GB2149054A

GB2149054A - Method and construction for reducing the drag of bodies arranged in a flow

Info

Publication number: GB2149054A
Application number: GB08426660A
Authority: GB
Inventors: Tamas Lajos; Elemer Litvai; Laszlo Preszler; Attila Darazs
Original assignee: KOEZPONTI VALTO HITELBANK
Current assignee: KOEZPONTI VALTO HITELBANK
Priority date: 1983-10-25
Filing date: 1984-10-22
Publication date: 1985-06-05
Also published as: HUT36033A; GB8426660D0; JPS60110575A; HU192821B; DE3438953A1; GB2149054B

Abstract

Element for reducing the resistance caused by a body in a fluid flow, arranged such that 0.4</=b/H</=0.3 (see Fig. 4), where H is the smaller dimension of the bodies front wall, and such that the angle alpha between the outer boundary of the element and the direction of the flow, is less than 60 DEG (see Fig. 12). <IMAGE>

Description

SPECIFICATION Method and construction for reducing the drag of bodies arranged in a flow The invention relates to a method and construction for reducing the resistance of flow of bodies arranged in the flow, wherein the construction is arranged on the frontal plate facing the flow, optionally forming one unit therewith.

When in motion, public-, railway-, water-or airborne vehicles are alike influenced by the resistance of flow, the magnitude of which considerably influences fuel consumption of the vehicle, i.e. fuel costs. As the force represented by the resistance of flow affecting the bodies arranged in the flow forms a rather considerable part of the competent load in certain cases, when it is reduced, among others, the load on the body becomes also less, accordingly, e.g. investment costs can be also reduced. Such constructions are e.g. constructions and buildings subjected to the effect of wind or streaming water. As nowdays special attention is paid to the reduction of fuel, energy or building costs, we may meet more and more experiments and technical solutions, the aim of which is to reduce resistance of flow in some form.According to technical literature it can be stated, that in particular with vehicles having been designed according to functional and not fluid-mechanical points of view, so e.g. autobusses, lorries, trucks as well as with structures and buildings, with which edges with a small radius of curvature are formed around the surfaces facing the direction of flow, a considerable part of the force induced by the resistance of flow, in a given case even the majority, is a result of the overpressure arising on the frontal surface facing the flow. This phenomenon is described in the work of Hoche: Aerodynamics of autocars. Majority of the experiments and tests aimed at the reduction of resistance of flow, deal with the matter of reducing the force arising on the front-wall, as a result of presure, i.e. of the resistance on the frontwall.This force can be reduced by reducing the presure prevailing on the front-wall.

Methods serving for the reduction of the pressure arising on the front-wall can be subdivided fundamentally in two large groups. According to one of the methods the front-wall as well as the edges and corners surrounding the front-wall are rounded i.e. formed with a spherical surface. According to the other method before the front-wall-parallel or approximately parallel therewith-a plate is arranged. When applying the first method, as it is seen in Fig. 1, at the curved surfaces i.e. of the spherically formed place flow becomes accelerated, simultaneously pressure drops.

The larger the extent of rounding, on the more smaller part of the front-wall 2, where an overpressure will arise, which is higher than external pressure, the greater will be the part of the frontal surface, where a pressure-being lower, than the external pressure-i.e.

depression will arise. Depression induces a force acting against the flow and directed forwards, that means that it reduces force of resistance.

The other method as illustrated in Fig. 2.

According to the illustrated arrangement before the front-plate 2 of the element having been arranged in the flow, parallel or approximately parallel thereto, there is the plate 4 to be found; the cross-section of this plate 4 is smaller than the cross-section of the frontplate. On the front-plate, in the space behind the plate low pressure resulted, inducing a forward directed force, that means, that the force caused by the resistance of flow is reduced. In this way said force is able to compensate the force downstream, arising on the frontal plate of the plate 4 arranged before the front-wall 2. As a result it may happen that common frontal resistance of the plate 4 and the construction before which the plate 4 is arranged, can be reduced to a really low value in a given case.This method can be advantageously used everywhere where it is possible to place a baffle plate before the frontal wall. As an example let us mention the baffle plate mounted onto the driver's cabin of trucks, reducing the force of resistance affecting the storage space. Theoretically both methods can be used for reducing the resistance on the front-wall, however, in practice we may be confronted with several difficulties reducing their cation, i.e. allowing only a restricted application.

