Recent Submissions

  • Achieving Super Sensitivity in Capacitive Strain Sensing by Electrode Fragmentation

    Nesser, Hussein; Lubineau, Gilles (ACS Applied Materials & Interfaces, American Chemical Society (ACS), 2021-07-26) [Article]
    Accurate wireless strain monitoring is critical for many engineering applications. Capacitive strain sensors are well suited for remote sensing but currently have a limited sensitivity. This study presents a new approach for improving the sensitivity of electrical capacitance change-based strain sensors. Our technology is based on a dielectric elastomer layer laminated between two fragmented electrodes (i.e., carbon nanotube papers) that, by design, experiences a significant change in resistance (from Ω to MΩ) when stretched and makes the sensor behave as a transmission line, a well-known structure in telecommunication engineering. The strain-dependent voltage attenuation over the structure length results in a large variation of the effective capacitance (gauge factor exceeding 37 at 3% strain).
  • Flame flow field interaction in non-premixed CH4/H2 swirling flames

    Elbaz, Ayman M.; Mannaa, Ossama; Roberts, William L. (International Journal of Hydrogen Energy, Elsevier BV, 2021-07-17) [Article]
    Alternative fuels and stocks like biomass or chemical and refinery waste, may potentially be used in gas turbines and industrial applications after gasification. Thus, understanding the role of hydrogen in these fuels is critical to the broader aim of utilising alternative fuels for power generation. In this work, the interaction between the flame and the flow field was studied in a quarl-stabilised swirl non-premixed flame burning CH4 and H2–enriched CH4. Simultaneous high-speed OH-PLIF/PIV imaging at 5 kHz was carried out on these flames to explore the flame-flow interaction. The instantaneous flow fields in the CH4 or CH4+H2 flames showed a small scale vortical structure near the shear layers, which were not apparent in the time-averaged flow fields. Increasing H2% in the fuel jet was observed to dampen the velocity fluctuations. The fuel composition affected the spatial location of the reaction zone; in the CH4 flames, the axial position of the reaction zone is seen to track the relatively large-magnitude axial velocity fluctuations while remaining in locally low-speed regions of the flow. In contrast, in H2-enriched flames, where the flame is more robust, the reaction zone was able to survive longer, in terms of axial distance, in the vicinity of high swirling jet velocity, with less sensitivity to velocity fluctuations. With increasing the H2%, the reaction zone steadily leaves the IRZ towards the swirling jet flow and localised between its outer and inner vortices. This acts as a stabilisation factor where the internal vortices convect hot product towards the fresh mixture. Moreover, the flame curvatures, the vorticity and compressive strain fields interactions with the reaction zone are presented and discussed. This article outlines results that yield more in-depth insight into hydrogen-enriched hydrocarbon non-premixed swirling flames' combustion, which is essential to accelerate the fuel switching from hydrocarbons to hydrogen.
  • A comprehensive combustion chemistry study of n-propylcyclohexane

    Ahmed, Ahfaz; Corrubia, Julius A.; Al-lehaibi, Moaz; Farid, Farinaz; Wang, Heng; Wang, Zhandong; Chen, Bingjie; Roberts, William L.; Miller, David L.; Farooq, Aamir; Cernansky, Nicholas P.; Sarathy, Mani (Combustion and Flame, Elsevier BV, 2021-07-11) [Article]
    Alkylated cycloalkanes are vital components in gasoline, aviation, and diesel fuels; however, their combustion chemistry has been less investigated compared to other hydrocarbon classes. In this work, the combustion kinetics of n-propylcyclohexane (n-Pch) was studied across a range of experiments including pressurized flow reactor (PFR), jet stirred reactor (JSR), shock tube (ST), and rapid compression machine (RCM). These experiments cover a wide range of conditions spanning low to intermediate to high temperatures, low to high pressures at lean to rich equivalence ratios. Stable intermediate species were measured in PFR over a temperature range of 550–850 K, pressure of 8.0 bar, equivalence ratio (ϕ) of 0.27, and constant residence time of 120 ms. The JSR was utilized to measure the speciation during oxidation of n-Pch at ϕ of 0.5–2.0, at atmospheric pressure, and across temperature range of 550–800 K. Ignition delay times (IDTs) for n-Pch were measured in the RCM and ST at temperatures ranging from 650 to 1200 K, at pressures of 20 and 40 bar, at ϕ = 0.5, 1.0. In addition, a comprehensive detailed chemical kinetic model was developed and validated against the measured experimental data. The new kinetic model, coupled with the breadth of data from various experiments, provides an improved understanding of n-Pch combustion.
  • Snap-back Instability of Double Cantilever Beam with bridging

