Steady Pumping Research Articles

Hyperfine interaction in InAs/GaAs self-assembled quantum dots: dynamical nuclear polarization versus spin relaxation

We report on the influence of the hyperfine interaction on the optical orientation of singly charged excitons X ± in self-assembled InAs/GaAs quantum dots. All measurements were carried out on individual quantum dots studied by micro-photoluminescence at low temperature. We show that the hyperfine interaction leads to an effective partial spin relaxation, under 50 kHz modulated excitation polarization, which becomes, however, strongly inhibited under steady optical pumping conditions because of dynamical nuclear polarization. This optically created magnetic-like nuclear field can become very strong (up to ∼ 4 T ) when it is generated in the direction opposite to a longitudinally applied field, and exhibits then a bistability regime. This effect is very well described by a theoretical model derived in a perturbative approach, which reveals the key role played by the energy cost of an electron spin flip in the total magnetic field. Finally, we emphasize the similarities and differences between X + and X − trions with respect to the hyperfine interaction, which turn out to be in perfect agreement with the theoretical description. To cite this article: O. Krebs et al., C. R. Physique 9 (2008).

Approximation of Well Function for Large Diameter Wells

A computationally simple and accurate algebraic approximation of the well function for large diameter wells is proposed. This approximation can be used to calculate drawdown in a large diameter well due to steady pumping. Using this approximation, a method is proposed for calculating drawdowns in large diameter wells due to unsteady pumping discharge. The proposed method is also applicable for calculating residual drawdowns in large diameter wells. In principle, the proposed method that uses the approximation of the well function should yield more accurate results than the previously proposed kernel methods.

A Comprehensive Analysis of Nonlinear Corrections to the Classical Ekman Pumping

The role of each nonlinear term in the vorticity and divergence equations derived from the primitive equations to the steady nonlinear Ekman pumping is examined for uni-directional flows with a small Rossby number comprehensively. For uni-directional flows with a constant horizontal shear, the perturbation analysis in the first order of a small Rossby number show that the contribution from the nonlinear divergence term in the vorticity equation to the Ekman pumping is the most dominant. That from the vertical advection term in the vorticity equation is the second dominant. The nonlinear terms from the divergence equation are less important for the nonlinear corrections to the classical Ekman pumping. In addition, the effect of the nonlinear divergence term to the Ekman pumping is opposite from that of the other nonlinear terms. The latter result also can be verified by qualitative discussions. Furthermore, the analytical solutions for constant horizontal shear flows can be utilized even for uni-directional flows with general horizontal shear. In this case, it is found that the contribution from the nonlinear divergence and horizontal advection terms in the vorticity equation to the Ekman pumping is the most dominant. That from the vertical advection and tilting terms in the vorticity equation is the second dominant. Some comments on the pre-existing approximate models of the nonlinear Ekman pumping are made.

Two‐Stage Aquifer Pumping Subject to Slow Desorption and Persistent Sources

This work focuses on improving pump-and-treat remediation by optimizing a two-stage operational scheme to reduce volumes extracted when confronted with nonequilibrium desorption, low-permeability units, and continuous contaminant sources such as non-aqueous phase liquids (NAPL). Q1 and Q2 are the initial short-term high pumping rate and later long-term low pumping rate, respectively. A two-dimensional ground water flow and transport management model was used to test the proposed strategy for plumes developed from finite (NAPL-free) and continuous (NAPL-driven) contaminant sources in homogeneous and nonhomogeneous (zoned) aquifers. Remediation scenarios were simulated over durations of 2000, 6000, and 15,000 d to determine (1) the optimal time to switch from a preset Q1 to Q2 and (2) the value of Q2. The problem was constrained by mass removal requirements, maximum allowable downgradient concentrations, and practical bounds on Q2. Q1 was fixed at preset values 50% to 200% higher than the single-stage pumping rates (i.e., steady pumping rates during entire remediation period) necessary to achieve a desired cleanup level and capture the plume. Results for the NAPL-free homogeneous case under nonequilibrium desorption conditions achieved the same level of cleanup as single-stage pumping, while reducing extracted volumes by up to 36%. Comparable savings were obtained with NAPL-driven sources only when the source concentration was reduced by at least 2 orders of magnitude. For the zoned aquifer, the proposed strategy provided volume savings of up to 24% under NAPL-free and reduced source conditions.

