Showing papers on "Velocity gradient published in 2012"

Redshift-space distortion of the 21-cm background from the epoch of reionization – I. Methodology re-examined

Abstract: The peculiar velocity of the intergalactic gas responsible for the cosmic 21cm background from the epoch of reionization and beyond introduces an anisotropy in the three-dimensional power spectrum of brightness temperature fluctuations. Measurement of this anisotropy by future 21cm surveys is a promising tool for separating cosmology from 21cm astrophysics. However, previous attempts to model the signal have often neglected peculiar velocity or only approximated it crudely. This paper presents a detailed treatment of the effects of peculiar velocity on the 21cm signal. (1) We show that properly accounting for finite optical depth eliminates the unphysical divergence of 21cm brightness temperature in the IGM overdense regions found in previous work that employed the usual optically-thin approximation. (2) We show that previous attempts to circumvent this divergence by capping the velocity gradient result in significant errors in the power spectrum on all scales. (3) We further show that the observed power spectrum in redshift-space remains finite even in the optically-thin approximation if one properly accounts for the redshift-space distortion. However, results that take full account of finite optical depth show that this approximation is only accurate in the limit of high spin temperature. (4) We also show that the linear theory for redshift-space distortion results in a ~30% error in the power spectrum at the observationally relevant wavenumber range, at the 50% ionized epoch. (5) We describe and test two numerical schemes to calculate the 21cm signal from reionization simulations which accurately incorporate peculiar velocity in the optically-thin approximation. One is particle-based, the other grid-based, and while the former is most accurate, we demonstrate that the latter is computationally more efficient and can achieve sufficient accuracy. [Abridged]

Blow Up Criterion for Compressible Nematic Liquid Crystal Flows in Dimension Three

Abstract: In this paper, we consider the short time strong solution to a simplified hydrodynamic flow modeling compressible, nematic liquid crystal materials in dimension three. We establish a criterion for possible breakdown of such solutions at a finite time in terms of the temporal integral of both the maximum norm of the deformation tensor of the velocity gradient and the square of the maximum norm of the gradient of a liquid crystal director field.

Laboratory Investigation of Turbulent Flow Structure around a Bed-Mounted Cube at Multiple Flow Stages

Abstract: This research compares the spatially distributed turbulent wake characteristics associated with a submerged bed-mounted cube at three flow stages. The experiments were conducted in a large laboratory flat-bed flume at high Reynolds numbers. The velocity gradient under each flow condition was kept near constant, which yielded novel data that isolated the effect of flow depth on turbulent flow structure in the wake. Spatially distributed instantaneous three-dimensional velocities were recorded using multiple acoustic Doppler velocimeters (ADVs). With decreasing relative depth, marked variations in the spatial distributions of mean and turbulence statistics were observed that have important implications for local sediment mobility and fish habitat.

Instabilities in wormlike micelle systems. From shear-banding to elastic turbulence.

Abstract: Shear-banding is ubiquitous in complex fluids. It is related to the organization of the flow into macroscopic bands bearing different viscosities and local shear rates and stacked along the velocity gradient direction. This flow-induced transition towards a heterogeneous flow state has been reported in a variety of systems, including wormlike micellar solutions, telechelic polymers, emulsions, clay suspensions, colloidal gels, star polymers, granular materials, or foams. In the past twenty years, shear-banding flows have been probed by various techniques, such as rheometry, velocimetry and flow birefringence. In wormlike micelle solutions, many of the data collected exhibit unexplained spatio-temporal fluctuations. Different candidates have been identified, the main ones being wall slip, interfacial instability between bands or bulk instability of one of the bands. In this review, we present experimental evidence for a purely elastic instability of the high shear rate band as the main origin for fluctuating shear-banding flows.

