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At least 811 records · Page 45
All sourcesNASA NTRSDOE OSTI
NASA NTRS2012

Slow Magnetosonic Waves and Fast Flows in Active Region Loops

Recent extreme ultraviolet spectroscopic observations indicate that slow magnetosonic waves are present in active region (AR) loops. Some of the spectral data were also interpreted as evidence of fast (approx 100-300 km/s) quasiperiodic flows. We have performed three-dimensional magnetohydrodynamic (3D MHD) modeling of a bipolar AR that contains impulsively generated waves and flows in coronal loops. The model AR is initiated with a dipole magnetic field and gravitationally stratified density, with an upflow-driven steadily or periodically in localized regions at the footpoints of magnetic loops. The resulting flows along the magnetic field lines of the AR produce higher density loops compared to the surrounding plasma by injection of material into the flux tubes and the establishment of siphon flow.We find that the impulsive onset of flows with subsonic speeds result in the excitation of damped slow magnetosonic waves that propagate along the loops and coupled nonlinearly driven fast-mode waves. The phase speed of the slow magnetosonic waves is close to the coronal sound speed. When the amplitude of the driving pulses is increased we find that slow shock-like wave trains are produced. When the upflows are driven periodically, undamped oscillations are produced with periods determined by the periodicity of the upflows. Based on the results of the 3D MHD model we suggest that the observed slow magnetosonic waves and persistent upflows may be produced by the same impulsive events at the bases of ARs.

Magnetohydrodynamics-Sun Activity↗
NASA NTRS1990

Observations of nonlinear effects in directional spectra of shoaling gravity waves

The spatial evolution of a directionally spread wave field on a near-planar natural beach is examined using data from longshore arrays of pressure sensors and wave staffs at depths 10.3 and 4.1 m. High-resolution frequency-directional spectra from the deeper array are used to initialize a linear refraction model, and the resulting model predictions are compared with frequency-directional measurements at the shallow array. Linear theory inaccurately predicts both the shapes of directional spectra in shallow water and the total variances in some frequency bands. The discrepancies are largest for frequencies associated with maxima in the bicoherence spectrum, suggesting the importance of nonlinear effects.

Freilich, M. H.↗
NASA NTRS2000

Acceleration of the Fast Solar Wind by Solitary Waves in Coronal Holes

The purpose of this investigation is to develop a new model for the acceleration of the fast solar wind by nonlinear, time-dependent multidimensional MHD simulations of waves in solar coronal holes. Preliminary computational studies indicate that solitary-like waves are generated in coronal holes nonlinearly by torsional Alfven waves. These waves in addition to thermal conduction may contribute considerably to the accelerate the solar wind. Specific goals of this proposal are to investigate the generation of nonlinear solitary-like waves and their effect on solar wind acceleration by numerical 2.5D MHD simulation of coronal holes with a broad range of plasma and wave parameters; to study the effect of random disturbances at the base of a solar coronal hole on the fast solar wind acceleration with a more advanced 2.5D MHD model and to compare the results with the available observations; to extend the study to a full 3D MHD simulation of fast solar wind acceleration with a more realistic model of a coronal hole and solar boundary conditions. The ultimate goal of the three year study is to model the fast solar wind in a coronal hole, based on realistic boundary conditions in a coronal hole near the Sun, and the coronal hole structure (i.e., density, temperature, and magnetic field geometry) that will become available from the recently launched SOHO spacecraft.

Ofman, Leon↗
NASA NTRS1995

A Zonal Approach for Prediction of Jet Noise

A zonal approach for direct computation of sound generation and propagation from a supersonic jet is investigated. The present work splits the computational domain into a nonlinear, acoustic-source regime and a linear acoustic wave propagation regime. In the nonlinear regime, the unsteady flow is governed by the large-scale equations, which are the filtered compressible Navier-Stokes equations. In the linear acoustic regime, the sound wave propagation is described by the linearized Euler equations. Computational results are presented for a supersonic jet at M = 2. 1. It is demonstrated that no spurious modes are generated in the matching region and the computational expense is reduced substantially as opposed to fully large-scale simulation.

