Vijay Modi

The African Millennium Villages

We describe the concept, strategy, and initial results of the Millennium Villages Project and implications regarding sustainability and scalability. Our underlying hypothesis is that the interacting crises of agriculture, health, and infrastructure in rural Africa can be overcome through targeted public-sector investments to raise rural productivity and, thereby, to increased private-sector saving and investments. This is carried out by empowering impoverished communities with science-based interventions. Seventy-eight Millennium Villages have been initiated in 12 sites in 10 African countries, each representing a major agroecological zone. In early results, the research villages in Kenya, Ethiopia, and Malawi have reduced malaria prevalence, met caloric requirements, generated crop surpluses, enabled school feeding programs, and provided cash earnings for farm families.

Estimation of daily total and diffuse insolation in India from weather data

Abstract Insolation and weather data for a large number of cities in India is analysed and correlated. Correlations based on a citywise regression analysis indicate that daily total insolation correlates best with sunshine duration, all clouds and precipitation. However these relations are not useful for predicting insolation at locations where this data is not measured. Monthwise correlations which are valid over a region are more useful. Hence such correlations have been developed for Indian conditions. In order to increase the accuracy of prediction of these correlations, India is divided into two regions on the basis of the climatic characteristics of the winter monsoon. Finally the Liu and Jordan model for predicting daily diffuse radiation from daily total radiation has been tested and found to be applicable for Indian conditions. However the numerical values obtained are very different from those obtained for conditions in the United States.

Optimum plane diffusers in laminar flow

The problem of determining the profile of a plane diffuser (of given upstream width and length) that provides the maximum static pressure rise is solved. Two-dimensional, incompressible, laminar flow governed by the steady-state Navier-Stokes equations is assumed through the diffuser. Recent advances in computational resources and algorithms have made it possible to solve the ‘direct’ problem of determining such a flow through a body of known geometry. In this paper, a set of ‘adjoint’ equations is obtained, the solution to which permits the calculation of the direction and relative magnitude of change in the diffuser profile that leads to a higher pressure rise. The direct as well as the adjoint set of partial differential equations are obtained for Dirichlet-type inflow and outflow conditions. Repeatedly modifying the diffuser geometry with each solution to these two sets of equations with the above boundary conditions would in principle lead to a diffuser with the maximum static pressure rise, also called the optimum diffuser. The optimality condition, that the shear stress all along the wall must vanish for the optimum diffuser, is also recovered from the analysis. It is postulated that the adjoint set of equations continues to hold even if the computationally inconvenient Dirichlet-type outflow boundary condition is replaced by Neumann-type conditions. It is shown that numerical solutions obtained in this fashion do satisfy the optimality condition.

Electrokinetic flow control for composition modulation in a microchannel

A numerical and experimental study of the injection into a microchannel of a fluid with a spatially modulated composition is presented. The investigation employs test structures constructed in polydimethylsiloxane by standard replica molding. Fluid-flow simulations are compared to flow results obtained by fluorescence microscopy experiments. Results show that for a given channel dimension, the desired modulation of the solution composition is only possible below a threshold frequency. The value of the threshold frequency is dependent on channel size as well as flow rate. Experimental results are in accord with numerical simulations and theoretical considerations.

A microfabricated wall shear-stress sensor with capacitative sensing

A silicon-based micromachined, floating-element sensor for low-magnitude wall shear-stress measurement has been developed. Sensors over a range of element sizes and sensitivities have been fabricated by thin-wafer bonding and deep-reactive ion-etching techniques. Detailed design, fabrication, and testing issues are described in this paper. Detection of the floating-element motion is accomplished using either direct or differential capacitance measurement. The design objective is to measure the shear-stress distribution at levels of O(0.10 Pa) with a spatial resolution of approximately O(100 /spl mu/m). It is assumed that the flow direction is known, permitting one to align the sensor appropriately so that a single component shear measurement is a good estimate of the prevalent shear. Using a differential capacitance detection scheme these goals have been achieved. We tested the sensor at shear levels ranging from 0 to 0.20 Pa and found that the lowest detectable shear-stress level that the sensor can measure is 0.04 Pa with an 8% uncertainty on a 200 /spl mu/m/spl times/500 /spl mu/m floating element plate.

Gas Jet–Workpiece Interactions in Laser Machining

Laser machining efficiency and quality are closely related to gas pressure, nozzle geometry, and standoff distance. Modeling studies of laser machining rarely incorporate gas effects in part because of the complex structure and turbulent nature of jet flow. In this paper, the interaction of a supersonic, turbulent axisymmetric jet with the workpiece is studied. Numerical simulations are carried out using an explicit, coupled solution algorithm with solution-based mesh adaptation. The model is able to make quantitative predictions of the pressure, mass flow rate as well as shear force at the machining front. Effect of gas pressure and nozzle standoff distance on structure of the supersonic shock pattern is studied. Experiments are carried out to study the effect of processing parameters such as gas pressure and standoff distance. The measured results are found to match and hence validate the simulations. The interaction of the oblique incident shock with the normal standoff shock is found to contribute to a large reduction in the total pressure at the machining front and when the nozzle pressure is increased beyond a certain point. The associated reduction in flow rate, fluctuations of pressure gradient and shear force at the machining front could lower the material removal capability of the gas jet and possibly result in a poorer surface finish. The laser cutting experiments show that the variation of cut quality are affected by shock structures and can be represented by the mass flow rate. @S1087-1357~00!01702-0#

