Adiabatic Simulations Research Articles

Multi-stage oxidation of a CH2F2/air mixture examined by weak flames in a micro flow reactor with a controlled temperature profile

Oxidation of CH2F2 (R32), a widely used refrigerant, was investigated experimentally and computationally using a micro flow reactor with a controlled temperature profile (MFR). Specifically, we examined weak flames of a stoichiometric CH2F2/air mixture in MFR observed at low flow velocities. In the case of the maximum wall temperature of 1300 K, weak flames showed two luminous regions at 1240 and 1290 K, implying two-stage oxidation. Computations also indicated two-stage heat release. Computational weak flame positions also showed good agreement with experimentally obtained results. To assess the validity of chemical kinetics in detail, species measurements of CH2F2, CO, CO2, H2O, HF, and CF2O for a stoichiometric CH2F2/air weak flame were conducted using Fourier Transform Infrared spectrometry (FTIR). The present species measurements elucidated the CH2F2/air weak flame structure at 800–1300 K and remaining intermediates of H2O and CF2O at the end of the reactor. Computational results also showed good agreements with measurement, indicating the validity of the computations using the chemical kinetics. For further analysis for CH2F2 weak flame at 1300 K, the reaction path and rate-of-production analyses revealed a major reaction at the first-stage heat release: the formation of HF and intermediates, and that at the second stage is the CO2 formation from CO. Computations also showed intermediates of H2O and CF2O remain at the end of the reactor. An additional numerical experiment for a CH2F2/air mixture at the maximum wall temperature of 2000 K, demonstrated that the reaction of H2O and CF2O proceeded slowly at temperatures higher than 1300 K, with complete oxidation of CH2F2/air mixture. In conclusion, results show that the overall reactivity of CH2F2 decreases because of the low reactivity intermediates such as CF2O and H2O if oxidation occurs in a non-adiabatic system such as MFR. Such specific characteristics of the CH2F2/air mixture are concealed in the common zero-dimensional adiabatic simulations. Results demonstrated that the CH2F2 reactivity depend strongly on the temperature history during oxidation.

Transport on flexible Rydberg aggregates using circular states

Assemblies of interacting Rydberg atoms show promise for the quantum simulation of transport phenomena, quantum chemistry and condensed matter systems. Such schemes are typically limited by the finite lifetime of Rydberg states. Circular Rydberg states have the longest lifetimes among Rydberg states but lack the energetic isolation in the spectrum characteristic of low angular momentum states. The latter is required to obtain simple transport models with few electronic states per atom. Simple models can however even be realized with circular states, by exploiting dipole-dipole selection rules or external fields. We show here that this approach can be particularly fruitful for scenarios where quantum transport is coupled to atomic motion, in adiabatic excitation transport or quantum simulations of electron-phonon coupling in light harvesting. Additionally, we explore practical limitations of flexible Rydberg aggregates with circular states and to which extent interactions among circular Rydberg atoms can be described using classical models.

Effects of surface condensation in an idealised steam-driven screw expander

Condensation and its effects on turbo machinery operation are well understood and have been widely investigated. However, little scientific work on condensation in rotary positive displacement machines has been published. Since the robustness of screw expanders allows for expansion of slightly superheated vapours or even two-phase fluids, condensation on machine parts needs to be considered during design and simulation of these machines. In this paper the general effects of surface condensation of water on the machine parts of an idealised screw expander are discussed. Diabatic chamber model simulation is used for the thermodynamic simulation of operational behaviour. The effect of surface condensation on energy conversion and the delivered mass flow rate is analysed. Furthermore, a comparison of adiabatic and diabatic simulation of steam expansion in screw expanders is given in order to quantify condensation losses. Typical operating parameters are widely varied in simulation so as to identify influential factors on the condensation process. It is found that surface condensation, which is driven by heat exchange from the working fluid to adjacent machine parts, slightly raises the mass flow rate of the machine. For low expander speeds a reduction in isentropic efficiency can be expected due to a condensation induced pressure drop during the expansion phase.

