1、 Introduction and overview of wind tunnel
Wind tunnel:
Wind tunnel experiment is an indispensable part of aircraft development. It not only plays an important role in the research and development of aviation and aerospace engineering, but also is indispensable in transportation, housing construction, wind energy utilization and other fields with the development of industrial aerodynamics. In this method, the flow conditions are easy to control, and the important basis is the principle of relativity of motion. In the experiment, the model or real object is often fixed in the windblown land to obtain the experimental data economically. In order to make the experimental results accurate, the flow in the experiment must be similar to the actual flow state, that is, it must meet the requirements of the similarity law. However, due to the limitation of wind tunnel size and dynamics, it is difficult to simulate all similar parameters in a wind tunnel at the same time. Usually, some parameters with the greatest impact are selected for simulation according to the subject to be studied. In addition, the flow field quality of the wind tunnel test section, such as the uniformity of air flow velocity distribution, the size of the average air flow direction deviating from the wind tunnel axis, the pressure gradient along the wind tunnel axis, the uniformity of section temperature distribution, the turbulence and noise level of the air flow, must meet certain standards and be checked and measured regularly.
Introduction and principle of wind tunnel test:
Wind tunnel experiment in fluid mechanics refers to an aerodynamic experiment method to place aircraft or other object models in the wind tunnel, study the gas flow and its interaction with the model, and understand the aerodynamic characteristics of actual aircraft or other objects; In terms of insect chemical ecology, it is an experiment to observe the behavior reaction of living insects to odorants in a rectangular space with circulating air. In short, it is based on the principle of relativity of motion to fix the model or object of the aircraft in the artificial environment on the ground and artificially create airflow to simulate various complex flight conditions in the air and obtain test data. This is a "green channel" for the development, finalization and production of modern aircraft, missiles and rockets. To put it simply, wind tunnel is to artificially create a "sky" on the ground.
The wind tunnels used in this system are as follows:
The wind tunnel is a low speed reflux wind tunnel. The test section is made of transparent acrylic glass, with a size of 2.4m × 2.4m × 4m, the maximum speed can reach 60m/s, and the structure is shown in Figure 1
1. Fixed section 2. Honeycomb 3. Damping net 4. Contraction section 5. Model 6. Wind tunnel balance 7. Experimental section 8. Pressure balance hole 9. Diffuser section 10. Motor 11. Fan 12. Counter twist deflector 13. Rectifier 14. Return section 15. Corner 16. Deflector
2、 Overview of experimental principles
（1） Test and calibration of wind speed
1. Wind speed measurement principle and device
This system uses laser Doppler velocimeter
Characteristics of Laser Doppler Velocimetry
a. No contact measurement, no interference to the flow field during measurement;
b. High spatial resolution, suitable for the measurement of boundary layer, thin layer fluid and narrow channel fluid;
c. It is a new method to study turbulence and measure instantaneous velocity because of its fast dynamic response and instantaneous measurement;
d. The measuring accuracy is high. The basic principle of the instrument is an accurate physical expression, which is basically independent of other characteristics of the fluid;
E. Speed is measured by measuring whisker difference. Since whisker difference and speed are a linear relationship from low speed to high speed, no correction is required, and the measurable speed range of frequency shift is large;
f. With good directional sensitivity, it is convenient to measure the velocity component in any direction.
Limitations of Laser Doppler Velocimetry
limitations
a. The measured fluid shall have a certain light transmittance, and the pipe shall have a transparent window. Generally, transparent plexiglass is used in the test section, but optical glass is usually used if high measurement accuracy is required;
b. When measuring pure water or air, it is necessary to manually add appropriate particles as the scattering center, and have certain requirements on particle size;
c. When the flow rate is very high, the laser output power is required to be increased, and the signal processing is difficult due to the high signal frequency;
d. The price is expensive, and there are certain requirements for shockproof.
1. Instrument principle:
Laser Doppler anemometer (LDA): two beams of light with the same intensity output by the Bragg unit, one of which is added with a frequency shift. These two beams of light enter the optical fiber through focusing, and then are transmitted to the probe. The light passes through a focusing lens and intersects at a point in the detector body.
In the detection body, due to the interference phenomenon of light, the intensity of light is adjusted to produce a group of interference fringes, and the direction of the fringes is parallel to the angular bisector of two incident light beams. It can be seen from the mutual interference results of two columns of phase interference plane waves that the distance between interference fringes is determined by the wavelength of the laser and the angle of the two beams:
Where λ—— Wavelength of incident light;
θ—— The included angle of the incident beam.
When the tracer particles pass through the fringes with the fluid at the speed V in the direction perpendicular to the interference fringes, if the particles are located in the bright area of the fringes, the light blocking and scattering are maximum, on the contrary, the light blocking and scattering are minimum. Therefore, the scattered light contains a Doppler frequency shift, and the optical signal received after the stripe is a modulation quantity, which is proportional to the velocity component perpendicular to the bisector of the two beams.
