Recent Submissions

  • Revisiting low temperature oxidation chemistry of n-heptane

    Xie, Cheng; Lailliau, Maxence; Issayev, Gani; Xu, Qiang; Chen, Weiye; Dagaut, Philippe; Farooq, Aamir; Sarathy, Mani; Wei, Lixia; Wang, Zhandong (Combustion and Flame, Elsevier BV, 2022-05-17) [Article]
    Benefitting from the rapid development of instrumental analysis methods, intermediate products that were difficult to probe in the past can now be measured and quantified in complex reaction systems. To understand low temperature reactions of interest for combustion applications, and reduce the deviations between model predictions and experimental measurements, constant advancement in understanding low temperature oxidation process is necessary. This work examines the oxidation of n-heptane in jet-stirred reactors at atmospheric pressure, with an initial n-heptane mole fraction of 0.005, equivalence ratio of 0.5, a residence time of 1s, and over a temperature range of 500-800 K. Reaction products were analyzed using synchrotron ultra-violet photoionization mass spectrometry, gas chromatography, and Fourier-transform infrared spectroscopy. Ignition delay times of n-heptane/O2/CO2 mixture were measured in a rapid compression machine at 20 and 40 bar over a 600-673 K temperature range. Based on the experimental results, a comprehensive kinetic model of n-heptane low temperature oxidation was developed by considering the sub-mechanisms of keto-hydroperoxide, cyclic ether, heptene isomers, and the third O2 addition reaction, and by updating the rate constants of keto-hydroperoxide decomposition and second oxygen addition reactions. The combination of reaction mechanism development and evaluation of the rate constants of key reactions enabled the model to effectively predict the species concentrations and ignition delay times of n-heptane low temperature oxidation, providing additional insight into alkane low temperature oxidation chemistry.
  • Evolution of the Seebeck effect in nanoparticle-percolated networks under applied strain

    Xin, Yangyang; Nesser, Hussein; Zhou, Jian; Lubineau, Gilles (Applied Materials Today, Elsevier BV, 2022-05-16) [Article]
    Nanoparticle clouds feature a unique evolution of the Seebeck coefficient under applied strain. At low strain, the Seebeck effect is very stable while, when a critical strain threshold is reached, it sharply decreases to zero. The evolution mechanism of this phenomenon that contributes to the development of various strain-insensitive sensors has yet to be documented. We elucidate here this evolution by proposing a phenomenological model supported by strong experimental evidence. We realize a full study of the Seebeck effect evolution for a network of nanoparticles under variable strain. We describe the evolution of this network as three subsequent stages: completely connected, partially connected, and disconnected. Based on the conductance-weighted formula, we theoretically analyze the evolution of the Seebeck coefficient in each stage. The Seebeck coefficient’s initial stability is attributed to the connected pathways that exist in the first two stages. We validated these theoretical results by constructing percolated networks with embedded silver nanowires in elastomers and calculating the Seebeck coefficient under various stretching conditions. Finally, using the theoretical results as a guide, we created a temperature sensor that is highly resistant to mechanical damage.
  • The impact of gasoline formulation on turbulent jet ignition

    Gorbatenko, Inna; Nicolle, Andre; Silva, Mickael; Im, Hong G.; Sarathy, Mani (Elsevier BV, 2022-05-13) [Article]
    Turbulent jet ignition (TJI) is a promising technology for burning ultra-lean mixtures; the process is comprised of hot reactive jets issuing from a pre-chamber (PC) and initiating combustion in the main chamber (MC). The present study employs a simplified zero-dimensional (0D) partially stirred reactor (PaSR) model to describe the complex mixing and reaction progress within the PC and its subsequent impact on the MC combustion in terms of combustion efficiency and pollutant formation characteristics. Full three-dimensional (3D) computational fluid dynamics (CFD) data are used to calibrate the PC model, which is subsequently linked to predict the MC combustion behavior. We propose a model to predict the effects of the fuel formulations with varying research octane number (RON) and octane sensitivities (OS) on the TJI performance. After a careful parametric study, a dedicated merit function for identifying the optimal TJI operating conditions was proposed to assess multiple fuel properties and their influence on MC combustion. The model properly accounts for micro-mixing effects in the early jet expansion phase, and represents the effects of a PC jet on enhancing flammability and pollutant mitigation. It was demonstrated that aromatic content affects not only the progress of the thermokinetic runaway, but also the importance of NO formation paths in MC (N2O vs NNH routes), and the effect of the PC jet on MC flammability limits. Among the jet active species, OH and NO exhibited the greatest chemical impact on MC reactivity, while the chemical effects of CO2 and H2O remained limited. The overall fuel TJI merit function showed optimum performance for fuels with 2 < OS < 6 and high RON, similar to the requirements for spark-ignited engine operation beyond motor octane number (MON) conditions, fuel lean advanced compression ignition operation, and spark-induced compression ignition.
  • The effect of ash, water vapor, and heterogeneous chemistry on the evolution of a Pinatubo-size volcanic cloud

