Roland Gårdhagen

Simulation of phase contrast MRI of turbulent flow

Phase contrast MRI is a powerful tool for the assessment of blood flow. However, especially in the highly complex and turbulent flow that accompanies many cardiovascular diseases, phase contrast MRI may suffer from artifacts. Simulation of phase contrast MRI of turbulent flow could increase our understanding of phase contrast MRI artifacts in turbulent flows and facilitate the development of phase contrast MRI methods for the assessment of turbulent blood flow. We present a method for the simulation of phase contrast MRI measurements of turbulent flow. The method uses an Eulerian‐Lagrangian approach, in which spin particle trajectories are computed from time‐resolved large eddy simulations. The Bloch equations are solved for each spin for a frame of reference moving along the spins trajectory. The method was validated by comparison with phase contrast MRI measurements of velocity and intravoxel velocity standard deviation (IVSD) on a flow phantom consisting of a straight rigid pipe with a stenosis. Turbulence related artifacts, such as signal drop and ghosting, could be recognized in the measurements as well as in the simulations. The velocity and the IVSD obtained from the magnitude of the phase contrast MRI simulations agreed well with the measurements. Magn Reson Med, 2010.

Quantifying turbulent wall shear stress in a subject specific human aorta using large eddy simulation

In this study, large-eddy simulation (LES) is employed to calculate the disturbed flow field and the wall shear stress (WSS) in a subject specific human aorta. Velocity and geometry measurements using magnetic resonance imaging (MRI) are taken as input to the model to provide accurate boundary conditions and to assure the physiological relevance. In total, 50 consecutive cardiac cycles were simulated from which a phase average was computed to get a statistically reliable result. A decomposition similar to Reynolds decomposition is introduced, where the WSS signal is divided into a pulsating part (due to the mass flow rate) and a fluctuating part (originating from the disturbed flow). Oscillatory shear index (OSI) is plotted against time-averaged WSS in a novel way, and locations on the aortic wall where elevated values existed could easily be found. In general, high and oscillating WSS values were found in the vicinity of the branches in the aortic arch, while low and oscillating WSS were present in the inner curvature of the descending aorta. The decomposition of WSS into a pulsating and a fluctuating part increases the understanding of how WSS affects the aortic wall, which enables both qualitative and quantitative comparisons.

Quantifying turbulent wall shear stress in a stenosed pipe using large eddy simulation.

Large eddy simulation was applied for flow of Re=2000 in a stenosed pipe in order to undertake a thorough investigation of the wall shear stress (WSS) in turbulent flow. A decomposition of the WSS into time averaged and fluctuating components is proposed. It was concluded that a scale resolving technique is required to completely describe the WSS pattern in a subject specific vessel model, since the poststenotic region was dominated by large axial and circumferential fluctuations. Three poststenotic regions of different WSS characteristics were identified. The recirculation zone was subject to a time averaged WSS in the retrograde direction and large fluctuations. After reattachment there was an antegrade shear and smaller fluctuations than in the recirculation zone. At the reattachment the fluctuations were the largest, but no direction dominated over time. Due to symmetry the circumferential time average was always zero. Thus, in a blood vessel, the axial fluctuations would affect endothelial cells in a stretched state, whereas the circumferential fluctuations would act in a relaxed direction.

Feasibility of patient specific aortic blood flow CFD simulation

Patient specific modelling of the blood flow through the human aorta is performed using computational fluid dynamics (CFD) and magnetic resonance imaging (MRI). Velocity patterns are compared between computer simulations and measurements. The workflow includes several steps: MRI measurement to obtain both geometry and velocity, an automatic levelset segmentation followed by meshing of the geometrical model and CFD setup to perform the simulations follwed by the actual simulations. The computational results agree well with the measured data.

Large Eddy Simulation of Flow Through a Stenosed Pipe

A majority of all deaths in the developed world are related to atherosclerosis, i.e. obstruction of blood vessels caused by growth of the vessel wall. Hemodynamic phenomena, especially wall shear stress, are since several decades thought to influence the risk to develop atherosclerosis; hence simulation of blood flow, either in order to elucidate the relation between the hemodynamic and disease initiation or to study the flow pattern, is an area of intense research [1,2].

Large Eddy Simulation of Pulsating Flow before and after CoA Repair: CFD for Intervention Planning

Large eddy simulation was applied to investigate hemodynamics in a model with coarctation of the aorta (CoA) and poststenotic dilatation. Special focus was put on the role of hemodynamics for success of CoA repair. Several parameters previously identified as related to cardiovascular disease were studied. Known risk factors were observed both with CoA and after repair, and the restoration of the anatomy seems to be crucial for a successful result.

Geometrical Considerations in Patient Specific Models of a Human Aorta with Stenosis and Aneurysm

The most important artery in the human body is the aorta that supplies the rest of the body with blood. Lesions in the aorta can cause serious complications, which can even lead to death. How the flow is behaving in lesions, what causes the problems and which lesions are dangerous are highly interesting to determine. Laminar, stationary CFD calculations are performed on two geometrically different models of the human aorta created from the same set of patient MRI (Magnetic Resonance Imaging) data. Differences in the CFD results due to different geometries are evaluated. Overview results e.g. pressure variations throughout the artery are not dependent on an exact description of the geometry. If absolute and local values e.g. wall shear stress are sought more robust geometry creation procedure is needed in order to get more reliable results.

Development of a Cockpit-Pilot Model for Thermal Comfort Optimization During Long-Mission Flight

The thermal comfort of a pilot is of crucial importance to maintain a high level ofconcentration and awareness during the entire ight mission. In this work a model for thethermal environment of the ...

Aerodynamics of an Unloaded Timber Truck-A CFD Investigation

Reducing energy consumption and emissions are ongoing challenges for the transport sector. The increased number of goods transports emphasize these challenges even more, as greenhouse gas emissions ...

Aerodynamic Drag Reduction of a Light Truck - from Conceptual Design to Full Scale Road Tests

Considerable amounts of the everyday goods transports are done using light trucks. In the last ten years (2005-2015), the number of light trucks has increased by 33 % in Sweden. The majority of the ...