Actual Boundary Conditions Research Articles

Asymmetric Conduction in an Infinite Functionally Graded Cylinder: Two-Dimensional Exact Analytical Solution Under General Boundary Conditions

Abstract This paper focuses on using an analytical method to obtain an exact solution for a long cylindrical vessel made of functionally graded materials (FGMs). Heat conduction equations are assumed to be in both radial and circumferential directions. The conduction coefficients are considered as different power-law functions of the radius. The general linear boundary conditions are adopted to make the solution applicable to the full range of problems. The obtained solution is successfully validated. Through solving illustrative test examples, the effects of material constants and boundary conditions on temperature distribution are studied. The obtained formulation can be utilized for tailoring of FGM based on the actual sophisticated thermal boundary conditions in the production process. The current analytical findings can help to manage the temperature distribution in FGMs which is an essential parameter in controlling the thermal stresses.

Evaluation and Optimization of the Effective Parameters on the Shield TBM Performance: Torque and Thrust—Using Discrete Element Method (DEM)

Cutterhead torque and thrust are the two main designing parameters of the shield TBM, which have to be evaluated and optimized, based on the interaction between the TBM and excavated material. This study employs the discrete element method (DEM) to simulate the tunneling procedure of the line-7 of Tehran underground urban train tunnel utilizing the micro mechanical parameters calculated from back analysis on direct shear tests. The resultant torque and thrust are then compared against the actual parameters and then with those estimated from the numerical analyses. This result shows that DEM is able to estimate the TBM torque and thrust applying actual boundary conditions and material properties for the model. Furthermore, the impact of ground properties, overburden height, linear and radial velocity on cutterhead torque and thrust and its performance are evaluated and optimized.

Determining the mass-related reaction effectiveness factor of large, nonspherical fuel particles for bridging between intrinsic and apparent combustion kinetics

The utilization of challenging solid fuels in the energy industry is urged by environmental requirements. The combustion kinetics of these fuel particles differs markedly from that of pulverized coal, mainly because of their larger sizes, irregular (nonspherical) shapes, and versatile internal pore structures. Although the intrinsic reaction kinetic measurements on very small amounts of finely ground samples of these particles are mostly available, a bridge toward their apparent reaction modeling is not evident. In this study, a method is introduced to build this bridge, the goodness of which was proved on the example of an industrially relevant biofuel. To do this, the results of a macroscopic combustion measurement with real samples in a well-modelable environment have to be used, and for considering some not negligible effects, 3D CFD modeling of the experimental environment is also to be applied. The outcome is the mass-related reaction effectiveness factor as a function of the rate of conversion. This variable can be considered as the active fraction of the entire particle mass on its periphery, and it can be used as the crucial element in modeling the combustion process of the same particle under other circumstances by including the actual boundary conditions. Another advantage of this method is its covering inherently the entire combustion process (water and volatile release, and char combustion) and also its applicability for reactors utilizing bigger particles like fluidized bed combustors.

Post-test simulations for the NACIE-UP benchmark by STH codes

This paper illustrates the results obtained in the last phase of the NACIE-UP benchmark activity foreseen inside the EU SESAME Project. The purpose of this research activity, performed by system thermal–hydraulic (STH) codes, is finalized to the improvement, development and validation of existing STH codes for Heavy Liquid Metal (HLM) systems. All the participants improved their modelling of the NACIE-UP facility, respect to the initial blind simulation phase, adopting the actual experimental boundary conditions and reducing as much as possible sources of uncertainty in their numerical model. Four different STH codes were employed by the participants to the benchmark to model the NACIE-UP facility, namely: CATHARE for ENEA, ATHLET for GRS, RELAP5-3D© for the “Sapienza” University of Rome and RELAP5/Mod3.3(modified) for the University of Pisa. Three reference tests foreseen in the NACIE-UP benchmark and carried out at ENEA Brasimone Research Centre were analysed from four participants.The data from the post-test analyses, performed independently by the participant using different STH codes, were compared together and with the available experimental results and critically discussed.

EFFECT OF TRANSIENT THERMAL GRADIENT ON STRESSES IN COMPOSITE BRIDGES

Composite steel-concrete bridges experience higher thermally induced stresses than their concrete and steel cousins. These thermal stresses which can result from support restraints, debris accumulating in expansion joints, or from non-uniform thermal gradients, can lead to significant damage in the concrete deck. Conventional heat transfer theory in solids, in three-dimensional finite element formulation, is used to perform a sequentially coupled thermal-stress analysis in a selected single-span case study bridge. Actual environmental boundary conditions for a selected geographical region are used to develop the thermal profile. The vertical thermal gradient is shown to be largely non-linear as opposed to existing models, such as AASHTO. The thermally induced tensile stresses in the concrete are shown to be significant compared to service load stresses and constitute 60% of the tensile strength of the concrete deck.

