85 research outputs found
The implication of the osteolysis threshold and interfacial gaps on periprosthetic osteolysis in cementless total hip replacement
No full textOsteolysis around joint replacements may develop due to migration of wear particles from the joint space into gaps between the interface bone and the implant where they can accumulate in high concentrations to cause tissue damage. Osteolysis may appear in various postoperative times and morphological shapes which can be generalized into linear and focal. However, there are no clear explanations on the causes of such variations. Patients’ degree of sensitivity to polyethylene particles (osteolysis thresholds), the local particle concentration and the access route provided by the interface gaps have been described as determining factors. To study their effects, a 2D computational fluid dynamics model of the hip joint capsule in communication with an interfacial gap and the surrounding bone was employed. Particles were presented using a discrete phase model (DPM). High capsular fluid pressure was considered as the driving force for particle migration. Simulations were run for different osteolysis thresholds ranging from 5×1085×108 to 1×10121×1012 particle number per gram of tissue and fibrous tissue generation in osteolytic lesion due to particles was simulated for the equivalent of ten postoperative years. In patients less sensitive to polyethylene particles (higher threshold), osteolysis may be linear and occur along an interfacial gap in less than 5% of the interfacial tissue. Focal osteolysis is more likely to develop in patients with higher sensitivity to polyethylene particles at distal regions to an interfacial gaps where up to 80% of the interfacial tissue may be replaced by fibrous tissue. In these patients, signs of osteolysis may also develop earlier (third postoperative year) than those with less sensitivity who may show very minor signs even after ten years. This study shows the importance of patient sensitivity to wear particles, the role of interfacial gaps in relation to morphology and the onset of osteolysis. Consequently, it may explain the clinically observed variation in osteolysis development
Efficient strategy for low nox combustor design in the spatial domain using multi-fidelity solutions
No full textParametric geometry exploration of the human carotid artery bifurcation
No full textA parametric computational model of the human carotid artery bifurcation is employed to demonstrate that it is only necessary to simulate approximately one-half of a single heart pulse when performing a global exploration of the relationships between shear stress and changes in geometry. Using design of experiments and surface fitting techniques, a landscape is generated that graphically depicts these multi-dimensional relationships. Consequently, whilst finely resolved, grid and pulse independent results are traditionally demanded by the computational fluid dynamics (CFD) community, this strategy demonstrates that it is possible to efficiently detect the relative impact of different geometry parameters, and to identify good and bad regions of the landscape by only simulating a fraction of a single pulse. Also, whereas in the past comparisons have been made between the distributions of appropriate shear stress metrics, such as average wall shear stress and oscillatory shear index, this strategy requires a figure of merit to compare different geometries. Here, an area-weighted integral of negative time-averaged shear stress, , is used as the principal objective function, although the discussion reveals that the extent as well as the intensity of reverse flow may be important. Five geometry parameters are considered: the sinus bulb width, the angles and the outflow diameters of the internal carotid artery (ICA) and external carotid artery (ECA). A survey of the landscape confirms that bulb shape has the dominant effect on with maximum occurring for large bulb widths. Also, it is shown that different sets of geometric parameters can produce low values of by either relatively small intense areas, or by larger areas of less intense reverse flow. <br/
The impact of imperfect frame deployment and rotational orientation on stress within the prosthetic leaflets during transcatheter aortic valve implantation
No full textTAVI devices are manufactured with cylindrical frames. However, the frames are rarely cylindrical post-deployment since deformation due to localised under expansion can be induced by calcified material on the native valve leaflets exerting irregular forces upon the frame. Consequently, the leaflets within a deformed TAVI device may undergo elevated stress during operation, which may lead to premature device failure.Using computational analysis a complete TAVI device model was simulated undergoing deployment into an aortic root model derived from CT data for a patient with severe calcific aortic stenosis, followed by a pressure simulated cardiac cycle. The complete analysis was performed eight times, each with the device at a different rotational orientation relative to the native valve, with an increment spacing of 15°.The TAVI device frames consistently featured significant distortions associated with bulky calcified material at the base of the non-coronary sinus. It was found that the average von Mises stress in the prosthetic valves was only increased in one of the cases relative to an idealised device. However, the maximum von Mises stress in the prosthetic valves was elevated in the majority of the cases.Furthermore, it was found that there were preferable orientations to deploy the prosthetic device, in this case, when the prosthetic leaflets were aligned with the native leaflets. As device orientation deviated from this orientation, the stresses in the valve increased because the distance between the prosthetic commissures decreased. This potentially could represent a sufficient increase in stress to induce variation in device lifespan.<br/
Geometry parameterization and multidisciplinary constrained optimization of coronary stents
No full textCoronary stents are tubular type scaffolds that are deployed, using an inflatable balloon on a catheter, most commonly to recover the lumen size of narrowed (diseased) arterial segments. A common differentiating factor between the numerous stents used in clinical practice today is their geometric design. An ideal stent should have high radial strength to provide good arterial support post-expansion, have high flexibility for easy manoeuvrability during deployment, cause minimal injury to the artery when being expanded and, for drug eluting stents, should provide adequate drug in the arterial tissue. Often, with any stent design, these objectives are in competition such that improvement in one objective is a result of trade-off in others. This study proposes a technique to parameterize stent geometry, by varying the shape of circumferential rings and the links, and assess performance by modelling the processes of balloon expansion and drug diffusion. Finite element analysis is used to expand each stent (through balloon inflation) into contact with a representative diseased coronary artery model, followed by a drug release simulation. Also, a separate model is constructed to measure stent flexibility. Since the computational simulation time for each design is very high (approximately 24 h), a Gaussian process modelling approach is used to analyse the design space corresponding to the proposed parameterization. Four objectives to assess recoil, stress distribution, drug distribution and flexibility are set up to perform optimization studies. In particular, single objective constrained optimization problems are set up to improve the design relative to the baseline geometry—i.e. to improve one objective without compromising the others. Improvements of 8, 6 and 15% are obtained individually for stress, drug and flexibility metrics, respectively. The relative influence of the design features on each objective is quantified in terms of main effects, thereby suggesting the design features which could be altered to improve stent performance. In particular, it is shown that large values of strut width combined with smaller axial lengths of circumferential rings are optimal in terms of minimizing average stresses and maximizing drug delivery. Furthermore, it is shown that a larger amplitude of the links with minimum curved regions is desirable for improved flexibility, average stresses and drug deliver
