Fig. 1.

(A and B) Unit cell representations of two contrasting ordered hexagonal arrays of posts used in the flow experiments. In A, the posts are staggered along the $x$ direction in which the flow is imposed. The post radius is $R$, and lattice spacing is $S$. Rotating the array by $\mathrm{30}°$ aligns the posts in the flow direction (B). Disordered aligned arrays are generated by the random displacement of each post within a hexagon of circumradius $\beta S$, as described in ref. 28. (C and D) Streamlines determined by flow velocimetry (Materials and Methods) with a Newtonian fluid in the staggered (C) and aligned (D) arrays at $\text{Re}\approx 10^{-3}$. The red crossed circles in C and D indicate the locations of the leading and trailing-edge stagnation points on one of the circular posts.