    Abdelkader, Mohamed; Stenchikov, Georgiy L.; Pozzer, Andrea; Tost, Holger; Lelieveld, Jos (Copernicus GmbH, 2022-05-13) [Preprint]
    We employ the atmospheric chemistry general circulation model (EMAC) with gas phase, heterogeneous chemistry, and detailed aerosol microphysics to simulate the 1991 Pinatubo volcanic cloud. We explicitly account for the interaction of simultaneously injected SO2, volcanic ash, and water vapor and conducted multiple ensemble simulations with different injection configurations to test the simulated SO2, SO42-, ash masses, stratospheric aerosol optical depth, surface area density (SAD), and the stratospheric temperature response against available observations. We find that the SO2, SO42- masses and stratospheric aerosol optical depth (SAOD) are sensitive to the initial height of the volcanic cloud. The volcanic cloud interacts with tropopause and stratopause, and its composition is shaped by heterogeneous chemistry coupled with the ozone cycle. The height of the volcanic cloud in our simulations is also affected by dynamic processes within the cloud, i.e., heating and lofting of volcanic products. The mass of the injected water vapor has a moderate effect on the cloud evolution when volcanic materials are released in the lower stratosphere because it freezes and sediments as ice crystals. However, the injected water vapor at a higher altitude accelerates the oxidization of SO2 which is sensitive to the injected water vapor mass (via hydroxyl production and reaction rate). The coarse ash comprises 98 % of ash injection mass, which sediments within a few days, but aged sub-micron ash could stay in the stratosphere for a few months providing SAD for heterogeneous chemistry. The presence of ash accelerates the SO2 oxidation that leads to a faster formation of the sulfate aerosol layer in the first two months after the eruption and has to be accounted for in modeling the impact of large-scale volcanic injections on climate and stratospheric chemistry.
  • The effect of hydrogen bonding on the reactivity of OH radicals with prenol and isoprenol: a shock tube and multi-structural torsional variational transition state theory study

    Mohamed, Samah; Monge Palacios, Manuel; Giri, Binod; KHALED, Fethi; Liu, Dapeng; Farooq, Aamir; Sarathy, Mani (Physical Chemistry Chemical Physics, Royal Society of Chemistry (RSC), 2022-05-11) [Article]
    The presence of two functional groups (OH and double bond) in C5 methyl-substituted enols (i.e., isopentenols), such as 3-methyl-2-buten-1-ol (prenol) and 3-methyl-3-buten-1-ol (isoprenol), makes them excellent biofuel candidates as fuel additives. As OH radicals are abundant in both combustion and atmospheric environments, OH-initiated oxidation of these isopentenols over wide ranges of temperatures and pressures needs to be investigated. In alkenes, OH addition to the double bond is prominent at low temperatures (i.e., below ∼700 K), and H-atom abstraction dominates at higher temperatures. However, we find that the OH-initiated oxidation of prenol and isoprenol displays a larger role for OH addition at higher temperatures. In this work, the reaction kinetics of prenol and isoprenol with OH radicals was investigated over the temperature range of 900–1290 K and pressure of 1–5 atm by utilizing a shock tube and OH laser diagnostic. To rationalize these chemical systems, variational transition state theory calculations with multi-structural torsional anharmonicity and small curvature tunneling corrections were run using a potential energy surface characterized at the UCCSD(T)/jun-cc-pVQZ//M06-2X/6-311++G(2df,2pd) level of theory. A good agreement was observed between the experiment and theory, with both predicting a non-Arrhenius behavior and negligible pressure effects. OH additions to the double bond of prenol and isoprenol were found to be important, with at least 50% contribution to the total rate constants even at temperatures as high as 700 and 2000 K, respectively. This behavior was attributed to the stabilizing effect induced by hydrogen bonding between the reacting OH radical and the OH functional group of isopentenols at the saddle points. These stabilizing intermolecular interactions help mitigate the entropic effects that hinder association reactions as temperature increases, thus extending the prominent role of addition pathways to high temperatures. The site-specific rate constants were also found to be slower than their analogous reactions of OH + n-alkenes.
  • Computational assessment of effects of throat diameter on combustion and turbulence characteristics in a pre-chamber engine

