Effects of finite poloidal gyroradius, shaping, and collisions on the zonal flow residual
Xiao Yong; Catto, Peter J.; Dorland, William
2007-05-15
Zonal flow helps reduce and regulate the turbulent transport level in tokamaks. Rosenbluth and Hinton have shown that zonal flow damps to a nonvanishing residual level in collisionless [M. Rosenbluth and F. Hinton, Phys. Rev. Lett. 80, 724 (1998)] and collisional [F. Hinton and M. Rosenbluth, Plasma Phys. Control. Fusion 41, A653 (1999)] banana regime plasmas. Recent zonal flow advances are summarized including the evaluation of the effects on the zonal flow residual by plasma cross-section shaping, shorter wavelengths including those less than an electron gyroradius, and arbitrary ion collisionality relative to the zonal low frequency. In addition to giving a brief summary of these new developments, the analytic results are compared with GS2 numerical simulations [M. Kotschenreuther, G. Rewoldt, and W. Tang, Comput. Phys. Commun. 88, 128 (1991)] to demonstrate their value as benchmarks for turbulence codes.
Generation of zonal flow and magnetic field by finite-amplitude waves in the ionospheric E-layer
NASA Astrophysics Data System (ADS)
Kahlon, Laila; Kaladze, Tamaz
2016-07-01
We review the generation of zonal flow and magnetic field by coupled electromagnetic (EM) ULF waves in the Earth's ionospheric E layer. It is shown that under the typical ionospheric E-layer conditions different planetary low-frequency waves can couple with each other. Propagation of coupled internal-gravity-Alfvén (CIGA), coupled Rossby-Khantadze (CRK) and coupled Rossby-Alfvén-Khantadze (CRAK) waves is revealed and studied. A set of appropriate equations describing the nonlinear interaction of such waves with sheared zonal flow is derived. The conclusion on the instability of short wavelength turbulence of such coupled waves with respect to the excitation of low-frequency and large-scale perturbation of the sheared zonal flow and sheared magnetic field is deduced. The nonlinear mechanism of the instability is based on the parametric triple interaction of finite amplitude coupled waves leading to the inverse energy cascade toward the longer wavelength. The possibility of generation of the intense mean magnetic field is shown. Obtained growth rates are discussed for each case of the considered coupled waves.
Collisional damping of zonal flows due to finite Larmor radius effects
NASA Astrophysics Data System (ADS)
Ricci, Paolo; Rogers, B. N.; Dorland, W.
2010-07-01
The collisional damping of seeded E ×B zonal flows on the ion Larmor radius scale is studied using a gyrokinetic model. The focus is on flow damping due to finite Larmor radius effects, which cause a v∥/v anisotropy of the ion distribution function that is damped by ion-ion collisions. The gyrokinetic equations are solved in a slab geometry with no gradients or curvature, and a gyroaveraged Lorentz collision operator that conserves particle number, momentum, and energy is used. The solution of the gyrokinetic equations explores the dependence of the damping rate on the wavelength of the flows and the impact of the collisions on the ion distribution function. These numerical results can be used as a benchmark test during the implementation of finite Larmor radius effects in the collision operator of gyrokinetic codes.
Low-frequency intraseasonal variability in a zonally symmetric aquaplanet model
NASA Astrophysics Data System (ADS)
Das, Surajit; Sengupta, Debasis; Chakraborty, A.; Sukhatme, Jai; Murtugudde, Raghu
2016-04-01
We use the aquaplanet version of the community atmospheric model, with perpetual spring equinox forcing and zonally symmetric sea surface temperature (SST), to study tropical intraseasonal oscillations (ISOs). In the first two experiments, we specify zonally symmetric SST profiles that mimic observed climatological July and January SSTs as surface boundary conditions. In the January SST simulation, we find a zonal wavenumber 1 mode with dominant period of 60 days, moving east at about 6 m s-1. This mode, which resembles the Madden-Julian oscillation (MJO), is absent in the July SST case, although convectively coupled Kelvin waves are prominent in both experiments. To further investigate the influence of tropical SST on ISO and convectively coupled equatorial waves, we conduct experiments with idealised symmetric SST profiles having different widths of warm ocean centered at the equator. In the narrowest SST experiment, the variance of moist activity is predominantly in weather-scale Kelvin waves. When the latitudinal extent of warm SST is comparable to or larger than the equatorial Rossby radius, we find a dominant low frequency (50-80 days) eastward mode that resembles the MJO, as in the January SST experiment. We also find westward propagating waves with intraseasonal (30-120 days) periods and zonal wavenumber 1-3; the structure of these signals projects onto equatorially trapped Rossby waves with meridional mode numbers 1, 3 and 5, associated with convection that is symmetric about the equator. In addition, the model generates 30-80 days westward moving signals with zonal wavenumber 4-7, particularly in the narrow SST experiment. Although these waves are seen in the wavenumber-frequency spectra in the equatorial region, they have largest amplitude in the middle and high latitudes. Thus, our study shows that wider, meridionally symmetric SST profiles support a strong MJO-like eastward propagation, and even in an aquaplanet setting, westward propagating Rossby
The global atmospheric response to low-frequency tropical forcing: Zonally averaged basic states
NASA Technical Reports Server (NTRS)
Li, Long; Nathan, Terrence R.
1994-01-01
The extratropical response to localized, low-frequency tropical forcing is examined using a linearized, non-divergent barotropic model on a sphere. Zonal-mean basic states characterized by solid-body rotation or critical latitudes are considered. An analytical analysis based on WKB and ray tracing methods shows that, in contrast to stationary Rossby waves, westward moving, low-frequency Rossby waves can propagate through the tropical easterlies into the extratropics. It is shown analytically that the difference between the stationary and low-frequency ray paths is proportional to the forcing frequency and inversely proportional to the zonal wavenumber cubed. An expression for the disturbance amplitude is derived that shows the ability of the forced waves to maintain their strength well into middle latitudes depends on their meridional wave scale and northward group velocity, both of which are functions of the slowly varying background flow. A local energetics analysis shows that the combination of energy dispersion from the forcing region and energy extraction from the equatorward flank of the midlatitude jet produces disturbances that have the greatest impact on the extratropical circulation. Under the assumption that the forcing amplitude is independent of frequency, this impact is largest when the tropical forcing period is in the range 10-20 days.
Ghizzo, A.; Palermo, F.
2015-08-15
We address the mechanisms underlying low-frequency zonal flow generation in turbulent system and the associated intermittent regime of ion-temperature-gradient (ITG) turbulence. This model is in connection with the recent observation of quasi periodic zonal flow oscillation at a frequency close to 2 kHz, at the low-high transition, observed in the ASDEX Upgrade [Conway et al., Phys. Rev. Lett. 106, 065001 (2011)] and EAST tokamak [Xu et al., Phys. Rev. Lett 107, 125001 (2011)]. Turbulent bursts caused by the coupling of Kelvin-Helmholtz (KH) driven shear flows with trapped ion modes (TIMs) were investigated by means of reduced gyrokinetic simulations. It was found that ITG turbulence can be regulated by low-frequency meso-scale zonal flows driven by resonant collisionless trapped ion modes (CTIMs), through parametric-type scattering, a process in competition with the usual KH instability.
NASA Astrophysics Data System (ADS)
Ghizzo, A.; Palermo, F.
2015-08-01
We address the mechanisms underlying low-frequency zonal flow generation in turbulent system and the associated intermittent regime of ion-temperature-gradient (ITG) turbulence. This model is in connection with the recent observation of quasi periodic zonal flow oscillation at a frequency close to 2 kHz, at the low-high transition, observed in the ASDEX Upgrade [Conway et al., Phys. Rev. Lett. 106, 065001 (2011)] and EAST tokamak [Xu et al., Phys. Rev. Lett 107, 125001 (2011)]. Turbulent bursts caused by the coupling of Kelvin-Helmholtz (KH) driven shear flows with trapped ion modes (TIMs) were investigated by means of reduced gyrokinetic simulations. It was found that ITG turbulence can be regulated by low-frequency meso-scale zonal flows driven by resonant collisionless trapped ion modes (CTIMs), through parametric-type scattering, a process in competition with the usual KH instability.
Finite frequency tomography: the checkerboard test revisited
NASA Astrophysics Data System (ADS)
Mercerat, E. D.; Zaroli, C.; Nolet, G.
2011-12-01
We address some consequences of the application of finite frequency theory for seismic tomography by revisiting the classical checkerboard test. We use a simple borehole-to-borehole experiment set-up in order to have complete control of the situation and to avoid complicating factors such as crustal corrections that still hamper global tomography. We are particularly interested in the feasibility of using ray-based finite frequency kernels in the inversion of travel time perturbations measured by crosscorrelation, in the cross-dependence between S wave velocity perturbations and the measured P travel times, and in the benefits of using finite-frequency theory on one or multiple frequency bands. We have done a 3D checkerboard test to assess the influence of these issues. Full-waveform synthetic seismograms are calculated using the spectral elements method up to 2 kHz maximum frequency. The computational domain extends 200 m x 120 m x 120 m and the target velocity model is a checkerboard with 12 m x 12 m x 12 m blocks of velocities 5% slower and faster than the background (homogeneous, Vp=6 km/s) model. First, we make a comparison between finite-frequency kernels calculated by ray theory with those based on the spectral elements method (adjoint technique), in terms of resolution, accuracy, but also computational cost. From synthetic seismograms calculated for the 3D checkerboard model as well as for the homogeneous model, we measure crosscorrelation travel times at different frequency bands and invert them with classical ray theory as well as with finite frequency theory. Several interesting features are highlighted in our multi-band data set, such as the wavefront healing effect. For instance, we observe that the delay times, in absolute value, are usually larger at short (0.5 ms) than long (4 ms) periods. This can be explained by the presence of the "doughnut hole" along the geometrical ray path in the sensitivity kernels, whose diameter is proportional to the
Ghizzo, A.; Palermo, F.
2015-08-15
Collisionless trapped ion modes (CTIMs) turbulence exhibits a rich variety of zonal flow physics. The coupling of CTIMs with shear flow driven by the Kelvin-Helmholtz (KH) instability has been investigated. The work explores the parametric excitation of zonal flow modified by wave-particle interactions leading to a new type of resonant low-frequency zonal flow. The KH-CTIM interaction on zonal flow growth and its feedback on turbulence is investigated using semi-Lagrangian gyrokinetic Vlasov simulations based on a Hamiltonian reduction technique, where both fast scales (cyclotron plus bounce motions) are gyro-averaged.
Detection of Zero-Mean-Frequency Zonal Flows in the Core of a High-Temperature Tokamak Plasma
Gupta, D. K.; Fonck, R. J.; McKee, G. R.; Schlossberg, D. J.; Shafer, M. W.
2006-09-22
A low-frequency, spectrally broad ({delta}f{approx}10 kHz) poloidal flow structure that peaks near zero frequency is observed in time-resolved measurements of the turbulence velocity field in the core region (r/a{approx}0.6-0.9) of DIII-D tokamak plasmas. These flows exhibit a long poloidal wavelength (low m) and a short radial coherence length comparable to the ambient turbulence decorrelation length. Characteristics of these observed poloidal flows are consistent with the theoretically predicted residual or zero-mean-frequency zonal flows.
Broadband Finite Frequency Ambient Noise Tomography
NASA Astrophysics Data System (ADS)
Zhao, K.; Yang, Y.; Luo, Y.; Xie, J.
2015-12-01
Ambient noise tomography (ANT) has become a popular method to study the crustal and uppermost mantle structure of the earth in recent years due to its exclusive capability to extract short-period surface wave signals. Most of ANT are based on ray theory that assumes interstation surface waves from ambient noise are mainly sensitive to a narrow zone alone the ray path from one station to the other. Recently, many studies have demonstrated that long-period Rayleigh wave signal with high SNR can be obtained from cross-correlation of ambient noise data and could be used to do long period surface ware tomography. In order to obtain accurate phase velocity maps using long period surface waves from ambient noise, frequency effects must be considered in tomography. In this study, we investigate the feasibility of finite frequency ANT by calculating 2-D phase sensitivity kernel based on Born approximation. In calculating 2D sensitivity kernels for empirical Green's functions extracted from cross-correlations between a pair of stations, one station is regarded as receiver and the other as virtual source. Based on the 2D finite frequency sensitivity kennels, we develop a finite frequency ambient noise tomography method to construct Rayleigh wave phase velocity maps. To demonstrate the feasibility of our developed method, we apply the method to empirical Green's functions extracted from cross-correlations of USArray noise data to construct phase velocity maps at 20-150 sec periods. Our resulting phase velocity maps are very similar to earthquake-based phase velocity maps with almost zero means and 20-30 m/s stand deviations of differences. Major tectonic features in USA are well revealed in our phase velocity maps.
Surface Consistent Finite Frequency Phase Corrections
NASA Astrophysics Data System (ADS)
Kimman, W. P.
2016-04-01
Static time-delay corrections are frequency independent and ignore velocity variations away from the assumed vertical ray-path through the subsurface. There is therefore a clear potential for improvement if the finite frequency nature of wave propagation can be properly accounted for. Such a method is presented here based on the Born approximation, the assumption of surface consistency, and the misfit of instantaneous phase. The concept of instantaneous phase lends itself very well for sweep-like signals, hence these are the focus of this study. Analytical sensitivity kernels are derived that accurately predict frequency dependent phase shifts due to P-wave anomalies in the near surface. They are quick to compute and robust near the source and receivers. An additional correction is presented that re-introduces the non-linear relation between model perturbation and phase delay, which becomes relevant for stronger velocity anomalies. The phase shift as function of frequency is a slowly varying signal, its computation therefore doesn't require fine sampling even for broadband sweeps. The kernels reveal interesting features of the sensitivity of seismic arrivals to the near surface: small anomalies can have a relative large impact resulting from the medium field term that is dominant near the source and receivers. Furthermore, even simple velocity anomalies can produce a distinct frequency dependent phase behaviour. Unlike statics, the predicted phase corrections are smooth in space. Verification with spectral element simulations shows an excellent match for the predicted phase shifts over the entire seismic frequency band. Applying the phase shift to the reference sweep corrects for wavelet distortion, making the technique akin to surface consistent deconvolution, even though no division in the spectral domain is involved. As long as multiple scattering is mild, surface consistent finite frequency phase corrections outperform traditional statics for moderately large
Surface consistent finite frequency phase corrections
NASA Astrophysics Data System (ADS)
Kimman, W. P.
2016-07-01
Static time-delay corrections are frequency independent and ignore velocity variations away from the assumed vertical ray path through the subsurface. There is therefore a clear potential for improvement if the finite frequency nature of wave propagation can be properly accounted for. Such a method is presented here based on the Born approximation, the assumption of surface consistency and the misfit of instantaneous phase. The concept of instantaneous phase lends itself very well for sweep-like signals, hence these are the focus of this study. Analytical sensitivity kernels are derived that accurately predict frequency-dependent phase shifts due to P-wave anomalies in the near surface. They are quick to compute and robust near the source and receivers. An additional correction is presented that re-introduces the nonlinear relation between model perturbation and phase delay, which becomes relevant for stronger velocity anomalies. The phase shift as function of frequency is a slowly varying signal, its computation therefore does not require fine sampling even for broad-band sweeps. The kernels reveal interesting features of the sensitivity of seismic arrivals to the near surface: small anomalies can have a relative large impact resulting from the medium field term that is dominant near the source and receivers. Furthermore, even simple velocity anomalies can produce a distinct frequency-dependent phase behaviour. Unlike statics, the predicted phase corrections are smooth in space. Verification with spectral element simulations shows an excellent match for the predicted phase shifts over the entire seismic frequency band. Applying the phase shift to the reference sweep corrects for wavelet distortion, making the technique akin to surface consistent deconvolution, even though no division in the spectral domain is involved. As long as multiple scattering is mild, surface consistent finite frequency phase corrections outperform traditional statics for moderately large
Finite frequency global P wave tomography
NASA Astrophysics Data System (ADS)
Montelli, R.; Nolet, G.; Masters, G.; Dahlen, F. A.; Hung, S.-H.
2003-04-01
The travel time of a finite frequency wave is sensitive to velocity structure off the geometrical ray within a volume known as the Fresnel zone. We compute 3D travel time sensitivity efficiently by using the paraxial approximation in conjunction with ray theory and the Born approximation (Dahlen et al., 2000) to invert global travel times of long-period compressional waves. Our data set consists of 67540 P and 20266 PP-P travel times measured by cross-correlation. The sensitivity of a broad-band P arrival time resembles a hollow-banana surrounding the unperturbed path with sensitivity being zero on the ray. Typical widths of sensitivity kernels at the turning point are about 1000 km and 1300 km for a P wave at 60o and 80o epicentral distance, respectively. The region of insensitivity around the geometrical ray is small near the source and the receiver but can extend to about 400 km near the turning point for a P wave at 80o epicentral distance. Because of the minimax nature, surface reflected PP waves show a much more complicated shape of the sensitivity region, with the banana-doughnut shape replaced by a saddle-shaped region upon passage of a caustic. Not surprisingly, the introduction of such complicated sensitivity has consequences for the final tomographic images. We compare tomographic models inverted with the new method and with the more standard technique of ray theory for the same data fit (i.e. same χ2) and each smoothed to resolve very similar length scales. Depending on depth and size of the anomaly, amplitudes of the velocity perturbations in finite frequency images are on average 30%-60% higher than those obtained with ray theory. This demonstrates a major shortcoming of ray theory. It is not possible to neglect wavefront healing effect, as ray theory does. The images obtained by inverting long-period waves provide unambiguous evidence that a limited number of hot-spots are fed by plumes originating in the lower mantle. To better constrain the P wave
Chen, R.; Xie, J. L. Yu, C. X.; Liu, A. D.; Lan, T.; Li, H.; Liu, W. D.; Zhang, S. B.; Kong, D. F.; Hu, G. H.
2015-01-15
Low-frequency zonal flow (ZF) has been observed in a linear magnetic plasma device, exhibiting significant intermittency. Using the conditional analysis method, a time-averaged fluctuation-induced particle flux was observed to consistently decrease as ZF increased in amplitude. A dominant fraction of the flux, which is driven by drift-wave harmonics, is reversely modulated by ZF in the time domain. Spectra of the flux, together with each of the related turbulence properties, are estimated subject to two conditions, i.e., when potential fluctuation series represents a strong ZF intermittency or a very weak ZF component. Comparison of frequency-domain results demonstrates that ZF reduces the cross-field particle transport primarily by suppressing the density fluctuation as well as decorrelating density and potential fluctuations.
NASA Astrophysics Data System (ADS)
Mathew, Tiju Joseph; Prabhakaran Nayar, S. R.
F-region zonal plasma drifts near the magnetic equator around sunset period have been observed at multiple altitudes using the HF Doppler radar. The pattern of the plasma flow is such that it starts with a westward drift at the pre-sunset times followed by an eastward drift shortly after the E-region sunset. The striking feature of the zonal drift is the altitude dependence of the zonal drift and the presence of a vertical shear around the post sunset period at the F-region. The shear region is found to be in the altitude range of 200-300 km where the F and E -region compete for dominance. The negative gradient in the vertical drift and shear in the zonal drift are the deterministic features of the evening equatorial ionosphere to maintain the curl-free nature of the electric field. The simultaneous observation of the vertical and zonal plasma drifts suggests the existence of a post sunset velocity vortex over the equator.
Low frequency sound radiation from finite stiffened plates
NASA Astrophysics Data System (ADS)
Keltie, Richard F.
1993-07-01
The purpose of the research effort reported herein was to assess the feasibility of developing efficient low frequency acoustic radiators using flexural vibration of submerged stiffened plates. Candidate radiator geometries were identified at NUWC using an infinite plate model. A finite plate implementation of these models was then examined by the author using an analysis capability previously developed. The purpose of this examination was to study the extent to which infinite plate results could be achieved by a finite radiator, and to obtain an estimate of the effects of plate size and number of attached ribs on the radiation characteristics.
Finite difference modeling of Biot's poroelastic equations atseismic frequencies
Masson, Y.J.; Pride, S.R.; Nihei, K.T.
2006-02-24
Across the seismic band of frequencies (loosely defined as<10 kHz), a seismic wave propagating through a porous material willcreate flow in the pore space that is laminar; that is, in thislow-frequency "seismic limit," the development of viscous boundary layersin the pores need not be modeled. An explicit time steppingstaggered-grid finite difference scheme is presented for solving Biot'sequations of poroelasticity in this low-frequency limit. A key part ofthis work is the establishment of rigorous stability conditions. It isdemonstrated that over a wide range of porous material properties typicalof sedimentary rock and despite the presenceof fluid pressure diffusion(Biot slow waves), the usual Courant condition governs the stability asif the problem involved purely elastic waves. The accuracy of the methodis demonstrated by comparing to exact analytical solutions for both fastcompressional waves and slow waves. Additional numerical modelingexamples are also presented.
Finite frequency tomography shows a variety of plumes
NASA Astrophysics Data System (ADS)
Nolet, G.; Montelli, R.; Masters, G.; Dahlen, F. A.; Hung, S.
2003-04-01
The new technique of finite-frequency tomography (see abstract by Montelli et al., this meeting) is very powerful in imaging objects of small dimension in the lower mantle. The first global images of P velocity anomalies obtained by using this technique to invert a small but very accurate data set of long period P arrivals bottoming in the lower mantle show 18 low velocity anomalies in excess of -0.5%, all but two of which are associated with a known hotspot at the surface, and they serve as an unprecented glimpse into the deep mechanisms that give rise to hotspots. The following synopsis is given under the caveat that we have not yet incorporated high frequency waves into the interpretation, nor completed a full resolution analysis at the time of writing of this abstract (both will be presented at the meeting). We observe six or seven hotspots fed by a plume extending to the core-mantle boundary: Cap Verde, Easter Island, Hawaii, Kerguelen, St Helena, Tahiti, and perhaps also Azores. Several hotspots, among which are Bouvet, Bowie, and Mount Erebus, seem to originate at mid-mantle depth, while others (Afar, Ascension, Galapagos, Iceland, la Reunion and others) seem to be mostly confined to the upper mantle. Many renowned hotspots (such as Eifel, Samoa and Yellowstone) have only very weak low velocity anomalies at depth and may be the result of superficial processes confined to the top of the upper mantle. We confirm the existence of the two superplumes which both have Δ V_P < -0.5% extending as high as 2000 km depth. It is clear that no one plume/hotspot model can explain the variety in deep expressions of hotspots in the mantle. If midmantle plume origins represent originally deep plumes in their end stage, while the two unidentified anomalies are either beginning new plumes (Greenland) or plumes cut off in their initial ascent (W. Pacific), the large number of plumes caught in this phase would point to lengthy rise times of the order of tens of millions of
Kirk, R.L.
1987-01-01
Thermal evolution of Ganymede from a hot start is modeled. On cooling ice I forms above the liquid H/sub 2/O and dense ices at higher entropy below it. A novel diapiric instability is proposed to occur if the ocean thins enough, mixing these layers and perhaps leading to resurfacing and groove formation. Rising warm-ice diapirs may cause a dramatic heat pulse and fracturing at the surface, and provide material for surface flows. Timing of the pulse depends on ice rheology but could agree with crater-density dates for resurfacing. Origins of the Ganymede-Callisto dichotomy in light of the model are discussed. Based on estimates of the conductivity of H/sub 2/ (Jupiter, Saturn) and H/sub 2/O (Uranus, Neptune), the zonal winds of the giant planets will, if they penetrate below the visible atmosphere, interact with the magnetic field well outside the metallic core. The scaling argument is supported by a model with zonal velocity constant on concentric cylinders, the Lorentz torque on each balanced by viscous stresses. The problem of two-dimensional photoclinometry, i.e. reconstruction of a surface from its image, is formulated in terms of finite elements and a fast algorithm using Newton-SOR iteration accelerated by multigridding is presented.
Reprint of : Finite-frequency noise in a topological superconducting wire
NASA Astrophysics Data System (ADS)
Valentini, Stefano; Governale, Michele; Fazio, Rosario; Taddei, Fabio
2016-08-01
In this paper we study the finite-frequency current cross-correlations for a topological superconducting nanowire attached to two terminals at one of its ends. Using an analytic 1D model we show that the presence of a Majorana bound state yields vanishing cross-correlations for frequencies larger than twice the applied transport voltage, in contrast to what is found for a zero-energy ordinary Andreev bound state. Zero cross-correlations at high frequency have been confirmed using a more realistic tight-binding model for finite-width topological superconducting nanowires. Finite-temperature effects have also been investigated.
On the validation of seismic imaging methods: Finite frequency or ray theory?
Maceira, Monica; Larmat, Carene; Porritt, Robert W.; Higdon, David M.; Rowe, Charlotte A.; Allen, Richard M.
2015-01-23
We investigate the merits of the more recently developed finite-frequency approach to tomography against the more traditional and approximate ray theoretical approach for state of the art seismic models developed for western North America. To this end, we employ the spectral element method to assess the agreement between observations on real data and measurements made on synthetic seismograms predicted by the models under consideration. We check for phase delay agreement as well as waveform cross-correlation values. Based on statistical analyses on S wave phase delay measurements, finite frequency shows an improvement over ray theory. Random sampling using cross-correlation values identifiesmore » regions where synthetic seismograms computed with ray theory and finite-frequency models differ the most. Our study suggests that finite-frequency approaches to seismic imaging exhibit measurable improvement for pronounced low-velocity anomalies such as mantle plumes.« less
On the validation of seismic imaging methods: Finite frequency or ray theory?
Maceira, Monica; Larmat, Carene; Porritt, Robert W.; Higdon, David M.; Rowe, Charlotte A.; Allen, Richard M.
2015-01-23
We investigate the merits of the more recently developed finite-frequency approach to tomography against the more traditional and approximate ray theoretical approach for state of the art seismic models developed for western North America. To this end, we employ the spectral element method to assess the agreement between observations on real data and measurements made on synthetic seismograms predicted by the models under consideration. We check for phase delay agreement as well as waveform cross-correlation values. Based on statistical analyses on S wave phase delay measurements, finite frequency shows an improvement over ray theory. Random sampling using cross-correlation values identifies regions where synthetic seismograms computed with ray theory and finite-frequency models differ the most. Our study suggests that finite-frequency approaches to seismic imaging exhibit measurable improvement for pronounced low-velocity anomalies such as mantle plumes.
Finite element model calibration using frequency responses with damping equalization
NASA Astrophysics Data System (ADS)
Abrahamsson, T. J. S.; Kammer, D. C.
2015-10-01
Model calibration is a cornerstone of the finite element verification and validation procedure, in which the credibility of the model is substantiated by positive comparison with test data. The calibration problem, in which the minimum deviation between finite element model data and experimental data is searched for, is normally characterized as being a large scale optimization problem with many model parameters to solve for and with deviation metrics that are nonlinear in these parameters. The calibrated parameters need to be found by iterative procedures, starting from initial estimates. Sometimes these procedures get trapped in local deviation function minima and do not converge to the globally optimal calibration solution that is searched for. The reason for such traps is often the multi-modality of the problem which causes eigenmode crossover problems in the iterative variation of parameter settings. This work presents a calibration formulation which gives a smooth deviation metric with a large radius of convergence to the global minimum. A damping equalization method is suggested to avoid the mode correlation and mode pairing problems that need to be solved in many other model updating procedures. By this method, the modal damping of a test data model and the finite element model is set to be the same fraction of critical modal damping. Mode pairing for mapping of experimentally found damping to the finite element model is thus not needed. The method is combined with model reduction for efficiency and employs the Levenberg-Marquardt minimizer with randomized starts to achieve the calibration solution. The performance of the calibration procedure, including a study of parameter bias and variance under noisy data conditions, is demonstrated by two numerical examples.
NASA Astrophysics Data System (ADS)
Zhang, Ke; Jiang, Bin; Shi, Peng; Xu, Jinfa
2014-08-01
The design of a multi-constrained full-order fault estimation observer (FFEO) with finite frequency specifications is studied for continuous-time systems. By constructing an augmented system, a multi-constrained FFEO in finite frequency domain is proposed to achieve fault estimation. Meanwhile, the presented FFEO can avoid the overdesign problem generated by the entire frequency domain by the generalised Kalman-Yakubovich-Popov lemma. Furthermore, by introducing slack variables, improved results on FFEO design in different frequency domains are obtained such that different Lyapunov matrices can be separately designed for each constraint. Simulation results are presented to demonstrate the effectiveness and potentials of the proposed techniques.
Approximate solution for frequency synchronization in a finite-size Kuramoto model
NASA Astrophysics Data System (ADS)
Wang, Chengwei; Rubido, Nicolás; Grebogi, Celso; Baptista, Murilo S.
2015-12-01
Scientists have been considering the Kuramoto model to understand the mechanism behind the appearance of collective behavior, such as frequency synchronization (FS) as a paradigm, in real-world networks with a finite number of oscillators. A major current challenge is to obtain an analytical solution for the phase angles. Here, we provide an approximate analytical solution for this problem by deriving a master solution for the finite-size Kuramoto model, with arbitrary finite-variance distribution of the natural frequencies of the oscillators. The master solution embodies all particular solutions of the finite-size Kuramoto model for any frequency distribution and coupling strength larger than the critical one. Furthermore, we present a criterion to determine the stability of the FS solution. This allows one to analytically infer the relationship between the physical parameters and the stable behavior of networks.
NASA Astrophysics Data System (ADS)
Itoh, Kimitaka
2005-10-01
This talk describes an overview of zonal flow physics, covering the theory, simulation and experiment. The zonal flows are excited nonlinearly by drift wave fluctuations, and suppress the turbulence and transport, so as to realize a self-regulating state for turbulence and mesoscale structure. This recognition is the central of recent paradigm shift in plasma physics, i.e., the preceding linear, local and deterministic pictures of instability and transport have been taken over by the new nonlinear, nonlocal (in real and wavenumber spaces) and statistical pictures of them. The zonal flow phenomenon, i.e., the global axial vector fields are generated by the release of global free energy in scalar fields through exciting turbulence, is a typical example of the fundamental issues in modern physics. In this review, the progresses made by theory and simulations, such as the linear damping rate, nonlinear mechanisms for growth and saturation, law of energy partition between turbulence and flow, life time of zonal flow, and so on, are explained. The transport by drift wave fluctuations, which are dressed by zonal flows, is discussed. Then experimental observations and verifications, which have been piled up rapidly in basic plasma experiments and confinement research, are explained, highlighting the integration with theory and simulation. Generalization to include magnetic field (zonal field) is addressed, in the light of the study of dynamo. Zonal flows in both laboratory and planetary-solar circumstances are discussed as well. This presentation illustrates the fast evolution of the physics of turbulence and structure formation of plasmas in the nature and laboratory. In collaboration with S.-I. Itoh, P. H. Diamond, T. S. Hahm, A. Fujisawa, G. R. Tynan and M. Yagi.
Finite frequency effects on global S diffracted traveltimes
NASA Astrophysics Data System (ADS)
To, Akiko; Romanowicz, Barbara
2009-12-01
Many seismic observations have shown that strong heterogeneities exist in the bottom few hundreds kilometres of the mantle. Among different seismic phases, this region, that is, the D'' layer, can be most globally sampled by diffracted waves along the core mantle boundary. Here, we assess the amplitude and distribution of S-wave velocity variations in the D'' layer of an existing tomographic model. We compare observed SHdiff traveltime anomalies to synthetic ones obtained using (1) the coupled spectral element method (CSEM), which is our reference exact method, (2) non-linear asymptotic coupling theory (NACT) and (3) 1-D ray theory. Synthetic waveforms are calculated down to 0.057 Hz with a corner frequency at 0.026 Hz. In the first part of this paper, we compare the traveltime anomaly predictions from the three different methods. The anomalies from CSEM and NACT are obtained by taking cross-correlations of the 3-D and 1-D synthetic waveforms. Both NACT and standard ray theory, which are used in other recent tomographic models, suffer from biases in traveltime predictions for vertically varying structure near the core-mantle boundary: NACT suffers from saturation of traveltimes, due to the portion in the kernel calculation that is based on the reference 1-D model, while ray theory suffers from wave front healing effects in the vertical plane, exacerbated in the presence of thin low velocity layers. In the second part, we compare observed traveltime anomalies and predictions from CSEM. The data consists of 506 Sdiff traveltime anomalies from 15 events, obtained form global seismograph network records. The tomographic model does a good job at predicting traveltimes of Sdiff phases especially when the path mostly samples fast S velocity regions at the base of the mantle, such as beneath India, China, North America and Northern Pacific. The underprediction of the positive observed traveltime anomalies seems to occur in regions where the paths sample close to the border
Feng, Xiaobing
1996-12-31
A non-overlapping domain decomposition iterative method is proposed and analyzed for mixed finite element methods for a sequence of noncoercive elliptic systems with radiation boundary conditions. These differential systems describe the motion of a nearly elastic solid in the frequency domain. The convergence of the iterative procedure is demonstrated and the rate of convergence is derived for the case when the domain is decomposed into subdomains in which each subdomain consists of an individual element associated with the mixed finite elements. The hybridization of mixed finite element methods plays a important role in the construction of the discrete procedure.
A conservative treatment of zonal boundaries for Euler equation calculations
NASA Technical Reports Server (NTRS)
Rai, M. M.
1984-01-01
Finite-difference calculations require the generation of a grid for the region of interest. A zonal approach, wherein the given region is subdivided into zones and the grid for each zone is generated independently, makes the grid-generation process for complicated topologies and for regions requiring selective grid refinement a fairly simple task. This approach results in new boundaries within the given region, that is, zonal boundaries at the interfaces of the various zones. The zonal-boundary scheme (the integration scheme used to update the points on the zonal boundary) for the Euler equations must be conservative, accurate, stable, and applicable to general curvilinear coordinate systems. A zonal-boundary scheme with these desirable properties is developed in this study. The scheme is designed for explicit, first-order-accurate integration schemes but can be modified to accommodate second-order-accurate explicit and implicit integration schemes. Results for inviscid flow, including supersonic flow over a cylinder, blast-wave diffraction by a ramp, and one-dimensional shock-tube flow are obtained on zonal grids. The conservative nature of the zonal-boundary scheme permits the smooth transition of the discontinuities associated with these flows from one zone to another. The calculations also demonstrate the continuity of contour lines across zonal boundaries that can be achieved with the present zonal scheme.
A Conservation Treatment of Zonal Boundaries for Euler Equation Calculations
NASA Technical Reports Server (NTRS)
Rai, Man Mohan
1986-01-01
Finite-difference calculations require the generation of a grid for the region of interest. A zonal approach, wherein the given region is subdivided into zones and the grid for each zone is generated independently, makes the grid-generation process for complicated topologies and for regions requiring selective grid refinement a fairly simple task. This approach results in new boundaries within the given region, that is, zonal boundaries at the interfaces of the various zones. The zonal-boundary scheme (the integration scheme used to update the points on the zonal boundary) for the Euler equations must be conservative, accurate, stable, and applicable to general curvilinear coordinate systems. A zonal-boundary scheme with these desirable properties is developed in this study. The scheme is designed for explicit, first-order-accurate integration schemes but can be modified to accommodate second-order-accurate explicit and implicit integration schemes. Results for inviscid flow, including supersonic flow over a cylinder, blast-wave diffraction by a ramp, and one-dimensional shock-tube flow are obtained on zonal grids. The conservative nature of the zonal-boundary scheme permits the smooth transition of the discontinuities associated with these flows from one zone to another. The calculations also demonstrate the continuity of contour lines across zonal boundaries that can be achieved with the present zonal scheme.
Low-frequency scaling applied to stochastic finite-fault modeling
NASA Astrophysics Data System (ADS)
Crane, Stephen; Motazedian, Dariush
2014-01-01
Stochastic finite-fault modeling is an important tool for simulating moderate to large earthquakes. It has proven to be useful in applications that require a reliable estimation of ground motions, mostly in the spectral frequency range of 1 to 10 Hz, which is the range of most interest to engineers. However, since there can be little resemblance between the low-frequency spectra of large and small earthquakes, this portion can be difficult to simulate using stochastic finite-fault techniques. This paper introduces two different methods to scale low-frequency spectra for stochastic finite-fault modeling. One method multiplies the subfault source spectrum by an empirical function. This function has three parameters to scale the low-frequency spectra: the level of scaling and the start and end frequencies of the taper. This empirical function adjusts the earthquake spectra only between the desired frequencies, conserving seismic moment in the simulated spectra. The other method is an empirical low-frequency coefficient that is added to the subfault corner frequency. This new parameter changes the ratio between high and low frequencies. For each simulation, the entire earthquake spectra is adjusted, which may result in the seismic moment not being conserved for a simulated earthquake. These low-frequency scaling methods were used to reproduce recorded earthquake spectra from several earthquakes recorded in the Pacific Earthquake Engineering Research Center (PEER) Next Generation Attenuation Models (NGA) database. There were two methods of determining the stochastic parameters of best fit for each earthquake: a general residual analysis and an earthquake-specific residual analysis. Both methods resulted in comparable values for stress drop and the low-frequency scaling parameters; however, the earthquake-specific residual analysis obtained a more accurate distribution of the averaged residuals.