    Li, Xiaole; Lu, Shuai; Lubineau, Gilles (International Journal of Solids and Structures, Elsevier BV, 2021-07) [Article]
    Adhesive bonding community shows a continued interest in using bridging mechanisms to toughen the interface of secondary bonded joints, especially in the case of laminated composites. Due to snap-back instability that occurs during fracture, confusions may exist when identifying the toughening effect experimentally. The true toughening effect may be overestimated by lumping all energy contributions (kinetic energy included) in an overall effective toughness. Here, fundamentals for bridging to enhance fracture resistance are explored through the theoretical analysis of the delamination of a composite double cantilever beam (DCB) with bridging. Specifically, we establish a theoretical framework on the basis of Timoshenko beam theory and linear elastic fracture mechanics to solve the fracture response of DCB in the presence of discrete bridging phases. We elucidate the crack trapping and the snap-back instability in structural response during the crack propagation. We identify the contribution to the overall toughness observed numerically/experimentally of both the physical fracture energy and other types of dissipation. The associated toughening mechanisms are then unveiled. Furthermore, we study the effects of property of the bridging phases on the snap-back instability, based on which, we propose a dimensionless quantity that can be deployed as an indicator of the intensity of snap-back instability. Finally, we identify the role of geometrical properties, i.e. the substrate thickness and the arrangement spacing of the bridging phases, in the snap-back instability and the macroscopic fracture toughness of a DCB. This work provides, from a theoretical point of view, an essential insight into the physics related to the structural response of DCB with discrete toughening elements.
  • Shock Fitting For Converging Cylidrical Shocks In Hydrodynamics And Ideal Magnetohydrodynamics

    Arshad, Talha (2021-07) [Thesis]
    Advisor: Samtaney, Ravi
    Committee members: Farooq, Aamir; Parsani, Matteo; Bakhsh, Abeer
    Converging shocks have long been a topic of interest in theoretical fluid mechanics, and are of prime importance in inertial confinement fusion. However, tracking converging shocks in numerical schemes poses several challenges. Numerical schemes based on shock capturing inherently diffuse out shocks to multiple grid cells, making it hard to track the shock. Converging shocks are significantly harder to track, as this numerical smearing is much more significant when converging shocks approach the axis of convergence. To mitigate this problem, we transform the conservation laws to a non-inertial frame of reference in which the accelerating shock is stationary. A system of equations is derived based on the transformed conservation laws coupled to the shock speed obtained from jump conditions and a characteristic-based derivation of a relation governing shock acceleration. We solve these equations using a finite volume method. Our numerical results compare favorably with the analytical value of Guderley exponent for self-similarly converging cylindrical hydrodynamic shocks. Results for fast magnetosonic shock in MHD are also presented and compared with results from geometrical shock dynamics (GSD). Results from our shock fitting method, developed without any approximation to the original ideal magnetohydrodynamics equations, provide further credibility to GSD applied to converging fast magnetosonic shocks. This sort of shock fitting is a precursor to future multidimensional stability analysis of imploding shocks.
  • Risk assessment of oil spills in the NEOM region

    Carrasco Franco, Ana Kenia (2021-07) [Thesis]
    Advisor: Knio, Omar
    Committee members: Hoteit, Ibrahim; Thoroddsen, Sigurdur T
    This study aims to assess the risk from oil spills in the NEOM region based on marine traffic, with the aid of Modelo Hidrodinâmico (MOHID), oil spill model driven by the outputs of a validated regional met-ocean data set. The region is classified into two sub-regions: the immediate region, extending 50 km in both directions (north and south) along the coastline from the pinpoint location of NEOM; and the extended region, covering as additional 50 km coastal segments in both directions. A total of 15 spill locations are selected in the regions of high marine traffic density, and for each location a total of 48 instantaneous spill events are considered, triggered at the beginning of each month during the period 2013-2016. An independent simulation is conducted for each event, tracking the evolution of the spill over a 30 day period. Simulation results are analyzed to estimate three hazard metrics, namely the volume beached at the end of the month, arrival time to the coast, and the rise time of the beached volume profile. Based on these metrics and historical data on the oceanic-atmosphericconditions, oil spill risk maps are generated, signaling hot spots. Also, an analysis of the seasoned circulation effects on the fate of the oil spills is conducted. The results of this study provide useful information for assessing the impact of an oil spill contamination, and designing monitoring and mitigation measures.
  • Bio-inspired adhesive joint with improved interlaminar fracture toughness

