Stability Considerations Research Articles

Numerical modeling and analysis of induced thermal stress for a non-isothermal wellbore strengthening process

Wellbore strengthening (WBS) methods are broadly applied in the field to combat lost circulation by plugging drilling induced fractures using wellbore strengthening materials (WSM). While changes in formation temperature during drilling have been identified as an important factor in wellbore stability considerations, the performance of WBS in a non-isothermal setting is unclear and remains as a barrier to successful designs of the types and sizes of WSMs. To understand the problem, a 3D finite element analysis was conducted to investigate the state of stresses in near-wellbore region from the interaction between induced thermal stresses and fractures based on different WBS plugging mechanisms. The model developed in this study coupled fluid and heat flow in fractured thermal-poroelastic rock formations and enabled flow restrictions in the fracture for simulating wellbore strengthening processes. By implementing different WBS mechanisms in the simulations, the induced thermal stresses from fluid leakage into the formations were analyzed, and the effectiveness of different WBS strategies in preventing fracture propagation under thermal effects was evaluated.The results disclosed how stresses redistribute around a fractured wellbore as non-isothermal mud invasion and WSM plugging take places. They show that temperature decreases of formation rock due to mud invasion can significantly facilitate fracture propagation and diminish the effectiveness of WBS. Such information is important to the successful management of lost circulation by taking thermal effects into consideration for better WSM design and WBS implementations.

A comparison of the effectiveness of voltage stability indices in an optimal power flow

Voltage stability is very important, as without it voltage collapse will occur. There is therefore a need to incorporate the consideration of voltage stability into the optimal power flow (OPF). This paper presents a comparison of the effectiveness of five voltage stability indices (VSIs) when incorporated in an OPF. The improved particle swarm optimization (IPSO) algorithm is used to solve the OPF problem. The five VSIs investigated are the L‐index, fast voltage stability index (FVSI), line stability index (Lmn), online voltage stability index (LVSI), and voltage collapse proximity indicator (VCPI). The effectiveness of each VSI is considered in the terms of the generation cost, emission, transmission loss, and voltage stability. The IEEE standard 14‐, 30‐, 57‐, and 118‐bus test systems are used for the comparison. It is found that considering each voltage stability index as part of the objective function for the OPF provided different values of generation cost, emission, transmission loss, and maximum loadability. The results suggest that FVSI and Lmn provide the minimum generation cost values for all test systems, while VCPI and LVSI provide lower emission values for most systems. VCPI gives the lowest transmission losses for all systems, and the L‐index provides the maximum loadability for most systems. Incorporation of these indices into the OPF can provide the optimal values in different terms, and the most suitable voltage stability index will depend on the situation and size of the system, which needs to be judiciously chosen. © 2018 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Linear Active Disturbance Rejection Control From the Practical Aspects

In this paper, important issues that could come up in practice of active disturbance rejection control (ADRC) due to a common assumption and some inevitable practical restrictions are investigated. The main idea behind ADRC is modeling effects of both internal uncertainties and external disturbances as an extra state called the total disturbance which is timely estimated. In the majority of recent works, it is assumed that time derivative of total disturbance is bounded. First we prove that, beside the system characteristics, the validity of this assumption strongly depends on the tuning parameters, even in the case of simple classes of linear second-order systems. Thus, one should be careful upon making this common assumption independent from the tuning parameters. Then, the effect of imperfections imposed by sensor dynamics, measurement noise, actuator saturation, and discrete implementation are thoroughly studied. It is shown that these imperfections could crucially affect the control performance. Furthermore, a tuning method, with a higher degree of freedom, is proposed involving the stability considerations in its design procedure. The given theoretical analyses are supported via both simulation and experimental results. The discussed issues are recommended to be carefully considered by experts and practitioners of ADRC.

Modified Polymer Matrix in Pharmaceutical Hot Melt Extrusion by Molecular Interactions with a Carboxylic Coformer.

Hot melt extrusion (HME) has become an essential technology to cope with an increasing number of poorly soluble drug candidates. However, there is only a limited choice of pharmaceutical polymers for obtaining suitable amorphous solid dispersions (ASD). Considerations of miscibility, stability, and biopharmaceutical performance narrow the selection of excipients, and further technical constraints arise from needed pharmaceutical processing. The present work introduces the concept of molecularly targeted interactions of a coformer with a polymer to design a new matrix for HME. Model systems of dimethylaminoethyl methacrylate copolymer, Eudragit E (EE), and bicarboxylic acids were studied, and pronounced molecular interactions were demonstrated by 1H, 13C NMR, FTIR spectroscopy, as well as by different techniques of microscopic imaging. A difference was shown between new formulations exploiting specifically the targeted molecular interactions and a common drug-polymer formulation. More specifically, a modified matrix with malic acid exhibited a technical extrusion advantage over polymer alone, and there was a benefit of improved physical stability revealed for the drug fenofibrate. This model compound displayed greatly enhanced dissolution kinetics from the ASD formulations. It can be concluded that harnessing molecularly designed polymer modifications by coformers has much potential in solid dispersion technology and in particular regarding HME processing.