Thomas, Edward Jr.; Fisher, Ross; Merlino, Robert L.
2007-12-15
An experiment has been performed to study the behavior of dust acoustic waves driven at high frequencies (f>100 Hz), extending the range of previous work. In this study, two previously unreported phenomena are observed--interference effects between naturally excited dust acoustic waves and driven dust acoustic waves, and the observation of finite dust temperature effects on the dispersion relation.
Witteveen, Jeroen A.S. Bijl, Hester
2009-10-01
The Unsteady Adaptive Stochastic Finite Elements (UASFE) method resolves the effect of randomness in numerical simulations of single-mode aeroelastic responses with a constant accuracy in time for a constant number of samples. In this paper, the UASFE framework is extended to multi-frequency responses and continuous structures by employing a wavelet decomposition pre-processing step to decompose the sampled multi-frequency signals into single-frequency components. The effect of the randomness on the multi-frequency response is then obtained by summing the results of the UASFE interpolation at constant phase for the different frequency components. Results for multi-frequency responses and continuous structures show a three orders of magnitude reduction of computational costs compared to crude Monte Carlo simulations in a harmonically forced oscillator, a flutter panel problem, and the three-dimensional transonic AGARD 445.6 wing aeroelastic benchmark subject to random fields and random parameters with various probability distributions.
P-Wave Velocity Structure Beneath Eastern Eurasia From Finite Frequency Seismic Tomography
NASA Astrophysics Data System (ADS)
Yang, T.; Shen, Y.; Yang, X.
2005-12-01
Eastern Eurasia is one of the most tectonically complex regions in the world. While the evolution history of continental lithosphere has been well recognized, the fine structure associated with the complicated deformation in this region is far from clear, and deep mantle processes that accompanied shallower lithosphere deformations are poorly understood. In order to improve the resolution of the velocity structure in the region, we applied the newly-developed Finite Frequency Seismic Tomography (FFST) method, which utilizes the 3D Frechet-Born sensitivity kernels of the travel times of finite frequency seismic waves to account for wavefront healing and off-ray scattering, to eastern Eurasia. In addition to the new technique, we obtained a comprehensive finite-frequency body wave travel time data set from cross-correlation of broadband waveforms. Datasets used in this study include waveforms from the publicly accessible sources (e.g. IRIS, GSN, PASSCAL, and IMS stations) and other seismic networks in the region such as the Japanese Broadband Seismograph Network (F-net), the Japanese International Seismic Network (JISNET), the Taiwan Broadband Seismic Network and China National Digital Seismic Network. Taking advantage of broadband waveforms, we measured relative delays times by waveform cross-correlation in three frequency bands between 0.03 to 2 Hz for P waves. The travel times in the three frequency bands were inverted jointly to take advantage of the `data fusion' made possible by the finite-frequency kernels and separately to understand the resolving power of each data set. Preliminary results are comparable to the velocity models obtained in previous tomographic studies.
Finite element modeling of truss structures with frequency-dependent material damping
NASA Technical Reports Server (NTRS)
Lesieutre, George A.
1991-01-01
A physically motivated modelling technique for structural dynamic analysis that accommodates frequency dependent material damping was developed. Key features of the technique are the introduction of augmenting thermodynamic fields (AFT) to interact with the usual mechanical displacement field, and the treatment of the resulting coupled governing equations using finite element analysis methods. The AFT method is fully compatible with current structural finite element analysis techniques. The method is demonstrated in the dynamic analysis of a 10-bay planar truss structure, a structure representative of those contemplated for use in future space systems.
NASA Technical Reports Server (NTRS)
Lee, L. C.
1976-01-01
The cross correlation of the intensity fluctuations between different frequencies and finite bandwidth effects on the intensity correlations based on the Markov approximation were calculated. Results may be applied to quite general turbulence spectra for an extended turbulent medium. Calculations of the cross-correlation function and of finite bandwidth effects are explicitly carried out for both Gaussian and Kolmogorov turbulence spectra. The increases of the correlation scale of intensity fluctuations are different for these two spectra and the difference can be used to determine whether the interstellar turbulent medium has a Gaussian or a Kolmogorov spectrum.
Generation of zonal flow and magnetic field in the ionospheric E-layer
NASA Astrophysics Data System (ADS)
Kahlon, L. Z.; Kaladze, T. D.
2015-10-01
> We review the generation of zonal flow and magnetic field by coupled electromagnetic ultra-low-frequency waves in the Earth's ionospheric E-layer. It is shown that, under typical ionospheric E-layer conditions, different planetary low-frequency waves can couple with each other. Propagation of coupled internal-gravity-Alfvén, coupled Rossby-Khantadze and coupled Rossby-Alfvén-Khantadze waves is revealed and studied. A set of appropriate equations describing the nonlinear interaction of such waves with sheared zonal flow is derived. The conclusion on the instability of short-wavelength turbulence of such coupled waves with respect to the excitation of low-frequency and large-scale perturbation of the sheared zonal flow and sheared magnetic field is deduced. The nonlinear mechanism of the instability is based on the parametric triple interaction of finite-amplitude coupled waves leading to the inverse energy cascade towards longer wavelength. The possibility of generation of an intense mean magnetic field is shown. Obtained growth rates are discussed for each case of the considered coupled waves.
Numerical experiments on the drift wave-zonal flow paradigm for nonlinear saturation
Waltz, R. E.; Holland, C.
2008-12-15
This paper confirms that ExB shearing from toroidally symmetric (toroidal mode number n=0) 'radial modes' provides the dominant nonlinear saturation mechanism for drift wave (n{ne}0) turbulence, which in turn nonlinearly drives the modes. In common usage, this is loosely referred to as the 'drift wave-zonal flow paradigm' for nonlinear saturation despite the fact that radial modes have several components distinguished in this paper: a residual or zero mean frequency 'zonal flow' part and an oscillatory 'geodesic acoustic mode' (GAM) part. Linearly, the zonal flows (and GAMs) are weakly damped only by ion-ion collisions, while the GAMs are strongly Landau damped only at low safety factor q. At high q the Hinton-Rosenbluth residual flow from an impulse vanishes and only the weakly damped GAMs remain. With the linear physics and driving rates of the finite-n transport modes unchanged, this paper argues that GAMs are only somewhat less effective than the residual zonal flows in providing the nonlinear saturation, and in some cases ExB shearing from GAMs (or at least the GAM physics) appears to dominate: transport appears to be nearly linear in the GAM frequency. By deleting the drift wave-drift wave nonlinear coupling, it is found that drift wave-radial mode nonlinear coupling triads account for most of the nonlinear saturation. Furthermore, the ExB shear components of the radial modes nonlinearly stabilize the finite-n modes, while the diamagnetic components nonlinearly destabilize them. Finally, from wave number spectral contour plots of the time average nonlinear entropy transfer function (and rates), it is shown that the peak in entropy generation coincides with the peak in transport production, while entropy dissipation (like Landau damping) is spread equally over all n modes (including n=0). Most of these conclusions appear to hold about equally well for all types of drift wave turbulence.
Robust fault-tolerant H∞ control of active suspension systems with finite-frequency constraint
NASA Astrophysics Data System (ADS)
Wang, Rongrong; Jing, Hui; Karimi, Hamid Reza; Chen, Nan
2015-10-01
In this paper, the robust fault-tolerant (FT) H∞ control problem of active suspension systems with finite-frequency constraint is investigated. A full-car model is employed in the controller design such that the heave, pitch and roll motions can be simultaneously controlled. Both the actuator faults and external disturbances are considered in the controller synthesis. As the human body is more sensitive to the vertical vibration in 4-8 Hz, robust H∞ control with this finite-frequency constraint is designed. Other performances such as suspension deflection and actuator saturation are also considered. As some of the states such as the sprung mass pitch and roll angles are hard to measure, a robust H∞ dynamic output-feedback controller with fault tolerant ability is proposed. Simulation results show the performance of the proposed controller.
A fault detection observer design for LPV systems in finite frequency domain
NASA Astrophysics Data System (ADS)
Chen, Jianliang; Cao, Yong-Yan; Zhang, Weidong
2015-03-01
This paper addresses the fault detection observer design problem for linear parameter-varying systems. Two finite frequency performance indexes are introduced to measure the fault sensitivity and the disturbance robustness. First, the H- index fault sensitivity condition in finite frequency domain is obtained by generalised Kalman-Yakubovich-Popov lemma and new linearisation techniques. Then, with the aid of Kalman-Yakubovich-Popov lemma and projection lemma, the stability and robustness conditions are derived. It turns out that the non-convexity problem which is caused by dealing with the above three conditions can be translated into a bilinear matrix inequality optimisation problem by increasing the dimensions of slack variable matrix. An iterative linear matrix inequality algorithm is proposed to get the solution. The effectiveness of the filter is shown via three numerical examples.
Fault detection in finite frequency domain for Takagi-Sugeno fuzzy systems with sensor faults.
Li, Xiao-Jian; Yang, Guang-Hong
2014-08-01
This paper is concerned with the fault detection (FD) problem in finite frequency domain for continuous-time Takagi-Sugeno fuzzy systems with sensor faults. Some finite-frequency performance indices are initially introduced to measure the fault/reference input sensitivity and disturbance robustness. Based on these performance indices, an effective FD scheme is then presented such that the generated residual is designed to be sensitive to both fault and reference input for faulty cases, while robust against the reference input for fault-free case. As the additional reference input sensitivity for faulty cases is considered, it is shown that the proposed method improves the existing FD techniques and achieves a better FD performance. The theory is supported by simulation results related to the detection of sensor faults in a tunnel-diode circuit. PMID:24184791
Effect of axial finiteness on electron heating in low-frequency inductively coupled plasmas
Aman-ur-Rehman; Pu, Y.-K.
2006-10-15
Total power absorption inside the plasma (by taking the thermal motion of the electrons into account) has been calculated using different inductively coupled plasma models. The comparison shows that in the low-frequency region the results of the semi-infinite plasma models are different from those of the finite-length plasma models. The semi-infinite plasma models show net reduction of heating in the low-frequency region, due to thermal motion of the electrons from inside the skin region to outside the skin region. The finite-length plasma models on the other hand (due to change in the skin depth owing to the boundary condition of E=0 at z=L, and reflection of electrons from the plasma boundary) show that the decrease in heating due to the motion of the electrons from inside the skin depth to outside the skin depth is recovered by the reflection of the electrons from the plasma boundary. Hence, it is concluded that the results of the semi-infinite plasma models presented by Tyshetskiy et al. [Phys Rev. Lett. 90, 255002 (2003)] can be misleading (in the low-frequency region), since they overlooked the effect of axial finiteness of the plasma.
NASA Technical Reports Server (NTRS)
Baumeister, Kenneth J.; Kreider, Kevin L.
1996-01-01
An explicit finite difference iteration scheme is developed to study harmonic sound propagation in aircraft engine nacelles. To reduce storage requirements for large 3D problems, the time dependent potential form of the acoustic wave equation is used. To insure that the finite difference scheme is both explicit and stable, time is introduced into the Fourier transformed (steady-state) acoustic potential field as a parameter. Under a suitable transformation, the time dependent governing equation in frequency space is simplified to yield a parabolic partial differential equation, which is then marched through time to attain the steady-state solution. The input to the system is the amplitude of an incident harmonic sound source entering a quiescent duct at the input boundary, with standard impedance boundary conditions on the duct walls and duct exit. The introduction of the time parameter eliminates the large matrix storage requirements normally associated with frequency domain solutions, and time marching attains the steady-state quickly enough to make the method favorable when compared to frequency domain methods. For validation, this transient-frequency domain method is applied to sound propagation in a 2D hard wall duct with plug flow.
P-Wave Velocity Structure beneath Eastern Eurasia from Finite Frequency Seismic Tomography
NASA Astrophysics Data System (ADS)
Yang, T.; Shen, Y.; Yang, X.
2006-05-01
Despite the recent extensive seismic studies, the detailed lithospheric structure and deep mantle dynamic processes beneath eastern Eurasia remain poorly constrained. In this study, we applied the Finite Frequency Seismic Tomography (FFST) method, which utilizes the 3D Fréchet sensitivity kernels of the travel times of finite frequency seismic waves to account for wavefront healing and off-ray scattering, to eastern Eurasia. Taking advantage of the broadband feature of seismic records, we measured P wave relative delays times by waveform cross-correlation in three frequency bands (0.03-0.1Hz, 0.1-0.5 Hz and 0.5 to 2.0 Hz), which were inverted jointly to constrain velocity heterogeneities with different distances from the central geometric rays. The effect of strong variations in crustal structure beneath this region on travel time data was removed by conducting a frequency dependent crustal correction. A comprehensive dataset, including waveforms from the publicly accessible sources and other seismic networks in the region, were collected for this study. Our preliminary results are consistent with the velocity models obtained in previous tomographic studies. A more complete dataset will further improve the resolution of the velocity structure beneath eastern Eurasia.
NASA Astrophysics Data System (ADS)
Kolstrup, M. L.; Maupin, V.
2015-10-01
We present a data-processing routine to compute relative finite-frequency travel time residuals using a combination of the Iterative Cross-Correlation and Stack (ICCS) algorithm and the Multi-Channel Cross-Correlation method (MCCC). The routine has been tailored for robust measurement of P- and S-wave travel times in several frequency bands and for avoiding cycle-skipping problems at the shortest periods. We also investigate the adequacy of ray theory to calculate crustal corrections for finite-frequency regional tomography in normal continental settings with non-thinned crust. We find that ray theory is valid for both P and S waves at all relevant frequencies as long as the crust does not contain low-velocity layers associated with sediments at the surface. Reverberations in the sediments perturb the arrival times of the S waves and the long-period P waves significantly, and need to be accounted for in crustal corrections. The data-processing routine and crustal corrections are illustrated using data from a~network in southwestern Scandinavia.
NASA Astrophysics Data System (ADS)
Kolstrup, M. L.; Maupin, V.
2015-07-01
We present a data processing routine to compute relative finite-frequency travel time residuals using a combination of the Iterative Cross-Correlation and Stack (ICCS) algorithm and the MultiChannel Cross-Correlation method (MCCC). The routine has been tailored for robust measurement of P and S wave travel times in several frequency bands and for avoiding cycle-skipping problems at the shortest periods. We also investigate the adequacy of ray theory to calculate crustal corrections for finite-frequency regional tomography in normal continental settings with non-thinned crust. We find that ray theory is valid for both P and S waves at all relevant frequencies as long as the crust does not contain low-velocity layers associated with sediments at the surface. Reverberations in the sediments perturb the arrival times of the S waves and the long-period P waves significantly, and need to be accounted for in crustal corrections. The data processing routine and crustal corrections are illustated using data from a network in southwestern Scandinavia.
NASA Technical Reports Server (NTRS)
Baumeister, K. J.; Kreider, K. L.
1996-01-01
An explicit finite difference iteration scheme is developed to study harmonic sound propagation in ducts. To reduce storage requirements for large 3D problems, the time dependent potential form of the acoustic wave equation is used. To insure that the finite difference scheme is both explicit and stable, time is introduced into the Fourier transformed (steady-state) acoustic potential field as a parameter. Under a suitable transformation, the time dependent governing equation in frequency space is simplified to yield a parabolic partial differential equation, which is then marched through time to attain the steady-state solution. The input to the system is the amplitude of an incident harmonic sound source entering a quiescent duct at the input boundary, with standard impedance boundary conditions on the duct walls and duct exit. The introduction of the time parameter eliminates the large matrix storage requirements normally associated with frequency domain solutions, and time marching attains the steady-state quickly enough to make the method favorable when compared to frequency domain methods. For validation, this transient-frequency domain method is applied to sound propagation in a 2D hard wall duct with plug flow.
Finite element solution of a Schelkunoff vector potential for frequency domain, EM field simulation
NASA Astrophysics Data System (ADS)
Kordy, M. A.; Wannamaker, P. E.; Cherkaev, E.
2011-12-01
A novel method for the 3-D diffusive electromagnetic (EM) forward problem is developed and tested. A Lorentz-gauge, Schelkunoff complex vector potential is used to represent the EM field in the frequency domain and the nodal finite element method is used for numerical simulation. The potential allows for three degrees of freedom per node, instead of four if Coulomb-gauge vector and scalar potentials are used. Unlike the finite-difference method, which minimizes error at discrete points, the finite element method minimizes error over the entire domain cell volumes and may easily adapt to complex topography. Existence and uniqueness of this continuous Schelkunoff potential is proven, boundary conditions are found and a governing equation satisfied by the potential in weak form is obtained. This approach for using a Schelkunoff potential in the finite element method differs from other trials found in the literature. If the standard weak form of the Helmholtz equation is used, the obtained solution is continuous and has continuous normal derivative across boundaries of regions with different physical properties; however, continuous Schelkunoff potential components do not have continuous normal derivative, divergence of the potential divided by (complex) conductivity and magnetic permeability is continuous instead. The weak form of governing equation used here imposes proper boundary conditions on the solution. Moreover, as the solution is continuous, nodal shape functions are used instead of edge elements. Magnetotelluric (MT) simulation results using the new method are compared with those from other MT forward codes
Stochastically forced zonal flows
NASA Astrophysics Data System (ADS)
Srinivasan, Kaushik
This thesis investigates the dynamics of multiple zonal jets, that spontaneously emerge on the barotropic beta-plane, driven by a homogenous and rapidly decorrelating forcing and damped by bottom drag. Decomposing the barotropic vorticity equation into the zonal-mean and eddy equations, and neglecting the eddy-eddy interactions, defines the quasi-linear (QL) system. Numerical solution of the QL system shows zonal jets with length scales comparable to jets obtained by solving the nonlinear (NL) system. Starting with the QL system, one can construct a deterministic equation for the evolution of the two-point single-time correlation function of the vorticity, from which one can obtain the Reynolds stress that drives the zonal mean flow. This deterministic system has an exact nonlinear solution, which is a homogenous eddy field with no jets. When the forcing is also isotropic in space, we characterize the linear stability of this jetless solution by calculating the critical stability curve in the parameter space and successfully comparing this analytic result with numerical solutions of the QL system. But the critical drag required for the onset of NL zonostrophic instability is up to a factor of six smaller than that for QL zonostrophic instability. The constraint of isotropic forcing is then relaxed and spatially anisotropic forcing is used to drive the jets. Meridionally drifting jets are observed whenever the forcing breaks an additional symmetry that we refer to as mirror, or reflexional symmetry. The magnitude of drift speed in our results shows a strong variation with both mu and beta: while the drift speed decreases almost linearly with decreasing mu, it actually increases as beta decreases. Similar drifting jets are also observed in QL, with the same direction (i.e. northward or southward) and similar magnitude as NL jet-drift. Starting from the laminar solution, and assuming a mean-flow that varies slowly with reference to the scale of the eddies, we obtain
Application of a finite-element model to low-frequency sound insulation in dwellings.
Maluski, S P; Gibbs, B M
2000-10-01
The sound transmission between adjacent rooms has been modeled using a finite-element method. Predicted sound-level difference gave good agreement with experimental data using a full-scale and a quarter-scale model. Results show that the sound insulation characteristics of a party wall at low frequencies strongly depend on the modal characteristics of the sound field of both rooms and of the partition. The effect of three edge conditions of the separating wall on the sound-level difference at low frequencies was examined: simply supported, clamped, and a combination of clamped and simply supported. It is demonstrated that a clamped partition provides greater sound-level difference at low frequencies than a simply supported. It also is confirmed that the sound-pressure level difference is lower in equal room than in unequal room configurations. PMID:11051501
Imaging seismic velocity structure beneath the Iceland hot spot: A finite frequency approach
NASA Astrophysics Data System (ADS)
Hung, Shu-Huei; Shen, Yang; Chiao, Ling-Yun
2004-08-01
Tomographic models based on hypothetically infinite frequency ray interpretation of teleseismic travel time shifts have revealed a region of relatively low P and S wave speeds extending from shallow mantle to 400 km depth beneath Iceland. In reality, seismic waves have finite frequency bandwidths and undergo diffractive wave front healing. The limitation in ray theory leaves large uncertainties in the determinations of the magnitude and shape of the velocity anomaly beneath Iceland and its geodynamic implications. We developed a tomographic method that utilizes the banana-shaped sensitivity of finite frequency relative travel times from the paraxial kernel theory. Using available seismic data from the ICEMELT and HOTSPOT experiments, we applied the new method to image subsurface velocity structure beneath Iceland. Taking advantage that the sensitivity volume of broadband waveforms varies with frequency, we measured relative delay times in three frequency ranges from 0.03 to 2 Hz for P and 0.02 to 0.5 Hz for S waves. Given similar fit to data, the kernel-based models yield the root-mean-square amplitudes of P and S wave speed perturbations about 2-2.8 times those from ray tomography in the depths of 150-400 km. The kernel-based images show that a columnar low-velocity region having a lateral dimension of ˜250-300 km extends to the base of the upper mantle beneath central Iceland, deeper than that resolved by the ray-based studies. The improved resolution in the upper mantle transition zone is attributed to the deeper crossing of broad off-path sensitivity of travel time kernels than in ray approximation and frequency-dependent wave front healing as an intrinsic measure of the distance from velocity heterogeneity to receivers.
Finite-frequency noise in a non-interacting quantum dot
NASA Astrophysics Data System (ADS)
Zamoum, Redouane; Lavagna, Mireille; Crépieux, Adeline
2016-05-01
We calculate the non-symmetrized finite-frequency NS-FF noise for a single-level quantum dot connected to reservoirs in the spinless non-interacting case. The calculations are performed within the framework of the Keldysh Green’s function formalism in the wide band approximation limit. We establish the general formula for NS-FF noise for any values of temperature, frequency and bias voltage. The electron transfer processes from one to the other reservoir act via the transmission amplitude and transmission coefficient depending on the energy. By taking the symmetrized version of this expression, we show that our result coincides with the expression of the finite frequency noise obtained by Büttiker using the scattering theory. We also give the explicit analytical expression for the NS-FF noise in the zero temperature limit. Finally, by performing numerical calculations, we discuss the evolution of the NS-FF noise spectrum with varying temperature, dot energy level, and coupling strength to the reservoirs, revealing a large variety of behaviors such as different symmetry properties and changes of sign in the excess noise.
Kuramoto model with uniformly spaced frequencies: Finite-N asymptotics of the locking threshold.
Ottino-Löffler, Bertrand; Strogatz, Steven H
2016-06-01
We study phase locking in the Kuramoto model of coupled oscillators in the special case where the number of oscillators, N, is large but finite, and the oscillators' natural frequencies are evenly spaced on a given interval. In this case, stable phase-locked solutions are known to exist if and only if the frequency interval is narrower than a certain critical width, called the locking threshold. For infinite N, the exact value of the locking threshold was calculated 30 years ago; however, the leading corrections to it for finite N have remained unsolved analytically. Here we derive an asymptotic formula for the locking threshold when N≫1. The leading correction to the infinite-N result scales like either N^{-3/2} or N^{-1}, depending on whether the frequencies are evenly spaced according to a midpoint rule or an end-point rule. These scaling laws agree with numerical results obtained by Pazó [D. Pazó, Phys. Rev. E 72, 046211 (2005)PLEEE81539-375510.1103/PhysRevE.72.046211]. Moreover, our analysis yields the exact prefactors in the scaling laws, which also match the numerics. PMID:27415267
Bora, B. Soto, L.
2014-08-15
Capacitively coupled radio frequency (CCRF) plasmas are widely studied in last decades due to the versatile applicability of energetic ions, chemically active species, radicals, and also energetic neutral species in many material processing fields including microelectronics, aerospace, and biology. A dc self-bias is known to generate naturally in geometrically asymmetric CCRF plasma because of the difference in electrode sizes known as geometrical asymmetry of the electrodes in order to compensate electron and ion flux to each electrode within one rf period. The plasma series resonance effect is also come into play due to the geometrical asymmetry and excited several harmonics of the fundamental in low pressure CCRF plasma. In this work, a 13.56 MHz CCRF plasma is studied on the based on the nonlinear global model of asymmetric CCRF discharge to understand the influences of finite geometrical asymmetry of the electrodes in terms of generation of dc self-bias and plasma heating. The nonlinear global model on asymmetric discharge has been modified by considering the sheath at the grounded electrode to taking account the finite geometrical asymmetry of the electrodes. The ion density inside both the sheaths has been taken into account by incorporating the steady-state fluid equations for ions considering that the applied rf frequency is higher than the typical ion plasma frequency. Details results on the influences of geometrical asymmetry on the generation of dc self-bias and plasma heating are discussed.
Kuramoto model with uniformly spaced frequencies: Finite-N asymptotics of the locking threshold
NASA Astrophysics Data System (ADS)
Ottino-Löffler, Bertrand; Strogatz, Steven H.
2016-06-01
We study phase locking in the Kuramoto model of coupled oscillators in the special case where the number of oscillators, N , is large but finite, and the oscillators' natural frequencies are evenly spaced on a given interval. In this case, stable phase-locked solutions are known to exist if and only if the frequency interval is narrower than a certain critical width, called the locking threshold. For infinite N , the exact value of the locking threshold was calculated 30 years ago; however, the leading corrections to it for finite N have remained unsolved analytically. Here we derive an asymptotic formula for the locking threshold when N ≫1 . The leading correction to the infinite-N result scales like either N-3 /2 or N-1, depending on whether the frequencies are evenly spaced according to a midpoint rule or an end-point rule. These scaling laws agree with numerical results obtained by Pazó [D. Pazó, Phys. Rev. E 72, 046211 (2005), 10.1103/PhysRevE.72.046211]. Moreover, our analysis yields the exact prefactors in the scaling laws, which also match the numerics.
NASA Astrophysics Data System (ADS)
Bora, B.
2015-10-01
In recent years, dual capacitively coupled radio frequency (CCRF) glow discharge plasma has been widely studied in the laboratory because of its simpler design and high efficiency for different material processing applications such as thin-film deposition, plasma etching, sputtering of insulating materials etc. The main objective of studies on dual frequency CCRF plasma has been the independent control of ion energy and ion flux using an electrical asymmetry effect (EAE). Most studies have been reported in electrode configurations that are either geometrically symmetric (both electrodes are equal) or completely asymmetric (one electrode is infinitely bigger than the other). However, it seems that most of the laboratory CCRF plasmas have finite electrode geometry. In addition, plasma series resonance (PSR) and electron bounce resonance (EBR) heating also come into play as a result of geometrical asymmetry as well as EAE. In this study, a dual frequency CCRF plasma has been studied in which the dual frequency CCRF has been coupled to the lumped circuit model of the plasma and the time-independent fluid model of the plasma sheath, in order to study the effect of finite geometrical asymmetry on the generation of dc-self bias and plasma heating. The dc self-bias is found to strongly depend on the ratio of the area between the electrodes. The dc self-bias is found to depend on the phase angle between the two applied voltage waveforms. The EAE and geometrical asymmetry are found to work differently in controlling the dc self-bias. It can be concluded that the phase angle between the two voltage waveforms in dual CCRF plasmas has an important role in determining the dc self-bias and may be used for controlling the plasma properties in the dual frequency CCRF plasma.
Comments on finite Larmor radius models for ion cyclotron range of frequencies heating in tokamaks
Phillips, C.K.; Wilson, J.R.; Hosea, J.C.; Majeski, R.; Smithe, D.N.
1994-06-01
The accuracy of standard finite Larmor radius (FLR) models for wave propagation in the ion cyclotron range of frequencies (ICRF) is compared against full hot plasma models. For multiple ion species plasmas, the FLR model is shown to predict the presence of a spurious second harmonic ion-ion type resonance between the second harmonic cyclotron layers of two ion species. It is shown explicitly here that the spurious resonance is an artifact of the FLR models and that no absorption occurs in the plasma as a result of this ``resonance.``
Comments on finite Larmor radius models for ion cyclotron range of frequencies heating in tokamaks
Phillips, C.K.; Wilson, J.R.; Hosea, J.C.; Majeski, R. ); Smithe, D.N. )
1994-12-01
The accuracy of standard finite Larmor radius (FLR) models for wave propagation in the ion cyclotron range of frequencies (ICRF) is compared against full hot plasma models. For multiple ion species plasmas, the FLR model is shown to predict the presence of a spurious second harmonic ion--ion type resonance between the second harmonic cyclotron layers of two ion species. It is shown explicitly here that the spurious resonance is an artifact of the FLR models and that no absorption occurs in the plasma as a result of this resonance.''
Random shearing by zonal flows and transport reduction
Kim, Eun-jin; Diamond, P.H.
2004-12-01
The physics of random shearing by zonal flows and the consequent reduction of scalar field transport are studied. In contrast to mean shear flows, zonal flows have a finite autocorrelation time and can exhibit complex spatial structure. A random zonal flow with a finite correlation time {tau}{sub ZF} decorrelates two nearby fluid elements less efficiently than a mean shear flow does. The decorrelation time is {tau}{sub D}=({tau}{sub {eta}}/{tau}{sub ZF}{omega}{sub rms}{sup 2}){sup 1sol2} ({tau}{sub {eta}} is the turbulent scattering time, and {omega}{sub rms} is the rms shear), leading to larger scalar field amplitude with a slightly different scaling ({proportional_to}{tau}{sub D}/{omega}{sub rms}), as compared to the case of coherent shearing. In the strong shear limit, the flux scales as {proportional_to}{omega}{sub rms}{sup -1}.
Application of generalized Snoek's law over a finite frequency range: A case study
NASA Astrophysics Data System (ADS)
Rozanov, Konstantin N.; Koledintseva, Marina Y.
2016-02-01
Generalized Snoek's law proposed in an integral form by Acher and coauthors is a useful tool for investigation of high-frequency properties of magnetic materials. This integral law referred to as Acher's law allows for evaluating the ultimate performance of RF and microwave devices which employ magnetic materials. It may also be helpful in obtaining useful information on the structure and morphology of the materials. The key factor in practical application of Acher's law is an opportunity to employ either measured or calculated data available over a finite frequency range. The paper uses simple calculations to check the applicability of Acher's law in cases when the frequency range is limited and the magnetic loss peak is comparatively wide and has a distorted shape. The cases of large magnetic damping, pronounced skin effect, and inhomogeneity of the material are considered. It is shown that in most cases calculation of the integral through fitting of actual magnetic frequency dispersion by the Lorentzian dispersion law results in accurate estimations of the ultimate high-frequency performance of magnetic materials.
Brigham, John C; Aquino, Wilkins; Aguilo, Miguel A; Diamessis, Peter J
2011-01-15
An approach for efficient and accurate finite element analysis of harmonically excited soft solids using high-order spectral finite elements is presented and evaluated. The Helmholtz-type equations used to model such systems suffer from additional numerical error known as pollution when excitation frequency becomes high relative to stiffness (i.e. high wave number), which is the case, for example, for soft tissues subject to ultrasound excitations. The use of high-order polynomial elements allows for a reduction in this pollution error, but requires additional consideration to counteract Runge's phenomenon and/or poor linear system conditioning, which has led to the use of spectral element approaches. This work examines in detail the computational benefits and practical applicability of high-order spectral elements for such problems. The spectral elements examined are tensor product elements (i.e. quad or brick elements) of high-order Lagrangian polynomials with non-uniformly distributed Gauss-Lobatto-Legendre nodal points. A shear plane wave example is presented to show the dependence of the accuracy and computational expense of high-order elements on wave number. Then, a convergence study for a viscoelastic acoustic-structure interaction finite element model of an actual ultrasound driven vibroacoustic experiment is shown. The number of degrees of freedom required for a given accuracy level was found to consistently decrease with increasing element order. However, the computationally optimal element order was found to strongly depend on the wave number. PMID:21461402
Bouayed, Kaïss; Hamdi, Mohamed-Ali
2012-08-01
This paper presents numerical and experimental validation of results obtained by a shell finite element, which has been developed for modeling of the dynamic behavior of sandwich multilayered structures with a viscoelastic core. The proposed shell finite element is very easy to implement in existing finite element solvers, since it uses only the displacements as degrees of freedom at external faces and at inter-layer interfaces. The displacement field is linearly interpolated in the thickness direction of each layer, and analytical integration is made in the thickness direction in order to avoid meshing of each sandwich layer by solid elements. Only the two dimensional mid-surface of reference is meshed, facilitating the mesh generation task. A simplified modal approach using a real modal basis is also proposed to efficiently calculate the dynamic response of the sandwich structure. The proposed method reduces the memory size and computing time and takes into account the frequency-dependence of the polymer core mechanical properties. Results obtained by the proposed element in conjunction with the simplified modal method have been numerically and experimentally validated by comparison to results obtained by commercial software codes (MSC/NASTRAN and ESI/RAYON-VTM), and to measurements done on automobile windscreens. PMID:22894198
Brigham, John C.; Aquino, Wilkins; Aguilo, Miguel A.; Diamessis, Peter J.
2010-01-01
An approach for efficient and accurate finite element analysis of harmonically excited soft solids using high-order spectral finite elements is presented and evaluated. The Helmholtz-type equations used to model such systems suffer from additional numerical error known as pollution when excitation frequency becomes high relative to stiffness (i.e. high wave number), which is the case, for example, for soft tissues subject to ultrasound excitations. The use of high-order polynomial elements allows for a reduction in this pollution error, but requires additional consideration to counteract Runge's phenomenon and/or poor linear system conditioning, which has led to the use of spectral element approaches. This work examines in detail the computational benefits and practical applicability of high-order spectral elements for such problems. The spectral elements examined are tensor product elements (i.e. quad or brick elements) of high-order Lagrangian polynomials with non-uniformly distributed Gauss-Lobatto-Legendre nodal points. A shear plane wave example is presented to show the dependence of the accuracy and computational expense of high-order elements on wave number. Then, a convergence study for a viscoelastic acoustic-structure interaction finite element model of an actual ultrasound driven vibroacoustic experiment is shown. The number of degrees of freedom required for a given accuracy level was found to consistently decrease with increasing element order. However, the computationally optimal element order was found to strongly depend on the wave number. PMID:21461402
Low-frequency finite-element modeling of the gerbil middle ear.
Elkhouri, Nidal; Liu, Hengjin; Funnell, W Robert J
2006-12-01
The gerbil is a popular species for experimental middle-ear research. The goal of this study is to develop a 3D finite-element model to quantify the mechanics of the gerbil middle ear at low frequencies (up to about 1 kHz). The 3D reconstruction is based on a magnetic resonance imaging dataset with a voxel size of about 45 microm, and an x-ray micro-CT dataset with a voxel size of about 5.5 microm, supplemented by histological images. The eardrum model is based on moiré shape measurements. Each individual structure in the model was assumed to be homogeneous with isotropic, linear, and elastic material properties derived from a priori estimates in the literature. The behavior of the finite-element model in response to a uniform acoustic pressure on the eardrum of 1 Pa is analyzed. Sensitivity tests are done to evaluate the significance of the various parameters in the finite-element model. The Young's modulus and the thickness of the pars tensa have the most significant effect on the load transfer between the eardrum and the ossicles and, along with the Young's modulus of the pedicle and stapedial annular ligament, on the displacements of the stapes. Overall, the model demonstrates good agreement with low-frequency experimental data. For example, (1) the maximum footplate displacement is about 35 nm; (2) the umbo/stapes displacement ratio is found to be about 3.5; (3) the motion of the stapes is predominantly piston-like; and (4) the displacement pattern of the eardrum shows two points of maximum displacement, one in the posterior region and one in the anterior region. The effects of removing or stiffening the ligaments are comparable to those observed experimentally. PMID:17043944
Zonal flow as pattern formation
NASA Astrophysics Data System (ADS)
Parker, Jeffrey B.; Krommes, John A.
2013-10-01
Zonal flows are well known to arise spontaneously out of turbulence. We show that for statistically averaged equations of the stochastically forced generalized Hasegawa-Mima model, steady-state zonal flows, and inhomogeneous turbulence fit into the framework of pattern formation. There are many implications. First, the wavelength of the zonal flows is not unique. Indeed, in an idealized, infinite system, any wavelength within a certain continuous band corresponds to a solution. Second, of these wavelengths, only those within a smaller subband are linearly stable. Unstable wavelengths must evolve to reach a stable wavelength; this process manifests as merging jets.
Zonal flow as pattern formation
Parker, Jeffrey B.; Krommes, John A.
2013-10-15
Zonal flows are well known to arise spontaneously out of turbulence. We show that for statistically averaged equations of the stochastically forced generalized Hasegawa-Mima model, steady-state zonal flows, and inhomogeneous turbulence fit into the framework of pattern formation. There are many implications. First, the wavelength of the zonal flows is not unique. Indeed, in an idealized, infinite system, any wavelength within a certain continuous band corresponds to a solution. Second, of these wavelengths, only those within a smaller subband are linearly stable. Unstable wavelengths must evolve to reach a stable wavelength; this process manifests as merging jets.
3D frequency-domain finite-difference modeling of acoustic wave propagation
NASA Astrophysics Data System (ADS)
Operto, S.; Virieux, J.
