Method for preparing and testing resistance-reducing small balls based on solid liquid drops

Abstract

The invention discloses a preparation and test method of a resistance-reducing ball based on solid liquid drops, and relates to the technical field of underwater resistance reduction, the preparation method comprises the following steps: carrying out decontamination treatment on the surface of a steel ball, preparing a PDMS solution, pouring the PDMS solution into a polytetrafluoroethylene mold with small holes with different curvature radiuses, carrying out vacuum defoaming, curing, and stripping to obtain the resistance-reducing ball based on the solid liquid drops. And solid-state liquid drop structures with different curvature radiuses are obtained and bonded to the bottom of the steel ball. By regulating the curvature radius (2.5-10.8 mm) of the solid droplet and the water entry speed, the change rule of sputtering crown formation, air entrainment and cavitation bubble closing modes in the water entry process is observed by utilizing a high-speed camera shooting technology, and the influence of the curvature radius of the solid droplet on the cavitation bubble form and the drag reduction performance is verified in combination with quantitative analysis of the underwater cavitation bubble volume and the resistance coefficient. The gas film capturing efficiency is optimized through the solid liquid drop structure, the pressure difference resistance and friction resistance of the bluff body are remarkably reduced, the preparation process is simple, the stability is high, and the method is suitable for development of the resistance reduction technology of the underwater vehicle.

Description

Technical Field

[0001] The present invention relates to the technical field of underwater drag reduction, in particular to a method for preparing and testing drag reduction balls based on solid droplets. Background Art

[0002] Underwater unmanned vehicles can replace humans to perform dangerous or monotonous tasks in extreme environments. They have excellent maneuverability, strong environmental adaptability and survivability. At the same time, they avoid the risk of casualties and have relatively low manufacturing and maintenance costs. The military and national defense fields have always been the main application directions of underwater unmanned vehicles. With the intensification of global security threats and the continuous development of modern naval forces, the military investment in underwater vehicle technology by various countries has continued to increase. When a vehicle operates underwater, the existence of flow resistance not only reduces the speed of the vehicle but also increases its energy consumption. Therefore, how to effectively reduce the flow resistance suffered by the vehicle during underwater navigation is of great research significance for reducing energy loss and increasing the speed of the vehicle.

[0003] The flow resistance suffered by a vehicle during underwater movement is the result of the combined action of various resistances, which can be mainly divided into the following categories: wave-making resistance, pressure drag, and frictional resistance. Wave-making resistance is the resistance formed by waves generated due to the movement of the hull when the vehicle moves on or near the water surface. Wave-making resistance is related to the speed of the vehicle, the shape of the hull, and the water depth. Pressure drag is the resistance generated due to the uneven pressure distribution before and after the vehicle moves in water. Pressure drag is closely related to the shape of the vehicle, so it is also called form drag. Frictional resistance is the resistance generated due to the viscous action between the surface of the vehicle and the water. The magnitude of frictional resistance is related to the roughness of the vehicle surface, the viscosity of the water, the sailing speed, and the surface area of the vehicle. For an object moving underwater, if it is a streamlined body, frictional resistance dominates; while for a blunt body (non-streamlined body), pressure drag is much greater than frictional resistance. Therefore, how to effectively reduce these two resistances, especially the pressure drag of blunt bodies, has become the core issue in the research of underwater drag reduction technology.

[0004] An effective drag reduction method is the cavitation drag reduction technology. By constructing cavitation on the surface of a blunt body and introducing gas between the solid and the liquid, a lubricating layer is formed. This method not only effectively isolates the direct contact between the solid and the liquid but also transforms the solid-liquid interface into a gas-solid and gas-liquid interface. Since the density and viscosity of air are much lower than those of the liquid, the frictional resistance is significantly reduced. In addition, when the gas adheres to the surface of the blunt body, the hydrostatic pressure will cause the cavitation to discharge bubbles from the tail during the underwater movement, and the shape of the cavitation will gradually be modified into a streamlined shape, thus greatly reducing the pressure drag.