    Wagih, A.; Tao, Ran; Lubineau, Gilles (Composites Part A: Applied Science and Manufacturing, Elsevier BV, 2021-06-29) [Article]
    Herein, a novel adhesive joint that mimics the structure of the bondline of two biological systems that have excellent adhesion, gecko and mytilus californianus, was designed. Sacrificial cracks were embedded inside the adhesive layers to activate new dissipative mechanisms and thus increase the effective interlaminar fracture toughness. A finite element model (FEM) that considered the progressive damage in the adhesive layer and decohesion at both adhesive/adherend interfaces was used to understand the damage mechanisms of the bio-inspired adhesives. Contrary to classical joints that experience interfacial failure, wherein failure occurs at one of the substrate/adherend interfaces, our bio-inspired joints are characterized by crack bifurcation between both sides of the bondline. Thus the developed toughening ligaments cause the propagation of secondary and backward cracks at the lower and upper interfaces under and over the sacrificial cracks, respectively. Such crack branching and fragmentation together with the energy required to break these ligaments improve the effective toughness of the adhesive. Increasing the sacrificial crack width and gap between two successive cracks reduces the toughness through reduction in the allowed surface for secondary and backward crack propagation. The adhesive properties significantly affect this toughening effect that increases with adhesive strength and failure strain.
  • The effect of oxygen content on the turbulent flame speed of ammonia/oxygen/nitrogen expanding flames under elevated pressures

    Wang, Shixing; Elbaz, Ayman M.; Wang, Zhihua; Roberts, William L. (Combustion and Flame, Elsevier BV, 2021-06-25) [Article]
    Ammonia is a promising, but weakly reactive, carbon-free fuel and a promising hydrogen carrier for renewable energy. In this study, the turbulent flame speed (ST) of stoichiometric ammonia/oxygen/nitrogen mixtures under oxygen enrichment conditions was investigated at elevated pressures using a fan-stirred constant volume combustion chamber. Turbulent flame speed is found to increase with oxygen content at all pressures and turbulent intensities (u′) studied. In contrast, the turbulent-to-laminar flame speed ratio (ST/SL) was found to decrease with the oxygen content, mainly due to SL increasing faster than ST. The self-similar propagation characteristics of ammonia flame as flame radius develops are similar to other fuels and scale as a one-half power law, but it bends down as oxygen content increases. Several correlations of turbulent flame speed are validated against the present data, and it is found that a fitting relationship including pressures and turbulent intensities is independent of fuel type and performs best among literature proposed correlations. Moreover, the correlations of ammonia get similar results with other fuels when turbulent length scale adopted same definition of laminar flame thickness, in contrast, the ST/SL is larger for ammonia compared to methane at the same Reynolds number. Two new correlations of turbulent flame speed based on Karlovitz number (Ka) and Damköhler number (Da) are presented and narrowed the data scatter. Ammonia/oxygen/nitrogen mixture with ηO2 = 0.4 get similar turbulent flame speed as methane/air. A flame surface wrinkling analysis and stretch sensitivity analysis showed that the sensitivity of the higher stretch rate combined with the wrinkling acceleration was responsible for the observed increase in ST/SL ratio.
  • Chemical effects of anisole and toluene addition to n‑heptane on PAH characteristics in laminar premixed flames by LIF measurement and kinetic model

