A Coordinate Transformation for Unsteady Boundary Layer Equations

This paper presents a new coordinate transformation for unsteady, incompressible boundary layer equations that applies to both laminar and turbulent flows. A generalization of this coordinate transformation is also proposed. The unsteady boundary layer equations are subsequently derived. In addition, the boundary layer equations are derived using a time linearization approach and assuming harmonically varying small disturbances

Experimental investigation of the critical Reynolds number for bubbly two-phase flow

The critical Reynolds number for dispersed bubbly two-phase flows, gas in liquid, was experimentally investigated. In this experiment, the gas was air and the liquid was water. Water flowed upward through a vertical, translucent pipe, and air was introduced into the water prior to the test section by means of a basswood microbubbler. Dye was injected into the water to indicate when the flow transitioned from laminar to turbulent. Mixtures with gas volume fractions up to 10% were tested, and the Reynolds numbers for which the flow transitioned from laminar to turbulent were recorded. The data showed that the critical Reynolds number fell to roughly one-half of its original single-phase liquid value once the gas volume fractions exceeded 0.1%. These results indicate that the presence of even small amounts of bubbles causes pre-mature transition to turbulence

A method to generate computationally efficient reduced order models

A new method is presented to generate reduced order models (ROMs) in Fluid Dynamics problems. The method is based on the expansion of the flow variables on a Proper Orthogonal Decomposition (POD) basis, calculated from a limited number of snapshots, which are obtained via Computational Fluid Dynamics (CFD). Then, the POD-mode amplitudes are calculated as minimizers of a properly defined overall residual of the equations and boundary conditions. The residual can be calculated using only a limited number of points in the flow field, which can be scattered either all over the whole computational domain or over a smaller projection window. This means that the process is both computationally efficient (reconstructed flow fields require less than 1% of the time needed to compute a full CFD solution) and flexible (the projection window can avoid regions of large localized CFD errors). Also, various definitions of the residual are briefly discussed, along with the number and distribution of snapshots, the number of retained modes, and the effect of CFD errors, to conclude that the method is numerically robust. This is because the results are largely insensitive to the definition of the residual, to CFD errors, and to the CFD method itself, which may contain artificial stabilizing terms. Thus, the method is amenable for practical engineering applications

Measurement of Organophosphate Pesticides, Organochlorine Pesticides, and Polycyclic Aromatic Hydrocarbons in Household Dust from Two Rural Villages in Nepal

Although there are few published studies of residential exposures to environmental contaminants in Nepal, there may be substantial exposures to multiple contaminants in Nepali households. Pesticides, which can be harmful to human health, are often used by Nepali farmers, and many farmers lack an understanding of the appropriate procedures for the safe use, handling and storage of pesticides. In addition, many Nepali families use wood burning stoves, leading to the potential for exposure to polycyclic aromatic hydrocarbons from wood smoke. This study measured the levels of four organophosphate pesticides, 22 organochlorine pesticides, and over thirty polycyclic aromatic hydrocarbons in house dust from two rural Nepali villages. Floor dust samples were collected in early summer from a total of 18 households, including nine households in the village of Keraghari, in Kavrepalanchok District, and nine homes in the village of Kafaldanda, in Lalitpur District. These villages have similar environmental features and are located at an altitude of approximately 2,000 meters. In these two villages, many of the homes have improved cookstoves to reduce smoke levels in the homes. The dust samples were collected using pre-ashed glass fiber filter cloths saturated with isopropyl alcohol. In both villages, the organochlorine pesticide that was present in the highest concentrations was the DDT metabolite p,p’-dichlorodiphenyldichloroethylene (4,4’-DDE). Among the organophosphates, methyl parathion accounted for much of the organophosphate mass detected. Across both villages, the median total organochlorine pesticide value (287 ng/m2) was 5-fold higher than the median total organophosphate pesticide value (54.3 ng/m2). The median total polycyclic aromatic hydrocarbon concentration in house dust from both villages was 14,700 ng/m2. Interventions are needed to improve safe handling and use of pesticides in Nepali villages. Additional studies are also needed to assess the extent to which improved cookstoves reduce polycyclic aromatic hydrocarbon exposure, during both winter and summer