[0005] The generation of cavitation bubbles is a key link in the cavitation drag reduction technology. Currently, the common cavitation bubble generation methods are mainly divided into two categories: one is the natural cavitation method, and the other is the ventilated cavitation method. The natural cavitation method induces the spontaneous formation of cavitation bubbles in the liquid through conditions such as high temperature, high speed, or local low pressure. Its core mechanism is cavitation. Under higher speed conditions or when the temperature rises to the critical temperature, when the local pressure of the liquid drops below the saturated vapor pressure, a phase change occurs inside the liquid to form vapor or gas cavitation bubbles. The ventilated cavitation method is to change the chemical properties of the object surface or construct a micro-structure on the object surface, thereby affecting the liquid film sputtering when the object impacts the water surface, and then capturing cavitation bubbles. However, these two methods have disadvantages such as high energy input, poor mechanical properties, high maintenance costs, and complex processing technology. Therefore, it is very important to find a simple, effective, and universal method to generate cavitation bubbles and maintain their stability.

[0006] The formation and stability of cavitation bubbles are not determined by a single factor, but the result of the synergistic effect of multiple physical parameters and structural characteristics. Surface wettability directly affects the contact state at the liquid-solid interface. A superhydrophobic surface promotes the formation of a gas film by reducing the solid-liquid contact area, while a hydrophilic surface is easily wetted by the liquid and requires a higher energy input to maintain cavitation bubbles; the micro-structure and roughness can regulate the formation and growth of cavitation bubbles by affecting the pinning of the triple line to capture gas; the macroscopic shape and size determine the flow field pressure distribution and cavitation bubble dynamics behavior, and the design of cavitation bubbles with a specific aspect ratio can optimize the gas film coverage range. The coupling effect of multiple factors under dynamic conditions is particularly crucial. Therefore, it is necessary to consider multi-scale parameters to systematically optimize the cavitation drag reduction technology. Summary of the Invention

[0007] To achieve the above object, the present invention provides a preparation and testing method of a drag reduction ball based on a solid-state droplet, which solves the above technical problems.

[0008] The present invention provides a preparation and testing method of a drag reduction ball based on a solid-state droplet. The preparation of the drag reduction ball based on a solid-state droplet includes the following steps:

[0009] (a) Decontaminate the surface of the metal ball;

[0010] (b) Prepare a PDMS solution, fill it into a mold, remove air bubbles by vacuum, cure it, and then peel it off to form a solid-state droplet;

[0011] (c) Bond the solid-state droplet to the bottom of the metal ball to obtain a drag reduction ball based on a solid-state droplet.

[0012] Further, the PDMS solution is mixed by the main agent and the curing agent in a mass ratio of 10:1 and is subjected to defoaming treatment.

[0013] Further, the curvature radius of the solid-state droplet is 2.5 - 10.8 mm.

[0014] Further, the decontamination treatment includes ultrasonic cleaning with n-hexane and absolute ethanol in sequence, and drying with nitrogen gas.

[0015] Further, the curing condition is heating in an oven at 50 °C for 3 hours.

[0016] Further, the mold is made of polytetrafluoroethylene, with evenly distributed small holes and a mold release agent sprayed on its surface.

[0017] Further, the test method for the drag reduction ball based on solid-state droplets includes: releasing the ball into water and controlling the water entry speed by adjusting the falling height, and observing the cavitation formation process and the underwater moving cavitation morphology using high-speed photography technology; analyzing the influence of the curvature radius of the solid-state droplet on the drag reduction performance based on the cavitation volume and the drag coefficient.

[0018] Further, the high-speed photography technology uses dual-camera synchronous shooting, with a frame rate of 5000 - 10000 frames per second, and the shooting angles include the horizontal direction and the inclined direction at 30° to the horizontal plane; the cavitation volume and the drag coefficient are negatively correlated, and when the cavitation volume reaches a stable threshold, the drag coefficient tends to the minimum value.