A simple thermal diffusivity measurement technique for polymers and particulate composites

Polymers and particulate composites are being increasingly used in heat insulation and dissipation applications. Accurate measurement of their thermal diffusivity is critical for reliable thermal designs. Commercially available methods, developed for homogeneous materials, rely on localized heating of small specimens. The localized area may not truly represent the bulk in heterogeneous materials like particulate composites. This paper presents a simple inexpensive technique to measure the thermal diffusivity of polymers and particulate composites. An analytical solution for the temperature distribution in a slab is obtained incorporating actual boundary conditions employing modified Angstrom’s method. From the resulting implicit relationships and temperature measurements at three easily accessible surfaces, thermal diffusivity is estimated. Proposed method circumvents prominent issues faced like heat loss due to imperfect contact by employing non-contact heating/cooling. This avoids special specimen preparations such as use of graphite coating and gold sputtering. Estimated thermal diffusivity for four polymeric solids and two particulate composites compare well with values from Laser flash method and literature.

ОПРЕДЕЛЕНИЕ ГРАНИЧНЫХ УСЛОВИЙ ДЛЯ РАСЧЁТА ТЕРМОНАПРЯЖЕННОГО СОСТОЯНИЯ ПОРШНЯ

The paper deals with the study of the influence of the working process parameters of the two-stroke opposed piston engine like D100 (20.7/2×25.4), especially the heat exchange between the working substance and the wall of the combustion chamber (CC) – cylinder and pistons on temperature and stress-strain state of the piston. To make an estimation of the effect of a working process on the boundary condition we considered the internal heat balance and specific features of gas dynamic loading of main parts of the cylinder-piston group. To calculate the temperature fields, the actual boundary conditions of non-stationary thermal loading were replaced with the equivalent steady-state ones, obtained from the condition that the amount of heat perceived by the piston surface in real and conditionally equivalent processes are equal. Equivalent parameters of heat transfer are calculated by the condition of conservation of the amount of heat passing through the walls of the CS. It was performed the validation of the calculation of equivalent heat exchange parameters. It is shown that in case of an error in specifying the initial conditions, for example, temperature per 100K, the temperature of the piston CC surface may change by 5K in the first 5 operating cycles. It is shown that the developed model of the workflow can be adjusted according to the available experimental data and used to model the boundary conditions. The authors made corrections to the dependence obtained by prof. Rosenblit, to determine the current heat transfer coefficient from the working fluid to the walls of the CC by the total heat removal for the cycle, equal to 20%. It was obtained the average coefficient of heat transfers from the working fluid to the piston and the temperature of the cycle for the nominal mode, which are 3500 W/(m2•K) and 835 K respectively. It was carried out the simulation of the thermal properties of the gap between the piston ring and the groove filled with combustion products. It is shown that the conditions of heat transfer through annular grooves and rings require clarification in modeling, which is associated with the conditions of heat transfer in the gaps, and the gap can be replaced by a gasket with appropriate thermal properties.

A Cross-Regulated Closed-Loop Control for Hybrid L-Z Source Inverter

Recently, switched inductor-impedance source inverter (L-ZSI) based hybrid converter was reported as a potential candidate for the dual output (dc and ac) applications. This converter has two operating modes based on inductor current nature: 1) continuous current mode (CCM); and 2) discontinuous current mode (DCM). However, the converter performs well only during CCM. To get rid of problems present in hybrid L-ZSI, modified hybrid L-ZSI (MHLZSI) was reported. To demarcate the CCM and DCM, boundary condition was derived based on the average inductor current. However, the actual boundary condition is dependent on the combined effect of inductor current ripple and average inductor current. In this paper, theoretical analysis is presented by taking into account the effect of inductor current ripple. Furthermore, it is proved that MHLZSI is capable of achieving higher gain when operated in DCM. In addition, a closed-loop control for MHLZSI is designed. State space analysis is presented to derive the output to control transfer function for both dc and ac loads. Both the controllers are designed based on PI control to ensure cross regulation. Theoretical analysis is validated by experimental analysis.