    Zhang, Yiran; Jiao, Anqi; Li, Youping; Liu, Peng; Yang, Guofeng; Zhan, Reggie; Roberts, William L.; Huang, Zhen; Lin, He (Fuel, Elsevier BV, 2021-06-18) [Article]
    Anisole is a candidate renewable fuel that displays satisfying combustion characteristics, but its sooting characteristics are not well known. The goal of this study is to investigate the chemical effects of anisole and toluene on PAH formation in n-heptane laminar premixed flames, using LIF measurement and chemical kinetic simulation. To focus on chemical effects, the equivalence ratios, dilution ratios, and flame temperatures were kept nearly unchanged when anisole and toluene were blended separately into n-heptane flame. LIF experimental results indicated that PAH formation was promoted with the addition of anisole and toluene, and the effect of toluene was stronger than the promotional effect of anisole. The chemical kinetic model predicted the observed PAH tendencies well in the LIF experiments. Based on this model, reaction pathway and sensitivity analyses were performed to interpret the chemical effects. Results revealed that, due to their different molecular structures, the difference in chemical effects between anisole and toluene was notable in PAH growth processes. Anisole decomposed first via the O-CH3 bond dissociation reaction, and then proceeded to a CO elimination reaction to yield an important PAH precursor, C5H5, which contributed to PAH formation via styrene and indene reaction networks. In the toluene added flame, PAH formation was enhanced because of ring expansion reaction on the dehydrogenated branch of the toluene.
  • Robust, Long-Term, and Exceptionally Sensitive Microneedle-Based Bioimpedance Sensor for Precision Farming

    Bu Khamsin, Abdullah; Moussi, Khalil; Tao, Ran; Lubineau, Gilles; Blilou, Ikram; Salama, Khaled N.; Kosel, Jürgen (Advanced Science, Wiley, 2021-06-17) [Article]
    Precision farming has the potential to increase global food production capacity whilst minimizing traditional inputs. However, the adoption and impact of precision farming are contingent on the availability of sensors that can discern the state of crops, while not interfering with their growth. Electrical impedance spectroscopy offers an avenue for nondestructive monitoring of crops. To that end, it is reported on the deployment of impedimetric sensors utilizing microneedles (MNs) that can be used to pierce the waxy exterior of plants to obtain sensitive impedance spectra in open-air settings with an average relative noise value of 3.83%. The sensors are fabricated using a novel micromolding and release method that is compatible with UV photocurable and thermosetting polymers. Assessments of the quality of the MNs under scanning electron microscopy show that the replication process is high in fidelity to the original design of the master mold and that it can be used for upward of 20 replication cycles. The sensor's performance is validated against conventional planar sensors for obtaining the impedance values of Arabidopsis thaliana. As a change is detected in impedance due to lighting and hydration, this raises the possibility for their widespread use in precision farming.
  • Local flame displacement speeds of hydrogen-air premixed flames in moderate to intense turbulence

    Yuvraj,; Song, Wonsik; Dave, Himanshu; Im, Hong G.; Chaudhuri, Swetaprovo (arXiv, 2021-06-15) [Preprint]
    Comprehensive knowledge of local flame displacement speed, $S_d$, in turbulent premixed flames is crucial towards the design and development of hydrogen fuelled next-generation engines. Premixed hydrogen-air flames are characterized by significantly higher laminar flame speed compared to other conventional fuels. Furthermore, in the presence of turbulence, $S_d$ is enhanced much beyond its corresponding unstretched, planar laminar value $S_L$. In this study, the effect of high Karlovitz number ($Ka$) turbulence on density-weighted flame displacement speed, $\widetilde{S_d}$, in a H$_2$-air flame is investigated. Recently, it has been identified that flame-flame interactions in regions of large negative curvature govern large deviations of $\widetilde{S_d}$ from $S_L$, for moderately turbulent flames. An interaction model for the same has also been proposed. In this work, we seek to test the interaction model's applicability to intensely turbulent flames characterized by large $Ka$. To that end, we investigate the local flame structures: thermal, chemical structure, the effect of curvature, along the direction that is normal to the chosen isothermal surfaces. Furthermore, relative contributions of the transport and chemistry terms to $\widetilde{S_d}$ are also analyzed. It is found that, unlike the moderately turbulent premixed flames, where enhanced $\widetilde{S_d}$ is driven by interactions among complete flame structures, $\widetilde{S_d}$ enhancement in high $Re_t$ and high $Ka$ flame is predominantly governed by local interactions of the isotherms. It is found that enhancement in $\widetilde{S_d}$ in regions of large negative curvature occurs as a result of these interactions, evincing that the interaction model is useful for high $Ka$ turbulent premixed flames as well.
  • Experimental and kinetic modeling study of α-methyl-naphthalene pyrolysis: Part II. PAH formation

