Recent Submissions

  • Analysis of Fuel Properties on Combustion Characteristics in a Narrow-throat Pre-chamber Engine

    Hlaing, Ponnya; Marquez, Manuel Echeverri; Burgos, Paula; Cenker, Emre; Ben Houidi, Moez; Johansson, Bengt (The Society of Automotive Engineers, 2021-04-14) [Conference Paper]
    In this study, the authors investigated the effect of fuel properties on the combustion characteristics by employing methane, methanol, ethanol, and primary reference fuels (PRFs) as the main chamber fuel while using methane for the pre-chamber. Global excess air ratios (λ) from 1.6 to lean limit were tested, while 13% of total fuel energy supplied to the engine was delivered via the pre-chamber. The gaseous methane was injected into the pre-chamber at the gas exchange top-dead-center (TDC). Port fuel injection was tested with both open and closed inlet valves. The pre-chamber assembly was designed to fit into the diesel injector pocket of the base engine, which resulted in a narrow throat diameter of 3.3 mm. The combustion stability limit was set at 5% of the coefficient of variation of gross IMEP, and the knock intensity limit was set at 10 bar. GT-Power software was used to estimate the composition of pre-chamber species and was used in heat release analysis of the two chambers. It was found that the rich limit was controlled by engine knock. Hence a higher reactivity fuel (lower octane) had to be operated leaner. However, with the increasing reactivity, the lean limit was also extended, while the peak efficiency was also obtained with a leaner mixture. With PRF 90, the lean limit was at global-λ = 3.0, while the limit was 2.3 with methane. The alcohol fuels exhibited a different behavior from the methane and the PRFs. Ethanol has the same lean limit as PRF100, but methanol could be operated up to global-λ = 3.2. The pre-chamber combustion did not change much with the different fuels in the main chamber, so the combustion stability trends must be related to the transition from burning jets to ignition of the main chamber charge and its subsequent combustion.
  • Revisiting Reynolds and Nusselt numbers in turbulent thermal convection

    Bhattacharya, Shashwat; Verma, Mahendra K.; Samtaney, Ravi (Physics of Fluids, AIP Publishing, 2021-01-01) [Article]
    In this paper, we extend Grossmann and Lohse’s (GL) model [S. Grossmann and D. Lohse, “Thermal convection for large Prandtl numbers,” Phys. Rev. Lett. 86, 3316 (2001)] for the predictions of Reynolds number (Re) and Nusselt number (Nu) in turbulent Rayleigh–Bénard convection. Toward this objective, we use functional forms for the prefactors of the dissipation rates in the bulk and boundary layers. The functional forms arise due to inhibition of nonlinear interactions in the presence of walls and buoyancy compared to free turbulence, along with a deviation of the viscous boundary layer profile from Prandtl–Blasius theory. We perform 60 numerical runs on a three-dimensional unit box for a range of Rayleigh numbers (Ra) and Prandtl numbers (Pr) and determine the aforementioned functional forms using machine learning. The revised predictions are in better agreement with the past numerical and experimental results than those of the GL model, especially for extreme Prandtl numbers
  • Soot formation in turbulent flames of ethylene/hydrogen/ammonia

    Boyette, Wesley; Steinmetz, Scott A.; Guiberti, Thibault; Dunn, Matthew J.; Roberts, William L.; Masri, Assaad R. (Combustion and Flame, Elsevier BV, 2020-12-29) [Article]
    This paper presents an experimental study of turbulent non-premixed jet flames of ethylene/nitrogen where the nitrogen is substituted with different proportions of hydrogen and/or ammonia. The focus is largely on the effects of hydrogen and ammonia on soot production in turbulent flames. A combination of pointwise, laser-induced fluorescence in the visible and UV bands (LIF-UV–visible), and laser-induced incandescence (LII) is used to measure soot precursors and soot along the flame centerline. All signals are collected at a repetition rate of 10 Hz with fast photomultiplier tubes to resolve the time decay. In a separate experiment, joint imaging of LIF-OH[sbnd]CH is also performed at a repetition rate of 10 kHz. Hydrogen substitution is found to increase the production of soot, whereas ammonia substitution inhibits soot formation. The peak mean soot volume fraction is almost a factor of 3 lower in the 25% ammonia case in comparison to the 25% nitrogen case. The mean signal decay time constant decreases with ammonia substitution, implying the formation of smaller soot nanoparticles. The mean signal decay time constant remains unaffected with hydrogen substitution. Measured peaks in LIF-CH and LIF-OH are reduced with ammonia substitution but only in regions upstream of where soot is formed. Further downstream in the sooting region, neither OH nor CH appear to be affected by the substitution of N2 with H2 or NH3.
  • Exergy loss characteristics of DME/air and ethanol/air mixtures with temperature and concentration fluctuations under HCCI/SCCI conditions: A DNS study

