New wind tunnel tester aerodynamic measuring instrument requirements principle
A device used to measure wind tunnel airflow characteristics and various aerodynamic parameters of airflow acting on the model. The early wind tunnels were only used for static experiments. The measured parameters were small, the amount was small, and the range was narrow. Most of them were dial-type instruments or optical qualitative analysis instruments. Since the 1960s, the original wind tunnel has been used for dynamic experiments, and new wind tunnels have been put into use continuously. The number of measured parameters has increased dramatically, and wind tunnel experimental techniques and test instruments have also undergone major changes: from individual items. Measure to comprehensive measurements, from static to dynamic, from macro to micro, from qualitative analysis to quantitative measurement, and more.
History development
Since the 1960s, the original wind tunnel has been used for dynamic experiments, and new wind tunnels have been put into use continuously. The number of measured parameters has increased dramatically, and wind tunnel experimental techniques and test instruments have also undergone major changes: from individual items. Measure to comprehensive measurements, from static to dynamic, from macro to micro, from qualitative analysis to quantitative measurement, and more. Therefore, the wind tunnel test instrument is required to have high sensitivity, high resolution, fast response time, good reliability, anti-overload capability, strong impact resistance and electromagnetic field interference, good shock resistance, wide measurement range and high precision. In addition, the instrument should be as versatile as possible to meet the requirements of computer-centric automated acquisition, detection, control and processing systems. The wind tunnel test instrument can be divided into contact type and non-contact type according to whether it is in contact with the measured object. According to the test content, it can be divided into aerodynamic and torque measuring instruments, pressure measuring instruments, temperature, heat flow and total enthalpy (see ç„“Measurement instruments, flow field density and density change measurement and display instruments, airflow velocity measuring instruments and patrol detection devices.
Aerodynamic measuring instrument requirements
The device that directly measures the aerodynamic forces and moments acting on the model in the wind tunnel (see aerodynamics) is mainly a wind tunnel balance. Usually the balance is designed to measure the forces along three axes in the Cartesian coordinate system and the moments around the three axes (or only one or two of the forces and moments). Wind tunnel balances evolved from general analytical balances, and early line-type balances (Fig. 1) were similar to analytical balances. It suspends the model with metal wires, and the load applied to the model is sent to the balance lever element through the metal wire, and the weight is added and subtracted to balance it. However, the analytical balance can only measure one force at a time, and the direction and point of action of the required force are known. Wind tunnel balances can measure several components of aerodynamic forces at the same time. The point and direction of aerodynamic forces are generally to be measured. Therefore, the structure of the wind tunnel balance is also different from the general analytical balance. There are many classification methods for wind tunnel balances. According to the measurement principle, it can be divided into mechanical balance, strain balance, piezoelectric crystal balance and electromagnetic suspension.
Mechanical balance
A wind tunnel balance that transmits and decomposes aerodynamic forces on a model through mechanical structural systems (including various force transmission struts, hinges, force platforms, moment platforms, etc.) and is measured by sensing components that sense displacement. The hanging line type is a mechanical balance. Due to the large interference resistance, the wire-type balance is easily broken and is rarely used and replaced by a pole-type mechanical balance. When using a pole balance, the model is supported by a strut that changes the angle of attack of the model and the angle of the slip (the angle between the speed vector of the aircraft and the plane of symmetry of the aircraft). The aerodynamic force of the model is transmitted from the struts to the linkage system, which is broken down into individual components and recorded by the measuring components. According to the different assembly forms of the connecting rod system, the pole type mechanical balance can be divided into a tower balance, a table balance and a yoke balance. Although the measurement accuracy of such mechanical balances is quite high, strain gauges are generally used in high speed wind tunnels because of the difficulty in rapid measurement.
Strain gauge balance
An instrument for measuring force and moment by a change in an output signal generated by deformation under the action of aerodynamic forces by a strain gauge attached to an elastic member. Typically, two strain gauges are placed side by side on the stretched and compressed surfaces of the elastic member. Then connect the four strain gauges to the bridge and add voltage to measure. The strain gauge balance is divided into a floating frame type and a composite type according to the component arrangement form. The floating frame balance is to fix the model on the sleeve, and the detachable measuring elements and the struts are mounted in the sleeve, and the components are measured by two parallel (parallel) load cells. The individual load cells of the compound balance consist of a series-parallel hybrid structure. Most strain gauges on strain balances use resistance wires. It is made of very thin wires or very thin metal foil. Semiconductor materials are sometimes used.
