Patent Publication Number: US-8113469-B2

Title: Passive micro-roughness array for drag modification

Description:
This application claims priority to U.S. Provisional Application Nos. 60/775,397, filed on Feb. 21, 2006, which is incorporated in its entirety in this document by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to an improved apparatus for reducing or enhancing the skin friction drag of an aerodynamic or hydrodynamic surface, and in particular to an improved micro-array surface design for reducing or enhancing the skin friction drag coefficient and/or heat transfer rate of aerodynamic or hydrodynamic surfaces. 
     BACKGROUND 
     The promise of drag reduction over solid surfaces in high Reynolds number flows is one that has captured the attention of researchers for years, yet has remained illusive. In the past, numerous approaches have used both passive and active methods to control the flow in a turbulent boundary layer. In one exemplary approach, it is relatively well known that the aerodynamic drag of a surface may be reduced by applying a microscopic “texture” to the otherwise smooth surface. Although the exact fluid dynamic mechanism at work in this drag reduction is not well understood, it is speculated that the reduction relates to controlling the turbulent vortices in the boundary layer adjacent to the surface. The microscopic texture reduces the skin friction drag of solids moving through fluids (e.g., aircraft, ships, cars, etc.), and of fluids moving along solids (e.g., pipe flow, etc.). 
     One well known geometric form for a microscopic, friction-reducing texture is known as “riblets.” Conventionally, riblets are positioned on a surface to form an integrated series of groove-like peaks and valleys with V-shaped cross-sections. Normally, the riblets are positioned to extend along the aerodynamic surface of the object in the direction of fluid flow. In one example, the height of the riblets and the spacing between the riblets are usually uniform and on the order of 0.001 to 0.01 inches for most applications. 
     Dimensionless units, sometimes referred to as wall units, are conventionally utilized in describing fluid flows of this type. The wall unit h+ is the non-dimensional distance away from the wetted surface or more precisely in the direction normal to the surface, extending into the fluid. Thus h+ is a non-dimensional measurement of the height of the riblets. The wall unit s+ is the non-dimensional distance tangent to the local surface and perpendicular to the flow direction, thus the non-dimensional distance between the riblets. In the prior art riblets, h+ and s+ are in the range between 10 and 20. Exemplary riblet designs can comprise an adhesive film applied to a smooth solid surface or alternatively, with advanced manufacturing techniques, the same shapes may be directly formed and integrated into the structure of the aerodynamic surface. 
     The interaction of riblets with the structure of the turbulent boundary layer of the fluid reduces the skin friction drag coefficient (Cdf) of the surface by approximately 6% compared to an identical smooth surface without riblets. This reduction occurs despite the significant increase in “wetted area” (the surface area exposed to the fluid stream) of a riblet-covered surface over a smooth surface. In attempts to further reduce the Cdf, modifications to conventional V-shaped riblets have been proposed. Examples include rounding of the peaks and/or valleys of the respective riblets, as well as even smaller V-shaped notches in the sides of the larger V-shaped riblets. 
     Further examples of improved riblet designs that decreases skin friction drag with less concomitant increase in wetted area than conventional riblets include the use of a series of parallel riblets that extend longitudinally from a smooth surface. In this example, the riblets have a triangular cross-section in the transverse direction in which the apex of the cross-section defines a continuous, undulated ridge with peaks and valleys that causes an effective reduction in Cdf. The wetted area of this exemplary design is increased less than with conventional riblets. 
     SUMMARY 
     Embodiments of this invention provide a surface of an object that is configured to provide for either drag reduction or enhancement, with the latter being beneficial in applications where increased turbulent mixing is desired such as in heat transfer applications. 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 roughness elements disposed on and extending therefrom the surface. In one example, the interaction of the roughness elements with a boundary layer of fluid can act to delay transition to reduce the skin friction drag coefficient of the surface over an identical smooth surface without the roughness elements. 
