Macro Instability Research Articles

Revealing hot deformation behavior of Ti2C-Ti composite and associated dynamic recrystallization mechanism of ceramic matrix

Titanium carbide based composites have attracted much attention owing to their light weight, high elastic modulus, and excellent wear resistance. However, the hot working of these composites has been facing great challenges, especially for the coordinated deformation of brittle phase. Here, soft Ti phase was introduced to improve the thermal deformation ability. In particular, the dynamic recrystallization (DRX) mechanism, crack propagation, and dislocation motion behavior of brittle Ti2C matrix were unveiled in this study. Firstly, a Ti2C-Ti composite was hot compressed at 770–1000 °C with strain rates of 0.012–0.0013 s−1. The results showed that the optimal processing temperature was 950 °C, at which the microstructure can be refined by DRX behavior without the occurrence of macro instability. In the later softening stage of hot compression, the recrystallized Ti2C grains were formed through both discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX). The intergranular Ti phase, which acts as a binder, coordinated the deformation of the Ti2C-Ti composite, and the phase boundaries attracted dislocations on the side of the brittle Ti2C phase. This dislocation motion behavior mitigated stress concentration but inhibited the DRX behavior of the Ti2C phase near the binary phase interfaces. The lamellar Ti precipitates within Ti2C grains facilitated the occurrence of CDRX by promoting the formation of subgrains. The subgrain boundaries were formed when the average distance between dislocation on the dislocation wall was lower than 8.68 nm. Compared with the as-sintered composite, the nanohardness of the Ti2C matrix in the composite deformed at 950 °C increased by 17%. The enhanced hardness was mainly attributed to grain boundary and dislocation strengthening effect, as well as the reduction of Ti precipitates.

Study on instability strengthening of flow field in stirred tank

BackgroundLow-frequent instabilities within flow pattern of stirred tank (ST) which well-known as macroinstability (MI) further as “flow field instability” has significant additional effect to common turbulence concerning effectivity of mixing process. MethodsIn this work, numerical simulation methods were used to study the impact of baffle on flow field instability. Significant FindingsThe results showed that flow field instability in baffled stirred tank (BST) was full of global tank compared with unbaffled stirred tank (Un-BST), and the instability of near baffle zone was stronger than that in near impeller zone. By comparing the power consumption in ST equipped with different structure baffles, find that the punched baffle stirred tank (Pun-BST) and isolated baffle stirred tank (Iso- BST) system have lower power consumption than the traditional baffle stirred tank (Trad-BST) system. Also, the degree of global instability decreased partly. In addition, the instability intensity of near baffle zone in the Pun-BST system was lifted locally, and the maximum increase can be as much as 4 times, compared with Un-BST.

Research and analyze the risks in business activities of Vietnamese enterprises in the context of international economic integration

Over the years, Vietnam has witnessed difficulties and challenges due to many subjective and objective reasons, including the Covid-19 pandemic, which has complicated developments that have greatly affected the health and people's lives, seriously affecting the economy and all aspects of social life. Meanwhile, the risks of macro instability and financial instability are increasing. Inflationary pressure increased due to cost-push factors and prolonged monetary policy. The economy contracted and the asset market corrected sharply, while budget spending on economic stimulus packages increased. With the impact of science and technology, the contemporary world has become a flat world, a global world. As a result, there are events that take place in a certain region or country that will also create great changes with other regions and countries, requiring us to have a very scientific view. learn to come up with guidelines, policies, and ways to take advantage of opportunities as well as overcome challenges that are not small. References Tien, N. H., & Minh, H. T. T. (2019). Challenges for Vietnamese Business Leaders in the Era of International Economic Integration and Industrial Revolution 4.0.“. International Journal of Commerce and Management Research, 5(5), 19-24. Linh, P. N. M., & Huong, N. T. T. (2020). The Supply Chain and Logistics of Vietnam in the context of international economic integration. International Business Research, 13(7), 27-44. Thanh, V. T. (2005). Vietnam's trade liberalization and international economic integration: evolution, problems, and challenges. ASEAN Economic Bulletin, 75-91. Tien, N. H., Hung, N. T., Vu, N. T., & Bien, B. X. (2020). Risks of Vietnamese enterprises in trade relations with China. International Journal of Research in Finance and Management, 3(1), 1-6. Tien, N. H., & Ngoc, N. M. (2019). Comparative Analysis of Advantages and Disadvantages of the Modes of Entrying the International Market. International journal of advanced research in engineering and management, 5(7), 29-36. Thanh, V. T. (2015). Vietnam's perspectives on regional economic integration. Journal of Southeast Asian Economies, 106-124. Nguyen, H. T. N., & Enderwick, P. (2016). The perceptions and responses of Vietnamese firms towards deeper regional economic integration: case studies from the food processing industry. Journal of Southeast Asian Economies, 65-82. Tien, N. H., & Minh, H. T. T. (2019). Cultural risk management in the integration process of Vietnamese enterprises. Int J Commerce Manage Res, 5(5), 25-29.