Steady State Passive Nonlinear Energy Pumping in Coupled Oscillators: Theoretical and Experimental Results

We study theoretically and experimentally the effect that anonlinear energy sink (NES) has on the steady state dynamics of a weaklycoupled system. The NES possesses essentially nonlinear(nonlinearizable) stiffness nonlinearity of the third degree. We findthat, in contrast to the classical linear vibration absorber, the NES iscapable of absorbing steady state vibration energy from the linearoscillator over a relatively broad frequency range. This results inlocalization of the steady state vibration in the NES, away from thedirectly forced subsystem. For a forward frequency sweep the localizedbranch of steady state motions is suddenly eliminated by a jump to alinearized low-amplitude motion, whereas, for a backward frequency sweepa reverse jump occurs. The difference in the frequencies of the twojumps introduces a nonlinear hysteresis loop. This work extends to thesteady state case of earlier transient passive energy pumping results.The notion of passively transferring vibration energy to an a prioridetermined NES, weakly attached to a main structure, is novel. The useof nonlinear energy sinks for passively absorbing energy from a linearmain structure can form the basis of relatively simple and modularvibration and shock isolation designs of mechanical systems.

Multilayer pebbles for application to new divertor systems

A `pebble drop' divertor concept is proposed for future commercial fusion reactors. The marked feature of this system is the use of multilayer pebbles that consist of a central kernel and some coating layers. By using multilayer pebbles the pebble drop divertor has advantages such as steady state wall pumping with low bulk tritium retention. The performance of the whole divertor system depends on the characteristics of the multilayer pebble. In particular, the maximum heat load of the system is determined by the dimensions, the layer structure and the material of the kernel. The kernel also has an important role in determining surface temperature, which affects the wall pumping efficiency. Numerical results for the maximum allowable heat load and the surface temperature of the divertor pebble are presented. From the numerical estimation of thermal stress and surface temperature, it is found that the radius of a divertor pebble with a ceramic kernel should be 0.5-1 mm.

Analysis of continuous and pulsed pumping of a phreatic aquifer

In a phreatic aquifer, fresh water is withdrawn by pumping from a recovery well. As is the case here, the interfacial surface (air/water) is typically assumed to be a sharp boundary between the regions occupied by each fluid. The pumping efficiency depends on the method by which the fluid is withdrawn. We consider the efficiency of both continuous and pulsed pumping. The maximum steady pumping rate, above which the undesired fluid will break through into the well, is defined as critical pumping rate. This critical rate can be determined analytically using an existing solution based on the hodograph method, while a Boundary Element Method is applied to examine a high flow rate, pulsed pumping strategy in an attempt to achieve more rapid withdrawal. A modified kinematic interface condition, which incorporates the effect of capillarity, is used to simulate the fluid response of pumping. It is found that capillarity influences significantly the relationship between the pumping frequency and the fluid response. A Hele-Shaw model is set up for experimental verification of the analytical and numerical solutions in steady and unsteady cases for pumping of a phreatic aquifer. When capillarity is included in the numerical model, close agreement is found in the computed and observed phreatic surfaces. The same model without capillarity predicts the magnitude of the free surface fluctuation induced by the pulsed pumping, although the phase of the fluctuation is incorrect.

Steady state wall pumping performance of pebble drop divertor

A pebble drop divertor concept is proposed for future fusion reactor. By using multi-layer coated pebbles, this system can be expected to provide the fuel gas pumping function. In this paper, the pumping performance of the pebble drop divertor is estimated. The transient behavior of hydrogen retained in the surface layer of pebbles in divertor irradiation is calculated using a mass balance equation of hydrogen atoms in the graphite surface. In this calculation, the incident hydrogen flux of 1×10 23 atoms/m 2 s (1.5 keV) and the heat flux of 35 MW/m 2, and the width of strike zone of 5 cm are assumed. Range distribution of 1.5 keV hydrogen and ion-induced detrapping cross-section is calculated by TRIM code. From the calculated dynamic hydrogen retention, it is shown that the pebble drop divertor using the pebble with graphite surface layer and BeO kernel can pump out hydrogen by 4.3×10 22−2.2×10 23 atoms/s from the ITER scale fusion reactor.

