CONTROL OF FLUID FLOW

Abstract: To modify a turbulent boundary layer flow over a surface to reduce its drag, a series of streamwise riblet projections is provided on the surface. These are arranged in a pattern consisting of spaced larger height projections (R) between which are a plurality of smaller height projections (r). The latter inhibit turbulent eddy motions from penetrating deep into the grooves between them and so displace the turbulent motion away from the wall. The larger projections control the longitudinal vortices or streaks that are characteristic of turbulent boundary layer flow and promote energy transfer to the smaller scale secondary vortices associated with the smaller projections.

Description: This invention is concerned with the modification of turbulent boundary layer flows over aerodynamic and hydrodynamic surfaces. There have been many investigations in recent years of the effects that small-geometry surface configurations can have on turbulent boundary layers. Particular attention has been paid to the provision of so-called riblet surfaces in which an array of small longitudinal ribs extend over the turbulent boundary layer region of a surface in the direction of fluid flow over the surface, and experimental results indicate that net or surface drag reductions of up to about 7% can be achieved. In a paper "Drag Characteristics of V-Groove and Transverse Curvature Riblets" (presented by M.J. Walsh at the Symposium on Viscous Drag Reduction, Dallas, Texas, November 7-8, 1979) reference is made to an early investigation (1966) by Liu, Kline, and Johnston in which drag reductions of 3-4% were obtained by reducing the turbulent bursting rate (i.e. the rate of break-up of the low speed longitudinal vortices or "streaks" that are characteristically formed in a turbulent boundary layer flow close to a wall) by employing rectangular fins. Walsh's paper reports the investigation of a number of alternative rib profiles and states that he was able to obtain a maximum drag reduction of 7% using V-groove riblets, this representing substantially the greatest reduction of skin friction drag that experimenters have been able to obtain up to the present. The drag reduction may be associated with the ability of the riblets to limit random spanwise movements of the streaks, as has been suggested by a number of sources, e.g. R.E. Falco (AIAA-83-0377, AIAA 21st Aerospace Sciences Meeting, January 10-13 1983, Reno, Nevada). Johansen and Smith (AIAA-85-0547, AIAA-Shear Flow Control Conference, March 12-14, 1985, Boulder, Colorado) have shown that cylindrical riblets of a smaller height than the V-groove riblets with which Walsh obtained his optimum results, have the effect of anchoring and/or creating sites of low-speed streaks in a limited region above the wall surface, but their experiments also showed drag increases of 3% to 8%. More recently, S.P. Wilkinson has attempted to follow Johansen and Smith by using blade riblets to anchor or create low-speed streaks by the use of riblets and then use V-groove riblets between the streak anchors to control bursting, but it is not yet reported whether a net drag reduction has been achieved thereby. The results reported from these and other previous investigations all show the effect of riblets is rather limited, and that has led to the search for alternative solutions. It has been proposed (U.S. patent application 686959 of M.J. Walsh et al - NASA case LAR-13286-1) to reduce skin friction more effectively by providing a riblet surface with superimposed large-eddy break-up devices (LEBU's) as manipulators for the outer region of the boundary layer but such an arrangement adds considerable complexity and is susceptible to wear and especially accidental damage. In another proposal ("On the Drag Reduction of the Shark Skin", Bechert, Hoppe and Reif, AIAA-85-0546) it has been suggested that it may be appropriate to use so-called vortex generator surfaces which are configured so as to produce tangential mixing using local flow conditions over a ridge-pattern structure analogous to the configuration of scales on a shark's skin. It is claimed that this could lead to greater drag reduction, but it would also be inherently complicated and difficult to build up a simulation of the scaly surface that the investigators have concluded is responsible for the low drag characteristic. In contrast to these attempts to reduce skin friction by rather complex means, riblet surfaces are less susceptible to damage and they can be formed relatively straightforwardly, e.g. by machining or pressing or by the application of a preformed, e.g. extruded, layer. It would clearly be preferable to employ such surfaces if they were able to give greater reductions of skin friction drag. The mechanisms by which riblets influence the turbulent boundary layer over a wall is not yet fully understood. If it can be postulated that they bring a turbulent flow closer to laminar flow conditions their efficiency is low, bearing in mind that in equivalent turbulent and laminar flow regions the skin friction drag with laminar flow would be 80% less than that with turbulent flow. If a more significant proportion of this potential improvement could be achieved, the use of riblet surfaces could offer marked advantages over the complex drag reduction systems described above. According to the present invention, there is provided an aerodynamic or hydrodynamic wall surface which is provided with a series of elongate projections extending