2006-12-01
We present a 3D frequency-domain finite-difference method for acoustic wave propagation modeling. This method is developed as a tool to perform 3D frequency-domain full-waveform inversion of wide-angle seismic data. For wide-angle data, frequency-domain full-waveform inversion can be applied only to few discrete frequencies to develop reliable velocity model. Frequency-domain finite-difference (FD) modeling of wave propagation requires resolution of a huge sparse system of linear equations. If this system can be solved with a direct method, solutions for multiple sources can be computed efficiently once the underlying matrix has been factorized. The drawback of the direct method is the memory requirement resulting from the fill-in of the matrix during factorization. We assess in this study whether representative problems can be addressed in 3D geometry with such approach. We start from the velocity-stress formulation of the 3D acoustic wave equation. The spatial derivatives are discretized with second-order accurate staggered-grid stencil on different coordinate systems such that the axis span over as many directions as possible. Once the discrete equations were developed on each coordinate system, the particle velocity fields are eliminated from the first-order hyperbolic system (following the so-called parsimonious staggered-grid method) leading to second-order elliptic wave equations in pressure. The second-order wave equations discretized on each coordinate system are combined linearly to mitigate the numerical anisotropy. Secondly, grid dispersion is minimized by replacing the mass term at the collocation point by its weighted averaging over all the grid points of the stencil. Use of second-order accurate staggered- grid stencil allows to reduce the bandwidth of the matrix to be factorized. The final stencil incorporates 27 points. Absorbing conditions are PML. The system is solved using the parallel direct solver MUMPS developed for distributed
Spectrum of Finite Frequency Pump Kinetic Alfvén Wave in the Solar Wind
NASA Astrophysics Data System (ADS)
Modi, K. V.; Sharma, R. P.; Gaur, Nidhi
2016-01-01
The nonlinear interaction between the kinetic Alfvén wave (KAW) and the slow magnetosonic wave is studied. The dynamical equation for the slow magnetosonic wave, in the presence of a ponderomotive force due to finite frequency KAW (ω0<ω_{ci}, where ω0 is the frequency of the KAW and ω_{ci} is the ion gyro frequency) is developed and then numerically solved for the solar wind parameters around 1 AU. Three different propagation angles of the slow magnetosonic wave (θ = 70°, 75°, and 85°) are considered. Our results reveal that due to the nonlinear interplay between the waves, the nature of the formation of localised structures becomes complex and depends on the different propagation angles of the slow magnetosonic wave. The power spectrum of a KAW shows the Kolmogorov scaling in larger scales but exhibits steepening in smaller scales. The scaling index of the power spectrum of the KAW depends on the propagation angles of the slow magnetosonic wave. Therefore, the heating of plasma particles in the solar wind may show such dependence. The present results are consistent with the observation of the Cluster spacecraft for the solar wind around 1 AU.
Finite-difference modeling of Biot's poroelastic equations across all frequencies
Masson, Y.J.; Pride, S.R.
2009-10-22
An explicit time-stepping finite-difference scheme is presented for solving Biot's equations of poroelasticity across the entire band of frequencies. In the general case for which viscous boundary layers in the pores must be accounted for, the time-domain version of Darcy's law contains a convolution integral. It is shown how to efficiently and directly perform the convolution so that the Darcy velocity can be properly updated at each time step. At frequencies that are low enough compared to the onset of viscous boundary layers, no memory terms are required. At higher frequencies, the number of memory terms required is the same as the number of time points it takes to sample accurately the wavelet being used. In practice, we never use more than 20 memory terms and often considerably fewer. Allowing for the convolution makes the scheme even more stable (even larger time steps might be used) than it is when the convolution is entirely neglected. The accuracy of the scheme is confirmed by comparing numerical examples to exact analytic results.
Lu, Yujie; Zhu, Banghe; Shen, Haiou; Rasmussen, John C; Wang, Ge; Sevick-Muraca, Eva M
2010-08-21
Fluorescence molecular imaging/tomography may play an important future role in preclinical research and clinical diagnostics. Time- and frequency-domain fluorescence imaging can acquire more measurement information than the continuous wave (CW) counterpart, improving the image quality of fluorescence molecular tomography. Although diffusion approximation (DA) theory has been extensively applied in optical molecular imaging, high-order photon migration models need to be further investigated to match quantitation provided by nuclear imaging. In this paper, a frequency-domain parallel adaptive finite element solver is developed with simplified spherical harmonics (SP(N)) approximations. To fully evaluate the performance of the SP(N) approximations, a fast time-resolved tetrahedron-based Monte Carlo fluorescence simulator suitable for complex heterogeneous geometries is developed using a convolution strategy to realize the simulation of the fluorescence excitation and emission. The validation results show that high-order SP(N) can effectively correct the modeling errors of the diffusion equation, especially when the tissues have high absorption characteristics or when high modulation frequency measurements are used. Furthermore, the parallel adaptive mesh evolution strategy improves the modeling precision and the simulation speed significantly on a realistic digital mouse phantom. This solver is a promising platform for fluorescence molecular tomography using high-order approximations to the radiative transfer equation. PMID:20671350
Equatorial zonal circulations: Historical perspectives
NASA Astrophysics Data System (ADS)
Hastenrath, Stefan
2007-04-01
The changing perceptions on zonal circulations in the equatorial belt are traced for (a) stratospheric wind regimes, and (b) vertical-zonal circulation cells in the troposphere. (a) Observations from the Krakatoa eruption 1883 and Berson's 1908 expedition to East Africa, along with later soundings over Batavia (Jakarta) led to the notion of "Krakatoa easterlies" around 30 km (10 mb) and "Berson westerlies" around 20 km (50 mb). Prompted by contrary observations since the late 1950s, this dogma was replaced by the notion of easterlies alternating with westerlies in the equatorial stratosphere at a rhythm of about 26 months. (b) Stimulated by Bjerknes' postulate of a "Walker circulation" along the Pacific Equator, a multitude of such cells have been hypothesized at other longitudes, in part from zonal contrasts of temperature and cloudiness. Essential for the diagnosis of equatorial zonal circulation cells is the continuity following the flow between the centers of ascending and subsiding motion. Evaluation of the recent NCEP-NCAR and ECMWF Reanalysis upper-air datasets reveals equatorial zonal circulation cells over the Pacific all year round, over the Atlantic only in boreal winter, and over the Indian Ocean only in autumn, all being seasons and oceanic longitudes with strong zonal flow in the lower troposphere.
Chillara, Vamshi Krishna; Ren, Baiyang; Lissenden, Cliff J
2016-04-01
This article describes the use of the frequency domain finite element (FDFE) technique for guided wave mode selection in inhomogeneous waveguides. Problems with Rayleigh-Lamb and Shear-Horizontal mode excitation in isotropic homogeneous plates are first studied to demonstrate the application of the approach. Then, two specific cases of inhomogeneous waveguides are studied using FDFE. Finally, an example of guided wave mode selection for inspecting disbonds in composites is presented. Identification of sensitive and insensitive modes for defect inspection is demonstrated. As the discretization parameters affect the accuracy of the results obtained from FDFE, effect of spatial discretization and the length of the domain used for the spatial fast Fourier transform are studied. Some recommendations with regard to the choice of the above parameters are provided. PMID:26746160
Effect of Finite Pulse Length and Laser Frequency Chirp on HGHG and EEHG Seeding
Stupakov, G.; /SLAC
2011-11-18
Theoretical studies of high-gain harmonic generation (HGHG) and echo-enabled harmonic generation (EEHG) often start from a simplified model in which the beam is assumed infinitely long and longitudinally uniform and the laser induced energy modulation is perfectly sinusoidal and of infinite duration. In such a model the resulting seed has a spectrum consisting of a collection of delta-functions (of zero width) located at the harmonics of the laser frequency. Being a useful tool for study of the seed bunching amplitudes, such a model cannot be used for realistic analysis the spectral properties of the seed. In this paper we take into account the finite duration of the laser pulse as well as some possible laser phase errors to study their effect on the spectrum of the seed.
NASA Astrophysics Data System (ADS)
Gardner, Bryce K.; Shorter, Philip J.; Bremner, Paul G.
2002-11-01
At low frequencies, vibroacoustic systems exhibit a dynamic response characterized by spatially correlated motion with low modal density. These systems are typically modeled with deterministic methods. While at high frequencies, the dynamic response is characterized by weak spatial correlation and a large number of modes with high modal overlap. These systems are typically modeled with statistical methods. However many vibroacoustic systems have some regions with high modal density and some regions with low modal density. Such systems require a midfrequency solution technique. One such method has been developed based on a hybrid approach combining finite element analysis (FE) in the low modal density regions and statistical energy analysis (SEA) in the high modal density regions. This method is called RESOUND [Langley and Bremner, J. Acoust. Soc. Am. 105, 1657-1671 (1999)]. Recent developments of RESOUND have focused on predicting the appropriate dynamic interactions and mechanisms for energy flow between the FE and the SEA regions. By including these effects, RESOUND can predict the dynamic response of systems having regions with low modal densities and regions with high modal densities. This paper will provide an overview of recent developments.
Finite-frequency traveltime tomography of San Francisco Bay region crustal velocity structure
Pollitz, F.F.
2007-01-01
Seismic velocity structure of the San Francisco Bay region crust is derived using measurements of finite-frequency traveltimes. A total of 57 801 relative traveltimes are measured by cross-correlation over the frequency range 0.5-1.5 Hz. From these are derived 4862 'summary' traveltimes, which are used to derive 3-D P-wave velocity structure over a 341 ?? 140 km2 area from the surface to 25 km depth. The seismic tomography is based on sensitivity kernels calculated on a spherically symmetric reference model. Robust elements of the derived P-wave velocity structure are: a pronounced velocity contrast across the San Andreas fault in the south Bay region (west side faster); a moderate velocity contrast across the Hayward fault (west side faster); moderately low velocity crust around the Quien Sabe volcanic field and the Sacramento River delta; very low velocity crust around Lake Berryessa. These features are generally explicable with surface rock types being extrapolated to depth ???10 km in the upper crust. Generally high mid-lower crust velocity and high inferred Poisson's ratio suggest a mafic lower crust. ?? Journal compilation ?? 2007 RAS.
Finite-frequency sensitivity kernels of seismic waves to fault zone structures
NASA Astrophysics Data System (ADS)
Allam, A. A.; Tape, C.; Ben-Zion, Y.
2015-12-01
We analyse the volumetric sensitivity of fault zone seismic head and trapped waves by constructing finite-frequency sensitivity (Fréchet) kernels for these phases using a suite of idealized and tomographically derived velocity models of fault zones. We first validate numerical calculations by waveform comparisons with analytical results for two simple fault zone models: a vertical bimaterial interface separating two solids of differing elastic properties, and a `vertical sandwich' with a vertical low velocity zone surrounded on both sides by higher velocity media. Establishing numerical accuracy up to 12 Hz, we compute sensitivity kernels for various phases that arise in these and more realistic models. In contrast to direct P body waves, which have little or no sensitivity to the internal fault zone structure, the sensitivity kernels for head waves have sharp peaks with high values near the fault in the faster medium. Surface wave kernels show the broadest spatial distribution of sensitivity, while trapped wave kernels are extremely narrow with sensitivity focused entirely inside the low-velocity fault zone layer. Trapped waves are shown to exhibit sensitivity patterns similar to Love waves, with decreasing width as a function of frequency and multiple Fresnel zones of alternating polarity. In models that include smoothing of the boundaries of the low velocity zone, there is little effect on the trapped wave kernels, which are focused in the central core of the low velocity zone. When the source is located outside a shallow fault zone layer, trapped waves propagate through the surrounding medium with body wave sensitivity before becoming confined. The results provide building blocks for full waveform tomography of fault zone regions combining high-frequency head, trapped, body, and surface waves. Such an imaging approach can constrain fault zone structure across a larger range of scales than has previously been possible.
NASA Technical Reports Server (NTRS)
Smith, Wayne Farrior
1973-01-01
The effect of finite source size on the power statistics in a reverberant room for pure tone excitation was investigated. Theoretical results indicate that the standard deviation of low frequency, pure tone finite sources is always less than that predicted by point source theory and considerably less when the source dimension approaches one-half an acoustic wavelength or greater. A supporting experimental study was conducted utilizing an eight inch loudspeaker and a 30 inch loudspeaker at eleven source positions. The resulting standard deviation of sound power output of the smaller speaker is in excellent agreement with both the derived finite source theory and existing point source theory, if the theoretical data is adjusted to account for experimental incomplete spatial averaging. However, the standard deviation of sound power output of the larger speaker is measurably lower than point source theory indicates, but is in good agreement with the finite source theory.
Finite beta effects on low- and high-frequency magnetosonic waves in a two-ion-species plasma
Toida, Mieko; Aota, Yukio
2013-08-15
A magnetosonic wave propagating perpendicular to a magnetic field in a two-ion-species plasma has two branches, high-frequency and low-frequency modes. The finite beta effects on these modes are analyzed theoretically on the basis of the three-fluid model with finite ion and electron pressures. First, it is shown that the Korteweg-de Vries (KdV) equation for the low-frequency mode is valid for amplitudes ε<ε{sub max}, where the upper limit of the amplitude ε{sub max} is given as a function of β (β is the ratio of the kinetic and magnetic energy densities), the density ratio, and the cyclotron frequency ratio of two ion species. Next, the linear dispersion relation and KdV equation for the high-frequency mode are derived, including β as a factor. In addition, the theory for heavy ion acceleration by the high-frequency mode pulse and the pulse damping due to this energy transfer in a finite beta plasma are presented.
Multiscale finite-frequency Rayleigh wave tomography of the Kaapvaal craton
NASA Astrophysics Data System (ADS)
Chevrot, S.; Zhao, L.
2007-04-01
We have measured phase delays of fundamental-mode Rayleigh waves for 12 events recorded by the Southern Africa Seismic Experiment at frequencies between 0.005 and 0.035 Hz. A novel multiscale finite-frequency tomographic method based on wavelet decomposition of 3-D sensitivity kernels for the phase of Rayleigh waves is used to map the shear velocities in the upper mantle beneath southern Africa. The kernels are computed by summing coupled normal modes over a very fine grid surrounding the seismic array. To estimate and minimize the biases in the model resulting from structures outside the tomographic grid, a jackknife inversion method is implemented. The contribution of heterogeneities outside the target volume is significant, but produces artefacts in the tomographic model that are easily identified and discarded before interpretation. With structures on length scales as short as 100 km retrieved beneath the array, the deep structure of the Kaapvaal craton is revealed with unprecedented detail. Outside the array, the corresponding resolution is 200 km. High velocity cratonic roots are confined to the Archean craton, and extend to depths of at least 250 km. Confirming earlier surface structural studies, we recognize two distinct units in the Kaapvaal craton. The eastern Witwatersrand block and the western Kimberley block are separated by a major near-vertical translithospheric boundary which coincides with the Colesberg Lineament. Lower than average velocities south and east of the Kaapvaal craton reveal extensive metasomatism and heating of the lithosphere, probably related to the Karoo magmatic event and to the opening of the South Atlantic Ocean.
Finite element model correlation of a composite UAV wing using modal frequencies
NASA Astrophysics Data System (ADS)
Oliver, Joseph A.; Kosmatka, John B.; Hemez, François M.; Farrar, Charles R.
2007-04-01
The current work details the implementation of a meta-model based correlation technique on a composite UAV wing test piece and associated finite element (FE) model. This method involves training polynomial models to emulate the FE input-output behavior and then using numerical optimization to produce a set of correlated parameters which can be returned to the FE model. After discussions about the practical implementation, the technique is validated on a composite plate structure and then applied to the UAV wing structure, where it is furthermore compared to a more traditional Newton-Raphson technique which iteratively uses first-order Taylor-series sensitivity. The experimental testpiece wing comprises two graphite/epoxy prepreg and Nomex honeycomb co-cured skins and two prepreg spars bonded together in a secondary process. MSC.Nastran FE models of the four structural components are correlated independently, using modal frequencies as correlation features, before being joined together into the assembled structure and compared to experimentally measured frequencies from the assembled wing in a cantilever configuration. Results show that significant improvements can be made to the assembled model fidelity, with the meta-model procedure producing slightly superior results to Newton-Raphson iteration. Final evaluation of component correlation using the assembled wing comparison showed worse results for each correlation technique, with the meta-model technique worse overall. This can be most likely be attributed to difficultly in correlating the open-section spars; however, there is also some question about non-unique update variable combinations in the current configuration, which lead correlation away from physically probably values.
FDFD: A 3D Finite-Difference Frequency-Domain Code for Electromagnetic Induction Tomography
NASA Astrophysics Data System (ADS)
Champagne, Nathan J.; Berryman, James G.; Buettner, H. Michael
2001-07-01
A new 3D code for electromagnetic induction tomography with intended applications to environmental imaging problems has been developed. The approach consists of calculating the fields within a volume using an implicit finite-difference frequency-domain formulation. The volume is terminated by an anisotropic perfectly matched layer region that simulates an infinite domain by absorbing outgoing waves. Extensive validation of this code has been done using analytical and semianalytical results from other codes, and some of those results are presented in this paper. The new code is written in Fortran 90 and is designed to be easily parallelized. Finally, an adjoint field method of data inversion, developed in parallel for solving the fully nonlinear inverse problem for electrical conductivity imaging (e.g., for mapping underground conducting plumes), uses this code to provide solvers for both forward and adjoint fields. Results obtained from this inversion method for high-contrast media are encouraging and provide a significant improvement over those obtained from linearized inversion methods.
NASA Astrophysics Data System (ADS)
Liang, Xiaofeng; Chen, Yun; Tian, Xiaobo; Wang, Minling; Xu, Tao; Sun, Changqing; Si, Shaokun; Lan, Haiqiang; Teng, Jiwen
2015-04-01
Combining the new collected teleseismic body waves recorded by TIBET-31N passive seismic array with waveforms from several previous temporary seismic arrays, we carried out finite-frequency tomographic inversions to image three-dimensional velocity structures beneath southern-central Tibet to examine the roles of the upper mantle in the formation of the Tibetan plateau. Strong low P- and S-wave velocity anomalies that extend from the lower crust to about 200 km depth beneath the Comei rift, Yadong-Gulu rift, Tangra Yum Co rift, suggesting that rifting in southern Tibet is probably a process that involves the entire lithosphere. At the same time there is only the low velocity close to Yadong-Gulu rift extending further north and connecting with the massive upper mantle low velocity beneath central Tibet, and moreover, the other two are limited in southern Tibet. This observation implies that the previous proposed fragmentation of underthrusting Indian lithosphere might not happen underneath all the north-south trending rifts. Instead, it only happens close to Yadong-Gulu rift, then hot temperature upwelling materials fill up this lithospheric crack and might stuff the other weak zones in shallow depths beneath southern Tibet. Continuous high velocities are observed beneath Himalayas and Lhasa Terrance with a moderate northward inclination angle. We interpret this anomaly as the subducting/underthrusting Indian continental lithosphere.
NASA Astrophysics Data System (ADS)
Yang, F.; Huang, J.
2009-12-01
In this study, we applied the finite-frequency seismic tomography(FFST) to teleseismic waveform data to determine 3-D P-wave velocity structure of the upper mantle under the Chinese capital region. The seismic waveform data from more than 300 teleseismic events recorded by the Chinese digital Capital Seismic Network during the period from September 2003 to December 2005 was used in this study. We obtained 18499 high accuracy P-wave relative travel-times by filtering these waveform data on the vertical component into high-, intermediate-, low-frequency bands (1.0-2.0, 0.1-1.0 and 0.05-0.1 hz, respectively) and the multi-channel waveform cross correlation measurement. The 3-D Fréchet sensitivity kernels were calculated by paraxial approximation for each frequency band. We established observation equations with these measured relative travel-times and 3-D Fréchet sensitivity kernels and then determined the 3-D velocity structure by inverting the observation equations. Our results show there are distinct differences of deep velocity structure down to 150 km depth under the four tectonic units of present study region. The Yanshan uplift exhibited the high velocity(high-V) feature. Under the Taihangshan uplift, broad low velocity(low-V) are visible, but it also shows up as small high-V anomalies. A large scale prominent low-V anomaly was revealed in the shallow upper mantle under the North China basin and Bohai bay. In the North China basin the low-V anomaly generally extend from 50 km to 150 km depth, but in the Bohai bay, this low-V anomaly gradually extend down to 200 km depth. The depth of this low-V anomaly is 50-70 km under the North China basin and Bohai bay, which is consistent with the depth of high conductivity layer in the upper mantle determined by the measurement of magnetotelluric sounding and heat flow. This result shows lithosphere thinning in the North China basin and Bohai bay. Most of large earthquakes occurred in the Zhangjiakou-Penglai fault zone
A finite-difference frequency-domain code for electromagnetic induction tomography
Sharpe, R M; Berryman, J G; Buettner, H M; Champagne, N J.,II; Grant, J B
1998-12-17
We are developing a new 3D code for application to electromagnetic induction tomography and applications to environmental imaging problems. We have used the finite-difference frequency- domain formulation of Beilenhoff et al. (1992) and the anisotropic PML (perfectly matched layer) approach (Berenger, 1994) to specify boundary conditions following Wu et al. (1997). PML deals with the fact that the computations must be done in a finite domain even though the real problem is effectively of infinite extent. The resulting formulas for the forward solver reduce to a problem of the form Ax = y, where A is a non-Hermitian matrix with real values off the diagonal and complex values along its diagonal. The matrix A may be either symmetric or nonsymmetric depending on details of the boundary conditions chosen (i.e., the particular PML used in the application). The basic equation must be solved for the vector x (which represents field quantities such as electric and magnetic fields) with the vector y determined by the boundary conditions and transmitter location. Of the many forward solvers that could be used for this system, relatively few have been thoroughly tested for the type of matrix encountered in our problem. Our studies of the stability characteristics of the Bi-CG algorithm raised questions about its reliability and uniform accuracy for this application. We have found the stability characteristics of Bi-CGSTAB [an alternative developed by van der Vorst (1992) for such problems] to be entirely adequate for our application, whereas the standard Bi-CG was quite inadequate. We have also done extensive validation of our code using semianalytical results as well as other codes. The new code is written in Fortran and is designed to be easily parallelized, but we have not yet tested this feature of the code. An adjoint method is being developed for solving the inverse problem for conductivity imaging (for mapping underground plumes), and this approach, when ready, will
Zonal flow dynamics in the double tearing mode with antisymmetric shear flows
Mao, Aohua; Li, Jiquan; Liu, Jinyuan; Kishimoto, Yasuaki
2014-05-15
The generation dynamics and the structural characteristics of zonal flows are investigated in the double tearing mode (DTM) with antisymmetric shear flows. Two kinds of zonal flow oscillations are revealed based on reduced resistive magnetohydrodynamics simulations, which depend on the shear flow amplitudes corresponding to different DTM eigen mode states, elaborated by Mao et al. [Phys. Plasmas 20, 022114 (2013)]. For the weak shear flows below an amplitude threshold, v{sub c}, at which two DTM eigen states with antisymmetric or symmetric magnetic island structure are degenerated, the zonal flows grow oscillatorily in the Rutherford regime during the nonlinear evolution of the DTMs. It is identified that the oscillation mechanism results from the nonlinear interaction between the distorted islands and the zonal flows through the modification of shear flows. However, for the medium shear flows above v{sub c} but below the critical threshold of the Kelvin-Helmholtz instability, an oscillatory growing zonal flow occurs in the linear phase of the DTM evolution. It is demonstrated that the zonal flow oscillation originates from the three-wave mode coupling or a modulation instability pumped by two DTM eigen modes with the same frequency but opposite propagating direction. With the shear flows increasing, the amplitude of zonal flow oscillation increases first and then decreases, whilst the oscillation frequency as twice of the Doppler frequency shift increases. Furthermore, impacts of the oscillatory zonal flows on the nonlinear evolution of DTM islands and the global reconnection are also discussed briefly.
Zonal flow dynamics in the double tearing mode with antisymmetric shear flows
NASA Astrophysics Data System (ADS)
Mao, Aohua; Li, Jiquan; Liu, Jinyuan; Kishimoto, Yasuaki
2014-05-01
The generation dynamics and the structural characteristics of zonal flows are investigated in the double tearing mode (DTM) with antisymmetric shear flows. Two kinds of zonal flow oscillations are revealed based on reduced resistive magnetohydrodynamics simulations, which depend on the shear flow amplitudes corresponding to different DTM eigen mode states, elaborated by Mao et al. [Phys. Plasmas 20, 022114 (2013)]. For the weak shear flows below an amplitude threshold, vc, at which two DTM eigen states with antisymmetric or symmetric magnetic island structure are degenerated, the zonal flows grow oscillatorily in the Rutherford regime during the nonlinear evolution of the DTMs. It is identified that the oscillation mechanism results from the nonlinear interaction between the distorted islands and the zonal flows through the modification of shear flows. However, for the medium shear flows above vc but below the critical threshold of the Kelvin-Helmholtz instability, an oscillatory growing zonal flow occurs in the linear phase of the DTM evolution. It is demonstrated that the zonal flow oscillation originates from the three-wave mode coupling or a modulation instability pumped by two DTM eigen modes with the same frequency but opposite propagating direction. With the shear flows increasing, the amplitude of zonal flow oscillation increases first and then decreases, whilst the oscillation frequency as twice of the Doppler frequency shift increases. Furthermore, impacts of the oscillatory zonal flows on the nonlinear evolution of DTM islands and the global reconnection are also discussed briefly.
Global Rayleigh wave phase-velocity maps from finite-frequency tomography
NASA Astrophysics Data System (ADS)
Liu, Kui; Zhou, Ying
2016-04-01
We report global phase-velocity maps of fundamental-mode Rayleigh waves at periods between 25 and 100 s based on finite-frequency tomography. Rayleigh wave dispersion measurements are made using a multitaper technique for both minor-arc and major-arc wave trains. The global phase-velocity maps confirm many features associated with surface tectonics including the ocean-continent dichotomy and the signature of lithospheric cooling in oceanic plates. In addition, the high-resolution phase-velocity maps reveal a major change in the distribution of small-scale anomalies in the Pacific at different wave periods. We calculate the global average of Rayleigh wave phase velocity in major tectonic regions and show that large discrepancies exist between our model and global crustal and mantle models: (1) In oceanic regions, short-period (<˜40 s) Rayleigh waves are faster than calculations based on models CRUST2.0 and S40RTS. The discrepancies could be explained by a thinner crust or faster wave speeds in the crust or upper mantle. The implementation of model CRUST1.0 significantly improves the agreement, with phase-velocity discrepancies less than 0.5 per cent on average. (2) In Archean cratons, Rayleigh wave phase velocities in our model are faster than calculations based on model S40RTS at periods longer than ˜40 s; and the global average in orogenic belts is ˜1-2 per cent slower than CRUST1.0 at periods shorter than ˜50 s.
Factors Controlling the Evolution of Anatolia: Clues from Teleseismic Finite-Frequency Tomography
NASA Astrophysics Data System (ADS)
Biryol, C. B.; Beck, S. L.; Zandt, G.; Ozacar, A. A.
2010-12-01
The complex and sinusoidal pattern of subduction zones of the Mediterranenan region plays an important role in controlling the current tectonic framework of the Alpine-Himalayan orogenic belt. The Anatolian region is part of this belt and it displays the complex characteristics of the interplay between continent collision in the east and subduction-rollback related backarc extension in the west. The ongoing northward subduction of the African Plate beneath the Anatolian Plate contributes significantly to the emergence of the current tectonic setting of this region. Despite its crucial effect on the tectonics of Anatolia, there are only a few studies that focus on the deeper extent of this zone. In this study we provide higher resolution tomographic images of the subducting African lithosphere beneath Anatolia. Our approach is based on analysis of teleseismic body-wave travel-time data using a finite-frequency seismic tomography algorithm. The data for our analysis comes from multiple permanent and temporary networks deployed in the region. A major part of our dataset is formed by the multiple frequency-band picks of P-wave arrival times recorded at more than 100 broadband and short-period seismic stations of the National Earthquake Monitoring Center and 39 broadband seismic stations of the North Anatolian Passive Seismic Experiment network. The results of our analysis indicates the presence of large and smaller scale gaps in the subducting African Lithosphere, that are interpreted as slab tears. The most significant tear is located beneath western Anatolia with a maximum width of ~250 km. This tear is marked by lack of intermediate to deep seismicity and is associated with slow seismic speed perturbations that we interpret as ascending hot, buoyant asthenosphere. The configuration of the edges of this gap at depths between 50 to 200 km provides clues about how the impediments on the subducting seafloor could have an influence on rates of roll-back on both sides
Finite Frequency Measurements of Conventional and Core-diffracted P-waves (P and Pdiff)
NASA Astrophysics Data System (ADS)
Hosseini, K.; Sigloch, K.; Stähler, S. C.
2014-12-01
Core-diffracted waves are body waves that dive deep enough to sense the core, and by interaction with this wave guide become dispersive. They sample the core-mantle boundary and the lower third of the mantle extensively. In ray theoretical modeling, the deepest part of the ray starts to graze the core at around 97 degrees distance, but ray theory is a very poor approximation to propagation of core-diffracted waves. In reality, finite-frequency waves with their spatially extend sensitivity regions start to sense the core at significantly smaller distances already. The actual, non-ray-like sensitivities have been difficult to model, as have been the associated synthetic seismograms. Core-diffracted waves have therefore not been used in tomography, despite abundant observations of these phases on modern broadband seismograms. Hence current global body-wave tomographies illuminate the lower third of the mantle much less well than the upper and especially the middle third. This study aims for broadband, global waveform tomography that seamlessly incorporates core-diffracted phases alongside conventional, teleseismic waves as well as regional body-waves. Here, we investigate the properties of P-diffracted waves in terms of waveform characteristics and travel-time measurements as compared to teleseismic P-wave measured by the same methods. Travel time anomalies, the primary data for tomography, are measured by waveform cross-correlation of data with synthetics, where the synthetics are calculated from fully numerical wave propagation in a spherically symmetric background model. These same numerical tools will be used to calculate the associated sensitivity kernels for tomography (figure, top). Demonstrating the extent to which waveform modeling can fit real data, we assemble and discuss a global data set of 851,905 Pdiff and 2,368,452 P-wave multi-frequency cross-correlation travel times. Findings are summarized in the Pdiff travel time map (figure, bottom) in which most
Finite-Frequency Tomographic Images of Subducting Slabs in the Southeast Caribbean
NASA Astrophysics Data System (ADS)
Bezada, M. J.; Levander, A.; Schmandt, B.
2009-12-01
We present a tomographic P-wave velocity model for the eastern Caribbean - South America plate boundary in northern Venezuela. The data were collected by the BOLIVAR (Broadband Ocean and Land Investigation of Venezuela and the Antilles arc Region) passive seismic array that included the 39 stations from the Venezuelan National Seismological Network (VNSN) as well as a temporary deployment of 35 IRIS-PASSCAL, 8 Rice broadband stations and 11 OBSIP ocean bottom seismographs. The temporary stations were deployed for a period of ~17 months. The array covers the eastern part of Venezuela with typical station spacing of ~50-100 km. Coverage is more sparse in the west of the study area and relies on the stations of the VNSN and an ongoing deployment of 8 PASSCAL and Rice instruments. Stations were located from south of the Orinoco river at latitude 6°N, across the northern Guayana Shield, to the Caribbean Sea at latitudes up to 14°N. We used data from 86 stations, and inverted traveltime residuals from 462 teleseismic events with good azimuthal and epicentral distance distribution. Traveltime delays were obtained by cross-correlation of waveforms in frequency bands centered on 1.0, 0.5 and 0.3 Hz. A total of 6619 delays were used in the inversion. Crustal corrections were computed using a 3-D crustal velocity model constructed from five, boundary normal, 2-D active source seismic refraction models and gravity data. The inversion algorithm we applied uses approximate finite-frequency sensitivity kernels and calculates static event and station terms. The tomographic image shows the subducting Atlantic slab very clearly to a depth of > 500 km in the east of the study area. The location of the southern edge of the slab corresponds roughly to the plate boundary in the surface down to ~400 km depth and extends southwards at greater depths. West of the Atlantic slab, the Caribbean mantle is dominated by low velocity anomalies, while the Guayana Shield is characterized by small
NASA Technical Reports Server (NTRS)
Dinyavari, M. A. H.; Friedmann, P. P.
1984-01-01
Several incompressible finite-time arbitrary-motion airfoil theories suitable for coupled flap-lag-torsional aeroelastic analysis of helicopter rotors in hover and forward flight are derived. These theories include generalized Greenberg's theory, generalized Loewy's theory, and a staggered cascade theory. The generalized Greenberg's and staggered cascade theories were derived directly in Laplace domain considering the finite length of the wake and using operational methods. The load expressions are presented in Laplace, frequency, and time domains. Approximate time domain loads for the various generalized theories, discussed in the paper, are obtained by developing finite state models using the Pade approximant of the appropriate lift deficiency functions. Three different methods for constructing Pade approximants of the lift deficiency functions were considered and the more flexible one was used. Pade approximants of Loewy's lift deficiency function, for various wake spacing and radial location parameters of a helicopter typical rotor blade section, are presented.
NASA Technical Reports Server (NTRS)
Liu, C. H.; Wong, T. C.; Kandil, O. A.
1988-01-01
The two-dimensional flow over a blunt leading-edge plate is simulated on the basis of an Euler/Navier-Stokes zonal scheme. The scheme uses an implicit upwind finite-volume scheme, which is based on the van Leer flux-vector splitting. It is shown that the Euler/Navier-Stokes zonal scheme with downstream boundary-layer compatibility conditions is accurate and efficient.
NASA Astrophysics Data System (ADS)
Kaladze, Tamaz; Kahlon, Laila
Nonlinear dynamics of coupled internal-gravity (IG) and alfven electromagnetic planetary waves in the weakly ionized ionospheric E-layer is investigated. Under such coupling new type of alfven waves is revealed. It is shown that such short wavelength turbulence of IG and alfvén waves is unstable with respect to the excitation of low-frequency and large-scale perturbations of the zonal flow and magnetic field. A set of coupled equations describing the nonlinear interaction of coupled IG and alfven waves with zonal flows is derived. The nonlinear mechanism of the instability is driven by the advection of vorticity and is based on the parametric excitation of convective cells by finite-amplitude coupled IG and alfven waves leading to the inverse energy cascade toward the longer wavelength. The growth rates of the corresponding instability and the conditions for driving them are determined. The possibility of generation of the intense mean magnetic field is shown.
Nonlinear excitation of zonal flows and streamers in plasmas
Benkadda, S.; Klochkov, D. N.; Popel, S. I.; Izvekova, Yu. N.
2011-05-15
Nonlinear excitation of zonal flows and streamers in plasmas is considered. The emphasis is given to the nonlinear interaction of low- and high-frequency drift waves which can result in the excitation of zonal flows and streamers in a plasma of fusion devices. For this purpose, an inhomogeneous nonisothermal plasma in a strong external magnetic field whose characteristic frequencies are lower than the ion Langmuir frequency but higher than the collision frequency is studied. The excitation of a long-wavelength low-frequency drift wave during the development of the nonlinear modulational interaction of a high-frequency drift pump wave is investigated. The growth rates of the modulational instability are obtained, and the conditions for its development are determined. Self-organized structures described by solutions of evolutionary equations for the modulational interaction are associated with zonal flows and streamers. A possible relation of the modulational interaction in Earth's ionospheric plasma to the formation of dust flows and transport of dust particles in the ionosphere is also discussed. It is shown that one of the ways of transport of dust particles in the ionosphere is vertical flows (streamers), which are generated by dust vortices as a result of development of the modulational instability.
Seismic imaging beneath southwest Africa based on finite-frequency body wave tomography
NASA Astrophysics Data System (ADS)
Youssof, Mohammad; Yuan, Xiaohui; Tilmann, Frederik; Heit, Benjamin; Weber, Michael; Jokat, Wilfried; Geissler, Wolfram; Laske, Gabi
2016-04-01
We present a seismic model of southwest Africa from teleseismic tomographic inversion of the P- and S- wave data recorded by an amphibious temporary seismic network. The area of study is located at the intersection of the Walvis Ridge with the continental margin of northern Namibia, and extends into the Congo craton. Utilizing 3D finite-frequency sensitivity kernels, we invert traveltime residuals of the teleseismic body waves to image seismic structures in the upper mantle. To test the robustness of our tomographic imaging, we employed various resolution assessments that allow us to inspect the extent of smearing effects and to evaluate the optimum regularization weights (i.e., damping and smoothness). These tests include applying different (ir)regular parameterizations, classical checkerboard and anomaly tests and squeezing modeling. Furthermore, we performed different kinds of weighing schemes for the traveltime dataset. These schemes account for balancing between the picks data amount with their corresponding events directions. Our assessment procedure involves also a detailed investigation of the effect of the crustal correction on the final velocity image, which strongly influenced the image resolution for the mantle structures. Our model can resolve horizontal structures of 1° x 1° below the array down to 300-350 km depth. The resulting model is mainly dominated by the difference in the oceanic and continental mantle lithosphere beneath the study area, with second-order features related to their respective internal structures. The fast lithospheric keel of the Congo Craton reaches a depth of ~250 km. The orogenic Damara Belt and continental flood basalt areas are characterized by low velocity perturbations down to a depth of ~150 km, indicating a normal fertile mantle. High velocities in the oceanic lithosphere beneath the Walvis Ridge appear to show signatures of chemical depletion. A pronounced anomaly of fast velocity is imaged underneath continental NW
NASA Astrophysics Data System (ADS)
Angus, D.; Hammond, J. O.; Kendall, J.; Wookey, J.
