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Wind velocities within a plant canopy are much more strongly skewed than those of the air flow above. We have examined the governing Eulerian equations for the velocity products u
i, u
j uk using data from a wind tunnel study with an artificial canopy consisting of an array of 5 cm lengths of monofilament fishing line, and from measurements in corn (Zea mays L).Simple parameterizations for pressure-velocity correlations, and for the quadruple velocity products allowed reasonably accurate calculations of the third moments using measured profiles of the mean velocity, variance and covariance fields. Comparisons of individual terms in the rate equations for ovu
i, u
j
u
krevealed that diffusion (from above) and mean shear were most important in creating large skewness in the canopy. A drag term also contributed but was of lesser importance. These terms were balanced by return-to-isotropy and a turbulence interaction term. A quasi-Gaussian approximation considerably underestimated the magnitude of the fourth moments within the canopy. 相似文献
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Ido Seginer 《Boundary-Layer Meteorology》1972,3(1):87-97
Several attempts have been made to calculate the drag coefficient of a windbreak from the known flow field around it. Here the complete horizontal-momentum balance equation, rather than parts of it, is used to calculate the drag from Nägeli's field data. The vertical velocity, the horizontal shearing stress and the static pressure are all estimated from the horizontal velocity field.The results show the horizontal and vertical advection of horizontal momentum to be the largest components of the balance. The shear stress and static pressure are of lesser magnitude, but are not negligible. The results also show previous methods to be inadequate and suggest that direct wind-load measurement should be preferred whenever possible. 相似文献
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Abstract Various patterns of sprinkling assemblies constructed to simulate natural design storms in the field are described. A design storm is that storm causing the most severe erosion losses out of all storms in the period of design. Adequate means of conservation or protection should be devised to meet such a storm. The objective in the use of a rainfall simulator is to considerably shorten the study period, without having to wait for a natural design storm to occur. The simulated rain is applied by various overlapping patterns of two-arm rotating sprinklers positioned 2 metres above the ground. Results of performance tests of the simulator in relation to natural storms are summarized as follows: Uniformity of application was reproduced within a 10% range lower than natural storms. Intensities can be applied between 6 and 120 mm/hr within 10% of designed intensity. Angles of impact of simulator drops were found comparable to those of natural rain with wind velocities of 10 to 20 kph. The mode diameter of simulator drop size distribution was found to be 0.5–1.0 mm lower than that of natural rain. Kinetic energy was 60 to 75% and momentum 70 to 80% of those in a natural storm. Collection and measurement of runoff and erosion are described. 相似文献
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Flow around a windbreak in oblique wind 总被引：2，自引：0，他引：2
Ido Seginer 《Boundary-Layer Meteorology》1975,9(2):133-141
Wind speed was measured near the surface, along a line normal to a single, 50 % porous windbreak. The results for near-neutral atmospheric conditions were strongly dependent on the incidence angle of the wind, a, and slightly dependent on the roughness of the surface. A measure of the shelter effect - the protected distance - was found to decrease faster than cos . The drag coefficient of the windbreak decreased proportionally with cos . The effect of increased roughness was to increase the wind speed near the surface in the lee of the windbreak. 相似文献
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Ido Mizrachi Yossi Loya Esti Kramarski-Winter Aldo Shemesh 《Geochimica et cosmochimica acta》2010,74(24):7021-7030
The build up of the isotopic signal in corals was followed by sampling the newly formed skeleton at a monthly resolution for a period of two years in order to establish the interrelations between the calcification processes and the skeletal isotopic composition. We deployed two underwater sampling schemes, which provide a monitor of the changes in water temperature and δ18O and in the corresponding newly accreted skeleton of undisturbed Porites lutea colonies under natural conditions and four transplanted colonies, which maintained the genetic identity throughout the experiment. The results indicate that δ18O of the newly accreted skeleton does not correlate with ambient temperature although the seasonal temperature variability at the site (winter to summer) is in the order of 6 °C and δ18O of seawater is constant throughout the year. In contrast to the newly formed surface skeleton, the isotopic compositions of the deep and older parts of the skeleton show the predicted annual isotopic pattern with highly significant correlation between δ18Os and SST. The transformation between temperature-independent to temperature-dependent isotopic signal occurs several months after the skeleton was formed at the surface. The position of the skeleton in relation to the open sea may generate the difference between δ18Os of the surface skeleton and that of the skeleton previously accreted further down the tissue layer. Our data support the general model of a multi-step skeletogenesis process, where the temperature independent skeleton is entails the first step, the production of skeletal scaffold, and the environmental temperature signature is captured by the next two other steps: the thickening and the periodic abrupt uplift occurring at the depth of the tissue layer. However, re-examination and development of the current isotopic models for coral calcification are required in order to explain the observed different temperature dependency during the growth’s sequence. 相似文献
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Elhanan Almoznino Noah Brosch Ido Finkelman Hagai Netzer Erez R. Yacoby Jeremy Topaz Nachman Saar 《Astrophysics and Space Science》2009,320(4):321-341
We describe in detail the procedures used for the ground testing and calibration of the flight model of the TAUVEX space telescope.