    Zhang, Jiabo; Luong, Minh Bau; Pérez, Francisco E.Hernández; Han, Dong; Im, Hong G.; Huang, Zhen (Combustion and Flame, Elsevier BV, 2020-12-29) [Article]
    The exergy loss characteristics of combustion processes under homogeneous-charge compression ignition (HCCI) and stratified-charge compression ignition (SCCI) conditions are numerically investigated by analyzing two-dimensional (2-D) direct numerical simulation (DNS) data. Two fuels, dimethyl ether and ethanol, together with the initial conditions of different mean temperatures, and levels of temperature and concentration fluctuations relevant to HCCI/SCCI conditions were investigated. It is found that the prevalent deflagration mode significantly decreases the maximum exergy loss rates and spreads out the exergy loss rate for all the cases regardless of fuel types, temperature regimes, and temperature and/or concentration fluctuations. The primary irreversible sources of exergy loss are also identified. The chemical reaction is found to be the primary contributor to the total exergy loss, followed by heat conduction and mass diffusion, regardless of the fluctuation levels. It is also found that the relative change of exergy loss due to chemical reactions, ELchemrel, correlates strongly with the heat release fraction by deflagration. The maximum ELchemrel is found to be less than 10%. Chemical pathway analysis reveals that the exergy loss induced by low-temperature reactions, represented by the decomposition of hydroperoxy–alkylperoxy and the H-abstraction reactions of the fuel molecule, is much lower under the SCCI conditions than that under the HCCI conditions. Generally, the dominant reactions contributing to the exergy loss in the high-temperature regime are nearly identical for the HCCI and SCCI combustion. Key reactions, including the H2O2 loop reactions, the reactions of the H2–O2 mechanism, and the conversion reaction of CO to CO2, CO+OH=CO2+H, are found to contribute more than 50% of the total exergy loss. Due to locally higher reactivities by temperature and concentration fluctuations inducing deflagration dominance, these reactions occur at a relatively higher temperature (1600 K–1900 K) compared with the homogeneous zero-dimensional cases (∼1400 K), resulting in a net reduction in exergy loss.
  • Mathematical Modeling of Solar Energy based Thermal Energy Storage for House Heating in Winter

    Soni, Neelesh; Sharma, Debojit; Rahman, Mustafa M.; Hanmaiahgari, Prashanth R.; Reddy, V. Mahendra (Journal of Energy Storage, Elsevier BV, 2020-12-25) [Article]
    A novel solar thermal energy storage (TES) system for house heating purposes is modeled in the present study. The solar parabolic collector acts as a heat source to charge the TES using compressed CO2. The thermal energy in terms of sensible heat is stored in mild steel (MS) block wrapped in the thermal insulation material and buried in the ground at a certain depth. The stored energy will be used for house heating in the winter season. A mathematical model is developed to calculate the quantity of stored energy, consumption, and loss from TES along with the optimal storage volume of the block to fulfill the energy demand for house heating. Computational analysis of the 3D model is executed by adopting stipulated boundary conditions. Quantitative and qualitative studies of numerical results have been investigated for two cases: Initial charging and year-round performance. Numerical results revealed that the initial charging of the block takes in 55.2 days with maximum block temperature and stored energy as 324.2 °C and 2604 MJ, respectively. The average exergy efficiency and exergetic effectiveness of TES are observed as 49.9 % and 0.50, respectively. The partial charging of the block continued to compensate the heat losses after initial charging. The year-round performance of TES exhibits the reduction of stored energy by 71.6% at the end of January due to increased heating demand. Meanwhile, during a cyclic year, the average exergy efficiency and exergetic effectiveness of TES are noticed as 56.12 % and 0.76 respectively. The proposed TES has enough potential for year-round house heating since 28.4 % of net stored energy exists with the least block temperature of 106 °C. The rational economic study endorsed the proposed system 42.5% less expensive than the traditional heating methods for 50 years of utilization in addition to mitigating the average CO2 production of 5.655 Tons/year.
  • Prediction of mean radical concentrations in lean hydrogen-air turbulent flames at different Karlovitz numbers adopting a newly extended flamelet-based presumed PDF

