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摘要:
在附面层测量中，需对微小尺度的高速气流变化场进行瞬态测量。数字化的干涉测量方法能定量地解算出流场的密度场，是一种重要的应用。介绍了一种共路干涉的环路剪切干涉方法，对震动不敏感，无需参考面，适合附面层测量使用。采用基于空间位相调制的快速算法，配以脉冲激光器和同步控制系统，可实时地对扰流密度场进行定量测量。该系统采集分辨率200 pixels × 200 pixels，采集频率可达每秒1000帧以上。系统的波前重构方法经过计算机仿真，检测结果优于1/20λ。在0.6 m风洞对圆柱体尾部附面层进行测量试验，结果表明，在一定风速下，该系统能抑制振动干扰，显著地区分出圆柱体尾部扰流信号和振动噪声，具有良好的应用前景。
Abstract:Transient measurements of high-speed airflow field are needed in the measurement of boundary layer. Digital interferometry can measure flow field quantitatively to obtain density information, which is very necessary in flow field measurement. In this paper, a common-path shearing interferometry method is introduced. It is insensitive to vibration and does not need a reference plane. It is suitable for flow field measurement. A fast algorithm based on spatial phase modulation, coupled with a pulse laser and a synchronous control system, is used to measure the disturbance density field quantitatively in real time. The acquisition resolution of the system is 200 pixels × 200 pixels, and the acquisition frequency can reach more than 1000 frames per second. The wavefront reconstruction method of the system has been simulated by computer, and the detection result is better than 1/20λ. The experimental results in a 0.6 m wind tunnel show that the system can restrain the vibration interference and distinguish the disturbance signal and the vibration noise remarkably. It has good application prospects.
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Key words:
- radial shear interferometry /
- density field /
- flow field /
- dynamic measurement
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Overview: "Boundary layer" refers to a thin flow layer with a non-negligible viscous force close to the aircraft surface. Its thickness is only a few millimeters of the model surface in the wind tunnel test. It shows typical characteristics of small scale random disturbance, small size and fast change.
The current methods of boundary layer flow display include particle tracer method, oil flow display method and optical measurement method, but they all have their own shortcomings. Particle tracer method is to add smoke generator into the air and show the flow track by observing the density of smoke particles in the flow field. As the size of the smoke particles is much larger than that of the gas molecules, this method will change the composition of the gas. Oil flow display technology is the reaction of the flowing air in the boundary layer to the friction stress on the wall surface. The optical measurement method has no contact and can directly reflect the integral of density difference along the optical path, but the schlieren and shadow methods can only display and cannot calculate the density field quantitatively.
Based on phase difference, the density field of flow field can be calculated quantitatively. However, the traditional interferometry method based on time phase modulation cannot detect a transient change field the interference image is easily interfered due to the influence of vibration generated by high-speed airflow, which is difficult to solve, so it cannot be applied in the boundary layer measurement.
This paper proposed a measurement system based on loop radial shear interference, with high wavefront detection accuracy, good anti-noise and anti-interference performance, and suitable for use in boundary layer measurement. The system adopts the fast transform method based on spatial phase modulation, which loads the information of shear wave surface onto the carrier, and an image can quickly recover the wavefront by using the fast Fourier transform method, avoiding the influence of dynamic changes of measured wavefront and realizing real-time dynamic detection. For the complex wavefront of the boundary layer, the iterative method is used to improve the wavefront reconstruction accuracy. The simulation results show that the residual root mean square (RMS) value is better than 1/20λ. This paper introduces the realization of hardware system and software process in detail. The principle of the algorithm is also presented. The experimental results in a 0.6 m wind tunnel show that the system can restrain the vibration interference and distinguish the disturbance signal and the vibration noise remarkably. The proposed method has broad application prospects in real-time boundary layer measuring.
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图 1 剪切层气动光学效应测量原理图
Figure 1. Principle diagram of aero-optical effect measurement in shear layer
图 2 环路径向剪切干涉测量风洞流场原理
Figure 2. Principle diagram of cyclic radial shearing interferometry
图 5 第一类波前仿真。(a)原始波前；(b)扩大波前；(c)相位差波前；(d)重建波前；(e)残差
Figure 5. Simulation analysis of the first type wavefront. (a) Original wavefront; (b) Extended wavefront; (c) Wavefront of phase difference; (d) Reconstructed wavefront; (e) Residual
图 6 第二类波前仿真。(a)原始波前；(b)扩大波前；(c)相位差波前；(d)重建波前；(e)残差
Figure 6. Simulation analysis of the second type wavefront. (a) Original wavefront; (b) Extended wavefront; (c) Wavefront of phase difference; (d) Reconstructed wavefront; (e) Residual
图 8 有模型扰流时密度图(单位：kg/m3)。(a) 1.2 s；(b) 3.6 s；(c) 4.5 s；(d) 6.3 s；(e) 7.8 s
Figure 8. Density diagram with model disturbance. (a) 1.2 s; (b) 3.6 s; (c) 4.5 s; (d) 6.3 s; (e) 7.8 s
图 9 空风洞时密度图(单位：kg/m3). (a) 1.3 s；(b) 3.4 s；(c) 4.7 s；(d) 6.9 s；(e) 7.7 s
Figure 9. Time density map of an empty wind tunnel. (a) 1.3 s; (b) 3.4 s; (c) 4.7 s; (d) 6.9 s; (e) 7.7 s
表 1 多幅图像相位RMS比值对比
Table 1. Comparison of phase RMS ratios of multiple images
马赫数 信噪比 0.4 3.68 0.5 2.72 0.6 2.13 0.7 1.79 0.8 1.43 0.9 1.25 1.0 1.09 
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