[1] Tropical Rainfall Measuring Mission (TRMM) satellite estimates of summertime rainfall over the southeast U.S. are found on average to be significantly higher during the middle of the work week than on weekends, attributable to a midweek intensification of afternoon storms and an increase in area with detectable rain. TRMM radar data show a significant midweek increase in the echo-top heights reached by afternoon storms. Weekly variations in model-reanalysis wind patterns over the region are consistent with changes in convection implied by the satellite data. Weekly variations in rain gauge averages are also consistent with the satellite estimates, though possibly smaller in amplitude. A midweek decrease of rainfall over the nearby Atlantic is also seen. EPA measurements of surface particulate concentrations show a midweek peak over much of the U.S. These observations are consistent with the theory that anthropogenic air pollution suppresses cloud-drop coalescence and early rainout during the growth of thunderstorms over land, allowing more water to be carried above the 0°C isotherm, where freezing yields additional latent heat, invigorating the storms and producing large ice hydrometeors. The enhanced convection induces regional convergence, uplifting and an overall increase of rainfall. Compensating downward air motion suppresses convection over the adjacent ocean areas. Pre-TRMM-era data suggest that the weekly cycle only became strong enough to be detectable beginning in the 1980's. Rain-gauge data also suggest that a weekly cycle may have been detectable in the 1940's, but with peak rainfall on Sunday or Monday, possibly explained by the difference in composition of aerosol pollution at that time. This ''weekend effect'' may thus offer climate researchers an opportunity to study the regional climate-scale impact of aerosols on storm development and monsoon-like circulation.

Generalized linear models with random effects are often used to explain the serial dependence of longitudinal categorical data. Marginalized random effects models (MREMs) permit likelihood-based estimations of marginal mean parameters and also explain the serial dependence of longitudinal data. In this paper, we extend the MREM to accommodate multivariate longitudinal binary data using a new covariance matrix with a Kronecker decomposition, which easily explains both the serial dependence and time-specific response correlation. A maximum marginal likelihood estimation is proposed utilizing a quasi-Newton algorithm with quasi-Monte Carlo integration of the random effects. Our approach is applied to analyze metabolic syndrome data from the Korean Genomic Epidemiology Study for Korean adults.

The basic operation of hybrid hydraulic actuators involves high frequency bi-directional operation of an active material that is converted to uni-directional motion of hydraulic fluid using valves. A hybrid actuator was developed using magnetostrictive material Terfenol-D as the driving element and hydraulic oil as the working fluid. Two different lengths of Terfenol-D rod, 51 and 102 mm, with the same diameter, 12.7 mm, were used. Tests with no load and with load were carried out to measure the performance for uni-directional motion of the output piston at different pumping frequencies. The maximum no-load flow rates were 24.8 cm3 s−1 and 22.7 cm3 s−1 with the 51 mm and 102 mm long rods respectively, and the peaks were noted around 325 Hz pumping frequency. The blocked force of the actuator was close to 89 N in both cases. A key observation was that, at these high pumping frequencies, the inertial effects of the fluid mass dominate over the viscous effects and the problem becomes unsteady in nature. In this study, we also develop a mathematical model of the hydraulic hybrid actuator in the time domain to show the basic operational principle under varying conditions and to capture phenomena affecting system performance. Governing equations for the pumping piston and output shaft were obtained from force equilibrium considerations, while compressibility of the working fluid was taken into account by incorporating the bulk modulus. Fluid inertia was represented by a lumped parameter approach to the transmission line model, giving rise to strongly coupled ordinary differential equations. The model was then used to calculate the no-load velocities of the actuator at different pumping frequencies and simulation results were compared with experimental data for model validation.

No abstract

A high efficiency design was explored for meso-scale magnetorheological (MR) valves (< 25 mm OD with an annular gap < 1 mm). The objective of this paper is to miniaturize the MR valve while maintaining the maximum performance of the MR effect in the valve. The main design issues in the MR valve involve the magnetic circuit and nonlinear fluid mechanics. The performance of the MR valve is limited by saturation phenomenon in the magnetic circuit and by the finite yield stress of the MR fluid. When field is applied to the magnetic circuit in the MR valve, a semisolid plug (as a result of particle chain formation) forms perpendicular to the flow direction through the valve, and a finite yield stress is developed as a function of field. The resulting plug thickness is used to control flow rate through, and pressure drop across, the MR valve. The nondimensional plug thickness is evaluated as a basis for evaluating valve efficiency. Design parameters of the MR valve are studied and an optimal performance was designed using steel (Permalloy) material in the magnetic circuit. A maximum magnetic flux density at the gap was achieved in the optimized valve design based on a constraint on the outer diameter limitation. Valve performance was verified with simulation. A flow mode bypass damper system was fabricated and was used to experimentally validate valve performance.

A systematic study of the magnetic and rheological properties of magneto-rheological (MR) fluids containing micron-size and nano-size iron particles is presented. The MR fluids were prepared with hydraulic oil as the carrier liquid and lecithin as an effective surfactant medium that promotes uniform particle dispersion. Magnetic measurements on micron-, hybrid-(nano + micron), and nano-MR fluids clearly indicate that the partial replacement of the micro-size particles by nanoparticles results in a better suspension and robust chain formation under applied external magnetic fields. For nano-MR fluids, the measured yield stress was found to be lower than micron-MR fluids. However, better flow properties and sharper magnetic switching make nanoparticle-based MR fluids appealing for microfluidics device applications where higher yield stress is not required.

No abstract

No abstract