In-house Finite Element Method Research Articles

Extraction of Thermal Properties of Organic Ablative Materials Using Molecular Dynamics Simulations

In this study, molecular dynamics method is used for extracting thermal properties of materials relevant to the ablative type of thermal protection system. The methodology and results are discussed for extracting thermal conductivity and specific heat capacity of ablative materials for their virgin and char states in a manner that is defined in the experimental data sets. To select a suitable nonreactive force field, Green–Kubo simulations are performed over phenol, toluene, and benzene materials using Amber, Dreiding, and all-atom optimized potentials for liquid simulations (OPLSAA) force fields. Next, crosslinked and non-crosslinked polymers are created, and pyrolysis simulations are performed over them using ReaxFF to obtain virgin and char states of the polymers. Thermal conductivity is extracted using the Green–Kubo method, whereas specific heat capacity is extracted by using the enthalpy gradient obtained by performing isobaric simulations over virgin and char materials. Later, by introducing the concept of an artificial composite material, the extracted thermal properties are used in macroscale one-dimensional heat conduction simulations with pyrolysis using an in-house finite element method (FEM)-based thermal response solver.

Influence of Boundary Conditions on Numerical Homogenization of High Performance Concrete.

Concrete is the most widely used construction material nowadays. We are concerned with the computational modelling and laboratory testing of high-performance concrete (HPC). The idea of HPC is to enhance the functionality and sustainability of normal concrete, especially by its greater ductility as well as higher compressive, tensile, and flexural strengths. In this paper, the influence of three types (linear displacement, uniform traction, and periodic) of boundary conditions used in numerical homogenization on the calculated values of HPC properties is determined and compared with experimental data. We take into account the softening behavior of HPC due to the development of damage (micro-cracks), which finally leads to failure. The results of numerical simulations of the HPC samples were obtained by using the Abaqus package that we supplemented with our in-house finite element method (FEM) computer programs written in Python and the homogenization toolbox Homtools. This has allowed us to better account for the nonlinear response of concrete. In studying the microstructure of HPC, we considered a two-dimensional representative volume element using the finite element method. Because of the random character of the arrangement of concrete’s components, we utilized a stochastic method to generate the representative volume element (RVE) structure. Different constitutive models were used for the components of HPC: quartz sand—linear elastic, steel fibers—ideal elastic-plastic, and cement matrix—concrete damage plasticity. The numerical results obtained are compared with our own experimental data and those from the literature, and a good agreement can be observed.

Numerical investigation on fluid forces of piggyback circular cylinders in tandem arrangement at low Reynolds numbers

Numerical simulations of flows past the piggyback circular cylinders in tandem arrangement are performed by solving the variational multiscale formulation of the incompressible Navier–Stokes equations using in-house finite element method (FEM) codes. The effects of the gap-spacing-to-diameter (G/D) and the two diameter ratio (d/D) on the flow characteristics and the reductions of the root-mean-square (RMS) drag and lift coefficients are considered for Reynolds numbers (Res) are 100 and 200. The validation shows the fluid force coefficients obtained by the in-house FEM codes are in good agreement with the results in the existing literatures. The obtained results show that, with a proper placement of the smaller cylinder ( $$d/D=0.2$$ ) behind the larger cylinder, the RMS drag and lift coefficients largely decrease compared to those of the single circular cylinder. When $$d/D=0.2$$ , the largest reductions of the RMS lift coefficient of the larger cylinder and the RMS total lift coefficient appear at $$G/D=1.2$$ as Re $$=100$$ and at $$G/D=1.0$$ as Re $$=200$$ . It is observed that the proper placement of the smaller cylinder causes the surrounding vorticity to take opposite sign with the vorticity in the outer region so as to suppress and postpone the vortex shedding in the wake, and that the different positions of the vortex shedding at two Res cause that the largest reductions of the RMS lift coefficient of the larger cylinder and the RMS total lift coefficient appear at different G/D as Re is different. When d/D varies, the variation of the RMS total lift coefficient behaves differently at two Res. It decreases with d/D increasing at Re $$=100$$ , while it no longer monotonously varies with d/D, but reaches a minimum in the considered range of d/D at Re $$=200$$ . Moreover, the larger d/D results in stronger suppression and postponement of the vortex shedding in the wake. The effects of the gap-spacing-to-diameter and the two-diameter ratio on the flow characteristics and the reductions of the root-mean-square (RMS) drag and lift coefficients are investigated by stabilized finite element method. With a proper placement of the smaller cylinder behind the larger cylinder, the RMS drag and lift coefficients are smaller than those of the single circular cylinder. The proper placement of the smaller cylinder causes the surrounding vorticity to take opposite sign with the vorticity in the outer region so as to suppress and postpone the vortex shedding in the wake.

