Trond Kvamsdal

Computational vascular fluid–structure interaction: methodology and application to cerebral aneurysms

A computational vascular fluid–structure interaction framework for the simulation of patient-specific cerebral aneurysm configurations is presented. A new approach for the computation of the blood vessel tissue prestress is also described. Simulations of four patient-specific models are carried out, and quantities of hemodynamic interest such as wall shear stress and wall tension are studied to examine the relevance of fluid–structure interaction modeling when compared to the rigid arterial wall assumption. We demonstrate that flexible wall modeling plays an important role in accurate prediction of patient-specific hemodynamics. Discussion of the clinical relevance of our methods and results is provided.

Determination of Wall Tension in Cerebral Artery Aneurysms by Numerical Simulation

Background and Purpose— Cerebral artery aneurysms rupture when wall tension exceeds the strength of the wall tissue. At present, risk-assessment of unruptured aneurysms does not include evaluation of the lesions shape, yet clinical experience suggests that this is of importance. We aimed to develop a computational model for simulation of fluid-structure interaction in cerebral aneurysms based on patient specific lesion geometry, with special emphasis on wall tension. Methods— An advanced isogeometric fluid-structure analysis model incorporating flexible aneurysm wall based on patient specific computed tomography angiogram images was developed. Variables used in the simulation model were retrieved from a literature review. Results— The simulation results exposed areas of high wall tension and wall displacement located where aneurysms usually rupture. Conclusion— We suggest that analyzing wall tension and wall displacement in cerebral aneurysms by numeric simulation could be developed into a novel method for individualized prediction of rupture risk.

Error estimation based on Superconvergent Patch Recovery using statically admissible stress fields

Norges teknisk-naturvitenskapelige universitet REPORT NTNU Institutt for konstruksjonsteknikk Title Date Author Sign. In this paper we investigate an approach for a posteriori error estimation based on recovery of an improved stress eld. The qualitative properties of the recovered stress eld necessary to obtain a conservative error estimator, i.e. an upper bound on the true error, are given. A speciic procedure for recovery of an improved stress eld is then developed. The procedure can be classiied as superconvergent patch recovery (SPR) enhanced with approximate satisfaction of the interior equilibrium and the natural boundary conditions. Herein the interior equilibrium is satissed a priori within each nodal patch. Compared to the original SPR-method, which usually underestimates the true error, the present approach gives a more conservative estimate. The performance of the developed error es-timator is illustrated by investigating two plane strain problems with known closed-form solutions. Abstract In this paper we investigate an approach for a posteriori error estimation based on recovery of an improved stress eld. The qualitative properties of the recovered stress eld necessary to obtain a conservative error estimator, i.e. an upper bound on the true error, are given. A speciic procedure for recovery of an improved stress eld is then developed. The procedure can be classiied as superconvergent patch recovery (SPR) enhanced with approximate satisfaction of the interior equilibrium and the natural boundary conditions. Herein the interior equilibrium is satissed a priori within each nodal patch. Compared to the original SPR-method, which usually underestimates the true error, the present approach gives a more conservative estimate. The performance of the developed error estimator is illustrated by investigating two plane strain problems with known closed-form solutions.

Goal oriented error estimators for Stokes equations based on variationally consistent postprocessing

In many flow problems, and in particular when fluid-structure interaction is considered, the important unknowns are the forces acting on the structure in certain areas. Hence, accurate values for these local quantities is essentially what one wants to get out of the flow computations. By means of variationally consistent postprocessing, where forces are computed using the weak form of the equations, we recover the requested forces. Goal oriented local error indicators are provided by solving an auxiliary problem. At the end numerical examples are presented that illustrate how this goal oriented strategy gives improved efficiency compared to traditional methods. The fluid flow is assumed to be governed by the Stokes equations.

