Transient Numerical Analysis Research Articles

Experimental Identification of Overall Structural Damping of System

The dynamic behaviour of large and complex structures largely depends on damping resistance in the structure. A portion of the structural energy is lost to deformations in material, friction between the contact surfaces, and relative motion within the structure. Often, in an analysis of numerical models, before the dynamic analysis of transient events (transient analysis), the damping resistance is adopted on the basis of recommendations, which implies an error of transient response (introduced by frequencies, logarithmic decrements and maximal amplitudes). Decreasing amortized vibratory movement is dependent on the extent of the structural damping. This paper presents the importance of structural damping in structural analysis and shows the experimental and theoretical procedure for identifying G values of the structural damping coefficient. A model for determining the G coefficient is shown in the example of a real tower crane structure. The experimentally obtained values were then used in the transient numerical FEM analysis, as the basis for adopting the conclusions about the dynamic behaviour of this class of structures (transportation machines). The effect of the external perturbation force of trapezoidal impulse form (lifting and quickly lowering of load) is introduced and the dynamic task, as an example of the use of the G coefficient G, is solved. The experimentally determined damping (theoretically isolated for tall truss structures) can be used in similar transient analyses.

Numerical transient state analysis of partly obstructed haemodynamics using FSI approach

Abstract Blood interacts in compliance with the vessels it flows in, determining effects of fluid-structure interaction (FSI). Using this approach, the present paper analyses the blood flow and its pathological conditions before diagnostic (Doppler catheters) or pathological (stenoses) obstacles. Catheters are concentric cylinders with defined vessels, whereas stenoses are shaped as triangles, trapeziums or hemi-ellipses. Rather than a basic/rigid case, three models were adopted to describe the artery wall dynamics: elastic (Hooke), hyper-elastic Mooney-Rivley 1 (MR1) and hyper-elastic Mooney-Rivley 2 (MR2). Blood has non-Newtonian rheological properties (Casson fluid. The numerical study used the CFD code Comsol Multiphysics, version 3.4. The FSI effect was studied through 3 physical parameters with reference to the blood flow: minimum velocity (re-circulation), maximum velocity (the mechanical stress on the obstacle) and the disturbance length. Results show a strong dependence of the FSI effect on the characteristics of the obstacle considered and on the kind of elastic/hyper-elastic model chosen. In a few cases, however, the results of the old rigid-vessel approach were reasonably accurate.

원심펌프의 캐비테이션에 대한 수치해석

As the operating range of centrifugal pump is expanded recently, the various suction conditions are demanded . The most important problem in the suction conditions is cavitation. In this study, to analyze the characteristics for such the cavitation, first the validity of the numerical analysis was certified through comparison with the experimental result of performance curve according to flow rate for the industrial centrifugal pump. At this time, the transient numerical analysis for the full type model of the centrifugal pump was performed to get more accurate results. The numerical analysis on the cavitation of centrifugal pump were conducted on the two-phase flow as the same method of one-phased flow.

Dynamic behavior of dc SQUIDs in time-division multiplexing readout schemes

We have investigated the transient response of superconducting quantum interference devices (SQUIDs) used in time-division multiplexing schemes for the readout from arrays of superconducting transition-edge sensors (TES). In time-division multiplexing the per channel settling time is an important parameter since it limits the multiplexing frequency as well as the signal-to-noise ratio. Although SQUIDs in general are fast cryoelectronic devices operating at a nanosecond scale, experimental evidence shows that the settling time in bias-switched SQUID multiplexers can, in the case of a low impedance input circuit, reach several microseconds. We have confirmed this behavior through numerical transient analysis using a Josephson circuit simulator. Within the simulation we have found that the transient response can be reduced using an extra inductor in series with the input coil of the dc SQUID. This result in turn has been confirmed experimentally. We furthermore discuss the conflicting constraints that a desired high signal bandwidth places on the parameters of the multiplexer, the readout SQUIDs, and the TES.

Numerical Analysis of Heavy Oil-Water Flow and Leak Detection in Vertical Pipeline

Pipeline is a conventional, efficient and economic way for oil transportations. The use of a good system for detecting and locating leaks in pipeline contribute significantly to operational safety and cost saving in petroleum industry. This paper aims to study the heavy oil-water flow in vertical ducts including leakage. A transient numerical analysis, using the ANSYS-CFX? 11.0 commercial software is performed. The mathematical modeling considers the effect of drag and gravitational forces between the phases and turbulent flow. Mass flow rate of the phases in the leaking orifice, the pressure drop as a function of the time and the velocity distributions are presented and discussed. We can conclude that volumetric fraction of phases and fluid mixture velocity affect pressure drop and mass flow rate at the leak hole.

