Blob Sizes Research Articles

Blob interactions in 2D scrape-off layer simulations

Interaction of coherent structures known as blobs in the scrape-off layer of magnetically confined plasmas is investigated. Isolated and interacting seeded blobs, as well as full plasma turbulence, are studied by two-dimensional numerical simulations. The features of the blobs (position, size, amplitude) are determined with a blob tracking algorithm, which identifies them as coherent structures with amplitudes above a chosen particle density threshold, and their velocities are compared to a conventional center of mass approach. We find that the theoretical velocity-size scaling dependence for isolated blobs is correctly resolved by the blob tracking method. The benchmarked approach is then extended to a population of interacting plasma blobs with statistically distributed amplitudes, sizes, and initial positions for different levels of blob interaction. We observe a correlation between the level of blob interaction and the number of blobs deviating from size–velocity scaling laws of perfectly isolated blobs. This is found to be caused by the interaction of blobs with the electrostatic potential of one another, leading to higher average blob velocities. We introduce a model specific intermittency parameter, quantifying the degree of blob interaction. For interacting blobs, we estimate the deviation from the picture of perfectly isolated blobs as a function of the intermittency parameter. For full plasma turbulence simulations, we observe a strong correlation between the blob amplitudes, sizes, and velocities estimated by the blob tracking algorithm.

(Keynote) Preparation and Structural Investigation of Linear and 4-Arm Poly(ethylene glycol) (2 x 4 PEG) Gels

Gel is a 3D polymer network holding solvent molecules inside its network. There are two major heterogeneities in gels. One is spatial heterogeneity, and the other is topological heterogeneity. The spatial heterogeneity is introduced by random cross-linking, so the network structure has both sparse and dense region. The spatial heterogeneity is observed as excess scattering in a light, X-ray, or neutron scattering experiment. On the other hand, the topological heterogeneity is related to the presence of dangling chains, loop structure, and entanglements inside the network, and is investigated by viscoelasticity and/or swelling experiments, or by other means, such as NMR. Because of these inhomogeneities, gels have seldom been applied to structural materials because of their poor mechanical properties. Hence, realization of ideal model networks free from heterogeneities has been strongly anticipated. Sakai et al. succeeded in fabrication of near ideal polymer networks called “Tetra-PEG gel”.[1] A Tetra-PEG gel is synthesized from two complementary end-functionalized poly(ethylene glycol) (PEG) which is able to react each other. It is pointed out that the Tetra-PEG gel has insignificant small spatial homogeneity and negligible topological heterogeneity, respectively by small angle scattering[2] and mechanical test.[3] Recently, we extended the above-mentioned method and succeeded in fabrication of multiple model network gels consisting of alternating tetra-functional poly(ethylene glycol)s (PEGs) and bis-functional linear PEGs (2 x 4 gels), where the molecular weight of linear PEG was varied from 200 Da to 20000 Da while that of tetra-functional PEG component was fixed to be 20000 Da.[4] The same level of homogeneities in the network structure as those of Tetra-PEG gels has been confirmed by dynamic light scattering (DLS) and dynamic viscoelastic measurements. Advantages of 2 x 4 gels are that one can independently tune the cross-link density (by the concentration of tetra-functional PEG) and the total polymer concentration. By using a series of 2 x 4 gels, we examined the concentration/cross-link density dependence of the correlation blob, the elastic blob, and geometric blob. We will discuss the scaling rules of these blob sizes as a function of polymer/cross-linker concentrations. Sakai, T. et al, Macromolecules 2008, 41, 5379.Matsunaga, T. et al, Macromolecules 2009, 42, 1344.Akagi, Y. et al, Macromolecules 2010, 43, 488.Tsuji, Y. et al., Gels 2018, 4, 50.

Evaluation of Mesh Size in Model Polymer Networks Consisting of Tetra-Arm and Linear Poly(ethylene glycol)s.

The structure and mechanical properties of model polymer networks consisting of alternating tetra-functional poly(ethylene glycol)s (PEGs) and bis-functional linear PEGs were investigated by dynamic light scattering and rheological measurements. The sizes of the correlation blob () and the elastic blob () were obtained from these measurements and compared to the theoretical mesh size, the geometric blob (), calculated by using the tree-like approximation. By fixing the concentration of tetra-PEGs and tuning the molecular weight of linear-PEGs, we systematically compared these blob sizes in two cases: complete network (Case A) and incomplete network (Case B). The correlation blob, , obtained by dynamic light scattering (DLS) was found to obey the well-known concentration dependence for polymer solutions in semidilute regime () irrespective of the Cases. On the other hand, the was strongly dependent on the Cases: For Case A, was weakly dependent on the molecular weight of linear-PEGs () while for Case B was a strong increasing function of (). However, both of them are different from the geometric blob (theoretical mesh) of the gel networks. In addition, interesting relationships between and , , , were obtained for Cases A and B, respectively.