    Jin, Hanfeng; Hao, Junyu; Yang, Jiuzhong; Guo, Junjun; Zhang, Yan; Cao, Chuang Chuang; Farooq, Aamir (Combustion and Flame, Elsevier BV, 2021-06-13) [Article]
    α-Methyl-naphthalene plays an important role as a functional material in petrochemical industries and as a precursor of soot particles. The formation chemistry of polycyclic aromatic hydrocarbons (PAHs) from α-methyl-naphthalene, therefore, warrants detailed investigations. In this work, we studied PAH formation from its pyrolysis using experiments and kinetic models. Flow reactor pyrolytic experiments at low and atmospheric pressures (30 and 760 Torr) were performed using synchrotron vacuum ultraviolet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). A kinetic model was then developed to predict PAH formation from α-methyl-naphthalene. According to the kinetic analysis of the proposed model, naphth-1-yl-methyl, benzo-fulvenallene, and benzo-fulvenallenyl are three critical intermediates in the formation of large PAHs. Other than the traditional H-abstraction acetylene-/vinylacetylene-addition mechanisms, three prototypical PAH formation pathways are identified in α-methyl-naphthalene pyrolysis: 1) addition and cyclization reactions of naphth-1-yl-methyl and naphth-1-yl radicals; 2) recombination of resonance stabilized radicals (indenyl, benzo-fulvenallenyl, phenalenyl, etc.) and the subsequent ring expansion reactions; 3) sequential propargyl addition reactions.
  • Plateau–Rayleigh Instability Induced Self-Assembly of Nano-Cubes in Stretched DNA Molecules

    Zhang, Peng; Yang, Zi Qiang; Thoroddsen, Sigurdur T; Di Fabrizio, Enzo (Submitted to MNE2021 - 47th international conference on Micro and Nano Engineering, 2021-06-13) [Preprint]
  • Furan formation pathways exploration in low temperature oxidation of 1,3-butadiene, trans-2-butene, and cis-2-butene

    Chen, Bingjie; Liu, Peng; Li, Zepeng; Hansen, Nils; Roberts, William L.; Pitsch, Heinz (Combustion and Flame, Elsevier BV, 2021-06-11) [Article]
    Furan is one of the smallest organic compounds with heterocycle ring. With this particular molecular structure, furan is considered as a highly toxic and carcinogenic combustion pollutant, and furan may contribute to the formation of oxygenated soot. In this work, furan formation pathways from 1,3-butadiene, trans-2-butene and cis-2-butene were comprehensively explored. The potential energy surfaces, reaction rate coefficients, and thermodynamics were calculated by quantum chemistry using high level of theories including the CCSD (T) and G3 methods. The proposed reaction pathways were then implemented into the AramcoMech 3.0 model uniformly or independently to examine the model performance with the experimental data. The oxidation experiments of 1,3-butadiene, trans-2-butene and cis-2-butene were performed in a jet stirred reactor (JSR) in the low temperature regime (500–830 K). The JSR is coupled with time-of-flight molecular beam mass spectrometry (ToF-MBMS) using synchrotron radiation as photon ionization source for species identification and quantification. Compared with experiments, both updated models (the independent and uniform model) showed better prediction of furan than the base AramcoMech 3.0 model, which highlighted the contribution of the proposed pathways. Reaction pathway analyses reveal that in the proposed reaction pathway, both reactions C4H6 + OH ⇌ S1–4 (H2C[dbnd]CH-ĊH–CH2OH, but‑1-en-3-yl-4-ol) and C4H6 + HO2 ⇌ C4H61–3OOH4 (H2C[dbnd]CH-ĊH–CH2OOH, but‑1-en-3-yl-4-peroxide) not only contribute to furan formation, but also to fuel consumption. Furthermore, the kinetic uncertainty from activation energy calculated by the CCSD series methods, CBS-ANPO, and G4 methods was evaluated for reaction C4H6 + HO2 ⇌ C4H61–3OOH4. Instead of developing a new kinetic model, this work aims at proposing and validating new reaction pathways to advance the understanding of furan formation chemistry in low temperature oxidation, and provide guidance for future model development.
  • Chemiluminescence signature of premixed ammonia-methane-air flames

