Influence Of Geometrical Changes Research Articles

An Analytical Model for Almost Conformal Spherical Contact Problems: Application to Total Hip Arthroplasty with UHMWPE Liner

Due to its high relevance for designing ball joints in mechanical engineering and (artificial) hip joints in biomechanics, the almost conformal elastic contact between a sphere and a spherical cup represents an important contact problem of current research. As no closed-form analytical solution to the problem has been found to date, full computational methods such as the finite element method are needed for analysis. However, they often require incredibly long, unacceptable calculation times, making parameter studies hardly practicable. For this reason, approximate analytical and semi-analytical models are applied, capable of predicting quantities of interest with sufficient accuracy. In the present work, a very simple model based on a radially directed Winkler foundation is presented, which provides (approximate) closed-form analytical solutions for both the pressure distribution and the dependencies between macroscopic contact quantities such as normal force and indentation depth. To ensure an optimal mapping of a specific contact problem, only the foundation modulus must be defined in a suitable way. As an example, the proposed model has been successfully adapted to adequately simulate the frictionless normal contact for hard-on-soft hip implants. For this purpose, the foundation modulus was approximated with the aid of a finite element analysis instead of adopting it from already well-established models, as the latter produce clearly erroneous results for large liner thicknesses and large Poisson’s ratios. By a comparison with extensive parameter studies of finite element simulations, it is demonstrated that the proposed model provides acceptable results for all commonly used hard-on-soft hip implants. On this basis, the influence of geometrical changes of the femoral head and the acetabular cup on the maximum pressure as well as the half-contact angle is discussed, and consequences on the wear behavior are deduced.

Influence of Geometrical Changes in an Adiabatic Portion on the Heat Transfer Performance of a Two-Phase Closed Thermosiphon System

In this study, a modified non-uniform adiabatic section in a Two-Phase Closed Thermosiphon (TPCT) is proposed where the uniform section was replaced by convergent and divergent (C-D) sections. The heat transfer analysis was performed on the modified TPCT and their findings were compared with standard TPCT. The deionized water (DI) in the proportion of 30 vol% is filled in both the TPCTs. Further, the heat transfer performance analysis was carried out for three different orientations, such as 0°, 45° and 90°, and heat input was varied from 50 to 250 W. The effect of these geometrical changes and inclination angles on the heat transfer performance of both the TPCT were evaluated to compare the thermal resistance, wall temperature variation and heat transfer coefficient. The non-dimensional numbers such as Weber (WE), Bond (BO), Condensation (CO) and Kutateladze (KU) were investigated based on heat fluxes for both TPCTs. By introducing the convergent-divergent section nearer to the condenser, the pressure before and after the C-D section was increased and decreased. This enhances the heat transfer in the evaporator slightly up to 2% and 1.4% at horizontal and 45° orientation, respectively, in Non-Uniformed Adiabatic Section (NUAS) TPCT when compared to Uniformed Adiabatic Section (UAS) TPCT. The thermal resistance of NUAS TPCT was reduced by up to 4.5% relative to UAS TPCT in horizontal and 45°. The results of the non-dimensional number also confirmed that NUAS TPCT provided better performance by enhancing 2% more pool boiling characteristics, interaction forces and condensate returns. Several factors such as gravity assistance, fluid accumulation, pressure drop and thermal resistance exert an influence on the heat transfer performance of the proposed NUAS TPCT at various orientation angles. However, different type of cross-sectional variations subjected to orientation changes may also get influenced by several other parameters that in turn affect the heat transfer performance distinctly.

Comparative structural stiffness: Exploiting 3D-printing

The teaching of structural stiffness is one of the keystones of the undergraduate curriculum in mechanics and the strength of materials. Standard linear theory, going back to Hooke's law, has proven to be very successful in predicting the performance of elastic structures under load. Many courses in basic mechanics have a conventional laboratory component often involving a universal testing machine and extensometer. However, the advent of 3D printing presents an appealing pedagogical opportunity mid-way between theory and a formal lab experience. The material contained in this paper focuses on using the 3D printing of relatively simple, flexible cantilever structures. The relatively high resolution of modern 3D printers facilitates the production of slender (elastically deformable) structures, and thus provides an opportunity to exploit geometric parametric variations to enhance a practical understanding of fundamental mechanics concepts such as stiffness. This approach has proved successful in initial inclusion in both the classroom via demonstration models, as well as in the lab in which elementary facilities can be utilized to acquire data. The boundary conditions associated with a cantilever, and the application of a point force are especially simple to produce in practice, and provide an effective tactile demonstration of the influence of geometrical changes on the relation between force and deflection, i.e., stiffness.