Numerical investigation of mining-induced strata disturbances using a discontinuous deformation and displacement coupling method

Strata movement and ground subsidence caused by mining have a significant influence on the safety and stability of the goaf. By taking the mesoscopic heterogeneity of rock masses into account, the discontinuous deformation and displacement (DDD) method was applied to simulate the whole failure process of overlying strata after coal mining under different advancing distances of the working face. The results show that key strata would not be broken when the working face was advanced by 30 m. When the advancing distance increased to 50 m, the first key stratum was broken; when it went up to 70 m, both the first and the second key strata were broken and collapsed successively. Moreover, the bed separation of strata had become particularly obvious and a three-zone phenomenon—including collapsed zone, fractured zone, and continuous deformation zone—presented throughout the whole rock mass when the distance rose to 90 m. As the working face is advanced by increasing distances, there are increases in the ranges of deformation, disturbance, and damage in the overlying strata, as well as surface settlement. Meanwhile, the gradual collapse process of the strata is further analyzed in terms of the background-stress evolution; i.e., stress accumulation, stress shadow, and stress transference. Furthermore, it has been proven that the DDD method is an effective approach to reproduce the whole process of strata collapse and macro instability from small deformation, crack initiation, and propagation to block translation, rotation, and contact, which is crucial for predicting strata movement and surface subsidence.

Dynamics of prestressed elastic lattices: Homogenization, instabilities, and strain localization

A lattice (or ‘grillage’) of elastic Rayleigh rods (possessing a distributed mass density, together with rotational inertia) organized in a parallelepiped geometry can be axially loaded up to an arbitrary amount without distortion and then be subject to incremental time-harmonic dynamic motion. At certain threshold levels of axial load, the grillage manifests instabilities and displays non-trivial axial and flexural incremental vibrations. Including every possible structural geometry and for an arbitrary amount of axial stretching, Floquet–Bloch wave asymptotics is used to homogenize the in-plane mechanical response, so to obtain an equivalent prestressed elastic solid subject to incremental time-harmonic vibration, which includes, as a particular case, the incremental quasi-static response. The equivalent elastic solid is obtained from its acoustic tensor, directly derived from homogenization and shown to be independent of the rods’ rotational inertia. Loss of strong ellipticity in the equivalent continuum coincides with macro-bifurcation in the lattice, while micro-bifurcation remains undetected in the continuum and corresponds to a vibration of vanishing frequency of the lowest dispersion branch of the lattice, occurring at finite wavelength. Dynamic homogenization reveals the structure of the acoustic branches close to ellipticity loss and the analysis of forced vibrations (both in physical space and Fourier space) shows low-frequency wave localizations. A perturbative approach based on dynamic Green’s function is applied to both the lattice and its equivalent continuum. This shows that only macro-instability corresponds to localization of incremental strain, while micro-instabilities occur in modes which spread throughout the whole lattice with an ‘explosive’ character. In particular, extremely localized mechanical responses are found both in the lattice and in the solid, with the advantage that the former can be easily realized, for instance via 3D printing. In this way, features such as shear band inclination, or the emergence of a single shear band, or competition between micro and macro instabilities become all designable features. The comparison between the mechanics of the lattice and its equivalent solid shows that the homogenization technique allows an almost perfect representation, except when micro-bifurcation is the first manifestation of instability. Therefore, the presented results pave the way for the design of architected cellular materials to be used in applications where extreme deformations are involved.

