Turbulence friction resistance reduction device

Abstract

A turbulent frictional resistance reduction device is provided that can improve the frictional resistance reduction rate in a flow where a fluid (water, air) comes into contact with a wall surface by using a method different from conventional methods. [Solution] The wall surface of an object in contact with a fluid is provided with rectangular holes that suck in longitudinal vortex structures formed in wall turbulence, the longitudinal direction of the holes being perpendicular to the direction of fluid flow, and the speed at which the longitudinal vortex structures are sucked in is 0.8 to 1.2 times the friction speed. When the object is a ship, it is preferable to provide the holes in the bottom wall surface of the ship, and this turbulent frictional drag reduction device can be suitably used in aircraft as well as ships.

Description

[Technical Field]

[0001] The present invention relates to reducing the friction resistance between fluids (water and air) and wall surfaces. [Background technology]

[0002] The longitudinal vortex structures formed in wall turbulence are paired vortex structures with axes in the flow direction, and in the case of water flows, they develop from the bottom wall toward the water surface. This vortex structure is a universal structure in water flows and air flows that involve walls, and because its motion plays a major role in generating turbulence, vigorous efforts are being made to control turbulence, which can reduce frictional resistance.

[0003] There are two types of turbulence control methods: passive control and active control. Passive control is a method that requires only initial processing and setting of the system. A representative example of this method is riblets, which are inspired by shark skin and involve creating tiny grooves on the wall surface in the hope of suppressing the oscillation of streak structures and reducing resistance (see Non-Patent Document 1). This is used in yachts and aircraft and is known to reduce total resistance by about 2% (see Non-Patent Document 2).

[0004] There is also a method for reducing frictional resistance in pipe flows by injecting a polymer solution to suppress the rotational motion of longitudinal vortex structures through the Toms effect. In recent years, a method has been devised for reducing frictional resistance in ships by covering the bottom of the hull with microbubbles generated at the bow. This method has achieved energy savings of 17% in laboratory model experiments and a net 2% on an actual ship (see non-patent documents 3 and 4).

[0005] On the other hand, active control methods require the exchange of energy between the system and the outside world while the system is running. A representative example of this method is the active cancellation method, which eliminates the rotational motion of the longitudinal vortex structure by blowing and suctioning air from the wall in a numerical simulation. This has been shown to reduce drag by approximately 30% (see Non-Patent Document 5). Experimentally, an active control system created from microsensors, actuators, and a controller in a wind tunnel experiment has succeeded in reducing frictional drag by 6% (see Non-Patent Document 6).

[0006] Another method for reducing turbulent frictional resistance is to provide small plates of a specific shape for reducing frictional resistance in the turbulent boundary layer of the fluid flowing along the wall surface, spaced slightly from the wall surface, approximately parallel to the wall surface, and with their longitudinal direction approximately perpendicular to the main flow direction (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 7-165156 [Non-patent literature]

[0008] [Non-Patent Document 1] Walsh, MT,: “Riblets” in viscous drag reduction in boundary, Bushnell, D., and Hefner, J. eds., Prog. Astronautics Aeronautics, 123, AIAA, pp. 203-261, 1990. [Non-patent document 2] N. Kasagi and Y. Suzuki: Intelligent Control of Turbulence, Transactions of the Institute of Systems, Control and Information Engineers, Vol., No. 4, pp. 131-137, 2004. [Non-patent document 3] Toshiaki Hirata, Nobuo Shiba, Keiji Kuramoto, Masaru Tominaga, and Kuryu Huang: Study on the Reduction of Frictional Resistance of Ships (3rd Report) Development of a Ship with Reduced Frictional Resistance, Western Industrial Technology Center Research Report No. 44, 2001 [Non-patent document 4] Tetsuro Matsunaga et al.: Full-scale experiment on frictional resistance reduction by microbubbles using the Seiun Maru, Journal of the Society of Naval Architects of Japan, No. 196, pp. 15-28, 2002. [Non-Patent Document 5] Choi, H., Moin, P., and Kim, J.,: Active turbulence control for drag reduction in wall-bounded flows, J. Fluid Mech, Vol. 262, pp. 75-110, 1994. [Non-patent document 6] N. Kasagi: Feedback Control of Wall Turbulence, Journal of the Japan Society of Fluid Mechanics, Nagare 25, pp. 13-22, 2006. Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a turbulent frictional drag reduction device that can improve the frictional drag reduction rate in a flow where a fluid (water, air) comes into contact with a wall surface by using a method different from conventional methods. [Means for solving the problem]