Virtual chemistry for temperature and CO prediction in LES of non-adiabatic turbulent flames

Flames stabilization and pollutant formation in confined combustion chambers are affected by heat losses. Reliable numerical simulations must account for such complex phenomena at a reduced CPU cost. The aim of the present work is the development of a comprehensive reduced chemistry model able to account for the impact of heat losses on chemical flame structure. The recently developed virtual chemistry concept has shown promising capabilities to capture detailed chemistry effects with a reduced set of virtual species and kinetic reactions in adiabatic flame conditions. The approach consists in : (i) using a virtual main chemical mechanism coupled to the flow solver equations to predict temperature and heat release, (ii) designing satellite sub-mechanisms dedicated to the description of pollutant formation phenomena. The virtual mechanism is trained to recover the properties of an ensemble of target flames. Here burner-stabilized flamelets are introduced in the reference database to capture the influence of heat-losses on flame heat release and pollutant formation. The present approach is validated in 1D laminar burner stabilized flames and radiative freely propagating flame configurations. Finally the non adiabatic virtual chemistry is used to perform LES of a premixed turbulent combustion chamber submitted to wall heat losses. Comparison against experiment shows that non adiabatic simulation captures well the flame shape, the temperature and the CO prediction.

On the transient response of the turbulent boundary layer inception in compressible flows

The behavioural characteristics of thermal boundary layer inception dictate the efficiency of heat exchangers and the operational limits of fluid machinery. The specific time required by the thermal boundary layer to be established is vital to optimize flow control strategies, as well as the thermal management of systems exposed to ephemeral phenomena, typically on the millisecond scale. This paper presents the time characterization of the momentum and thermal boundary layer development in transient turbulent compressible air flows. We present a new framework to perform such estimations based on detailed unsteady Reynolds averaged Navier–Stokes simulations that may be extended to higher fidelity simulations. First of all, the aerodynamic boundary layer initiation is described using adiabatic simulations. Additional numerical calculations were then performed by setting the isothermal wall condition to evaluate the additional time required by the thermal boundary layer to establish after the aerodynamic boundary layer reaches its steady state. Finally, full conjugate simulations were executed to compute the warm up effect of the solid during the blowdown of a hot fluid over a colder metallic test model. The transient performance of the turbulent thermal and momentum boundary layers is quantified through numerical simulations of air blowdown over a flat plate for different mainstream flow conditions. The effects of Reynolds number, free stream velocity, transient duration, test article length and free stream temperature were independently assessed, to then define a mathematical expression of the momentum boundary layer settlement. This paper presents a novel numerical correlation of the additional time required by the thermal boundary layer to be stablished after the settlement of the momentum boundary layer. The time scales of the aerodynamic and thermal boundary layers are presented as a function of relevant non-dimensional numbers, as well as the description of the response of the near wall flow to sudden free stream changes. The characterization of the boundary layer mechanisms discussed in this paper contribute to the establishment of an evidence-based foundation for advances in the field of flow control.

Cosmic voids in evolving dark sector cosmologies: the high-redshift universe

We compare the evolution of voids formed under the standard cosmological model and two alternative cosmological models. The two models are a quintessence model ($\phi$CDM) and a Coupled Dark Matter-Dark Energy (CDE) model, both of which have evolving and interacting dark sectors. From $N$-body adiabatic hydrodynamical simulations of these models, we measure the statistics and quantify the properties of voids over the redshift range $z=1.5-12$: these include their population size, volumes, shapes and average densities. We find that the latter property has potential as a probe of cosmology, particularly dark energy, as significant differences in average void densities exist between the alternative models and the standard model. We postulate that this signature arises from an increased evacuation rate of particles out of voids, or an earlier start to void evacuation, in the alternative models as a direct consequence of the dynamical scalar field, which also leads to greater void merger rates. Additionally, differences between the two alternative models are likely due to the drag force arising from dark sector coupling, acting on dark matter particles in our coupled model.

