Gravitational Fragmentation in Expanding Shells

1997 ◽  
Vol 166 ◽  
pp. 409-412
Author(s):  
Ch. Theis ◽  
S. Ehlerová ◽  
J. Palouš ◽  
G. Hensler
Keyword(s):  
Density Gradient ◽  
Time Scale ◽  
Sound Speed ◽  
Gravitational Instability ◽  
Ambient Medium ◽  
Injection Rate ◽  
Gas Density ◽  
Typical Size ◽  
Evolving Systems ◽  
Expanding Shells

AbstractWe investigate the gravitational fragmentation in expanding shells by applying an instability ’thermometer’ similar to the Toomre parameter for instabilities in self-gravitating disks. For Sedov–like evolving systems the onset of instability is mainly depending on the density of the ambient medium and the sound speed of the shell matter, whereas the energy injection rate is less important. Shells evolve towards gravitational instability, if the density gradient of the ambient medium is shallower than an isothermal profile, otherwise they become more stable. For density gradients flatter than ∝ r−1, the fragmentation becomes non-linear on the same time scale as the gravitational instability needs to start. In a homogeneous ambient medium the typical size of gravitationally unstable shells is 1 kpc for a gas density of n = 1 cm−3 and decreases to 10 pc for n = 104 cm−3.

Synoptic time-scale variability of sediment temperature profile and sound speed in shallow seas derived from in situ observations

2020 ◽  
Vol 244 ◽  
pp. 106932
Author(s):  
Guang-Bing Yang ◽  
Quanan Zheng ◽  
Xiaomin Hu ◽  
De-Jing Ma ◽  
Zhao Chen ◽  
...  
Keyword(s):  
Temperature Profile ◽  
Time Scale ◽  
Sound Speed ◽  
Sediment Temperature ◽  
In Situ Observations ◽  
Shallow Seas

The effect of compaction of a porous material confiner on detonation propagation

2017 ◽  
Vol 834 ◽  
pp. 434-463 ◽  
Author(s):  
Mark Short ◽  
James J. Quirk
Keyword(s):  
Relaxation Time ◽  
Porous Material ◽  
Time Scale ◽  
Sound Speed ◽  
Volume Fraction ◽  
Relaxation Rates ◽  
Slab Geometry ◽  
Local Flow ◽  
Detonation Propagation ◽  
Detonation Speed

The fluid mechanics of the interaction between a porous material confiner and a steady propagating high explosive (HE) detonation in a two-dimensional slab geometry is investigated through analytical oblique wave polar analysis and multi-material numerical simulation. Two HE models are considered, broadly representing the properties of either a high- or low-detonation-speed HE, which permits studies of detonation propagating at speeds faster or slower than the confiner sound speed. The HE detonation is responsible for driving the compaction front in the confiner, while, in turn, the high material density generated in the confiner as a result of the compaction process can provide a strong confinement effect on the HE detonation structure. Polar solutions that describe the local flow interaction of the oblique HE detonation shock and equilibrium state behind an oblique compaction wave with rapid compaction relaxation rates are studied for varying initial solid volume fractions of the porous confiner. Multi-material numerical simulations are conducted to study the effect of detonation wave driven compaction in the porous confiner on both the detonation propagation speed and detonation driving zone structure. We perform a parametric study to establish how detonation confinement is influenced both by the initial solid volume fraction of the porous confiner and by the time scale of the dynamic compaction relaxation process relative to the detonation reaction time scale, for both the high- and low-detonation-speed HE models. The compaction relaxation time scale is found to have a significant influence on the confinement dynamics, with slower compaction relaxation time scales resulting in more strongly confined detonations and increased detonation speeds. The dynamics of detonation confinement by porous materials when the detonation is propagating either faster or slower than the confiner sound speed is found to be significantly different from that with solid material confiners.

scholarly journals Star Formation in Expanding Shells: When and Where

1997 ◽  
Vol 166 ◽  
pp. 413-416
Author(s):  
S. Ehlerová ◽  
J. Palouš ◽  
Ch. Theis ◽  
G. Hensler
Keyword(s):  
Star Formation ◽  
Interstellar Medium ◽  
Analytical Model ◽  
Computer Simulations ◽  
Sound Speed ◽  
Finite Thickness ◽  
Gaseous Disk ◽  
Expanding Shells

AbstractThe fragmentation of expanding shells and subsequent star formation are analyzed using an analytical model and computer simulations. We discuss the role of the sound speed in the ambient interstellar medium and the influence of the finite thickness of the gaseous disk.

scholarly journals Gravitational instability of expanding shells

2001 ◽  
Vol 374 (2) ◽  
pp. 746-755 ◽  
Author(s):  
R. Wünsch ◽  
J. Palouš
Keyword(s):  
Gravitational Instability ◽  
Expanding Shells

scholarly journals On the Acoustical Implications of Vortex Shedding from an Exhaust Pipe

1981 ◽  
Vol 103 (4) ◽  
pp. 378-384 ◽  
Author(s):  
S. W. Rienstra
Keyword(s):  
Sound Speed ◽  
Ambient Medium ◽  
Jet Flow ◽  
Edge Condition ◽  
Exhaust Pipe ◽  
Ambient Flow ◽  
Reflected Wave ◽  
Radiated Sound ◽  
Subsonic Jet ◽  
Radiated Sound Power

Asymptotic approximations for small Strouhal number are derived for the solution of the problem of the interaction between an acoustic wave and a subsonic jet flow issuing from a semi-infinite pipe. Density and sound speed differences between the jet flow and the (slowly moving) ambient medium, and a general edge condition are included. The approximations relate to the field inside the jet flow, to the far field, to the reflection coefficient, end-impedance and end correction for the reflected wave inside the pipe, and to the transmitted and radiated sound power. Within the range of parameters considered, the effect of the density and sound speed differences and ambient flow is found to be appreciable, although the character of the solution is not changed. However, the choice of the edge condition does have important implications; specifically, the phase of the reflected wave is most sensitive to only slight deviations from the Kutta condition.