Explicit synchronous partitioned algorithms for interface problems based on Lagrange multipliers

Traditional explicit partitioned schemes exchange boundary conditions between subdomains and can be related to iterative solution methods for the coupled problem. As a result, these schemes may require multiple subdomain solves, acceleration techniques, or optimized transmission conditions to achieve sufficient accuracy and/or stability. We present a new synchronous partitioned method derived from a well-posed mixed finite element formulation of the coupled problem. We transform the resulting Differential Algebraic Equation (DAE) to a Hessenberg index-1 form in which the algebraic equation defines the Lagrange multiplier as an implicit function of the states. Using this fact we eliminate the multiplier and reduce the DAE to a system of explicit ODEs for the states. Explicit time integration both discretizes this system in time and decouples its equations. As a result, the temporal accuracy and stability of our formulation are governed solely by the accuracy and stability of the explicit scheme employed and are not subject to additional stability considerations as in traditional partitioned schemes. We establish sufficient conditions for the formulation to be well-posed and prove that classical mortar finite elements on the interface are a stable choice for the Lagrange multiplier. We show that in this case the condition number of the Schur complement involved in the elimination of the multiplier is bounded by a constant. The paper concludes with numerical examples illustrating the approach for two different interface problems.

Bank Competition and Firm Credit Availability: Firm-Bank Evidence From Europe

We examine the impact of bank competition on firms' access to credit using a large panel of 900 banks matched to almost 60.000 firms across the euro area over the period 2010-2016. Results provide empirical support for the market power hypothesis whereby low inter-bank competition worsens firms' credit conditions. Specifically, we find that higher bank market power is associated with lower short and long-term bank credit, higher reliance on trade credit and higher funding costs for corporate borrowers. Furthermore, the effect of bank competition is heterogeneous across firms and banks. On the one hand, high bank market power is especially detrimental for small and opaque firms, suggesting that low competition exacerbates the financial constraint of borrowers most exposed to information problems. On the other hand, the reduction in credit availability associated with high market power is attenuated for firms that borrow from small and local community banks, a finding consistent with the information hypothesis, whereby low competition increases banks’ incentive to supply relationship loans. On balance, however, the predominance of medium-large commercial banks in our sample determines that the overall effect of low inter-bank competition on credit conditions is unequivocally adverse for most firms, i.e. the market power effect outweighs the information effect. This evidence contributes to previous research on bank competition, firms’ credit constraint and relationship lending, and has implications for competition policy. Indeed, with respect to the prospect of greater banking consolidation in the European Union, our results suggest that considerations of efficiency and financial stability should be weighed against the potential negative consequences in terms of firms’ access to credit.

Review Essay - Democratic Leviathan: Defending First-Past-the-Post in Canada

This review essay examines a number of recent books claiming to offer a defence of Canada's traditonal first past the post voting system. The works can be divided into two camps, one Conservative, the other liberal, though their logic, arguments, and evidence are surprisingly similar. Through a detailed engagement with each work, this review argues that both versions ultimately defend first past the post as an effective ‘democratic leviathan’ in that the voting system tends to produce a strong, single party legislative majority government that can rule unhindered while it remains in office. Thus, for these authors, considerations of stability and legislative efficiency trump all other concerns e.g. representation, diversity, majority rule, electoral competitiveness, etc. However, in making their case, the contributors largely fail to seriously engage opposing views or the relevant academic literatures, particularly relevant Canadian work.

A dynamics and stability framework for avian jumping take-off.

