SURFACE WITH REDUCED DRAG

In times of environmental problems due to energy consumption, such as the global warming issues, there is an increased attention to reduce energy consumption.

One of the factors determining energy consumption is friction. One area in which friction exists is in the area of fluids flowing along a surface. For example gasses or liquids which are transported through pipes experience friction from the wall, but also vehicles like cars and airplanes, experience friction when traveling through the atmosphere. It is now an object of the invention to provide a surface, in which this kind of friction is at least reduced.

This object is achieved by a surface substantially cylindrical, having a plurality of parallel helical grooves comprising a common longitudinal axis, substantially coinciding with the longitudinal axis of the cylindrical surface, and a circular cross-section, wherein each groove surface is provided with a plurality of parallel helical grooves having a common longitudinal axis, substantially coinciding with the longitudinal axis of the groove. When a fluid, like a gas or liquid, flows along a flat surface, a boundary layer will be generated because the fluid tends to adhere to the flat surface. This causes friction.

Now by providing helical grooves in the surface the boundary layer will be directed by the helical grooves. As each helical groove is provided with its own set of helical grooves, the boundary layer within a helical groove will be forced to rotate creating a vortex. These vortexes provide a

boundary layer which has a small influence on the main stream of the fluid flowing along the surface as a result of which the drag of the surface is reduced.

In addition, the vortexes provide an isolation layer which reduces energy transfer from the fluid to the surface. This isolation property is provided as a result of the lower pressure present in the center of a vortex. Also the temperature within the vortex will be reduced as a result of centripetal acceleration. This acceleration provides for an adiabatic process as a result of which the temperature drops.

A further reduction of the drag, which the vortexes experience, is eliminated by having the vortexes running along a wave and with such a speed that the fluϊdum glides over the peaks of the waves. In order to achieve this result with air, it is necessary that either the peak distance is small or the speed of the air is high. By providing vortexes within the main vortex, the speed of the air is increased and the desired result is achieved.

In a further embodiment of the surface according to the invention, the pattern of helical grooves within a groove is repeated like an illusion of infinity. Each helical groove comprises its own set of helical grooves, which each have in turn their own set of helical grooves and this pattern is repeated until the limitations of manufacturing. Furthermore the surface according to the invention is either concave or convex. A concave cylindrical surface is for example the inner wall of a tube, whereas a convex cylindrical surface could be the hull of a plane.

These and other advantages of the invention will be elucidated in conjunction with the accompanying drawings.

Figure 1 shows a cross-sectional view of a tube having an inner surface according to the invention.

Figure 2 is a spread out view of the tube according to figure 1.

Figure 1 shows a cross-sectional view of a tube 1 having an outer cylindrical surface 2 and an inner cylindrical surface 3. The inner cylindrical surface 3 comprises a number of helical grooves 4. Figure 2 shows a spread out view of the inner surface 3 of the tube 1. Figure 2 clearly shows that the grooves 4 are helical. In each groove 4 further helical grooves 5 are arranged. When a fluid is pumped through the tube 1 the fluid has the tendency to adhere to the inner surface 3 of the tube 1. However due to the helical grooves 4 small vortexes 6 will be generated into the grooves 5. These vortexes 6 provide for a boundary layer such that the main stream of fluid F cannot adhere to the inner surface 3. These vortexes 6 provide for some kind of bearing reducing the drag of the inner surface 3.

In order to further reduce the drag for the vortexes 6, secondary grooves 5 are arranged into the primary grooves 4. The cross-section of the grooves 4 is semi-circular and comparable to a tube 1. As a result small secondary vortexes will be generated in the surface of the groove 4 also providing some kind of a bearing for the vortexes 6. As a result of the generated vortexes 6 small vortexes 7 will develop and which have a rotation opposite to the rotation direction of the vortexes 6.