Structure of unsteady stably stratified turbulence with mean shear.

The statistics of unsteady turbulence with uniform stratification N (Brunt–Väisälä frequency) and shear α(=dU1/dx3) are analysed over the entire time range (00 and \it Ri>0.25 respectively, oscillatory momentum and positive and negative density fluxes develop. Above a critical value of \it Ri\scriptsize\it crit(∼0.3), their average values are persistently countergradient. This structural change in the turbulence is the primary mechanism whereby stable stratification reduces the fluxes and the production of variances. It is quite universal and differs from the energy and stability mechanisms of Richardson (1926) and Taylor (1931). The long-time asymptotics of the energy ratio ER(=\it PE/VKE) of the potential energy to the vertical kinetic energy generally decreases with \it Ri(≥0.25), reaching the smallest value of 3/2 when there is no shear (\it Ri→∞). For strong mean shear (\it Ri<0.25), RDT significantly overestimates ER since (as in unstratified shear flow) it underestimates the vertical kinetic energy VKE. The RDT results show that the asymptotic values of the energy ratio ER and the normalized vertical density flux are independent of the initial value of ER, in agreement with DNS. This independence of the initial condition occurs because the ratios of the contributions from the initial values PE0 and KE0 are the same for PE and VKE and can be explained by the linear processes. Stable stratification generates buoyancy oscillations in the direction of the energy propagation of the internal gravity wave and suppresses the generation of turbulence by mean shear. Because the shear distorts the wavenumber fluctuations, the low-wavenumber spectrum of the vertical kinetic energy has the general form E33(k)∝(αtk)−1, where (LXαt)−1≪k≪L−1X (LX: integral scale). The viscous decay is controlled by the shear, so that the components of larger streamwise wavenumber k1 decay faster. Then, combined with the spectrum distortion by the shear, the energy and the flux are increasingly dominated by the small-k1 components as time elapses. They oscillate at the buoyancy period π/N because even in a shear flow the components as k1→0 are weakly affected by the shear. The effects of stratification N and shear α at small scales are to reduce both VKE and PE. Even for the same \it Ri, larger N and α reduce the high-wavenumber components of VKE and PE. This supports the applicability of the linear assumption for large N and α. At large scales, the stratification and shear effects oppose each other, i.e. both VKE and PE decrease due to the stratification but they increase due to the shear. We conclude that certain of these unsteady results can be applied directly to estimate the properties of sheared turbulence in a statistically steady state, but others can only be applied qualitatively

The role of dust in "active" and "passive" low-metallicity star formation

We investigate the role of dust in star formation activity of extremely metal-poor blue compact dwarf galaxies (BCDs). Observations suggest that star formation in BCDs occurs in two different regimes: "active" and "passive". The "active" BCDs host super star clusters (SSCs), and are characterised by compact size, rich H2 content, large dust optical depth, and high dust temperature; the "passive" BCDs are more diffuse with cooler dust, and lack SSCs and large amounts of H2. By treating physical processes concerning formation of stars and dust, we are able to simultaneously reproduce all the above properties of both modes of star formation (active and passive). We find that the difference between the two regimes can be understood through the variation of the "compactness" of the star-forming region: an "active" mode emerges if the region is compact (with radius \la 50 pc) and dense (with gas number density \ga 500 cm$^{-3}$). The dust, supplied from Type II supernovae in a compact star-forming region, effectively reprocesses the heating photons into the infrared and induces a rapid H2 formation over a period of several Myr. This explains the high infrared luminosity, high dust temperature, and large H2 content of active BCDs. Moreover, the gas in "active" galaxies cools (\la 300 K) on a few dynamical timescales, producing a "run-away" star formation episode because of the favourable (cool) conditions. The mild extinction and relatively low molecular content of passive BCDs can also be explained by the same model if we assume a diffuse region (with radius \ga 100 pc and gas number density \la 100 cm$^{-3}$). We finally discuss primordial star formation in high-redshift galaxies in the context of the "active" and "passive" star formation scenario.Comment: Astronomy and Astrophysics, in press, 16 pages, 8 figure

The size--density relation of extragalactic HII regions

We investigate the size--density relation in extragalactic HII regions, with the aim of understanding the role of dust and different physical conditions in the ionized medium. First, we compiled several observational data sets for Galactic and extragalactic HII regions and confirm that extragalactic HII regions follow the same size (D)--density (n) relation as Galactic ones. Motivated by the inability of static models to explain this, we then modelled the evolution of the size--density relation of HII regions by considering their star formation history, the effects of dust, and pressure-driven expansion. The results are compared with our sample data whose size and density span roughly six orders of magnitude. We find that the extragalactic size--density relation does not result from an evolutionary sequence of HII regions but rather reflects a sequence with different initial gas densities (``density hierarchy''). Moreover, the size of many HII regions is limited by dust absorption of ionizing photons, rather than consumption by ionizing neutral hydrogen. Dust extinction of ionizing photons is particularly severe over the entire lifetime of compact HII regions with typical gas densities of greater than 10^3 cm^{-3}. Hence, as long as the number of ionizing photons is used to trace massive star formation, much star-formation activity could be missed. Such compact dense environments, the ones most profoundly obscured by dust, have properties similar to ``maximum--intensity starbursts''. This implies that submillimeter and infrared wavelengths may be necessary to accurately assess star formation in these extreme conditions both locally and at high redshift.Comment: 18 pages, 11 figures, accepted for publication in Astronomy and Astrophysic