Transient Three-dimensional Numerical Analysis of Forced Convection Flow and Heat Transfer in a Curved Pipe

A three-dimensional transient numerical study of a constant property Newtonian fluid in curved pipe under laminar flow conditions is presented for a uniform wall temperature boundary condition. Numerical solutions were obtained using the control volume method described by Patankar for the range of . The working fluid was water. The transient flow pattern and the temperature distribution on the tube section were derived for different values of the Reynolds number. Graphical results for velocity and temperature are presented and analyzed. Results have shown that the maximum velocity in center of velocity profile increase with increasing of Reynolds number. In curved pipes, time averaged results exhibited Dean circulation and a strong velocity and temperature stratification in the radial direction. Flow and heat transfer were strongly asymmetric, with higher values near the outer pipe bend.

Tecnoeconomic Analysis of Medium and Large-sacle Desalination Plants Driven by Concentrated Solar Systems in the Mena Region

Although medium and large scale solar desalination systems have been long been discussed and proposed as a sustainable root to offset water shortage in water-scarce areas, commercial units have not been introduced to the market, mainly due to inherent difficulties of part load operation under transient conditions and high costs of solar systems compared to conventional fossil-fired desalination. This study presents transient numerical analysis andtechno economic assessment of a concentrated solar thermal (CST) system to drive a Multi-Stage Flash (MSF) desalination plant with 5000 m3/day capacity. The dynamic performance of the CST system over a wide range of operation conditions is simulated and evaluated under varying weather conditions over one year for Mersa Matruhcity on the Mediterranean coast of Egypt, as a case study for the MENA region. A simplified cost analysis has been performed to explore the potential of CST-driven MSF and additionally for Reverse Osmosis (RO) desalination technology driven by concentrated solar power (CSP) in comparison with fossil-fired desalination. It has been found that the desalinated water cost of CSP-RO is competitive with fossil-fired systems, while the associated cost of CST- MSF is approximately 3 times more costly than both CSP-RO and fossil-fired systems, without the cost of CO2 capturing and storage for fossil-fired systems.

Methane vibrational relaxation lifetimes at low temperatures

We have made photoacoustic measurements for the ν4 bending mode lifetime in methane in self collision over a temperature range of 146–293K. We analyzed these results through a new theory using finite differences. Via a numerical transient analysis and a statistical evaluation of data we obtained similar results as those of earlier experimenters. Having proved the capabilities of our equipment we will direct future experiments to the study of methane deactivation by major gases present in the atmospheres of the large planets of the Solar System.

Transient Electromagnetic Field Numerical Analysis of Self-Excited Retarder

Power generation performance of a self-excited retarder determines whether the design goal of its braking torque could be achieved. The numerical analysis of transient electromagnetic field took magnetic vector potential as variable and made six reasonable assumptions to simplify calculation. Based on above, definite solution about transient electromagnetic field and the finite element received a detailed description. The external circuit and end effect of the power generation unit were taken into account so as to improve the calculation accuracy. Let the finite element equation and external circuit equation combine to form field-circuit coupled equations and then using Crank Nicolson method established its space-time discrete model. After verification by experiment, theoretical performance of the power generation unit was basically consistent to the experimental result for a 500Nm prototype. This showed that the numerical analysis as well as the model was feasible and the power generation unit could satisfy the requirement of braking unit for excitation current.

Numerical investigation of transient natural convection in a vertical channel-chimney system symmetrically heated at uniform heat flux

In the present numerical investigation, a transient numerical analysis for natural convection in air, between two vertical parallel plates (channel), heated at uniform heat flux, with adiabatic parallel plates downstream (chimney), is carried out by means of the finite volume method. The analyzed transient problem is two-dimensional and laminar. The computational domain is made up of the channel-chimney system, and two reservoirs, placed upstream the channel and downstream the chimney. The reservoirs are important because they simulate the thermal and fluid dynamic behaviors far away from the inflow and outflow regions. Results are presented in terms of wall temperature and air velocity profiles. They are given at different Rayleigh number and expansion ratios (chimney gap/channel gap) for a fixed channel aspect ratio (channel height/channel gap) equal to 10 and extension ratio (channel-chimney height/channel height) equal to 2.0. Wall temperature profiles over a period show the presence of overshoots and undershoots. The comparison among the maximum wall temperatures shows that the simple channel is the most critical configuration at steady state condition, but it is the best configuration during the transient heating at the first overshoot. As indicated by the temperature profiles, average Nusselt number profiles over a period of consideration show minimum and maximum values and oscillations before the steady state. Stream function fields allow to observe the development of fluid dynamic structures inside the channel-chimney system, particularly how and when the cold inflow is present and develops.

