems by creating barriers of adversity and mistrust between students and educators. In addition, this Article offers recommendations to address the disproportionate use of security measures on low-income and minority students and to curb violence more effectively. It urges school officials and policymakers to support programs that build trust and collective responsibility instead of providing grants for strict security measures. Further, it recommends that the Department of Education’s Office of Civil Rights play a more active role in addressing the disproportionate use of strict security measures on minority students.

Effects of anthropogenic disturbances on the diversity and composition of the butterfly fauna of sites in the Sango Bay and Iriiri areas, Uganda: implications for conservation

AbstractIn assessing environmental change, butterflies have been proven as replicable indicators of biodiversity and functional integrity that can be monitored at a range of scales. Butterflies have been identified as important bio‐indicators for assessing biodiversity and monitoring ecosystem responses to environmental perturbations. The objectives of this study were to determine changes in the butterfly fauna of differing sites in the Sango Bay Area (SBA) and Iriiri (Karamoja) in comparison with data collected 10 years ago, and to investigate the impact of different degrees of habitat disturbance on butterflies. The general butterfly diversity was determined by trapping and sweep netting along transect lines and by random sweeping. The impact of human‐induced disturbance was assessed by comparing species richness and composition between the sites and regressing the weighted disturbances against species diversity per site. There was a marked decrease in species diversity and varied species composition between the two studies and between the habitat types. Charcoal burning and grazing had significant negative correlations with diversity within forest sites (r2 = 0.825, P < 0.05), whereas cultivation and tree cutting/logging had significant negative correlations with diversity of open savannas (r2 = 0.718, P < 0.05 and r2 = 0.999, P < 0.05, respectively).

One Good Punch (review)

Reviewed by: One Good Punch Elizabeth Bush Wallace, Rich One Good Punch. Knopf, 2007114p Library ed. ISBN 978-0-375-91352-5$18.99 Trade ed. ISBN 978-0-375-81352-8$15.99 R Gr. 9-12 Michael Kerrigan knows what he needs to do to break free of life in Scranton, Pennsylvania—land a track scholarship for college, use his part-time job at the Scranton Observer (writing obituaries) to leverage his way into journalism, and generally avoid the senior-year antics that too often drag his teammates down. All his efforts unravel, though, over the course of a week when a random drug sweep turns up four joints in his locker, placed there by one-time grade school pal Joey Onager, who stashes books in Michael's locker between classes. Everyone advises Michael to simply tell the truth and rely on his clean record to convince the administration and police of his innocence, but he's reluctant to turn Joey in. For good reason—he had asked Joey to get him the joints in time for a weekend date with his maybe girlfriend, Shelly. He stubbornly resists telling the one small lie that will transfer all the blame to Joey, but the truth costs him his track career, his job, and his high-school diploma, and it may doom him to the future he most fears—becoming an unemployed loser in Scranton, PA. Wallace uses his limited page count effectively, cagily delaying the revelation that Michael isn't quite the innocent victim we first think him to be and crafting for Michael a narrative voice that reflects his obituary-writing style. The grave consequences that result from what appears to be a relatively minor misstep are chillingly convincing. Teens in the mood for a short but provocative title will realize a deliciously troubling payoff for a modest investment of reading time. Copyright © 2007 The Board of Trustees of the University of Illinois

Lagrangian intermittencies in dynamic and static turbulent velocity fields from direct numerical simulations

Three temporal velocity signals are analyzed from direct numerical simulations of the Navier–Stokes (N–S) equations. The three signals are: (i) the velocity of fluid particles transported by the time-evolving solution (Eulerian velocity field) of the N–S equations, referred to as the dynamic case; (ii) the velocity of fluid particles transported by a solution of the N–S equations at some fixed time, referred to as the static case; and (iii) the time evolution of the solution of the N–S equations at some fixed positions, referred to as the Eulerian case. The comparison of these three signals aims at elucidating the importance of the overall spacetime evolution of the flow on Lagrangian statistics. It is observed that the static case is, to some extent, similar to the Eulerian case; a feature that can be understood as an ergodicity property of homogeneous and isotropic turbulence and can be related to the process of random sweeping. The dynamic case is clearly different. It bears the signature of the time evolution of the flow. This study emphasizes the importance of the global dynamics of the flow and points out the existence of long-time correlations in the fluid-particle dynamics in the Lagrangian description.

