Cerebral aneurysms, characterized by abnormal bulges in the walls of blood vessels within the brain, present a significant risk to those affected by these potentially life-threatening conditions. The assessment of blood flow dynamics within aneurysms is of paramount importance for medical professionals in determining the risk of rupture, devising appropriate treatment strategies, and evaluating the effectiveness of medical devices. Blood, a complex non-Newtonian fluid, exhibits unique properties, resulting in challenging flow behavior to model and analyze.

The remarkable capabilities of STAR-CCM+, a powerful computational fluid dynamic (CFD) and multi-physics simulation software, will be showcased in the context of non-Newtonian blood flow simulation within cerebral aneurysms. A step-by-step guide will be provided, encompassing the entire process, from the acquisition of patient-specific aneurysm geometries to the analysis and visualization of simulation results.

The aim of this blog is to unveil the potential of STAR-CCM+ and equip readers with valuable insights and techniques for the successful execution of cerebral aneurysm blood flow simulations. Researchers, medical professionals, and biomedical engineers alike will find this blog beneficial in harnessing the power of STAR-CCM+ to advance their understanding of cerebral aneurysm hemodynamics. Join us on this captivating journey as we discover how STAR-CCM+ can revolutionize cerebral aneurysm research and patient care.

Pre-processing: Importing and Preparing a Cardiovascular Model in STAR-CCM+

Acquiring a Patient-Specific Geometry:

The first step in simulating blood flow dynamics is to obtain a patient-specific cardiovascular model. Medical imaging techniques such as computed tomography (CT) or magnetic resonance imaging (MRI) scans are commonly used to capture the complex geometry of the cardiovascular system. These scans can be processed using specialized software, to create a 3D geometry in the form of a CAD file format compatible with STAR-CCM+.

Importing the Cardiovascular Model:

Once you have your patient-specific cardiovascular model, launch STAR-CCM+ and create a new simulation. To import the model, go to “File” > “Import” > “Geometry,” and select the appropriate file format of your model. After the import is complete, the cardiovascular geometry will be displayed in the graphics window.

Defining Blood as a Non-Newtonian Fluid and Setting Physical Properties:

Blood is a complex fluid with unique properties, and it’s crucial to accurately define these properties in the simulation. In STAR-CCM+, you can define blood as a non-Newtonian fluid by selecting the appropriate material model, such as the Carreau-Yasuda or the Casson model, which accounts for blood’s shear-thinning behavior.

To define blood as a material, go to the “Continua” folder in the “Simulation Tree” and select “Physics” > “Models” > “Material.” Create a new material named “Blood” and choose the appropriate non-Newtonian model. Set the necessary physical properties, such as density, viscosity parameters, and reference temperature, based on available literature or experimental data.

Meshing: Generating a High-Quality Mesh for Blood Flow Simulation

Importance of Mesh Quality for Blood Flow Simulations

A high-quality mesh is crucial for obtaining accurate and reliable results in blood flow simulations. The mesh should be fine enough to capture the intricate geometries of the cardiovascular model and resolve critical flow features like boundary layers, recirculation zones, and areas with high velocity gradients.

Mesh Generation Techniques in STAR-CCM+

STAR-CCM+ offers a range of mesh generation techniques, including:

Surface meshing: Generates a mesh on the geometry’s surface.

Volume meshing: Fills the geometry’s volume with cells.

Boundary layer meshing: Generates a fine mesh near the walls to accurately capture flow behavior near the walls.

Creating a High-Quality Mesh for the Cardiovascular Model

To create a high-quality mesh for blood flow simulation, follow these steps:

Start by generating a surface mesh on the imported cardiovascular model using the “Surface Remesher” tool. Adjust the surface mesh size to balance computational cost and accuracy.

Create a volume mesh using the “Polyhedral Mesher” or “Trimmed Cell Mesher.” Polyhedral meshing is generally more efficient for blood flow simulations, as it produces fewer cells and provides better flow resolution.

Add boundary layer meshing using the “Prism Layer Mesher.” Adjust the number of layers, growth rate, and total thickness to ensure that the boundary layer is well-resolved, especially near the vessel walls where wall shear stress is an important parameter.

Refine the mesh in regions with complex flow features or high velocity gradients using local mesh controls like “Volume Refinement” or “Surface Refinement.”

Physics Setup: Defining Boundary Conditions and Solvers for Blood Flow Simulation

Setting Up Boundary Conditions

Boundary conditions define how the fluid interacts with the model’s boundaries. For blood flow simulations, typical boundary conditions include:

• Inlet boundary conditions: Prescribe either a constant flow rate or a pulsatile flow profile based on physiological data.
• Outlet boundary conditions: Apply pressure or flow rate conditions, depending on the simulation’s goal and available data.

