  A net force can arise on objects which lie in systems with complex energy
partitions, even if the system is on average stationary. These forces are
usually called fluctuation forces, as they arise due to the objects modifying
the character of the fluctuations within the system. We continue the
investigation of Spandan \emph{et al.}, \textit{Sci. Adv., 6(14), eaba0461}
(2020), who found an attractive fluctuation force between two parallel square
plates in homogeneous isotropic turbulence (HIT). We conduct simulations which
systematically vary the plate size and Reynolds number. At $Re_\lambda=100$
small plates show a monotonic force dependence, with a maximum force for the
smallest plate separations, while medium and large plates show a non-monotonic
behaviour of the force with maximum attractive force at intermediate
separations. We find that energy-related statistics cannot explain the
dependence on plate separation of the force, but that statistics related to
vorticity do show qualitative variations around the plate separation
corresponding to the maximum force. This suggests that the role of plates in
affecting intense vorticity structures is critical to the behaviour of the
force. By decreasing $Re_\lambda$, we show that removing vortex stretching
decreases the attractive force, but does not completely eliminate it, and find
that the local maximum at intermediate distances becomes a local minimum. This
confirms that the attractive force is related to vorticity, while suggesting
that a second mechanism is present -- supporting the proposal for a two-fold
origin from earlier work: the plates both restrict the presence of energy
structures in the slit and pack intense vortical structures which stretch each
other causing the pressure to drop.