Three Systems of Workers' Compensation

Only three countries in the world maintain sub-national workers' compensation systems:Australia, Canada, and the United States. Three models are used to organize the insuranceresponsibilities for making the payments to injured or ill workers: private market, exclusivepublic insurer, and mixed (although the three models do not correspond exactly with the threecountries).All 10 Canadian provinces, 6 U.S. states, and 3 jurisdictions in Australia use the exclusive public insurer approach; the remaining 44 U.S. states and 4 jurisdictions in Australiause a predominately private market approach; and 3 Australian states use a mixed approach, in hich the public fund bears the underwriting risk, but private firms collect and disburse themoney.We will look at each of the three models, one from each country.1 Studying differentjurisdictions, even though from different nations, follows the tradition of using the "laboratory ofthe states" to inform policy decisions. We will describe the essential features of each system and then review system performance

The Michigan Disability Prevention Study: Research Highlights

This 3-year collaborative research project was designed to provide empirical evidence to substantiate the impact of various employer policies and practices on the prevention and management of workplace disability. It studied a random sample of 220 Michigan establishments with more than 100 employees from seven different industries who responded to a mail survey in the first half of 1991. The study correlates differences in employer-reported levels of achievement on policy and practice dimensions with performance on disability outcome measures, while controlling for a set of establishment characteristics in a multivariate regression analysis. There are three sets of policy and practice interventions evaluated here. First is safety intervention, that is, the attempt to prevent injuries from happening at all (measured as Safety Diligence, Ergonomic Solutions, and Safety Training). Second is disability management, the set of strategies to minimize the disability consequences of a given injury or disease arising from the workplace (measured as Disability Case Monitoring and Proactive Return-to-Work Program). Third is health promotion, which represents an attempt to intervene directly with individuals to encourage more healthy lifestyles, in the expectation that this will reduce the likelihood of a workplace accident or disease, or reduce the lost worktime resulting from a given injury or disease (measured as Wellness Orientation). In addition, a fourth dimension was included to capture the general environment of the firm and the orientation of its management in areas relevant to the study (measured as People Oriented Culture and Active Safety Leadership). These interventions and the general environment of the firm were scored on this set of eight variables which represent self-rated firm achievement of the policy and practice dimensions. The marginal effect of these interventions is determined by comparing firm performance on the incidence of work-related disability (Lost Workday Cases and Workers' Compensation Wage-Loss Claims), the duration of disability (Lost Workdays per Case), and overall disability prevention and management performance (Total Lost Workdays). Our results show that a higher self-rating on Safety Diligence is strongly associated with better performance on disability outcomes, varying with the specific measure. Higher self-rating on Proactive Return-to-Work Programs is also strongly associated with better performance outcomes. Safety Training and Active Safety Leadership is shown to have significant effects on the number of Lost Workday Cases. For example, on the summary measure of total Lost Workdays per 100 Employees, 10 percent better self-rating on Safety Diligence translates into 17 percent fewer lost workdays, and 10 percent better self-rating on Proactive Return-to-Work Programs translates into 7 percent fewer lost workdays. Thus, the twin strategies of trying to prevent injuries in the first place, and working to ameliorate their disability effects through disability management techniques, are both shown to be productive in reducing workplace disability in those establishments that have implemented them rigorously. Disability Case Monitoring could not be shown to have significant effects; in fact, Disability Case Monitoring had negative impacts in some cases. This probably reflects the controlling aspects of Disability Case Monitoring, as we speculate that these practices can be viewed by the employees as negative and interfering if they do not emanate from a supportive company human resource climate. Ergonomic Solutions and Wellness Orientation generally do not perform significantly, and this is attributed to their indirect connection to the performance outcomes used here or ineffective measurement of these dimensions in the study. Site visits were made to a subsample of 32 firms selected from the larger, random sample in order to confirm the quantitative survey findings and gain operational understanding of successful policies and practices contributing to low disability rates. Companies were generally found to be most advanced in their safety efforts, very active in injury management, and had implemented at least some form of return-to-work. Health promotion strategies to prevent specific work injuries have not yet been fully developed. The disability prevention efforts of successful firms use data effectively to measure performance, identify problems, guide actions taken, and motivate active support and participation of management, supervisors and line employees. Successful firms rigorously investigate injuries and communicate their commitment by immediately responding to risks when they are identified. In these low-disability firms, safety and disability management are viewed as components of quality, productivity and financial stability. Working relationships have been developed with responsive health care providers to assure effective injury management, but firms also maintain an active role in case management themselves. Their return-to-work process is systematic, yet flexible to respond to individual needs. Innovative firms have also implemented ergonomic principles to prevent risks. Nearly all companies visited reported increasing incidence and costs due to cumulative trauma and repetitive motion disorders. Additional strategies are needed to resolve and prevent these disabilities. This study demonstrates that many employers have moved aggressively to policies and practices designed to reduce the incidence and the costs of disability in their workplace. The project concludes that disability can be prevented and managed; and those who do it well can expect to be rewarded with lower disability costs, more satisfied workers, higher productivity and, ultimately, higher profits.disability, workers', compensation, Michigan, Hunt, Habeck