Piezoelectric crystal balance
The working principle is: using the piezoelectric effect of piezoelectric material, the aerodynamic force is converted into an electrical signal, and the output signal can be directly measured by a charge amplifier, or can be measured by an electrostatic amplifier or follower with a high input impedance. This type of balance is mostly used for shock wave tunnels with short working hours.
Electromagnetic suspension balance
The working principle is: the model made of soft iron is suspended in the wind tunnel by electromagnetic force, and any displacement of the model will cause the luminous flux of the photocell to change, and then the servo feedback control system generates a magnetic force against the dynamic force on the model, so that The model returns to the correct position. The aerodynamic force is converted from the amount of current of each magnetic coil or the magnetic flux of the magnetic field. The biggest advantage of this type of balance is that it is not affected by the interference of the model support.
Each of these balances has a calibration problem. Balance calibration is divided into static school and dynamic school. Static calibration of the balance using a calibration device is called a day of calm. The purpose of the calm school is to prove how much the balance can be loaded; to determine the calibration coefficient and sensitivity of each component; to determine the interference and deformation of the balance; to verify the repeatability of the load data, to determine the accuracy and stiffness of the balance using the formula and the balance. And strength. In the wind tunnel, the standard model is installed on the balance of the static school. The experiment is called the balance motion school. The purpose is to check the performance of the balance and determine the accuracy of the balance.
Pressure measuring instrument
Early in the wind tunnel, the pressure is measured by a liquid pressure gauge (such as a U-tube pressure gauge), and the multi-point pressure gauge is used to measure the multi-point pressure. The dial gauge is mainly used to monitor the pressure of the wind tunnel. These pressure gauges have gradually been replaced by wind tunnel pressure sensors due to the need to see, record or photograph, and the response time is long. Wind tunnel pressure sensors convert pressure into current or voltage signals for measuring static and dynamic pressures in various wind tunnels.
There are many types of pressure sensors, which can be divided into the following six forms according to the working principle:
1 resistance pressure sensor: it measures the pressure according to the principle that the resistance changes with pressure;
2 strain pressure sensor: the pressure is converted into strain by the elastic element, and the strain gauge stuck on the elastic element converts the strain into an electrical signal;
3 crystal pressure sensor: it uses the effect of the crystal to generate a charge when a specific axial force is applied to measure the pressure;
4 capacitive pressure sensor: it senses the pressure through one of the plates of the capacitor and converts its change into a change in capacitance;
5 electromagnetic pressure sensor: it converts the change of pressure into the change of magnetoresistance or inductance to measure the pressure, which can be divided into two types: magnetoresistive type and inductive type;
6 Resonant Pressure Sensor: It converts the change in pressure into a change in the natural frequency of the elastic element to measure the pressure.
See the pressure sensor for the detailed principle and structure of the various types of pressure sensors described above.
In the mid-1960s, a pressure scanning valve and a small pressure sensor were used to form a pressure measuring device. During the rotation of the rotor of the scanning valve, the pressure measuring points are connected to the sensors in turn, so that one sensor can measure the multi-point pressure, so that the number of sensors is greatly reduced, which creates conditions for improving the experimental precision. In the mid-1970s, in order to meet the increase in pressure points and increase the pressure measurement speed of large wind tunnels, an electronic sampling pressure component consisting of a small pressure sensor, an analog signal multiplexer and a pneumatic switch was used.
