Bibliothek

feed icon rss

Ihre E-Mail wurde erfolgreich gesendet. Bitte prüfen Sie Ihren Maileingang.

Leider ist ein Fehler beim E-Mail-Versand aufgetreten. Bitte versuchen Sie es erneut.

Vorgang fortführen?

Exportieren
  • 1
    Digitale Medien
    Digitale Medien
    Oxford, UK : Blackwell Publishing Ltd
    BJOG 92 (1985), S. 0 
    ISSN: 1471-0528
    Quelle: Blackwell Publishing Journal Backfiles 1879-2005
    Thema: Medizin
    Notizen: Summary. Thirty patients, thought clinically to have ovarian tumours, were studied prospectively by pre-operative computed tomographic (CT) scans of the abdomen and pelvis. In six patients (20%) small metastases in mesentery, omentum and on subdiaphragmatic peritoneum were not detected by the scans. CT did not improve the accuracy of staging or assist the surgeons by drawing their attention to disease which they might otherwise have missed. Although CT gives an elegant demonstration of anatomy, it is not an alternative to extended laparotomy in patients with ovarian tumours.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
  • 2
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 8225-8237 
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The motion of single- and two-cavitation bubbles generated by laser beams directly beneath a free surface is studied experimentally, using high-speed photography, and theoretically using the highly accurate boundary integral method. Favorable comparisons of bubble shape history and centroid motion are observed while the numerical calculations provide information on the pressure field surrounding the bubbles. A range of responses, including the null impulse state, is obtained for the two bubbles depending on the bubble size ratio and the interbubble and bubble-free surface distances, although in all cases reported in this article, the bubble nearest the free surface yields a high-speed liquid jet directed away from the free surface. It is also found that when the free-surface–bubble interaction is strong, a fast free-surface spike is formed for both the single- and two-bubble cases. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
  • 3
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Localized bursty plasma waves are detected by spacecraft in many space plasmas. The large spatiotemporal scales involved imply that beam and other instabilities relax to marginal stability and that mean wave energies are low. Stochastic wave growth occurs when ambient fluctuations perturb the system, causing fluctuations about marginal stability. This yields regions where growth is enhanced and others where damping is increased; bursts are associated with enhanced growth and can occur even when the mean growth rate is negative. In stochastic growth, energy loss from the source is suppressed relative to secular growth, preserving it far longer than otherwise possible. Linear stochastic growth can operate at wave levels below thresholds of nonlinear wave-clumping mechanisms such as strong-turbulence modulational instability and is not subject to their coherence and wavelength limits. These mechanisms can be distinguished by statistics of the fields, whose strengths are lognormally distributed if stochastically growing and power-law distributed in strong turbulence. Recent applications of stochastic growth theory (SGT) are described, involving bursty plasma waves and unstable particle distributions in type III solar radio sources, the Earth's foreshock, magnetosheath, and polar cap regions. It is shown that when combined with wave–wave processes, SGT also accounts for associated radio emissions. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
  • 4
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 133-144 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The formation, collapse and arrest of lower-hybrid wave packets are investigated analytically. The three-dimensional structure of the wave packet is incorporated in the analysis and its polarization is studied for the first time. Nonlinear collapse thresholds are obtained via a Hamiltonian formulation and are used in calculating the probability distribution of collapsing wave packet structures as a function of their polarization. Transit-time interaction theory is then used to calculate the arrest scale at which collapse is halted as the waves are damped. It is found that collapse thresholds are lowest for circularly polarized packets, but that nearly linearly polarized ones predominate in collapse because of their greater numbers in the linear phase of the evolution. It is argued that subsonic collapse persists until very near arrest, in accord with recent numerical simulations. Time scale analysis shows that the parallel field structure has difficulty in attaining its self-similar form in the available collapse time, also in accord with simulations. Transit-time theory implies that electrons travelling roughly parallel to the ambient magnetic field can arrest collapse at a scale comparable to that previously estimated for ions; which process dominates depends on the electron and ion temperatures and packet geometry. The resulting arrest scales are found to be in accord with the simulations. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
  • 5
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 122-132 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Study of Langmuir collapse thresholds shows that they have little polarization dependence and that moving packets have the lowest thresholds in the undamped case. However, incorporation of damping into the density response inhibits collapse of packets moving at more than a small fraction of the sound speed. Investigation of energy transfer to packets localized in density wells—the nucleation process—shows that at most a few trapped states can exist and that energy transfer is most effective when there is a single barely-trapped state. Coupled with an argument that closely packed wave packets have lower collapse thresholds, this argument yields an estimate of the number density of localized nucleating states in a turbulent plasma. It also leads to a simple and direct semiquantitative estimate of the collapse threshold. All these results are in accord with previous numerical simulations incorporating ion-sound damping, which show a preponderance of slow-moving or stationary packets with little or no intrinsic polarization dependence of thresholds. Likewise, the number densities obtained are in good agreement with simulation values, and the simple estimate of the threshold is semiquantitatively correct. The extent of the agreement supports the nucleation scenario with close-packed nucleation sites in the turbulent state. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
