Showing papers on "Pressure gradient published in 2005"
TL;DR: In this paper, the authors proposed a wall function formulation applicable to any RANS turbulence model, based on the assumption of wall layer universality, applied to the entire model and implemented via tables for the turbulence quantities and the friction velocity.
354 citations
TL;DR: In this paper, an experimental investigation of the flow induced by the collapse of a column of granular material (glass beads of diameter d) over a horizontal surface was conducted, where two different setups were used, namely, a rectangular channel and a semicircular tube, allowing us to compare two-dimensional and axisymmetric flows, with particular focus on the internal flow structure.
Abstract: We report the results of an experimental investigation of the flow induced by the collapse of a column of granular material (glass beads of diameter d) over a horizontal surface. Two different setups are used, namely, a rectangular channel and a semicircular tube, allowing us to compare two-dimensional and axisymmetric flows, with particular focus on the internal flow structure. In both geometries the flow dynamics and the deposit morphologies are observed to depend primarily on the initial aspect ratio of the granular column a=Hi∕Li, where Hi is the height of the initial granular column and Li its length along the flow direction. Two distinct regimes are observed depending on a: an avalanche of the column flanks producing truncated deposits for small a and a column free fall leading to conical deposits for large a. In both geometries the characteristic time scale is the free fall of the granular column τc=Hi∕g. The flow initiated by Coulomb-like failure never involves the whole granular heap but remains localized in a surface layer whose size and shape depend on a and vary in both space and time. Except in the vicinity of the pile foot where the flow is pluglike, velocity profiles measured at the side wall are identical to those commonly observed in steady granular surface flows: the velocity varies linearly with depth in the flowing layer and decreases exponentially with depth in the static layer. Moreover, the shear rate is constant, γ=0.3g∕d, independent of the initial aspect ratio, the flow geometry, position along the heap, or time. Despite the rather complex flow dynamics, the scaled deposit height Hf∕Li and runout distance ΔL∕Li both exhibit simple power laws whose exponents depend on a and on the flow geometry. We show that the physical origin of these power laws can be understood on the basis of a dynamic balance between acceleration, pressure gradient, and friction forces at the foot of the granular pile. Two asymptotic behaviors can be distinguished: the flow is dominated by friction forces at small a and by pressure forces at large a. The effect of the flow geometry is determined primarily by mass conservation and becomes important only for large a.
261 citations
TL;DR: The essential results are that many parameters depend on the inlet pressure, hence on the flow rate, and while reproducibility of results obtained at constant pressure and flow rate will not be affected, method development becomes far more complex because of the pressure dependence of everything.
199 citations
TL;DR: It is shown that an appropriate density profile brings significant improvement to the self-compression by filamentation and under an optimal pressure gradient, the pulse duration is reduced to the single optical cycle limit over a long distance, allowing easy extraction into an interaction chamber.
Abstract: We calculate pulse self-compression of a 30 fs laser pulse traversing gas with different pressure gradients. We show that an appropriate density profile brings significant improvement to the self-compression by filamentation. Under an optimal pressure gradient, the pulse duration is reduced to the single optical cycle limit over a long distance, allowing easy extraction into an interaction chamber.
175 citations
TL;DR: This bubble production technique was used to produce monodisperse aqueous foam in two-dimensional and three-dimensional microchannels and showed good results on bubble formation in microfluidic devices.
Abstract: This Brief Report presents experimental and computational results on bubble formation in microfluidic devices. Bubbles are generated at the right-angle intersection of four identical square microchannels. When the pressure gradient generated by the liquid flow dominates the pressure gradient generated by gas flow, the length of the produced confined bubbles follows a law based on the channel size and fluid volume fraction. This bubble production technique was used to produce monodisperse aqueous foam in two-dimensional and three-dimensional microchannels.
142 citations
TL;DR: In this article, the authors examined the plasma profiles across the separatrix and scrape-off layer (SOL) in Alcator C-Mod for a range of plasma densities, currents and magnetic fields.