The gas density gradient for three dark interstellar clouds

1984 ◽  
Vol 287 ◽  
pp. 723 ◽  
Author(s):  
S. A. Fulkerson ◽  
F. O. Clark
Keyword(s):  
Density Gradient ◽  
Gas Density ◽  
Interstellar Clouds

scholarly journals The Fragmentation of Expanding Shells: Cloud Formation Rates and Masses

2002 ◽  
Vol 207 ◽  
pp. 675-677 ◽  
Author(s):  
Jan Palouš ◽  
Richard Wünsch ◽  
Soňa Ehlerová
Keyword(s):  
Analytical Solution ◽  
Gravitational Instability ◽  
Homogeneous Medium ◽  
Mass Function ◽  
Initial Mass Function ◽  
Cloud Formation ◽  
Initial Mass ◽  
Linear And Nonlinear ◽  
Expanding Shells

The gravitational instability of expanding shells is discussed. Linear and nonlinear terms are included in an analytical solution in the static and homogeneous medium. We discuss the interaction of modes and give the time needed for fragmentation. Masses of individual fragments are also estimated and their formation rates and the initial mass function are derived. Results of simulations are compared to observation.

scholarly journals Seasonal Mesoscale and Submesoscale Eddy Variability along the North Pacific Subtropical Countercurrent

2014 ◽  
Vol 44 (12) ◽  
pp. 3079-3098 ◽  
Author(s):  
Bo Qiu ◽  
Shuiming Chen ◽  
Patrice Klein ◽  
Hideharu Sasaki ◽  
Yoshikazu Sasai
Keyword(s):  
Kinetic Energy ◽  
Density Gradient ◽  
North Pacific ◽  
Time Scale ◽  
Surface Density ◽  
Energy Cascade ◽  
Growth Time ◽  
Dominant Wavelength ◽  
Subtropical Countercurrent ◽  
Positive Vorticity

Abstract Located at the center of the western North Pacific Subtropical Gyre, the Subtropical Countercurrent (STCC) is not only abundant in mesoscale eddies, but also exhibits prominent submesoscale eddy features. Output from a ° high-resolution OGCM simulation and a gridded satellite altimetry product are analyzed to contrast the seasonal STCC variability in the mesoscale versus submesoscale ranges. Resolving the eddy scales of >150 km, the altimetry product reveals that the STCC eddy kinetic energy and rms vorticity have a seasonal maximum in May and April, respectively, a weak positive vorticity skewness without seasonal dependence, and an inverse (forward) kinetic energy cascade for wavelengths larger (shorter) than 250 km. In contrast, the submesoscale-resolving OGCM simulation detects that the STCC eddy kinetic energy and rms vorticity both appear in March, a large positive vorticity skewness with strong seasonality, and an intense inverse kinetic energy cascade whose short-wave cutoff migrates seasonally between the 35- and 100-km wavelengths. Using a 2.5-layer, reduced-gravity model with an embedded surface density gradient, the authors show that these differences are due to the seasonal evolution of two concurring baroclinic instabilities. Extracting its energy from the surface density gradient, the frontal instability has a growth time scale of O(7) days, a dominant wavelength of O(50) km, and is responsible for the surface-intensified submesoscale eddy signals. The interior baroclinic instability, on the other hand, extracts energy from the vertically sheared STCC system. It has a slow growth time scale of O(40) days, a dominant wavelength of O(250) km, and, together with the kinetic energy cascaded upscale from the submesoscales, determines the mesoscale eddy modulations.

scholarly journals Self-Regulated Star-Formation in Galaxies

1996 ◽  
Vol 171 ◽  
pp. 401-401
Author(s):  
J. Köppen ◽  
Ch. Theis ◽  
G. Hensler
Keyword(s):  
Star Formation ◽  
Time Scale ◽  
Star Formation Rate ◽  
Numerical Study ◽  
Ambient Medium ◽  
Formation Rate ◽  
Self Regulation ◽  
Physical Explanation ◽  
Star Formation In Galaxies ◽  
Gas Consumption

In the chemodynamical models of galaxies the energy input from massive stars into the ambient medium results in a self-regulation of the star formation rate (SFR). A thorough analytical and numerical study of this model shows that there is always a strong and negative feed-back, and the SFR becomes almost independent of the assumed stellar birth function (SBF). The time-scale to reach this equilibrium is much shorter than the gas consumption time-scale, hence the models evolve along this solution for most of the time. This mechanism provides a physical explanation for a quadratic dependence of the SFR on gas density. For more details cf. Köppen et al. (1995), A&A 296, 99 and in preparation.

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