Three‐dimensional anisotropic seismic wave modelling in spherical coordinates by a collocated‐grid finite‐difference method

Abstract: SUMMARY To simulate seismic wave propagation in the spherical Earth, the Earth’s curvature has to be taken into account. This can be done by solving the seismic wave equation in spherical coordinates by numerical methods. In this paper, we use an optimized, collocated-grid finite-difference scheme to solve the anisotropic velocity–stress equation in spherical coordinates. To increase the efficiency of the finite-difference algorithm, we use a non-uniform grid to discretize the computational domain. The grid varies continuously with smaller spacing in low velocity layers and thin layer regions and with larger spacing otherwise. We use stress-image setting to implement the free surface boundary condition on the stress components. To implement the free surface boundary condition on the velocity components, we use a compact scheme near the surface. If strong velocity gradient exists near the surface, a lower-order scheme is used to calculate velocity difference to stabilize the calculation. The computational domain is surrounded by complex-frequency shifted perfectly matched layers implemented through auxiliary differential equations (ADE CFS-PML) in a local Cartesian coordinate. We compare the simulation results with the results from the normal mode method in the isotropic and anisotropic models and verify the accuracy of the finite-difference method.

Near-Wake Turbulent Flow Structure and Mixing Length Downstream of a Fractal Tree

Abstract: In order to study the turbulence structure behind a multiscale tree-like element in a boundary layer, detailed particle image velocimetry measurements are carried out in the near-wake of a fractal-like tree. The tree is a pre-fractal with five generations, each consisting of three branches and a scale-reduction factor of 1/2 between consecutive generations. Detailed mean velocity and turbulence stress profiles are documented, as well as their downstream development. Scatter plots of mean velocity gradient (transverse shear in the wake) and Reynolds shear stress exhibit a good linear relation at all locations in the flow. Therefore, in the transverse direction of the wake evolution, the data support the Boussinesq eddy-viscosity concept. The measured mixing length increases with streamwise distance, in agreement with classic wake expansion rates. Conversely, the measured eddy viscosity and mixing length in the transverse direction decrease with increasing elevation, which differs from the behaviours measured in the vertical direction in traditional boundary layers or in canopy flows studied before. In order to find an appropriate single length scale to describe the wake evolution behind a multiscale object, two models are proposed, based on the notion of superposition of scales. One approach is based on the radial spectrum of the object while the second is based on its length-scale distribution evaluated using fractal geometry tools. Both proposed models agree well with the measured mixing length. The results suggest that information about multiscale clustering of branches must be incorporated into models of the mixing length for flows through single or sparse canopies of multiscale trees.

Shock structure in shock-turbulence interactions

Abstract: The structure of a shock wave interacting with isotropic turbulence is investigated. General principles of similarity scaling show that consistency with known physical limiting behavior requires incomplete similarity solutions where the governing non-dimensional parameters, namely, the Reynolds, convective, and turbulent Mach numbers (Rλ, M, and Mt, respectively), can be combined to reduce the number of similarity parameters that describes the phenomenon. An important parameter is found to be K = Mt/Rλ1/2(M − 1) which is proportional to the ratio of laminar shock thickness to the Kolmogorov length scale. The shock thickness under turbulent conditions, on the other hand, is essentially a random variable. Under a quasi-equilibrium assumption, shown to be valid when K2 ≪ 1, analytical results are obtained for the first and second moments of the turbulent shock thickness, velocity gradient, and dilatation at the shock. It is shown that these quantities exhibit universal behavior in the parameter K with correc...

The high-mass disk candidates NGC7538IRS1 and NGC7538S

Abstract: Context: The nature of embedded accretion disks around forming high-mass stars is one of the missing puzzle pieces for a general understanding of the formation of the most massive and luminous stars. Methods: Using the Plateau de Bure Interferometer at 1.36mm wavelengths in its most extended configuration we probe the dust and gas emission at ~0.3",corresponding to linear resolution elements of ~800AU. Results: NGC7538IRS1 remains a single compact and massive gas core with extraordinarily high column densities, corresponding to visual extinctions on the order of 10^5mag, and average densities within the central 2000AU of ~2.1x10^9cm^-3 that have not been measured before. We identify a velocity gradient across in northeast-southwest direction that is consistent with the mid-infrared emission, but we do not find a gradient that corresponds to the proposed CH3OH maser disk. The spectral line data toward NGC7538IRS1 reveal strong blue- and red-shifted absorption toward the mm continuum peak position. The red-shifted absorption allows us to estimate high infall rates on the order of 10^-2 Msun/yr. Although we cannot prove that the gas will be accreted in the end, the data are consistent with ongoing star formation activity in a scaled-up low-mass star formation scenario. Compared to that, NGC7538S fragments in a hierarchical fashion into several sub-sources. While the kinematics of the main mm peak are dominated by the accompanying jet, we find rotational signatures from a secondary peak. Furthermore, strong spectral line differences exist between the sub-sources which is indicative of different evolutionary stages within the same large-scale gas clump.