2008-12-01
As part of the Ethiopian Afar Geoscientific Lithospheric Experiment (EAGLE) 79 seismic stations were deployed, for up to 18 months, in the Main Ethiopian Rift (MER). Many indicators of melt were observed leading to the idea that magma was driving the rifting process in this region. Some of the best evidence for melt came from observations of anisotropy in studies of surface waves and shear-wave splitting. The shear- wave splitting shows fast directions which change abruptly from being rift parallel on the rift flanks to magmatic-segment parallel in the rift valley. This was interpreted in terms of melt-induced anisotropy. The abrupt change in splitting parameters over small lateral distances suggests that the source of anisotropy is shallow. To further constrain the location of the anisotropy and study the ability of shear-wave splitting to identify sharp lateral changes in anisotropy, we model finite-frequency waveforms for a suite of model representations of the rift zone. This allows us to determine the lateral and vertical extent of the melt-induced anisotropy. The results show how a simple model with two regimes of anisotropy can explain the variability across the rift, in both delay time and shear-wave polarization, over short length scales of the order 20- 40 km. Our models have enabled us to constrain the anisotropic characteristics beneath the MER. Our best model has a 9% anisotropy on the western rift margin, with fast directions of 30°, a 100 km wide rift zone with fast direction of 20° inside the rift zone and with 9% anisotropy close to the western margin, 7% elsewhere, and 7% anisotropy on the eastern margin with fast directions of 30°. In all regions of the model we constrain anisotropy to begin at a depth of 90 km. The depth of anisotropy co-incides with the proposed depth of melt initiation beneath the region, based on geochemistry. Also the elevated splitting beneath the western margin supports evidence of low velocities and highly conductive
Theory of turbulence regulation by oscillatory zonal flows
Kim, Eun-jin
2006-02-15
The theory of turbulence regulation by oscillatory zonal flows is presented for passive scalar field models. Zonal flows are assumed to have linear spatial variation of the form U=-x{omega}(t)y, where {omega}(t) has amplitude {omega}{sub m} and frequency {omega}{sub z}. The flux and fluctuation levels are found to scale as 1/|k{sub y}U{sub m}| and {tau}{sub *}/|k{sub y}U{sub m}|, respectively, for {omega}{sub m}>{omega}{sub z}. Here, {tau}{sub *}={tau}{sub {eta}}({omega}{sub z}/{omega}{sub m}){sup 2} is the effective decorrelation time, {tau}{sub {eta}}={tau}{sub *}({omega}=0), U{sub m}=x{omega}{sub m}, and k{sub y} is the typical poloidal wave number of the turbulence. The effect of stochasticity of oscillatory zonal flows on shear decorrelation is discussed. The results complement the theory of turbulence regulation by low-frequency random zonal flows [E. Kim and P. H. Diamond, Phys. Rev. Lett 91, 075001 (2003)].
Generation of magnetoacoustic zonal flows by Alfven waves in a rotating plasma
Mikhailovskii, A. B.; Lominadze, J. G.; Churikov, A. P.; Erokhin, N. N.; Tsypin, V. S.; Smolyakov, A. I.; Galvao, R. M. O.
2007-08-15
Analytical theory of nonlinear generation of magnetoacoustic zonal flows in a rotating plasma is developed. As the primary modes causing such a generation, a totality of the Alfven waves are considered, along with the kinetic, inertial, and rotational. It is shown that in all these cases of the Alfven waves the generation is possible if the double plasma rotation frequency exceeds the zonal flow frequency.
NASA Astrophysics Data System (ADS)
Zhao, L.; Chevrot, S.
2007-12-01
Numerical modeling experiments of wave propagation have shown that the traveltime and amplitude anomalies of a finite-frequency seismic wave are influenced by the heterogeneities in the first Fresnel zone of the wave, a region surrounding its ray path. This leads to the so-called 'banana-doughnut' sensitivity (Fr{é}chet) kernels for finite-frequency waves whose values vary in the first Fresnel zone, rather than concentrate only on the ray path. Therefore, if finite-frequency effect is not taken into account in seismic tomography, the unrealistic sensitivity kernels will limit the resolution to roughly the widths of the first Fresnel zones of the seismic waves, even if all the other aspects are perfect. For the same reason, to realize the full potential of finite-frequency approach in seismic tomography, the spatial sampling in computing the Fr{é}chet kernels and in discretizing the structural model must be sufficiently small so that there are enough sampling points within the width of the first Fresnel zone. This introduces a high demand in computational resources including memory, CPU time, disk storage and the input/output (I/O) operations. To facilitate the practice of finite-frequency high-resolution tomography, we develop an efficient algorithm for computing the Fr{é}chet kernels based on the normal-mode theory in spherically symmetric earth models. The strain Green tensors (SGTs) for a spherically symmetric reference model are computed by normal-mode summation on a dense depth-distance grid by normal-mode summation. This normal-mode SGT database can then be used to calculate all the wavefield quantities needed in seismic studies including synthetic seismograms, partial derivatives with respect to source parameters for the inversion of CMT solutions, and the Fr{é}chet kernels for various kinds of model parameters for the inversions of anelastic and anisotropic structures. The SGT database approach provides the necessary efficiency for calculating the Fr
Validity of the Rytov Approximation in the Form of Finite-Frequency Sensitivity Kernels
NASA Astrophysics Data System (ADS)
Xu, Wenjun; Xie, Xiao-Bi; Geng, Jianhua
2015-06-01
The first-order (or linear) Rytov or Born approximation is the foundation for formulation of wave-equation tomography and waveform inversion, so the validity of the Rytov/Born approximation can substantially affect the applicability of these theories. However, discussions and research reported in literature on this topic are insufficient or limited. In this paper we introduce five variables in scattering theory to help us discuss conditions under which the Rytov approximation, in the form of the finite frequency sensitivity kernels (RFFSK), the basis of waveform inversion and tomography, is valid. The five variables are propagation length L, heterogeneity scale a, wavenumber k, anisotropy ratio ξ, and perturbation strength ɛ. Combined with theoretical analysis and numerical experiments, we conclude that varying the conditions used to establish the Rytov approximation can lead to uninterpretable or undesired results. This conclusion has two consequences. First, one cannot rigorously apply the linear Rytov approximation to all theoretical or practical cases without discussing its validity. Second, the nonlinear Rytov approximation is essential if the linear Rytov approximation is not valid. Different from previous literature, only phase (or travel time) terms for the whole wavefield are discussed. The time shifts of two specific events between the background and observed wavefields measured by cross-correlation will serve as a reference for evaluation of whether the time shifts predicted by the FFSKs are reasonably acceptable. Significantly, the reference "cross-correlation" should be regarded as reliable only if the condition "two specific similar signals" is satisfied. We cannot expect it to provide a reasonable result if this condition is not met. This paper reports its reliability and experimental limitations. Using cross-correlation (CC) samples as the X axis and sensitivity kernel (SK) or ray tracing (RT) samples as the Y axis, a chart of cross validation
Localized Single Frequency Lasing States in a Finite Parity-Time Symmetric Resonator Chain
NASA Astrophysics Data System (ADS)
Phang, Sendy; Vukovic, Ana; Creagh, Stephen C.; Sewell, Phillip D.; Gradoni, Gabriele; Benson, Trevor M.
2016-02-01
In this paper a practical case of a finite periodic Parity Time chain made of resonant dielectric cylinders is considered. The paper analyzes a more general case where PT symmetry is achieved by modulating both the real and imaginary part of the material refractive index along the resonator chain. The band-structure of the finite periodic PT resonator chains is compared to infinite chains in order to understand the complex interdependence of the Bloch phase and the amount of the gain/loss in the system that causes the PT symmetry to break. The results show that the type of the modulation along the unit cell can significantly affect the position of the threshold point of the PT system. In all cases the lowest threshold is achieved near the end of the Brillouin zone. In the case of finite PT-chains, and for a particular type of modulation, early PT symmetry breaking is observed and shown to be caused by the presence of termination states localized at the edges of the finite chain resulting in localized lasing and dissipative modes at each end of the chain.
NASA Astrophysics Data System (ADS)
Hosseini, Kasra; Sigloch, Karin; Staehler, Simon C.
2014-05-01
In its lowermost 200-300 km, the mantle has a complex structure resulting from accumulations of downwellings (subducted slabs), upwellings (LLSVPs and plumes), and probably phase transitions; seismic velocities and density show large variations but are not tightly constrained. Core-diffracted body waves are the seismic phases that sample the lowermost mantle extensively and are prime candidates to be used in tomography for enhancing resolution in this depth range. Since they are diffracted along the core-mantle boundary, their behavior is highly dispersive and cannot be modeled satisfactory using ray theory, nor early versions of finite-frequency modeling. Hence they have rarely been used for tomography so far, and where they have been, large imaging blur can be expected. We present a processing scheme to measure finite-frequency travel-time anomalies of arbitrary seismic body-wave phases in a fully automated way, with an initial focus on core-diffracted P waves. The aim is to extract a maximum of information from observed broadband seismograms using multi-frequency techniques. Using a matched-filtering approach, predicted and observed waveforms are compared in a cross-correlation sense in eight overlapping frequency passbands, with dominant periods ranging between 30 and 2.7sec. This method was applied to a global data set of ≡2000 teleseismic events in our waveform archive, which resulted in 1,616,184 P and 536,190 Pdiff usable multi-frequency measurements of high cross-correlation coefficient (≥ 0.8). The measurements are analyzed statistically in terms of goodness of fit, effects of epicentral distance, and frequency-dependent behavior of P and Pdiff phases. The results for Pdiff waves are displayed by projecting the measured travel time anomalies onto the phase's nominal grazing segments along the core-mantle boundary.
Zonal flow formation in the Earth's core.
Miyagoshi, Takehiro; Kageyama, Akira; Sato, Tetsuya
2010-02-11
Zonal jets are very common in nature. Well-known examples are those in the atmospheres of giant planets and the alternating jet streams found in the Earth's world ocean. Zonal flow formation in nuclear fusion devices is also well studied. A common feature of these zonal flows is that they are spontaneously generated in turbulent systems. Because the Earth's outer core is believed to be in a turbulent state, it is possible that there is zonal flow in the liquid iron of the outer core. Here we report an investigation at the current low-viscosity limit of numerical simulations of the geodynamo. We find a previously unknown convection regime of the outer core that has a dual structure comprising inner, sheet-like radial plumes and an outer, westward cylindrical zonal flow. We numerically confirm that the dual-convection structure with such a zonal flow is stable under a strong, self-generated dipole magnetic field. PMID:20148036
NASA Astrophysics Data System (ADS)
Joglekar, D. M.; Mitra, M.
2015-11-01
A breathing crack, due to its bilinear stiffness characteristics, modifies the frequency spectrum of a propagating dual-frequency elastic wave, and gives rise to sidebands around the probing frequency. This paper presents an analytical-numerical method to investigate such nonlinear frequency mixing resulting from the modulation effects induced by a breathing crack in 1D waveguides, such as axial rods and the Euler-Bernoulli beams. A transverse edge-crack is assumed to be present in both the waveguides, and the local flexibility caused by the crack is modeled using an equivalent spring approach. A simultaneous treatment of both the waveguides, in the framework of the Fourier transform based spectral finite element method, is presented for analyzing their response to a dual frequency excitation applied in the form of a tone-burst signal. The intermittent contact between the crack surfaces is accounted for by introducing bilinear contact forces acting at the nodes of the damage spectral element. Subsequently, an iterative approach is outlined for solving the resulting system of nonlinear simultaneous equations. Applicability of the proposed method is demonstrated by considering several test cases. The existence of sidebands and the higher order harmonics is confirmed in the frequency domain response of both the waveguides under investigation. A qualitative comparison with the previous experimental observations accentuates the utility of the proposed solution method. Additionally, the influence of the two constituent frequencies in the dual frequency excitation is assessed by varying the relative strengths of their amplitudes. A brief parametric study is performed for bringing out the effects of the relative crack depth and crack location on the degree of modulation, which is quantified in terms of the modulation parameter. Results of the present investigation can find their potential use in providing an analytical-numerical support to the studies geared towards the
Numerical analysis of curved frequency selective surface by finite-difference time-domain
NASA Astrophysics Data System (ADS)
Chen, Xin-yi; Wang, Jian-bo; Chen, Gui-bo; Sun, Guan-cheng; Lu, Jun
2011-08-01
Frequency selective surface is a monolayer or multilayer 2D periodic structure which is composed of multiple resonance units scattering by a two-dimensional periodic array on dielectric layer. FSS can't absorb radio frequency energy, but can filter the frequency which is therefore applied in microwave technique or stealth technology. The relative research on curved FSS is relatively scarce since the curved FSS structure can be obtained only when FSS is attached on the materials surfaces of curved structures in engineering application. However, curved FSS is widely applied in practical engineering; therefore, the research on curved FSS structure has important significance. In this paper, a curved FSS structure model of Y-pore unit is established and numerical simulated by means of FDTD. The influence of curvature on FSS transmission characteristics is studied according to the analysis on the changing of radar cross section (RCS). The results show: the center frequency point of the plane band pass FSS structure drifts after the curve surface deformation of the structure; the center frequency point of the curved band pass FSS structure drifts with the changing of the curvature radius, i. e. with the decreasing of curvature radius, the frequency point drifts towards high points and the transmittance decreases. The design of FSS radome demands of accurate and stable center resonance frequency; therefore, the actual situation of curved surface should be considered in practical engineering application when band pass FSS is made into frequency selection filtering radome. The curvature radius should be long enough to avoid center frequency drifting and transmittance deceasing.
NASA Astrophysics Data System (ADS)
Yamamoto, Mitsuru; Inoue, Takeshi; Shiba, Hiroshi; Kitamura, Yuta
2009-07-01
In recent ocean investigations using underwater sonar transducers, low-frequency and wideband long-range sonar systems have been demanded for strong acoustic radiation and improved detective resolution capability in shallow-sea regions. We developed a disk bender transducer with a dual radiation surface as a miniaturized, light weight, low-frequency, and high-power transducer. However, there were problems in that the fractional bandwidth was small because the radiation surface was far smaller than the radiated wavelength, and the acoustic load per unit radiation area was small. Therefore, we suggest a technique to enable a wideband sonar array using differential connections of multiple disk bender transducers with different resonance frequencies to solve these problems. In this paper, we report results that endorse the above-mentioned technique obtained by finite-element method (FEM) analysis. The results confirm that this technique produces a wideband transducer array with low-frequency and high-power characteristics. We found that a wideband characteristic of more than 100% could be achieved with as a 6 dB fractional bandwidth by differential connection of disk bender transducers with three different resonance frequencies. In addition, we found that a superior horizontally oriented directivity was provided by locating the transducers in a plane symmetrical to the horizontal plane.
Multi-frequency, finite-wavelength and dc-augmentation effects in large area capacitive sources
NASA Astrophysics Data System (ADS)
Kushner, Mark
2009-10-01
The scaling of high frequency, multi-frequency capacitively coupled plasmas (CCPs) to large areas has many challenges. It has been well established that electromagnetic (EM) effects become increasingly more important as the frequency of excitation increases while the diameter of the substrate also increases. The complexity of the system increases with the addition of dc-augmentation. Although much as been learned about EM effects, scaling laws are difficult to develop because the discharge characteristics are functions of the frequency dependence of the conductivity, the response of the electron energy distribution (EED) to the electric fields that penetrate into the plasma, the geometry of the reactor, gas mixture, pressure and dc augmentation power. In the case of multi-frequency excitation, the coupling of low and high frequencies through surface waves and through the bulk plasma is also an issue. In this talk we will discuss results from a computational investigation of multi- and high- frequency (up to 200 MHz) excitation of CCPs having diameters up to 450 mm, with and without dc augmentation. The model used in this study includes a full time-domain solution of Maxwell's equations that enables investigation of coupling between frequencies. A Monte Carlo simulation is used to predict EEDs as a function of position and ion energy distributions to the substrate. Gas mixtures (e.g., Ar and Ar/CF4), pressures (10 mTorr to 100 mTorr) and geometry (gap size) are investigated. Methods to minimize EM effects will be discussed by using variable conductivity and shaped electrodes; and segmented electrodes in which the electrical path from the generator to any point in the plasma is made as consistent as possible.
Rapid Frequency Chirps of TAE mode due to Finite Orbit Energetic Particles
NASA Astrophysics Data System (ADS)
Berk, Herb; Wang, Ge
2013-10-01
The tip model for the TAE mode in the large aspect ratio limit, conceived by Rosenbluth et al. in the frequency domain, together with an interaction term in the frequency domain based on a map model, has been extended into the time domain. We present the formal basis for the model, starting with the Lagrangian for the particle wave interaction. We shall discuss the formal nonlinear time domain problem and the procedure that needs to obtain solutions in the adiabatic limit.
NASA Astrophysics Data System (ADS)
De Gersem, Hilde; Moens, David; Desmet, Wim; Vandepitte, Dirk
2005-12-01
This work introduces a numerical algorithm to calculate frequency response functions of damped finite element models with fuzzy uncertain parameters. Part 1 of this paper focusses on the numerical procedure for the solution of the underlying interval finite element problem, based on the undamped procedure and the principle of Rayleigh damping. Part 2 of this paper illustrates the applicability of the methodology through four case studies. The concepts of the interval and the fuzzy finite element frequency response function analysis are illustrated for different types of uncertainties. The obtained results are compared with the results of Monte Carlo simulations.
NASA Astrophysics Data System (ADS)
Li, Y.; Han, B.; Métivier, L.; Brossier, R.
2016-09-01
We investigate an optimal fourth-order staggered-grid finite-difference scheme for 3D frequency-domain viscoelastic wave modeling. An anti-lumped mass strategy is incorporated to minimize the numerical dispersion. The optimal finite-difference coefficients and the mass weighting coefficients are obtained by minimizing the misfit between the normalized phase velocities and the unity. An iterative damped least-squares method, the Levenberg-Marquardt algorithm, is utilized for the optimization. Dispersion analysis shows that the optimal fourth-order scheme presents less grid dispersion and anisotropy than the conventional fourth-order scheme with respect to different Poisson's ratios. Moreover, only 3.7 grid-points per minimum shear wavelength are required to keep the error of the group velocities below 1%. The memory cost is then greatly reduced due to a coarser sampling. A parallel iterative method named CARP-CG is used to solve the large ill-conditioned linear system for the frequency-domain modeling. Validations are conducted with respect to both the analytic viscoacoustic and viscoelastic solutions. Compared with the conventional fourth-order scheme, the optimal scheme generates wavefields having smaller error under the same discretization setups. Profiles of the wavefields are presented to confirm better agreement between the optimal results and the analytic solutions.
Wareing, Robin R; Davy, John L; Pearse, John R
2016-01-01
Current theories for predicting the sound insulation of orthotropic materials are limited to a small range of infinite panels. This paper presents a method that allows for the prediction of the sound insulation of a finite size orthotropic panel. This method uses an equation for the forced radiation impedance of a finite size rectangular panel. This approach produces an equation that has three nested integrals. The long numerical calculation times were reduced by using approximate formulas for the azimuthally averaged forced radiation impedance. This reduced the number of nested integrals from three to two. The resulting predictions are compared to results measured using two sample sizes of four different thicknesses of plywood and one sample size of another three different thicknesses of plywood. Plywood was used for all the tests because it is somewhat orthotropic. It was found during testing that the Young's moduli of the plywood were dependent on the frequency of excitation. The influence of the frequency dependent Young's moduli was then included in the prediction method. The experimental results were also compared with a simple isotropic prediction method. PMID:26827045
Model of intermittent zonal flow structure formation
Anderson, Johan; Kim, Eun-jin
2008-11-01
We present a theory the PDF tails of the zonal flow formation by assuming that a modon (a bipolar vortex) drives a zonal flow through the generalized Reynolds stress. We show that the PDF tails of zonal flow formation have exponential behavior {approx_equal}e{sup -{xi}}{sup {phi}{sub Z}{sub F}{sup 3}}, with the overall amplitude {xi} severely quenched by strong flow shear. It is found that stronger zonal flows are generated in ITG turbulence than Hasegawa-Mima (HM) turbulence as well as further from marginal stability. This suggests that although ITG turbulence has a higher level of heat flux, it also more likely generates stronger zonal flows, leading to a self-regulating system. It is also shown that shear flows can significantly reduce the PDF tails of structure formation.
A new paradigm for plasma transport and zonal flows
Sen, A.K.; Sokolov, V.; Wei, X.
2006-05-15
Most tokamak experimental results indicate dependence of the ion thermal conductivity on the isotopic mass close to {chi}{sub perpendicular}{approx}m{sub i}{sup -0.5}, i.e., inverse gyro-Bohm. This is in stark contradiction to most present theoretical models predicting Bohm (m{sub i}{sup 0}) or gyro-Bohm (m{sub i}{sup 0.5}) scaling. A basic physics isotopic scaling experiment [V. Sokolov and A. K. Sen, Phys. Rev. Lett. 89, 095001 (2002)] on the anomalous ion thermal conduction due to ion temperature gradient (ITG) instabilities in two different gases (hydrogen and deuterium) closely confirms the tokamak results. Another series of experiments designed to explore the physics basis of this scaling appears to lead to a new model for this scaling based on 3-wave coupling of two ITG radial harmonics and an IA wave. The resulting isotopic scaling of transport is {approx}m{sub i}{sup -0.5} dictated primarily by the IA damping. This basic physics may be extrapolated to tokamaks resolving the paradox [V. Sokolov and A. K. Sen, Phys. Rev. Lett. 92, 165002 (2004)]. Last, the much discussed theoretical role of zonal flows in transport regulation is critically examined by another set of experiments. A novel diagnostic has been developed on the basis of the observation that the effect of zonal flow can be seen in the FM modulation (at zonal flow frequency) of the carrier frequency of the large equilibrium Doppler shift frequency of ITG modes both in tokamaks and in the Columbia Linear Machine [V. Sokolov, X. Wei, and A. K.Sen, APS DPP meeting, Savannah (2004)]. The present results indicate zonal flow levels close to the theoretical prediction, but its shear is much lower than that predicted by theory for transport regulation.
NASA Astrophysics Data System (ADS)
Morency, C.; Tromp, J.
2008-12-01
The mathematical formulation of wave propagation in porous media developed by Biot is based upon the principle of virtual work, ignoring processes at the microscopic level, and does not explicitly incorporate gradients in porosity. Based on recent studies focusing on averaging techniques, we derive the macroscopic porous medium equations from the microscale, with a particular emphasis on the effects of gradients in porosity. In doing so, we are able to naturally determine two key terms in the momentum equations and constitutive relationships, directly translating the coupling between the solid and fluid phases, namely a drag force and an interfacial strain tensor. In both terms, gradients in porosity arise. One remarkable result is that when we rewrite this set of equations in terms of the well known Biot variables us, w), terms involving gradients in porosity are naturally accommodated by gradients involving w, the fluid motion relative to the solid, and Biot's formulation is recovered, i.e., it remains valid in the presence of porosity gradients We have developed a numerical implementation of the Biot equations for two-dimensional problems based upon the spectral-element method (SEM) in the time domain. The SEM is a high-order variational method, which has the advantage of accommodating complex geometries like a finite-element method, while keeping the exponential convergence rate of (pseudo)spectral methods. As in the elastic and acoustic cases, poroelastic wave propagation based upon the SEM involves a diagonal mass matrix, which leads to explicit time integration schemes that are well-suited to simulations on parallel computers. Effects associated with physical dispersion & attenuation and frequency-dependent viscous resistance are addressed by using a memory variable approach. Various benchmarks involving poroelastic wave propagation in the high- and low-frequency regimes, and acoustic-poroelastic and poroelastic-poroelastic discontinuities have been
Finite Difference Time Domain Electromagnetic Scattering from Frequency-Dependent Lossy Materials
NASA Technical Reports Server (NTRS)
Luebbers, Raymond J.; Beggs, John H.
1991-01-01
During this effort the tasks specified in the Statement of Work have been successfully completed. The extension of Finite Difference Time Domain (FDTD) to more complicated materials has been made. A three-dimensional FDTD code capable of modeling interactions with both dispersive dielectric and magnetic materials has been written, validated, and documented. This code is efficient and is capable of modeling interesting targets using a modest computer work station platform. However, in addition to the tasks in the Statement of Work, a significant number of other FDTD extensions and calculations have been made. RCS results for two different plate geometries have been reported. The FDTD method has been extended to computing far zone time domain results in two dimensions. Finally, the capability to model nonlinear materials has been incorporated into FDTD and validated. The FDTD computer codes developed have been supplied, along with documentation, and preprints describing the other FDTD advances have been included with this report as attachments.
Low frequency eddy current finite element model validation and benchmark studies
Cherry, M.; Knopp, J.; Mooers, R.; Boehnlein, T.; Aldrin, J. C.; Sabbagh, H. A.
2011-06-23
A finite element method (FEM) model was created to calculate the change in impedance of a coil due to the presence of a notch in a plate. The rectangular notches were created via electrical discharge machining (EDM) in a thick aluminum plate and were positioned at normal and oblique angles (10, 20, and 30 degrees) with respect to the vertical axis of the coil. The FEM method was chosen for this model due to its ability to solve problems in complicated geometries with the use of irregular mesh elements to discretize the solution domain. The change in impedance was calculated from the field variables in the simulation for each probe position along the parallel axis of the plate. The error between the model and the experimental data was approximately 5% for the majority of cases. The validated model was used to investigate more complex problems.
NASA Astrophysics Data System (ADS)
Corrado, Charles N., Jr.
The measurements and analyses were conducted over a mid-frequency range of 2 < ka < 12 corresponding to about 1/2 to 3 times the ring frequency of the empty shell. The measurements were all conducted with the use of wide-band pulses yielding good time resolution of propagating waves. Various time and frequency domain representations of the scattered field are presented to illustrate the evolution of observed backscattering processes. Although the field measured at all aspect angles is reviewed, emphasis is placed on interpretation of the backscatter observed over a range within 30 degrees of beam aspect where phase matched (coincident) excitation of membrane waves occurs. Coincident shear wave radiation is the dominant source of backscatter generated by the empty shell at oblique angles of incidence within 30^circ of beam aspect. Peak levels of backscatter are generally found at combinations of aspect angle and frequency where coincidence and peak levels of length-scale modulation coexist. Coincident back radiation of shear waves remains evident in the backscatter of the ring stiffened shell, but the backscatter is smeared in time and frequency because the rings directly scatter energy to the acoustic medium, as well as from one membrane wave type to another, and to subsonic flexural waves. The decay rate of the empty shell backscatter exceeds that of the ring stiffened shell by a factor of 2-3 because the rings scatter energy to poorly radiating waves. Although details of the backscatter produced by the empty and ring stiffened shells differ, peak levels of target strength consistently fall within a range of -20 to -15 dB re 1 m. The internal loading further impairs coincident radiation but increases the target strength by about 2 dB for ka > 5.5. The damping provided by the resilient mounts increases backscatter decay rates by roughly 1.2 to 1.4 relative to those of the ring stiffened shell. Bistatic measurements of the internally loaded shell also demonstrate
ITG sideband coupling models for zonal flows
Stransky, M.
2011-05-15
Four-wave interaction model between ITG mode and zonal flow was derived using fluid equations. In this model, the zonal flow is excited non-linearly by ITG turbulence via Reynolds stress. Numerical simulations show that the system allows for a small range above the ITG threshold where the zonal flow can stabilize an unstable ITG mode, effectively increasing {eta}{sub i} threshold, an effect which has been called the Dimits shift. However, the shift is smaller than in known cases such that in the Cyclone base.
Turbulence regulation by stochastic zonal flows in dynamical models
Kim, Eun-jin
2005-09-15
A theory of turbulence reduction by zonal flows is presented in the interchange turbulence model. Zonal flows with a finite correlation time {tau}{sub ZF} are shown to lead to a significant reduction in particle transport and turbulence amplitude, with the scalings {upsilon}{sub x}{proportional_to}{tau}{sub D}{omega}{sub eff}{sup -1}{proportional_to}{omega}{sub eff}{sup -3/2}, n{sup 2}{proportional_to}{tau}{sub D}{proportional_to}{omega}{sub eff}{sup -1/2}, and {upsilon}{sub x}{sup 2}{proportional_to}{tau}{sub D}{omega}{sub eff}{sup -2}{proportional_to}{omega}{sub eff}{sup -5/2}. Here, {omega}{sub eff}={tau}{sub ZF}{omega}{sub rms}{sup 2}, {tau}{sub D}=({tau}{sub {eta}}/{omega}{sub eff}){sup 1/2}, and {tau}{sub {eta}} are the effective shearing rate, effective decorrelation time, and diffusive turbulent scattering time, respectively. Compared to the transport of passive scalar fields [E. Kim and P. H. Diamond, Phys. Plasmas, 11, L77 (2004)], the reduction is much more severe due to the suppression of turbulent velocity. However, the overall transport and turbulence amplitude are still larger compared with the case of coherent shearing because shearing by random zonal flows with a finite correlation time is less efficient, with a longer decorrelation time {tau}{sub D} than ({tau}{sub {eta}}/{omega}{sup 2}){sup 1/3} in the case of coherent shearing.
Finite size effect on spread of resonance frequencies in arrays of coupled vortices
Vogel, Andreas; Drews, André; Im, Mi-Young; Fischer, Peter; Meier, Guido
2011-01-25
Dynamical properties of magnetic vortices in arrays of magnetostatically coupled ferromagnetic disks are studied by means of a broadband ferromagnetic-resonance (FMR) setup. Magnetic force microscopy and magnetic transmission soft X-ray microscopy are used to image the core polarizations and the chiralities which are both found to be randomly distributed. The resonance frequency of vortex-core motion strongly depends on the magnetostatic coupling between the disks. The parameter describing the relative broadening of the absorption peak observed in the FMR transmission spectra for a given normalized center-to-center distance between the elements is shown to depend on the size of the array.
NASA Astrophysics Data System (ADS)
Nowacki, A.; Walker, A. M.; Wookey, J.; Kendall, J.
2012-12-01
The core-mantle boundary (CMB) region is the site of the largest change in properties in the Earth. Moreover, the lowermost mantle above it (known as D″) shows the largest lateral variations in seismic velocity and strength of seismic anisotropy below the upper mantle. It is therefore vital to be able to accurately forward model candidate structures in the lowermost mantle with realistic sensitivity to structure and at the same frequencies at which observations are made. We use the spectral finite-element method to produce synthetic seismograms of ScS waves traversing a model of D″ anisotropy derived from mineralogical texture calculations and show that the seismic discontinuity atop the lowermost mantle varies in character laterally purely as a function of the strength and orientation of anisotropy. The lowermost mantle is widely anisotropic, shown by numerous shear wave splitting studies using waves of dominant frequency ~0.2-1 Hz. Whilst methods exist to model the finite-frequency seismic response of the lowermost mantle, most make the problem computationally efficient by imposing a certain symmetry to the problem, and of those which do not, almost none allow for completely general elasticity. Where low frequencies are simulated to reduce computational cost, it is uncertain whether waves of that frequency have comparable sensitivity to D″ structure as those observed at shorter periods. Currently, therefore, these computational limitations precludes the ability to interpret our observations fully. We present recent developments in taking a general approach to forward-modelling waves in D″. We use a modified version of SPECFEM3D_GLOBE, which uses the spectral finite-element method to model seismic wave propagation in a fully generally-elastic (i.e., 3D-varying, arbitrarily anisotropic) Earth. The calculations are computationally challenging: to approach the frequency of the observations, up to 10,000 processor cores and up to 2 TB of memory are needed. The
Aguinaga, Iker; Fierz, Basil; Spillmann, Jonas; Harders, Matthias
2010-12-01
The behavior, performance, and run-time of mechanical simulations in interactive virtual surgery depend heavily on the type of numerical differential equation solver used to integrate in time the dynamic equations obtained from simulation methods, such as the Finite Element Method. Explicit solvers are fast but only conditionally stable. The condition number of the stiffness matrix limits the highest possible time step. This limit is related to the geometrical properties of the underlying mesh, such as element shape and size. In fact, it can be governed by a small set of ill-shaped elements. For many applications this issue can be solved a priori by a careful meshing. However, when meshes are cut during interactive surgery simulation, it is difficult and computationally expensive to control the quality of the resulting elements. As an alternative, we propose to modify the elemental stiffness matrices directly in order to ensure stability. In this context, we first investigate the behavior of the eigenmodes of the elemental stiffness matrix in a Finite Element Method. We then propose a simple filter to reduce high model frequencies and thus allow larger time steps, while maintaining the general mechanical behavior. PMID:20869390
Stoykov, Nikolay S; Kuiken, Todd A; Lowery, Madeleine M; Taflove, Allen
2003-09-01
We present what we believe to be the first algorithms that use a simple scalar-potential formulation to model linear Debye and Lorentz dielectric dispersions at low frequencies in the context of finite-element time-domain (FETD) numerical solutions of electric potential. The new algorithms, which permit treatment of multiple-pole dielectric relaxations, are based on the auxiliary differential equation method and are unconditionally stable. We validate the algorithms by comparison with the results of a previously reported method based on the Fourier transform. The new algorithms should be useful in calculating the transient response of biological materials subject to impulsive excitation. Potential applications include FETD modeling of electromyography, functional electrical stimulation, defibrillation, and effects of lightning and impulsive electric shock. PMID:12943277
A hybrid absorbing boundary condition for frequency-domain finite-difference modelling
NASA Astrophysics Data System (ADS)
Ren, Zhiming; Liu, Yang
2013-10-01
Liu and Sen (2010 Geophysics 75 A1-6 2012 Geophys. Prospect. 60 1114-32) proposed an efficient hybrid scheme to significantly absorb boundary reflections for acoustic and elastic wave modelling in the time domain. In this paper, we extend the hybrid absorbing boundary condition (ABC) into the frequency domain and develop specific strategies for regular-grid and staggered-grid modelling, respectively. Numerical modelling tests of acoustic, visco-acoustic, elastic and vertically transversely isotropic (VTI) equations show significant absorptions for frequency-domain modelling. The modelling results of the Marmousi model and the salt model also demonstrate the effectiveness of the hybrid ABC. For elastic modelling, the hybrid Higdon ABC and the hybrid Clayton and Engquist (CE) ABC are implemented, respectively. Numerical simulations show that the hybrid Higdon ABC gets better absorption than the hybrid CE ABC, especially for S-waves. We further compare the hybrid ABC with the classical perfectly matched layer (PML). Results show that the two ABCs cost the same computation time and memory space for the same absorption width. However, the hybrid ABC is more effective than the PML for the same small absorption width and the absorption effects of the two ABCs gradually become similar when the absorption width is increased.
Frequency analysis of finite beams on nonlinear Kelvin-Voight foundation under moving loads
NASA Astrophysics Data System (ADS)
Ansari, M.; Esmailzadeh, E.; Younesian, D.
2011-03-01
The vibration of an Euler-Bernoulli beam, resting on a nonlinear Kelvin-Voight viscoelastic foundation, traversed by a moving load is studied in the frequency domain. The objective is to obtain the frequency responses of the beam and the effects of different parameters on the system response. The parameters include the magnitude and speed of the moving load and the foundation nonlinearity and its damping coefficient. The solution is obtained by using the Galerkin method in conjunction with the multiple scales method (MSM). The governing nonlinear partial differential equations of motion are discretized into sets of nonlinear ordinary differential equations. Subsequently, the solution is calculated for different harmonics by using the MSM as one of the powerful perturbation techniques. The steady-state responses of the main harmonic as well as its two super-harmonics are then obtained. As a case study, a conventional railway track is dynamically simulated and the jump phenomenon in the response is observed for three harmonics. Moreover, a thorough stability analysis of the system is carried out.
NASA Astrophysics Data System (ADS)
Kaladze, T. D.; Horton, W.; Kahlon, L. Z.; Pokhotelov, O.; Onishchenko, O.
2013-12-01
waves and vortices in the weakly ionized ionospheric E layer are dominated by the Hall conductivity that couples the Rossby and Alfvén dynamics giving rise to what are called Rossby-Alfvén-Khantadze electromagnetic structures. At finite amplitudes we show that the nonlinearities in the dynamics generate sheared zonal-flow velocities and zonal magnetic field fluctuations. The zonal-flow mechanism is based on the parametric excitation of the zonal variations through three-wave mode coupling in the planetary-scale waves. The coupled dynamics of the nonlinear 3-D incompressible flows and the magnetic field fluctuations are derived and used to derive the structure and growth rates for the zonal flows and zonal magnetic fields. Large-amplitude planetary waves are shown to drive up magnetic fluctuations up to 100 nT.