     Other systems, methods, features, and advantages of the passive micro-array system will be or become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the passive micro-array system, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate certain aspects of the instant invention and together with the description, serve to explain, without limitation, the principles of the invention. Like reference characters used therein indicate like parts throughout the several drawings. 
         FIG. 1  shows a schematic flow model for a drag enhancing d-type surface roughness, in which downwash is shown between the counter-rotating vertex pair and upwash, that would occur on either side, is shown on the front region of the surface roughness. 
         FIG. 2  shows a schematic flow model for a drag reducing d-type surface roughness, in which outflow, as depicted by the arrows, from the upstream cavity to the adjacent neighboring downstream cavity occurs through the valleys in the saw tooth geometry of the formed ridges. 
         FIG. 3  shows a schematic front elevational view of one embodiment of a ridge of an array of roughness elements of the present invention. In one aspect, for drag reduction, the elements can be aligned such that the peaks of the roughness elements of each adjacent ridge can be staggered and can be spaced at about half the peak height of the roughness element. In this view, flow will encounter the ridge by moving into the figure. In one exemplary aspect, the spacing between the peaks of the adjoined roughness elements is on the order of about 30 viscous length scales at close to maximum velocity for the fluid passing over the wall surface. 
         FIG. 4  is a side elevational schematic view of the exemplary micro-array of roughness elements shown in  FIG. 3 , showing the tops of the roughness elements of  FIG. 3  and showing the formation of counter-rotating streamwise vortices due to the staggered alignment of adjacent rows of the roughness elements in the drag enhancing case. The flow of fluid is directed into the figure. 
         FIG. 5  is a top elevational schematic view of exemplary vertex structures that form within the transversely extending cavities of an exemplary micro-array of roughness elements of  FIG. 3  of the present invention, showing fluid flow moving from the bottom to the top of the figure and showing dark short lines correspond to the peaks of the roughness element in  FIG. 3 . 
         FIG. 6  is a perspective view of one embodiment of a roughness element of a micro-array of the present invention, showing riblets formed on a front, upstream surface of the roughness element. 
         FIG. 7  is a side elevational view of the roughness element of  FIG. 6 . 
         FIG. 8  is a top elevational view of the roughness element of  FIG. 6 . 
         FIG. 9  is front, upstream elevational view of a plurality of adjoined roughness elements of  FIG. 6  that form a ridge, and showing a plurality of channels formed between portions of the respective bases and the bottom portions of the peripheral edges of the respective adjoined roughness elements. 
         FIG. 10  is a perspective view of a portion of a micro-array of the present invention, showing a plurality of staggered rows of the formed ridges of adjoined roughness element of  FIG. 8 , and showing the approximate spacing between the rows of ridges to be approximately half the height of a roughness element. 
         FIG. 11  is a schematic diagram of cavity flow of representative fluid flow between the tops of roughness elements of  FIG. 6  and across one “valley,” the roughness elements being positioned in adjacent ridges or rows. In this diagram, fluid flow over the surface is from left to right. 
         FIG. 12  is a top elevational schematic view of exemplary vertex structures that form an exemplary micro-array of roughness elements of  FIG. 6  of the present invention, showing fluid flow moving from the left to the right of the figure. The vortices represent the outer vortices shown in  FIG. 11  and may have small counter-rotating vortices superimposed on the outer-vortices that make the flow field consistent to its neighboring vortices. In the exemplified aspect with three riblets on the front face of the roughness element, two counter-rotating vortices would form with an upwelling between them and a downwash to the flow at the sides. These vortices are also known as Taylor-Gortler vortices. The vortex tubes represent the vortex cores to the vortex array that link all the individual outer cavity vortices together. 
         FIG. 13  is a graphical illustration of a two-dimensional computational fluid dynamics (CFD) numerical calculation through a line of symmetry over the peaks and valleys; of the roughness elements in drag reduction mode. The cavity Re for this calculation is 2000, and the formation of stable cavity vortices is observed. 