    Silva, Mickael Messias; Liu, Xinlei; Hlaing, Ponnya; Sanal, Sangeeth; Cenker, Emre; Chang, Junseok; Johansson, Bengt; Im, Hong G. (Applied Thermal Engineering, Elsevier BV, 2022-05-06) [Article]
    Towards fundamental investigation of key physical aspects of pre-chamber combustion, the current work utilizes computational fluid dynamics to comprehend the effect of the throat diameter in an engine operated with methane. Previous studies showed that this parameter is dominant in pressure build-up and flow pattern inside the pre-chamber, suggesting that a detailed characterization is necessary. This pre-chamber type is composed of an upper conical part that lodges the spark plug and fuel injector, followed by a straight and tubular region called throat, which tip accommodates the nozzles responsible for the charge exchange between pre and main chambers. Two types of pre-chamber having distinct throat diameters are investigated, while utilizing consistent experimental data for validation of the model. The combustion process is modeled with the G-Equation model; the laminar flame speed was tabulated from a methane oxidation mechanism reduced from the GRI 3.0; the turbulent flame speed was computed using Peters' relation. The simulations were run for a full cycle, starting at exhaust valve opening. A homogeneous charge of methane is considered at the intake port, maintaining a global λ = 1.8, while 3% of total energy fuel is added through the pre-chamber. The results show that the throat changes the flow field inside the pre-chamber, impacts the air-fuel ratio, stratification, turbulence, jet dynamics, and ultimately the pre and main chambers combustion processes and heat fluxes. The combustion regime according to the Borghi-Peters diagram were found to lay in the thin reaction zone and in the flamelet regime.
  • A New Method to Measure the Spatial Distribution of Pressure Oscillations in Engine Knock Using Optical Diagnostics

    Shi, Hao; Tang, Qinglong; Uddeen, Kalim; Magnotti, Gaetano; Turner, James W. G. (Elsevier BV, 2022-05-02) [Preprint]
    Engine knock is one of the major obstacles limiting the thermal efficiency of spark-ignition (SI) engines. The in-cylinder spatial distribution of the pressure oscillation is of great importance to investigate the knock initiation and development. Using multiple pressure sensors to detect the local pressure oscillations is expensive and has a low spatial resolution. This study proposes a new method to measure the pressure oscillation distribution by monitoring the fluctuations of the local natural flame luminosity (NFL) during engine knocking combustion. To validate this method, simultaneous six-point pressure measurements and high-speed NFL imaging are implemented on an optical engine. The results indicate that end-gas auto-ignition leads to local fluctuations of both pressure and natural flame intensity. The local NFL oscillation phasing is 0.5 crank angle degrees earlier than those of the pressure signals, possibly due to the different response delays of high-speed imaging and pressure data acquisition systems. After applying a time offset and amplitude normalization, the flame luminosity oscillations could reproduce the pressure oscillations with very similar phase and frequency spectra. Based on the six channels of pressure signals, the distributions of pressure oscillation amplitude in the cylinder are exhibited by a fitted two-dimensional contour; the pressure oscillation distribution can be well predicted by the six flame monitors at the same locations. The prediction accuracy is affected by the flame monitor size and the optimal radius is between 5 and 8 mm. More flame detectors can present more details of the knock-induced pressure oscillations. A proper monitor number can maintain the measurement accuracy while keeping an appropriate computation load. The proposed new method provides a non-intrusive way to measure the spatial distribution of the knock-induced pressure oscillations, which could be directly applied in optical engines or metal engines through optical fibers.
  • Effects of non-thermal plasma on turbulent premixed flames of ammonia/air in a swirl combustor