Jumping take-off in birds is an explosive behaviour with the goal of providing a rapid transition from ground to airborne locomotion. An effective jump is predicated on the need to maintain dynamic stability through the acceleration phase. The present study concerns understanding how birds retain control of body attitude and trajectory during take-off. Cursory observation suggests that stability is achieved with relatively little cost. However, analysis of the problem shows that the stability margins during jumping are actually very small and that stability considerations play a significant role in the selection of appropriate jumping kinematics. We use theoretical models to understand stability in prehensile take-off (from a perch) and also in non-prehensile take-off (from the ground). The primary instability is tipping, defined as rotation of the centre of gravity about the ground contact point. Tipping occurs when the centre of pressure falls outside the functional foot. A contribution of the paper is the development of graphical tipping stability margins for both centre of gravity location and acceleration angle. We show that the nose-up angular acceleration extends stability bounds forward and is hence helpful in achieving shallow take-offs. The stability margins are used to interrogate simulated take-offs of real birds using published experimental kinematic data from a guinea fowl (ground take-off) and a diamond dove (perch take-off). For the guinea fowl, the initial part of the jump is stable; however, simulations exhibit a stuttering instability not observed experimentally that is probably due to the absence of compliance in the idealized joints. The diamond dove model confirms that the foot provides an active torque reaction during take-off, extending the range of stable jump angles by around 45°.

Diversity, stability, and social contract theory

The topic of moral diversity is not only prevalent in contemporary moral and political philosophy, it is also practically relevant. Moral diversity, however, poses a significant challenge for moral theory building. John Thrasher (Synthese, forthcoming), in his discussion of public reason theory, which includes social contract theory, argues that if one seriously considers the goal of moral constructivism and considerations of representation and stability, then moral diversity poses an insurmountable problem for most public reason theories. I agree with Thrasher that moral diversity poses a significant challenge for orthodox multistage social contract theories. In fact, I even add a further problem for such theories under the assumption of deep moral diversity. Nevertheless, I argue that my (Moehler, Minimal morality: a multilevel social contract theory, Oxford University Press, Oxford, 2018) recently developed multilevel social contract theory overcomes these problems. I focus on some of the underexplored features of this theory to show that multilevel social contract theory offers one conceptually coherent and plausible way to render social contract theory viable and relevant for modern diverse societies.

Orthognathic surgery Deteriorates the osseointegration of dental implants: A propensity-matched multicentre cohort study.

This study aimed to investigate the possible influence of the regional acceleratory phenomenon (RAP) on dental implant osseointegration. Orthognathic surgery was set as an intervention for RAP, and a multicentre cohort study of two groups was undertaken. Group O included patients with single implant placement at least 4months after orthognathic surgery and functional loading for more than 1year, while controls (Group C) were without any major surgery. Clinical and radiographic assessments of implants, including changes in marginal bone levels, were analysed at baseline, 6- and 12-month follow-up. Bivariate analysis of two groups with propensity score matching was performed. After propensity score matching, all 10 confounding variables had acceptable standardised difference scores (<20%), indicating that the matching procedure had efficiently balanced the two groups. Following the propensity score adjustment, the marginal bone loss was significantly higher in Group O than the control at 6months (1.66±1.05mm vs 0.59±0.64mm, P<0.001) and 12months (2.30±1.27mm vs 0.82±0.78mm, P<0.001). Compared to Group C, subjects in Group O had a higher incidence of peri-implant mucositis and implantitis (11.8% vs 1.5%, P=0.033). Impaired osseointegration of dental implants was associated with orthognathic surgery. Special considerations for peri-implant soft and hard tissue stability should be addressed to obtain ideal treatment results and prognosis for patients who have had prior orthognathic surgery.

Considerations of long-term stability of orthodontic treatment results

Considerations of long-term stability of orthodontic treatment results

Multiple Resonances Mitigation of Paralleled Inverters in a Solid-State Transformer (SST) Enabled AC Microgrid

Solid-state transformer (SST) becomes a promising technology in smart grid applications due to its multiple functionalities and control flexibility compared to line-frequency transformers. In a SST enabled ac microgrid, the grid impedance represented by the SST output impedance is frequency-dependent, which is different from the inductive grid impedance in a conventional ac microgrid. The frequency-dependent SST output impedance can interact with those of paralleled inverters and result in multiple resonances locating in a wide frequency range. In this paper, a combination of a lead-lag compensator and a negative impedance feedback control has been proposed to enhance the system stability. With proposed methods, the multiple resonances are decoupled and thus more feasible to be mitigated. The negative effect of digital control delay on the high frequency resonance damping is attenuated with proposed methods. In addition, the tradeoff design of virtual impedance is also analyzed with consideration of both system-level resonance mitigation function and single inverter stability. Consequently, the stability of paralleled inverters in the SST-enabled ac microgrid has been improved. Finally, an OPAL-RT-based hardware-in-the-loop verification has been provided to prove the validity of proposed methods.

