Signal-enhanced and bi-directional interferometric Rayleigh scattering velocimetry using an asymmetry cavity

Interferometric Rayleigh scattering technique is commonly employed to measure single-point velocity fluctuation and its standard deviation in a high-speed flow due to many benefits, such as high accuracy, easy data interpretation, and high sampling rate. However, this technique suffers from a severe problem often referred to as the weak Rayleigh scattering signal, especially in the supersonic and hypersonic flow with an extremely low gas molecule density. An asymmetry cavity structure that could cost-effectively improve the Rayleigh scattering (RS) signal of interest is designed and used in the interferemetric Rayleigh scattering technique. The ZEMAX simulations suggest that the parallel beam can be repeatedly reflected in the resonant cavity and can be focused in a measurement region with the order of 0.67 mm×1.31 mm. The number of propagating rays inside the cavity can reach about 50. The fidelity of this proposed cavity is then verified by the Rayleigh scattering imaging experiments. Results show that this cavity allows the laser beam to reflect several times in the resonant cavity, and the RS signal intensity in the major axis can be 10.4 times larger than that of the incident laser. The cavity is finally employed under realistic supersonic flow velocity measurements, where the results conclusively illustrate that the Rayleigh scattering signal of interest in a single direction can be improved by a factor of 4∼5. In addition, the bi-directional (both the axial and radial directions) velocity parameters can also be obtained simultaneously. The axial velocity and its standard deviation are similar to conventional single-line ones.