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11.
Power spectral density estimation is often employed as a method for signal detection. For signals which occur randomly, a frequency domain kurtosis estimate supplements the power spectral density estimate and, in some cases, can be employed to detect their presence. This has been verified from experiments with real data of randomly occurring signals. In order to better understand the detection of randomly occurring signals, sinusoidal and narrow-band Gaussian signals are considered, which when modeled to represent a fading or multipath environment, are received as non-Ganssian in terms of a frequency domain kurtosis estimate. Several fading and muitipath propagation probability density distributions of practical interest are considered, including Rayleigh and log-normal. The model is generalized to handle transient and frequency modulated signals by taking into account the probability of the signal being in a specific frequency range over the total data interval. It is shown that this model produces kurtosis values consistent with real data measurements. The ability of the power spectral density estimate and the frequency domain kurtosis estimate to detect randomly occurring signals, generated from the model, is compared using the deflection criterion. It is shown, for the cases considered, that over a large range of conditions, the power spectral density estimate is a better statistic based on the deflection criterion. However, there is a small range of conditions over which it appears that the frequency domain kurtosis estimate has an advantage. The real data that initiated this analytical investigation are also presented. 相似文献
12.
In an active sonar system setting, a novel method is evaluated that extracts range and Doppler information from a Doppler-spread active sonar echo. The Doppler spreading is based on H. Van Trees' (1971) Gaussian amplitude modulating model, and the new method is based on the Fourier transform of a special case of the fourth-order cumulant. Specifically, from the envelope of the Gaussian amplitude modulated echo of a transmitted coded pulse train, the second-order spectrum and the Fourier transform of a special case of the fourth-order cumulant are derived and analyzed for this ability to extract range and Doppler information. It is shown that the method can theoretically extract range and Doppler information without degradation. The reason for this result is that a special case of the fourth-order cumulant is independent of the covariance of the Gaussian amplitude modulating function. These methods are also simulated and compared with the simulated results of the range-Doppler ambiguity function. This shows that the ambiguity function and the second-order spectrum are degraded due to the Gaussian amplitude modulation. The results are further demonstrated by simulating the three range-Doppler extraction methods for the received echo in noise 相似文献
13.
Summary The edge function method is considered as an alternative to conventional numerical schemes for the solution of plane problems in rock mechanics. The essence of the approach is the approximation of the solution by a linear combination of solutions of the field equations. The unknowns in the linear combination are obtained from a system of equations which follows from the approximation of the boundary conditions by a boundary Galerkin energy method. No mesh generation is required over the domain or boundary of the problem. Previous edge function work in anisotropic elasticity is enhanced by the incorporation of a special solution for the effect of gravity. Examples are presented to illustrate the applicability of the method in determining stresses in various rock mechanics problems. A high level of accuracy is achieved with a relatively small number of degrees of freedom. Convergence is rapid because of the inclusion of special analytic solutions to model stress concentrations. The inclusion of the gravity force does, however, lead to a small increase in the number of degrees of freedom needed to achieve acceptable results. The optimum use of the edge function method, at present, may be as a special element within more general finite element or discrete element codes. 相似文献
14.