         FIG. 14  is a graphical illustration of the velocity profiles in the boundary layer forming over the surface in  FIG. 13  above the third and eighth cavities. These profiles are compared to that of a flat plate boundary layer, known as the Blasius solution. One can observe the non-zero velocity over the surface of the cavities due to the embedded cavity vortex. One skilled in the art will appreciate that one can obtain the momentum thickness of the two boundary layers, which is proportional to the total drag coefficient on the plate from the leading edge to that corresponding downstream distance, by integrating these velocity profiles. In one example, the momentum thickness over the third cavity is 16.09% of the momentum thickness of the flat plate Blasius solution, while at the eighth cavity the percentage of the momentum thickness of the surface with cavities with respect to the flat plate solution is 23.91%. Thus, at the third and eighth cavity, the drag coefficient is reduced by 84% and 76% correspondingly. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention can be understood more readily by reference to the following detailed description, examples, drawings, and claims, and their previous and following description. However, before the present devices, systems, and/or methods are disclosed and described, it is to be understood that this invention is not limited to the specific devices, systems, and/or methods disclosed unless otherwise specified, as such can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting. 
     The following description of the invention is provided as an enabling teaching of the invention in its best, currently known embodiment. To this end, those skilled in the relevant art will recognize and appreciate that many changes can be made to the various aspects of the invention described herein, while still obtaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be obtained by selecting some of the features of the present invention without utilizing other features. Accordingly, those who work in the art will recognize that many modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances and are a part of the present invention. Thus, the following description is provided as illustrative of the principles of the present invention and not in limitation thereof. 
     As used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a roughness element” includes arrays of two or more such roughness elements, and the like. 
     Ranges can be expressed herein as from “about” one particular value, and/or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. It will be further understood that the endpoints of each of the ranges are significant both in relation to the other endpoint, and independently of the other endpoint. It is also understood that there are a number of values disclosed herein, and that each value is also herein disclosed as “about” that particular value in addition to the value itself. For example, if the value “10” is disclosed, then “about 10” is also disclosed. It is also understood that when a value is disclosed that “less than or equal to” the value, “greater than or equal to the value” and possible ranges between values are also disclosed, as appropriately understood by the skilled artisan. For example, if the value “10” is disclosed the “less than or equal to 10” as well as “greater than or equal to 10” is also disclosed. It is also understood that throughout the application, data is provided in a number of different formats and that this data represents endpoints and starting points, and ranges for any combination of the data points. For example, if a particular data point “10” and a particular data point 15 are disclosed, it is understood that greater than, greater than or equal to, less than, less than or equal to, and equal to 10 and 15 are considered disclosed as well as between 10 and 15. It is also understood that each unit between two particular units are also disclosed. For example, if 10 and 15 are disclosed, then 11, 12, 13, and 14 are also disclosed. 
     As used herein, the terms “optional” or “optionally” mean that the subsequently described event or circumstance may or may not occur, and that the description includes instances where said event or circumstance occurs and instances where it does not. 
     The present invention may be understood more readily by reference to the following detailed description of embodiments of the invention and the Examples included therein and to the Figures and their previous and following description. 
     Referring to  FIG. 1 , an array of roughness elements  10  with the induced flow field is illustrated. As shown, spanwise or transverse cavities  16  defined between the ridges  12  that are exemplarily formed from adjoined roughness elements  20  that are positioned substantially transverse to the flow of the fluid over the surface  2 , which results in a series of cavity flows, each containing a re-circulating flow field. In the exemplary embodiment illustrated in  FIGS. 1 and 2 , roughness elements  20  are integrally connected together to form individual ridges  12  that are positioned on and extend from the surface  2  substantially transverse to the flow of fluid across the surface  2 . In one aspect, the ridges  12  are spaced substantially uniform and, optionally can be variably spaced. 
