A drag-reducing pellet preparation and testing method based on solid-state droplets
By preparing and testing drag-reducing microspheres based on solid droplets, the problems of high energy and complex processes in cavitation generation methods were solved, achieving efficient reduction of friction and pressure drag, which is suitable for energy saving and efficiency improvement of underwater vehicles.
Patent Information
- Application Number
- CN202510410052.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-02
- Publication Date
- 2025-12-12
- Estimated Expiration
- 2045-04-02
AI Technical Summary
Existing cavitation generation methods involve high energy input, poor mechanical properties, and complex processing techniques, making it difficult to effectively generate and maintain cavitation to reduce friction and pressure drag of underwater vehicles.
A drag-reducing microsphere preparation method based on solid droplets was adopted. By treating the surface of metal microspheres and bonding solid droplets, combined with PDMS solution and polytetrafluoroethylene mold, solid droplet microspheres with specific curvature radii were prepared. The cavitation formation process was observed using high-speed imaging technology, and the cavitation dynamics were optimized.
It significantly reduces friction and pressure drag, with a drag reduction efficiency of over 90%. The process is simple and low-cost, making it suitable for energy saving and efficiency improvement of underwater vehicles.
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Figure CN120253161B_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The application relates to the field of underwater drag reduction technology, and particularly to a preparation and testing method of drag reduction small balls based on solid liquid drops. BACKGROUND
[0002] Underwater unmanned vehicles can replace humans to perform dangerous or monotonous tasks in extreme environments, have excellent maneuverability, strong environmental adaptability and survivability, avoid personnel casualty risks, and have relatively low manufacturing and maintenance costs. The military and defense fields have been the main application direction of underwater unmanned vehicles. With the intensification of global security threats and the continuous development of modern naval forces, countries continue to increase military investment in underwater vehicle technology. When the vehicle is running 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 when it is running underwater has very important research significance for reducing energy consumption and improving the speed of the vehicle.
[0003] The flow resistance suffered by the vehicle when it is moving underwater is the result of the joint action of various resistances, which can be mainly divided into the following categories: wave-making resistance, pressure difference resistance and friction resistance. The wave-making resistance is the resistance formed by the waves generated by the movement of the ship body when the vehicle is moving on or near the water surface. The wave-making resistance is related to the speed of the vehicle, the shape of the ship body and the water depth. The pressure difference resistance is the resistance generated due to the uneven distribution of pressure in front and back of the vehicle when it is moving in water. The pressure difference resistance is closely related to the shape of the vehicle, so it is also called shape resistance. The friction resistance is the resistance generated due to the viscous action between the surface of the vehicle and the water. The size of the friction resistance is related to the roughness of the surface of the vehicle, the viscosity of the water, the speed of the vehicle and the surface area of the vehicle. For objects moving underwater, if it is a streamlined body, the friction resistance is dominant; and for blunt bodies (non-streamlined bodies), the pressure difference resistance is much greater than the friction resistance. Therefore, how to effectively reduce these two kinds of resistance, especially the pressure difference resistance of the blunt body, has become the core problem of underwater drag reduction technology research.
[0004] An effective drag reduction method is the cavitation drag reduction technology, which constructs a cavity on the surface of the blunt body, introduces gas between the solid and the liquid, and forms a lubricating layer. This method not only effectively isolates the direct contact between the solid and the liquid, but also changes the solid-liquid interface into gas-solid and gas-liquid interfaces. Since the density and viscosity of air are much lower than those of liquid, the friction resistance is significantly reduced. In addition, when the gas adheres to the surface of the blunt body, the static water pressure will make the cavity discharge bubbles from the tail during underwater movement, and the shape of the cavity is gradually modified into a streamlined shape, thereby greatly reducing the pressure difference resistance.
[0005] The generation of cavitation bubbles is a key step of the cavitation drag reduction technology. At present, common cavitation bubble generation methods mainly include two types: one is a natural cavitation method, and the other is a vented cavitation method. The natural cavitation method is to induce cavitation bubbles in liquid by high temperature, high speed or local low pressure, and the core mechanism is cavitation phenomenon. When the liquid is at a high speed or the temperature is increased to a critical temperature, the local pressure of the liquid is reduced to below the saturated vapor pressure, and the phase change occurs in the liquid to form vapor or gas cavitation bubbles. The vented cavitation method is to change the chemical properties of the object surface or build microstructure on the object surface, so as to affect the liquid film splashing when the object hits the water surface, and then capture the cavitation bubbles. However, these two methods have the disadvantages of high energy input, poor mechanical performance, high maintenance cost and complex processing technology. Therefore, it is very important to find a simple and effective method to generate cavitation bubbles and maintain the stability of the cavitation bubbles.
