Measurement of flow field density of wind tunnel test instrument

Optical instruments are commonly used in wind tunnel to display and measure the flow field, such as shadow instrument, schlieren instrument and Mach-Zehnder interferometer (M-Z interferometer for short). An optical instrument for observing the change rate of inhomogeneity of transparent media by using the principle of shadow method. In wind tunnel or ballistic target, it is often used to observe the position and shape of the density gradient change of the flow field when the model and gas are moving relative to each other. As shown in Figure 2, the collimator L turns the divergent light of the point light source S into parallel light and emits it, which passes through the experimental part D and reaches the screen Q. If the flow field density gradient in D is zero or uniform, the parallel light will not deflect or deflect in the same direction (dεy is the same), and the illumination on the screen Q will be uniform; If the gas density in D changes unevenly in the flow field, the deflection of light passing through the flow field will be different, some light will converge and some light will diverge, and a shadow image with different light and dark will appear on the screen, reflecting the change of gas density gradient in the flow field. The second derivative of density perpendicular to the direction of incident light in the same flow field is proportional to the integral value of the product of distance to the screen. If the deviation of light in the disturbance range of flow field can be ignored, the above relationship can be used for quantitative analysis. But it can only be used for qualitative analysis. There are two kinds of shadow instruments made by the principle of shadow method: parallel light column type and divergent light cone type. The point light source often uses electric spark or laser pulse light source, and the photosensitive film is placed in the Q area of the screen, which is recorded or imaged by the optical system. Shadow method has simple equipment and intuitive image, and can obtain clear images of surrounding shock waves and vortices in the wake when the model moves at high speed relative to the air. The position of the boundary layer transition zone and the flow in the turbulent zone can also be observed.

Fig. 2 Shadow method schematic diagram and experimental photos An optical instrument for determining the density gradient in a transparent medium by observing the change of refractive index in an inhomogeneous transparent medium and converting it into the change of illumination on the recording plane. In the wind tunnel experiment, the density change of the flow field around the model is displayed by schlieren instrument, and the area and position of shock wave, expansion wave, boundary layer and wake are observed. The word schlieren comes from German, which means that grooves appear in transparent materials due to impure components. In 1859, J.-B.-L. Foucault proposed to use the knife edge as the diaphragm to inspect the quality of optical parts. 1886a.j.i. Toppler observed schlieren with an optical system for the first time, and studied the flow phenomena such as sparks and explosions. Schlieren method is sometimes called the Toppler method. As shown in Figure 3, the light source S (usually a slit) is imaged on the knife-edge plane K, and the objects in the experimental part are imaged on the screen Q through the mirror M2 and the photographic objective L. When the medium in the experimental section is uniform, a single light source image is formed on the knife-edge plane, and the illumination on the screen is uniform. When the medium density in the local area of the experimental cross section is uneven, the light passing through this area will be deflected, and the deflection angle is proportional to the refractive index gradient. A deviated light source image is formed on the blade plane, and the illumination of the corresponding area on the screen changes. The change of illumination is directly proportional to the integral value of the first derivative of refractive index change perpendicular to the knife edge direction along the optical path in the medium. According to the relationship between refractive index and density of gas medium, the density gradient of medium can be obtained. In wind tunnel experiments, schlieren is usually used to display the qualitative flow field. Color interference schlieren can be obtained by using color ribbon, grating and polarizing prism, which improves sensitivity and is suitable for quantitative research. The schlieren instrument can take high-speed schlieren photos by combining high-speed photography and microscopy. The laser light source used in schlieren can not only shorten the exposure time and obtain high-speed transient schlieren, but also form a holographic system to "freeze" the time and space of the experiment and reproduce it for quantitative study of three-dimensional space.

Fig. 3 Schematic diagram of schlieren optical path

M-z interferometer

An optical instrument that uses the coherence principle of light to determine the refractive index value in a transparent medium. It can be used to measure the local density change of flow field in wind tunnel experiment. 1878, E. Mach used Yaman double-mirror double-beam interferometer to study gas dynamics. 189 1 and 1892, respectively, Zendel made four-mirror double-beam interferometers, that is, M-Z interferometers commonly used in wind tunnels. As shown in fig. 4, the light emitted by a monochromatic point light source 1 is collimated by a beam splitter 3 and divided into a reference beam (through 3, 4 and 7) and an experimental beam (through 3, 5 and 7). When the experimental beam passes through the experimental part 6 from the outside at different densities, the speed changes and a phase shift occurs. When it encounters a reference beam in space, it will interfere. When the experimental cross-section density is uniform, straight interference fringes are formed; When the density is uneven, the stripes will bend. The relative displacement of fringes is proportional to the change of refractive index. According to the shape and spacing of interference fringes, the refractive index of corresponding points on the experimental section can be accurately obtained, and the density distribution can be calculated accordingly. In the case of isentropic flow, the pressure and velocity distribution of the flow field can be obtained by measuring the parameters of the gas flow at rest. In the case of plasma, the electron density and its change can be measured quantitatively. M-Z interferometer needs high-quality optical elements and precise adjustment mechanism, which is technically difficult. Since 1967, the application of M-Z interferometer in wind tunnel has gained new vitality with laser as light source.

Fig. 4 M-Z interferometer