in the direction of fluid flow relative to the surface for modification of a turbulent boundary layer on said surface, said projections comprising spaced larger height projections between each successive pair of which are a plurality of smaller height projections. The projections preferably form a substantially continuous spanwise array. It is also preferred to form both the larger and smaller projections with sharp-peaked profiles. The smaller height projections of a configuration according to the present invention correspond to the riblets of recent researchers. They function to inhibit turbulent eddy motions in the boundary layer from penetrating to all parts of the surface, in particular deep into the grooves defined by adjacent riblets, resulting in a displacement of the turbulent motion away from the wall. Depending upon their profile, they may also have some effect in inhibiting the spanwise gradients associated with the formation of the longitudinal vortices or "streaks" in the boundary layer and initiating secondary, smaller-scale longitudinal eddies which take energy from the larger scale streaks. Such riblets however, have hitherto only shown limited results, and it seems that this is because they have functioned wholly or primarily in a passive manner with little influence on the evolution of the eddies themselves. In that case they are largely unable to moderate the mixing and momentum transfer that the eddies encourage and that is the major factor in the increase of boundary layer drag of turbulent flow in comparison with laminar flow. If, however, in accordance with the present invention the larger height projections are provided as, preferably individual, riblets set at appropriate intervals between series of smaller height riblets, the longitudinal vortices or streaks, which otherwise shift in a random manner transversely across the surface, can not only be constrained laterally but can be moved further apart than their natural spacing. By increasing the disparity between the spanwise scale of the streaks and that of the smaller riblets, the larger riblets can promote a more rapid energy transfer from the streaks to the smaller-scale secondary vortices that these smaller riblets induce, and they can produce effects that permeate to the region above the smaller riblets and so improve the effectiveness of these riblets in that these reduce eddy activity there. In this sense both the larger and smaller riblets are able to function in an active manner. In a preferred configuration according to the present invention, at least the larger height projections have side faces that are inclined at an angle that varies over the height of the projections, each said side face having an intermediate region at a lesser steepness than the adjacent regions above and below said region and forming the lower part of a concave profile peak of the associated projection. For best effect the transition to the concave-sided peaks should be positioned at or close to the virtual wall height established by the projections. The region below the transition preferably has substantially planar faces so as to form a series of steep-sided V-grooves between the projections in their lowest reaches. These can act in the manner already described to displace the turbulent motion away from the wall surface. The longitudinal vortices displaced above the virtual wall surface are then controlled by the concave peaks projecting into this region which, by virtue of their different profile, can act more strongly on the vortices. The transition to the concave profile may be substantially sharp-­ edged, but it is also possible to have a radiussed or chamfered transition. The concave faces are preferably continuously curved to a relatively sharp-edged tip but it is also possible to build them up from a group of straight and/or curved sections and for convenience the tip itself may be flattened or radiussed. By way of example, reference will now be made to the accompanying drawings, in which: Fig. 1 illustrates in schematic form a transverse cross-section through a wall surface formed in accordance with the invention, perpendicular to the flow direction over the surface, and Figs. 2 to 7 illustrate a number of modifications that can be applied individually or in any combination to the configuration of Fig. 1. In Fig. 1, the wall surface is continuously ribbed in a regular pattern spanwise which consists of parallel groups of 8 uniform, smaller riblets r between individual larger riblets R, the riblets running in the stream direction over the surface in the turbulent boundary layer region, and preferably starting from the transition region of the flow over the surface. The pattern extending streamwise preferably continues over all the turbulent boundary layer area of the surface. It is not essential that the riblets be aligned precisely with the fluid stream and deviations of up to 10°-15° can be tolerated. The riblets are all shown with triangular profiles, but other profiles can be used, another preferred profile having a sharp-peaked form (apex included angle preferably not greater than about 20°) with concave valleys between successive peaks, as is illustrated with the riblets R′, r′ in Fig. 2. Furthermore, while Fig. 1 shows the riblets with base widths equal to their heights it may be preferred to increase the base widths to about twice the heights of the respective riblets; this is particularly relevant to cusp-shaped profiles shown in Fig. 2. The preferred