Frequency chirping in the Alfvén continuum

A new kinetic code, CHIRP, shows how long range frequency chirping emerges from the energetic particle (EP) excitation of Alfvénic instability. Both a perturbative toroidal Alfvén eigenmode (TAE) and a non-perturbative energetic particle mode (EPM) can be driven unstable due to the EP pressure gradient. These instabilities produce phase space chirping structures in the form of holes or clumps. The code calculates the nonlinear resonant EP response of the poloidally coupled Alfvén wave equations, while the background plasma is assumed to respond linearly to the generated fields. At moderate EP pressure gradient, only TAEs are excited, that produce chirping structures that have only moderate frequency change which do not reach the continuum frequency. Larger pressure gradients excite an EPM mode that emerges in the lower continuum, which then chirps rapidly downward with its amplitude growing appreciably in time. An adiabatic theory was developed that accurately describes the late time evolution of the rapid frequency chirping. The strong correlation between the adiabatic theory and simulation is compatible with the view that the bulk of the EPM-trapped particles evolve in accordance with the conservation of their action.

Flame–wall interaction effects on the flame root stabilization mechanisms of a doubly-transcritical LO2/LCH4 cryogenic flame

High-fidelity numerical simulations are used to study flame root stabilization mechanisms of cryogenic flames, where both reactants (O2 and CH4) are injected in transcritical conditions in the geometry of the laboratory scale test rig Mascotte operated by ONERA (France). Simulations provide a detailed insight into flame root stabilization mechanisms for these diffusion flames: they show that the large wall heat losses at the lips of the coaxial injector are of primary importance, and require to solve for the fully coupled conjugate heat transfer problem. In order to account for flame–wall interaction (FWI) at the injector lip, detailed chemistry effects are also prevalent and a detailed kinetic mechanism for CH4 oxycombustion at high pressure is derived and validated. This kinetic scheme is used in a real-gas fluid solver, coupled with a solid thermal solver in the splitter plate to calculate the unsteady temperature field in the lip. A simulation with adiabatic boundary conditions, an hypothesis that is often used in real-gas combustion, is also performed for comparison. It is found that adiabatic walls simulations lead to enhanced cryogenic reactants vaporization and mixing, and to a quasi-steady flame, which anchors within the oxidizer stream. On the other hand, FWI simulations produce self-sustained oscillations of both lip temperature and flame root location at similar frequencies: the flame root moves from the CH4 to the O2 streams at approximately 450 Hz, affecting the whole flame structure.

Introducing CGOLS: The Cholla Galactic Outflow Simulation Suite

Abstract We present the Cholla Galactic OutfLow Simulations (CGOLS) suite, a set of extremely high resolution global simulations of isolated disk galaxies designed to clarify the nature of multiphase structure in galactic winds. Using the GPU-based code Cholla, we achieve unprecedented resolution in these simulations, modeling galaxies over a 20 kpc region at a constant resolution of 5 pc. The simulations include a feedback model designed to test the effects of different mass- and energy-loading factors on galactic outflows over kiloparsec scales. In addition to describing the simulation methodology in detail, we also present the results from an adiabatic simulation that tests the frequently adopted analytic galactic wind model of Chevalier & Clegg. Our results indicate that the Chevalier & Clegg model is a good fit to nuclear starburst winds in the nonradiative region of parameter space. Finally, we investigate the role of resolution and convergence in large-scale simulations of multiphase galactic winds. While our largest-scale simulations show convergence of observable features like soft X-ray emission, our tests demonstrate that simulations of this kind with resolutions greater than 10 pc are not yet converged, confirming the need for extreme resolution in order to study the structure of winds and their effects on the circumgalactic medium.

