CN108955956A - Frictional resistance measuring system and method based on flexible micro- beam - Google Patents

Abstract

A frictional resistance measurement system based on flexible microbeams, including a medium platform filled with a fluid medium, the bottom of the medium platform is provided with an opening, and a sample to be tested is placed inside the opening, and the fluid medium flows through the sample to be tested The measurement module is located below the sample to be measured, the measurement module includes a flexible microbeam and a floating plate, the flexible microbeam supports the floating plate, and the sample to be measured is fixed on the floating plate, because the surface of the sample to be measured is subjected to fluid in the fluid medium The frictional resistance causes the sample to be tested and the microbeam to generate a displacement along the flow direction, and the measurement module is used to measure the displacement; the data processing module displays the displacement and determines the frictional resistance according to the displacement. According to the displacement along the flow direction of the sample to be tested and the microbeam, the invention can measure the frictional resistance of the sample to be tested, has high sensitivity, high dynamic response, less interference by external temperature and pressure, easy calibration, and high measurement accuracy. advantage.

Description

Friction resistance measurement system and method based on flexible microbeams

technical field

The invention relates to a system for measuring frictional resistance near a wall, in particular to a system and method for measuring frictional resistance based on flexible microbeams.

Background technique

In fluid media, for example, the frictional resistance near the wall in the water flow field accounts for about 70% of the total resistance. Therefore, the characteristics of drag reduction and cavitation of underwater moving objects are important factors in hydrodynamics, fluid engineering and biofluid mechanics. The in-situ accurate measurement of near-wall frictional resistance is of great significance to the study of hydrodynamic characteristics and drag reduction mechanism. In recent years, the measurement technology of near-wall frictional resistance in the flow field has attracted widespread attention from all over the world. DARPA in the United States has set up a special project to develop in-situ frictional resistance measurement devices. my country also provided special support for the research on near-wall frictional resistance measurement in 2013. Although scholars at home and abroad have proposed a variety of measurement methods for the frictional resistance near the wall of the flow field, the current drag reduction measurement method still has many shortcomings, mainly concentrated in: (1) The measurement element has high requirements on the measurement surface properties, and it is difficult to use in complex (2) The measurement system has poor anti-interference performance, and it is difficult to eliminate errors caused by local temperature and pressure drift; (3) The rigid structure of the measurement unit is easily damaged and the measurement range is limited.

At present, researchers mostly use MEMS-based wall shear stress sensors, which mainly include two types: one is a heat-sensitive wall shear stress microsensor, and the other is a floating wall shear stress microsensor. The thermal-sensitive wall shear stress sensor based on MEMS is an indirect measurement method, which is greatly affected by temperature. Fluid temperature changes are likely to cause errors in the results, and the uniformity of the flow field temperature is not easy to control. In short, the heat-sensitive wall shear stress sensor is affected by many links, its measurement accuracy is limited, and its calibration is relatively complicated.

MEMS-based direct-measurement sensors generally have floating elements that can be displaced by force. The shear stress acts on the floating unit, causing it to generate a displacement proportional to the magnitude of the shear stress, which is converted into a measurable electrical signal. However, since the calibration process of the MEMS-based floating wall shear stress sensor needs to exert a concentrated force on the floating unit, which is equivalent to the effect of shear stress, the small size of the floating unit limits the loading of the concentrated force, which greatly reduces the Measurement accuracy, and the rigid silicon-based microstructures processed by MEMS are prone to brittle fracture, limiting the measurement range.

Contents of the invention

(1) Technical problems to be solved

The object of the present invention is to provide a frictional resistance measurement system and method based on flexible microbeams to solve at least one of the above technical problems.

(2) Technical solutions

An aspect of the embodiments of the present invention provides a frictional resistance measurement system based on flexible microbeams, including:

The medium platform is filled with a fluid medium, the bottom of the medium platform is provided with an opening, the inside of the opening is placed with a sample to be tested, and the fluid medium flows through the sample to be tested;

The measurement module is located below the sample to be measured, and the measurement module includes a flexible microbeam and a floating plate, the flexible microbeam supports the floating plate, and the sample to be measured is fixed on the floating plate, since the surface of the sample to be measured is in a fluid medium The sample to be tested and the microbeam are displaced along the flow direction by the frictional resistance of the fluid, and the measurement module is used to measure the displacement;

The data processing module is used to display the displacement and determine the frictional resistance according to the displacement.

