Measurement of flow density field by cyclic radial shearing interferometer

Pu Hongyu; Li Dahai; Luo Peng; Zhang Chen

doi:10.12086/oee.2020.190390

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Pu H Y, Li D H, Luo P, et al. Measurement of flow density field by cyclic radial shearing interferometer[J]. Opto-Electron Eng, 2020, 47(4): 190390. doi: 10.12086/oee.2020.190390

Citation:

Pu H Y, Li D H, Luo P, et al. Measurement of flow density field by cyclic radial shearing interferometer[J]. Opto-Electron Eng, 2020, 47(4): 190390. doi: 10.12086/oee.2020.190390

Measurement of flow density field by cyclic radial shearing interferometer

1.
School of Electronics and Information, Sichuan University, Chengdu, Sichuan 610065, China
2.
High Speed Institute, China Aerodynamics Research and Development Center, Mianyang, Sichuan 621000, China

Fund Project: Supported by National Natural Science Foundation of China (11732016, 11402286)

More Information

^*Corresponding author: Li Dahai, E-mail:lidahai@scu.edu.cn

Received Date 08 July 2019

Revised Date 04 November 2019

Published Date 01 April 2020

Abstract

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.
- radial shear interferometry /
- density field /
- flow field /
- dynamic measurement

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References

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Overview

Overview

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.