This paper addresses the difficulties in achieving full-field and real-time measurement of the plasma temperature field evolution process produced by the electric explosion of metal bridge foils, both domestically and internationally. The interferometric method is adopted to measure the plasma temperature field evolution process. This method is widely used in industrial applications because it is highly efficient, high in precision, and can achieve dynamic real-time full-field measurement. Firstly, an experimental laser interferometer measurement system is established based on the Mach-Zehnder interferometric measurement system. Different physical images of the plasma at different times are obtained through experiments. The changes in the interference pattern generated in the plasma are analyzed to obtain the refractive index distribution of the plasma. A model for the refractive index and plasma temperature distribution is then established. The rationality and applicability of this model are theoretically analyzed and discussed, from which the temperature of the plasma produced after the metal bridge foil electric explosion can be deduced. The experimental results show that during the metal bridge foil electric explosion, the highest temperature in the entire flow field concentrates in the center of the plasma, reaching up to 7000K. There are also multiple temperature gradients, which remain continuously between 280 and 8000K.
Based on the photon Doppler velocimetry technology, this paper studies high-speed targets' three-dimensional (3D) velocimetry technology, reproduces the movement trajectory of high-speed targets through 3D coordinates and broadens the photon Doppler velocimetry technology application scope. First, design the multiplexed photonic Doppler velocimetry optical path structure and analyze the multiplexed photonic Doppler velocimetry principle. Then, the target 3D velocity extraction method is studied, including the fiber optic probe's spatial arrangement and the calculated method of target motion trajectory. Finally, the detection sensitivity of velocity in the direction of the 3D coordinate axis is analyzed. In order to verify the reliability of 3D velocimetry technology, the paper is described in two parts: software simulation and experiment. MATLAB is used to simulate the principle of 3D velocimetry technology. The simulation results show that 3D velocimetry technology can be realized by processing Doppler frequency shift voltage signals. In the experimental part, the high-speed target is detected through fixed-angle optical fiber probes, and four sets of Doppler frequency shift voltage signals are collected. After data processing, the target's 3D velocity is extracted, and the target's motion trajectory is reproduced. Therefore, this paper provides an idea for reproducing the target's motion trajectory by obtaining the high-speed target's 3D velocity.
When detecting ultrahigh speed micro targets, because the photon Doppler signal will be seriously interfered with by high-frequency noise, it is difficult to accurately extract the beat frequency information, resulting in serious speed demodulation error. Therefore, we propose a beat frequency extraction method for high-speed and high-noise Doppler signals based on double correction, which can accurately correct the beat frequency signals of high-frequency noise interference. First, the ratio correction method is used for the first spectrum correction to screen out the normalized spectrum of high-frequency noise interference; then, according to the degree of noise interference, the energy barycenter correction method or linear interpolation method is used for the second spectrum correction. Numerical simulation results show that this method can reduce the spectral error by 1.6% to 2.1%. We used this method to process the Doppler signal from the detonation small flyer experiment in Initiating explosive device testing. The results show that it can reduce the beat frequency error by 1.83 times at most under high-frequency noise interfered, equivalent to the instantaneous velocity error of 1955 m/s can be decreased after velocity demodulation. Therefore, the beat frequency extraction method based on double correction has good noise resistance and reliability. We corrected the different regions’ frequency spectrums according to the noise interference intensity, which provides an idea for the beat frequency extraction of high-speed and high-noise Doppler signals.
Schlieren technology can realize the visualization of flow field testing. The transmission schlieren device as a model, a laser light source with Gaussian distribution is used to pass through a cylindrical area with uniform refractive index distribution to obtain a simulated schlieren image and a flow field visualized diffraction fringe. Based on the theory of Fourier optics and applying Fresnel diffraction optics technology, the Fourier characteristics of the lens are derived, and the light field distribution at the focal plane of the lens is calculated. The Heaviside step function is combined to express the light field distribution on the back surface of the knife-edge device, and then the light field intensity expression of the imaging plane is obtained. The results show that the transmission schlieren method can obtain a high imaging quality light field intensity distribution map containing the parameters of the test object. When the cutting position of the knife edge is half of the light field spectrum surface, the imaging plane display effect is the best. In the comparative experiment, when the knife-edge device cuts the spectrum surface 2/5~1/2, the test object parameter information contained in the light field intensity map is the most complete. The intensity of the part that is shielded by the knife edge device is 0, and the intensity of the unshielded part is constant, which is consistent with the performance result of ray optics. This algorithm can provide theoretical basis and experimental reference for the visualization of typical flow field.
