Determination of prestressing levels for cable trusses as a function of their stability
The stability of prestressed cable trusses subjected to static and dynamic loads is analysed in the paper. The stiffness of the elastic cable truss system was varied by changing the prestressing force. Modelling results for different levels of tension show that, in terms of satisfying dynamic resistance conditions, a much greater tensile force is required for cable grids with diagonal members, compared to those with vertical rods. The conclusions derived constitute recommendations for the use of calculation methods, as well as for the determination of prestressing forces under which stability criteria are fulfilled.
Evaluation of two atmospheric models for wind–wave modelling in the NW Mediterranean
Abstract The NW Mediterranean experiences, as illustrated by the last decade, strong and rapidly varying storms with severe waves and winds. This has motivated a continuous validation of models and the efforts to improve wave and wind predictions. In this paper we use two atmospherics models, MASS (from SMC-Meteorological Office of Catalunya) and ARPEGE (from Meteo-France), to force two third generation wave models: WAM and SWAN. The evaluation and comparison has been carried out for two severe storms registered in November 2001 and March–April 2002. The ARPEGE and MASS models predicted higher 10 m wind speeds than coastal meteorological stations, a fact attributed to local land influences. Regarding the 10 m wind direction, models do not present large differences, although considerable deviations from recorded data were found during some dates. ARPEGE presents less scatter and lower errors than MASS when compared with QuikSCAT data. The 10m wind fields from both atmospheric models were used to force the two selected wave models and analyse the errors and sensitivities when predicting severe wave storms. The wave model simulations show some interesting results; during the storm, the spatial wave pattern using ARPEGE showed a higher maximum, although the values of significant wave height at the buoys were lower than the ones forced by MASS (with both WAM and SWAN). The SWAN simulations show a better agreement in predicting the growing and waning of the storm peaks. The prediction of mean period was improved when using the ARPEGE wind field. However the underestimation by SWAN due to the large energy at high frequencies was evident. Validation of spectral shape predictions showed that it still has considerable error when predicting the full frequency spectra. The storms showed bimodal spectral features which were not always reproduced by wave models and are likely to be responsible for part of the discrepancies.
Dependence of Whitecap Coverage on Wind and Wind-Wave Properties
Using Phillips equilibrium range theory and observational data, we show first that the total rates of wave energy dissipation estimated by the Hasselmann and Phillips dissipation models are substantially consistent with each other, though their original forms are different. Both are proportional to the cube of air friction velocity, u*3, with a weak dependence on wave age. As a direct manifestation of the wave energy dissipation processes, we reanalyze previous observational data of whitecap coverage and find that it has greater correlation with the wind speed or friction velocity than the wave period or wave age. However, the data scatter decreases remarkably when the breaking-wave parameter RB= u*2/νωp is used, where ν is the kinematic viscosity of air, and ωp, the wind-wave spectral peak frequency. Physical interpretation of RB with some related issues, and a discussion of the probability models of whitecap coverage in terms of a threshold mechanism, are also presented. We conclude that RB is a good parameter to effectively express the overall wave breaking behavior for the case of wind-waves in local equilibrium with the wind. Since RB can be expressed as the product of u*3 and the wave age, this result demonstrates a stronger dependence of whitecap coverage on wave age than expected by the previous description by power-laws of u* and by the two theoretical models. Our conclusion suggests that current dissipation models should also be modified to represent full properties of wind-wave breaking.