Building an adiabatic quantum computer simulation in the classroom

We present a didactic introduction to adiabatic quantum computation (AQC) via the explicit construction of a classical simulator of quantum computers. This constitutes a suitable route to introduce several important concepts for advanced undergraduates in physics: quantum many-body systems, quantum phase transitions, disordered systems, spin-glasses, and computational complexity theory.

A phenomenological HCCI combustion model in 0D and 3D-CFD

A phenomenological combustion model for HCCI applications – the 3-Stage heat release model – which predicts low, intermediate and high temperature heat release (LTR, ITR and HTR) is developed and validated for six automotive surrogate fuels. The contribution of all three combustion phases to the overall heat release and their durations are modeled based on rapid compression and expansion (RCEM) single-zone adiabatic simulations with detailed chemical kinetic mechanisms for all six fuels. This allows “pure” chemical effects on combustion to be included in the proposed 3-Stage heat release model, avoiding any combustion chamber related uncertainties such as blow-by, heat losses or trapped mass in crevices. Agreement between the 3-Stage heat release model and virtual RCEM (VRCEM) reference data is good. In a second step, a zero-dimensional HCCI model (0D model) is presented. To predict the onset of combustion, the low temperature ignition term of a previously developed and validated 3-Arrhenius auto-ignition model is employed in conjunction with the ignition integral of Livengood and Wu. Thereafter the 3-Stage heat release model predicts chemical heat release. Results show that the 0D model gives excellent predictions of ignition delays and combustion progress compared to the single-zone RCEM simulations using detailed chemical reaction mechanisms. Furthermore, including ITR in the 0D model is shown to be of essential importance to correctly predict high temperature ignition delay. As a final step a 3D HCCI model (3D model) is presented. The onset of combustion is again predicted by the low temperature ignition term of the 3-Arrhenius auto-ignition model, implemented via the source term of the transport equation into the 3D-CFD code. To predict heat release, the source term of the enthalpy equation is used to incorporate the 3-Stage heat release model into the 3D-CFD code. A simplified RCEM geometry is used for the simulations. Validation of the 3D model is made by a comparison to a chemical kinetic mechanism that is directly integrated into the 3D-CFD code. Combustion progress, temperature stratification in the combustion chamber and ignition delay times agree well with the reference data. The combination of the 3-Arrhenius auto-ignition model and 3-stage heat release model, their integration into 0D or 3D-CFD as a fully predictive HCCI model and their very low computational cost suggest the proposed model constitutes a promising approach for the development of future HCCI combustion engines.

Aspherical Supernovae: Effects on Early Light Curves

Abstract Early light from core-collapse supernovae, now detectable in high-cadence surveys, holds clues to a star and its environment just before it explodes. However, effects that alter the early light have not been fully explored. We highlight the possibility of nonradial flows at the time of shock breakout. These develop in sufficiently nonspherical explosions if the progenitor is not too diffuse. When they do develop, nonradial flows limit ejecta speeds and cause ejecta–ejecta collisions. We explore these phenomena and their observational implications using global, axisymmetric, nonrelativistic FLASH simulations of simplified polytropic progenitors, which we scale to representative stars. We develop a method to track photon production within the ejecta, enabling us to estimate band-dependent light curves from adiabatic simulations. Immediate breakout emission becomes hidden as an oblique flow develops. Nonspherical effects lead the shock-heated ejecta to release a more constant luminosity at a higher, evolving color temperature at early times, effectively mixing breakout light with the early light curve. Collisions between nonradial ejecta thermalize a small fraction of the explosion energy; we will address emission from these collisions in a subsequent paper.