NASA Astrophysics Data System (ADS)
Panayappan, Kadappan
With the advent of sub-micron technologies and increasing awareness of Electromagnetic Interference and Compatibility (EMI/EMC) issues, designers are often interested in full- wave solutions of complete systems, taking to account a variety of environments in which the system operates. However, attempts to do this substantially increase the complexities involved in computing full-wave solutions, especially when the problems involve multi- scale geometries with very fine features. For such problems, even the well-established numerical methods, such as the time domain technique FDTD and the frequency domain methods FEM and MoM, are often challenged to the limits of their capabilities. In an attempt to address such challenges, three novel techniques have been introduced in this work, namely Dipole Moment (DM) Approach, Recursive Update in Frequency Domain (RUFD) and New Finite Difference Time Domain ( vFDTD). Furthermore, the efficacy of the above techniques has been illustrated, via several examples, and the results obtained by proposed techniques have been compared with other existing numerical methods for the purpose of validation. The DM method is a new physics-based approach for formulating MoM problems, which is based on the use of dipole moments (DMs), as opposed to the conventional Green's functions. The absence of the Green's functions, as well as those of the vector and scalar potentials, helps to eliminate two of the key sources of difficulties in the conventional MoM formulation, namely the singularity and low-frequency problems. Specifically, we show that there are no singularities that we need to be concerned with in the DM formulation; hence, this obviates the need for special techniques for integrating these singularities. Yet another salutary feature of the DM approach is its ability to handle thin and lossy structures, or whether they are metallic, dielectric-type, or even combinations thereof. We have found that the DM formulation can handle these
EQUATORIAL ZONAL JETS AND JUPITER's GRAVITY
Kong, D.; Liao, X.; Zhang, K.; Schubert, G.
2014-08-20
The depth of penetration of Jupiter's zonal winds into the planet's interior is unknown. A possible way to determine the depth is to measure the effects of the winds on the planet's high-order zonal gravitational coefficients, a task to be undertaken by the Juno spacecraft. It is shown here that the equatorial winds alone largely determine these coefficients which are nearly independent of the depth of the non-equatorial winds.
Long-range correlations induced by the self-regulation of zonal flows and drift-wave turbulence
Manz, P.; Ramisch, M.; Stroth, U.
2010-11-15
By means of a unique probe array, the interaction between zonal flows and broad-band drift-wave turbulence has been investigated experimentally in a magnetized toroidal plasma. Homogeneous potential fluctuations on a magnetic flux surface, previously reported as long range correlations, could be traced back to a predator-prey-like interaction between the turbulence and the zonal flow. At higher frequency the nonlocal transfer of energy to the zonal flow is dominant and the low-frequency oscillations are shown to result from the reduced turbulence activity due to this energy loss. This self-regulation process turns out to be enhanced with increased background shear flows.
Experimental Evidence of a Zonal Magnetic Field in a Toroidal Plasma
Fujisawa, A.; Itoh, K.; Shimizu, A.; Nakano, H.; Ohshima, S.; Iguchi, H.; Matsuoka, K.; Okamura, S.; Minami, T.; Yoshimura, Y.; Nagaoka, K.; Ida, K.; Toi, K.; Takahashi, C.; Kojima, M.; Nishimura, S.; Isobe, M.; Suzuki, C.; Akiyama, T.; Nagashima, Y.
2007-04-20
A zonal magnetic field is found in a toroidal plasma. The magnetic field has a symmetric bandlike structure, which is uniform in the toroidal and poloidal directions and varies radially with a finite wavelength of mesoscale, which is analogous to zonal flows. A time-dependent bicoherence analysis reveals that the magnetic field should be generated by the background plasma turbulence. The discovery is classified as a new kind of phenomenon of structured magnetic field generation, giving insight into phenomena such as dipole field generation in rotational planets.
NASA Technical Reports Server (NTRS)
2000-01-01
This movie is a manipulated sequence showing motions in Jupiter's atmosphere over the course of five days beginning Oct. 1, 2000, as seen by a camera on NASA's Cassini spacecraft, using a blue filter.
Beginning with seven images taken at uneven time intervals, this sequence was made by using information on wind speeds derived from actual Jupiter images to create evenly spaced time steps throughout. The final result is a smooth movie sequence consisting of both real and false frames.
The view is of the opposite side of the planet from Jupiter's Great Red Spot. The region shown reaches from 50 degrees north to 50 degrees south of Jupiter's equator, and extends 100 degrees east-to-west, about one-quarter of Jupiter's circumference. The smallest features are about 500 kilometers (about 300 miles) across.
Towards the end of the sequence, a shadow appears from one of Jupiter's moons, Europa.
The movie shows the remains of a historic merger that began several years ago, when three white oval storms that had existed for 60 years merged into two, then one. The resulting oval is visible in the lower left portion of the movie.
The movie also shows zonal jets that circle the planet on constant latitudes. Winds seen moving toward the left (westward) correspond to features that are rotating a little slower than Jupiter's magnetic field, and winds moving the opposite direction correspond to features that are rotating a little faster than the magnetic field. Since Jupiter has no solid surface, the rotation of the magnetic field is the point of reference for the rotation of the planet.
Cassini is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini mission for NASA's Office of Space Science, Washington, D.C.
NASA Astrophysics Data System (ADS)
Matharu, G.; Bostock, M. G.; Christensen, N. I.; Tromp, J.; Peter, D. B.
2012-12-01
The Leech River Complex (LRC) of southern Vancouver Island is part of a once continuous belt of Cretaceous sandstone, mudstone and volcanics that formed an accretionary wedge along the northwestern margin of North America. Metamorphism at 50 Ma to prehnite-pumpellyite, greenschist, amphibolite and blueschist facies produced pervasive foliations with strong phyllosilicate lattice preferred orientations. Laboratory measurements and in-situ S-wave splitting analysis of tectonic tremor wavetrains indicate that this fabric produces substantial S-wave anisotropy of up to 30%. In this study we seek to gain further understanding on the nature of anisotropy within the LRC using high signal to noise ratio low frequency earthquake (LFE) templates and 3-D simulations from the spectral element method (SEM). The LFEs are characterized by impulsive, double couple, point sources and lie along a surface between 27 and 37 km depth that is inferred to be the plate boundary, immediately underlying the LRC. The SEM modelling employs a regional mesh that incorporates realistic topography, bathymetry and a 3-D tomographic P-wave velocity model of southern Vancouver Island. It allows us to readily simulate wave propagation in general anisotropic media with up to 21 independent elastic constants. We will investigate the orientation and distribution of anisotropy within the LRC by employing sensitivity kernels determined using adjoint methods in conjunction with SEM.
Zonal-flow-driven nonlinear energy transfer in experiment and simulation
Holland, C.; Tynan, G. R.; Fonck, R. J.; McKee, G. R.; Candy, J.; Waltz, R. E.
2007-05-15
Using a newly developed algorithm, the nonlinear transfer of internal fluctuation energy vertical bar n-tilde vertical bar{sup 2} due to convection of drift-wave turbulence by a geodesic acoustic mode (GAM, a finite-frequency zonal flow) has now been measured directly in a high-temperature plasma. By combining spatially resolved density fluctuation measurements obtained via an upgraded beam emission spectroscopy system in the edge region of the DIII-D tokamak [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] with a velocity inference algorithm, the convection of turbulent fluctuations by the GAM has been measured. Taken together, the results strongly suggest that GAM convection of turbulence leads to a transfer of internal fluctuation energy from low to high frequencies, in agreement with expectations from theory and simulation. In addition, the GAM is found to modulate the intensity of the density fluctuations. Calculations of the measured nonlinear interactions in the gyrokinetic code GYRO are found to be in good qualitative agreement with the experimental observations.
NASA Astrophysics Data System (ADS)
Ma, Zhitu; Masters, Guy; Mancinelli, Nicholas
2016-01-01
In this study, we obtain a set of 2-D global phase velocity and attenuation maps for Rayleigh waves between 5 and 25 mHz. Correcting the effect of focusing-defocusing is crucial in order to obtain reliable attenuation structure. Great circle linearized ray theory, which has been used to date, can give useful predictions of this effect if careful attention is paid to how the phase velocity model is smoothed. In contrast, predictions based on the 2-D finite-frequency kernels are quite robust in this frequency range and suggest that they are better suited as a basis for inversion. We use a large data set of Rayleigh wave phase and amplitude measurements to invert for the phase velocity, attenuation, source and receiver terms simultaneously. Our models provide 60-70 per cent variance reduction to the raw data though the source terms are the biggest contribution to the fit of the data. The attenuation maps show structures that correlate well with surface tectonics and the age progression trend of the attenuation is clearly seen in the ocean basins. We have also identified problematic stations and earthquake sources as a by-product of our data selection process.
Luo, Y.; Xia, J.; Xu, Y.; Zeng, C.; Liu, J.
2010-01-01
Love-wave propagation has been a topic of interest to crustal, earthquake, and engineering seismologists for many years because it is independent of Poisson's ratio and more sensitive to shear (S)-wave velocity changes and layer thickness changes than are Rayleigh waves. It is well known that Love-wave generation requires the existence of a low S-wave velocity layer in a multilayered earth model. In order to study numerically the propagation of Love waves in a layered earth model and dispersion characteristics for near-surface applications, we simulate high-frequency (>5 Hz) Love waves by the staggered-grid finite-difference (FD) method. The air-earth boundary (the shear stress above the free surface) is treated using the stress-imaging technique. We use a two-layer model to demonstrate the accuracy of the staggered-grid modeling scheme. We also simulate four-layer models including a low-velocity layer (LVL) or a high-velocity layer (HVL) to analyze dispersive energy characteristics for near-surface applications. Results demonstrate that: (1) the staggered-grid FD code and stress-imaging technique are suitable for treating the free-surface boundary conditions for Love-wave modeling, (2) Love-wave inversion should be treated with extra care when a LVL exists because of a lack of LVL information in dispersions aggravating uncertainties in the inversion procedure, and (3) energy of high modes in a low-frequency range is very weak, so that it is difficult to estimate the cutoff frequency accurately, and "mode-crossing" occurs between the second higher and third higher modes when a HVL exists. ?? 2010 Birkh??user / Springer Basel AG.
NASA Astrophysics Data System (ADS)
Chen, Y.; Gu, Y. J.; Hung, S. H.
2014-12-01
Based on finite-frequency theory and cross-correlation teleseismic relative traveltime data from the USArray, Canadian National Seismograph Network (CNSN) and Canadian Rockies and Alberta Network (CRANE), we present a new tomographic model of P-wave velocity perturbations for the lithosphere and upper mantle beneath the Cordillera-cration transition region in southwestern Canada. The inversion procedure properly accounts for the finite-volume sensitivities of measured travel time residuals, and the resulting model shows a greater resolution of upper mantle velocity heterogeneity beneath the study area than earlier approaches based on the classical ray-theoretical approach. Our model reveals a lateral change of P velocities from -0.5% to 0.5% down to ~200-km depth in a 50-km wide zone between the Alberta Basin and the foothills of the Rocky Mountains, which suggests a sharp structural gradient along the Cordillera deformation front. The stable cratonic lithosphere, delineated by positive P-velocity perturbations of 0.5% and greater, extends down to a maximum depth of ~180 km beneath the Archean Loverna Block (LB). In comparison, the mantle beneath the controversial Medicine Hat Block (MHB) exhibits significantly higher velocities in the uppermost mantle and a shallower (130-150 km depth) root, generally consistent with the average depth of the lithosphere-asthenosphere boundary beneath Southwest Western Canada Sedimentary Basin (WCSB). The complex shape of the lithospheric velocities under the MHB may be evidence of extensive erosion or a partial detachment of the Precambrian lithospheric root. Furthermore, distinct high velocity anomalies in LB and MHB, which are separated by 'normal' mantle block beneath the Vulcan structure (VS), suggest different Archean assembly and collision histories between these two tectonic blocks.
NASA Astrophysics Data System (ADS)
Fourn, Cédric; Lasquellec, Sophie; Brosseau, Christian
2007-12-01
There has been much recent interest in how morphological descriptors may affect the electromagnetic wave transport in particulate composite mediums. In this work, we present results of finite-element simulations that model the permittivity of two-dimensional (or cross sections of infinite three-dimensional parallel, infinitely long, identical, circular cylinders, where the properties and characteristics are invariant along the perpendicular cross-sectional plane) three-phase heterostructures made of a multilayered discoidal particle. While strictly valid only in a direct current situation, our analysis can be extended to treat electric fields that oscillate with time provided that the wavelengths and attenuation lengths associated with the fields are much larger than the microstructure dimension in order that the homogeneous (effective medium) representation of the composite structure makes sense. From simulations over a range of parameters, our analysis evaluates the effect of the surface fraction of inclusion, the conductivity, and thickness (relative to the particle radius) of the particle conductive coating on the effective complex permittivity of isotropic heterostructures in which the filler particles have a core-shell structure. Four main effects are found. First, the importance of the surface fraction of inclusion on the effective complex permittivity at high frequencies (from microwave to infrared) is illustrated over a broad range of coating thicknesses and conductivities. Second, the encapsulation phase (metallic coating) conductivity is identified as the key property controlling the dielectric relaxation due to interfacial polarization. Third, a simple parametrization of the high-frequency effective permittivity spectrum allowed us to obtain a reliable modelization of the Debye-type relaxation processes. From the least-squares fit of the effective complex permittivity data, we extract information on these relaxation processes, i.e., relaxation
The Galileo probe Doppler wind experiment: Measurement of the deep zonal winds on Jupiter
NASA Astrophysics Data System (ADS)
Atkinson, David H.; Pollack, James B.; Seiff, Alvin
1998-09-01
During its descent into the upper atmosphere of Jupiter, the Galileo probe transmitted data to the orbiter for 57.5 min. Accurate measurements of the probe radio frequency, driven by an ultrastable oscillator, allowed an accurate time history of the probe motions to be reconstructed. Removal from the probe radio frequency profile of known Doppler contributions, including the orbiter trajectory, the probe descent velocity, and the rotation of Jupiter, left a measurable frequency residual due to Jupiter's zonal winds, and microdynamical motion of the probe from spin, swing under the parachute, atmospheric turbulence, and aerodynamic buffeting. From the assumption of the dominance of the zonal horizontal winds, the frequency residuals were inverted and resulted in the first in situ measurements of the vertical profile of Jupiter's deep zonal winds. A number of error sources with the capability of corrupting the frequency measurements or the interpretation of the frequency residuals were considered using reasonable assumptions and calibrations from prelaunch and in-flight testing. It is found that beneath the cloud tops (about 700 mbar) the winds are prograde and rise rapidly to 170 m/s at 4 bars. Beyond 4 bars to the depth at which the link with the probe was lost, nearly 21 bars, the winds remain constant and strong. Corrections for the high temperatures encountered by the probe have recently been completed and provide no evidence of diminishing or strengthening of the zonal wind profile in the deeper regions explored by the Galileo probe.
Generation of zonal flows by electrostatic drift waves in electron-positron-ion plasmas
Kaladze, T. D.; Shad, M.; Tsamalashvili, L. V.
2010-02-15
Generation of large-scale zonal flows by comparatively small-scale electrostatic drift waves in electron-positron-ion plasmas is considered. The generation mechanism is based on the parametric excitation of convective cells by finite amplitude drift waves having arbitrary wavelengths (as compared with the ion Larmor radius of plasma ions at the plasma electron temperature). Temperature inhomogeneity of electrons and positrons is taken into account assuming ions to be cold. To describe the generation of zonal flow generalized Hasegawa-Mima equation containing both vector and two scalar (of different nature) nonlinearities is used. A set of coupled equations describing the nonlinear interaction of drift waves and zonal flows is deduced. Explicit expressions for the maximum growth rate as well as for the optimal spatial dimensions of the zonal flows are obtained. Enriched possibilities of zonal flow generation with different growth rates are revealed. The present theory can be used for interpretations of drift wave observations in laboratory and astrophysical plasmas.
NASA Astrophysics Data System (ADS)
Youssof, M.; Thybo, H.; Artemieva, I. M.; Levander, A.
2015-06-01
We present a 3D high-resolution seismic model of the southern African cratonic region from teleseismic tomographic inversion of the P- and S-body wave dataset recorded by the Southern African Seismic Experiment (SASE). Utilizing 3D sensitivity kernels, we invert traveltime residuals of teleseismic body waves to calculate velocity anomalies in the upper mantle down to a 700 km depth with respect to the ak135 reference model. Various resolution tests allow evaluation of the extent of smearing effects and help defining the optimum inversion parameters (i.e., damping and smoothness) for regularizing the inversion calculations. The fast lithospheric keels of the Kaapvaal and Zimbabwe cratons reach depths of 300-350 km and 200-250 km, respectively. The paleo-orogenic Limpopo Belt is represented by negative velocity perturbations down to a depth of ˜ 250 km, implying the presence of chemically fertile material with anomalously low wave speeds. The Bushveld Complex has low velocity down to ˜ 150 km, which is attributed to chemical modification of the cratonic mantle. In the present model, the finite-frequency sensitivity kernels allow to resolve relatively small-scale anomalies, such as the Colesberg Magnetic Lineament in the suture zone between the eastern and western blocks of the Kaapvaal Craton, and a small northern block of the Kaapvaal Craton, located between the Limpopo Belt and the Bushveld Complex.
NASA Astrophysics Data System (ADS)
Chevrot, Sébastien; Martin, Roland; Komatitsch, Dimitri
2012-12-01
Wavelets are extremely powerful to compress the information contained in finite-frequency sensitivity kernels and tomographic models. This interesting property opens the perspective of reducing the size of global tomographic inverse problems by one to two orders of magnitude. However, introducing wavelets into global tomographic problems raises the problem of computing fast wavelet transforms in spherical geometry. Using a Cartesian cubed sphere mapping, which grids the surface of the sphere with six blocks or 'chunks', we define a new algorithm to implement fast wavelet transforms with the lifting scheme. This algorithm is simple and flexible, and can handle any family of discrete orthogonal or bi-orthogonal wavelets. Since wavelet coefficients are local in space and scale, aliasing effects resulting from a parametrization with global functions such as spherical harmonics are avoided. The sparsity of tomographic models expanded in wavelet bases implies that it is possible to exploit the power of compressed sensing to retrieve Earth's internal structures optimally. This approach involves minimizing a combination of a ℓ2 norm for data residuals and a ℓ1 norm for model wavelet coefficients, which can be achieved through relatively minor modifications of the algorithms that are currently used to solve the tomographic inverse problem.
Self-organized zonal flow in the flute-mode turbulence of a plasma
Kodama, Y.; Pavlenko, V.P.
1988-04-11
Flute-mode turbulence has a forward spectral cascade unlike the case of drift-wave turbulence. Therefore the linear flute instability may be reduced by this energy cascading toward large wave numbers. As a consequence of three-wave cascade processes derivable from model equations including the effects of density gradient and finite ion Larmor radius the formation of zonal flows in flute mode turbulence is predicted.
Measurements of Zonal Winds on Titan from Millimeter Interferometric Observations
NASA Astrophysics Data System (ADS)
Moreno, R.; Marten, A.
2003-05-01
Narrow emission lines of HC3N (cyanoacetylene) and CH3CN (acetonitrile) were observed on Titan with the Plateau-de-Bure Interferometer of IRAM (France) in February-March 2003. Using the most extended configuration of the six-antenna array, an angular resolution of 0.6 arc sec was obtained at the working frequencies (227.4 and 220.7 GHz, respectively), permitting a disk-resolved investigation of the nitrile composition and the zonal wind flow in the upper atmosphere. Observing details and composition results are reported in an accompanying presentation (Marten and Moreno, BAAS, 2003). Our second objective was reached by recording the spectra at a very high spectral resolution of 40 kHz. A sufficient signal-to-noise ratio (greater than 10 for the equatorial measurements) was achieved after 8 hours of integration time on Titan. Examination of the contribution functions calculated for the strongest lines used in our analysis shows that the probed altitudes are in the low mesosphere around 400 km (0.01-mbar pressure level). The Doppler shifts of the lines measured between the east and west limbs provide evidence for a prograde wind direction. The retrieved maps focusing on the zonal wind speeds determined across Titan's disk will be presented.
Penetrative Convection and Zonal Flow on Jupiter
Zhang; Schubert
1996-08-16
Measurements by the Galileo probe support the possibility that the zonal winds in Jupiter's atmosphere originate from convection that takes place in the deep hydrogen-helium interior. However, according to models based on recent opacity data and the probe's temperature measurements, there may be radiative and nonconvective layers in the outer part of the jovian interior, raising the question of how deep convection could extend to the surface. A theoretical model is presented to demonstrate that, because of predominant rotational effects and spherical geometry, thermal convection in the deep jovian interior can penetrate into any outer nonconvective layer. These penetrative convection rolls interact nonlinearly and efficiently in the model to generate and sustain a mean zonal wind with a larger amplitude than that of the nonaxisymmetric penetrative convective motions, a characteristic of the wind field observed at the cloud level on Jupiter. PMID:8688074
Laboratory Exploration of Multiple Zonal Jet Regimes
NASA Astrophysics Data System (ADS)
Smith, C. A.; Speer, K. G.; Griffiths, R. W.
2012-12-01
The differentially heated, rotating annulus has classically been used to study wave interactions within a single, baroclinic jet. At high rotation rates, the baroclinic instability of the flow leads to a transition to a turbulent, eddy-dominated regime. In the presence of a topographic beta effect, the flow has been observed to produce multiple, meandering zonal jets that are qualitatively similar to those found in planetary atmospheres and in the Antarctic Circumpolar Current (ACC). Our study builds on previous annulus experiments [1] by making observations further within this new regime. We observe with PIV and other techniques how the structure of the flow responds to changes in various parameters such as tank geometry, gradient in the Coriolis parameter, rotation rate, and differential thermal forcing. By not employing the more typical direct forcing of small scales, but by applying a large scale forcing over the annulus gap width, this study allows the varying effects of eddy scale selection, enstrophy cascade, etc. to naturally generate flow that more closely resembles planetary atmospheres and the ACC. We seek nondimensional parameters that significantly control zonation in a real fluid. These observations will provide a metric for the comparison of various theoretical models for multiple zonal jet formation. Other properties of the jets, such as their migration, meandering, bifurcation, and merging, can also be observed in an idealized situation and compared to numerical simulations. Ultimately, this will aid the testing and development of sub-grid-scale parameterizations for the multiple zonal jet regime that remain robust in the face of multiple forcing parameters. [1] Wordsworth, R. D., Read, P. L., & Yamazaki, Y. H. (2008). Turbulence, waves, and jets in a differentially heated rotating annulus experiment Physics of Fluids, 20(12), 126602.Streak photograph of suspended particles visualizing the flow representative of multiple zonal jets
NASA Astrophysics Data System (ADS)
Kaladze, T. D.; Kahlon, L. Z.; Tsamalashvili, L. V.; Kaladze, D. T.
2012-11-01
Nonlinear dynamics of coupled internal-gravity (IG) and alfvén electromagnetic planetary waves in the weakly ionized ionospheric E-layer is investigated. Under such coupling new type of alfvén waves is revealed. It is shown that such short wavelength turbulence of IG and alfvén waves is unstable with respect to the excitation of low-frequency and large-scale perturbations of the zonal flow and magnetic field. A set of coupled equations describing the nonlinear interaction of coupled IG and alfvén waves with zonal flows is derived. The nonlinear mechanism of the instability is driven by the advection of vorticity and is based on the parametric excitation of convective cells by finite-amplitude coupled IG and alfvén waves leading to the inverse energy cascade toward the longer wavelength. The growth rates of the corresponding instability and the conditions for driving them are determined. The possibility of generation of the intense mean magnetic field is shown.
Generalized Quasilinear Approximation: Application to Zonal Jets
NASA Astrophysics Data System (ADS)
Marston, J. B.; Chini, G. P.; Tobias, S. M.
2016-05-01
Quasilinear theory is often utilized to approximate the dynamics of fluids exhibiting significant interactions between mean flows and eddies. We present a generalization of quasilinear theory to include dynamic mode interactions on the large scales. This generalized quasilinear (GQL) approximation is achieved by separating the state variables into large and small zonal scales via a spectral filter rather than by a decomposition into a formal mean and fluctuations. Nonlinear interactions involving only small zonal scales are then removed. The approximation is conservative and allows for scattering of energy between small-scale modes via the large scale (through nonlocal spectral interactions). We evaluate GQL for the paradigmatic problems of the driving of large-scale jets on a spherical surface and on the beta plane and show that it is accurate even for a small number of large-scale modes. As GQL is formally linear in the small zonal scales, it allows for the closure of the system and can be utilized in direct statistical simulation schemes that have proved an attractive alternative to direct numerical simulation for many geophysical and astrophysical problems.
NASA Technical Reports Server (NTRS)
Vlahopoulos, Nickolas
2005-01-01
The Energy Finite Element Analysis (EFEA) is a finite element based computational method for high frequency vibration and acoustic analysis. The EFEA solves with finite elements governing differential equations for energy variables. These equations are developed from wave equations. Recently, an EFEA method for computing high frequency vibration of structures either in vacuum or in contact with a dense fluid has been presented. The presence of fluid loading has been considered through added mass and radiation damping. The EFEA developments were validated by comparing EFEA results to solutions obtained by very dense conventional finite element models and solutions from classical techniques such as statistical energy analysis (SEA) and the modal decomposition method for bodies of revolution. EFEA results have also been compared favorably with test data for the vibration and the radiated noise generated by a large scale submersible vehicle. The primary variable in EFEA is defined as the time averaged over a period and space averaged over a wavelength energy density. A joint matrix computed from the power transmission coefficients is utilized for coupling the energy density variables across any discontinuities, such as change of plate thickness, plate/stiffener junctions etc. When considering the high frequency vibration of a periodically stiffened plate or cylinder, the flexural wavelength is smaller than the interval length between two periodic stiffeners, therefore the stiffener stiffness can not be smeared by computing an equivalent rigidity for the plate or cylinder. The periodic stiffeners must be regarded as coupling components between periodic units. In this paper, Periodic Structure (PS) theory is utilized for computing the coupling joint matrix and for accounting for the periodicity characteristics.
NASA Technical Reports Server (NTRS)
Young, Donald R.; Orne, David
1976-01-01
The influence of pretwist, nonuniformities in mass and flexural stiffness, rotatory inertia and shear deformation on the natural frequencies of intact bones is evaluated by means of a linear elastic, finite-element model which has been programmed for solution on the digital computer. Theoretical results are compared to the results on the forced vibration of intact canine radii obtained experimentally by Thompson. Surprisingly, inclusion of fairly large pretwist angles (from -14 to 12 deg for one specimen) had little affect on the first three frequencies of transverse vibration in either the cranial or lateral directions. Inclusion of shear deformation reduced the third-mode frequency in the stiffest (lateral) direction by about six percent, otherwise shear deformation played a minor role in determining natural frequencies. Similarly. rotatory inertia had negligible influence up to the third natural frequency. The predominant influence on the first three natural frequencies of transverse vibration could be attributed to the variations in mass and flexural stiffness along the length of the test specimens. Different effective moduli of elasticity are required to yield correct absolute values for the frequencies which correspond to experimental findings. thus implying the presence of some inhomogeneities in material properties around the bone cross-section and/or along its length.
NASA Astrophysics Data System (ADS)
Moens, David; Vandepitte, Dirk
2005-12-01
This work introduces a numerical algorithm to calculate frequency-response functions (FRFs) of damped finite element (FE) models with fuzzy uncertain parameters. Part one of this paper describes the numerical algorithm for the solution of the underlying interval finite element (IFE) problem. First, the IFE procedure for the calculation of undamped envelope FRFs is discussed. Starting from the undamped procedure, a strategy is developed to analyse damped structures based on the principle of Rayleigh damping. This is achieved by analysing the effect of the proportional damping coefficients on the subsequent steps of the undamped procedure. This finally results in a procedure for the calculation of fuzzy damped FRFs based on an analytical extension of the undamped algorithm. Part one of this paper introduces the numerical procedure. Part two of this paper illustrates the application of the methodology on four numerical case studies.
Gyroaverage effects on chaotic transport by drift waves in zonal flows
Martinell, J.; Del-Castillo-Negrete, Diego B
2013-01-01
Finite Larmor radius (FLR) effects on E x B test particle chaotic transport in the presence of zonal flows is studied. The FLR effects are introduced by the gyro-average of a simplified E x B guiding center model consisting of the linear superposition of a non-monotonic zonal flow and drift waves. Non-monotonic zonal flows play a critical role on transport because they exhibit robust barriers to chaotic transport in the region(s) where the shear vanishes. In addition, the non-monotonicity gives rise to nontrivial changes in the topology of the orbits of the E x B Hamiltonian due to separatrix reconnection. The present study focuses on the role of FLR effects on these two signatures of non-monotonic zonal flows: shearless transport barriers and separatrix reconnection. It is shown that, as the Larmor radius increases, the effective zonal flow profile bifurcates and multiple shearless regions are created. As a result, the topology of the gyro-averaged Hamiltonian exhibits very complex separatrix reconnection bifurcations. It is also shown that FLR effects tend to reduce chaotic transport. In particular, the restoration of destroyed transport barriers is observed as the Larmor radius increases. A detailed numerical study is presented on the onset of global chaotic transport as function of the amplitude of the drift waves and the Larmor radius. For a given amplitude, the threshold for the destruction of the shearless transport barrier, as function of the Larmor radius, exhibits a fractal-like structure. The FLR effects on a thermal distribution of test particles are also studied. In particular, the fraction of confined particles with a Maxwellian distribution of gyroradii is computed, and an effective transport suppression is found for high enough temperatures.
Gyroaverage effects on chaotic transport by drift waves in zonal flows
Martinell, Julio J.; Castillo-Negrete, Diego del
2013-02-15
Finite Larmor radius (FLR) effects on E Multiplication-Sign B test particle chaotic transport in the presence of zonal flows is studied. The FLR effects are introduced by the gyro-average of a simplified E Multiplication-Sign B guiding center model consisting of the linear superposition of a non-monotonic zonal flow and drift waves. Non-monotonic zonal flows play a critical role on transport because they exhibit robust barriers to chaotic transport in the region(s) where the shear vanishes. In addition, the non-monotonicity gives rise to nontrivial changes in the topology of the orbits of the E Multiplication-Sign B Hamiltonian due to separatrix reconnection. The present study focuses on the role of FLR effects on these two signatures of non-monotonic zonal flows: shearless transport barriers and separatrix reconnection. It is shown that, as the Larmor radius increases, the effective zonal flow profile bifurcates and multiple shearless regions are created. As a result, the topology of the gyro-averaged Hamiltonian exhibits very complex separatrix reconnection bifurcations. It is also shown that FLR effects tend to reduce chaotic transport. In particular, the restoration of destroyed transport barriers is observed as the Larmor radius increases. A detailed numerical study is presented on the onset of global chaotic transport as function of the amplitude of the drift waves and the Larmor radius. For a given amplitude, the threshold for the destruction of the shearless transport barrier, as function of the Larmor radius, exhibits a fractal-like structure. The FLR effects on a thermal distribution of test particles are also studied. In particular, the fraction of confined particles with a Maxwellian distribution of gyroradii is computed, and an effective transport suppression is found for high enough temperatures.
Stochastic magnetic field driven charge transport and zonal flow during magnetic reconnection
Ding, W. X.; Brower, D. L.; Craig, D.; Chapman, B. E.; Ennis, D.; Fiksel, G.; Gangadhara, S.; Den Hartog, D. J.; Mirnov, V. V.; Prager, S. C.; Sarff, J. S.; Terry, P. W.; Svidzinski, V.; Yates, T.
2008-05-15
Magnetic fluctuation-induced charge transport, resulting from particle transport that is not intrinsically ambipolar, has been measured in the high-temperature interior of a reversed-field pinch plasma. It is found that global resistive tearing modes and their nonlinear interactions lead to significant charge transport, equivalent to the perpendicular Maxwell stress, in the vicinity of the resonant surface for the dominant core resonant mode during magnetic reconnection. Finite charge transport can result in a zonal flow associated with locally strong radial electric field and electric field shear. In the presence of stochastic magnetic field, radial electric field is expected to be balanced by radial electron pressure gradient. Direct measurement of local density gradient is consistent with the formation of radial electric field and the zonal flow.
Predictability of Zonal Means During Boreal Summer
NASA Technical Reports Server (NTRS)
Schubert, Siegfried; Suarez, Max J.; Pegion, Philip J.; Kistler, Michael A.; Kumar, Arun; Einaudi, Franco (Technical Monitor)
2001-01-01
This study examines the predictability of seasonal means during boreal summer. The results are based on ensembles of June-July-August (JJA) simulations (started in mid May) carried out with the NASA Seasonal-to-Interannual Prediction Project (NSIPP-1) atmospheric general circulation model (AGCM) forced with observed sea surface temperatures (SSTS) and sea ice for the years 1980-1999. We find that the predictability of the JJA extra-tropical height field is primarily in the zonal mean component of the response to the SST anomalies. This contrasts with the cold season (January-February-March) when the predictability of seasonal means in the boreal extratropics is primarily in the wave component of the El Nino/Southern Oscillation (ENSO) response. Two patterns dominate the interannual variability of the ensemble mean JJA zonal mean height field. One has maximum variance in the tropical/subtropical upper troposphere, while the other has substantial variance in middle latitudes of both hemispheres. Both are symmetric with respect to the equator. A regression analysis suggests that the tropical/subtropical pattern is associated with SST anomalies in the far eastern tropical Pacific and the Indian Ocean, while the middle latitude pattern is forced by SST anomalies in the tropical Pacific just east of the dateline. The two leading zonal height patterns are reproduced in model runs forced with the two leading JJA SST patterns of variability. A comparison with observations shows a signature of the middle latitude pattern that is consistent with the occurrence of dry and wet summers over the United States. We hypothesize that both patterns, while imposing only weak constraints on extratropical warm season continental-scale climates, may play a role in the predilection for drought or pluvial conditions.
ZASPE: Zonal Atmospheric Stellar Parameters Estimator
NASA Astrophysics Data System (ADS)
Brahm, Rafael; Jordan, Andres; Hartman, Joel; Bakos, Gaspar
2016-07-01
ZASPE (Zonal Atmospheric Stellar Parameters Estimator) computes the atmospheric stellar parameters (Teff, log(g), [Fe/H] and vsin(i)) from echelle spectra via least squares minimization with a pre-computed library of synthetic spectra. The minimization is performed only in the most sensitive spectral zones to changes in the atmospheric parameters. The uncertainities and covariances computed by ZASPE assume that the principal source of error is the systematic missmatch between the observed spectrum and the sythetic one that produces the best fit. ZASPE requires a grid of synthetic spectra and can use any pre-computed library minor modifications.
Zonal flow formation in the presence of ambient mean shear
Hsu, Pei-Chun; Diamond, P. H.
2015-02-15
The effect of mean shear flows on zonal flow formation is considered in the contexts of plasma drift wave turbulence and quasi-geostrophic turbulence models. The generation of zonal flows by modulational instability in the presence of large-scale mean shear flows is studied using the method of characteristics as applied to the wave kinetic equation. It is shown that mean shear flows reduce the modulational instability growth rate by shortening the coherency time of the wave spectrum with the zonal shear. The scalings of zonal flow growth rate and turbulent vorticity flux with mean shear are determined in the strong shear limit.
NASA Technical Reports Server (NTRS)
Picaut, Joel; Camusat, Bruno; Busalacchi, Antonio J.; Mcphaden, Michael J.
1990-01-01
The applicability of satellite altimeter data for estimating zonal current variability at the equator is assessed using the meriodionally differenced form of the geostrophic balance. Estimates of geostrophic zonal flow anomalies in the equatorial Pacific have been deduced from 17-day collinear altimeter data during the first year of the Geosat Exact Repeat Mission. Altimeter-derived geostrophic estimates agree well with in situ zonal current variability. Comparison of flow-frequency near-surface zonal current observed from equatorial moorings at 165 deg E, 140 deg W, and 110 deg W yield correlations of 0.83, 0.85, and 0.51, respectively, with a mean rms difference of 23 cm/sec. The inclusion of up to 11 ascending and descending Geosat tracks within the 9-deg band for every 17-day repeat effectively reduced the temporal sampling interval to 1.5 days at 165 deg E and 140 deg W. The 6.8-km along track spacing of the altimeter measurements provides sufficient resolution for the effective filtering of small-scale meridional noise, both instrumental and oceanic.
NASA Astrophysics Data System (ADS)
Margolis, R. E.; Thurner, S.; Levander, A.
2014-12-01
Here we present a 3D shear velocity model for the lower crust and upper mantle beneath the Great Plains region in the central United States using finite frequency Rayleigh wave travel time tomography. We use USArray Transportable Array (TA) vertical component recording of teleseismic Rayleigh waves that we first invert for phase velocity using the modified two-plane wave method with finite frequency kernels. We then invert the resulting dispersion curves for shear velocity structure. Our analysis includes a characterization of the lithospheric structure in this tectonically transitional regime to illuminate the differences between the actively deforming western US and the stable continental interior of the northeastern Great Plains. The west is defined by slow velocities and thin lithosphere, whereas the east has fast velocities and thick lithosphere, with the thickest lithosphere in the northeast, representing the southwestern keel of the Superior craton. The Great Plains, which abut the Rocky Mountain Front, have an unusual elevation profile that possesses a much broader region of uplifted elevation and lower relief than other orogenic systems (Eaton 2009). From our tomography and regional heat flow data, we infer warm temperatures in the west and suggest that the asthenospheric mantle contributes to anomalously high elevation of the westernmost Great Plains with some secondary contribution due to crustal effects.