Effect of Cylinder Arrangement on Fluid Flow and Heat Transfer Characteristics past Four Elliptic Cylinders

Fluid flow and heat transfer characteristics past four elliptic cylinders arranged in square configuration are numerically investigated in this study. An in-house finite element method code based on streamline upwind Petrov - Galerkin algorithm has been used for simulations of two-dimensional, incompressible and laminar flow of air. Analyses are carried out by changing geometric parameters such as spacing ratio, axis ratio and incident angle. Results in terms of vorticity and temperature contours, Strouhal number, drag and lift coefficients and Nusselt number are studied for all cases. These results show that fluid flow patterns are strongly influenced by geometric parameters, based on which flow can be categorized into different types. A critical spacing ratio is found for some of the configurations, where a steep increase in drag coefficient is encountered. From this study, it is observed that incident angle 22.5 degree gives a higher heat transfer rate compared to the other two configurations (0 and 45 degree). Also, elliptic cylinders (axis ratio of 0.8 and 0.5) have similar Nusselt number value with less drag force compared to circular cylinders (axis ratio of 1.0) which justifies the utilization of elliptic cylinders as an alternative for circular ones.

Temperature Uniformity in Cross-Flow Double-Layered Microchannel Heat Sinks

An in-house finite element method (FEM) procedure is used to carry out a numerical study on the thermal behavior of cross-flow double-layered microchannel heat sinks with an unequal number of microchannels in the two layers. The thermal performance is compared with those yielded by other more conventional flow configurations. It is shown that if properly designed, i.e., with several microchannels in the top layer smaller than that in the bottom layer, cross-flow double-layered microchannel heat sinks can provide an acceptable thermal resistance and a reasonably good temperature uniformity of the heated base with a header design that is much simpler than that required by the counter-flow arrangement.

Deformation and damage of nuclear power station fuel elements under cyclic loading

Deformation and damage of nuclear power station fuel element shells under irradiation and cyclic loading due vibrations are studied. Constitutive equations include dependencies for a creep-damage equation with a scalar damage parameter, as well as terms for thermal and irradiation creep strains, elastic, thermal, and swelling strains. The acceleration of the creep-damage process due to cyclic variation of internal pressure is considered with a dynamic creep model, for which constitutive equations are derived using the method of asymptotic expansions and averaging over a period of cyclic loading. Stress and strain states in the fuel element shell are determined by use of an in-house finite element method creep-damage code for shells of revolution. Results show the essential variation in the initially symmetric fuel element shell form, as well as the acceleration of creep-damage processes due to the cyclic pressure.

Quasi-Static Thermal Modeling of Multiscale Sliding Contact for Unlubricated Brush Seal Materials

Prediction of contact temperature between two materials in high-speed rubbing contact is essential to model wear during unlubricated contact. Conventionally, assumptions of either a steady or an annular heat source are used for slow and high speed rotation, respectively. In this paper, a rotating heating source is solved using an in-house finite element method code. This captures the full geometry and rotating speed of the rubbing bodies. Transient heat transfer is modeled quasi-statically, eliminating the need for a transient 3D simulation. This model is shown to be suitable for contact temperature prediction over a wide range of rotating speeds, anisotropic thermal conductivity, and nonuniform thermal boundary conditions. The model calculates heat partition accurately for a thin rotating disk and short pin combination, which cannot be predicted using the existing analytical solutions. The method is validated against ansys mechanical and experimental infrared thermography. Results demonstrate that the annular source assumption significantly underpredicts contact temperature, especially at the rubbing interface. Explicit modeling of a thin disk results in higher heat partition coefficients compared with the commonplace semi-infinite length assumption on both static and rotating components. The thermal anisotropy of tuft-on-disk configurations is evaluated and compared to a uniform pin-on-disk configuration. Despite the effective thermal conductivity (ETC) in the bristle tuft being approximately 1 order of magnitude lower than along the bristle length (treating the bristle pack as a porous medium), its impact on heat partition and contact temperature is shown to be limited.

Creep effects on crack initiation and propagation in reinforced concrete walls

The present paper presents an analysis of creep cracking on base-constrained reinforced concrete walls. Crack initiation and propagation in reinforced concrete walls due to creep are simulated using an in-house finite element method developed by the authors. The original model was extended and modified to account for creep strain in the numerical modelling. Crack widths and pattern, and degree of restraint are analysed and compared without creep. Concrete is modelled as nonlinear material, and smeared crack approach, tension stiffening, shear retention and stress degradation of concrete are included in the model. It was found that creep influences the time of first cracking in reinforced concrete walls restrained at the base; creep causes to delay crack initiation and propagation. It was also found that the cracks in the base-restrained wall were found to be the most numerous in the base level and they decrease with distance above the base.

Modeling framework for free edge effects in laminates under thermo-mechanical loading

Abstract In this paper, a novel Quasi-2D (Q-2D) plane strain formulation for predicting interlaminar stresses in multi-directional laminates is developed and implemented within the finite element method framework. In particular, analyses of free edge stresses in composite laminates subjected to uniform axial and/or thermal loading are conducted. The Q-2D modeling approach presented here is validated by comparing the predicted interlaminar stresses with corresponding 3D models and previously published data. Computational time required for determining the interlaminar stresses using Q-2D model is approximately 30 times lower than the 3D analysis of the same laminate. Also, the Q-2D model is implemented within a commercially available software by modifying a 3D model to behave like a 2D model. This is particularly advantageous over developing an in-house finite element method code to capture the complex free edge stress states in multi-directional composites. Hence, the current framework can potentially be used as a computational tool for efficiently predicting the interlaminar stresses in different laminates subjected to thermo-mechanical loading, which can assist in determining interlaminar regions susceptible to free edge delamination.

