Gradient Scale Research Articles

Effect of external axial magnetic field on the early stage instabilities in magnetized cylindrical liners

Based on the resistive MHD theory, the dispersion relations of instabilities are derived in the liner geometry, where a uniform current flows over the cross section. Both the external axial magnetic field and the electrothermal effect are taken into account. It is found that instabilities act as electrothermal instabilities (ETIs) below a critical wavelength, which is increased by the axial magnetic field. Beyond the critical wavelength, the growth rate increases with the wavelength quickly and approaches the maximum which depends on the gradient scale length of the azimuthal magnetic field. When the axial magnetic field is applied, the m = 1 mode has a higher growth rate than the m = 0 mode although the difference between them is relatively small.

Multiscale fluid–particle thermal interaction in isotropic turbulence

We use direct numerical simulations to investigate the interaction between the temperature field of a fluid and the temperature of small particles suspended in the flow, employing both one- and two-way thermal coupling, in a statistically stationary, isotropic turbulent flow. Using statistical analysis, we investigate this variegated interaction at the different scales of the flow. We find that the variance of the carrier flow temperature gradients decreases as the thermal response time of the suspended particles is increased. The probability density function (PDF) of the carrier flow temperature gradients scales with its variance, while the PDF of the rate of change of the particle temperature, whose variance is associated with the thermal dissipation due to the particles, does not scale in such a self-similar way. The modification of the fluid temperature field due to the particles is examined by computing the particle concentration and particle heat fluxes conditioned on the magnitude of the local fluid temperature gradient. These statistics highlight that the particles cluster on the fluid temperature fronts, and the important role played by the alignments of the particle velocity and the local fluid temperature gradient. The temperature structure functions, which characterize the temperature fluctuations across the scales of the flow, clearly show that the fluctuations of the carrier flow temperature increments are monotonically suppressed in the two-way coupled regime as the particle thermal response time is increased. Thermal caustics dominate the particle temperature increments at small scales, that is, particles that come into contact are likely to have very large differences in their temperatures. This is caused by the non-local thermal dynamics of the particles: the scaling exponents of the inertial particle temperature structure functions in the dissipation range reveal very strong multifractal behaviour. Further insight is provided by the flux of temperature increments across the scales. Altogether, these results reveal a number of non-trivial effects, with a number of important practical consequences.

Edge flame propagation statistics in igniting monodisperse droplet-laden mixtures

The effects of droplet diameter, overall (i.e., liquid+gaseous phases) equivalence ratio, and turbulence intensity on the edge flame propagation statistics for localized forced ignition of uniformly dispersed n-heptane droplet-laden mixtures under homogeneous isotropic decaying turbulence have been analyzed based on direct numerical simulations data. It has been found that the edge flame structure becomes increasingly prominent for large overall equivalence ratios and droplet diameters. Although the mean edge flame speed has been found to be positive and its most probable value remains comparable to the theoretical value for laminar edge flames in purely gaseous mixtures, the mean values have been found to decrease and the probabilities of finding locally negative edge flame speeds have been found to increase with increasing turbulence intensity. The marginal probability density function and curvature and strain rate dependences of the edge flame speed have been found to be principally governed by the displacement speed of the fuel mass fraction isosurface intersecting the stoichiometric mixture fraction isosurface. The displacement speed of the stoichiometric mixture fraction isosurface has also been found to influence the local scalar gradient dependences of the edge flame speed in this configuration, especially for large droplets. The displacement speed of the fuel mass fraction isosurface Sd has been found to be principally governed by leading order contributions of the reaction and molecular diffusion components and the evaporation contribution remains weak in comparison to these leading order contributors. The local edge flame speed exhibits nonlinear curvature and strain rate dependences and its variation with the magnitudes of both fuel mass fraction and mixture fraction gradients has been found to be nonmonotonic for all cases considered here. The correlations of the edge flame speed with curvature, strain rate, and scalar gradient have been found to be qualitatively similar to the corresponding statistics reported in the existing literature for edge flames in purely gaseous mixtures. Additionally, the curvature and tangential strain rate dependences of the edge flame speed have been found to be dependent on the droplet size and overall equivalence ratio, and these dependences become weak for cases with large droplets.