     In one aspect, due to the spacing of the saw tooth peaked roughness elements  20 , an on average streamwise vortex forms in the flow above each cavity, such as found in the case of drag enhancing riblets. In one aspect, it is contemplated that the cavities would comprise vortices of alternating sign as this would appear to provide the most stable flow regime. In this aspect, and as illustrated, neighboring vortices contribute to upwashes and downwashes in an alternating manner across the spanwise direction. 
     One skilled in the art will also appreciate that alternative shapes of the roughness elements  20  are contemplated. Exemplary alternative shapes can comprise, but are not meant to be limited to, a blade-like thin peak, which allows the formation of an increased number of vortices in a predetermined spanwise dimension, a trapezoidal cross-sectional shape with a flat portion of the ridge over which the vortices will form, and the like. 
     Independent of the ideal shape of the ridges  12 , the overall characteristics of the flow field remains unchanged. In operation, and referring to  FIG. 1 , a fluid particle would enter from the left at some distance above the surface  2 , such as exemplary shown as a flat plate. As the fluid particle approaches the surface it feels the presence more of the counter-rotating vortex pair and is pulled downward into a region of downwash. As it enters this downwash, the fluid particle enters&#39; the cavity  16  and is spun around, in an almost slingshot type motion, and injected back out above the surface through an upwash region of the channels. From a heat transfer standpoint, the proposed surface causes fluid particles far away from the surface to come in contact (or very near) to the surface for a short period of time and then to be pushed out again far above the surface. With this “on average” flow field, the burst/sweep process has been accentuated and controlled to take place in an organized manner. Thus, in one aspect, the exemplary array of roughness elements  10  provides an efficient manner by which a turbulent boundary layer flow can be optimized for convective heating/cooling purposes over a solid surface. 
     In one aspect of the invention, in order to cause as much fluid as possible to come in contact with the “rough” surface  2 , the spacing between the transverse cavities  16  should be minimized. However, if the spacing became too small, the mass flow rate pumped through the cavities would decrease due to viscous effects. In one exemplary aspect, the average height of the ridges (h + ) is substantially equal to the width of the cavity (w + ), or is about a one to one height to width ratio (h + ≈w + ). In another aspect, with respect to the average height of the cavities, it can be greater than about half the peak-to-peak amplitude of the saw tooth pattern along the ridges. In an exemplary aspect, the amplitude for riblet spacing would be about and between 10s +  to 20s + . In another example, the amplitude would be about 15s + . In this aspect, this would also be the average height of the ridges, with the minimum valley point of the ridges located at an elevation of s +  that is about 7.5 (±2.5) above the bottom of the cavity, and maximum peak located at s +  that is about 22.5 (±2.5). 
     In a further aspect, the wavelength of the saw tooth pattern can be about λ + =40, based on the size of a typical vortex mentioned previously of s +  being about 30. This would be sufficient to hold a vortex between the peaks. Of course, it will be appreciated that these dimensions are exemplary only and are not meant to be limiting. Further, one will appreciate that the exemplary dimensions can be scaled as desired. 
     Referring now to  FIG. 2 , an exemplary flow field through the drag reducing roughness element  20  is illustrated. It has been demonstrated that a series of transverse cavities  16  with substantially constant ridge height is prone to a random efflux/influx of fluid due to the high shear region located above the cavities. This high shear region results in the formation of streamwise vortices and low speed streaks above the cavities such as found in the smooth surface case. It is likely that the peak velocity may be larger for cavities  16  formed by a series of transverse blades, but would more than likely still be a large enough percentage below the freestream that streamwise vortices would still be formed due to a high shear region above the cavities. As shown in  FIG. 2 , to prevent and/or reduce the efflux/influx process out/into the cavities, a saw tooth geometry is defined by the respective roughness elements  20  that form the ridges of the array of roughness elements. 