Total measuring instrument
In the measurement of the airflow temperature, the total temperature probe with the thermocouple as the temperature sensing element is widely used for the airflow with the total temperature below 2000. The temperature of the airflow measured is the total temperature (ie, the temperature at which the airflow velocity at the probe surface is zero). The total temperature measured by the probe is usually lower than the actual total temperature of the airflow, which is mainly caused by the heat conduction from the thermocouple junction to the support and the outward thermal radiation. The temporary wind tunnel also uses a thin film resistance thermometer to measure the temperature. The thin film resistance thermometer (referred to as a thin film meter) is a sensor that measures the instantaneous temperature change or heat flow density of the object surface according to the physical properties of the resistance of the metal film with temperature. The temperature rise ΔT of the metal film is proportional to its relative resistance increment, and inversely proportional to the temperature coefficient of resistance α. The steady or unsteady heat flux of the object can be determined by the relationship between the instantaneous temperature rise of the object surface and the heat flux density. . The film meter consists of a platinum film (on the order of submicron thickness) deposited on the surface of an insulating substrate (usually glass) and a silver wire, hence the name platinum film resistance thermometer. The advantages of the film meter are high sensitivity, fast response and small size. The measurement method commonly used for the airflow temperature of 2000 to 3000 is the emission absorption method according to the principle of radiation temperature measurement; for the airflow temperature of 4000 to 10000, the spectrum method commonly used in plasma diagnosis is often used. The absolute intensity of the line or the intensity of a certain two lines is measured by using a spectrometer or a spectrophotometer of a suitable wavelength range using a gas and a spectrum of the impurity molecular or atomic spectrum contained in the gas and the temperature. Ratio, thereby estimating the temperature. The total temperature of the gas flow in a thin gas wind tunnel is about 1000 to 3000, but due to the low airflow density, electron beam temperature measurement is used. The surface temperature is below several hundred open, and is usually measured by thermistor and temperature sensitive paint. The surface temperature is lower than 1200 and can be measured by an infrared thermometer and an infrared camera. For the surface temperature of the model from 1200 to 4000, the radiation temperature measuring instruments such as radiation pyrometer, photoelectric pyrometer and colorimeter pyrometer are usually used. For the ablation material model whose emissivity is difficult to measure in advance, the spectral energy distribution can be measured by a fast scanning infrared spectrophotometer, and then the emissivity and the real temperature can be calculated. It can also be compared with a set of black body energy distribution curves at different temperatures. The maximum brightness temperature is obtained. This maximum brightness temperature can be used as a more accurate measurement.
The total helium probe is a device for calorimetric measurement of the total local velocity of the gas stream and can be used to measure the total enthalpy. Early probes were water-cooled and appeared in the early 1960s. Subsequently, a transient helium probe was developed. In the 1970s, high pressure and high pressure diluted helium probes were developed.
When the total helium probe is working, the airflow to be measured is taken up by the sampling tube and cooled, and the heat absorption rate of the heat exchange medium in the probe, the flow rate of the sample to be aspirated and the temperature and pressure when leaving the heat exchanger are measured, and then the energy is pressed. The conservation relationship calculates the total enthalpy of the gas sample. The water-cooled probe uses high-pressure water as a heat exchange medium, and the body adopts a double-layer water jacket structure. Eliminate the effects of ambient heating with the difference in heat absorption rate of the cooling water when sampling and not sampling. Its response time is longer, from a few seconds to a dozen seconds. It can work in airflows up to 15,000 inches with high measurement accuracy. The heat exchanger of the transient helium probe is an elongated thin-walled platinum (or nickel) tube. The heat absorption rate is calculated using its resistance change rate. Its response time is on the order of milliseconds for transient total enthalpy measurements.
The high-pressure dilution type ç„“ probe uses a cold air to mix with the gas sample at the inlet to improve the severe heating state of the inlet section of the sampling tube, thereby greatly improving the measurement range. This probe has a diluent supply and flow measuring device, and the structure is complicated.
Shadow meter
An optical instrument that uses the principle of the shadow method to observe the rate of change of the unevenness of a transparent medium. In wind tunnels or ballistic targets, it is often used to observe the position and shape of the gradient of the flow field density when the model and gas move relative to each other. As shown in FIG. 2, the collimator lens L converts the divergent light of the point source S into parallel light, and passes through the experimental section D to reach the screen Q. If the flow density gradient in D is zero or the density gradient is uniform, the parallel light is not deflected or deflected in the same direction (dεy is the same), and the illumination on the screen Q is uniform; if the density of the gas in the flow field in D is uneven, then There are also differences in the deflection of the light passing through the flow field. Some of the light converges, some of the light diverge, and different shades of light appear on the screen, reflecting the change in the flow density gradient of the flow field. The illuminance on the screen is proportional to the integral of the density of the vertical incident light in the flow field and the integral value of the product to the screen distance. If the amount of deviation of the light in the flow field disturbance range is negligible, the above relationship can be used for quantitative analysis; and generally can only be used for qualitative analysis. Shading instruments made by the shadow method have two types: parallel light column type and divergent light cone type. The point source usually uses an electric spark or a laser pulse source, and the photosensitive film is placed in the Q area of ​​the screen for recording or imaged by an optical system. The shadow method is simple in equipment and intuitive in image. A clear image of the vortex around the shock wave and wake in the high-speed motion of the model can be obtained. The position of the boundary layer transition zone and the flow in the turbulent zone can also be observed.
