Fluid Code Research Articles

Analysis of neutral particle recycling and pedestal fueling in a H-mode DIII-D discharge

A detailed analysis of neutral atom recycling and pedestal fueling in a DIII-D [J. Luxon, Nucl. Fusion 42, 614 (2002)] high-confinement mode discharge is presented. Experimental data and two-dimensional (2D) edge plasma fluid code calculations are employed to provide ion wall recycling and recombination neutral sources and background edge plasma parameters for a 2D edge neutral code calculation of detailed neutral density, ionization, and charge-exchange distributions throughout the edge pedestal, scrape-off layer and surrounding halo region, divertor, and private flux regions. The effectiveness of the different neutral sources for fueling the confined plasma is evaluated.

Kinetic Particle Simulation Study of Parallel Heat Transport in Scrape-off Layer Plasmas over a Wide Range of Collisionalities

Fluid models are not generally applicable to fusion edge plasmas without external provision of kinetic factors: closure parameters and boundary conditions inside the sheath region. We explain the PARASOL-1D simulation, a particle-in-cell code with a binary collision Monte-Carlo model, and use it to determine four kinetic factors commonly needed in fluid codes. These are the electron and ion heat flux limiting factors, αe and αi, the ion adiabatic index, γA, and the electron and ion temperature anisotropy, T∥/T⊥. We survey these factors over a wide range of collisionalities and find that, as predicted, the conductive heat flux is accurately described by the Spitzer-Harm expression in the collisional limit and asymptotes to a constant value in the collisionless limit. However, unique behavior occurs in the weakly collisional regime when the ratio of the mean free path to connection length is 0.1 < λmfp/L∥< 10, when the SOL is between the conduction- and sheath-limited regimes. We find that αe can peak, becoming larger than the collisionless limit, γA is less than unity, and only the ions are anisotropic. The effects of electron energy radiation and Langevin heating are explored. Finally, the strong deviations of the energy distribution function from Maxwellian in the weakly collisional and collisionless regimes are explained.

2-D lid-driven cavity flow of nematic polymers: an unsteady sea of defects

A classical benchmark for viscous and viscoelastic fluid codes is the lid-driven cavity, a model problem which highlights effects arising from a recirculating, geometrically confined flow. Here we simulate a square 2-D cavity for nematic liquid crystalline polymers (LCPs), which adds the physics of ordered phases and distortional elasticity to earlier cavity simulations. The range of local flow types within the cavity create ideal conditions for strong orientational conflicts: nematic polymers are prone to steady alignment in extension-dominated flow, yet sustained dynamic responses (tumbling, wagging, kayaking) in shear-dominated flow. Orientational conflicts are mediated by defects, or disordered phases, whose genesis and evolution in fully confined 2-D flows are explored here for the first time. Our algorithm extends previous LCP-flow solvers by implementation of physical boundary conditions on velocity and the orientational distribution at all four walls. We impose uniform orientational anchoring at the cavity walls (parallel on top and bottom, normal on sides) to suppress corner defect anomalies, and apply the method of Shen (J. Shen, J. Comp. Phys., 1991, 95, 228–245) for smoothing of the corner flow singularities due to the standard no-slip flow condition on solid walls. The same code simulates a viscous fluid by decoupling the orientational stress from the Navier–Stokes equations. We choose a Reynolds number for which the viscous simulation develops a stationary flow structure with a central rotating eddy and three corner eddies. We then simulate the Doi–Hess–Marrucci–Greco orientation tensor model for nematic polymers coupled to the flow equations, and apply level-set detection, tracking and graphics of defect domains (X. Yang, M. G. Forest, W. Mullins and Q. Wang, J. Rheology, 2009, 53 (3), 589–615). The viscous flow structure is qualitatively preserved, albeit with weaker corner vortices; the orientation field, however, consists of a highly transient, defect-laden texture. Our diagnostics continuously detect and track defect domains through strong fluctuations in topology as defects spawn, propagate, merge, collide and annihilate ad infinitum.