Optimizing separate phase light hydrocarbon recovery from contaminated unconfined aquifers

A modeling approach is presented that optimizes separate phase recovery of light non-aqueous phase liquids (LNAPL) for a single dual-extraction well in a homogeneous, isotropic unconfined aquifer. A simulation/regression/optimization (S/R/O) model is developed to predict, analyze, and optimize the oil recovery process. The approach combines detailed simulation, nonlinear regression, and optimization. The S/R/O model utilizes nonlinear regression equations describing system response to time-varying water pumping and oil skimming. Regression equations are developed for residual oil volume and free oil volume. The S/R/O model determines optimized time-varying (stepwise) pumping rates which minimize residual oil volume and maximize free oil recovery while causing free oil volume to decrease a specified amount. This S/R/O modeling approach implicitly immobilizes the free product plume by reversing the water table gradient while achieving containment. Application to a simple representative problem illustrates the S/R/O model utility for problem analysis and remediation design. When compared with the best steady pumping strategies, the optimal stepwise pumping strategy improves free oil recovery by 11.5% and reduces the amount of residual oil left in the system due to pumping by 15%. The S/R/O model approach offers promise for enhancing the design of free phase LNAPL recovery systems and to help in making cost-effective operation and management decisions for hydrogeologists, engineers, and regulators.

Dynamics of human milk extraction: A comparative study of breast feeding and breast pumping

We describe a mathematical model of the flow and deformation in a human teat. Our aim is to compare the theoretical milk yield during infant breast feeding with that obtained through the use of a breast pump. Infants use a peristaltic motion of the tongue, along with some suction, to extract milk, whereas breast pumps use a cyclic pattern of suction only. Our model is based on quasi-linear poroelasticity whereby the teat is modelled as a cylindrical porous elastic material saturated with fluid. We impose a cyclic axial suction pressure difference across the teat and impose a radial compressive force moving along the teat which mimics infant suckling. This is compared to the case of cyclic and steady pumping only which models the action of breast pumps. The results illustrate that there is an optimal time to apply the compressive force during the suction cycle that will increase the flow rate in our theoretical teat. The model and results may be of use in the future design of effective breast pumps.

Effect of subsurface heterogeneity on free-product recovery from unconfined aquifers

Free-product record system designs for light-hydrocarbon-contaminated sites were investigated to evaluate the effects of subsurface heterogeneity using a vertically integrated three-phase flow model. The input stochastic variable of the areal flow analysis was the log-intrinsic permeability and it was generated using the Turning Band method. The results of a series of hypothetical field-scale simulations showed that subsurface heterogeneity has a substantial effect on free-product recovery predictions. As the heterogeneity increased, the recoverable oil volume decreased and the residual trapped oil volume increased. As the subsurface anisotropy increased, these effects together with free- and total-oil contaminated areas were further enhanced. The use of multiple-stage water pumping was found to be insignificant compared to steady uniform pumping due to reduced recovery efficiency and increased residual oil volume. This observation was opposite to that produced under homogeneous scenarios. The effect of subsurface heterogeneity was enhanced at relatively low water pumping rates. The difference in results produced by homogeneous and heterogeneous simulations was substantial, indicating greater attention should be paid in modeling free-product recovery systems with appropriate subsurface heterogeneity.

The Sensitivity of Remedial Strategies to Design Criteria

AbstractThe specific criteria for design of systems for the remediation of contaminated ground water can affect the cost, configuration, and feasibility of the remedial system. Two important design criteria are the degree of remediation required, as measured by water concentration, and the time by which compliance must be achieved. In this paper the impact of these criteria on remedial design is studied for a hypothetical pump‐and‐treat remedial system applied to a simplified model of a large organic solvents plume in Tucson, Arizona. The analysis is performed by combining two‐dimensional numerical flow and transport modeling with nonlinear optimization. The use of optimization provides a means of systematically varying design criteria in an experimental framework. Based on the data for this site, variation of the concentration standard over one order of magnitude can affect the optimal cost by as much as 220%. Change in the time to achieve compliance over less than one order of magnitude can affect optimal cost by as much as 175% and in some cases can produce problems which are infeasible due to physical or technical limitations. Under some conditions, a time to achieve compliance can be identified which has minimum cost. Changes in the design criteria are also shown to have significant impact on the distribution of steady pumping rates among the individual wells.