We present the measurements done for this purpose, the various assumptions adopted during the ground calibration process,
the results obtained, and provide a first approximation to the real performance of the experiment.
The results of the ground-based calibration are that the system complies with the predicted performance in all aspects except
for the total throughput, which is a factor of ∼5 smaller than expected. However, the instrument sensitivity measurement is
highly uncertain due to known limitations of the available equipment and a more accurate calibration will be performed in
orbit. 相似文献
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Aerodynamic roughness of vegetated surfaces 总被引：2，自引：0，他引：2
Ido Seginer 《Boundary-Layer Meteorology》1974,5(4):383-393
Available experimental results indicate that as the density of roughness elements over a horizontally homogeneous surface is varied, the roughness length, z
0, varies in a manner that exhibits a maximum at intermediate density values. In an attempt to explain this behaviour, the available analytical solutions for the wind profile inside dense homogeneous canopies were reviewed. The review indicated that the variation of z
0 with density depends on the interrelationship between the leaf density, a, and the mixing length, l. In view of this finding, a numerical model was devised based on a simple rule for constructing mixing-length profiles in the canopy. The rule states that the actual value of l is the maximum possible under the two constraints: l l
i and ¦dl/dz¦ k, where k is the von Karman constant and the intrinsic mixing length, l
i, is a function of the local internal structure of the canopy. The model which ensures a smooth transition from dense to thin canopy, was used to reproduce the observed maximum of z
0. The model is also capable of handling vertically non-homogeneous canopies. 相似文献
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Ido Seginer 《Boundary-Layer Meteorology》1975,8(3-4):383-400
Wind speed was measured near the surface, along a line normal to a single, 50% porous windbreak, to determine its wind-reducing effect. Simultaneously, undisturbed wind and temperature profiles were measured to obtain the atmospheric stability. In some of the runs, the drag on a section of the windbreak was also measured. A systematic and significant effect of the atmospheric stability was found. The relative wind speed at any distance from the windbreak could be expressed for unstable conditions as an empirical function of the Richardson number. As an extreme example, the reduction of the surface shear, at a downwind distance 7.5 times the height of the windbreak, was 86 % in adiabatic conditions, and only 62 % when the gradient Richardson number at the windbreak height was ?1.0. The minimum relative effect was found in the neighborhood of the windbreak, where the windbreak-induced turbulence was dominant. The maximum absolute effect was found at a downwind distance of about ten times the height of the windbreak. The drag coefficient of the windbreak was found to bec d =0.77 in a neutral atmosphere, increasing slightly with instability. This trend is contrary to the trend of the surface-shear-reduction coefficient, which decreases significantly with instability. 相似文献
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The fate of the OP nerve agent isopropyl methylphosphonofluoridate (sarin) on granular activated and metal‐impregnated activated carbons that are used in gas‐mask filters was investigated by means of 31P magic‐angle‐spinning (MAS) NMR spectroscopy. The results show that most of the adsorbed sarin on extensively dried carbons decomposes with a half‐life of 5–12 days. A MAS‐NMR signal of the degradation product isopropyl methylphosphonic acid (IMPA) appears in the spectra of sarin on non‐impregnated carbons. The IMPA signal is not visible in the spectra from metal‐impregnated carbons, probably due to strong binding of the acid molecule to paramagnetic Cr3+ and Cu2+ ions. Exposure of BPL and ASC carbons to air of 53% relative humidity shortens the degradation time by approximately an order of magnitude. Wetting shortens the half‐life of sarin on BPL carbon to approximately 2 hours. 相似文献
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