Drift velocity

About: Drift velocity is a research topic. Over the lifetime, 6897 publications have been published within this topic receiving 129602 citations. The topic is also known as: drift speed.

Dynamics of solids in the midplane of protoplanetary disks: implications for planetesimal formation

Abstract: We present local two-dimensional and three-dimensional hybrid numerical simulations of particles and gas in the midplane of protoplanetary disks (PPDs) using the Athena code. The particles are coupled to gas aerodynamically, with particle-to-gas feedback included. Magnetorotational turbulence is ignored as an approximation for the dead zone of PPDs, and we ignore particle self-gravity to study the precursor of planetesimal formation. Our simulations include a wide size distribution of particles, ranging from strongly coupled particles with dimensionless stopping time τ s ≡ Ωt stop = 10–4 (where Ω is the orbital frequency, t stop is the particle friction time) to marginally coupled ones with τ s = 1, and a wide range of solid abundances. Our main results are as follows. (1) Particles with τ s 10–2 actively participate in the streaming instability (SI), generate turbulence, and maintain the height of the particle layer before Kelvin-Helmholtz instability is triggered. (2) Strong particle clumping as a consequence of the SI occurs when a substantial fraction of the solids are large (τ s 10–2) and when height-integrated solid-to-gas mass ratio Z is super-solar. We construct a toy model to offer an explanation. (3) The radial drift velocity is reduced relative to the conventional Nakagawa-Sekiya-Hayashi (NSH) model, especially at high Z. Small particles may drift outward. We derive a generalized NSH equilibrium solution for multiple particle species which fits our results very well. (4) Collision velocity between particles with τ s 10–2 is dominated by differential radial drift, and is strongly reduced at larger Z. This is also captured by the multi-species NSH solution. Various implications for planetesimal formation are discussed. In particular, we show that there exist two positive feedback loops with respect to the enrichment of local disk solid abundance and grain growth. All these effects promote planetesimal formation.

An investigation of steady-state velocity overshoot in silicon

Abstract: Electron dynamics in silicon is investigated by means of improved momentum- and energy-balance equations including particle diffusion and heat flux. The resulting system of partial differential equations is numerically solved in a variety of field configurations including strong discontinuities, in order to enhance velocity overshoot effects. It is found that diffusion, usually neglected in previous studies, plays a major role, and considerably modifies the features of the velocity vs distance curve, leading to an increase of the carrier drift velocity in the low-field region, i.e. before experiencing the effect of the strong field. In addition, it is found that, in order to take full advantage of velocity overshoot effects in MOSFET's, a structure must be designed having the strongest possible field at the source-end of the channel, where carrier density is controlled by the gate.

Predicted electron transport coefficients and operating characteristics of CO2–N2–He laser mixtures

Abstract: Calculations have been made of transport coefficients of electrons in gas mixtures for ratios CO2:N2:He of 1:1:8, 1:2:3, 1:7:30, and 1:0.25:3. New cross sections for CO2 derived from swarm experiments are used together with previously published cross sections for N2 and He. Curves are presented of the predicted electron drift velocity, transverse and longitudinal diffusion coefficients, and ionization and attachment coefficients for E/N values ranging from 10−18 to 1 × 10−15 V cm2; E is the electric field strength and N the gas number density. Examples are given of derived distribution functions and comparisons are made with a Maxwellian distribution function. The percentage of the input electrical power which excites vibrational processes coupled to the 001 upper laser level of CO2 is given as a function of E/N. The maximum efficiency from these calculations increases for increasing ratios of N2:CO2, because the proportion of energy used to excite the bending and stretching modes of CO2 is then reduced. ...

Generation of small-scale irregularities in the equatorial electrojet.

Abstract: A theory that accounts for the generation of short-wavelength irregularities in the equatorial electrojet for an electron drift velocity below the ion-acoustic speed is presented. Long-wavelength (tens of meters) horizontally propagating waves driven unstable by the ambient ionization density gradient are found not to steepen very much, even though they are nondispersive. However, the local vertical velocity and horizontal density gradient associated with these waves are capable of generating the meter-wavelength obliquely propagating irregularities detected in radar observations.