The Equilibrium Orientation of Scalar Gradient in a Lagrangian Model of Three-dimensional Turbulence

The kinematics of scalar gradient in three-dimensional turbulence is revisited using stochastic Lagrangian modelling. The equilibrium direction is computed numerically. It noticeably departs from the compressional direction, and is the attracting -- and preferential -- alignment for the scalar gradient.

The Application of Theta Method to Potential Field Gradient Tensor Data for Edge Detection of Complex Geological Structures

Edge detection of magnetized structures is an important application of magnetic filters for geological interpretations. In this way, one can mention methods of the potential field derivatives which are categorized into: methods based on scalar potential field and potential field gradient tensor (PGT) data. Using five independent signal components, methods based on the PGT matrices have higher accuracy than methods based on scalar potential field data. Investigations of various methods of edge detection show that as geologic conditions become more complex, they greatly lose their efficacy and sometimes suffer from distortions. These distortions probably result from the Gibbs phenomenon which occurs in the process of computing the potential field data derivatives. This phenomenon creates false edges and incorrect interpretations of data. The thetaPGT method addresses this phenomenon by combining the PGT matrix and the previously derived theta map method. The maximum value of thetaPGT delineates the horizontal edge of geologic contacts. Achieved results by applying the thetaPGT method on simple and then complex synthetic magnetic models demonstrates higher accuracy of the proposed method in comparison with the analytic signal, theta and horizontal directional theta (HDT) methods. The new method has no sensitivity to depth and strike, and also doesn’t create additional edges. Furthermore, it has lower noise sensitivity and significantly reduces the effect of the Gibbs phenomenon. Finally, applying this method on real magnetic data from the Varzeghan area, northwest Iran, with respect to other methods reveals more details of structures in the area and characterized false faults resulting from the Gibbs phenomenon.

Computational Investigation of Weakly Turbulent Flame Kernel Growths in Iso-Octane Droplet Clouds in CVC Conditions

Numerical simulations of turbulent flame kernel growths in monodisperse clouds of iso-octane liquid droplets are conducted in conditions relevant to constant volume combustors. The simulations make use of a low-Mach number Navier-Stokes solver and a thermodynamic pressure evolution model has been implemented to reproduce the pressure variation that may be issued from either experiments or from a standard (i.e., analytical) compression law. Chemistry is described with a representative skeletal mechanism featuring 29 species and 48 elementary reaction steps. The computational results clearly confirm the enhancement of flame propagation in constant volume combustion conditions. The impact of the droplet diameter on the turbulent flame development is scrutinized for two distinct values of the Stokes number St equal to 0.1 and 1.0. Significant influence on the flame dynamics is put into evidence. This is a direct outcome of the equivalence ratio and temperature heterogeneities, which are themselves very sensitive to the choice of the Stokes number value. Then, small-scale turbulence-scalar interactions (TSI) are studied by analyzing the fields of the scalar gradients and strain-rate. Their dynamics is investigated for both non-reactive and reactive two-phase flows conditions. The TSI analysis is performed on the basis of time evolution equations written for quantities that characterize the couplings between the velocity gradient tensor and scalar gradients vectors. Special emphasis is placed on the possible influence of mass exchange terms between the liquid and gaseous phases.

Effect of vanadium micro-alloying on the microstructure evolution and mechanical properties of 718H pre-hardened mold steel

The effects of different contents of vanadium (V) (0.1, 0.2, and 0.3 wt%) on the microstructure evolution and mechanical properties of 718H steel were investigated. The precipitate was characterized by means of atom probe tomography (APT) and bright-field transmission electron microscopy (TEM). The increase in V content has great benefits for strength, but has an adverse effect on impact toughness. The strength increase can be attributed to the influence of V addition on dislocation density, misorientation gradient, and fine scale MC precipitates. Precipitation strengthening mainly contributes to the V-added steel by analyzing various strengthening mechanisms. However, fine scale MC precipitates can pin dislocation leading to a decrease in its mobility. A large number of immovable dislocations will increase the dislocation accumulation, internal stress and brittle cracking, resulting in a gradual decrease in impact toughness with the V addition. In addition, compared with V-free steel, the dissolved V content in austenite decreases the grain boundary energy and inhibits the diffusion of the C atoms, ultimately reducing the transformation range of pearlite (P).

