Group velocity

About: Group velocity is a research topic. Over the lifetime, 10051 publications have been published within this topic receiving 204007 citations. The topic is also known as: group speed.

On the displacement height in the logarithmic velocity profile

Abstract: The displacement height appears in the logarithmic velocity profile for rough-wall boundary layers as a reference height for the vertical co-ordinate. It is shown that this height should be regarded as the level at which the mean drag on the surface appears to act. The equations of motion then show that this also coincides with the average displacement thickness for the shear stress.A simple analytical model, experimental results and dimensional analysis are all used to indicate how the displacement height depends upon the detailed geometry of the roughness elements.

An introduction to wavelet analysis in oceanography and meteorology - With application to the dispersion of Yanai waves

Abstract: Wavelet analysis is a relatively new technique that is an important addition to standard signal analysis methods. Unlike Fourier analysis that yields an average amplitude and phase for each harmonic in a dataset, the wavelet transform produces an instantaneous estimate or local value for the amplitude and phase of each harmonic. This allows detailed study of nonstationary spatial or time-dependent signal characteristics. The wavelet transform is discussed, examples are given, and some methods for preprocessing data for wavelet analysis are compared. By studying the dispersion of Yanai waves in a reduced gravity equatorial model, the usefulness of the transform is demonstrated. The group velocity is measured directly over a finite range of wavenumbers by examining the time evolution of the transform. The results agree well with linear theory at higher wavenumber but the measured group velocity is reduced at lower wavenumbers, possibly due to interaction with the basin boundaries.

Linear Pulse Propagation in an Absorbing Medium

Abstract: The pulse velocity in the linear regime in samples of GaP: N with a laser tuned to the bound $A$-exciton line is measured with use of a picosecond time-of-flight technique. The pulse is seen to propagate through the material with little pulse-shape distortion, and with an envelope velocity given by the group velocity even when the group velocity exceeds 3\ifmmode\times\else\texttimes\fi{}${10}^{10}$ cm/sec, $\mathrm{equals}\ifmmode\pm\else\textpm\fi{}\ensuremath{\infty}$, or becomes negative. The results verify the predictions of Garrett and McCumber.

Real-Space Observation of Ultraslow Light in Photonic Crystal Waveguides

Abstract: We show the real-space observation of fast and slow pulses propagating inside a photonic crystal waveguide by time-resolved near-field scanning optical microscopy. Local phase and group velocities of modes are measured. For a specific optical frequency we observe a localized pattern associated with a flat band in the dispersion diagram. During at least 3 ps, movement of this field is hardly discernible: its group velocity would be at most c/1000. The huge trapping times without the use of a cavity reveal new perspectives for dispersion and time control within photonic crystals.