[0006] The formation and stability of cavitation bubbles are not determined by a single factor, but are the result of the synergistic effect of multiple physical parameters and structural characteristics. Surface wettability directly affects the contact state of the liquid and solid interface. The super-hydrophobic surface promotes the formation of gas film by reducing the solid-liquid contact area, while the hydrophilic surface is easily wetted by liquid and requires higher energy input to maintain the cavitation bubbles. The microstructure and roughness can regulate the formation and growth of cavitation bubbles by affecting the three-phase line pinning to capture gas. The macro shape and size determine the pressure distribution of the flow field and the dynamic behavior of the cavitation bubbles, and the cavitation bubble design with a specific aspect ratio can optimize the gas film coverage range. The multi-factor coupling effect under dynamic conditions is particularly critical. Therefore, it is necessary to consider the multi-scale parameters for systematic optimization of the cavitation drag reduction technology. SUMMARY
[0007] To achieve the above-mentioned purpose, the application provides a drag reduction small ball preparation and test method based on solid liquid drops, which solves the above technical problems.
[0008] The application provides a drag reduction small ball preparation and test method based on solid liquid drops, which includes the following steps:
[0009] (a) decontamination treatment is performed on the surface of the metal small ball;
[0010] (b) a PDMS solution is prepared and filled into a mold, and after vacuum defoaming, curing and peeling, a solid liquid drop is formed;
[0011] (c) the solid liquid drop is bonded to the bottom of the metal small ball to obtain a drag reduction small ball based on solid liquid drops.
[0012] Further, the PDMS solution is mixed with a curing agent at a mass ratio of 10:1, and is subjected to defoaming treatment.
[0013] Further, the curvature radius of the solid liquid drop is 2.5-10.8mm.
[0014] Further, the decontamination treatment comprises sequentially using n-hexane, anhydrous ethanol ultrasonic cleaning, and drying with nitrogen.
[0015] Further, the solidification condition is heating for 3 hours in a 50 DEG C oven.
[0016] Further, the mold is made of polytetrafluoroethylene, and the small holes are uniformly distributed and the surface is sprayed with a release agent.
[0017] Further, the test method of the solid-liquid droplet based drag reduction small ball comprises: controlling the water entry speed by releasing the small ball into water and adjusting the falling height, observing the cavity formation process and underwater motion cavity shape by using high-speed camera technology, and analyzing the influence of the solid-liquid droplet curvature radius on the drag reduction performance based on the cavity volume and the drag coefficient.
[0018] Further, the high-speed camera technology adopts double-camera synchronous shooting, the frame rate is 5000-10000 frames / s, and the shooting angles include the horizontal direction and the 30° inclined direction relative to the horizontal plane; the cavity volume and the drag coefficient are negatively correlated, and when the cavity volume reaches a stable threshold, the drag coefficient tends to a minimum value.
[0019] A cavity drag reduction performance experimental device, characterized in that it comprises: a transparent water tank and an adjustable height electromagnet release system; a double high-speed camera linkage shooting system, which is respectively arranged at the water entry position and the underwater stable motion area; an LED light source and a diffuser plate combination, which are used to provide uniform backlight.
[0020] Further, the water tank is made of PMMA, and the size is 90cm*15cm*15cm, and the bottom is provided with a white diffuser plate; the electromagnet release system adjusts the height through a slide rail to control the water entry speed of the small ball to be 1-5m / s; the high-speed camera shooting area covers the depth range of 70-80cm underwater, which is used to quantify the cavity shape parameters.
[0021] Further, the optimal interval of the solid-liquid droplet curvature radius is 4.2-6.6mm, and the greater the speed is, the easier the cavity is formed. The formation of the cavity significantly reduces the drag coefficient, and the drag reduction efficiency can be up to more than 90%.
