Semi-empirical Theoretical Model Research Articles

Effect of slug characteristics on the nonlinear dynamic response of a long flexible fluid-conveying cylinder

Hydrocarbon flows in a marine riser may appear in multiple phases with varying flow patterns, among which, slug flows are known to exhibit complex flow characteristics due to fluctuations in multiphase mass, velocities, and pressure changes. Bulk of the literature concerning internal single and two-phase flow induced vibrations have largely focused on the use of a linearized tension beam model. In this study, the fluid-structure interaction phenomena of a submerged long flexible cylinder conveying two-phase slug flows is investigated taking into account the geometric and hydrodynamic nonlinearities. A semi-empirical theoretical model which consists of nonlinear structural equations of coupled out-of-plane, in-plane and axial structural motions is presented for the analysis and prediction of two-phase slug flow induced vibrations (SIV). The model includes equations involving centrifugal and Coriolis forces to capture mass variations in the slug flow regime. A numerical space-time finite difference approach combined with a frequency domain analysis is used for analyzing the highly nonlinear three-dimensional responses. The model assumes constant geometric properties across the span of the cylinder and utilizes an idealized slug unit concept, wherein slug properties are considered to be fully developed and undisturbed by pipe oscillations. Model validations are performed through comparisons with published experimental internal flow-induced vibration results. Parametric study captures the effects of several key slug characteristics on the vibration responses of a fluid-conveying cylinder. The results demonstrate amplitude-modulation response, mean displacements, three-dimensional cylinder displacements due to geometric nonlinear couplings and the influence of internal flow velocities. Higher dominant modes and oscillation frequencies were observed when the cylinder experiences large amplitude motions at specific slug formations. A new dimensionless parameter is introduced to predict scenarios of high amplitude oscillations for long flexible cylinders carrying two-phase slug flows.

High precision measurement of spectroscopic parameters of H<sub>2</sub>S in 6320—6350 cm<sup>–1</sup> band

As a highly corrosive and highly toxic gas, hydrogen sulfide (H<sub>2</sub>S) is an important intermediate product or pollutant in many fields such as chemical industry, energy and environment. Accurate online measurement of its concentration is of great significance for process control and production safety. Tunable diode laser absorption spectroscopy (TDLAS), as a quantitative absorption spectroscopy technique, is suitable for high-precision on-line measurement of H<sub>2</sub>S concentration in atmospheric environmental monitoring and industrial processes control. Considering that most of the spectroscopic parameters of H<sub>2</sub>S in the HITRAN2020 database are mainly calculated based on semi-empirical theoretical model and the experimental data to support them are lacking. In this work, direct absorption spectroscopy (DAS) method is firstly used to measure the absorption spectra of H<sub>2</sub>S in the band of 6320–6350 cm<sup>–1</sup>. Six groups of characteristic lines with strong absorption and relative independence are selected as the target transitions for experimental measurement. Then, the wavelength modulation-direct absorption (WM-DAS) method with no calibration and high signal-to-noise ratio is used to measure the absorbances of the six groups of transitions under different pressures. Voigt, Raution and quadratic speed-dependent Voigt profiles fit the measured absorbances by least squares method in order to obtain the spectroscopic parameters such as the collision broadening coefficient, line strength and Dicke narrowing coefficient. And the minimum standard deviation of residual error of absorbances is 7×10<sup>–5</sup>. The measurement uncertainty of each line strength is less than 2%, and the uncertainty of collision broadening coefficients, Dicke narrowing coefficients and the speed-dependent coefficients are all less than 10%. This work is helpful in improving the H<sub>2</sub>S spectral database and providing the spectral data basis for the high-precision measurement of H<sub>2</sub>S concentration.

Numerical simulation of cryogenic cavitating flow in LRE oxygen turbopump inducer

Turbopump is a critical component of modern liquid rocket engine (LRE), which employs cryogen as propellant. Its working performance is closely relevant to the reliability of LRE. The performance improvement of turbopump is limited by cryogenic cavitation condition. While the cryogenic cavitating flow characteristic is not well understood till now. A novel numerical method for cryogenic cavitation simulation was developed based on a transport-based cavitation model in the present investigation. The real thermodynamic properties of fluids and an additional energy source item which takes the heat transfer during phase change into account were imbedded into the code, and the empirical constants of the cavitation model are unnecessary to alter. It is found that a moderate reduction of released heat by vapor condensation in the energy source item provides better prediction accuracy for temperature distributions, and an optimal reduction proportion has been suggested. The cavitating flow features inside a three-bladed inducer using liquid oxygen as working fluid have been investigated in detail based on the proposed method, thermal effect reduces both the cavitation regions and the vapor volume fraction inside cavities remarkably, producing less blocking effect on the blade flow channels, which delays the head breakdown. Higher temperature shows stronger thermal effect and better cavitation performance. A classical semiempirical theoretical model is introduced to quantitatively predict the influence of thermal effect on cavitation performance. The predicted results are in consistent with the calculated results. The developed numerical model shows favorable universality to act as a reliable tool for cryogenic turbopump design.

