DE3534268A1 - Surface designed to avoid flow separation on a body around which a fluid flows - Google Patents

Abstract

To avoid flow separation, a surface 1 of a body around which a fluid flows is provided with a multiplicity of elements 2 which project from the plane of the surface 1 into the flow and have such a shape that, with intermixing in a boundary layer, energy is supplied to the boundary layer near the edge. The elements 2 are on the one hand adjustably arranged relative to the plane of the surface 1 and on the other hand have such a shape that they cause intermixing when flow separation threatens but simulate the plane of the surface 1 in the case of close-lying flow while resistance is reduced. <IMAGE>

Description

The invention relates to a to avoid Flow separations formed surface of a flow Body with a variety of elements coming from the Level of the surface provided protruding into the flow are and have such a shape that under Mixing in a boundary layer of the boundary layer close to the wall Energy is supplied.

The flow on the rear side of the flow around bodies tends to detach from the surface. That leads too strongly increased flow resistance. Flow separation occurs, however not only at the rear of blunt bodies, e.g. B. cars, on, but also on wings at high angles of attack. Avoiding flow separation can not only Resistance can be saved, but also the buoyancy of Wings are raised. Flow separation occurs when the flow around a body with such a strong Pressure increase (= strong positive pressure gradient) connected is that the flow slowed down by friction in the The proximity of the body (boundary layer) is no longer sufficient kinetic Has energy around that when current is present resulting pressure (potential energy) on the Build up the back of the body.

In a known surface of the type described above, elements protruding from the plane of the surface into the flow are provided in order to better mix the high-energy external flow with the decelerated flow near the surface and thereby to supply energy to the boundary layer near the wall, so that the flow remains in contact for a longer time or can flow against higher positive pressure gradients. The elements arranged on the surface of the flowed-around body can have different shapes, but in each case it is aimed at increasing the mixing in a boundary layer. The elements can be arranged as rows of small wings on the top of the wings of aircraft in order to produce longitudinal vortices in the direction of flow ( FIG. 1). Another possibility, however, with a similar effect, is a wedge-shaped design of the elements with a corresponding arrangement in the flow direction ( FIG. 2). The increased mixing is achieved in both cases by generating longitudinal vortices. However, these fixed elements are only useful as long as the flow is near the separation. In an aircraft wing, this is e.g. B. during the approach. In other flight conditions, e.g. B. during cruise, the known elements for generating longitudinal vortices are not necessary and disadvantageously generate additional resistance.

It is known (D.W. Bechert, G. Hoppe and W.-E. Reif: "On the drag reduction of the shark skin", AIAA paper 85-0546 (1985).) That the elements for producing Longitudinal vortices can also be made very small, if they cover the surface tightly. In a turbulent Boundary layer, they just need something out of the viscous Protrude lower layer. The viscous underlayer is a very thin layer, very close to the wall, in which the toughness forces dominate the flow. With technical Applications, this layer is usually a small one A fraction of a millimeter thick. Form in this layer already without elements protruding into the current very small longitudinal vertebrae that mix control the entire boundary layer (S.J. Kline, W.C. Reynolds, FA. Screw, P.W. Runstadler: "The structure of turbulent boundary layers ", J. Fluid Mech. 30 (1967), Pages 741-773).  

Through very fine longitudinal grooves (D.W. Bechert, G. Hoppe and W.-E. Mature: "On the drag reduction of the shark skin", AIAA paper 85-0546 (1985)) can be used for training this vortex hinder necessary cross flow. The leads to a lower energy exchange in the turbulent Boundary layer, leading to the reduction of turbulent Wall shear stress (below the value for a smooth plate) can be used. This appears to be usable for direct Reduction in resistance for long bodies without risk of detachment, e.g. B. aircraft fuselages.

On the other hand, by reinforcing the small longitudinal vertebrae with small elements, the mixing is also strong lift. Although this leads to an increase in the wall shear stress, but also to avoid flow separation. By attachment e.g. B. in the tail area of an aircraft fuselage or in the rear part of a wing can thus by separation suppression both the total resistance decreased as well as the buoyancy can be increased. On an aircraft surface can also use different elements come. However, an optimal effect can only be achieved for develop a flow state.

The invention has for its object a surface of the type described in the introduction such that it adapts to changing flow conditions or can at least be adjusted.

