CN109883605B - Method for measuring impact wave pressure intensity in liquid medium and detection device - Google Patents
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Abstract
The invention provides a method for measuring impact wave pressure in a liquid medium, which comprises the following steps: generating cavitation shock waves by the liquid medium through laser induction; adjusting the distance d between the laser-induced cavitation and the free liquid level by means of a mobile platformnWherein n is the number of measurements and n isAn integer number; the maximum liquid level bulge height h is measured by shooting a liquid level bulge imagenThe included angle between the connecting line from the edge of the spherical crown to the center of the sphere and the normal line of the liquid level
And the included angle theta from the edge of the spherical crown to the top point of the spherical crown and the liquid levelnCalculating transient additional pressure P of the liquid level of the bulgesn(ii) a Calculating the average value C; and calculating the impact wave pressure according to the l value. The invention converts the interface deformation of the cavitation excitation shock wave at the free liquid level into the pressure of the shock wave, and the complete shock wave transmission equation in the liquid medium away from the bubble center can be obtained by using the value.
Description
Technical Field
The invention relates to the field of liquid shock waves, in particular to a method and a device for measuring shock wave pressure in a liquid medium.
Background
In a flow field where a hydrofoil, a propeller, a water turbine blade and the like are located, a large amount of cavitation collapse generates local high-energy impact load, the surface of a material is damaged by cavitation erosion, and the accumulation effect of pressure pulses can also induce problems of strong vibration, noise and the like. In the field of high-speed hydrodynamics, shock waves generated by cavitation collapse can be said to be the first enemy in engineering application. However, the high pressure generated by collapse of the cavitation bubbles can also be used by us. The laser-induced cavitation bubbles are formed by the interaction of pulse laser and a liquid medium, when the laser energy threshold is higher than the liquid medium breakdown threshold, a plasma cavity is formed at a breakdown point, and expansion pulsation and collapse of the plasma cavity form shock waves to act on a material, so that material modification is realized. The shape and shock wave characteristics of the vacuole are controlled by adjusting laser parameters, and the material characteristics are predictably modified. The method for processing the material by the laser cavitation induced shock wave keeps the characteristics of high efficiency, high quality and non-contact of laser processing, fully utilizes the strong pressure of the cavitation shock wave and effectively prevents the defect that the material is excessively ablated by the direct action of the laser.
The key for effectively utilizing the cavitation impact wave modified material and preventing the cavitation impact wave from being damaged is to test the impact wave pressure. The existing common mode is to use a hydrophone for measurement, but the hydrophone is easy to damage, and the cost is high, so that the hydrophone cannot be popularized and used on a large scale, and the measurement work of the impact wave pressure intensity is difficult to be effectively carried out.
Disclosure of Invention
Aiming at the defects in the prior art, the invention provides a method for measuring the pressure intensity of shock waves in a liquid medium, which converts the size of interface deformation caused by cavitation excitation shock waves at a free liquid level into the pressure intensity of the shock waves at the position, and can obtain a complete shock wave transmission equation in the liquid medium away from the bubble center by using the value.
The present invention achieves the above-described object by the following technical means.
A method of measuring shock wave pressure in a liquid medium, comprising the steps of:
generating cavitation shock waves by the liquid medium through laser induction;
adjusting the distance d between the laser-induced cavitation and the free liquid level by means of a mobile platformnWherein n is the number of measurements and n is an integer;
the maximum liquid level bulge height h is measured by shooting a liquid level bulge imagenThe included angle between the connecting line from the edge of the spherical crown to the center of the sphere and the normal line of the liquid level
And the included angle theta from the edge of the spherical crown to the top point of the spherical crown and the liquid levelnCalculating transient additional pressure P of the liquid level of the bulgesn；
By the formula
Calculating the average value of C, wherein,
p is the pressure of the liquid medium at a distance l from the center of the plasma cavity, and P is equal to Psn，l＝dn；
ρ0Is the liquid initial density;
b is a constant related to the pressure and the liquid medium;
and calculating the impact wave pressure according to the l value.
