KR20070067760A - The vehicle moving in water with drag reduction device using ultrasonic forcing - Google Patents

Abstract

A vehicle moving in water with a drag reduction unit using ultrasonic wave is provided to reduce the frictional resistance by generating minute air bubbles by attaching an ultrasonic vibrator to a surface of the vehicle making contact with the water surface. A vehicle moving in water with a drag reduction unit using ultrasonic wave includes a unit for reducing the frictional force by generating minute air bubbles by attaching an ultrasonic vibrator to a surface of the vehicle making contact with the water surface. If an electric signal corresponding to the natural frequency of a vibrator is applied from an ultrasonic wave oscillating unit(2), the vibrator generates resonance and converts the electrical signal to vibration which is a mechanical signal.

Description

Vehicle moving in contact with water with drag reduction device using ultrasonic wave {The vehicle moving in water with drag reduction device using ultrasonic forcing}

1 is a view for explaining the principle of the ultrasound excitation equipment,

Figure 2 shows a schematic diagram of an experiment for demonstrating the present invention using the ultrasonic wave excitation

3 is a view showing a change in the frictional resistance after the ultrasonic wave obtained through the experiment to prove the present invention

4 is a view showing a change in the flow direction velocity after the ultrasonic wave obtained through the experiment to prove the present invention

5 is a view showing the change in the vertical velocity of the wall surface after the ultrasonic wave obtained through the experiment to prove the present invention

*** Explanation of major symbols in drawings ***

1: tank 11: basin

2: Vehicle Model 3: Exciter

4: shooting unit

The present invention relates to a vehicle moving in contact with water having a drag reduction device using ultrasonic waves in order to reduce frictional resistance generated in an object moving in a fluid such as a ship.

Objects moving in a fluid create a boundary layer at the surface and move under frictional resistance through interaction with the fluid. In particular, friction resistance accounts for 80% of the total resistance in ships moving in contact with water. Recently, the coherent structure inside the turbulent boundary layer is revealed through direct numerical simulation (DNS), which has been actively developed with the development of computers, and the research is aimed at reducing the frictional resistance of the turbulent boundary layer by controlling it. Is actively underway. Robinson (1991) has shown through DNS that surface friction in the turbulent boundary layer is related to the velocity towards the wall of a streamwise vortex located very close to the wall. Therefore, in many fields, such as aerospace, ship propulsion, and noise control, effective friction control of wired vortices in the turbulent flow can reduce surface friction. Direct numerical simulation applied inhalation / injection with a phase opposite to the vertical velocity component near the wall, which weakened streamwise vorticity and consequently reduced surface friction (Choi et al. (1994)). ). However, this control is still technically difficult to realize because it requires infinitely small detectors and actuators. For practical use in the field, passive control methods such as riblets and flow walls have been performed. However, this manual control method has the advantage that the method is simple, but can not cope with changes in the flow field, the result of the control is different depending on the flow conditions. In order to secure such a problem, researches to control the flow field using local vibration have been actively conducted.

Representative method of reducing frictional resistance of boundary layer by using local vibration is the method of injecting microairbubble into boundary layer. McCormick and Bhattacharyya (1973) reduced drag by 10–30% using air bubbles created by electrolysis of water by placing copper wires on the surface of the specimen in water. Pal et al. (1988), Kato et al. (2000), and Kodama et al. (2000) show that the lower the flow rate and the greater the amount of inflow air, the greater the frictional resistance decrease, and the amount of micro air near the wall It was confirmed that it is an important factor. Xu et al. (2002) confirmed that the smaller the size of air bubbles entering the flow field, the greater the frictional resistance reduction, and the smaller the air bubbles, the more streak structures on the wall and the longer the distance between streaks. Ferrante and Elghobashi (2004) show that the vertical velocity component of the wall is greatly increased after micro air bubbles are injected into the flow field through DNS, and the lateral distance of the streaks causing the velocity component motion in the wall is increased. It was confirmed to increase. We also confirmed that the peak position of Reynolds stress production moved away from the wall, resulting in a smaller production of turbulent kinetic energy.

In order to use the method of directly injecting the air bubbles into the flow field as described above, air bubbles must be introduced directly into the flow field by drilling a slot or a hole in the surface. Since this method requires a lot of changes on the surface of the ship, there are many limitations in applying it to the solid line.

