When self-consistency makes a difference

Compound semiconductor power RF and microwave device modeling requires, in many cases, the use of selfconsistent electrothermal equivalent circuits. The slow thermal dynamics and the thermal nonlinearity should be accurately included in the model; otherwise, some response features subtly related to the detailed frequency behavior of the slow thermal dynamics would be inaccurately reproduced or completely distorted. In this contribution we show two examples, concerning current collapse in HBTs and modeling of IMPs in GaN HEMTs. Accurate thermal modeling is proved to be be made compatible with circuit-oriented CAD tools through a proper choice of system-level approximations; in the discussion we exploit a Wiener approach, but of course the strategy should be tailored to the specific problem under consideratio

Preferential Paths of Air-water Two-phase Flow in Porous Structures with Special Consideration of Channel Thickness Effects.

Accurate understanding and predicting the flow paths of immiscible two-phase flow in rocky porous structures are of critical importance for the evaluation of oil or gas recovery and prediction of rock slides caused by gas-liquid flow. A 2D phase field model was established for compressible air-water two-phase flow in heterogenous porous structures. The dynamic characteristics of air-water two-phase interface and preferential paths in porous structures were simulated. The factors affecting the path selection of two-phase flow in porous structures were analyzed. Transparent physical models of complex porous structures were prepared using 3D printing technology. Tracer dye was used to visually observe the flow characteristics and path selection in air-water two-phase displacement experiments. The experimental observations agree with the numerical results used to validate the accuracy of phase field model. The effects of channel thickness on the air-water two-phase flow behavior and paths in porous structures were also analyzed. The results indicate that thick channels can induce secondary air flow paths due to the increase in flow resistance; consequently, the flow distribution is different from that in narrow channels. This study provides a new reference for quantitatively analyzing multi-phase flow and predicting the preferential paths of immiscible fluids in porous structures

The nonlinear viscoelastic behavior of polypropylene

A series of tensile relaxation tests is performed on isotactic polypropylene in the sub-yield and post-yield regions at room temperature. Constitutive equations are derived for the time-dependent response of a semicrystalline polymer at isothermal loading with small strains. Adjustable parameters in the stress-strain relations are found by fitting experimental data. It is demonstrated that the growth of the longitudinal strain results in an increase in the relaxation rate in a small interval of strains in the sub-yield domain. When the strain exceeds some critical value which is substantially less than the apparent yield strain, the relaxation process becomes strain-independent.Comment: 20 pages, 6 figure

Computational Fluid Dynamics Methods for Gas Pipeline System Control

At the present level of development of long, branched gas transmission networks (GTN), solving the problems of improving safety, efficiency and environmental soundness of operation of industrial pipeline systems calls for the application of methods of numerical simulation. The development of automated devices for technical inspection and process control, and availability of high-performance computer hardware have created a solid technical basis to introduce numerical simulation methods into the industrial practice of GTN analysis and operation. One of the promising approaches for numerical analysis of GTN operating is the development and application of high-accuracy computational fluid dynamics (CFD) simulators of modes of gas mixture transmission through long, branched pipeline systems (CFD-simulator) (Seleznev, 2007). Actually, a CFD-simulator is a special-purpose software simulating, in online and real time modes with a high similarity and in sufficient detail, the physical processes of gas mixture transmission through a particular GTN. The development of a CFD-simulator focuses much attention to correctness of simulation of gas flows in the pipelines and to the impact produced by operation of relevant GTN gas pumping equipment (including gas compressor unit (GCU), valves, gas pressure reducers, etc.) and the environment upon the physical processes under study. From the standpoint of mathematical physics, a CFD-simulator performs numerical simulation of steady and transient, non-isothermal processes of a gas mixture flow in long, branched, multi-line, multi-section gas pipeline network. Such simulation is aimed at obtaining high-accuracy estimates of the actual distribution (over time and space) of fluid dynamics parameters for the full range of modes of gas mixture transmission through the specific GTN in normal and emergency conditions of its operation, as well as of the actual (temporal) distribution of main parameters of GTN equipment operation, which can be Document type: Part of book or chapter of boo

CFD simulation of highly transient flows

This thesis describes the fundamental extension and extensive testing of a robust CFD model for predicting outflow following the failure of pressurised hydrocarbon pipelines. The main thrust of the study involves the extension of the basic outflow model to account for complex pipeline systems, improvements of the theoretical basis and numerical stability. The basic model, based on the numerical solution of conservation equations using the method of characteristics, incorporates a suitable equation of state to deal with pipelines containing pressurised multi-component hydrocarbon mixtures. It utilises the homogeneous equilibrium flow (HEM) assumption, where the constituent phases in a two-phase mixture are assumed to be at thermal and mechanical equilibrium. The first part of the study focuses on the development of an outflow model to simulate the failure of multi-segment pipelines incorporating valves and fittings passing through terrains of different inclinations. In the absence of real data, the numerical accuracy of the model is assessed based on the calculation of a mass conservation index. The results of a case study involving the comparison of the simulated outflow data based on the failure of a multi-segment pipeline as opposed to an equivalent single segment pipeline containing gas, liquid or two-phase inventories are used to highlight the impact of pipeline complexity on the simulated data. The development and extensive testing of two models, namely the Hybrid Model and the Modified Homogeneous Equilibrium Model (MHEM) each addressing a principal limitation of the HEM are presented next. The Hybrid Model deals with the failure of the HEM in predicting post-depressurisation outflow for inclined pipelines containing two-phase mixtures or liquids through its coupling with a hydraulic flow model. The MHEM on the other hand addresses the failure of the HEM to accurately predict the discharge rates of flashing/ two-phase fluids discharging through an orifice. Finally, the dilemma of the appropriate choice of the size and duration of the numerical discritisation steps expressed in terms of the Courant, Friedrichs and Lewy (CFL) criterion on the stability and computational workload of the pipeline failure model is investigated for different classes of hydrocarbon inventories. These include gas, liquid (flashing and incompressible) and two-phase mixtures