     In this example, the substantially transverse cavities formed between the adjacent ridges help with the stability of the flow field as the flow through the cavities is given a longer distance (two cavity widths as opposed to one) by which it is exposed and pulled along by the flow directly above. As a result of the exemplary geometry, the estimated peak velocity achieved is in a range between about 5 to 40 percent of the freestream flow. Second, the jets formed through the cavities are substantially tangent to the flow above so that very little vertical velocity component is formed. If one were looking down onto the surface, the formed jets would appear to be a periodic array of suction and blowing at a smooth wall. Finally, the flow acting on the bottom of the cavities results in a shear stress that provides thrust to the surface. In this case the effect is such that it may act to cancel out a large percentage of the skin friction losses due to the momentum change in the flow over the vertical walls of the cavities. It is contemplated that this effect is more pronounced as higher peak velocities in the jets (and thus closer to the bottom surface of the cavities) are achieved. Thus, in one example, the width of the cavities  16  can be increased or maximized (such that the stable flow field in  FIG. 2  is maintained) so as to decrease the number of spanwise channels over a given surface area. 
     In this aspect, considering an averaged streamline through the roughness element, a fluid particle that starts from the left close to the surface would approach a transverse cavity in the array and upon entering the cavity be captured by the cavity vortex and travel around in a spiral motion before being passed through another cavity just to enter the neighboring cavity and repeat the previous motion. In this example, all fluid near the ridge stays near the ridge and there is little or no on average vertical velocity component away from the cavities of the array. Given the flow model as stated, and that the cavities are dimensionally small enough such that viscous effects dominate, it is contemplated that the net skin friction drag over such an exemplary surface could start to approach that of a laminar flat plate boundary layer. 
     In one aspect, the formed “rough” surface can be categorized as a series of trapezoidal channels (d-type roughness geometry) that are orientated in the spanwise direction (transverse to the flow of fluid across the array), but, in one exemplary aspect, with a saw tooth geometry of alternating peaks along the ridges of the channels giving the surface a three-dimensional, yet repeatable, pattern. The alignment of the peaks in the streamwise direction of the flow of fluid is proposed to increase drag, while the alternation of the peaks in the streamwise direction will decrease drag. In one aspect, the spacing between the ridges in the streamwise direction can vary from ½ to a full value of the peak height (or amplitude) of the ridges with respect to the bottom of the cavities. In another aspect, the distance between adjacent successive ridges can be in a range of between about 40 to 60% of the peak longitudinal height or amplitude of the roughness elements that form the respective ridges. Optionally, the distance between adjacent successive ridges can be in a range of between about 45 to 55% of the peak longitudinal height or amplitude of the roughness elements that form the respective ridges 
     In an alternative embodiment of the invention, and referring now to  FIGS. 3-12 , the micro-array  10  can comprise a plurality of roughness elements  20  that can extend from the surface and be positioned in spaced ridges  12  along the surface  2 . In this aspect, it is contemplated that each roughness element  20  has a front, upstream surface  22  and an opposing rear, downstream surface  24 . Further, each roughness element has a peripheral edge  26  that has an upper portion  28  that tapers to a top  29  and a bottom portion  30  that tapers to a base  31 . As one would appreciate, the base is configured to be connected to the underling surface  2  of the object. In one exemplified aspect, the roughness elements  20  are positioned on the underlying surface  2  substantially transverse to the flow of the fluid across the surface. In another aspect, the roughness elements extend substantially normal to the underlying surface. For example, and not meant to be limiting, the transverse longitudinal height of the roughness elements can be between about 0.001 to 2.00 cm. 
     In one aspect of the invention, a plurality of roughness elements  20  can be positioned transverse to the flow of fluid across the surface such that a distance between a medial portion  32  of the peripheral edges of adjacent and aligned roughness elements  20  is less than the distance between the respective tops  29  of the roughness elements and is less than the distance between the respective bases  31  of the roughness elements. In a further aspect of the inventions, adjacent and aligned roughness elements can be connected at some selected portion of the respective peripheral edges of the roughness elements. In this aspect, a channel  34  is defined therebetween portions of the bases and the bottom portions of the peripheral edges of the adjacent and adjoined roughness elements. In one exemplary aspect, it is contemplated that the formed channels would extend longitudinally substantially co-axial to the flow of the fluid across the surface. In an alternative aspect of the invention, the adjoining roughness elements can be connected together such that no channel is formed therebetween the respective adjoining elements. In a further aspect, the adjoined roughness elements can form a “saw tooth” ridge that extends substantially transverse to the fluid flow. 
