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Abstract The breakdown and separation or reattachment of boundary layers adjoining a mainstream are studied in the three related situations (i)-(iii) of the title. For (i) the classical steady boundary layer generally admits a logarithmic singularity in the displacement when breakdown occurs on a downstream-moving surface whereas the corresponding singularity for an upstream-moving surface can be logarithmic or of minus-one-sixth form. Conversely, the breakdown can be delayed to the onset of zero mainstream flow, in which case the displacement singularity is again logarithmic. In certain flows these singularities prove to be removable locally, yielding a breakaway separation or reattachment and including the first known successes of a classical strategy in describing large-scale separation. Other flows, by contrast, require an interactive strategy. Again, even on a fixed surface a breakdown different from Goldstein's can be produced if there is a moving section of surface further upstream. The application to (ii), semi-similar unsteady boundary layers, e.g. near an impulsively started wedge-like trailing edge, then follows readily and predicts analogous forms of singularity. The corresponding singularity in displacement predicted for fully unsteady classical boundary layers, (iii), occurs within a finite time and, like (i) (usually) and (ii), a three-tiered breakdown is involved at first. Subsequently interaction comes into play. Comparisons with numerical and/or earlier work are noted. In all three situations (i)-(iii), although the dynamics involved near breakdown, separation or reattachment are predominantly inviscid, the presence of small viscosity is of significance in enforcing smoothness of the local velocity profiles. 相似文献
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《Astronomy& Geophysics》2009,50(3):3.38-3.38
Associate of the RAS, mapper of the large-scale structure of the local universe, and popular astronomy author and advocate, well-known as Director of Cape Town Planetarium. 相似文献
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Up-scaling of SEBAL derived evapotranspiration maps from Landsat (30 m) to MODIS (250 m) scale 总被引：2，自引：0，他引：2
Remotely sensed imagery of the Earth’s surface via satellite sensors provides information to estimate the spatial distribution of evapotranspiration (ET). The spatial resolution of ET predictions depends on the sensor type and varies from the 30–60 m Landsat scale to the 250–1000 m MODIS scale. Therefore, for an accurate characterization of the regional distribution of ET, scaling transfer between images of different resolutions is important. Scaling transfer includes both up-scaling (aggregation) and down-scaling (disaggregation). In this paper, we address the up-scaling problem.The Surface Energy Balance Algorithm for Land (SEBAL) was used to derive ET maps from Landsat 7 Enhanced Thematic Mapper Plus (ETM+) and Moderate Resolution Imaging Spectroradiometer (MODIS) images. Landsat 7 bands have spatial resolutions of 30–60 m, while MODIS bands have resolutions of 250, 500 and 1000 m. Evaluations were conducted for both “output” and “input” up-scaling procedures, with aggregation accomplished by both simple averaging and nearest neighboring resampling techniques. Output up-scaling consisted of first applying SEBAL and then aggregating the output variable (daily ET). Input up-scaling consisted of aggregating 30 m Landsat pixels of the input variable (radiance) to obtain pixels at 60, 120, 250, 500 and 1000 m before SEBAL was applied. The objectives of this study were first to test the consistency of SEBAL algorithm for Landsat and MODIS satellite images and second to investigate the effect of the four different up-scaling processes on the spatial distribution of ET.We conclude that good agreement exists between SEBAL estimated ET maps directly derived from Landsat 7 and MODIS images. Among the four up-scaling methods the output simple averaging method produced aggregated data and aggregated differences with the most statistically and spatially predictable behavior. The input nearest neighbor method was the least predictable but was still acceptable. Overall, the daily ET maps over the Middle Rio Grande Basin aggregated from Landsat images were in good agreement with ET maps directly derived from MODIS images. 相似文献
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W. M. Richter 《洁净——土壤、空气、水》1984,12(6):601-607
This paper represents the documentation of phragmites stocks by means of photographic records with a panoramic camera (120°/45° angular coverage, f 28 mm, 1 : 2.8). There are demonstrated the changes in the stock of reed due to grazing, boat traffic, entering from the bank and passing at the back as well as the weakening of stocks under the influence of increasing nutrient contents in the water. The method allows a very efficient inspection and documentation of the hatching strips and their changes. 相似文献
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The aim of this paper is to point out the advantages of multipoles for the exploration of the very near subsurface (0–3 m) by continuous profiling. We propose a new geometry with eight poles for a MUltipole Continuous Electrical Profiling (MUCEP) measuring system, where the array has a V-shape and is thus called ‘Vol-de-canards’. A series of criteria including 3D numerical simulations are performed (direct and inverse modelling) to determine the optimal geometry and to compare its performance (in terms of depth of investigation and resolution of the geometry of the targets) with the other arrays (quadrupoles or rectangular-type multipoles). This multipole was built together with a real-time acquisition system. The multidepth maps obtained confirm the characteristics predicted by numerical simulations. 相似文献
