Quantitative determination device and calculation method for laser-induced cavitation impact load in near-wall area

Abstract

The invention relates to the technical field of cavitation and cavitation erosion, and provides a quantitative determination device and a calculation method for laser-induced cavitation impact load in a near-wall area, the quantitative determination device comprises a loading mobile platform used for bearing a water tank, a cushion block is arranged in the water tank, and a tested material is arranged on the cushion block; the transmitting end of the laser cavitation system faces the upper part of the tested material, and the laser cavitation system is used for transmitting laser to the upper part of the tested material; the schlieren display device is arranged on one side of the object carrying moving platform and is used for recording and capturing the collapse process of cavitation bubbles of the air-containing holes near the rigid wall surface; and the laser baffle plate moving platform is movably arranged between the laser cavitation system and the loading moving platform. The cavitation bubbles are subjected to a series of expansion and contraction stages under the action of internal and external huge pressure difference, and ultimately collapse occurs along with the wall-approaching effect of the cavitation bubbles. Shock waves and microjets caused by collapse of the cavitation bubbles can cause damage to the tested material, and the cavitation erosion effect of the cavitation bubbles on the tested material can be measured by measuring the damage degree of the material.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of cavitation erosion, and in particular to a device and method for quantitatively measuring laser-induced cavitation impact load in a near-wall region. BACKGROUND

[0002] Material surface cavitation erosion damage has strong transient and time accumulation effects. The strong transient is that cavitation erosion caused by a single cavitation collapse usually occurs on a nanosecond / microsecond time scale; the time accumulation effect is that the cavitation collapse on the material is a continuous and repeated process with increasing damage. The existing several cavitation erosion experimental measurement methods, such as the Venturi tube experiment method, the magnetostrictive vibration cavitation erosion experiment method, and the rotating disc cavitation erosion experiment method, all produce a large range of cavitation groups. Due to the complexity and uncertainty of cavitation group collapse, the above methods are difficult to control experimental variables and cannot guarantee experimental repeatability, so it is difficult to explain the quantitative explanation of material cavitation damage causes and core influencing factors, and it is even more difficult to answer the basic problems such as the mechanism of cavitation erosion and abrasion on a micro level.

[0003] In order to study the cavitation erosion resistance of different materials and the mechanism of cavitation erosion and abrasion, it is necessary to develop a high-precision, high-repeatability, and quantitative observation of cavitation damage process experimental measurement system. With the development of laser-induced cavitation technology, using laser focusing to generate a single cavitation near the material surface has become an important method for cavitation erosion research. Laser-induced cavitation technology has the advantages of strong controllability, high repeatability, and large adjustable scale, which can accurately control and adjust the formation and evolution process of cavitation. At the same time, the controllability of experimental conditions makes the experimental results more repeatable, and accurate quantitative analysis and comparative study can be carried out, which provides good experimental conditions for quantitative study of the cavitation erosion resistance of different materials and the mechanism of cavitation erosion and abrasion. SUMMARY

[0004] The embodiment of the present application provides a kind of laser-induced cavitation impact load quantitative determination device in near-wall region, can realize the quantitative determination of cavitation to different material cavitation erosion effect.

[0005] The embodiment of the present application provides a kind of laser-induced cavitation impact load quantitative determination device in near-wall region, comprising:

[0006] The object moving platform is used to carry the water tank, and the water tank is provided with a pad, and the measured material is arranged on the pad;

[0007] The laser cavitation system is arranged on the object moving platform, and the emission end of the laser cavitation system is arranged towards the top of the measured material, for emitting laser towards the top of the measured material to form cavitation above the measured material;

[0008] A schlieren display device is arranged on one side of the object moving platform and used for recording and capturing the collapse process of the cavitation bubble containing the gas cavity near the rigid wall surface.

[0009] A laser baffle moving platform is movably arranged between the laser cavitation system and the object moving platform and used for shielding at least part of the laser beam to avoid the laser beam from forming burning on the measured material.

[0010] According to an embodiment of the present application, the schlieren display device comprises:

[0011] An illumination light source is arranged on one side of the object moving platform and used for illuminating the water tank.

[0012] An industrial camera is arranged on one side of the object moving platform and used for capturing the propagation of the cavitation bubble collapse shock wave.

[0013] A hydrophone is arranged in the water tank and used for measuring the collapse period of the cavitation bubble and the far-field pressure of the shock wave.

[0014] According to an embodiment of the present application, the laser cavitation system comprises a pulse laser and a beam expander, and the emission end of the pulse laser is directed to the top of the measured material through the beam expander.

[0015] According to an embodiment of the present application, the pulse laser has a single pulse energy of 10-200 mJ, a pulse width of 5 ns, and a working frequency of 1-10 Hz.

[0016] According to an embodiment of the present application, the straight line motor has a fisheye hole on the extending rod, and one end of the first connecting rod is rotatably connected in the fisheye hole.

[0017] According to an embodiment of the present application, the quantitative measuring device for the laser-induced cavitation bubble impact load near the wall region further comprises a synchronization system electrically connected with the pulse laser, the illumination light source, the hydrophone and the industrial camera, and used for controlling the trigger interval of the pulse laser, the illumination light source, the hydrophone and the industrial camera.

