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A micro-array surface that provides for drag reduction. In one aspect, an aerodynamic or hydrodynamic wall surface that is configured to modify a fluid boundary layer on the surface comprises at least one array of micro-cavities formed therein the surface. In one example, the interaction of the micr

A micro-array surface that provides for drag reduction. In one aspect, an aerodynamic or hydrodynamic wall surface that is configured to modify a fluid boundary layer on the surface comprises at least one array of micro-cavities formed therein the surface. In one example, the interaction of the micro-cavities with the boundary layer of the fluid can delay transition of the fluid over an identical smooth surface without the micro-cavities.

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1. An aerodynamic or hydrodynamic wall surface configured to modify the interaction of a boundary layer of a fluid flowing across the wall surface, comprising: at least one array of roughness elements disposed on and extending from the wall surface,wherein each roughness element extends outwardly fr

1. An aerodynamic or hydrodynamic wall surface configured to modify the interaction of a boundary layer of a fluid flowing across the wall surface, comprising: at least one array of roughness elements disposed on and extending from the wall surface,wherein each roughness element extends outwardly from the wall surface at one or more selected angles relative to the flow of fluid across the wall surface such that a preferential flow direction surface is formed,wherein each roughness element has a front, upstream surface and an opposing rear, downstream surface and a peripheral edge that has an upper portion that tapers to a top and a bottom portion that tapers to a base, which is connected to the wall surface,wherein a plurality of roughness elements are positioned substantially transverse to the flow of fluid across the wall surface such that a distance between a medial portion of the peripheral edges of adjacent and aligned roughness elements is less than the distance between the respective tops of the roughness elements and is less than the distance between the respective bases of the roughness elements,wherein the array of roughness elements defines a plurality of cavities,wherein the thickness of the boundary layer is at least 20% of a cavity height of each cavity such that shear layer instabilities of cavity vortexes that form in the plurality of cavities are reduced,wherein each roughness element comprises at least one riblet extending outwardly therefrom the front, upstream surface of the roughness element that is configured to aid in the formation and stability of cavity flows embedded between the roughness elements, andwherein each roughness element comprises at least one riblet extending outwardly from the rear, downstream surface of the roughness element, and wherein each riblet extends substantially longitudinally. 2. The wall surface of claim 1, wherein each formed cavity vortex has a Reynolds number (Re), relative to the cavity height, velocity of the fluid over the wall surface, and the kinematic viscosity of the fluid, in the range of between 100 and 20,000, such that the instability of the formed cavity vortexes are suppressed. 3. The wall surface of claim 1, wherein each formed cavity vortex has a Reynolds number (Re), relative to the cavity height, velocity of the fluid over the wall surface, and the kinematic viscosity of the fluid, in the range of between 1,000 and 5,000, such that the instability of the formed cavity vortexes are suppressed. 4. The wall surface of claim 1, wherein each formed cavity vortex has a Reynolds number (Re), relative to the cavity height, velocity of the fluid over the wall surface, and the kinematic viscosity of the fluid, in the range of between 9,000 and 11,000, such that the instability of the formed cavity vortexes are suppressed. 5. The wall surface of claim 1, wherein the at least one selected angle comprises acute angles. 6. The wall surface of claim 5, wherein the at least one selected angle comprises a normal angle such that at least one of the roughness elements extends substantially normal to the wall surface. 7. The wall surface of claim 6, wherein the array of roughness elements are positioned in successive ridges of roughness elements, wherein each ridge of roughness elements is positioned substantially transverse to the flow of fluid across the wall surface, and wherein each ridge of roughness elements forms a substantially saw tooth pattern of roughness elements having a selected wavelength. 8. The wall surface of claim 7, wherein one cavity of the plurality of cavities is formed between adjacent successive ridges of roughness elements. 9. The wall surface of claim 8, wherein the distance between adjacent successive ridges is in a range between about 40 to 60% of the peak longitudinal height of the roughness elements. 