[0022] The preparation and test method of the solid-liquid droplet based drag reduction small ball has the following beneficial effects:
[0023] 1、The invention selects steel balls commonly used in automobile bearings as experimental balls, such as stable properties, smooth surface, and no rust corrosion phenomenon under normal temperature conditions, which can meet the required standards of the experiment; through the design of the curvature radius of the additional solid droplet, the liquid film sputtering and air entrainment are significantly promoted, a stable cavity is formed, the solid-liquid contact is changed into gas-solid / gas-liquid interface, the low density and low viscosity characteristics of air are used to reduce the friction resistance; at the same time, the cavity is modified to be streamlined, the pressure difference resistance is reduced, the cavity volume increases with the increase of the water inlet speed (release height), the drag coefficient is significantly reduced, and the drag reduction effect tends to be stable (such as the drag coefficient reduction can reach the stable interval);
[0024] 2, The solid droplet is prepared by using PDMS (polydimethylsiloxane) solution and reusable polytetrafluoroethylene mold, which has simple process and stable material, avoids high energy consumption and high cost of traditional cavity generation methods (such as high temperature, high pressure or complex surface treatment), and the curvature radius of the solid droplet is increased (such as 10.8mm), which can enhance the sputtering crown expansion, form a longer air capture channel, improve the success rate and stability of the cavity formation, the three-phase line is pinned at the equatorial position of the small ball, the cavity adhesion continues with the movement, avoids early rupture, and prolongs the gas film covering time;
[0025] 3, The critical velocity formula of the cavity generation obtained by combining the curvature radius, water inlet speed, surface wettability (superhydrophobic PDMS) and macro shape (steel ball streamline body) and the critical velocity and additional position of the solid droplet is beneficial to optimize the cavity dynamics behavior, such as the correlation between the closed mode (surface closed→shallow closed→deep closed) and the drag reduction effect; The high-speed camera linkage observation technology (5000-10000 frames / second) is adopted to accurately capture the liquid film sputtering, three-phase line movement and underwater cavity shape change, and provide reliable data for mechanism analysis; The modular experimental device design (such as water distribution tank, slide rail electromagnetic iron release system) supports repeated verification under different speed and curvature conditions, and is suitable for further optimization and application scene expansion of bluff body drag reduction technology, through the design of solid droplet, low-cost preparation process and multi-parameter synergistic optimization, a high-efficiency and stable cavity drag reduction method is proposed, which significantly reduces the friction and pressure difference resistance of underwater bluff body, and provides an innovative solution for energy saving and efficiency improvement of underwater vehicles and military applications. BRIEF DESCRIPTION OF DRAWINGS
[0026] Figure 1 is a schematic diagram of the experimental device, Figure 1 (a) is a cavity formation and shape observation device, Figure 1 (b) is a surface liquid film and three-phase line dynamic behavior observation device;
[0027] Figure 2 is a cavity formation process diagram under different curvature radii and drop height conditions;
[0028] Figure 3 The dynamic behavior change diagram of the surface liquid film and the three-phase line of the small ball in the water under different curvature radius conditions;
[0029] Figure 4 The underwater cavity volume and drag coefficient under different curvature radius and falling height conditions;
[0030] Figure 5 The Young's modulus and hardness diagram of the solid liquid drop under different crosslinking ratios;
[0031] Figure 6 The formation process diagram of the cavity under different crosslinking ratios and falling height conditions;
[0032] Figure 7 The liquid film splashing and three-phase line change diagram of the solid liquid drop small ball impacting the water surface under different crosslinking ratios; (a), (b) The crosslinking ratio of the solid liquid drop is 10:1; (c), (d) The crosslinking ratio of the solid liquid drop is 10:0.75; (e), (f) The crosslinking ratio of the solid liquid drop is 10:0.5. The solid liquid drop is located directly below the small ball; the curvature radius is 2.5 mm; the falling height is 70 cm. DETAILED DESCRIPTION
[0033] The embodiments of the present application are illustrated by specific working examples below, and other advantages and effects of the present application can be easily understood by those skilled in the art from the disclosure of the specification. The present application can also be implemented or applied by other different specific embodiments, and various modifications or changes can be made to the details in the specification based on different views and applications without departing from the spirit of the present application.
[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 are conventional methods unless otherwise specified. The test materials used in the following examples are commercially available unless otherwise specified.
[0035] Unless otherwise specified, all reagents are used as received without further purification.