Velocity and velocity gradient based properties of a turbulent plane mixing layer

Abstract: Experiments were carried out in a turbulent mixing layer designed to match, as closely as possible, the conditions of the temporally evolving direct numerical simulation of Rogers & Moser (Phys. Fluids, vol. 6, 1994, pp. 903–922). Two Reynolds numbers, based on the local momentum thickness in the self-similar region of the mixing layer, were investigated: and . Measurements were also made in the mixing layer in the pre-mixing transition region where . The three velocity components and their cross-stream gradients were measured with a small 12-sensor hot-wire probe that traversed the mixing layer. Taylor’s hypothesis was used to estimate the streamwise gradients of the velocity components so that reasonably good approximations of all the components of the velocity gradient tensor would be available. The signal from a single-sensor probe at a fixed position in the high-speed free stream flow provided a reference to the phases of the passage of large-scale, coherent, spanwise-oriented vortices past the 12-sensor probe. The velocity and velocity gradient data were analysed to determine turbulence statistical characteristics, including moments, probability density functions and one-dimensional spectra of the velocity and vorticity fields. Although the velocity statistics obtained from the experiment agree well with those from the direct numerical simulation of Rogers & Moser, there are significant differences in the vorticity statistics. The phase reference from the fixed single-sensor probe permitted phase averaging of the 12-sensor probe data so that the spanwise ‘roller’ vortices could be separated from the small-scale, more random turbulence, as had been previously demonstrated by Hussain & Zaman (J. Fluid Mech., vol. 159, 1985, pp. 85–104). In this manner, the data could be conditionally averaged to examine the spatial distributions, with respect to the roller vortices, of interesting and important characteristics of the turbulence, such as the turbulent kinetic energy production and dissipation rate, enstrophy and vorticity component covariances.

Subcritical fluctuations and suppression of turbulence in differentially rotating gyrokinetic plasmas

Abstract: Differential rotation is known to suppress linear instabilities in fusion plasmas. However, numerical experiments show that even in the absence of growing eigenmodes, subcritical fluctuations that grow transiently can lead to sustained turbulence, limiting the ability of the velocity shear to suppress anomalous transport. Here transient growth of electrostatic fluctuations driven by the parallel velocity gradient (PVG) and the ion temperature gradient (ITG) in the presence of a perpendicular (E × B) velocity shear is considered. The maximally simplified (but, as numerical simulations suggest, most promising for transport reduction) case of zero magnetic shear is treated in the framework of a local shearing box approximation. In this case there are no linearly growing eigenmodes, so all excitations are transient. In the PVG-dominated regime, the maximum amplification factor is found to be eN with N ∝ q/ϵ (safety factor/inverse aspect ratio), the maximally amplified wavenumbers perpendicular and parallel to the magnetic field are related by kyρi ≈ (ϵ/q)1/3k∥vthi/S, where ρi is the ion Larmor radius, vthi the ion thermal speed and S the E × B shear. In the ITG-dominated regime, N is independent of wavenumber and N ∝ vthi/(LTS), where LT is the ion-temperature scale length. Intermediate ITG–PVG regimes are also analysed and N is calculated as a function of q/ϵ, LT and S. Analytical results are corroborated and supplemented by linear gyrokinetic numerical tests. Regimes with N ≲ 1 for all wavenumbers are possible for sufficiently low values of q/ϵ (≲7 in our model); ion-scale turbulence is expected to be fully suppressed in such regimes. For cases when it is not suppressed, an elementary heuristic theory of subcritical PVG turbulence leading to a scaling of the associated ion heat flux with q, ϵ, S and LT is proposed; it is argued that the transport is much less ‘stiff’ than in the ITG regime.