Investigation of the thermal stability of MoOx as hole-selective contacts for Si solar cells

The stoichiometry and work function of molybdenum oxide (MoOx) are of crucial importance for its performance as hole selective contact for crystalline silicon solar cells. Hydrogenated amorphous silicon (a-Si:H) is typically used as an interface passivation layer in combination with MoOx to reduce surface recombination. As the fabrication process of a solar cell typically contains subsequent high-temperature processes, the consideration of thermal stability of MoOx with and without a-Si:H becomes critical. In this work, in situ x-ray spectroscopy (XPS)/ultraviolet photoelectron spectroscopy and Fourier transform infrared spectroscopy in the temperature range from 300 K to 900 K are used to investigate the thermal stability of MoOx with and without a-Si:H. In addition, both the passivation and contact performance are studied by evaluating the surface saturation current density J0s, carrier lifetime τeff, and contact resistivity ρc. The XPS results reveal that the as-evaporated MoOx on top of both c-Si and a-Si:H is sub-stoichiometric, and the work function of both films is higher than 6 eV. While after in situ annealing, the evolution of MoOx phase on top of a-Si:H shows a different behavior compared to it on c-Si which is attributed to H diffusion from a-Si:H after 600 K, whereas the work function shows a similar trend as a function of the annealing temperature. The J0s of a p-type Si symmetrically passivated by MoOx is found to be 187 fA/cm2 and the ρc is ∼82.5 mΩ·cm2 in the as-evaporated state. With a-Si interface passivation layer, J0s is significantly lower at 5.39 fA/cm2. The J0s and the ρc increase after post-deposition annealing. The evolution of these functional properties can be attributed to the material properties.

Generalized transient simulation of two-phase heat exchangers using zeotropic fluid mixtures

Transient simulation techniques for the modeling of two-phase heat exchangers employed in thermally driven heat pump systems utilizing a binary fluid mixture of ammonia and water as the working fluid are presented. Model formulation is presented using unsteady forms of conservation equations. The systems of equations for the discretized finite volume (FV) method and the moving-boundary (MB) method are described. The resulting systems of coupled, non-linear equations form a set of differential-algebraic equations (DAE), and are solved at each simulation time step. The solver implementation for DAE systems is discussed in detail and considerations for stability and accuracy are discussed. The schemes presented in this paper ensure conservation of mass and energy on an integral basis over the entire component for all time steps. Comparisons of computational efficiency and accuracy between the two approaches are presented for different test cases encountered in an ammonia-water condenser.

Geotechnical characterization of mining rock waste dumps in central Evia, Greece

Open pit mining using drill and blast has been taking place in central Evia, Greece for many decades. This is so, due to the presence of ferronickel ores in the area. As a consequence of the mining activity, large volumes of overburden Upper Cretaceous limestone are being annually extracted and deposited at predefined sites, creating rock waste dumps or embankments. In the context of optimum land surface use and better stability conditions, a geotechnical characterization of these waste dumps has taken place and is presented in the current paper. First, image analyses are performed on fragmented limestone muckpiles and the degree and intensity of rock fragmentation due to blasting is assessed. Results compare reasonably well with computations based on the Kuz-Ram empirical model for fragmentation analysis. Overall, muckpiles on excavation faces consist of boulders and cobbles by 80–90% per weight. Following, a laboratory investigation program is executed on samples retrieved from the waste dumps. Based on the results, the dumps consist of heavily coarse grain material, mostly classified as GC. According to modified Proctor tests and in-situ density tests using the Troxler device, compaction on the crest of the dumps is characterized as very high, while permeability is empirically considered as medium to high. As far as the friction angle of the waste dump material is concerned, this is estimated based on observations (i.e. recorded inclinations of existing dumps), combined with an empirical model from the literature. Generally, it is considered that the friction angle varies considerably with the overburden pressure within the dump’s body. Nonetheless, if a single-value approach is necessary for simplicity purposes and stability considerations, then a friction angle of about 40° (characteristic value) is suggested as a reasonable assumption.

Quantified Design Guidelines of a Compact Transformerless Active EMI Filter for Performance, Stability, and High Voltage Immunity

This paper proposes and analyzes a compact transformerless common mode active electromagnetic interference (EMI) filter (AEF) using a push-pull amplifier. Quantified design guidelines are then rigorously derived from the analysis with consideration for performance, stability, and high voltage transient immunity. The proposed AEF is manufactured based on the design guidelines, and its performance is validated by measurements using a vector network analyzer. The AEF is then implemented into a real 2.2 kW current resonant inverter, and the conducted emissions are reduced by 5-25 dB at a frequency range from 150 kHz to 6 MHz. In addition, the AEF's immunity against high voltage transients is demonstrated by lightning surge tests.