So, e.g. rounding of the front-wall of autobusses or trucks reduces useful volume, simultaneously results in difficulties in technology and costs are also increased.

Application of the plate before the frontwall, being parallel or approximately parallel therewith, is also restricted, introduction is not always possible, partly for the reason of aesthetics, partly from the point of view of economicalness.

The invention is based on the recognition, in so far as, if by using proper structural elements and methods differing from the known ones on a part of the front-wall low pressure can be induced, resistance of flow will be reduced to the same extent as if we used the known methods, however, disadvantages can be mostly eliminated.

The basic idea lies in that such means and structures are placed into the flow near to the front-wall, which cause low pressure or the front-wall, accordingly, they reduce resistance of flow and pressure on the front-wall as well.

In order to facilitate understanding of the basic idea of the invention, it is worth the trouble to examine the flow characteristics on the front-wall. This is to be seen in Fig. 3. On the front-wall 2 there is a point, the so-called point T of accumulation, in which pressure reaches its maximal value, at the same time velocity of flow equals to zero. Leaving the point T of accumulation on the front-wall 2 flow velocity gradually increases, at the same time pressure drops gradually. In the vicinity of the front-wall 2 a flow-being approximately parallel with the front-wall-is formed, the direction of which is pointing outwards from the point of accumulation T.

The fundamental idea of our invention has been that into the flow, near to the front-wall, elements are to be placed which induce a low pressure at the front-wall 2, thus reducing pressure on the front-wall 2, i-e-, reducing resistance of the front-wall. It is a well known fact that the boundary layer formed on the surface of obtuse-not streamlined-bodies becomes detached on the decelerated section of flow and behind the body a trace of low velocity, the so-called bubble of detachment-appears, in which the prevailing pressure is less than the external pressure. The pressure within the bubble of detachment depends on the velocity of the undisturbed flow-indicated with V in the figure -, on the magnitude of the angle enclosed by the direction of the streamline confining the bubble and the direction of the undisturbed flow, as well as on the geometry of the solid body.

Based on this recognition a process was developed for reducing the resistance of flow of bodies placed into the flow, with at least one baffle element; the essence of the process lies in that on the frontal surface/being perpendicular or near perpendicular to the flow/ of the body having been arranged in the flow, along the periphery which is less than the periphery of said surface, or along a partial periphery at least reduced pressure-inducing element forming a partially open cavity is formed.The construction according to the invention is formed on the frontal surface of the body in the flow, forming optionally a unit therewith; in accordance with the invention at least one element inducing reduced pressure is arranged directly on the frontal surface; the boundary surface of the inducing element facing the edge of the front-wall forms in axial section an angle '60 with the direction of the undisturbed flow, while the distance between the extreme edge and the edge of the front-wall is in accordance with the correlation 0,04'b/H'0,3, wherein H stands for the smaller dimension of the front-wall.

With a preferred embodiment of the construction according to the invention between the element forming the cavity and the frontwall there is an air -gap formed of the size h = 0,05 H.

The pressure-reducing element is expediently rounded towards the outer edges of the front-wall, with a radius of curvature RIO, 15 H. It is also considered as advantageous, if the height of the pressure reducing element corresponds to the 0,4 times the smaller side of the front-wall, i.e. Z'0,4 H.

With a further preferred embodiment of the structure according to the invention the pressure reducing element is formed of a plurality of concentric flanges.

It is also advantageous, if the pressure reducing element is formed by a flange running parallel with one side of the front-wall. It may be advantageous if the pressure reducing element is composed of several flange-like elements.