Adam Smith (Virtue Ethics) Against Jeremy Bentham(Utilitarian Ethics): Two Completely Different, Conflicting Types of Capitalism Result From Their Distinct Ethical and Economic Positions

Adam Smith's ethical concerns and recommendations for behavior, based on a rock-solid foundation of Aristotelian Virtue Ethics, leads to a completely different, conflicting type of capitalism then that advocated by Jeremy Bentham's egoistic, act utilitarianism, which forms foundation for Benthamite Utilitarian ethics. The ethical and economic divide separating Bentham and Smith is so wide that no compromise or synthesis is possible regarding their two systems. Both Bentham and Smith recognized this. Bentham's two 1787 books, Defence of Usury and The Principles of Morals and Legislation, represent a direct, but subtle, attack on Adam Smith. Smith responded to Bentham's attack on The Theory of Moral Sentiments by spending all of his energy over last three years of his life strengthening and revising his The Theory of Moral Sentiments in order to strengthen Part 6 of that book in order to withstand, counter and repel Bentham's attack. Bentham, always conniving, devious, cunning, sly, and tricky master of intrigue that resulted from his lifelong commitment and service to British East India Company, on surface would always compliment Adam Smith as one who was the father of political economy, a great master, and a writer of consummate genius. Bentham's attacks on Smith's foundation of virtue ethics, which was based on impartial spectator and role of sympathy, take place in chapter 2 on pp.8-25 of The Principles of Morals and Legislation. Bentham cleverly misrepresents Smith's emphasis on sympathy by criticizing all systems based on sentiments relating to sympathy and antipathy. Smith's system has nothing to do with antipathy. At end of chapter 2 in The Principles of Morals and Legislation, Bentham makes it clear that no other concept except utility, can ever serve as foundation for ethics. Bentham's attack on Wealth of Nations takes place in his Defence of Usury, where Bentham attempts to challenge Smith's demonstration that major cause of macro instability, bubbles, deflation, and inflation is economic power of some members of upper-income class that Smith described as prodigals, projectors, and imprudent risk takers. The prodigals, projectors, and imprudent risk takers, such as John Law, British East India Company, and Bentham himself, were able to obtain bank loans in order to carry out their speculative plans involving financial manipulation of land or real estate. Smith's analysis of Ayr bank collapse in 1772 showed how massive loans made by bank directors to British East India Company connected individuals for land speculation led to a four-year depression in Scotland. Smith thus separated prudent, sober, middle class merchants and tradesmen, who produced and consumed physical goods and services, from upper-class speculators and financial manipulators associated with British East India Company, by far most powerful economic force in world in 17th and 18th centuries, whose get rich quick schemes were purely speculative and involved financial manipulation. The prodigals, projectors, and imprudent risk takers thus represented an internal, endogenous threat to macroeconomy. This directly conflicted with Bentham's pendulum model, based on constant oscillations (negative feedback) between happiness and unhappiness that only allowed for exogenous, external threats to macroeconomy, as well as creating havoc with Bentham's representation of all consumers as utility (preferences, tastes) maximizing individuals and all producers as profit (capital, labor technology, entrepreneurship) maximizing individuals. Smith, as he did with Sir James Steuart, effectively refutes Bentham's attack on sentiments-impartial spectator-sympathy approach without mentioning Bentham by name. The real advocate of Invisible Hand-self adjusting, pendulum model is Bentham and not Smith. G Kennedy has forcefully demonstrated that an Invisible Hand economy can't have any prodigals, projectors, and imprudent risk takers in it or it will break down at macro level. The existence of such individuals simply means that there is an internal, endogenous impact, which generates positive feedback that moves economy farther away from any optimal equilibrium over time. All other writers on Adam Smith have mistakenly followed Jevons, Sedgwick, and Viner in foisting a pendulum model on Smith that Smith entirely rejected. Any claim that Smith's system of ethics and economics is based on an Invisible Hand, resulting in a self-correcting, self-adjusting pendulum model, where market prices and wages automatically convert private greed into a social macroeconomic optimum for all participants, has badly confused Smith with Bentham.