Further, the method also comprises the following steps: and (5) constructing an experimental device, and adjusting the focal length and the laser parameters.
Further, the adjusting laser parameters specifically include: according to the distance d between the laser-induced cavitation and the free liquid surfacenAnd the breakdown threshold of the liquid medium, selecting the laser energy J of the laser-induced cavitation bubble: j is not less than J0+J1Wherein J0The lowest energy required to form cavitation bubbles; j. the design is a square1Other losses in the laser transmission process.
Further, according to the maximum liquid level bulge height hnThe included angle between the connecting line from the edge of the spherical crown to the center of the sphere and the normal line of the liquid level
The included angle theta from the edge of the spherical crown to the top point of the spherical crown and the liquid levelnCalculating transient additional pressure P of the liquid level of the bulgesnThe method specifically comprises the following steps:
transient additional pressure P of convex liquid surfaceSnHeight h from maximum liquid levelnThe relationship of (1) is:
wherein γ is a surface tension coefficient.
Further, the distance d between the laser-induced cavitation and the free liquid surfacenThe range of (A) is as follows: d is not less than 1mmn≤100mm。
A detection device for impact wave pressure intensity in a liquid medium comprises a pulse laser emitting device, a high-speed camera and a control system; the pulse laser emitting device is used for focusing pulse laser into a liquid medium; the high-speed camera is used for shooting a liquid level bulge image; and a moving platform is arranged at the bottom of the container for containing the liquid medium, and the control system controls the pulse laser device, the high-speed camera and the moving platform.
Further, the pulse laser emitting device comprises a pulse laser, a reflecting mirror and a focusing lens, wherein the pulse laser is used for generating pulse laser, and the pulse laser is focused into the liquid medium through the reflecting mirror and the focusing lens.
Further, the liquid medium is a transparent medium.
The invention has the beneficial effects that:
1. the method for measuring the shock wave pressure in the liquid medium utilizes laser to excite cavitation bubbles in the liquid medium, and measures the shock wave pressure caused by the cavitation bubbles.
2. According to the method for measuring the impact wave pressure in the liquid medium, the impact wave pressure is measured without using expensive and fragile hydrophones, so that the measurement cost is greatly reduced.
3. The method for measuring the impact wave pressure in the liquid medium can repeatedly measure the shot interface deformation picture for many times, thereby greatly reducing the measurement error.
Drawings
Fig. 1 is a schematic structural diagram of a device for detecting shock wave pressure in a liquid medium according to the present invention.
FIG. 2 is a schematic view of the cavitation and the liquid level protrusion of the free liquid surface of the present invention.
FIG. 3 is a graph of measured liquid level elevations for the example.
In the figure:
1-a control system; 2-a pulsed laser; 3-a mirror; 4-a focusing lens; 5-a container; 6-liquid medium; 7-a high-speed camera; 8-a mobile platform; 9-vacuole; 10-free liquid level; 11-a three-dimensional mobile platform control system; 12-liquid level convex.
Detailed Description
The invention will be further described with reference to the following figures and specific examples, but the scope of the invention is not limited thereto.
As shown in FIG. 1, the device for detecting the impact wave pressure in a liquid medium comprises a pulse laser emitting device, a container 5, a high-speed camera 7 and a control system 1; the container 5 is filled with a liquid medium 6; the pulse laser focusing device comprises a pulse laser 2, a reflecting mirror 3 and a focusing lens 4, wherein the reflecting mirror 3 and the focusing lens 4 are positioned on a light path of the pulse laser 2, and the control system 1 is used for controlling and adjusting the light beam output of the pulse laser 2. The pulsed laser 2 is used to generate pulsed laser light which is focused into a liquid medium 6 by means of a mirror 3 and a focusing lens 4. In order to facilitate the focusing of the pulse laser, the laser focusing device further comprises a moving platform 8, the container 5 is placed on the moving platform 8, and a three-dimensional moving platform control system 11 is used for controlling the moving platform 8 to move. The liquid medium 6 is a transparent liquid, preferably water. The free liquid surface 10 is shown in the figure as being perpendicular to the incident laser light.
