Compression Front Research Articles

Observations of the emergence of detonation from a turbulent flame brush

Experimental schlieren images and pressure records are presented from tests where a detonation was observed to emerge from a turbulent flame bursh. This paper describes how an initial flame front formed following autoignition behind a reflected shock wave at a point away from the reflecting end wall. The test gas was stoichiometric ethylene/oxygen diluted with 75% argon. Typical relfected shock pressures and temperatures were 110 kPa and 947 K, respectively. This paper then describes, in detail the sequence of flame propagation, compression wave formation, and eventual onset of detonation events that were observed. Estimates of flame front velocity and changes in the velocity of unburned ahead, of the flame gas are estimated from the schlieren images and found to be of the order of serveral hundreds of meters per second, respectively. Just prior to ignition, the flame accelerates rapidly forming a steep compression front at which point a detonation is seen to form. This paper discusses factors that could contribute to weak or non-ideal ignition away from the wall and considers how chemical and gas dynamic processes could contribute to the final transition to, detonation event. Possible factors considered are spatial variations in extent of pre-exothermic multistep chemical reactions and propagation of a flame front through such a region.

Blind hood effects on the compression wave generated by a train entering a tunnel

The running of high-speed trains on railway networks creates strong transient flows in tunnels. A large number of solutions have been proposed to reduce the amplitude of the pressure gradients in tunnels and one of the most efficient solutions consists in the addition of a hood before the tunnel entrance. The present paper provides a detailed experimental study on the effects of blind hood (hood without any perforation) with constant section on the generation of a compression wave. An axisymmetric reduced scale (1/140th) experimental apparatus was used and the train Mach number was in the range from 0.06 to 0.15. Results showed that the initial compression front can be split into several smaller fronts in the presence of a blind hood. The number of fronts depends on the respective length of the hood to the train nose length and only slightly on the train Mach number. The characteristics of each of these fronts (amplitude and gradient) can be predicted for given train, tunnel and hood. Furthermore the present analysis showed that, for a fixed train/tunnel blockage ratio, there exists a unique optimum hood for which the pressure gradient can be minimized. For the usual train/tunnel blockage ratio (around 0.2), the maximum reduction of the pressure gradient was proved not to exceed 3. The exact value of the optimum pressure gradient depends on both the train/tunnel blockage ratio and the train nose geometry.

Structure and kinematics of the South Iberian paleomargin and its relationship with the Flysch Trough units: extensional tectonics within the Gibraltar Arc fold-and-thrust belt (western Betics)

The superposition of structures observed in a selected area of the external zones of the Betic orogen, situated in the westernmost segment of the peri-Mediterranean mountain belt, reveals an alternation of compressional and extensional events during the overall convergence of the African and Eurasian plates that bound the system. An Early Miocene compressional event produced the formation of the so-called Gibraltar Arc fold-and-thrust belt, formed by the units derived from the South Iberian paleomargin and the Flysch Trough. This wedge constitutes the footwall of the Gibraltar Thrust, whose hanging wall corresponds to the Alboran Domain. The outward migration of the compressional front (W-to-NWward in the South Iberian segment) was followed by the outward migration of the extensional locus, related to the Middle Miocene rifting that led to the Alboran Basin subsidence. The tectonic inversion of the Gibraltar Thrust was accompanied by normal faulting of the innermost part of the paleomargin and the Flysch Trough units. Finally, the Alboran region underwent N–S-to-NW–SE compression from Late Miocene onwards, which caused the uplift of the Gibraltar Arc area.

Where Are the High-Velocity Clouds?

Recent observations of high velocity clouds (HVCs) have revealed compression fronts and tail shaped features of HI suggesting that they are interacting with external medium. We perform 3-D hydro-dynamical simulations of HVCs moving through a diffuse hot gaseous component, investigating the behaviour of both extra-galactic dark matter dominated HVCs and nearby pure gas clouds that may be accreting onto the Galactic disk via a Galactic fountain. Both scenarios can give rise to similar features as observed if the external medium has a density $>10^{-4}$ cm$^{-3}$. Observations suggest that this may be too high for a hot ionised halo or intergalactic gas and supports the Galactic fountain origin, a model that can also account for the high fraction of HVCs with tails and asymmetrical morphologies.