    Zhu, Xuren; Khateeb, Abdulrahman A.; Roberts, William L.; Guiberti, Thibault (Combustion and Flame, Elsevier BV, 2021-06-07) [Article]
    This paper reports on the chemiluminescence footprint of premixed ammonia-methane-air flames. The chemiluminescence spectrum of laminar twin-flames stabilized with a counterflow burner were measured between 200 and 457 nm for large ranges of equivalence ratio (0.60 ≤ ϕ ≤ 1.30), ammonia fraction (pure methane to pure ammonia), and strain rate (55/s ≤ a ≤ 300/s). Relevant excited radicals were identified, namely, NO*, OH*, NH*, CN*, CH*, and CO2* and evolutions of their chemiluminescence intensity were analyzed as a function of equivalence ratio, ammonia fraction, and strain rate. These measurements produced an unprecedented database on the chemiluminescence of ammonia-methane-air flames, which could be used in the future for model validation. A total of 15 ratios of chemiluminescence intensity were also considered and 5 ratios showing promise for the development of chemiluminescence-based flame sensors were identified. The CN*/OH* ratio is a potential surrogate for equivalence ratio even if the ammonia fuel fraction in the fuel blend is not known accurately – as long as it exceeds 0.3 by volume. The CN*/NO* ratio is another possible surrogate for equivalence ratio if the ammonia fraction in the fuel blend is below 0.5. The OH*/CH* ratio, often used to sense equivalence ratio in hydrocarbons flames, is not recommended for ammonia-methane flames. The NH*/CN* ratio is a potential surrogate for the ammonia fraction in the fuel blend if equivalence ratio is larger than ϕ = 0.7 and if the ammonia fraction in the fuel blend is below 0.4. Other ratios may be combined to provide a simultaneous measure of equivalence ratio and ammonia fraction in the fuel blend with an extended range of validity, for example NH*/OH* and CN*/NH*.
  • On the origins of lubricity and surface cleanliness in ethanol-diesel fuel blends

    Hong, Frank T.; Singh, Eshan; Sarathy, Mani (Fuel, Elsevier BV, 2021-06-04) [Article]
    Ethanol is the most used bio-derived fuel additive. However, adding ethanol in diesel fuel may negatively impact lubricity or surface cleanliness, which is critical for high-pressure fuel injection systems employed in compression ignition engines. This work investigates surfaces lubricated by ethanol–diesel blends. Adding 5 wt% ethanol in diesel showed negligible changes in fuel lubricity, while blending 10, 20, and 40 wt% ethanol increased wear rates by 46, 81, and 239% respectively. These increases in wear rates (with increases in ethanol by wt%) correlate with the evolution of electrical contact resistance (ECR) values over time. As more ethanol was added, the ECR values signaled thinner fuel films, more metal-to-metal contacts, and a delayed onset of frictional product growth. Raman spectra showed that forming frictional species produced by tribochemical reactions enhanced fuel lubricity. The absence of some frictional species in ethanol lubricated surfaces points to simultaneously improved surface cleanliness and reduced lubricity.
  • Magnetic sensors – A review and recent technologies

    Khan, Mohammed Asadullah; Sun, Jian; Li, Bodong; Przybysz, Alexander; Kosel, Jürgen (Engineering Research Express, IOP Publishing, 2021-06-04) [Article]
    Magnetic field sensors are an integral part of many industrial and biomedical applications, and their utilization continues to grow at a high rate. The development is driven both by new use cases and demand like internet of things as well as by new technologies and capabilities like flexible and stretchable devices. Magnetic field sensors exploit different physical principles for their operation, resulting in different specifications with respect to sensitivity, linearity, field range, power consumption, costs etc. In this review, we will focus on solid state magnetic field sensors that enable miniaturization and are suitable for integrated approaches to satisfy the needs of growing application areas like biosensors, ubiquitous sensor networks, wearables, smart things etc. Such applications require a high sensitivity, low power consumption, flexible substrates and miniaturization. Hence, the sensor types covered in this review are Hall Effect, Giant Magnetoresistance, Tunnel Magnetoresistance, Anisotropic Magnetoresistance and Giant Magnetoimpedance.
  • Statics and Dynamics of V-shaped Micro-beams Under Axial Forces