    Lipatnikov, A. N.; Sabelnikov, V. A.; Hernandez Perez, Francisco; Song, W.; Im, Hong G. (Combustion and Flame, Elsevier BV, 2020-12-23) [Article]
    A recent analysis (Lipatnikov et al., 2020) of complex-chemistry direct numerical simulation (DNS) data obtained from lean hydrogen-air flames associated with corrugated-flame (case A), thin-reaction-zone (case B), and broken-reaction-zone (case C) regimes of turbulent burning has shown that the flamelet concept (i) can predict mean concentrations of various species in those flames if the probability density function (PDF) for the fuel-based combustion progress variable c is extracted from the DNS data, but (ii) poorly performs for the mean rate W¯c of product creation. These results suggest applying the concept to evaluation of mean species concentration (but not the mean rate) in combination with another closure relation for W¯c whose predictive capabilities are better. This proposal is developed in the present paper whose focus is placed on studying a new flamelet-based presumed PDF P(c) for predictions of mean concentration of radicals in engineering computational fluid dynamics (CFD) applications. Analysis of the DNS data shows that (i) the flamelet PDF performs well at intermediate values of c in cases A and B, but should be truncated at small and large c, (ii) modeling P(c) in the radical recombination zone (i.e., at large c) is of importance for predicting mean concentrations of H,O, and OH. Accordingly, the flamelet PDF is truncated and combined with a uniform P(c) at large c. Moreover, the mean rate W¯c extracted from the DNS data is used to calibrate the PDF (the rate is considered to be given by another model). Assessment of the approach against the DNS data shows that it well predicts mean density, temperature, and concentrations of reactants, product, and the aforementioned radicals in cases A and B. In case C, the approach performs worse for OandOH at large c¯ and moderately underestimates the mean concentration of H in the entire flame brush.
  • Analysis of Thermally Induced Breakup of Ultrasonically Emulsified Heavy Fuel Oil using Dynamic Mode Decomposition

    Guida, Paolo; Saxena, Saumitra; Roberts, William L. (International Journal of Heat and Mass Transfer, Elsevier BV, 2020-12-16) [Article]
    Clean and efficient processing of heavy fuels is a major challenge for several combustion driven prime movers like internal combustion engines, used in marine or power generation sectors. Emulsification was recognized in the past as practical technology for heavy fuels combustion since it engenders an enabling phenomenon called micro-explosion that proceeds during the spray process. Micro-explosions allow finer secondary break-up, leading to improved mixing, and subsequent cleaner and fuller burning. However, the translation of this technology to real applications is still not fully exploited due to lack of basic understanding and characterization of the evaporation process which includes both micro-explosions and puffing. Ultrasonically induced cavitation is a promising technology for the production of water-in-oil emulsions at industrial scale. Fundamental research performed in the field of liquid fuels gasification and combustion mostly regards ideal or simple mixtures and not all the considerations made in these cases apply for real fuels. In this work, we investigated the evaporation characteristics of ultrasonically produced heavy fuel oil (HFO) emulsions with a set of newly developed methodologies. We characterized the emulsions by using a state-of-the-art microscopy technique, the Cryogenic Scanning Electron Microscopy, Cryo- SEM and obtained accurate droplet size distribution up to nano-scale. We tested the fuel emulsion in a suspended droplet experiment and reconstructed the interface from the obtained images. The normalized squared diameter profile is not representative of the complex physics involved in heavy fuel evaporation; therefore, it was substituted with the normalized distance of the interface from the centroid of the droplet. By using this procedure, it is possible to highlight both evaporation and stochastic events like puffing and ejections. A dimensionality reduction algorithm, the dynamic mode decomposition (DMD), was then performed on the evolving interface to highlight the main modes describing the emulsion system and the dynamics. The overall objective was to develop a strategy for optimizing emulsions for improved combustion performance. From the experimental data, it was observed that a water concentration of 5% by mass decreases the vaporization time of the mixture and that the presence of water favors puffing and ejections with different intensity depending on the percentage of water enhancing the volatilization of the fuel.
  • Enhanced mode II fracture toughness of secondary bonded joints using tailored sacrificial cracks inside the adhesive