Weakly nonlinear internal wave fronts trapped in contractions

The propagation of weakly nonlinear, long internal wave fronts in a contraction is considered in the transcritical limit as a model for the establishment of virtual controls. It is argued that the appropriate equation to describe this process is a variable coefficient Korteweg–de Vries equation. The solutions of this equation are then considered for compressive and rarefaction fronts. Rarefaction fronts exhibit both normal and virtual control solutions. However, the interaction of compressive fronts with contractions is intrinsically unsteady. Here the dynamics take two forms, interactions with the bulk of the front and interactions with individual solitary waves separating off from a front trapped downstream of the contraction.

Plio-Quaternary paleostress regimes and relation to structural development in the Kertch–Taman peninsulas (Ukraine and Russia)

In the Kertch–Taman peninsulas, located between the Crimean and NW-Caucasus Mountains, deep troughs developed since Oligocene time. Sediments were subsequently folded. Folding was associated with thrusting. Paleostress analyses allowed constraint of the tectonic evolution of the area. An extensional event occurred before folding, perhaps related to the trough development. This event was followed by compression associated with Plio-Quaternary folding. Stress tensors determined in older rocks in the Crimean and NW-Caucasus belts are correlated with this regional Plio-Quaternary compression. In detail, N–S compression followed NW–SE compression. This change in direction could be explained by a change of the direction of convergence of the Black Sea Plate with the Scythian Plate. The NW–SE compression induced NE–SW folds in the southern part of the peninsulas. The N–S compression corresponds to E–W folds of the northern part, suggesting a northward propagation of the deformation front. The structural evolution of the Kertch–Taman region is controlled by (1) pre-existing structures, (2) a change in the direction of compression related to a change of the direction of convergence of the Black Sea with the Scythian Plate and (3) the northward propagation of the compressional front with the continuing indentation of the Black Sea block.

Tight-Binding Molecular Dynamics of Shock Waves in Methane

The behavior of shock-compressed methane at high temperatures and pressures is studied using nonequilibrium molecular dynamics and linear-scaling tight-binding electronic structure theory in simulations containing as many as 1728 molecules. For certain piston velocities, a chemical dissociation wave evolves that lags behind the compressive shock front. At about 1 ps, the dissociation region consists mainly of molecular hydrogen and hydrocarbon polymers. Shock wave experiments, which access much longer time scales, suggest that the hydrocarbons ultimately decompose into elemental carbon.

On the processes in the terrestrial magnetosheath: 2. Case study

We test a new scheme to study the magnetosheath. The scheme uses the solar wind measurements as the input into the gasdynamic convected field model, and the model output is compared with magnetosheath observations. In our four test cases there is a significant overall success in the model prediction. This scheme works better than other methods in magnetosheath studies and is potentially useful for space weather forecasts and nowcasts. The direction of the magnetic field is modeled most accurately. The prediction of the size of the magnetosphere is accurate within a few percent. The predicted thickness of the magnetosheath is accurate up to 90%. With a double‐normalization procedure developed in this study, we are able to separate the processes intrinsic in the magnetosheath from those due to large‐scale upstream temporal variations. The test cases confirm the existence of a compressional front one third of the distance from the magnetopause to the bow shock near the stagnation streamline. The magnetosheath density profile near the stagnation streamline is consistent with the models that add a compressional front between the two depletion processes described by the plasma depletion model. A major unexpected feature is that the magnetosheath flow pattern is very different from that described by the model and maybe by most other models, including MHD models. The magnetosheath flow near the stagnation streamline does not slow down gradually toward the stagnation point. It moves rapidly until reaching a very small region near the magnetopause.