[0019] An experimental device for cavitation drag reduction performance, characterized by comprising: a transparent water tank and an adjustable-height electromagnet release system; a dual high-speed camera linkage shooting system, respectively configured at the water entry position and the underwater stable movement area; an LED light source and a diffuser plate combination for providing uniform backlighting.

[0020] Further, the water tank is made of PMMA, with dimensions of 90 cm × 15 cm × 15 cm, and a white diffuser plate is provided at the bottom; the electromagnet release system adjusts the height through a slide rail to control the water entry speed of the ball to be 1 - 5 m / s; the shooting area of the high-speed camera covers a depth range of 70 - 80 cm underwater for quantifying cavitation morphology parameters.

[0021] Further, the optimal range of the curvature radius of the solid-state droplet is 4.2–6.6 mm, and the larger the speed, the easier it is to form cavitation. The formation of cavitation significantly reduces the drag coefficient, and the maximum drag reduction efficiency can reach more than 90%.

[0022] A preparation and test method for a drag reduction ball based on solid-state droplets proposed by the present invention has the following beneficial effects:

[0023] 1. The steel balls commonly used in automotive bearings are selected as the experimental balls in this invention. Such steel balls have stable properties, smooth surfaces, and will not rust or corrode when placed in air at room temperature, meeting the required standards for experiments. By designing the curvature radius of the attached solid droplets, the sputtering of the liquid film and air entrainment are significantly promoted, forming stable cavities. The cavities transform the solid-liquid contact into a gas-solid / gas-liquid interface. Utilizing the low density and low viscosity characteristics of air, the frictional resistance is reduced. At the same time, the cavities are modified into a streamlined shape to reduce the pressure drag. The volume of the cavities increases with the increase of the water entry speed (release height), and the drag coefficient is significantly reduced, and the drag reduction effect tends to be stable (such as the drag coefficient reduction can reach a stable range).

[0024] 2. PDMS (polydimethylsiloxane) solution and reusable polytetrafluoroethylene molds are used to prepare solid droplets. The process is simple and the materials are stable, avoiding the high energy consumption and high cost of traditional cavity generation methods (such as high temperature, high pressure, or complex surface treatment). The increase in the curvature radius of the solid droplets (such as 10.8 mm) can enhance the expansion of the sputtering crown, forming a longer air capture channel, improving the success rate and stability of cavity formation. The triple-phase line is pinned at the equatorial position of the small ball, and the cavity attachment continues with the movement, avoiding early rupture and prolonging the gas film coverage time.

[0025] 3. Combining the multi-factor synergistic effects of the curvature radius, water entry speed, surface wettability (superhydrophobic PDMS), and macroscopic shape (steel ball streamlined body), and combining the critical speed of cavity formation and the attachment position of the solid droplets, the derived critical speed formula for cavity generation is beneficial to optimizing the cavity dynamics behavior, such as the correlation between the closing mode (surface closure → shallow closure → deep closure) and the drag reduction effect. The high-speed camera linkage observation technology (5000 - 10000 frames per second) is used to accurately capture the liquid film sputtering, the movement of the triple-phase line, and the morphological changes of the underwater cavities, providing reliable data for mechanism analysis. The modular experimental device design (such as a water tank, a slide rail electromagnet release system) supports repeated verification under different speed and curvature conditions, and is suitable for the further optimization of the blunt body drag reduction technology and the expansion of application scenarios. Through the solid droplet design, low-cost preparation process, and multi-parameter collaborative optimization, an efficient and stable cavity drag reduction method is proposed, significantly reducing the friction and pressure drag of underwater blunt bodies, providing an innovative solution for the energy saving and military applications of underwater vehicles. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic diagram of the experimental device. Figure 1 (a) is the device for observing the formation and morphology of cavities. Figure 1 (b) is the device for observing the dynamic behavior of the surface liquid film and the triple-phase line.

[0027] Figure 2 It is a diagram showing the formation process of cavities under different curvature radii and falling heights.

