Test case of horizontal shear flows. Upper panel: 3D visualization of the initial spatial distribution of the shear flow in our simulation. Arrows mark the direction of shear flow, and the total flow velocity magnitude is mapped to colors (see the color bar). Middle panels: solid red line shows υ_x/υ_x0 vs. R, i.e., normalized shear velocity as a function of global radius of the local box, showing growth of shear velocity as collapse proceeds, closely matches the analytical expectation (dashed black lines). The result of υ_y is identical to that of υ_x and is not shown here. Lower panel: time evolution of relative error err(υ_x).