According to the invention this is achieved in that the elements on the one hand, adjustable relative to the level of the surface are arranged and on the other hand such a shape have that the impending flow separation Mix, but with the flow present reproduce the level of the surface while reducing the resistance. It is thought that the elements at different Flow states different relative positions ingesting or ingesting to the surface  at least in such a way that at least in one position increased mixing avoided the impending flow separation and in a different position with a secure fit Flow a decrease in resistance occurs. The adjustability on the one hand and the design of the shape of the elements, i.e. their shape and dimensioning as well as arrangement, affect both desired effects out. At least two flow conditions in the Optimization should be considered, if possible of course more than two flow states. That's the way it is in particular, the adjustment and design of the Form elements so that with respect to a variety of flow conditions a more or less continuous Decrease in one effect and increase in the other Effect occurs.

The elements can be designed to be resilient and / or articulated to the surface. The elements can be arranged to be adjustable by the flow forces themselves and / or by external forces. It is of course particularly advantageous if the elements are adjusted by the flow forces themselves, so that a more or less continuous adaptation to the different flow conditions takes place automatically and any external force adjustment devices are not required. On the one hand, the adjustment can be a more global reaction to the flow conditions due to slowly reacting large elements. On the other hand, especially when a liquid flows around the body, there is also the possibility that the elements can be made particularly small and can react quickly to the current flow structure, as a result of which the boundary layer is influenced to an even greater extent. Wedge-shaped elements, as they are in the solid state of the art ( Fig. 2), can be easily pulled into the plane of the surface or stored so that they can be adjusted so that they can be adapted to the respective flow state by means of an external force adjustment device. There are various options for adjusting the elements by the flow itself:

The elements can be resilient, ie correspondingly be elastic so that it is reproducible Have provision. The adjustment or bending of the elements is useful due to the shear stress controlled the flow. The following connection is used used: with safe flow the wall shear stress is high and the elements are stressed by this wall shear stress due to their resilient training or also suspension in a flat surface adapted to the surface Position held at a distance from the Recreate this surface approximately parallel and the resulting surface of the elements largely is closed or smooth, so that the desired Reduction in resistance occurs. With less Shear stress, on the other hand, in the event of impending flow separation, the elements align themselves with their resilience or by spring force and create a strong mix the boundary layer. This makes the flow close held. Due to reduced flow velocity, e.g. B. when approaching an aircraft also decreases the wall shear stress. But this is in most Cases also include the area where the flow is held close must become.

On the other hand, it is possible to use spring forces to do without and an articulated storage or Realize arrangement of the elements relative to the surface. The elements are adjusted in such a way Fall purely through the fluid mechanics, that of the forces acting on the elements. Currents near the detachment have a sharp increase in pressure in the direction of flow. This pressure increase creates in Cavity under the surface  a secondary flow that is opposite to the flow direction the main flow is directed. This secondary flow can be used to display the elements in sensible way to adjust. It is also possible to do both to combine the options shown and for example in addition to an articulated suspension of the elements use a very soft suspension.

In a preferred embodiment, the elements L- shaped cross section and are with one, the surface facing legs approximately perpendicular to the surface arranged while the other leg is approximately parallel extends to the surface in the direction of flow. This is at least when there is current and high shear stress the case where the elements are collapsed, so to speak and cause no additional frictional resistance.

The one that extends approximately perpendicular to the surface Leg of the elements has a cross to the direction of flow extending area training on that one opposite to the direction of flow secondary flow Offers resistance. By attacking this secondary flow on the corresponding surface elements of the the elements are attached to one leg of the elements low shear stress and almost detached flow. This erection is only possible if the flow conditions exist near the flow separation if So the shear stress is small and if it increases Flow pressure occurs. Such an adjustment mechanism leads to an equilibrium of the Elements, according to the shear stresses and pressures, both roughly proportional to the flow rate are. That means setting the items independently of the flow velocity and only of it is affected whether the flow near the detachment is or not. With a safe current with high  Wall shear stress, the elements are folded and set practically a flow parallel to the surface of the Body-extending smooth surface that produces only a small frictional resistance.

The other, which extends approximately in the direction of flow Legs of the elements can taper in the direction of flow be trained to in the extended position bring about the desired mixing in the boundary layer.

On the surface of the other exposed to the current Legs of the elements can run in the direction of flow, resistance-reducing longitudinal grooves with sharp Burrs may also be provided. This is for reduction the frictional resistance in turbulent flow in the folded state of the elements. The size of the elements is not fixed in itself. An effectiveness is even when they are very small and theirs Dimensions close to the thickness of the viscous underlayer lie. In this case there is also an interaction with the already existing longitudinal vertebrae in the viscous lower layer possible. The features of claims 1 to 7 are on directed a passive surface.