Photodetectors convert the fluctuations of light intensity into electrical signals, namely Doppler pulses. The Doppler pulse is filtered and amplified in the signal processor, and then the Doppler frequency is determined through frequency analysis (such as fast Fourier transform), so as to obtain the velocity information of particles.
Laser Doppler effect:
When the light source and the moving object are in relative motion, the light scattered from the moving object will produce Doppler frequency shift, which is related to the speed of the moving object, the angle between the incident light and the speed direction.
2. Test system
A complete LDV usually consists of the following parts: optical path system, signal processing system, recording and display part.
The laser, beam expander, collimating lens and beam splitter are installed away from the wind tunnel. The polarizer and the focusing lens are placed on the X-Y measuring frame that can make axial and transverse measurements through the wind tunnel test section, and the beam from the beam splitter is projected onto the measuring frame from the first surface of the dielectric film plane reflector. The output or receiving optical system is installed on the opposite side of the wind tunnel. The pinhole diaphragm and the collimating lens are installed on the precision fine adjustment frame. All optical systems are installed on the X-Y measuring frame, which matches the displacement of the input optical measuring frame.
2. Measuring methods and steps
Experiment steps:
1、 Instrument startup
(1) Open the cooling water valve of argon ion laser;
(2) Turn on the power supply of the cooling water circulating pump;
(3) Turn on the "key" switch on the laser control panel, and adjust the laser power after preheating for 10 minutes after the laser is emitted from the laser outlet (this is very important). Generally, the laser intensity is adjusted by adjusting the power, generally about 0.5w-0.6w;
(4) Turn on the power of the PDPA system processor and turn on the switch;
(5) Turn on the power of the three-dimensional coordinate frame (Lightweight);
(6) Turn on the system computer.
2、 BSA Flow Software Operation
(1) Open BSA Flow, as shown in Figure 3.
(2) Create a new file, select BSA F/P Application, name it, and click OK to enter the operation interface.
(3) Connect the processor, as shown in Figure 4. Right click the Processor in the tree menu under the Device List, and left click Connect to Processor. If the connection is wrong, check whether the cross cable between the processor and the computer host is correctly connected.
(4) Connect the three-dimensional coordinate frame, similar to step (3). Right click Traverse System, and left click Connect to Traverse.
(5) Call up the system monitor, right-click anywhere on the operation interface, and select System Monitor.
(6) Adjust the attitude of the laser emission lens (including the detector) on the coordinate frame so that the incident angle of the laser is convenient for subsequent photodetectors to capture the modulation signal. After adjustment, fix the laser emission lens on the coordinate frame.
(7) Right click Traverse System, select Traverse Controller in the right-click menu, call up the controller of the three-dimensional coordinate frame, and adjust the coordinate frame to the point to be measured. In case of multi-point measurement, generally first adjust the laser focus to the origin of the measuring point grid.
(8) Call up the right-click menu of the Processor, click Configuration Manager in the configuration manager, and a dialog box as shown in Figure 6 pops up. You can select the type of test items. For example, 1D LDA refers to measuring one-dimensional velocity, and 1D PDA refers to measuring the diameter of tracer particles while measuring one-dimensional velocity. After selecting, click Finish, and the system will automatically re initialize the configuration.
(10) After step 9 is completed, observe whether there is a stable burst signal in each channel of the system monitor, which is consistent with the characteristics of the Doppler frequency shift signal. If the burst signal does not appear, fine tune the position and attitude of the detector of the photoelectric signal, and repeatedly debug until the burst signal on the system monitor is stable and meets the general characteristics of the Doppler signal. At the same time Validation and so on have reached a high level, as shown in Figure 8.
(11) Signal acquisition. Click the Run button on the toolbar to collect data.
3、 Export data. Click File/Export... on the menu bar, select the save path and File name, and click Save.
3. Calibration of wind speed in wind tunnels
During the experiment, two points need to be aligned:
Mechanical alignment of the two displacement directions and alignment of the optical system.
Mechanical alignment includes the positioning of the displacement platform relative to the window of the test section so that the measured displacement can contain the window of the test section, and then leveling and aligning them so that the moving measuring point can maintain an equal distance from the window of the test section when moving horizontally. And make the moving measuring point perpendicular to the window when crossing horizontally. The input optical plane reflector is aligned so that the intersection point (measuring volume) of the three input beams is kept in the horizontal plane during displacement. The volume measurement experiment is located on the vertical centerline of the test section (Y=0) by relying on the input side measuring frame and the input focusing lens.