Time course of cytochrome oxidase blob plasticity in the primary visual cortex of adult monkeys after retinal laser lesions.

We studied the time course of changes of cytochrome oxidase (CytOx) blob spatial density and blob cross-sectional area of deprived (D) and nondeprived (ND) portions of V1 in four capuchin monkeys after massive and restricted retinal laser lesions. Laser shots at the border of the optic disc produced massive retinal lesions, while low power laser shots in the retina produced restricted retinal lesions. These massive and restricted retinal lesions were intended to simulate glaucoma and diabetic retinopathy, respectively. We used a Neodymium-YAG dual frequency laser to make the lesions. We measured Layer III blobs in CytOx-reacted tangential sections of flat-mounted preparations of V1. The plasticity of the blob system and that of the ocular dominance columns (ODC) varied with the degree of retinal lesions. We found that changes in the blob system were different from that of the ODC. Blob sizes changed drastically in the region corresponding to the retinal lesion. Blobs were larger and subjectively darker above and below the non deprived ODC than in the deprived columns. With restricted lesions, blobs corresponding to the ND columns had sizes similar to those from non-lesioned areas. In contrast, blobs corresponding to the deprived columns were smaller than those from nonlesioned areas. With massive lesions, ND blobs were larger than the deprived blobs. Plastic changes in blobs described here occur much earlier than previously described.

Coalescence Model for Crumpled Globules Formed in Polymer Collapse.

The rapid collapse of a polymer, due to external forces or changes in solvent, yields a long-lived "crumpled globule." The conjectured fractal structure shaped by hierarchical collapse dynamics has proved difficult to establish, even with large simulations. To unravel this puzzle, we study a coarse-grained model of in-falling spherical blobs that coalesce upon contact. Distances between pairs of monomers are assigned upon their initial coalescence, and do not "equilibrate" subsequently. Surprisingly, the model reproduces quantitatively the dependence of distance on segment length, suggesting that the slow approach to scaling is related to the wide distribution of blob sizes.

History effects on nonwetting fluid residuals during desaturation flow through disordered porous media.

We investigate experimentally the sweeping of a nonwetting fluid by a wetting one in a quasi-two-dimensional porous medium consisting of random obstacles. We focus primarily on the resulting phase distributions and the residual nonwetting phase saturation as a function of the normalized wetting fluid flow rate-the capillary number Ca-at steady state. The wetting liquid is then flowing in the medium partially saturated by immobile nonwetting liquid blobs. The decrease of the nonwetting saturation is an irreversible process that depends strongly on flow history and more specifically on the highest value of Ca reached in the past. At lower Ca values, when capillary forces are dominant, the residual steady state saturation depends significantly on the initial phase configuration. However, at higher Ca, the saturation becomes independent of the history and thus follows a master curve that converges to an asymptotic residual value. Blob sizes range over four orders of magnitude in our experimental domain, following a probability distribution function P that scales with the blob size s as P(s)∝s(-2) for blob sizes larger than the typical pore size. It also exhibits a maximum size cutoff s(max), that decreases as s(max)∝Ca(-1). To determine the flow properties, we have measured the pressure drop (B) versus the flow rate (Ca). In the ranges of low and high Ca values, the relationship between Ca and B is found to be linear, following Darcy's law (B∝Ca). In the intermediate regime, the progressive mobilization of blobs leads to a nonlinear dependence B∝Ca(0.65), due to an increase of the available flow paths.

The influence of finite ion temperature on plasma blob dynamics

In the scrape-off layer of magnetically confined fusion devices, the ion temperature is at least as high as the electron temperature and usually even much higher. The effects of the finite ion temperature enhance the blob drive and modify the vorticity. Recently developed scaling laws for blob velocity independent of its size, based on the full drift-interchange-Alfvén fluid equations are compared with recent experiments on the ASDEX Upgrade tokamak and gyrofluid simulations, showing remarkable agreement for the blob sizes and reasonable agreement for the blob velocities.

Method for estimating the propagation direction of a coherent plasma structure using a one-dimensional diagnostic array.