Shih, S. H.↗
NASA NTRS1989

Interactions of Tollmien-Schlichting waves and Dean vortices. Comparison of direct numerical simulation and a weakly nonlinear theory

Direct numerical simulation is used to evaluate a weakly nonlinear theory describing the interaction of Tollmien-Schlichting waves with Dean vortices in curved channel flow. The theory and the simulation agree for certain combinations of parameters, but the two approaches give conflicting results for other combinations. Some possibilities for these discrepancies are discussed.

Singer, Bart A.↗
NASA NTRS1988

Dynamics of localized ion-acoustic waves in a magnetized plasma

The evolution of negative potential pulses in a magnetized plasma is studied. A three-dimensional nonlinear ion-acoustic wave equation, including nonstationary effects of reflected electrons, has been derived from the Poisson-Vlasov equations with uniform magnetic field. The studies show that a negative potential pulse can be enhanced by drifting electrons. The growing pulse develops asymmetrically with an oscillatory precursor and a local potential jump resembling the early phase of weak double layer formation.

Qian, S.↗
NASA NTRS1980

Mechanisms of wave transformation in finite-depth water

Mechanisms of wave transformation in finite-depth water are investigated. The linear mechanisms examined are percolation, bottom motion, shoaling, and refraction. The nonlinear mechanisms examined are wave-wave interaction and bottom friction. New exact computations of the nonlinear transfer for finite-depth waves are presented for some directional wave spectra. These mechanisms are found to explain satisfactorily wave decay observations obtained at several sites with different bottom sediment properties. The decay rates at these sites are found to be dominated by different mechanisms which are determined by the bottom conditions. As an example, detailed calculations are presented for data obtained at the Jonswap site.

Shemdin, O. H.↗
NASA NTRS1988

Evolution of nonlinear polarization in localized and finite amplitude Alfven waves

Theoretical and computational study for a polarization change of localized and finite amplitude Alfven waves propagating parallel to an applied magnetic field is presented using both reductive perturbation theory and numerical simulations. In the magnetohydrodynamic limit, where right-hand and left-hand circularly polarized waves are degenerate, one of the transverse components of the circularly polarized Alfven wave is stable as it propagates, but the other component is unstable to either self-focusing or diffraction effects. Consequently, the wave changes its polarization from circular to linear. In the high frequency regime, where two circularly polarized waves (right-hand and left-hand circularly polarized waves) are not degenerate, two transverse components of the circularly polarized Alfven wave are strongly coupled to each other, and there is almost no polarization change.

Hoshino, M.↗
NASA NTRS2003

Numerical Simulation of Waves Driven by Plasma Currents Generated by Low-Frequency Alfven Waves in a Multi-Ion Plasma

When multi-ion plasma consisting of heavy and light ions is permeated by a lowfrequency Alfien (LFA) wave, the EXB and the polarization drifts of the different ion species and the electrons could be quite different. The relative drifts between the charged-particle species drive waves, which energize the plasma. Using 2.5-D particle-in-cell simulations, we study this process of wave generation and its nonlinear consequences in terms of acceleration and heating plasma. Specifically we study the situation for LFA wave frequency being lower than the heavyion cyclotron frequency in a multi-ion plasma. We impose such a wave to the plasma assuming that its wavelength is much larger than that of the waves generated by the relative drifts. For better understanding, the LFA-wave driven simulations are augmented by those driven by initialized ion beams.

Singh, Nagendra↗
NASA NTRS2004

Numerical Simulation of Waves Driven by Plasma Currents Generated by Low-Frequency Alfven Waves in a Multi-Ion Plasma