Abstract: Plasma profiles across the separatrix and scrape-off layer (SOL) in Alcator C-Mod are examined for a range of plasma densities, currents and magnetic fields in Ohmic L-mode discharges and for a subset of conditions in Ohmic H-mode discharges. In all plasmas, electron pressure gradient scale lengths ( ) exhibit a minimum value just outside the separatrix (i.e. in the near SOL), forming the base of a weak (strong) pedestal in L-mode (H-mode) plasmas. Over a wide range of conditions in Ohmic L-mode discharges, at this location are found to track with a monotonic function of electron collision frequency, when this quantity is normalized according to the framework of electromagnetic fluid drift turbulence (EMFDT) theory. Moreover, at fixed values of normalized collisionality parameter (characterized as the 'diamagnetic parameter', ?d), electron pressure gradients in the near SOL increase with plasma current squared, holding the MHD ballooning parameter, ?MHD, unchanged. Thus, the state of the near SOL is restricted to a narrow region within this two-parameter phase-space. An implication is that cross-field heat and particle transport are strong functions of these parameters. Indeed, as ?d is decreased below ~0.3, cross-field heat convection increases sharply and competes with parallel heat conduction along open field lines, making high plasma density regions of ?MHD??d space energetically inaccessible. These observations are consistent with the idea that the operational space of the edge plasma, including boundaries associated with the tokamak density limit, is controlled by EMFDT.
135 citations
TL;DR: In this article, it was shown from a symmetry in the gyrokinetic equation that for up-down symmetric tokamak equilibria, the transport of parallel momentum can be written as the sum of a diffusive and a pinch contribution with no off-diagonal terms due to temperature and pressure gradients.
Abstract: It is shown from a symmetry in the gyrokinetic equation that for up–down symmetric tokamak equilibria and for uϕ⪢ρυthi∕r (where uϕ is the toroidal velocity, υthi is the thermal ion velocity, ρ is the Larmor radius, and r is the radius of the flux surface), the transport of parallel momentum can be written as the sum of a diffusive and a pinch contribution with no off-diagonal terms due to temperature and pressure gradients. The measured parallel velocity gradient in ASDEX Upgrade [O. Gruber, H.-S. Bosch, S. Gunter et al., Nucl. Fusion 39, 1321 (1999)] is insufficient to drive the parallel velocity shear instability. The parallel velocity is then transported by the ion temperature gradient mode. The diffusive contribution to the transport flux is investigated using a linear gyrokinetic approach, and it is found that the diffusion coefficient for parallel velocity transport divided by the ion heat conductivity coefficient is close to 1, and only weakly dependent on plasma parameters.
134 citations
TL;DR: In this paper, the geodesic curvature and toroidicity-induced coupling between shear Alfven waves and acoustic modes are investigated. But the authors focus on the low frequency part of the shear wave dispersion relation.
Abstract: Tokamak plasmas with reversed magnetic shear are prone to the excitation of Alfven cascade (AC) eigenmodes by energetic particles. These modes exhibit a quasiperiodic pattern of predominantly upward frequency sweeping. Observations also reveal that the AC spectral lines sometimes bend at low frequencies, which is a significant deviation from the shear Alfven wave dispersion relation. This paper shows that the underlying reasons for such bending are the finite pressure of the plasma and the geodesic curvature that precludes shear Alfven perturbations from being strictly incompressible. In addition to the geodesic effect, there are two other pressure effects on shear Alfven waves, which are the convection in the presence of an equilibrium pressure gradient and the toroidicity-induced coupling between shear Alfven waves and acoustic modes. An analytical treatment of the problem enables a parametric comparison of all three mechanisms. The key distinction between the geodesic compressibility and the acoustic coupling is that geodesic compression occurs without plasma displacement along the magnetic-field lines. As a result, the mode phase velocity is greater than the ion thermal velocity even in an isothermal plasma, which allows the mode to avoid a strong ion Landau damping. Plasma temperature diagnostics via magnetohydrodynamic spectroscopy employing the low-frequency part of the ACs is suggested.