[0018] According to an embodiment of the present application, the quantitative measuring device for the laser-induced cavitation bubble impact load near the wall region further comprises a control system electrically connected with the synchronization system, the laser cavitation system and the schlieren display device.

[0019] The quantitative determination device of the laser-induced cavity impact load in the near-wall region according to one embodiment of the present application comprises a laser baffle moving platform, wherein the laser baffle moving platform comprises:

[0020] a baffle and a moving mechanism, wherein the baffle is arranged on the moving mechanism, and the moving mechanism is used for controlling the baffle to shield at least part of the laser beam.

[0021] The present application further provides a calculation method of the cavity energy distribution by using the quantitative determination device of the laser-induced cavity impact load in the near-wall region, comprising the following steps:

[0022] integrating all pressure profiles at a fixed distance at all times to obtain a calculation formula of the shock wave energy, wherein the calculation formula of the shock wave energy corresponds to the shock wave energy at a certain distance from the center of the spherical shock wave;

[0023] based on the measured shock wave pressure signal, the calculation formula of the shock wave energy and the calculation formula of the entering cavity energy, fitting and determining the cavity energy distribution in the near-wall region.

[0024] According to the calculation method of the cavity energy distribution according to one embodiment of the present application, the calculation formula of the shock wave energy E s is expressed as:

[0025]

[0026] wherein ρ is the density of the solution, G is the gain, the complete voltage signal of the multiple collapse shock wave is U(t), a and b are regarded as positive free parameters, r is the distance from the center of the shock wave, c is the sound speed in water, and p(t) is the pressure profile of the shock wave front surface at the position r.

[0027] According to the calculation method of the cavity energy distribution according to one embodiment of the present application, the calculation formula of the entering cavity energy E b is expressed as:

[0028]

[0029] wherein p0 is the static water pressure, p v is the saturated vapor pressure, and R is the radius of the cavity.

[0030] The quantitative determination device of the laser-induced cavity impact load in the near-wall region and the calculation method provided by the present application, since the cavity experiences a series of expansion and contraction stages under the action of a huge pressure difference between the inside and the outside, accompanied by the wall-seeking effect of the cavity, and finally collapses. The shock wave and microjet caused by the collapse of the cavity can cause damage to the measured material, and the damage degree of the material can be measured to determine the cavitation erosion effect of the cavity on the measured material. BRIEF DESCRIPTION OF DRAWINGS

[0031] In order to make the technical solutions of the embodiments of the present application or the prior art clearer, the accompanying drawings needed in the embodiments or prior art description will be briefly introduced. Obviously, the accompanying drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort based on these drawings.

[0032] Figure 1 is a schematic diagram of the overall structure of the quantitative determination device of the laser-induced cavitation bubble impact load near the wall provided by an embodiment of the present application;

[0033] Figure 2 is a schematic diagram of the partial structure of the quantitative determination device of the laser-induced cavitation bubble impact load near the wall provided by an embodiment of the present application;

[0034] Figure 3 is a schematic diagram of the object moving platform provided by an embodiment of the present application;

[0035] Figure 4 is a flowchart of the calculation method of the cavitation bubble energy distribution of the quantitative determination device of the laser-induced cavitation bubble impact load near the wall provided by an embodiment of the present application;

[0036] Figure 5 is a detection image of the first collapse period of the hydrophone signal recording of the free-field spherical cavitation bubble provided by an embodiment of the present application;

[0037] Figure 6 is a detection image of the light breakdown shock wave of the hydrophone signal recording of the free-field spherical cavitation bubble provided by an embodiment of the present application;

[0038] Figure 7 is a detection image of the collapse shock wave of the hydrophone signal recording of the free-field spherical cavitation bubble provided by an embodiment of the present application;

[0039] Figure 8 is a cavitation bubble collapse process photograph of the process of the cavitation bubble collapse when γ≈2.1 and α≈0.7 provided by an embodiment of the present application;

[0040] Figure 9 is a detailed process of the multiple shock waves generated in the cavitation bubble collapse process near the rigid wall containing air holes photographed by an embodiment of the present application;

[0041] Figure 10 is a detection image of the first collapse period of the far-field pressure recording of the shock wave when γ≈2.1 and α≈0.7 provided by an embodiment of the present application;

[0042] Figure 11is the detection image of the shock wave of the far-field pressure record of the laser-induced shock wave when γ≈2.1 and α≈0.7 provided by an embodiment of the present application;

[0043] Figure 12 is the detection image of the shock wave of the far-field pressure record of the laser-induced shock wave when γ≈2.1 and α≈0.7 provided by an embodiment of the present application;

[0044] Figure 13 is the image of the damage caused by the action of the bubble collapse and the laser ablation on the sample at the same time when the laser baffle is not used provided by an embodiment of the present application.

[0045] Reference signs:

[0046] 1, object moving platform; 10, water tank; 11, measured material; 2, laser cavitation system; 21, pulsed laser; 22, beam expander; 3, schlieren display device; 31, illumination light source; 32, industrial camera; 33, hydrophone; 4, laser baffle moving platform; 5, synchronization system; 6, control system. DETAILED DESCRIPTION

[0047] The embodiments of the present application will be further described below in conjunction with the drawings and examples. The following examples are used to illustrate the present application, but cannot be used to limit the scope of the present application.

