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One Difficulty (one + difficulty)

Distribution by Scientific Domains
Engineering40%

Selected Abstracts

Review of the Integrated Groundwater and Surface-Water Model (IGSM)

GROUND WATER, Issue 2 2003
Eric M. LaBolle
Development of the finite-element-based Integrated Groundwater and Surface-Water Model (IGSM) began in the 1970s. Its popularity grew in the early 1990s with its application to California's Central Valley Groundwater Surface-Water Model in support of the Central Valley Project Improvement Act. Since that time, IGSM has been applied by federal, state, and local agencies to model a number of major basins in California. Our review of the recently released version 5.0 of IGSM reveals a solution methodology that deviates from established solution techniques, potentially compromising its reliability under many circumstances. One difficulty occurs because of the semi-explicit time discretization used. Combined with the fixed monthly time step of IGSM, this approach can prevent applications from accurately converging when using parameter values typically found in nature. Additionally, IGSM fails to properly couple and simultaneously solve ground water and surface water models with appropriate mass balance and head convergence under the reasonable conditions considered herein. As a result, IGSM-predicted streamflow is error prone, and errors could exceed 100%. IGSM does not inform the user that there may be a convergence problem with the solution, but instead generally reports good mass balance. Although our review touches on only a few aspects of the code, which exceeds 17,000 lines, our experience is that similar problems arise in other parts of IGSM. Review and examples demonstrate the potential consequences of using the solution methods in IGSM for the prediction, planning, and management of water resources, and provide perspective on the roles of standards and code validation in ground water modeling. [source]

Occurrence and quantity of precipitation can be modelled simultaneously

INTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 10 2004
Peter K. Dunn
Abstract Many statistical models exist for modelling precipitation. One difficulty is that two issues need to be addressed: the probability of precipitation occurring, and then the quantity of precipitation recorded. This paper considers a family of distributions for modelling the quantity of precipitation, including those observations in which exactly no precipitation is recorded. Two examples are then discussed showing the distributions model the precipitation patterns well. Copyright © 2004 Royal Meteorological Society [source]

Inf,sup control of discontinuous piecewise affine systems

INTERNATIONAL JOURNAL OF ROBUST AND NONLINEAR CONTROL, Issue 13 2009
J. Spjřtvold
Abstract This paper considers the worst-case optimal control of discontinuous piecewise affine (PWA) systems, which are subjected to constraints and disturbances. We seek to pre-compute, via dynamic programming, an explicit control law for these systems when a PWA cost function is utilized. One difficulty with this problem class is that, even for initial states for which the value function of the optimal control problem is finite, there might not exist a control law that attains the infimum. Hence, we propose a method that is guaranteed to obtain a sub-optimal solution, and where the degree of sub-optimality can be specified a priori. This is achieved by approximating the underlying sub-problems with a parametric piecewise linear program. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Joint line elevation in revision TKA leads to increased patellofemoral contact forces

JOURNAL OF ORTHOPAEDIC RESEARCH, Issue 1 2010
Christian König
Abstract One difficulty in revision total knee arthroplasty (TKA) is the management of distal femoral bone defects in which a joint line elevation (JLE) is likely to occur. Although JLE has been associated with inferior clinical results, the effect that an elevated joint line has on knee contact forces has not been investigated. To understand the clinical observations and elaborate the potential risk associated with a JLE, we performed a virtual TKA on the musculoskeletal models of four subjects. Tibio- and patellofemoral joint contact forces (JCF) were calculated for walking and stair climbing, varying the location of the joint line. An elevation of the joint line primarily affected the patellofemoral joint with JCF increases of as much as 60% of the patient's body weight (BW) at 10-mm JLE and 90% BW at 15-mm JLE, while the largest increase in tibiofemoral JCF was only 14% BW. This data demonstrates the importance of restoring the joint line, as it plays a critical role for the magnitudes of the JCFs, particularly for the patellofemoral joint. JLE caused by managing distal femoral defects with downsizing and proximalizing the femoral component could increase the patellofemoral contact forces, and may be a contributing factor to postoperative complications such as pain, polyethylene wear, and limited function. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:1,5, 2010 [source]

Theological Pragmatism: A Critical Evaluation

THE HEYTHROP JOURNAL, Issue 2 2000
Mikael Stenmark
Theological pragmatists like Daly, Kaufman, McFague and Reuther claim that the God we should believe in and the kind of images we should use to express our religious faith should be evaluated primarily on the basis of the consequences they have for the maintenance of certain political or moral values. These views are presented and critically evaluated. One difficulty is that their pragmatism seems to clash with our intuition and experience that there is no automatic fit between our moral aspirations and political visions, on the one hand, and how the world is actually structured, on the other. Their strong emphasis on political and moral considerations is, therefore, questionable and only plausible under certain specific circumstances. [source]