Sensitivity analysis of predicted night cooling performance to internal convective heat transfer modelling

The potential of night cooling, a passive cooling technique of growing interest, is typically investigated by numerical means. In particular multi-zone energy simulation is currently appraised for building design. Unfortunately, in addition to the inaccurate approximation of an ideally mixed room, the implemented empirical convective heat transfer coefficients (CHTCs) only apply to specific flow regimes - forcing to use arbitrary correlations and, thus, possibly limiting the usefulness of the simulation results. Therefore, the authors of this paper investigate the sensitivity of the night cooling performance to convection algorithms. First, the authors examine the applicability of convection correlations for real building enclosures, extracted from literature. Subsequently, simulations of a night cooled office room during summertime of a moderate climate (Belgium) are carried out in TRNSYS, using different convection correlations in addition to varying design parameters. The results show that the choice of the convection algorithm strongly affects the energy and thermal comfort predictions. More importantly, the convection algorithm is of the same importance as the design parameters - making an exact definition of the CHTC crucial. Therefore, additional research by experiments or airflow codes, based on fluid dynamics, is regarded necessary.     

FinFET for high sensitivity ion and biological sensing applications

A double-gate (DG) fin field effect transistor (FinFET) is discussed as new label-free ion and biological sensor. Simulations as function of channel doping, geometrical dimensions, operation point and materials investigated the device response to an external potential difference which provides a body threshold voltage modulation. The simulation results presented in this work clearly state the key features for an ultrasensitive FET based sensor: an enhancement low doped and partially gated transistor operating in weak-moderate inversion regime. The optimized sensitivity, obtained when the width of the fin is equal to the gate height (w_NW ∼ h_g), reaches a value of 85% for an extraction current, I_d, of 0.1 μA. These results pave the way for the fabrication process of an innovative CMOS compatible sensing system.     

High-resolution imaging of dielectric profiles by using a time-domain ultra wideband radar sensor

This paper describes the development of an imaging instrument that capitalizes on high-resolution phenomenon in inverse scattering using a time-domain ultra wideband (UWB) sensor. The image reconstruction algorithm that accounts for the band-limited view of the UWB data is based upon the TM-mode wave equation, the Born approximation, and the adjoint method for computing the Fréchet derivatives. The computation of the sensitivity function requires the forward propagation of the UWB wavefield, as well as the reverse propagation of the residual wavefield. The electromagnetic and adjoint fields are calculated using the finite-difference time-domain (FDTD) method, implementing the first and second orders Mur’s absorbing boundaries. The overall performance of the instrumentation system is demonstrated using computer simulations and experimental measurements. Results indicate that the equipment can reconstruct fairly complicated dielectric profiles at near millimeter resolution even with the presence of large amount of noise.     

Deformation characteristics at contact interface in ring compression

The main objective of this study is to provide the deformation characteristics in detail in ring compression, especially at the tool/workpiece interface. The surface flow patterns at the contact boundary are analyzed in terms of surface expansion, surface expansion velocity, pressure distributions exerted on the die surface, relative sliding velocity between die and workpiece, and the sliding distance along the die surface. Movement of neutral positions and folding phenomenon is also been investigated to see its effect on the flow pattern, that is geometrical change, which is important to measure the frictional condition at the interface using calibration curves.     

杆件失效时张拉整体结构的敏感性分析

张拉整体结构含有大量的杆件,具有高度的不确定性。然而,大量杆件对结构整体性是至关重要的,任何杆件的破坏都能严重降低结构强度。当这些杆件突然破坏时,其承受的荷载动态地传给整个结构,导致更严重的破坏。考虑几何和材料非线性,对杆件逐渐及突然破坏时的张拉整体结构的敏感性进行数值研究,参数还包括:自应力水平、支柱长细比及结构阻尼比。本研究结果给这种系统的设计提供了建议。     