A comparison of the shear stress distribution in the bottom boundary layer of experimental density and turbidity currents

Abstract: The internal stress distribution within weakly depositional turbidity currents has often been assumed to be similar to saline gravity currents. This assumption is investigated by analyzing a series of experiments to quantify and compare the shear stress distribution in the bottom boundary layer (BBL) of saline and particle-laden gravity currents. Vertical profiles of Reynolds stresses, viscous stresses and turbulent kinetic energy (TKE) were obtained from the mean downstream velocity profiles and turbulent velocity fluctuations, and were broadly similar in both flow types, suggesting that saline gravity currents are a good analogue to turbidity currents. Maximum positive Reynolds stresses occur where the velocity gradient is largest in the BBL but below this maximum, the Reynolds stresses decrease significantly and are balanced by an increase of viscous stresses. The bulk drag coefficients C D is defined for both flows using three methods, (i) a log-fit method based on the law of the wall, (ii) the observed maximum total stress and (iii) direct measurements of turbulent velocities. The C D values of both flow types were broadly similar but each method led to C D values of different orders of magnitude. The log-fit method yielded the largest drag coefficients of O ( 1 0 − 2 ) whereas measurements of turbulent velocities gave relatively small values of O ( 1 0 − 4 ) . The best correlation with drag coefficients observed in field measurements of O ( 1 0 − 3 ) was obtained by using the maximum total stresses next to the wall. The variation of C D is discussed in relation to parameterization methods in experimental and numerical modeling.

Simulation of a propelled wake with moderate excess momentum in a stratified fluid

Abstract: Direct numerical simulation is used to simulate the turbulent wake behind an accelerating axisymmetric self-propelled body in a stratified fluid. Acceleration is modelled by adding a velocity profile corresponding to net thrust to a self-propelled velocity profile resulting in a wake with excess momentum. The effect of a small to moderate amount of excess momentum on the initially momentumless self-propelled wake is investigated to evaluate if the addition of excess momentum leads to a large qualitative change in wake dynamics. Both the amount and shape of excess momentum are varied. Increasing the amount of excess momentum and/or decreasing the radial extent of excess momentum was found to increase the defect velocity, mean kinetic energy, shear in the velocity gradient and the wake width. The increased shear in the mean profile resulted in increased production of turbulent kinetic energy leading to an increase in turbulent kinetic energy and its dissipation. Slightly larger vorticity structures were observed in the late wake with excess momentum although the differences between vorticity structures in the self-propelled and 40 % excess momentum cases was significantly smaller than suggested by previous experiments. Buoyancy was found to preserve the doubly inflected velocity profile in the vertical direction, and similarity for the mean velocity and turbulent kinetic energy was found to occur in both horizontal and vertical directions. While quantitative differences were observed between cases with and without excess momentum, qualitatively similar evolution was found to occur.

Dispersion in electroosmotic flow generated by oscillatory electric field interacting with oscillatory wall potentials

Abstract: An analytical study is presented in this article on the dispersion of a neutral solute released in an oscillatory electroosmotic flow (EOF) through a two-dimensional microchannel. The flow is driven by the nonlinear interaction between oscillatory axial electric field and oscillatory wall potentials. These fields have the same oscillation frequency, but with disparate phases. An asymptotic method of averaging is employed to derive the analytical expressions for the steady-flow-induced and oscillatory-flow-induced components of the dispersion coefficient. Dispersion coefficients are functions of various parameters representing the effects of electric double-layer thickness (Debye length), oscillation parameter, and phases of the oscillating fields. The time–harmonic interaction between the wall potentials and electric field generates steady as well as time-oscillatory components of electroosmotic flow, each of which will contribute to a steady component of the dispersion coefficient. It is found that, for a thin electric double layer, the phases of the oscillating wall potentials will play an important role in determining the magnitude of the dispersion coefficient. When both phases are zero (i.e., full synchronization of the wall potentials with the electric field), the flow is nearly a plug flow leading to very small dispersion. When one phase is zero and the other phase is π, the flow will be sheared to the largest possible extent at the center of the channel, and such a sharp velocity gradient will lead to the maximum possible dispersion coefficient.

The Discrete Duality Finite Volume Method for Stokes Equations on Three-Dimensional Polyhedral Meshes

Abstract: We develop a discrete duality finite volume method for the three-dimensional steady Stokes problem with a variable viscosity coefficient on polyhedral meshes. Under very general assumptions on the mesh, which may admit nonconforming polyhedrons, we prove the stability and well-posedness of the scheme. We also prove the convergence of the numerical approximation to the velocity, velocity gradient, and pressure and derive a priori estimates for the corresponding approximation error. Final numerical experiments confirm the theoretical predictions.