Theoretical and numerical studies on a five-ray flexible pectoral fin during labriform swimming

Natural fish have evolved with an excellent swimming performance after millions of years. Based on the flexible features of the pectoral fin, this paper focuses on the kinematics and hydrodynamics of the fin when fish are swimming stably in still water in labriform mode. The locomotion mechanism based on the morphology of the pectoral fin is applied to establish a kinematic model composed of five rays and membranes, which is adopted to control the pectoral fin to reach deformation in approximately the same way as the labriform mode. A semi-empirical theoretical model based on the kinematics is proposed to calculate the hydrodynamic force. In order to study the flow field, the numerical simulation of fluid–structure interaction is carried out and the results are validated by the present semi-empirical model, which also verifies the feasibility of the semi-empirical theoretical model for describing the dynamics of the pectoral fin under a complex water environment. In addition, the relationship between propulsion performance and locomotion parameters (e.g. frequency of motion, amplitude of flapping and rowing angle, and phase lag between flapping and rowing) of the multi-degree of freedom flexible pectoral fin is also revealed. It is found that the frequency and amplitude of the flapping angle have a significant influence on the hydrodynamic thrust, while the rowing angle and phase lag have little effect. The established models and the results provide effective tools and significant reference for the design of bionic pectoral fins.

Study of Thermodynamic Cavitation Effects in an Inducer

To investigate the effect of water temperature on the cavitating flow characteristics inside a rotating inducer, a series of experiments have been conducted based on a model inducer in a newly developed visualization test facility using heated water as a working fluid. A practical image processing method was developed to analyze the visualization results quantitatively. The cavitation tests were carried out both before and after deaeration at room temperature. It was found that air content has little influence on the cavitation performance, though higher air content distinctly facilitates the cavitation inception. Comparison of the cavitation performance and cavitating structures at different temperatures shows that the intensity of the thermodynamic effect is closely related to the working condition. At a lower cavitation number and flow coefficient (, ), the thermodynamic effect is stronger to suppress the cavitation development inside the inducer, and thus the breakdown point is delayed. But at a larger flow coefficient (), higher temperatures show promotion effects on the cavitation breakdown. A semi-empirical theoretical model was employed to quantitatively predict the influence of the thermodynamic effect on the cavitation performance, and the predicted results match well with the experimental results.

Characteristics and simulation of snow interception by the canopy of primary spruce-fir Korean pine forests in the Xiaoxing'an Mountains of China.

Snow interception by the forest canopy is an important control on the forest hydrological cycle in the Xiaoxing'an Mountains within the northern temperate region of China. In this study, the effects of snowfall characteristics and stand structures on the snowfall redistribution of the canopies within primary spruce‐fir Korean pine forests are analyzed at the forest stand scale. Characteristics of snowfall, through‐canopy snowfall, and stand structure are continuously measured using positioning observations. A semiempirical theoretical model is used to conduct snow interception simulations in the Xiaoxing'an Mountain region. The results indicate that the snowfall, canopy density, slope gradient, and tree height have a significant effect on the through‐canopy snowfall. The interception efficiency gradually decreases with an increase in the amount of snowfall and is particularly sensitive to the snowfall and canopy density, although it shows no significant correlation with average diameter at breast height, tree height, basal area, canopy height, canopy width, leaf area, or slope gradient. Very similar results have been observed in Canada and Switzerland, suggesting the transferability of the results between North America, Western Europe, and China. However, although model results provide a satisfactory simulation of snow interception, further studies are required to optimize the model in this region.