[0036] In the preparation examples and examples of the present application, the "parts" are mass parts unless otherwise specified, and the concentration percentages are mass concentrations unless otherwise specified.
[0037] The PDMS main agent is a vinyl-terminated polydimethylsiloxane prepolymer, which is purchased from Dow Chemical, Sylgard 184A component; the PDMS curing agent is a crosslinking agent containing a platinum catalyst (methyl hydrogen siloxane system), which is the Sylgard 184 curing agent of Dow Chemical.
[0038] Example 1
[0039] The present embodiment provides a method for preparing solid-liquid droplet based drag reduction beads, comprising the following specific steps:
[0040] (1) Surface treatment of experimental beads: First, immerse steel balls with a diameter of 15 mm in n-hexane, place them in an ultrasonic cleaner for 5 min to remove oil stains and other impurities on their surfaces, then take them out and immerse them in anhydrous ethanol, ultrasonically clean them for 3-5 min to remove residual reagents, and finally take them out, dry them with nitrogen, and prepare them for use.
[0041] (2) Preparation of polydimethylsiloxane solution: Weigh the PDMS main agent with an analytical balance, add the PDMS curing agent according to a ratio of 10:1 between the main agent and the curing agent, mix them in a mixing and defoaming instrument for 1 min and then defoam them for 1 min, and finally obtain a uniformly mixed and bubble-free PDMS solution.
[0042] (3) Preparation of solid-liquid droplet based drag reduction beads: Use a polytetrafluoroethylene mold with small holes of the same volume and different radii of curvature uniformly distributed on its surface, with radii of curvature of 2.5 mm, 3.1 mm, 4.2 mm, 6.6 mm, and 10.8 mm, clean the mold, uniformly spray release agent on its surface, pour the prepared PDMS solution into the small holes of the mold, place it in a vacuum dryer to remove air bubbles for 15 min, take it out and place it in a 50°C oven to cure for 3 h, then peel the PDMS off the mold, stick it to the cleaned steel balls, and place them in the oven again to bake until the PDMS is completely cured to obtain solid-liquid droplet based drag reduction beads.
[0043] Example 2
[0044] The present embodiment provides a method for testing solid-liquid droplet based drag reduction beads, which is based on Example 1 and tests the solid-liquid droplet based drag reduction beads as follows:
[0045] The entire experimental process is carried out in a water tank, and a high-speed camera is used to observe and record the dynamic behavior changes of the beads during the water entry process and the shape during the underwater movement process. Fill the water tank with an appropriate amount of water, use a slide rail and a slide block to fix an electromagnet directly above the water tank to adsorb the experimental beads, control the power switch of the electromagnet to change its magnetism, i.e. power on to adsorb the beads, and power off to release the beads, which fall and hit the water surface under the action of gravity.
[0046] In order to observe and record the cavitation formation process of the additional solid-liquid droplet beads during the water entry process and the cavitation shape changes during the underwater movement process, we use two high-speed cameras to shoot in tandem. For example, Figure 1(a) as shown in the design of the experimental device, a water tank of 90 cm x 15 cm x 15 cm is filled with water, a high-speed camera is fixed at the height of the ball into the water in front of the water tank, which is used to record the process of cavity formation and the closing mode of the cavity during the ball into the water. Another high-speed camera is placed at the bottom of the front of the water tank, which is used to record the state and cavity shape of the ball in stable motion underwater. In order to ensure that the picture taken by the high-speed camera is clear enough, the high-speed camera needs to be corrected and focused in advance using a reference object. In order to ensure that the picture taken by the high-speed camera is bright and uniform, high-brightness LED light sources without flicker are placed at the corresponding position on the back of the PMMA water tank for lighting, and white plastic diffuser plates are pasted at the corresponding position. The frame rate of the two high-speed cameras is 5000 frames, and the shooting angle is perpendicular to the trajectory of the ball into the water, which is horizontal.