Implementing componentwise Hastings algorithms

Markov chain Monte Carlo (MCMC) routines have revolutionized the application of Monte Carlo methods in statistical application and statistical computing methodology. The Hastings sampler, encompassing both the Gibbs and Metropolis samplers as special cases, is the most commonly applied MCMC algorithm. The performance of the Hastings sampler relies heavily on the choice of sweep strategy, that is, the method by which the components or blocks of the random variable X of interest are visited and updated, and the choice of proposal distribution, that is the distribution from which candidate variates are drawn for the accept–reject rule in each iteration of the algorithm. We focus on the random sweep strategy, where the components of X are updated in a random order, and random proposal distributions, where the proposal distribution is characterized by a randomly generated parameter. We develop an adaptive Hastings sampler which learns from and adapts to random variates generated during the algorithm towards choosing the optimal random sweep strategy and proposal distribution for the problem at hand. As part of the development, we prove convergence of the random variates to the distribution of interest and discuss practical implementations of the methods. We illustrate the results presented by applying the adaptive componentwise Hastings samplers developed to sample multivariate Gaussian target distributions and Bayesian frailty models.

Application of Fluid Modeling To Determine Threshold Leak Size for Liquid Foods

In this study, the mechanism by which a package defect converts to a leaker was examined in an effort to develop a relationship between threshold leak size and loss of package sterility. The threshold leak size is the hole size at which the onset of leakage occurs. The threshold pressure is the pressure required to initiate a leak. Leak initiation was studied in terms of the interaction between three components: liquid attributes of liquid food products, defect size, and pressures required to initiate liquid flow. Liquid surface tension, viscosity, and density values were obtained for 16 liquids. The imposed pressures required to initiate flow through microtubes with interior diameters of 0, 2, 5, 7, 10, 20, and 50μm were measured with the use of 63 test cells filled with safranin red dye, tryptic soy broth, and distilled water with surface tensions of 18.69, 44.09, and 64.67 mN/m, respectively. Significant differences (P < 0.05) between threshold pressures observed for safranin red dye, tryptic soy broth, and distilled water were found. Liquids with low surface tensions, such as safranin red dye, required significantly lower threshold imposed pressures than did liquids with high surface tensions, such as distilled water (P < 0.05). An equation to quantify the relationship between liquid surface tension, threshold imposed pressure, and defect size was developed. Threshold pressures observed were not significantly different (P > 0.05) from those predicted by the equation. Imposed pressures and vacuums generated within packages during random vibration and sweep resonance tests were measured for brick-style aseptic packages (250 ml), metal cans (76.2 by 114.3mm [425 ml]), 1-qt gable-top packages (946 ml), 0.5-gal gable-top packages (1.89 liters), and 1-gal milk jugs (4.25 liters). Significant differences between packages were found with respect to observed generated pressures during vibration testing (P < 0.05). An equation to calculate threshold size on the basis of liquid surface tension and imposed pressure was established.

Random-sweeping hypothesis for passive scalars in isotropic turbulence

The hypothesis of the small scales being passively swept along by the large-scale motions in turbulent flow is extended to passive scalars in isotropic turbulence. A theory based on strong mutual cancellation between local and advective derivatives and other assumptions is shown to capture the Reynolds and Schmidt number dependence of time scales characterizing Eulerian and Lagrangian rates of change. Agreement with direct numerical simulation data improves systematically with increasing Reynolds number. In accordance with the physics of random sweeping, the Eulerian frequency spectrum is very similar in shape to the wavenumber spectrum, but is broadened at higher frequencies compared to its Lagrangian counterpart. Overall the hypothesis appears to be even more valid for transported scalars than for the velocity field, which gives support to the use of Lagrangian approaches in the study of turbulent mixing.

