DE3609541A1 - REDUCED FLOW RESISTANCE BY LOWERING THE WALL SHEAR VOLTAGE OF THE SURFACE OF A TURBOLENT OVERFLOWED BODY - Google Patents

Abstract

The surface of a body over which a fluid flows in a turbulent manner has a reduced flow resistance due to reduced wall shearing stress. It has channels which run in the flow direction and are separated from one another by sharp-edged ribs (5). The ribs (5) are provided in a multiplicity of lines (9, 10, etc.). Each rib line (9, 10) consists of a multiplicity of ribs (5) arranged next to one another at a distance apart transversely or obliquely to the flow direction. The ribs of rib lines (9, 10) following one another in the flow direction (3) are staggered laterally relative to the flow direction. The ribs of the individual rib lines extend a short distance in the flow direction (3). <IMAGE>

Description

The invention relates to a reduced Flow resistance due to reduced Wall shear stress surface of a turbulent body, with in Flow direction through grooves sharp-edged ribs separated from each other are. These fine ribs reduce the Flow resistance in turbulent flow at the Surface. The reduction in resistance is achieved by Reduction of the turbulent wall shear stress reached.

A surface of the type described at the outset is made Walsh, M. J .: Turbulent boundary layer drag reduction using riblets, AIAA paper 82-0169 (1982). The resistance-reducing ribs used however, extend over a very long length in the direction of flow. Unless the ribs are in a suitable mutual distance and with a sharp-edged shape their upper edge, it is possible thereby reducing turbulent To achieve wall shear stress of approx. 7 to 8%.

According to an earlier application (P 34 41 554.8-53) has already been suggested to surface out assemble a large number of individual elements, the ribs facing the current and have grooves. Here are the individual elements arranged and / or designed relative to one another, that there are slits on the side facing the current arise. The slots are through below that  Flow facing surface of the individual elements arranged channels connected to each other. The Individual elements are arranged so that the grooves and Ribs of the various elements in the direction of flow in line with each other.

Furthermore, it is known from Kline, SJ / Reynolds, WC, Schraub, FA / Runstadler, PW: The structure of turbulent boundary layers, J. Fluid Mech, 30, 4, (1967), pp. 741-773 that in the wall-near layer of a turbulent boundary layer form strips of low flow speed ("slow strips" = "low speed streaks") which extend in the direction of flow (see FIG. 1). These are due to slowly rotating longitudinal vortices, which transport heavily braked fluid from the wall into the flow. The longitudinal vertebrae are in Jang, PS / Benney, DJ / Gran, RL: On the origin of streamwise vortices in an turbulent boundary layer, Report DT- 8154-06 (1985), Dynamics Technology Inc., 21311 Hawthorne Blvd., Torrance, California 90503, USA, has been explained as a flow resonance of the layer close to the wall. The wavelength λ _z transverse to the direction of flow corresponds to twice the distance between the individual opposing longitudinal vortices (see Fig. 1). These longitudinal vortices also have a wavelength g _x in the direction of flow, which is, however, considerably larger than the lateral vortex distance. The typical dimensions are λ ≈ 90 and g . λ ⁺ is a dimensionless length, with the definition

Here, t _{o is} the wall shear stress of the undisturbed turbulent boundary layer flow over a smooth plate. ρ is the density of the fluid and ν is the kinematic toughness. After a few experiments, λ may be higher than the given values.

The effect of the longitudinal ribs is based on the fact that the flow of the longitudinal vortices is impeded transversely to the main flow direction. As a result, the "slow streaks" become less intense, the flow remains more stable and the degree of turbulence decreases (Hooshmand, D./Youngs, R./Wallace, JM: An experimental study of changes in the structure of a turbulent boundary layer due to surface geometry changes, AIAA paper 83-0230 (1983)). Overall, this leads to a reduction in the turbulent wall shear stress, which is known to be based on the momentum exchange of the turbulent flow. However, the longitudinal ribs only work optimally if they are very sharp and if their lateral distance is of the order of half the longitudinal vertebral distance or less. This results in a rib spacing s , which should be s ⁺≈ λ / 4 ≈ 22 or less. In fact, all measurements with rib surfaces that have brought about a reduction in resistance also show this dimension of the rib spacing (Walsh, MJ: Turbulent boundary layer drag reduction using riblets, AIAA paper 82-0169 (1982)). This rib spacing is usually very small and is z. B. for an application on a commercial aircraft at about 0.05 mm.