In other words, for the introduction of the polymer and the air bubbles are being actively studied, a porous surface or a slot must be installed on the surface of the ship. Such a surface change of the ship requires many changes in the existing manufacturing process, there are many problems in the practical application.

The present invention uses ultrasonic excitation as part of reducing the frictional resistance with water by generating micro air bubbles in the flow field in order to reduce the drag resistance of the vehicle moving in contact with water. The advantages of using an ultrasonic excitation are that it does not require much change on the wall and the strength of the excitation can be easily adjusted. This device uses the principle of the ultrasonic cleaner which is widely used. The ultrasonic cleaner generates cavitation by generating a pressure change inside the fluid. When the cavitation occurs, micro air bubbles are generated in the flow field. It can be used as an alternative to putting it directly in the.

The technical problem to be achieved by the present invention is to develop a method that is more suitable for the application of a vehicle such as a ship by minimizing the change in the surface of the vehicle, which is a problem with the frictional resistance reduction techniques of a vehicle moving in contact with water such as an existing ship. It is. Porous surface or slot should be installed on the surface of the vessel for the introduction of the polymer and the air bubbles which are being actively studied. Such a surface change of the ship requires many changes in the existing manufacturing process, there are many problems in the practical application. However, when the ultrasonic excitation is used, cavitation is generated by changing the applied electric signal into vibration, which is a mechanical signal, and through this, numerous air bubbles are generated in the flow field. Therefore, since small air bubbles can be easily generated in the flow field without any significant change on the surface of the ship, the disadvantages of the existing methods can be compensated for.

Ultrasound refers to sound waves with frequencies higher than the audible frequency that humans can hear. Ultrasound is now widely used in real life. Imaging equipment of medical devices, sonar of submarines, and non-destructive testing equipment are typical examples. Ultrasonic wave excitation equipment used in a vehicle moving in contact with water having a drag reduction device using ultrasonic waves according to the present invention is to remove foreign substances by vaporizing a liquid medium using cavitation to generate a high speed micro air droplets. It uses the principle of an ultrasonic cleaner.

Figure 1 illustrates the principle of the ultrasonic cleaner used in the present invention, the operation principle is that when the electric signal corresponding to the natural frequency of the oscillator is applied from the ultrasonic oscillator, the vibrator causes resonance while the electrical signal is a mechanical signal vibration Convert. This converted mechanical signal is transmitted to the medium in the water, which causes the kinetic energy of the medium to increase and the sound pressure increases and decreases repeatedly. When the repetitive change in sound pressure becomes lower than the saturated steam pressure of the medium, the cavitation core is generated as the medium vaporizes. The cavitation core expands with repeated increase and decrease of pressure and explode as the ambient pressure exceeds the surface tension. At this time, very small particles of air bubbles are generated at a very high speed. This process is called cavitation.

Experiments were carried out in a water bath to measure the reduction in frictional resistance of a vehicle moving in contact with water through ultrasonic excitation. 2 shows a schematic of an experiment demonstrating the present invention. The experiment was conducted with a model 2 of the vehicle moving in contact with water in a recirculating open water channel 1 where the water is circulated with a centrifugal pump (not shown). The tank 1 used in the experiment was an open (open) channel 250 mm wide, 250 mm high and 1000 mm long. Here, the vehicle model 2 equipped with the ultrasonic excitation device 3 is formed in a structure having a width of 150 mm and a length of 1000 mm, and installed at a height of 200 mm from the bottom of the water tank 11. Ultrasonic excitation device (3) is located 460mm away from the tip of the vehicle model (2), the origin of the coordinate axis is located at the central end of the ultrasonic excitation device (3), the flow direction in the x-axis, the wall vertical direction in the y-axis, The coordinate axis was set so that the horizontal direction would be the z axis. The ultrasonic wave excitation device 2 mounted on the vehicle model 2 used a general ultrasonic cleaning device. The ultrasound used in the experiment was 27.5 kHz and 6 vibrators were used.

SPIV was performed using two cameras to analyze the three-dimensional change of the flow field due to ultrasonic excitation. The size of the imaging section 4 is 90 x 90 mm and the experiment was conducted at -80 mm <x <240 mm to broadly examine the effect of excitation. Therefore, the experiment was divided into four sections.

