🩸 Comparative Study of Newtonian and Non-Newtonian Blood Flow Models in a Bifurcating Artery Under Pulsatile Conditions 

 🧠 Project Overview  

This study explores how the representation of blood viscosity—Newtonian versus Non-Newtonian (Carreau)—influences the behavior of blood flow in a 3D bifurcating artery subjected to pulsatile (unsteady) conditions. The analysis focuses on two critical hemodynamic parameters: wall shear stress (WSS) and pressure distribution, which are strongly linked to vascular health and disease development.

The simulations were conducted using ANSYS Fluent 2023 R2, and all post-processing was carried out in CFD-Post. The study highlights key physiological implications of shear-thinning blood behavior, especially in complex vascular regions such as bifurcations.

🧪 Simulation Setup 

A 3D bifurcating artery geometry was used for both cases. The inlet was driven by a pulsatile velocity profile over a 0.5-second flow cycle, and both outlets were maintained at zero pressure. The mesh and geometry remained unchanged between the two cases to ensure direct comparability.

Two viscosity models were compared:

• A Newtonian model with constant viscosity.
  
  

• A Non-Newtonian Carreau model, which incorporates shear-thinning effects—more realistic for human blood, whose viscosity decreases with increasing shear rate.
  
  

The key variables monitored were wall shear stress and static pressure, both of which were evaluated at specific points and over the domain.

📍 Wall Shear Stress at the Bifurcation Apex

The maximum wall shear stress was found at the bifurcation apex—a physiologically sensitive location prone to endothelial damage, atherosclerosis, and vessel remodeling. A probe was placed at this critical point, and the WSS was tracked over time for both models.

At approximately 0.15 seconds, during the acceleration phase of the pulsatile flow, WSS peaked in both simulations. However, the Non-Newtonian model produced a higher peak (around 33 pascals) compared to the Newtonian case (around 31 pascals).

📌 Why this matters: Although Non-Newtonian fluids are often assumed to reduce shear due to their shear-thinning behavior, this result shows the opposite at the bifurcation. The increased effective viscosity in moderate-shear regions appears to steepen the velocity gradient near the wall, thereby increasing WSS during the most dynamic phase of the cardiac cycle.

This is especially important because bifurcation apexes are hotspots for vascular disease, and a model that underpredicts WSS here could lead to misinterpretation of risk.

🩺 Inlet Pressure Evolution and Its Connection to WSS

• Static pressure was monitored at the center of the inlet throughout the flow cycle. Both models showed a pulsatile pressure curve, with a steady rise followed by a sharp drop around the same time WSS peaked.

• However, the Non-Newtonian model maintained slightly higher inlet pressure than the Newtonian one—especially during the deceleration phase of the flow.

• 📌 Interpretation: This elevated pressure indicates that the Non-Newtonian model encountered greater viscous resistance, particularly in regions of moderate shear where the fluid remains more viscous. This additional resistance likely contributed to the higher wall shear stress at the bifurcation, as the elevated pressure would steepen the wall velocity gradient even further.

• This finding reinforces the idea that Non-Newtonian effects are essential to capture not only global resistance but also local stress amplification, especially in transient, branching arterial flows.

🔻 Pressure Drop Between Inlet and Outlets

• To assess global flow resistance, pressure probes were placed at the inlet center and at the center of each outlet branch. Pressure drop was calculated throughout the flow cycle.

• Preliminary results suggest that the Non-Newtonian model experienced slightly higher pressure loss, indicating greater total resistance. This aligns with the higher inlet pressure and higher WSS observed earlier.

📌 While the absolute pressure differences were modest, their timing and location—occurring alongside peak wall shear stress at the bifurcation—emphasize how even small viscosity-related changes can impact critical stress patterns and energy loss in the system.

🧬 Conclusions

This study demonstrates that modeling blood as a Non-Newtonian fluid can have a significant impact on both wall shear stress and pressure dynamics, especially in bifurcating arteries under unsteady flow.

• The Non-Newtonian model produced higher WSS at the bifurcation apex, contradicting the assumption that shear-thinning always leads to stress reduction.
  
  

• It also showed higher inlet pressure, particularly during the deceleration phase of flow, implying increased resistance in certain flow regimes.
  
  

• These effects combined to generate steeper near-wall gradients, amplifying shear at the very location most prone to vascular disease.

🔬 Key takeaway: Using a Newtonian approximation may overlook important hemodynamic features—especially near bifurcations—by underestimating wall stress and resistance during pulsatile events. Incorporating Non-Newtonian models is essential for more accurate and physiologically meaningful simulation results in vascular biomechanics.