Zonal flow modes in a tokamak plasma with dominantly poloidal mean flows
Zhou Deng
2010-10-15
The zonal flow eigenmodes in a tokamak plasma with dominantly poloidal mean flows are theoretically investigated. It is found that the frequencies of both the geodesic acoustic mode and the sound wave increase with respect to the poloidal Mach number. In contrast to the pure standing wave form in static plasmas, the density perturbations consist of a standing wave superimposed with a small amplitude traveling wave in the poloidally rotating plasma.
EXPERIMENTAL CHARACTERIZATION OF COHERENT, RADIALLY-SHEARED ZONAL FLOWS IN THE DIII-D TOKAMAK
MCKEE,GR; FONCK,RJ; JAKUBOWSKI,M; BURRELL,KH; HALLATSCHEK,K; MOYER,RA; NEVINS,W; PORTER,GD; RUDAKOV,DL; XU,X
2002-11-01
A271 EXPERIMENTAL CHARACTERIZATION OF COHERENT, RADIALLY-SHEARED ZONAL FLOWS IN THE DIII-D TOKAMAK. Application of time-delay-estimation techniques to two-dimensional measurements of density fluctuations, obtained with beam emission spectroscopy in DIII-D plasmas, has provided temporally and spatially resolved measurements of the turbulence flow-field. Features that are characteristic of self-generated zonal flows are observed in the radial region near 0.85 {<=} r/a {<=} 1.0. These features include a coherent oscillation (approximately 15 kHz) in the poloidal flow of density fluctuations that has a long poloidal wavelength, possibly m = 0, narrow radial extent (k{sub r}{rho}{sub I} < 0.2), and whose frequency varies monotonically with the local temperature. The approximate effective shearing rate, dv{sub {theta}}/dr, of the flow is of the same order of magnitude as the measured nonlinear decorrelation rate of the turbulence, and the density fluctuation amplitude is modulated at the frequency of the observed flow oscillation. Some phase coherence is observed between the higher wavenumber density fluctuations and low frequency poloidal flow fluctuations, suggesting a Reynolds stress contribution. These characteristics are consistent with predicted features of zonal flows, specifically identified as geodesic acoustic modes, observed in 3-D Braginskii simulations of core/edge turbulence.
NASA Astrophysics Data System (ADS)
Yin, Y.; Hung, S.
2007-12-01
Seismic tomographic imaging has played a key component to unravel the deep processes that caused the surface morphology and rift magmatism in the southwest United States. Several studies used teleseismic body- wave arrivals recorded by the La Ristra experiment, a dense broadband array of 950-km in length deployed during 1999-2001 and run through the Great Plains, the Rio Grande Rift, and the Colorado Plateau, to construct a 2-D tomographic image of the upper mantle structure beneath this linear array (e.g., Gao et al., 2004). However, because of the inevitable smoothing and damping imposed in the tomographic model, the resulting velocity contrast is too weak to explain distinct P and S waveform changes across the array (Song and Helmberger, 2007). In this study, we include all the data from the La Ristra and available nearby arrays and reexamine finite- frequency travel time delays measured by inter-station cross correlation of waveforms at both high- (0.3-2 Hz for P and 0.1-0.5 Hz for S) and low-frequencies (0.03-0.125 Hz for P and 0.03-0.1 Hz for S). Differing from the previous models that rely on classical ray theory and simple grid parameterization, our inversion considers more realistic 3-D sensitivity kernels for relative travel-time delays and a wavelet-based, multi-scale parameterization that enables to yield robust features with spatially-varying resolutions. Our preliminary P-wave model reveals a prominent low-velocity zone extending from near surface to the depth of 300 km beneath the Rio Grande Rift, while the upper mantle which underlies the Great Plains and the Colorado Plateau is seismically fast. We will demonstrate the difference and improvement of 3-D tomographic models through the use of finite-frequency kernels and multi-scale parameterization.
Collisionless Zonal Flow Saturation for Weak Magnetic Shear
NASA Astrophysics Data System (ADS)
Lu, Zhixin; Wang, Weixing; Diamond, Patrick; Ashourvan, Arash; Tynan, George
2015-11-01
The damping of the zonal flow, either collisional or collisionless, plays an important role in regulating the drift wave-zonal flow system, and can affect the transport and confinement. The tertiary instability, e.g., a generalized Kelvin-Helmholtz (KH) instability driven by flow shear, has been suggested theoretically as a possible damping mechanism [Rogers 2000 PRL, Diamond 2005 PPCF]. The sensitivity of the tertiary mode to magnetic shear has not been quantified, especially in weak magnetic shear regimes. In this work, parametric scans using gyrokinetic simulation demonstrate that the zonal electric field energy normalized by the turbulence electric field energy decreases as magnetic shear decreases. With ITG drive artificially eliminated, the time evolution of the zonal structure indicates that the zonal electric field damps more rapidly at weak shear. This suggests larger collisionless zonal flow damping or larger effective turbulent viscosity at weak magnetic shear. The effects of the zonal components of specific variables, e.g., the parallel shear flow and the radial electric field, on tertiary instability, are also studied. Quantitative studies on the magnetic shear scaling of tertiary instability excitation and the collisionless zonal flow saturation are ongoing.
ZONAL FLOWS AND LONG-LIVED AXISYMMETRIC PRESSURE BUMPS IN MAGNETOROTATIONAL TURBULENCE
Johansen, A.; Youdin, A.; Klahr, H. E-mail: youd@cita.utoronto.ca
2009-06-01
We study the behavior of magnetorotational turbulence in shearing box simulations with a radial and azimuthal extent up to 10 scale heights. Maxwell and Reynolds stresses are found to increase by more than a factor of 2 when increasing the box size beyond two scale heights in the radial direction. Further increase of the box size has little or no effect on the statistical properties of the turbulence. An inverse cascade excites magnetic field structures at the largest scales of the box. The corresponding 10% variation in the Maxwell stress launches a zonal flow of alternating sub- and super-Keplerian velocity. This, in turn, generates a banded density structure in geostrophic balance between pressure and Coriolis forces. We present a simplified model for the appearance of zonal flows, in which stochastic forcing by the magnetic tension on short timescales creates zonal flow structures with lifetimes of several tens of orbits. We experiment with various improved shearing box algorithms to reduce the numerical diffusivity introduced by the supersonic shear flow. While a standard finite difference advection scheme shows signs of a suppression of turbulent activity near the edges of the box, this problem is eliminated by a new method where the Keplerian shear advection is advanced in time by interpolation in Fourier space.
B-spline methods and zonal grids for numerical simulations of turbulent flows
NASA Astrophysics Data System (ADS)
Kravchenko, Arthur Grigorievich
1998-12-01
A novel numerical technique is developed for simulations of complex turbulent flows on zonal embedded grids. This technique is based on the Galerkin method with basis functions constructed using B-splines. The technique permits fine meshes to be embedded in physically significant flow regions without placing a large number of grid points in the rest of the computational domain. The numerical technique has been tested successfully in simulations of a fully developed turbulent channel flow. Large eddy simulations of turbulent channel flow at Reynolds numbers up to Rec = 110,000 (based on centerline velocity and channel half-width) show good agreement with the existing experimental data. These tests indicate that the method provides an efficient information transfer between zones without accumulation of errors in the regions of sudden grid changes. The numerical solutions on multi-zone grids are of the same accuracy as those on a single-zone grid but require less computer resources. The performance of the numerical method in a generalized coordinate system is assessed in simulations of laminar flows over a circular cylinder at low Reynolds numbers and three-dimensional simulations at ReD = 300 (based on free-stream velocity and cylinder diameter). The drag coefficients, the size of the recirculation region, and the vortex shedding frequency all agree well with the experimental data and previous simulations of these flows. Large eddy simulations of a flow over a circular cylinder at a sub-critical Reynolds number, ReD = 3900, are performed and compared with previous upwind-biased and central finite-difference computations. In the very near-wake, all three simulations are in agreement with each other and agree fairly well with the PIV experimental data of Lourenco & Shih (1993). Farther downstream, the results of the B- spline computations are in better agreement with the hot- wire experiment of Ong & Wallace (1996) than those obtained in finite-difference simulations
NASA Astrophysics Data System (ADS)
Mobley, Joel; Waters, Kendall R.; Miller, James G.
2005-07-01
Kramers-Kronig (KK) analyses of experimental data are complicated by the extrapolation problem, that is, how the unexamined spectral bands impact KK calculations. This work demonstrates the causal linkages in resonant-type data provided by acoustic KK relations for the group velocity (cg) and the derivative of the attenuation coefficient (α') (components of the derivative of the acoustic complex wave number) without extrapolation or unmeasured parameters. These relations provide stricter tests of causal consistency relative to previously established KK relations for the phase velocity (cp) and attenuation coefficient (α) (components of the undifferentiated acoustic wave number) due to their shape invariance with respect to subtraction constants. For both the group velocity and attenuation derivative, three forms of the relations are derived. These relations are equivalent for bandwidths covering the entire infinite spectrum, but differ when restricted to bandlimited spectra. Using experimental data from suspensions of elastic spheres in saline, the accuracy of finite-bandwidth KK predictions for cg and α' is demonstrated. Of the multiple methods, the most accurate were found to be those whose integrals were expressed only in terms of the phase velocity and attenuation coefficient themselves, requiring no differentiated quantities.
ON THE VARIATION OF ZONAL GRAVITY COEFFICIENTS OF A GIANT PLANET CAUSED BY ITS DEEP ZONAL FLOWS
Kong Dali; Zhang Keke; Schubert, Gerald E-mail: kzhang@ex.ac.uk
2012-04-01
Rapidly rotating giant planets are usually marked by the existence of strong zonal flows at the cloud level. If the zonal flow is sufficiently deep and strong, it can produce hydrostatic-related gravitational anomalies through distortion of the planet's shape. This paper determines the zonal gravity coefficients, J{sub 2n}, n = 1, 2, 3, ..., via an analytical method taking into account rotation-induced shape changes by assuming that a planet has an effective uniform density and that the zonal flows arise from deep convection and extend along cylinders parallel to the rotation axis. Two different but related hydrostatic models are considered. When a giant planet is in rigid-body rotation, the exact solution of the problem using oblate spheroidal coordinates is derived, allowing us to compute the value of its zonal gravity coefficients J-bar{sub 2n}, n=1,2,3,..., without making any approximation. When the deep zonal flow is sufficiently strong, we develop a general perturbation theory for estimating the variation of the zonal gravity coefficients, {Delta}J{sub 2n}=J{sub 2n}-J-bar{sub 2n}, n=1,2,3,..., caused by the effect of the deep zonal flows for an arbitrarily rapidly rotating planet. Applying the general theory to Jupiter, we find that the deep zonal flow could contribute up to 0.3% of the J{sub 2} coefficient and 0.7% of J{sub 4}. It is also found that the shape-driven harmonics at the 10th zonal gravity coefficient become dominant, i.e., {Delta}J{sub 2n}>=J-bar{sub 2n} for n {>=} 5.
Dimbylow, P J
1987-01-01
This paper presents three-dimensional finite difference calculations of induced current densities in a grounded, homogeneous, realistically human-shaped phantom. Comparison is made with published experimental values of current density at 60 Hz, measured in conducting saline manikins with their arms down by the side. The congruence between calculation and experiment gives confidence in the applicability of the numerical method and phantom shape to other configurations. The effect of raising both arms above the head is to reduce the current densities in the head and neck by approximately 50% and to increase those from the thorax downwards by 20-30%. A sensitivity analysis was performed on the shape and dimensions of the phantom, from a 45-kg, 1.5-m-tall person to a 140-kg, 1.9-m-tall person. When the phantom is grounded through both feet the current densities range from 50 to 90 microAm-2 in the head (all values for a 60-Hz, 1-kVm-1, vertical applied field), 70 to 140 microAm-2 in the thorax, 150 to 440 microAm-2 at the crotch, and 500 to 2,230 microAm-2 in the ankle. When grounded through only one foot the current densities at the crotch range from 400 to 1,000 microAm-2 and from 1,000 to 4,400 microAm-2 in the ankle of the grounded leg. Scale transformations of the short-circuit current with phantom height, weight, and surface area are confirmed. PMID:3122768
Mondal, Mintu; Kamlapure, Anand; Ganguli, Somesh Chandra; Jesudasan, John; Bagwe, Vivas; Benfatto, Lara; Raychaudhuri, Pratap
2013-01-01
The persistence of a soft gap in the density of states above the superconducting transition temperature Tc, the pseudogap, has long been thought to be a hallmark of unconventional high-temperature superconductors. However, in the last few years this paradigm has been strongly revised by increasing experimental evidence for the emergence of a pseudogap state in strongly-disordered conventional superconductors. Nonetheless, the nature of this state, probed primarily through scanning tunneling spectroscopy (STS) measurements, remains partly elusive. Here we show that the dynamic response above Tc, obtained from the complex ac conductivity, is highly modified in the pseudogap regime of strongly disordered NbN films. Below the pseudogap temperature, T*, the superfluid stiffness acquires a strong frequency dependence associated with a marked slowing down of critical fluctuations. When translated into the length-scale of fluctuations, our results suggest a scenario of thermal phase fluctuations between superconducting domains in a strongly disordered s-wave superconductor. PMID:23446946
NASA Astrophysics Data System (ADS)
Xu, Fuming; Wang, Bin; Wei, Yadong; Wang, Jian
2013-10-01
Orbital-free density functional theory (OFDFT) replaces the wavefunction in the kinetic energy by an explicit energy functional and thereby speeds up significantly the calculation of ground state properties of the solid state systems. So far, the application of OFDFT has been centered on closed systems and less attention is paid on the transport properties in open systems. In this paper, we use OFDFT and combine it with non-equilibrium Green's function to simulate equilibrium electronic transport properties in silicon nanostructures from first principles. In particular, we study ac transport properties of a silicon atomic junction consisting of a silicon atomic chain and two monoatomic leads. We have calculated the dynamic conductance of this atomic junction as a function of ac frequency with one to four silicon atoms in the central scattering region. Although the system is transmissive with dc conductance around 4 to 5 e2/h, capacitive-like behavior was found in the finite frequency regime. Our analysis shows that, up to 0.1 THz, this behavior can be characterized by a classic RC circuit consisting of two resistors and a capacitor. One resistor gives rise to dc resistance and the other one accounts for the charge relaxation resistance with magnitude around 0.2 h/e2 when the silicon chain contains two atoms. It was found that the capacitance is around 5 aF for the same system.
NASA Astrophysics Data System (ADS)
Yu, Y.; Chen, Y. J.; An, M.; Feng, Y.; Liang, X.; Dong, S.
2013-12-01
The east Qinling orogenic belt is located between the North China Craton and the South China Block, and is also at the northeastern boundary of Tibetan Plateau. A temporal seismic array of over 110 portable seismic stations was deployed by Peking University and the Chinese Academy of Geological Sciences (CAGS) from July 2011 to October 2013 to study the complex tectonics of this region. We used earthquake data recorded at 65 stations from this array between July 2011 and October 2012 to image the seismic velocity variations of the region using finite-frequency tomography method. The travel times used in the inversion contain 10876 P-waves and 5945 S-waves at 3 different frequencies. Preliminary results show that velocity structures of P-waves and S-waves are quite similar. The upper mantle velocities under the east Qinling orogenic belt are higher in general in the east than that in the west at depth around 80 km. A higher velocity anomaly is observed under the southern Ordos plateau from 40 km to 360km deep and a low velocity anomaly is seen beneath the Taihang uplift from the depth of 40km to the depth of 200km. These velocity anomalies in the lithosphere and the upper mantle will be interpreted with the geologic observations and tectonic process of the region.
Kinefuchi, K.; Funaki, I.; Shimada, T.; Abe, T.
2012-10-15
Under certain conditions during rocket flights, ionized exhaust plumes from solid rocket motors may interfere with radio frequency transmissions. To understand the relevant physical processes involved in this phenomenon and establish a prediction process for in-flight attenuation levels, we attempted to measure microwave attenuation caused by rocket exhaust plumes in a sea-level static firing test for a full-scale solid propellant rocket motor. The microwave attenuation level was calculated by a coupling simulation of the inviscid-frozen-flow computational fluid dynamics of an exhaust plume and detailed analysis of microwave transmissions by applying a frequency-dependent finite-difference time-domain method with the Drude dispersion model. The calculated microwave attenuation level agreed well with the experimental results, except in the case of interference downstream the Mach disk in the exhaust plume. It was concluded that the coupling estimation method based on the physics of the frozen plasma flow with Drude dispersion would be suitable for actual flight conditions, although the mixing and afterburning in the plume should be considered depending on the flow condition.
Zhang, Hai-Feng; Ding, Guo-Wen; Lin, Yi-Bing; Chen, Yu-Qing
2015-05-15
In this paper, the properties of acceptor mode in two-dimensional plasma photonic crystals (2D PPCs) composed of the homogeneous and isotropic dielectric cylinders inserted into nonmagnetized plasma background with square lattices under transverse-magnetic wave are theoretically investigated by a modified finite-difference frequency-domain (FDFD) method with supercell technique, whose symmetry of every supercell is broken by removing a central rod. A new FDFD method is developed to calculate the band structures of such PPCs. The novel FDFD method adopts a general function to describe the distribution of dielectric in the present PPCs, which can easily transform the complicated nonlinear eigenvalue equation to the simple linear equation. The details of convergence and effectiveness of proposed FDFD method are analyzed using a numerical example. The simulated results demonstrate that the enough accuracy of the proposed FDFD method can be observed compared to the plane wave expansion method, and the good convergence can also be obtained if the number of meshed grids is large enough. As a comparison, two different configurations of photonic crystals (PCs) but with similar defect are theoretically investigated. Compared to the conventional dielectric-air PCs, not only the acceptor mode has a higher frequency but also an additional photonic bandgap (PBG) can be found in the low frequency region. The calculated results also show that PBGs of proposed PPCs can be enlarged as the point defect is introduced. The influences of the parameters for present PPCs on the properties of acceptor mode are also discussed in detail. Numerical simulations reveal that the acceptor mode in the present PPCs can be easily tuned by changing those parameters. Those results can hold promise for designing the tunable applications in the signal process or time delay devices based on the present PPCs.
Metric-discontinuous zonal grid calculations using the Osher scheme
NASA Technical Reports Server (NTRS)
Rai, M. M.; Hessenius, K. A.; Chakravarthy, S. R.
1984-01-01
Computations on zonal grids - in particular, grids with metric discontinuities resulting from the interspersion of highly clustered regions with coarse regions - are possible using a fully conservative form of the Osher upwind scheme. These zonal grids can result from an abrupt clustering of points near solution discontinuities or near other flow features that require improved resolution. The zonal approach is shown to capture shocks with almost 'shock-fitting' quality but with minimal effort. Results for inviscid flow, including quasi-one-dimensional nozzle flow, supersonic flow over a cylinder, and blast-wave diffraction by a ramp, are presented. These calculations demonstrate the powerful capabilities of the Osher scheme used in conjunction with zonal grids in simulating flow fields with complex shock patterns.
NASA Astrophysics Data System (ADS)
Tsekhmistrenko, Maria; Sigloch, Karin; Hosseini, Kasra; Barruol, Guilhem
2016-04-01
From 2011 to 2014, the RHUM-RUM project (Reunion Hotspot Upper Mantle - Reunions Unterer Mantel) instrumented a 2000x2000km2 area of Indian Ocean seafloor, islands and Madagascar with broadband seismometers and hydrophones. The central component was a 13-month deployment of 57 German and French Ocean Bottom Seismometers (OBS) in 2300-5600 m depth. This was supplemented by 2-3 year deployments of 37 island stations on Reunion, Mauritius, Rodrigues, the southern Seychelles, the Iles Eparses and southern Madagascar. Two partner projects contributed another 30+ stations on Madagascar. Our ultimate objective is multifrequency waveform tomography of the entire mantle column beneath the Reunion hotspot. Ideally we would use all passbands that efficiently transmit body waves but this meets practical limits in the noise characteristics of ocean-bottom recordings in particular. Here we present the preliminary data set of frequency-dependent P-wave traveltime measurements on seismometers and hydrophones, obtained by cross-correlation of observed with predicted waveforms. The latter are synthesized from fully numerical Green's functions and carefully estimated, broadband source time functions. More than 200 teleseismic events during the 13-month long deployment yielded usable P-waveform measurements. We present our methods and discuss data yield and quality of ocean-bottom versus land seismometers, and of OBS versus broadband hydrophones. Above and below the microseismic noise band, data yields are higher than within it, especially for OBS. The 48 German OBS, equipped with Guralp 60 s sensors, were afflicted by relatively high self-noise compared to the 9 French instruments equipped with Nanometrics Trillium 240 s sensors. The HighTechInc (model HTI-01 and HTI-04-PCA/ULF) hydrophones (100 s corner period) functioned particularly reliably but their waveforms are relatively more challenging to model due to reverberations in the water column. We obtain ~15000 combined cross
Joint inversion of normal-mode and finite-frequency S-wave data using an irregular tomographic grid
NASA Astrophysics Data System (ADS)
Zaroli, Christophe; Lambotte, Sophie; Lévêque, Jean-Jacques
2015-12-01
Global-scale tomographic models should aim at satisfying the full seismic spectrum. For this purpose, and to better constrain isotropic 3-D variations of shear velocities in the mantle, we tackle a joint inversion of spheroidal normal-mode structure coefficients and multiple-frequency S-wave delay times. In all previous studies for which normal modes were jointly inverted for, with body and/or surface waves, the mantle was laterally parametrized with uniform basis functions, such as spherical harmonics, equal-area blocks and evenly spaced spherical splines. In particular, spherical harmonics naturally appear when considering the Earth's free oscillations. However, progress towards higher resolution joint tomography requires a movement away from such uniform parametrization to overcome its computational inefficiency to adapt to local variations in resolution. The main goal of this study is to include normal modes into a joint inversion based upon a non-uniform parametrization that is adapted to the spatially varying smallest resolving length of the data. Thus, we perform the first joint inversion of normal-mode and body-wave data using an irregular tomographic grid, optimized according to ray density. We show how to compute the projection of 3-D sensitivity kernels for both data sets onto our parametrization made up of spherical layers spanned with irregular Delaunay triangulations. This approach, computationally efficient, allows us to map into the joint model multiscale structural informations from data including periods in the 10-51 s range for body waves and 332-2134 s for normal modes. Tomographic results are focused on the 400-2110 km depth range, where our data coverage is the most relevant. We discuss the potential of a better resolution where the grid is fine, compared to spherical harmonics up to degree 40, as the number of model parameters is similar. Our joint model seems to contain coherent structural components beyond degree 40, such as those related
Zonal Flow Dynamics and Size-scaling of Anomalous Transport
Liu Chen; Roscoe B. White; F. Zonca
2003-07-30
Nonlinear equations for the slow space-time evolution of the radial drift wave envelope and zonal flow amplitude have been self-consistently derived for a model nonuniform tokamak equilibrium within the coherent 4-wave drift wave-zonal flow modulation interaction model of Chen, Lin, and White [Phys. Plasmas 7 (2000) 3129]. Solutions clearly demonstrate turbulence spreading due to nonlinearly enhanced dispersiveness and, consequently, the device-size dependence of the saturated wave intensities and transport coefficients.
Deriving Saturn's Zonal Winds from Cassini Radio Occultations
NASA Astrophysics Data System (ADS)
Flasar, F. Michael; Schinder, Paul J.
2015-11-01
Tracking cloud features from visible images have provided detailed maps of the meridional variation of the mean zonal winds on the giant planets, including Saturn. Filters at different wavelengths can provide information on the vertical structure of the zonal winds, but that is approximate, and the altitudes of winds observed with a given filter generally vary with location, because cloud heights do. Radio occultations provide vertical profiles of refractivity, pressure, and temperature vs. altitude. Zonal winds can be derived from the assumption of gradient wind balance, which relates the zonal wind to the change of geopotential height with latitude along an isobar. Occultations have the advantage that vertical profiles of winds can be obtained in the troposphere and stratosphere. There are, however, complicating factors. In general, the meridional distribution of occultation soundings is limited and unevenly distributed. Moreover, one needs to know the geometry of the occulting atmosphere to correctly account for the path of the refracted radio signal. The zonal winds matter, because they distort isobaric surfaces. For example, an inversion that includes Saturn's oblateness from uniform rotation, based on the Voyager System III period, would yield equatorial temperature profiles that are shifted by ~ 2 K relative to one that also includes the differential rotation associated with the cloud-tracked zonal winds. In retrieving vertical profiles of atmospheric variables from occultation soundings, one also needs an additional symmetry assumption to make the inversions tractable. Typically one uses the zonal winds based on cloud-tracking studies, and assumes they are axisymmetric and barotropic, so that both the gravitational and centrifugal forces are derivable from a potential, and the surfaces of constant geopotential height, pressure, and temperature coincide. This forms the basis for an iterative approach. The pressures and temperatures so retrieved from the
Pulsation-driven mean zonal and meridional flows in rotating massive stars
NASA Astrophysics Data System (ADS)
Lee, Umin; Mathis, Stéphane; Neiner, Coralie
2016-04-01
Zonal and meridional axisymmetric flows can deeply impact the rotational and chemical evolution of stars. Therefore, momentum exchanges between waves propagating in stars, differential rotation, and meridional circulation must be carefully evaluated. In this work, we study axisymmetric mean flows in rapidly and initially uniformly rotating massive stars driven by small amplitude non-axisymmetric κ-driven oscillations. We treat them as perturbations of second order of the oscillation amplitudes and derive their governing equations as a set of coupled linear ordinary differential equations. This allows us to compute 2D zonal and meridional mean flows driven by low frequency g and r modes in slowly pulsating B stars and p modes in β Cephei stars. Oscillation-driven mean flows usually have large amplitudes only in the surface layers. In addition, the kinetic energy of the induced 2D zonal rotational motions is much larger than that of the meridional motions. In some cases, meridional flows have a complex radial and latitudinal structure. We find pulsation-driven and rotation-driven meridional flows can have similar amplitudes. These results show the importance of taking wave - mean flow interactions into account when studying the evolution of massive stars.
The Congo basin zonal overturning circulation
NASA Astrophysics Data System (ADS)
Neupane, Naresh
2016-06-01
The Gulf of Guinea in the equatorial Atlantic is characterized by the presence of strong subsidence at certain times of the year. This subsidence appears in June and becomes well established from July to September. Since much of theWest African monsoon flow originates over the Gulf, Guinean subsidence is important for determining moisture sources for the monsoon. Using reanalysis products, I contribute to a physical understanding of what causes this seasonal subsidence, and how it relates to precipitation distributions across West Africa. There is a seasonal zonal overturning circulation above the Congo basin and the Gulf of Guinea in the ERA-Interim, ERA-40, NCEP2, and MERRA reanalyses. The up-branch is located in the Congo basin around 20°E. Mid-tropospheric easterly flows constitute the returning-branch and sinking over the Gulf of Guinea forms the down-branch, which diverges at 2°W near the surface, with winds to the east flowing eastward to complete the circulation. This circulation is driven by surface temperature differences between the eastern Gulf and Congo basin. Land temperatures remain almost uniform, around 298 K, throughout a year, but the Guinean temperatures cool rapidly from 294 K in May to about 290 K in August. These temperature changes increase the ocean/land temperature contrast, up to 8 K, and drive the circulation. I hypothesize that when the overturning circulation is anomalously strong, the northward moisture transport and Sahelian precipitation are also strong. This hypothesis is supported by ERA-Interim and PERSIANN-CDR (Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks-Climate Data Record) data.
NASA Astrophysics Data System (ADS)
Liang, Xiaofeng; Chen, Yun; Tian, Xiaobo; Chen, Yongshun John; Ni, James; Gallegos, Andrea; Klemperer, Simon L.; Wang, Minling; Xu, Tao; Sun, Changqing; Si, Shaokun; Lan, Haiqiang; Teng, Jiwen
2016-06-01
We perform a finite-frequency tomographic inversion to image 3D velocity structures beneath southern and central Tibet using teleseismic body-wave data recorded by the TIBET-31N passive seismic array as well as waveforms from previous temporary seismic arrays. High-velocity bodies dip ∼40° northward beneath the Himalaya and the Lhasa Terrane. We interpret these high-velocity anomalies as subducting Indian Continental Lithosphere (ICL). The ICL appears to extend further north in central Tibet than in eastern Tibet, reaching 350 km depth at ∼31°N along 85°E but at ∼30°N along 91°E. Low P- and S-wave velocity anomalies extend from the lower crust to ≥180 km depth beneath the Tangra Yum Co Rift, Yadong-Gulu Rift, and the Cona Rift, suggesting that rifting in southern Tibet may involve the entire lithosphere. The anomaly beneath Tangra Yum Co Rift extends down to about 180 km, whereas the anomalies west of the Yadong-Gulu Rift and east of the Cona Rift extend to more than 300 km depth. The low-velocity upper mantle west of the Yadong-Gulu Rift extends furthest north and appears to connect with the extensive upper-mantle low-velocity region beneath central Tibet. Thus the northward-subducting Indian Plate is fragmented along north-south breaks that permit or induce asthenospheric upwellings indistinguishable from the upper mantle of northern Tibet.
Centrilobular zonal necrosis as a hallmark of a distinctive subtype of autoimmune hepatitis
Abe, Hiroshi; Sugita, Tomonori; Seki, Nobuyoshi; Chuganji, Yoshimichi; Furumoto, Youhei; Sakata, Akihiko
2016-01-01
Background and aim Centrilobular zonal necrosis (CZN) is a known histological variant of autoimmune hepatitis (AIH). However, the significance of CZN is yet to be fully elucidated. This study aimed to determine whether CZN is a hallmark of a distinctive subtype of AIH. Methods Histological changes in the centrilobular zones of liver biopsies from 113 AIH patients were assessed by a single pathologist and classified into three categories: typical zonal necrosis defined as CZN (15 patients); other necroinflammatory change (NIC; 24 patients); and absence of necrosis (non-NIC; 74 patients). The clinicopathological features and immunogenetic background of CZN patients were then assessed. Results The clinicopathological features of AIH with CZN were distinct from other types of AIH, including a higher frequency of acute onset, lower frequency of antinuclear antibodies, lower antinuclear antibody titers, lower serum immunoglobulin G levels, lower grade interface hepatitis, less prominent lymphoplasmacytic infiltration, and lower AIH score. Increased and decreased frequencies of HLA-DR9 and HLA-DR4, respectively, were identified as immunogenetic features of AIH with CZN. Conversely, the clinicopathological characteristics of AIH with NIC were similar to those of non-NIC AIH, including the majority of the AIH patients. The therapeutic outcomes of AIH with CZN were excellent when precise diagnoses were made without delay. Conclusion The clinicopathological features and immunogenetic background of AIH with CZN differed from AIH without CZN. CZN may be a hallmark of a distinct subtype of AIH. PMID:26657454
Numerical simulation on zonal disintegration in deep surrounding rock mass.
Chen, Xuguang; Wang, Yuan; Mei, Yu; Zhang, Xin
2014-01-01
Zonal disintegration have been discovered in many underground tunnels with the increasing of embedded depth. The formation mechanism of such phenomenon is difficult to explain under the framework of traditional rock mechanics, and the fractured shape and forming conditions are unclear. The numerical simulation was carried out to research the generating condition and forming process of zonal disintegration. Via comparing the results with the geomechanical model test, the zonal disintegration phenomenon was confirmed and its mechanism is revealed. It is found to be the result of circular fracture which develops within surrounding rock mass under the high geostress. The fractured shape of zonal disintegration was determined, and the radii of the fractured zones were found to fulfill the relationship of geometric progression. The numerical results were in accordance with the model test findings. The mechanism of the zonal disintegration was revealed by theoretical analysis based on fracture mechanics. The fractured zones are reportedly circular and concentric to the cavern. Each fracture zone ruptured at the elastic-plastic boundary of the surrounding rocks and then coalesced into the circular form. The geometric progression ratio was found to be related to the mechanical parameters and the ground stress of the surrounding rocks. PMID:24592166
Numerical Simulation on Zonal Disintegration in Deep Surrounding Rock Mass
Chen, Xuguang; Wang, Yuan; Mei, Yu; Zhang, Xin
2014-01-01
Zonal disintegration have been discovered in many underground tunnels with the increasing of embedded depth. The formation mechanism of such phenomenon is difficult to explain under the framework of traditional rock mechanics, and the fractured shape and forming conditions are unclear. The numerical simulation was carried out to research the generating condition and forming process of zonal disintegration. Via comparing the results with the geomechanical model test, the zonal disintegration phenomenon was confirmed and its mechanism is revealed. It is found to be the result of circular fracture which develops within surrounding rock mass under the high geostress. The fractured shape of zonal disintegration was determined, and the radii of the fractured zones were found to fulfill the relationship of geometric progression. The numerical results were in accordance with the model test findings. The mechanism of the zonal disintegration was revealed by theoretical analysis based on fracture mechanics. The fractured zones are reportedly circular and concentric to the cavern. Each fracture zone ruptured at the elastic-plastic boundary of the surrounding rocks and then coalesced into the circular form. The geometric progression ratio was found to be related to the mechanical parameters and the ground stress of the surrounding rocks. PMID:24592166
NASA Astrophysics Data System (ADS)
Zhong, W. L.; Shi, Z. B.; Xu, Y.; Zou, X. L.; Duan, X. R.; Chen, W.; Jiang, M.; Yang, Z. C.; Zhang, B. Y.; Shi, P. W.; Liu, Z. T.; Xu, M.; Song, X. M.; Cheng, J.; Ke, R.; Nie, L.; Cui, Z. Y.; Fu, B. Z.; Ding, X. T.; Dong, J. Q.; Liu, Yi; Yan, L. W.; Yang, Q. W.; Liu, Y.; HL-2A Team
2015-09-01
The oscillations of poloidal plasma flows induced by radially sheared zonal flows are investigated by newly developed correlation Doppler reflectometers in the HL-2A tokamak. The non-disturbing diagnostic allows one to routinely measure the rotation velocity of turbulence, and hence the radial electric field fluctuations. With correlation Doppler reflectometers, a three-dimensional spatial structure of geodesic acoustic mode (GAM) is surveyed, including the symmetric feature of poloidal and toroidal Er fluctuations, the dependence of GAM frequency on radial temperature and the radial propagation of GAMs. The co-existence of low-frequency zonal flow and GAM is presented. The temporal behaviors of GAM during ramp-up experiments of plasma current and electron density are studied, which reveal the underlying damping mechanisms for the GAM oscillation level.
Ion gyroradius effects on zonal flows in extended Hasegawa-Mima models
NASA Astrophysics Data System (ADS)
Gallagher, Stephen; Hnat, Bogdan; Connaughton, Colm; Nazarenko, Sergey
2012-10-01
Zonal flows are important in fusion plasma where they regulate drift wave turbulence and improve plasma confinement. Two mechanisms can lead to the creation of zonal flows: an inverse cascade of energy, similar to that observed for 2D turbulence, and a coupling between wave modes known as the modulational instability. This work focused on the modulational instability; a four mode truncation of the extended Hasegawa-Mima system was derived to model this. The extended Hasegawa-Mima model is more appropriate for tokamaks than its predecessors as it decouples global flows from the flux surface averaged potential of the system. In addition to this truncated model a linearised set of equations for the system has been derived and used to produce a dispersion relation. Finite difference simulations of the whole system have been used to check these models. Previous work, which has largely considered the case where the ion gyroradius has been taken to its limits, has been expanded upon to show how the ion gyroradius can effect the behaviour of drift waves. It has been shown that the ion gyroradius can be used to change the strength of the nonlinearity of the system leading to changes in behaviour that have previously been demonstrated by altering the initial amplitude of the drift wave.
Measurement of a zonal wind profile on Titan by Doppler tracking of the Cassini entry probe
NASA Technical Reports Server (NTRS)
Atkinson, D. H.; Pollack, J. B.; Seiff, A.
1990-01-01
A program, called the Cassini mission, intended to study the Saturn system by utilizing a Saturn orbiter and a probe descending to the surface of Titan, is discussed. Winds are expected to cause perturbations to the probe local horizontal velocity, resulting in an anomalous drift in the probe location and a shift in the frequency of the probe telemetry, due to the Doppler effect. By using an iterative algorithm, in which the time variation of the probe telemetry frequency is monitored throughout the descent, and the probe trajectory is updated to reflect the effect of wind on the probe location, a highly accurate relative wind profile can be recovered. By adding a single wind velocity, measured by independent means, an absolute wind profile can be obtained. However, the accuracy of the zonal winds recovery is limited by errors in trajectory, and frequency.