[0028] Figure 3 It is a diagram of the dynamic behavior changes of the surface liquid film and the triple phase line during the process of a small ball entering water under conditions of different curvature radii;

[0029] Figure 4 It is the underwater cavitation volume and drag coefficient under conditions of different curvature radii and falling heights;

[0030] Figure 5 It is a diagram of the Young's modulus and hardness of solid droplets with different crosslinking ratios;

[0031] Figure 6 It is a diagram of the cavitation formation process under conditions of different crosslinking ratios and falling heights;

[0032] Figure 7 It is a diagram of the liquid film splash and triple phase line changes when a small ball of solid droplets with different crosslinking ratios impacts the water surface; (a), (b) The crosslinking ratio of the solid droplet is 10:1; (c), (d) The crosslinking ratio of the solid droplet is 10:0.75; (e), (f) The crosslinking ratio of the solid droplet is 10:0.5. The solid droplet is located directly below the small ball; the curvature radius is 2.5 mm for all; the falling height is 70 cm for all. Specific implementation manners

[0033] The following uses specific specific examples to illustrate the implementation manners of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific implementation manners. Various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0034] Unless otherwise specified, the examples and comparative examples are parallel tests with the same components, component contents, preparation steps, and preparation parameters. The test methods in the following examples, unless otherwise specified, are all conventional methods; the test materials used in the following examples, unless otherwise specified, are all obtained by purchasing from commercial channels.

[0035] Unless otherwise specified, all reagents are used as they are without further purification.

[0036] In the preparation examples and examples of the present invention, unless otherwise specified, the "parts" are all parts by mass, and the concentration percentages, unless otherwise specified, are all mass concentrations.

[0037] The PDMS main agent is a vinyl-terminated polydimethylsiloxane prepolymer, purchased from Dow Chemical, Sylgard 184 A component; the PDMS curing agent is a crosslinking agent containing a platinum catalyst (methylhydrosiloxane system), which is the Dow Chemical Sylgard 184 curing agent.

[0038] Example 1

[0039] This embodiment provides a method for preparing a drag reduction ball based on solid droplets, comprising the following specific steps:

[0040] (1) Surface treatment of the experimental balls: First, a steel ball with a diameter of 15 mm was immersed in n-hexane and placed in an ultrasonic cleaner for 5 min to remove the oil and other impurities on its surface. Then, the steel ball was taken out and immersed in anhydrous ethanol and ultrasonically cleaned for 3 to 5 min to remove the residual reagent. Finally, the steel ball was taken out, dried with nitrogen and set aside.

[0041] (2) Preparation of polydimethylsiloxane solution: Weigh the PDMS main agent using an analytical balance, add the PDMS curing agent in a ratio of 10:1, place the solution in a mixing and defoaming apparatus, mix for 1 min, and then defoam for 1 min to obtain a uniformly mixed PDMS solution without bubbles.

[0042] (3) Preparation of drag-reducing spheres based on solid droplets: A polytetrafluoroethylene mold with small holes of the same volume and different curvature radii evenly distributed on the surface was used. The curvature radii of the small holes were 2.5 mm, 3.1 mm, 4.2 mm, 6.6 mm, and 10.8 mm, respectively. The mold was cleaned and a release agent was evenly sprayed on its surface. The prepared PDMS solution was poured into the small holes of the mold and placed in a vacuum dryer for 15 min to remove bubbles. The solution was taken out and placed in a 50°C oven for curing for 3 h. The PDMS was then peeled off the mold and adhered to a clean steel ball. The solution was placed in an oven and baked again until the PDMS was completely cured to obtain a drag-reducing sphere based on solid droplets.