133 citations
TL;DR: The study shows that high shear levels are confined to small regions within the channel cross-section, but the circulatory nature of the flow causes an increased percentage of blood elements to pass through the high sheer regions, and increases the likelihood of cell damage.
Abstract: This study presents a numerical investigation of the flow field in a screw pump designed to circulate biological fluid such as blood. A simplified channel flow model is used to allow analysis using a Cartesian set of coordinates. Finite analytic (FA) numerical simulation of the flow field inside the channel was performed to study the influence of Reynolds number and pressure gradient on velocity distribution and shear stresses across the channel cross-section. Simulation results were used to predict flow rates, circulatory flow and the shear stresses, which are known to be related to the level of red blood cell damage (hemolysis) caused by the pump. The study shows that high shear levels are confined to small regions within the channel cross-section, but the circulatory nature of the flow causes an increased percentage of blood elements to pass through the high shear regions, and increases the likelihood of cell damage.
124 citations
TL;DR: In this article, the competition between drift wave and interchange physics in general E-cross-B drift turbulence is studied with computations in three-dimensional tokamak flux tube geometry.
Abstract: The competition between drift wave and interchange physics in general E-cross-B drift turbulence is studied with computations in three-dimensional tokamak flux tube geometry. For a given set of background scales, the parameter space can be covered by the plasma β and drift wave collisionality. At large enough plasma β the turbulence breaks out into ideal ballooning modes and saturates only by depleting the free energy in the background pressure gradient. At high collisionality it finds a more gradual transition to resistive ballooning. At moderate β and collisionality it retains drift wave character, qualitatively identical to simple two-dimensional slab models. The underlying cause is the nonlinear vorticity advection through which the self-sustained drift wave turbulence supersedes the linear instabilities, scattering their structure apart before they can grow, imposing its own physical character on the dynamics. This vorticity advection catalyses the gradient drive, while saturation occurs solely throu...
124 citations
TL;DR: In this paper, a high-resolution upper-ocean climatology was used to determine the alongshore pressure gradient and altimeter and mooring observations to derive an eddy-induced Reynolds stress.
Abstract: The Leeuwin Current is a poleward-flowing eastern boundary current off the western Australian coast, and alongshore momentum balance in the current has been hypothesized to comprise a southward pressure gradient force balanced by northward wind and bottom stresses. This alongshore momentum balance is revisited using a high-resolution upper-ocean climatology to determine the alongshore pressure gradient and altimeter and mooring observations to derive an eddy-induced Reynolds stress. Results show that north of the Abrolhos Islands (situated near the shelf break between 28.2° and 29.3°S), the alongshore momentum balance is between the pressure gradient and wind stress. South of the Abrolhos Islands, the Leeuwin Current is highly unstable and strong eddy kinetic energy is observed offshore of the current axis. The alongshore momentum balance on the offshore side of the current reveals an increased alongshore pressure gradient, weakened alongshore wind stress, and a significant Reynolds stress exerte...
TL;DR: In this article, a detailed analysis of the energy transfer system between ExB turbulence and zonal flows is given, and a statistical equilibration between turbulence, zonal flow and sidebands, and additionally the various poloidally asymmetric parallel dynamical subsystems are shown.
Abstract: A detailed analysis of the energy transfer system between ExB turbulence and zonal flows is given. Zonal flows, driven by the ExB Reynolds stress of the turbulence, are coupled to pressure disturbances with sinusoidal poloidal structure in toroidal geometry through the geodesic curvature. These pressure 'sidebands' are nonlinearly coupled not only back to the turbulence, but also to the global Alfv?n oscillation whose rest state is the Pfirsch?Schl?ter current in balance with the pressure gradient. The result is a statistical equilibration between turbulence, zonal flows and sidebands, and additionally the various poloidally asymmetric parallel dynamical subsystems. Computations in three-dimensional flux surface geometry show this geodesic transfer effect to be the principal mechanism which limits the growth of zonal flows in tokamak edge turbulence in its usual parameter regime, by means of both control tests and statistical analysis. As the transition to the magnetohydrodynamic (MHD) ballooning regime is reached, the Maxwell stress takes over as the main drive, forcing the Reynolds stress to become a sink.
