External Pressure Field Research Articles

A more general exergy function and its application to the definition of exergy efficiency

An availability function which includes the usual functions exergy and flow exergy as special cases is defined and called exergy. The new definition of exergy is based on the concept of pressure energy, namely the work that must be done on a stationary external pressure field to build inside it the region of space occupied by the system. Then, the definition of exergy efficiency is written in a more explicit form, where the same availability function is employed for both steady-flow processes and non-flow processes.

Room temperature giant baroresistance and magnetoresistance and its tunability in Pd doped FeRh

We report room temperature giant baroresistance (≈128%) in Fe49(Rh0.93Pd0.07)51. With the application of external pressure (P) and magnetic field (H), the temperature range of giant baroresistance (≈600% at 5 K, 19.9 kilobars and 8 T) and magnetoresistance (≈ −85% at 5 K and 8 T) can be tuned from 5 K to well above room temperature. It is shown that under external pressure, antiferromagnetic state is stabilized at room temperature and shows giant magnetoresistance (≈−55%). Due to coupled magnetic and lattice changes, the isothermal change in resistivity at room temperature under pressure (at constant H) as well as magnetic field (at constant P) can be scaled together to a single curve, when plotted as a function of X = T + 12.8 × H − 7.3 × P.

Dynamics of Gas Bubbles Encapsulated by a Viscoelastic Fluid Shell Under Acoustic Fields

The dynamics of a gas-filled microbubble encapsulated by a viscoelastic fluid shell immersed in a Newtonian liquid and subject to an external pressure field is theoretically studied. The problem is formulated by considering a nonlinear Oldroyd type constitutive equation to model the rheological behavior of the fluid shell. Heat and mass transfer across the surface bubble have been neglected but radiation losses due to the compressibility of the surrounding liquid have been taken into account. Bubble collapse under sudden increase of the external pressure as well as nonlinear radial oscillations under ultrasound fields are investigated. The numerical results obtained show that the elasticity of the fluid coating intensifies oscillatory collapse and produces a strong increase of the amplitudes of radial oscillations which may become chaotic even for moderate driving pressure amplitudes. The role played by the elongational viscosity has also been analyzed and its influence on both, bubble collapse and radial oscillations, has been recognized. According to the theoretical predictions provided in the present work, a microbubble coated by a viscoelastic fluid shell is an oscillating system that, under acoustic driving, may experience volume oscillations of large amplitude, being, however, more stable than a free bubble. Thus, it could be expected that such a system may have a suitable behavior as an echogenic agent.

Modeling the interaction of microbubbles: Effects of proximity, confinement, and excitation amplitude

The interaction of closely spaced microbubbles (MBs) exposed to a transient external pressure field is relevant for a variety of industrial and medical applications. We present a computational framework employing an interface tracking approach to model the transient dynamics of multiple, interacting, insonated MBs in arbitrary settings. In particular, this technique allows studying the effects of mutual proximity, confinement, and variations in excitation amplitude on the translatory motion of pairs of differently sized MBs. Domains of mutual repulsion or attraction are observed for closely spaced MBs in the investigated range of excitation frequencies. The repulsion domain widens and shifts to lower frequencies with increasing excitation pressure amplitude. When the MBs are confined in rigid tubes of decreasing diameters, we observe a shift of the translatory patterns towards lower frequencies, accompanied by a change in relative strength of the two translation modes. This effect is correlated to a decrease of the resonance frequency due to confinement which causes changes in oscillation amplitude and phase shift between the bubble vibrations. Coupling to the viscous host liquid gives rise to phenomena such as collective MB drift, non-symmetric attraction or repulsion, and reversal of translation direction. A system comprising six MBs inside a narrow tube highlights the potential of the computational framework to treat complex setups with multiple bubbles.

Elastic stresses in an encapsulated gas bubble

The main objective of the present work is to study the elastic behaviour and the non-linear radial oscillations of a thin spherical shell encapsulating a perfect gas under the influence of an external acoustic pressure field. Our basic motivation is to derive a full elastic description of the above conjugate fluid mechanics problem. The mathematical model is reduced to solve a modified Rayleigh–Plesset equation to predict the external radius together with a Cauchy equation needed to predict the deformations and elastic stresses of the spherical thin shell. We impose conjugate boundary and initial conditions to predict the radius, elastic deformations and stresses and the resulting governing equations are solved numerically. Using dimensionless variables, we introduce, among others, three characteristic parameters that dictate the dynamic response of the encapsulated bubble: β that represents the competition between acoustic forces and induced stresses into the elastic solid, πx is the ratio between the equilibrium pressure and the elasticity modulus of the shell and βA defines the competition between the characteristic pressures of the system given lines below. The numerical predictions reveal that this last dimensionless parameter βA regulates clearly a pronounced cavitation limit and a non-linear oscillating behaviour. The most relevant physical variables of the problem such as the external radius of the shell, the elastic deformations and stresses are obtained as functions of time and different values of the above dimensionless parameters.