The invention relates to a method for measuring the impact wave pressure in a liquid medium, which comprises the following steps:
the first step, preparation stage: pouring the liquid medium 6 into the container 5 to enable the distance between the bottom of the container 5 and the free liquid level to be D;
and secondly, constructing an experimental device, adjusting parameters including focal length adjustment, laser parameters and the like, and generating cavitation shock waves by laser induction.
Adjusting the focal length: the three-dimensional moving platform control system 11 adjusts the moving platform 8, and indirectly adjusts the distance d between the laser-induced vacuole 9 and the free liquid level 10 in order to adjust the distance between the focusing plane of the focusing lens 4 and the free liquid leveln；
Adjusting laser parameters: according to dnAnd selecting the laser energy J of the laser-induced cavitation bubble according to the breakdown threshold value of water.
The laser processing energy J meets the following requirements: all laser energy ranges that enable convex deformation of the free liquid surface 10 without fracture of the free liquid surface 10. The focusing lens 4 has a focusing plane distance from the free liquid level height dnThe range of (A) is as follows: d is not less than 1mmn≤100mm。
Thirdly, as shown in fig. 2, the liquid level protrusion image is shot, the maximum liquid level protrusion height is measured, and the included angle between the connecting line from the edge of the spherical cap to the center of the sphere and the normal of the liquid level is measured
And from the edge of the spherical cap to the apex of the spherical capAngle theta of liquid surfacenCalculating transient additional pressure P of the liquid level of the bulgesn；
Pulse laser emitted by the pulse laser 2 is incident into the liquid medium 6, at a focusing point, the laser energy density exceeds the breakdown threshold of the liquid medium 6, a luminous plasma cavity is formed in a breakdown area, and the plasma has strong light absorption capacity, so that the plasma continuously absorbs the laser energy, the pressure inside the cavity is greater than that in the liquid medium 6, the cavity is rapidly expanded, and a cavity 9 is formed; the pulsation of the cavitation bubbles 9 radiates shock waves into the liquid, the shock waves deform the free liquid surface after reaching the free liquid surface 10, the deformation condition of the free liquid surface at the moment is shot through the high-speed camera 7, and the underwater shock wave pressure at the moment is obtained according to the force required by the deformation of the corresponding degree.
And fourthly, calculating a parameter C, changing the position of a laser focusing point, and testing the relation between the impact wave pressure and the distance at the liquid level position for n times. C is related to the type of liquid medium and the speed of sound in the liquid.
By the formula
Calculating the average value of C, wherein,
p is the pressure of the liquid medium at a distance l from the center of the cavity (9), and P is equal to Psn，l＝dn；
ρ0Is the liquid initial density;
b is a constant related to the pressure and the liquid medium;
and fifthly, calculating the pressure at any position l. Substituting the value of l into
And obtaining the impact wave pressure at the corresponding position.
The specific embodiment is as follows:
the laser beam direction is perpendicular to the free liquid surface 10. Selecting water as the liquid medium 6 and pouring the water into the container 5, so that the height D of the bottom of the container from the water surface is 5 mm; the container 5 is placed on a moving platform 8, and the laser-induced vacuoles 9 and the moving platform 8 are adjusted to move by controlling the moving platform 8 to moveDistance d between free liquid surfaces 10nAnd n is 1,2,3,4,5 and 6. I.e. d1＝2.5mm，d2＝2.6mm，d3＝2.7mm，d4＝2.8mm，d5＝2.9mm，d63mm, the laser energy J is more than or equal to J0+J1Wherein J0＝0.6mJ；J 15 mJ. Here, J is 25 mJ.