Active stress along the ne external margin of the Apennines: the Ferrara arc, northern Italy

We have analysed borehole breakout data from 12 deep wells in order to constrain the direction of the minimum and maximum horizontal stress in a part of the Po Plain, northern Italy, characterised by a ∼N–S prevailing compressional stress regime, and in order to shed light on the regional state of stress and on the correlation between the active stress field and the orientation of tectonic structures. The results have been compared with seismological data relating to 1988–1995 crustal seismicity (2.5< M d<4.8) and to the 1983 Parma ( M s=5.0) and the 1996 Reggio Emilia ( M s=5.1) events. Plio-Pleistocene mesostructural data are also described in order to better define the present-day stress field and to understand the active tectonic processes in particular stress provinces. The borehole breakout analysis, in accordance with the seismicity and mesostructural data, shows the presence of a predominant compression area, characterised by approximately N–S maximum horizontal stress, along the outer thrust of the Ferrara arc. Particularly, the breakout analysis indicates a minimum horizontal stress, N81W±22° relative to a total of eleven analysed wells, with 3746 m cumulative total length of breakout zones. Among these, nine wells are located in the same tectonic structure, consisting of an arc of asymmetric folds overthrust towards the NE. The breakout results for these wells are quite similar in terms of minimum horizontal stress direction (∼E–W oriented). The other two wells are located in the outside sector of the arc and one of them shows a different minimum horizontal stress direction, probably distinctive of another tectonic unit. On the basis of these new reliable stress indicators, the active compressive front in this area is located along the termination of the external northern Apenninic arc.

Recent tectonic evolution and present stress in the Northern Apennines (Italy)

The present‐day stress field and its recent tectonic evolution in the Northern Apennines are reconstructed from borehole breakout analysis and focal mechanisms of crustal earthquakes and through the comparison with paleostress data. We have considered 86 wells for breakout analysis, with depths down to 6–7 km, 125 fault plane solutions of crustal earthquakes with M&lt;5 that occurred between 1988 and 1995 in the Northern and Central Apennines, and data of stronger earthquakes (M≤6) reported in other studies. The Tyrrhenian coastal region and the Apenninic belt are characterized by Shmin direction mainly trending NE–SW, with predominantly normal fault plane solutions. Along the outer front of the belt and the Adriatic offshore, Shmin is oriented NW–SE, and focal solutions are thrust or strike‐slip, with maximum compression around NE–SW. Conversely, south of 43°N, breakouts evidence an orthogonal direction of horizontal compression (NW–SE), following the Southern Apennine trend, where a widespread NE–SW extension was recognized by previous investigations. Comparing these results to the recent tectonic evolution inferred from structural geology, we argue that the extension‐compression pair, characteristic of the post‐Tortonian evolution of the mountain belt, has been migrating in time from late Miocene to Present only in the northern sector of the arc, whereas the southern sector underwent a generalized extension, at least since middle Pleistocene. The striking correspondence between the active compression front and the region with evidence of a remnant subducted slab suggests that the migrating extension‐compression pair has been controlled by progressive retreat of the slab.