The invention has for its object a Surface of the type described in the introduction to further develop that the flow resistance at turbulent overflow is further reduced becomes. In addition, this surface shape must be like this be trained that the surface also comparatively easy to manufacture.

According to the invention this is achieved in that the Ribs are provided in a variety of seasons  that each rib season from a multitude cross or diagonally to the direction of flow next to each other at a distance arranged ribs is that the ribs in Direction of flow of successive rib squadrons offset laterally to the flow direction are arranged, and that the ribs of each Rib staggering short extensions in the direction of flow exhibit. In general, the individual ribs each rib season also of the same length; however, this must don't be like that. The main characteristic is the staggered Arrangement of the ribs, i.e. the arrangement of several Rib squadrons, each from season to season Flow direction offset from each other are. Of course, the ribs must have sharp edges be trained and the geometric conditions, as will be explained below be respected.

The obstruction of the transverse flow of the longitudinal vortices also depends on how far the ribs protrude into the flow perpendicular to the wall. However, as our own calculations and measurements have shown, this protrusion cannot be increased arbitrarily by making the ribs higher. The decisive penetration depth is given by the distance between the top edge of the ribs and the "virtual zero point" of the viscous flow close to the wall. The virtual zero point is defined as follows: If the ribbed surface is replaced by a smooth surface at the height of the virtual zero point, the same average viscous flow results above the smooth surface as somewhat above the ribs of the ribbed surface. The virtual zero point is naturally between the top edge of the ribs and the bottom of the groove between the ribs. The virtual zero point can also be understood as the center of gravity of the shear stress on the ribbed surface. The "penetration depth" d is the difference in height between the top edge of the rib and the virtual zero point. The depth of penetration d of the ribs is the decisive factor for influencing the longitudinal vortex cross flow.

The calculation of the viscous flow around long fins shows that the penetration depth d cannot be increased above 22.1% of the fin spacing s . The specified limit

was determined both analytically and experimentally. This limit cannot be changed by high ribs or any shape variants of the long ribs. From this limit, the previous considerations ( s ⁺ ≈ 22) are used to calculate a dimensionless depth of penetration of a maximum of d ⁺ ≈ 5. This limits the possibility of influencing such a longitudinal vortex flow, whose vortex cores are much higher above the surface, namely at a dimensionless height of y ⁺ ≈ 35. This explains the limitation to the reduction in resistance achieved so far.

The only way to increase the depth of penetration and thus the reduction in resistance is a radical departure from long ribs. The individual ribs or rib series must be short in the direction of flow and a lateral offset of rib series to rib series following in the flow direction is required. This offset means that the ribs can effectively be moved apart comparatively twice as far to the side. You get twice the depth of penetration. On the other hand, the staggered arrangement of the ribs is also suitable for preventing the formation of longitudinal vortices if the ribs and the subsequent space are shorter than the wavelength in the flow direction g _x .

It when the ribs in is particularly advantageous Direction of flow of successive rib squadrons  offset by half a division are arranged so that on a groove between two Ribs of a rib season in the flow direction Rib and vice versa follows. The ribs can be rectangular be limited, the depth of penetration approximately doubled. But it is also possible that the aligned opposite to the flow direction The ends of the ribs are chamfered or rounded are trained. The leading edges of the ribs are so beveled or rounded. Such circular section or fin-shaped rib structure fluid mechanically appears even cheaper and enables even greater penetration depths. So that's it possible the 8% limits of resistance reduction to fall even further from the state of the art.