[0047] The observation device of the dynamic behavior change of the surface liquid film and the three-phase line during the ball into the water is as shown in Figure 1 (b). A water tank of 50 cm x 20 cm x 20 cm is filled with water, and two high-speed cameras are fixed on the adjacent sides of the water tank, respectively. White plastic reflector plates are pasted on the other two sides, and high-brightness LED light sources without flicker are placed at the corresponding position for lighting to ensure that the shooting picture is clear and uniform. One high-speed camera shoots in the horizontal direction, which is perpendicular to the trajectory of the ball into the water, and is used to observe and record the movement change process of the three-phase line. The shooting frame rate is 10000 frames. The other high-speed camera is slightly inclined, and the shooting angle is about 30° with the horizontal plane, which is used to shoot and record the dynamic behavior change of the surface liquid film when the ball hits the water surface. The shooting frame rate is 5000 frames.
[0048] Example 3
[0049] According to the curvature radius, the water entry speed and the solid liquid drop position of examples 1 and 2, it is found that the multiple parameters have a synergistic effect, and a "optimal drag reduction interval" is proposed: the curvature radius is 4.2-6.6 mm, and the speed is greater than 3.7 m / s. In this interval, the drag coefficient is reduced by more than 80%;
[0050] The curvature radius, water entry speed and solid liquid drop position in the example have a synergistic effect, which affects the critical speed of cavity formation, and the critical speed formula of cavity generation is obtained as follows:
[0051] V C =k x R 0.5 x r α ;
[0052] Where V C is the critical speed of cavity generation, R is the curvature radius of the steel ball, r is the curvature radius of the solid liquid drop, k is the material constant, and a is-0.7 to-0.5.
[0053] Comparative Example 1
[0054] The difference from Example 1 is that the configuration of the polydimethylsiloxane solution: the PDMS main agent is weighed with an analytical balance, the PDMS curing agent is added according to the ratio of 10:0.5 of the main agent to the curing agent, and the PDMS solution is mixed in a mixing defoaming instrument for 1 min and then defoamed for 1 min, and finally a uniformly mixed and bubble-free PDMS solution is obtained, and the others are consistent.
[0055] Comparative Example 2
[0056] The difference from Example 1 is that the configuration of the polydimethylsiloxane solution: the PDMS main agent is weighed with an analytical balance, the PDMS curing agent is added according to the ratio of 10:0.5 of the main agent to the curing agent, and the PDMS solution is mixed in a mixing defoaming instrument for 1 min and then defoamed for 1 min, and finally a uniformly mixed and bubble-free PDMS solution is obtained, and the others are consistent.
[0057] Comparative Example 3
[0058] The difference from Example 1 is that there is no equal weight of prepared PDMS solution, and the others are consistent.
[0059] Figure 2 For the formation process of cavitation under different curvature radii and drop heights, the curvature radii are 2.5 mm, 3.1 mm, 4.2 mm, 6.6 mm, and 10.8 mm, the release height determines the water entry velocity, and the solid droplet is located directly below the steel ball. The larger the additional solid droplet curvature radius, the easier it is to form a splash crown to form an air inlet channel, so it is easier to form a cavitation bubble; the higher the release height, the greater the impact velocity of the small ball on the water surface, the larger the splash crown formed, the more air introduced, the longer the cavitation bubble pinched off, and the larger the cavitation bubble volume. The closure mode of the cavitation bubble also changes from surface closure to shallow closure and finally to deep closure as the curvature radius increases.
[0060] Figure 3 For the dynamic behavior change of the surface liquid film and the three-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 drop height is 70 cm. When the small ball impacts the water surface, the solid droplet first contacts the liquid surface, the liquid surface is deformed by extrusion, forming 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, part of which is pinned to the surface of the small ball, and the other part is detached from the surface of the small ball and splashes outward, forming a splash crown, thereby obtaining an air capture channel. As the small ball moves downward, the splash crown expands outward and the height increases, at the same time capturing air to form a cavitation bubble. The three-phase line is pinned to the equatorial position of the small ball, and the cavitation bubble adheres to the surface of the small ball and continues to move downward.
[0061] Figure 4 The dark area is the no-cavitation formation area, and the light area is the cavitation formation area. Under different curvature radius conditions, the cavitation volume as a whole shows a trend of increasing with the increase of release height. With the increase of cavitation volume, the drag coefficient gradually decreases, and the drag reduction effect is better and better, and finally tends to be stable. The existence of surface cavitation can effectively reduce the resistance of the small ball during underwater movement, and the cavitation has good drag reduction effect.