    Kim, Gyeong Taek; Park, Jeong; Chung, Suk Ho; Yoo, Chun Sang (Fuel, Elsevier BV, 2022-04-29) [Article]
    The effects of non-thermal plasma (NTP) induced by a dielectric barrier discharge (DBD) reactor on the stability of turbulent lean and rich premixed ammonia/air flames and the NOx emission characteristics in a modified model gas turbine combustor are experimentally investigated by varying the applied AC voltage, , frequency, , and the mean mixture velocity, . Applying NTP to ammonia/air flames augments the flame stability such that the stable flame regime is extended to lower equivalence ratio, , under fuel-lean conditions or higher under fuel-rich conditions. NTP is found to significantly reduce the amount of NOx emission for both lean and rich premixed ammonia/air flames and the amount of NOx emission is well correlated with . The reduction of NOx emission through NH2 reactions is also identified by measuring NH chemiluminescence.
  • Model for Estimating Time-Varying Properties of an Inductively Coupled Plasma

    Georg, Robin; Chadwick, Ashley R.; Dally, Bassam; Herdrich, Georg (IEEE Transactions on Plasma Science, Institute of Electrical and Electronics Engineers (IEEE), 2022-04-25) [Article]
    A developing application of inductively coupled plasmas is in the field of electrodeless (propellant-flexible) electric propulsion. A significant issue facing this application is the need for diagnostic techniques that do not disturb the plasma (are nonintrusive), are propellant-agnostic, can resolve time variance, and are suitable for use in-flight. A new technique meeting these criteria is presented in this work. The technique makes use of the transformer model of inductive coupling to estimate the plasma impedance from the antenna current and resonant frequency, both of which can be measured nonintrusively. Having an estimate of the plasma impedance, it is possible to estimate a variety of plasma properties under the assumption of a uniform tubular plasma volume. Starting with a circuit representation of a high-power inductive plasma source, governing equations are derived and a solution method is described. Experimental data from the plasma source showing transient behavior (fluctuations within 300-Hz cycle) in oxygen plasmas with various input powers and flow rates are analyzed to demonstrate the technique and investigate trends. The technique produces results that are self-consistent and align well with previous theoretical work.
  • On the Mechanism Responsible for Extreme Turbulence Intensity Generation in the Hi-Pilot Burner

    Boxx, Isaac G.; Skiba, Aaron W.; Carter, Campbell D.; Ceschin, Alberto; Pérez, Francisco E. Hernández; Im, Hong G. (Flow, Turbulence and Combustion, Springer Science and Business Media LLC, 2022-04-21) [Article]
    In this study, we apply particle image velocimetry (PIV), hot-wire anemometry (HWA), and large-eddy simulation (LES) to identify and characterize a key mechanism by which high-intensity turbulence measured in the “Hi-Pilot” burner is generated. Large-scale oscillation of the high-velocity jet core about its own mean axial centerline is identified as a dominant feature of the turbulent flow field produced by this piloted Bunsen burner. This oscillation is linked to unsteady flow separation along the expanding section of the reactant nozzle and appears stochastic in nature. It occurs over a range of frequencies (100–300 Hz) well below where the turbulent kinetic energy (TKE) spectrum begins to follow a – 5/3 power law and results in a flow with significant scale separation in the TKE spectrum. Although scale separation and intermittency are not unusual in turbulent flows, this insight should inform analysis and interpretation of previous, and future studies of this unique test case
  • Laser-based pretreatment of composite T-joints for improved pull-off strength and toughness

    Hashem, Mjed H.; Wagih, A; Lubineau, Gilles (Composite Structures, Elsevier BV, 2022-04-14) [Article]
    This study demonstrates the efficiency of two CO surface laser treatment strategies for improving the strength and toughness of the adhesive bonding of T-joints. The first strategy involves uniform laser treatment to the surface, and the second strategy involves laser patterning by alternatively applying high- and low-power laser irradiation to the T-joint stiffener and skin surfaces. All the laser-treated T-joints showed progressive failure modes during pull-off test, unlike the PP surface-treated joints that exhibited brittle failure. The uniform laser-treated joints exhibited improved strength and toughness because contaminants were removed and the fibers were exposed to be directly bonded with adhesive. The joints subjected to the laser-patterning strategy demonstrated a substantial improvement in terms of the strength and toughness, reaching values of 2.2 and 12 times those of the PP surface-treated T-joints, respectively. This is because of the crack arresting at the transition between the two treatments at the stiffener interface, inducing crack migration from the stiffener-adhesive interface to the skin-adhesive interface while forming adhesive ligaments. The formation, deformation, and breakage of these ligaments dissipated more energy, hence increasing the effective joint toughness and strength.
  • A consistent soot nucleation model for improved prediction of strain rate sensitivity in ethylene/air counterflow flames