Analysis of Aerodynamic Characteristics in the Bottom of a Car

The aerodynamic characteristics of ordinary vehicle have been studied by lots of scholars, while few people pay enough attention to the aerodynamic characteristics in the bottom of the car. As the requirements of regulations for the performance of the car continues to increase, Analysis of Aerodynamic Characteristics of the car is much more necessary now. In this paper , by using CFD method and getting benefit from the established CFD software, we gave external velocity field and pressure field of the car. The data was analyzed and summarized and the computational results are obtained. In the final analysis for the flatness in the bottom of the car influence on aerodynamic characteristics.This paper has done some useful attempts, and it will provide specific reference significance to the numerical simulation on design of the car in the future.

Forming Microgears by Micro-FAST Technology

The use of microcomponents as well as intensive competition on manufacturing cost led to the requirement for cost-effective production of those components without much compromise on their final quality. To address this issue, a novel microforming technology, named as micro-field-activated sintering technology (Micro-FAST), was introduced for the fabrication of microcomponents, in which loose powders were pressed and sintered to form microgears via electroheating loop sintering and microforming. The whole process was realized in a Gleeble-1500D thermal simulation machine. A relative density of 98.7% was achieved after sintering microgears with eight teeth on it and 1.6 mm pitch diameter at 600°C, heating rate of 50°C/s, pressure of 100 MPa, and electroheating loop of five times. Being different from a conventional FAST process, the Micro-FAST process is of different characteristics largely due to the size effect. Effects of the external electric field, thermal field, and pressure field on the Micro-FAST densification process were analyzed. It was revealed that the deformation of the particles and fusion at the contact interfaces between particles are two important parameters to the densification of the green body and, hence, to the quality of the final component.

Peculiar magnetism of transition metal cluster compounds

We discuss a class of solids which have in their structure fairly separated tetrahederal clusters of transition metals: vanadium and molybednum. With decreasing temperature these compounds undergo a structural transition from cubic to rhombohederal phase at a temperature Ts, followed by a transition to a ferromagnetic phase at a lower temperarure Tc. Magnetic measurements indicate the presence of moments of spin half per cluster. Between Tc and Ts susceptibility shows large departures from Curie-Weiss behavior and specific heat has an unusually large non-phononic contribution. We present a model in which each cluster acts as a unit associated with a magnetic moment of spin half and a structural degree of freedom related to distortions of the tetrahedron. Our model accounts for all the observed features including effects due to external pressure and magnetic field.

Dynamic interaction of multiple buoyant jets

AbstractAn array of closely spaced round buoyant jets interact dynamically due to the pressure field induced by jet entrainment. Mutual jet attraction can result in a significant change in jet trajectories. Jet merging also leads to overlapping of the passive scalar fields associated with the individual jets, resulting in mixing characteristics that are drastically different from those of an independent free jet. A general semi-analytical model for the dynamic interaction of multiple buoyant jets in stagnant ambient conditions is proposed. The external irrotational flow field induced by the buoyant jets is computed by a distribution of point sinks with strength equal to the entrainment per unit length along the unknown jet trajectories and accounting for boundary effects. The buoyant jet trajectories are then determined by an iterative solution of an integral buoyant jet model by tracking the changes in the external entrainment flow and dynamic pressure fields. The velocity and concentration fields of the jet group are obtained by momentum or kinetic energy superposition for merged jets and plumes, respectively. The modelling approach is supported by numerical solution of the Reynolds-averaged Navier–Stokes equations. The model shows that jet merging and mixing can be significantly affected by jet interactions. Model predictions of the multiple jet trajectories, merging height, as well as the centreline velocity and concentration of the buoyant jet group are in good agreement with experimental data for: (i) a clustered momentum jet group; (ii) a turbulent plume pair; and (iii) a rosette buoyant jet group. Dynamic interactions between a jet group are shown to decrease with the addition of an ambient cross-flow.

ChemInform Abstract: Polymorphism in Spin—Crossover Systems

AbstractReview: 129 refs.