New Analytical Method to Evaluate Casing Integrity during Hydraulic Fracturing of Shale Gas Wells

An accurate analysis of casing stress distribution and its variation regularities present several challenges during hydraulic fracturing of shale gas wells. In this paper, a new analytical mechanical‐thermal coupling method was provided to evaluate casing stress. For this new method, the casing, cement sheath, and formation (CCF) system was divided into three parts such as initial stress field, wellbore disturbance field, and thermal stress field to simulate the processes of drilling, casing, cementing, and fracturing. The analytical results reached a good agreement with a numerical approach and were in‐line with the actual boundary condition of shale gas wells. Based on this new model, the parametric sensitivity analyses of casing stress such as mechanical and geometry properties, operation parameters, and geostress were conducted during multifracturing. Conclusions were drawn from the comparison between new and existing models. The results indicated that the existing model underestimated casing stress under the conditions of the geostress heterogeneity index at the range of 0.5–2.25, the fracturing pressure larger than 25 MPa, and a formation with large elastic modulus or small Poisson’s ratio. The casing stress increased dramatically with the increase of in situ stress nonuniformity degree. The stress decreased first and then increased with the increase of fracturing pressure. Thicker casing, higher fluid temperature, and cement sheath with small modulus, large Poisson’s ratio, and thinner wall were effective to decrease the casing stress. This new method was able to accurately predict casing stress, which can become an alternative approach of casing integrity evaluation for shale gas wells.

Effect Of The Engine Speed And Loading On The Heat Transfer Within Exhaust Valves

The aim of the present paper is to investigate numerically the heat transfer within exhaust valves by considering the actual boundary conditions provided from an internal combustion engine at differentload and speed. For this purpose, the valve is subdivided into seven adequate subdivisions to better assess the effect of each engine parts, therefore, an average value of the transient heat transfer coefficient (HTC) and the adiabatic wall temperature (AWT) for each subdivisions are evaluated during one cycle. These two parameters are introduced as boundary condition in a FEM model. The simulations are done at diverse engine regime, and therefore, the trend of the real boundary condition in term of HTC and AWT are given versus engine speed at different load. The findings show that the HTC increases linearly with engine speed however, the AWT decrease slightly at partial load and increase in the case of full engine load. The obtained model is used to highlight the temperature map, which will certainly help to avoid any damage to the exhaust valve.

Characteristics of the wake formation and force distribution of the bistable flow on two cylinders side-by-side

This numerical study with experimental support examines the bistable phenomenon along two parallel smooth cylinders submitted to a uniform turbulent cross-flow. The spacing ratio P/D of the cylinders is 1.26, being D the cylinder diameter and P the distance between centers. The computational domain and the experimental tests section are equivalents. The Reynolds number based on the free mean velocity and cylinder diameter was Re = 2.2 × 104. For the numerical analysis, the Navier–Stokes equations were solved using finite volumes method for a URANS simulation (unsteady Reynolds averaged Navier–Stokes) with the kω-SST-SAS (shear stress tensor-scale adaptive simulation) turbulence model. Measurements were performed by means of hot wire anemometry technique, firstly to determine actual boundary conditions of flow velocity and turbulence intensity at the test section entrance, secondly to obtain velocity and velocity fluctuation at the wake of the cylinders in different positions along the cylinder height to corroborate the numerical results. By means of the numerical results of velocity, the process of the wake formation is described, showing that the bistability does not occur simultaneously along the cylinder, corroborated by a delay on the switching process of the simultaneously measured velocities and the cross-correlation function. This effect appears also in the results of cylinder wall pressure distribution as well as in lift and drag coefficients.

Out-of-plane structural identification of a masonry infill wall inside beam-column RC frames

Field observations after seismic events have shown that out-of-plane collapses are one of the predominant modes of infill masonry wall failure leading to life-safety hazards. This type of mechanism depends on the geometrical and mechanical features of the structure, but it is substantially affected by the constraints along the structural frame. However, the actual boundary conditions are very difficult to define, especially in the case of an existing building, for which information is lacking, a visual screening can be inefficient, and deterioration may have occurred due to the design conditions.In this paper, a procedure for the structural identification of the out-of-plane behaviour of infill masonry walls using a simple and cost effective innovative procedure based on an in situ experimental dynamic test and a consequent process of updating is proposed. The procedure, composed of the in situ test and the updating of the numerical model, was implemented for a case study to confirm that it is effective in defining the mechanical characteristics of the masonry, the absence of collaboration between the two brick leaves of the infill wall, and the out-of-plane constraints along the perimeter.