Joint mass transfer of two components associated with the spontaneous interfacial convection in the liquid-liquid extraction system

This article focuses on the experimental work supported by theoretical model of the multicomponent mass transfer during extraction driven by spontaneous interfacial convection (SIC). The regularities of individual and joint unidirectional and reciprocal mass transfer of components with different surface-active properties in the benzene-water extraction system at a flat phase interface are considered. The article demonstrates that both the diffusion and convective hydrodynamics of the flow affect the intensity of unidirectional and counter-mass transfer of the components under the conditions of SIC. Marangoni instability at the interface causes local convection, which enhances the mass transfer rate considerably. Experimental studies have shown that the joint unidirectional mass transfer of acetic acid and iodine from organic phase to aqueous phase under the conditions of SIC can be characterised by an increase in the iodine mass transfer coefficient in comparison with its individual mass transfer. It has found that with the acetic acid the unidirectional mass transfer of iodine can be increased by more than 20 times compared to its mass transfer without acid. However in case of counter-transfer of acetic acid and iodine, both the acetic acid as well as iodine mass transfer coefficients decrease compared to their individual mass transfer situations. The experimental data are in accord with the theoretical justification. A simplistic semi empirical theoretical model is developed for joint mass transfer, to estimate the coefficient of mass transfer of the components. The satisfactory convergence of the experimental and calculated data are shown.

Tailoring thermal conductivity of AlN films by periodically aligned surface nano-grooves

Low thermal conductivity in condensed matter is critical to a diverse range of technologies, such as high efficient thermoelectrics and thermal insulation. It is thus important to fabricate, grow, or assemble structures that can reach a low limit. For III-nitride with high intrinsic thermal conductivity, how to utilize periodic nanostructures to manipulate phonons and achieve controllable low thermal conductivity is rarely studied. Recently, periodically self-organized arrays of nano-grooves on AlN (0001) surface have been observed experimentally. Inspired by this, we perform non-equilibrium molecular dynamics simulations to explore the thermal transport in such structures. The dependence of thermal conductivity on the periodic length of the nano-grooves and the angle of the side wall is systematically studied. Remarkably, results show that the thermal conductivity has a minimum value for a critical periodic length, which is one order of magnitude lower than the counterpart bulk value. The intrinsic high thermal conductivity of AlN can be robustly manipulated to the desired value by rationally designing the periodic nano-groove structure on the surface of the film. The detailed mechanism is provided by the frequency dependent heat current spectrum and phonon polarization analysis, along with the semi-empirical theoretical model. The understanding gained from this study offers an alternative route for tailoring the thermal conductivity of nanofilms by periodically patterned nano-grooves, which has large impact in relevant applications involving thermal transport.

Three-dimensional VIV prediction model for a long flexible cylinder with axial dynamics and mean drag magnifications

This study presents a three-dimensional phenomenological model for the analysis and prediction of vortex-induced vibration (VIV) of a long flexible circular cylinder in uniform flows, by taking into consideration the two salient effects of cylinder axial dynamics and mean drag magnifications. A semi-empirical theoretical model consists of structural nonlinear equations of coupled cross-flow, in-line and axial motions excited by vortex-induced hydrodynamic lift and drag forces. These forces are governed by two distributed and coupled wake oscillators describing the variations of lift and drag coefficients. This reduced-order model contains cubic and quadratic nonlinearities associated with geometric displacement couplings and fluid-structure interactions. A numerical finite difference approach combined with a frequency domain and modal analysis is used for analyzing the highly nonlinear three-dimensional VIV responses. Several key features of flexible cylinder VIV are captured in the case of varying flow velocities. In particular, the dynamic responses exhibit a transition from standing to travelling wave characteristics, spatial dual 2:1 resonances with multi modal contributions, variable figure-of-eight trajectories, higher harmonic components, mean in-line/axial displacements and mean stresses as a result of VIV-amplified mean drag forces. Model validations are also performed through comparisons with published experimental results. Parametric studies highlight the individual and combined effects of geometric and hydrodynamic nonlinearities together with the axial dynamics which contribute to the space-time varying responses, frequencies, dominant modes and modal contributions, transverse bending and longitudinal stresses. Both quantitative and qualitative discrepancies are noticeable when the prediction model disregards these effects.

Device simulation studies on latch-up effects in CMOS inverters induced by microwave pulse

This paper presents the simulated results of latch-up induced by microwave pulse with respect to pulse parameters, bias voltages and pulse injection conditions. A semi-empirical theoretical model based on the simulated results to access the pulse width and frequency effects on inducing latch-up quantitatively was proposed and agreed with the experimental results reported in the literature. Simulated results also indicated that pulse repetition frequency (PRF) must be far greater than the reciprocal of the minority carrier lifetime to lower the power threshold causing latch-up. Furthermore, excess carrier effects were believed to the primary reason of latch-up and be attributed to the difference between microwave pulse injection conditions.

