Direct Numerical Simulation Study of Turbulence and Flame Structure in Transverse Jets Analysed in Jet-Trajectory Based Coordinates
Abstract not provided
42 ENGINEERING↗
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Phase contrast imaging measurements and numerical simulations of turbulent density fluctuations in gas-fuelled ECRH discharges in Wendelstein 7-X
The fundamental nature of turbulent density fluctuations in standard Wendelstein 7-X (W7-X) stellarator discharges is investigated experimentally via phase contrast imaging (PCI) in combination with gyrokinetic simulations with the code GENE. We find that density fluctuations are ion-temperature-gradient-driven and radially localised in the outer half of the plasma. It is shown that the line-integrated PCI measurements cover the right range of wavenumbers and a favourable toroidal and poloidal location to capture some of the strongest density fluctuations in W7-X. Due to the radial localisation of fluctuations, measured wavenumber–frequency spectra exhibit a dominant phase velocity, which can be related to the E × B rotation velocity at the radial position of a well in the neoclassical radial electric field. The match is robust against variations of heating power and line-integrated density, which is partly due to the localisation of fluctuations and partly due to effects of the radial gradient in the E × B velocity profile on the wavenumber–frequency spectrum. The latter effect is studied with a newly built synthetic PCI diagnostic and global gyrokinetic simulations with GENE-3D.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Theoretical Model and Numerical Simulation of Adsorption and Deformation in Flexible Metal–Organic Frameworks
We report that metal–organic frameworks (MOFs) have recently attracted considerable attention as new nanoporous materials with applications to separation of fluid mixtures, catalysis, gas capture and storage, and drug delivery. A subclass of MOFs, the MIL-53 (with Al or Cr) family, exhibits a complex structural phase transition, often referred to as the breathing transition, which occurs when gas molecules are adsorbed in its pore space. In this phenomenon, the material morphology oscillates between two distinct phases, usually referred to as the narrow-pore (np) and large-pore (lp) phases. We describe a statistical mechanical model based on the energetics of the system that couples the adsorbates with the host and the ensuing structural deformation of the materials. The Hamiltonian of the system consists of the elastic energy of the MOF, the fluid phase energy, and the interaction energy between the gas and MOFs. Minimizing the total energy with respect to both the gas density and the displacement field in the material yields the governing equations for both. The model is used to study the adsorption of CO 2 and CH 4 in MIL-53(Al). Here, the results demonstrate that all reported experimental features of the phenomenon are produced by the model. In particular, consistent with experimental data, the model predicts that the material experiences contraction in the np phase, whereas it undergoes swelling in the lp phase, and that the two phases coexist in the transition zone. The model is, however, completely general and is applicable to any type of guest-responsive material that undergoes deformation as a result of its interaction with the guest molecules.
36 MATERIALS SCIENCE↗
The Mechanism of Methane Gas Migration Through the Gas Hydrate Stability Zone: Insights From Numerical Simulations
Free gas migration through the gas hydrate stability zone (GHSZ) and subsequent gas seepage at the seabed are characteristic features in marine gas hydrate provinces worldwide. The biogenic or thermogenic gas is typically transported along faults from deeper sediment strata to the GHSZ. Several mechanisms have been proposed to explain free gas transport through the GHSZ. While inhibition of hydrate formation by elevated salinities and temperatures have been addressed previously in studies simulating unfocused, area-wide upward advection of gas, which is not adequately supported by field observations, the role of focused gas flow through chimney-like structures has been underappreciated in this context. Our simulations suggest that gas migration through the GHSZ is, fundamentally, a result of methane gas supply in excess of its consumption by hydrate formation. The required high gas flux is driven by local overpressure, built up from gas accumulating below the base of the GHSZ that fractures the overburden when exceeding a critical pressure, thereby creating the chimney-like migration pathway. Initially rapid hydrate formation raises the temperature in the chimney structure, thereby facilitating further gas transport through the GHSZ. As a consequence, high hydrate saturations form preferentially close to the seafloor, where temperatures drop to bottom water values, producing a prominent subsurface salinity peak. Over time, hydrates form at a lower rate throughout the chimney structure, while initial temperature elevation and salinity peak dissipate. Thus, our simulations suggest that the near-surface salinity peak and elevated temperatures are a result of transient high-flux gas migration through the GHSZ.
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Parameterizations of Entrainment-Mixing Mechanisms and their Effects on Cloud Droplet Spectral Width Based on Numerical Simulations
Entrainment-mixing mechanisms significantly affect cloud droplet number concentration, radius, and spectral shape. Quantitative examination of entrainment-mixing effects on cloud droplet spectral width is lacking. Here, we examine the effects of entrainment-mixing processes on cloud microphysics by 12,218 different setups, each simulated 10 times using the Explicit Mixing Parcel Model (EMPM) driven by the observational data from the Third Tibetan Plateau Atmospheric Scientific Experiment (TIPEX-III) campaign. Parameterizations of entrainment-mixing mechanisms are developed by relating homogeneous mixing degree to transition scale number that depends on the dissipation rate and droplet evaporation time scale. The correlation between relative dispersion of cloud droplet size distribution and homogeneous mixing degree changes from negative to positive with the decreasing homogeneous mixing degree. The different relationships are closely related to the competition between complete and partial droplet evaporation and the number concentration of small droplets, which are quantitatively described by two newly introduced dimensionless numbers. The competition is significantly affected by relative humidity and mixing fraction of entrained air as well as turbulence dissipation rate, but not much by cloud droplet number concentration. Especially, when relative humidity and dissipation rate are high, there is only a negative correlation. This study sheds new light on generalizing the homogeneous/inhomogeneous concept by considering relative dispersion, and also provides parameterizations of entrainment-mixing processes and relative dispersion for atmospheric models.