4. Error analysis
In order to further improve the speed measurement accuracy of the laser Doppler velocimeter, we must consider all aspects of the impact in the actual measurement process to achieve the best effect. Therefore, the laser with narrow linewidth should be used in the experiment; The detector with smaller aperture shall be selected; When the condition of heterodyne interference is satisfied, the angle between the optical axes of two beams is appropriately increased; On the premise of ensuring the signal strength, appropriately increase the distance from the point to be measured to the center of the effective spot on the semi transparent semi reflective mirror to reduce the measurement error of the system. When selecting the included angle H between the direction of the central light of the signal light and the direction of particle motion, the error caused by the approximation of the principle formula and the size of the detector aperture, as well as the error caused by the finite transit time, should be considered.
The error caused by the size of the detector aperture is relatively obvious, so the smaller included angle can be appropriately selected; On the other hand, small included angle will reduce the signal strength, so this parameter needs to be debugged repeatedly during the experiment to select the best value. When choosing the beam spot radius, it is also necessary to consider the error caused by Gaussian beam interference and finite transit time, as well as the laser quality.
（2） Test and calibration of air volume
1. Air volume measurement principle and device
This system uses a Verabar flowmeter
Performance characteristics of Weiweiba flowmeter:
Measurement accuracy: ± 1% Repetition accuracy: ± 0.1%
Applicable pressure: 0~40MPa Applicable temperature: - 180 ℃~+550 ℃
Upper limit of measurement: it depends on the lower limit of probe strength measurement: it depends on the minimum differential pressure requirement of measurement
Range ratio: greater than 10:1
Applicable pipe diameter: 38mm ～ 9000mm round pipe and square pipe
Applicable medium: full pipe, unidirectional flow, unidirectional gas, steam, and liquid with viscosity no more than 10 centipoises. Valbar is widely used for measuring various gases, liquids, and steam. The following are typical application mediums.
Gas/liquid/steam
Natural gas/cooling water/saturated steam
Compressed air/boiler water/superheated steam
Fuel gas/demineralized water
Gaseous hydrocarbons/liquid hydrocarbons
Hot air/cryogenic liquid
Generator gas/heat-conducting liquid
Instrument principle
When the fluid flows through the probe, a high-pressure distribution area is generated in its front, and the pressure in the high-pressure distribution area is slightly higher than the static pressure of the pipe. According to the principle of Bernoulli equation, when the fluid flows through the probe, the speed is accelerated, and a low pressure distribution area is generated at the back of the probe. The pressure in the low pressure distribution area is slightly lower than the static pressure of the pipe. After the fluid flows through the probe, a partial vacuum is generated at the back of the probe, and vortices appear on both sides of the probe. The section shape, surface roughness and the position of low pressure tap hole of the average velocity flow probe are the key factors to determine the probe performance. The stability and accuracy of the low voltage signal play a decisive role in the accuracy and performance of the velocity averaging probe. The Wilbur average velocity flow probe can accurately detect the average differential pressure generated by the average velocity of the fluid. The Verabar average velocity flow probe has several pairs of pressure tapping holes arranged according to certain rules in the high and low pressure areas, making it possible to accurately measure the average velocity
measuring principle
Verabar flowmeter is a plug-in flow measuring instrument. Insert a Weibar sensor into the pipe. When the fluid flows through the sensor, a high pressure distribution area will be generated in the upstream direction of the sensor, and a low pressure distribution area will be generated in the rear. In the high and low pressure areas, there are many pairs of pressure taps arranged in a certain rule, generally three pairs. The total pressure of the fluid is measured separately, including static pressure, average velocity pressure Pl and static pressure P2. Introduce P1 and P2 into the differential pressure transmitter respectively, measure the differential pressure △ P=P1-P2, and △ P reflects the average velocity of the fluid, so as to calculate the fluid flow.
2. Measuring methods and steps
1 Installation steps:
① The sensor is installed correctly:
After the sensor is installed on the pipeline, it must be carefully checked before putting into operation. It is required that the welding is firm, the direction is correct, there is no leakage, and the insertion depth is appropriate.
② Instrument calibration:
The supporting instruments of the sensor include differential pressure transmitter and intelligent flow totalizer (pressure transmitter and temperature transmitter may also be available). They can be put into use only after being inspected and adjusted. The measuring range of the instrument shall meet the requirements of the sensor and the measured medium. For example, the maximum flow of the measured air Qmax=5000m3/h, and the maximum differential pressure generated by the sensor is calculated
△ Pmax=0.6Kpa, then the measurement range of the differential pressure transmitter should be adjusted to 0~0.6KPa, corresponding to the output of 4~20mADC current signal. For the general flow totalizer, the real-time flow range, differential pressure range, medium density, temperature, pressure, flow calculation requirements, etc. should be programmed and configured in advance to input the totalizer, so that the totalizer can correctly calculate and display the flow.
③ Instrument wiring is correct:
Sensor, differential pressure transmitter, flow totalizer, etc