sizes and dimensional relationships of the riblets may be expressed in the non-dimensional form of so-called "law of the wall" variables, in which actual distance values are multiplied by a "wall unit" scalar quantity defined as: where τ is the wall shear stress, ℓ is the fluid density, and ν is the fluid kinematic viscosity. In the example, the smaller riblets r have a non-dimensionalised height h⁺ of 15 units and are set at a pitch s⁺ also of 15 units. The larger riblets R are three times this size, with height h L ⁺ and width w L ⁺ at 45 units, giving a pitch s L ⁺ of 165 units. Because of the presence of the smaller riblets r, a virtual surface is established for the turbulent boundary layer flow some three-quarters of their height above the wall surface itself. The effective height (h Le ⁺) of the larger riblets R, i.e. their height above the virtual surface, is therefore approximately 33 units. The larger riblets act in the region above the virtual surface to control the long regions of streamwise vorticity or streaks that form in the boundary layer by inhibiting their random spanwise movements, increasing their natural spanwise spacing, and encouraging the transfer of energy from the streaks to the smaller scale secondary vortices induced by the smaller riblets. In this flow mechanism, the actions of two forms of riblet reinforce each other in that the larger riblets influence the boundary layer to produce flow patterns which are associated with inherent reductions in drag and at the same time are more suited to manipulation by the smaller riblets, while the latter make their own contribution to the reduction of skin friction by moving the turbulent boundary layer flow some way up their height. It is also possible that the presence of the smaller riblets, by changing the virtual wall position, so reduces the wall shear stress that the streaks are weakened and are therefore more amenable to control. Fig. 3 illustrates another riblet pattern which has only a pair of smaller riblets r between successive larger riblets R, and in which successive riblets are separated by flat valleys. In the particular example of Fig. 3, the smaller riblets have h⁺ and s⁺ each of 15 wall units. The larger riblets have a height h L ⁺ of 30 units and their pitch s L ⁺, which is of course the pattern repeat, is 45 units. It may be preferred, especially when using rounded or flat-­ bottomed valley configurations as in Fig. 2 and 3, to reduce the heights of the smaller riblets in relation to their pitch. Fig. 4 illustrates an example in which with seven smaller riblets r between successive larger riblets R, with h⁺ = ½s⁺ = 3.5 wall units, h L ⁺ = 10 units and s L ⁺ = 49 units. Many other variations of the sizes and profiles of the riblets are possible within the scope of the invention. The ratio h:s is preferably not substantially less than 1:2 nor substantially greater than 2:1 as regards the smaller riblets, although higher aspect ratios h:w can be more easily tolerated for the large riblets to the extent of giving them a blade-like profile. As far as concerns sizes, the following are the preferred size ranges in non-dimensional wall units for low speed flow: for the smaller riblets r, 2 ≦ h⁺ ≦ 20, and preferably 5 ≦ h⁺ ≦ 15 6 ≦ s ⁺ ≦ 40, and preferably 10 ≦ s⁺ ≦ 20 for the larger riblets R, 8 ≦ h Le ⁺ ≦ 45, and preferably 15≦ h Le ⁺ ≦ 35 40 ≦ s L ⁺ ≦ 240, and preferably 80≦ s S ⁺≦160 In general, the base width w L ⁺ of the larger riblets will be kept to less than 60 wall units. Variations within the ranges specified may be made independently for each parameter but the larger riblets must of course project above the smaller riblets and a minimum projection of eight wall units is desirable. Furthermore, the relationship of s⁺ to s L ⁺ must always allow at least two smaller riblets to be interposed between successive larger riblets. Fig. 5 illustrates another wall surface riblet pattern within the scope of the invention with modified riblet profiles. Like the example in Fig. 1, the smaller riblets r of this illustrative example have a non-­ dimensionalised height h⁺ of 15 units above the wall surface and are set at a pitch s⁺ also of 15 units. The larger riblets R have a height h L ⁺ and width w L ⁺ of 45 units, so that their pitch s L ⁺ is 165 units. The smaller riblets r establish a virtual surface for the turbulent boundary layer flow some ¾h⁺ above the wall surface itself and below that level all the riblets have planar inclined side faces. At the level of the virtual surface there is a transition t to a smaller inclination and the riblets continue upwards from that point of inflection with concave side faces to a sharp-peaked tip, in analogy to the concave form of riblet illustrated in Fig. 2. Above the virtual wall surface, the larger riblets R have a height (h Le ⁺) of 33 wall units, while the corresponding height (h e ⁺) of the smaller riblets is 3 units. The differently profiled regions are adapted to different functions. The regions below the virtual wall surface form the relatively deep V-grooves that are better able to inhibit penetration of turbulent eddy motions, while the concave faces above the virtual wall surface can more effectively promote very small scale longitudinal vortices which drain energy from the larger streaks in the turbulent boundary layer flow. To perform their functions effectively the concave faces should form relatively sharp-edged peaks. It will be appreciated that the effects described can be achieved, at least in