Turbulent kinetic energy budgets from a large-eddy simulation of airflow above and within a forest canopy 总被引：3，自引：2，他引：3
The output of a large-eddy simulation was used to study the terms ofthe turbulent kinetic energy (TKE) budget for the air layers above andwithin a forest. The computation created a three-dimensional,time-dependent simulation of the airflow, in which the lowest third ofthe domain was occupied by drag elements and heat sources to representthe forest. Shear production was a principal source of TKE in theupper canopy, diminishing gradually above tree-top height and moresharply with depth in the canopy. The transfer of energy to subgridscales (dissipation) was the main sink in the upper part of the domainbut diminished rapidly with depth in the canopy. Removal ofresolved-scale TKE due to canopy drag was extremely important,occurring primarily in the upper half of the forest where the foliagedensity was large. Turbulent transport showed a loss at the canopytop and a gain within the canopy. These general features have beenfound elsewhere but uncertainty remains concerning the effects ofpressure transport. In the present work, pressure was calculateddirectly, allowing us to compute the pressure diffusion term. Wellabove the canopy, pressure transport was smaller than, and opposite insign to, the turbulent transport term. Near the canopy top andbelow, pressure transport acted in concert with turbulent transport toexport TKE from the region immediately above and within the uppercrown, and to provide turbulent energy for the lower parts of theforest. In combination, the transport terms accounted for over half ofthe TKE loss near the canopy top, and in the lowest two-thirds of thecanopy the transport terms were the dominant source terms in thebudget. Moreover, the pressure transport was the largest source ofturbulent kinetic energy in the lowest levels of the canopy, beingparticularly strong under convective conditions. These resultsindicate that pressure transport is important in the plant canopyturbulent kinetic energy budget, especially in the lowest portion ofthe stand, where it acts as the major driving force for turbulentmotions. 相似文献
15.
Smith C.W. Ness N.F. Burlaga L.F. Skoug R.M. McComas D.J. Zurbuchen T.H. Gloeckler G. Haggerty D.K. Gold R.E. Desai M.I. Mason G.M. Mazur J.E. Dwyer J.R. Popecki M.A. Möbius E. Cohen C.M.S. Leske R.A. 《Solar physics》2001,204(1-2):227-252
We present ACE observations for the six-day period encompassing the Bastille Day 2000 solar activity. A high level of transient
activity at 1 AU, including ICME-driven shocks, magnetic clouds, shock-accelerated energetic particle populations, and solar
energetic ions and electrons, are described. We present thermal ion composition signatures for ICMEs and magnetic clouds from
which we derive electron temperatures at the source of the disturbances and we describe additional enhancements in some ion
species that are clearly related to the transient source. We describe shock acceleration of 0.3–2.0 MeV nucl−1 protons and minor ions and the relative inability of some of the shocks to accelerate significant energetic ion populations
near 1 AU. We report the characteristics of < 20 MeV nucl−1 solar energetic ions and < 0.32 MeV electrons and attempt to relate the release of energetic electrons to particular source
regions. 相似文献
16.
Geographers have assessed the success and failure of the U.S. Civil Rights Movement in terms of the African American struggle for justice, social identity, and economic survival. Conspicuously absent from the geographic literature are pedagogically oriented studies of the historical geography of the Civil Rights era. The Movement's popular image has congealed into a celebratory collection of names and dates, the sum of which is a vague, nearly mythic retelling that students might recognize but not necessarily care about. As a result, the Movement is at once contemptuously familiar yet bewilderingly strange for our students. This article offers a sympathetic critique of conventional Movement narratives, introducing the notion of empathetic pedagogy and presenting a case study of the Montgomery bus boycott. Our pedagogical approach stresses the role of empathy, both as a factor in shaping the actual sociospatial development of the Movement, as well as a strategy for encouraging students to appreciate the everyday courage and sacrifice that animated so many of its participants. Our study brings together two burgeoning literatures that have the potential to cultivate empathy among students: the critical reevaluation of mobility and explorations of subjectivity from a psychoanalytic perspective. Here mobility is understood in both its literal and figurative sense: in the case of the bus boycott, the intricate network established to literally move African Americans around the city, as well as the figurative movement of sympathy and solidarity that “moved” people to support their efforts and now informs popular, selective understandings of the protest. 相似文献
17.
