Plane turbulent mixing between two streams of different gases (especially nitrogen and helium) was studied in a novel apparatus. Spark shadow pictures showed that, for all ratios of densities in the two streams, the mixing layer is dominated by large coherent structures. High-speed movies showed that these convect at nearly constant speed, and increase their size and spacing discontinuously by amalgamation with neighbouring ones. The pictures and measurements of density fluctuations suggest that turbulent mixing and entrainment is a process of entanglement on the scale of the large structures; some statistical properties of the latter are used to obtain an estimate of entrainment rates. Large changes of the density ratio across the mixing layer were found to have a relatively small effect on the spreading angle; it is concluded that the strong effects, which are observed when one stream is supersonic, are due to compressibility effects, not density effects, as has been generally supposed.

The authors claim that their data do not confirm the results of Gordon and Witting [1977], who noted that velocity fluctuations measured in a tidal boundary layer reflected contributions from a large-scale, quasi-ordered motion and a 'background of relatively isotropic turbulence of smaller scale.' Gordon and Witting's observation is similar to numerous observations made in the laboratory turbulent boundary layer. The back or upstream side of the large-scale motion is easily distinguishable by a relatively sudden change [e.g., Chen and Blackwelder, 1978; Phong-anant et al., 1980] in temperature in boundary, layers with neutral and nonzero stratification. This been apphed. In this latter reference, significant correlation was found between the instantaneous shear stress at the smooth wall and u'w' at higher levels; it seems reasonable to expect a similar correlation between the instantaneous boundary shear stress that controls the sediment movement and u'w' at other levels in the river. At least six negative excursions in u'w' of the order of 20-30 u'w', appear simultaneously on all four traces of Figure 15 but the time scale in this figure (and in Figure 14) is inadequate to establish vertical coherence for these excursions and establish a relation with the large-scale motion. A time scale similar to that used in FigUre 16 would sudden change accompanies a rapid change in fluctuations of have been more appropriate for Figures 14 and 15. On the u', w', or the product u'w'. It also seems likely that the bursting phenomenon observed near a smooth wall and other 'events' mentioned by the authors are perhaps best interpreted as the signature of the large-scale motion. It is difficult to determine the precise contribution of this motion to u'w' as this determination would require accurate estimates of both the frequency and of the ensemble averaged distribution of u'w' associated with the large-scale motion. The correlations in Figure 16 are not inconsistent with the notion of a large-scale motion inclined in the direction of tl'ie shear to the river bottom, in as much as u' at higher levels leads u' at levels nearer the river bottom. There is also little doubt that enhanced correlations between u' (or w', u'w') at different levels would be obtained if an enhancement technique similar to that used by Brown and Thomas [1977] had Copyright ¸ 1981 by the American Geophysical Union. basis of the evidence presented, it is difficult to accept the claim by the authors that their data do not confirm the results of Gordon and Witting [1977] and others. REFERENCES Chen, C.-H. P., and R. F. Blackwelder, Large-scale motion in a turbulent boundary layer: A study using temperature contamination, J.

The physical mechanisms for vortex breakdown which, it is proposed here, rely on the production of a negative azimuthal component of vorticity, are elucidated with the aid of a simple, steady, inviscid, axisymmetric equation of motion. Most studies of vortex breakdown use as a starting point an equation for the azimuthal vorticity (Squire 1960), but a departure in the present study is that it is explored directly and not through perturbations of an initial stream function. The inviscid equation of motion that is derived leads to a criterion for vortex breakdown based on the generation of negative azimuthal vorticity on some stream surfaces. Inviscid predictions are tested against results from numerical calculations of the NavierStokes equations for which breakdown occurs.

A turbulent mixing layer in a water channel was observed at Reynolds numbers up to 3 x los. Flow visualization with dyes revealed (once more) large coherent structures and showed their role in the entrainment process; observation of the reaction of a base and an acid indicator injected on the two sides of the layer, respectively, gave some indication of where molecular mixing occurs. Autocorrelations of streamwise velocity fluctuations, using a laser-Doppler velocimeter (LDV) revealed a fundamental periodicity associated with the large structures. The surprisingly long correlation times suggest time scales much longer than had been supposed; it is argued that the mixing-layer dynamics at any point are coupled to the large structure further downstream, and some possible consequences regarding the effects of initial conditions and of the influence of apparatus geometry are discussed.

No abstract

The properties of axisymmetric accretion ows of cold adiabatic gas with zero total energy in the vicinity of a Newtonian point mass are characterized by a single dimensionless parameter, the thickness of incoming ow. In the limit of thin accretion ows with vanishing thickness, we show that the governing equations become self-similar, involving no free parameters. We study numerically thin accretion ows with nite thickness as well as those with vanishing thickness. Mass elements of the incoming ow enter the computational regime as thin rings. In the case with nite thickness, after a transient period of initial adjustment, an almost steady-state accretion shock with a small oscillation amplitude forms, con rming the previous work by Molteni, Lanzafame, & Chakrabarti (1994). The gas in the region of vorticity between the funnel wall and the accretion shock follows closed streamlines, forming a torus. This torus, in turn, behaves as an e ective barrier to the incoming ow and supports the accretion shock which re ects the incoming gas away from the equatorial plane. The postshock ow, which is further accelerated by the pressure gradient behind the shock, goes through a second shock which then re ects the ow away from the symmetry axis to form a conical outgoing wind. As the thickness of the in owing layer decreases (or if the ratio of the half thickness to the distance to the funnel wall along the equatorial plan is smaller than 0:1), the ow becomes unstable. In the case with vanishing thickness, the accretion shock formed to stop the incoming ow behind the funnel wall oscillates quasi-periodically with an amplitude comparable to the thickness. The structure between the funnel wall and the accretion shock is destroyed as the shock moves inwards toward the central mass and re-generated as it moves outwards. We suggest a possible explanation for the instability. The phenomenon may be related to the quasi-periodic oscillations observed in accreting galactic sources.

No abstract

The effects of trace amounts of Fe and Cu in p-and n-type silicon were investigated with microwave photoconductance decay and surface photovoltage. The wafers received controlled amounts of surface contamination of Fe and Cu that are relevant for ultralarge scale integrated technologies. The substrate doping type has a strong impact on the effect of the metallic impurities. Fe, as expected, strongly degrades the minority carrier lifetime of p-type substrates. On the other hand, the impact of Fe on n-type silicon is at least one order of magnitude lower than on p-type. In contrast, Cu is highly detrimental to n-type material, but has no significant impact on the minority carrier properties of p-type silicon for the contamination levels studied.