Squirmers that are bottom-heavy experience a torque that aligns them along the vertical so that they swim upwards. In a suspension of many squirmers, they also interact hydrodynamically via flow fields that are initiated by their swimming motion and by gravity. Swimming under the combined action of flow field vorticity and gravitational torque is called gyrotaxis. Using the method of multi-particle collision dynamics, we perform hydrodynamic simulations of a many-squirmer system floating above the bottom surface. Due to gyrotaxis they exhibit pronounced cluster formation with increasing gravitational torque. The clusters are more volatile at low values but compactify to smaller clusters at larger torques. The mean distance between clusters is mainly controlled by the gravitational torque and not the global density. Furthermore, we observe that neutral squirmers form clusters more easily, whereas pullers require larger gravitational torques due to their additional force-dipole flow fields. We do not observe clustering for pusher squirmers. Adding a rotlet dipole to the squirmer flow field induces swirling clusters. At high gravitational strengths, the hydrodynamic interactions with the no-slip boundary create an additional vertical alignment for neutral squirmers, which also supports cluster formation.

底部较重的蠕虫会经历一个扭矩，使它们沿着垂直方向对齐，以便它们向上游动。在许多蠕虫的悬浮液中，它们还通过由它们的游泳运动和重力引发的流场进行流体动力学相互作用。在流场涡量和重力力矩的共同作用下游泳称为回旋。使用多粒子碰撞动力学方法，我们对漂浮在底面上方的多蠕虫系统进行了流体动力学模拟。由于回转性，它们随着重力扭矩的增加表现出明显的簇形成。簇在低值时更易挥发，但在较大扭矩下会压缩成较小的簇。星团之间的平均距离主要受重力力矩而非全局密度控制。此外，我们观察到中性蠕虫更容易形成集群，而牵引器由于它们额外的力偶极流场而需要更大的重力扭矩。我们没有观察到 pusher squirmers 的聚类。在 squirmer 流场中添加一个 rotlet 偶极子会引起旋转簇。在高重力强度下，与无滑移边界的流体动力学相互作用为中性蠕虫创造了额外的垂直排列，这也支持集群的形成。