54 ENVIRONMENTAL SCIENCES↗
Numerical Simulations of the Geospace Response to the Arrival of an Idealized Perfect Interplanetary Coronal Mass Ejection
Abstract Previously, Tsurutani and Lakhina (2014, https://doi.org/10.1002/2013GL058825 ) created estimates for a “perfect” interplanetary coronal mass ejection and performed simple calculations for the response of geospace, including . In this study, these estimates are used to drive a coupled magnetohydrodynamic‐ring current‐ionosphere model of geospace to obtain more physically accurate estimates of the geospace response to such an event. The sudden impulse phase is examined and compared to the estimations of Tsurutani and Lakhina (2014, https://doi.org/10.1002/2013GL058825 ). The physics‐based simulation yields similar estimates for Dst rise, magnetopause compression, and equatorial values as the previous study. However, results diverge away from the equator. values in excess of 30 nT/s are found as low as magnetic latitude. Under southward interplanetary magnetic field conditions, magnetopause erosion combines with strong region one Birkeland currents to intensify the response. Values obtained here surpass those found in historically recorded events and set the upper threshold of extreme geomagnetically induced current activity at Earth.
79 ASTRONOMY AND ASTROPHYSICS↗
Spontaneous imbibition in a square tube with corner films: theoretical model and numerical simulation
Spontaneous imbibition in an angular tube with corner films is a fundamental problem in many scientific and engineering processes. In this paper, a modified interacting capillary bundle model is developed to describe the liquid imbibition dynamics in a square tube with corner films. The square tube is decomposed into several interacting sub-capillaries and the local capillary pressure in each sub-capillary is derived based on the specific shape of its meniscus. The conductance of each sub-capillary is calculated using single-phase lattice Boltzmann simulation. The modified interacting capillary bundle model and color-gradient lattice Boltzmann method are used to simulate the liquid imbibition dynamics in the square tube with different fluid properties. The predictions by the modified interacting capillary bundle model match well with the lattice Boltzmann simulation results for different conditions, demonstrating the accuracy and robustness of the interacting capillary bundle model to describe the imbibition dynamics with corner films. In addition, the interacting capillary bundle model is helpful to investigate the mechanisms during spontaneous imbibition and the influences of fluid viscosity, surface tension, wetting phase contact angle and gravity on imbibition dynamics. Finally, a universal scaling law of imbibition dynamics for the main meniscus is developed and the scaling law for arc meniscus is also analyzed.
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Long-lived laser-induced microwave plasma guides in the atmosphere: Self-consistent plasma-dynamic analysis and numerical simulations
A detailed model of plasma dynamics, which self-consistently integrates plasma-kinetic, Navier-Stokes, electron heat conduction, and electron-vibration energy transfer equations, is used to quantify the limitations on the lifetime of microwave plasma waveguides induced in the atmosphere through filamentation with high-intensity ultrashort laser pulses further sustained by long laser pulses. We demonstrate that a near-infrared or midinfrared laser pulse can tailor plasma decay in the wake of a filament, efficiently suppressing, through electron temperature increase, the attachment of electrons to neutral species and dissociative recombination, thus substantially increasing the plasma-guide lifetime and facilitating long-distance transmission of microwaves.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Numerical simulations of electrical asymmetry effect on electronegative plasmas in capacitively coupled rf discharge
Recently a so-called electrical asymmetry effect (EAE), which could achieve high-degree separate control of ion flux and energy in dual-frequency capacitively coupled radio-frequency (CCRF) discharges, was discovered theoretically by Heil et al. [J. Phys. D: Appl. Phys. 41, 165202 (2008)] and was confirmed by experiments and theory/numerical simulations later on for electropositive argon discharges. In this work simulations based on particle-in-cell/Monte Carlo collision are performed to study the EAE on electronegative oxygen plasmas in geometrically symmetric CCRF discharges. Dual frequency discharges operating at 13.56 and 27.12 MHz are simulated for different pressures and the results are compared with those of electropositive argon discharges at the same conditions. It is found that in general the EAE on oxygen discharges has similar behavior as on argon discharge: The self-bias voltage {eta} increases monotonically and almost linearly with the increase in the phase angle {theta} between the two driving voltages in the range 0<{theta}<90 deg. , and the maximum ion energy varies by a factor of 3 by adjusting {theta}. However, the ion flux varies with {theta} by {+-}12% for low pressure and by {+-}15% for higher pressure, due primarily to an enhanced plasma series resonance, which then leads to dramatic changes in plasma density, power absorption and consequently the electronegativity. This may place a limitation for achieving separate control of ion energy and flux for electronegative plasma via the EAE.