This article proposes a new method to evaluate basic characteristics of the dynamics of a coherent plasma structure (blob). With this method, one can evaluate the propagation angle of a blob in a two-dimensional plasma cross section as well as the blob velocity, size, and amplitude from one-dimensional data. The method is applied to blob measurements from the Lithium beam emission spectroscopy system in ASDEX-Upgrade. Statistical features of the observed blob velocities, angles of propagation, blob sizes, and amplitudes are discussed. The validity of the method is examined by comparing two values of the propagation angle that are evaluated in an independent manner.

Magnetic field dependence of the blob dynamics in the edge of ASDEX upgrade L-mode plasmas

The magnetic field dependence of intermittently expelled density filaments (blobs) is investigated in the scrape-off layer of ASDEX Upgrade low confinement (L-mode) plasmas. It is demonstrated that lithium beam emission spectroscopy can be used to determine the frequency, radial size and velocity of the blobs. The measured radial blob sizes depend only weakly on magnetic field B. Normalizing the blob sizes to the drift parameter ρs ∝ B−1 results in a large variation beneficial for a quantitative comparison with theoretical blob scaling laws. The blob velocity scales inversely proportional to the square of the blob size in agreement with analytic models for blobs in the sheath-connected regime. The measurements point towards an influence of finite ion temperature on radial blob transport.

Plasma blob formation by ion kinetic Kelvin–Helmholtz and interchange instabilities

The near-edge region of a tokamak is observed to generate radially propagating, coherent filamentary structures (‘blobs’), which transport particles and heat from the confined region and across the scrape-off layer. The distribution of blob sizes may include a currently unresolvable population with radii comparable to the ion gyro-radius. Here, we conduct large-scale numerical simulations to study mechanisms for the creation of ion gyro-scale blobs via the ion kinetic Kelvin–Helmholtz and interchange instabilities, using a hybrid (kinetic ion, fluid electron) model. We present statistics of the sizes of blobs created by these instabilities, and radial particle displacement data. We find that ion gyro-scale blobs constitute a significant portion of the blob population, and that an increase in ion gyro-radius results in an increase in radial transport. Results are contrasted for pure proton plasmas and for 50 : 50 deuterium–tritium mix, relevant to burning plasmas. We conclude that ion kinetic physics plays a significant role in the transport of energy and particles by ion gyro-scale blobs in the near-edge region of low-field tokamaks.

Blob sizes and velocities in the Alcator C-Mod scrape-off layer

Gas-puff imaging data is used to investigate the dynamics of blob-like structures in the scrape-off layer of Alcator C-Mod. A blob-tracking algorithm is developed and used to compute blob propagation velocities and cross-field sizes for a series of discharges where the line-averaged particle density n¯e is varied. We find that the mean radial blob velocity increases from 150ms-1 for n¯e/nG=0.15 to 450ms-1 for n¯e/nG=0.48, where is the nG the Greenwald density. The cross-field size of the blobs is independent of the line-averaged particle densities. With increasing n¯e/nG the observed blob velocities become larger than predicted by simple blob models for sheath-connected plasma filaments.

Microbial Enhanced Oil Recovery in Fractional-Wet Systems: A Pore-Scale Investigation

Microbial enhanced oil recovery (MEOR) is a technology that could potentially increase the tertiary recovery of oil from mature oil formations. However, the efficacy of this technology in fractional-wet systems is unknown, and the mechanisms involved in oil mobilization therefore need further investigation. Our MEOR strategy consists of the injection of ex situ produced metabolic byproducts produced by Bacillus mojavensis JF-2 (which lower interfacial tension (IFT) via biosurfactant production) into fractional-wet cores containing residual oil. Two different MEOR flooding solutions were tested; one solution contained both microbes and metabolic byproducts while the other contained only the metabolic byproducts. The columns were imaged with X-ray computed microtomography (CMT) after water flooding, and after MEOR, which allowed for the evaluation of the pore-scale processes taking place during MEOR. Results indicate that the larger residual oil blobs and residual oil held under relatively low capillary pressures were the main fractions recovered during MEOR. Residual oil saturation, interfacial curvatures, and oil blob sizes were measured from the CMT images and used to develop a conceptual model for MEOR in fractional-wet systems. Overall, results indicate that MEOR was effective at recovering oil from fractional-wet systems with reported additional oil recovered (AOR) values between 44 and 80%; the highest AOR values were observed in the most oil-wet system.

