Figure 7: Numerical simulations of a sphere with a sheet.
From: Passive appendages generate drift through symmetry breaking
We model a freely falling object (shown in grey colour in frames a–c) by imposing a free stream in the x-direction and allowing translation in y and z directions, as well as rotation around y axis. In a the u=−0.1U streamwise-velocity isosurface (red) shows the existence of a back-flow region, whereas u=1.1U isosurface (blue) illustrates the asymmetric flow field around the sphere. The asymmetric wake is also observed in b, where the vorticity component in the y-direction is shown (blue and red correspond to ωy=3.0 and ωy=−3.0, respectively). The direction of movement of the body is depicted with a black arrow. In contrast, in the xy-plane shown in c there is no drift and the wake is symmetric (blue and red corresponding to ωz=3.0 and ωz=−3.0, respectively). The time evolution of the turn angle (θ) and the drift angles (α,γ) for the sphere alone and the sphere with a sheet are reported in d and e. When a sheet is appended to the body it rotates an angle of θ=−8.5 degrees and drifts in zx-plane by α=4.5 degrees. See also Supplementary Movies 6 and 7.