A. C. Hack R. C. Dwyer G. Phillips S. Whalan H.-Q. Huang 《Australian Journal of Earth Sciences》2013,60(7-8):1071-1096
AbstractMagmatic-textured zircon from medium- to high-K calc-alkaline Warraweena Volcanics (WV) in two drill holes have yielded concordant U–Pb dates of 417?±?3.5 and 414?±?4.0?Ma and are interpreted as maximum emplacement ages. The Warraweena volcanics were previously considered to be either Neoproterozoic or Macquarie arc equivalents. Whole-rock εNdt values of these volcanics are +4.5 and +4.8. Along strike of the drill holes, Devonian zircon U–Pb ages (411?±?5.5?Ma) were obtained from coherent S-type rhyolite flows that have highly negative εNdt values (–7.9 and –7.8). These are a component of the Oxley volcanics. The ages of the Warraweena and Oxley volcanics are identical within uncertainty.The Oxley volcanics (OV) are interbedded with predominantly fine- to medium-grained metasedimentary and so imply a Lower Devonian deposition age for these host rocks. Based on their geophysical characteristics, the metasediments are widely distributed. These metasedimentary rocks yield a wide range of maximum depositional ages, from Early Devonian to earliest Ordovician–latest Cambrian, similar to the Cobar Basin. The absence of complex fabric development typical of Ordovician supracrustal rocks in the region, and conformity with the OV where observable suggest the widespread sedimentation was synchronous with rift-related volcanism in the Early Devonian.Regionally, the WV is temporally, geochemically and isotopically (εNd values) similar to the calc-alkaline Louth Volcanics located over 100?km to the southwest of the WV. Louth Volcanics define a complexly folded belt in geophysical data. Other potentially correlative Early Devonian igneous rocks occur in the nearby Cobar Superbasin and elsewhere in the eastern Lachlan Orogen and are considered to represent the products of a post-orogenic, nascent continental back-arc rift system. 相似文献
18.
R. C. Dwyer W. J. Collins A. C. Hack R. Hegarty H.-Q. Huang 《Australian Journal of Earth Sciences》2013,60(7-8):1049-1069
AbstractRe-evaluation of geochemical and geophysical datasets, and analysis of magmatic and detrital zircons from drill-core samples extracted from the Louth region of the southern Thomson Orogen (STO), augmented by limited field samples, has shown that two temporally and compositionally distinct igneous groups exist. The older Lower Devonian, calc-alkaline group corresponds to complexly folded, high-intensity curvilinear magnetic anomalies in the Louth region (Louth Volcanics) and are probable equivalents to Lower Devonian volcanics in the northern Lachlan Orogen. A younger Permo-Triassic alkaline assemblage forms part of an E–W corridor of diatremes that appears to relate to focussed lithospheric extension associated with the later stages of the Hunter–Bowen Orogeny in the New England Orogen. The alkaline group includes gabbros previously considered as Neoproterozoic, but all magmatic rocks, including alkaline basalts, contain an unusual number of xenocrystic zircons. The age spectra of the xenocrystic zircons mimic detrital zircons from Cobar Basin sedimentary rocks and/or underlying Ordovician turbidites, suggesting incorporation of upper crustal zircons into the alkaline basaltic magmas. A distinct difference of detrital zircon age spectra from central Thomson Orogen metasediments indicates the STO metasediments have greater affinities to the Lachlan Orogen, but both orogens probably began in the Early Ordovician during widespread backarc extension and deposition of turbidites in the Tasmanides. A surprising result is that Ordovician, Devonian and Permo-Triassic basaltic rocks from the STO and elsewhere in the Tasmanides, all yield the same Nd-model ages of ca 960–830 Ma, suggesting that Neoproterozoic subcontinental lithospheric mantle persisted throughout the evolution of the Tasmanide orogenic system. 相似文献
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Until recently, vessel collisions with small cetaceans were presumed rare, mainly as a consequence of limited reporting. Observations on dolphin wound healing from propeller strike injuries also remain scarce. We present an extreme case of a bottlenose dolphin (Tursiops truncatus) with multiple propeller wounds, including a penetration to the bone, where survival was possible for at least 23 days post injury. We used photographic records in conjunction with field observations to describe wound progression in the absence of treatment. Considering the severity of the wounds, it was surprising that the injuries were not immediately fatal. A practical solution remains to be found for the problem of odontocete vessel collisions, in particular for small highly mobile species using neritic waters. In view of the ongoing problem in the Hauraki Gulf and, indeed, worldwide, we recommend continued evaluation of cases and reporting of incidents, in addition to investigations into mechanisms that may reduce the risk of occurrence of vessel collisions with cetaceans. 相似文献