Streamwise variable height riblets for reducing skin friction drag of surfaces

An aerodynamic surface, comprising:

• a leading edge, a trailing edge, an x-axis extending longitudinally between the leading and trailing edges, a y-axis perpendicular to the x-axis, and a z-axis perpendicular to both the x-axis and the y-axis, wherein the surface is adapted to be subjected to fluid flow in a substantially x-direction;
• and
• a plurality of parallel, microscopic riblets on the surface extending substantially in the x-direction for reducing skin friction drag, each of the riblets having a continuous longitudinal profile in an x-z plane with a plurality of peaks separated by valleys, wherein the peaks have a z-dimension that is greater than a z-dimension of the valleys.

In an aerodynamic surface having a leading edge, a trailing edge, an x-axis extending longitudinally between the leading and trailing edges, a y-axis perpendicular to the x-axis, and a z-axis perpendicular to both the x-axis and the y-axis, wherein the surface is subjected to fluid flow in a substantially x-direction, an improved apparatus for reducing skin friction drag, comprising:

• a plurality of parallel, microscopic riblets on the surface extending continuously along the x-axis, each of the riblets having a generally triangular cross-section in a y-z-plane, a continuous ridge with a plurality of peaks and a plurality of valleys, a first side surface with a first longitudinal edge, a second side surface with a second longitudinal edge, wherein the first longitudinal edges of the riblets intersect the second longitudinal edges of adjacent ones of the riblets;
• and wherein
• each riblet has an undulated cross-section in an x-z-plane such that the peaks of the ridge have a z-dimension that is greater than a z-dimension of the valleys of the ridge.