A parametric model for studying the aorta hemodynamics by means of the computational fluid dynamics

Perturbed aorta hemodynamics, as for the carotid and the coronary artery, has been identified as potential predicting factor for cardiovascular diseases. In this study, we propose a parametric study based on the computational fluid dynamics with the aim of providing information regarding aortic disease. In particular, the blood flow inside a parametrized aortic arch is computed as a function of morphological changes of baseline aorta geometry. Flow patterns, wall shear stress, time average wall shear stress and oscillatory shear index were calculated during the cardiac cycle. The influence of geometrical changes on the hemodynamics and on these variables was evaluated. The results suggest that the distance between inflow and aortic arch and the angle between aortic arch and descending trunk are the most influencing parameters regarding the WSS-related indices while the effect of the inlet diameter seems limited. In particular, an increase of the aforementioned distance produces a reduction of the spatial distribution of the higher values of the time average wall shear stress and of the oscillatory shear index independently on the other two parameters while an increase of the angle produce an opposite effect. Moreover, as expected, the analysis of the wall shear stress descriptors suggests that the inlet diameter influences only the flow intensity. As conclusion, the proposed parametric study can be used to evaluate the aorta hemodynamics and could be also applied in the future, for analyzing pathological cases and virtual situations, such as pre- and/or post-operative cardiovascular surgical states that present enhanced changes in the aorta morphology yet promoting important variations on the considered indexes.

Influence of Geometric Changes in the Thoracic Aorta due to Arterial Switch Operations on the Wall Shear Stress Distribution.

Background:The transposition of the great arteries (TGA) is one of the most severe congenital heart diseases. The arterial switch operation (ASO) is the preferred procedure to treat TGA. Although numerous reports have shown good results after ASOs, some patients suffer from circulatory system problems following the procedure. One reason for problems post-ASO is the local changes in the curvature and torsion of the thoracic aorta.Objective:The influence of these geometric changes on the blood flow field needs to be investigated in detail to consider possible cardiovascular problems after an ASO.Method:In this study, we conduct blood flow simulations in the thoracic aorta post-ASO, evaluate geometric changes in the aorta due to the ASO in terms of curvature and torsion, and consider the effect of geometric changes on blood flow in the aorta.Results:It was found that a large curvature near the aortic root causes an increase in the maximal wall shear stress value in the middle systole. Moreover, a large torsion results in a circumferential change in the maximal wall shear stress region. It was also found that the maximal wall shear stress in the post-ASO models is significantly higher than that in the normal models. This indicates that the aortic aneurysm initiation risk for a post-ASO artery may be higher than that of a normal artery.Conclusion:To reduce the risk of initiating an aneurism, it is suggested that the curvature near the aortic root should be decreased during the ASO.

In silico evaluation of geometry variations with respect to the thermal spread during coagulation of egg white using bipolar vessel sealing instruments.

BackgroundBipolar vessel sealing is an efficient electrosurgical procedure for the occlusion of blood vessels particularly during minimally invasive surgery. Reliable knowledge of the thermal spread is crucial for a safe application of bipolar vessel sealing instruments when operating close to thermo-sensitive structures, such as nerves. The evolution of the thermal spread over time and space depends on a variety of parameters, such as the biological tissue, the energy applied to the tissue, and the geometry of the vessel sealing instrument. Mathematical modeling has proven useful for the prediction of the thermal spread. It is, thus, a promising tool for the systematic analysis of the influence of geometrical changes on the thermal spread.ResultsWe present an experimentally validated in silico study to evaluate the impact of geometry variations on the progression of chicken egg white coagulation and the final shape of coagulated egg white as an approximation of the temporal and spatial evolution of the thermal spread during bipolar vessel sealing. Egg white has similar thermal and electrical properties to human tissue, with the advantage being that the spatial and temporal evolution of the thermal spread can be visually gauged. The simulations were performed using a mathematical model based on the finite element analysis of chicken egg white. The progression of egg white coagulation was predicted for two different peak voltages and various electrode geometries. Starting with two planar electrodes, one electrode was gradually changed to adopt a wedge shape. These changes to the geometry showed a distinct influence on the progression of egg white coagulation in the simulations. The predictions were successfully validated using an experimental setup with two different electrodes representing the extreme geometries.DiscussionThe predicted spatial temperature distributions were experimentally validated for two geometries. Our simulation study shows that the geometry has a pronounced influence on the thermal spread and, thus, is a suitable parameter to reduce thermal damage. The in silico optimization of instrument designs is a suitable tool to accelerate the development of new vessel sealing instruments, with only a few promising designs having to be tested as prototypes.