Fluid–structure interaction of a two-dimensional membrane in a flow with a pressure gradient with application to convertible car roofs

The flow-induced deformation of a membrane in a flow with a pressure gradient is studied. The investigation focuses on the deformation of aerodynamically loaded convertible car roofs. A computational methodology is developed with a line-element structural model that incorporates initial slackness of the flexible roof material. The computed flow–structure interaction yields stable solutions, the flexible roof settling into static equilibrium. The interaction converges to a static deformation within 1% difference in the displacement variable after three iterations between fluid and structural codes. Reasonably accurate predictions, to within 7%, are possible using only a single iteration between the fluid and the structural codes for the model problem studied herein. However, the deformation results are shown to be highly dependent on the physical parameters that are used in the calculation. Accurate representation of initial geometry, material properties and slackness should be found before the predictive benefits of the fluid–structure computations are sought. The iterative methodology overcomplicates the computation of deformation for the relatively small displacements encountered for the model problem studied herein. Such an approach would be better suited to applications with large amplitude displacements such as those encountered in sail design or deployment of a parachute.

LABORATORY EXPERIMENTS, NUMERICAL SIMULATIONS, AND ASTRONOMICAL OBSERVATIONS OF DEFLECTED SUPERSONIC JETS: APPLICATION TO HH 110

Collimated supersonic flows in laboratory experiments behave in a similar manner to astrophysical jets provided that radiation, viscosity, and thermal conductivity are unimportant in the laboratory jets, and that the experimental and astrophysical jets share similar dimensionless parameters such as the Mach number and the ratio of the density between the jet and the ambient medium. Laboratory jets can be studied for a variety of initial conditions, arbitrary viewing angles, and different times, attributes especially helpful for interpreting astronomical images where the viewing angle and initial conditions are fixed and the time domain is limited. Experiments are also a powerful way to test numerical fluid codes in a parameter range where the codes must perform well. In this paper we combine images from a series of laboratory experiments of deflected supersonic jets with numerical simulations and new spectral observations of an astrophysical example, the young stellar jet HH 110. The experiments provide key insights into how deflected jets evolve in 3-D, particularly within working surfaces where multiple subsonic shells and filaments form, and along the interface where shocked jet material penetrates into and destroys the obstacle along its path. The experiments also underscore the importance of the viewing angle in determining what an observer will see. The simulations match the experiments so well that we can use the simulated velocity maps to compare the dynamics in the experiment with those implied by the astronomical spectra. The experiments support a model where the observed shock structures in HH 110 form as a result of a pulsed driving source rather than from weak shocks that may arise in the supersonic shear layer between the Mach disk and bow shock of the jet's working surface.

Two-point analysis of the numerical modelling of detached divertor plasmas

A two-point model connecting plasma parameters at the divertor target and upstream is considered which is exact in the framework of the equations solved by the 2D multi-species plasma fluid code B2. The two-point formulae allow verification and quantitative analysis of the results of numerical modelling. The definition of the loss factors proposed here allows one to relate directly the free parameters of conventional two-point models with the simulation results. An example for the B2-EIRENE modelling of an ASDEX-Upgrade plasma with detached divertor is discussed. The new code analysis tool is applied to analyse the phenomenon of roll-over of the target incident particle flux as it appears in the simulations.

TOKAM-3D: A 3D fluid code for transport and turbulence in the edge plasma of Tokamaks

We present a new code aiming at giving a global and coherent approach for transport and turbulence issues in the edge plasma of Tokamaks. The TOKAM-3D code solves 3D fluid drift equations in full-torus geometry including both closed field lines and SOL physics. No scale separation is assumed so that interactions between large scale flows and turbulence are coherently treated. Moreover, the code can be run in transport regimes ranging from purely anomalous diffusion to fully established turbulence. Specific numerical schemes have been developed which can solve the model equations whether the presence of a limiter in the plasma is taken into account or not. Example cases giving an overview of the field of application of the code as well as verification results are also presented.

