Kinds Of Fronts Research Articles

Physics of ablative Rayleigh–Taylor and Landau–Darrieus instabilities

An analysis of the linear instability of Rayleigh–Taylor instability in ablation fronts and of Landau–Darrieus instability in laminar flames is performed by means of a physical model that allows for identifying the mechanisms that control the stability of both kinds of fronts. The stability behavior of each front is shown to be determined by the particular process of energy transport that drives it. The evolution of perturbations due to the instability is found to always lead to a change in the temperature gradients, but this effect gives place to a restoring force only if the driving mechanism is sensitive to this change, such as happens in ablation fronts driven by thermal conduction. In flame fronts, the driving mechanism is not sensitive to perturbations in the temperature gradient, but, instead, it is sensitive to the temperature perturbations. The latter give place to a force that induces the well known instability in the flame front even in the absence of a gravitational field. The force driving the flame instability as well as the force providing stabilization to an ablation front, are both obtained from the same theoretical framework. The stabilizing role of the lateral thermal conduction for short perturbation wavelengths is also analyzed.

Opady frontowe na polskim wybrzeżu Bałtyku

The climate of the Baltic coastline is strongly affected by prevailing cyclones and frontal activities. Frontal precipitation is shown to dominate in the Baltic area, with about two–thirds of rainfall events being frontal in origin (Walther and Bennartz, 2006). The aim of the work described here was to study precipitation totals in relation to selected atmospheric fronts and days without fronts. The analysis took in the daily precipitation totals recorded during both the cold seasons (October–March) and the warm ones (April-September) in the years 1998–2003, at seven stations along the Baltic coast (i.ee. Szczecin, Świnoujście, Koszalin, Łeba, Hel, Gdynia and Elbląg). The lower weather charts from the years 1998–2003 published by wetterzentrale (www.wetterzentrale.de/topkarten) formed a basis upon which to state the occurrence of different kinds of fronts. Irrespective of kind, the occurrence of a front causes an increase in precipitation totals, while there is a decrease on days without fronts (Table 1). The t-Student test shows that the influence of cold fronts on the precipitation pattern is more important in a warm season than in a cold one, while the effect of warm fronts is greater in a cold season. The highest mean daily precipitation totals along the Polish coast relate to the passage of an occluded front (Tab. 1, Tab. 2). Extreme high totals for precipitation are associated with cold fronts. There are single events with very high totals on days without fronts (Fig. 1) – 27.1 mm was the maximum daily precipitation total (mean for seven stations) recorded for days analyzed in the study. It was noted on September 13th 1999 and was associated with convective rainfall in conditions of a barometric trough. The breakdown of precipitation totals was also studied by size category, with the frequency of occurrence of daily totals within selected size intervals being presented (Fig. 2). The most frequent daily totals were of between 0.01 and 0.50 mm. The probability of moderate precipitation totals (in the 1.01–5.00 range) is higher in the presence of occluded fronts than when warm or cold fronts are passing through. Mean numbers of stations experiencing precipitation as selected fronts pass through were also calculated, as was the frequency of occurrence of precipitation at all stations in days with or without fronts (Figs. 3, 4).

AT WAR

Technological supremacy means that the West can avoid getting up close and personal to kill. Our legacies of imperial power ensure that our wars are fought ‘over there’, and men in post-industrial societies are disassociated from the death embracing culture of the warrior. However, in the aftermath of September 11, we have entered a new era of global militarization – the shadowed world of the War on Terror. It confronts us with the historical changes in men's relationship to war, and how masculinity has been shaped by the processes of formalization and informalization. It opens up a new kind of front, with an enemy who is claimed to be everywhere, and a military-market complex that is denationalizing war in its promotion of a neo-liberal imperialism.

Modelling evaporation fronts with reactive Riemann solvers

This work deals with the modelling of permeable fronts and the building of a numerical method allowing the multi-dimensional propagation of such fronts. A particular attention is given to evaporation waves that appear in cavitating systems. These ones are considered as discontinuities through which a non-equilibrium liquid turns to a liquid–vapor mixture at thermodynamic equilibrium. Such transformation occurs at finite rate. In order to determine this kinetics, the evaporation front is assumed to propagate at the maximum admissible speed corresponding to the Chapman–Jouguet deflagration point [J.R., Simões-Moreira, J.E., Shepherd, Evaporation waves in superheated dodecane, J. Fluid Mech. 382 (1999) 63–86]. Using this particular kinetic relation, Rankine–Hugoniot relations are closed at such fronts. Then it is possible to solve the associated reactive Riemann problem. However, another difficulty is present to solve the multi-dimensional propagation of permeable fronts. This kind of front is subsonic and a conventional averaging scheme (such as Godunov scheme) is inappropriate. To overcome this difficulty, the reactive Riemann problem solution is embedded into the discrete equations method (DEM) [R., Abgrall, R., Saurel, Discrete equations for physical and numerical compressible multiphase mixtures, J. Comp. Phys. 186 (2003) 361–396; R., Saurel, S., Gavrilyuk, F., Renaud, A multiphase model with internal degrees of freedom: application to Shock–Bubble Interaction, J. Fluid. Mech., 495 (2003) 283–321]. This numerical method necessitates deep extensions that are detailed herein. Numerical results are shown and validated over experimental data. Some examples show that the same method may be applied to the propagation of detonation fronts.