Global gyrokinetic simulations of intrinsic rotation in ASDEX Upgrade Ohmic L-mode plasmas

Non-linear, radially global, turbulence simulations of ASDEX Upgrade (AUG) plasmas are performed and the nonlinear generated intrinsic flow shows agreement with the intrinsic flow gradients measured in the core of Ohmic L-mode plasmas at nominal parameters. Simulations utilising the kinetic electron model show hollow intrinsic flow profiles as seen in a predominant number of experiments performed at similar plasma parameters. In addition, significantly larger flow gradients are seen than in a previous flux-tube analysis (Hornsby et al 2017 Nucl. Fusion 57 046008). Adiabatic electron model simulations can show a flow profile with opposing sign in the gradient with respect to a kinetic electron simulation, implying a reversal in the sign of the residual stress due to kinetic electrons. The shaping of the intrinsic flow is strongly determined by the density gradient profile. The sensitivity of the residual stress to variations in density profile curvature is calculated and seen to be significantly stronger than to neoclassical flows (Hornsby et al 2017 Nucl. Fusion 57 046008). This variation is strong enough on its own to explain the large variations in the intrinsic flow gradients seen in some AUG experiments. Analysis of the symmetry breaking properties of the turbulence shows that profile shearing is the dominant mechanism in producing a finite parallel wave-number, with turbulence gradient effects contributing a smaller portion of the parallel wave-vector.

Preparation and thermal hazard evaluation of 1,3,3,5-tetra(1H-tetrazol-5-yl)-pentane

A novel multicyclo-tetrazoles energetic material, 1,3,3,5-tetra(1H-tetrazol-5-yl)-pentane (TTZP), was prepared by 1,3,3,5-tetra(cyano)-pentane and sodium azide. The crystal structure is characterized by X-ray diffraction. The thermal stabilities and the kinetic parameters of TTZP were studied by differential scanning calorimetry (DSC) and adiabatic calorimetry (Phi-TEC). Starink method was employed to calculate the activation energy based on the DSC data. Due to the complex reaction mechanism, the traditional method for the adiabatic data which using an nth-order reaction was modified based on the Kinetics Committee recommendations of the International Confederation for Thermal Analysis and Calorimetry. Avrami–Erofeev equation which was selected for the adiabatic simulation has good performance on the calculation of activation energy and pre-exponential factor. Finally, the important safety parameters (self-accelerating decomposition temperature) was calculated and discussed.

Simulating a topological transition in a superconducting phase qubit by fast adiabatic trajectories

The significance of topological phases has been widely recognized in thecommunity of condensed matter physics. The well controllable quantum systemsprovide an artificial platform to probe and engineer various topologicalphases. The adiabatic trajectory of a quantum state describes thechange of the bulk Bloch eigenstates with the momentum, and this adiabatic simulation method is however practicallylimited due to quantum dissipation. Here we apply the “shortcut to adiabaticity” (STA)protocol to realize fast adiabatic evolutions in the system of a superconducting phase qubit.The resulting fast adiabatic trajectories illustratethe change of the bulk Bloch eigenstates in the Su-Schrieffer-Heeger (SSH) model. A sharp transitionis experimentally determined for the topological invariant of a winding number.Our experiment helps identify the topological Chern number of a two-dimensionaltoy model, suggesting the applicability of the fast adiabatic simulationmethod for topological systems.

Nitrogen Injection in Molten Aluminum in a Tank Degasser

The injection of nitrogen in molten aluminum through a static impeller in a tank degassing unit is studied. Using basic principles of fluid mechanics, it is analyzed the influence of the nozzle diameter on the bubble diameter and the mean residence time of the bubbles in the molten aluminum. By means of transient isothermal 2D Computational Fluid Dynamics (CFD) simulations, the influence of the nitrogen volumetric flow rate on the phase distributions and the tank degasser dynamics is studied. Finally, an adiabatic CFD simulation is carried out in order to elucidate the changes in the molten aluminum temperature due to the injection of nitrogen at ambient temperature. This simulation shows that molten aluminum does not suffer drastic temperature reductions given that, in spite that the nitrogen is fed at ambient temperature, the mass of nitrogen is relatively small compared with the mass of aluminum.

Energy dissipation to tungsten surfaces upon hot-atom and Eley-Rideal recombination of H2.