    Wagih, A.; Lubineau, Gilles (Composites Science and Technology, Elsevier BV, 2020-12-13) [Article]
    The mode II fracture toughness of secondary bonded joints can be improved by creating tailored sacrificial cracks inside the adhesive. To this end, we inserted a PTFE film inside the adhesive bondline during the bonding process to create sacrificial cracks inside the adhesive. We demonstrated the efficiency of this technique through ENF tests that characterize mode II fracture toughness of adhesive-bonded CFRP adherends. We ascribed the improvement in toughness to the reduction of the strain concentration at the crack tip, which delayed the crack propagation and thus improved joint initiation fracture toughness, GIIi, and the maximum load capacity, Pmax. Moreover, after crack propagation, sacrificial cracks arrested the crack propagation at the upper interface, grew secondary backward cracks at the lower interface, and created adhesive ligaments. These three damage mechanisms dissipated more energy during propagation, which improved the propagation fracture toughness, GIIc. The improvement rates depend on the sacrificial crack width and the gap between two successive cracks reaching 96%, 98%, and 25% for GIIi, GIIc and Pmax, respectively, for a 2 mm sacrificial crack width and 5 mm gap. Our approach works well for both thin and thick adhesives and is a simple technique to substantially enhance the toughness of secondary bonded joints.
  • Self-excited noise generation from laminar methane/air premixed flames in thin annular jets

    Jin, Seong Ho; Joung, Jae Hoon; Chung, Suk Ho (Experimental Thermal and Fluid Science, Elsevier BV, 2020-12-10) [Article]
    Self-excited noise generation from laminar flames in thin annular jets of premixed methane/air has been investigated experimentally. Various flame shapes were observed in this flow configuration, including conical shaped flames, ring shaped flames, steady crown shaped flames, and oscillating crown shaped flames. Self-excited noise with a total sound pressure level of about 70 dB was generated from the oscillating crown shaped flames for equivalence ratios>0.95. Sound pressure and CH* chemiluminescence were measured by using a microphone and a photomultiplier tube. The frequency of generated noise was measured as a function of equivalence ratio and premixture velocity. A frequency doubling phenomenon has also been observed. The measured CH* chemiluminescence data were analysed and which the corresponding sound pressure has been calculated. By comparing the measured and calculated sound pressures, the noise source can be attributed to the flame front fluctuation near the edge of the oscillating crown shaped flames. The flame stability regime was influenced strongly by the mass flow rate of air through the inner tube.
  • Calcium Looping: On the Positive Influence of SO2 and the Negative Influence of H2O on CO2 Capture by Metamorphosed Limestone-Derived Sorbents

    Homsy, Sally Louis; Moreno, Joseba; Dikhtiarenko, Alla; Gascon, Jorge; Dibble, Robert W. (ACS Omega, American Chemical Society (ACS), 2020-12-07) [Article]
    The CO2 capture performance of sorbents derived from three distinct limestones, including a metamorphosed limestone, is studied under conditions relevant for calcium looping CO2 capture from power plant flue gas. The combined and individual influence of flue gas H2O and SO2 content, the influence of textural changes caused by sequential calcination/carbonation cycles, and the impact of CaSO4 accumulation on the sorbents’ capture performance were examined using bubbling fluidized bed reactor systems. The metamorphosed limestone-derived sorbents exhibit atypical capture behavior: flue gas H2O negatively influences CO2 capture performance, while limited sulfation can positively influence CO2 capture, with space time significantly impacting CO2 and SO2 co-capture performance. The morphological characteristics influencing sorbents’ capture behavior were examined using imaging and material characterization tools, and a detailed discussion is presented. This insight into the morphology responsible for metamorphosed limestone-derived sorbent’s anomalous capture behavior can guide future sorbent selection and design efforts.
  • Fatigue crack growth in laser-treated adhesively bonded composite joints: An experimental examination

