Numerical Analysis of Orientational Behavior of Particles Exhibiting Rheotaxis in Unsteady Uniform Flows

The flow-induced structure of suspensions of particles exhibiting rheotaxis in a uniform flow was numerically analyzed. A startup flow and a flow with step changes in the flow direction were considered. The particles were self-propelled and rotated in the flow. The computational model for particle rotation includes rotation in the direction opposite to the flow, alignment with the average direction of surrounding particles within an effective region, and random rotation. The effect of the intensity of both rheotaxis and random motion of particles on their orientation in a uniform flow was analyzed. The particles move to orient themselves in the direction opposite to the flow with time, increasing the orientation order of the particle suspension. The orientation order increases as the effect of rheotaxis becomes stronger. Furthermore, it is evident that robustness of the orientational behavior to noise intensifies even for weak rheotaxis. In a flow in which the flow direction changes, the director rotates more slowly for systems exhibiting weaker rheotaxis and the particles rotate in a more synchronized manner when rheotaxis is strong.