Key Solvers in STAR-CCM+ for Blood Flow Simulations

STAR-CCM+ offers various solvers to handle different physics involved in blood flow simulations, such as:

Segregated Flow Solver: Solves the Navier-Stokes equations for blood flow as a non-Newtonian fluid in a segregated manner. This solver is suitable for steady-state and transient simulations and is computationally less expensive compared to the coupled solver.

Laminar Model: Assumes that the blood flow is laminar, which is a reasonable approximation for low Reynolds number flows, commonly found in microcirculation or certain cardiovascular regions.

Selecting Appropriate Solver Settings

Select the “Segregated Flow” solver for blood flow simulation and set the fluid properties to match those of blood, considering its non-Newtonian behavior.

Choose the “Laminar Model” to simulate blood flow without turbulence, particularly when the Reynolds number is low and the flow is primarily laminar in nature.

Manually set the time step based on the desired Courant number (equal to 1) to ensure numerical stability and accuracy. The time step can be calculated using the formula: Δt = Courant Number * (minimum cell size) / (maximum flow velocity).

Following these steps will ensure that your blood flow simulation is set up with appropriate solver settings, resulting in accurate and reliable results. Keep in mind that, depending on the specific cardiovascular region and flow conditions, the laminar model may not always be the most suitable choice. In such cases, consider using a turbulence model, as described in the previous version of the text.

Running the Simulation and Monitoring Convergence

Initiating the Simulation

To initiate the blood flow simulation in STAR-CCM+, follow these steps:

Open the “Run” menu in the top toolbar.

Click on “Initialize” to set the initial conditions for the simulation.

Click on “Run” to start the simulation.

Monitoring Convergence in Blood Flow Simulations

Monitoring convergence is essential to ensure the accuracy and reliability of blood flow simulations. Convergence is achieved when the solution reaches a steady state or when transient results no longer change significantly over time. Monitor the residuals of key variables (e.g., velocity, pressure) and check that they decrease and reach a plateau or fall below a specified tolerance.

Troubleshooting Convergence Issues

If convergence issues arise during the blood flow simulation, consider the following tips:

Check the mesh quality and refine it if necessary, especially in regions with complex flow features. Adjust solver settings, such as under-relaxation factors or time step size, to improve stability.

Verify that boundary conditions and fluid properties are set correctly. Try using a different turbulence model if applicable, or switch to a coupled solver for better convergence behavior.

Post-processing: Analyzing and Visualizing Blood Flow Results

Visualization Tools in STAR-CCM+ for Blood Flow Analysis

STAR-CCM+ offers various visualization tools for blood flow analysis, such as:

Streamlines: Display the flow trajectory and help identify flow patterns and recirculation zones.

Velocity vectors: Show the direction and magnitude of the flow at specific points in the domain.

Wall shear stress: Visualize the shear stress distribution on the vessel walls, which is important for understanding the development of cardiovascular diseases.

Analyzing Results

To analyze the results of the blood flow simulation, follow these steps:

Open the “Scenes” menu in STAR-CCM+ and create new scenes for each desired visualization tool (e.g., streamlines, velocity vectors, wall shear stress). Configure the visualization settings and apply appropriate color maps, legends, and labels.

Inspect the pressure, velocity, and wall shear stress distributions to identify any regions of interest or potential issues, such as high-pressure zones or regions with high wall shear stress.

Applications and Case Studies

Blood Flow Simulations in Cardiovascular Disease Prevention and Treatment

Blood flow simulations can help researchers better understand the development of cardiovascular diseases, such as atherosclerosis and aneurysms, by identifying regions of abnormal flow and high wall shear stress. This knowledge can aid in early diagnosis and treatment planning.

Medical Device Design

Blood flow simulations are valuable for designing medical devices, such as stents, artificial heart valves, and ventricular assist devices. These simulations can be used to optimize device geometry, materials, and performance, ensuring patient safety and device efficacy.

Surgical Planning

Simulations of blood flow can assist surgeons in pre-operative planning by providing insights into patient-specific hemodynamics. This information can be used to determine the most appropriate surgical approach and minimize potential complications. By exploring these applications and case studies, you can discover the potential of STAR-CCM+ in blood flow simulations and cardiovascular research.

Author:

Dimitris Lampropoulos – Project Engineer / CFD expert Physicist – Ph.D. e-mail: dslabrop@feacomp.com LinkedIn Profile: Dimitrios S. Lampropoulos