Background Oriented Schlieren Method as an Optical Method to Study Shock Waves

Background oriented schlieren method is applied in diagnostics of shock waves in air. The method can be used for visualization of shock waves that are generated after explosion or due to motion at ultrasonic speeds. Experimental data make it possible to observe propagation of a shock wave in space, estimate the asymmetry of energy liberation in explosion, and determine parameters of shock wave.     

Reconstruction of the density field using the Colored Background Oriented Schlieren Technique (CBOS)

In this paper the improved Background Oriented Schlieren technique called CBOS (Colored Background Oriented Schlieren) is described and used to reconstruct density fields of three-dimensional flows. The Background Oriented Schlieren technique (BOS) allows to measure the light deflection caused by density gradients in a compressible flow. For this purpose the local image displacements of the image of a background pattern observed through the flow is used. In order to increase the performance of the conventional Background Oriented Schlieren technique, the monochromatic background is replaced by a colored dot pattern. The different colors are treated separately using suitable correlation algorithms. Therefore, the accuracy and the spatial resolution can be increased. A tomographic reconstruction method is then used to reconstruct the density field in three-dimensional flows from CBOS measurements.     

Sensitive high-resolution white-light Schlieren technique with a large dynamic range for the investigation of ablation dynamics

We developed a modified Hoffman contrast technique with a 12 ns pulsed incoherent extended white-light source that enables an easily interpretable visualization of ablation plumes with high resolution, a large dynamic range, and color information. By comparison, a conventional dark-field setup with a slitlike laser light source provides large sensitivity but a small dynamic range, and it is difficult to interpret the filtered images.     

Schlieren method with narrow screen for electron density determination in a plasma

The modification of schlieren method using a narrow screen with changeable width in focus and the electron density calculations in current sheath inhomogeneities and in current sheath compression by choosing the simple models of electron density are described.     

大型彩色纹影系统像面均匀性分析

采用无大口径窗口玻璃光路系统设计了Φ1 200mm口径彩色纹影系统,并建模分析了该大型彩色纹影系统在重力、温度、气体冲击等复杂工作状况下的纹影像面照度均匀性.针对2.0mm切1/4、2.0mm切1/2、0.5mm切3/4这3种典型狭缝宽度和刀口切割量的组合,分别分析了该系统在理想状态下和考虑系统综合误差时系统像面照度的均匀性.结果表明:在考虑系统综合误差时,纹影像面均匀性较理想情况降低,在较低灵敏度下,整个像面照度不均匀性范围为-2.44%~2.45%;在中等灵敏度下,不均匀性范围为-3.43%~3.45%;在较高灵敏度下,不均匀性范围为-5.76%~5.81%,说明该系统能满足实际工程应用要求.     

Temperature measurement of air convection using a Schlieren system

In this work, we have developed a procedure for full-field measurement of temperature of a fluid flow by using the schlieren technique. The basic idea is to relate the intensity level of each pixel in a schlieren image to the corresponding knife-edge position measured at the exit focal plane of the system. The method is applied in the measurement of temperature fields of the air convection caused by a heated rectangular metal plate (7.3 cm×12 cm). Our tests are carried out at plate temperatures of 50 °C and 80 °C. To validate the proposed method, the schlieren temperature results are compared to those obtained by a thermocouple. Thermocouple data are obtained along two mutually perpendicular directions (one direction along the optical axis, z-direction, and other direction along the x-axis, which is perpendicular to the optical axis) at points located on a 9×9 grid with a variable spacing. The thermocouple measurements were integrated along the z-axis in order to be compared with the measurements obtained by the schlieren technique. The results from the two methods show good agreement between them.     

Schlieren visualization of ultrasonic wave fields with high spatial resolution

The visualization of ultrasonic wave fields in optically transparent liquids using the acousto-optic interaction is a well proven tool for the experimental investigation of wave propagation including wave field interaction effects with certain discontinuities and obstacles like reflection, refraction, and diffraction effects as well as for transducer testing and design. For high resolution visualization of wave fields including pulsed waveforms, pulsed light sources and sensitive optical imaging sensors with certain specifications are needed. In this paper the technical requirements of optical and electronic components for high resolution visualization of ultrasound wave fields will be presented. Also, specifications and operation results of a new designed, inexpensive Schlieren optical system will be presented, which is capable of pulsed wave field visualization in the MHz frequency range. The spatial resolution is high enough, not only for accurate beam shape and wave pattern visualization, but also for a gray-scaled display of wave amplitudes including amplitude zero crossings in ultrasound pulses. Consequently, ultrasonic wavelengths can be visualized quantitatively as well as wavelength changes of the ultrasound pulses while traveling through transparent media with different sound velocities. Results to be presented will include 2 MHz and 10 MHz experiments using single transducers as well as linear arrays of commercial medical scanners during their standard operation showing the system beamforming characteristics.     

