Patent ID: 12214863

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

FIG.1is a perspective view of a riblet structure according to an embodiment of the present invention.FIG.2is a plan view showing the riblet structure in a partially enlarged manner.FIG.3is a side view showing the riblet structure in a partially enlarged manner.

A riblet structure1includes a plurality of wave-shaped riblets3, which cyclically change, on a surface2. In other words, each riblet3is formed into a wave shape that cyclically changes as viewed from above the surface2(upper surface). Here, the wave shape that cyclically changes typically refers to a sinusoidal shape.

The riblet structure1is provided to an object such as an aircraft, ship, pipe arrangement of plants, and a pipeline such that a ridge line4or the like of the riblet3comes into contact with a fluid. In the aircraft, ship, or the like that moves through a fluid such as air or water, the riblets3are provided to an outer surface of the object so as to be directed outward. In the pipe arrangement of plants, the pipeline, or the like, through which liquid or gas flows, the riblets3are provided to an inner surface of the object so as to be directed inward. In both cases, each riblet3is provided to a surface of the object such that a direction of the center line of a displacement of the wave shape, which cyclically changes, of each riblet3, typically a sinusoidal shape, coincides with a flow direction of a fluid.

The riblet structure1is typically configured by attaching a sheet, which includes a large number of riblets3formed on the surface2, to the surface of the object. The technology associated with the above configuration is disclosed in Patent Literature 2. As a matter of course, the riblets3may be formed on the surface of the object itself or may have other forms.

The plurality of riblets3are formed in a first direction that is an advance direction of the waves of the wave-shaped riblets3. Typically, the plurality of riblets3are formed along a flow direction x of a fluid flowing on the surface2. The plurality of riblets3may be formed in the direction of the center line of a displacement of the wave shape, which cyclically changes, of each riblet3, typically a sinusoidal shape.

Adjacent riblets3are formed to be parallel at predetermined intervals in a direction z orthogonal to the x direction, for example, if the object is an aircraft, at intervals of approximately 100 μm.

Hereinafter, a single riblet3will be described, but each riblet3has an identical shape.

The riblet3typically has a triangular cross-sectional shape. For example, in the riblet3, a y-z cross-section orthogonal to the flow direction x of the fluid is an isosceles triangle having a top angle of around 45°. In order to enhance surface friction resistance reducing performance, a smaller top angle is desirable. However, for example, when the technology of Patent Literature 2 already mentioned is adopted, it has been favorable to form the riblets3from coating and, in that case, to select 45° because of the trade-off with an aspect of manufacture. As a matter of course, the present invention is not limited to such a top angle. If the tip of the top is not acute-angled and is somewhat rounded, that is, deformed into a trapezoid having a flat-surface, a curved surface such as a circle or an ellipse, or the like when viewed in an enlarged manner, this is included in the technical range of the present invention. In this specification, the continuous line of the top of the riblet3is assumed as the ridge line4.

The riblet3has a lower height H as an angle β formed between the ridge line4and the flow direction x of the fluid becomes larger. In other words, the height H of the riblet3is not constant; is the lowest at a position where the angle β formed between the ridge line4and the flow direction x of the fluid is the largest; is the highest at a position where the angle β formed between the ridge line4and the flow direction x of the fluid is zero; and is a height that gradually continuously changes between those positions. In other words, the riblet3has the height H that changes cyclically in a wavelike manner.

The riblet3has a smaller width W between peak bases5and6in the direction z orthogonal to the flow direction x of the fluid as the angle β formed between the ridge line4and the flow direction x of the fluid becomes larger. In other words, the width W is not constant; is the narrowest at a position where the angle β formed between the ridge line4and the flow direction x of the fluid is the largest, that is, a position where the height H of the riblet3is the lowest; is the widest at a position where the angle β formed between the ridge line4and the flow direction x of the fluid is zero, that is, a position where the height H of the riblet3is the highest; and is a width that gradually continuously changes between those positions. Note that the continuous lines of the peak bases5and6of the riblet3are assumed as the peak base lines7and8, respectively.

FIG.4is a diagram showing the A-A cross-section and the B-B cross-section ofFIG.2.