A self-contained composition space solution method for strained and curved premixed flamelets

Solving flames in scalar or composition space usually requires the modeling of scalar dissipation rates, or scalar gradients, which appear in the Jacobian of the transformation from the physical-space coordinates to the scalar space coordinates attached to the flame surface. Recently, Scholtissek et al. (2019) have discussed a self-contained solution for freely propagating premixed flames, in which the gradient distribution of the coordinate is also solved in scalar space. This approach is here extended to include curvature, strain and effects of unsteadiness. The resulting method is applied to a constant pressure and homogeneous ignition case, a stagnation flame, a tubular flame and a spherical expanding flame, capturing accurately effects of unsteadiness, strain and curvature. The results are systematically validated against physical-space solutions and experiments. In addition to the possibility of exploring these various canonical problems with a single set of equations, it is shown that another major outcome of this new formulation lies in the possibility of studying steady flames subjected to negative strain.

Asymmetry of vertical buoyancy gradient in stratified turbulence

We consider the asymmetry of the buoyancy field in the vertical direction in stratified turbulence. While this asymmetry is known, its causes are not well understood, and it has not been systematically quantified previously. Using theoretical arguments, it is shown that both stratified turbulence and isotropic turbulence in the presence of a mean scalar gradient will become positively skewed, as a direct consequence of the presence of stratification and mean scalar gradient, respectively. Assuming a rapid adjustment of isotropic turbulence to a stable stratification on a time scale $\unicode[STIX]{x1D70F}\sim N^{-1}$, where $N$ is the Brunt–Väisälä frequency, a scaling for the skewness of the vertical buoyancy gradient is obtained. Direct numerical simulations of stratified turbulence with forcing are performed and the positive skewness of $\unicode[STIX]{x2202}b/\unicode[STIX]{x2202}z$ is confirmed ($b$ is the buoyancy). Both the volume-averaged dimensional skewness, $\langle (\unicode[STIX]{x2202}b/\unicode[STIX]{x2202}z)^{3}\rangle$, and the non-dimensional skewness, $S$, are computed and compared against the theoretical predictions. There is a good agreement for $\langle (\unicode[STIX]{x2202}b/\unicode[STIX]{x2202}z)^{3}\rangle$, while there is a discrepancy in the behaviour of $S$. The theory predicts $S\sim 1$ and a constant skewness, while the direct numerical simulations confirm that the skewness is $O(1)$ but with a remaining dependence on the Froude number. The results are interpreted as being due to the concurrent action of linear and nonlinear processes in stratified turbulence leading to $S>0$ and to the formation of layers and interfaces in vertical profiles of buoyancy.

Does depth-cue combination yield identical biases in perception and grasping?

Grasping critically depends on stereo information. We previously found that binocular disparities yield a distorted visual space, in which objects close to the observer are grasped and perceived as if they were more elongated than farther objects. Such lack of shape constancy results from the inaccurate estimate of the viewing distance, which affects the estimated depth-to-width ratio of an object. This is because (1) depth from binocular disparities scales with the square of the distance and (2) width from retinal size scales linearly with distance. Conversely, depth from monocular cues (i.e., motion and texture gradients) scales linearly with distance, hence the overall shape from these signals should not be affected by errors in egocentric estimates of object location. We therefore reasoned that adding these cues to stereo information should improve shape constancy. Contrary to expectations, in four experiments we found that stereo-texture and stereo-motion stimuli appeared even more distorted than stereo stimuli. More remarkably, results revealed that grasping execution showed identical biases, which were corrected only late in the movement through online control mechanisms, but only if both grasping digits could be visually guided on their respective contact locations. On the contrary, when the index was occluded by the object, biases in shape estimation continued to affect grasping execution until movement completion. Moreover, while the initial part of the grasp showed evidence of collision avoidance, a control experiment suggested that the above biases could have emerged as early as at movement planning, consistent with previous evidence. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Growth of Rayleigh instability in a Hall thruster channel having dust in exit region

In a Hall thruster, the axial movement of ions towards the exit of the chamber and the divergence of ion beam sputter the wall and thus the maximum erosion of wall of the exit region takes place. This way the dust is generated, which is largely confined near the exit of the channel. By considering dust particles with negative charge, we derive and solve the dispersion equation for the growth of Rayleigh instability in the thruster plasma and examine its growth rate for different density, charge, mass and temperature of the dust particles. We also talk about the role of density gradient scale length on the growth of the instability.