General Reduced-Order Model to Design and Operate Synthetic Jet Actuators

Synthetic jets are used in various applications from flow control to thermal management of electronics. Controlling the jet operating point using a simple voltage to velocity calibration becomes unreliable in case of external pressure field disturbances or varying actuator characteristics. This paper presents a general lumped parameter model for a synthetic jet actuator with an electromagnetic or piezoelectric driver. The fluidic model accurately predicts the synthetic jet operating point (i.e., Reynolds number and stroke length) based on the measured cavity pressure. The model requires only two empirical coefficients characterizing nozzle fluid damping and inertia. These can be obtained via calibration or estimated from pressure loss correlations and the governing acoustic radiation impedance. The model has been validated experimentally for circular and rectangular orifices. The effect of nozzle damping on the nonlinear system response is discussed. Analytical expressions are given for the two resonance frequencies characterizing the system response as a function of the diaphragm and Helmholtz resonance frequencies. The optimal design of an impinging synthetic jet actuator is discussed in terms of the thermal and acoustic efficiencies. Guidelines for selecting the optimum combination of diaphragm and Helmholtz resonance frequency are presented and compared with previous studies.

Large variations in the magnetic ordering behavior of EuCu2As2 with the application of external pressure and magnetic field

The influence of external pressure on the electrical transport and magnetic properties of EuCu2As2, crystallizing in a ThCr2Si2-type structure, is reported. The system is known to be an antiferromagnet below TN ≈ 15 K in the absence of external magnetic fields. We find that there is a gradual reduction of TN with the application of a magnetic field with an extrapolated value of the critical field of around 18 kOe which can drive TN to zero. Electrical resistivity under pressure (<11 GPa) reveals that the magnetic ordering temperature is pushed up dramatically to higher temperatures which is quite interesting if compared with the behavior in isostructural FeAs-based systems containing Eu. Above 7 GPa, the pressure-induced state appears to be ferromagnetic. The results thus reveal interesting changes in the magnetic ordering behavior of this compound with increasing pressure and magnetic fields.

Polymorphism in spin-crossover systems

The occurrence of spin-crossover (SCO) highly depends on external influences, i.e. temperature, pressure, light irradiation or magnetic field, this electronic switching phenomenon is accompanied by drastic changes in magnetic and optical properties, dielectric constants, colour and structures. Thus, SCO materials are particularly attractive for potential applications in molecular sensing, switching, data storage, display, and other electronic devices at nanometric scale. Polymorphism is widely encountered in the studies of crystallization, phase transition, materials synthesis, biomineralization, and in the manufacture of drugs. Because different crystal forms of the same substance can possess very different properties and behave as different materials, so they are particularly meaningful for investigating SCO phenomena. Studying polymorphism of SCO compounds is therefore important for better understanding the structural factors contributing to spin transition and the structure-function relationship. This critical review is aimed to provide general readers with a comprehensive view of polymorphism in SCO systems. The article is generally structured according to specific metal ions and the dimensionality of compounds in the field. This paper is addressed to readers who are interested in multifunctional materials and tuning magnetic properties through supramolecular chemistry principles (129 references).

Forced oscillations of two gas bubbles in a fluid in the vicinity of bubble contact

For a theoretical derivation of bubble coalescence conditions, nonlinear forced oscillations of two closely spaced spherical bubbles subjected to the action of a periodic external pressure field are considered. The equations, asymptotic with respect to a small distance between the bubble surfaces, are derived to describe the approach of the bubbles under the action of (i) the Bjerknes attraction force averaged over the oscillation period and (ii) the viscous drag. It is shown that due to nonlinear interaction of the viscous drag with the radial and translational oscillations of the bubbles a unidirectional repulsive force is generated, which prevents the approach of the bubbles. The coalescence of the bubbles is possible when the nondimensional parameter combined from the amplitude and frequency of the external pressure field, the bubble radius, and the fluid viscosity is greater than a certain critical value. The obtained coalescence condition is qualitatively confirmed by experiments.

The effects of external acoustic pressure fields on a free-running supercavitating projectile

Proliferation of supercavitating torpedoes has motivated research on countermeasures against them as well as on the fluid phenomenon which makes them possible. The goal of this research was to investigate an envisaged countermeasure, an acoustic field capable of slowing or diverting the weapon by disrupting the cavitation envelope. The research focused on the interactions between high pressure amplitude sound waves and a supercavity produced by a small free-flying projectile. The flight dynamics and cavity geometry measurements were compared to control experiments and theoretical considerations were made for evaluating the effects. Corrugations on the cavity/water interface caused by the pressure signal have been observed and characterized. Results also show that the accuracy of a supercavitating projectile can be adversely affected by the sound signal. This research concludes with results that indicate that it is acoustic cavitation in the medium surrounding the supercavity, caused by the high pressure amplitude sound, that is responsible for the reduced accuracy. A hypothesis has been presented addressing the means by which the acoustic cavitation could cause this effect.