One skilled in the art would assume that the division of a Rib into a variety of staggered ribs would inevitably bring an increase in resistance. In a normal parallel flow with constant That would also be the case with speed. In a Viscous flow close to the wall can, however, be achieved with a relatively simple calculation show that here no adverse effect occurs. Technologically the advantages of previous groove surfaces remain receive.

The dimensionless distance of the ribs from each other in a rib season should be of the order of about s ⁺ = 10. . . 45 lie. In a typical application in a commercial aircraft, the rib spacing would be approximately 0.1 mm.

The extension of the ribs of the individual ribs Staggering in the direction of flow corresponds approximately to one up to six groove widths. The staggered ribs can also have different lengths and they can also partially overlap. Also one  oblique arrangement to the flow direction of the ribs or Rib staggering is possible, taking the single rib but must be arranged in the direction of flow. The Ribs are all relatively easy to get through Manufacture hot rolling or casting of plastic films. Such plastic films can then on the surface e.g. B. an aircraft.

It is also possible to use the invention staggered arrangement of the ribs with other known ones to combine measures to reduce resistance. For example, a combination with under the Slits connected to the surface possible Show outlet openings in the direction of flow. Furthermore, the combination with in the turbulent Boundary layer introduced rectification wings according to Anders, J. B./Watson, R. D .: Airfoil large-eddy breakup devices for turbulent drag reduction, AIAA- Paper 85-0520 (1985) possible, which is also for itself and in the combination then to a reinforced one Reduction of the turbulent wall shear stress to lead.

Different embodiments and representations to illustrate the invention are in the drawings are shown and are described below. It demonstrate:

Fig. 1 is an illustration of stripes of lower flow velocity in a near-wall layer due to occurrence of longitudinal vortices,

FIG. 2, illustrating some geometrical parameters,

Fig. 3 shows a first embodiment of the inventive surface with staggered ribs arranged,

Fig. 4 shows another embodiment with staggered ribs in circular segment shape,

Fig. 5 shows another embodiment of the surface with staggered ribs in the form of fins,

Fig. 6 is a plan view of a further embodiment of the surface with different length-scales of ribs,

Fig. 7 shows an embodiment with a relatively large overlap of the rib scales,

Fig. 8 shows a further embodiment of the rib seasons and

Fig. 9 shows a last embodiment of the rib squadrons with oblique arrangement.

In Fig. 1, the known from the prior art is shown fact that a plate (2) be formed on a surface (1) in the flow direction (3) in the near-wall layer of a turbulent boundary layer stripes of lower flow velocity. These are slowly rotating longitudinal vortices ( 4 ) which rotate in opposite directions and transport strongly decelerated fluid from the surface ( 1 ) into the flow. The wavelength λ _z transverse to the direction of flow corresponds to twice the distance between the individual opposing longitudinal vortices ( 4 ). The longitudinal vortices ( 4 ) also have a wavelength λ _x in the direction of flow, which is, however, considerably larger than the lateral vortex distance.

In Fig. 2, part of a rib series of the surface according to the invention is shown in high magnification in order to clarify some sizes. From the surface ( 1 ) protrude individual ribs ( 5 ), which face the flow by sharp edges ( 6 ), are aligned parallel to each other and arranged parallel to the direction of flow. It is not harmful if the ribs ( 5 ) are rounded at the base, ie at the transition point to the surface ( 1 ). The ribs ( 5 ) have a relatively short extension parallel to the direction of flow. The rib spacing s is the distance between two ribs ( 5 ), which are adjacent in a rib series, measured transversely to the direction of flow. The virtual zero point ( 7 ) of the viscous flow near the wall is also shown. This virtual zero point ( 7 ) lies at a penetration depth d from the plane of the edges ( 6 ) towards the surface ( 1 ). Two speed profiles are shown, namely at the location of the rib ( 5 ) and an intermediate groove ( 8 ). The speed distributions are to be understood as averages over time.