     In one embodiment, the roughness element  20  has a substantially diamond cross-sectional shape, as shown in  FIG. 3 . Alternatively, and as shown in  FIG. 6 , the roughness element  20  can have a substantially oval shape. Of course, one skilled in the art will appreciate that other geometric shapes are contemplated and that the aspects illustrated are merely exemplary. 
     Referring now to  FIGS. 6-10 , in one aspect, it is contemplated that the front, upstream surface  22  of the roughness element  20  has a curved, convex cross-sectional shape relative to the flow of fluid across the surface  2  of the object. In another aspect, it is contemplated that the rear, downstream surface  24  of the roughness element has a curved, concave cross-sectional shape relative to the flow of fluid to promote the recirculation of the flow within the cavity, and to act as a streamlining effect in both stabilizing and promoting the embedded vortex flow field. In one aspect, this slight concavity in the rear surface  24  of the roughness element also acts to position the tops  29  of the roughness elements at a slight, acute angle relative to the underlying surface such that the tops of the roughness elements do not protrude into the fluid flow normal to the flow direction. In one aspect, it is contemplated that the radius of curvature of the rear surface  24  of the roughness element is less than the radius of curvature of the front surface  22  of the roughness element. 
     In a further aspect of the present invention, each roughness element  20  can have at least one riblet  40  extending outwardly therefrom the front surface  22  of the roughness element. In one aspect, the riblet  40  extends longitudinally from at or near the bottom portion  30  of the roughness element, proximate the base  31 , to at or near the top  29  of the roughness element. That is, in one aspect, the riblet extends substantially transverse to the underlying surface. If a plurality of riblets are used, it is contemplated that the ribs can be spaced apart substantially equal or at varying distances. Of course, the number of riblets  40  may vary in number, but typical values would be that from 1 to 7 per each longer wavelength of the saw tooth pattern of the formed ridge of the micro-array. In one aspect, the number of riblets is 1, 3, 5, or 7. 
     The presence of the riblets  40  formed to either the front surface  22 , or, optionally, to both sides of the roughness element, act to give a streamlining effect that is conductive to the formation and stability of the cavity flows (or vortices) embedded within the cavities formed between adjacent ridges or rows of the roughness elements. In one aspect, the addition of the riblets to the roughness elements micro-geometry help to increase drag reduction, such as, for example, with higher speed flows. In a further aspect, the riblets  40  act to excite counter-rotating vortices within the outer vortex structure that when in even numbers (formed by an odd number of riblets) promote the stability of the vortex array in the surface. 
     Further, in another aspect, it is contemplated that a trough  42  is defined therebetween adjacent riblets  40  that is recessed from the respective tips  44  of the riblets. In one aspect, the trough may be formed by a smooth, curved surface. Of course, it is contemplated that the surface of each of the troughs in the respective roughness element can have a substantially equal radius of curvature or can vary as desired. 
     In another aspect, the riblets  40  have an edge surface  46  that extends between the respective riblets that are adjacent to the sides of the roughness element. In one aspect, the edge surface  46  can be substantially planar. Alternatively, at least a portion of the edge surface can be curved. In the curved aspect, it is contemplated that the radius of curvature of the edge surface can be greater than the radius of curvature of the troughs  42  of the roughness elements. 
     It is further contemplated that the geometry of the formed surface of the present invention can be altered as a function of the thickness of the boundary layer adjacent to the surface. For example, in regions where the boundary layer is thicker, the tops  29  of the roughness elements  20  may also comprise an additional saw tooth pattern of shorter wavelength superimposed on the larger wavelength saw tooth pattern. This is of importance in regions far downstream from the leading edge of a body where the boundary layer is thicker, yet the flow outside the boundary layer and above the surface is of high velocity. 