Red blood cell migration in microvessels
No full textRed blood cell (RBC) migration effects and RBC–plasma interactions occurring in microvessel blood flow have been investigated numerically using a shear-induced particle migration model. The mathematical model is based on the momentum and continuity equations for the suspension flow and a constitutive equation accounting for the effects of shear-induced RBC migration in concentrated suspensions. The model couples a non-Newtonian stress/shear rate relationship with a shear-induced migration model of the suspended particles in which the viscosity is dependent on the haematocrit and the shear rate (Quemada model). The focus of this paper is on the determination of the two phenomenological parameters, Kc and K?, in a diffusive flux model when using the non-Newtonian Quemada model and assuming deformable particles. Previous use of the diffusive flux model has assumed constant values for the diffusion coefficients which serve as tuning parameters in the phenomenological equation. Here, previous data [1 and 16] is used to develop a new model in which the diffusion coefficients depend upon the tube haematocrit and the dimensionless vessel radius for initially uniform suspensions. This model is validated through previous publications and close agreement is obtained
Design optimisation of coronary artery stent systems
No full textIn recent years, advances in computing power and computational methods have made it possible to perform detailed simulations of the coronary artery stenting procedure and of related virtual tests of performance (including fatigue resistance, corrosion and haemodynamic disturbance). Simultaneously, there has been a growth in systematic computational optimisation studies, largely exploiting the suitability of surrogate modelling methods to time-consuming simulations. To date, systematic optimisation has focussed on stent shape optimisation and has re-affirmed the complexity of the multi-disciplinary, multi-objective problem at hand. Also, surrogate modelling has predominantly involved the method of Kriging. Interestingly, though, optimisation tools, particularly those associated with Kriging, haven't been used as efficiently as they could have been. This has especially been the case with the way that Kriging predictor functions have been updated during the search for optimal designs. Nonetheless, the potential for future, carefully posed, optimisation strategies has been suitably demonstrated, as described in this review. </p
An evaluation of the RANS method for the prediction of steady ship rudder performance compared to wind tunnel measurements
No full textUsing Reynolds-Averaged Navier-Stokes (RANS) methods detailed investigations are carried out, focussing on different cases of steady rudder flow. During the introducing two-dimensional cases the hybrid meshing scheme is optimized for rudder use, and grid parameters are studied. Also the effect of solver settings and the capabilities of implemented turbulence models are investigated. Two different verification strategies are applied and compared - eventually concentrating on parametric grid independence studies, rather than using global systematic grid refinement.The three-dimensional investigation concentrates on the all-movable square tipped NACA0020 rudder tested within the large wind tunnel of the University of Southampton (3.5m x 2.5m). This case is modeled in free stream as well as within the wind tunnel, using the Spalart-Allmaras and the ?-? RNG turbulence models. The numerical prediction of the tunnel blocking effect is investigated. In particular, the tip vortex flow with its effect on the rudder performance is studied and how this is influenced by the numerical discretization. Here the span-wise distribution of the normal force coefficient is an informative tool. The three-dimensional grids consist of between 2 - 10 to the power of 6 to 4.5 - 10 to the power of 6 cells.Verification studies are carried out and results are validated against experimental data as far as available. Generally, the lift prediction turned out to show closer agreement with experimental benchmark (within 10%) than the drag, which used to deviate more (within 30%). The prediction of the tip vortex shows the expected characteristics. The tip peak in the span-wise normal force distribution is found to be captured well
VaMpy: A Python Package to Solve 1D Blood Flow Problems
No full textFinite-differences methods such as the Lax-Wendroff method (LW) are commonly used to solve 1D models of blood flow. These models solve for blood flow and lumen area and are useful in disease research, such as hypertension and atherosclerosis, where flow and pressure are good indicators for the presence of disease. Despite the popularity of the LW method to solve the blood flow equations, no implementation of a LW solver for these equations has been published and made publicly available. This leads to the reimplementation of the same methods within different research groups and makes verification of results more difficult. The Vascular Modelling in Python (VaMpy) toolkit is a Python package that aims to fill this gap. It implements Richtmyer’s two-step Lax-Wendroff scheme to solve 1D model equations of blood flow in arterial trees and aims at facilitating the solution of blood flow problems for various medical applications. Funding statement: The development of this software was supported by an EPSRC Doctoral Training Centre grant (EP/G03690X/1)
Response surface model evolution
No full textMethods are presented for reducing time and effort when performing aerodynamic optimisation using response surface models. Significant time savings are made possible by monitoring the convergence of computational fluid dynamics simulations and omitting regions of poor designs. In so doing, optimal regions of the design space can be highlighted and surface refinement commenced early in the convergence of the design point set. A strategy employing surface updates with new data at points of maximum expected improvement is shown to perform more efficiently than reducing the design space to the region of the optimum. The response surface evolution methods are demonstrated through an example two parameter optimisation of a flap track fairing on a commercial airliner wing
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