Realistic Heterogeneous Model Research Articles

Pore-scale natural convection in cavities filled with randomly distributed heat-conducting particles

Three-dimensional numerical simulations were carried out to analyze natural convection in a differentially heated cubic cavity filled with spherical particles. Three models were considered: a fluid-only case for reference, a homogeneous porous media approximation, and a heterogeneous model that considered the actual structure of the solid particles. DEM (discrete element method) simulations were used to generate a realistic particle distribution for the construction of volumetric meshes for the heterogeneous model. The impact of thermal conduction through the solid particles was evaluated by varying the solid to fluid ratio ( K * ) within the range 0.02 − 50 for Rayleigh numbers between 103 and 105. Additionally, different number and diameter of the particles inside the cavity were evaluated. The results of the homogeneous approximation indicate a continuous decrease in the Nusselt number (Nu) as K * increases, associated with a higher effective thermal conductivity and the pressure drop. However, the more realistic heterogeneous model predicted the opposite trend. For K * <1, solid particles acted as thermal insulators, impeding fluid flow, while for K * > 1 conduction heat transfer between the isothermal walls and the particles was highly efficient and the particles surface area acted like extensions of the isothermal walls, facilitating heat transfer to the fluid phase, especially at lower Rayleigh values.

Optimizing GPR time‐zero adjustment and two‐way travel time wavelet measurements using a realistic three‐dimensional numerical model

ABSTRACTTime‐zero adjustment or the true ground surface for ground penetrating radar (GPR) applications is a very important aspect and an essential factor in order to position subsurface targets, especially those located at shallow depths, at their true position in depth. As the transmitted and received signals from GPR antennas are affected by the presence of different materials with varying electromagnetic properties, adjusting the time zero appropriately is important, but often not straightforward to position accurately. This paper uses a realistic three‐dimensional numerical model of a GPR transducer in order to examine where is the best location for time zero on a GPR trace. It is shown that, in order to establish a robust and consistent time‐zero position, careful consideration is also needed around how the two‐way travel time of the reflected GPR wavelet is estimated. Starting with a simple homogeneous model with a set of different targets, a better process of time‐zero adjustment and time picking of the GPR wavelets is put forward, that is verified using further more complex and realistic heterogeneous models. Further verification is obtained by using experimental data.

Kinetics and reactor modelling of fatty acid epoxidation in the presence of heterogeneous catalyst

Epoxidized fatty acids and fatty acid esters are used as chemical intermediates and environmentally friendly bio-lubricants. The epoxidation is carried out by the Prileschajew reaction, which implies the use of hydrogen peroxide as oxidation agent and a percarboxylic acid as the oxygen carrier. The epoxidation process is slow in the two-phase system (aqueous phase and oil phase), but it can be considerably enhanced by incorporating a heterogeneous acid catalyst. The results for the epoxidation of a model compound, oleic acid were presented. A cation-exchange resin (Amberlite IR-120) was used as the heterogeneous catalyst and acetic acid was the reaction carrier. Experimental data on oleic acid epoxidation were obtained at 40–50 °C under conventional heating and microwave irradiation. The experimental set was focused on studying the catalytic effect of the solid resin. The kinetic results revealed that the presence of the heterogeneous catalyst completely masked the enhancing effect of microwave radiation. An extensive set of kinetic data obtained from a laboratory-scale loop reactor was modelled with a realistic heterogeneous model taking into account the co-existence of three phases in the chemically complex reaction system. Rate equations were presented for all the reaction steps involved: the perhydrolysis, epoxidation and ring-opening steps. The rate equations were included in the three-phase reactor model and the kinetic parameters of the system were determined by non-linear regression analysis.

Updated breast CT dose coefficients (DgNCT ) using patient-derived breast shapes and heterogeneous fibroglandular distributions.