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Numerical simulation and analysis of plasma turbulence the Large Plasma Device
Turbulence calculations with a 3D collisional fluid plasma model demonstrate qualitative and semi-quantitative similarity to experimental data in the Large Plasma Device [W. Gekelman et al., Rev. Sci. Inst. 62, 2875 (1991)], in particular for the temporal spectra, fluctuations amplitude, spatial correlation length, and radial particle flux. Several experimentally observed features of plasma turbulence are qualitatively reproduced, and quantitative agreement is achieved at the order-of-magnitude level. The calculated turbulence fluctuations have non-Gaussian characteristics, however the radial flux of plasma density is consistent with Bohm diffusion. Electric polarization of density blobs does not appear to play a significant role in the studied case. Turbulence spectra exhibit direct and inverse cascades in both azimuthal and axial wavenumbers and indicate coupling between the drift instability and Kelvin-Helmholtz mode.
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Experimental Analysis and Numerical Simulation of Tensile Behaviour of TiNi Shape Memory Alloy Fibres Reinforced Epoxy Matrix Composite at High Temperatures
The shape memory alloys (SMA) possess both sensing and actuating functions due to their shape memory effect, pseudo-elasticity, high damping capability and other remarkable properties. Combining the SMA with other materials can create intelligent or smart composites. The epoxy resin composites filled with TiNi alloys fibres were fabricated and their mechanical properties have been investigated. In this study, stress/strain relationships for a composite with embedded shape memory materials (SMA) fibres are presented. The paper illustrates influence of the SMA fibres upon changes in mechanical behaviour of a composite plate with the SMA components, firstly and secondly, the actuating ability and reliability of shape memory alloy hybrid composites.
36 MATERIALS SCIENCE↗
Publisher's Note: ''An extended hybrid magnetohydrodynamics gyrokinetic model for numerical simulation of shear Alfven waves in burning plasmas''[Phys. Plasmas 18, 052504 (2011)]
No abstract prepared.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Numerical simulation of thin-shell direct drive DHe3-filled capsules fielded at OMEGA
Thin-shell deuterium-helium-3 (DHe{sup 3}) filled glass capsules on the Omega laser provide a fast-implosion experimental platform for developing separate time-resolved measurements of ion, electron, and radiation temperatures in nonequilibrium plasmas. Dynamically significant non-local thermodynamic equilibrium (NLTE) conditions are created by the addition of xenon dopant to the DHe{sup 3} gas fill, in quantities sufficient to have an impact on yields, compression, and cooling rates. The high-Z dopant dramatically increases the radiative cooling rate in the plasma, allowing it to collapse in compressions that can be an order of magnitude higher than in undoped capsules. A baseline LASNEX simulation model using detailed configuration accounting NLTE atomic physics shows very good agreement with the data for doped as well as undoped capsules, while other models either underpredict or overpredict the radiative cooling enhancement. The baseline model captures the behavior of the capsule when the D:He{sup 3} ratio is varied well away from equimolar, suggesting no yield anomaly with either nearly pure deuterium or He{sup 3} fills. Variation of the electron-ion coupling in the baseline simulation model shows agreement with the data for a coupling multiplier that is within 20% of unity. Reliably inferring electron-ion coupling strength from the data is complicated by uncertainties in the hydrodynamic mix and other parameters, but many of these can be mitigated in follow-on experiments at the National Ignition Facility.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Numerical simulation of operation modes in atmospheric pressure uniform barrier discharge excited by a saw-tooth voltage
The characteristics of dielectric barrier discharge excited by a saw-tooth voltage are simulated in atmospheric pressure helium based on a one-dimensional fluid model. A stepped discharge is obtained per half voltage cycle with gas gap width less than 2 mm by the simulation, which is different to the pulsed discharge excited by a sinusoidal voltage. For the stepped discharge, the plateau duration increases with increasing the voltage amplitude and decreasing the gas gap. Therefore, uniform discharge with high temporal duty ratio can be realized with small gap through increasing the voltage amplitude. The maximal densities of both electron and ion appear near the anode and the electric field is almost uniformly distributed along the gap, which indicates that the stepped discharge belongs to a Townsend mode. In contrast to the stepped discharge with small gas gap, a pulsed discharge can be obtained with large gas gap. Through analyzing the spatial density distributions of electron and ion and the electric field, the pulsed discharge is in a glow mode. The voltage-current (V-I) characteristics are analyzed for the above mentioned discharges under different gas gaps, from which the different discharge modes are verified.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Design and numerical simulations of a high power 0.15 THz oscillator
An overmoded rectangularly corrugated relativistic surface wave oscillator is designed and theoretically analyzed to produce the high power terahertz radiation with a TM{sub 01} mode. The design is fully verified and optimized by 2.5D particle-in-cell simulations. A high power terahertz wave is obtained with the output power of 285 MW at the frequency of 0.148 THz with conversion efficiency 20% when beam voltage is about 400 kV, current 3.8 kA, and the magnetic field 6 T.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