Is the Power Threshold of 0.8 Applicable to Surgical Science?—Empowering the Underpowered Study

BackgroundMany articles in the surgical literature were faulted for committing type 2 error, or concluding no difference when the study was “underpowered”. However, it is unknown if the current power standard of 0.8 is reasonable in surgical science. MethodsPubMed was searched for abstracts published in Surgery, JAMA Surgery, and Annals of Surgery and from January 1, 2012 to December 31, 2016, with Medical Subject Heading terms of randomized controlled trial (RCT) or observational study (OBS) and limited to humans were included (n = 403). Articles were excluded if all reported findings were statistically significant (n = 193), or if presented data were insufficient to calculate power (n = 141). ResultsA total of 69 manuscripts (59 RCTs and 10 OBSs) were assessed. Overall, the median power was 0.16 (interquartile range [IQR] 0.08-0.32). The median power was 0.16 for RCTs (IQR 0.08-0.32) and 0.14 for OBSs (IQR 0.09-0.22). Only 4 studies (5.8%) reached or exceeded the current 0.8 standard. Two-thirds of our study sample had an a priori power calculation (n = 41). ConclusionsHigh-impact surgical science was routinely unable to reach the arbitrary power standard of 0.8. The academic surgical community should reconsider the power threshold as it applies to surgical investigations. We contend that the blueprint for the redesign should include benchmarking the power of articles on a gradient scale, instead of aiming for an unreasonable threshold.

Scale invariance of BMP signaling gradients in zebrafish

In both vertebrates and invertebrates, spatial patterning along the Dorsal-ventral (DV) embryonic axis depends on a morphogen gradient of Bone Morphogenetic Protein signaling. Scale invariance of DV patterning by BMPs has been found in both vertebrates and invertebrates, however the mechanisms that regulate gradient scaling remain controversial. To obtain quantitative data that can be used to address core questions of scaling, we introduce a method to tune the size of zebrafish embryos by reducing varying amounts of vegetal yolk. We quantified the BMP signaling gradient in wild-type and perturbed embryos and found that the system scales for reductions in cross-sectional perimeter of up to 30%. Furthermore, we found that the degree of scaling for intraspecies scaling within zebrafish is greater than that between Danioninae species.

Multi-particle model of coarse-grained scalar dissipation rate with volumetric tensor in turbulence

A multi-particle model is proposed for a coarse-grained scalar dissipation rate, where a coarse-grained quantity is defined with an ensemble average of spatially-distributed fluid particles within a finite volume. The model computes the coarse-grained scalar dissipation rate from the coarse-grained scalar gradient with a subgrid scale model of the scalar dissipation rate, which requires length-scale estimation for particle distribution. A volumetric tensor that characterizes the particle distribution is used in the model for computing the length scale and the coarse-grained scalar and velocity gradients from the particles. The model is examined in a priori and posteriori tests. A priori test with direct numerical simulation (DNS) databases of turbulent planar jets shows that the present model works well for a wide range of the length scale of particle distribution when the number of particles NM is about 10-16. The model with NM≲10 overestimates the coarse-grained scalar dissipation rate, while NM≳16 causes stronger dependence of the model on the length scale of particle distribution. The proposed model is tested in hybrid large-eddy-simulation/Lagrangian-particle-simulation (LES/LPS) of planar jets, where the coarse-grained scalar dissipation rate appears as an unknown variable in a mixing volume model that computes a molecular diffusion term based on a multi-particle interaction. LES/LPS and DNS yield a similar profile of the root-mean-squared scalar fluctuation, which strongly depends on the scalar dissipation rate. Comparison of the mean scalar dissipation rate between the model and the DNS shows that the present model applied to the LES/LPS well predicts the coarse-grained scalar dissipation rate at various jet Reynolds numbers.