TL;DR: In this paper, a 1.4-mm hydraulic diameter multiport minichannel tube was used for adiabatic two-phase flow of HFC refrigerants and the experimental runs were carried out at mass velocities ranging from 200 to 1400 kg/(m2 s) depending on the refrigerant under test, at constant value of vapour quality.
19 Aug 2005
TL;DR: In this paper, a Serrin-type regularity criterion on the gradient of pressure for weak solutions to the Navier-Stokes equations in R 3 was established, and it was shown that if the gradient belongs to L α,γ with 2/a + 3/γ ≤ 3, 1 ≤ γ ≤ ∞, then the weak solution is actually regular.
Abstract: In this paper we establish a Serrin-type regularity criterion on the gradient of pressure for the weak solutions to the Navier-Stokes equations in R 3 . It is proved that if the gradient of pressure belongs to L α,γ with 2/a + 3/γ ≤ 3, 1 ≤ γ ≤ ∞, then the weak solution is actually regular. Moreover, we give a much simpler proof of the regularity criterion on the pressure, which was showed recently by Berselli and Galdi (Proc. Amer. Math. Soc. 130 (2002), no. 12, 3585-3595).
TL;DR: In this article, the authors assess errors in soil respiration fluxes of CO2 obtained using the closed dynamic chamber method, focusing on small pressure gradients between the chamber headspace and the external environment that may induce mass flow of soil air.
TL;DR: In this paper, a model of multiphase transport in a porous medium coupled with large deformation of the porous matrix is developed and applied to the process of bread baking, which is based on energy conservation and mass conservation of water, water vapor, and CO2 produced during baking.
Abstract: A model of multiphase transport in a porous medium coupled with large deformation of the porous matrix is developed and applied to the process of bread baking. Transport-governing equations are based on energy conservation and mass conservation of water, water vapor, and CO2 produced during baking. Deformation is caused by the pressure gradient from internal evaporation and CO2 generation. Temperature, moisture, and pressure changes in turn are affected by deformation. Bread is assumed to be viscoelastic, mechanical properties of which are functions of temperature. Geometric nonlinear effects are considered in the mechanics problem. Results are compared with those from baking experiments and literature data. Vapor pressure inside the matrix is likely to be lower than the equilibrium vapor pressure. Convective heat transfer is small compared to heat conduction and evaporation–condensation of water vapor promotes heat transfer to the inside. Rate of CO2 generation, mechanical properties of dough, and gravity together determine the final shape of the bread. © 2005 American Institute of Chemical Engineers AIChE J, 2005
TL;DR: In this article, phase inversion and its effect on pressure gradient during the dispersed flow of two immiscible liquids have been studied for two pipe materials (steel and acrylic) and two pipe sizes (60 and 32 mm ID).
TL;DR: Skin pressure distributions and magnitudes applied by GCSs were significantly influenced by the locations of testing points in terms of height and direction, which are determined by the specific anatomic structure and body shape of individual human legs and potentially influenced byThe pressure sensor and testing methods.
Abstract: Background. The beneficial effects of graduated elastic compression stockings (GCSs) have been demonstrated. However, their pressure performances are variable and unstable in practical applications owing to many influencing factors. Comprehensive assessment of skin pressure profiles may help elucidate the mechanisms of action of compression stockings.
Objective. The objective of this study was to quantify and objectively evaluate the magnitude and distribution of skin pressure applied by different GCSs and to analyze the possible reasons influencing the stocking pressure performances.
Methods. Six healthy females were required to wear eight kinds of GCSs with different pressure levels while standing upright. The skin pressures of 16 different positions located in four heights and four directions of the lower limb were measured and recorded by FlexiForce interface pressure sensors (Tekscan, Inc., Boston, MA, USA) and a multichannel measuring system.