Modeling and visualization of the activation wavefront propagation to improve understanding the QRS complex changes indicating left ventricular hypertrophy

Activation wavefront propagation was computed and visualized in a geometrical heart model for pathological cases of reduced velocity of propagation, left ventricular hypertrophy and their combination. Selected parameters of a multiple dipole equivalent heart generator were computed and compared for three heart geometries and several degrees and extents of reduction of propagation velocity.First, the influence of geometrical changes modeling the left ventricular hypertrophy at reference propagation velocity was compared with reduction of the propagation velocity in the reference heart geometry. Reduced propagation velocity yielded similar or greater changes of the magnitude of the (electrical) heart vector representing the activation wavefront than the geometrical changes. Observations of the wavefront propagation with reduced velocity revealed longer presence of a large extent of the wavefront during depolarization which resulted in increased magnitude of the heart vector. The duration of depolarization was significantly prolonged only when the propagation velocity was decreased to 25% of its normal value. Changes of the direction of the maximal heart vector were dependent on the position of the region where the propagation velocity was reduced.Then the combination of the left ventricular hypertrophy and reduced propagation velocity was studied. Such combination enhanced the enlargement of the electrical heart vector and significantly prolonged the duration of depolarization. The influence of reduced activation velocity on the observed parameters was greater than the effect of the enlargement of the left ventricular mass.The presented study showed that intramyocardial conduction disturbances might cause increase of the actual surface area of propagation wavefront leading to changes of the amplitudes of ECG signals comparable with the changes resulting from the left ventricular hypertrophy. Intramyocardial conduction disturbances, as well as the modeled 50% increase of the thickness of the left ventricular wall, did not cause prolongation of the QRS complex out of normal range. Considerable prolongation of the QRS complex duration was observed only for transmural slowing of the propagation velocity to 25% of its reference value in large ventricular areas or for combination of such slowing with the left ventricular hypertrophy.

Aerodynamic Structure and Development of Formula 1 Racing Car Wakes

For the modern Formula 1 racing car, the degradation in aerodynamic performance when following another car is well documented. The problem can be broken into two parts; firstly the wake flow generated by these vehicles and the subsequent interaction a following car has with this field. Previous research [1, 2 & 3] has focused upon investigating the later without completely characterizing the former. This paper seeks to address this deficiency with initial data from a newly commissioned 30% scale Formula One wind tunnel model built to the 2011 technical regulations. Experimentation was carried out in the Industrial Wind-Tunnel (IWT) at RMIT University. In the absence of a rolling road an elevated ground plane was implemented; the results obtained show good agreement with the limited published material available. Using a high frequency response, four-hole pressure probe the aft body flow was investigated at multiple downstream locations. Time-averaged velocity, turbulence and secondary flow vectors were plotted. These data were supplemented through flow visualization techniques to better understand the salient aerodynamic features. The influence of geometric changes to the aerodynamic devices upon the rear of the car (i.e. drag reduction system currently used within Formula 1) was also investigated.

Influencia de la geometría en la resistencia a la flexión de los engranajes plásticos

En el presente trabajo se analiza el tema relacionado con las modificaciones geométricas del perfil del diente de engranajes plásticos cilíndricos de dientes rectos mediante el método de los elementos finitos y su influencia en la resistencia a la flexión. El objetivo fundamental del estudio es, desarrollar un procedimiento para determinar la influencia de la asimetría y la corrección del dentado en la resistencia a la flexión de los dientes de los engranajes plásticos. En esta investigación se ha desarrollado el cálculo de los esfuerzos de flexión, mediante procedimientos matemáticos y el uso del software con el que se ha logrado incrementar la resistencia a la flexión debido a la corrección del dentado (para un factor de corrección de 0 a 0,7) y asimismo la determinación del factor de forma en los engranajes cilíndricos de dientes rectos asimétricos que constituye una innovación científica. Los resultados alcanzados satisfacen a los objetivos generales y específicos del estudio, con el cual se demuestra que las innovaciones contribuirán a una mejorar el desarrollo tecnológico de la industria y del país.