When multi-ion plasma consisting of heavy and light ions is permeated by a low-frequency Alfven (LFA) wave, the crossed-electric-and-magnetic field (E x B), and the polarization drifts of the different ion species and the electrons could be quite different. The relative drifts between the charged-particle species drive waves, which energize the plasma. Using 2.5-dimensional (2.5-D) particle-in-cell simulations, we study this process of wave generation and its nonlinear consequences in terms of acceleration and heating plasma. Specifically, we study the situation for LFA wave frequency being lower than the heavy-ion cyclotron frequency in a multi-ion plasma. We impose such a wave to the plasma assuming that its wavelength is much larger than that of the waves generated by the relative drifts. For better understanding, the LFA-wave driven simulations are augmented by those driven by initialized ion beams. The driven high-frequency (HF) wave modes critically depend on the heavy ion density nh; for small values of nh, the lower hybrid (LH) waves dominate. On the other hand, for large nh a significantly enhanced level of waves occurs over a much broader frequency spectrum below the LH frequency and such waves are interpreted here as the ion Bernstein (IB) mode near the light ion cyclotron harmonics. Irrespective of the driven wave modes, both the light and heavy ions undergo significant transverse acceleration, but for the large heavy-ion densities, even the electrons are significantly accelerated in the parallel direction by the waves below the LH frequency. Even when the LFA wave drive is maintained, the ion heating leads to the cessation of HF wave excitation just after a few cycles of the former wave. On the basis of marginal stability seen in the simulations, an empirical relation for LFA wave amplitude, frequency and ion temperature is given.

Singh, Nagendra↗
NASA NTRS1995

Hybrid simulations of the saturated Farley-Buneman instability in the ionosphere

Numerical simulations of the Farley-Buneman instability in 2-1/2 dimensions using particle ions and fluid electrons show the growth, saturation and nonlinear behavior of two-stream waves. This hybrid technique models the saturated state of the instability for a much longer period of time than the pure particle codes that preceded it. While focusing principally on modeling the topside E region equatorial electrojet, many of these results apply to the auroral two-stream instability as well. The following features are seen in all our hybrid simulations: (1) wave growth at an angle offset from the electron drift direction where the angle depends on the strength of the driving electric field, (2) nonlinear coupling to waves traveling perpendicular to the propagation direction of the principal two-stream waves, (3) a saturated wave phase velocity at or above the sound speed but well below the velocity predicted by linear theory and (4) phase velocities which remain almost constant as a simulated radar sweeps from a horizontal direction to nearly vertical. The nonlinear electron motion dominates the behavior of these waves. Further, these simulations indicate that ion kinetic effects are not essential for the saturation of the instability and that electron temperature effects have a minor impact on the final saturated state.

Oppenheim, Meers↗
DOE OSTI2024

High-Fidelity Numerical Wave Tank Verification & Validation Study: Wave Generation Through Paddle Motion: Preprint

This paper presents a numerical benchmark study of wave propagation due to a paddle motion using different high-fidelity numerical models, which are capable of replicating the nearly actual physical wave tank testing. A full time series of the measured wave generation paddle motion which was used to generate wave propagation in the physical wave tank will be utilized in each of the models contributed by IEA OES Task 10's participants, which includes both computational fluid dynamics (CFD) and smooth hydrodynamic particle (SPH). The high-fidelity simulations of the physical wave testcase will allow for the evaluation of the initial transient effects from wave ramp-up and its evolution in the wave tank over time for two representative regular waves with varying levels of nonlinearity. A couple of interesting metrics like the predicted wave surface elevation at select wave probes, wave period, and phase-shift in time will be assessed to evaluate the relative accuracy of numerical models versus experimental data within specified time intervals. These models will serve as a guide for modelers in the wave energy community and provide a base case to allow further and more detailed numerical modeling of the fixed Kramer Sphere Cases under wave excitation force wave tank testing.

HYDRO ENERGY,TIDAL AND WAVE POWER↗
NASA NTRS1988

Subcritical dispersive shock waves upstream of planetary bow shocks and at Comet Giacobini-Zinner

The nonlinear evolution of ULF (magnetosonic) waves is studied using electromagnetic simulations. The waves were observed upstream of the planetary bow shocks at Comet Giacobini-Zinner. It is shown that as ULF waves generated by the resonant ion beam instability steepen, their polarization becomes linear. A high-frequency wave packet is generated by the steepening process. The steepened wave has a structure and a behavior similar to a subcritical dispersive shock. It is suggested that Comet Giacobini-Zinner did not have a single bow shock. It is concluded that the comet had a transition region consisting of a series of shocks which were convected by the solar wind.