10. The wall surface of claim 7, wherein a portion of the respective peripheral edges of the adjacent and aligned roughness elements in a ridge of roughness elements are connected and define a channel between portions of the bases and the bottom portions of the peripheral edges of the adjacent and adjoined roughness elements. 11. The wall surface of claim 10, wherein each channel extends longitudinally substantially co-axial to the flow of the fluid across the wall surface. 12. The wall surface of claim 7, wherein the front, upstream surface of each roughness element has a curved, convex cross-sectional shape relative to the flow of fluid toward the front surface. 13. The wall surface of claim 12, wherein the rear, downstream surface of each roughness element has a curved, concave cross-sectional shape relative to the flow of fluid toward the rear surface, the rear surface being configured to promote the recirculation of the flow within the cavity and to act as a streamlining effect in both stabilizing and promoting an embedded vortex flow field. 14. The wall surface of claim 13, wherein the top of each roughness element is positioned at an acute angle relative to the wall surface such that the tops of the roughness elements do not protrude into the fluid flow substantially normal to the flow direction. 15. The wall surface of claim 7, wherein the roughness elements in adjacent ridges of the array are positioned offset from each other relative to the flow of fluid across the surface. 16. The wall surface of claim 1, wherein the at least one riblet extending from the front, upstream surface and/or the at least one riblet extending from the rear, downstream surface comprise a plurality of riblets. 17. The wall surface of claim 16, wherein a trough is defined therebetween adjacent riblets that are recessed from the respective tips of the riblets. 18. The wall surface of claim 1, wherein adjacent roughness elements within a ridge of roughness elements have different scaled dimensions, such that the formed ridge has a staggered saw tooth appearance. 19. The wall surface of claim 1, wherein a juncture of two adjoining roughness elements acts as a center for each cavity vortex and is configured to allow for a secondary pair of vortices to form inside the larger cavity vortex, wherein the secondary pair of vortices interlock a plurality of formed cavity flows, formed between the respective roughness elements, together in a substantially chain-link type array of streamlines that are relatively stable. 20. An aerodynamic or hydrodynamic wall surface configured to modify the interaction of a boundary layer of a fluid flowing across the wall surface, comprising: at least one array of roughness elements disposed on and extending from the wall surface, the array of roughness elements are positioned in successive ridges of roughness elements, wherein each ridge of roughness elements is positioned substantially transverse to the flow of fluid across the wall surface, and wherein each ridge of roughness elements forms a substantially saw tooth pattern of roughness elements having a selected wavelength,wherein each roughness element extends outwardly from the wall surface at one or more selected angles relative to the flow of fluid across the wall surface such that a preferential flow direction surface is formed, the at least one selected angle comprises acute angles,wherein each roughness element has a front, upstream surface and an opposing rear, downstream surface and a peripheral edge that has an upper portion that tapers to a top and a bottom portion that tapers to a base, which is connected to the wall surface, the front, upstream surface having a curved, concave cross-sectional shape relative to the flow of fluid toward the front surface, and the rear, downstream surface having a curved, concave cross-sectional shape relative to the flow of fluid toward the rear surface that is configured to promote recirculation of the flow within the cavity and to act as a streamlining effect in both stabilizing and promoting an embedded vortex flow field,wherein a plurality of roughness elements are positioned substantially transverse to the flow of fluid across the wall surface such that a distance between a medial portion of the peripheral edges of adjacent and aligned roughness elements is less than the distance between the respective tops of the roughness elements and is less than the distance between the respective bases of the roughness elements,wherein the array of roughness elements defines a plurality of cavities,wherein the preferential flow direction surface is configured for passively resisting flow adjacent the wall surface that is flowing in a direction toward the rear surface,wherein each roughness element comprises at least one riblet extending outwardly from the front, upstream surface, the at least one riblet being configured to aid in the formation and stability of cavity flows embedded between the roughness elements, andwherein each roughness element comprises at least one riblet extending outwardly from the rear, downstream surface of the roughness element, and wherein each rear surface extending riblet extends substantially longitudinally.