Results. The pressure gradients, pressure levels, and testing locations significantly influenced skin pressure (p < .001). All tested GCSs exerted the highest pressure at the ankle region, and the pressure gradually decreased toward the thigh, which follows the gradient design of GCSs. However, many of them failed to produce the perfect pressure gradients from the ankle region to the calf region. Obviously, reversed pressure gradients occurred in the medial side of the leg, where the pressure at the ankle region was lower than that on the prominent part of the calf by 36% on average. GCSs with higher pressure levels applied higher skin pressure on the lower limb. The pressure at the anterior side was far higher than that in the medial and lateral directions. The distribution patterns of skin pressure at transverse sections were similar to the anatomic outlines of cross sections of the leg. The measured average ankle pressure of all tested GCSs did not reach the pressures specified by the manufacturers.
Conclusion. Skin pressure distributions and magnitudes applied by GCSs were significantly influenced by the locations of testing points in terms of height and direction, which are determined by the specific anatomic structure and body shape of individual human legs and potentially influenced by the pressure sensor and testing methods.
TL;DR: In this paper, the authors present a solution based on enriching the pressure shape functions on the elements cut by the interface to enable the pressure gradient to be discontinuous at the interface, thus improving the ability to simulate the behavior of fluids with different density under a gravitational force.
Abstract: In this paper we present a problem we have encountered using a stabilized finite element method on fixed grids for flows with interfaces modelled with the level set approach. We propose a solution based on enriching the pressure shape functions on the elements cut by the interface. The enrichment is used to enable the pressure gradient to be discontinuous at the interface, thus improving the ability to simulate the behaviour of fluids with different density under a gravitational force. The additional shape function used is local to each element and the corresponding degree of freedom can therefore be condensed prior to assembly, making the implementation quite simple on any existing finite element code. Copyright © 2005 John Wiley & Sons, Ltd.
TL;DR: In this paper, the experimental heat transfer coefficients measured during condensation of R134a and R410A inside multiport minichannels are presented, and the frictional pressure gradient was also measured during adiabatic two-phase flow.
Abstract: In this paper, the experimental heat transfer coefficients measured during condensation of R134a and R410A inside multiport minichannels are presented. The frictional pressure gradient was also measured during adiabatic two-phase flow. The need for experimental research on condensation inside multiport minichannels comes from the wide use of those channels in automotive air-conditioners. The perspective for the adoption of similar channels in the residential air conditioning applications also calls for experimental research on new high pressure refrigerants, such as R410A. Experimental data are compared against models to show the accuracy of the models in the prediction of heat transfer coefficients and pressure drop inside minichannels.
TL;DR: In this paper, the authors investigated the pressure drop characteristics during the simultaneous flow of a kerosene-water mixture through a horizontal pipe of 0.025 m diameter, and measured the pressure gradient for different combinations of phase superficial velocities ranging from 0.03-2 m/s.
Abstract: The present study is aimed at an investigation of the pressure drop characteristics during the simultaneous flow of a kerosene-water mixture through a horizontal pipe of 0.025 m diameter. Measurements of pressure gradient were made for different combinations of phase superficial velocities ranging from 0.03-2 m/s such that the regimes encountered were smooth stratified, wavy stratified, three layer flow, plug flow and oil dispersed in water, and water flow patterns. A model was developed, which considered the energy minimization and pressure equalization of both phases.
TL;DR: In this paper, the authors report the experimental microstability analyses of ITBs from an international multi-machine database, the International Tokamak Physics Activity database, accessible on the web, and show that? is a relevant parameter of ITB physics that should be taken into account in interpretative and predictive one-dimensional transport codes.
Abstract: In plasmas exhibiting an internal transport barrier (ITB), locally very high pressure gradient (?P) is obtained. It induces high values of the magnetohydrodynamic ? parameter (? = ?q 2 ?R?P/P, with R the major radius, q the safety factor, P the pressure, ? the radial gradient and ? the ratio between kinetic and magnetic pressure). Similarly to low or negative magnetic shear (s), high ? reduces the curvature and ? B drifts driving curvature-type microinstabilities. Therefore, high values of ? can stabilize part of the microturbulence, which leads to higher pressure gradient and to even higher ?. This possibility for entering a positive feedback loop is very attractive to sustain ITBs in high performance plasmas. Indeed, ? scales favourably with higher pressure and does not require any external momentum input. In this paper, after having discussed the ? stabilization mechanism in detail, we report the experimental microstability analyses of ITBs from an international multi-machine database?the International Tokamak Physics Activity database, accessible on the web. We show that ? is indeed a relevant parameter of ITB physics that should be taken into account in interpretative and predictive one-dimensional transport codes.