COMPRESSIVE NEOGENE‐QUATERNARY TECTONICS IN THE HINTERLAND AREA OF THE NORTHERN APENNINES

The NW‐SE trending Northern Apennine Mountains consist of a series of allochthonous units which were thrust generally to the NE during Neogene crustal shortening, in the direction of a foreland basin to the east andNE. On the hinterland (internal) side of this fold‐and‐thrust belt, a series of small‐scale sedimentary basins developed from the late Miocene and were deformed at the same time. The late Tortonian‐Pleistocene evolution of the Northern Apennines has previously been considered by most authors in terms of a classical model of a NE‐migrating compressional front, which was followed in time and space by a hinterland extensional regime related to the development of the Tyrrhenian Basin.This paper presents new structural data from both the external parts of the Northern Apennines and the late Tortonian‐Pleistocene basins located in the internal sector. In the Northern Apennine thrust belt, reactivation and out‐of‐sequence geometries for the thrust faults have been recorded. In the hinterland basins, compressional deformation has been documented and is usually associated with thrust ramps and regional unconformities. The timing of both thrust reactivation and of the major compressional phases affecting the hinterland basins is closely correlated with periods of magmatic quiescence, and with compressional phases detected in the external margin of the Northern Apennines (the Padan‐Adriatic foredeep).Data presented in this paper indicate that compressional deformation has played a major role in the recent evolution of the Northern Apennines. The mechanism envisaged to explain this tectonic framework takes account of the piggy‐back emplacement of basement thrusts from internal to external sectors, which occurred in post‐Serravallian time. Activity on basement thrusts may have caused reactivation of thrusts in the internal cover sequence, giving rise to out‐of‐sequence geometries and controlling the development and/or deformation of the hinterland basins. This type of structural evolution has resulted in a complex geometry for the thrust sheets, and this must be taken into consideration during re‐interpretation of the structure of the Northern Apennines. It may also have important implications for petroleum exploration.

Nonequilibrium Thermodynamics and Cosmological Pancake Formation

We investigate the influence of non equilibrium thermodynamics on cosmological structure formation. In this paper, we consider the collapse of planar perturbations usually called Zel'dovich pancakes. We have developed for that purpose a new two fluids (gas and dark matter) hydrodynamical code, with three different thermodynamical species: electrons, ions and neutral particles (T_e\ne T_i \ne T_n). We describe in details the complex structure of accretion shock waves. We include several relevant processes for a low density, high temperature, collisional plasma such as non-equilibrium chemical reactions, cooling, shock heating, thermal energy equipartition between electrons, ions and neutral particles and electronic conduction. We find two different regions in the pancake structure: a thermal precursor ahead of the compression front and an equipartition wave after the compression front where electrons and ions temperatures differ significantly. This complex structure may have two interesting consequences: pre-heating of unshocked regions in the vicinity of massive X-ray clusters and ions and electrons temperatures differences in the outer regions of X-rays clusters.

Cover thrust reactivations related to internal basement involvement during Neogene‐Quaternary evolution of the northern Apennines

Up to recently the Neogene‐Quaternary evolution of the northern Apennines (Italy) has been described by the classic model of a migrating eastward, compressive external front, with an extensional regime in the back areas connected with the Tyrrhenian basin formation. However, in the last few years, new structural data have been collected in the internal marine and continental episutural basins, and in the external exposed thrust belt. A complex structural evolution has now been reconstructed, with coeval main tectonic phases that affect both areas with stress field change. Four main tectonic phases have been identified since Late Tortonian times; these are dated as Messinian, late Pliocene, middle, and late Pleistocene. The thrust belt has a complex evolution, detected in a wide external area of the chain after the first emplacement of the main thrust sheets, with reactivations and out of sequence thrusting. These reactivation phases fit very well with the compressive phases affecting the sediments of the internal basins, suggesting a direct relationship within a single evolutive model. The increased knowledge of the deep structure of the northern Apennines through geophysical and subsurface data acquired in the last few years indicates the basement involvement at least for the internal side of the northern Apennines. This basement involvement also played an important role in the tectonic evolution of the external sector of the Apennines. In this paper a new model is proposed that integrates field data, geophysical evidence, and geodynamic constraints. The thrust reactivations and the out‐of‐sequence structures of the external area are related to internal crustal thrust activity. The deformed sediments of the Neogene‐Quaternary basins are dating the thrust activity. All the evidences point to a late Neogene rejuvenation of the tectonic evolution, but some inferences can be drawn on the development of the foredeep.