The purpose of this work was to generate uncompressed heterogeneous breast phantom models using size-dependent fibroglandular distributions derived from a large cohort of breast CT (bCT) datasets, and to compare differences in normalized glandular dose coefficients for bCT "DgNCT " when the realistic heterogeneous model is considered relative to the simple, homogeneous model used in the past. A cohort of 274 segmented bCT datasets were used to quantify the fibroglandular tissue distribution within the breast parenchyma. Each dataset was interpolated to an isotropic voxel size of 0.25mm and the breast center-of-mass was aligned for all coronal slices. Each aligned dataset was converted into two binarized volumes representing voxels containing only glandular tissue "G(x,y,z)" and voxels containing glandular or adipose tissue "AG(x,y,z)". The datasets were classified by volume in accordance with previously reported size-dependent, breast-shaped phantoms. Within the five groups - each containing on average 55 datasets, all G(x,y,z) and AG(x,y,z) volumes were summed separately representing the cumulative distribution of glandular tissue or breast parenchyma (glandular+adipose), respectively. G(x,y,z) was divided by AG(x,y,z) on a voxel-by-voxel basis resulting in a glandular fraction "GF(x,y,z)" distribution for each phantom size. The GF(x,y,z) distributions were used to construct heterogeneous mathematical phantoms for the small, median, and large breast sizes with a 1.5mm skin thickness - based on previously reported measurements from bCT, and a 5mm skin thickness for comparison with outdated assumptions of skin thickness. A subset of 15 bCT datasets from the cohort (five for each breast size) were used to construct voxelized patient models for validation of the heterogeneous phantom models. Monte Carlo techniques were used to estimate monoenergetic DgN(E)C T values for photon energies from 9 to 70keV (in 1keV intervals) using the mathematical phantoms composed of either heterogeneous or homogeneous breast parenchyma. Polyenergetic (pDgNCT ) coefficients were determined by weighting the DgN(E)C T values by x-ray spectra tuned to the beam characteristic of breast CT. Dose coefficients were compared between the two breast compositions for each volume class, breast density, and skin thickness. For photon energies ≲45keV, the homogeneous model overestimates DgN(E) values relative to the realistic heterogeneous model sorted into five volume classes. The 5mm skin thickness underestimates DgN(E) values relative to the realistic 1.5mm thickness for lower energies and the differences diminish up to 70keV. Averaged across all phantom sizes the homogeneous model overestimates pDgNCT by 5.7% and 23.3% for the 60kV W/Cu and 49kV W/Al spectra, respectively. The heterogeneous model was also found to be in agreement with the voxelized bCT patient models with pDgNCT differences less than 2.3% and 5.2% for the 60kV W/Cu and 49kV W/Al spectra, respectively, across all phantom sizes. Anatomically accurate heterogeneous phantom models were developed using bCT image-derived fibroglandular tissue distributions. These new models improve the accuracy of bCT dosimetry, and in conjunction with previous models for mammography, may help in providing a more universally accepted breast dosimetry model.

SU-GG-T-37: Monte Carlo Dose Comparison Assessing Material Inhomogeneity Effects in Breast Brachytherapy

Purpose: To quantify the effect of material heterogeneity on dosimetry in single‐entry breast brachytherapy devices such as the MammoSite, Contura, and SAVI in comparison to the assumed homogeneity of the TG‐43 formalism, with an emphasis on devices with a close proximity to the skin. Method and Materials: A simple phantom was constructed in the Penelope Monte Carlo code consisting of a sphere of varying size and position situated within a larger cube surrounded by air. The cube was always composed of water while the sphere could be water or air in order to compare a more realistic heterogeneous model with the homogenous model used in the TG‐43 formalism. Spacing between the sphere and the cube surface was brought as close as 7mm to test the clinical limits of the SAVI device as pertains to skin spacing. Seeds were represented as point sources with a single band or energy at 365 keV (approximating an iridium 192 source) and simulated to be either at the air‐water interface or in the center of the sphere. Results: Simulations indicated that dose to the water — in the heterogeneous case for >2mm from the air‐water interface — was different by <10% maximum and ∼3% on average, from dose calculated for the homogenous case. At <2mm from the air‐water interface, however, the difference in dose for the heterogeneous case was ∼30% of the homogenous case on average and could be as high as 230% for close (7mm) spacing. Conclusions: Although the dose differs between the air‐water and water‐water interfaces, the discrepancy was limited to 1–2mm from the interface and could be reduces in some situation. Further exploration needs to be done to assess the actual dose being given as opposed to what has been calculated for all brachytherapy devices with air‐tissue interfaces.