The effects of using different type of inlet vents on the thermal characteristics of the automobile cabin and the human body during cooling period

A three-dimensional (3-D) transient numerical analysis was performed inside an automobile cabin during cooling period. A three-dimensional vehicle cabin including glazing surfaces was modelled by using the real dimensions of a car. A virtual manikin with real dimensions and physiological shape was added to the model of the vehicle cabin, and it was assumed that the manikin surfaces were subjected to constant temperature. The virtual manikin was divided into 17 parts in standing posture to evaluate the local heat transfer characteristics of the human body during transient cooling period. We considered three different cases that the cooling capacity of the automobile cabin was same for all cases. Three-dimensional fluid flow, temperature distribution and heat transfer characteristics inside the automobile cabin were calculated with different type of inlet vents. Comparisons of the numerical results were presented and discussed.

Virtual welding equipment for simulation of GMAW processes with integration of power source regulation

A two dimensional transient numerical analysis and computational module for simulation of electrical and thermal characteristics during electrode melting and metal transfer involved in Gas-Metal-Arc-Welding (GMAW) processes is presented. Solution of non-linear transient heat transfer equation is carried out using a control volume finite difference technique. The computational module also includes controlling and regulation algorithms of industrial welding power sources. The simulation results are the current and voltage waveforms, mean voltage drops at different parts of circuit, total electric power, cathode, anode and arc powers and arc length. We describe application of the model for normal process (constant voltage) and for pulsed processes with U/I and I/I-modulation modes. The comparisons with experimental waveforms of current and voltage show that the model predicts current, voltage and electric power with a high accuracy. The model is used in simulation package SimWeld for calculation of heat flux into the work-piece and the weld seam formation. From the calculated heat flux and weld pool sizes, an equivalent volumetric heat source according to Goldak model, can be generated. The method was implemented and investigated with the simulation software SimWeld developed by the ISF at RWTH Aachen University.

Transient numerical analysis of airflow and heat transfer in a vehicle cabin during heating period

This paper describes a three-dimensional transient numerical analysis to determine the airflow and heat transfer characteristics inside the vehicle cabin during a heating period. A three-dimensional vehicle cabin including glazed surfaces and pertinent physical and thermal properties of the enclosure was modelled by using the real dimensions of a car. A virtual manikin was added to the model of the vehicle cabin to determine the heat transfer between the human body and the vehicle cabin ambient. The analysis takes into account all the possible thermal load of the cabin. Governing equations were solved by an implicit, time marching finite volume numerical scheme under varying inlet boundary conditions of the vehicle air conditioning system. The results of numerical calculations were in good agreement with the available experimental and theoretical data in the literature.

Evaluation of Heat Transfer Characteristics in an Automobile Cabin with a Virtual Manikin During Heating Period

In this study, a three-dimensional transient numerical analysis was performed inside the automobile cabin during heating period. A three-dimensional vehicle cabin including glazing surfaces was modeled by using the real dimensions of a car. A virtual manikin with real dimensions and physiological shape was added to the model of the vehicle cabin. It was assumed that the manikin surfaces were subjected to either constant heat flux or constant temperature. Three-dimensional fluid flow, temperature distribution, and heat transfer characteristics inside the cabin were calculated. Experimental measurements were also conducted. Comparisons of the results were presented and discussed. The results of numerical calculations were in good agreement with the experimental and theoretical data in the literature.

Coupling system dynamics and contact behaviour: Modelling bearings subjected to environmental induced vibrations and ‘false brinelling’ degradation

During the last decades the increase in power of mechanical systems and the demand for increasing service life leads mechanical components of a system to work in extreme conditions. Moreover, actual mechanical systems include surfaces in sliding contact that are subjected to wear if exposed to high vibration. In fact, the vibration of components in contact results in large oscillations of the local contact stresses, due to the local deformation of the components at the contact interfaces. To approach correctly tribological problems, the coupling between the scale of the mechanism (system dynamics) and the scale of the contact needs to be accounted for. This paper presents an analysis concerning the influence of the vibrations induced by aircraft engines on the contact stresses of rolling bearings of the bleed system valves. To study the wear, resulting from false brinelling at the contact surfaces between balls and races of the bearings, it is then necessary to determine the forces due to the system vibrations and acting at the bearing connections with the structure. In order to perform a numerical transient analysis of the system dynamics a nonlinear simplified model of the valve (mechanism scale) is developed. The model is validated by comparing the numerical results with experimental tests. The time behaviour of the global forces on the bearings, and the respective displacements between the contact surfaces, are then used as inputs for a finite element model of the bearings (contact scale). The model is used to calculate and analyze the behaviour in time of the local contact constraints between race and balls. This analysis, developed in the framework of a European project, is an example of the proposed general approach to contact problems, by coupling the analysis of the mechanism and contact scales.

