Cascade model for particle concentration and enstrophy in fully developed turbulence with mass-loading feedback.

A cascade model is described based on multiplier distributions determined from three-dimensional (3D) direct numerical simulations (DNS) of turbulent particle laden flows, which include two-way coupling between the phases at global mass loadings equal to unity. The governing Eulerian equations are solved using psuedospectral methods on up to 512(3) computional grid points. DNS results for particle concentration and enstrophy at Taylor microscale Reynolds numbers in the range 34-170 were used to directly determine multiplier distributions on spatial scales three times the Kolmogorov length scale. The multiplier probability distribution functions (PDFs) are well characterized by the beta distribution function. The width of the PDFs, which is a measure of intermittency, decreases with increasing mass loading within the local region where the multipliers are measured. The functional form of this dependence is not sensitive to Reynolds numbers in the range considered. A partition correlation probability is included in the cascade model to account for the observed spatial anticorrelation between particle concentration and enstrophy. Joint probability distribution functions of concentration and enstrophy generated using the cascade model are shown to be in excellent agreement with those derived directly from our 3D simulations. Probabilities predicted by the cascade model are presented at Reynolds numbers well beyond what is achievable by direct simulation. These results clearly indicate that particle mass loading significantly reduces the probabilities of high particle concentration and enstrophy relative to those resulting from unloaded runs. Particle mass density appears to reach a limit at around 100 times the gas density. This approach has promise for significant computational savings in certain applications.

Scaling properties of particle density fields formed in simulated turbulent flows.

Direct numerical simulations of particle concentrations in fully developed three-dimensional turbulence were carried out in order to study the nonuniform structure of the particle density field. Three steady-state turbulent fluid fields with Taylor microscale Reynolds numbers (Re(lambda)) of 40, 80, and 140 were generated by solving the Navier-Stokes equations with pseudospectral methods. Large-scale forcing was used to drive the turbulence and maintain temporal stationarity. The response of the particles to the fluid was parametrized by the particle Stokes number St, defined as the ratio of the particle's stopping time to the mean period of eddies on the Kolmogorov scale (eta). In this paper, we consider only passive particles optimally coupled to these eddies (St approximately 1) because of their tendency to concentrate more than particles with lesser or greater St values. The trajectories of up to 70x10(6) particles were tracked in the equilibrated turbulent flows until the particle concentration field reached a statistically stationary state. The nonuniform structure of the concentration fields was characterized by the multifractal singularity spectrum f(alpha), derived from measures obtained after binning particles into cells ranging from 2eta to 15eta in size. We observed strong systematic variations of f(alpha) across this scale range in all three simulations and conclude that the particle concentration field is not statistically self-similar across the scale range explored. However, spectra obtained at the 2eta, 4eta, and 8eta scales of each flow case were found to be qualitatively similar. This result suggests that the local structure of the particle concentration field may be flow independent. The singularity spectra found for 2eta-sized cells were used to predict concentration distributions in good agreement with those obtained directly from the particle data. This singularity spectrum has a shape similar to the analogous spectrum derived for the inertial-range energy dissipation fields of experimental turbulent flows at Re(lambda)=110 and 1100. Based on this agreement, and the expectation that both dissipation and particle concentration are controlled by the same cascade process, we hypothesize that singularity spectra similar to the ones found in this work provide a good characterization of the spatially averaged statistical properties of preferentially concentrated particles in higher Re(lambda) turbulent flows.

Double think and double talk.

The public health debate on population growth and child mortality continues, fuelled by the hypothesis that in allowing more children to survive until reproductive age, programmes such as the Diarrhoeal Diseases Control Programme of the World Health Organization contribute to long-term human misery by overburdening the carrying capacity of the planet. A significant part of the solution put forward is to withhold public health services to children in developing countries. This argument is here refuted on socioeconomic, ethical and humanitarian grounds. An alternative approach is offered, which takes into account the economic and social obligations of the industrialized nations.