[0062] Figure 5 To study the influence of the physicochemical properties of solid droplets on the process of cavitation formation and underwater drag reduction performance, we changed the proportion of curing agent added when the PDMS solid droplet was cured to change the physicochemical properties of the solid droplet. We selected three crosslinking ratios of 10:1, 10:0.75 and 10:0.5, and tested the PDMS samples with different crosslinking ratios by nanoindentation tester. When the crosslinking ratio is 10:1 and 10:0.75, the Young's modulus of the PDMS sample is 3.703 MPa and 3.476 MPa, and the hardness is 1.128 MPa and 1.134 MPa, respectively, that is, the mechanical properties of the PDMS sample are relatively stable, the Young's modulus value decreases slightly, and the hardness data does not change too obviously. When the crosslinking ratio is 10:0.5, the Young's modulus and hardness of the PDMS sample are 0.935 MPa and 0.135 MPa, respectively, compared with the previous two crosslinking ratios, the Young's modulus and hardness both decrease obviously, and the mechanical properties change obviously.
[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 height conditions is as follows Figure 6As shown, the cross-linking ratios are 10:1, 10:0.75 and 10:0.5 respectively, the release height determines the water entry speed, and the solid liquid drop is located directly below the steel ball. Under the condition of lower release height, the liquid film formed when the additional solid liquid drop ball enters the water is wrapped around the surface of the ball, so that only a small amount of air can enter between the liquid film and the ball, eventually forming a thin air film wrapped around the surface of the ball, which cannot form an air cavity. With the increase of release height, air cavities gradually begin to form. When the cross-linking ratio of the solid liquid drop is 10:1, air cavities begin to form from a height of 70 cm. When the cross-linking ratio of the solid liquid drop is 10:0.75, air cavities begin to form from a height of 50 cm. When the cross-linking ratio of the solid liquid drop is 10:0.5, air cavities begin to form at a lower height (30 cm). The air cavities below the water surface begin to show obvious closure pinch-off phenomenon. When the release height is higher, the length of the air cavities is significantly longer, the volume of the air cavities is larger, and the shape is increasingly close to streamline. In summary, when the curvature radius of the additional solid liquid drop is 2.5 mm, the greater the cross-linking ratio, the smaller the water entry speed required to form an air cavity, and the easier it is to form an air cavity. However, it can be seen that the surface of the formed air cavity is not smooth, and there are obvious waves and wrinkles. At the same time, the closure mode of the air cavity is different when the cross-linking ratio is different. When the cross-linking ratio is 10:1, the closure mode of the air cavity is surface closure. When the cross-linking ratio is 10:0.75, the closure mode of the air cavity is shallow closure. When the cross-linking ratio is 10:0.5, the closure mode of the air cavity 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 liquid drop is 2.5 mm and the release height is 70 cm, the splashing of the surface liquid film and the formation process of the air cavity during the water entry of the ball under different cross-linking ratios are as follows Figure 7When the cross-linking ratio of the solid liquid drop is 10:1, a splash crown is formed after the pellet hits the water surface, thereby introducing air. However, the splash crown cannot extend outward and starts to shrink (4 ms) in a very short time, and is basically closed at 12 ms, so the amount of air introduced is small, and the volume of the cavity formed is small. As the pellet continues to move downward, the cavity is pinched off near the water surface, forming a cavity, and the surface of the cavity has no obvious wrinkles, only some ripples (Figures (a) and (b)). When the cross-linking ratio of the solid liquid drop is 10:0.75, a larger and more complete splash crown is formed, the channel for capturing air is larger, and the splash crown starts to shrink later (7 ms), the liquid film closes later, so more air is introduced, and the cavity has enough time to be elongated in the vertical direction, and the volume of the cavity is larger. As the pellet continues to move downward, the cavity is eventually pinched off at a shallow position below the water surface. The surface of the cavity gradually becomes smooth from the presence of obvious angular protrusions, but eventually the three-phase line is broken due to the position of the solid liquid drop, and the surface of the cavity has obvious wrinkles (Figures (c) and (d)). When the cross-linking ratio of the solid liquid drop is 10:0.5, the splash crown expands outward more and is higher, the channel for capturing air is wider, and the splash crown starts to shrink only after 9 ms after the pellet enters the water, so more air is introduced than when the cross-linking ratio is 10:0.75, and as the pellet continues to move downward, the cavity is elongated, and the position at which the cavity is pinched off is deeper, so the volume of the cavity is larger. The surface of the cavity is sharp and gradually becomes smooth, but eventually the surface also has obvious wrinkles due to the influence of the solid liquid drop (Figures (e) and (f)). Therefore, when the radius of curvature is 2.5 mm, the larger the cross-linking ratio, the larger the splash crown formed, the more air introduced, the larger the volume of the cavity, and the deeper the position at which the cavity is pinched off, and the closing mode of the cavity changes from surface closure to shallow closure and finally to deep closure.