Fluid dynamic interaction between water hammer and centrifugal pumps

Centrifugal pumps generate in piping systems noticeable pressure pulsations. In this paper the dynamic interaction between water hammer and pressure pulsations is presented. The experimental investigations were performed at a piping system with nominal diameter DN 100 (respectively NPS 4) and 75 m total length, built at the Institute for Process Technology and Machinery. Different measurements at this testing facility show that pulsating centrifugal pumps can damp pressure surges generated by fast valve closing. It is also shown that 1-dimensional fluid codes can be used to calculate this phenomenon. Furthermore it is presented that pressure surges pass centrifugal pumps almost unhindered, because they are hydraulic open.

Summary of reports presented to magnetic confinement theory and modelling (TH) section: main ideas and achievements

This is a summary of the reports presented to the 22nd IAEA Fusion Energy Conference, Magnetic confinement theory and modelling section (Geneva, October 2008). Many of the papers are devoted to the investigation of transport processes, in particular to the toroidal momentum transport. Simulation by gyrokinetic codes has been improved in many countries, and the number of available codes reaches several tens. Numerical developments tend to follow the same trend as improvements in the computation power. The timescale for plasma simulations is now comparable to the ion–ion collision time. To improve the predictions for ITER, the near future advances are the combination of gyrokinetic and fluid codes. Reports on stellarators confirm that in these devices the neoclassical transport dominates, but the influence of turbulent transport can play a role in improved confinement regimes and in the resilience of pressure profiles. The resonant magnetic perturbations, mitigating the ELMs, could brake the plasma rotation, increasing the danger of disruption. The problems on the scrape-off layer and the divertor attract a large number of theoretical works that could lead to a better understanding of periphery plasma processes. ITER and reactor studies have been presented, and calculations confirm that ITER can achieve Q = 10 or larger. It has also been shown that the alpha-particle diffusion due to drift driven ITG turbulence will be relatively small in ITER, uncertainty remains in the magnitude of alpha-particle diffusion due to Alfvén waves.

Observation of reduced core electron temperature fluctuations and intermediate wavenumber density fluctuations in H-mode plasmas

In this paper, we report observations of reduced core electron temperature and intermediate-scale density fluctuations in H-mode. Electron temperature fluctuation levels are observed to decrease from L-mode levels ( for kθρs < 0.5 as measured by correlation electron cyclotron emission radiometry) by at least a factor of four in H- and quiescent H-mode regimes in the DIII-D tokamak (r/a = 0.7). Linear stability calculations (using the trapped gyro-Landau fluid (TGLF) code) indicate that the observed temperature fluctuations are associated with L-mode ITG turbulence which is shear-stabilized at the L- to H-mode transition. Recent results from DIII-D provide the first experimental evidence that, in addition, intermediate-scale turbulence (0.5 < kθρs ⩽ 3) is reduced at the L–H transition. A 30–40% prompt reduction (r/a ⩾ 0.7) has been found at the L–H transition in co-injected medium density plasmas, with a larger decrease (⩾75%) observed near the pedestal top. Experimental results and TGLF calculations indicate that intermediate/small scale turbulence persists in H-mode at a reduced amplitude (0.6 ⩽ r/a ⩽ 1) and may substantially contribute to the residual anomalous H-mode electron heat transport.

The Effects of Electrode Structures on the Luminous Efficacy of Micro Dielectric Barrier Discharges

A high-efficacy alternating-current plasma display panel utilizing various electrode structures is simulated using a 3-D fluid code for the investigation of the effects of cell structures on the discharge characteristics of micro dielectric barrier discharges. For the modification of the conventional coplanar electrode structure, three suggested electrode structures were simulated, namely, a patterned bus electrode, a multilayer bus electrode, and a two-electrode facial discharge structure. The time evolutions of infrared radiation images for the three structures compare well with the experimental results of test panels. The ultraviolet and visible-light emission profiles, as well as the time evolution of wall charge profiles, are analyzed in order to explain the role of the electrode structure for the improvement of luminous efficacy.