Schlieren diagnostics of the plasma focus using a mode-locked dye laser

By means of schlieren techniques the discharge characteristics of a 1 kJ plasma focus device were investigated. Different inner electrodes were used with diameters ranging from 4 mm to 10 mm and the working gases were hydrogen and neon. A spatial resolution in the m region and a temporal resolution in the ps region could be achieved using a mode-locked dye laser as schlieren light source. During the run-down and collapse phase of the discharge the spatially and temporally resolved electron density was determined from the schlieren pictures by means of numerical simultations of the beam deflection by the plasma. During the focus phase the temporal development of the plasma could be observed.     

Quantitative Schlieren measurements of a high energy electromagnetic transducer acoustic shock field

Schlieren photography has been used to analyse quantitatively the acoustic field of an electromagnetic acoustic transducer (EMAT). By measuring the angle through which the rays are refracted it is possible to compute the refractive index gradient and thus determine both the absolute and complex pressure related structures of the images. Using this method, planar and focused shock transients generated by the EMAT have been evaluated and compared with transducer derived pressure measurements. The peak pressure in the unfocused shock was found to be 3.2 MPa and 4.6 MPa for the Schlieren and piezoelectric transducer measurements respectively. Corresponding values for the focused shock-wave agreed to within experimental error at about 19 MPa.     

Influence of high-order dispersion on self-focusing. I. Qualitative investigation

The self-focusing, described by the nonlinear Schrödinger equation with a higher-order derivative term, appearing from dispersive corrections, is considered. A qualitative investigation shows that this term, even if it is small, may play an important role in the final state of the self-focusing. Depending on the sign of a coefficient before this term, it may lead either to a tunneling of the self-trapped radiation, which finally results in defocusing, or to a steady homogeneous wave beam.     

双频光栅纹影剪切干涉法对温度场的诊断

用双频光栅纹影剪切干涉法对无限热平板的自然对流温度场进行诊断。纹影系统光线从轴外入射造成的系统象散、彗差对剪切干涉图的系统误差可以通过减小入射角、增大纹影仪球反镜的曲率半径以及调节双频光栅的位置控制在给定精度范围内。     

Schlieren visualization of micro gas turbine exhaust plume with different shapes of nozzle

Journal of Visualization - Schlieren visualization of the plume ejected from the microgas turbine nozzle was conducted to understand infrared signal characteristics of various shapes of the exhaust...     

Schlieren imaging of nano-scale atom-surface inelastic transition using a Fresnel biprism atom interferometer

Surface-induced exo-energetic inelastic transitions among atomic Zeeman states in a magnetic field (“van der Waals – Zeeman” transitions) are useable as tuneable beam splitters. A transversally coherent atom beam impinging a pair of opposite surfaces (e.g. 2 edges of a slit or of an ensemble of periodic slits) gives rise to two coherently diffracted wave packets. Within the wave packet overlap, non-localised interference fringes of the Young-slit type are predicted. From the diffraction pattern observed in the Fraunhofer regime (Schlieren image), detailed information about the transition amplitude on a scale of a few nanometers should be derived.     

High-speed visualization of cavitation evolution around a marine propeller

Journal of Visualization - Nowadays, a lot of types of digital high-speed video camera are available with a wide range of image rates. The new camera technology has made it possible to observe the...     

Stereoscopic PIV measurements of flow around a marine propeller

A stereoscopic PIV (Particle Image Velocimetry) technique has been employed to measure the 3 dimensional flow structure of turbulent wake behind a marine propeller with 5 blades. The out-of-plane velocity component was measured using particle images captured simultaneously by two CCD cameras installed in the angular displacement configuration. 400 instantaneous velocity fields were acquired for each of four different blade phases of 0°, 18°, 36° and 54°. They were ensemble averaged to investigate the spatial evolution of propeller wake in the near wake region up to one propeller diameter (D) downstream. The phase-averaged velocity fields show clearly the viscous wake formed by the boundary layers developed along both surfaces of the blade. Tip vortices were generated periodically and the slipstream contraction occurs in the near-wake region. The out-of-plane velocity component has large values at the locations of tip and trailing vortices. With going downstream, the axial turbulence intensity and the strength of tip vortices were decreased due to the viscous dissipation, turbulence diffusion and blade-to-blade interaction. The difference in the mean velocity fields measured by SPIV and 2-D PIV methods was about 5% ≈ 10%. However, the 2-D PIV results also give sufficient information on propeller wake beyond the region of X/D=0.2.