Large-eddy simulation and 3D proper orthogonal decomposition of the hydrodynamics in a stirred tank

The present study attempts to characterize the time-dependent three-dimensional hydrodynamics in a stirred tank reactor using computational fluid dynamics (CFD). The rotating propellers produce a complex, unsteady, three-dimensional turbulent flow field. This results in efficient mixing and is therefore widely used in various process engineering applications. The time-dependent turbulent single-phase flow is computed using large eddy simulation, relying on the sliding mesh approach. The unresolved subgrid scales are treated using the Smagorinsky-Lilly model. The dominant coherent flow structures are characterized in the entire three-dimensional computational domain using the 3D proper orthogonal decomposition (POD) technique. The design of the propeller is evaluated in a separate POD analysis in a rotating frame which encloses the propeller. The most energetic POD modes characterize the organized large scale structures, the so-called coherent flow structures, while the higher modes correspond to the small-scale disorganized turbulence. It was found that the dynamics of the main flow structures can be reconstructed using only 3 modes corresponding to 98% of the overall energy in the entire 3D inner rotating domain and 21 modes are necessary for the same amount of energy in the outer stationary region.Furthermore, the macro instability (MI) was characterized by monitoring the velocity in more than 1 million computational cells, as well as using FFT analyses of the three-dimensional POD temporal coefficients. In the outer stationary domain, both approaches showed characteristic frequencies around one-eighth and one-fifth of the blade passage frequency.

Chaotic characteristics enhanced by impeller of perturbed six-bent-bladed turbine in stirred tank

The fundamental way of improving the mixing efficiency is to induce the chaotic flow in a stirred vessel. The impeller form plays an important role for changing the structure of flow field and realizing chaotic mixing. Based on the velocity time series acquired by the experiment of particle image velocimetry (PIV), with the software Matlab, the macro-instability (MI), largest Lyapunov exponent (LLE), and Kolmogorov entropy in the water stirred tank is investigated respectively with the impeller of perturbed six-bent-bladed turbine (6PBT). The results show that the MI characteristics are obvious and two peak values of MI frequency are observed at the speed N=60rpm. With the increasing speed (more than 100rpm), the peak characteristics of MI frequency disappear and a multi-scale wavelet structure of characterizing the chaotic flow field appears. Moreover, under the speed N=60rpm, the LLE is less than 0 and Kolmogorov entropy is 0, which means that the flow field is in the periodic moving state. As the speed is increased to more than 100rpm, the LLE and Kolmogorov entropy are all more than 0, which indicates that the flow field goes into the chaotic mixing. When the speed reaches up to about 210rpm, both of the LLE and Kolmogorov entropy achieve the optimum values, which will result in an excellent chaos with the highest mixing efficient. So it is feasible that the MI frequency, the LLE and the Kolmogorov entropy can be used to analyze the flow field characteristics in a stirred tank. The research results promote the understanding of the chaotic mixing mechanism and provide a theoretical reference for the development of new type impeller.

Banking Overleveraging and Macro Instability: A Model and VSTAR Estimations

Abstract:Overleveraging of the banking sector has been considered one of the main causes of the 2007–09 financial crisis and the subsequent great recession. It was also of major concern for the subsequent BIS regulatory policies resulting in Basel III and its request for higher capital requirements. It has now become highly relevant for the planned European banking union. Overleveraging of the banking sector exposes the financial sector and the macroeconomy to vulnerabilities, but also, as critics state, seems to constrain credit flows to the private sector. We present here a measure of overleveraging, defined as the difference between actual and sustainable debt, conduct an empirical study on overleveraging for 40 banks in Europe, and study the vulnerabilities and credit contractions that can arise subsequently. Before the year 2004 overleveraging had not been a serious problem as leverage was on a sustainable level. However, in the run-up to the financial crisis, actual and optimal debt spread apart and the banking sector began to suffer from overleveraging. We use a non-linear Vector STAR model to evaluate the hypothesis that periods of increasing debt levels are accompanied by more severe credit constraints than periods of low leveraging. We demonstrate this for country groups across Europe.

