High-Density Suspensions Formed Under Waves

We performed a series of laboratory experiments to investigate the interactions of a turbulent wave boundary layer with a predominantly silt-size sediment bed. Quasi-steady, turbulent, high-density suspensions (HDS) formed over a wide range of wave conditions and had near-bed (~1 mm above bed) concentrations ranging from 17 to 81 g/l scaling roughly with the wave orbital velocity. HDS were defined by the presence of a lutocline, an abrupt change in vertical concentration gradient. Despite the initial bed being 70% silt and 20% sand, HDS had significant near-bed sand fractions ranging from 27 to 78%. Winnowing of the bed caused more concentrated HDS to be coarser grained, which in turn caused the suspensions to be thinner because of the greater settling velocity of the sediment. Our experiments are consistent with a dynamic feedback model where suspended sediment is limited through sediment-induced stratification expressed with a bulk Richardson number. However, our computed values of the bulk Richardson number converge to a value that is an order of magnitude less than the critical value of 0.25 that is typically assumed. The experimental wave orbital velocities (15–60 cm/s) and periods (3–8 s), as well as the characteristics of the HDS and the bed in our experiments, were comparable to observations made on the Eel shelf, California, during storm conditions when fluid mud has been observed

Master\u27s and Doctoral Thesis Citations: Analysis and Trends of a Longitudinal Study

This article reports the results of a longitudinal study of over 9100 citations from 629 master’s and doctoral theses written between 1973 and 1992 at a large mid-western landgrant university. The results of this study suggest that graduate students writing theses favor current research regardless of disciplinary affiliation. The length of theses increased over time and the number of citations in thesis bibliographies varied by discipline. Implications of the results for collection development and scholarship as well as areas for future research are discussed

Turbulent structure of high-density suspensions formed under waves

We performed a series of laboratory experiments on the interactions between turbulent wave boundary layers and a predominantly silt-sized sediment bed. Under a wide range of wave conditions similar to those observed on storm-dominated midshelf environments we produced quasi-steady high-density benthic suspensions. These suspensions were turbulent, while containing large near-bed concentrations of suspended sediment (17–80 g/L), and were separated from the upper water column by a lutocline. Detailed measurements of the vertical structure of velocity, turbulence, and sediment concentration revealed that the wave boundary layer, while typically >1 cm thick in sediment-free conditions, was reduced substantially in size, often to <3 mm, with the addition of suspendible sediment. This likely resulted from sediment-induced stratification that limited vertical mixing of momentum. Despite boundary layer reduction the flows were able to support high-density suspensions as thick as 8 cm because turbulent energy was transported upward from this thin but highly energetic near-bed region. Standard formulations of the Richardson number for shear flows are not applicable to our experiments since the suspensions were supported from transported rather than locally produced turbulence

Heterogeneity in Short Gamma-ray Bursts

We analyze the Swift/BAT sample of short gamma-ray bursts, using an objective Bayesian Block procedure to extract temporal descriptors of the bursts' initial pulse complexes (IPCs). The sample comprises 12 and 41 bursts with and without extended emission (EE) components, respectively. IPCs of non-EE bursts are dominated by single pulse structures, while EE bursts tend to have two or more pulse structures. The medians of characteristic timescales - durations, pulse structure widths, and peak intervals - for EE bursts are factors of ~ 2-3 longer than for non-EE bursts. A trend previously reported by Hakkila and colleagues unifying long and short bursts - the anti-correlation of pulse intensity and width - continues in the two short burst groups, with non-EE bursts extending to more intense, narrower pulses. In addition we find that preceding and succeeding pulse intensities are anti-correlated with pulse interval. We also examine the short burst X-ray afterglows as observed by the Swift/XRT. The median flux of the initial XRT detections for EE bursts (~ 6 x 10^-10 erg cm^-2 s^-1) is ~> 20 x brighter than for non-EE bursts, and the median X-ray afterglow duration for EE bursts (~ 60,000 s) is ~ 30 x longer than for non-EE bursts. The tendency for EE bursts toward longer prompt-emission timescales and higher initial X-ray afterglow fluxes implies larger energy injections powering the afterglows. The longer-lasting X-ray afterglows of EE bursts may suggest that a significant fraction explode into more dense environments than non-EE bursts, or that the sometimes-dominant EE component efficiently powers the afterglow. Combined, these results favor different progenitors for EE and non-EE short bursts.Comment: 30 pages, 11 figures, 3 tables; accepted to The Astrophysical Journa

Hesperian-Amazonian Transition Mid-Latitude Valleys: Markers of a Late Martian Climate Optima?

Recently the inventory of fluvial features that have been dated to the late Hesperian to early Amazonian epoch has increased dramatically, including a reassessment of the ages of the large alluvial fans and deltas (e.g., Eberswalde) to this time period. Mid-latitude Valleys (MLVs) are distinct from the older, more integrated Noachian-Hesperian Valley Networks which are deeply dissected, are generally of much larger spatial extent, and are more degraded. Although some MLVs involve rejuvenation of older Valley Networks, many MLVs are carved into smooth or rolling slopes and intercrater terrain. The MLVs range from a few meters to < 300 m in width, with nearly parallel valley walls and planforms that are locally sinuous. Although the MLVs in Newton and Gorgonum basins extend from the basin rims up to 75 km into the basin interior, most MLVs are shorter and often discontinuous. The occurrence of widespread MLVs suggest the possibility of their formation during one or perhaps more regional to global climatic episodes, possibly due to melting of seasonal to long-term accumulations of snow and ice. Temperatures warm enough to cause extensive melting may have occurred during optimal orbital and obliquity configurations, perhaps in conjunction with intensive volcanism releasing moisture and greenhouse gasses, or as a result of a brief episode of warming from a large impact. The concentration of MLVs to the northern and western basin slopes of Newton and Gorgonum basins suggests a possible aspect control to ice accumulation or melting. MLV activity occurred about at the same time as formation of the major outflow channels. A possible scenario is that delivery of water to the northern lowlands provided, through evaporation and sublimation, water that temporarily accumulated in the mid-southern latitudes as widespread ice deposits whose partial melting formed the MLVs and small, dominantly ice-covered lakes