Fig. 3 shows a free liquid level protrusion diagram when D is 5 mm. (a) D corresponds to (b), (c), (d), (e) and (f) respectivelyn(n-1, 2,3,4,5,6) is 2.5, 2.6, 2.7, 2.8, 2.9, 3.0mm, and h is measured respectivelyn、θnAnd
according to the formula:
calculate to obtain Psn；
By the formula
Calculating the average value of C, and taking the average value to be 2.87, wherein,
p is the pressure of the liquid medium at a distance l from the center of the cavity (9), and P is equal to Psn，l＝dn；
ρ0Is the liquid initial density;
b is a constant related to the pressure and the liquid medium; b in water was 2.07.
The present invention is not limited to the above-described embodiments, and any obvious improvements, substitutions or modifications can be made by those skilled in the art without departing from the spirit of the present invention.
Claims (7)
1. A method of measuring shock wave pressure in a liquid medium, comprising the steps of:
generating cavitation shock waves by the liquid medium through laser induction;
the distance d between the laser-induced cavitation (9) and the free liquid level is adjusted by means of the mobile platform (8)nWherein n is the number of measurements and n is an integer;
the maximum liquid level bulge height h is measured by shooting a liquid level bulge imagenThe included angle between the connecting line from the edge of the spherical crown to the center of the sphere and the normal line of the liquid level
And the included angle theta from the edge of the spherical crown to the top point of the spherical crown and the liquid levelnCalculating transient additional pressure P of the liquid level of the bulgesn(ii) a The method specifically comprises the following steps: transient additional pressure P of convex liquid surfaces nHeight h from maximum liquid levelnThe relationship of (1) is:
wherein γ is the surface tension coefficient;
by the formula
Calculating the average value of C, wherein,
p is the pressure of the liquid medium at a distance l from the center of the cavity (9), and P is equal to Psn，l＝dn；
ρ0Is the liquid initial density;
b is a constant related to the pressure and the liquid medium;
and calculating the impact wave pressure according to the l value.
2. A method of measuring shock wave pressure in a liquid medium according to claim 1, further comprising the steps of: and (5) constructing an experimental device, and adjusting the focal length and the laser parameters.
3. Method for measuring shock wave pressure in a liquid medium according to claim 2, characterized in that the adjusting laser parameters are in particular: according to the distance d between the laser-induced cavitation (9) and the free liquid surfacenAnd the breakdown threshold of the liquid medium, selecting the laser energy J of the laser-induced cavitation bubble: j is not less than J0+J1Wherein J0The lowest energy required to form the cavitation bubbles (9); j. the design is a square1Other losses in the laser transmission process.
4. Method for measuring shock wave pressure in a liquid medium according to claim 1, characterized in that the distance d between the laser induced cavity (9) and the free liquid surfacenThe range of (A) is as follows: d is not less than 1mmn≤100mm。
5. A detection device for a method of measuring shock wave pressure in a liquid medium according to claim 1, characterized by comprising a pulsed laser emitting device, a high speed camera (7) and a control system (1); the pulse laser emitting device is used for focusing pulse laser into a liquid medium (6); the high-speed camera (7) is used for shooting a liquid level protrusion image; and a moving platform (8) is arranged at the bottom of the container for containing the liquid medium, and the control system (1) controls the pulse laser emitting device, the high-speed camera (7) and the moving platform (8).
6. Detection apparatus for a method of measurement of shock wave pressure in a liquid medium according to claim 5, characterized in that the pulsed laser emitting means comprises a pulsed laser (2), a mirror (3) and a focusing lens (4), the pulsed laser (2) being adapted to generate pulsed laser light, which is focused into the liquid medium (6) by the mirror (3) and the focusing lens (4).
7. Detection apparatus for a method of measurement of shock wave pressure in a liquid medium according to claim 5, characterised in that the liquid medium is a transparent medium.
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