In some embodiments of the present invention, the media platform includes a gravity-type circulating water tunnel.

In some embodiments of the present invention, at least one opening is provided at the bottom of the experiment section of the gravity-type circulating water tunnel, and a sample to be tested and a measurement module are arranged in each opening, and the measurement module includes a micrometer beam force measuring unit and an optical displacement measuring unit, wherein,

The microbeam force measuring unit includes: the flexible microbeam and the floating plate;

The optical displacement measuring unit includes: an optical encoder and a scale, wherein the scale is used to receive the laser light emitted by the optical encoder and reflect the laser light to the optical encoder; the optical encoder receives the reflected laser light and A quadrature differential signal related to displacement is generated.

In some embodiments of the present invention, the data processing module includes a data acquisition card connected to the optical encoder and a host computer, wherein the host computer is used to send instructions to the data acquisition card, and the data acquisition card receives instructions Sampling the quadrature differential signal, obtaining and outputting the sampled pulse signal to the host computer for processing, thereby obtaining the displacement in the form of a digital signal, and storing the displacement as data.

In some embodiments of the present invention, the host computer is also used to calculate and display the frictional resistance, and the frictional resistance satisfies the formula F=4Edtw ³ /L ³ ,

Wherein, L is the length of the flexible microbeam, w is the width of the flexible microbeam, t is the height of the flexible microbeam, E is the modulus of elasticity of the flexible microbeam, and d is the displacement.

In some embodiments of the present invention, E ranges from 625KPa to 2GPa.

In some embodiments of the present invention, the flexible microbeams are 3D printed microbeams.

In some embodiments of the present invention, there is a gap between the flexible micro-beam and the optical displacement measuring unit.

In some embodiments of the invention, wherein:

The output frequency of the optical encoder ranges from 0.225MHz to 7.2MHz; and/or

The input frequency of the data acquisition card is not greater than 2MHz.

Another aspect of the embodiments of the present invention also provides a flexible microbeam-based friction resistance measurement method, which is applied to any of the flexible microbeam-based friction resistance measurement systems described above.

(3) Beneficial effects

Compared with the prior art, the frictional resistance measurement system and method based on the flexible microbeam of the present invention has at least the following advantages:

(1) The present invention designs flexible microbeams, optimizes and integrates 3D printing formulas and processes, prints microbeam structures with different elastic moduli, and prepares prototype devices that meet the design goals, so that it has high sensitivity and high dynamic response. Features, greatly improving the frictional resistance measurement range of the micro-beam structure;

(2) The flexible microbeam of the present invention, compared with the traditional rigid microbeam sensor, the flexible microbeam force measuring unit is not easy to fracture and damage, and it is easier to reflect frictional resistance, which greatly improves the feasibility;

(3) The present invention can directly and accurately measure the flow friction resistance of the near-wall region of the complex function surface, has the characteristics of less interference by external temperature and pressure, easy calibration, high measurement accuracy, wide measurement range, etc., and can solve the problem of traditional experimental methods. It can only measure the problem of smooth surface, filling the blank of accurate measurement technology of frictional resistance near the wall;

(4) The present invention can solve the technical problem for researchers to accurately measure the frictional resistance of the surface with complex topography near the wall, and provide a test platform for accurately evaluating the drag reduction performance of the surface with complex topography. At the same time, the particle image velocimetry system (PIV) can be combined with the present invention to analyze the fine flow field in the boundary layer of the surface with complex structure and function, so as to comprehensively reveal the flow drag reduction mechanism of the surface with complex shape.

(5) The present invention has the advantages of strong environmental adaptability, good robustness, high measurement accuracy, and is not easy to be damaged. It is flexible in deployment and can meet the needs of in-situ resistance measurement for surfaces with different complex shapes.

Description of drawings

Fig. 1 is the structural representation of the frictional resistance measuring system based on flexible microbeam of a specific embodiment of the present invention;

Fig. 2 is the specific schematic diagram of the gravity type circulating water tunnel in Fig. 1;

Fig. 3 is the specific schematic diagram of microbeam force measuring unit in Fig. 1;

FIG. 4 is a specific schematic diagram of the data processing module in FIG. 1 .

