2,703 research outputs found
River monitoring from satellite radar altimetry in the Zambezi River basin
Satellite radar altimetry can be used to monitor surface water levels from space. While current and past altimetry missions were designed to study oceans, retracking the waveforms returned over land allows data to be retrieved for smaller water bodies or narrow rivers. The objective of this study is the assessment of the potential for river monitoring from radar altimetry in terms of water level and discharge in the Zambezi River basin. Retracked Envisat altimetry data were extracted over the Zambezi River basin using a detailed river mask based on Landsat imagery. This allowed for stage measurements to be obtained for rivers down to 80m wide with an RMSE relative to in situ levels of 0.32 to 0.72m at different locations. The altimetric levels were then converted to discharge using three different methods adapted to different data-availability scenarios: first with an in situ rating curve available, secondly with one simultaneous field measurement of cross-section and discharge, and finally with only historical discharge data available. For the two locations at which all three methods could be applied, the accuracies of the different methods were found to be comparable, with RMSE values ranging from 4.1 to 6.5% of the mean annual in situ gauged amplitude for the first method and from 6.9 to 13.8% for the second and third methods. The precision obtained with the different methods was analyzed by running Monte Carlo simulations and also showed comparable values for the three approaches with standard deviations found between 5.7 and 7.2% of the mean annual in situ gauged amplitude for the first method and from 8.7 to 13.0% for the second and third methods
The steady state in noncollinear magnetic multilayers
There are at least two different putative steady state solutions for current
across noncollinear magnetic multilayers; one has a discontinuity in the spin
current at the interface the other is continuous. We compare the resistance of
the two and find the solution with the continuous spin currents is lower. By
using the entropic principle we can state that this solution is a better
estimate of the resistance for a noncollinear magneticComment: 14 pages, 4 figures,Submitted to Physical Review
Extending Bauer's corollary to fractional derivatives
We comment on the method of Dreisigmeyer and Young [D. W. Dreisigmeyer and P.
M. Young, J. Phys. A \textbf{36}, 8297, (2003)] to model nonconservative
systems with fractional derivatives. It was previously hoped that using
fractional derivatives in an action would allow us to derive a single retarded
equation of motion using a variational principle. It is proven that, under
certain reasonable assumptions, the method of Dreisigmeyer and Young fails.Comment: Accepted Journal of Physics A at www.iop.org/EJ/journal/JPhys
Wake losses optimization of offshore wind farms with moveable floating wind turbines
In the future, floating wind turbines could be used to harvest energy in deep offshore areas where higher
wind mean speeds are observed. Currently, several floating turbine concepts are being designed and
tested in small scale projects; in particular, one concept allows the turbine to move after installation. This
article presents a novel layout optimization framework for wind farms composed of moveable floating
turbines. The proposed framework uses an evolutionary optimization strategy in a nested configuration
which simultaneously optimizes the anchoring locations and the wind turbine position within the mooring
lines for each individual wind direction. The results show that maximum energy production is
obtained when moveable wind turbines are deployed in an optimized layout. In conclusion, the framework
represents a new design optimization tool for future offshore wind farms composed of moveable
floating turbines
Mitral annuloplasty in patients with ischemic versus dilated cardiomyopathy
Objective: Mitral regurgitation is a frequent finding in patients with end-stage cardiomyopathy predicting poor survival. Conventional treatment consists medical treatment or cardiac transplantation. However, despite severely decreased left ventricular function, mitral annuloplasty may improve survival and reduce the need for allografts. Methods: From January 1996 to July 2002, 121 patients with severe end-stage dilated (DCM) or ischemic cardiomyopathy (ICM), mitral regurgitation ≥2, and left ventricular ejection fraction ≤30% underwent mitral valve annuloplasty using a flexible posterior ring. DCM was diagnosed in 30 patients (25%), whereas ICM was found in 91 patients (75%). Concomitant tricuspid valve repair was performed in 14 (46.6%) patients in the DCM, and in 11 (12%) in the ICM group (P=0.0001), coronary artery bypass grafting in three (10%) in the DCM, and in 78 patients (86%) in the ICM group (P<0.00001). The mean follow-up time was 567±74 days in the DCM and 793±63 days in the ICM group (ns). Results: Early mortality was 6.6% (8/121), and was equal for both groups. Improvement in NYHA class (DCM 3.3+0.1-1.8±0.16; ICM from 3.2+0.04 to 1.7±0.07) were equal between groups after 1 year. Seventeen (15%) late deaths occurred during the follow-up period. There was no difference in the 2-year actuarial survival between groups (DCM/ICM 0.93/0.85). Risk factors for mitral reconstruction failure, defined as regurgitation ≥2 after 1 year, were preoperative NYHA IV in the DCM group (P=0.03), a preoperative posterior infarction (P=0.025), decreased left ventricular function (P=0.043), larger ring size (P=0.026) and preoperative renal failure (P=0.05) in the ICM group. Risk factors for death were larger ring size (P=0.02) and an increased LVEDD (P=0.027) in the DCM group and the postoperative use of IABP (P=0.002), renal failure (P=0.001), and a larger preoperative LVESD (P=0.035) in the ICM group. Conclusion: Mitral reconstruction with a posterior annuloplasty using a flexible ring is effective in patients with severely depressed left ventricle function and has an acceptable operative mortality. Mid-term results are superior to medical treatment alone and comparable to cardiac transplantatio
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