[0043] Example 2

[0044] This embodiment provides a method for testing a drag reduction ball based on solid liquid droplets. On the basis of Embodiment 1, a drag reduction ball based on solid liquid droplets is tested as follows:

[0045] The entire experiment was conducted in a water tank, and a high-speed camera was used to observe and record the dynamic behavior changes of the ball during its entry into the water and its morphology during underwater movement. An appropriate amount of water was added to the water tank, and an electromagnet was fixed directly above the water tank using a slide rail and a slider to adsorb the experimental ball. The magnetism of the electromagnet was changed by controlling the power switch of the electromagnet, that is, the ball was adsorbed when the power was turned on, and released when the power was turned off. The ball fell and hit the water surface under the action of gravity.

[0046] In order to observe and record the cavitation formation process of the solid liquid droplet when entering the water and the changes in the cavitation morphology during underwater movement, we used two high-speed cameras to shoot. Figure 1As shown in (a), when designing and building the experimental device, an appropriate amount of water was filled into a water tank with dimensions of 90 cm × 15 cm × 15 cm. A high-speed camera was fixed at the position of the water entry height of the small ball in front of the water tank to record the cavitation formation process and the cavitation closure mode of the small ball during the water entry process. Another high-speed camera was placed at the bottom position in front of the water tank to record the state and cavitation morphology of the underwater small ball during stable movement. To ensure that the images captured by the high-speed cameras are clear enough, it is necessary to calibrate and focus the high-speed cameras in advance using a reference object. To ensure that the images captured by the high-speed cameras are bright and uniform, a high-brightness, non-flickering LED light source was placed at the corresponding position on the back of the PMMA water tank for lighting, and a white plastic diffuser was pasted at the corresponding position. The frame rate of both high-speed cameras was 5000 frames per second, and the shooting angle was perpendicular to the water entry movement trajectory of the small ball, which was in the horizontal direction.

[0047] The observation device for the dynamic behavior changes of the surface liquid film and the triple line during the water entry process of the small ball is as Figure 1 shown in (b). An appropriate amount of water was filled into a water tank with dimensions of 50 cm × 20 cm × 20 cm. Two high-speed cameras were respectively fixed on the adjacent sides of the water tank, and white plastic reflectors were pasted on the other two sides. High-brightness, non-flickering LED light sources were placed at the corresponding positions for supplementary lighting to ensure that the captured images are clear and uniform. One high-speed camera was used to shoot in the horizontal direction, perpendicular to the water entry movement trajectory of the small ball, to observe and record the movement and change process of the triple line, with a shooting frame rate of 10,000 frames per second. The other high-speed camera was slightly tilted, and the shooting angle was about 30° with respect to the horizontal plane, used to shoot and record the dynamic behavior changes of the surface liquid film when the small ball impacts the water surface, with a shooting frame rate of 5000 frames per second.

[0048] Example 3

[0049] Based on the curvature radius, water entry speed, and solid droplet position in Examples 1 and 2, it was found that multiple parameters have a synergistic effect, and an "optimal drag reduction range" was proposed: the curvature radius is 4.2 - 6.6 mm, and the speed is greater than 3.7 m / s. Within this range, the drag coefficient is reduced by more than 80%.

[0050] In the examples, the multiple parameters of curvature radius, water entry speed, and solid droplet position have a synergistic effect, affecting the critical speed of cavitation formation. The critical speed formula for cavitation generation is obtained as:

[0051] V C = k × R 0.5 × r α ;

[0052] where V C is the critical speed of cavitation generation, R is the curvature radius of the steel ball, r is the curvature radius of the solid droplet, k is the material constant, and α ranges from -0.7 to -0.5.

[0053] Comparative Example 1

[0054] The difference from Example 1 lies in the preparation of the polydimethylsiloxane solution: Weigh the PDMS main agent with an analytical balance, add the PDMS curing agent according to the ratio of the main agent to the curing agent of 10:0.75, put it into a mixing and defoaming instrument to mix for 1 minute and then defoam for 1 minute, and finally obtain a uniformly mixed PDMS solution without bubbles, and the others are the same.

[0055] Comparative Example 2

[0056] The difference from Example 1 lies in the preparation of the polydimethylsiloxane solution: Weigh the PDMS main agent with an analytical balance, add the PDMS curing agent according to the ratio of the main agent to the curing agent of 10:0.5, put it into a mixing and defoaming instrument to mix for 1 minute and then defoam for 1 minute, and finally obtain a uniformly mixed PDMS solution without bubbles, and the others are the same.