[0048] In the description of the embodiments of the present application, the terms "first", "second" are only used for the purpose of description, and cannot be understood as indicating or implying relative importance.

[0049] In the description of the embodiments of the present application, it should be noted that, unless otherwise explicitly specified and limited, the terms "connected", "connected" should be understood in a broad sense, for example, it can be fixedly connected, or it can be detachably connected, or integrally connected; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium. For those skilled in the art, the specific meaning of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

[0050] In the description of the present application, the description of the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In the present application, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine the different embodiments or examples described in the present application and the features of the different embodiments or examples without contradiction.

[0051] The present application provides a kind of near-wall zone laser-induced cavitation shock load quantitative determination device, as shown in the figure, the near-wall zone laser-induced cavitation shock load quantitative determination device includes: object moving platform 1, laser cavitation system 2, schlieren display device 3 and laser baffle moving platform 4. Figures 1 to 3

[0052] In this embodiment, object moving platform 1 is used to carry water tank 10, which is provided with a cushion block, and the measured material 11 is arranged on the cushion block. The edge ring is above the measured material 11, and the measured material 11 and the cushion block are pressed tightly by the pressing force of the edge ring to ensure that the material remains stable during the test. The emission end of the laser cavitation system 2 is directed upward of the measured material, for emitting laser to the upper side of the measured material. The power and pulse frequency of the laser cavitation system 2 can be adjusted to meet the testing needs of different materials. The laser pulses emitted by the laser cavitation system 2 form cavities above the measured material 11, and the formation and collapse process of the cavities are the main object of study. The schlieren display device 3 is arranged on one side of the object moving platform 1, for recording and capturing the collapse process of the cavities near the rigid wall containing gas holes. The laser baffle moving platform 4 is movably arranged between the laser cavitation system 2 and the object moving platform 1, for shielding at least part of the laser beam to avoid the laser beam from forming burning on the measured material, and generating ablation defects that negatively affect the measurement results.

[0053] During the test, the measured material 11 is placed on the cushion block and pressed tightly with the edge ring. Adjust the emission end of the laser cavitation system 2 to ensure that the laser pulses can accurately hit the upper side of the measured material 11. Start the laser cavitation system 2 to form cavities above the measured material 11 by emitting laser pulses. At the same time, start the schlieren display device 3 to record the collapse process of the cavities. During the test, if it is necessary to shield the laser beam, the baffle can be quickly moved to the appropriate position by the laser baffle moving platform 4. After the test is completed, the laser cavitation system 2 and the schlieren display device 3 are turned off, and the measured material 11 is taken out for analysis.

[0054] The calculation formula for the shock wave energy is obtained by integrating all pressure profiles at a fixed distance at all times, which corresponds to the shock wave energy at a certain distance from the center of the spherical shock wave; based on the shock wave pressure signals measured by the schlieren display device 3, the calculation formula for the shock wave energy and the calculation formula for the energy entering the cavity, the energy distribution of the near-wall zone cavity is determined by fitting, and the effect of cavity erosion on different materials can also be quantitatively measured by measuring the damage degree of the measured material 11.

[0055] In some embodiments, as shown in Figures 1 to 3 ​As shown, the schlieren display device 3 includes an illumination light source 31, an industrial camera 32, and a hydrophone 33. The illumination light source 31 is disposed on one side of the object moving platform 1 for illuminating the water tank 10; the industrial camera 32 is disposed on one side of the object moving platform 1 for capturing the propagation of the cavitation bubble collapse shock wave; and the hydrophone 33 is disposed in the water tank 10 for measuring the collapse period of the cavitation bubble and the far-field pressure of the shock wave.

[0056] In this embodiment, the illumination light source 31 is a Q-switched Nd:YAG laser with a wavelength of 532 nm and a pulse width of 5 ns, which is located on one side of the object moving platform 1 to provide high-quality illumination for the entire experiment. This laser can provide high-energy, short-pulse-width laser pulses to ensure sufficient illumination during the formation and collapse of the cavitation bubble.

[0057] The industrial camera 32 uses a high-resolution CCD industrial camera and a macro lens to capture the propagation of the shock wave during the collapse of the cavitation bubble. The industrial camera 32 can record high-frame-rate, high-resolution images, thereby capturing subtle changes in the propagation of the shock wave. In order to eliminate the interference of other wavelengths of light, a narrow-band filter with a center wavelength of 532 nm is placed in front of the CCD camera lens.

[0058] The hydrophone 33 is disposed in the water tank 10, and its design features include a hemispherical head, a small sensitive diameter, an extremely short rise time, and a large measurement range. These features enable the hydrophone 33 to accurately measure the collapse period of the cavitation bubble and the far-field pressure of the shock wave. The hydrophone 33 is connected to an oscilloscope with a sampling frequency of 2 GHz to ensure real-time data acquisition and accurate recording. The hydrophone 33 is placed 5 mm above the focal point center, which is approximately 4.5 times the maximum cavitation bubble radius Rmax. At this position, the hydrophone 33 can capture the main changes of the shock wave during the collapse of the cavitation bubble. According to the experimental conditions, the time for the shock wave to propagate to the hydrophone is about 3 μs. This setting ensures that the hydrophone 33 can measure the pressure changes of the shock wave within the appropriate time range.