Revisiting the finance–growth nexus: the Turkish case, 1980–2010

ABSTRACTIn Turkey, the empirical results on the link between financial development and economic growth are mixed. The existing studies do not take into account the fact that Turkey has experienced endemic political and economic instabilities over extended periods. This study aims to analyse the role of macroeconomic instability and public borrowing on the finance–growth nexus in Turkey by using time series econometric techniques over the 1980–2010 period. In doing so, we attempt to extend the existing literature by taking into account the role of macroeconomic instability as well as public borrowing. Our results reveal that there are additional – albeit indirect – channels between finance and growth via the effects of macro instability and public borrowing on financial development and economic growth. After taking into account the effects of overall instability and public borrowing, we found that growth–financial development relationship is bidirectional and permanent. In other words, in Turkish case, economic growth and financial development are jointly determined. Thus, our results shed some light on the ambiguity of the evidence on the link between financial development and economic growth for Turkey.

Numerical simulation on macro-instability of coupling flow field structure in jet-stirred tank

The velocity field macro-instability (MI) can help to improve the mixing efficiency. In this work, the MI features of flow field induced by jet-stirred coupling action is studied by using computational fluid dynamics (CFD) simulations. The numerical simulation method of jet-stirred model was established based on standard turbulent equations, and the impeller rotation was modeled by means of the Sliding Mesh (SM) technology. The numerical results of test fluid (water) power consumption were compared with the data obtained by power test experiments. The effects of jet flow velocity and impeller speed on MI frequency were analyzed thoroughly. The results show that the calculated values of power consumption agree well with the experiment measured data, which validates the turbulent model, and the flow structure and MI frequency distribution are affected by both impeller speed and jet flow rate. The amplitude of MI frequency increases obviously with the increasing rotation speed of impeller and the eccentric jet rate, and it can be enhanced observably by eccentric jet rate, in condition of comparatively high impeller speed. At this time, the MI phenomenon disappears with the overall chaotic mixing.

Investigations of the Gas-Liquid Multiphase System Involving Macro-Instability in a Baffled Stirred Tank Reactor

Bubble Sauter Mean Diameter (SMD) in gas-liquid multiphase system is of particular interest and the quantification of gas characteristics is still a challenge today. In this contribution, multiphase Computational Fluid Dynamic (CFD) simulations are combined with Population Balance Model (PBM) to investigate the bubble SMD in baffled stirred tank reactor (STR). Hereby, special attention is given to the phenomenon known as the fluid macro-instability (MI), which is a large-scale low-frequency fluid velocity variation in baffled STRs, since the fluid MIs have a dominating influence on the bubble breakage and coalescence processes. The simulations, regarding the fluid velocity, are validated with Laser Doppler Anemometry (LDA) experiments, in which the instant radial velocity is analyzed through Fast Fourier Transform (FFT) spectrum. The frequency peaks of the fluid MIs are found both in the simulation and in the experiment with a high degree of accuracy. After the validation, quantitative predictions of overall bubble SMD with and without MIs are carried out. Due to the accurate prediction of the fluid field, the influence of the fluid MI to bubble SMD is presented. This result provides more adequate information for engineers working in the field of estimating bubble SMDs in baffled STRs.

Modeling Extreme Instability

We construct a coherent theory of extreme macro instability, an important macro phenomenon most recently experienced in the United States in 2008-09. The model is then used to identify significant policymaking lessons from the Great Recession. The analysis is part of the ongoing GEM Project that focuses on the generalization of rational price-mediated exchange from the marketplace to the workplace, uniquely microfounding causation from adverse nominal demand disturbances to involuntary job loss. It further draws upon two recent contributions to the literature that we believe have received inadequate attention. In the first, Nancy Stokey (2009) demonstrates that, as investors/lenders become uncertain about macro prospects, inaction becomes rational. In the second, Roger Farmer (2010a, 2010b) has usefully repackaged an old idea: Independent of economic fundamentals, investor confidence can influence, and is influenced by, the behavior of prices on asset exchanges.