Next-order structure-function equations

Kolmogorov’s equation [Dokl. Akad. Nauk SSSR 32, 16 (1941)] relates the two-point second- and third-order velocity structure functions and the energy dissipation rate. The analogous next higher-order two-point equation relates the third- and fourth-order velocity structure functions and the structure function of the product of pressure-gradient difference and two factors of velocity difference, denoted Tijk. The equation is simplified on the basis of local isotropy. Laboratory and numerical simulation data are used to evaluate and compare terms in the equation, examine the balance of the equation, and evaluate components of Tijk. Atmospheric surface-layer data are used to evaluate Tijk in the inertial range. Combined with the random sweeping hypothesis, the equation relates components of the fourth-order velocity structure function. Data show the resultant error of this application of random sweeping. The next-order equation constrains the relationships that have been suggested among components of the fourth-order velocity structure function. The pressure structure function, pressure-gradient correlation, and mean-squared pressure gradient are related to Tijk. Inertial range formulas are discussed.

Experimental characterization of nonlinear rubber isolators in a multi-degree-of-freedom system configuration

Experimental characterization has been investigated for three rubber isolators placed in a multi-degree-of-freedom system configuration. The configuration consisted of a rigid mass mounted to a flexible support beam via each of the rubber isolators. Random, sinusoidal, and sine sweep excitations were applied to the rigid mass and accelerations were measured. Data revealed the dependence of the isolators’ properties on amplitude and type of excitation. Modeling based on the continuous system theory resulted in quasi-linear representations of the experimental system. Discrete system models consisting of nonlinear polynomial equations for describing the nonlinearities of the isolators were also developed. In addition, sound pressure levels were measured during sine sweep excitation for one of the isolators. Increased sound levels were observed at off-resonant regions of the vibration response. Finally, single-degree-of-freedom static and dynamic experiments were conducted. Discrepancies in the isolators’ properties were found between the single- and multi-degree-of-freedom experimental results.

Information bounds for Gibbs samplers

If we wish to estimate efficiently the expectation of an arbitrary function on the basis of the output of a Gibbs sampler, which is better: deterministic or random sweep? In each case we calculate the asymptotic variance of the empirical estimator, the average of the function over the output, and determine the minimal asymptotic variance for estimators that use no information about the underlying distribution. The empirical estimator has noticeably smaller variance for deterministic sweep. The variance bound for random sweep is in general smaller than for deterministic sweep, but the two are equal if the target distribution is continuous. If the components of the target distribution are not strongly dependent, the empirical estimator is close to efficient under deterministic sweep, and its asymptotic variance approximately doubles under random sweep.

Numerical Simulation of Randomly Forced Turbulent Flows

Several authors have proposed studying randomly forced turbulent flows (e.g., E. A. Novikov,Soviet Physics JETP,20(5), 1290 1965). More recently, theoretical investigations (e.g., renormalization group) have focused on white-noise forced Navier–Stokes equations (V. Yakhot and S. A. Orszag,J. Sci. Comput.1(1), 3 1986). The present article aims to provide an appropriate numerical method for the simulation of randomly forced turbulent systems. The spatial discretization is based on the classical Fourier spectral method. The time integration is performed by a second-order Runge–Kutta scheme. The consistency of an extension of this scheme to treat additive noise stochastic differential equations is proved. The random number generator is based on lagged Fibonacci series. Results are presented for two randomly forced problems: the Burgers and the incompressible Navier–Stokes equations with a white-noise in time forcing term characterized by a power-law correlation function in spectral space. A variety of statistics are computed for both problems, including the structure functions. The third-order structure functions are compared with their exact expressions in the inertial subrange. The influence of the dissipation mechanism (viscous or hyperviscous) on the inertial subrange is discussed. In particular, probability density functions of velocity increments are computed for the Navier–Stokes simulation. Finally, for both Burgers and Navier–Stokes problems, our results support the view that random sweeping is the dominant effect of the large-scale motion on the small-scales.

Convergence properties of the Gibbs sampler for perturbations of Gaussians

The exact second eigenvalue of the Markov operator of the Gibbs sampler with random sweep strategy for Gaussian densities is calculated. A comparison lemma yields an upper bound on the second eigenvalue for bounded perturbations of Gaussians which is a significant improvement over previous bounds. For two-block Gibbs sampler algorithms with a perturbation of the form $\chi(g_1(x^{(1)}) + g_2(x^{(2)}))$ the derivative of the second eigenvalue of the algorithm is calculated exactly at $\chi = 0$, in terms of expectations of the Hessian matrices of $g_1$ and $g_2$.