    Bello, Idris; Alowayed, Yasir; Albinmousa, Jafar; Lubineau, Gilles; Merah, Nesar (International Journal of Adhesion and Adhesives, Elsevier BV, 2020-12-04) [Article]
    Carbon fiber-reinforced polymers (CFRPs) are commonly used in structures in which weight and strength determine energy efficiency, such as automobiles and aircraft. CFRPs are light in weight, and they provide design flexibility, low thermal expansion, and high specific strength. Bonding CFRPs to make larger structures, however, is problematic. Recently, laser pretreatment has been used to improve bonding of CFRPs. This study investigates the effects of uniform surface pretreatment with a laser on the resistance of secondary bonded carbon fiber reinforced structures to mode-I fatigue crack growth. The joint fatigue limit was characterized by carrying out displacement-controlled cyclic testing on treated double cantilever beam specimens. A mid-infrared-range CO2 pulse laser with a 10.6 μm wavelength was used to treat the CFRP substrates with the aim to increase the substrates’ fatigue limit and thereby expand the design envelope for secondary bonding. The cured substrates were treated uniformly with either high (laser-ablated) or low (laser-cleaned) CO2 laser irradiation and compared with specimens that were treated with a baseline Teflon film treatment such that their bonding surfaces were smooth. Our results show that uniform laser treatment increased the fatigue limit evaluated at a threshold crack growth rate of 10−5 mm/cycle. Furthermore, the laser-ablated specimens, in which the carbon fibers were totally exposed, achieved triple the strain energy release rate threshold value of the laser-cleaned samples.
  • Strength-induced peridynamic modeling and simulation of fractures in brittle materials

    Wang, Yongwei; Han, Fei; Lubineau, Gilles (Computer Methods in Applied Mechanics and Engineering, Elsevier BV, 2020-12-04) [Article]
    This paper presents a hybrid peridynamics and classical continuum mechanics modeling approach for brittle fracture, in which peridynamics is activated according to a strength criterion. The proposed approach represents a new way of simulating the fracture process, including elastic deformation and crack nucleation and propagation. Classical continuum mechanics and peridynamics are coupled into a closed equation system, and an adaptive algorithm is developed to solve it. The algorithm initially employs classical continuum mechanics throughout the entire structure to describe its mechanical response. Once the stress state of a point reaches the strength of the material, peridynamics is adaptively activated in the vicinity of that point to further describe the crack nucleation and propagation processes. Two-dimensional numerical examples illustrate that successful fracture simulations of complex structure can be achieved by this approach.
  • Partial coalescence of a drop on a larger-viscosity pool

    Alhareth, Abdullah A.; Thoroddsen, Sigurdur T (Physics of Fluids, AIP Publishing, 2020-12-01) [Article]
    When a low-viscosity drop coalesces with a pool surface of the same liquid, it often portrays partial coalescence, where it pinches off a daughter droplet from its top. Such partial coalescence can also occur for a drop spreading on a strongly hydrophilic solid surface. Herein, we investigate the partial coalescence of a low-viscosity drop with a pool surface, when the viscosity of the miscible pool is changed from low to very high, in other words, spanning the conditions from a pool to a solid surface. We find that above a certain pool viscosity, the partial coalescence transitions to second-stage coalescence with a much smaller satellite droplet. This occurs because higher pool viscosity prevents drainage from the drop into the pool, which, in turn, increases the axial curvature in the neck connecting the primary satellite to the drop, thereby preventing the first-stage pinch-off.
  • First aromatic ring formation by the radical-chain reaction of vinylacetylene and propargyl

    Jin, Hanfeng; Xing, Lili; Liu, Dapeng; Hao, Junyu; Yang, Jiuzhong; Farooq, Aamir (Combustion and Flame, Elsevier BV, 2020-12-01) [Article]
    Recent investigations illustrated that clustering of hydrocarbons by radical-chain reaction (CHRCR) mechanism provides key mechanistic steps for the rapid synthesis of polycyclic aromatic hydrocarbons (PAHs) and soot. Resonance-stabilized radicals (RSRs) play critical roles in this mechanism, and non-benzene first-ring species have attracted considerable attention as precursors of larger aromatic hydrocarbons. C7H7 RSRs, such as benzyl, tropyl, vinyl-cyclopentadienyl, are particularly stable and are thus quite important in the growth of PAHs. The addition of vinylacetylene to propargyl radical, a prototypical CHRCR reaction, provides a facile route to C7H7 RSRs. We have directly investigated the reaction of propargyl and vinylacetylene in isomer-resolved elementary experiments by synchrotron vacuum ultra-violet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). In good agreement with theoretical predictions, vinyl-cyclopentadienyl is found to be the major product of vinylacetylene and propargyl reaction while benzyl is minor. This work demonstrates a feasible CHRCR pathway, not proceeding through benzene, for PAH formation.
  • Current status of the high-temperature kinetic models of silane: Part I. Pyrolysis