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The coprecipitation method is widely used for the preconcentration of trace metal ions prior to their determination by flame atomic absorption spectrometry (FAAS). A simple and sensitive method based on coprecipitation of Fe(III) and Ni(II) ions with Cu(II)‐4‐(2‐pyridylazo)‐resorcinol was developed. The analytical parameters including pH, amount of copper (II), amount of reagent, sample volume, etc., were examined. It was found that the metal ions studied were quantitatively coprecipitated in the pH range of 5.0–6.5. The detection limits (DL) (n = 10, 3s/b) were found to be 0.68 µg L?1 for Fe(III) and 0.43 µg L?1 for Ni(II) and the relative standard deviations (RSD) were ≤4.0%. The proposed method was validated by the analysis of three certified reference materials (TMDA 54.4 fortified lake water, SRM 1568a rice flour, and GBW07605 tea) and recovery tests. The method was successfully applied to sea water, lake water, and various food samples. 相似文献
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Yu Chen Yan‐peng Mao Hai‐song Zhu Jing‐yi Cheng Xiang‐li Long Wei‐kang Yuan 《洁净——土壤、空气、水》2010,38(7):601-607
The wet ammonia (NH3) desulfurization process can be retrofitted to remove nitric oxide (NO) and sulfur dioxide (SO2) simultaneously by adding soluble cobalt(II) salt into the aqueous ammonia solution. Activated carbon is used as a catalyst to regenerate hexaminecobalt(II), Co(NH3), so that NO removal efficiency can be maintained at a high level for a long time. In this study, the catalytic performance of pitch‐based spherical activated carbon (PBSAC) in the simultaneous removal of NO and SO2 with this wet ammonia scrubbing process has been studied systematically. Experiments have been performed in a batch stirred cell to test the catalytic characteristics of PBSAC in the catalytic reduction of hexaminecobalt(III), Co(NH3). The experimental results show that PBSAC is a much better catalyst in the catalytic reduction of Co(NH3) than palm shell activated carbon (PSAC). The Co(NH3) reduction reaction rate increases with PBSAC when the PBSAC dose is below 7.5 g/L. The Co(NH3) reduction rate increases with its initial concentration. Best Co(NH3) conversion is gained at a pH range of 2.0–6.0. A high temperature is favorable to such reaction. The intrinsic activation energy of 51.00 kJ/mol for the Co(NH3) reduction catalyzed by PBSAC has been obtained. The experiments manifest that the simultaneous elimination of NO and SO2 by the hexaminecobalt solution coupled with catalytic regeneration of hexaminecobalt(II) can maintain a NO removal efficiency of 90% for a long time. 相似文献
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A collaborative structural analysis (CSA) system is developed, which is capable of performing highly sophisticated structural analyses utilizing beneficial features of existing individual structural analysis programs. In the system, the global equations of motion for the overall structural system are formulated in the host program. Some substructures, whose behaviors are relatively simple, are directly solved in the host program, whereas those having complex behavior are analyzed by the station programs. A time‐consuming static condensation procedure is needed for the substructures analyzed by the station programs if adopting an implicit integration scheme. The operator splitting (OS) method, which does not require tangential stiffness, can be used to improve the system efficiency. To this end, a hybrid formulation of the Newmark‐β and OS methods is proposed, and a CSA scheme based on the hybrid formulation is developed. In the CSA system adopting the hybrid formulation, the degrees of freedom whose tangential stiffness are unavailable are formulated by the OS method, whereas the rest are still formulated by the commonly used Newmark‐β method. Using the system, analyses of a three‐story‐braced steel moment‐resisting frame are conducted. In the analyses, the column bases are analyzed using the commercial finite element method software ABAQUS, and the remaining structural elements are analyzed using a frame analysis program called NETLYS. Results suggest that the hybrid formulation is very effective for the CSA system. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
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A destructive earthquake of magnitude Mw = 6.8 hit the region of Boumerdes and Algiers (Algeria) on May 21, 2003. This is among the strongest seismic events of the mediterranean region and the most important event in the capital Algiers since 1716. It caused a widespread damage in the epicentral region, claimed 2271 human lives, injured 10000, about 20000 housing units affected and left about 160000 homeless. The main shock was felt about 250 km far from the epicenter and triggered sea waves of 1–3 m in amplitude in Balearic islands (Spain). Based on field observations and press report an intensity IX (MSK scale) is attributed to the epicentral area. The main shock was followed by many aftershocks among them several are of magnitude greater than 5.0, which added panic to inhabitants. The main shock triggered ground deformation, particularly liquefaction whose features are in different forms and sizes and caused damage and collapse of roads. The focal mechanism determined by worldwide institutions yield a pure reverse faulting with a compressional axis striking NE-SW. The epicenter is located offshore about 7 km from the Boumerdes-Dellys coast. Field observations show 0.7 m of coseismic uplift of shoreline between Boudouaou and Dellys. This uplift is about a half of the extracted coseismic slip from the seismic moment. On the other hand there is no clear surface break onshore, confirming hence, that the causative active fault is offshore. However, the rupture may propagate onshore to the SE near the Boudouaou region where ground cracks showing reverse faulting are observed a long a corridor of about 1 km wide. These fissures may correspond to a diffuse coseismic deformation. 相似文献