The zero-turbulence manifold in fusion plasmas

Abstract: The transport of heat that results from turbulence is a major factor limiting the temperature gradient, and thus the performance, of fusion devices. We use nonlinear simulations to show that a toroidal equilibrium scale sheared flow can completely suppress the turbulence across a wide range of flow gradient and temperature gradient values. We demonstrate the existence of a bifurcation across this range whereby the plasma may transition from a low flow gradient and temperature gradient state to a higher flow gradient and temperature gra- dient state. We show further that the maximum temperature gradient that can be reached by such a transition is limited by the existence, at high flow gradient, of subcritical turbulence driven by the parallel velocity gradient (PVG). We use linear simulations and analytic calculations to examine the properties of the transiently growing modes which give rise to this subcritical turbulence, and conclude that there may be a critical value of the ratio of the PVG to the suppressing perpendicular gradient of the velocity (in a tokamak this ratio is equal to q/{\epsilon} where q is the magnetic safety factor and {\epsilon} the inverse aspect ratio) below which the PVG is unable to drive subcritical turbulence. In light of this, we use nonlinear simulations to calculate, as a function of three parameters (the perpendicular flow shear, q/{\epsilon} and the temperature gradient), the surface within that parameter space which divides the regions where turbulence can and cannot be sustained: the zero- turbulence manifold. We are unable to conclude that there is in fact a critical value of q/{\epsilon} below which PVG-driven turbulence is eliminated. Nevertheless, we demonstrate that at low values of q/{\epsilon}, the maximum critical temperature gradient that can be reached without generating turbulence is dramatically increased.

Deformation field in indentation of a granular ensemble.

Abstract: An experimental study has been made of the flow field in indentation of a model granular material. A granular ensemble composed of spherical sand particles with average size of 0.4 mm is indented with a flat ended punch under plane-strain conditions. The region around the indenter is imaged in situ using a high-speed charge-coupled device (CCD) imaging system. By applying a hybrid image analysis technique to image sequences of the indentation, flow parameters such as velocity, velocity gradient, and strain rate are measured at high resolution. The measurements have enabled characterization of the main features of the flow such as dead material zones, velocity jumps, localization of deformation, and regions of highly rotational flow resembling vortices. Implications for validation of theoretical analyses and applications are discussed.

Hydrodynamic diffusion of a suspension of elastic capsules in bounded simple shear flow

Abstract: A suspension of red blood cells in a flow undergoes hydrodynamic or shear-induced diffusion. It is known from experiments that deoxygenated or stiffer red blood cells have a lower hydrodynamic diffusion coefficient compared to oxygenated or softer red blood cells. In this paper, we numerically calculate the hydrodynamic diffusion coefficients of a suspension of elastic capsules of viscosity ratio unity and as a function of volume fraction, elastic capillary number and channel height in a bounded simple shear flow. We show that the time required for the suspension to reach the diffusive regime in the direction perpendicular to the shear plane decreases with channel height. In a narrow channel, the effect of capsule elasticity is to delay the approach to a diffusive regime. However, the motion in the direction parallel to the velocity gradient is always subdiffusive. We show that the hydrodynamic diffusion coefficient in the direction perpendicular to the shear plane varies linearly with capsule volume fraction up to about 25%. In addition, it does not increase monotonously with elastic capillary number but drops when the capsules become sufficiently soft. Finally, it displays a weak dependence on the channel height.

The Extremely High-Velocity Outflow from the Luminous Young Stellar Object G5.89-0.39

Abstract: We have imaged the extremely high velocity outflowing gas in CO (2-1) and (3-2) associated with the shell-like ultracompact H II region G5.89–0.39 at a resolution of ~3'' (corresponding to ~4000 AU) with the Submillimeter Array. The integrated high-velocity (45 km s–1) CO emission reveals at least three blueshifted lobes and two redshifted lobes. These lobes belong to two outflows, one oriented N-S, the other NW-SE. The NW-SE outflow is likely identical to the previously detected Brγ outflow. Furthermore, these outflow lobes all clearly show a Hubble-like kinematic structure. For the first time, we estimate the temperature of the outflowing gas as a function of velocity with large velocity gradient calculations. Our results reveal a clear increasing trend of temperature with gas velocity. The observational features of the extremely high velocity gas associated with G5.89–0.39 qualitatively favor the jet-driven bow shock model.