으로 고정되었으며 초음파 가진부가 끝나는 지점, x⁺=0에서 모멘텀 두께(

)는 3.633mm였으며 이를 근거로 한 레이놀즈 수는

이다. 초음파 가진을 유동장에 가했을 때 가진후류에서의 유동방향에 따른 벽면마찰계수변화를 도 3에서 보여주고 있다. 벽면마찰계수는 가진이 없을 때의 벽면 마찰계수와의 비(

)로 무차원화 했기 때문에 초음파 가진 후 벽면마찰계수가 변한 비를 알 수 있다. 이 후로 유동방향은 x=0에서 가진이 없을 때의 모멘텀 두께인

, 벽면수직방향은 x=0에서 가진이 없을 때의 벽면마찰속도인

를 이용하여 무차원화가 되었다. 모든 속도 성분은 자유단의 속도(

)로 무차원화 되었다. 따라서

=0은 진동부의 끝부분을 의미한다. 초음파 가진 후 벽면마찰계수는 급격히 감소한 후 서서히 가진이 없을 때의 유동장으로 수렴해간다. 벽면마찰계수는

<5에서 최대 60%이상 감소한다. 대략

>45부근에서 벽면마찰계수의 비가 1보다 약간 큰 값을 보이며 서서히 가진이 없을 때의 마찰계수로 수렴한다. 도 3을 통하여 초음파 가진으로 생성된 미소공기방울을 이용하여 벽면마찰저항을 효과적으로 감소시킬 수 있다는 것을 알 수 있다. 따라서 초음파 가진을 이용함으로 미소공기방울을 직접 유동장에 유입하는 방법을 효과적으로 대체할 수 있다. The free flow velocity in the experiment

And the momentum thickness at x ⁺ = 0 where the ultrasonic excitation ends

) Was 3.633 mm and the Reynolds number based on that

to be. Figure 3 shows the change of the wall friction coefficient according to the flow direction in the wake after the ultrasonic excitation is applied to the flow field. The wall friction coefficient is the ratio with the wall friction coefficient in the absence of excitation (

Because it is dimensionless, the ratio of wall friction coefficient changed after ultrasonic wave is known. Since then, the flow direction is the momentum thickness without excitation at x = 0.

, The wall vertical direction is the wall friction velocity in the absence of excitation at x = 0.

It is dimensionless using. All velocity components are the speed of the free end (

) To be dimensionless. therefore

= 0 means the end of the vibration part. After ultrasonic excitation, the wall friction coefficient decreases rapidly and gradually converges to the flow field when there is no excitation. The wall friction coefficient

<5% reduction at maximum. about

In the vicinity of> 45, the ratio of the wall friction coefficient is slightly larger than 1 and gradually converges to the friction coefficient without excitation. 3, it can be seen that the wall frictional resistance can be effectively reduced by using the micro air bubbles generated by the ultrasonic excitation. Therefore, the use of ultrasonic excitation can effectively replace the method of directly injecting micro air bubbles into the flow field.

=24, 56에서 약간의 불일치를 보이지만 유동장의 변화를 잘 나타낸다. 가진이 없을 때의 유동방향의 분포를 보면 벽면에 대해 수평하게 분포하며 후류로 가면서 경계층이 서서히 성장하는 것을 볼 수 있다. 등강도의 폭의 변화를 보면 벽면부근에서 폭이 좁고 위로 올라갈수록 등강도의 폭이 넓어지게 되며

>200인 영역에서는 모두 같은 등강도 분포를 보이는 것을 알 수 있다. 벽면영역에서 등강도의 폭이 좁은 것은 그 만큼 속도가 벽면부근에서 상대적으로 큰 기울기를 가지는 것을 의미한다. 초음파 가진이 가해진 후 유동방향평균 속도가 감소하는 것을 도 4를 통해서 볼 수 있다. 특히

<24인 영역에서 유동방향 속도가 크게 감소한다.