Contemporaneous extension and compression in the Northern Apennines from earthquake fault-plane solutions

SUMMARY The Northern Apenninic arc belongs to the deformation zones surrounding the Adriatic plate, which behaves as a rigid block. It is a NE-verging fold-and-thrust belt, which developed since the Miocene with the migration of an extensional-compressional pair. Previous seismological data are roughly in agreement with this deformation picture, although the outer compressional front was not clearly defined by the few available solutions. In this work we confirm the existence of two adjacent zones of contemporaneous extension and compression in the Northern Apennines, defining the extent of these two zones and their interim1 deformation better than was previously done. This is accomplished through the analysis of 125 new focal mechanisms of earthquakes (2.6 < Md < 4.8) recorded by the national network of the lstituto Nazionale di Geofisica in the period 1988-1995. The two deformation zones are clearly separated and lie very close to each other with a partial overlapping in the Emilian-Tuscan Apennines. Strikeslip events are scattered in most of the study area. Thrust-faulting earthquakes are located in the internal part of the most recently active thrust front (middle-upper Pleistocene). There is a general trend of the compressional P-axes of thrust-fault and strike-slip solutions to be perpendicular to the Apenninic direction in the external zone, while the tensional axes of the normal-fault and strike-slip solutions in the belt do not show a uniform orientation. However, clearly evident is the -ENE direction of extension in the peri-Tyrrhenian region, consistent with the direction of Shmln in the Quaternary volcanoes of this region inferred from borehole breakouts and microearthquake fault-plane solutions. We determined the orientation of the stress tensor in the two main deformation zones by inversion of the 125 fault-plane solutions determined in this study. We found that the external belt is characterized by NE-SW compression (ol horizontal, oriented N45E), while a normal-faulting regime with o3 - E-W oriented is characteristic of the Umbria-Tuscany (back-arc) region.

Paleomagnetic evidence for Cenozoic clockwise rotation of the external Albanides

A paleomagnetic study of 750 samples obtained from 55 late Eocene to middle Pliocene sedimentary sites demonstrates a clockwise rotation of about 45° of the external zones of Albania. This rotation occurred either in two phases of roughly equal amplitude during the early-middle Miocene and Plio-Pleistocene or in a single phase with a clear acceleration of the movement during the Plio-Pleistocene. The good agreement of these results with those from southern Albania and northwestern Greece indicates that a large geographical area is involved in the rotation. The active ‘Aegean’ compressive front thus extends much farther north than hitherto believed and the rotational pattern is not significantly distorted by the tectonic discontinuities recorded by geological studies. The external zones of the Albano-Hellenic Belt appear to have rotated virtually as a single entity from the Peloponnesos to northern Albania over a deep decollement level probably involving the basement itself.

Anatomy of late orogenic extension: The Northern Apennines case

Late orogenic extensional structures overprint the compressional structures of the Northern Apennines fold and thrust belt. The Tyrrhenian Sea is the result of major continental extension and rifting which started in the thickened zone of the Apulian/ Corso-Sardinian collision. It is believed to have commenced as early as in the Late Oligocene or Early Miocene in the Corsica basin. Extension post-dates the collision by approximately 20 Ma. The extensional stress field observed in the internal part of the Northern Apennines co-exists with compression in the foreland part of the belt. Field, borehole and refraction and reflection data were used to investigate the geometry of extension and how it affected the compressional abric in the Northern Apennines. Analysis of the collected data has revealed a complex geometry for the extension across the belt. A geological cross-section from the island of Elba, to the west of the Italian Peninsula, to Ancona, in the east, as well as a geodynamic model for the evolution of the Northern Apennines is presented and discussed. From the Early Miocene stretching of the collision-thickened crust created a N-S rift which evolved into the present Northern Tyrrhenian Sea. Typically, two sets of extensional faults, a low-angle E-dipping and a W-dipping one are observed across the chain. Usually the W-dipping faults have similar attitudes to those of the older reverse faults, reactivating some of the thrusts as normal faults. The E-dipping normal faults, however, cut down-section through the pre-existing thrusts and, in the internal part of the belt, accommodate most of the displacement associated with extension. A change in fault polarity occurs in the Tuscan region and major faults dip towards the west. A-type subduction of the Apulian crust under the Corso-Sardinian block occurred after collision leading to the delamination of the Apulian lithospheric mantle away from the crust. Collision-induced delamination of the mantle lithosphere away from the zone of collision resulted in mechanical thinning of the lithosphere beneath the orogenic belt. This process induced late orogenic crustal stretching in the internal zone of the Northern Apennines. As the convergence between the Apulian and Corso-Sardinian blocks continued the extensional and compressional fronts, accompanied by the delamination, migrated from the hinterland towards the foreland of the orogen.