substance, with modified forms of the inclined side faces shown. In particular, it may be desirable for practical reasons to blunt the points of inflection or the peaks of the riblet faces; also, these faces may comprise straight and/or curved portions both below and above the virtual wall surface without unduly changing the general character of the profiles required for the functions described. The number of smaller riblets between successive larger riblets can be greater or smaller than the group of 8 shown and the profiles of both series of riblets may be varied. If the larger riblets serve only to locate the streaks in position, their peaks may be flattened or rounded to reduce their inherent drag without affecting their function. If, however, they are employed to reduce the strength of the streaks, the top peaks will be retained and may be further emphasised by other changes of form. Similarly, the profile of the smaller riblets can be varied in accordance with their function. It may improve performance, if, at their leading edges, the riblets are faired into the wal surface to avoid drag increments associated with the flow meeting a bluff leading face. For reducing wetted area it may also be possible to have short spanwise-extending gaps (less than the boundary layer thickness δ) in the streamwise direction between riblet areas (themselves greater than 2δ in streamwise length) without sacrificing the advantages offered by the riblets. Reference will be made here to the subject matter of our two simultaneously filed patent applications (Case II - "Control of Boundary Layer Flow" and Case III - "Boundary Layer Flow Control") since these other configurations can be employed jointly or separately in combination with that of this patent application (Case I). In Case II, a reduction of the skin friction of a turbulent boundary layer is obtained by the use of a series of elongate projections disposed to a repeated spanwise pattern in which the adjacent projections in each pattern project to a different height. Such a configuration can be used in conjunction with the present invention, as is illustrated in Fig. 6, groups of the smaller height projections r₁, r₂ between successive larger projections R being given different heights so as to operate more effectively on the inner boundary layer region between larger projections and so gain a reduction in skin friction that is additional to that resulting from the presence of said larger projections. This concept may also be realised in a riblet pattern in which there are groups of adjacent uniform height projections between projections of two different heights, e.g. in the example of Fig. 6, all the projections r₂ save the central one in the pattern being replaced by smaller projections r₁, this last arrangement having the advantage that wetted area is reduced. To retain their functional effect, the exclusion of any of the intermediate length riblets should be limited to the extent that it does not leave any group of the smallest riblets with a span of more than 50 wall units. The function of the groups of smaller projections of different heights, as is described in that other application, is to displace the virtual wall surface further than is possible with uniform height smaller projections. The larger projections alternating with the groups of varying height smaller projections are intended to exercise the streak control functions described above. For that purpose they must extend significantly higher above the virtual wall surface than any of the smaller height projections. In our Case II the example is shown of two heights of V-groove riblets in alternation, the smaller riblets r₁ of height h⁺ establishing a virtual wall position which is the datum for effective height of the larger riblets r₂. If the effective height of the riblets r₂ is here termed h le ⁺ in distinction from the effective height h Le ⁺ of the larger height riblets R described earlier in this application, the new virtual wall position H⁺ is at a height in the region of 0.75h₁⁺, that is to say, 0.75 (h le ⁺ + 0.75h⁺) ≦ H⁺ ≦ (0.75h le ⁺ + 0.75h⁺) The effective height h LE ⁺ of the larger height riblets above the virtual wall position H⁺ is at least 8 wall units greater, i.e. their height h L ⁺ from the original wall surface is ≧ 8 + (0.75 (h le ⁺ + 0.75h⁺) In Case III the elongate projections provided on a wall surface for modifying the turbulent boundary layer are given progressively increasing heights with distance along the wall surface in the direction of fluid flow, as is illustrated in Fig. 7. This can be done with the configurations of projections described above, whether they are composed of pluralities of uniformly smaller height projections between successive larger projections or whether the smaller projections are themselves of different heights at any particular station in the direction of the stream. It will be understood that the other features disclosed in our Cases II and III not directly referred to above can be incorporated in any combination of the different configurations. Reference can be made directly to the specifications of our Cases II and III for further details of the subject matter of those applications. The riblet dimensions referred to above appear appropriate from the results of experiments carried out at relatively low free-stream velocities (U) and for Mach numbers (M) below about 0.3. However, it has been found that modification of the dimensions, in particular the riblet heights, is