In the riblet3according to this embodiment, the cross-sectional shape in the direction orthogonal to the flow direction x of the fluid is similar at any position. Further, in the riblet structure1according to this embodiment, all the riblets3have the same shape. Therefore, in all the riblets3, the cross-sectional shapes in the direction orthogonal to the flow direction x of the fluid are similar. In other words, in adjacent riblets3, the cross-sectional shapes in the direction orthogonal to the flow direction x of the fluid are similar at any position.

The inventors of the present invention have proposed in Patent Literature 1 the technology in which the riblets are configured to be formed into a wave shape and the height H of the riblets is also changed along the wave shape, thus enhancing resistance reducing performance. The method of manufacturing such riblets is typically disclosed in Patent Literature 2. This technology is for preparing a sheet, in which a material constituting the riblet structure1is applied onto a water-soluble resin on which the surface shape of the riblet structure1is transferred, attaching the sheet to a surface of an object such as an aircraft body of an aircraft, and removing the water-soluble resin from the surface. The water-soluble resin on which the surface shape of the riblet structure1is transferred is formed by transfer using a roller having, on one surface thereof, irregularities corresponding to the surface shape of the riblet structure1.

The riblet3according to this embodiment is configured such that the width W between the peak bases5and6in the direction orthogonal to the flow direction x of the fluid becomes smaller as the angle β formed between the ridge line4and the flow direction x of the fluid becomes larger; and in the cross-sectional shape in the direction z orthogonal to the flow direction of the fluid, an angle α formed between a slope5a(6a) of the peak and the surface2at the peak base5(6) is identical at any position. Further, in the riblet structure1according to this embodiment, all the riblets3have the same shape, and thus all the riblets3are configured such that the width W between the peak bases5and6in the direction orthogonal to the flow direction x of the fluid becomes smaller as the angle β formed between the ridge line4and the flow direction x of the fluid becomes larger; and in the cross-sectional shape in the direction z orthogonal to the flow direction of the fluid, the angle α formed between the slope5a(6a) of the peak and the surface2at the peak base5(6) is identical. In other words, an adjacent riblet3is configured such that the width W between peak bases5and6in the direction orthogonal to the flow direction x of the fluid becomes smaller as an angle β formed between the ridge line4and the flow direction x of the fluid becomes larger; and in the cross-sectional shape in the direction z orthogonal to the flow direction of the fluid, an angle α formed between a slope5a(6a) of a peak and the surface2at a peak base5(6) is identical at any opposed position.

Therefore, irregularities, by which the tip of the riblet3can be formed to have an acute angle, can be formed in the surface of the roller by only moving a tool bit, and the riblets3transferred using such a roller do not require the operation of cutting the tip thereof as in conventional riblets. Thus, the riblet structure1according to this embodiment can be easily manufactured, and additionally, the tip of the riblet3has an acute angle at any position, so that the surface friction resistance can be reduced and the resistance reducing performance can be enhanced.

Note that the irregularities formed in the surface of the roller can also be formed by endmills, not by a tool bit. In this case as well, irregularities, by which the tip of the riblet3can be formed to have an acute angle, can be formed in the surface of the roller by only moving endmills. In this case, the riblet3has a curved surface between the slope5a(6a) of the peak and the surface2at the peak base5(6) in the cross-sectional shape in the direction z orthogonal to the flow direction of the fluid, but the curvature of those curved surfaces only needs to be identical at any position.

Example of Riblet Structure

FIG.5is a diagram showing the ridge line4, the peak base lines7and8, and the center line9between the peak bases5and6of the riblet3according to an example of the riblet structure1by using x-y-z axes.FIG.6is a graph showing the peak base lines7and8and the center line9between the peak bases5and6of the riblet3by using the x-z coordinates and showing the height (ridge line) of the riblet by using the x-y coordinates. Note that the x direction is the flow direction of the fluid, and y is the height direction of the riblet3.