24에서부터 가진의 영향이 서서히 약해지면서 크게 감소한 유동방향속도가 서서히 가진의 영향이 약해지면서 가진이 없을 때의 유동장으로 수렴해간다. 또한 속도가 감소한 영역에서 유동방향속도의 등강도 폭들이 가진이 없을 때에 비하여 넓어진다. 등 강도의 폭이 넓어진 것을 통하여 속도의 기울기가 감소한 것을 알 수 있다. 이렇게 벽면에서 속도의 등강도 폭이 넓어지는 것을 확인함으로서 초음파 가진이 벽면 마찰을 감소시키는 것을 확인할 수 있다. 또한 가진이 없을 때 벽면과 평행하게 보인 등강도분포가

<24에서 벽면에서 약간의 기울기를 가지며 떠오르는 것을 통하여 경계층의 두께가 두꺼워 지는 것을 볼 수 있다. 가진의 영향이

<24에서 강하게 나타나며 유동장이 큰 변화를 보이기 때문에 이 후로의 등강도 분포는 0<

<24인 부분에 대하여 자세히 살펴보겠다. The wall friction coefficient is proportional to the slope of the average velocity in the flow direction on the wall. Therefore, in order to examine the influence of the excitation on the average velocity in the flow direction, as shown in FIG. This is because the average velocity distribution of three experimental photographs is connected.

There is a slight discrepancy at = 24, 56, but it shows a good change in the flow field. The distribution of the flow direction in the absence of excitation shows that the boundary layer grows horizontally with respect to the wall and goes downstream. If you look at the change in the width of the isometric intensity, the width is narrower near the wall, and the width of the isometric width becomes wider as you go up.

It can be seen that in the region of> 200, all have the same intensity distribution. The narrow width of the equal intensity in the wall region means that the velocity has a relatively large slope near the wall. It can be seen from FIG. 4 that the average velocity in the flow direction decreases after the ultrasonic excitation is applied. Especially

In the region of <24, the flow direction velocity is greatly reduced.

From the 24th, the effect of the excitation gradually diminishes, and the greatly reduced flow direction velocity gradually converges to the flow field in the absence of excitation, with the weakening of the excitation. It is also wider than in the absence of excitation of the isotropic widths of the flow direction velocity in the region of reduced velocity. It can be seen that the slope of the speed is reduced through the widening of the back intensity. By confirming that the width of the uniform intensity of the velocity is widened on the wall surface, it can be confirmed that the ultrasonic excitation reduces the wall friction. Also, when there is no excitation,

At <24, we can see that the thickness of the boundary layer becomes thicker by rising with a slight slope on the wall. Having influence

Since it is strong at <24 and the flow field shows a large change, the distribution of isotensities thereafter is 0 <

Let's take a closer look at <24.

In order to see the effect of the excitation on the wall vertical velocity, the isotropic distribution of the average velocity in the vertical direction of the wall with and without the ultrasonic excitation is shown in FIG. In both cases, the difference in velocity was so large that the level of the uniform intensity distribution was applied differently. The flow field in the absence of excitation shows less than 1/30 of the flow field in the presence of excitation. Because of the turbulent boundary layer flow, in the absence of ultrasonic excitation, the wall vertical velocity is very small, less than 0.5% of the free end velocity. However, after ultrasonic excitation, the magnitude of the wall vertical velocity component increases significantly. After excitation, the maximum value of the wall vertical velocity increases to about 15% of the maximum free end velocity. Ultrasonic excitation shows that the vertical velocity component increases more than 30 times. This is due to the influence on the micro air bubbles generated by the ultrasonic wave excitation. Ferrante and Elghobashi (2004) confirmed that the vertical velocity of the wall increases to about 6% of the free end velocity when micro air droplets are introduced into the flow field. Increasing the wall vertical velocity reduces the flow velocity component and decreases the wall friction. Also in this experiment, when the microscopic air bubbles are generated in the flow field by ultrasonic excitation through Figs. 4 and 5, the velocity component in the wall vertical direction of the flow field greatly increases, so the flow velocity near the wall decreases and the wall friction decreases. have.

As a means of transportation in contact with the water having a drag reduction device according to the present invention having the configuration as described above is a vessel is the most common application and can also be applied to submarines and torpedoes to reduce drag.

As described above, the present invention minimizes the change in the surface of the vehicle, which is a problem with the frictional resistance reduction techniques of the vehicle moving in contact with water, such as a conventional vessel, and is suitable for practical application in a vessel. By using the ultrasonic excitation, only the ultrasonic vibrator can be attached to the surface to generate a myriad of micro air bubbles in the flow field. Therefore, it is possible to easily generate micro air bubbles in the flow field without large change on the surface of the ship, and to increase the vertical velocity component of the wall to reduce the flow velocity on the wall, thereby reducing the frictional resistance.

Claims (1)

A vehicle that moves in contact with water having an ultrasonic vibrator attached to a surface in contact with water to generate micro air bubbles in a flow field, and having a means for reducing frictional resistance.

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