COMPUTATIONAL SIMULATIONS OF FLOW OVER THE SURFACE OF A FORMED THROMBUS IN A BACKWARD-FACING STEP

Open Access
Author:
Long, George Douglas
Graduate Program:
Bioengineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Keefe B Manning, Thesis Advisor
Keywords:
  • Thrombosis
  • embolization
  • backward-facing step
  • sudden expansion
  • CFD
  • simulation
  • clot
Abstract:
Thrombosis and thromboembolization remain a major impediment to the successful use of cardiovascular devices, such as ventricular assist devices, arterial stents, and artificial heart valves. Separated flow produced by these devices is known to be a major contributor to these phenomena. In order to better characterize these effects, a computational fluid dynamics (CFD) study is performed. First, an in vitro flow loop consisting of a backward-facing step is used to generate an experimentally formed thrombus. Its geometry is captured using magnetic resonance imaging, and CFD simulations are then performed with a reconstructed, computer rendering of the thrombus. Two-dimensional studies at an average Reynolds number of 525 and a sinusoidal velocity-inlet waveform are performed on the in vitro model’s geometry. Three-dimensional simulations using a physiologic velocity-inlet waveform taken from a human iliac artery are performed at an average Reynolds number of 1000, with and without the reconstructed thrombus, to observe the flow and surfaces stresses. The frequency of the velocity-inlet waveform is varied from 50 to 100 bpm to observe the effects of Strouhal number. From these simulations, wall shear stress magnitudes are shown to increase with Strouhal number, with the 100 bpm case producing values up to 53% higher than the 50 bpm case, approaching 35 dynes/cm2. Flow instabilities are more pronounced in the 100 bpm case compared to the lower frequency cases, with Taylor cells visible during periods of flow deceleration. From these results, predictions into the likelihood of thrombosis and embolization are made, with larger thrombi being more likely in the 50 bpm case than the higher frequency cases, due to the larger recirculation regions observed past the step and the reduced wall washing. While significant embolization is not likely to occur at the current flow conditions, at higher Reynolds numbers, the 100 bpm case is predicted to be more susceptible, due to an increase in wall shear stress along the clot’s surface, with shear stresses of 50 dyne/cm2 observed for a Reynolds number of 1000.