TL;DR: In this paper, the authors evaluated the applicability of the Reynolds analogy for turbine flows using experimental data collected in a low-speed wind tunnel and found that the Reynolds approximation factor increases with turbulence level.
Abstract: The application of Reynolds analogy (2St/c f ≅1) for turbine flows is critically evaluated using experimental data collected in a low-speed wind tunnel. Independent measurements of St and c f over a wide variety of test conditions permit assessments of the variation of the Reynolds analogy factor (i.e., 2St/c f ) with Reynolds number, freestream pressure gradient, surface roughness, and freestream turbulence. While the factor is fairly independent of Reynolds number, it increases with positive (adverse) pressure gradient and decreases with negative (favorable) pressure gradient. This variation can be traced directly to the governing equations for momentum and energy which dictate a more direct influence of pressure gradient on wall shear than on energy (heat) transfer. Surface roughness introduces a large pressure drag component to the net skin friction measurement without a corresponding mechanism for a comparable increase in heat transfer. Accordingly, the Reynolds analogy factor decreases dramatically with surface roughness. Freestream turbulence has the opposite effect of increasing heat transfer more than skin friction, thus the Reynolds analogy factor increases with turbulence level. Physical mechanisms responsible for the observed variations are offered in each case. Finally, synergies resulting from the combinations of pressure gradient and freestream turbulence with surface roughness are evaluated.
TL;DR: In this paper, the authors diagnose the momentum balance over the West Florida Shelf using observations of currents, bottom pressures, temperatures, winds, and coastal sea levels, along with hydrographic data from 32 monthly cruises spanning summer 1998 to winter 2001.
TL;DR: In this article, it was shown that using the value of the surface-thermal capacity determined at a spherical interface, the energy transported by thermocapillary convection and thermal conduction at a cylindrical interface is sufficient to evaporate the liquid at the observed rate.
Abstract: When evaporation occurs at a spherical water-vapor interface maintained at the circular mouth of a small funnel, studies of the energy transport have indicated that thermal conduction alone does not provide enough energy to evaporate the liquid at the observed rate. If the Gibbs model of the interface is adopted and the "surface-thermal capacity" is assigned a value of 30.6+/-0.8 kJ/(m2 K), then for evaporation experiments with the interfacial temperature in the range -10 degrees C< or =TLV< or =3.5 degrees C and Marangoni number (Ma) in the range 100
TL;DR: In this article, it was shown that stress gradient balance regions are characterized by an intrinsic hierarchy of scaling layers (analogous to the inner and outer domains), filling out the stress gradient imbalance region except for locations near the wall.
Abstract: Steady Couette and pressure-driven turbulent channel flows have large regions in which the gradients of the viscous and Reynolds stresses are approximately in balance (stress gradient balance regions). In the case of Couette flow, this region occupies the entire channel. Moreover, the relevant features of pressure-driven channel flow throughout the channel can be obtained from those of Couette flow by a simple transformation. It is shown that stress gradient balance regions are characterized by an intrinsic hierarchy of 'scaling layers' (analogous to the inner and outer domains), filling out the stress gradient balance region except for locations near the wall. The spatial extent of each scaling layer is found asymptotically to be proportional to its distance from the wall. There is a rigorous connection between the scaling hierarchy and the mean velocity profile. This connection is through a certain function A(y+) defined in terms of the hierarchy, which remains O(1) for all y+. The mean velocity satisfies an exact logarithmic growth law in an interval of the hierarchy if and only if A is constant. Although A is generally not constant in any such interval, it is arguably almost constant under certain circumstances in some regions. These results are obtained completely independently of classical inner/ outer/overlap scaling arguments, which require more restrictive assumptions. The possible physical implications of these theoretical results are discussed. © 2005 Cambridge University Press.