[0059] In some embodiments, as Figures 1 to 3 As shown, the laser cavitation system 2 includes a pulsed laser 21 and a beam expander 22, and the emission end of the pulsed laser 21 is directed towards the top of the material being measured through the beam expander 22. The parameters of the pulsed laser of the pulsed laser 21 are as follows: single-pulse energy of 10-200 mJ, pulse width of 5 ns, operating frequency of 1-10 Hz, and wavelength of 1064 nm.

[0060] In this embodiment, the pulsed laser 21, as the core component of the ignition laser, is a Q-switched Nd:YAG laser with a wavelength of 1064 nm and a pulse width in the nanosecond range. This type of laser can generate high-energy, short-pulse laser pulses, making it very suitable for laser-induced cavitation generation. The pulsed laser parameters of the pulsed laser 21 are: single-pulse energy adjustable in the range of 10–200 mJ, pulse width fixed at 5 ns, and operating frequency adjustable between 1–10 Hz. These parameters can be adjusted according to different experimental requirements to obtain the best cavitation generation effect.

[0061] The beam expander 22 is used to change the diameter and divergence angle of the laser beam. In this embodiment, the diameter of the ignited laser beam is approximately on the order of millimeters. After passing through the beam expander 22, which amplifies the beam by 6 times, the diameter of the laser beam is significantly increased.

[0062] To further focus the laser beam, achromatic cemented doublet lenses and meniscus lenses were also used. These lens combinations effectively focus the laser beam above the test material 11, ensuring that the laser energy is concentrated in the area where cavitation bubbles need to form. Through adjustments to the beam expander 22 and the lens combination, the laser beam is precisely focused above the test material 11. This focusing method not only ensures concentrated laser energy but also helps control the size and position of the cavitation bubbles. Furthermore, the use of a 1064nm laser, with its longer wavelength, allows for stronger penetration in liquids, which is beneficial for forming larger and more stable cavitation bubbles.

[0063] In some embodiments, such as Figures 1 to 3 As shown, the quantitative measurement device for laser-induced cavitation impact load in the near-wall region also includes: a synchronization system 5, which is electrically connected to the pulsed laser 21, the illumination source 31, the hydrophone 33 and the industrial camera 32, and is used to control the trigger interval when the pulsed laser 21, the illumination source 31, the hydrophone 33 and the industrial camera 32 are working.

[0064] Specifically, the synchronization system 5 is mainly composed of an 8-channel or more digital delay pulse generator. This generator has a time resolution of picoseconds, which can accurately control the trigger interval between each device. The synchronization system 5 can accurately control the trigger time of the pulsed laser 21, ensuring that the laser pulse forms a cavity at the exact moment above the measured material 11. After the laser pulse forms a cavity, the synchronization system 5 triggers the illumination light source 31 to provide sufficient illumination for the industrial camera 32 to capture the detailed process of the cavity collapse. The synchronization system 5 also synchronizes the starting time of the hydrophone 33, ensuring that the hydrophone 33 can accurately measure the pressure change of the shock wave when the cavity collapses. One of the most critical functions of the synchronization system 5 is to control the trigger time of the industrial camera 32. By accurately controlling the trigger time of the camera, we can ensure that the camera can capture key frames during the cavity collapse process, thereby obtaining detailed information about the propagation of the shock wave. Due to the use of a digital delay pulse generator with a time resolution of picoseconds, the synchronization system 5 can ensure that the trigger interval between each device reaches a very high accuracy. This accuracy is crucial for capturing the detailed process of the cavity collapse, measuring the pressure change of the shock wave, and analyzing the distribution of the shock load.

[0065] In some embodiments, as shown in FIG. 1, the near-wall region laser-induced cavity shock load quantitative measurement device further comprises a control system 6. The control system 6 is electrically connected with the synchronization system 5, the laser cavitation system 2, and the schlieren display device 3. Figures 1 to 3

[0066] Specifically, the control system 6 is mainly composed of a computer and its related software. It communicates with the synchronization system 5, the laser cavitation system 2, and the schlieren display device 3 through electrical signals, realizing centralized control of these systems. The control system 6 controls the trigger time of the pulsed laser 21, the illumination light source 31, the hydrophone 33, and the industrial camera 32 by sending instructions to the synchronization system 5. It can accurately set the trigger interval between each device according to experimental needs, ensuring that they work cooperatively at the same time point. The control system 6 can control the working parameters of the pulsed laser 21 in the laser cavitation system 2, such as single-pulse energy, pulse width, and working frequency. By adjusting these parameters, accurate control of the cavity generation process can be achieved. At the same time, the control system 6 can receive image data captured by the industrial camera 32 in the schlieren display device 3 and process and analyze it. It can display the propagation process of the shock wave in real time and extract key information such as the propagation speed of the shock wave and the pressure distribution. Using a computer as the core of the control system 6 makes the entire experimental process more intelligent and automated. By writing corresponding software programs, accurate control of the experimental process and data acquisition can be achieved. In addition, the computer can also process and analyze experimental data in real time, extracting useful information to facilitate the interpretation of experimental results. ​

[0067] In some embodiments, as shown in Figures 1 to 3 The laser baffle moving platform 4 includes a baffle and a moving mechanism, the baffle is arranged on the moving mechanism, and the moving mechanism is used for controlling the baffle to shield at least part of the laser beam.