Microwave Power Absorption in a Iviultilayered Cylindrical Model of Man Near a Flat Reflector

There are usually objects in the vicinity when a person is exposed to electromagnetic waves. It is exceedingly difficult to calculate the whole-body average specific absorption rate (SAR) in the microwave frequency region using a realistic heterogeneous model of man. In this paper, we use a multi-layered cylinder model to numerically examine the average SAR of an adult standing near a flat reflector exposed to microwave energy. We also offer a comparison with existing data from a realistic model for an E-polarized wave below 600MHz.

Chaotic ray behaviour in regional seismology

SUMMARY Rays propagating through strongly laterally varying media exhibit chaotic behaviour. This means that initially close rays diverge exponentially, rather than according to a power law. This chaotic behaviour is especially pronounced if the medium contains laterally varying interfaces. By studying simple 2-D and 3-D versions of models with laterally varying interfaces, the importance of chaotic ray behaviour is determined. A model of the Moho below Germany produces sharp variations with epicentral distance of the number of arrivals. In addition, the number of caustics grows dramatically: up to 1200 caustics are present between a distance of 0 and 800 km. Using the theory of Hamiltonian systems, a more in-depth study of the chaotic character of the ray equations is obtained. It is found that for realistic heterogeneous models most of the relevant rays will exhibit chaotic behaviour. The degree of chaos is quantified in terms of predictability horizons. Beyond the predictability horizons ray tracing cannot be carried out accurately. For the models under consideration, the length from the source to the predictability horizon has an order of magnitude of 1000 km. The chaotic behaviour of the rays makes it necessary to use extensions of asymptotic ray theory, such as Maslov theory, to compute seismic waveforms. It is shown that pseudo-caustics, an important obstacle in computing Maslov synthetics, are a generic feature of the 2-D laterally varying models that are studied. Eventually, the use of asymptotic methods is restricted because of the inaccuracy in the computation of the ray paths.

Electromagnetic absorption in the human head and neck for mobile telephones at 835 and 1900 MHz

The authors have used the finite-difference time-domain method and a new millimeter-resolution anatomically based model of the human to study electromagnetic energy coupled to the head due to mobile telephones at 835 and 1900 MHz. Assuming reduced dimensions characteristic of today's mobile telephones, the authors have obtained SAR distributions for two different lengths of monopole antennas of lengths /spl lambda//4 and 3/spl lambda//8 for a model of the adult male and reduced-scale models of 10- and 5-year-old children and find that peak one-voxel and 1-g SARs are larger for the smaller models of children, particularly at 835 MHz. Also, a larger in-depth penetration of absorbed energy for these smaller models is obtained. The authors have also studied the effect of using the widely disparate tissue properties reported in the literature and of using homogeneous instead of the anatomically realistic heterogeneous models on the SAR distributions. Homogeneous models are shown to grossly overestimate both the peak 1-voxel and 1-g SARs. Last, the authors show that it is possible to use truncated one-half or one-third models of the human head with negligible errors in the calculated SAR distributions. This simplification will allow considerable savings in computer memory and computation times.

Comparison of homogeneous and heterogeneous tissue models for coil optimization in neural stimulation

In stimulation of peripheral nerves by strong pulsed magnetic fields, it is desirable to produce a well‐focused activating function. Previous analyses and an experiment have indicated that quadruple coils provide many advantages. An analytical solution has been obtained for a homogeneous cylindrical volume conductor, and a finite difference numerical solution for a realistic heterogeneous model of a human forearm. In both models it has been assumed that nerves are straight and long, and therefore the activating function is an induced field derivative in the neuron direction. The magnitudes of the activating function for various coil geometries are compared for the two models.

Finite-difference time-domain calculations of SAR in a realistic heterogeneous model of the head for plane-wave exposure from 600 MHz to 3 GHz

Presents finite-difference time-domain (FDTD) calculations of the specific energy absorption rate (SAR) in a fine-scaled, heterogeneous, realistic model of the head for frequencies ranging from 600 MHz to 3 GHz. The phantom has been derived from an atlas of cross-sectional anatomy. The cell size is 3.2 mm which results in a 120000 cell head model comprising brain, bone/fat, muscle, skin, blood, air and eye humour, lens, and sclera. Irradiation from the front and side for plane-wave exposure of an adult and an infant are considered.