desirable for higher velocities and Mach Nos. It would appear that the streamwise length of the projections has a significant influence on the optimum dimensions especially at Mach Nos. greater than about 0.4M. Wind tunnel tests with uniform arrays of single-height V-groove riblets having a streamwise length of about 1 metre exemplify the change of dimensions in terms of wall units that is required to give optimum results at higher velocities. The following table, giving the riblet dimensions at the leading edge of the riblets, puts the data from these tests (3), (4), (5) at Mach Nos. in the range of 0.4-0.9M in context with previous low speed results (1), (2) in the range of 0.1M: R eL being the length Reynolds No = , L being the length of the riblets. These results indicate that for optimum effect, at the leading edge of the low speed riblets an increase of height in wall units of some 50% is required for a ten-fold increase of R eL relative to the low speed riblet height at R eL = 1x10⁶. The factors that determine the optimum are many and complex. The form of the underlying body, which may have streamwise and/or spanwise curvatures or discontinuities, and the possibility of non-uniform U values over the extent of a riblet array introduce further influences and an empirical evaluation of the optimum riblet dimensions may be necessary. For general purposes however, if the compensation for R eL is obtained by increasing the leading edge height of the riblets, and possibly also their spacing, it is suggested that in relation to the lower speed values given in the discussion of the illustrated examples above, an appropriate scaling factor of the leading edge height of the riblets in wall units in relation to R eL (h⁺ : R eL ) should be in the range 1:20 to 1:6. It is also desirable to scale the increase of riblet dimensions along the length of the array irrespective of R eL , as is discussed in more detail in our further patent application, Case III, filed simultaneously herewith. The invention is generally applicable to the wall surfaces of bodies in which a reduction of skin friction is sought. It may thus provide a means of reducing drag on the external surfaces of vehicles, including aircraft, land vehicles and watercraft. It can also be used in machines, e.g. the blading, both rotating and stationary of turbo-­ machines, and for the internal surfaces of pipes and conduits. It will be understood that the invention includes the possibilities of forming the surface projections integrally with the bodies concerned and also of providing surface elements, e.g. as sheets or tapes, having the projections, for application to bodies to provide the required surface configurations thereon.

Claims: 1. An aerodynamic or hydrodynamic wall surface which is provided with a series of elongate projections extending in the direction of fluid flow relative to the surface for modification of a turbulent boundary layer on said surface, characterised in that said projections comprise spaced larger height projections between each successive pair of which are a plurality of smaller height projections. 2. A surface according to claim 1 wherein the projections form a substantially continuous spanwise array. 3. A surface according to claim 1 or claim 2 wherein the projections extend streamwise at least over the length of the turbulent boundary layer region. 4. A surface according to any one of claims 1 to 3 wherein the smaller and/or the larger projections are sharp-peaked. 5. A surface according to any one of claims 1 to 4 wherein the projections have a V-form profile. 6. A surface according to any one of claims 1 to 4 wherein the projections have a profile which comprises concavely curved valleys between the peaks. 7. A surface according to any one of the preceding claims wherein flat-bottomed valleys are provided between at least some of the projections. 8. A surface according to any one of the preceding claims wherein the larger projections have an effective height of at least eight wall units above the smaller projections. 9. A surface according to any one of the preceding claims wherein there are smaller projections of at least two different heights between successive large projections. 10. A surface according to any one of the preceding claims wherein the heights of the projections increase in the direction of the stream. 11. A surface according to any one of the preceding claims wherein at least the larger height projections have side faces that are inclined at an angle that varies over the height of the projections, each said side face having an intermediate region at a lesser steepness than the adjacent regions above and below said regions and forming the lower part of a concave profile peak of the associated projection. 12. A surface according to claim 11 wherein all the projections extending above a virtual wall height having side walls of said varying inclination, with said intermediate region positioned substantially at the virtual wall height. 13. A surface according to any one of the preceding claims wherein there is at least one gap between lengths of said projections, the gap width not being substantially greater than half the lengths of the adjacent projections. 14. A body having an aerodynamic or hydrodynamic wall surface provided with a series of projections according to any one of the preceding claims.

Inventors: SAVILL, ANTHONY MARK
Applicants: ROLLS-ROYCE PLC
Application year: 1987
Main classification: B64C21
IPC codes: B63B1, B64C21