Large Eddy Simulation of the influence of solids on macro instability frequency in a stirred tank

The mixing flow in a cylindrical tank stirred with a PBT was simulated with a LES model with Eulerian–Lagrangian approach, in order to study the effect of varying amounts of suspended solids on the frequency and amplitude of macro instabilities (MI). The dispersed phase was reassembled solid glass particles of two different sizes (1 mm, 2 mm) and varying concentrations (up to 3% by volume). The continuous phase velocities were sampled at 18 different locations and were evaluated with the Lomb algorithm to obtain frequency spectra of the flow. The results showed that the MI could be detected in all suspensions and that the frequency is not influenced by the addition of solids. However, the MI became less prominent with increasing number of particles. A periodic fluctuation of the baffle jet could be observed with identical frequency. All results compare fairly well to previously obtained experimental data.

The Interspinous Spacer: A New Posterior Dynamic Stabilization Concept for Prevention of Adjacent Segment Disease

Introduction. Posterior Dynamic stabilization using the interspinous spacer device is a known to be used as an alternative to rigid fusion in neurogenic claudication patients in the absence of macro instability. Actually, it plays an important in the management of adjacent segment disease in previously fused lumbar spine. Materials and Method. We report our experience with posterior dynamic stabilization using an interspinous spacer. 134 cases performed in our institution between September 2008 and August 2012 with different lumbar spine pathologies. The ages of our patients were between 40 and 72 years, with a mean age of 57 years. After almost 4 years of follow up in our patient and comparing their outcome to our previous serious we found that in some case the interspinous distracter has an important role not only in the treatment of adjacent segment disease but also in its prevention. Results and Discussion. Clinical improvement was noted in ISD-treated patients, with high satisfaction rate. At first, radicular pain improves with more than 3/10 reduction of the mean score on visual analog scale (VAS). In addition, disability score as well as disc height and lordotic angle showed major improvement at 3 to 6 months post operatively. And, no adjacent segment disease was reported in the patient operated with interspinous spacer. Conclusion. The interspinous spacer is safe and efficient modality to be used not only as a treatment of adjacent segment disease but also as a preventive measure in patients necessitating rigid fusion.

Stiffnites. Part II

The dynamics of a stiffnite are here inferred. A stiffnite is a sheet-shaped, gravity-driven submarine sediment flow, with a fabric made up of marine ooze. To infer stiffnite dynamics, order of magnitude estimations are used. Field deposits and experiments on materials taken from the literature are also used. Stiffnites can be tens or hundreds of kilometers wide, and a few centimeters/ meters thick. They move on the sea slopes over hundreds of kilometers, reaching submarine velocities as high as 100 m/s. Hard grain friction favors grain fragmentation and formation of triboelectrically electrified particles and triboplasma (i.e., ions + electrons). Marine lipids favor isolation of electrical charges. At first, two basic assumptions are introduced, and checked a posteriori: (a) in a flowing stiffnite, magnetic dipole moments develop, with the magnetization proportional to the shear rate. I have named those dipoles as Ambigua. (b) Ambigua are ‘vertically frozen’ along stiffnite streamlines. From (a) and (b), it follows that: (i) Ambigua create a magnetic field (at peak, >1 T). (ii) Lorentz forces sort stiffnite particles into two superimposed sheets. The lower sheet, L+, has a sandy granulometry and a net positive electrical charge density. The upper sheet, L–, has a silty muddy granulometry and a net negative electrical charge density; the grains of sheet L– become finer upwards. (iii) Faraday forces push ferromagnetic grains towards the base of a stiffnite, so that a peak of magnetic susceptibility characterizes a stiffnite deposit. (iv) Stiffnites harden considerably during their motion, due to magnetic confinement. Stiffnite deposits and inferred stiffnite characteristics are compatible with a stable flow behavior against bending, pinch, or other macro instabilities. In the present report, a consistent hypothesis about the nature of Ambigua is provided.