    Chatelain, Karl P.; He, Yizhuo; Alharbi, Reham; Mével, Rémy; Petersen, Eric L.; Lacoste, Deanna (Combustion and Flame, Elsevier BV, 2020-12) [Article]
    The present work compares the performance of seven reaction models with respect to a large experimental dataset relevant to the high-temperature pyrolysis of both silane (SiH) and disilane (SiH). Their performances were established based on different validation criteria that account for the shape and the amplitude of the validation profile. Then, the model performances were quantified with a global error, which accounts for the experimental uncertainties. The most satisfactory model has a global error as low as 3.1 (i.e., meaning 3.1 times higher than the experimental uncertainty) and the highest fraction (74%) of criteria with a low error (), while most of the models have large discrepancies with the validation dataset, global error near 8 and up to 110 for the less accurate model. The origins of these discrepancies are identified with reaction pathway and sensitivity analyses. Among the seven tested model, three main decomposition pathways are evidenced, including one more specific to the models presenting the lowest errors. Based on the global error values, the ability to reproduce all the experimental conditions, and the model analyses, the reaction pathways relevant to the high-temperature pyrolysis of silane and disilane are determined. In addition, the present study provides experimental and numerical guidance for the future developments of silicon hydride reaction models. The limited performance of most of the oldest reaction models may have a significant impact on our current understanding of the pyrolysis and oxidation kinetics of silane.
  • Current status of the high-temperature kinetic models of silane: Part II. Oxidation

    Chatelain, Karl P.; He, Yizhuo; Javoy, Sandra; Mével, Rémy; Petersen, Eric L.; Lacoste, Deanna (Combustion and Flame, Elsevier BV, 2020-12) [Article]
    The present study is the second part of our work on the current status of high-temperature kinetic models of silane. Except Slakman’s model, all the models tested in the first part of the study, restricted to the pyrolysis dataset, are now compared against a large validation dataset (230 conditions) for silane oxidation. This large validation dataset is composed of both new and literature data, mainly representative of the highly-diluted and high-temperature oxidation of silane with different oxidizers ( and NO) and diluents (Ar and ) over an extensive range of temperature ( = [801– 2955 K]) and pressure ( = [50 – 629 kPa]) conditions. The new experimental data are limited to --Ar mixtures, obtained in a double-diaphragm shock tube equipped with an Atomic Resonance Absorption Spectroscopy (ARAS) detection technique. Experimental results present the temporal evolution of the total absorption signal, considering the absorption of O, Si, and . The performance of the models is assessed based on the same five validation criteria and objective function calculation, as presented in the first part of the study. The model of Chatelain and of Mével present good performances with a global error of 2.4, i.e. meaning an average error of 2.4 fold above the experimental uncertainty, and high fraction (70 %) of criteria predicted within two times the experimental uncertainty. Although the reference reaction models performed better on the oxidation dataset compared to the pyrolysis dataset (part I), their global error is still 50 to 125 % higher than the two most accurate reaction models. Rate of production and sensitivity analyses revealed that the origin of the discrepancy of the least performing models can be attributed to the reaction pathways consuming/producing Si and O atoms and to some kinetic rates that must be updated.
  • Direct numerical simulations of turbulent reacting flows with shock waves and stiff chemistry using many-core/GPU acceleration

    Desai, Swapnil; Kim, Yu Jeong; Song, Wonsik; Luong, Minh Bau; Hernandez Perez, Francisco; Sankaran, Ramanan; Im, Hong G. (Computers and Fluids, Elsevier BV, 2020-11-30) [Article]
    Compressible reacting flows may display sharp spatial variation related to shocks, contact discontinuities or reactive zones embedded within relatively smooth regions. The presence of such phenomena emphasizes the relevance of shock-capturing schemes such as the weighted essentially non-oscillatory (WENO) scheme as an essential ingredient of the numerical solver. However, these schemes are complex and have more computational cost than the simple high-order compact or non-compact schemes. In this paper, we present the implementation of a seventh-order, minimally-dissipative mapped WENO (WENO7M) scheme in a newly developed direct numerical simulation (DNS) code called KAUST Adaptive Reactive Flows Solver (KARFS). In order to make efficient use of the computer resources and reduce the solution time, without compromising the resolution requirement, the WENO routines are accelerated via graphics processing unit (GPU) computation. The performance characteristics and scalability of the code are studied using different grid sizes and block decomposition. The performance portability of KARFS is demonstrated on a variety of architectures including NVIDIA Tesla P100 GPUs and NVIDIA Kepler K20X GPUs. In addition, the capability and potential of the newly implemented WENO7M scheme in KARFS to perform DNS of compressible flows is also demonstrated with model problems involving shocks, isotropic turbulence, detonations and flame propagation into a stratified mixture with complex chemical kinetics.
  • Implicitly coupled phase fraction equations for polydisperse flows