Comparison of Reynolds Averaged Navier-Stokes Based Simulation and Large-eddy Simulation for One Isothermal Swirling Flow

Abstract: The flow structure of one isothermal swirling case in the Sydney swirl flame database was studied using two numerical methods. Results from the Reynolds-averaged Navier-Stokes (RANS) approach and large eddy simulation (LES) were compared with experimental measurements. The simulations were applied in two different Cartesian grids which were investigated by a grid independence study for RANS and a post-estimator for LES. The RNG k-ɛ turbulence model was used in RANS and dynamic Smagorinsky-Lilly model was used as the sub-grid scale model in LES. A validation study and cross comparison of ensemble average and root mean square (RMS) results showed LES outperforms RANS statistic results. Flow field results indicated that both approaches could capture dominant flow structures, like vortex breakdown (VB), and precessing vortex core (PVC). Streamlines indicate that the formation mechanisms of VB deducted from the two methods were different. The vorticity field was also studied using a velocity gradient based method. This research gained in-depth understanding of isothermal swirling flow.

Role of mixed boundaries on flow in open capillary channels with curved air-water interfaces.

Abstract: Flow in unsaturated porous media or in engineered microfluidic systems is dominated by capillary and viscous forces. Consequently, flow regimes may differ markedly from conventional flows, reflecting strong interfacial influences on small bodies of flowing liquids. In this work, we visualized liquid transport patterns in open capillary channels with a range of opening sizes from 0.6 to 5.0 mm using laser scanning confocal microscopy combined with fluorescent latex particles (1.0 μm) as tracers at a mean velocity of ∼0.50 mm s(-1). The observed velocity profiles indicate limited mobility at the air-water interface. The application of the Stokes equation with mixed boundary conditions (i.e., no slip on the channel walls and partial slip or shear stress at the air-water interface) clearly illustrates the increasing importance of interfacial shear stress with decreasing channel size. Interfacial shear stress emerges from the velocity gradient from the adjoining no-slip walls to the center where flow is trapped in a region in which capillary forces dominate. In addition, the increased contribution of capillary forces (relative to viscous forces) to flow on the microscale leads to increased interfacial curvature, which, together with interfacial shear stress, affects the velocity distribution and flow pattern (e.g., reverse flow in the contact line region). We found that partial slip, rather than the commonly used stress-free condition, provided a more accurate description of the boundary condition at the confined air-water interface, reflecting the key role that surface/interface effects play in controlling flow behavior on the nanoscale and microscale.

The scattering of Lyα radiation in the intergalactic medium: numerical methods and solutions

Abstract: Two methods are developed for solving the steady-state spherically symmetric radiative transfer equation for resonance line radiation emitted by a point source in the intergalactic medium, in the context of the Wouthuysen–Field mechanism for coupling the hyperfine structure spin temperature of hydrogen to the gas temperature. One method is based on solving the ray and moment equations using finite differences. The second uses a Monte Carlo approach incorporating methods that greatly improve the accuracy compared with previous approaches in this context. Several applications are presented serving as test problems for both a static medium and an expanding medium, including inhomogeneities in the density and velocity fields. Solutions are obtained in the coherent scattering limit and for Doppler RII redistribution with and without recoils. We find generally that the radiation intensity is linear in the cosine of the azimuthal angle with respect to radius to high accuracy over a broad frequency region across the line centre for both linear and perturbed velocity fields, yielding the Eddington factors fν ≃ 1/3 and gν ≃ 3/5. The radiation field produced by a point source divides into three spatial regimes for a uniformly expanding homogeneous medium. The regimes are governed by the fraction of the distance r from the source in terms of the distance r* required for a photon to redshift from line centre to the frequency needed to escape from the expanding gas. For a standard cosmology, before the Universe was reionized r* takes on the universal value independent of redshift of 1.1 Mpc, depending only on the ratio of the baryon to dark matter density. At r/r* 1, the diffusion approximation breaks down and the decline of the mean intensity near line centre and the scattering rate approach the geometric dilution scaling 1/r2. The mean intensity and scattering rate are found to be very sensitive to the gradient of the velocity field, growing exponentially with the amplitude of the perturbation as the limit of a vanishing velocity gradient is approached near the source. We expect the 21-cm signal from the epoch of reionization to thus be a sensitive probe of both the density and the peculiar velocity fields. The solutions for the mean intensity are made available in machine-readable format.