Stiffnites. Part I

The dynamics of a stiffnite are here inferred. A stiffnite is a sheet-shaped, gravity-driven submarine sediment flow, with a fabric made up of marine ooze. To infer stiffnite dynamics, order of magnitude estimations are used. Field deposits and experiments on materials taken from the literature are also used. Stiffnites can be tens or hundreds of kilometers wide, and a few centimeters/ meters thick. They move on the sea slopes over hundreds of kilometers, reaching submarine velocities as high as 100 m/s. Hard grain friction favors grain fragmentation and formation of triboelectrically electrified particles and triboplasma (i.e., ions + electrons). Marine lipids favor isolation of electrical charges. At first, two basic assumptions are introduced, and checked a posteriori: (a) in a flowing stiffnite, magnetic dipole moments develop, with the magnetization proportional to the shear rate. I have named those dipoles as Ambigua. (b) Ambigua are ‘vertically frozen’ along stiffnite streamlines. From (a) and (b), it follows that: (i) Ambigua create a magnetic field (at peak, >1 T). (ii) Lorentz forces sort stiffnite particles into two superimposed sheets. The lower sheet, L+, has a sandy granulometry and a net positive electrical charge density. The upper sheet, L–, has a silty muddy granulometry and a net negative electrical charge density; the grains of sheet L– become finer upwards. (iii) Faraday forces push ferromagnetic grains towards the base of a stiffnite, so that a peak of magnetic susceptibility characterizes a stiffnite deposit. (iv) Stiffnites harden considerably during their motion, due to magnetic confinement. Stiffnite deposits and inferred stiffnite characteristics are compatible with a stable flow behavior against bending, pinch, or other macro instabilities. In the present report, a consistent hypothesis about the nature of Ambigua is provided.

Influence of solids on macro-instabilities in a stirred tank

Measurements were conducted in a cylindrical tank stirred with a PBT in order to study the effect of varying amounts of suspended solids, up to 11.8% by volume, on the frequency and amplitude of macro-instabilities (MI). Solid glass particles of three different sizes were used in order to investigate the influence of the particle Stokes number. Measurements were made at 18 different locations in the vessel using laser Doppler anemometry (LDA) and were evaluated with the Lomb algorithm to obtain the frequency spectrum of the liquid flow.The results showed that the MI frequency is not influenced by the addition of solids. However, the MI amplitude was reduced by the addition of the solid phase although still detectable up to the highest concentration measured (11.8vol.%). In the studied system there seems to be a difference dependent on the particle Stokes number.

Scalar mixing in a turbulent stirred tank with pitched blade turbine: Role of impeller speed perturbation

Mixing of a passive scalar inside a pitched blade turbine (PBT) impeller stirred tank (STR) is studied using large-eddy simulation (LES) coupled with the immersed boundary method (IBM) for resolving moving interfaces. Mixing time is calculated based on the 95% homogenization of the scalar over the entire tank volume. Growth rate of the unmixed tracer volume is observed in order to identify the effects of low frequency macroinstability (MI) oscillations. Mixing time is significantly reduced when the STR flow is perturbed using a step-change in the impeller speed with a specific MI frequency. The enhancement in turbulent kinetic energy and changes in mean flow field due to the perturbation is observed. The spatio-temporal behavior of the large-scale mixing structures for the fixed impeller-speed case and the perturbed case are compared. The mechanism of mixing enhancement is further explored by observing dynamic changes in the concentration distribution and the velocity field over a perturbation cycle. Penalty in power requirement due to perturbation is calculated.

Experimental Study and Detached Eddy Simulation of the Macro-Instability in an Eccentric Stirred Tank

The turbulent flow in stirred tank is highly complicated and anisotropic, especially when the macro-instability (MI) are involved. In this work, the numerical simulation method of the eccentric agitation was established based on the detached eddy simulation (DES) model to study the MI in an eccentric stirred tank. The turbulent flow in the eccentrically located Rushton turbine stirred tank was numerically investigated. The rotation of the impeller was simulated by the transient sliding mesh (SM) method. The effect of eccentricity, impeller Reynolds number and impeller-tank diameter ratio were studied in order to quantify the MI frequency. PIV experiments were performed to validate the DES results and frequency analyses were applied to the obtained time series of the velocity recordings. It was found that the flow field in eccentrically stirred tank are highly unsteady and is subject to MI with varying period less than 10 blade passage period. Good agreements have been found between the DES and PIV results, both indicate that the dominant frequency of MI increases linearly with the Reynolds number, increases with the impeller-tank diameter ratio and decreases with the eccentricity. According to the agreements between the experimental and simulation results, it can be concluded that the combination of DES and SM is suitable for the prediction of the MI phenomenon in stirred tanks.