[0057] Comparative Example 3

[0058] The difference from Example 1 is the lack of an equal weight portion of the prepared PDMS solution, and the others are the same.

[0059] Figure 2 It shows the formation process of cavitation under different curvature radii and falling heights. The curvature radii are 2.5 mm, 3.1 mm, 4.2 mm, 6.6 mm, and 10.8 mm respectively. The release height determines the water entry speed, and the solid droplet is located directly below the steel ball. The larger the curvature radius of the additional solid droplet, the easier it is to form a sputtering crown and thus form a channel for air to enter, so it is easier to form cavitation; the higher the release height, the greater the speed of the small ball hitting the water surface, the larger the formed sputtering crown, the more air is introduced, the longer the pinched cavitation is, and the volume of the cavitation becomes larger and larger. The closing mode of the cavitation also changes from surface closing to shallow closing and finally to deep closing as the curvature radius increases.

[0060] Figure 3 It shows the dynamic behavior changes of the surface liquid film and the triple phase line during the water entry process of the small ball under different curvature radii; (a), (b) the curvature radius of the solid droplet is 3.1 mm; (c), (d) the curvature radius of the solid droplet is 6.6 mm; (e), (f) the curvature radius of the solid droplet is 10.8 mm; the falling height is 70 cm. When the small ball hits the water surface, the solid droplet first contacts the liquid surface, and the liquid surface is squeezed and deformed to form a thin liquid film. The liquid film climbs along the surface of the small ball to a position above the equator of the small ball, a part of it is pinned on the surface of the small ball, and the other part breaks away from the surface of the small ball and splashes outward to form a sputtering crown, thus obtaining a channel for capturing air. As the small ball moves downward, the sputtering crown expands outward and the height increases. At the same time, air is captured to form cavitation. The triple phase line is pinned at the equator position of the small ball, and the cavitation adheres to the surface of the small ball and continues to move downward with the small ball.

[0061] Figure 4 are the underwater cavitation volume and drag coefficient under different curvature radii and falling heights. The dark area is the region without cavitation formation, and the light area is the region with cavitation formation. Under different curvature radii conditions, the cavitation volume generally shows a trend of increasing with the increase of the release height. As the cavitation volume increases, the drag coefficient gradually decreases, and the drag reduction effect becomes better and better, and finally tends to be stable. The existence of surface cavitation can effectively reduce the resistance suffered by the small ball during underwater movement, and the cavitation has a good drag reduction effect.

[0062] Figure 5 To study the influence of the physical and chemical properties of solid droplets on the cavitation formation process and underwater drag reduction performance, we changed the curing agent ratio added during the curing of PDMS solid droplets to change the physical and chemical properties of the solid droplets. We selected three crosslinking ratios of 10:1, 10:0.75, and 10:0.5, and tested PDMS samples with different crosslinking ratios through a nanoindentation tester. When the crosslinking ratio was 10:1 and 10:0.75, the Young's moduli of the PDMS samples were 3.703 MPa and 3.476 MPa respectively, and the hardnesses were 1.128 MPa and 1.134 MPa respectively. That is, at this time, the mechanical properties of the PDMS samples were relatively stable, the Young's modulus value decreased slightly, and the hardness data did not change significantly. When the crosslinking ratio was 10:0.5, the Young's modulus and hardness of the PDMS sample were 0.935 MPa and 0.135 MPa respectively. Compared with the previous two crosslinking ratios, both the Young's modulus and hardness decreased significantly, and the mechanical properties changed significantly.