  • 6
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 8 (2001), S. 428-440 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The three-wave decay of beam-driven Langmuir waves into ion-sound waves, and backscattered Langmuir waves is analyzed. Realistic approximations are used for the spectra of Langmuir and ion-sound waves to enable calculations of growth rates of ion-sound and backscattered Langmuir waves in terms of a single well behaved integral. This integral is evaluated numerically, and approximated analytically and the results are compared with previous estimates. The case of intense beam-driven Langmuir waves decaying amid a weak thermal background of pre-existing Langmuir and ion-sound waves is examined in detail and the analytical approximation in this case is found to reproduce the numerical results accurately. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
  • 7
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 3057-3065 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The effect of ambient density fluctuations on Langmuir wave collapse and strong Langmuir turbulence is investigated. Hamiltonian analysis of the collapse threshold implies that fluctuations with scales near those of nucleating wave packets can disrupt them before they can accumulate enough energy to collapse, provided the ambient fluctuation level is greater than that generated ponderomotively by the Langmuir waves. If packet disruption is effective, Langmuir energy cannot be dissipated via wave collapse and burnout, but must be scattered off density fluctuations directly to high wave numbers, as predicted by previous analyses. Numerical simulations of strong Langmuir turbulence confirm these predictions, with sudden transitions occurring from a strong-turbulence regime to one dominated by scattering or one with relatively rare wave collapses as a result of disruption of nascent wave packets. A corresponding sudden drop in Langmuir energy density is observed. Simulations of individual wave packets near the threshold for collapse show that such packets are easily disrupted by fluctuations with wavelengths near their linear scale, and confirm previous analytic disruption criteria. © 1999 American Institute of Physics.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
  • 8
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 498-510 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A Fokker–Planck theory is developed to describe the diffusion in momentum space of a beam of relativistic electrons due to multiple transit-time interactions with an ensemble of coherent Langmuir wave packets. The theory incorporates two ingredients: a perturbed-orbit calculation of the momentum change of a test particle during a single transit-time interaction, and an ensemble average of the resulting Fokker–Planck coefficients based on the statistical properties of strong Langmuir turbulence. An approximate analytic solution of the Fokker–Planck equation is obtained for the case of a strongly collimated beam, and is used to interpret measurements of energy and pitch-angle scattering in relativistic-electron-beam (REB) experiments. Fokker–Planck coefficients are also calculated for a weakly collimated beam. It is shown that the theory correctly predicts the amount of energy scattering in REB experiments, but underestimates the pitch-angle scattering regardless of the distribution of wave packet orientations and the degree of collimation of the beam. This discrepancy may be a product of the approximate wave-packet structure assumed in the analysis, or of systematic errors in the experimental data; alternatively, it may imply that a non-transit-time process is responsible for part of the pitch-angle scattering observed. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
  • 9
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 192-201 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A two-component model of strongly nonlinear wave turbulence is developed for a broad class of systems in which high-frequency electrostatic waves interact with low-frequency sound-like waves. In this model coherent nonlinear wave packets form and collapse amid a sea of incoherent background waves. It is shown that three classes of turbulence exist, typified by Langmuir, lower-hybrid, and upper-hybrid turbulence. Balance between power input to incoherent waves, and dissipation at the end of collapse determines power-law scalings of turbulent electrostatic energy density, density fluctuations, length and time scales. Knowledge of the evolution of collapsing packets enables probability distributions of the magnitudes of electric fields and density fluctuations to be calculated, yielding power-law dependences. Wavenumber spectra of collapsing waves and associated density fluctuations are also calculated and shown to have power-law forms. Applications to Langmuir, lower-hybrid, and upper-hybrid waves are discussed. In the Langmuir case the results agree with earlier theory and simulations, with one exception, which is consistent only with earlier simulations. In the lower-hybrid and upper-hybrid cases, the results are consistent with the few simulations to date. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
  • 10
    Digitale Medien
    Digitale Medien
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 3 (1996), S. 149-159 
    ISSN: 1089-7674
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The coalescence of Langmuir waves to produce electromagnetic waves at twice the plasma frequency is considered. A simplified expression for the rate of production of second harmonic electromagnetic waves is obtained for a broad class of Langmuir spectra. In addition, two different analytic approximations are considered. The validity of the commonly used head-on approximation is explored, in which the two coalescing Langmuir waves are assumed to approach from opposite directions. This approximation breaks down at low Langmuir wavenumbers, and for narrow Langmuir wave spectra. A second, more general, approximation is introduced, called the narrow-spectrum approximation, which requires narrow spectral widths of the Langmuir spectra. The advantages of this approximation are that it does not break down at low Langmuir wavenumbers, and that it remains valid for relatively broad Langmuir wave spectra. Finally, the applicability of these approximations in treating harmonic radiation in type III solar radio bursts is discussed. © 1996 American Institute of Physics.
    Materialart: Digitale Medien
    Bibliothek Standort Signatur Band/Heft/Jahr Verfügbarkeit
    BibTip Andere fanden auch interessant ...
Schließen ⊗
Diese Webseite nutzt Cookies und das Analyse-Tool Matomo. Weitere Informationen finden Sie hier...