Pattern machine
An optical instrument for determining the internal density gradient of a transparent medium by observing a change in the refractive index of the uneven transparent medium and converting it into a change in illuminance on the recording plane. In the wind tunnel experiment, the schlieren is used to display the density variation around the flow field around the model, and to observe the region and location of the shock wave, the expansion wave, the boundary layer, and the wake. The term schlieren comes from the German Schliere, which means that grooves appear in the transparent material due to impure composition. In 1859, J.-B.-L. Foucault proposed using a knife edge as a diaphragm to check the quality of optical parts. In 1886, AJI Tepler first observed the schlieren with an optical system to study the flow phenomena such as sparks and explosions. The schlieren method is sometimes called the Töppler method. As shown in Fig. 3, the light source S (usually a slit) is imaged on the plane K of the knife edge, and the object of the experimental section is imaged on the screen Q via the mirror M2 and the photographic objective lens L. When the medium of the experimental section is uniform, a single light source image is formed on the plane of the knife edge, and the illumination on the screen is uniform. When the density of the medium in the local area of ​​the experimental section is not uniform, the light passing through the area is deflected, and the deflection angle is proportional to the refractive index gradient. An off-going light source image is formed on the plane of the knife edge, and the illuminance of the corresponding area on the screen changes. The change in illuminance is proportional to the integral value of the refractive index change in the direction perpendicular to the knife edge in the medium along the integrated value of the optical path. The density gradient of the medium can be obtained from the relationship between the refractive index and the density of the gaseous medium. In the wind tunnel experiment, the schlieren is generally used as a qualitative flow field display. The schlieren diaphragm can use color bands, gratings, polarizing prisms, etc. to obtain color interference schlieren images, which can improve sensitivity and be suitable for quantitative research. The schlieren combines high-speed photography with microscopic technology to capture high-speed and micro-grain photos. The laser light source is used in the lithography instrument to not only shorten the exposure time, but also obtain a high-speed transient schlieren image, and can form a holographic system to "freeze" the experimental time and space, and reproduce it for quantitative research in three-dimensional space. .
Air velocity measuring instrument
There are mainly pitot-static tubes, hot wire anemometers and laser Doppler velocimeters.
Pito-static tube
The most commonly used instrument for measuring airflow velocity is derived from the pitot tube. The pitot tube is a cylindrical tube with one end open and the other end attached to a pressure gauge to measure the total air pressure. This tube was used by H. Pito in 1872 to measure the water depth and flow rate of a river. In addition to the pitot tube, the pitot-static tube can sense the total pressure of the gas stream and simultaneously measure the static pressure of the gas stream. Figure 5 is a schematic view of a typical pitot-static tube used in low subsonic speeds. It has an inner tube and an outer tube. The inner tube measures the total pressure. The static pressure hole is opened on the outer tube at a certain distance from the head. According to the Bernoulli equation (see Bernoulli's theorem), the pressure difference measured by the total and static pressure holes is converted to obtain the flow rate. It can be used for speed measurements from 1 to 2 m/s to below the critical speed. The front end of such a tube is mostly hemispherical, and the total pressure hole is on the axis, which is insensitive to the shape of the tube. The static pressure holes are greatly affected by the ends and the rear struts. Because of the opposite effects of both, this effect can be reduced by careful design. In order to reduce the influence of the deflection of the airflow direction, it is sometimes possible to open a plurality of static pressure holes in the circumferential direction. In order to avoid errors caused by design and processing, corrections should be made before use.