1‐D Self‐consistent Fluid Modelling of the Pulsed Magnetron Discharge

AbstractA 1‐D fluid code simulation of the region above the racetrack in a magnetron discharge has been developed to yield predictions for the temporal evolution of the axial plasma parameters during pulsed DC operation. To take into account anomalous cross‐field diffusion transport rates, an effective magnetic field profile has been calculated based on the real experimental values, to yield ratios in the effective electron gyro and collision frequencies comparable with Bohm diffusion. Pulse frequencies from 100 to 500 kHz are considered, with gas pressures ranging from 0.25 to 0.65 Pa. The model results for the plasma potential, electron and ion densities, during the pulse cycle agree well with experimental measurements made in the Liverpool magnetron using time‐resolved Langmuir and emissive probes. In the bulk plasma, the predictions for electron temperature are close to the measured values; however, in the sheath region they are artificially high due to the model assumption that only Maxwellian distributed electrons exist in all regions of the discharge. This inadequacy in the model and the methods to improve has been discussed. Results for DC magnetrons are also shown.

Fluid code simulations of radial electric field in the scrape-off layer of JET

Recent simulations of JET and ASDEX Upgrade plasmas with EDGE2D and SOLPS, respectively, showed that upstream radial electric field in the scrape-off layer (SOL) is substantially below values expected from simple estimates, based on the effects of the potential Debye sheath drop at the target and thermoelectric force (Chankin et al 2007 Nucl. Fusion 47 479). In this paper, dedicated EDGE2D-Nimbus modelling of JET plasmas aimed at identification of key mechanisms responsible for the formation of the radial electric field (Er) in the main SOL of high recycling divertor plasmas is described. Code runs with different upstream density and input power levels were carried out. The results of the modelling identify the main reason why the predicted Er values in the SOL are always too low compared with the simple estimates, which is the contribution from the −∇‖pe/nee term in the equation for parallel electric field (pe and ne—electron pressure and density). At the same time, it still remains unclear why experimental measurements in tokamak SOLs reveal fairly large Er values that are in reasonable agreement with the simple estimates. This discrepancy might be attributed to missing elements in the present 2D edge fluid codes, such as the role of plasma fluctuations or non-local kinetic effects of parallel plasma transport from the SOL to the divertor.

A New Modular Approach for Tightly Coupled Fluid/Structure Analysis

Static aeroelastic computations are made using a C++ executable developed for closely coupled fluid-structure analyses. The fluid flow is modeled using Euler/Navier-Stokes equations and the structure is modeled using finite elements. The FORTRAN-based fluid and structure codes are integrated in a C++ environment in which the flow and structural solvers are treated as separate object files, and the data exchange between them is accomplished using I/O. Use of this technique to solve transonic flow over a partially flexible surface is presented, and its ability to accurately predict flow-induced deformations associated with nonlinear structures is demonstrated.

Design, R&D and assessment of performance of the JT-60SA upper divertor

Modification of JT-60 to a fully superconducting coil tokamak (JT-60 Super Advanced) has been started last year. Divertor plasma performance for high triangularity and low aspect ratio plasma in two types of divertor geometries was studied and compared by using 2D plasma fluid (SOLDOR) and neutral Monte-Carlo (NEUT2D) codes. ‘Vertical target with deep V corner’ is effective to reduce heat flux even in low density operation without additional gas puffing, but large pumping speed is required to change particle balance. On the other hand, the particle balance in ‘W-shaped with shallow V’ can be changed with small pumping speed and larger main plasma can be obtained due to its compactness. Performance of water-cooled mono-block carbon fiber composite (CFC) divertor target has been confirmed by heat load cycle test of small mockup. Obvious performance degradation was not found during 1450 cycles at 15 MW m −2 and 600 cycles at 20 MW m −2.