Probing Metal-Organic Framework Design for Adsorptive Natural Gas Purification.

Parent and amine-functionalized analogues of metal-organic frameworks (MOFs), UiO-66(Zr), MIL-125(Ti), and MIL-101(Cr), were evaluated for their hydrogen sulfide (H2S) adsorption efficacy and post-exposure acid gas stability. Adsorption experiments were conducted through fixed-bed breakthrough studies utilizing multicomponent 1% H2S/99% CH4 and 1% H2S/10% CO2/89% CH4 natural gas simulant mixtures. Instability of MIL-101(Cr) materials after H2S exposure was discovered through powderX-ray diffraction and porosity measurements following adsorbent pelletization, whereas other materials retained their characteristic properties. Linker-based amine functionalities increased H2S breakthrough times and saturation capacities from their parent MOF analogues. Competitive CO2 adsorption effects were mitigated in mesoporous MIL-101(Cr) and MIL-101-NH2(Cr), in comparison to microporous UiO-66(Zr) and MIL-125(Ti) frameworks. This result suggests that the installation of H2S binding sites in large-pore MOFs could potentially enhance H2S selectivity. In situ Fourier transform infrared measurements in 10% CO2 and 5000 ppm H2S environments suggest that framework hydroxyl and amine moieties serve as H2S physisorption sites. Results from this study elucidate design strategies and stability considerations for engineering MOFs in sour gas purification applications.

Surface chemistry of as-synthesized and air-oxidized PbS quantum dots

This paper reports theoretical and experimental studies that explain how the size-dependent shape and elemental composition of PbS quantum dots (QDs) affect the arrangement and density of adsorbed ligands by QDs surfaces, and their related susceptibility to oxidation. By considering the differences of Pb:S ratios of the particles, we predict a size-dependent Pb:S ratio that is consistent with our measurements, and those reported in the literature. We further connect the shape and facet Pb:S composition to surface ligand densities, and relate this to the particle-size dependence of oxidation when exposed to air. While the surfaces of smaller particles (d < 2.7 nm) are entirely covered by Pb terminated (1 1 1) facets, bigger QDs (2.7 < d < 7.5 nm) are truncated at their (1 0 0) facets to form cuboctahedron-shape particles. The (1 1 1) facets representing the largest portion of both smaller and bigger particle’s surfaces can be well-passivated by anionic oleate/hydroxide ligands. Investigation on the surface chemistry of PbS QDs by the means of FT-IR, NMR, XPS and combustion elemental analyses, as well as steric stability considerations suggested that hydroxide to oleate atomic ratio in (1 1 1) facet of PbS QDs is about 1:5. On the other side, (1 0 0) surface facets have a checkerboard arrangement of lead and sulfur atoms and are coordinated by oleic acid ligands. Low density and weak binding energy of such ligands make the (1 0 0) facets vulnerable to oxidation in an air atmosphere. XPS investigations indicate that the oxide includes both PbSO3 and PbSO4 components on the surface of PbS QDs.

Targeting Financial Stability: Macroprudential or Monetary Policy?

This paper explores monetary-macroprudential policy interactions in a simple, calibrated New Keynesian model incorporating the possibility of a credit boom precipitating a financial crisis and a loss function reflecting financial stability considerations. Deploying the countercyclical capital buffer (CCyB) improves outcomes significantly relative to when interest rates are the only instrument. The instruments are typically substitutes, with monetary policy loosening when the CCyB tightens. We also examine when the instruments are complements and assess how different shocks, the effective lower bound for monetary policy, market-based finance and a risk-taking channel of monetary policy affect our results.

Globally Divergence-Free Discontinuous Galerkin Methods for Ideal Magnetohydrodynamic Equations

Ideal magnetohydrodynamic (MHD) equations are widely used in many areas in physics and engineering, and these equations have a divergence-free constraint on the magnetic field. In this paper, we propose high order globally divergence-free numerical methods to solve the ideal MHD equations. The algorithms are based on discontinuous Galerkin methods in space. The induction equation is discretized separately to approximate the normal components of the magnetic field on elements interfaces, and to extract additional information about the magnetic field when higher order accuracy is desired. This is then followed by an element by element reconstruction to obtain the globally divergence-free magnetic field. In time, strong-stability-preserving Runge–Kutta methods are applied. In consideration of accuracy and stability of the methods, a careful investigation is carried out, both numerically and analytically, to study the choices of the numerical fluxes associated with the electric field at element interfaces and vertices. The resulting methods are local and the approximated magnetic fields are globally divergence-free. Numerical examples are presented to demonstrate the accuracy and robustness of the methods.