[Symbol Description]

1. Dielectric platform 2. Micro beam force measuring unit

3. Optical displacement measurement unit 4. Data processing module

5. Left opening 6. Right opening

7. Floating plate 8. Flexible micro-beam

9. Hoarding 10. Sample to be tested

11. Glue 12. Sample table

13. Bolt 14. Ruler

15. Glass sheet 16. Optical encoder

17. Host computer 18. Data acquisition card

19. Optical encoder connector 20. Data transfer cable connector

21. Adapter socket

Detailed ways

Most of the existing technologies use wall shear stress sensors based on MEMS, which have the defects of limited measurement accuracy, complicated calibration, and limited measurement range. In view of this, the present invention provides a friction resistance measurement system and method based on flexible microbeams. According to the displacement along the flow direction of the sample to be tested and the microbeam, the invention can measure the frictional resistance of the sample to be tested, has high sensitivity, high dynamic response, less interference by external temperature and pressure, easy calibration, and high measurement accuracy. At the same time, it can solve the problem that the traditional friction resistance measurement method can only measure smooth surfaces, and meets the needs of in-situ resistance measurement for samples to be tested on surfaces with different complex shapes.

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be described in further detail below in conjunction with specific embodiments and with reference to the accompanying drawings.

An aspect of the embodiments of the present invention provides a frictional resistance measurement system based on flexible microbeams, including:

The medium platform 1 is filled with a fluid medium, the bottom of the medium platform 1 is provided with an opening, the inside of the opening is placed with a sample 10 to be tested, and the fluid medium flows through the sample 10 to be tested;

The measurement module is located below the sample to be measured, the measurement module includes a flexible microbeam 8 and a floating plate 7, the flexible microbeam 8 supports the floating plate 7, and the sample 10 to be measured is fixed on the floating plate 7, because the to be measured The surface of the sample 10 is subjected to fluid frictional resistance in the fluid medium, so that the sample 10 to be tested and the flexible microbeam 8 generate a displacement along the flow direction, and the measurement module is used to measure the displacement;

The data processing module 4 is used to display the displacement and determine the frictional resistance according to the displacement.

Wherein, the flexible microbeam 8 can be obtained by 3D printing or prepared by other materials with specific flexible properties, and its shape and preparation method are not limited in the present invention. It should be noted that the elastic modulus E of the flexible microbeam 8 preferably ranges from 625KPa to 2GPa, thus, the displacement of the flexible microbeam 8 can be more easily and accurately measured.

The fluid medium in the present invention can be water, air and the like. In some embodiments of the present invention, the medium platform 1 may be based on a gravity circulating water tunnel or other medium platforms. Next, the gravity-type circulating water tunnel is taken as a specific embodiment and described in conjunction with FIG. 1 .

Fig. 2 is the specific schematic diagram of the gravity type circulating water hole in Fig. 1, as shown in Fig. 2, in the embodiment of the present invention, the bottom of the experimental section of the gravity type circulating water hole is provided with two left and right symmetrical openings, namely Left opening 5 and right opening 6. In fact, the number and position of the holes are not limited, and each hole is provided with a sample 10 to be tested and a measurement module. Wherein, the roughness of the sample to be tested 10 in each opening can be partly the same, so that the relationship between the sample size or shape and frictional resistance can be detected; the roughness of the sample to be tested 10 in each opening can be different are not the same, while maintaining the same size and shape, so that the relationship between the roughness of the sample and the frictional resistance can be detected. In the embodiment of the present invention, the sample 10 to be tested may have a complex shape with a rough surface or a simple shape with a smooth surface, and the present invention can measure the frictional resistance it receives.

FIG. 3 is a specific schematic diagram of the microbeam force measuring unit in FIG. 1 . As shown in FIG. 3 , the measurement module includes two units: the microbeam force measuring unit 2 and the optical displacement measuring unit 3 . The sample 10 to be tested is connected to the sample stage 12 through glue 11 ; the sample stage 12 is fixed on the microbeam force measuring unit 2 by bolts 13 . In order to ensure that the microbeam 8 is not in contact with the optical displacement measuring unit 3, a gap is left in the middle, thereby preventing the friction force generated after the microbeam 8 contacts the optical displacement measuring unit 3, and affecting the measurement of the frictional resistance of the fluid medium on the flexible microbeam 8 .

