An experimental study on the density characteristics of a zero-pressure-gradient flat plate turbulent boundary layer at Ma=3.0 is performed by the density field measurement method based on Nano-tracer planar laser scattering (NPLS) technology. The mean and the fluctuating characteristics of the density field of the boundary layer are analyzed. And the spectrum analyses of density fluctuations are performed by utilizing Taylor's hypothesis to convert spatial measurements into pseudo-temporal measurements. The mean density profile increases away from the wall, which accords well with the density profile deduced from the mean velocity distribution by using the adiabatic Crocco-Busemann relation. The root mean square (RMS) of the density fluctuations increases in the logarithmic region with a peak value of 0.2ρ∞, and its probability density distribution follows a normal distribution. However, the RMS of density fluctuations decreases in the outer region of the boundary layer. According to the spectrum analysis, the density fluctuations are characterized in a wide range of frequencies throughout the boundary layer, with the maximum frequency on the order of 1 MHz. The low frequency fluctuations are predominant near the wall and in the outer region of the turbulent boundary layer. However, the proportion of high-frequency fluctuations is nearly equal to that of low-frequency fluctuations in the logarithmic region. The combined NPLS and PIV technique provide a simultaneous density and velocity measurements of the present turbulent boundary layer. The high frequency fluctuations in the supersonic turbulent boundary layer may be induced by the density fluctuations, which are caused by the convection of the turbulent structures with nonuniform density distributions. And the contribution of the velocity fluctuations only to the low frequency fluctuations is observed. There are good similarities between the density fluctuations and the mass flux fluctuations for both the probability density distribution and the spectrum characteristics. On the contrary, a large difference between the fluctuations of velocity and density is identified. Therefore, the strong density fluctuations inside supersonic turbulent boundary layers, as well as its difference between the velocity fluctuations, should be one of the most important differences between compressible and incompressible turbulent boundary layers.

Studying the aero-optical effects induced by turbulent structures with different scales helps determine the capture scale of turbulent structures in experiments/calculations and improve the turbulence breakup device. In this paper, the density field of a supersonic turbulent boundary layer at M a = 3.0 was measured based on the nano-tracer plane laser scattering technique. Two-dimensional orthogonal wavelet multi-resolution analysis was applied to obtain information about different flow scales. The ray-tracing method simulates the propagation of a Gaussian plane beam through the nonuniform flow field at different resolutions. The results show that the turbulent boundary layer thickness and its calculation method lead to the difference in scaling calculation results among the existing experiments. The turbulent structures about 0.7 δ contribute most to aero-optical effects. With the reduction of the resolution, the contribution of small turbulent structures to aero-optical effects reduces obviously. When the minimum scale of turbulent structures captured is larger than 0.072 δ , the resolution can no longer reflect the real aero-optics results of turbulent structures. The smallest optically active scale predicted is 0.017 δ in Mani’s theory. The turbulent structures smaller than 0.018 δ have little effect on optical path difference (OPD), and the higher-order quantities change significantly around 0.009 δ ∼ 0.018 δ . According to experimental results, it is promising to improve the aero-optical suppression effects by breaking the large eddy into the turbulent structures smaller than 0.018 δ , or even 0.009 δ .