PIP: Dr. Maurice King has predicted that Third-World societies will collapse as a result of their growing demographic entrapment. Although Dr. King acknowledges that a lack of economic connections is strongly related to entrapment, he fails to call for economic solutions or interventions aimed at increasing the carrying capacity of an ecosystem (which would also lead to economic growth and, thus, provide a prerequisite for slowing population growth). Instead, Dr. King proposes withholding support from child survival programs. Since the current mortality rate for children under 5 years old in least developed countries (150-300/1000 live births) is held in check by improved economic and social conditions as well as by child survival interventions, these public health measures only reduce mortality among 15-30% of all children. Therefore, preventing 50% of the deaths which now occur would only increase the population by 10%. Instead of asking children to bear the brunt of the problem, it would be more humane and reasonable to provide better family planning (FP) programs. Also, curtailing existing programs for child survival would only lead to an insignificant reduction in financial allotments as compared to those devoted to such activities as military support. In addition, Dr. King's argument that communities should make their own decisions about whether or not to accept proposed programs of international aid has 4 fallacies. 1) It is unlikely that communities would choose to sacrifice their children for the promise of a better tomorrow. 2) Decision-making implies having viable options. Offering a community a "decision" without offering the social, economic, and technological choices available in developed countries would be unjust. 3) If FP were the only possible alternative to entrapment, societies would have to limit couples to one child or no children. 4) Even if communities opted to withhold public health services which were safe and effective, it would be wrong for the world community to endorse such unethical behavior. Dr. King also misses the point that child survival strategies are also FP strategies (extended breast feeding, for example) and that the best interests of community development are served by better FP and by better health care for children, which are complementary rather than competitive. Dr. King is also incorrect when he maintains that children are receiving the highest priority. Defending the previously-neglected plight of children does not preclude searching for the best balance of developmental strategies for a particular country. Promoting development at the expense of children, however, is a radical position which upsets this balance.

Diarrhoeal disease control: reviews of potential interventions.

PIP: Diarrheal diseases are a major cause of illness and death in young children in most developing countries, with recent estimates attributing nearly 5 million deaths/year to children under 5 in developing countries excluding China. Because diarrheal disease mortality can be effectively reduced at reasonable cost by oral rehydration and possibly other measures, it is a priority target for primary health care programs in many countries. Other interventions are needed in addition to oral rehydration to lessen the impact of probable operational constraints on oral rehydration programs, to reduce mortality from chronic or dysenteric diarrheas in which oral rehydration is of limited use, and to reduce diarrhea morbidity rates. The Diarrhoeal Diseases Control (CDD) Programme of the World Health Organization (WHO) advocates a 4-part strategy for diarrhea control consisting of improved case management, improved maternal and child health care, improved use and maintenance of drinking water and sanitation facilities and improved food hygiene, and detection and control of epidemics. The CDD program has undertaken a systematic review of the effectiveness, feasibility, and cost of available antidiarrheal interventions and has developed a classification of interventions to guide the review process. Each intervention will be reviewed using a standard format and assigned to 1 of 3 categories: 1) those shown to be effective, feasible, and affordable, for which the CDD program will develop guidelines for implementation within primary health care programs and promote any additional operational research needed; 2) those believed on theoretical grounds to be effective but in which insufficient field experience has been gained will receive further field testing; and 3) those shown to be too costly, ineffective, or unfeasible will not be recommended by the CDD. The classification of possible interventions has 4 categories: 1) case management, including oral rehydration therapy at home or at a medical facility, promoting appropriate feeding during diarrheal episodes, and chemotherapy at home or in a medical facility; 2) increasing host resistence to infection through various programs of maternal nutrition, child nutrition, immunization, or chemoprophylaxis; 3) reducing transmission of the pathogenic agents through control of water supply and excreta disposal, personal and domestic hygiene, food hygiene, control of zoonotic reservoirs, or fly control; and 4) controlling or preventing diarrhea epidemics.^ieng

Surveillance projects for selected diseases.

Disease surveillance systems constitute the foundation of appropriate health plans. Surveillance data collection components at best are dependent on a series of contingencies. In developing countries scattered static health centres may result in biased and practically unusable data. A system is here described for collecting data from a sample of persons chosen for their statistical representation of the entire population. Monthly interviews can provide both prevalence and incidence data at low cost using paramedical personnel. Continuing work in this area could provide developing countries with alternative methods of defining their population-nutrition-communicable disease problems.