[0068] For example, in the embodiment, the baffle is made of stainless steel material, which not only has sufficient strength and stability, but also can maintain good corrosion resistance in long-term work. The thickness of the baffle is designed to be 3mm-5mm, which ensures that the baffle can stably shield the laser beam, and at the same time, will not introduce too much weight and inertia due to too large thickness. The baffle is placed before the achromatic doublet lens, and is used to shield the upper half of the laser beam. By shielding part of the laser beam, the energy distribution of the laser on the target area can be adjusted, and then the precise control of the cavitation bubble generation and collapse process can be realized. The moving mechanism is responsible for controlling the movement of the baffle. By precisely controlling the moving speed and position of the baffle, the precise adjustment of the laser beam shielding area can be realized. This flexibility enables the device to adapt to different experimental requirements and optimize the experimental parameters to obtain more accurate results.

[0069] The embodiment of the present application also provides a method for calculating the cavitation energy distribution of a device for quantitatively measuring the laser-induced cavitation bubble impact load in the near-wall region, as shown in Figure 4 The method comprises the following steps:

[0070] Step S410: integrating all pressure profiles at a fixed distance at all times to obtain a calculation formula of the shock wave energy, which represents the shock wave energy at a distance from the center of the spherical shock wave.

[0071] Step S420: based on the measured shock wave pressure signal, the calculation formula of the shock wave energy and the calculation formula of the entering cavitation energy, the cavitation energy distribution in the near-wall region is determined by fitting.

[0072] Specifically, the method for calculating the cavitation energy distribution in the near-wall region based on the hydrophone integrates all pressure profiles at a fixed distance r at all times to obtain the shock wave energy, and represents the shock wave energy at a distance from the center of the spherical shock wave as:

[0073]

[0074] Here, r is the distance from the center of the shock wave, c is the sound speed in water, and p(t) is the pressure profile of the shock wave front at position r. When p(t) represents the measured shock wave pressure signal at the hydrophone, the far-field energy of the shock wave can be calculated. Assuming that the complete voltage signal of the multiple collapse shock waves stored in the oscilloscope is U(t) with a gain G, then U(t)=Gp(t), by which formula (1) can be adjusted as:

[0075]

[0076] The exponential b > 0 is used to compensate for the nonlinear dissipation of the shock energy during propagation, whose relative effect increases with pressure. Equation (2) can be rewritten as:

[0077]

[0078] Umaxis the maximum value of U(t). Here a and b are treated as positive free parameters. These parameters are then fitted to satisfy two conditions simultaneously: (1) the ratio of the shock energy E s1 (initial stage, due to the high-energy laser focused in a portion of the water, the water is broken down to produce a shock wave, the energy is calculated by equation (3)) to the energy E b (post optical breakdown, the potential energy of the cavity when the cavity expands to the maximum radius, calculated by equation (4)) is about 1.64:1; (2) the ratio of the total energy E s2 (collapse stage, due to the shock wave produced by the violent compression of the water around the cavity when the cavity collapses, the energy is calculated by equation (3)) to the energy E b of the cavity E b is calculated using the following formula:

[0079]

[0080] Here, p0is the static water pressure, and pvis the saturated vapor pressure. At the same time, the value of b is fitted using the hydrophone signal of the spherical cavitation bubble in the free field. The energy Ebinto the cavity is calculated using equation (4), and the voltage signals U(t) of the optical breakdown shock wave and the collapse shock wave are integrated. Under the above conditions, the values of a and b are fitted, and then the energy distribution of the near-wall region cavity is obtained.