    Keser, Robert; Ceschin, Alberto; Battistoni, Michele; Im, Hong G.; Jasak, Hrvoje (International Journal for Numerical Methods in Fluids, Wiley, 2020-11-28) [Article]
    This work presents the implementation, verification and the validation of an incompressible Eulerian multi-fluid model for polydisperse flows. The proposed model uses a novel monolithic, i.e. implicitly coupled phase continuity equation for an arbitrary number of fluids, where the breakup source and sink terms are handled implicitly in the block-system. The implemented model is tested for an upward bubbly flow inside a large vertical pipe. The selected flow conditions exhibit both breakup and coalescence. The grid refinement study is conducted on four structured grids with varying levels of refinement. In the validation section, the numerical results are compared to the TOPFLOW experimental measurements. The last presented test examines the performance of the novel implicitly coupled phase continuity equation to the corresponding segregated formulation and the standard segregated formulation. The performance is evaluated by comparing the conservation error over the non-linear iterations. The presented model exhibits good agreement with the experimental measurements and gives stable results on various grids with different levels of refinement. Moreover, the implicit coupling reduces the conservation error during the calculation.
  • Evolution of a shock generated by an impulsively accelerated, sinusoidal piston

    Shen, Naijian; Pullin, D. I.; Samtaney, Ravi; Wheatley, V. (Journal of Fluid Mechanics, Cambridge University Press (CUP), 2020-11-26) [Article]
    We consider the evolution of a shock wave generated by an impulsively accelerated, two-dimensional, almost planar piston with a sinusoidally corrugated surface of amplitude. We develop a complex-variable formulation for a nonlinear theory of generalized geometrical shock dynamics (GGSD) (Best, Shock Waves, vol. 1, issue 4, 1991, pp. 251–273; Best, Proc. R. Soc. Lond. A, vol. 442, 1993, pp. 585–598) as a hierarchical expansion of the Euler equations that can be closed at any order. The zeroth-order truncation of GGSD is related to the equations of Whitham’s geometrical shock dynamics (GSD), while higher-order corrections incorporate non-uniformity of the flow immediately behind the piston-driven shock. Numerical solutions to GGSD systems up to second order are coupled to an edge-detection algorithm in order to investigate the hypothesized development of a shock-shape curvature singularity as the rippled shock evolves. This singular behaviour, together with the simultaneous development of a Mach-number discontinuity, is found at all orders of the GGSD hierarchy for both weak and strong shocks. The critical time at which a curvature singularity occurs converges as the order of the GGSD system increases at fixed , and follows a scaling inversely proportional to at sufficiently small values. This result agrees with the weakly nonlinear GSD analysis of Mostert et al. (J. Fluid Mech., vol. 846, 2018, pp. 536–562) for a general Mach-number perturbation on a planar shock, and suggests that this represents the universal behaviour of a slightly perturbed, planar shock.
  • A Highly Sensitive and Wide-Range Resonant Magnetic Micro-Sensor Based on A Buckled Micro-Beam

    Alcheikh, Nouha; Mbarek, Sofiane; Younis, Mohammad I. (Research Square, 2020-11-25) [Preprint]
    Abstract We experimentally demonstrate a miniature highly sensitive wide-range resonant magnetic Lorentz-force micro-sensor. The concept is demonstrated based on the detection of the resonance frequency of an in-plane electrothermally heated straight resonator operated near the buckling point. The frequency shift is measured with optical sensing and the device is operating at atmospheric pressure. The magnetometer demonstrates a sensitivity (S) of 33.9/T, which is very high compared to the state of the art. In addition, the micro-sensor shows a good linearity in wide range and low power consumption around 0.2 mW. The above performances make the proposed micro-sensor promising for various low-cost magnetic applications.

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