If the peak of the riblet is a triangle, assuming that the middle line of a valley formed with a riblet3adjacent to one riblet3is za, the center line9between the peak bases5and6is zc, the height of the peak of the riblet3is H, one peak base line7is zb1, and the other peak base line8is Zb2, curved lines represented by the following functions are set.
za=A·sin(2πx/λ1)
zc=za+s/2=A·sin(2πx/λ1)+s/2
H=(h−a)−a·cos(2πx/λ2)where λ1: a wave-shaped wavelength when the riblet3is viewed from above the surface2(upper surface),λ2: a wavelength of the height of the riblet3λ2=0.5λ1,A: an amplitude on the x-z plane of the middle line of the valley,s: intervals between riblets,h: the maximum height of the peak, anda: an amplitude of the height of the peak,
zb1=zc−Htanθ
=(A·sin(2πx/π1)+s/2)−((h−a)−a·cos(2πx/λ2))tanθ
zb2=zc+Htanθ
=(A·sin(2πx/λ1)+s/2)+((h−a)−a−cos(2πx/λ2))tanθ

In order to confirm the resistance reducing performance of the riblet structure1according to this example, analysis results obtained by the inventors of the present invention are shown inFIG.7.FIG.7shows a resistance reducing rate of various riblets obtained by a wind tunnel test. Note that all riblets have an apex angle of 45°. The resistance reducing rate (DRtotal) used herein is for a wall index s+of the peak intervals (corresponding to the intervals s) of the riblets.
s+=s(uτ/v)where s: an interval (m) between the riblets,uτ: a friction speed (m/s), andv: a kinetic viscosity (m2/s),

Resistance reducing rate (DRtotal)={(total resistance on riblet surface)−(total resistance on smooth surface)}/(total resistance on smooth surface). The smooth surface refers to a flat surface with no riblets, and the total resistance on the smooth surface is the total surface friction resistance.

As shown inFIG.7, when the wave-shaped riblet according to the present invention is compared with a wave-shaped riblet in which the top is trimmed and a peak height is adjusted, it is found that the wave-shaped riblet according to the present invention has better resistance reducing performance, though there is a difference depending on an airflow condition Reτ. It is found that the resistance can be reduced by approximately 8% at a maximum.

It is found fromFIG.7that the resistance reducing rate is smallest in the vicinity of s+=17. In addition, it is found that, even in values other than the vicinity of s+=17, the wave-shaped riblet according to this example has an improved resistance reducing effect as compared with the conventional wave-shaped riblet. For example, if the riblets are mounted on an aircraft, since the flight cannot be necessarily performed in the vicinity of the optimal conditions of the riblets, robustness to the value of s+other than the optimal conditions is important. In other words, it can be said that the wave-shaped riblet according to this example improves the robustness of the resistance reducing effect as compared with the conventional wave-shaped riblets.

<Others>

The present invention is not limited to the embodiment described above, and various modifications or applications may be made thereto without departing from the range of the technical idea. The range of implementation is also encompassed in the technical range of the present invention.

In the embodiment described above, the cross-section of the riblet3has a triangular shape, and the cross-sectional shape in the direction orthogonal to the flow direction x of the fluid is similar at any position, but if the shape at the vicinity of the top is similar at any position in the cross-sectional shape in the direction orthogonal to the flow direction of the fluid, the cross-sectional shape is not limited to that shape. The vicinity of the top refers to, if it is defined purposely, a region lower than the top by the minimum height of the riblet.

In the embodiment described above, the apex angle of the top of the riblet3is approximately 45°. However, of course, the apex angle may be made smaller than that angle to enhance the resistance reducing effect. The apex angle of the top of the riblet3may be made larger than approximately 45° to enhance the strength.

The wave shape in the embodiment described above typically has a sinusoidal shape, but it means that, if curved surfaces other than the sinusoidal shape are continuous to some extent, those curved surfaces are also included.

In the present invention, all the riblets constituting the riblet structure may be the wave-shaped riblets according to the present invention, but part of the region may be the wave-shaped riblets.

The present invention can be applied to various technical fields. For example, the present invention can be applied to pipe arrangement of plants, pipelines, and the like, so that fluid transport efficiency and the like can be improved. If the present invention is applied to the field of fluid machinery, surface friction resistance can be reduced.

REFERENCE SIGNS LIST

1: riblet structure2: surface3: riblet4: ridge line5: peak base6: peak base7: peak base line8: peak base lineH: height of ribletθ: half apex angle of top of ribletα: angle formed between slope of peak and surface at peak baseβ: ridge line and flow direction of fluidW: width between peak bases