Non-linear oscillations of an elastic shell controlled by the interface pressure

In the present work, we study analytically and numerically the non-linear radial oscillations and unitary deformations of a thin elastic shell surroundings a perfect gas under an external ultrasound pressure field. The model comprehends a Rayleigh–Plesset kind equation (RP) to predict the liquid radius, which in turn surrounds the elastic shell. In this manner, both media are coupled through the boundary conditions and the resulting governing equations are solved simultaneously. Using dimensionless variables, we use basically three main parameters: β which represents the quasi steady-state deformations of the solid, π x is the ratio between the ambient pressure and the elasticity modulus of the shell and β A that represents the competition between the driving pressure and the dynamic pressure. The numerical results predict the oscillations of the liquid and the pressure, also. For the solid, we have obtained a weak oscillatory and irregular behavior which can conduct to the final collapse of the solid. The most relevant physical variables of the problem such as the displacement of the shell, interface pressure, deformation rate and radius of the shell are obtained as functions of time and different values of the above dimensionless parameters.

Pulsating gas bubbles in liquid: The conditions of merging

INTRODUCTIONAs is well known, two balls pulsating in a cophasedstate undergo the action of the Bjorknes attractionforce. However, experiments show that pulsating bubbles do not always merge. Usually, for theoretical derivation of bubblemerging conditions, stimulatednonlinear oscillations are considered for two sphericalbubbles moving in close vicinity to each other underthe action of an external periodic pressure field. It wasshown that as a result of the nonlinear interaction ofthe viscous force with bubble radial and translationaloscillations, there arises a unidirectional repulsionforce quadratic with respect to viscosity. This forcestops the approach of bubbles. The condition obtainedfor bubble merging is confirmed qualitatively byexperimental data.The interaction of two pulsating balls, i.e., of ballsperiodically changing their radii

Physical Basis for Creating Energy and Resource-Saving Rheo-Technology in Oil Production

In a previous paper we presented the results of our investigations on the application of externally imposed temperature fields for the improvement of the non-Newtonian properties of raw oil in the well and also on the possible use of electric and magnetic fields in the water flooding process of the oil layer. In this article, some additional results are presented with regards to the application of external pressure fields to the same purpose, with the goal of increasing the well output and improve the efficiency of gaslift wells and oil pipelines. The possibility of regulating the gas-liquid system saturation pressure is discussed as well, to expand the opportunity of varying the well operating regime.

Disproportionation and electronic phase separation in parent manganiteLaMnO3

Nominally pure undoped parent manganite LaMnO_3 exhibits a puzzling behavior inconsistent with a simple picture of an A-type antiferromagnetic insulator (A-AFI) with a cooperative Jahn-Teller ordering. We do assign its anomalous properties to charge transfer instabilities and competition between insulating A-AFI phase and metallic-like dynamically disproportionated phase formally separated by a first-order phase transition at T_{disp}=T_{JT}\approx 750 K. The unconventional high-temperature phase is addressed to be a specific electron-hole Bose liquid (EHBL) rather than a simple "chemically" disproportionated R(Mn^{2+}Mn^{4+})O_3 phase. New phase does nucleate as a result of the charge transfer (CT) instability and evolves from the self-trapped CT excitons, or specific EH-dimers, which seem to be a precursor of both insulating and metallic-like ferromagnetic phases observed in manganites. We arrive at highly frustrated system of triplet (e_g^2)^3A_{2g} bosons moving in a lattice formed by hole Mn^{4+} centers. Starting with different experimental data we have reproduced a typical temperature dependence of the volume fraction of high-temperature mixed-valent EHBL phase. We argue that a slight nonisovalent substitution, photo-irradiation, external pressure or magnetic field gives rise to an electronic phase separation with a nucleation or an overgrowth of EH-droplets. Such a scenario provides a comprehensive explanation of numerous puzzling properties observed in parent and nonisovalently doped manganite LaMnO_3 including an intriguing manifestation of superconducting fluctuations.

Open-loop control of fully nonlinear self-excited oscillations

Experiments conducted using a low-density axisymmetric jet reveal that the self-excited nature can be altered by modulating the external pressure field using an acoustic driver. A region exists in the forcing frequency-amplitude space where the self-sustaining frequency is entirely absent and solely replaced by the forcing frequency and harmonics thereof. Inside this region, the centerline streamwise velocity disturbance level can be significantly increased or decreased. Visual evidence suggests these alterations coincide with dramatic changes in jet spreading and demonstrates that forcing can provide an effective means of controlling mixing in self-excited jets. Results support theoretical predictions made by Pier [B. Pier, Proc. R. Soc. London, Ser. A 459, 1105 (2003)].