Sudden Breakup Research Articles

Structures induced by periodic acoustic excitation of a diffusion flame

Signal recovery techniques have been used to measure the periodic structures formed in a coannular diffusion flame due to acoustic excitation of the flow from upstream. The flow response was sensed using the quantitative schlieren technique, and consistent results were obtained on the basis of both axial and transverse optical beam deflections and appropriate analysis of signal records. The two dominant modes of response were identified as a series of alternating disturbances along the flow axis at the higher frequency, and a series of alternating ring disturbances containing on axis disturbances of opposite sign at the lower frequency. The former mode is essentially associated with the fuel jet shear layer, while the latter is associated with the outer annular shear layer surrounding the air flow from the outer nozzle. The strength of disturbances was consistent with mixing fluctuations between cold fuel gas and the products of combustion. Structures induced in the outer shear layer weakened rapidly with distance from the nozzle, indicating a relatively sudden breakup of coherent structures caused by excitation near to the nozzle.

A Study of Finite Amplitude Barotropic Instability

We report cases with a series of disturbances tilted upshear along the shear zone during the Mei-Yu season. The scale involved is much smaller than the local Rossby radius of deformation; we hypothesize the relevance of barotropic instability, and have explored the nonlinear evolution of barotropic instability. The small and finite amplitude theories are reviewed; their relevance to the observations are briefly discussed. Eigenvalues of ideal three and four region models are calculated analytically. A new Fourier Chebyshev nondivergent barotropic model is constructed. With the initial value problem approach, our experiments on barotropic instability with a vorticity strip either have hyperbolic tangent or Bickerly jet type of wind profiles. We studied the time evolution of the shear layers in terms of the formation of fundamental eddies and successive pairing or merging of eddies. The mutual intensification of counter-propagating Rossby waves (vorticity gradient waves) across the vorticity strip, breakdown of the vorticity strip, local concentration of vorticity and vortex mergering processes were simulated. While the barotropic instability is an efficient way to concentrate vorticity in a small region, the later vortex merging can enlarge the vortex sizes but not the intensity of the resultant vortex. We propose that the concentration and the merging of vortices can create a favorable localized environment within the shear zone for the moist baroclinic processes to operate. As far as the intensity, shape and evolution of the individual vortices are concerned, they are very sensitive to initial background noise(1/100 of mean vorticity). In other words, there is no predictability in the nonlinear evolution. However, the maximum growth rate and the dominant wavelength of vortex can be predicted from the linear analysis. The Bickerly jet possesses higher growth rate than the hyperbolic tangent case on the f plane. We document chaotic behavior of sudden breakup of shear zone and associated vortex merging after a couple of regular cycles of wave-mean flow interaction with shear zone maintained. In the thirty-day integration, the vortices on the β plane become disorganized and scattered while they still remain well organized on the f plane.

A rotating, midday auroral event with northward interplanetary magnetic field

We present ground‐based observations of an isolated, daytime auroral event that was detected above Svalbard by means of an all‐sky TV camera and a multichannel, meridian scanning photometer. This 5‐min event occurred near magnetic noon, poleward of a stable cusp aurora. It emitted ∼20 kR of 557.7‐nm light, with little if any admixture of 630.0‐nm radiation. The structure rotated rapidly then underwent a sudden breakup. The spectral distribution of emitted light indicates that electrons responsible for this auroral structure were accelerated though several kilovolts between the magnetosheath and the ionosphere. Such a potential structure requires that the precipitating, arc electrons constitute a field‐aligned current out of the ionosphere. Current continuity and Ohm's law require radial electric fields within the arc. From the structure's rotational speed, we estimate that the accelerated electrons carried an upward field‐aligned current of order 100µA/m². Satellite observations of polar cap convection patterns suggest that the interplanetary magnetic field had turned northward prior to the event. The auroral structure may be explained as resulting from a transitory magnetic merging of interplanetary and lobe magnetic field lines at and/or a penetration of plasma across the high‐latitude magnetopause.