If the elements are not strong when the flow is present another interaction is possible here, which - similar to the arrangement of the fine longitudinal grooves with sharp burrs - to reduce the shear stress current flows:

There may be slits in the surface between the elements for the application of resistance-reducing polymer additives be provided under the elements. Through this Slots between the elements can be fluid in one moment and locally occurring low pressure area be blown in from the space below the surface. This means that the  particularly slow flow can be accelerated. This leads to a stabilization of the flow and this in turn to reduce the frictional resistance. The The mobility of the small, adjustable elements is increased this effect. Roughing calculations have shown that the achievable effect, especially when flowing around Bodies with liquid is possible. Even the slower ones large-scale fluctuations in turbulent boundary layers lead to sub-surface currents and to Adjustments of the elements. If fluid through the slots blown out between the elements, it always will blown out in the direction of flow and preferably on such Place where pressure and speed at the moment Surface are small. This alone leads to a flow profile that fits the flow longer holds.

When using the new surface of the flowed body in liquids there is also the possibility known resistance-reducing polymer additives feed into the cavity under the elements. Through the resulting breakthroughs between the elements these polymer additives then leak out and become another Reduce resistance.

The invention is based on a few exemplary embodiments further explained and described. Show it:

Fig. 1 is arranged a surface with fixed, wing-like elements (prior art),

Fig. 2 is arranged a surface with fixed, wedge-shaped elements (prior art),

Fig. 3 is a surface having the appearance of a single, resiliently formed element,

FIG. 4 shows a sectional view through the surface with a large number of adjustable elements according to FIG. 3 with the flow applied and high shear stress, FIG.

Fig. 5 is a sectional view of the surface shown in FIG. 4, but at low shear stress and almost separated flow,

Fig. 6 shows a surface with a single, articulated element and

Fig. 7 is a cross-sectional view of the surface with a plurality of elements according to Fig. 6 with low shear stress and almost detached flow.

In Fig. 1 in a perspective view a piece of a surface 1 is shown an airfoil, the exposed on its flow side of the surface having 1 elements 2 are fixed to be immobile. The elements 2 are wing-like and are arranged at an angle to the flow direction 3 , so that they cause longitudinal vortices 4 in a boundary layer, which also rotate in opposite directions due to the opposite, obliquely arranged arrangement, as indicated by the directional arrows 5 .

Fig. 2 shows a fundamentally similar design and arrangement, ie also with fixed elements 2 , which here have a wedge-shaped shape and are arranged aligned in the direction of flow 3 , so that longitudinal vortices 4 corresponding to the directional arrows 5 also arise in the overflow.

Surfaces 1 with fixed elements 2 according to FIGS. 1 and 2 belong to the prior art.

Fig. 3, the single figure shows a section of the new surface 1 with the representation of an individual element 2, but which is formed resiliently. The shape of element 2 is also new. The element 2 has a first leg 6 and another leg 7 , which are connected to one another at an angle of approximately 90 ° so that there is approximately an L-shaped cross section. Here, the surface 1 facing leg 6 is approximately perpendicular or slightly inclined to the surface 1 is arranged, while the other leg 7 about located at a distance parallel to the plane of the surface. 1 The leg 7 tapers in or out in the direction of flow 3 . The solid line shows the position that results from the current and high shear stress. Here, the element 2 is resiliently bent or bent in the region of the leg 6 , so that the other leg 7 results in a relative position approximately parallel to the plane of the surface 1 . In the event of a drop in the shear stress, as is typical for an almost detached flow, the element 2 more or less straightens up to its springback-free normal position, which is indicated in dashed lines. A spring travel 8 is covered from the tip of the leg 7 . While the leg 6 also extends more or less perpendicular to the surface 1 in this position, the other leg 7 is no longer parallel to the plane of the surface 1 , but projects in the direction of flow, so that a rough second surface is created here , which could be called a secondary surface. This rough surface contributes to increased mixing in the boundary layer area. The element 2 can be made of thin, leaf spring-like material, so that a reproducible reset occurs depending on the flow conditions and a more complex adjustment device with external force is avoided. It goes without saying that not only a single element 2 , but rather a plurality or at least a plurality thereof are provided on a surface 1 , as can be seen from the sectional illustration in FIGS . 4 and 5. At high shear stress ( Fig. 4), the elements 2 are bent by the forces acting on them against their return spring force so that the other legs 7 lie on one another or are supported against one another, so that, as it were, the plane of the surface 1 at a distance again is reproduced. However, it is a comparatively smooth level, so that it generates a correspondingly low resistance. With low shear stress and almost detached flow, however, the relative position shown in FIG. 5 results. The forces acting on the elements 2 become smaller, so that the resilience of the resilient material becomes so noticeable that the individual elements 2 are erected. As a result, the legs 7 come into a scale-like relative position to one another, which means a rough surface with appropriate mixing in the boundary layer. The elements 2 are arranged in the same direction and, as can be seen from the drawings, are provided aligned in the direction of flow 3 .

