Leaflet Stresses During Full Device Simulation of Crimping to 6 mm in Transcatheter Aortic Valve Implantation, TAVI

Background With continuing growth in transcatheter aortic valve implantation for the treatment of a failing aortic valve, there is increasing interest in prosthetic valve durability and the potential damage caused to leaflets by stress. Whilst most available research into the computational prediction of leaflet stresses using finite element analysis, FEA, has focussed on variations during dynamic loading, very little appears to have been reported for the impact of crimping, even though awareness of this effect is widespread. Potentially, this has been due to the difficulty of performing full model simulations of crimping to clinically meaningful diameters. Method A full model comprising a self-expanding frame, skirt and leaflets has been developed and crimped to a final diameter of 6 mm. A detailed description is provided of the FEA setup, emphasising the importance of the skirt definition needed to successfully crimp to this small diameter. Then, an analysis of leaflet folding and stresses is presented, particularly with respect to the differences produced between leaflet thicknesses of 0.20, 0.25 and 0.30 mm and for bioprosthetic and polymeric leaflet material models. Results In all cases, peak stresses occurred close to the modelled suture lines joining the leaflets and the skirt and high stresses were also present along axially aligned folds in the leaflets. Stresses were lower for the polymeric leaflets. Conclusion Successful simulation of crimping requires a finely resolved skirt mesh. Leaflet stresses during crimping are dependent on leaflet thickness, material properties and the ratio of leaflet volume to the available volume inside the crimped valve

Predictive haemodynamics in a one-dimensional human carotid artery bifurcation. Part 1: application to stent design

A diagnostic technique is proposed to identify patients with carotid stenosis who could most benefit from angioplasty followed by stent implantation. This methodology involves performing a parametric study to investigate the haemodynamic behavior due to alterations in the stenosis shapes in the internal carotid artery (ICA). A pulsatile 1-D Navier-Stokes solver incorporating fluid-wall interactions for a Newtonian fluid which predicts pressure and flow in the human carotid artery bifurcation is used for the numerical simulations. In order to assess the performance of each individual geometry, we introduce pressure variation factor as a metric to directly compare the global effect of variations in the geometry. It is shown that the probability of an overall catastrophic effect is higher when the stenosis is present in the upstream segment of the ICA. Furthermore, maximum pressure is used to quantify the local effects of geometry changes. The location of the peak and extent of stenosis are found not to influence maximum pressure. We also show how these metrics respond after stent deployment into the stenosed part of the ICA. In particular, it is found that localized pressure peaks do not depend on the length of a stent. Finally, we demonstrate how these metrics may be applied to cost-effectively predict the benefit of stenting

Computational fluid dynamics simulation of a rim driven thruster

An electric rim driven thruster is a relatively new marine propulsion device that uses a motor in its casing to drive a propeller by its rim and the fluid dynamics associated with their operation have not been fully investigated. There are many interacting flow features that make up the flow field of a rim driven thruster that pose a number of challenges when it comes to simulating the device using computational fluid dynamics. The purpose of this work is to develop a computational fluid dynamics solution process that accurately simulates features including vortex generation and behaviour, radial pumping and rotor-stator interaction while attempting to minimise computational costs. This will enable the method to be used to calculate an objective function, typically the thrust or propulsive efficiency of the device, in a design optimisation study. Implementation within a design optimisation study also requires the numerical methods to be easily repeatable and robust in both mesh generation and solution.Mesh generation was performed using snappyHexMesh, a meshing program that is part of OpenFOAM, and a thorough mesh verification procedure has been conducted. Validation of the computational fluid dynamics solution of a standard series propeller, as a baseline case with good experimental data from MARIN, using the open source Reynolds-Averaged Navier-Stokes solver MRFSimpleFoam (part of the OpenFOAM software) has been performed. Results show a great sensitivity to computational domain size that suggest that similar previous works may have used an insuffcient domain size. In particular, it is shown that a number of boundary conditions may be used if the domain is large enough. Also, comparisons are made between the Re-Normalisation Group (RNG) k-e and k-w Shear Stress Transport (SST) turbulence models (the most widely reported models in the literature), and the k-w SST model is found to be robust due to its better handling of the separation that occurs at low propeller advance ratios. Validation against experimental data for the standard series propeller shows good agreement to within 5%.The validated solution method is then applied to a rim driven thruster and key design areas are highlighted by the results. The rim is found to be an important region of the flow, the drag on which comprises almost half of the torque losses in the device. Interaction between the rotors and the stators is also a key area, with both thrust and torque changing as the position of the blades is varied