Hot wire anemometer
An instrument that measures airflow velocity, temperature, and density based on the principle of non-electrical electrical measurement has been used for more than 70 years. Its sensor (commonly known as a probe) is a thin wire with a length much larger than the diameter, referred to as a hot wire, or a very thin metal film, referred to as a hot film. During the measurement, the hot wire or the hot film is placed in the airflow to be tested, and at the same time connected to one arm of the bridge, and heated by current, so that the temperature of the hot wire or the hot film itself is higher than the temperature of the airflow medium to be tested. The change in the state of the airflow causes a change in the heat transfer between the hot wire or the hot film and the gas flow medium, thereby changing the voltage across the hot wire or the hot film, thereby measuring the average value of the velocity, temperature or density of the gas stream and Instantaneous value. There are two types of circuits for the hot wire anemometer: one is a constant temperature type that maintains the temperature of the hot line; the other is a constant current type that maintains the constant current of the hot line. The voltage change at both ends of the hot wire is generally amplified and compensated before measurement. The electrical signals measured in the past were all processed by electrical simulation. In recent years, the electric signals measured by hot wires or hot films are input to an electronic computer for processing, so that the measurement accuracy is higher, and thus the application range is wider. The hot wire has a diameter of only 1 to 5 microns; the length is only 0.5 to 1 mm. The thickness of the hot film is only 5 to 10 nm. The hot wire material is platinum or tungsten, or a platinum-rhodium alloy wire containing bismuth, or a silver-coated Wollaston wire. The hot film material is mostly platinum or nickel, and sometimes a layer of 2 to 5 micrometers of quartz is also sprayed thereon for measurement in conductive liquids.
Laser Doppler tachometer
A device that uses a laser as a light source to measure the velocity of gases, liquids, and solids using the Doppler shift effect of light. In 1842, the Austrian physicist C. Doppler discovered the Doppler effect of sound waves. In 1905, A. Einstein pointed out in the special theory of relativity that the Doppler effect can also occur in light waves. When light is scattered onto the moving particles, the frequency of the scattered light changes with respect to the frequency of the incident light. The offset of the frequency is proportional to the velocity of the moving particles. When the diameter of the scattering particles in the flow field is of the same order of magnitude as the wavelength of the incident light, and the weight of the scattering particles is close to the weight of the surrounding flow field particles, the velocity of the scattering particles substantially represents the local flow velocity of the flow field. The United States Y. Y. and H. Cummings first reported in 1964 the use of laser Doppler shift effect for fluid velocity measurement.
Laser Doppler tachometers include optical systems and signal processing systems. The optical system illuminates the laser beam onto the particles following the movement of the fluid and converges the scattered light of the measured point (volume) into the photoreceiver. The optical system can be classified into a forward scattering type, a backward scattering type, and a mixed scattering type in such a manner as to receive scattered light. According to the optical structure, it can be divided into reference light type, double scattering type, stripe type and polarized light type. Figure 6 is a schematic diagram of a forward double scattering type. The photoreceiver (photomultiplier tube, silicon photodiode, etc.) receives two scattered light waves that change with time. After mixing, the frequency of the output signal is the frequency difference between the two parts of the light wave, which is proportional to the flow rate. Signal processing systems are used to detect the true information reflecting the flow rate from various noises and convert them into analog or digital quantities for further processing or display. Commonly used signal processors include frequency analyzers, frequency trackers, and counting processors. In principle, the laser Doppler tachometer is the only means of measuring speed directly. It can be used in wind tunnel experiments to measure local velocity, average velocity, turbulence intensity, velocity pulsation, etc. It is suitable for studying the separation interference zone of the shock and boundary layers, the rotor velocity field, the blasting boundary layer and the high temperature flow. The speed measuring instrument or device has a speed range from 0.05 cm/sec to 2000 m/sec. When measuring high speed, it is limited by the frequency response range of the optoelectronic device. In experiments, it is sometimes necessary to incorporate particles of different sizes into the gas stream using specialized particle dissemination devices. Due to the influence of the inertia of the scattering particles, the velocity of the particles lags behind the fluid, so the speed measurement accuracy is low, and the accuracy is lower when the turbulence is high.
Tour detection device
Multiple voltage or current signals (called analog quantities) are acquired in a certain order or randomly, and these analog quantities are converted into binary or decimal digital devices (referred to as detection devices).