High Schmidt mass transfer in a laminar impinging slot jet flow

Abstract In this paper, high Schmidt-number mass transfer to a line electrode in laminar impinging slot-jet flows is investigated experimentally and numerically. Slot-based Reynolds numbers from 220 to 690 are considered. The mass-transfer measurements, made by the electrochemical method on 100 μm electrodes, are compared to the computed wall shear via an established analytical relationship. The local shear is obtained from steady, two-dimensional flow-field simulations of the Navier–Stokes equations. The use of an isolated line electrode and a small electrode size (100 μm) makes it possible to resolve the sharp variations in shear and mass transfer in the stagnation region. Both the experimental and theoretical results show that the peak values in Nusselt number occur at a point one-half to one jet width away from the stagnation point. Earlier experimental studies of heat transfer (Prandtl number of 0.72) reported by Gardon and Akfirat [R. Gardon, J.C. Akfirat, Heat transfer characterstics of impinging two-dimensional air jets, ASME Journal of heat Transfer 101 (1966) 101–108] (referred to in the figures as GA66) and mass transfer by Alkire and Ju [R., Alkire, J. Ju, High speed selective electroplating with impinging two-dimensional slot jet flow, J. Electrochemical Society 134 (2) (1987) 294–299] (referred to in the figures as AJ87) were not able to observe this behavior near the stagnation point.

Gas Dynamic Effects on Laser Cut Quality

Abstract The presence of a gas jet plays an important role in laser cutting. Both the cutting efficiency and cut quality are very sensitive to gas pressure and nozzle standoff distance because of the complex nature of shock fronts and associated phenomena in a supersonic gas jet impinging on a workpiece. An idealized case is considered first, where the cut is assumed to be a circular hole directly underneath and concentric with the gas jet nozzle. A more realistic case of an axisymmetric nozzle impinging on a plate with a linear cut is considered next. Unlike the idealized case, the problem now is three-dimensional. Simple experiments to measure the through-kerf mass flow rate were carried out for both geometries. The two important forces exerted by the gas jet for melt ejection, namely, shear force and pressure gradient, show the same trend as that of the mass flow rate with varying gas pressure and standoff. The mass flow rate for the three-dimensional case shows the same behavior as that of the axisymmetric case, indicating the basic shock structures of the axisymmetric case are applicable to the real cutting cases. Laser cutting of mild steels under the corresponding conditions was performed, and the cut quality characterized by roughness, dross attachment, and recast layer thickness was analyzed. The deterioration of cut quality with the gas pressure and standoff is found to closely match reductions in through-kerf mass flow rate. It is thus verified that the shock structure of the gas jet and the associated mass flow rate have a direct impact on laser cutting as predicted.

Micromachined silicon structures for free-convection PEM fuel cells

This paper presents details on the design, fabrication, testing and modeling of micromachined gas diffusion media (GDM) for micro proton exchange membrane (PEM) fuel cell applications. Two-tiered mesh structures were thru-etched into silicon wafers and subsequently assembled with membrane electrode assemblies (MEAs) and tested with hydrogen fuel and ambient air as the oxidizer. These silicon structures doubled as gas diffusion layers and supports for thermally evaporated gold current collection layers that mated with commercially available MEAs of the catalyst-on-membrane variety. In general, the cell V?I performance curves approached that of conventional GDM-based free-convection cells for current densities less than 75 mA cm?2 on an iR-free basis. At higher current densities, the cells operability became less stable as product water flooded the micromesh structures as evidenced by stereoscopic images during cell operation. Single- and two-phase flow modelings of the fuel cell operating in free-convection mode were also developed and the simulations support the experimental results that water accumulation significantly reduces the maximum current density achievable for such micro fuel cells. Improved water management approaches are proposed.

Optimum design of minimum drag bodies in incompressible laminar flow using a control theory approach

The problem of modifying the shape of a two-dimensional body to reduce its drag while maintaining its section area (volume per unit span) constant is addressed. Two-dimensional, incompressible, laminar flow governed by the steady-state Navier-Stokes equations is assumed about the body. In this paper, a set of “adjoint” equations is obtained, the solution to which permits the calculation of the direction and relative magnitude of change in the body profile that leads to a lower viscous drag. The direct as well as the adjoint set of partial differential equations are obtained for Dirichlet-type far-field conditions. Repeatedly modifying the body shape with each solution to these two sets of equations with the above boundary conditions, would lead to a body with minimum drag, for a specified section area. For such a body it is shown that the product of shear and the “adjoint” shear is constant everywhere along the body. Even though the viscous terms are retained in the direct and the adjoint equations, in or...