Streamlines (black) and velocity profile $u(x=0)$ on the vertical centerline (blue) for $Str=1$ and $Re=St=500$ shown during the acceleration phase of the lid in the positive x direction. Full and dashed lines indicate negative (clockwise rotation) and positive stream functions (counterclockwise rotation), respectively. The phase $ϕ=2π(t mod 1)$ is indicated for each subfigure. (a) ϕ = 1.625π, (b) ϕ = 1.75π, and (c) ϕ = 1.9π.