    Quadarella, Erica; Guo, Junjun; Im, Hong G. (Aerosol Science and Technology, Informa UK Limited, 2022-04-12) [Article]
    An improved consistent soot nucleation model was proposed and tested on ethylene counterflow flames at different strain rates. The main objective of the proposed model is to capture the correct strain rate sensitivity and broaden the applicability of the aerosol part of the soot model with different gas-phase kinetic mechanisms. Due to the uncertainties associated with experimental measurements of quantitative soot volume fraction (SVF), the approach’s effectiveness is mainly investigated on qualitative behavior in terms of strain rate sensitivity. Starting from a dimer-based nucleation model available in literature, soot inception is described as heterogeneous collisions of polycyclic aromatic hydrocarbons (PAHs) forming an intermediate dimer. Such a model justifies the inclusion of small precursors that conciliate a satisfactory reproduction of SVF sensitivity to strain rate, while retaining the quantitative accuracy of SVF prediction. The nucleation and condensation rates sensitivities are found to be regulated by the presence of the dimer to maintain the right balance with the upstream dimerization process. The heterogeneous collision model helps generalize the procedure and makes the model more adaptable to different kinetic mechanisms. Details about the inclusion of temperature-dependent sticking coefficients are also provided and found to be pivotal for a correct synergistic prediction of SVF trends and PAHs sensitivities to strain rate. The integration of important features in the soot nucleation model allows a generalized soot model free of empirical corrective factors, capturing the correct sensitivity to strain rates. Its ease of implementation and low computational cost make it suitable for turbulent flame simulations.
  • Toughening adhesive joints through crack path engineering using integrated polyamide wires

    Tao, Ran; Li, Xiaole; Yudhanto, Arief; Alfano, Marco; Lubineau, Gilles (Composites Part A: Applied Science and Manufacturing, Elsevier BV, 2022-04-09) [Article]
    Ensuring the progressivity of failure of adhesively-bonded composite joints is necessary to guarantee safety and to optimize maintenance operations. In our previous work, we proposed a novel surface patterning strategy to stop crack propagation by triggering bridging of adhesive ligaments. However, the brittle failure of classical bridging ligaments still releases a large amount of stored elastic energy, leading to a snap-slip crack propagation or even catastrophic sudden fracture of bonded joints. Such technology could be further improved by integrating ductile structures within the adhesive layer, but the detailed failure mechanisms require systematic investigation. In this work, we integrated thermoplastic polyamide structures within the epoxy adhesive layer of double cantilever beams to guide this transition from brittle failure to a stable softening behavior. Weak polyamide/epoxy adhesion and their embedded area fractions were critical since they affected the damage mechanisms and determined energy dissipation within bonded joints.
  • Double compression-expansion engine (DCEE) fueled with hydrogen: Preliminary computational assessment

    Babayev, Rafig; Andersson, Arne; Dalmau, Albert Serra; Im, Hong G.; Johansson, Bengt (Transportation Engineering, Elsevier BV, 2022-04-08) [Article]
    Hydrogen (H2) is currently a highly attractive fuel for internal combustion engines (ICEs) owing to the prospects of potentially near-zero emissions. However, the production emissions and cost of H2 fuel necessitate substantial improvements in ICE thermal efficiency. This work aims to investigate a potential implementation of H2 combustion in a highly efficient double compression-expansion engine (DCEE). DICI nonpremixed H2 combustion mode is used for its superior characteristics, as concluded in previous studies. The analysis is performed using a 1D GT-Power software package, where different variants of the DICI H2 and diesel combustion cycles, obtained experimentally and numerically (3D CFD) are imposed in the combustion cylinder of the DCEE. The results show that the low jet momentum, free jet mixing dominated variants of the DICI H2 combustion concept are preferred, owing to the lower heat transfer losses and relaxed requirements on the fuel injection system. Insulation of the expander and removal of the intercooling improve the engine efficiency by 1.3 and 0.5%-points, respectively, but the latter leads to elevated temperatures in the high-pressure tank, which makes the selection of its materials harder but allows the use of cheaper oxidation catalysts. The results also show that the DCEE performance is insensitive to combustion cylinder temperatures, making it potentially suitable for other high-octane fuels, such as methane, methanol, ammonia, etc. Finally, a brake thermal efficiency of 56% is achieved with H2 combustion, around 1%-point higher than with diesel. Further efficiency improvements are also possible with a fully optimized H2 combustion system.
  • Effects of DC Electric Fields on Flickering and Acoustic Oscillations of an M-shape Premixed Flame