The effects of pulsed pumping on land subsidence in the Santa Clara Valley, California

Land subsidence caused by pulsed and constant pumping schemes was calculated, and the results were compared to determine the possible advantages of pulsed pumping in reducing land subsidence. Pulsed pumping refers to pumping strategies involving alternation between periods of pumping and recovery in a well. Subsidence was calculated using a numerical model based on the Santa Clara Valley, CA. The model accounts for delay in the release of water from storage in compressible interbeds as well as flow within aquifers. For these tests, the ratio of pumping period length to recovery period length was either 1 1 or 1 3 . Pulsing interval length (length of one pumping period plus one recovery period) ranged from 4 to 90 days. Pumping rates were adjusted so that pulsed pumping and constant pumping removed the same volume of water over the period under consideration. Pulsed pumping has the potential to ‘concentrate’ subsidence around the pumping well. For all simulations, pulsed pumping caused greater subsidence near the test well than constant pumping. Under some conditions a transition point was reached, beyond which pulsed pumping caused less subsidence than steady pumping. The distance from the test well to the transition point decreases for (a) decreasing aquifer transmissivity, (b) decreasing ratio of pumping period to recovery period, and (c) increasing pulsing interval length. The concentration of subsidence was reduced considerably when delay in release of water from storage in compressible aquitards was neglected, indicating the importance of including delay effects in subsidence studies. All pulsed pumping strategies caused a greater total volume of subsidence than steady pumping.

Stepwise pumping approach to improve free phase light hydrocarbon recovery from unconfined aquifers

A stepwise, time-varying pumping approach is developed to improve free phase oil recovery of light non-aqueous phase liquids (LNAPL) from a homogeneous, unconfined aquifer. Stepwise pumping is used to contain the floating oil plume and obtain efficient free oil recovery. The graphical plots. The approach uses ARMOS ©, an areal two-dimensional multiphase flow, finite-element simulation model. Systematic simulations of free oil area changes to pumping rates are analyzed. Pumping rates are determined that achieve LNAPL plume containment at different times (i.e. 90, 180 and 360 days) for a planning period of 360 days. These pumping rates are used in reverse order as a stepwise (monotonically increasing) pumping strategy. This stepwise pumping strategy is analyzed further by performing additional simulations at different pumping rates for the last pumping period. The final stepwise pumping strategy is varied by factors of −25% and +30% to evaluate sensitivity in the free oil recovery process. Stepwise pumping is compared to steady pumping rates to determine the best free oil recovery strategy. Stepwise pumping is shown to improve oil recovery by increasing recoveredoil volume (11%) and decreasing residual oil (15%) when compared with traditional steady pumping strategies. The best stepwise pumping strategy recovers more free oil by reducing the amount of residual oil left in the system due to pumping drawdown. This stepwise pumping pproach can be used to enhance free oil recovery and provide for cost-effective design and management of LNAPL cleanup.

Stream depletion by cyclic pumping of wells

The rate of stream depletion caused by cyclic pumping from a well located in a connecting groundwater aquifer was studied. Equations were developed by applying superposition principles to analytical solutions for steady continuous pumping. Dimensionless plots based on these equations have been developed; one for dimensionless volume of stream depletion over a pumping cycle and another for maximum rate of stream depletion at a practical state of dynamic equilibrium. These plots provide a way to quickly determine the time at which a practical state of dynamic equilibrium is reached and the error in stream depletion rates produced by neglecting the nonuniform pumping rate within a pumping cycle. Analysis showed that the volume of stream depletion over a cycle ending at time t, is the same as the volume of stream depletion between the start of pumping and time t by a single period of pumping. Analysis also showed that under some circumstances, approximating the effects of cyclic pumping by a steady, continuous pumping at the equivalent cycle‐average rate is not adequate.

Characterization of Fracture Permeability with High‐Resolution Vertical Flow Measurements During Borehole Pumping

ABSTRACTThe distribution of fracture permeability in granitic rocks was investigated by measuring the distribution of vertical flow in boreholes during periods of steady pumping. Pumping tests were conducted at two sites chosen to provide examples of moderately fractured rocks near Mirror Lake, New Hampshire and intensely fractured rocks near Oracle, Arizona. A sensitive heat‐pulse flowmeter was used for accurate measurements of vertical flow as low as 0.2 liter per minute. Although boreholes were spaced at intervals ranging from 10 to 50 meters, acoustic televiewer logs showed little direct continuity of individual fractures from borehole to borehole in either the moderately fractured rocks or intensely fractured rocks. Results indicated that nearly all inflow and outflow to boreholes occurred by means of one or two discrete fractures in both cases. These fractures did not appear very different from other prominent fractures indicated on televiewer and resistivity logs for these boreholes. Hydraulic connections between boreholes apparently were composed of conduits formed by the most permeable portions of intersecting fractures. Most flow in the moderately fractured rocks occurred at isolated fractures at a depth of about 45 meters indicating a nearly horizontal zone of fracture permeability composed of orthogonal, steeply dipping fractures. Previous studies have identified a zone of horizontal permeability in the lower part of the boreholes in the intensely fractured rocks, but flowmeter tests indicated that flow also entered and exited individual boreholes by means of one or two steeply dipping fractures. These results indicate zones of fracture permeability in crystalline rocks are composed of irregular conduits that cannot be approximated by planar fractures of uniform aperture, and that the orientation of permeability zones may be unrelated to the orientation of individual fractures within those zones.