Wherein, the microbeam force measuring unit 2 includes: four flexible microbeams 8 ; and a floating plate 7 connected with the flexible microbeams 8 and the sample 10 to be tested. in. The micro-beam force measuring unit 2 also includes a coaming plate 9 for fixing the flexible micro-beam 8 and the floating plate 7, wherein the coaming plate 9 and the floating plate 7 are all rigid materials, and the modulus of elasticity of the two can also be 2GPa . It should also be noted that the number of flexible micro-beams 8 can be 4, or other numbers in other embodiments, and the number can be selected according to the actual situation so as to determine the arrangement of the micro-beam load-measuring units 2 .

Under the flow shear of the frictional resistance F of the water flow in the gravity-type circulating water tunnel, the sample 10 to be tested will be displaced. Since the floating plate 7 is connected with the sample 10 to be tested, the floating plate 7 will also be displaced. As a result, the top of the flexible micro-beam 8 is also displaced.

In order to measure the displacement of the flexible microbeam 8 conveniently, the optical displacement measuring unit 3 includes an optical encoder 16 and a scale 14 (the accuracy of which can reach 78nm). Specifically, the optical encoder 16 emits laser light, and the scale 14 opposite the optical encoder 16 receives the laser light and reflects the laser light back to the optical encoder 16 . When the optical encoder 16 receives the reflected laser light, it will also generate a quadrature differential signal related to the displacement, and then process the quadrature differential signal to obtain the magnitude of the displacement.

Wherein, the maximum output frequency range of the optical encoder 16 is preferably 0.225MHz-7.2MHz, which can match the speed of the data acquisition card, thereby accurately measuring the displacement.

The optical displacement measuring unit 3 can also include a glass sheet 15 between the optical encoder 16 and the scale 14, which can prevent water from entering the square cavity under the glass sheet 15, and can transmit light at the same time, which is conducive to accurate displacement measurement.

Fig. 4 is the specific schematic diagram of the data processing module in Fig. 1, as shown in Fig. 4, in order to facilitate subsequent processing of the orthogonal differential signal, the data processing module 4 includes a data acquisition card 18 connected with the optical encoder 16; And upper computer 17. The optical encoder connector 19 is used for connecting the optical encoder 16 and the transfer socket 21 , and the data transfer cable connector 20 is used for connecting the data acquisition card 18 and the transfer socket 21 . Both the optical encoder connector 19 and the data transfer cable connector 20 are fixed on the transfer socket 21 . Generally speaking, the input frequency of the data acquisition card 18 is not greater than 2MHz, so as to avoid the inaccurate pulses collected and the cases of missing transmissions.

The data acquisition card 18 receives the instruction sent by the host computer 17, samples the orthogonal differential signal output by the optical encoder 16 according to the instruction, obtains and outputs the sampled pulse signal (including the number of pulses) and outputs it to the host computer 17. The host computer 17 processes the pulse signal to obtain the displacement in the form of a digital signal, and stores the displacement as data, thereby determining the displacement of the flexible microbeam 8 .

After obtaining the displacement, the host computer 17 also needs to calculate the frictional resistance according to the displacement. Assuming that the sample 10 under test undergoes a displacement of d under the shear of the frictional resistance F, the length of each flexible microbeam 8 is L, the width w, the height t, and the elastic modulus E. Then, the displacement of the floating plate 7 (the displacement of the top of the flexible microbeam 8 structure) is also d. Then the moment of inertia I of a single flexible microbeam:

I=tw ³ /12

Under the action of frictional resistance F, the two flexible microbeams 8 at the corresponding positions on both sides of the floating plate 7 undergo antisymmetric deformation, and the two flexible microbeams 8 on the same side of the floating plate 7 undergo symmetrical deformation, and each flexible microbeam bears Force of F/4. For a flexible microbeam, one side is a fixed end, and the other side bears the flow shear force of F/4, and there is a bending moment M to satisfy the condition that the rotation angle is 0. According to the equation of the rotation angle being 0, M =FL/8, under the combined action of F/4 and M, the displacement of each flexible micro-beam end is the displacement d=FL ³ /(48EI) of the floating plate 7, thus F= 4Edtw ³ /L ³ .