Observations of zonal flows in electrode biasing experiments on the Joint Texas Experimental tokamak
NASA Astrophysics Data System (ADS)
Shen, H. G.; Lan, T.; Chen, Z. P.; Kong, D. F.; Zhao, H. L.; Wu, J.; Sun, X.; Liu, A. D.; Xie, J. L.; Li, H.; Ding, W. X.; Liu, W. D.; Yu, C. X.; Xu, M.; Sun, Y.; Liu, H.; Wang, Z. J.; Zhuang, G.
2016-04-01
Zonal flows (ZFs) are observed during the electrode biasing (EB) high confinement mode (H-mode) using Langmuir probe arrays on the edge of J-TEXT tokamak. The long-distance correlation characteristics of floating potentials and interactions with turbulence are studied. During positive biasing H-mode, either the geodesic acoustic mode or low frequency ZF increases. Strong suppression of radial transport by ZFs is found in the low frequency region. The components of the radial particle flux without and with EB are compared in the frequency domain. The interaction between ZFs and ambient turbulence is also discussed. The results show that the rate of ZFs' shear is comparable with that of E × B shear, suggesting that ZFs could be the trigger of the biasing H-mode.
Magnetic Field Generation and Zonal Flows in the Gas Giants
NASA Astrophysics Data System (ADS)
Duarte, L.; Wicht, J.; Gastine, T.
2013-12-01
The surface dynamics of Jupiter and Saturn is dominated by a banded system of fierce zonal winds. The depth of these winds remains unclear but they are thought to be confined to the very outer envelopes where hydrogen remains molecular and the electrical conductivity is negligible. The dynamo responsible for the dipole dominated magnetic fields of both Gas Giants, on the other hand, likely operates in the deeper interior where hydrogen assumes a metallic state. We present numerical simulations that attempt to model both the zonal winds and the interior dynamo action in an integrated approach. Using the anelastic version of the MHD code MagIC, we explore the effects of density stratification and radial electrical conductivity variations. The electrical conductivity is assumed to remain constant in the thicker inner metallic region and decays exponentially towards the outer boundary throughout the molecular envelope. Our results show that the combination of stronger density stratification (Δρ≈55) and a weaker conducting outer layer is essential for reconciling dipole dominated dynamo action and a fierce equatorial zonal jet. Previous simulations with homogeneous electrical conductivity show that both are mutually exclusive, with solutions either having strong zonal winds and multipolar magnetic fields or weak zonal winds and dipole dominated magnetic fields. The particular setup explored here allows the equatorial jet to remain confined to the weaker conducting region where is does not interfere with the deeper seated dynamo action. The equatorial jet can afford to remain geostrophic and reaches throughout the whole shell. This is not an option for the additional mid to higher latitude jets, however. In dipole dominated dynamo solutions, appropriate for the Gas Giants, zonal flows remain very faint in the deeper dynamo region but increase in amplitude in the weakly conducting outer layer in some of our simulations. This suggests that the mid to high latitude jets
NASA Astrophysics Data System (ADS)
Jheng, Y.; Hung, S.; Zhou, Y.; Chang, Y.
2012-12-01
Surface wave travel-time tomography has been widely used as a powerful strategy to image shear wave velocity structure of the Earth's crust and upper mantle, providing comparable information other than body wave tomography. Traditionally, lateral variations of dispersive phase velocities are first obtained at multiple frequencies and then used to invert for shear wave velocity with 1-D depth-dependent sensitivity kernels. However, this approach runs short on considering the directional- and depth-dependence of scattering while surface wave propagating through laterally heterogeneous Earth. To refrain from these shortcomings, we here provide a fully 3-D finite-frequency method based on the Born scattering theory formulated with surface wave mode summation, and apply it to regional fundamental Rayleigh wave travel-time tomography in central Tibet. Our data were collected from Project Hi-CLIMB, which deployed an N-S trending linear array of over 100 broadband seismic stations with a large aperture of 800 km and very dense spacing of ~3-8 km across the Lhasa and Qiangtang terranes during 2004-2005. We follow a standard procedure of ambient noise cross correlation to extract empirical Green's functions of fundamental Rayleigh waves at 10-33 s between station pairs. A multi-taper method is employed to measure the phase differences as a function of period between observed and synthetic Rayleigh waves as well as the corresponding sensitivity kernels for the measured phase delays to 3-D shear wave velocity perturbations in a spherically-symmetric model suitable for central Tibet. A wavelet-based, multi-scale parameterization is invoked in the tomographic inversion to deal with the intrinsically multi-scale nature of unevenly distributed data and resolve the structure with data-adaptive spectral and spatial resolutions. The preliminary result shows that to the north of the Banggong-Nujiang suture (BNS), the crustal shear wave velocity beneath the Qiangtang terrane is
Zonal flows and turbulence in fluids and plasmas
NASA Astrophysics Data System (ADS)
Parker, Jeffrey Bok-Cheung
In geophysical and plasma contexts, zonal flows are well known to arise out of turbulence. We elucidate the transition from statistically homogeneous turbulence without zonal flows to statistically inhomogeneous turbulence with steady zonal flows. Starting from the Hasegawa--Mima equation, we employ both the quasilinear approximation and a statistical average, which retains a great deal of the qualitative behavior of the full system. Within the resulting framework known as CE2, we extend recent understanding of the symmetry-breaking 'zonostrophic instability'. Zonostrophic instability can be understood in a very general way as the instability of some turbulent background spectrum to a zonally symmetric coherent mode. As a special case, the background spectrum can consist of only a single mode. We find that in this case the dispersion relation of zonostrophic instability from the CE2 formalism reduces exactly to that of the 4-mode truncation of generalized modulational instability. We then show that zonal flows constitute pattern formation amid a turbulent bath. Zonostrophic instability is an example of a Type I s instability of pattern-forming systems. The broken symmetry is statistical homogeneity. Near the bifurcation point, the slow dynamics of CE2 are governed by a well-known amplitude equation, the real Ginzburg-Landau equation. The important features of this amplitude equation, and therefore of the CE2 system, are multiple. First, the zonal flow wavelength is not unique. In an idealized, infinite system, there is a continuous band of zonal flow wavelengths that allow a nonlinear equilibrium. Second, of these wavelengths, only those within a smaller subband are stable. Unstable wavelengths must evolve to reach a stable wavelength; this process manifests as merging jets. These behaviors are shown numerically to hold in the CE2 system, and we calculate a stability diagram. The stability diagram is in agreement with direct numerical simulations of the quasilinear
A zonally symmetric model for volcanic influence upon atmospheric circulation
NASA Technical Reports Server (NTRS)
Schatten, K. H.; Mayr, H. G.; Harris, I.; Taylor, H. A., Jr.
1984-01-01
The effects of volcanic activity upon zonal wind flow in a model atmosphere are considered. A low latitude volcanic eruption could lower the tropospheric pole to equator temperature difference and thereby affect the atmospheric motions. When the temperature contrast decreases, the zonal wind velocities at high altitudes are reduced. To conserve angular momentum, the velocities in the lower atmosphere near the surface must increase, thus providing a momentum source for ocean currents. It is suggested that this momentum source may have played a role as a trigger for inducing the 1982-83 anomalous El Nino and possibly other climate changes.
The modulational instability in the extended Hasegawa-Mima equation with a finite Larmor radius
Gallagher, S.; Hnat, B.; Rowlands, G.; Connaughton, C.; Nazarenko, S.
2012-12-15
The effects of the finite Larmor radius on the generation of zonal flows by the four-wave modulational instability are investigated using an extended form of the Hasegawa-Mima equation. Growth rates of the zonal mode are quantified using analytical predictions from a four-mode truncated model, as well as from direct numerical simulation of the nonlinear extended Hasegawa-Mima equation. We not only consider purely zonal flows but also examine the generic oblique case and show that, for small Larmor radii, off-axis modes may become dominant. We find a key parameter M{sub {rho}} which characterises the behaviour of the system due to changes in the Larmor radius. We find that, similarly to previous results obtained by changing the driving wave amplitude, two separate dynamical regimes can be accessed. These correspond to oscillatory energy transfer between zonal flows and a driving wave and the fully saturated zonal flow.
The modulational instability in the extended Hasegawa-Mima equation with a finite Larmor radius
NASA Astrophysics Data System (ADS)
Gallagher, S.; Hnat, B.; Connaughton, C.; Nazarenko, S.; Rowlands, G.
2012-12-01
The effects of the finite Larmor radius on the generation of zonal flows by the four-wave modulational instability are investigated using an extended form of the Hasegawa-Mima equation. Growth rates of the zonal mode are quantified using analytical predictions from a four-mode truncated model, as well as from direct numerical simulation of the nonlinear extended Hasegawa-Mima equation. We not only consider purely zonal flows but also examine the generic oblique case and show that, for small Larmor radii, off-axis modes may become dominant. We find a key parameter Mρ which characterises the behaviour of the system due to changes in the Larmor radius. We find that, similarly to previous results obtained by changing the driving wave amplitude, two separate dynamical regimes can be accessed. These correspond to oscillatory energy transfer between zonal flows and a driving wave and the fully saturated zonal flow.
Navier-Stokes simulation of transonic wing flow fields using a zonal grid approach
NASA Technical Reports Server (NTRS)
Chaderjian, Neal M.
1988-01-01
The transonic Navier-Stokes code was used to simulate flow fields about isolated wings for workshop wind-tunnel and free-air cases using the thin-layer Reynolds-averaged Navier-Stokes equations. An implicit finite-difference scheme based on a diagonal version of the Beam-Warming algorithm was used to integrate the governing equations. A zonal grid approach was used to allow efficient grid refinement near the wing surface. The flow field was sensitive to the turbulent transition model, and flow unsteadiness was observed for a wind-tunnel case but not for the corresponding free-air case. The specification of experimental pressure at the wind-tunnel exit plane is the primary reason for the difference of these two numerical solutions.
NASA Technical Reports Server (NTRS)
Lou, Y. Q.
1987-01-01
This paper considers two-dimensional nonlinear MHD waves of large horizontal spatial scales for a thin magnetofluid layer on the surface of a rotating sphere. The 'shallow fluid' hydrodynamic equations are generalized to include the effects of magnetic fields, and it is shown that the resulting MHD equations can be reduced to a single scalar equation for a stream function involving several free functions. For special choices of these free functions, two kinds of finite-amplitude MHD waves are obtained, propagating in the azimuthal direction relative to the uniformly rotating background atmosphere in the presence of a background zonal magnetic field and a steady differential zonal flow. These two kinds of MHD waves are fundamentally due to the joint effects of the uniform rotation of the background atmosphere and background magnetic field; the first is an inertial wave of the Rossby (1939) and Haurwitz (1940) type, modified by the presence of the background zonal magnetic field, while the second is a magnetic Alfven-like wave which is modified by the uniform rotation of the background atmosphere.
Balanced Data Assimilation For Improving Zonal Equatorial Currents
NASA Astrophysics Data System (ADS)
Burgers, G.; Balmaseda, M. A.; Vossepoel, F. C.; van Oldenborgh, G. J.; van Leeuwen, P. J.
Assimilation schemes that are used for seasonal prediction can have a problem in estimating zonal velocities near the equator. This is the case for OI schemes that use density information for updating only the model density field. In some situations, this leads to a detoriation of the zonal velocity field around the equator. The problem is studied first for the assimilation of height observations in a simple linear 1.5 layer shallow-water model of the equatorial Pacific. It is found that equa- torial zonal velocities can be degraded if velocity is not updated in the assimilation procedure, even if the assimilation increments for height are spread over time. Adding updates to the zonal velocity which are related by geostrophic balance to the height updates is shown to be a simple remedy for the shallow-water model. A straightforward generalisation of the balanced data assimilation method has been implemented in the ocean circulation model of the ECMWF seasonal forecasting sys- tem. First tests are encouraging: upper-ocean surface currents are improved, and cou- pled hindcasts are improved if balanced assimilation is used for the ocean analyses.
Impact of Stratospheric Ozone Zonal Asymmetries on the Tropospheric Circulation
NASA Technical Reports Server (NTRS)
Tweedy, Olga; Waugh, Darryn; Li, Feng; Oman, Luke
2015-01-01
The depletion and recovery of Antarctic ozone plays a major role in changes of Southern Hemisphere (SH) tropospheric climate. Recent studies indicate that the lack of polar ozone asymmetries in chemistry climate models (CCM) leads to a weaker and warmer Antarctic vortex, and smaller trends in the tropospheric mid-latitude jet and the surface pressure. However, the tropospheric response to ozone asymmetries is not well understood. In this study we report on a series of integrations of the Goddard Earth Observing System Chemistry Climate Model (GEOS CCM) to further examine the effect of zonal asymmetries on the state of the stratosphere and troposphere. Integrations with the full, interactive stratospheric chemistry are compared against identical simulations using the same CCM except that (1) the monthly mean zonal mean stratospheric ozone from first simulation is prescribed and (2) ozone is relaxed to the monthly mean zonal mean ozone on a three day time scale. To analyze the tropospheric response to ozone asymmetries, we examine trends and quantify the differences in temperatures, zonal wind and surface pressure among the integrations.
Can zonally symmetric inertial waves drive an oscillating mean flow?
NASA Astrophysics Data System (ADS)
Seelig, Torsten; Harlander, Uwe
2016-04-01
In the presentation [5] zonal mean flow excitation by inertial waves is studied in analogy to mean flow excitation by gravity waves [3] that plays an important role for the quasi-biennial oscillation in the equatorial atmosphere. In geophysical flows that are stratified and rotating, pure gravity and inertial waves correspond to the two limiting cases: gravity waves neglect rotation, inertial waves neglect stratification. The former are more relevant for fluids like the atmosphere, where stratification is dominant, the latter for the deep oceans or planet cores, where rotation dominates. In the present study a hierarchy of simple analytical and numerical models of zonally symmetric inertial wave-mean flow interactions is considered and the results are compared with data from a laboratory experiment [4]. The main findings can be summarised as follows: (i) when the waves are decoupled from the mean flow they just drive a retrograde (eastward) zonal mean flow, independent of the sign of the meridional phase speed; (ii) when coupling is present and the zonal mean flow is assumed to be steady, the waves can drive vertically alternating jets, but still, in contrast to the gravity wave case, the structure is independent of the sign of the meridional phase speed; (iii) when coupling is present and time-dependent zonal mean flows are considered the waves can drive vertically and temporarily oscillating mean flows. The comparison with laboratory data from a rotating annulus experiment shows a qualitative agreement. It appears that the experiment captures the basic elements of the inertial wave mean flow coupling. The results might be relevant to understand how the Equatorial Deep Jets can be maintained against dissipation [1, 2], a process currently discussed controversially. [1] Greatbatch, R., Brandt, P., Claus, M., Didwischus, S., Fu, Y.: On the width of the equatorial deep jets. J. Phys. Oceanogr. 42, 1729-1740 (2012) [2] Muench, J.E., Kunze, E.: Internal wave
NASA Astrophysics Data System (ADS)
Yasui, Takashi; Hasegawa, Koji; Hirayama, Koichi
2016-07-01
The finite-difference time-domain (FD-TD) method using a staggered grid with the collocated grid points of velocities (SGCV) was formulated for elastic waves propagating in anisotropic solids and for a rectangular SGCV. Resonant frequency analysis of Lamé-mode resonators on a quartz plate was carried out to confirm the accuracy and validity of the proposed method. The resonant frequencies for the fundamental and higher-order Lamé-modes calculated by the proposed method agreed very well with their theoretical values.
Response of the intertropical convergence zone to zonally asymmetric subtropical surface forcings
NASA Astrophysics Data System (ADS)
Shaw, Tiffany A.; Voigt, Aiko; Kang, Sarah M.; Seo, Jeongbin
2015-11-01
The energetic framework predicts no shift of the zonal mean Intertropical Convergence Zone (ITCZ) in response to zonally asymmetric forcings (zonal warming and cooling regions with zero zonal mean) assuming radiative feedbacks are linear. Here we show the ITCZ shifts southward in response to a zonally asymmetric forcing in the Northern Hemisphere subtropics in a slab ocean aquaplanet model. The southward shift is consistent with decreased zonal mean energy input to the atmosphere due to cloud radiative effect changes in the cooling region. When cloud-radiative feedbacks are disabled the ITCZ shifts northward consistent with changes in the warming region where increased energy input via surface heat fluxes and stationary Rossby-wave transport dominate. Competition between cooling and warming regions leads to changes in gross moist stability. Our results show rectification of zonally asymmetric forcings play an important role in zonal mean ITCZ dynamics and highlight the importance of assessing the momentum budget when interpreting ITCZ shifts.
Zonal flow generation from trapped electron mode turbulence
NASA Astrophysics Data System (ADS)
Wang, Lu; Hahm, T. S.
2009-11-01
Most existing zonal flow generation theory [1,2] has been developed with a usual assumption of qrρiθ<<1 (qr is the radial wave number of zonal flow, and ρiθ is the ion poloidal gyroradius). However, recent nonlinear gyrokinetic simulations of trapped electron mode (TEM) turbulence exhibit a relatively short radial scale of the zonal flows with qrρiθ˜1 [3,4,5]. This work reports an extension of zonal flow growth calculation to this short wavelength regime via the wave kinetics approach. A generalized expression for the polarization shielding for arbitrary radial wavelength [6] which extends the Rosenbluth-Hinton formula in the long wavelength limit [7] is applied. The electron nonlinearity effects on zonal flow are investigated by using GTC simulation. This work was supported by the China Scholarship Council (LW), U.S. DoE Contract No. DE--AC02--09CH11466 (TSH, LW), the U. S. DOE SciDAC center for Gyrokinetic Particle Simulation of Turbulent Transport in Burning Plasmas, and the U. S. DOE SciDAC-FSP Center for Plasma Edge Simulation (TSH). [1] P. H. Diamond et al., IAEA-CN-69/TH3/1 (1998). [2] L. Chen, Z. Lin, and R. White, Phys. Plasmas 7, 3129 (2000). [3] Z. Lin et al., IAEA-CN-138/TH/P2-8 (2006). [4] D. Ernst et al., Phys. Plasmas 16, 055906 (2009). [5] Y. Xiao and Z. Lin, ``Turbulent transport of trapped electron modes in collisionless plasmas'', submitted to Phys. Rev. Lett. (2009). [6] Lu Wang and T.S. Hahm, Phys. Plasmas 16, 062309 (2009). [7] M. N. Rosenbluth and F. L. Hinton, Phys. Rev. Lett. 80, 724 (1998).
Changes in the zonal propagation of El Niño-related SST anomalies: a possible link to the PDO
NASA Astrophysics Data System (ADS)
Antico, Pablo L.; Barros, Vicente R.
2016-03-01
Long-term variability of El Niño (EN) cycle has been the topic of several studies, mainly because of its impacts on climate around the globe. This variability has been mainly described by changes in the intensity and frequency of EN events. In this study, interdecadal changes in the zonal evolution of EN-related sea surface temperature anomalies (SSTA) and their possible link with a well-known mode of Pacific interdecadal variability are analyzed. EN events are classified according to the sense of zonal propagation of SSTA along the equatorial Pacific during the period 1900-2012. As a result, two types of EN are defined: eastward-directed and westward-directed EN. It is found that EN-related SSTA preferably evolves to the east (west) during the warm (cold) phase of the Pacific Decadal Oscillation. Hence, this study offers new insights into the possible causes of long-term EN changes.
NASA Astrophysics Data System (ADS)
Regi, Mauro; De Lauretis, Marcello; Redaelli, Gianluca; Francia, Patrizia
2016-04-01
Recent experimental results suggest that changes in the atmospheric conductivity, due to energetic electrons precipitation, as well as high latitude potential variations, both associated to geomagnetic activity driven by the solar wind, can affect the atmospheric dynamics. In this work we present an investigation of the correspondence of temperature/zonal wind velocity fluctuations in the stratosphere and troposphere with geomagnetic ULF power fluctuations and polar cap potential difference during the solar cycle 23. Daily values of the ERA-Interim temperature and zonal wind over Antarctica are compared with the daily geomagnetic ULF power, in the Pc5 (1-7 mHz) and Pc1-2 (100 mHz-1 Hz) frequency ranges, at Terra Nova Bay (Antarctica, corrected geomagnetic latitude λ~ 80°S) and with solar wind data.
Response of the Earth system to zonal tidal forcing examined by VLBI based dUT1 variations
NASA Astrophysics Data System (ADS)
Boehm, S.; Schuh, H.
2011-10-01
The VLBI group at the Institute of Geodesy and Geophysics of Vienna University of Technology is developing the software VieVS (Vienna VLBI software) for the analysis of geodetic VLBI data. VieVS incorporates the most recent models recommended by the IERS Conventions and in contrast to other VLBI software uses a parameterization with piece-wise linear offsets at integer hours. Thus it provides more flexibility for combination or comparison with time series from other space geodetic techniques or of geophysical origin. We employed this new software to re-process all available geodetic VLBI sessions from 1984 till 2010, suitable for the determination of the Earth rotation parameters (ERP), i.e. dUT1 (UT1-UTC) and the polar motion coordinates xp and yp. Zonal tidal signals with periods from 5 to 35 days in the derived dUT1 long-time series were then used to estimate the so-called zonal response coefficient κ defined by Agnew and Farrell (1978). The frequency dependent zonal response coefficient is an extension to the concept of the Love number k2 which allows for a response of the Earth to tidal forcing, deviating from purely elastic behaviour and thus taking into account effects of ocean tides, a fluid core and mantle anelasticity. A tidally induced change of the rotation rate of the Earth and consequently of dUT1 is proportional to the tide-generating potential through the zonal response coefficient κ. The values estimated for κ for different tidal frequencies from VLBI observations of dUT1 were compared to theory and to the results of previous determinations of κ from observations of space geodetic techniques.
NASA Astrophysics Data System (ADS)
Fetzer, Eric J.; Gille, John C.
1996-02-01
Zonal-mean gravity wave variance in the Limb Infrared Monitor of the Stratosphere (LIMS) temperature data is seen to correlate strongly with the residual term in the LIMS zonal-mean momentum budget throughout much of the observed mesosphere. This momentum residual is attributed to gravity wave momentum transport at scales that cannot be directly sampled by the LIMS instrument Correlation is highest in the vicinity of the fall and winter mesospheric jets, where both gravity wave variance and momentum residual reach their largest values. Correlation is also high in the Southern Hemisphere subtropical mesophere, where gravity wave variance and the momentum residual have broad temporal maxima during the easterly acceleration of the stratopause semi-annual oscillation (SAO). This subtropical correlation has important implications for the SAO eastward acceleration, which several studies suggest is forced by gravity wave momentum flux divergence. Correlation between gravity wave variance and inferred gravity wave momentum flux divergence is unexpected because variance is dominated by large scales and long periods (inertio-gravity waves), while both theoretical arguments and ground-based observations indicate that momentum transport is dominated by periods under 1 h. The results of this study suggest a broadband gravity wave field experiencing forcing and loss processes, which are largely independent of frequency.
Diffusion of Zonal Variables Using Node-Centered Diffusion Solver
Yang, T B
2007-08-06
Tom Kaiser [1] has done some preliminary work to use the node-centered diffusion solver (originally developed by T. Palmer [2]) in Kull for diffusion of zonal variables such as electron temperature. To avoid numerical diffusion, Tom used a scheme developed by Shestakov et al. [3] and found their scheme could, in the vicinity of steep gradients, decouple nearest-neighbor zonal sub-meshes leading to 'alternating-zone' (red-black mode) errors. Tom extended their scheme to couple the sub-meshes with appropriate chosen artificial diffusion and thereby solved the 'alternating-zone' problem. Because the choice of the artificial diffusion coefficient could be very delicate, it is desirable to use a scheme that does not require the artificial diffusion but still able to avoid both numerical diffusion and the 'alternating-zone' problem. In this document we present such a scheme.
Statistical properties of Charney-Hasegawa-Mima zonal flows
NASA Astrophysics Data System (ADS)
Anderson, Johan; Botha, G. J. J.
2015-05-01
A theoretical interpretation of numerically generated probability density functions (PDFs) of intermittent plasma transport events in unforced zonal flows is provided within the Charney-Hasegawa-Mima (CHM) model. The governing equation is solved numerically with various prescribed density gradients that are designed to produce different configurations of parallel and anti-parallel streams. Long-lasting vortices form whose flow is governed by the zonal streams. It is found that the numerically generated PDFs can be matched with analytical predictions of PDFs based on the instanton method by removing the autocorrelations from the time series. In many instances, the statistics generated by the CHM dynamics relaxes to Gaussian distributions for both the electrostatic and vorticity perturbations, whereas in areas with strong nonlinear interactions it is found that the PDFs are exponentially distributed.
Statistical properties of Charney-Hasegawa-Mima zonal flows
Anderson, Johan; Botha, G. J. J.
2015-05-15
A theoretical interpretation of numerically generated probability density functions (PDFs) of intermittent plasma transport events in unforced zonal flows is provided within the Charney-Hasegawa-Mima (CHM) model. The governing equation is solved numerically with various prescribed density gradients that are designed to produce different configurations of parallel and anti-parallel streams. Long-lasting vortices form whose flow is governed by the zonal streams. It is found that the numerically generated PDFs can be matched with analytical predictions of PDFs based on the instanton method by removing the autocorrelations from the time series. In many instances, the statistics generated by the CHM dynamics relaxes to Gaussian distributions for both the electrostatic and vorticity perturbations, whereas in areas with strong nonlinear interactions it is found that the PDFs are exponentially distributed.
Global variations of zonal mean ozone during stratospheric warming events
NASA Technical Reports Server (NTRS)
Randel, William J.
1993-01-01
Eight years of Solar Backscatter Ultraviolet (SBUV) ozone data are examined to study zonal mean variations associated with stratospheric planetary wave (warming) events. These fluctuations are found to be nearly global in extent, with relatively large variations in the tropics, and coherent signatures reaching up to 50 deg in the opposite (summer) hemisphere. These ozone variations are a manifestation of the global circulation cells associated with stratospheric warming events; the ozone responds dynamically in the lower stratosphere to transport, and photochemically in the upper stratosphere to the circulation-induced temperature changes. The observed ozone variations in the tropics are of particular interest because transport is dominated by zonal-mean vertical motions (eddy flux divergences and mean meridional transports are negligible), and hence, substantial simplifications to the governing equations occur. The response of the atmosphere to these impulsive circulation changes provides a situation for robust estimates of the ozone-temperature sensitivity in the upper stratosphere.
Coherent structures in ion temperature gradient turbulence-zonal flow
Singh, Rameswar; Singh, R.; Kaw, P.; Gürcan, Ö. D.; Diamond, P. H.
2014-10-15
Nonlinear stationary structure formation in the coupled ion temperature gradient (ITG)-zonal flow system is investigated. The ITG turbulence is described by a wave-kinetic equation for the action density of the ITG mode, and the longer scale zonal mode is described by a dynamic equation for the m = n = 0 component of the potential. Two populations of trapped and untrapped drift wave trajectories are shown to exist in a moving frame of reference. This novel effect leads to the formation of nonlinear stationary structures. It is shown that the ITG turbulence can self-consistently sustain coherent, radially propagating modulation envelope structures such as solitons, shocks, and nonlinear wave trains.
First zonal harmonic component of cosmic ray neutron intensity
NASA Technical Reports Server (NTRS)
Takahashi, H.; Yahagi, N.; Chiba, T.
1985-01-01
Cosmic ray neutron data from the cosmic ray stations from the worldwide network in 1966, 1967 and 1969 are analyzed by means of the three dimensional analysis method by Nagashima. The variations of the north-south anisotropy, which is the first zonal harmonic component obtained from the analysis are studied. The result obtained confirms earlier findings. Relationship of the anisotropy to the interplanetary magnetic field sector polarity is also studied.
The residual zonal flows in anisotropic tokamak plasmas
NASA Astrophysics Data System (ADS)
Ren, Haijun
2016-06-01
The gyro-kinetic equation is analytically solved based on the anisotropic two-temperature distribution, in which the ions' parallel temperature is a flux function while the perpendicular temperature depends on the poloidal angle. The residual level of collisionless zonal flows (ZFs) is derived and calculated in the large aspect circular limit. Our result shows that the anisotropy plays a remarkable role in determining the residual value of ZFs. Even weak anisotropy can significantly change the residual level.
Nonstationary Gravity Wave Forcing of the Stratospheric Zonal Mean Wind
NASA Technical Reports Server (NTRS)
Alexander, M. J.; Rosenlof, K. H.
1996-01-01
The role of gravity wave forcing in the zonal mean circulation of the stratosphere is discussed. Starting from some very simple assumptions about the momentum flux spectrum of nonstationary (non-zero phase speed) waves at forcing levels in the troposphere, a linear model is used to calculate wave propagation through climatological zonal mean winds at solstice seasons. As the wave amplitudes exceed their stable limits, a saturation criterion is imposed to account for nonlinear wave breakdown effects, and the resulting vertical gradient in the wave momentum flux is then used to estimate the mean flow forcing per unit mass. Evidence from global, assimilated data sets are used to constrain these forcing estimates. The results suggest the gravity-wave-driven force is accelerative (has the same sign as the mean wind) throughout most of the stratosphere above 20 km. The sense of the gravity wave forcing in the stratosphere is thus opposite to that in the mesosphere, where gravity wave drag is widely believed to play a principal role in decelerating the mesospheric jets. The forcing estimates are further compared to existing gravity wave parameterizations for the same climatological zonal mean conditions. Substantial disagreement is evident in the stratosphere, and we discuss the reasons for the disagreement. The results suggest limits on typical gravity wave amplitudes near source levels in the troposphere at solstice seasons. The gravity wave forcing in the stratosphere appears to have a substantial effect on lower stratospheric temperatures during southern hemisphere summer and thus may be relevant to climate.
Computational fluid dynamics research in three-dimensional zonal techniques
NASA Technical Reports Server (NTRS)
Walters, Robert W.
1989-01-01
Patched-grid algorithms for the analysis of complex configurations with an implicit, upwind-biased Navier-Stokes solver were investigated. Conservative and non-conservative approaches for performing zonal interpolations were implemented. The latter approach yields the most flexible technique in that it can handle both patched and overlaid grids. Results for a two-dimensional blunt body problem show that either approach yield accurate steady-state shock locations and jump conditions. In addition, calculations of the turbulent flow through a hypersonic inlet on a three-zone grid show that the numerical prediction is in good agreement with the experimental results. Through the use of a generalized coordinate transformation at the zonal interface between two or more blocks, the algorithm can be applied to highly stretched viscous grids and to arbitrarily-shaped zonal boundaries. Applications were made to the F-18 aircraft at subsonic, high-alpha conditions, in support of the NASA High-Alpha Research Program. The calculations were compared to ground-based and flight test experiments and were used as a guide to understanding the ground-based tests, which are laminar and transitional, and their relationship to flight. Calculations about a complete reconnaissance aircraft were also performed in order to further demonstrate the capability of the patched-grid algorithm.
Cerebellar Zonal Patterning Relies on Purkinje Cell Neurotransmission
White, Joshua J.; Arancillo, Marife; Stay, Trace L.; George-Jones, Nicholas A.; Levy, Sabrina L.; Heck, Detlef H.
2014-01-01
Cerebellar circuits are patterned into an array of topographic parasagittal domains called zones. The proper connectivity of zones is critical for motor coordination and motor learning, and in several neurological diseases cerebellar circuits degenerate in zonal patterns. Despite recent advances in understanding zone function, we still have a limited understanding of how zones are formed. Here, we focused our attention on Purkinje cells to gain a better understanding of their specific role in establishing zonal circuits. We used conditional mouse genetics to test the hypothesis that Purkinje cell neurotransmission is essential for refining prefunctional developmental zones into sharp functional zones. Our results show that inhibitory synaptic transmission in Purkinje cells is necessary for the precise patterning of Purkinje cell zones and the topographic targeting of mossy fiber afferents. As expected, blocking Purkinje cell neurotransmission caused ataxia. Using in vivo electrophysiology, we demonstrate that loss of Purkinje cell communication altered the firing rate and pattern of their target cerebellar nuclear neurons. Analysis of Purkinje cell complex spike firing revealed that feedback in the cerebellar nuclei to inferior olive to Purkinje cell loop is obstructed. Loss of Purkinje neurotransmission also caused ectopic zonal expression of tyrosine hydroxylase, which is only expressed in adult Purkinje cells when calcium is dysregulated and if excitability is altered. Our results suggest that Purkinje cell inhibitory neurotransmission establishes the functional circuitry of the cerebellum by patterning the molecular zones, fine-tuning afferent circuitry, and shaping neuronal activity. PMID:24920627
Equatorial ionospheric zonal drift by monitoring local GPS reference networks
NASA Astrophysics Data System (ADS)
Ji, Shengyue; Chen, Wu; Ding, Xiaoli; Zhao, Chunmei
2011-08-01
The propagation of electromagnetic waves through the turbulent ionosphere produces scintillations through diffraction, and understanding the physical nature of scintillations is important for engineers and technologists as well as for scientists. In recent years, the establishment of the Global Positioning System (GPS) provided a new technique that can be used to study ionospheric scintillations. The usual way of doing that is the deployment of GPS receivers closely spaced in east-west magnetic direction and then estimating the zonal drift velocities based on the signal power observations. One of the weaknesses of this method is that high-rate sampling such as 20 Hz is required for close-spaced stations and generally no such data are available for studying ionospheric scintillation in the past years. In this research work, a scintillation monitoring method based on slant TEC (STEC) observations of local GPS Continuously Operating Reference Station (CORS) network is proposed. First, the past research works on the equatorial ionospheric drift velocities are summarized. Then, by comparing the scintillation pattern of the signal power and STEC observations of California local GPS reference network, we find that the STEC is a good choice for estimating the ionospheric zonal drift velocity. Then it is illustrated how to calculate the ionospheric scintillation velocity based on STEC. Finally, the proposed method is applied to Hong Kong GPS reference network and several cases of the calculated ionospheric zonal velocities are given.
Effects of Zonal Wind on Stratospheric Ozone Variations over Nigeria
NASA Astrophysics Data System (ADS)
Chidinma Okoro, Eucharia,
2016-07-01
The effects of zonal wind on stratospheric ozone variation over Nigeria have been studied. The areas covered in this study include; Maiduguri, Ikeja, Port-Harcourt, Calabar, Makurdi, Ilorin, Akure, Yola, Minna, Jos, Kano and Enugu in Nigeria, from 1986 to 2008. Zonal wind was computed from the iso-velocity map employing MATLAB software. The mean monthly variations of AAM and LOD at pressure levels of 20, 30 and 50 mb in the atmosphere depict a trend of maximum amplitude between April and September, and minimum amplitude between December and March. The trend observed in seasonal variation of O3 column data in the low latitude had maximum amount from May through August and minimum values from December through February. The mean monthly maximum O3 concentrations was found to be 284.70 Du (Kano) occurring in May 1989 while, an average monthly minimum O3 concentration was found to be 235.60 Du (Port-Harcourt and Calabar) occurring in January 1998. It has been established in this study that, the variation in atmospheric angular momentum (AAM) caused by variation of the universal time or length of day (LOD) transfer ozone (O3) by means of zonal wind from the upper troposphere to the lower stratosphere in the stations understudy. The strong effect of the pressure levels of the atmosphere on O3 variation could be attributed to its effect on the AAM and LOD. Variation in the LOD is significant in the tropics, suggesting that, the effects of the extra-tropical suction pump (ETSP) action is not the only driver responsible for O3 transportation from the tropics to extra-tropical zones. Consequently, these findings lead to a deduction that weather pattern alteration observed due to these changes could lead to climate change. Keywords: ozone variations; dynamical processes; harmattan wind; ETSP; and climatic variability
Magnetic flux concentration and zonal flows in magnetorotational instability turbulence
Bai, Xue-Ning; Stone, James M.
2014-11-20
Accretion disks are likely threaded by external vertical magnetic flux, which enhances the level of turbulence via the magnetorotational instability (MRI). Using shearing-box simulations, we find that such external magnetic flux also strongly enhances the amplitude of banded radial density variations known as zonal flows. Moreover, we report that vertical magnetic flux is strongly concentrated toward low-density regions of the zonal flow. Mean vertical magnetic field can be more than doubled in low-density regions, and reduced to nearly zero in high-density regions in some cases. In ideal MHD, the scale on which magnetic flux concentrates can reach a few disk scale heights. In the non-ideal MHD regime with strong ambipolar diffusion, magnetic flux is concentrated into thin axisymmetric shells at some enhanced level, whose size is typically less than half a scale height. We show that magnetic flux concentration is closely related to the fact that the turbulent diffusivity of the MRI turbulence is anisotropic. In addition to a conventional Ohmic-like turbulent resistivity, we find that there is a correlation between the vertical velocity and horizontal magnetic field fluctuations that produces a mean electric field that acts to anti-diffuse the vertical magnetic flux. The anisotropic turbulent diffusivity has analogies to the Hall effect, and may have important implications for magnetic flux transport in accretion disks. The physical origin of magnetic flux concentration may be related to the development of channel flows followed by magnetic reconnection, which acts to decrease the mass-to-flux ratio in localized regions. The association of enhanced zonal flows with magnetic flux concentration may lead to global pressure bumps in protoplanetary disks that helps trap dust particles and facilitates planet formation.