TL;DR: In this article, the authors demonstrate the release of an entrapped ganglion by vibrations in a computational fluid-dynamics simulation using digital photography, and demonstrate that the mobilization effect is proportional to the amplitude and inversely to the frequency of vibrations.
Abstract: [1] Elastic waves have been observed to increase productivity of oil wells, although the reason for the vibratory mobilization of the residual organic fluids has remained unclear. Residual oil is entrapped as ganglia in pore constrictions because of resisting capillary forces. An external pressure gradient exceeding an “unplugging” threshold is needed to carry the ganglia through. The vibrations help overcome this resistance by adding an oscillatory inertial forcing to the external gradient; when the vibratory forcing acts along the gradient and the threshold is exceeded, instant “unplugging” occurs. The mobilization effect is proportional to the amplitude and inversely proportional to the frequency of vibrations. We observe this dependence in a laboratory experiment, in which residual saturation is created in a glass micromodel, and mobilization of the dyed organic ganglia is monitored using digital photography. We also directly demonstrate the release of an entrapped ganglion by vibrations in a computational fluid-dynamics simulation.
TL;DR: This article showed that shear heating, also known as viscous dissipation, dramatically reduces the pressure gradient required for flow and concentrates shear in narrow zones along the conduit margin.
TL;DR: In this paper, a new method for seismic deghosting of towed streamer data acquired in rough seas is presented, which combines pressure recordings along one or several cables with an estimate of the vertical pressure gradient (or the vertical component of the particle velocity).
Abstract: A new method is presented for seismic deghosting of towed streamer data acquired in rough seas. The deghosting scheme combines pressure recordings along one or several cables with an estimate of the vertical pressure gradient (or the vertical component of the particle velocity). The estimation of the vertical pressure gradient requires continuous elevation measurements of the wave height directly above the receivers. The vertical pressure gradient estimate is obtained by spatially weighting the pressure field. Each spatial weight generally is the product of two weight functions. The first is a function of partial derivatives acting solely along the horizontal Cartesian coordinates. It can be implemented by finite-difference or Fourier derivative operations. The second is a function of the vertical Cartesian coordinate and accounts for the varying sea state. This weight can be changed from one receiver to the next, making the deghosting a local process. Integrated with the measured pressure field, the esti...
TL;DR: In this paper, a coupled model for one-dimensional time-dependent compressible flow and bubble expansion is developed to investigate fragmentation mechanisms of viscous bubbly magmas in shock tubes.
Abstract: [1] A coupled model for one-dimensional time-dependent compressible flow and bubble expansion is developed to investigate fragmentation mechanisms of viscous bubbly magmas in shock tubes. Initially a bubbly magma at a high pressure is separated from air at the atmospheric pressure by a diaphragm. As the diaphragm is ruptured, a shock wave propagates into the air, and a rarefaction wave propagates into the bubbly magma. As a result, the bubbly magma is decompressed and expands. Gas overpressure and hoop stress around expanding bubbles are calculated by applying the cell model. It is assumed that the magma fragments and the flow changes from bubbly flow to gas-pyroclast dispersion when the hoop stress or the gas volume fraction reaches a given threshold. Two types of fragmentation mechanisms are recognized: (1) high-viscosity magma fragments as the hoop stress reaches the tensile strength of the melt (stress fragmentation) and (2) the hoop stress does not grow in low-viscosity magma so that fragmentation occurs after bubble expansion when the gas volume fraction reaches a threshold (expansion fragmentation). During stress fragmentation a zone of steep pressure gradient forms just behind the fragmentation surface, which propagates into the magma together with the fragmentation surface. Analytical considerations suggest that the self-sustained stress fragmentation process can be described by a combination of a traveling-wave-type solution in the bubbly flow region and a self-similar solution in the gas-pyroclast flow region. Some simple formulae to predict the fragmentation speed (downward propagation velocity of the fragmentation surface) are derived on the basis of these solutions. The formulae are applied to recent experimental results using shock tube techniques as well as Vulcanian explosions in nature.