Dynamics of Cool Fronts in Optically Thin Hot Plasmas

Steady cool fronts supported by nonsaturated thermal conduction in optically thin plasmas are analyzed. Explicit relations for the pressure (p2), number density (n2), temperature (T2), and flow velocity (v2) behind the front are found as functions of the temperature T1 and the intake Mach number M1 ahead of the cool front. Weak cool fronts may exist only for intake velocities less than the isothermal sound speed. Behind these kinds of fronts, the flow retards, cools down, and is compressed. Strong cool fronts exist for intake Mach numbers greater than a threshold value depending on T1. In these fronts, two compression branches appear, a high-compression branch well below the isotherm for which n1/n2 < (γ - 1)/(γ + 1) and a weak-compression branch, close to (but below) the isotherm. In particular, in the high-compression branch for low values of M1 quasi-isobaric cool fronts occur when the ratio n1/n2 is close to the maximum value, where this branch just emerges, beyond which the pressure of the flow behind the front increases when n1/n2 decreases. The implications of these results on the formation of cool condensations in the hot interstellar medium gas and in the solar atmosphere are outlined.

Fronts and interfaces in bistable extended mappings

We study the time evolution of the interfaces in one-dimensional bistable extended dynamical systems with discrete time. The dynamics are governed by the competition between a local piecewise affine bistable mapping and any couplings given by the convolution with a function of bounded variation. We prove the existence of travelling wave interfaces, namely fronts, and the uniqueness of the corresponding selected velocity and shape. This selected velocity is shown to be the propagating velocity for any interface, to depend continuously on the couplings and to increase with the symmetry parameter of the local nonlinearity. We apply the results to several examples including discrete and continuous couplings, and the dynamics of planar fronts in multidimensional coupled map lattices. We use our technique to study the existence of other kinds of fronts. Finally we consider a more general class of bistable extended mappings for which the couplings are allowed to be nonlinear and the local map to be smooth.

Observations of sea-breeze fronts near the shoreline

Results from an observational study of sea-breeze fronts as they cross a shoreline are presented. Two kinds of fronts are analyzed, one with an offshore regional wind and one without. Their structure is found to be substantially different, the former being steeper and having stronger gradients. Measurements of the profiles of the vertical component of the wind speed, its standard deviation and the structure parameter for temperature are presented along with time series of the structure parameters for water vapor pressure and wind speed. The vertical wind component, w, is found to be of the order of 1.0–1.5 ms−1 in the front zone of the sharp front but only 5 as large in the weaker front. The usual height variation laws under convective conditions are found to apply for both the vertical velocity variance and the temperature structure parameter, which in conjunction with the appropriate spectra indicate that local equilibrium is re-established fairly quickly after the passage of the front. Substantial differences have also been noted in the values of the structure parameters before and after the front, especially in the water vapor pressure and wind speed, differences which are of dissimilar magnitude and sign for the two kinds of fronts.

On the Fields of Wind and Temperature over Japan and Adjacent Waters during Winter of 1950-1951

First, a mean cross section along the meridian 140° E during three months (Dec. 1950—Feb. 1951) is discussed. The main characteristics are: 1) An extremely strong subtropical jet (75 m sec −1 ) situated at 33° N near 190 mb; 2) A semi-stationary sloping surface which divides almost barotropic warm tropical air in the south from more or less baroclinic temperate air situated in the north; 3) A slight indication of a polar-front jet situated near 230 mb at 41° N; the weak intensity of this jet depends upon the strong variability in position of the polar front. 4) Much warmer temperature in the upper troposphere to the south, and much colder temperature in the lower troposphere north of the subtropical jet, as compared with mean conditions at 76° E and 80° W. The computed mean winds at 140° E are combined with the mean isotachs given by Yeh over China, at the 12-km level. Using the acceleration determined from the isotach pattern, the mean angle of cross-contour flow at 200 mb is determined to be about 8° (between longitudes 110° and 140° E), with a mean cross-contour component of 8 in sec −1 toward lower heights, in the center of the mean jet stream. Second, daily meridional cross sections through Japan are presented for the period 8 to 15 December 1950. These show an extremely strong individual jet stream in which the observed maximum wind speed reached a velocity of more than 110 m sec −1 , with near-zero absolute vorticity to the south. An associated nearly-isothermal layer underneath is much deeper (up to 4 and even 6 km) than in other parts of the globe. It is suggested that this unusual depth results from a combination of a stable layer associated with the semi-stationary subtropical jet, and the polar front, when the latter moves southward in east Asia. The behavior of the tropopause in the intermediate atmosphere south of the polar front during this series suggests subsidence of the tropopause and its transformation into the lower boundary of a deep stable layer which is transformed into some kind of front separating the real tropical air from a more temperate air moving in from the regions north of the Himalayas. DOI: 10.1111/j.2153-3490.1953.tb01066.x

What Kind of Front Are You?

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