RESEARCH PROGRESS OF LASER DIAGNOSTICS FOR VELOCIMETRY IN HYPERSONIC FLOWS

Coupling effects such as strong turbulence and turbulent fluctuation, boundary layer transition, shock wave-boundary layer interaction, and high-temperature real gas effects can exist in internal and/or external flows of vehicles under hypersonic flow conditions. Characterization of unsteady flow phenomena induced by these coupling effects becomes much more complicated with an increase in flight Mach number, which can have a significant impact on the performances of the hypersonic vehicle’s aerodynamic force, aerodynamic heat, and aero-optical effects, it is, therefore, a cutting-edge research area in hypersonic flows. Velocity is one of the most critical parameters to characterize flow processes, its field is also widely used to characterize the spatial distribution of kinetic energy, which accounts for most of the total energy of hypersonic flow fields. Hence, accurate velocity measurement is of importance to an in-depth understanding of the complex flow-transport mechanisms and design of hypersonic vehicles. So, the main objective of this paper is to review commonly-used, non-intrusive laser-based diagnostic technologies developed for velocimetry in hypersonic flow fields, such as particle imaging velocimetry (PIV) based on spatial method; Doppler-shift velocimetry with laser absorption spectroscopy (LAS), laser-induced fluorescence (LIF) and Rayleigh scattering (RS); molecular tagging velocimetry (MTV) based on time-of-flight method; and focused laser differential interference (FLDI) based on the gradient of refractive index. First, the basic principles of these laser-based diagnostic technologies for velocimetry are briefly introduced. Typical applications are then presented for those laser-based diagnostic technologies to determine velocity distributions and corresponding fluctuations in freestream, laminar/turbulent boundary layer, shock wave-boundary interaction, wake and/or other complex flow regions of hypersonic flows. Furthermore, the adaptabilities of test environment, limitations and challenges of these laser-based technologies are further discussed as well. Last, laser-based diagnostics are summarized, and the potential trends are also proposed in practical velocimetry of hypersonic flows.