    Xiong, Yuan; Lacoste, Deanna; Chung, Suk Ho; Cha, Min Suk (Flow, Turbulence and Combustion, Springer Science and Business Media LLC, 2022-04-07) [Article]
    This paper reports on the effect of DC electric fields on the dynamics of a premixed methane-air laminar flame, in a buoyant environment. DC electric fields can be capable of affecting both the buoyancy-driven flickering oscillation of the flame and the response of the flame to acoustic modulation of the flow. We conduct fast visualization of the emission of excited methylidyne radicals (CH*), representing the heat release rate of the flame. Such visualizations are also synchronized with electric current and voltage measurements. We notice that the suppression of buoyancy-driven flickering oscillations can be obtained by applying sub-critical negative DC voltages. Moreover, the current measured in the inter-electrodes area is analyzed for positive and negative DC applied voltages and we find that this quantity cannot be used as a tracer of heat release rate in a configuration where the flame location in the inter-electrodes gap varies with sub-critical electric fields. In addition, the effect of DC electric fields on the flame transfer function for acoustic modulation of the flow is reported and discussed.
  • Laser sensors for energy systems and process industries: Perspectives and directions

    Farooq, Aamir; Alquaity, Awad; Raza, Mohsin; Nasir, Ehson Fawad; Yao, Shunchun; Ren, Wei (Progress in Energy and Combustion Science, Elsevier BV, 2022-04-07) [Article]
    Sensors are perhaps the most important and integral components of our modern society. With global warming and environmental pollution garnering ever-increasing attention, as well as solutions for sustainabile and smart cities, the optimized performance of current and future energy systems and process industries is paramount. The accurate sensing and quantification of key parameters of such systems are essential for monitoring, controlling, and optimization efforts. In situ laser-based optical sensors are most suitable for achieving the desired characteristics of accuracy, sensitivity, selectivity, portability, speed, safety, and intelligence. In recent decades, significant progress has been made in the development and deployment of laser-based sensing solutions, although new challenges and opportunities continue to emerge rapidly. In this review paper, we focus on laser absorption spectroscopy (LAS)-based sensors owing to their simple architecture, easy implementation, and market penetration. We detail recent advancements made in LAS variants using new laser sources and techniques. A brief discussion on other laser-based sensing techniques, namely, photoacoustic spectroscopy, laser-induced fluorescence, coherent anti-Stokes Raman spectroscopy, and laser-induced breakdown spectroscopy, is provided to compare these strategies with LAS. The applications of laser-based sensors in various energy systemsincluding engines, turbines, power plants, furnaces, and boilers—as well as process industriessuch as petrochemical, semiconductor, natural gas leak detection, and corrosion detectionare presented, illustrating their many benefits and possible uses. A distinguishing aspect of this review paper is that we present the comparison of previous studies in tabular formats, making it easy to appreciate the recent progress in laser-based sensing solutions. Finally, suggestions on future directions and emerging technologies to pursue for the further enhancement, development, and deployment of laser-based sensors are proposed.
  • Bubble eruptions in a multilayer Hele-Shaw flow

    Al Brahim, Ahmed; Thoroddsen, Sigurdur T (Physical Review E, American Physical Society (APS), 2022-04-04) [Article]
    We study the dynamical rearrangement of gravitationally unstable multilayer fluid inside the narrow vertical gap of a Hele-Shaw cell. Four layers of immiscible fluids are superposed inside the cell, which is subsequently turned over. We vary the fluid properties and the relative thicknesses of the layers. One of the layers is air, the others are immiscible liquids: olive oil, water-glycerin mixture, and perfluorohexane. The concentration of the glycerin-water mixture is used to vary its viscosity. We classify various different dynamics of stirring and breakthrough of adjacent layers. We note a prominent phenomenon, where an air finger breaks through the high-viscosity layer to erupt as a hemisphere into the lower-viscosity perfluorohexane layer above it. These eruptions have a periodic neck pinch-off accompanied with high-speed airflow which breaks up some of the low-viscosity liquid to eject a spray of fine droplets. We use high-speed video to characterize the details of the eruptions and how wetting, contact lines and three-dimensionalities play a key role. We also investigate the center-of-mass trajectories for each layer and notice counterflows, where the center of some layers can temporarily move against buoyancy. The top and bottom layers can interchange by channeling through the intermediate layers, which subsequently overturn on longer timescales. We also point out some unexpected dynamics occurring in the triple- and four-phase interactions. Specifically, droplet motions are as much affected by local viscosity as by the density gradients.
  • Direct imaging of polymer filaments pulled from rebounding drops

