PASSIVE DRAG MODIFICATION SYSTEM

• US 20100108813A1
• Filed: 03/31/2008
• Published: 05/06/2010
• Est. Priority Date: 03/30/2007
• Status: Active Grant

• Current Assignee: Board of Trustees of The University of Alabama (University of Alabama System)
• Original Assignee: Board of Trustees of The University of Alabama (University of Alabama System)
• Inventors: Lang, Amy W.
• Granted Patent: US 8,794,574 B2
• Time in Patent Office: Days
• US Class Current: 244/130
• CPC Class Codes: B63B 1/36 using mechanical means
• CPC Class Codes: B64C 21/10 using other surface properties, e.g. roughness
• CPC Class Codes: B64C 2230/26 by using rib lets or hydrophobic surfaces
• CPC Class Codes: F15D 1/0045 oriented essentially perpendicular to the direction of flow
• CPC Class Codes: F15D 1/06 by influencing the boundary layer
• CPC Class Codes: Y02T 50/10 Drag reduction
• CPC Class Codes: Y02T 70/10 Measures concerning design or construction of watercraft hulls

Abstract

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.

Claims

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 therefrom the wall surface, wherein each roughness element extends outwardly from the wall surface at at least one selected angle 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, an upstream surface and an opposing rear, downstream surface, wherein each roughness element has 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, and 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 therein the plurality of cavities are reduced.

2. The wall surface of claim 1, wherein each formed cavity vortex has a 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 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 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 underlying 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 each roughness element has a front, upstream surface and an opposed, downstream surface, and wherein the front, upstream surface of each roughness element has a curved, convex cross-sectional shape relative to the flow of fluid across the wall 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 that is 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 13, 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.

16. The wall surface of claim 15, wherein each roughness element comprises at least one riblet extending outwardly therefrom the rear, downstream surface of the roughness element, and wherein each riblet extends substantially longitudinally.

17. The wall surface of claim 15, wherein the at least one riblet comprises a plurality of riblets.

18. The wall surface of claim 17, wherein a trough is defined therebetween adjacent riblets that are recessed from the respective tips of the riblets.

19. 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.

20. The wall surface of claim 7, wherein each ridge of roughness elements of the array has a saw tooth wavelength that is substantially equal to an optimal perturbation wavelength for the shear flow inside the boundary layer.

21. The wall surface of claim 1, wherein adjacent roughness elements within a ridge of roughness elements can have different scaled dimensions, such that the formed ridge has a staggered saw tooth appearance.

22. The wall surface of claim 1, further comprising a means of interlocking 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.

23. An aerodynamic or hydrodynamic wall surface configured to modify the interaction of a boundary layer of a fluid flowing across the wall surface, comprising:

a means for forming a partial slip condition, formed at the boundary separating the cavity flow fluid and the outer flow fluid, that results in reduced momentum thickness within the boundary layer, wherein the means for forming a partial slip condition comprises a microgeometry formed on the wall surface comprising a three-dimensional array of micro-cavities, wherein each micro-cavity has a substantially constant depth

24. The wall surface of claim 23, wherein the micro-cavities are rectangular in shape and are positioned adjacent to each other in rows that are aligned substantially transverse to the flow of fluid over the wall surface.

25. The wall surface of claim 24, wherein adjacent rows of the micro-cavities are positioned offset relative to each other.

26. The wall surface of claim 23, wherein the micro-cavities are hexangonal in shape and are positioned adjacent to each other in rows that are aligned substantially transverse to the flow of fluid over the wall surface.

27. The wall surface of claim 23, wherein each micro-cavity has a plurality of cavity walls and each cavity wall has a top edge that is in the generalized plane of the wall surface.

28. The wall surface of claim 27, further comprising a plurality of upwardly extending ribs that are connected to and extend outwardly from at least a portion of the top edges of the plurality of cavity walls that are substantially aligned with the flow of fluid over the wall surface.

29. The wall surface of claim 23, wherein each micro-cavity has a plurality of cavity walls and each cavity wall has a top edge, wherein portions of the plurality of cavity walls of the micro-cavities extend upwardly above the generalized plane of the wall surface to form wall extensions.

30. The wall surface of claim 29, wherein the wall extensions are formed on the cavity ails that are substantially aligned with the flow of fluid over the wall surface.

31. The wall surface of claim 30, wherein the wall extensions can extend at least a portion of the length of the cavity walls that are substantially aligned with the flow of fluid over the wall surface.

32. The wall surface of claim 31, wherein the height of the wall extensions above the generalized plane of the wall surface is a multiple of the depth of the cavity.

33. The wall surface of claim 32, wherein the multiple is in a range between about 0 to 4.

34. The wall surface of claim 23, wherein the cavity Re is in a range between about 1500 to 2500.

35. The wall surface of claim 34, wherein the cavity Re is about 2000.