     In a drag reduction mode, the saw tooth pattern on the tops  29  of the roughness elements  20  acts to inhibit the formation of the optimal perturbations that appear due to the instability of the shear flow (or boundary layer) above the roughness element and inside the boundary layer. At lower speeds this wavelength is larger. Conversely, at higher speeds this wavelength is smaller. In one exemplary aspect, the smaller wavelength superimposed on the larger saw tooth tops can vary from between about ⅓ to 1/7 that of the larger wavelength. The sizing is a function of the speed of the flow outside the boundary layer adjacent to the surface (U), the kinematic viscosity of the fluid (ν) and the maximum shear in the boundary layer ((du/dy) max ). It should be noted that as a body moves at higher speeds, the boundary layer at a particular point on the body will reduce in thickness and the maximum shear sustained in the boundary layer will increase. This corresponds to a decrease in the wavelength sizing required of the roughness element to act in drag reduction mode. 
     Regardless of whether a surface results in the formation of embedded vortices within the respective roughness elements or not, the “male protrusions” that result from the roughness elements and their sizing may be sufficient enough to delay the transition to turbulence in the boundary layer and thus still result in drag reduction. However, to maximize the drag reduction characteristic of the micro-array of roughness elements of the present invention would include both the formation of the embedded spanwise vortex array within the roughness element as well as the protrusion geometry of the roughness geometry, which leads to the damping of instabilities in the boundary layer that result in the transition to turbulence. 
     In addition, and as noted above, the downstream side of the roughness elements can, or can not, comprise a slightly concavity to the surface (see  FIG. 7 ) as well. This thickness to the peak of the formed ridge provides a smooth line of reattachment for the separated shear layer over the top of the cavity from the previous upstream roughness element and at the top of the roughness element provides for a tangential meeting of this outer flow with the next downstream embedded cavity vortex (again, see  FIG. 7 ). All of the elements listed here have to do with the effects of streamlining the micro-geometry to promote the formation of a stable, embedded cavity vortex within the roughness element. 
     Further, it is contemplated that the micro-array of roughness elements  10  on the surface  2  can comprise a plurality of micro-arrays of roughness elements  10  on the respective surface  2 . In this aspect, each micro-array can comprise a plurality of roughness elements, as described above, of a predetermined height and/or shape. Thus, it is contemplated that, the plurality of micro-arrays could comprise arrays of varying sized or shaped roughness elements. 
     In another aspect, each micro-array of roughness elements can comprises individual roughness elements that vary in respective scale and/or shape. For example and not meant to be limiting, adjacent roughness element could have different relative scaled dimensions. Thus, a “large” roughness element can adjoin a “small” roughness element, such that a front view would be of a line or ridge of the adjoining roughness elements that have a staggered saw tooth appearance. 
     In the arrays discussed above, the formed channel  34  between adjoining roughness elements  20  allows for some of the reversed flow at the bottom of the cavities between adjacent span-wise extending ridges of lines of the roughness elements to head back upstream to the adjacent, neighboring cavity through the channels between the roughness elements. In operation, a cavity flow may result such that fluid particles stay in the cavities to continue the circulatory pattern between the two cavities, i.e., entering the downstream cavity over the top of the valley to return back to the upstream cavity through the gap beneath the valley as shown in  FIG. 11 . The juncture of the two adjoining roughness elements acts as a center for each individual cavity vortex and may also allow for a secondary pair of vortices to form inside the larger cavity vortex, which is also shown in  FIG. 11 . Referring to  FIG. 12 , these vortices, one inside each transverse half cavity, provides a means of interlocking all of the cavity flows together in an almost chain-link type array of streamlines that are relatively stable and are not subject to cavity influx/efflux of flow, which leads to an increase in drag for the d-type surface. As noted above, the micro-geometrical patterning of a surface in this invention for maximum drag reduction mode results in the formation of an array of embedded cavity flows (or vortices) between the roughness elements. 