[0063] Figure 6 It can be seen that when the curvature radius of the additional polydimethylsiloxane solid droplet is 2.5 mm, the cavitation formation process under different crosslinking ratios and release heights is as Figure 6As shown, the crosslinking ratios are 10:1, 10:0.75, and 10:0.5 respectively. The release height determines the water entry speed, and the solid droplet is located directly below the steel ball. Under the condition of a lower release height, the liquid film formed when the additional solid droplet ball enters the water wraps around the surface of the sphere, allowing only a very small amount of air to enter between the liquid film and the ball. Eventually, a thin air film forms around the ball surface, and no cavitation can be formed. As the release height increases, cavitation gradually begins to form. When the crosslinking ratio of the solid droplet is 10:1, cavitation starts to form at a height of 70 cm. When the crosslinking ratio of the solid droplet is 10:0.75, cavitation starts to form at a height of 50 cm. When the crosslinking ratio of the solid droplet is 10:0.5, cavitation starts to form at an even lower height (30 cm), and obvious closure and pinching phenomena of the cavitation below the water surface start to appear. When the release height is higher, the length of the cavitation is significantly longer, the volume of the cavitation also becomes larger, and the shape becomes more streamlined. In summary, when the curvature radius of the additional solid droplet is 2.5 mm, the larger the crosslinking ratio, the smaller the water entry speed required to form cavitation, and the easier it is to form cavitation. However, it can be seen that the surface of the formed cavitation is not smooth, and obvious ripples and wrinkles exist. At the same time, the closure methods of the cavitation are different for different crosslinking ratios. When the crosslinking ratio is 10:1, the closure method of the cavitation is surface closure. When the crosslinking ratio is 10:0.75, the closure method of the cavitation is shallow closure. When the crosslinking ratio is 10:0.5, the closure method of the cavitation changes from shallow closure to deep closure at a height of 50 cm.

[0064] Figure 7 It can be seen that when the curvature radius of the solid droplet is 2.5 mm and the release height is 70 cm, the splashing of the surface liquid film and the formation process of cavitation during the water entry of the ball under different crosslinking ratio conditions are as Figure 7As shown. When the crosslinking ratio of the solid droplet is 10:1, a sputtering crown is formed after the small ball impacts the water surface, thereby introducing air. However, the sputtering crown cannot extend outward and begins to contract within a very short time (4 ms), and is basically closed at 12 ms. Therefore, the amount of introduced air is small, and the volume of the formed air bubble is small. As the small ball continues to move downward, the air bubble is pinched off at a position near the water surface, forming an air bubble. There are no obvious wrinkles on the surface of the air bubble, only some ripples (Figures (a) and (b)). When the crosslinking ratio of the solid droplet is 10:0.75, the formed sputtering crown is larger and more complete, the air capture channel is larger, and the sputtering crown starts to contract later (7 ms), and the liquid film closing time is later. Therefore, more air is introduced, and the air bubble has enough time to be stretched in the vertical direction, and the air bubble volume is larger. As the small ball continues to move downward, finally the air bubble is pinched off at a shallower position below the water surface. The surface of the air bubble gradually becomes smooth from having obvious prismatic protrusions, but finally, affected by the position of the solid droplet, the triple line is damaged, and there are obvious wrinkles on the surface of the air bubble (Figures (c) and (d)). When the crosslinking ratio of the solid droplet is 10:0.5, the sputtering crown expands outward more and is higher, the air capture channel is wider, and the sputtering crown does not start to contract until 9 ms after the small ball enters the water. Therefore, more air is introduced than when the crosslinking ratio is 10:0.75. As the small ball continues to move downward, the air bubble is stretched, and the position where the air bubble is pinched off is deeper, so the air bubble volume is larger. The surface of the air bubble gradually becomes smooth from being angular, but finally, affected by the solid droplet, there are also obvious wrinkles on the surface (Figures (e) and (f)). Therefore, when the curvature radius is 2.5 mm, the larger the crosslinking ratio, the larger the formed sputtering crown, the more gas is introduced, the larger the air bubble volume, the deeper the position where the air bubble is pinched off, and the air bubble closing mode changes from surface closing to shallow closing, and finally to deep closing.