Arterial Stiffness Is Regulated by Nitric Oxide and Endothelium-Derived Hyperpolarizing Factor During Changes in Blood Flow in Humans

Cytochrome-derived epoxyeicosatrienoic acids may be important endothelium-derived hyperpolarizing factors, opening calcium-activated potassium channels, but their involvement in the regulation of arterial stiffness during changes in blood flow in humans is unknown. In healthy volunteers, we measured arterial pressure, radial artery diameter, wall thickness, and flow (NIUS02) during hand skin heating in the presence of saline or inhibitors of NO synthase (N(G)-monomethyl-L-arginine), calcium-activated potassium channels (tetraethylammonium), and cytochrome epoxygenases (fluconazole). Arterial compliance and elastic modulus were calculated and fitted as functions of midwall stress to suppress the confounding influence of geometric changes. Under saline infusion, heating induced an upward shift of the compliance-midwall stress curve and a downward shift of the modulus-midwall stress curve demonstrating a decrease in arterial tone and stiffness when blood flow increases. These shifts were reduced by N(G)-monomethyl-L-arginine and abolished by the combinations of N(G)-monomethyl-L-arginine+tetraethylammonium and N(G)-monomethyl arginine+fluconazole. In parallel, in isolated mice coronary arteries, fluconazole and tetraethylammonium reduced the relaxations to acetylcholine. However, fluconazole did not affect the relaxations to the openers of calcium-activated potassium channels of small- and intermediate-conductance NS309 and of large-conductance NS1619 excluding a direct effect on these channels. Moreover, tetraethylammonium reduced the relaxations to NS1619 but not to NS309, suggesting that the endothelium-derived hyperpolarizing factor involved mainly acts on large-conductance calcium-activated potassium channels. These results show in humans that, during flow variations, arterial stiffness is regulated by the endothelium through the release of both NO and cytochrome-related endothelium-derived hyperpolarizing factor.

Studies of the reactivity of carbon nanotubes towards selected alkali cations and chlorides based on the HSAB theory

Polycyclic curved aromatic fragments (C 24H 12) have been employed as models of SWNTs (( n,0), where n varies from 4 up to 30) in order to investigate the relation between the curvature and reactivity of the sidewalls. The flat coronene structure has been chosen as a molecular fragment topologically resembling the honeycomb lattice, and it is treated as the reference one. The intrinsic chemical reactivity in terms of the Fukui function, applying the principle of hard and soft acids and bases (HSAB) in the framework of the density functional theory (DFT), was investigated. In the current study we take into account the interactions of hard acids (Li +, Na +, and K +) and isoelectronic ions (F − and Cl −, hard bases), treated as the probe molecules, with the exterior surface of mentioned above structures. The linear correlations between the adsorbate–adsorbent interactions and the both total as well as local hardness and/or molecular energy potential for studied ions were observed (increasing reactivity upon increasing curvature). Obtained results illustrate that the HSAB parameters can be a good measure for the reactivity if the influence of geometrical changes is considered.

The HSAB principle as a means to interpret the reactivity of carbon nanotubes

Polycyclic curved aromatic fragments (C 24H 12) have been employed as models of the single-walled carbon nanotubes ( n,0), where n varies from 4 up to 30. Those structures were chosen on the basis of the analysis of the strain energy values calculated for the models possessing various sizes. The flat coronene structure has been chosen as a molecular fragment topologically resembling the honeycomb lattice in order to investigate the relation between the curvature and reactivity of the sidewalls of SWNTs. In the current study we took into account the interaction of CO and NH 2 (treated as probe molecules) with the exterior surface of nanotubes. Obtained results illustrate that both total as well as local hardness and/or molecular electrostatic potential (MEP) can be a good measure for the reactivity if the influence of geometrical changes is considered. The systematic theoretical studies also show that the calculated interaction energies of sorbed CO on those models are related to the both types of hardness. On the other hand, in the case of amidogen sorbed on the nanotube surface the correlation between the binding energy and MEP is visible. Those differences can be explained by various kinds of the adsorption mechanism, i.e. physical or chemical adsorption, respectively.

Are Octahedral Ruthenium(II/III) and Osmium(II/III) Complexes Always Low-Spin?

The influence of geometrical changes on the spin multiplicity of the ground states of the octahedral ruthenium(II/III) and osmium(II/III) complexes is investigated using the TD-DFT and MRCI methods. On the example of the [RuCl6]4- complex, we show that only after the optimisation of the molecular geometry in a solvent (using a polarised continuum model), which shortens the M-L bond lengths by ~0.2 Å, is the correct order of spin states obtained (i.e. a singlet is correctly predicted to be the ground state). On the contrary, in terms of the in vacuo optimised geometries of this negatively charged species, both the DFT and MRCI calculations predict a quintet ground state. This finding is further analysed by calculating the low- and high-spin potential energy curves corresponding to an elongation of the M-L distance, which makes it possible to predict the critical point at which the crossing of the two spin states occurs. Finally, it is complemented by the TD-DFT calculations of the lowest excited states in each spin multiplicity for a series of prototypical ligands. It is demonstrated that the calculated results correlate well with the known strengths of the ligand field. The two findings presented in this work are a small contribution to our understanding of the electronic structure and properties of the octahedral ruthenium(II/III) and osmium(II/III) complexes, which are relevant both in biomolecular and material sciences.