    Ouakad, Hassen; Alcheikh, Nouha; Ben Mbarek, Sofiane; Rocha, Rodrigo Tumolin; Younis, Mohammad I. (Journal of Computational and Nonlinear Dynamics, ASME International, 2021-06-02) [Article]
    Abstract We present an investigation into the static and dynamic behaviors of electrostatically actuated in-plane micro-electro-mechanical V-shaped micro-beam under axial loads. The micro-beams are actuated with two separate electrodes of uniform air-gap across their length. The effects of the initial rise and DC bias voltage are examined while varying the axial loads ranging from compressive to tensile. The numerical analysis is based on a nonlinear equation of motion of a shallow V-shaped micro-beam. The static equation is solved using a reduced-order model based on the Galerkin procedure. Then, the eigenvalue problem of the structure is solved for various equilibrium positions. The analytical model is validated by comparing to an experimental case study. The results show rich and diverse static and dynamic behavior. It is shown that the micro-beam may exhibit only pull-in or snap-through and pull-in instabilities. Various multi-state and hysterics behaviors are demonstrated when varying the actuation forces and the initial rise. High tunability is demonstrated when varying the axial and DC loads for the first two symmetric vibration modes. Such rich behavior can be very useful for high performance micro-scale applications designs.
  • Prandtl number dependence of the small-scale properties in turbulent Rayleigh-Bénard convection

    Bhattacharya, Shashwat; Verma, Mahendra K.; Samtaney, Ravi (Physical Review Fluids, American Physical Society (APS), 2021-06-01) [Article]
    We analyze the Prandtl number (Pr) dependence of spectra and fluxes of kinetic energy, as well as the energy injection rates and dissipation rates, of turbulent thermal convection using numerical data. As expected, for a flow with Pr 1, the inertial-range kineticenergy flux is constant, and the kinetic-energy spectrum is Kolmogorov-like (k−5/3). More importantly, we show that the amplitudes of the kinetic-energy fluxes and spectra and those of structure functions increase with the decrease of Pr, thus exhibiting stronger nonlinearity for flows with small Prandtl numbers. Consistent with these observations, both the kinetic-energy injection rates and the dissipation rates increase with the decrease of Pr. Our results are in agreement with earlier studies that report the Reynolds number to be a decreasing function of Prandtl number in turbulent convection. On the other hand, the tail of the probability distributions of the local heat flux grows with the increase of Pr, indicating increased fluctuations in the local heat flux with Pr.
  • Studies of Preignition in Homogeneous Environments

    Figueroa Labastida, Miguel (2021-06) [Dissertation]
    Advisor: Farooq, Aamir
    Committee members: Roberts, William L.; Thoroddsen, Sigurdur T; Santamarina, Carlos; Heufer, Karl Alexander
    Preignition is an ignition event that happens before it is expected to happen and, many times, where it is not expected to happen. Understanding this phenomenon is of great importance as it influences the design and operation of modern downsized boosted internal combustion engines. To gain a fundamental understanding of preignition, homogeneous reactors like shock tubes and rapid compression machines may be used to decipher the influence of fuel chemical structure, temperature, pressure, equivalence ratio and bath gas on preignition. In this thesis, a comprehensive study of the preignition tendency of various chemical systems is presented. Firstly, renewable fuels like ethanol, methanol and a surrogate of conventional fuels, n-hexane, are characterized by traditional shock tube techniques, such as the measurements of ignition delay times and pressure-time histories, to identify thermodynamic conditions which promote non-ideal ignition behavior. Preignition pressure rise and the expedition of measured ignition delay times are identified as the indicators of non-homogeneous combustion. It is shown that preignition effects are more likely to be observed in mixtures containing higher fuel concentration and that preignition energy release is more pronounced at lower temperatures. High-speed imaging was implemented to visualize the combustion process taking place inside the shock tube. End-wall imaging showed that low-temperature ignition may be initiated from an individual hot spot that grows gradually, while high-temperatures ignition starts from many spots simultaneously which consume the reactive mixture almost homogeneously. Simultaneous lateral and endwall imaging was implemented in both low- and high-pressure shock tube facilities. All tested fuels exhibited localized ignition at low temperatures, and methanol showed a higher propensity than ethanol to ignite far from the endwall. Imaging experiments were also performed in a rapid compression machine to understand preignition at lower temperatures. Herein, ethanol showed non-homogeneous ignition while iso-octane and diethyl ether exhibited homogeneous ignition at the low-temperature conditions. Various criteria for the onset of preignition were tested against experimental observations to propose an adequate predictor of non-ideal ignition phenomena in practical applications. A non-dimensional number, relating the ignition delay sensitivity and laminar flame speed of the mixtures, was found to be the best criterion to elucidate ignition regimes.

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