[0065] Referring to the above experimental method, the results of Example 1 and Comparative Example 3 are shown in Table 1.

[0066] Table 1

[0067] Example 1 Comparative Example 3 <![CDATA[Bubble volume (cm 3 )]]> 12.2 1.8 Liquid film closing time (ms) 21.7 2.8 Drag coefficient (3.8 m / s) 0.04 0.56 Drag reduction efficiency (%) 92.9 0

[0068] The air bubble volume is the sum of the small ball volume and the air cavity volume. It can be seen from Table 1 that in Comparative Example 3, a stable air bubble cannot be formed when entering the water, the resistance coefficient is always at a high level, there is no triple line pinning effect after the liquid film is formed, and the liquid film quickly climbs and closes. At the same water entry speed of 3.8 m / s, the resistance coefficient of Comparative Example 3 is larger, and the resistance coefficient of Example 1 is smaller, and the drag reduction efficiency is higher than 90%.

[0069] The specific embodiments of the present invention have been described in detail above, but they are only examples, and the present invention is not limited to the specific embodiments described above. For those skilled in the art, any equivalent modifications and substitutions to the present invention are also within the scope of the present invention. Therefore, equivalent transformations and modifications made without departing from the spirit and scope of the present invention should all be covered within the scope of the present invention.

Claims

1. A method for preparing and testing a drag reduction small ball based on solid-state droplets, characterized in that, The preparation of the drag reduction ball based on solid-state droplets includes the following steps: (a) Decontaminate the surface of the metal ball; (b) Prepare a PDMS solution, fill it into a mold, remove air bubbles under vacuum, cure it, and then peel it off to form solid-state droplets; (c) Bond the solid-state droplets to the bottom of the metal ball to obtain the drag reduction ball based on solid-state droplets.

2. The method according to claim 1, wherein The PDMS solution is mixed by a main agent and a curing agent in a mass ratio of 10:1 and is subjected to defoaming treatment.

3. The method according to claim 1, characterized in that, The radius of curvature of the solid-state droplets is 2.5 - 10.8 mm.

4. The method according to claim 3, characterized in that The decontamination treatment includes ultrasonic cleaning with n-hexane and absolute ethanol in sequence, and drying with nitrogen.

5. The method according to claim 1, characterized in that, The curing condition is heating in an oven at 50 °C for 3 hours.

6. The method according to claim 1, wherein The mold is made of polytetrafluoroethylene, with evenly distributed small holes and a release agent sprayed on its surface.

7. The method according to claim 1, characterized in that The test method for the drag reduction ball based on solid-state droplets includes: releasing the ball into water and adjusting the falling height to control the water entry speed, and using high-speed photography technology to observe the process of cavitation formation and the morphology of the underwater moving cavitation; analyzing the influence of the radius of curvature of the solid-state droplets on the drag reduction performance based on the cavitation volume and the drag coefficient.

8. The method according to claim 7, characterized in that The high-speed photography technology uses dual-camera synchronous shooting, with a frame rate of 5000 - 10000 frames per second, and the shooting angles include the horizontal direction and the inclined direction at 30° to the horizontal plane; the cavitation volume and the drag coefficient are negatively correlated, and when the cavitation volume reaches a stable threshold, the drag coefficient tends to the minimum value.

9. An experimental device for cavitation drag reduction performance used in any of the methods of claims 1-8, characterized in that, It includes: A transparent water tank and an adjustable-height electromagnet release system; a dual high-speed camera linkage shooting system, respectively configured at the water entry position and the underwater stable movement area; A combination of an LED light source and a diffuser plate for providing uniform backlight.

10. The device according to claim 9, characterized in that, The water tank is made of PMMA, with dimensions of 90 cm × 15 cm × 15 cm, and a white diffuser plate is provided at the bottom; the electromagnet release system adjusts the height through a slide rail to control the water entry speed of the ball to be 1 - 5 m / s; the shooting area of the high-speed camera covers a depth range of 70 - 80 cm underwater for quantifying the cavitation morphology parameters.

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