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31.
Despite its importance for momentum and mass transfer across the air–sea interface, the dynamics of airflow over breaking
waves is largely unknown. To fill this gap, velocity and vorticity distributions above short-gravity breaking waves have been
measured in a wind-wave tank. A Digital Particle Image velocimetry technique (DPIV) was developed to accomplish these measurements
above single breaking waves, propagating in mechanically-generated wave groups and forced by the wind. By varying the wind
speed and initial characteristics of the groups, the airflow structure was captured over waves at different stages of the
breaking process, and breaking with various intensities. The instantaneous airflow that separates from a sharp breaking crest
is very similar to that occurring over a backward facing step. The separation bubble is however strongly unsteady: the steeper
the wave crest and the larger the Reynolds number based on the crest-height, the higher the separated layer and the farther
downwind the reattachment point. Instantaneous flow topology displays specific features of three-dimensional separation patterns.
The tangential stress above the wave profile does not exhibit spikes at reattachment but grows progressively downwind from
zero at reattachment to a value at the next crest approximately that found at the upwind breaking crest. Static pressure measurements
revealed that large pressure falls are generated by vortices in the separated layer, as found in separated flows over solids.
This study may provide useful data for theoretical and numerical modelling of the flow and associated phenomena. 相似文献
32.
A. Longinelli B. Stenni L. Genoni O. Flora C. Defrancesco G. Pellegrini 《Journal of Hydrology》2008,360(1-4):103-116
Discontinuous measurements of the isotopic composition of surface water samples of the Garda lake carried out between 1998 and 2006 showed almost constant δ18O, δD and d-excess values through time. During 2006 and 2007 monthly vertical profiles of water samples were collected in the northernmost section of the lake, not far from the main inflow (Sarca river) to check whether there was any detectable influence from this inflowing river and whether there was a vertical isotopic stratification of the lake water. The isotopic measurement of water samples from the vertical profiles yielded isotopic values which were almost equal to those obtained from surface waters showing no detectable effect of the inflowing river water and no isotopic vertical stratification. The attempt to evaluate the evaporation rate of lake water by means of current models was totally unsuccessful. Despite the marked summer warming of the surface layer no isotopic fractionation related to evaporation processes could be detected. This anomalous behaviour may be related to the large amount of spring and summer precipitation characteristic of this area. The water balance of the lake calculated according to the amount of the inflowing water (Sarca river water plus rain water on the lake plus 20% of the precipitations on the whole catchment basin) and to the amount of outflowing water (Mincio river) showed a large imbalance, the river outflow alone resulting on average, during the last decade, at least double the inflow. To explain this imbalance of the lake, a large recharge by concealed groundwater is suggested: its isotopic composition should be quite close to the mean isotopic composition of precipitations over that area. This would be in agreement with the almost constant isotopic composition of both surface and deep waters and with the lack of vertical isotopic stratification. A few measurements of the tritium concentration carried out on lake water show values that are considerably higher than modern tritium values either in precipitation or in the Sarca river water: these results are in good agreement with the hypothesis of a recharge of the lake by deep aquifers. 相似文献
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34.
An alternative approach for the seismic rehabilitation of existing RC buildings using seismic isolation 
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下载免费PDF全文 Usually, buildings with seismic isolation are designed to comply with an operational building performance level after strong earthquakes. This approach, however, may limit the use of seismic isolation for the seismic rehabilitation of existing buildings with low lateral strength or substandard details, because it often requires invasive strengthening measures in the superstructure or the use of expensive custom‐made devices. In this paper, an alternative approach for the seismic rehabilitation of existing buildings with seismic isolation, based on the acceptance of limited plastic deformations in the superstructure under strong earthquakes, is proposed and then applied to a real case study, represented by a four‐storey RC frame building. Nonlinear response‐time histories analyses of an accurate model of the case‐study building are carried out to evaluate the seismic performances of the structure, comparing different rehabilitation strategies with and without seismic isolation. Initial costs of the intervention and possible (future) repair costs are then estimated. Based on the results of this study, values of the behavior factor (i.e. response modification factor) higher than those adopted in the current codes for base‐isolated buildings are tentatively proposed. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
35.
Mele Lucia Chiaradonna Anna Lirer Stefania Flora Alessandro 《Bulletin of Earthquake Engineering》2021,19(10):3865-3893
In engineering practice, the liquefaction potential of a sandy soil is usually evaluated with a semi-empirical, stress-based approach computing a factor of safety in free field conditions, defined as the ratio between the liquefaction resistance (capacity) and the seismic demand. By so doing, an estimate of liquefaction potential is obtained, but nothing is known on the pore pressure increments (often expressed in the form of normalized pore pressure ratio ru) generated by the seismic action when the safety factor is higher than 1. Even though ru can be estimated using complex numerical analyses, it would be extremely useful to have a simplified procedure to estimate them consistent with the stress-based approach adopted to check the safety conditions. This paper proposes such a procedure with reference to both saturated and unsaturated soils, considering the latter as soils for which partial saturation has been artificially generated with some ground improvement technology to increase cyclic strength and thus tackle liquefaction risk. A simple relationship between the liquefaction free field safety factor FS, and ru(Sr) is introduced, that generalizes a previous expression proposed by Chiaradonna and Flora (Geotech Lett, 2020. https://doi.org/10.1680/jgele.19.00032) for saturated soils. The new procedure has been successfully verified against some experimental data, coming from laboratory constant amplitude cyclic tests and from centrifuge tests with irregular acceleration time histories for soils having different gradings and densities.
相似文献36.
Mariano Angelo Zanini Lorenzo Hofer Flora Faleschini Carlo Pellegrino 《Natural Hazards》2017,86(3):1247-1273
A tornado with severe intensity hit the municipalities of Pianiga, Dolo and Mira close to Venice, northeast Italy, causing damages on a wide number of residential and industrial buildings and destroying some historical villas. In this study, the authors show the results of the damage assessment survey performed in the first days after the occurrence of the extreme event. Limited literature deals with damage assessment of European building types due to wind actions, and the available one does not consider building vulnerability as key factor in the structural response of existing structures subject to tornado hazard. In this paper, structural damages surveyed in reinforced concrete frame structures and masonry buildings, representative of common Italian building types, are critically discussed. Additionally, this work provides a database of past tornado events in northeast Italy, evidencing how the analyzed area has been found to be quite prone to tornado hazard. 相似文献