Buoyancy reversal and cloud-top entrainment instability

In 1980, Randall suggested that cloud top entrainment instability, a runaway positive feedback between mixing, evaporative cooling and entrainment, may cause the transition from the subtropical stratocumulus layers, frequently observed over the cold water of the eastern oceans, to tradewind cumulus. Predicting how the mean position of this transition, and the associated large change in albedo, would vary with, and feed back on, a change in climate is important for any credible model of the climate system. We have used numerical and laboratory simulations, as well as field observations, to study entrainment in buoyancy-reversing systems. We consider a system in which an upper fluid stably overlies a second lower fluid; but some mixtures are less buoyant than either fluid. the laboratory results show that buoyancy reversal does not cause runaway ‘explosive entrainment’ unless D (the ratio of the density difference between the most cooled mixed parcel and the lower fluid to the density difference across the unmixed layers, a measure of CEI potential) exceeds 1.3, which is far greater than the range of D observed in stratocumulus. For D below this threshold, buoyancy reversal leads to weak circulations rather reminiscent of Rayleigh-Bénard convection in the lower layer, but only slightly distorts the interface. For D > 1.3 an entrainment tongue of upper fluid penetrates a sizable fraction of the depth of the lower layer and entrainment is explosively rapid. Numerical experiments suggest that even when 0 < D < 1.3, entrainment can break up idealized subtropical marine stratocumulus within about an hour if radiation and surface fluxes are removed, but comparison of numerical simulations with the laboratory experiments suggests that the numerically simulated entrainment rate may be substantially too large in this regime owing to the inability to accurately produce and resolve the entraining eddies. Our observations of stratocumulus from FIRE typically had a strong stable layer above the cloud top inversions, leading to some ambiguity in defining D, but clearly show that stable stratocumulus layers often have 0 < D < 0.2. We suggest that sudden breakup of stratocumulus due to runaway entrainment is unlikely, but evaporatively enhanced entrainment may aid the transition from stratocumulus to tradewind cumulus in conjunction with some other mechanism, such as short-wave absorption by liquid water, that can help dynamically decouple the cloud layer from the surface layer.

Buoyancy reversal and cloud‐top entrainment instability

AbstractIn 1980, Randall suggested that cloud top entrainment instability, a runaway positive feedback between mixing, evaporative cooling and entrainment, may cause the transition from the subtropical stratocumulus layers, frequently observed over the cold water of the eastern oceans, to tradewind cumulus. Predicting how the mean position of this transition, and the associated large change in albedo, would vary with, and feed back on, a change in climate is important for any credible model of the climate system.We have used numerical and laboratory simulations, as well as field observations, to study entrainment in buoyancy‐reversing systems. We consider a system in which an upper fluid stably overlies a second lower fluid; but some mixtures are less buoyant than either fluid. the laboratory results show that buoyancy reversal does not cause runaway ‘explosive entrainment’ unless D (the ratio of the density difference between the most cooled mixed parcel and the lower fluid to the density difference across the unmixed layers, a measure of CEI potential) exceeds 1.3, which is far greater than the range of D observed in stratocumulus. For D below this threshold, buoyancy reversal leads to weak circulations rather reminiscent of Rayleigh‐Bénard convection in the lower layer, but only slightly distorts the interface. For D &gt; 1.3 an entrainment tongue of upper fluid penetrates a sizable fraction of the depth of the lower layer and entrainment is explosively rapid. Numerical experiments suggest that even when 0 &lt; D &lt; 1.3, entrainment can break up idealized subtropical marine stratocumulus within about an hour if radiation and surface fluxes are removed, but comparison of numerical simulations with the laboratory experiments suggests that the numerically simulated entrainment rate may be substantially too large in this regime owing to the inability to accurately produce and resolve the entraining eddies. Our observations of stratocumulus from FIRE typically had a strong stable layer above the cloud top inversions, leading to some ambiguity in defining D, but clearly show that stable stratocumulus layers often have 0 &lt; D &lt; 0.2. We suggest that sudden breakup of stratocumulus due to runaway entrainment is unlikely, but evaporatively enhanced entrainment may aid the transition from stratocumulus to tradewind cumulus in conjunction with some other mechanism, such as short‐wave absorption by liquid water, that can help dynamically decouple the cloud layer from the surface layer.