Experimental Study on the Correlation Between Stress and P-Wave Velocity for Burst Tendency Coal-Rock Samples

Abstract: To explore the mechanism of rockburst,we established the experimental model between stress and P-wave velocity for burst tendency coal and rock samples from deep coal mine under the uniaxial loadings,and analyzed the parameters of the derived model.The results show that:1) the P wave velocity in rock sample changes more sensitively than that in coal sample under loadings,which means the P wave velocity in rock sample changes more quicker with the increase of stress;2) the velocity gradient of the sample is usually high at the elastic stage and then begins to level out at the plastic stage under the uniaxial loading,which shows that a power function is exist between the P-wave velocity and stress.The calculated correlation coefficients to measured value show that the experimental model has high fitness,and can be well used to describe the stress distribution and identify the risk level of rockburst in mines.

Seismic ground motion amplification in a 3D sedimentary basin: the effect of the vertical velocity gradient

Abstract: Ground motion amplification in sedimentary basins has been observed in some moderate or large earthquakes, such as the 1994 Northridge and 1999 Chi-Chi event. Many numerical studies with simplified 2D models have shown significant effects of the vertical velocity gradient of sediment on basin amplification. However, we need to consider a more realistic 3D model and solve wave equations with 3D numerical methods in order to improve our understanding of basin amplification. In this study, we extend a 2D pseudospectral and finite difference hybrid method to a 3D case and investigate the effects of the vertical velocity gradient for a 3D basin model. Numerical simulations were performed for four basin models with increasing vertical velocity gradients on a PC cluster using 64 processors for 67 108 864 discretized grids. The results show that the vertical velocity gradient enhances basin amplification through strong secondary surface waves and basin trapped waves. The 3D geometry of the basin causes a wave-front focusing effect that contributes significantly to a localized strong amplification with the maximum peak ground velocity in the basin. The results of this study suggest that it is important to consider the detailed properties of sedimentary basins in seismic ground motion studies.

The scattering of LyA radiation in the intergalactic medium: numerical methods and solutions

Abstract: Two methods are developed for solving the steady-state spherically symmetric radiative transfer equation for resonance line radiation emitted by a point source in the Intergalactic Medium. One method is based on solving the ray and moment equations using finite differences. The second uses a Monte Carlo approach incorporating methods that greatly improve the accuracy compared with previous approaches in this context. Several applications are presented serving as test problems for both a static medium and an expanding medium, including inhomogeneities in the density and velocity fields. Solutions are obtained in the coherent scattering limit and for Doppler RII redistribution with and without recoils. We find generally that the radiation intensity is linear in the cosine of the azimuthal angle with respect to radius to high accuracy over a broad frequency region across the line centre for both linear and perturbed velocity fields, yielding the Eddington factors f(nu) = 1/3 and g(nu) = 3/5. We show the radiation field produced by a point source divides into three spatial regimes for a uniformly expanding homogeneous medium: at radii r small compared with a characteristic radius r*, the mean intensity near line centre varies as 1/ r^(7/3), while at r > r* it approaches 1/ r^2; for r << r* it is modified by frequency redistribution. Before the reionization epoch, r* takes on the universal value 1.1 Mpc, independent of redshift. The mean intensity and scattering rate are found to be very sensitive to the gradient of the velocity field, growing exponentially with the amplitude of the perturbation as the limit of a vanishing velocity gradient is approached near the source. We expect the 21cm signal from the Epoch of Reionization to thus be a sensitive probe of both the density and the peculiar velocity fields.

Suppression of turbulence and subcritical fluctuations in differentially rotating gyrokinetic plasmas