The input analog quantity of the circuit detection device is sent by the sensing conversion device (such as sensor, force measurement equal) through the transmission line, and its output digital quantity is sent to computer processing or other recording equipment (such as printer, puncher, tape, etc.) for recording. . Its location in the wind tunnel test system is shown in Figure 7. The patrol detection device is generally composed of a sampler, a data amplifier, an analog-to-digital converter, a filter, a display, an interface, and a controller (Fig. 8). The sampler is a program-controlled electronic or mechanical switch that collects a continuous variable value at periodic intervals or at arbitrary intervals. The sampler is composed of a sampling switch, a channel counter, a channel decoder, a cycle number counter, a clock, and the like. The sampler works at speeds from tens of times per second to tens of thousands of times per second. A data amplifier is a component that amplifies an input signal. It typically amplifies a few millivolts into a few volts and then feeds it into an analog-to-digital converter. It also rejects the interfering signal and picks up the useful signal from it. The analog-to-digital converter (A/D) converts the measured voltage analog (continuous) into digital (discrete). There are many types, the most commonly used one is called feedback comparison analog-to-digital converter, by comparator, analog-to-digital converter (with decoding switch, resistor network, digital register), beat generator, conversion controller, voltage reference , pulse source and other components. The purpose of the filter is to filter out unwanted signals in the signal source. A filter composed of a resistor capacitor or an inductor capacitor is called a passive filter; a filter composed of a resistor capacitor and an amplifier is called an active filter; and a digital filter is processed by a computer to eliminate an interference signal. The display is a component that displays measurement parameters, consisting of a selection point display switch, a binary to decimal arithmetic unit, a decoder, and a digital tube. An interface is a handover part when two different devices are interconnected. The information transfer and coordination between all components in the detection device is performed by the controller.
The company's main stainless steel water collector, tank bottom weld vacuum test box, reading instrument, eight-stage air microbial sampler, relay comprehensive tester, dual-wavelength scanner, coating thickness gauge, soil mill, tempered glass surface Flatness tester, sound sensor, portable electric water level gauge, network mouth flow meter, corrosion rate meter, portable scratch tester, freezing point tester, water quality tester, online ammonia tester, coating thickness gauge, coating Thickness gauge, soil pulverizer, digital thermometer, gas sampling pump, ceramic impact tester, automatic crystallization point tester, drug freezing point tester, reed switch tester, constant temperature water bath, gasoline root turn, gas Sampling pump, tempered glass tester, water quality tester, PM2.5 tester, inhalable particulate matter detector, high frequency heat sealing machine, strain control triaxial instrument, milk somatic cell detector, helium gas concentration detector, soil moisture conductance Rate tester, field strength meter, collection box, color ratio tester, capillary water absorption time measuring instrument, redox potentiometer vibrometer, carbon monoxide carbon dioxide detection , CO2 analyzer, oscillopolarograph, slime content tester, car starter power supply, automatic potentiometric titrator, portable thermometer, zirconia analyzer, reed switch tester, precision conductivity meter, TOC water quality analysis Instrument, microcomputer plasticity tester, wind direction station, automatic spotting instrument, soil oxidation reduction potentiometer, digital thermometer, portable total phosphorus tester, corrosion rate meter, constant temperature water bath, residual chlorine detector, free expansion rate meter , centrifuge cup, concrete saturated vapor pressure device, particle strength tester, Gauss meter, automatic film coating machine, safety valve grinding tool, weather station, kinesthetic orientation instrument, dark adaptation instrument, smell collector, rain gauge, four in one Gas analyzer, emulsion concentration meter, dissolved oxygen meter, temperature measuring instrument, thin layer planking device, temperature recorder, aging meter, noise detector, constant temperature and humidity chamber, split resistivity tester, initial viscosity and holding viscosity Tester, infrared carbon dioxide analyzer, hydrogen lamp, kinesthetic orientation instrument, constant temperature animal operating table, cooling fan, grease acid value detector, viscosity meter, colony counter, Weather station, rain gauge, Kjeldahl nitrogen analyzer, fluorescent whitening agent, the company adhering to the "customer first, forge ahead" business philosophy, adhere to the "customer first" principle to provide our clients with quality services. Welcome patrons!
Medical Cart,Med Cart,Mobile Medical Carts,Medical Trolley Cart
Hebei Boxin Rehabilitation Equipment Co., Ltd , https://www.cnsmartcare.com