Carbon Dioxide Induced Silk Protein Gelation for Biomedical Applications

We present a novel method to fabricate silk fibroin hydrogels using high pressure carbon dioxide (CO(2)) as a volatile acid without the need for chemical cross-linking agents or surfactants. The simple and efficient recovery of CO(2) post processing results in a remarkably clean production method offering tremendous benefit toward materials processing for biomedical applications. Further, with this novel technique we reveal that silk protein gelation can be considerably expedited under high pressure CO(2) with the formation of extensive β-sheet structures and stable hydrogels at processing times less than 2 h. We report a significant influence of the high pressure CO(2) processing environment on silk hydrogel physical properties such as porosity, sample homogeneity, swelling behavior and compressive properties. Microstructural analysis revealed improved porosity and homogeneous composition among high pressure CO(2) specimens in comparison to the less porous and heterogeneous structures of the citric acid control gels. The swelling ratios of silk hydrogels prepared under high pressure CO(2) were significantly reduced compared to the citric acid control gels, which we attribute to enhanced physical cross-linking. Mechanical properties were found to increase significantly for the silk hydrogels prepared under high pressure CO(2), with a 2- and 3-fold increase in the compressive modulus of the 2 and 4 wt % silk hydrogels over the control gels, respectively. We adopted a semiempirical theoretical model to elucidate the mechanism of silk protein gelation demonstrated here. Mechanistically, the rate of silk protein gelation is believed to be a function of the kinetics of solution acidification from absorbed CO(2) and potentially accelerated by high pressure effects. The attractive features of the method described here include the acceleration of stable silk hydrogel formation, free of residual mineral acids or chemical cross-linkers, reducing processing complexity, and avoiding adverse biological responses, while providing direct manipulation of hydrogel physical properties for tailoring toward specific biomedical applications.

Scanning Tunneling Spectroscopy of Single DNA Molecules

We briefly present the results of recent experiments of transverse scanning tunneling spectroscopy of homogeneous poly(dG)-poly(dC) DNA molecules and discuss them in the light of theoretical investigation. A semiempirical theoretical model is adopted to describe the transverse tunneling current across a DNA molecule placed between a metallic gold substrate and a metallic STM tip. We show that the main trends in the positions and relative magnitudes of the conductance peaks can be explained by a minimal model of a double tunnel junction with the molecule-electrode couplings and the applied voltage explicitly taken into account.

Shock Hugoniot and temperature data for polystyrene obtained with quartz standard

Equation-of-state data, not only pressure and density but also temperature, for polystyrene (CH) are obtained up to 510 GPa. The region investigated in this work corresponds to an intermediate region, bridging a large gap between available gas-gun data below 60 GPa and laser shock data above 500 GPa. The Hugoniot parameters and shock temperature were simultaneously determined by using optical velocimeters and pyrometers as the diagnostic tools and the α-quartz as a new standard material. The CH Hugoniot obtained tends to become stiffer than a semiempirical chemical theoretical model predictions at ultrahigh pressures but is consistent with other models and available experimental data.

Correlation between sputtering parameters and composition of SmCo-based films for microelectromechanical system applications

About 3.0 μm thick SmCo-based films with additives of Fe, Cu, and Zr were deposited on Si substrates. Based on a developed semiempirical theoretical model, the dependence of the film composition on the sputtering parameters was discussed. The experimental results show that the Sm concentration increases with decreasing sputtering power or increasing Ar gas pressure, which are in good agreement with the calculated results when the preferential sputtering effect is disregarded. The effect of the sputtering parameters on the film composition provides an opportunity for the same composite target to fabricate films with Sm concentration varying from 13.8 to 17.3 at. %, which is reasonable for the permanent magnetic phase transformation (Sm2Co17→SmCo7→SmCo5). Furthermore, the observed TbCu7-type film shows a better crystal texture with a low remanence ratio for the hysteresis loops measured out plane to in plane of 0.08.

Behavior of normal and high-strength concrete cylinders confined with E-glass/epoxy composite laminates

This paper is aimed at studying the behavior of concrete cylinders with varying compressive strength wrapped with E-glass/epoxy fiber reinforced polymer (GFRP) jackets and subjected to uniaxial compressive loads. A comprehensive experimental program which involves 54 plain concrete cylinders was conducted in this study. The cylinders evaluated in this study, were divided into six groups, and each group contain a control cylinder without confinement to quantify the amount of gain obtained using the GFRP laminates. Experimental results indicated that the use of GFRP jackets substantially increases both the compressive strength and ductility of unreinforced concrete cylinders. In this paper, the influences of two parameters influencing the behavior of the GFRP confined cylinder is investigated. These parameters are: the number of composite plies (i.e. composite thickness) and concrete compressive strength. The results of this study showed that: (i) compressive strength and ductility of the concrete cylinders increases with number of composite layers; and (ii) effect of confinement is substantial for normal strength concrete and marginal for high-strength concrete. A semi-empirical theoretical model is also presented in order to predict stress–strain relationship of GFRP confined concrete cylinders. The model results showed an excellent agreement with experimental values.