[0010] The present invention provides a turbulent frictional drag reduction device, characterized in that the wall surface of an object in contact with a fluid is provided with rectangular holes that absorb longitudinal vortex structures formed in wall turbulence, the holes being arranged so that their longitudinal direction is perpendicular to the fluid flow direction, and the speed at which the longitudinal vortex structures are absorbed is 0.8 to 1.2 times the friction speed.

[0011] In the device for reducing turbulent frictional resistance according to the present invention, the object is a ship, and the hole is provided in a bottom wall surface of the ship.

[0012] The present invention relates to a ship or aircraft equipped with the turbulent friction resistance reduction device. [Effects of the Invention]

[0013] According to the present invention, it is possible to provide a turbulent frictional resistance reduction device that can improve the frictional resistance reduction rate in a flow where a fluid (water, air) comes into contact with a wall surface by using a method different from conventional methods. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a bottom view of a ship model with a suction function used in an experiment on a device for reducing turbulent frictional resistance according to the present invention, and FIG. 2 is a side view of the model. [Figure 2] FIG. 10 is a diagram for explaining a method for measuring water flow resistance used in an experiment related to the device for reducing turbulent frictional resistance according to the present invention. [Figure 3] 1 shows experimental results for a device for reducing turbulent frictional resistance according to the present invention, and is a result of measuring water flow resistance. DETAILED DESCRIPTION OF THE INVENTION

[0015] The device for reducing turbulent frictional resistance according to the present invention reduces frictional resistance by providing a suction function to the wall surface to absorb the longitudinal vortex structures that are formed on the wall surface and are believed to be the cause of fluid frictional resistance, and is an original method not found in existing methods for reducing fluid frictional resistance. The device for reducing turbulent frictional resistance according to the present invention will be described in detail below.

[0016] The turbulent frictional resistance reduction device according to the present invention is provided with rectangular holes in the wall surface of an object that comes into contact with a fluid, which absorb the longitudinal vortex structures formed in the wall turbulence, the holes being arranged so that their longitudinal direction is perpendicular to the direction of the fluid flow, and the speed at which the longitudinal vortex structures are absorbed is 0.8 to 1.2 times the friction speed.

[0017] Here, fluids are liquids such as water and seawater, and gases such as air. Objects are objects that move through fluids, such as ships that sail on water and aircraft that travel through the atmosphere, and can also be said to be objects that move relative to the fluid.

[0018] The holes that suck in the longitudinal vortex structure are provided on the wall surface of the object that comes into contact with the fluid. If the object is a ship, the wall surface that comes into contact with the fluid will be the wall surface below the waterline. However, since the draft of a ship varies depending on the amount of cargo carried, it is practically best to provide holes only on the bottom of the ship. If the object is an aircraft, the wall surface that comes into contact with the fluid will be the entire wall surface, and it is best to provide holes on the entire wing and fuselage surfaces.

[0019] The holes in the wall surface are rectangular, preferably elongated rectangular holes like slit-shaped holes. The holes are placed so that their longitudinal direction is perpendicular to the direction of movement of the object (see Figure 1). If the holes are positioned so that their longitudinal direction is perpendicular to the direction of movement of the object, the longitudinal direction of the holes is perpendicular to the fluid flow, so they can reliably suck in the roots of the longitudinal vortex structures formed on the wall surface (low speed streaks), suppressing the generation and development of longitudinal vortex structures.

[0020] The ratio of holes to the wall surface is preferably as large as possible, and it is preferable to provide holes over the entire wall surface. As will be shown in the examples below, frictional resistance basically decreases in proportion to the installation area of ​​the holes (size of the suction area).