A Dynamic Adam Based Deep Neural Network for Fault Diagnosis of Oil-Immersed Power Transformers

This paper presents a Dynamic Adam and dropout based deep neural network (DADDNN) for fault diagnosis of oil-immersed power transformers. To solve the problem of incomplete extraction of hidden information with data driven, the gradient first-order moment estimate and second-order moment estimate are used to calculate the different learning rates for all parameters with stable gradient scaling. Meanwhile, the learning rate is dynamically attenuated according to the optimal interval. To prevent over-fitted, we exploit dropout technique to randomly reset some neurons and strengthen the information exchange between indirectly-linked neurons. Our proposed approach was utilized on four datasets to learn the faults diagnosis of oil-immersed power transformers. Besides, four benchmark cases in other fields were also utilized to illustrate its scalability. The simulation results show that the average diagnosis accuracies on the four datasets of our proposed method were 37.9%, 25.5%, 14.6%, 18.9%, and 11.2%, higher than international electro technical commission (IEC), Duval Triangle, stacked autoencoders (SAE), deep belief networks (DBN), and grid search support vector machines (GSSVM), respectively.

How the turbulent/non-turbulent interface is different from internal turbulence

The average patterns of the velocity and scalar fields near turbulent/non-turbulent interfaces (TNTI), obtained from direct numerical simulations (DNS) of planar turbulent jets and shear free turbulence, are assessed in the strain eigenframe. These flow patterns help to clarify many aspects of the flow dynamics, including a passive scalar, near a TNTI layer, that are otherwise not easily and clearly assessed. The averaged flow field near the TNTI layer exhibits a saddle-node flow topology associated with a vortex in one half of the interface, while the other half of the interface consists of a shear layer. This observed flow pattern is thus very different from the shear-layer structure consisting of two aligned vortical motions bounded by two large-scale regions of uniform flow, that typically characterizes the average strain field in the fully developed turbulent regions. Moreover, strain dominates over vorticity near the TNTI layer, in contrast to internal turbulence. Consequently, the most compressive principal straining direction is perpendicular to the TNTI layer, and the characteristic 45-degree angle displayed in internal shear layers is not observed at the TNTI layer. The particular flow pattern observed near the TNTI layer has important consequences for the dynamics of a passive scalar field, and explains why regions of particularly high scalar gradient (magnitude) are typically found at TNTIs separating fluid with different levels of scalar concentration. Finally, it is demonstrated that, within the fully developed internal turbulent region, the scalar gradient exhibits an angle with the most compressive straining direction with a peak probability at around 20$^{\text{o}}$. The scalar gradient and the most compressive strain are not preferentially aligned, as has been considered for many years. The misconception originated from an ambiguous definition of the positive directions of the strain eigenvectors.

Few-cycle laser wakefield acceleration on solid targets with controlled plasma scale length

We measure the emission of energetic electrons from the interaction between relativistic-intensity ultrashort laser pulses and a solid density plasma with a tunable density gradient scale length. We detect an electron beam that only appears with few-cycle pulses (<10 fs) and large plasma scale lengths (L > λ0). Numerical simulations, in agreement with the experiments, reveal that these electrons are accelerated by a laser wakefield. Plasma waves are indeed resonantly excited by the few-cycle laser pulses in the near-critical density region of the plasma. Electrons are then injected by ionization into the plasma waves and accelerated to relativistic energies. In this laser wakefield acceleration regime, the plasma waves are rotated by the plasma density gradient, which results in the electrons not being emitted in the same direction as the driving laser pulse.

Breast MRI and X-ray mammography registration using gradient values.