Fluid modeling of an ELMing H-mode and a RMP H-mode

In this paper, we investigate the role of wall conditions during the density pump-out observed in Resonant Magnetic Perturbation (RMP) experiments. We use a 2D fluid code, coupled to a neutral Monte-Carlo code (SOLPS5) to model a reference ELMing H-mode. Next we modify either the recycling at the target plates or the pumping efficiency at the pump entrance. This causes a change in upstream profiles, that alone cannot account for the shape change observed in the RMP H-mode. Therefore, the radial transport must be altered to match the RMP H-mode. By comparing the transport model for the ELMing H-mode and the RMP H-mode, we show that inside the separatrix an increase in transport is necessary to model the effects of the applied perturbed field in RMP H-mode.

3D modelling of edge parallel flow asymmetries

The issue of parallel flows asymmetries in the edge plasma is tackled with a new first principle transport and turbulence code. TOKAM-3D is a 3D full-torus fluid code that can be used both in diffusive and turbulent regimes and covers either exclusively closed flux surfaces or both open and closed field lines in limiter geometry. Two independent mechanisms susceptible to lead to large amplitude asymmetric parallel flows are evidenced. Global ExB drifts coupled with the presence of the limiter break the poloidal symmetry and can generate large amplitude parallel flows even with poloidally uniform transport coefficients. On the other hand, turbulent transport in the edge exhibits a strong ballooning of the radial particle flux generating an up-down m = 1, n = 0 structure on the parallel velocity. The combination of both mechanisms in complete simulations leads to a poloidal and radial distribution of the parallel velocity comparable to experimental results.

Numerical simulations of internal solitary waves interacting with uniform slopes using an adaptive model

Two-dimensional, non-linear, Boussinesq, non-hydrostatic simulations of internal solitary waves breaking and running up uniform slopes have been performed using an adaptive, finite volume fluid code “Gerris”. It is demonstrated that the Gerris dynamical core performs well in this specific but important geophysical context. The “semi-structured” nature of Gerris is exploited to enhance model resolution along the slope where wave breaking and run-up occur. Comparison with laboratory experiments reveals that the generation of single and multiple turbulent surges (“boluses”) as a function of slope angle is consistently reproduced by the model, comparable with observations and previous numerical simulations, suggesting aspects of the dynamical energy transfers are being represented by the model in two dimensions. Adaptivity is used to explore model convergence of the wave breaking dynamics, and it is shown that significant cpu memory and time savings are possible with adaptivity.

Simulations of electron transport and ignition for direct-drive fast-ignition targets

The performance of high-gain, fast-ignition fusion targets is investigated using one-dimensional hydrodynamic simulations of implosion and two-dimensional (2D) hybrid fluid-particle simulations of hot-electron transport, ignition, and burn. The 2D/3D hybrid-particle-in-cell code LSP [D. R. Welch et al., Nucl. Instrum. Methods Phys. Res. A 464, 134 (2001)] and the 2D fluid code DRACO [P. B. Radha et al., Phys. Plasmas 12, 056307 (2005)] are integrated to simulate the hot-electron transport and heating for direct-drive fast-ignition targets. LSP simulates the transport of hot electrons from the place where they are generated to the dense fuel core where their energy is absorbed. DRACO includes the physics required to simulate compression, ignition, and burn of fast-ignition targets. The self-generated resistive magnetic field is found to collimate the hot-electron beam, increase the coupling efficiency of hot electrons with the target, and reduce the minimum energy required for ignition. Resistive filamentation of the hot-electron beam is also observed. The minimum energy required for ignition is found for hot electrons with realistic angular spread and Maxwellian energy-distribution function.

Comparaison des patients caucasiens et non caucasiens au cours de la maladie de Takayasu : étude rétrospective monocentrique de 82 cas

The main features of codes for inertial confinement fusion studies are outlined, and a few recent simulation results are presented. The two-dimensional Lagrangian fluid code DUED is used to study target evolution, including beam-driven compression, hydrodynamic stability, hot spot formation, ignition and burn. An electro-magnetic particle-in-cell (PIC) code is applied to the study of ultraintense laser–plasma interaction and generation of fast electron jets. A relativistic 3D collisionless fluid model addresses relativistic electron beam propagation in a dense plasma.