Blob current structures in TORPEX plasmas: experimental measurements and numerical simulations

In the simple magnetized torus TORPEX, field-aligned blobs originate from ideal interchange waves and propagate radially outward due to ∇B and curvature induced drifts. Time-resolved two-dimensional measurements of the field-aligned current density J∥ associated with blobs are obtained from conditionally sampled data from a single-sided Langmuir probe and a specially designed current probe. The profile of J∥ exhibits an asymmetric dipolar structure, which originates from the polarization of the blob and is consistent with sheath boundary conditions. The asymmetry results from the non-linear dependence of J∥ at the sheath edge upon the floating potential. Using internal measurements, we directly confirm the existence of two regimes, in which parallel currents to the sheath do or do not significantly damp charge separation and thus blob radial velocity. To investigate the effect of the observed asymmetry of J∥ on the blob motion, we carried out numerical simulations of seeded blobs, using a two-field fluid model, which evolves electron density and vorticity. Simulations are performed spanning a wide range of blob sizes covering both regimes. We use either the complete or a linearized form for the sheath dissipation term in the vorticity equation. The structure of the parallel current density and plasma potential is found to be different in the two cases. Asymmetric profiles are observed in simulations with the complete form, while symmetric profiles are obtained when a linearized form is used. Negligible effects are, however, observed in terms of blob radial velocity. The relevance of the present results for fusion devices is also discussed.

Electrostatic instabilities, turbulence and fast ion interactions in the TORPEX device

Electrostatic turbulence, related structures and their effect on particle, heat and toroidal momentum transport are investigated in TORPEX simple magnetized plasmas using high-resolution diagnostics, control parameters, linear fluid models and nonlinear numerical simulations. The nature of the dominant instabilities is controlled by the value of the vertical magnetic field, Bv, relative to that of the toroidal field, BT. For Bv/BT > 3%, only ideal interchange instabilities are observed. A critical pressure gradient to drive the interchange instability is experimentally identified. Interchange modes give rise to blobs, radially propagating filaments of enhanced plasma pressure. Blob velocities and sizes are obtained from electrostatic probe measurements using pattern recognition methods. The observed values span a wide range and are described by a single analytical expression, from the small blob size regime in which the blob velocity is limited by cross-field ion polarization currents, to the large blob size regime in which the limitation to the blob velocity comes from parallel currents to the sheath. As a first attempt at controlling the blob dynamical properties, limiter configurations with varying angles between field lines and the conducting surface of the limiter are explored. Mach probe measurements clearly demonstrate a link between toroidal flows and blobs. To complement probe data, a fast framing camera and a movable gas puffing system are installed. Density and light fluctuations show similar signatures of interchange activity. Further developments of optical diagnostics, including an image intensifier and laser-induced fluorescence, are under way. The effect of interchange turbulence on fast ion phase space dynamics is studied using movable fast ion source and detector in scenarios for which the development from linear waves into blobs is fully characterized. A theory validation project is conducted in parallel with TORPEX experiments, based on quantitative comparisons of observables that are defined in the same way in the data and in the output of numerical codes, including 2D and 3D local and global simulations.

Cross-Field Motion of Plasma Blobs in an Open Magnetic Field Line Configuration

The radial propagation of blobs generated from plasma instabilities is investigated in an open magnetic field line configuration. Blob cross-field velocities and sizes are obtained from internal probe measurements using pattern recognition. By varying the ion mass, the normalized vertical blob scale a[over] is scanned from a[over] < 1 to a[over] > 1. An analytical expression for the blob velocity including cross-field ion polarization currents, parallel currents to the sheath, and ion-neutral collisions is derived and shows good quantitative agreement with the experimental data. In agreement with previous theoretical studies, this scaling shows that, for a[over] < 1, the blob velocity is limited by cross-field ion polarization currents, while for a[over] > 1 it is limited by parallel currents to the sheath.

Measurement and Estimation of Organic-Liquid/Water Interfacial Areas for Several Natural Porous Media

The objective of this study was to quantitatively characterize the impact of porous-medium texture on interfacial area between immiscible organic liquid and water residing within natural porous media. Synchrotron X-ray microtomography was used to obtain high-resolution, three-dimensional images of solid and liquid phases in packed columns. The image data were processed to generate quantitative measurements of organic-liquid/water interfacial area and of organic-liquid blob sizes. Ten porous media, comprising a range of median grain sizes, grain-size distributions, and geochemical properties, were used to evaluate the impact of porous-medium texture on interfacial area. The results show that fluid-normalized specific interfacial area (A) and maximum specific interfacial area (Am) correlate very well to inverse median grain diameter. These functionalities were shown to result from a linear relationship between effective organic-liquid blob diameter and median grain diameter. These results provide the basis for a simple method for estimating specific organic-liquid/water interfacial area as a function of fluid saturation for a given porous medium. The availability of a method for which the only parameter needed is the simple-to-measure median grain diameter should be of great utility for a variety of applications.