Abstract: Differential rotation is known to suppress linear instabilities in fusion plasmas. However, numerical experiments show that even in the absence of growing eigenmodes, subcritical fluctuations that grow transiently can lead to sustained turbulence, limiting the ability of the velocity shear to suppress anomalous transport. Here transient growth of electrostatic fluctuations driven by the parallel velocity gradient (PVG) and the ion temperature gradient (ITG) in the presence of a perpendicular (E × B) velocity shear is considered. The maximally simplified (but most promising for transport reduction) case of zero magnetic shear is treated in the framework of a local shearing box approximation. In this case there are no linearly growing eigenmodes, so all excitations are transient. In the PVG-dominated regime, the maximum amplification factor is found to be e with N ∝ q/ǫ (safety factor/aspect ratio), the maximally amplified wavenumbers perpendicular and parallel to the magnetic field are related by kyρi ≈ (ǫ/q)k‖vthi/S, where ρi is the ion Larmor radius, vthi the ion thermal speed and S the E × B shear. In the ITGdominated regime, N is independent of wavenumber and N ∝ vthi/(LTS), where LT is the ion-temperature scale length. Intermediate ITG-PVG regimes are also analysed and N is calculated as a function of q/ǫ, LT and S. Analytical results are corroborated and supplemented by linear gyrokinetic numerical tests. Regimes with N . 1 for all wavenumbers are possible for sufficiently low values of q/ǫ (. 7 in our model); ion-scale turbulence is expected to be fully suppressed in such regimes. For cases when it is not suppressed, an elementary heuristic theory of subcritical PVG turbulence leading to a scaling of the associated ion heat flux with q, ǫ, S and LT is proposed; it is argued that the transport is much less “stiff” than in the ITG regime. PACS numbers: 52.30.Gz, 52.35.Qz, 52.35.Ra, 52.55.Fa Subcritical fluctuations in rotating plasmas 2

Extraction of shear viscosity in stationary states of relativistic particle systems.

Abstract: Starting from a classical picture of shear viscosity we construct a stationary velocity gradient in a microscopic parton cascade. Employing the Navier-Stokes ansatz we extract the shear viscosity coefficient η. For elastic isotropic scatterings we find an excellent agreement with the analytic values. This confirms the applicability of this method. Furthermore, for both elastic and inelastic scatterings with pQCD based cross sections we extract the shear viscosity coefficient η for a pure gluonic system and find a good agreement with already published calculations.

Flame tip traveling in boundary layer flow with flammable mixture along fuel-soaked ground

Abstract: From the point of view of a fundamental study on flame propagation along ground soaked with high volatile liquid fuels, the effect of an entrained flammable mixture by airflow toward theignition location on traveling of a flame tip along the fuel-spillage area was studied. The thickness of flammable mixture layer is about one tenth of that of velocity boundary layer ofthe airflow. After the onset of ignition, the flame tip travels through the thin flammable mixture layer in the airflow with large velocity gradient. The effect of airflow velocity on the traveling of flame tip (both of upstream and downstream) was investigated. The dependence of the flame tip traveling on the airflow velocity is classified into some regions by the ratio of the airflow velocity about 1 mm above the ground surface to the flame propagation velocity with no surrounding airflow. The critical airflow velocity for the blowoff of flame tip (upstream and downstream) strongly depends on the quantity of flammable mixture entrained tow...

Hydrodynamic-induced enantiomeric enrichment of self-assemblies: role of the solid-liquid interface in chiral nucleation and seeding.

Abstract: A simple hydrodynamic model has been developed to explain the experimentally observed chirality selection in stirred solutions of self-assembling achiral dyes. Selection depends on the stirring direction: the dichroic signal reverses its shape in clockwise or anti-clockwise rotations. Our model investigates the possible role of the liquid-solid interface in nucleating, growing, and transferring to the bulk of chiral seeds. The nucleation step requires a double modulation of the hydrodynamic field exhibiting different velocity along two orthogonal axes. Under a series of restrictions, such a condition is easily met at the solid-liquid interface and it is dictated by the boundary conditions and geometry of stirring. In stagnant conditions, growing helices made-up of self-assembled achiral dyes have no chiral preference forming a racemic mixture that contains identical amount of right-handed (R) and left-handed (L) configurations. The application of a hydrodynamic torque (related to the velocity gradient and width of the helix) breaks down the original symmetry, a further velocity gradient perpendicular to the first one ensures, after averaging, a slightly different population of R and L conformations. The yields of the hydrodynamic-induced chirality excess are extremely tiny, hence the suggested mechanism is significant only if next chirality amplification processes are efficient. Again, hydrodynamics provides a tool for the detachment of weakly bound aggregates once they have reached a critical length. Aggregates are transported in the bulk where the ripening process goes to completion. The efficiency of the surface catalytic effect strongly depends on the aggregate-surface sticking energy, reaching a maximum at intermediate sticking energies (of order of 10 kT). Numerical estimates show that the proposed mechanism is rather efficient, giving rise to entatiomeric excesses near (but smaller than) those experimentally found.