Unmixed Regions Research Articles

Increase in regularity by polymer addition during chaotic mixing in two-dimensional flows.

Experiments show that the addition of small amounts of polymer to a Newtonian fluid undergoing chaotic advection decreases the amount of chaos; regular or unmixed regions increase in size and squeeze out the regions of chaotic behavior while preserving underlying symmetries. The size of the islands increases with increasing elasticity and rate of stirring. Large effects are present at relatively low Deborah numbers

Variability of the Supersaturation in Cumulus Clouds

Abstract The quasi-steady supersaturation in cumulus clouds from the Cooperative Convective Precipitation Experiment of 1981 are calculated from measurements of vertical wind, cloud droplet size, temperature and pressure. Mean values and spectral characteristics of the supersaturation are presented. The supersaturation values in 147 cloud regions averaged near 0%, with standard deviations in the (10-m average) supersaturation values that ranged from 0.1% in unmixed regions to about 0.4% in regions about 80% diluted by entrainment. The Eulerian variance spectra for vertical wind, integral radius and supersaturation were determined, and a technique is described for estimating the Lagrangian spectra. The Lagrangian characteristic times for supersaturation were estimated to be about 50 s for mixed regions but substantially larger for some regions with strong updrafts. It is argued that the observed variability in supersaturation is comparable to that needed to account for typical dispersions in droplet size s...

Dose-dependent mixing of AlAs-GaAs superlattices by Si ion implantation

The effects of Si ion implantation and annealing on AlAs-GaAs superlattices are examined with secondary ion mass spectrometry (SIMS), Rutherford backscattering spectrometry (RBS), and transmission electron microscopy (TEM). Samples implanted with 180 keV 28Si+ of doses ranging from 3×1013 to 3×1015 cm−2 are examined before and after a 3-h 850 °C anneal. Both the TEM and RBS channeling data indicate the formation of a heavily damaged surface layer where diffusion of Al is inhibited even after thermal annealing. After annealing, however, significant mixing is observed at depths well beyond the implant range. Depth-dependent diffusion lengths of Al and Si are derived from the SIMS data. The diffusion coefficient of Si is markedly reduced in the unmixed regions with both the Si and Al concentrations exhibiting abrupt forward and rear diffusion fronts.

AN EVALUATION OF MODELS OF MIXING AND CHEMICAL REACTION WITH A TURBULENCE ANALOGY

Five mechanistic models of mixing and chemical reaction having an analogy with isotropic turbulent mixing are evaluated. The turbulence analogies, based on matching variance decay laws of models and turbulence theory, provide a physical basis for the models and a means of estimating their micromixing parameters apriori. Experimental data for single second order liquid phase reactions provide strong support for the analogies. However, it is demonstrated that in spite of their success for single reactions, the models may predict grossly different selectivities in the case of competing reactions in a plug flow reactor. This emphasizes the importance of certain structural features of the models which are independent of the existence of a turbulence analogy, such as: (i) reacting regions which are rich in each of the reactants of a two feedstream reactor, and (ii) unmixed regions in the reaction mixture. The importance of obtaining data for competing reactions in a highly segregated plug flow reactor for the p...

Dynamical-Microphysical Evolution of a Convective Storm in a Weakly-Sheared Environment. Part I: Microphysical Observations and Interpretation

Abstract Aircraft measurements of microphysical thermodynamic, and vertical air motion properties supplemented by radar measurements of reflectivity structure are used to investigate precipitation development throughout much of the life cycle of a moderately intense convective storm in northeast Colorado. There was considerable variability of cloud properties such as updraft speed at scales of a few hundred meters early in the life of the storm. Greater organization was evident in the later, more mature stage. The earliest radar return from a major cell came from particles larger than 1 mm diameter in concentrations less than 10 m−3. It is suggested that these particles must have already followed complicated growth trajectories even at this early stage of the storm, including more than one ascent in updraft. In the more mature stage of the storm millimetric water drops and partially melted and refreezing ice particles were observed at 1 to 2 km above cloud base in both mixed and unmixed regions of the mai...