    Yang, Zi Qiang; Zhang, Peng; Shi, Meng; Julaih, Ali Al; Mishra, Himanshu; Fabrizio, Enzo Di; Thoroddsen, Sigurdur T (arXiv, 2022-04-04) [Preprint]
    Polymer filaments form the foundation of biology from cell scaffolding to DNA. Their study and fabrication play an important role in a wide range of processes from tissue engineering to molecular machines. We present a simple method to deposit stretched polymer fibers between micro-pillars. This occurs when a polymeric drop impacts on and rebounds from an inclined superhydrophobic substrate. It wets the top of the pillars and pulls out liquid filaments which are stretched and can attach to adjacent pillars leaving minuscule threads, with the solvent evaporating to leave the exposed polymers. We use high-speed video at the microscale to characterize the most robust filament-forming configurations, by varying the impact velocity, substrate structure and inclination angle, as well as the PEO-polymer concentration. Impacts onto plant leaves or randomized nano-structured surface leads to the formation of a branched structure, through filament mergers at the free surface of the drop. SEM shows the deposition of filament bundles which are thinner than those formed by evaporation or rolling drops. Raman spectroscopy identifies mode B stretched DNA filaments from aqueous-solution droplets.
  • Control Design and Analysis for Reduced Gravity Atmospheric Flights

    Chen, Yi-Hsuan (2022-04) [Thesis]
    Advisor: Feron, Eric
    Committee members: Keyes, David E.; Park, Shinkyu
    Microgravity environments have a wide range of potential applications, such as astronaut training and scientific research in weightlessness or at partial-$g$ levels, which helps humans move toward space exploration. Parabolic flights are one way to simulate microgravity on Earth, which can be achieved by making aircraft follow specific flight trajectories. This work describes a kinematic and dynamic analysis of general partial gravity cases and develops a flight control framework for a zero-gravity flight using a proof-mass-tracking approach. During the zero-gravity parabola phase, aircraft will have a zero local (non-gravitational) acceleration and be in a state of free-fall, thus causing the sensation of weightlessness. Hence, the control objective is to simultaneously compensate for aerodynamic drag using thrust control and to minimize lift force by regulating the elevator. A triple-integral control structure is adopted to overcome unknown, quadratically increasing drag, based on an internal model principle. Furthermore, to avoid the non-minimum phase characteristics of aircraft longitudinal dynamics, the position deviation from the inertial reference is redefined such that the closed-loop system is minimum phase. Flight simulations are demonstrated to validate the proposed control strategy and are visualized in the open-source flight simulator FlightGear.
  • Investigation of Cryorgenic Nitrogen Injection Under Supercritical Condition Using Tabulated Real Fluid Modeling

    Zaihi, Abdullah (2022-04) [Thesis]
    Advisor: Im, Hong G.
    Committee members: Mishra, Himanshu; Turner, James W. G.
    This work intended to model the cryogenic nitrogen spray under supercritical conditions using tabulated real-gas fluid properties. To investigate the effects of various equations of states (EoSs), four tables were generated using different EoSs such as the idea gas (IG), Peng-Robinson (PR), Soave-Redlich-Kwong (SRK), and Redlich-Kwong-Peng-Robinson (RKPR). For validations, the tabulated fluid properties using the CoolProp library were taken as the baseline case. The modelling results demonstrated that all the real-gas EoSs predicted similar spray features except for the IG EoS, which significantly underpredicted the density gradient and led to less intense jet diffusion and shorter penetration length. Of the four EoSs, the PR EoS exhibited the best agreement with the baseline case. Furthermore, the effect of ambient pressure on the spray development was also examined, ranging from the subcritical to the supercritical conditions. A highly diffusive and longer jet was observed under supercritical conditions (4 and 5 MPa). Thereafter, the effect of ambient temperature was also evaluated. It was found that the higher chamber temperature led to a more rapid decrease in density due to the better evaporation process, which resulted in a longer jet breakup length.

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