Plane compression front steepening in nonlinear media forms both a shock and a reflected wave

An exact solution is given for the reflected wave formed in a nonlinear medium when a plane compression front steepens into a shock. The solution predicts both a shock and a reflected wave. In the small-amplitude limit the reflected wave strength varies as (ΔP)3, where ΔP is the strength of the initial wave front.

The unusual cometary star-forming region G110-13

We present far-IR, radio continuum, and spectral line observations of an unusual, highly elongated, comet-shaped molecular cloud, located about 100 pc from the Galactic plane. The presence of three late B-type stars embedded within, or adjacent to, this low-mass cloud implies a star-forming efficiency that may be as high as 30 percent. Several mechanisms that may have been responsible for its unusual morphology and high star-forming efficiency will be described and evaluated. Although ram-pressure resulting from the rapid motion of this cloud through the interstellar medium could explain its streamlined appearance, there is evidence that G110-13 is the compression front formed by a recent cloud collision.

On the form of the flow in the magnetosheath

The form of the flow field in the subsonic and transsonic magnetosheath has received little attention since the proposal of a depletion layer adjacent to the magnetopause by Zwan and Wolf. Recently, observations of density enhancements just upstream of the magnetopause which have been interpreted as slow mode compressional fronts appear to contradict the earlier suggestions. In this paper we describe the reason for the formation of an upstream compressional front standing near the magnetopause, describe its form and geometry, and explain the likely relationship between the compression region and a depletion layer. The model accounts for the variation of density and magnetic field strength with radial distance from the magnetopause and makes predictions regarding the orientation and geometry of the front.

Tertiary sedimentary history and structure of the Valencia trough (western Mediterranean)

We present here main results of the Common Depth Point (CDP) data acquired during the Valsis 2 Cruise in 1988 in the Valencia trough. The profiles are tied in with industrial well data and this correlation allows the sedimentary and structural history of the region to be deduced. The Valsis Cruise seismic profiles have been supplemented by a very dense grid of industrial seismic lines and these data permit us to establish an accurate depth to basement map. The formation of the initial grabens, coeval with those of the Gulf of Lions, is related to the Early Miocene opening of the northwestern Mediterranean basin and the Barcelona graben is filled by the same sedimentary layers, including evaporites, as that of the Provençal region. Nevertheless, the Valencia-Catalan grabens have been reactivated by young extensional tectonics which could be a consequence of the convergence of Africa relative to Europe. The Valencia trough is segmented by transfer faults which trend NW-SE. These faults, which have a more accentuated structural expression than the Valencia and Catalonia grabens, may act as transform faults separating the individual Balearic Islands. The transfer faults are in strike with volcanic ridges which have been sampled during the DSDP Leg 13. The dense seismic grid allows us to delineate several widespread volcanic features in the Valencia trough which have been active from the Early Miocene to the Pleistocene. However, we note that the volcanic features are mainly Miocene in age whereas the recent volcanism is restricted to a narrow zone (Columbretes Islands). The compressional tectonics which deformed the Balearic Islands does not appear to extend far towards the North. We delineate the compressional front north of Ibiza, but we failed to determine any thrust or fold north of Mallorca, whereas an extensional tectonics is evident.