Variation of Quench Propagation Velocities in YBCO Cables

We show by modelling that the quench propagation velocity is not constant in HTS coils but it changes during the quench. Due to the large temperature margin between the operation and the current sharing temperatures, the normal zone does not propagate with the temperature front. This means that the temperature will rise in a considerably larger volume when compared to the quenched volume. Thus, the evolution of the temperature distribution below current sharing temperature T c s after the quench onset affects the normal zone propagation velocity in HTS more than in LTS coils. This can be seen as an acceleration of the quench propagation velocities while the quench evolves when margin to T c s is high. In this paper, we scrutinize quench propagation in a stack of YBCO cables with an in-house finite element method software which solves the heat diffusion equation. We compute the longitudinal and transverse normal zone propagation velocities at various distances from the hot spot to demonstrate the distance-variation of these velocities. According to the results in our particular simulation case, the longitudinal normal zone propagation velocity is 30 % higher far away from the quench origin compared to its immediate vicinity when T op=4.2 K and T c s =15 K.

Predicting shot peening coverage using multiphase computational fluid dynamics simulations

A three-dimensional non-steady state shot peening simulation platform for coverage prediction is developed based on commercial Computational Fluid Dynamics (CFD) ANSYS-FLUENT software. To simulate the air–peen flow, the airflow is treated as a continuum phase governed by the Reynolds-averaged Navier–Stokes conservation equations, while the peen is taken as a discrete phase in which the trajectories of the peens are tracked individually. Two-way air–peen interactions are solved by exchanging mutual momentum during the peening process. User Defined Functions (UDFs) for particle loading distribution in the nozzle, particle dispersion, particle rebound, and nozzle movement are developed and incorporated into ANSYS-FLUENT. A numerical procedure based on the Eulerian–Lagrangian approaches is used in the solution. The air–peen simulation results based on the above model and numerical procedure is found to be in excellent agreement with the literature experimental data. For coverage prediction, both single dimple area and multiple impact coverage models are developed and validated by both experimental data and in-house Finite Element Method (FEM) simulation result. The obtained simulation results for coverage are in good agreement with existing experimental data. In addition, the simulations for two other configurations, that is, round-flat and airfoil NACA0012 specimens, are also performed to demonstrate the capability of the simulation platform. The present work shows that the multiphase shot peening simulation platform developed here can be used to accurately predict air–peen flow and coverage and to further optimize shot peening processes under real operating conditions.

Design and optimization of RF ICs with embedded linear macromodels of multiport MEMS devices

In this paper, we demonstrate efficient modeling approach for simulation, analysis, design, and optimization of multiport radio frequency microelectromechanical systems (RF MEMS) resonating structures embedded in RF circuits. An in-house finite element method (FEM) solver is utilized to develop accurate and efficient macromodels that capture all the essential characteristics of the device. Using the datasets generated from the FEM simulations, the artificial neural network models are trained for two-way mapping between the physical input and electrical output parameters. Realized model is implemented in a circuit simulator, enabling a simple yet accurate circuit simulator compatible modeling and optimization procedure instead of memory and time demanding FEM analysis. The derivation of dynamic macromodels with preserved electromechanical behavior of the multiport resonating structures is also presented. Capabilities of the proposed approach are demonstrated with several examples featuring capacitively actuated MEMS resonating structures: a clamped–clamped beam, a free–free beam, and a coupled clamped–clamped beam. © 2007 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2007.

A time integration method for simulations of tyre vehicles on flexible structures

The main objective was to perform simulations using a realistic model of a truck and trailer passing over a flexible bridge structure. A general multi-body dynamics program, DADS, and its tyre subroutines were interfaced with in-house finite element method (FEM) routines in order to create a vehicle/structure interaction computer program. Different time stepping algorithms from multi-body dynamics and FEM domains were integrated into a ‘common’ time stepping procedure. The bridge structure was simplified to a flat surface built up of shell finite elements. Tyre normal, lateral and longitudinal forces were applied to the bridge. Three driving cases were considered: straight forward driving with and without braking and a lane change manoeuvre. Braking moments were distributed to the wheel axles. The program ANSYS was used for post-processing of bridge stresses.

Influence of Delamination Growth and Contact Phenomena on the Compressive Behaviour of Composite Panels

A numerical analysis, providing a realistic insight on the compressive behaviour of composite panels with through-the-width delaminations, was performed by means of an in-house Finite Element Method (FEM) based code able to analyse the influence of delaminations' growth and contact phenomena on the load carrying capability of these kinds of structures. First of all a sensitivity analysis was carried out to underline the influence of delaminations' position along the thickness and their growth on panels' compressive behaviour. Then comparisons with experimental data available in bibliography were performed to test the effectiveness of new numerical techniques. Finally a panel with two delaminations was investigated to point out the need of a numerical tool able to take into account contacts between sublaminates in order to evaluate the real structure's response under compressive load.