Afterwards, in order to enable the user to obtain the frictional resistance intuitively, the host computer 17 may also display the frictional resistance, such as a document format or a screen display, which is not limited by the present invention.

Another aspect of the embodiments of the present invention also provides a flexible microbeam-based friction resistance measurement method, which is applied to the aforementioned flexible microbeam-based friction resistance measurement system.

In summary, the frictional resistance measurement system and method based on the flexible microbeam of the present invention can measure the frictional resistance of the sample to be tested according to the displacement along the flow direction of the sample to be tested and the microbeam, and has high sensitivity and high dynamic response , Less interference from external temperature and pressure, easy calibration, high measurement accuracy, etc., can solve the problem that the traditional friction resistance measurement method can only measure smooth surfaces, and meet the requirements of in-situ resistance measurement for samples with different complex surface shapes. need.

Unless known to the contrary, the numerical parameters set forth in the specification and appended claims are approximations that can vary depending upon the desired properties obtained through the teachings of the invention. Specifically, all numbers used in the specification and claims to represent the content of components, reaction conditions, etc., should be understood to be modified by the term "about" in all cases. In general, the expressed meaning is meant to include a variation of ±10% in some embodiments, a variation of ±5% in some embodiments, a variation of ±1% in some embodiments, a variation of ±1% in some embodiments, and a variation of ±1% in some embodiments ±0.5% variation in the example.

Furthermore, "comprising" does not exclude the presence of elements or steps not listed in a claim. "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

The specific embodiments described above have further described the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above descriptions are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (10)

1. a kind of frictional resistance measuring system based on flexible micro- beam, comprising:

Media stage, internal be full of have fluid media (medium), and the bottom of the media stage is provided with aperture, and the inside of the aperture is put It is equipped with sample to be tested, the fluid media (medium) flows through sample to be tested；

Measurement module is located at below the sample to be tested, and the measurement module includes flexible micro- beam and kickboard, the micro- beam branch of the flexibility Support kickboard, sample to be tested is fixed in kickboard, due to the sample to be tested surface in fluid media (medium) by fluid friction resistance, make Sample to be tested and micro- beam generate the displacement along flow direction, and the measurement module is for measuring the displacement；

Data processing module determines the frictional resistance for showing the displacement, and according to the displacement.

2. system according to claim 1, wherein the media stage includes gravity type circulating water tunnel.

3. system according to claim 2, wherein the experimental section bottom of the gravity type circulating water tunnel is provided at least one A aperture, a sample to be tested and a measurement module are provided in each aperture, and the measurement module includes a micro- beam dynamometry Unit and an optical location move unit, wherein

Micro- beam load cell includes: the micro- beam of the flexibility and kickboard；

It includes: optical encoder and scale that the optical location, which moves unit, wherein the scale is for receiving optical encoder transmitting Laser and by the laser reflection to the optical encoder；The laser and generation and position that the optical encoder reception is reflected Move relevant orthogonal differential signal.

4. system according to claim 3, wherein the data processing module includes being connected with the optical encoder Data collecting card and host computer, wherein for sending a command to data collecting card, data collecting card reception refers to the host computer Order samples the orthogonal differential signal, obtains and pulse signal to the host computer exported after sampling is handled, thus The displacement of digital signal form is obtained, and the displacement is subjected to data storage.

5. system according to claim 4, wherein the host computer is also used to be calculated and be shown frictional resistance, described to rub It wipes resistance and meets formula F=4Edtw³/L³,

Wherein, L is the length of flexible micro- beam, and w is the width of flexible micro- beam, and t is flexible micro- depth of beam, and E is flexible micro- beam Elasticity modulus, d are the displacement.

6. system according to claim 5, wherein the range of E is 625KPa~2GPa.

7. system according to claim 1, wherein the micro- beam of the flexibility is micro- beam that 3D printing obtains.

8. system according to claim 3, wherein there are skies between the micro- beam of flexibility and optical location shifting unit Gap.

9. system according to claim 4, in which:

The reference frequency output of the optical encoder is 0.225MHz~7.2MHz；And/or

The input frequency of the data collecting card is not more than 2MHz.

10. a kind of frictional resistance measurement method based on flexible micro- beam, applied to the base as described in any in claim 1 to 9 In the frictional resistance measuring system of flexible micro- beam.