A comparison of blob methods for vortex sheet roll-up

The motion of vortex sheets is susceptible to the onset of the Kelvin–Helmholz instability. There is now a large body of evidence that the instability leads to the formation of a curvature singularity in finite time. Vortex blob methods provide a regularization for the motion of vortex sheets. Instead of forming a curvature singularity in finite time, the curves generated by vortex blob methods form spirals. Theory states that these spirals will converge to a classical weak solution of the Euler equations as the blob size vanishes. This theory assumes that the blob method is the result of a convolution of the sheet velocity with an appropriate choice of a smoothing function. We consider four different blob methods, two resulting from appropriate choices of smoothing functions and two not. Numerical results indicate that the curves generated by these methods form different spirals, but all approach the same weak limit as the blob size vanishes. By scaling distances and time appropriately with blob size, the family of spirals generated by different blob sizes collapse almost perfectly to a single spiral. This observation is the next step in developing an asymptotic theory to describe the nature of the weak solution in detail.

Pore-Scale Characterization of Organic Immiscible-Liquid Morphology in Natural Porous Media Using Synchrotron X-ray Microtomography

The objective of this study was to quantitatively characterize the pore-scale morphology of organic immiscible liquid residing within natural porous media. Synchrotron X-ray microtomography was used to obtain high-resolution, three-dimensional images of solid and liquid phases in packed columns. The image data were processed to generate quantitative measurements of organic-liquid blob morphology. Three porous media, comprising a range of particle-size distributions, were used to evaluate the impact of porous-medium texture on blob morphology. The sizes and shapes of the organic-liquid blobs varied greatly, ranging from small spherical singlets (> or = 0.03 mm in diameter) to large, amorphous ganglia with mean lengths of 4-5 mm. The smaller blobs were composed primarily of singlets, which comprised approximately half of all blobs for all three media. Conversely, large, complex blobs comprising four or more bodies composed 11-24% of the total number of blobs. However, the majority of the total organic-liquid surface area and volume was associated with the largest blobs. The ratio of median blob size to median grain size was close to unity for all three systems. The distribution of blob sizes was greatest for the porous medium with the broadest particle-size and pore-size distributions. These results illustrate the utility of synchrotron X-ray microtomography for characterizing fluid distributions at the pore scale in natural porous media.

Growth of shocked gaseous interfaces in a conical geometry

The results of experiments on Richtmyer–Meshkov instability growth of multimode initial perturbations on an air–sulfur hexafluoride (SF6) interface in a conical geometry are presented. The experiments are done in a relatively larger shock tube. A nominally planar interface is formed by sandwiching a polymeric membrane between wire-mesh frames. A single incident shock wave ruptures the membrane resulting in multimode perturbations. The instability develops from the action of baroclinically deposited vorticity at the interface. The visual thickness δ of the interface is measured from schlieren photographs obtained in each run. Data are presented for δ at times when the interface has become turbulent. The data are compared with the experiments of Vetter [Shock Waves 4, 247 (1995)] which were done in a straight test section geometry, to illustrate the effects of area convergence. It is found from schlieren images that the interface thickness grows about 40% to 50% more rapidly than in Vetter’s experiments. Laser induced scattering is used to capture the air–helium interface at late times. Image processing of pictures is also used to determine the interface thickness in cases where it was not clear from the pictures and to obtain the dominant eddy–blob sizes in the mixing zone. Some computational studies are also presented to show the global geometry changes of the interface when it implodes into a conical geometry in both light–heavy and heavy–light cases.

Semi-automatic measurement of gap–blob sizes along tracks of relativistic ions for computer identification

A method for the identification of relativistic ions with low atomic numbers in nuclear emulsion was proposed for measurements with a semi-automatic microscope-computer system. In this method we used a simultaneous consideration of both blob and gap distributions over their sizes and a comparison of these distributions with Monte Carlo simulation on the basis of fluctuation theory of the photographic action of ionizing particles (Phys.-Math. Sci. Thesis, Moscow State University, 278pp). The applicability of the proposed method was tested for a set of ion tracks in nuclear emulsion of type R2.