Morphing of ‘flying’ shapes for autonomous underwater and aerial vehicles

Autonomous vehicles are energy poor and should be designed to minimise the power required to propel them throughout their mission. The University of Southampton’s School of Engineering Sciences is actively involved in the development of improved designs for aerial and maritime autonomous vehicles. The ability to adapt or ‘morph’ their shape in-flight offers an opportunity to extend mission range/duration and improve agility. The practical implementation of such systems at small scale requires detailed consideration of the number, mass and power requirements of the individual actuation elements. Three approaches for minimising actuation requirements are considered. The first uses a combination of push-pull actuators coupled with a snap-through composite lay-up to achieve alterations in shape. It is proposed that such a system could be applied to the trailing edge of an autonomous underwater glider wing instead of the more usual servo operated trailing edge flap. The anisotropy achieved through use of different composite ply orientations and stacking can also be used to generate bend-twist coupling such that fluid dynamic loads induce ‘passive’ shape adaptation. The third approach uses a detailed understanding of the structural response of buckled elements to applied control moments to deform a complete wing. At this stage of the research no definitive conclusions have been drawn other than that all three approaches show sufficient promise and can now be applied to one of the autonomous vehicles

Optimization using surrogate models and partially converged computational fluid dynamics simulations

Efficient methods for global aerodynamic optimization using computational fluid dynamics simulations should aim to reduce both the time taken to evaluate design concepts and the number of evaluations needed for optimization. This paper investigates methods for improving such efficiency through the use of partially converged computational fluid dynamics results. These allow surrogate models to be built in a fraction of the time required for models based on converged results. The proposed optimization methodologies increase the speed of convergence to a global optimum while the computer resources expended in areas of poor designs are reduced. A strategy which combines a global approximation built using partially converged simulations with expected improvement updates of converged simulations is shown to outperform a traditional surrogate-based optimization

Simulation of longitudinal stent deformation in a patient-specific coronary artery

In percutaneous coronary intervention (PCI), stent malapposition is a common complication often leading to stent thrombosis (ST). More recently, it has also been associated with longitudinal stent deformation (LSD) normally occurring through contact of a post balloon catheter tip and the protruding malapposed stent struts.The aim of this study was to assess the longitudinal integrity of first and second generation drug eluting stents in a patient specific coronary artery segment and to compare the range of variation of applied loads with those reported elsewhere. We successfully validated computational models of three drug-eluting stent designs when assessed for longitudinal deformation. We then reconstructed a patient specific stenosed right coronary artery segment by fusing angiographic and intravascular ultrasound (IVUS) images from a real case. Within this model the mechanical behaviour of the same stents along with a modified device was compared. Specifically, after the deployment of each device, a compressive point load of 0.3 N was applied on the most malapposed strut proximally to the models. Results indicate that predicted stent longitudinal strength (i) is significantly different between the stent platforms in a manner consistent with physical testing in a laboratory environment, (ii) shows a smaller range of variation for simulations of in vivo performance relative to models of in vitro experiments, and (iii) the modified stent design demonstrated considerably higher longitudinal integrity. Interestingly, stent longitudinal stability may differ drastically after a localised in vivo force compared to a distributed in vitro force

The development of a hybridized particle swarm for kriging hyperparameter tuning

Optimizations involving high-fidelity simulations can become prohibitively expensive when an exhaustive search is employed. To remove this expense a surrogate model is often constructed. One of the most popular techniques for the construction of such a surrogate model is that of kriging. However, the construction of a kriging model requires the optimization of a multi-model likelihood function, the cost of which can approach that of the high-fidelity simulations upon which the model is based. The article describes the development of a hybridized particle swarm algorithm which aims to reduce the cost of this likelihood optimization by drawing on an efficient adjoint of the likelihood. This hybridized tuning strategy is compared to a number of other strategies with respect to the inverse design of an airfoil as well as the optimization of an airfoil for minimum drag at a fixed lif

The influence of soot loading on weighted sum of grey gases solutions to the radiative transfer equation across mixtures of gases and soot

Different approaches to employing weighted sum of grey gases data in line of sight solutions to the radiative transfer equation (RTE) are compared for mixtures of combustion gases and soot. Responses to variations in soot loading are analysed across configurations comprising uniform and non-uniform properties and compositions. Relative to a differential banded transmissivity solution, a weighted sum of grey gases (WSGG) solution, which solves the RTE for each grey gas component, yields greater accuracy than a total property (TP) solution. The latter evaluates radiative properties on a cell-by-cell basis for application ina a single equation. However, accuracy of the TP solution is shown to improve with increasing soot loading

Parallelisation of the discrete transfer radiation model

The transfer of energy by thermal radiation represents an important component in many reacting flows. Thus, when radiative energy exchange is included in a complete, parallel CFD simulation of such systems, the relatively large computational expense of evaluating this complex process demands efficient parallelisation of the radiation sub-model. This article investigates the parallelisation of the discrete transfer radiation model, paying particular attention to different solutions to the radiative transfer equation. Speed-up efficiencies of a constant absorption coefficient solution, a weighted sum of gray gases solution, and a differential total absorptivity solution are compared across a non-uniform mixture of combustion gases and soot