Design of feedback controllers for paraplegic standing

The development, implementation and experimental evaluation of feedback systems for the control of the upright posture of paraplegic persons in standing is described. While the subject stands in a special apparatus, stabilising torque at the ankle joint is generated by electrical stimulation of the paralysed calf muscles of both legs using surface electrodes. This allows the subject to stand without the need to hold on to external supports for stability- this is termed 'unsupported standing'. Sensors in the apparatus allow independent measurement of left and right ankle moments together with measurement of the inclination angle. A nested loop structure for control of standing is implemented, where a high-bandwidth inner loop provides control of the ankle moments, while the angle controller in the outer loop regulates the inclination angle. A number of important modifications to a control strategy which was previously tested with both neurologically intact and paraplegic subjects are presented. The new strategy is described, and an experimental evaluation with intact subjects is reported. The experimental results show that the control system for unsupported standing performs reliably, and according to the design formulation. There aa-e a number of design choices, appropriate to different situations, and the practical effect of each is clear. This allows easy 'tuning' during an experimental session. This is important since the complete design procedure, from muscle dynamics identification to control design, has to be carried out as quickly as possible while the subject is standing in the apparatus. A number of recommendations are made regarding the preferred design choices for control of unsupported standing

Feedback control of unsupported standing

This paper presents the results of continuing work on feedback control of unsupported standing in paraplegia. Our experimental setup considers a situation in which all joints above the ankle are braced, and stabilising torque at the ankle is generated by stimulation of the plantarflexors. A previous study showed that short periods of unsupported standing with paraplegic subjects could be achieved. In order to improve consistency and reliability of unsupported standing we are currently investigating several modifications to the control strategy. The paper reports progress towards this goal

A simple algorithm for computing canonical forms

It is well known that all linear time-invariant controllable systems can be transformed to Brunovsky canonical form by a transformation consisting only of coordinate changes and linear feedback. However, the actual procedures for doing this have tended to be overly complex. The technique introduced here is envisioned as an on-line procedure and is inspired by George Meyer's tangent model for nonlinear systems. The process utilizes Meyer's block triangular form as an intermedicate step in going to Brunovsky form. The method also involves orthogonal matrices, thus eliminating the need for the computation of matrix inverses. In addition, the Kronecker indices can be computed as a by-product of this transformation so it is necessary to know them in advance

New results in feedback control of unsupported standing in paraplegia

The aim of this study was to implement a new approach to feedback control of unsupported standing and to evaluate it in tests with an intact and a paraplegic subject. In our setup, all joints above the ankles are braced and stabilizing torque at the ankle is generated by electrical stimulation of the plantarflexor muscles. A previous study showed that short periods of unsupported standing with a paraplegic subject could be achieved. In order to improve consistency and reliability and to prolong the duration of standing, we have implemented several modifications to the control strategy. These include a simplified control structure and a different controller design method. While the reliability of standing is mainly limited by the muscle characteristics such as reduced strength and progressive fatigue, the results presented here show that the new strategy allows much longer periods (up to several minutes) of unsupported standing in paraplegia

Workers' Compensation Insurance In North America: Lessons for Victoria?

Among the issues we will consider here are the following. Who carries the underwriting(insurance) risk for workers' compensation benefits? How is workers' compensation insuranceprices, and by whom? What fundamental principles guide the insurance pricing system? Whomonitors benefits for compliance with statutory requirements? Are the availability of coverageand the payment of insurers' claims obligations guaranteed? Is self-insurance allowed and, if so, for whom? How are incentives for prevention of accidents, and resulting workers' compensation claims, maintained? What is the performance of the overall system? In summary, how are these questions answered and what so the answers reveal about how these responsibilities are allocated among government agencies, other public entities and private firms