The process according to the invention and the element resulting depression for realizing the process will be described in detail by means of preferred embodiments serving as example, by the aid of the drawings, wherein: Figure 1 shows one of the known solutions for reducing the resistance of flow, with which resistance of flow is reduced by rounding and spherical formation, Figure 2 shows another known solution, with which before the front-wall of the body arranged into the flow, a further plate is arranged parallel or nearly parallel therewith for reducing the resistance of flow, Figure 3 shows the streamlines of the body in the flow with the point of accumulation indicated with T, Figure 4 is the arrangement of the element forming the pressure-reducing cavities. where said element bears up directly against the frontal plate, Figure 5 illustrates an embodiment of the element forming the pressure-reducing cavity according to the invention-serving as an example-wherein between the element inducing depression and the front-wall an air-gap is formed, Figure 6-12 are showing further preferred embodiments and arrangements of the element forming the reduced-pressure cavity.

An embodiment and arrangement of the element according to the invention forming the pressure-reducing (depression) cavity is to be seen in Fig. 4, where the element 3 forming the cavity is arranged directly on the frontal plate 2 of the body facing into the flow and in such a manner that the periphery of the element 3 is less than the smaller size H of the frontal plate 2. By this formation on the frontal plate 2 with an otherwise sharp flange a step was formed. Experiments performed with this embodiment of the element 3 confirmed that resistance of flow of the frontal plate 2 thus formed was less, as if we performed rounding-off of the frontal plate 2 with the characteristic dimensions of the step, i.e.

proportionally with the average of height and length. The surface of the element 3 fixed onto the frontal plate 2 and facing the flow can be optionally formed, it can be flat, conical or even convex; an example is shown in Fig. 5, with this embodiment a strip of plate was arranged perpendicularly to the front-wall 2 as a cavity forming element 3.

In order to obtain proper pressure reduction with the element 3, in connection with dimen sioning, the following are to be considered.

First of ali a minimal distance is to be ensured between the element 3 and the edge of the frontal plate 2, why, this is the part /A/ of the space, where pressure reduction as arise due to the flow. Indicating this distance- measured from the edge of the front-wall 2-with "b" and the smaller dimension of the frontal plate 2 lying perpendicularly to the stream with "H", it seems to be expedient to choose the value of b/H as follows: 0,3ab/H > 0,01. That means that if b/H equals to 0,3, essentially resistance of flow does not decrease further, that is, if it is below 0,01 change in resistance of flow will be influenced to a neglible extent only.

A further characteristic of the element 3 according to the invention forming the cavity lies in that greatest height ought not be more than 0,4 H and simultaneously the surface is also small, it may be even a flange. The importance lies in that plates of particularly large dimensions need not to be arranged before the frontal plate 2, accordingly no special requirements are to be met in respect to technology, at the same time the element can be subsequently mounted onto existing bodies, constructions, or equipments.

When designing the element 3, a further aspect is to be considered, namely, what angle should be formed by the outer planewhich is connected to the frontal plate 2-in axial section to the direction of the undisturbed flow. If the angle is too large, practically the same streamlines will be followed which would be otherwise formed on the rounded frontal plate /Fig. 1 and 3/, accordingly resistance of flow can not be reduced.

So the angle a, enclosed by the outer plane of the element 3 in the axial section and the direction of flow, must be less, than 60 .

The advantage of the element 3 accordingly to the invention forming the cavity lies in that the cavity can be formed even subsequently on already existing constructions, thus improving the previously disadvantageous value of resistance of flow; it can be considered, when planning equipment, at the same time special tech ologies, as e.g. chasing are not required, there is no hazard of accident.

In course of dimensioning and arranging the element according to the invention forming the cavity, the following correlations are to be considered: O, 32b/ H20, 04; Or0,4 H; RCOR15 H, wherein: H is the smallest dimension of the frontal plate, h stands for the distance between the element forming the cavity and the frontal plate, i.e. the size of the air-gap, b indicates the distance between the edge of the element and the edge of the frontal plate, a is the angle enclosed by the outer plane of the element forming the depression and by the direction of the undisturbed flow in axial section, R stands for the radius of rounding-off and Z indicates the height of the element forming the depression cavity.

Taking the aforementioned into consideration elements 3 forming the cavity were developed and experiments were performed in wind channels; it could be stated that by the solution according to the invention resistance of flow could be reduced in compliance with earlier solutions or even better, simultaneously the solution is far simpler and cheaper, functionally it can be better matched to the structural elements of existing equipments, e.g. of vehicles.

