[1] The measurement of mean water levels, roller geometries, and phase ensembleaveraged velocity and turbulence intensity fields under spilling and plunging waves breaking in a two-dimensional laboratory surf zone is presented. The velocities were measured using digital correlation image velocimetry, while water levels and roller geometries were determined through gray scale filtering of video images. The phase ensemble-averaged horizontal and vertical components of velocity and turbulence intensities are measured throughout the entire flow domain, including the wave roller area, by utilizing the aerated areas as part of the flow structure. The time-averaged horizontal velocities (undertow), turbulence intensities, and turbulent kinetic energies are determined by averaging across the wave phase. Turbulence magnitudes are found to compare favorably with existing laser Doppler anemometry measurements below the wave trough level, where such measurements have generally been confined because of aeration contamination effects. The significantly higher velocity and turbulence intensity magnitudes measured above the trough level in the present experiments highlight the novel nature of the present investigation for describing flow regimes in the surf zone. INDEX

[1] The measurement of turbulence dissipation rates and length scales associated with three-dimensional isotropic structures under spilling waves breaking in a laboratory surf zone is presented. Dissipation rates were estimated from the spectral characteristics of the turbulence velocities in the inertial subrange, and length scales were estimated using measurements of the turbulent kinetic energy and dissipation rate. The spatial velocity flow fields for the above analysis were measured using digital correlation image velocimetry. A unique set of measurements that spans the entire water column, including the aerated portion near the crest of the wave, is presented. Dissipation rates were found to reach a maximum above the effective trough level, with over 80% of the depth integrated dissipation occurring in this upper zone. The total depth integrated turbulence dissipation rate is found to be up to an order of magnitude smaller than the local rate of wave energy dissipation due to breaking, the primary turbulence production source. The length scale is found to increase in magnitude below the surface, consistent with the idea that turbulence production occurs above the trough level in the vicinity of the wave roller and is transported downward toward the bed.INDEX TERMS: 4546 Oceanography: Physical: Nearshore processes; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; 4558 Oceanography: Physical: Sediment transport; KEYWORDS: digital correlation image velocimetry, dissipation rates, length scales, turbulent kinetic energy, wave breaking, surf zone Citation: Govender, K., G. P. Mocke, and M. J. Alport (2004), Dissipation of isotropic turbulence and length-scale measurements through the wave roller in laboratory spilling waves,

Observations of coherent electrostatic ion cyclotron (EIC) waves associated with a strong, magnetized double layer are presented. The double layers are produced in a weakly ionized argon plasma by applying a positive potential to an electrode plate located in the diverging magnetic field region of a cylindrical plasma column. Ionization within the electrode sheath is essential to the formation of these double layers. The resulting V‐shaped potential structures have extended parallel, oblique and perpendicular (to B) electric field components. The frequency of the ion cyclotron instability is dependent upon the magnetic field strength at the position of the parallel potential structure. The properties of EIC waves in the presence of the double layer are discussed in relation to the possible excitation mechanisms (field‐aligned currents, ion and electron beams, and perpendicular E fields).

Ion dynamics in the plasma sheath following the application of a negative voltage pulse to a two-dimensional target are modeled using a time-dependent, two-fluid simulation. The target considered is a square bar of infinite length. We find that the sheath focuses ions near to, but not on, the corner of the bar, resulting in an enhancement of the dose received there.

The structure of the ion-matrix sheath surrounding a square bar is calculated. Equipotentials and field lines are presented, and the sheath width and eccentricity are calculated as a function of the bar width.