The effect of some additives on the closed bomb burning and ignitability of RDX/TNT (60/40)

Abstract The burning and ignition properties of Composition (Comp) B are studied so that formulation modification could be made that would reduce gun projectile in-bore thermal explosions. Comp B first burns on its surface like a gun propellant; then, at a pressure near 100 MPa, a sudden breakup burning phase develops. New surface area is created at a phenomenal rate, and this is the cause of the catastrophic reaction rates that it can experience. Coating its RDX constituent with a protectant significantly slows this reaction rate. It appears to do this by inhibiting the rate at which the RDX is released from the TNT matrix for burning, probably by infinitesimally delaying the ignition of the RDX in the particle ignition sequence. A new test that evaluates explosive ignitability as a function of pressure and energy indicates that an RDX protective coating may also increase the level of thermal ignition and delay the onset of a thermal explosion. Collectively, these three desirable properties, caused by a single modification, may significantly reduce the incidence of in-bore explosions where casting flaws would have precipitated this event.

Stratospheric mixing from radioactive fallout

The radioactive fall-out during 1959 shows that the Russian October 1958 debris dominated the northern hemisphere and came down with a residence time corresponding to about 8 months. The rest of the world had a steady fall-out rate of about 0.06 mc/mi/sup 2//in. of rain throughout the year, whereas values for the northern hemisphere came down to this value in the months of September, October, and November when the Russian debris was apparently exhausted. Thus. it appears that there is a world-wide fall-out rate of about 0.06 mc/mi/sup 2//in. of rain which is nearly independent of the Russian polar debris and which is probably from the equatorial tests. Using the estimate of ihe amount of material injected into the stratosphere, this world-wide rate of fall-out corresponds to a residence time of about 5 years for equatorial injections. Goldie suggested that the meridional circulation at levels well within the stratosphere consists of an up-welling of air at the equator. followed by a polar fiow, a down-welling in the polar region, and an equatorward movement above the troposphere at approximately 24 km. In addition, a seasonal perturbation of this circulation occurs in the long winter night near the pole. It results inmore » an accumulation of material at high levels near the pole, until the sun returns in the spring, when a sudden break-up occurs and the material is rapidly carried toward the troposphere. This model explains quite well most of the present observations on radioactive fall- out. The polar debris would Naturally have a shorter residence time because it would be introduced in the last steps of the cycle, the equatorial would have a longer residence time, having been introduced at the beginning of the cycle, and the spring breakup of the polar vortex would account for the well-known spring maximum in the rate of radioactive fall-out. The residence times of material injected near the equator and the pole, respectively, permit a quantitative estimate of the intensity and depth of the circulation to be made. The meridiomal velocity deduced for the equatorially directed low stratospheric current is about l to 3 miles per hour, whereas that for the poleward high stratospheric current is about one-tenth as large. This requires, of course, that 90% of the stratospheric air be concentrated in the high brand. (auth)« less

Military Occupation and German Revolution

In discussing the problems of military government, some writers have recalled critically the fact that in 1918 the armies of occupation did not coöperate with the workers' and soldiers' councils in Germany. Did this really mean that the commanding officers hampered Germany's transition from a half-absolutist monarchy to a democratic republic? What was the position of the workers' and soldiers' councils in the total population? How did the occupation armies act toward the democratic movements in Germany? As one of the leaders of the revolution against the Emperor, and as a close observer of the foreign armies in my home province, the Rhineland, I may be justified in presenting some facts and experiences.When on November 11 I read in Paragraph V of the armistice agreement that “the left bank of the Rhine shall be administered by the local authorities under the control of the troops of occupation,” I informed my friends in the Cologne workers' and soldiers' council that this undoubtedly meant the end of that council on the day of the arrival of the foreign troops. It was clear that we were no “local authorities.” Nowhere had the councils replaced the civil servant administration of the former régime. Why not? Simply because the social-democratic labor movement, which alone backed the councils, was in most places, on the left bank of the Rhine, a small minority. Only by using terror against a large majority of the population could we have eliminated the existing civil servant administration. Such terror was against the democratic principles and traditions of our movement. Incidentally, even the wildest radicals knew in those days of threatening chaos that a sudden break-up of a centuries-old civil servant administration would mean grave disaster, especially in shipping the necessary supplies to the exhausted population.