The measurement and visualization of transient three-dimensional (3-D) physical parameters (density and temperature) distribution of complex flow fields are critical technologies for the characteristics studies of flow fields in modern energy engineering. Among the optical computed tomography (OCT) methods, Moiré tomography has the advantages of simple optical path structure, strong anti-interference ability and wide measurement range, which is especially suitable for complex flow field measurement in noisy environments. Acquiring the transient phase information from the moiré projection is of great importance for the dynamic 3-D parameters reconstruction of complex flow fields. In this paper, the dynamic phase retrieve methods including Fourier and spatial phase-shifting in moiré tomographic are studied, respectively. In the Fourier method, an adaptive first-order spectrum extraction algorithm for Fourier transform moiré fringe and a phase calculation method are proposed. Through this, the projection phase can be obtained directly by multiplying the inverse Fourier transform of the positive first-order spectrum of deformed fringe with the inverse Fourier transform of the negative first-order spectrum of reference fringe. In spatial phase-shifting method, a spatial phase-shifting- interferometry-based moiré volume computed tomography (MVCT) method was proposed to extract the radial shearing phase distribution of grid moiré fringe. The measured results for the first-order partial derivative of the phase projection of a propane flame both by Fourier and spatial phase-shifting methods in the experimental moiré computed tomography systems are presented. The research will be valuable for monitoring the combustion state in energy engineering.
Researching the combustion process is important core contents in aviation, aerospace and energy engineering. And it has an important significance for the combustion researches to measure the physics parameters by combustion diagnosis and to acquire the 3D transient combustion distribution. Optical computerized tomography (OCT) applied in combustion diagnosis plays an important role in flow field diagnosis for its advantages of non-contact, real time, and 3D full-field measurements. The essence of CT is reconstructing the test field with projections from different directions. And the projections are independent to each other. Therefore, a multi-direction projections calibration should be performed before CT reconstruction, which can remap different projections to a unified coordinate to reduce the system errors from the installations and optical elements. Aiming at this problem, in this project, a multi-direction calibration method based on affine projection is proposed. By studying the affine projection theory, the calibration model was build based on single directional moiré tomographic system. Furthermore, the corresponding algorithm based on the calibration model is proposed. Finally, a multi-directional moiré tomographic system is built and combining with the calibration results for remapping the multi-directional projections, the 3D temperature distribution of the propane combustion flame was reconstructed.
Flame chemiluminescence tomography (FCT), which combines computerized tomography theory and multidirectional chemiluminescence emission measurements, can realize instantaneous three-dimensional (3-D) diagnostics for flames with high spatial and temporal resolutions. One critical step of FCT is to record the projections by multiple cameras from different view angles. For high accuracy reconstructions, it requires that extrinsic parameters (the positions and orientations) and intrinsic parameters (especially the image distances) of cameras be accurately calibrated first. Taking the focus effect of the camera into account, a modified camera calibration method was presented for FCT, and a 3-D calibration pattern was designed to solve the parameters. The precision of the method was evaluated by reprojections of feature points to cameras with the calibration results. The maximum root mean square error of the feature points’ position is 1.42 pixels and 0.0064 mm for the image distance. An FCT system with 12 cameras was calibrated by the proposed method and the 3-D CH* intensity of a propane flame was measured. The results showed that the FCT system provides reasonable reconstruction accuracy using the camera’s calibration results.
Moiré tomography has been considered as an effective tool in studying flow fields because of its advantages such as non-contact measurement, strong anti-disturbing capability, and wide measurement range. The spatial phase-shifting method, which can simultaneously obtain several phase-shifted interferograms, can be applied in the flow field measurements by the moiré tomography when the flow field varies rapidly. In this paper, we present a new spatial phase-shifting shearing interferometry. The optical structure of the interferometry, which only consists of a crossed grating and a linear grating, is very simple. With it six phase-shifted interferograms can be acquired simultaneously. Based on the scalar diffraction theory, the explicit forms of intensity distribution of the interferograms containing the phase information can be derived and a corresponding four-step phase-shifting algorithm is proposed to extract the first-order partial derivative of phase projection from the interferograms. Finally, the spatial phase shifting optical system is used to retrieve the first-order partial derivative of propane flame phase projection produced by plane incident wave. This work is crucial to accuracy reconstruction the physical parameter of the varied flow fields in moiré tomography.
Volume optical computerized tomography (VOCT), which can realize real 3D measurement rather than traditional 2D
OCT, has great superiority in quantitatively measuring the thermo physical parameters of transient flow field. Among the
refractive index reconstruction techniques, filtered back-projection (FBP) method performs better than algebraic
reconstruction techniques (ARTs) with higher accuracy and computationally efficient. In order to apply FBP to VOCT,
the radial second-order derivative of projection wave front passes through the tested phase object should be obtained
firstly. In this paper, a projection device with two circular gratings is established. In particular, owing to an inherent
phase shift exists between moiré fringes of +1 and -1 diffraction orders, a two-step phase-shifting algorithm is utilized to
extract the wave front’s radial first-order derivative which is contained in the moiré fringes. The reliability of the
two-step phase-shifting algorithm is proved by a computer simulation. Finally, the radial first-order derivative of wave
front passing through a propane flame is measured and retrieved by these methods.
Moiré tomography is an important technique to diagnose the flow field. However, the traditional moiré deflectometry cannot meet the requirements of Volume Moiré Tomography (VMT). In this Letter, an improved moiré deflected system based on double orthogonal gratings is introduced for real 3-D reconstruction. The proposed method could obtain the first-order partial derivatives in two vertical directions of the projection in one time. Comparing with the traditional moiré deflectometry, the proposed system is more effective and easier to realize the multi-direction data acquisition.
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