[0081] In one specific embodiment, as Figures 1 to 3A Q-switched Nd:YAG laser (Grace laser, TINY-200L) with a wavelength of 1064 nm and a pulse duration of 5 ns was used as the ignition laser. The diameter of the ignition laser beam was about 5 mm, and after passing through a 6x beam expander (DHC, GCO-2502), it was focused in the water tank using a doublet achromatic lens (f = 40 mm) and a meniscus lens (f = 100 mm). The high-intensity transient laser pulse focused in the medium leads to a local energy exceeding the critical threshold of the medium, causing the medium to ionize, locally heat, and evaporate, generating a cavitation bubble. The energy of the ignition laser ranged from 10 to 200 mJ, and the maximum radius of the generated cavitation bubble was about 0.5-3 mm. A laser shutter was placed between the beam expander and the doublet achromatic lens to block part of the laser, preventing the diverging laser from ablation on the measured material. The water tank was 5 x 5 x 10 cm in size and filled with ultrapure water. The distance between the tank wall and the laser focus position was far enough to ensure that it had no effect on the cavitation bubble. The sample block with gas-filled pores was placed directly below the focal point. A high-resolution (5120 x 5120 pixels) CCD industrial camera (DHC, MARS-1230-23U3M) and a macro lens were used to capture the propagation of the collapse shock wave. After calibration, the image resolution was determined to be 1.152 μm / pixel. To capture the transient images of the collapse shock wave, a Q-switched Nd:YAG laser (New Wave, Research Solo 200XT) with a wavelength of 532 nm and a pulse duration of 5 ns was used as the illumination source. At the same time, the fine structure of the collapse shock wave was observed using the transmission schlieren method based on the principle of light deflection caused by changes in fluid density. A narrow-band filter with a center wavelength of 532 nm was placed in front of the CCD camera lens to prevent damage to the camera from the light generated by the plasma. When the ambient light was blocked, the light-sensitive elements of the industrial camera could only receive light from the illumination laser. Therefore, the single exposure time of the image could be shortened to equal the pulse duration of the illumination laser. In this way, nanosecond transient single-frame images of the collapse shock wave could be captured. At the same time, a hydrophone (Müller-Platte Needle Probe, 100-100-1) was used to accurately measure the collapse period of the cavitation bubble and the far-field pressure of the shock wave. The head of the hydrophone was spherical with a small sensitive diameter (less than 0.5 mm) and a very short rise time (about 50 ns), and the measurement range was large (-10-150 MPa). It was placed 5 mm above the focal point center (about 4.5 times the maximum cavitation bubble radius Rmax), and the shock wave took about 3 μs to propagate to the hydrophone. The small volume of the hydrophone made us believe that its influence on the shock wave emission could be ignored. The hydrophone was connected to an oscilloscope (RIGOL, MSO2302A) with a sampling frequency of 2 GHz to observe and store the far-field pressure signals of the shock wave.An 8-channel digital delay pulse generator (DDPG, Quantum Composer 9520) was used to synchronize the firing laser, the illumination laser and the CCD camera. The time resolution of the DDPG was 250 ps, which allowed us to control the trigger interval between the devices precisely.

[0082] Figure 3 The size information of the sample block with gas hole and the relative position of the block and the cavity were depicted. The sample block with gas hole was made of transparent organic glass, which was used to observe the flow in the small hole. The size of the sample block was 30 x 30 x 30 mm, the wall thickness of the gas hole was 2 mm, and the other wall thickness was 3 mm. The sample block was filled with air to ensure that there was a gas-liquid interface directly below the cavity. We defined a dimensionless distance parameter γ = H / Rmax, which represents the distance from the cavitation bubble to the surface of the block with gas hole, and a dimensionless size parameter α = Rh / Rmax, which represents the hole radius relative to the maximum radius of the cavitation bubble. Where H represents the vertical distance from the center of the cavitation bubble to the surface of the block with gas hole, Rh represents the radius of the cavitation bubble, and Rmax represents the maximum radius of the cavitation bubble.

[0083] In the experiment, the firing laser energy was 50 mJ, which produced a cavitation bubble with a maximum radius of about 1.06 mm. The cavity radius Rh was 0.75 mm. The vertical distance from the center of the cavitation bubble to the rigid wall surface with gas hole H was 2.2 mm. Here, we analyzed the repeatability of the experiment. The key indicator for evaluating the repeatability of the experiment was the change in the maximum radius of the cavitation bubble. We used the hydrophone to continuously detect the collapse period of the cavitation bubble, and used formula (5) to calculate the corresponding maximum radius.

[0084]

[0085] Here, Tc represents half of the first collapse period of the cavitation bubble. We randomly recorded 10 groups of signal values of the cavitation bubble oscillation process using the hydrophone. The calculated maximum radius was in the range of 1066 ± 30 μm, indicating that the experiment had good repeatability.

[0086] The shock wave energy in the cavitation bubble collapse process of the rigid wall surface with gas hole was indirectly calculated using the energy distribution theory in the spherical cavitation bubble and the shock wave pressure signal measured by the hydrophone.

[0087] Figures 5 to 7 The hydrophone signal of the spherical cavitation bubble oscillation process in the free field was given. After the optical breakdown, the time of the shock wave propagating to the hydrophone was about 3.1 μs. The collapse time of the cavitation bubble was 281.9 μs, and the energy Eb entering the cavitation bubble was calculated to be 1448.4 μJ. The optical breakdown shock wave of Figure 6 and the shock wave energy of the cavitation bubble collapse process of the rigid wall surface with gas hole were calculated to be 0.5 μJ and 0.2 μJ, respectively. Figure 7The voltage signal U(t) of the collapse shock wave was integrated. Under the above conditions, the values of a and b were fitted. The results show that b is approximately 0.43, while a is a variable, and the goal is to reach a ratio of 1.64:1 between the shock wave energy and the cavity energy under different boundary conditions. This shows that the recorded pressure is affected by the nonlinear dissipation of the shock wave.