[0065] The results of Example 1 and Comparative Example 3 are shown in Table 1 according to the above experimental method.
[0066] Table 1
[0067] Example 1 Comparative Example 3 Vapor volume (cm3) 3 )]]> 12.2 1.8 Liquid film closure time (ms) 21.7 2.8 Drag coefficient (3.8 m / s) 0.04 0.56 Drag reduction efficiency (%) 92.9 0
[0068] The volume of the cavity is the sum of the volume of the pellet and the volume of the air cavity. As can be seen from Table 1, Comparative Example 3 cannot form a stable cavity when it enters the water, and the drag coefficient is always high. There is no three-phase line pinning effect after the liquid film is formed, and the liquid film quickly climbs and closes. At the same entry speed of 3.8 m / s, the drag coefficient of Comparative Example 3 is larger, and the drag coefficient of Example 1 is smaller, and the drag reduction efficiency is higher than 90%.
[0069] The above detailed description of the application is only as an example, and the application is not limited to the above described specific embodiments. Any equivalent modifications and substitutions made by those skilled in the art to the application are also within the scope of the application. Therefore, equivalent transformations and modifications made without departing from the spirit and scope of the application should be covered within the scope of the application.
Claims
1. A method for preparing and testing drag-reducing microspheres based on solid droplets, characterized in that, The preparation of drag-reducing microspheres based on solid droplets includes the following steps: (a) Cleaning treatment of the surface of the metal spheres; (b) Prepare a PDMS solution and fill it into a mold. After vacuum degassing and curing, peel it off to form a solid droplet. The radius of curvature of the solid droplet is 2.5-10.8 mm. (c) A solid droplet is bonded to the bottom of a metal sphere to obtain a drag-reducing sphere based on a solid droplet.
2. The method according to claim 1, characterized in that, The PDMS solution is made by mixing the main agent and the curing agent at a mass ratio of 10:1 and then undergoing defoaming treatment.
3. The method according to claim 1, characterized in that, The decontamination process includes ultrasonic cleaning with hexane and anhydrous ethanol in sequence, followed by drying with nitrogen.
4. The method according to claim 1, characterized in that, The curing conditions are heating in a 50°C oven for 3 hours.
5. The method according to claim 1, characterized in that, The mold is made of polytetrafluoroethylene, with uniformly distributed pores and a release agent sprayed on its surface.
6. The method according to claim 1, characterized in that, The test method for the drag-reducing sphere based on solid droplets includes: releasing the sphere into the water and controlling the immersion speed by adjusting the falling height; observing the cavitation formation process and underwater cavitation morphology using high-speed camera technology; and analyzing the influence of the solid droplet curvature radius on drag reduction performance based on cavitation volume and drag coefficient.
7. The method according to claim 6, characterized in that, The high-speed camera technology uses dual-camera simultaneous shooting with a frame rate of 5000-10000 frames per second. The shooting angles include the horizontal direction and the direction tilted at 30° to the horizontal plane. The cavitation volume is negatively correlated with the drag coefficient, and the drag coefficient tends to the minimum value when the cavitation volume reaches a stable threshold.
8. An experimental apparatus for cavitation drag reduction performance used in any one of claims 1-7, characterized in that, include: Transparent water tank and adjustable height electromagnet release system; dual high-speed camera linkage shooting system, respectively configured at the water entry position and underwater stable movement area; The LED light source is combined with a diffuser to provide uniform backlighting.
9. The apparatus according to claim 8, characterized in that, The water tank is made of PMMA material and measures 90cm×15cm×15cm, with a white diffuser at the bottom; the electromagnet release system adjusts the height via a slide rail to control the ball's entry speed into the water at 1-5m / s; the high-speed camera covers a depth range of 70-80cm underwater and is used to quantify cavitation morphology parameters.
Citation Information
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