Recent progress in the understanding of the ion composition in the magnetosphere and some major question marks

Observations of the energetic ion composition in the magnetosphere are reviewed with emphasis on the recent measurements by means of GEOS 1 and 2, ISEE 1 and 2, PROGNOZ 7, and SCATHA. The observations are compared with the predictions of the open magnetosphere model. One of the major conclusions is that there are processes in the magnetosphere which play a much larger part than the model, as hitherto presented, predicts. Direct ejection of ionospheric ions, in combination with acceleration, along closed as well as open field lines may even be the dominant source process for the ring current/inner plasma sheet in magnetic storms. In very disturbed conditions this ejection mechanism must work over most of the hemisphere poleward of, say, 50°. Circulation of the ionospheric ions through the tail of the magnetosphere is not likely to be of primary importance for the energization of these ions in very disturbed conditions. Other conclusions about the ring current/plasma sheet plasma are as follows: (1) The polar wind may be the dominant source of ring current ions in the keV range near the geostationary orbit in quiet conditions. (2) The detached plasma blobs from the plasmasphere do not generally appear to be an important source of hot ions in the inner magnetosphere. (3) In the inner ring current region (L < 4), charge exchange loss processes give rise to the dominance in this region of ions of ionospheric origin in quiet conditions and during the recovery phase of magnetic storms. (4) Acceleration both along and perpendicular to field lines affects the ejected ionospheric ions. (5) Mass‐dependent acceleration processes appear to be of importance; ion beams which may have traveled adiabatically from below a few thousand kilometers altitude to 5 RE along the field lines have been seen, but mostly, the beams are wide at great heights. (6) Energized electrons are sometimes found flowing along the same field tubes as the energized ions. (7) There appear to exist more or less simultaneously quite different source mechanisms in different parts of the magnetosphere. (8) There is a solar cycle variation in the ion composition. Conclusions from the recent investigations of the composition of the magnetospheric boundary layers by means of the PROGNOZ 7 satellite are as follows: (1) The dayside low‐latitude boundary layer/entry layer contains a structure of unmixed regions with plasma of either hot ‘magnetospherelike’ characteristics or less hot and more ‘magnetosheathlike’ properties within the boundary layer, but the magnetosheathlike plasma also contains ions of ionospheric origin. (2) The plasma mantle contains large, although generally not dominant, amounts of ions of ionospheric origin in disturbed conditions but frequently no such ions in quiet conditions. (3) The mantle frequently contains ‘inclusions’ of high‐density (β ≥ 1) magnetosheathlike plasma with sometimes strongly deviating magnetic field and even flow and with Birkeland currents at their edges. (4) The flank mantle frequently contains stagnant plasma with appreciable amounts of ionospheric ions of hard energy spectra and also strongly heated electrons. (5) Mostly, the mantle observations appear to be consistent with both acceleration processes which provide about equal velocity to the ions, irrespective of mass (thought to be associated with reconnection), and processes which give equal E/q to the ions, irrespective of mass (electrostatic potential differences), but they do not generally demonstrate which of these two kinds of processes, or any other, has accelerated the ions; cases have, however, been found in which the various ion species had equal E/q. (6) At least in disturbed conditions the plasma mantle is not the dominant source of the hot plasma in the ring current/plasma sheet. (7) The nightside magnetopause appears to be a more solid boundary for the heavy ions of ionospheric origin than is expected from the open magnetosphere model. (8) In other respects the observations in the deep nightside mantle (far from the flanks) are in general in reasonable agreement with the open magnetosphere model, if this is amended with ‘additional processes’ which frequently increase the ionospheric ion content in the mantle.

Ice Initiation in Unmixed Updraft Cores in Northeast Colorado Cumulus Congestus Clouds

Ice particle concentrations have been measured from the NOAA/NCAR Explorer sailplane in unmixed and mixed updraft regions within northeast Colorado cumulus congestus clouds, and compared with the concentrations predicted from measured ice nucleus spectra. The clouds investigated were “cold, continental” cumulus with droplet populations of ∼1000 cm−3 and cloud base temperatures between +7 and −7°C. The concentrations of ice particles within unmixed updraft regions, exclusive of ice particles so large that they almost certainly entered the unmixed region by sedimentation or recycling, are consistent with those expected on the basis of the ice nucleus spectra, suggesting that primary ice nucleation is the dominant mechanism active within these regions. Millimetric size ice particles found within unmixed updraft cores presumably enter through sedimentation or recycling. The data do not indicate an important role for ice multiplication in these clouds.