Longitudinal variation in zonal winds at subauroral regions: Possible mechanisms
NASA Astrophysics Data System (ADS)
Wang, Hui; Lühr, Hermann
2016-01-01
Longitudinal differences in thermospheric zonal winds (ΔUy) are investigated in the subauroral region for different seasons and under solar maximum and medium conditions by using Challenging Minisatellite Payload observations. Prominent wave-1 longitudinal and diurnal variations of ΔUy are observed, along with an antiphase relationship between the Northern and Southern Hemispheres. These structures persist over the whole year and are independent of solar activity. ΔUy values are greater at nighttime than at daytime, and values in the south are greater than those in the north in local summer and winter. Model simulations confirm observed results in large-scale structures, and the nonzero dipole tilt is found to be vital for the longitudinal variation of the zonal wind. The neutral air pressure gradient caused by the day-night difference in solar heating is a major contributor to the observed ΔUy. The pressure effects are larger at nighttime than at daytime and larger in the Southern Hemisphere than in the Northern Hemisphere. Ion drag reduces the compatibility between the modeled and observed ΔUy as expected, with larger effects at nighttime than at daytime. Viscous force also reduces the compatibility between the modeled and observed ΔUy with greater effects at daytime, except at nighttime in the Southern Hemisphere. Similarly, the Coriolis force makes the difference between the modeled and observed ΔUy larger. The sum of these factors can explain, in general, the observed local time and hemispheric asymmetry features in longitudinal variation of the zonal wind.
Magnetic Flux Concentration and Zonal Flows in Magnetorotational Instability Turbulence
NASA Astrophysics Data System (ADS)
Bai, Xue-Ning; Stone, James M.
2014-11-01
Accretion disks are likely threaded by external vertical magnetic flux, which enhances the level of turbulence via the magnetorotational instability (MRI). Using shearing-box simulations, we find that such external magnetic flux also strongly enhances the amplitude of banded radial density variations known as zonal flows. Moreover, we report that vertical magnetic flux is strongly concentrated toward low-density regions of the zonal flow. Mean vertical magnetic field can be more than doubled in low-density regions, and reduced to nearly zero in high-density regions in some cases. In ideal MHD, the scale on which magnetic flux concentrates can reach a few disk scale heights. In the non-ideal MHD regime with strong ambipolar diffusion, magnetic flux is concentrated into thin axisymmetric shells at some enhanced level, whose size is typically less than half a scale height. We show that magnetic flux concentration is closely related to the fact that the turbulent diffusivity of the MRI turbulence is anisotropic. In addition to a conventional Ohmic-like turbulent resistivity, we find that there is a correlation between the vertical velocity and horizontal magnetic field fluctuations that produces a mean electric field that acts to anti-diffuse the vertical magnetic flux. The anisotropic turbulent diffusivity has analogies to the Hall effect, and may have important implications for magnetic flux transport in accretion disks. The physical origin of magnetic flux concentration may be related to the development of channel flows followed by magnetic reconnection, which acts to decrease the mass-to-flux ratio in localized regions. The association of enhanced zonal flows with magnetic flux concentration may lead to global pressure bumps in protoplanetary disks that helps trap dust particles and facilitates planet formation.
The Zonal Satellite Problem. I. Near-Escape Flow
NASA Astrophysics Data System (ADS)
Mioc, V.; Stavinschi, M.
The study of the zonal satellite problem is continued by tackling the situation r-> infty. New equations of motion (for which the infinite distance is a singularity) and the corresponding first integrals of energy and angular momentum are set up. The infinity singularity is blown up via McGehee-type transformations, and the infinity manifold is pasted on the phase space. The fictitious flow on this manifold is described. Then, resorting to the rotational symmetry of the problem and to the angular momentum integral, the near-escape local flow is depicted. The corresponding phase curves are interpreted as physical motions.
A simple inertial model for Neptune's zonal circulation
NASA Technical Reports Server (NTRS)
Allison, Michael; Lumetta, James T.
1990-01-01
Voyager imaging observations of zonal cloud-tracked winds on Neptune revealed a strongly subrotational equatorial jet with a speed approaching 500 m/s and generally decreasing retrograde motion toward the poles. The wind data are interpreted with a speculative but revealingly simple model based on steady gradient flow balance and an assumed global homogenization of potential vorticity for shallow layer motion. The prescribed model flow profile relates the equatorial velocity to the mid-latitude shear, in reasonable agreement with the available data, and implies a global horizontal deformation scale L(D) of about 3000 km.
Zonal drifts of irregularities imparted by meridional winds.
NASA Technical Reports Server (NTRS)
Waldman, H.; Da Rosa, A. V.
1973-01-01
In a uniform ionosphere, meridional winds cause only meridional motions of irregularities. It is shown, however, that, if F-region irregularities are considered in a real ionosphere in which there is a highly conductive E-layer, zonal motions occur. During the day a substantial westward drift takes place, while at night the drift is eastward but smaller, owing to the much smaller E-layer conductivity. Thus, the effect of meridional winds is to impart a net westward drift to small irregularities in the ionization, provided such irregularities persist long enough.
Zonal flow regimes in rotating anelastic spherical shells (Invited)
NASA Astrophysics Data System (ADS)
Gastine, T.; Wicht, J.; Aurnou, J. M.; Heimpel, M. H.
2013-12-01
The surface zonal winds observed in the giant planets form a complex jet pattern with alternating prograde and retrograde direction. While the main equatorial band is prograde on the gas giants, both ice giants have a pronounced retrograde equatorial jet. The depth of these jets is however poorly known and highly debated. Theoretical scenarios range from "shallow models", that assume that these zonal flows are restricted to the outer stably stratified layer; to "deep models" that hypothesise that the surface winds are the signature of deep-seated convection. Most of the numerical models supporting the latter idea employed the Boussinesq approximation where compressibility effects are ignored. While this approximation is suitable for modelling the liquid iron core of terrestrial planets, this becomes questionable in the gas giants interiors, where density increases by several orders of magnitude. To tackle this problem, several numerical models using the "anelastic approximation" have been recently developed to study the compressibility effects while filtering out the fast acoustic waves. Here, we consider such anelastic models of rapidly-rotating spherical shells to explore the properties of the zonal winds in different regimes where either rotation or buoyancy dominates the force balance. We conduct several parameter studies to quantify the dependence of zonal flows on the background density stratification and the driving of convection. We find that the direction of the equatorial wind is controlled by the ratio of buoyancy and Coriolis force. The prograde equatorial band maintained by Reynolds stresses is found in the rotation-dominated regime. At low Ekman numbers, several alternating jets form at high latitude in a similar way to some previous Boussinesq calculations. In cases where buoyancy dominates Coriolis force, the angular momentum per unit mass is homogenised and the equatorial band is retrograde, reminiscent to those observed in the ice giants
Non-linear Paradigm for Drift Wave - Zonal Flow interplay: coherence, chaos and turbulence
NASA Astrophysics Data System (ADS)
Zonca, Fulvio
2003-10-01
Non-linear equations for the slow space-time evolution of the radial drift wave (DW) envelope and zonal flow (ZF) amplitude have been self-consistently derived for a model nonuniform tokamak equilibrium within the coherent 4-wave drift wave-zonal flow modulation interaction model of Chen, Lin and White(chen00). For the sake of simplicity, in this work we assume electrostatic fluctuations; but our formalism is readily extended to electromagnetic fluctuations(chen01). In the local limit, i.e. neglecting equilibrium profile variations, the coherent 4-wave DW-ZF modulation interaction model has successfully demonstrated spontaneous generation of ZFs and non-linear DW/ITG-ZF dynamics in toroidal plasmas(chen00). The present work is an extension of previous analyses to allow both (slow) temporal and spatial variations of the DW/ITG radial envelope; thus, it naturally incorporates the effects of equilibrium variations; i.e., turbulence spreading and size-dependence of the saturated wave intensities and transport coefficients(lin99). This approach makes it possible to treat equilibrium profile variations and non-linear interactions on the same footing, assuming that coupling among different DWs on the shortest non-linear time scale is mediated by ZF only. At this level, the competition between linear drive/damping, DW spreading due to finite linear (and nonlinear) group velocity(lin02,chen02,kim02) and non-linear energy transfer between DWs and ZF, determines the saturation levels of the fluctuating fields. Despite the coherence of the underlying non-linear dynamics at this level, this system exhibits both chaotic behavior and intermittency, depending on system size and proximity to marginal stability(chen02). The present model can be further extended to include longer time-scale physics such as 3-wave interactions and collisionless damping of zonal flows. 9 chen00 Liu Chen, Zhihong Lin and Roscoe White, Phys. Plasmas 7, 3129, (2000). chen01 L. Chen, Z. Lin, R.B. White and
Experimental studies of zonal flow and field in compact helical system plasma
Fujisawa, A.; Itoh, K.; Shimizu, A.; Nakano, H.; Ohshima, S.; Iguchi, H.; Matsuoka, K.; Okamura, S.; Minami, T.; Yoshimura, Y.; Nagaoka, K.; Ida, K.; Toi, K.; Takahashi, C.; Kojima, M.; Nishimura, S.; Isobe, M.; Suzuki, C.; Akiyama, T.; Ido, T.
2008-05-15
The experimental studies on zonal flows and turbulence have been carried out in Compact Helical System [K. Matsuoka, S. Kubo, M. Hosokawa et al., in Plasma Physics and Controlled Nuclear Fusion Research, Proc. 12th Int. Conf., Nice, 1988 (International Atomic Energy Agency, Vienna, 1989, Vol. 2, p. 411] using twin heavy ion beam probes. The paper presents the experimental observations of stationary zonal flow, nonlinear couplings between zonal flow and turbulence, and the role of zonal flow in the improved confinement, together with the recent discovery of zonal magnetic field. The presented experimental results strongly support the new paradigm that the plasma transport should be considered as a system of drift wave and zonal flows, and provides the first direct evidence for turbulence dynamo that the structured magnetic field can be really generated by turbulence.
Overview of gyrokinetic studies of finite-β microturbulence
NASA Astrophysics Data System (ADS)
Terry, P. W.; Carmody, D.; Doerk, H.; Guttenfelder, W.; Hatch, D. R.; Hegna, C. C.; Ishizawa, A.; Jenko, F.; Nevins, W. M.; Predebon, I.; Pueschel, M. J.; Sarff, J. S.; Whelan, G. G.
2015-10-01
Recent results on electromagnetic turbulence from gyrokinetic studies in different magnetic configurations are overviewed, detailing the physics of electromagnetic turbulence and transport, and the effect of equilibrium magnetic field scale lengths. Ion temperature gradient (ITG) turbulence is shown to produce magnetic stochasticity through nonlinear excitation of linearly stable tearing-parity modes. The excitation, which is catalyzed by the zonal flow, produces an electron heat flux proportional to β2 that deviates markedly from quasilinear theory. Above a critical beta known as the non-zonal transition (NZT), the magnetic fluctuations disable zonal flows by allowing electron streaming that shorts zonal potential between flux surfaces. This leads to a regime of very high transport levels. Kinetic ballooning mode (KBM) saturation is described. For tokamaks saturation involves twisted structures arising from magnetic shear; for helical plasmas oppositely inclined convection cells interact by mutual shearing. Microtearing modes are unstable in the magnetic geometry of tokamaks and the reversed field pinch (RFP). In NSTX instability requires finite collisionality, large beta, and is favored by increasing magnetic shear and decreasing safety factor. In the RFP, a new branch of microtearing with finite growth rate at vanishing collisionality is shown from analytic theory to require the electron grad-B/curvature drift resonance. However, gyrokinetic modeling of experimental MST RFP discharges at finite beta reveals turbulence that is electrostatic, has large zonal flows, and a large Dimits shift. Analysis shows that the shorter equilibrium magnetic field scale lengths increase the critical gradients associated with the instability of trapped electron modes, ITG and microtearing, while increasing beta thresholds for KBM instability and the NZT.
Physical mechanism behind zonal-flow generation in drift-wave turbulence.
Manz, P; Ramisch, M; Stroth, U
2009-10-16
The energetic interaction between drift-wave turbulence and zonal flows is studied experimentally in two-dimensional wave number space. The kinetic energy is found to be transferred nonlocally from the drift waves to the zonal flow. This confirms the theoretical prediction that the parametric-modulational instability is the driving mechanism of zonal flows. The physical mechanism of this nonlocal energetic interaction between and zonal flows and turbulent drift-wave eddies in relation to the suppression of turbulent transport is discussed. PMID:19905704
Application of zonal model on indoor air sensor network design
NASA Astrophysics Data System (ADS)
Chen, Y. Lisa; Wen, Jin
2007-04-01
Growing concerns over the safety of the indoor environment have made the use of sensors ubiquitous. Sensors that detect chemical and biological warfare agents can offer early warning of dangerous contaminants. However, current sensor system design is more informed by intuition and experience rather by systematic design. To develop a sensor system design methodology, a proper indoor airflow modeling approach is needed. Various indoor airflow modeling techniques, from complicated computational fluid dynamics approaches to simplified multi-zone approaches, exist in the literature. In this study, the effects of two airflow modeling techniques, multi-zone modeling technique and zonal modeling technique, on indoor air protection sensor system design are discussed. Common building attack scenarios, using a typical CBW agent, are simulated. Both multi-zone and zonal models are used to predict airflows and contaminant dispersion. Genetic Algorithm is then applied to optimize the sensor location and quantity. Differences in the sensor system design resulting from the two airflow models are discussed for a typical office environment and a large hall environment.
Zonal harmonics of the gravity field in DEF-variables.
NASA Astrophysics Data System (ADS)
Aparicio, I.; Floría, L.
In order to be in a position to take advantage of the linear and regular formulation and treatment of Celestial Mechanics problems, in a recent paper Sharaf & Saad (1997) have given an analytical expansion of the Earth's zonal potential in terms of Kustaanheimo-Stiefel (KS) regular elements (Kustaanheimo & Stiefel 1965; Stiefel & Scheifele 1971), putting special emphasis on the consideration of elliptic-type two-body orbits. In the present paper we carry out an application of the so-called focal method (Burdet 1969) to derive the expression, in terms of the linearizing DEF-variables (Deprit, Elipe & Ferrer 1994, S S 4.1), of any zonal harmonic of the gravitational field created by a central body, and obtain the corresponding equations of motion for any value of the eccentricity. To this end, we will follow a variant of the focal method canonical approach based on the (weakly) canonical extension of the projective-decomposition point-transformation proposed by these authors.
Asymmetric zonal shim coils for magnetic resonance applications.
Forbes, L K; Crozier, S
2001-08-01
A method is presented for the systematic design of asymmetric zonal shim coils for magnetic resonance applications. Fourier-series methods are used to represent the magnetic field inside and outside a circular cylinder of length 2L and radius a. The current density on the cylinder is also represented using Fourier series. Any desired field can be specified in advance on the cylinder's radius, over some nonsymmetric portion pL
Implementing Multidisciplinary and Multi-Zonal Applications Using MPI
NASA Technical Reports Server (NTRS)
Fineberg, Samuel A.
1995-01-01
Multidisciplinary and multi-zonal applications are an important class of applications in the area of Computational Aerosciences. In these codes, two or more distinct parallel programs or copies of a single program are utilized to model a single problem. To support such applications, it is common to use a programming model where a program is divided into several single program multiple data stream (SPMD) applications, each of which solves the equations for a single physical discipline or grid zone. These SPMD applications are then bound together to form a single multidisciplinary or multi-zonal program in which the constituent parts communicate via point-to-point message passing routines. Unfortunately, simple message passing models, like Intel's NX library, only allow point-to-point and global communication within a single system-defined partition. This makes implementation of these applications quite difficult, if not impossible. In this report it is shown that the new Message Passing Interface (MPI) standard is a viable portable library for implementing the message passing portion of multidisciplinary applications. Further, with the extension of a portable loader, fully portable multidisciplinary application programs can be developed. Finally, the performance of MPI is compared to that of some native message passing libraries. This comparison shows that MPI can be implemented to deliver performance commensurate with native message libraries.
Progress on the development of a zonal bimorph deformable mirror
NASA Astrophysics Data System (ADS)
Griffith, Mike S.; Laycock, Leslie C.; Archer, Nick; Myers, Richard; Doel, Peter; Birch, Rolf
2008-07-01
The Zonal Bimorph Deformable Mirror (ZBDM) is a new concept of adaptive mirror. It exploits the benefits normally associated with bimorph mirrors, namely simple rugged construction, low capacitance, and cost effectiveness, but in a significant departure from classical, edge supported bimorphs each element is supported from underneath. This results in a localised (zonal) response and enables the device to be scalable up to large aperture, multi-1000 element devices. Crucially, the combination of continuous support coupled with the use of flexi-circuit interconnect promotes the assembly of a high density 'tweeter' deformable mirror (DM) onto a lower density, high dynamic range 'woofer' DM to generate an integrated, dual-stage deformable mirror which can deliver both high resolution and high dynamic range simultaneously. Such a device has the potential to significantly simplify the design of astronomical adaptive optics (AO) systems. We present the progress made on the development of the ZBDM as part of a collaborative project funded by the newly formed UK Science and Technology Facilities Council.
Zonal Flows from Spontaneous Symmetry Breaking of Homogeneous Turbulence
NASA Astrophysics Data System (ADS)
Parker, Jeffrey; Krommes, John
2013-10-01
To study how zonal flows (ZF) arise, we examine one of the simplest possible models, the stochastically forced Hasegawa-Mima equation, which displays the bifurcation of steady ZFs from a state of homogeneous turbulence; thus a statistical treatment is required. Here an approach is adopted in which the ZFs are treated as mean fields that spontaneously break the background symmetry. The resulting inhomogeneous ensemble is treated self-consistently without assuming weak inhomogeneity. Closed statistical equations are obtained by ignoring the drift-wave self-interactions while fully retaining the drift-wave-ZF nonlinearities. We show that from the statistical point of view ZF generation can be understood as pattern formation. This leads to the surprising result that in a saturated turbulent state the ZF wavelength is not unique; a continuous band of ZF scales is allowed. Only those within a smaller sub-band are linearly stable. That stability is analyzed and the stability diagram in parameter space is calculated and successfully compared with simulations. The stability concept provides a way of interpreting the merging of zonal jets, a phenomenon commonly observed in observations and numerical studies. Work supported by U.S DOE Contract No DE-AC02-09CH11466 and by an NSF Graduate Research Fellowship.
Non-axisymmetric instabilities in discs with imposed zonal flows
NASA Astrophysics Data System (ADS)
Vanon, R.; Ogilvie, G. I.
2016-09-01
We conduct a linear stability calculation of an ideal Keplerian flow on which a sinusoidal zonal flow is imposed. The analysis uses the shearing sheet model and is carried out both in isothermal and adiabatic conditions, with and without self-gravity (SG). In the non-SG regime a structure in the potential vorticity (PV) leads to a non-axisymmetric Kelvin-Helmholtz (KH) instability; in the short-wavelength limit its growth rate agrees with the incompressible calculation by Lithwick (2007), which only considers perturbations elongated in the streamwise direction. The instability's strength is analysed as a function of the structure's properties, and zonal flows are found to be stable if their wavelength is ≳ 8H, where H is the disc's scale height, regardless of the value of the adiabatic index γ. The non-axisymmetric KH instability can operate in Rayleigh-stable conditions, and it therefore represents the limiting factor to the structure's properties. Introducing SG triggers a second non-axisymmetric instability, which is found to be located around a PV maximum, while the KH instability is linked to a PV minimum, as expected. In the adiabatic regime, the same gravitational instability is detected even when the structure is present only in the entropy (not in the PV) and the instability spreads to weaker SG conditions as the entropy structure's amplitude is increased. This eventually yields a non-axisymmetric instability in the non-SG regime, albeit of weak strength, localised around an entropy maximum.
NASA Astrophysics Data System (ADS)
2015-10-01
The world has agreed on 17 Sustainable Development Goals, to be adopted this week. This is great progress towards acknowledging that the planet's finite resources need to be managed carefully in the face of humanity's unlimited aspirations.
Chen, Z.
2001-01-01
The Spallation Neutron Source (SNS) is an accelerator-based neutron scattering research facility. The linear accelerator (linac) is the principal accelerating structure and divided into a room-temperature linac and a superconducting linac. The normal conducting linac system that consists of a Drift Tube Linac (DTL) and a Coupled Cavity Linac (CCL) is to be built by Los Alamos National Laboratory. The CCL structure is 55.36-meters long. It accelerates H- beam from 86.8 Mev to 185.6 Mev at operating frequency of 805 MHz. This side coupled cavity structure has 8 cells per segment, 12 segments and 11 bridge couplers per module, and 4 modules total. A 5-MW klystron powers each module. The number 3 and number 9 bridge coupler of each module are connected to the 5-MW RF power supply. The bridge coupler with length of 2.5 {beta}{gamma} is a three-cell structure and located between the segments and allows power flow through the module. The center cell of each bridge coupler is excited during normal operation. To obtain a uniform electromagnetic filed and meet the resonant frequency shift, the RF induced heat must be removed. Thus, the thermal deformation and frequency shift studies are performed via numerical simulations in order to have an appropriate cooling design and predict the frequency shift under operation. The center cell of the bridge coupler also contains a large 4-inch slug tuner and a tuning post that used to provide bulk frequency adjustment and field intensity adjustment, so that produce the proper total field distribution in the module assembly.
Detection of zonal flow spectra in DIII-D by a dual-channel Doppler backscattering system
Schmitz, L.; Wang, G.; Hillesheim, J. C.; Rhodes, T. L.; Peebles, W. A.; White, A. E.; Zeng, L.; Carter, T. A.; Solomon, W.
2008-10-15
Doppler backscattering (DBS) has been successfully used to measure the ExB flow velocity and local intermediate wavenumber density fluctuation levels in the DIII-D tokamak. Depending on the launch angle and the frequency of the probing beam, the signal backscattered from the plasma cut-off layer is sensitive to density fluctuations at a specific perpendicular wavenumber (1{<=}k{sub perpendicular}{rho}{sub s}{<=}4). Due to the localization and high time resolution for poloidal flow measurements, DBS is well suited to detect stationary and time-dependent shear flows [zonal flows (ZFs)]. We present a novel scheme to measure ZF spectra using a dual-channel DBS system capable of simultaneously probing two minor radii separated by a distance of 0.2 cm<{delta}r<3 cm. Frequency spectra of geodesic acoustic modes and low frequency ZFs (f{<=}10 kHz) have been obtained for 0.6
Effect of chlorofluoromethane infrared radiation on zonal atmospheric temperatures
NASA Technical Reports Server (NTRS)
Dickinson, R. E.; Donahue, T. M.; Liu, S. C.
1978-01-01
Estimates are made of changes in the atmospheric climate due to the radiative effects of 10 ppb of chlorofluoromethanes (CFM's). The estimates are derived on the basis of a 12-layer stratospheric general circulation model with a specified change of ocean temperature. Two tropical maxima in zonal average temperature change were observed: one in the upper troposphere and one centered at the tropopause. The temperature change exceeds the surface temperature change by a factor of at least two. If the 1975 CFM emission rate were to continue indefinitely, stratospheric water-vapor concentrations would increase by up to 60% due to CFM radiative effects. This would reduce ozone concentrations by an additional 4% of the natural ozone column.
Converging and Diverging Shocks in Space Plasmas: Zonal Flow Effect?
NASA Astrophysics Data System (ADS)
Ma, Z.; Hirose, A.
2011-12-01
Cluster observed converging and diverging shocks in magnetosphere at the same time1. No explanations have been proposed till now. We set up a self-similar, two-fluid model, and study the features of nonlinear waves propagating along geomagnetic field lines. Well-known three shapes of nonlinear ion-acoustic solitary structures (sinusoidal, sawtooth, and spiky or bipolar) in space plasmas were obtained in both Cartesian2 and cylindrical3 geometries via analytical and numerical calculations. Importantly, not only the observed two-type shocks are obtained simultaneously in the cylindrical frame, but accompanying characteristics are illustrated, such as, two reversely propagating nonlinear waves, density dips and humps, etc. The study4~6 exposes that field-aligned plasma beams contribute to highly structured magnetospheric electric fields; the fields bring about zonal flows which are the prime mover of Cluster-measured shocks.
Residual zonal flows in tokamaks and stellarators at arbitrary wavelengths
NASA Astrophysics Data System (ADS)
Monreal, Pedro; Calvo, Iván; Sánchez, Edilberto; Parra, Félix I.; Bustos, Andrés; Könies, Axel; Kleiber, Ralf; Görler, Tobias
2016-04-01
In the linear collisionless limit, a zonal potential perturbation in a toroidal plasma relaxes, in general, to a non-zero residual value. Expressions for the residual value in tokamak and stellarator geometries, and for arbitrary wavelengths, are derived. These expressions involve averages over the lowest order particle trajectories, that typically cannot be evaluated analytically. In this work, an efficient numerical method for the evaluation of such expressions is reported. It is shown that this method is faster than direct gyrokinetic simulations performed with the Gene and EUTERPE codes. Calculations of the residual value in stellarators are provided for much shorter wavelengths than previously available in the literature. Electrons must be treated kinetically in stellarators because, unlike in tokamaks, kinetic electrons modify the residual value even at long wavelengths. This effect, that had already been predicted theoretically, is confirmed by gyrokinetic simulations.
Zonal wave number variance spectra of stratospheric microwave brightness temperatures
NASA Technical Reports Server (NTRS)
Martin, Russell L.; Stanford, John L.
1986-01-01
Well-known theoretical predictions suggest that variances of large-scale atmospheric fluctuations, for scales somewhat shorter than those of the forcing mechanisms, should obey a power law, k exp -b, where k is the horizontal wave number. The invariance of the exponent b with season, hemisphere, and latitude is tested using satellite-measured brightness temperatures. Global grids of Tiros-N Microwave Sounding Unit channel 4 measurements, which closely approximate the 30- to 150-mbar layer mean temperature, are constructed for January, March, and August 1979. These grids are zonally Fourier-transformed, and the resulting spectra are averaged over four 18 deg-wide latitude bands. Fits of a power law to these spectra over wave numbers 10-36 and 10-26 give values of b which are independent of season, hemisphere, and latitude band, to within statistical uncertainties. The observed values of b are about 3.6, as compared to theoretical predictions of 3.
NASA Astrophysics Data System (ADS)
Veljović-Jovanović, S.; Vidović, M.; Morina, F.; Prokić, Lj.; Todorović, D. M.
2016-09-01
Green-white variegated leaves of Pelargonium zonale were studied using the photoacoustic method. Our aim was to characterize photosynthetically active green tissue and nonphotosynthetically active white tissue by the photoacoustic amplitude signals. We observed lower stomatal conductance and higher leaf temperature in white tissue than in green tissue. Besides these thermal differences, significantly higher absorbance in green tissue was based on chlorophyll and carotenoids which were absent in white tissue. However, optical properties of epidermal layers of both tissues were equal. The photoacoustic amplitude of white tissue was over four times higher compared to green tissue, which was correlated with lower stomatal conductance. In addition, at frequencies >700 Hz, the significant differences between the photoacoustic signals of green and white tissue were obtained. We identified the photoacoustic signal deriving from photosynthetic oxygen evolution in green tissue, using high intensity of red light modulated at 10 Hz. Moreover, the photoacoustic amplitude of green tissue increased progressively with time which corresponded to the period of induction of photosynthetic oxygen evolution. For the first time, very high frequencies (1 kHz to 5 kHz) were applied on leaf material.
A PV view of the zonal mean distribution of temperature and wind in the extratropical troposphere
NASA Technical Reports Server (NTRS)
Sun, De-Zheng; Lindzen, Richard S.
1994-01-01
The dependence of the temperature and wind distribution of the zonal mean flow in the extratropical troposphere on the gradient of pontential vorticity along isentropes is examined. The extratropics here refer to the region outside the Hadley circulation. Of particular interest is whether the distribution of temperature and wind corresponding to a constant potential vorticity (PV) along isentropes resembles the observed, and the implications of PV homogenization along isentropes for the role of the tropics. With the assumption that PV is homogenized along isentropes, it is found that the temperature distribution in the extratropical troposphere may be determined by a linear, first-order partial differential equation. When the observed surface temperature distribution and tropical lapse rate are used as the boundary conditions, the solution of the equation is close to the observed temperature distribution except in the upper troposphere adjacent to the Hadley circulation, where the troposphere with no PV gradient is considerably colder. Consequently, the jet is also stronger. It is also found that the meridional distribution of the balanced zonal wind is very sensitive to the meridional distribution of the tropopause temperature. The result may suggest that the requirement of the global momentum balance has no practical role in determining the extratropical temperature distribution. The authors further investigated the sensitivity of the extratropical troposphere with constant PV along isentropes to changes in conditions at the tropical boundary (the edge of the Hadley circulation). It is found that the temperature and wind distributions in the extratropical troposphere are sensitive to the vertical distribution of PV at the tropical boundary. With a surface distribution of temperature that decreases linearly with latitude, the jet maximum occurs at the tropical boundary and moves with it. The overall pattern of wind distribution is not sensitive to the change of
Constraining the depth of Saturn’s zonal winds by measuring thermal and gravitational signals
NASA Astrophysics Data System (ADS)
Liu, Junjun; Schneider, Tapio; Fletcher, Leigh N.
2014-09-01
Based on straightforward dynamical considerations, we show how available and upcoming measurements of Saturn’s thermal and gravitational signals can be used to constrain the depth to which its zonal winds penetrate. The dynamical considerations issue from the facts that Saturn has a strong intrinsic heat flux, rotates rapidly, and has negligible atmospheric viscosity. As a result, convective motions align with surfaces of constant specific angular momentum, which are, away from the equator, approximately cylinders concentric with the planet’s spin axis. Convective motions in the interior therefore tend to homogenize entropy in the direction of the spin axis, but not necessarily perpendicular to it. Using the assumption of interior entropy homogenization in the direction of the spin axis, we determine the zonal winds and their associated thermal and gravitational signals by combining thermal wind balance, the equation of state, the observed zonal winds at the cloud level, and estimates of the strength of the magnetohydrodynamic (MHD) drag that zonal winds experience in the deep interior. We find zonal winds likely extend deeply into Saturn, to a depth between about 0.63 and 0.83RS (with Saturn’s radius RS), or to pressures between 1.4 and 0.3 Mbar. The equation of state of hydrogen constrains zonal winds with strengths similar to the cloud level winds to be confined within the outer few percent of Saturn’s radius, with substantially weaker winds below, irrespective of where in the range of plausible estimates Saturn’s imprecisely known rotation rate falls. Depending on the rotation rate and the precise depth to which zonal winds penetrate, we estimate that the meridional equator-to-pole temperature contrasts in thermal wind balance with the inferred zonal winds increase with depth and reach 1-2 K at 1 bar and 2-4 K at 5 bar. They would be much larger if the cutoff radii of the zonal winds were much shallower than we estimate, but thermal observations by
The residual zonal flow in tokamak plasmas toroidally rotating at arbitrary velocity
Zhou, Deng
2014-08-15
Zonal flows, initially driven by ion-temperature-gradient turbulence, may evolve due to the neoclassic polarization in a collisionless tokamak plasma. In our previous work [D. Zhou, Nucl. Fusion 54, 042002 (2014)], the residual zonal flow in a tokamak plasma rotating toroidally at sonic speed is found to have the same form as that of a static plasma. In the present work, the form of the residual zonal flow is presented for tokamak plasmas rotating toroidally at arbitrary velocity. The gyro-kinetic equation is analytically solved for low speed rotation to give the expression of residual zonal flows, and the expression is then generalized for cases with arbitrary rotating velocity through interpolation. The zonal flow level decreases as the rotating velocity increases. The numerical evaluation is in good agreement with the former simulation result for high aspect ratio tokamaks.
NASA Astrophysics Data System (ADS)
Hopkins, Edward John
This investigation produces zonally and temporally averaged irradiance and illumination geometry data and their annual and long-term variability in greater detail than previously available. The computational algorithm (the HELIO-DAY algorithm) evaluates zonally averaged irradiance data and supporting information describing the latitudinally dependent illumination geometry (solar zenith angles and daylight duration) on a daily basis, provided an input of mean annual orbital elements applicable to the epoch are inserted; numerical summation of each day's irradiation within an interval generates multiday average irradiance for any zone. The accuracy of this methodology was tested with results consistent with theoretical evaluations. Using Berger's (1978) computational formulae to produce the time sequence of pertinent orbital elements, arrays of monthly and tri-monthly zonal average extra-atmospheric irradiance are produced by the HELIO-DAY algorithm from 500,000 B.P. to 1950.0 A.D. at 1000 yr time steps, for 10(DEGREES)-, 30(DEGREES) - and hemispheric-sized zones. Comparison of these tabulations is made with traditional astronomical and caloric half -year insolation chronologies. For the northern hemisphere 321 separate insolation chronologies are prepared, sorted according to a given multiday and zonal combination, and analyzed in both temporal and spectral domains. These frequency spectra for the 500,000 yr multiday-zonal irradiance time series reveal dominant periodicities: 40,600 yr associated with the obliquity of the ecliptic, the 23,600 and 18,900 yr with the precessional parameter (the longitude of perihelion with respect to the equinox of date, modulated by the eccentricity) and weak 385,000 and 100,000 yr periodicities associated with the eccentricity. This investigation shows that over the last 500,000 yr the monthly and tri-monthly average irradiance for individual 10(DEGREES) zones experience frequency spectra significantly different from those of the
The generation of zonal jets by large-scale mixing
NASA Astrophysics Data System (ADS)
Scott, R. K.; Tissier, A.-S.
2012-12-01
The development of zonal flows on a midlatitude β-plane subject to a time-varying topographic forcing is investigated in a series of numerical integrations in which the forcing is concentrated at large scales, and in which the usual two-dimensional inverse energy cascade is absent. In contrast to the case of small-scale forcing, where mixing of potential vorticity occurs largely through the action of small-scale eddies, mixing of potential vorticity in this case occurs predominantly in latitudinally localized Rossby wave critical layer regions, whose width grows continuously in time due to the entrainment of background fluid. The potential vorticity is found to organize into a piecewise constant staircase-like profile, monotonic in latitude, provided the ratio L_Rh/L_fgtrsim 1, where L_Rh is the usual Rhines scale and Lf is the scale of the forcing; this may be regarded as supplemental to the condition L_Rh/L_{\\varepsilon }gtrsim 6, where Lɛ = (ɛ/β3)1/5 and ɛ is the rate of energy input, obtained recently [R. K. Scott and D. G. Dritschel, "The structure of zonal jets in geostrophic turbulence," J. Fluid Mech. 711, 576-598 (2012), 10.1017/jfm.2012.410] for the case of small-scale forcing. The numerical results further suggest that the nature of the potential vorticity mixing is controlled by the ratio Lɛ/Lf, and occurs predominantly in critical layers when Lɛ/Lf ≲ 1/6. A combined condition for staircase formation may therefore be expressed as L_Rh/L_{\\varepsilon }gtrsim max lbrace 6,L_f/L_{\\varepsilon }rbrace. Finally, in a separate set of experiments it is shown that when forcing is represented by an additive source term in the evolution equation, as is common practice in numerical investigations of two-dimensional turbulence, the effect of non-conservation of potential vorticity may obscure the development of the staircase profile in the critical layer mixing dominated regime.
Recent pollen spectra and zonal vegetation in the western USSR
NASA Astrophysics Data System (ADS)
Peterson, G. M.
The relationship of modern pollen spectra to present-day vegetation is critical to the reconstruction of vegetation and climate from fossil pollen spectra. This study uses isopoll maps to illustrate the pollen-vegetation relationships in the Soviet Union west of 100°E and presents descriptive statistics for 544 modern samples of arboreal pollen and for 370 samples of herb pollen obtained from the Soviet palynological literature. Data are assembled from this large geographic region and presented in a standardized form on a scale which can be used to relate quantitative pollen data to zonal vegetation and climatic variables and to make comparisons with other regions. In order to show the relationship between pollen types and major ecotones in forested and non-forested areas, the pollen data are presented as percentages of a sum including both arboreal and non-arboreal pollen. Major pollen types which attain values of 10% or more in at least one vegetation zone include Betula (birch), Cyperaceae (sedges), Picea (spruce), Pinus (total pine), Pinus sibirica, Ericaceae (heath family), Gramineae (grasses), Artemisia (sage), and Chenopodiaceae (i.e., saltbush, Russian thistle, pigweed family). Samples from the tundra and forest-tundra have high values of Ericaceae (heath family), birch, alder, and sedge pollen. In the boreal forest, pine, spruce, and birch pollen predominate. In the mixed and deciduous forests, Tilia (linden), Quercus (oak), Ulmus (elm), and Corylus (hazel) pollen attain maximum values. In the forest-steppe and steppe zones, arboreal pollen decreases in importance and is replaced by non-arboreal pollen types. Pollen of Artemisia and Chenopodiaceae predominates in the semi-desert zones. In spite of variation in the pollen spectra arising from the use of different sediment types (soil, peat, and river sediments), and human disturbance of vegetation, the pollen spectra are clearly related to zonal vegetation. Pollen spectra from the western USSR show