Figs. 6 to 1 2 show some preferred embodiments of the invention. The solution according to the invention may be delimited from known methods-based on the rounded edges of the frontal surface i.e. on placing rounded-off elements onto the front-wall-by the fact that reduction of resistance on the front-wall is achieved by using the elements arranged on the unchanged front-wall, in the environment of the elements, by reducing the pressure arising on the front-wall and not by means of the depression induced on the subsequently arranged elements.Compared to the method with which resistance is reduced by means of a plate (baffle plate) arranged before the frontwall the difference lies in that while with the former method function of reducing the resistance is performed by a plate lying perpendicularly or nearly perpendicularly to the direction of undisturbed flow and arranged relatively far from the front-wall and the elements connecting the plate with the front-wall have strength functions only, with the solution proposed by us the surfaces being important from the point of view of resistance reduction are parallel or nearly parallel with the undisturbed flow; they are connected functionally/ and not for reason of strength/directly or through a gap of relatively small size. Due to this fact, while the plate arranged before the front-wall results in a detached flow on the whole front-wall and a simultaneous pressure reduction, the constructions according to the invention cause a pressure drop only on the edges of the front-wall. With the plate arranged before the front-wall in the space between the plate and the front-wall flow takes place, under the influence of which on the backside of the plate depression is somewhat higher than that on the front-wall. The proposed plate-being perpendicular or nearly perpendicular onto the front-wall-eliminates this disadvantage, the resistance factor increasing factor.

Claims

1. Process for reducing resistance of flow of bodies arranged in flow with at least one baffle element arranged on the frontal plate, characterized in that on the frontal surface-being perpendicular or nearly perpendicular to the flow-of the body arranged in the flow, along the periphery being less, than said surfacial periphery or along a partial periphery at least one depression inducing element being partially open is formed.

2. Construction for realizing the process according to the invention, which is formed on the frontal surface of the body arranged in the flow and forming optionally a unit therewith, characterized in that at least one element (3) inducing depression is arranged directly on the frontal surface (2) and the boundary surface of the depression inducing element (3) facing the edge of the front-wall (2) includes in the meridian section with the direction of the undisturbed stream an angle of s600, and the distance (b) between the extreme edge and the edge of the front-wall (2) is formed on basis of the correlation 0,04%b/Hr0,3, wherein H stands for the smaller dimension of the front-wall.

3. Construction as claimed in claim 2, characterized in that between the element (3) forming the depression cavity and the frontwall (2) there is an air-gap of the size S0,05 H formed.

4. Construction as claimed in claim 2 or 3, characterized in that the element (3) inducing depression is rounded towards the outer edges of the front-wall and the radius of rounding-off is Rio, 15 H.

5. Construction as claimed in any of the claims 2 to 4, characterized in that the height of the element (3) inducing depression corresponds to the 0,4-fold of the smaller size of the front-wall, i.e. Us0,4 H.

6. Construction as claimed in any of the claims 2 to 5, characterized in that the element forming the depression cavity is composed of a plurality of concentric flanges.

7. Construction as claimed in any of the claims 2 to 5, characterized in that the element (3) resulting depression is formed by a flange running parallel with at least one side of the front-wall (2).

8. Construction as claimed in any of the claims 2 to 5, characterized in that the element (3) inducing depression consists of several flange-like elements.

9. A device for reducing the resistance of a body that is moving relative to a fluid flow and that has a front wall facing the direction of relative motion between the body and the flow, which device is secured with respect to the front surface of the body and comprises at least one pressure-reducing element whose outer surface forms, in axial cross-section, an angle aS60 with the direction of relative motion between the body and the flow and wherein the distance, b, between the outer edge of the element and the nearest outer edge of the front surface is 0.04sb/H~0.3 where H is the smallest dimension of the front surface.

10. A device as claimed in claim 2 or claim 9, substantially as hereinbefore described in connection with and as illustrated in any one of Figs. 4 to 12 of the accompanying drawings.

11. A body fitted with a device as claimed in any one of claims 2 to 10.