Adiabatic and nonadiabatic quasi-classical molecular dynamics simulations are performed to investigate the role of electron-hole pair excitations in hot-atom and Eley-Rideal H2 recombination mechanisms on H-covered W(100). The influence of the surface structure is analyzed by comparing with previous results for W(110). In the two surfaces, hot-atom abstraction cross sections are drastically reduced due to the efficient energy exchange with electronic excitations at low incident energies and low coverage, while the effect on Eley-Rideal reactivity is negligible. As the coverage increases, the projectile energy is more efficiently dissipated into the other adsorbates. Consequently, the effect of electronic excitations is reduced. As a result, the reactivity and final energy distributions of the formed H2 molecules are similar for both abstraction mechanisms.

Efficient adiabatic hydrodynamical simulations of the high-redshift intergalactic medium

We present a post-processing tool for GADGET-2 adiabatic simulations to model various observed properties of the Ly$\alpha$ forest at $2.5 \leq z \leq 4$ that enables an efficient parameter estimation. In particular, we model the thermal and ionization histories that are not computed self-consistently by default in GADGET-2. We capture the effect of pressure smoothing by running GADGET-2 at an elevated temperature floor and using an appropriate smoothing kernel. We validate our procedure by comparing different statistics derived from our method with those derived using self-consistent simulations with GADGET-3. These statistics are: line of sight density field power spectrum, flux probability distribution function, flux power spectrum, wavelet statistics, curvature statistics, HI column density (${\rm N_{HI}}$) distribution function, linewidth ($b$) distribution and $b$ versus $\log {\rm N_{HI}}$ scatter. For the temperature floor of $10^4$ K and typical signal-to-noise of 25, the results agree well within 20 percent of the self-consistent GADGET-3 simulation. However, this difference is smaller than the expected $1\sigma$ sample variance for an absorption path length of $\sim 5.35$ at $z=3$. Moreover for a given cosmology, we gain a factor of $\sim N$ in computing time for modelling the intergalactic medium under $N \gg 1$ different thermal histories. In addition, our method allows us to simulate the non-equilibrium evolution of thermal and ionization state of the gas and include heating due to non-standard sources like cosmic rays and high energy $\gamma$-rays from Blazars.

Communication: Hot-atom abstraction dynamics of hydrogen from tungsten surfaces: The role of surface structure.

Adiabatic and non-adiabatic quasiclassical molecular dynamics simulations are performed to investigate the role of the crystal face on hot-atom abstraction of H adsorbates by H scattering from covered W(100) and W(110). On both cases, hyperthermal diffusion is strongly affected by the energy dissipated into electron-hole pair excitations. As a result, the hot-atom abstraction is highly reduced in favor of adsorption at low incidence energy and low coverages, i.e., when the mean free path of the hyperthermal H is typically larger. Qualitatively, this reduction is rather similar on both surfaces, despite at such initial conditions, the abstraction process involves more subsurface penetration on W(100) than on W(110).

Experimental and Kinetic Modeling Study on Self-Ignition of α-Methylnaphthalene in a Heated Rapid Compression Machine

As an important component of diesel and kerosene surrogates, the experimental study and chemical kinetic modeling of α-methylnaphthalene (AMN) are still very insufficient. The ignition delay of an AMN/O2/Ar mixture in a heated rapid compression machine (RCM) was measured in this study. The data were obtained for equivalence ratios of 0.7, 1, and 1.2, at pressures of 12, 15, and 20 bar, over the temperature range of 860–1040 K. A semi-detailed kinetic mechanism for the oxidation of AMN was established, which consists of 196 species and 1330 reactions. In comparison to the different previous mechanism, the new mechanism can more accurately predict the ignition delay of AMN in RCM and shock tube experiments. It can also accurately predict the experimental data obtained in a jet-stirred reactor from the literature. The ignition delays using adiabatic constant-volume simulation and RCM simulation were compared, which indicated that RCM simulation could better predict the experimental data. Sensitivity and reac...