Fig. 3 shows a first embodiment of the surface ( 1 ) with the ribs ( 5 ), which have a rectangular shape here. The first ribs ( 5 ) arranged transversely to the direction of flow ( 3 ) form a rib series ( 9 ). The ribs ( 5 ) following in the direction of flow ( 3 ) form a series of ribs ( 10 ) etc. It can be seen that the series of ribs ( 9, 10 etc.) which follow one another in the direction of flow ( 3 ) are each laterally offset from one another, especially by half a division.

Fig. 4 shows a further view in perspective. Here, too, ribs ( 5 ) arranged next to one another transversely to the direction of flow complement each other to form the various rib series ( 9, 10 etc.). The individual ribs here consist of a circular section shape, ie they are rounded.

Fig. 5 shows the arrangement of ribs ( 5 ) in the form of fins, each rib ( 5 ) beginning on its side facing the flow with a small height. Here too, the lateral offset of the ribs from one another can be seen.

As can be seen in FIG. 6, which shows a plan view of a further embodiment, the individual ribs ( 5 ) of the rib series ( 9, 10 , etc.) do not necessarily have to be of the same length. The ribs ( 5 ) of each rib series are of equal length to one another. However, the ribs of different rib seasons can have different lengths. It is also possible, as shown in FIG. 7, for the individual ribs ( 5 ) of the various rib series ( 9, 10 , etc.) to overlap. The rib staggered sections ( 9, 10 , etc.) are arranged transversely to the direction of flow ( 3 ).

The exemplary embodiments of FIGS. 8 and 9 show ribs ( 5 ) which are staggered obliquely to the direction of flow ( 3 ), the ribs ( 5 ) according to FIG. ( 8 ) being able to have the shape shown, that is to say pointed at the front and rear. Of course, the ribs ( 5 ) in all of the exemplary embodiments have the sharp edges ( 6 ) illustrated on the basis of FIG. 2, specifically at their end projecting into the flow. In all the embodiments shown so far, the ribs are offset by half a division in the flow direction of successive rib series ( 9, 10 , etc.). Fig. 9 shows an embodiment with obliquely staggered ribs, in which the lateral offset does not necessarily have to be half a division. The offset can be divided here in the ratio 1 / 3-2 / 3. Also, the length of the ribs in a rib season does not necessarily have to be constant.

• Reference symbol list: 1 = surface
  2 = plate
  3 = flow direction
  4 = longitudinal vertebrae
  5 = rib
  6 = edge
  7 = virtual zero point
  8 = groove
  9 = Rib season
  10 = Rib season

Claims (5)

1. Reduced flow resistance due to reduced wall shear stress surface of a turbulent flowed body, with grooves running in the flow direction, which are separated from one another by sharp-edged ribs, characterized in that the ribs ( 5 ) in a plurality of staggered ( 9, 10 , etc.) it is provided that each rib series ( 9, 10 , etc.) consists of a plurality of ribs ( 5 ) arranged transversely or obliquely to the direction of flow next to one another at a distance s , that the ribs in the flow direction of successive series of ribs ( 9, 10 , etc.) are laterally offset from each other to the direction of flow, and that the ribs of the individual rib series have short extents in the direction of flow ( 3 ). 2. Surface according to claim 1, characterized in that the ribs ( 5 ) in the flow direction ( 3 ) of successive rib seasons ( 9, 10 etc.) are arranged offset by half a pitch. 3. Surface according to claim 1 and 2, characterized in that the opposite to the flow direction ( 3 ) arranged ends of the ribs ( 5 ) are chamfered or rounded. 4. Surface according to claim 1 to 3, characterized in that the dimensionless distance s ⁺ of the ribs ( 5 ) from one another in a rib series ( 9 ) in the order of magnitude s ⁺ = 10. . . 45 lies. 5. Surface according to claim 1 to 4, characterized in that the extent of the ribs ( 5 ) of the individual rib series ( 9, 10 , etc.) in the flow direction ( 3 ) corresponds to about 1 to 6 groove widths.