A further embodiment of the elements 2 is shown with the aid of FIGS. 6 and 7. Although each element 2 also has a first leg 6 and a second leg 7 here , it is mounted on the surface 1 or in the surface with the aid of the joint 9 , so that here too - although different - an adjustment of the element 2 by the acting forces takes place. In Fig. 6 in a solid line the relative position at high shear stress and in dotted lines is illustrated at low shear stress. Both relative positions differ by the adjustment path 10 .

It is understood that only two relative positions are shown here are. According to the flow conditions a continuous or stepless adjustment according to the current flow conditions.

FIG. 7 illustrates the position of a plurality of elements 2 according to FIG. 6 with a low shear stress and an almost separated flow. Typical of this flow state is a pressure increase opposite to the flow direction 3 , which in turn is the cause of a secondary flow according to the arrows 11 . This secondary flow according to the arrows 11 acts on surface elements on the legs 6 which are not further illustrated and which for this purpose can have long, flat shafts. These shafts have a resistance in the secondary flow according to the arrows 11 , so that the elements 2 are adjusted about the joints 9 in this way. With a small shear stress and increasing pressure curve in the flow direction 3 , this adjustment mechanism leads to the establishment of an equilibrium state of the forces on the elements 2 , resulting from shear stresses and pressures, both of which are approximately proportional to the flow velocity. This means that the elements 2 are set independently of the flow rate.

In Fig. 5 it is shown that the surface 1 may be provided in each case with slits 12, through which a fluid can additionally be blown out. The blowing out always takes place approximately in the direction of flow 3 . Through the slots 12 , however, a polymer additive can also be applied, which contributes to a further reduction in resistance.

The construction of the variably adjustable elements 2 can take place in various ways, since the size of the elements 2 is not fixed from the outset. The elements 2 can be made of sheet metal or plastic and can also be assembled from individual parts, provided that the elements 2 are of the order of centimeters. In the case of very small structures made up of the elements 2 in the millimeter range and below, production techniques can be used as are known for weaving velvet, faux fur or the like. In all cases, the elements 2 are flat, uniformly oriented and taper as sharply as possible at the rear.

• Reference symbol list: 1 = surface
  2 = elements
  3 = flow direction
  4 = longitudinal vertebrae
  5 = directional arrows
  6 = leg
  7 = leg
  8 = travel
  9 = joint
  10 = adjustment path
  11 = arrow
  12 = slot

Claims (8)

1. To avoid flow separation formed surface of a flow-around body with a plurality of elements ( 2 ), which protrude from the plane of the surface ( 1 ) protruding into the flow, and have such a shape that with mixing in a boundary layer Energy is supplied to the boundary layer near the wall, characterized in that on the one hand the elements ( 2 ) are arranged so as to be adjustable relative to the plane of the surface ( 1 ) and on the other hand they are of such a shape that they cause mixing when there is an impending flow separation, but when the flow is present they reduce the resistance Recreate the surface level. 2. Surface according to claim 1, characterized in that the elements ( 2 ) are resilient and / or articulated to the surface ( 1 ). 3. Surface according to claim 1 and 2, characterized in that the elements ( 2 ) are arranged adjustable by the flow forces themselves and / or by external forces. 4. Surface according to claim 1 to 3, characterized in that the elements ( 2 ) have an L-shaped cross section and with the one, the surface ( 1 ) facing leg ( 6 ) are arranged approximately perpendicular to the surface ( 1 ), and that the other leg ( 7 ) extends approximately parallel to the surface ( 1 ) in the direction of flow ( 3 ). 5. Surface according to claim 4, characterized in that the approximately perpendicular to the surface ( 1 ) extending one leg ( 6 ) of the elements ( 2 ) has a transverse to the flow direction ( 3 ) extending surface formation which is opposite to the flow direction secondary flow Offers resistance. 6. Surface according to claim 4, characterized in that the approximately in the flow direction ( 3 ) extending other leg ( 7 ) of the elements ( 2 ) is tapered in the flow direction. 7. Surface according to claim 6, characterized in that on the surface exposed to the flow ( 1 ) of the other leg ( 7 ) of the elements ( 2 ) in the flow direction, resistance-reducing longitudinal grooves are provided with sharp burrs. 8. Surface according to claim 1 to 7, characterized in that in the surface ( 1 ) between the elements ( 2 ) slots ( 12 ) for applying resistance-reducing polymer additives are provided under the elements ( 2 ).