Exploring Demographic and Environmental Factors Related to Unintentional Pesticide Poisonings in Children and Adolescents in Texas

There is limited literature on the frequency and distribution of pesticide exposures, specifically with respect to demographic and environmental factors in the United States. The purpose of this exploratory study was to investigate geographic trends and factors associated with unintentional pesticide exposures in children and adolescents in Texas. The study used an ecological design with secondary data. A spatial scan statistic, based on a Poisson regression model, was employed to identify spatial clusters of unintentional pesticide-related poison center exposures. Next, logistic regression models were constructed to identify potential demographic and environmental factors associated with unintentional pesticide-related poison center exposures. There were 59,477 unintentional pesticide-related poison center exposures from 2000 to 2013. The spatial scan statistic found a change in the number of counties in the identified clusters (e.g. , aggregation of counties with higher than expected exposures) for two time periods (2000-2006; 2007-2013). Based on the logistic regression models, factors associated with unintentional pesticide-related poison center exposures were percent black or African American population, year structure built, and percent moved in the past 12 months. In conclusion, this study found certain demographic and environmental factors may be associated with unintentional pesticide-related poison center exposures. Through understanding trends and associated factors, public health professionals can design interventions for populations at higher risk of pesticide exposures. This study also supports the use of spatial methods being utilized to expand upon current analysis of poison center data. Future research should confirm and build upon these findings

Reduced order models based on local POD plus Galerkin projection

A method is presented to accelerate numerical simulations on parabolic problems using a numerical code and a Galerkin system (obtained via POD plus Galerkin projection) on a sequence of interspersed intervals. The lengths of these intervals are chosen according to several basic ideas that include an a priori estimate of the error of the Galerkin approximation. Several improvements are introduced that reduce computational complexity and deal with: (a) updating the POD manifold (instead of calculating it) at the end of each Galerkin interval; (b) using only a limited number of mesh points to calculate the right hand side of the Galerkin system; and (c) introducing a second error estimate based on a second Galerkin system to account for situations in which qualitative changes in the dynamics occur during the application of the Galerkin system. The resulting method, called local POD plus Galerkin projection method, turns out to be both robust and efficient. For illustration, we consider a time-dependent Fisher-like equation and a complex Ginzburg–Landau equation

Unexpected removal of the most neutral cationic pharmaceutical in river waters

Contamination of surface waters by pharmaceuticals is now widespread. There are few data on their environmental behaviour, particularly for those which are cationic at typical surface water pH. As the external surfaces of bacterio-plankton cells are hydrophilic with a net negative charge, it was anticipated that bacterio-plankton in surface-waters would preferentially remove the most extensively-ionised cation at a given pH. To test this hypothesis, the persistence of four, widely-used, cationic pharmaceuticals, chloroquine, quinine, fluphenazine and levamisole, was assessed in batch microcosms, comprising water and bacterio-plankton, to which pharmaceuticals were added and incubated for 21 days. Results show that levamisole concentrations decreased by 19 % in microcosms containing bacterio-plankton, and by 13 % in a parallel microcosm containing tripeptide as a priming agent. In contrast to levamisole, concentrations of quinine, chloroquine and fluphenazine were unchanged over 21 days in microcosms containing bacterio-plankton. At the river-water pH, levamisole is 28 % cationic, while quinine is 91–98 % cationic, chloroquine 99 % cationic and fluphenazine 72–86 % cationic. Thus, the most neutral compound, levamisole, showed greatest removal, contradicting the expected bacterio-plankton preference for ionised molecules. However, levamisole was the most hydrophilic molecule, based on its octanol–water solubility coefficient (K ow). Overall, the pattern of pharmaceutical behaviour within the incubations did not reflect the relative hydrophilicity of the pharmaceuticals predicted by the octanol–water distribution coefficient, D ow, suggesting that improved predictive power, with respect to modelling bioaccumulation, may be needed to develop robust environmental risk assessments for cationic pharmaceuticals