[0021] The speed at which the longitudinal vortex structures are sucked in through the holes in the wall is based on the friction speed, with the effective range having a lower limit of 0.8 times the friction speed and an upper limit of 1.2 times the friction speed. If the speed at which the longitudinal vortex structures are sucked in exceeds 1.2 times the friction speed, the surrounding high-speed flow will be collected, increasing the velocity near the wall, resulting in an increase in the velocity gradient and increasing frictional resistance. On the other hand, if the speed at which the longitudinal vortex structures are sucked in is below 0.8 times the friction speed, the longitudinal vortex structures cannot be sufficiently sucked in, and the rate of reduction in frictional resistance will be small.

[0022] The means for sucking the vertical vortex structure is not particularly limited, and a suction device such as a pump can be used if the fluid is a liquid, or a fan can be used if the fluid is a gas. It is also possible to use a pressure difference between the inside and outside of the wall surface.

[0023] As described above, the turbulent frictional resistance reduction device according to the present invention can be applied to water or air flows that involve walls. Therefore, by installing it on ships or aircraft, it is thought that frictional resistance can be reduced and energy savings can be achieved. [Example]

[0024] The following describes the results of experiments on the frictional resistance reducing device according to the present invention.

[0025] To demonstrate the effect of reducing frictional resistance through suction by a longitudinal vortex structure, a ship model with a suction function on the bottom, as shown in Figure 1, was created and experiments were conducted. The dimensions of the ship model, as shown in Figure 1, are 2200 mm in length, 210 mm in width, and 100 mm in height. A space is provided in the model, with three opening and closing valves at the top. In addition, holes measuring 3 mm in width and 190 mm in length are drilled at 10 mm intervals over a length of 2000 mm on the bottom of the ship model. In the following experiments, the holes on the bottom were partially closed with tape to change the suction area.

[0026] When this model is floated in flowing water and the valve at the top of the space is opened, water enters the space up to the draft depth, and at the same time, the vertical vortex structure formed on the bottom of the ship is sucked in. The suction speed was set to the friction speed of the flow. A Newton meter was used to measure the flow resistance acting on this ship model, as shown in Figure 2.

[0027] The results of the water flow resistance measurements are shown in Table 1. The actual measurement results are shown in Figure 3. In Table 1, F a indicates the reduction in flow resistance (%) due to suction relative to the total resistance (N).F f indicates the reduction in flow resistance (%) due to suction relative to friction resistance (N). In the experiment, the suction area was changed from 25 cm to 150 cm to see the effect of the suction area range.

[0028] [Table 1]

[0029] Looking at the experimental results for CaseFD1(f), where suction was performed within a range of 150 cm, F a 22.2% in F f The result was 28.1%, which exceeds the reduction achieved by previous experiments and actual equipment. f Looking at the value, this reduction rate is close to 30% in the numerical simulation using DNS.

[0030] Figure 3 shows the measurement results for Case FD1(f). The vertical axis is fluid force (N) and the horizontal axis is time (sec). The fluid force before suction is 4.5N. The vertical line at elapsed time 5 (s) represents the time when the valve was opened. Opening the valve causes the fluid force to temporarily increase, but it is clear that as time passes, the fluid force decreases to 3.5N as the vertical vortex structure is sucked in from the bottom. After that, the amount of suction is limited, so the fluid force returns to the level before suction.

[0031] This experiment was conducted to verify whether suction operation of the vertical vortex structure is effective in reducing frictional resistance, and it is expected that continuous suction will be performed using a pump or other device in order to sustain the reduction in frictional resistance.

Claims

1. a rectangular hole is provided on the wall of the object that comes into contact with the fluid, which absorbs the longitudinal vortex structure formed in the wall turbulence; The turbulent friction resistance reduction device is characterized in that the holes are arranged so that their longitudinal direction is perpendicular to the direction of fluid flow, and the speed at which the longitudinal vortex structure is sucked in is 0.8 to 1.2 times the friction speed.

2. 2. The device for reducing turbulent frictional resistance according to claim 1, wherein the object is a ship, and the hole is provided in a bottom wall surface of the ship.

3. A ship or aircraft equipped with the turbulent friction resistance reduction device according to claim 1.

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