The influence of geometrical changes on the dose distribution after I-125 seed implantation of the prostate

Purpose After prostate implantation, dose calculation is usually based on a single imaging session, assuming no geometrical changes occur during the months of dose accumulation. In this study, the effect of changes in anatomy and implant geometry on the dose distribution was investigated. Materials and methods One day, 1 month and 3 1 2 months after seed implantation, a combined TRUS-CT scan was made of 13 patients. Based on these scans changes in dose rate distribution were determined in prostate, urethra and bladder and a ‘geometry corrected’ dose distribution was estimated. Results When based on the day-1 scan, parameters representing high dose volumes in prostate and urethra were largely underestimated: V150 of the prostate 18 ± 10% and V120 of the urethra 47 ± 32%. The dose to a 2 cm 3 hotspot in the bladder wall (D2cc), however, was overestimated by 31 ± 35%. Parameters based on scans 1 month post-implant or later were all within ±5% of geometry corrected values. Conclusion Values meant to indicate the adequacy of dose coverage of the prostate, V100 and D90, were not influenced by geometrical changes and were independent of the post-implant scan date. Other parameters representing high dose volumes changed strongly within the first month after implantation.

Optimal Plastic Design of Shallow Curved Beams with Step-wise Varying Cross Sections1*

The problem of designing minimum weight thin curved beams with piece-wise constant thickness is studied. The material of the beam is assumed to be rigid-plastic that obeys the yield condition and associated flow law. The influence of geometric changes that occur in the post-yield stage are taken into account. The weight minimization is performed under conditions where the maximum deflection of the beam coincides with the deflection of a reference beam of constant thickness. Necessary optimality conditions are derived with the aid of variational methods from optimal control theory. Numerical results are presented for shallow circular beams with two, three, and four steps in thickness.

Overtopping Formulas for Caisson Breakwaters with Nonbreaking 3D Waves

From the analysis of an extensive series of two-dimensional and three-dimensional hydraulic model tests on the overtopping response of various types of caisson breakwaters, general design formulas and graphs have been derived. Exponential relationships relate the mean discharge and the percentage of overtopping waves with the relative freeboard. The parametric influence of geometrical changes and of wave obliquity and multidirectionality is described by reduction factors referred to the case of simple vertical structure under long-crested head-on wave attack. Comparisons are made with the three-dimensional overtopping response of sloping structures. Overtopping volumes per wave have also been measured and their distribution fitted with a universal probability function. Even the effects of overtopping waves on pedestrians behind the crownwall were statistically evaluated to allow an upgrading of the existing criteria for the admissible rates over breakwaters.

Reactivity of fullerenes. Quantum-chemical descriptors versus curvature

The relation between the reactivity of a fullerene towards a nucleophile and the curvature of the fullerene surface, reflecting the hybridisation of the carbon atoms, is investigated. Model 3-21G calculations on distorted ethenes show that the electronic part of the molecular electrostatic potential, V el, might be a good measure for the reactivity of a double bond towards a nucleophile when only the influence of geometrical changes is considered (increasing reactivity upon increasing curvature). STO-3G calculations on the fullerenes C 60, C 70 and C 76 again indicate the quality of V el as a reactivity descriptor. Both the regioselectivity in C 60, C 70 and C 76 (in line with local curvature), and the relative reactivity of C 60 and C 70 calculated by V el are in agreement with experimental data. The local hardness for these systems shows a fair correlation with the global hardness calculated in a finite difference approximation.

Orientation planning in continuous path applications for wrist partitioned manipulators

The paper presents an algorithm of trajectory planning for wrist partitioned robotic manipulators. The desired translational trajectory is defined as a continuous function in Cartesian co-ordinates. The desired orientation is defined as a continuous function of the Frenet frame. Therefore, orientation is planned without influence of geometric changes along the translational path. The path is generated as a sequence of elementary motions. Each new position is defined as a function of the previous one. After having derived a position, the orientation is planned using a maximising process. The present algorithm avoids singularities, because there is no need for using the inverse kinematics.

Planning and control of robot orientational path

A novel geometric method for the planning and controlling the orientation of a robot's end-effector as it moves along a prescribed translational path is presented. To eliminate the influence of geometric changes to the path and to be able to predict the orientation, the proposed method plans rotational paths with respect to a natural local coordinate frame formed by three unit vectors, namely, the tangent, normal, and binormal vectors, defined by the geometry of the path. A motion control strategy is developed to maintain the desired rotational speed or acceleration of the end effector. Examples are presented which show the practicality of the method.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>