I'm currently using ANSYS to simulate the large arteries of the cardiovascular system, specifically injecting nanoparticles in certain areas to see how many nanoparticles reach a certain area of the cardiovascular system. However, I'm getting results I don't fully understand. Before running simulations with my model, I created some simplest models to observe the behavior of the nanoparticles. I realized that having 3 pipes (like in a dumbbell topology) with same dimensions (same length and radius), the results of the simulation (i.e., where nanoparticles goes) are different if the pipes are placed at different angles (i.e. horizontal, vertical and/or diagonal).

For example, the dumbbell topology has 3 pipes as I explained above, with 1 inlet and 3 outlets. The pipe of outlet 1 is placed vertically (along -y axis), pipe of outlet 2 is placed diagonally (along -x and -y axis) and pipe of outlet 3 is placed horizontally (along -x axis). I injected 1000 gold nanoparticles with 1e-08m in diameter using the Stokes-Cunningham with Brownian motion model randomly distributed at the inlet surface, assuming laminar flow model. The results for this simulation is that 37.5% of nanoparticles escape through outlet 1, 58.2% through outlet 2 and 0% through outlet 3. I ran a second simulation, but this time, placing pipe of outlet 2 diagonally along the +x and -y axis and I got these results: 31.9% of nanoparticles escape through outlet 1, 28% escape through outlet 2 and 31.9% escape through outlet 3. My question is why now do I have some nanoparticles escaping from outlet 3 by only changing the angle of pipe of outlet 2? Can someone help me to understand this? Thanks!