[0088] The collapse process of a cavitation bubble near a rigid wall with a gas-filled cavity was captured in detail using a transient schlieren imaging system, and the evolution of the bubble shape was analyzed and discussed. The CCD camera has high image resolution but low frame rate. To overcome this limitation, an 8-channel digital delay pulse generator was used to synchronize and control the ignition laser, the illumination laser, and the CCD camera, enabling accurate capture of the transient process of the cavitation bubble collapse at a given time. Figure 8 The collapse process of a cavitation bubble with γ≈2.1 and α≈0.7 is shown. Due to the wall effect, the bubble no longer remains spherical. Before collapse, the side of the bubble far from the porous surface begins to indent towards the wall, forming a microjet. Subsequently, the bubble collapses at different times and locations, producing different forms of shock waves. During the collapse process, the cavitation bubble is compressed into a small ring-like structure. After the bubble collapses, the bubble begins to rebound and generates a reflected jet pointing away from the wall. The bubble then continues to expand towards the wall, and the reflected jet gradually disappears. The bubble takes on a heart shape, and another jet pointing towards the wall is generated.

[0089] Figure 8 The detailed process of multiple shock waves generated during the collapse of a bubble near a rigid wall with a gas-filled cavity is shown. At time t1, the upper surface of the bubble is completely concave, resembling a closed ring with a lower surface. At time t2, a point on the bubble ring collapses, generating multiple ring-surface collapse shock waves (labeled 1). At time t3, the jet breaks through the lower surface of the bubble, generating a strong jet shock wave (labeled 2). At time t4, the bubble ring continues to collapse, generating two more strong ring-surface collapse shock waves. It can be observed that the collapse process of the bubble is asymmetric, starting from a point on the bubble ring and gradually propagating along the bubble ring, generating multiple ring-surface collapse shock waves. At time t5, the wave fronts of the three strongest shock waves gradually merge upwards and continue to propagate outward.

[0090] Figure 9 The far-field pressure of the shock wave generated during the oscillation of a cavitation bubble near a rigid wall with a gas-filled cavity detected by a hydrophone is shown. The pressure p(t) is calculated as p(t) = U(t) / G based on the calibrated energy of the breakdown shock wave, where the gain G is defined as:

[0091]

[0092] The results show that at γ ≈ 2.1 and α ≈ 0.7, the shock front is detected by the hydrophone at 3.1 μs after the optical breakdown in the far field (r = 5 mm); the pressure is about 9.06 MPa, which is similar to the far field pressure of the free-field breakdown shock wave. This shows that the complex boundary conditions do not affect the energy distribution in the breakdown process. Subsequently, the shock wave fronts reflected from different boundaries are detected at 6.2 μs and 37.0 μs, respectively. Figure 12 The pressure of the collapse shock wave is shown. It can be observed that near the bubble collapse time, the hydrophone detects two shock wave signals, which are consistent with the various types of collapse shock waves captured in Figure 9 Figure 12 The shock wave marked 1 in is the toroidal collapse shock wave, and the far field pressure is about 2.0 MPa. The shock wave marked 2 is the jet shock wave with a far field pressure of about 5.8 MPa. The time interval between shock wave 1 and shock wave 2 is 140 ns. According to the speed of sound propagation in water, the distance between 1 and 2 is about 210 μm, which is the same as the distance between the two bubbles in Figure 9

[0093] In the experiment, 5 groups of hydrophone signals were collected at γ ≈ 2.1 and α ≈ 0.7, and combined with equations (3) and (4), we first calculated the partial energy of the cavitation bubble collapse near the rigid wall with gas-containing holes. The calculation results are shown in Table 1.

[0094] Table 1. Collapse shock wave energy at γ ≈ 2.1 and α ≈ 0.7

[0095]

[0096]

[0097] The results show that when γ ≈ 2.1 and α ≈ 0.7, the ratio of the shock wave energy Es generated in the bubble collapse process to the energy Eb entering the bubble after optical breakdown is about 0.7-0.8. This shows that when the bubble collapses near the rigid wall with gas-containing holes, the energy dissipation ratio through shock wave emission is less than that in the free field. This is mainly due to the energy dissipation in the form of micro-jet. Since the generation mechanism and shape of the micro-jet are highly complex, the energy of the jet is not calculated in this study. In summary, this method is based on certain assumptions and is a preliminary attempt to solve the energy distribution problem in the bubble collapse process under complex boundary conditions.

[0098] ​​This paper presents the detailed laser-induced single bubble collapse on a rigid wall with a gas-filled cavity using a nanosecond high-resolution transient imaging system. The micro-jet production and the shock wave emission during the collapse are analyzed. In addition, the energy distribution during the collapse of a single bubble near a rigid wall with a gas-filled cavity is calculated for the first time. The main conclusions are as follows: (1) When γ≈2.1 and α≈0.7, the bubble collapses at different times and locations, producing various forms of shock waves. The collapse of the bubble starts at a certain point on the torus and gradually advances along the torus, producing multiple torus collapse shock waves. Subsequently, the micro-jet penetrates the lower surface of the bubble, leading to the production of jet shock waves. (2) Considering the hydrophone gain and the nonlinear loss of shock wave energy during propagation, the energy distribution during the collapse of a single bubble near a rigid wall with a gas-filled cavity is calculated. The ratio of the shock wave energy Es produced during the collapse of the bubble to the energy Eb entering the bubble after optical breakdown is approximately between 0.7 and 0.8.

[0099] Figure 13 The surface image of the sample block is given when the residual laser still produces ablation on the right side of the sample block in addition to the damage to the sample block caused by bubble collapse without laser baffles.