     It is contemplated that the flow arranged by this roughness element is a series of micro-slip walls in which the orange ovals in  FIG. 12  denote each micro-slip wall. From another standpoint, it is contemplated that the roughness element of the present invention alters the no slip condition which the outside flow sees at the wall. Further, it is known that embedded cavity flow can be used as a means of separation control due to the alteration of the no-slip condition at the surface. It is contemplated that the roughness element described herein can be used in applications that would reduce the pressure drag associated with separated flows over surfaces. 
     In a further aspect of the “roughness” surface, the thickness of the boundary layer can be in a range of at least 10 to 30% of a cavity height of each cavity such that shear layer instabilities of cavity vortexes that form therein the plurality of cavities are reduced. Preferably, about at least 20% of the cavity height. Typically, cavity height would be measured from the surface  2  of the object to the peak or highest amplitude of the roughness elements that form the transversely disposed ridge. In one aspect, each formed cavity vortex can have 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. Optionally, each formed cavity vortex can have 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. 
     The micro-arrays of the roughness elements of the present invention would find applicability in drag reduction modalities, such as, for example and not meant to be limiting, on the surfaces of aircraft, submarines, ship hulls, high speed trains and the like. In the case of the flow over the hull of a ship, the micro-arrays of the roughness elements can impact the boundary layer formation over the hull and therefore affect the amount of air ingested below the water line, thereby altering the entire flow field of a ship&#39;s wake. It is also contemplated than the micro-arrays can be used in pipeline walls as well, which would result in a large reduction in the amount of energy saved to pump fluids from one point to another. 
     It is also contemplated that the micro-arrays of the present invention allows for the trapping of pockets of air inside the cavities such that, for example, in hydrodynamic applications, the working fluid for the micro-slip walls would consist of these air pockets. This would also reduce the skin friction for hydrodynamic applications and, in another aspect, can reduce cativation. 
     Still further, the micro-arrays of roughness element can act as a means of controlling separation. The effect of the arrays acts to reduce pressure drag over bluff bodies such as automobiles and trucks. It can also minimize separation over turbine blades, airfoils, and helicopter rotors as well as flow through serpentine ducts, which is often a requirement for inlet geometries for engines on an aircraft. Optionally, in a drag enhancement mode, a surface formed with the micro-array of roughness elements of the present invention allows for highly effective convective cooling to the surfaces of computer board components, which could greatly impact the performance of these devices. 
     It is also contemplated that the self-cleaning property of the roughness elements should be excellent due to the high shear rates resulting over the major portions of the surfaces of the roughness elements. However, it is also contemplated to use hydrophobic materials in constructing the roughness elements for hydrodynamic applications. 
     It is contemplated that a surface formed with a micro-array of roughness element as described above, could be formed for a saw tooth wavelength that corresponds to that of the optimal perturbation wavelength for the shear flow inside the boundary layer. In this example, the alignment or alternation of the peaks to achieve maximum heat transfer rates and maximum drag at a surface is considered. In one aspect, the alternation of the peaks forces the half-wavelength of the saw tooth amplitude to correspond to the optimal perturbation wavelength. Thus, it is contemplated that the formed drag reducing surface could become drag enhancing as the flow speed is increased. 
     The preceding description of the invention is provided as an enabling teaching of the invention in its best, currently known embodiment. To this end, those skilled in the relevant art will recognize and appreciate that many changes can be made to the various aspects of the invention described herein, while still obtaining the beneficial results of the present invention. It will also be apparent that some of the desired benefits of the present invention can be obtained by selecting some of the features of the present invention without utilizing other features. The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or acts for performing the functions in combination with other claimed elements as specifically claimed. 
     Accordingly, those who work in the art will recognize that many modifications and adaptations to the present invention are possible and can even be desirable in certain circumstances and are a part of the present invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. Thus, the preceding description is provided as illustrative of the principles of the present invention and not in limitation thereof. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.