压裂改造复合页岩气藏不稳定压力与产量分析方法

为了给页岩气现场开发提供理论依据,考虑页岩气扩散、黏性流、解吸等多种传质机理,建立了复合页岩气藏的综合流动数学模型。基质中考虑浓度差引起的非稳态流动,内外区裂缝中考虑达西流动,水力压裂主裂缝考虑为无限导流;引入了新的无因次量,在椭圆坐标系下综合运用拉氏变换、Mathieu函数、Stehfest数值反演等方法对数学模型进行了解析求解;分析了定产量条件下不稳定压力和定井底压力条件下产量的变化特征,基于不稳定压力曲线将页岩气流动划分为7个流动阶段,即井筒储集阶段、过渡流阶段、早期线性流阶段、基质向裂缝窜流阶段、早期径向流动阶段、第一径向流与第二径向流的过渡阶段、第二径向流动阶段,为复合页岩气藏生产动态分析提供了理论基础。研究结果表明:增大改造区域半径和渗透率可以提高页岩气产量;扩散系数越大、兰格缪尔体积和兰格缪尔压力越大,页岩气产量越大,气藏初始压力高对页岩气的开发具有积极的影响。结论认为,所建立的综合流动数学模型丰富了页岩气多级压裂水平井开发分析方法。     

Transverse Dispersion Caused by Secondary Flow in Curved Channels

A new theoretical equation is proposed to describe the streamwise variations of the transverse velocity along a curved channel with a constant curvature. Furthermore, based on this theoretical equation for the transverse velocity, a new equation for the transverse dispersion coefficient is developed to incorporate the effect of the secondary flow on the transverse dispersion in curved channels. The new equations for the transverse velocity and dispersion coefficient are verified with experimental data sets that were obtained from laboratory experiments conducted in two different channels. The results show that the proposed velocity equation properly describes the streamwise variations of the secondary flow developed in the curved channels. The reach-averaged values of the transverse dispersion coefficient calculated by the new equation are in relatively good agreement with the observed values from the laboratory channels. Sensitivity analysis reveals that both the secondary flow and the transverse dispersion coefficient are proportional to the roughness factor, and in inverse proportion to the aspect ratio of the channel.     

Evaluation of Undrained Shear Strength Using Full-Flow Penetrometers

Full-flow penetrometers (the T-bar and ball) are increasingly used on sites with thick deposits of soft clays, particularly prevalent offshore. Full-flow penetration tests were performed at five international well-characterized soft clay test sites to assess the use of full-flow penetrometers to estimate undrained shear strength. Field vane shear data were used as the reference undrained strength. Statistical analyses of strength factors indicates that full-flow penetrometers provide an estimate of undrained shear strength at a similar level of reliability compared to the piezocone. Relationships for estimating the strength factor and soil sensitivity using only full-flow penetrometer data obtained during initial penetration and extraction are developed. A strong dependence of the strength factor on sensitivity was identified and can be used for the estimation of undrained strength. The effectiveness and use of the developed correlations are demonstrated through their application at an additional test site.     

Acid-Gas Storage in a Deep Saline Aquifer: Numerical Sensitivity Study to Evaluate Parameter and Model Uncertainty

A modeling study is conducted to evaluate the sensitivity of acid gas evolution in a deep saline aquifer to expected variations of multiple geologic and engineering variables. Relative permeability hysteresis, aquifer heterogeneity variance, formation water salinity, permeability of caprock and leakage wells, injection rate, regional hydraulic gradient, and formation depth are evaluated as uncertain input parameters to a three-dimensional synthetic aquifer model with fully heterogeneous permeability. To understand the effect of conceptual model uncertainty on predicting gas flow and storage, permeability of the heterogeneous model is upscaled to equivalent permeability for three increasingly homogenized stratigraphic models: an eight-unit facies model, a three-unit depositional model, and a one-unit formation model. Two upscaling methods are used: a flow-based numerical method and an analytical averaging method. Over 120?years (20?years of injection and 100?years of monitoring), multiphase compositional simulation is conducted to model gas migration and trapping in the aquifer and its dissolution in the formation brine. Results suggest that among the variables evaluated, gas-relative permeability hysteresis, heterogeneity variance, and injection rate have the most significant impact predicting the total mobile gas in the storage system, whereas caprock permeability is the most important factor influencing the prediction of total gas leakage and thus the storage security. Over the simulation time scale, for the fixed amount of gas injected, regional hydraulic gradient, salinity, and formation depth have lesser impact on gas flow and storage predictions. Further, leakage through abandoned wells can occur when permeability of the wellbore is as low as 1?mdarcy (md), while caprock permeability becomes critical to storage security when it is more than 1×10-4??md, in which case significant leakage occurs during the monitoring phase. Compared to the predictions of the heterogeneous model, the greater the number of stratigraphic units in the upscaled models, the better its accuracy in predicting gas storage and plume sweep. However, the accuracy of the stratigraphic models depends on aquifer variance, upscaling method, and type of prediction outcome that is being evaluated.