SUPERCONVERGENT PATCH RECOVERY FOR PLATE PROBLEMS USING STATICALLY ADMISSIBLE STRESS RESULTANT FIELDS

In this paper, we study an approach for recovery of an improved stress resultant field for plate bending problems, which then is used for a posteriori error estimation of the finite element solution. The new recovery procedure can be classified as Superconvergent Patch Recovery (SPR) enhanced with approximate satisfaction of interior equilibrium and natural boundary conditions. The interior equilibrium is satisfied a priori over each nodal patch by selecting polynomial basis functions that fulfil the point-wise equilibrium equations. The natural boundary conditions are accounted for in a discrete least-squares manner. The performance of the developed recovery procedure is illustrated by analysing two plate bending problems with known analytical solutions. Compared to the original SPR-method, which usually underestimates the true error, the present approach gives a more conservative error estimate. Copyright

Numerical Analysis of NREL 5MW Wind Turbine: A Study Towards a Better Understanding of Wake Characteristic and Torque Generation Mechanism

With the increased feasibility of harvesting offshore wind energy, scale of wind turbines is growing rapidly and there is a trend towards clustering together higher number of turbines in order to harvest maximum yield and to leave a smaller footprint on the environment. This causes complex flow configurations inside the farms, the study of which is essential to making offshore wind energy a success. The present study focuses on NREL 5MW wind turbine with the following objectives (a)To compare Sliding Mesh Interface and Multiple Reference Frame modeling approaches and their predictive capabilities in reproducing the characteristics of flow around the full scale wind turbine. (b)To get a better insight into wake dynamics behind the turbine in near and far wake regions operating under different tip-speed-ratio and incoming turbulence intensities.

LES and RANS simulation of onshore Bessaker wind farm: analysing terrain and wake effects on wind farm performance

This work compares the predictive performance of RANS and LES solver in capturing the effect of terrain and wakes on the performance of the Bessaker wind farm. This 25 turbine wind farm is located in a highly complex terrain and is exposed to predominantly high westerly and south easterly winds. A one-equation sub-grid scale LES turbulence model has been used to help capture the wake dynamics: particularly the effects of wake meandering and wake-turbine interactions. A comparison between RANS and LES models highlights the influence of turbulence model on wake decay and its subsequent effect on prediction of power production. The LES model predicts delayed decay of the wake and more pronounced wake interference leading to a lower power production in wind farm than the RANS case. The RANS model overpredicts turbulence, which cause faster turbulent momentum diffusivity and faster wake recovery. This study has given some insights regarding the power production at Bessaker wind farm for neutral conditions and westerly flow.

Object-Oriented Programming in Field Recovery and Error Estimation

Abstract.This paper considers an object-oriented implementation of a generic program module for field recovery and recovery-based error stimation. The field recovery is based on the superconvergent patch recovery technique by Zienkiewicz and Zhu. The current implementation is problem independent, and is organized as a set of C++ classes based on the software library Diffpack. The program may be run stand-alone as a post-processor, reading finite element data and results from ASCII files. Alternatively, it may be linked into existing simulation codes through a Fortran interface, thereby enabling error estimation within the time-step loop of a transient simulation.The use of the field recovery and error estimation module is demonstrated on an isotropic linear elastic problem with known analytical solution, such that also the exact error, and not only the estimated error, may be computed. The computational efficiency of the object-oriented module is assessed by comparing the time consumption with a similar program implemented in Fortran.

Postprocessing of non-conservative flux for compatibility with transport in heterogeneous media

A conservative flux postprocessing algorithm is presented for both steady-state and dynamic flow models. The postprocessed flux is shown to have the same convergence order as the original flux. An ...

Parallel Methods for Fluid-Structure Interaction

A parallel CFD code capable of simulating flow within moving boundaries has been coupled to a beam element structural dynamics code. The coupled codes are used to simulate fluid- structure interaction for a class of applications involving long and slender structures, e.g. suspension bridges and offshore risers. Due to the difference in size and dimensionality of the 3D CFD problem on one side, and the essentially 1D structure problem on the other side, the bulk of the computations are carried out in the CFD code. The parallel efficiency of the coupled codes thus rest on the paralll performance of the CFD code, and on minimizing the amount of communication between the two codes. The CFD code uses implicit time stepping, and is parallelized by a multiblock technique based on a block-Jacobi iteration together with coarse grid correction. To reduce the amount of communication between the CFD code and the structure code, the mesh movement algorithm is split into two parts, where the most computationally intensive part is carried out in parallel within the CFD code. The resulting coupled system has a high parallel efficiency even if the structure code runs on a workstation and the CFD code runs on a parallel supercomputer provided that the size of the CFD problem is sufficiently large.