Semiempirical Model of Vibrational Relaxation for Estimating Absolute Rate Coefficients

A semiempirical theoretical model is developed for estimating the rates of collision-induced V−T vibrational relaxation from an initially excited vibrational state in a polyatomic molecule to a relatively dense field of destination vibrational states. The rationale has been to provide a means of estimating absolute relaxation rate coefficients, with a reasonable level of precision, for relaxation induced by a wide range of collision partners (atomic, diatomic, and polyatomic) and for a relatively wide range of temperatures. The model is based on calculating an efficiency P for the vibrational relaxation process by making use of relevant known molecular and intermolecular parameters. The inelastic rate coefficient for vibrational relaxation then emerges from a product of this efficiency and the classical Lennard-Jones elastic encounter rate. A feature of the model is that it uses no adjustable parameters. Comparisons are made between the predictions from the model and a number of experimental measurements of specific dependencies of vibrational relaxation rate coefficients (e.g. on collision partner properties, on the initial vibrational state, and on temperature) to characterize and illustrate the ingredients that deliver the estimate for P. A correlation between the predictions of the model and data from over 100 experimental systems and for a temperature range from 2−300 K invites the conclusion that the model is useful for estimating the absolute magnitude of state-to-field vibrational relaxation rate coefficients in the intermediate regime of final state densities to within an overall accuracy of 30% and with an average error of −10%.

Influence of indium on the electronic states in GaInNAs/GaAs quantum well structures

We use room-temperature photoreflectance spectroscopy to investigate the influence of indium on the electronic structure of Ga1−xInxNyAs1−y/GaAs multiple quantum wells. To fit our experimental data, a semiempirical theoretical model based on the band anticrossing Hamiltonian is successfully applied. Thus, we can extract some information about the Hamiltonian, in particular, the dependence of the coupling parameter CNM on the In concentration in GaInNAs. CNM is shown to decrease with increasing indium mole fraction, confirming theoretical predictions.

Experimental investigation of three-dimensional dynamic stall model oscillating in pitch

The ONERA semiempirical theoretical model for unsteady aerodynamics in the stall regime has been extended to three-dimensi onal flow. Parameter identification for the model is obtained from experimental dynamic stall data for a low AR rectangular wing with a NACA 0012 airfoil profile oscillating in pitch. Excellent correlations were obtained with the measured data for a three-dimensional linear aerodynamic model and reasonable agreement between theory and test data were obtained for a nonlinear (stalled) aerodynamic model. Qualitatively, the results obtained for this simple three-dimensional wing are similar to the corresponding data for the twodimensional airfoil.

Design trade-offs and evaluation of the performance: attainable by GaAs-Al/sub 0.3/Ga/sub 0.7/As asymmetric Fabry-Perot modulators

We use a semi-empirical theoretical model to evaluate the performance ultimately attainable by normally-on asymmetric Fabry-Perot modulators (AFPMs) that incorporate GaAs-Al/sub 0.3/Ga/sub 0.7/As multiple quantum wells (MQW). Various optimization criteria are employed and the calculations address issues that involve both the micro-resonator design and the quantum-well region. Thus we evaluate the minimum required barrier width in order to compare between different well widths, and examine the final AFPM performance in many respects, considering not only the modulation itself but also the tolerances and the speed of the device. Finally, we proceed to the design of example devices where some characteristics have been specifically optimized.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Use of a Mathematical Model to Predict Oxygen Transfer Rates in Hollow Fiber Membrane Oxygenators

A semi-empirical theoretical model of oxygen transfer is used to predict the rates of oxygen transfer to blood in hollow fiber membrane oxygenators over a wide range of inlet conditions. The predicted oxygen transfer rates are based on performance of the devices with water, which is more cost effective and easier to handle than blood forin vitroevaluations. Water experiments were conducted at three different flow rates to evaluate oxygen transfer performance in three commercially available membrane oxygenators. Data obtained from these experiments were used in a computer model to predict the rate of oxygen transfer to bovine blood at specified inlet conditions. Blood experiments were conducted at three different flow rates at a wide variety of inlet conditions, including different pH levels, hemoglobin concentrations, and oxyhemoglobin saturations for the three types of oxygenators. The measured and predicted oxygen transfer rates are closely correlated, which suggests that we have an accurate, reliable method for predicting oxygen transfer in hollow fiber membrane lungs.ASAIO Journal1994; 40:990-996.