Hyperhomocysteinemia, anticardiolipin antibody status, and risk for vascular access thrombosis in hemodialysis patients

Hyperhomocysteinemia, anticardiolipin antibody status, and risk for vascular access thrombosis in hemodialysis patients.Background. Vascular access failure is an important cause of morbidity in end-stage renal failure patients on hemodialysis. Currently, little is known about risk factors that predispose certain hemodialysis patients to recurrent access thrombosis. Hyperhomocysteinemia (common in patients with renal failure) predisposes people with normal renal function to recurrent and early-onset venous thrombosis, although the effect on vascular access thrombosis is currently unknown. Previous studies have suggested that high titers of IgG anticardiolipin antibody (IgG-ACA) predispose hemodialysis patients to access thrombosis. This cross sectional study was designed to assess for an association between two predictive variables, hyperhomocysteinemia and elevated titers of IgG-ACA, and vascular access thrombosis in patients undergoing chronic hemodialysis.Methods. Risk factors for vascular access thrombosis were documented, and the number of episodes of access thrombosis was recorded for the previous three years in patients undergoing hemodialysis. Midweek predialysis total homocysteine and IgG-ACA levels were measured in all subjects.Results. Of the 118 patients who were enrolled, 75.4% had a native arteriovenous fistula. Episodes of vascular access thrombosis were recorded for the previous three years; 34 (28.8%, 95% CI 20.9 to 37.9%) patients had 72 episodes of access thrombosis over the period of risk. Mean homocysteine levels were not significantly different between these 34 patients (28.6 μmol/liter, 95% CI 24.5 to 32.7) and the patients who had no episodes of graft thrombosis (29.8 μmol/liter, 95% CI 26.7 to 32.9). Sixty-seven unselected patients had IgG-ACA levels drawn for analysis, and all assays were negative. The only variable that was associated with a higher risk for graft thrombosis was the type of vascular access placed (odds ratio 4.0, 95% CI 1.6 to 9.6 for patients with a synthetic graft compared with those with an arteriovenous fistula).Conclusions. No association was found between homocysteine levels or anticardiolipin antibody and vascular access thrombosis in our patient population

Driving performance in older adults: Current measures, findings, and implications for roadway safety

BACKGROUND AND OBJECTIVES: Over 10,000 people a day turn 65 in the United States. For many older adults, driving represents an essential component of independence and is one of the most important factors in overall mobility. Recent survey studies in older adults suggest that up to 60% of older adult drivers with mild cognitive impairment, and up to 30% with dementia, continue to drive. The purpose of this review is to provide a comprehensive and detailed resource on the topics of cognition and driving for clinicians, researchers, and policymakers working on efforts related to older adult drivers. RESEARCH DESIGN AND METHODS: Publications on PubMed and Medline and discussions with experts working in geriatrics, technology, driving policy, psychology, and diverse aspects of driving performance were utilized to inform the current review. RESULTS: Research indicates that there is a complex and inverse correlation between multiple cognitive measures, driving performance, and risky driving behaviors. The fragmented nature of available peer-reviewed literature, and a reliance on correlative data, do not currently allow for the identification of the temporal and reciprocal nature of the interplay between cognition and driving endpoints. DISCUSSION AND IMPLICATIONS: There are currently no widely accepted definitions, conceptual models, or uniform set of analyses for conducting geriatric research that is focused on driving. Establishing conventions for conducting research that harmonizes the fields of geriatrics, cognition, and driving research is critical for the development of the evidence base that will inform clinical practice and road safety policy

Challenging race-based stereotypes about gay and bisexual men’s sexual behavior and perceived penis size and size satisfaction

Racial prejudice and stereotyping in gay and bisexual communities may be important contextual factors that contribute to racial disparities in HIV. In an effort to challenge race-based stereotypes regarding gay and bisexual men’s sexuality, we sought to determine the extent to which race and ethnicity were associated with (1) racial homophily (i.e., same-race partnerships), (2) sexual behavior (e.g., number of partners, condomless anal sex (CAS), sexual position (top/versatile/bottom)), and (3) perceived penis size and size satisfaction. Data were taken from a survey of 1,009 gay and bisexual men recruited using a street-intercept method at gay, lesbian, and bisexual community events in NYC in 2006—15% Black, 61% White, 18% Latino, and 6% Asian/Pacific Islander (mean age: 35.7). There was strong evidence of racial homophily among men who were in relationships, particularly for White and Black men. Race and ethnicity was largely unassociated with multiple dimensions of sexual behavior (e.g., number of partners, CAS, sexual positioning). Although we observed some racial and ethnic differences in perceived penis size that were consistent with stereotypes, the magnitudes of the differences were insufficient to justify the stereotype. As well, there were no significant differences with regard to satisfaction with penis size or lying to others about penis size. The disproportionate HIV prevalence among Black and Latino men does not appear to be as a result of differences in sexual behavior (e.g., CAS, number of partners) and race-based sexual stereotypes were largely unsupported by empirical data