Omidi, N.↗
DOE OSTI2025

Nonlinearities in Magnetic Confinement, Ionospheric Physics, and Population Explosion Leading to Profile Resilience Нелінійності в магнетному утриманні, фізиці іоносфери та процесі демографічного вибуху, які приводять до стійкості профілю

Nonlinearities play an important role in many fields. In the field of thermonuclear fusion, they are involved in questions such as profile resilience and fluid closure. A nonlinear phenomenon common to both fusion and astrophysical planets is the generation of zonal flows. These flows play a significant role in determining the level of turbulence and fluid closure in fusion. The effects of resonance broadening and nonlinearities are investigated, specifically focusing on the case of nonlinear instability that has appeared in drift waves. Similarities and differences between our systems are discussed, with population explosion and the dynamics of nonlinear systems for drift waves by different states in profile resilience described with great precision. The aim of our study is to put our fluid model for drift waves in tokamaks within the wider framework of statistical physics principles. This reinforces our belief in the broad application of our drift wave model, which encompasses current tokamaks, ITER, and the fusion pilot plant.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS1995

A Simple Theory of Capillary-Gravity Wave Turbulence

Employing a recently proposed 'multi-wave interaction' theory, inertial spectra of capillary gravity waves are derived. This case is characterized by a rather high degree of nonlinearity and a complicated dispersion law. The absence of scale invariance makes this and some other problems of wave turbulence (e.g., nonlinear inertia gravity waves) intractable by small-perturbation techniques, even in the weak-turbulence limit. The analytical solution obtained in the present work for an arbitrary degree of nonlinearity is shown to be in reasonable agreement with experimental data. The theory explains the dependence of the wave spectrum on wind input and describes the accelerated roll-off of the spectral density function in the narrow sub-range separating scale-invariant regimes of purely gravity and capillary waves, while the appropriate (long- and short-wave) limits yield power laws corresponding to the Zakharov-Filonenko and Phillips spectra.

Glazman, Roman E.↗
NASA NTRS1995

Low-frequency variability and wavenumber selection in models with zonally symmetric forcing

The authors consider a two-layer quasigeostrophic model with linear surface drag and forcing that relaxes to a zonal baroclinically unstable equilibrium state consisting of a meridionally confined temperature gradient. It is observed that the most energetic wave in the time-mean climate has near zero frequency and is not driven by upscale nonlinear energy transfers. This wave has a zonal scale near the long-wave cutoff of the equilibrium state, and its energy balance is mainly between baroclinic generation and dissipation. This maintenance mechanism is different from that suggested by beta-plane, two-dimensional, and quasigeostrophic turbulence arguments and may be relevant to the dynamics of zonally asymmetric low-frequency variability in the atmosphere, particularly in the Southern Hemisphere.

Whitaker, Jeffrey S.↗
NASA NTRS1995

Nonlinear Propagating Features in the Stratosphere of Jupiter Generated by the Impact of SL-9

Among the more intriguing features observed by HST after the impacts of the fragments of Shoemaker-Levy 9 were rings, encircling several of the impact sites, that expanded radially at constant velocities of order 450-500 m/s. One credible suggestion has been that the rings are stratospheric manifestations of linear gravity waves seated in the troposphere (Ingersoll and Kanamori, 1995). In this scenario the waves are made visible by a condensible material of unknown composition. Based on simulations of the comet impact by Zahnle and Mac Low (1995), we suggest an alternative hypothesis, namely that the rings are nonlinear stratospheric gravity waves which break and entrain impact debris material in the breaking wave front. The computations produce just such a feature, with the nonlinear breaking wave front in the stratosphere propagating radially away from the impact site with the correct speed. The computed ring of material is located in an altitude region where the static stability profile has a local maximum and therefore forms a waveguide. Because entrained impact debris forms the ring feature in the model, a condensate of some unknown composition is not required in order to view the wave front. However, there is one severe problem with the computed nonlinear wave: in contrast to a linear wave, it slows down. The computed feature stops within 1000 seconds, while the observed feature lasted at least 10,000 seconds. Since the nonlinear breaking wave front is a natural product of our impact simulations, we are now investigating the conditions that will enable the breaking wave front to propagate further, in accord with the observations. The results of these computations will be presented.

Young, R. E.↗