Optimizing the Rapid Evolution of Target Groundwater Potentiometric Surfaces

ABSTRACT -THE long-term availability of adequate groundwater for sustainable irrigated agricultural production can be assured by causing the evolution and maintenance of an appropriate potentiometric (piezometric) surface in utilized aquifers. By providing adequate saturated thickness and hydraulic gradients and heads, such a surface affords some protection from the effects of drought, economic hardship, and environmental or legal complications. Procedures for determining desirable or optimal regional steady-state groundwater potentiometric surfaces have been reported by several researchers. An existing potentiometric surface can be forced to evolve into a desirable 'target' surface if it is subjected through time to the proper combination of recharges and discharges. Depending on the difference between initial and target surfaces, steady pumping strategies may require extended periods to complete the evolutionary process. The evolutionary period may be significantly shortened by using a time-varying pumping strategy. Such a strategy may be calculated by combining goal programming and simulation. The presented technique maximizes attainment of target potentiometric surface elevations within a predetermined planning period. It is a valuable tool for a management agency that is seeking to determine how to most rapidly achieve desirable groundwater conditions in either an undeveloped region or a region that has an existing pattern of groundwater pumping.

DETERMINATION OF AQUIFER PARAMETERS USING REGRESSION ANALYSIS1

ABSTRACT: Transmissivity and storativity of an aquifer are usually determined by analysis of steady or nonsteady pumping test data. The classical methods of nonsteady pumping test analysis are mostly graphical in nature and are, therefore, subject to errors of judgment in curve fitting, interpolating, and reading graphs and charts. A method is described here which does not require construction of graphs or use of charts and tables. The transmissivity and the storativity are calculated using regression analysis of the nonsteady time drawdown field data. The calculations can readily be performed on a hand held calculator. The procedure is described using four examples, and the results are compared with those obtained from graphical techniques. It is shown that the method is a viable alternative to the type curve solution of Theis or Straight line solution of Jacob for nonleaky artesian aquifers. However, the regression method poses problem in the cases of leaky artesian and water table aquifers.

Sensitivity analysis in aquifer studies

A method is presented for determining in which areas detailed knowledge of aquifer characteristics and conditions is most critical to the success of a management plan. These questions are answered by using sensitivity analysis to determine how variations in parámeters and input data affect the optimal solution of a linear programing management model. The model uses either finite element or finite difference approximations of the groundwater equations as constraints. The optimal locations and discharge rates of wells were determined for dewatering a rectangular area to a specified level while minimizing steady state total pumping rate. The area is in a small aquifer having constant head boundaries. Sensitivity analysis showed that the optimal steady state solution is most sensitive to hydraulic conductivity at and near the aquifer boundaries parallel to the length of the dewatered area. Thus field exploration and testing should be concentrated on determination of hydraulic conductivity in those areas.

Solid state thermal control for spacecraft

The steady state heat pumping capabilities of thermoelectric devices are examined for spacecraft in Earth orbit, using a simple model. Thermoelectric parameters for a standard Bi 2Te 3 alloy are fitted with fourth order polynomials in temperature. Parameters are used which are the average for the temperature difference across the p− n couple. Internal spacecraft temperatures are varied from 270 to 330°K, and absorbed incident radiation fluxes are varied from zero to 700 W/m 2. It is found that heat rejection can be optimized with respect to thermoelectric device and radiator fin geometries, as well as with respect to input electrical power and that these devices are most useful on the surfaces with highest incident radiant power. Maximum heat fluxes which can be pumped out of the spacecraft are on the order of 50–160 W/m 2 for the various thermal environments examined. Input electrical powers corresponding to these maximum heat fluxes range from about 50–300 W/m 2. However, efficiency is greatly improved by operation at lower input power levels.