[0100] In summary, the beneficial effects of the present application are: (1) The present application uses a high-energy laser beam to break down the liquid medium and produce a cavitation bubble in the liquid medium. Due to the large pressure difference between the inside and outside of the bubble, the bubble undergoes a series of expansion and contraction stages, accompanied by the bubble's wall-seeking effect, and eventually collapses. The shock waves and micro-jets caused by the collapse of the bubble will cause damage to the measured material. By measuring the degree of material damage, the cavitation effect of the bubble on the measured material can be determined. The energy distribution of the near-wall bubble collapse can be calculated by measuring the signal with a hydrophone. (2) The laser baffle is placed behind the laser focal point close to the measured material, blocking the divergent laser from irradiating the measured material to produce ablation, making the measurement results more accurate. (3) Optical beam expansion changes the laser energy density by expanding the beam radius and changing the laser focusing angle in water, avoiding multiple breakdowns in water, making it easier to produce a single bubble, and facilitating subsequent experimental research.

[0101] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the present application, and not to limit them. Although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing examples, or make equivalent substitutions for some of the technical features. These modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

[0102] The above embodiments are only used for illustrating the present application, but not limiting the present application. Although the present application is explained in detail with reference to the embodiments, those skilled in the art should understand that various combinations, modifications or equivalent replacements of the technical solutions of the present application do not deviate from the spirit and scope of the present application, and should be covered in the scope of claims of the present application.

Claims

1. A quantitative measurement device for laser-induced cavitation impact load in the near-wall region, characterized in that, include: A material-carrying mobile platform is used to carry a water tank, the water tank is equipped with a pad, and the material to be tested is placed on the pad; A laser cavitation system, wherein the emitting end of the laser cavitation system is directed upward toward the material under test, for emitting a laser beam upward toward the material under test to form cavitation bubbles above the material under test; A schlieren display device is installed on one side of the moving platform for recording the collapse process of cavitation bubbles containing air cavities near a rigid wall surface. A laser baffle moving platform is movably disposed between the laser cavitation system and the material moving platform to block at least part of the laser beam to prevent the laser beam from burning the material under test.

2. The quantitative measurement device for laser-induced cavitation impact load in the near-wall region according to claim 1, characterized in that, The schlieren display device includes: A lighting source is provided on one side of the cargo moving platform to illuminate the water tank; An industrial camera, mounted on one side of the moving platform, is used to capture the propagation of the cavitation collapse shock wave. A hydrophone, installed in the water tank, is used to measure the collapse period of cavitation bubbles and the far-field pressure of shock waves.

3. The quantitative measurement device for laser-induced cavitation impact load in the near-wall region according to claim 2, characterized in that, The laser cavitation system includes a pulsed laser and a beam expander, wherein the emitting end of the pulsed laser is directed upward toward the material under test via the beam expander.

4. The quantitative measurement device for laser-induced cavitation impact load in the near-wall region according to claim 3, characterized in that, The pulsed laser parameters of the pulsed laser are: single pulse energy of 10-200 mJ, pulse width of 5 ns, and operating frequency of 1-10 Hz.

5. The quantitative measurement device for laser-induced cavitation impact load in the near-wall region according to claim 3, characterized in that, Also includes: A synchronization system, electrically connected to the pulsed laser, the illumination source, the hydrophone, and the industrial camera, is used to control the trigger interval when the pulsed laser, the illumination source, the hydrophone, and the industrial camera are operating.

6. The quantitative measurement device for laser-induced cavitation impact load in the near-wall region according to claim 5, characterized in that, Also includes: The control system is electrically connected to the synchronization system, the laser cavitation system, and the schlieren display device.

7. The quantitative measuring device for laser-induced cavitation impact load in the near-wall region according to any one of claims 1-6, characterized in that, The laser baffle moving platform includes: A baffle and a moving mechanism, wherein the baffle is disposed on the moving mechanism and the moving mechanism is used to control the baffle to block at least a portion of the laser beam.

8. A method for calculating the cavitation energy distribution using the quantitative determination device for near-wall laser-induced cavitation impact load according to any one of claims 1-7, characterized in that, include: The formula for calculating the shock wave energy is obtained by integrating all pressure profiles at a fixed distance at all times, which corresponds to the shock wave energy at a certain distance from the center of the spherical shock wave. Based on the measured shock wave pressure signal, the calculation formula for the shock wave energy, and the calculation formula for the cavitation energy, the cavitation energy distribution in the near-wall region is determined by fitting.

9. The method for calculating cavitation energy distribution according to claim 8, characterized in that, The shock wave energy E s The calculation formula is expressed as: Where ρ is the solution density, G is the gain, the complete voltage signal of the multiple collapse shock wave is U(t), a and b are considered as positive free parameters, r is the distance from the center of the shock wave, c is the speed of sound in water, and p(t) is the pressure profile of the shock wave front at position r.

10. The method for calculating cavitation energy distribution according to claim 8, characterized in that, The energy E entering the cavitation bubble b The calculation formula is expressed as: Where p0 is the hydrostatic pressure, p v R is the saturated vapor pressure, and R is the radius of the cavitation bubble.

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