Transient Natural Convection in Vertical Channels Symmetrically Heated at Uniform Heat Flux

A numerical transient analysis of natural convection in air between two vertical parallel plates, heated at uniform heat flux, is carried out. The problem is two-dimensional and laminar and the full Navier-Stokes and energy equations are employed. The control volume method is used to discretize the equations on a uniform grid. Results are given at different aspect ratio values and Rayleigh number values. The simulation allows detection of complex structures of the flow inside and outside the channel. Temperature profiles as a function of time show an overshoot and undershoot increase at the lowest aspect ratio and highest Rayleigh number. Inside the channel conductive and convective regimes as well as an inverse fluid motion are observed. Transient average Nusselt number presents oscillations before the steady-state.

Transient numerical analysis of a multi-layered porous heat exchanger including gas radiation effects

Thermal characteristics of a new type of multi-layered porous heat exchanger (PHE) are identified in the present study. This system works based on the energy conversion between gas enthalpy and thermal radiation. It has a five-layered structure consisting of two high temperature, two heat recovery and one low temperature sections. These sections are separated from each other by four quartz glass windows. In the two high temperature sections, the enthalpy of high temperature gas flow is converted to thermal radiation that is emitted toward the three adjacent layers where the low temperature air flows are effectively heated by the reverse conversion from thermal radiation into gas enthalpy. The gaseous radiation is also considered, such that in each section, a transient theoretical analysis is conducted for a one-dimensional system where convection, conduction and radiation take place simultaneously in both gas and solid phases. The coupled energy equations for the gas flows and porous layers based on the two-flux radiation model are solved numerically to identify the transient heat transfer characteristics of the system. It is shown that this type of porous heat exchanger has a very high efficiency especially when the porous layers have high optical thicknesses.

Numerical modeling of radiative heat transfer in Bridgman solidification of semi-transparent BaF 2 crystals

The radiative heat transfer during Bridgman solidification of semi-transparent barium fluoride (BaF 2) crystals is numerically investigated by using the commercial software FIDAP. This code uses the P-1 approximation for the participating media modeling. The thermal field and the solid–liquid interface shape are computed for an opaque melt-crystal sample, a semi-transparent grey sample and a semi-transparent non-grey sample. The transient numerical analysis of the latent heat influence on the interface deflection shows a significant effect on the interface shape. In the case of an opaque sample, this effect is huge because of the small thermal conductivity of the BaF 2 melt. The interface curvature is drastically reduced when the latent heat is taken into account, and the growth front, which has a convex shape, becomes flat when the growth rate increases. The latent heat effect is reduced in the case of the participating BaF 2 sample because the effective thermal conductivity of the melt is augmented by the internal radiative heat transfer. The internal radiative effect is small at low solidified fractions but becomes significant when the crystal length increases, leading to a more curved interface. When the growth rates are greater than a critical value, the interface becomes concave and a destabilization of the growth process can occur. These results are in agreement with previous experimental measurements of the interface curvature and analytical investigations of the factors affecting the interface deflection.

Numerical Investigation of Transient Natural Convection in a Horizontal Channel Heated from the Upper Wall

In this investigation a transient numerical analysis for laminar natural convection in air between two horizontal parallel plates, with the upper plate heated at uniform heat flux and the lower one adiabatic, is carried out by means of the finite-volume method. The model is assumed to be two-dimensional. The Navier-Stokes equations together with the continuity and energy equations are solved by a numerical scheme based on SIMPLE in an H-shaped domain. Results are presented in terms of air velocity and temperature profiles. Wall temperatures and the temporal behavior of several significant variables, such as the penetration length, are reported for different Rayleigh numbers and aspect ratio values. Correlations among penetration length, dimensionless time, Rayleigh number, and aspect ratio are evaluated for 102 ≤ Ra ≤ 104 and L/b = 5, 10, and 20. At steady state regime, correlations for average Nusselt numbers and maximum dimensionless wall temperature in terms of Rayleigh number and aspect ratio are given for 102 ≤ Ra ≤ 104 and 5 ≤ L/b ≤ 20.

FEM-based design of an induction motor's part winding to reduce the starting current

The goal of this research is designing the part winding of an induction motor in order to reduce the surge currents when starting. The analyzed motor is to be built for special compressor applications; therefore, the standard star-delta windings are inappropriate. Until recently, winding schemes have been chosen using different analytical and experiential criteria. In this paper, a design process is presented based on transient numerical analysis with the rotation of the rotor also taken into consideration. The developed numerical model enables a study of the motor's performance under arbitrary load conditions, and, thus, the entire torque characteristics of the motor can be predicted. Furthermore, the effects of eddy currents in the motor shaft material and their influence on the magnetic field and the motor's performance have been analyzed. The numerical results have been verified by laboratory measurements