Slender Roof Structures - Failure Reviews and a Qualitative Survey of Experienced Structural Engineers

Many slender roof structures have collapsed due to snow loading and instability. Although accurate stability calculations can be performed using theoretical models, these calculations may not always reflect the behaviour of real structures as a result of the uncertainties relating to e.g. loading, material behaviour, geometry, initial imperfections and the actual boundary conditions of the structure. As a result, the approach to stability design requires subjective decisions by the structural engineer concerning loading and modelling assumptions. This paper investigates the significance of these types of decisions made by structural engineers in designing slender roof structures. This investigation is based on a review of previous failure studies together with a survey of 17 experienced structural engineers. The results of this investigation indicate that most structural failures are a result of human errors; thus, a suitable strategy for avoiding errors is through quality control and design checking. Moreover, a significant discrepancy was observed concerning design assumptions made by the engineers in the study. Some of these assumptions, e.g. a non-conservative choice of the buckling length of a beam, have a significant negative impact to structural safety. It is therefore recommended that the structural engineers involved in the design of a structure have adequate experience and a holistic mindset. Another recommendation is that both drawings and design calculations are thoroughly reviewed before construction. Also, temporary bracing to be used during construction should be included in design. Finally, it is important that the communication between different partners in the design of a structure is satisfying.

Structural static and modal frequency simulations in a coffee harvester’s chassis

ABSTRACT Coffee harvesters are subject to stresses and vibrations in their structure, originating from engines and harvesting system. These structures must be designed to avoid rupturing of the components due to fragility of the materials, inadequate geometries, or the phenomenon of resonance, which increases the displacements/deformations of the components. In this scenario, the main objective of this study is to analyse the results of stresses and displacements from two static simulations and to present results of natural vibration frequencies from two modal simulations in a self-propelled coffee harvester. For this purpose, 20 modal shapes were generated, considering coffee harvester reservoir as empty and full. The simulations were carried out using the finite element method in which actual boundary conditions were applied to the motor chassis of the harvester. As results, stresses above the material’s yield strength were observed in some regions of the components. Greater displacements were observed at the rear of the chassis, suggesting a new positioning of the rear wheel to obtain a uniform load distribution. Some natural frequencies, which presented greater displacements/deformations, can be affected by the operation of the main motor, also associated with the vibrating system in the coffee harvest, which may cause rupture of components.

Boundary Conditions Accuracy Effect on the Numerical Simulations of the Thermal Performance of Building Elements

Numerical simulation is widely used in the field of computational building physics for the definition of the thermal performance of building elements. An integral component of numerical simulation using finite elements is the boundary conditions, which, in the case of simulating the thermal performance of a building element, are usually expressed in terms of the external surface temperature as a function of time. The purpose of this study is to examine the effect of the accuracy of the boundary conditions on the thermal performance simulation of building elements. The assumption that the temperature versus time is a sinusoidal function, applied in standard methods, is comparatively assessed with the actual function for diverse climatic conditions using finite elements simulation. The findings of the analysis indicate that the sinusoidal function fails to accurately simulate real boundary conditions. The originality of this study lies within the adoption of a signal reconstruction algorithm, which follows a novel approach by reconstructing the actual temperature versus time signal for the simulation of the actual boundary conditions.

Modal density of rectangular structures in a wide frequency range

A novel approach to investigate the modal density of a rectangular structure in a wide frequency range is presented. First, the modal density is derived, in the whole frequency range of interest, on the basis of sound transmission through the infinite counterpart of the structure; then, it is corrected by means of the low-frequency modal behavior of the structure, taking into account actual size and boundary conditions. A statistical analysis reveals the connection between the modal density of the structure and the transmission of sound through its thickness. A transfer matrix approach is used to compute the required acoustic parameters, making it possible to deal with structures having arbitrary stratifications of different layers. A finite element method is applied on coarse grids to derive the first few eigenfrequencies required to correct the modal density. Both the transfer matrix approach and the coarse grids involved in the finite element analysis grant high efficiency. Comparison with alternative formulations demonstrates the effectiveness of the proposed methodology.