Breast magnetic resonance imaging (MRI) and X-ray mammography are two image modalities widely used for early detection and diagnosis of breast diseases in women. The combination of these modalities, traditionally done using intensity-based registration algorithms, leads to a more accurate diagnosis and treatment, due to the capability of co-localizing lesions and susceptibles areas between the two image modalities. In this work, we present the first attempt to register breast MRI and X-ray mammographic images using intensity gradients as the similarity measure. Specifically, a patient-specific biomechanical model of the breast, extracted from the MRI image, is used to mimic the mammographic acquisition. The intensity gradients of the glandular tissue are directly projected from the 3D MRI volume to the 2D mammographic space, and two different gradient-based metrics are tested to lead the registration, the normalized cross-correlation of the scalar gradient values and the gradient correlation of the vectoral gradients. We compare these two approaches to an intensity-based algorithm, where the MRI volume is transformed to a synthetic computed tomography (pseudo-CT) image using the partial volume effect obtained by the glandular tissue segmentation performed by means of an Expectation-Maximization algorithm. This allows us to obtain the digitally reconstructed radiographies by a direct intensity projection. The best results are obtained using the scalar gradient approach along with a transversal isotropic material model, obtaining a target registration error (TRE), in millimeters, of 5.65 ± 2.76 for CC- and of 7.83 ± 3.04 for MLO-mammograms, while the TRE is 7.33 ± 3.62 in the 3D MRI. We also evaluate the effect of the glandularity of the breast as well as the landmark position on the TRE, obtaining moderated correlation values (0.65 and 0.77 respectively), concluding that these aspects need to be considered to increase the accuracy in further approaches.

Structure of the Current Sheet in the 11 July 2017 Electron Diffusion Region Event.

The structure of the current sheet along the Magnetospheric Multiscale (MMS) orbit is examined during the 11 July 2017 Electron Diffusion Region (EDR) event. The location of MMS relative to the X‐line is deduced and used to obtain the spatial changes in the electron parameters. The electron velocity gradient values are used to estimate the reconnection electric field sustained by nongyrotropic pressure. It is shown that the observations are consistent with theoretical expectations for an inner EDR in 2‐D reconnection. That is, the magnetic field gradient scale, where the electric field due to electron nongyrotropic pressure dominates, is comparable to the gyroscale of the thermal electrons at the edge of the inner EDR. Our approximation of the MMS observations using a steady state, quasi‐2‐D, tailward retreating X‐line was valid only for about 1.4 s. This suggests that the inner EDR is localized; that is, electron outflow jet braking takes place within an ion inertia scale from the X‐line. The existence of multiple events or current sheet processes outside the EDR may play an important role in the geometry of reconnection in the near‐Earth magnetotail.

The Updating of the Nautical Chart Number 69 on Java Sea Area for Safety Navigation of Sailing

Indonesia is the largest archipelagic country in the world that has become one of the main routes in international marine trade by contributing 40% of all international marine trade routes. The importance of waterways in international trading demands the existence of nautical chart for navigation. Time after time, the sea changes, therefore nautical chart needs to be update especially on the area that has a high marine traffic density such as Java Sea which currently has developed an electronic map for navigation, however paper charts are still needed for planning shipping line, the navigation on small boats and backup of charts on large ships. The aim to be achieved from this research is the availability of the latest paper chart which is appropriate with IHO standard S-4, INT1 (symbols, abbreviations and terms used on charts), INT2 (the boundary lines, gradient, grid and linear scale) and Chart Number 1. Updating chart number 69 was created by using data Electronic Navigation Chart (ENC) Northen part of Central Java region, data raster paper chart number 69 the ninth edition of the second expenditure, survey data 2017 in Cirebon and the Eastern part of Java Sea and Indonesian Notice to Mariners number 11-29 year 2017. Data obtained from the Pusat Hidrografi dan Oseanografi TNI-AL. The software used in production of nautical chart is Paper Chart Composer (PCC). Then it was added the updated data and information to the chart. The result of production a chart were corrected using digital and manual quality control analysis to eliminate errors in the process of production chart. As a result of updating nautical chart number 69 is that some of the objects that had been corrected have errors so it needs to be repaired until no more errors occur. The updating paper chart was already matched to standard IHO S-4 and INT1. The visualization of paper chart in accordance with the standard IHO INT2 and Chart number 1. The avaibility of paper chart number 69 with the update of data on the area of the Java Sea has been accomplished and can be used by mariner to navigation.