Numerical Investigation of Timber Log‐Haus Walls with Steel Dovetail Reinforcements under In‐Plane Seismic Loads

The paper investigates the structural response and vulnerability of timber log‐haus walls under in‐plane seismic loads. Careful consideration is given, in particular, to the structural efficiency of additional metal fasteners introduced within the thickness of traditional timber log‐walls. Log‐haus systems are in fact typically obtained by stacking multiple logs and generally used for residential or commercial buildings up to two levels. Their seismic characterization, however, still requires further investigations and studies, since current standards for timber structures (i.e., Eurocode 5 and Eurocode 8) do not provide specific recommendations for their seismic design. In this regard, the so‐called “steel dovetail profiles” investigated in this paper are aimed to improve the in‐plane stiffness and ultimate resistance of traditional log‐walls. In particular, taking advantage of past experiments carried out on small‐scale joint specimens, as well as past 3D numerical efforts for the same structural typology (unreinforced assemblies), full 3D solid models are described in ABAQUS to assess the potential of steel dovetail profiles, as well as to capture possible issues. Numerical simulations are proposed both for small‐scale specimens and full log‐haus assemblies, being representative of the actual loading and boundary conditions for log‐haus walls as part of a real building. As such, the effects of key input parameters and main influencing aspects are emphasized. In conclusion, it is shown that the examined reinforcing technique can offer rather stable initial stiffness and ultimate resistance increase, compared to unreinforced assemblies. At the same time, several aspects should be taken into account to properly optimize their benefits.

A modified model for spontaneous imbibition of wetting phase into fractal porous media

Fractal theory has been applied to characterize complex pore structure of sedimentary rocks and to model fluid flow through porous media. In recent years, several analytical models have been developed to simulate spontaneous imbibition of wetting phase into porous media with fractal geometry. In previous studies, spontaneous imbibition of wetting phase is considered as a piston-like displacement phenomenon which leads to similar rates of imbibition in capillaries with different diameters. However, imbibition rate of wetting phase is positively proportional to the square root of pore diameter based on the Lucas-Washburn equation. Therefore, we should expect faster imbibition of wetting phase in larger capillaries compared with smaller capillaries.In this paper, we extend the previous model by assuming non-piston-like imbibition in capillaries with different diameters. In our model, first the larger capillaries are filled by wetting phase, followed by smaller capillaries. We develop an analytical equation without considering hydrostatic pressure and a semi-analytical equation with considering hydrostatic pressure. We can apply actual boundary conditions in the proposed model, while the previous piston-like models are only valid before the wetting phase reaches top of the porous media in co-current spontaneous imbibition test. Moreover, the proposed model can be used to determine the minimum pore diameter of rock sample according to the equilibrium time in spontaneous imbibition tests.

The in-plane effective length of members in aluminum alloy reticulated shell with gusset joints

This paper presents an analytical research on the in-plane stability of members in single-layer reticulated latticed shells with aluminum alloy gusset (AAG) joints. The influence of two kinds of actual boundary conditions are considered, namely, the stiffness of the AAG joint (k1) and restraint of adjacent members (k2). In order to explore the influence of k1, an experimental program including 5 member specimens with AAG joints is introduced, and finite element (FE) models are established and verified. Based on the test and FE results, the effective length of the specimens are obtained. Subsequently, theoretical analysis is conducted to investigate the effect of k2, with accounting for the stiffness reduction caused by axial compressive force. Finally, the effective length factors of members in reticulated shell with AAG joints are derived and tabulated. Numerical examples are presented to illustrate the necessity of considering the influence of k1 and k2, and verify the good accuracy of the given effective length factors.

Boundary layer and fundamental problems of hydrodynamics (compatibility of a logarithmic velocity profile in a turbulent boundary layer with the experience values)

The compatibility of the semiempirical turbulence theory of L. Prandtl with the actual flow pattern in a turbulent boundary layer is considered in this article, and the final calculation results of the boundary layer is analyzed based on the mentioned theory. It shows that accepted additional conditions and relationships, which integrate the differential equation of L. Prandtl, associating the turbulent stresses in the boundary layer with the transverse velocity gradient, are fulfilled only in the near-wall region where the mentioned equation loses meaning and are inconsistent with the physical meaning on the main part of integration. It is noted that an introduced concept about the presence of a laminar sublayer between the wall and the turbulent boundary layer is the way of making of a physical meaning to the logarithmic velocity profile, and can be defined as adjustment of the actual flow to the formula that is inconsistent with the actual boundary conditions. It shows that coincidence of the experimental data with the actual logarithmic profile is obtained as a result of the use of not particular physical value, as an argument, but function of this value.