Nanoparticle-based planar laser scattering was used to measure the density distribution of the supersonic (Ma=3.0) turbulent boundary layer and the optical path difference (OPD), which is quite crucial for aero-optics study. Results were obtained using ray tracing. The influences of different layers in the boundary layer, turbulence scales, and light incident angle on aero-optics were examined, and the underlying flow physics were analyzed. The inner layer plays a dominant role, followed by the outer layer. One hundred OPD of the outer layer at different times satisfy the normal distribution better than that of the inner layer. Aero-optics induced by the outer layer is sensitive to the filter scale. When induced by the inner layer, it is not sensitive to the filter scale. The vortices with scales less than the Kolmogorov scale (=46.0  μm) have little influence on the aero-optics and could be ignored; the validity of the smallest optically active scale (=88.1  μm) proposed by Mani is verified, and vortices with scales less than that are ignored, resulting in a 1.62% decay of aero-optics; the filter with a width of 16-grid spacing (=182.4  μm) decreases OPD by 7.04%. With the increase of the angle between the wall-normal direction and the light-incident direction, the aero-optics becomes more serious, and the difference between the distribution of the OPD and the normal distribution increases. The difficulty of aero-optics correction is increased. Light tilted toward downstream experiences more distortions than when tilted toward upstream at the same angle relative to the wall-normal direction.

Fine structures of supersonic flow over a 5 mm high backward facing step (BFS), including expansion wave fan, reattachment shock, supersonic boundary layer were measured in a Ma=3.0 low-noise indraft wind tunnel. By varying the superficial roughness of the wall upstream from the step, supersonic laminar flow and supersonic turbulent flow could be formed over a BFS. Measurements on the spatiotemporal features of the holistic flow field and the fine structures in four typical regions were carried out using NPLS (nano-based planar laser scattering). Flow structures, including expansion wave fan, reattachment shock, supersonic boundary layer and its separation, reattachment and redevelopment are revealed by measuring the holistic structure of the transient flow field. Comparing the two time-averaged flow fields with each other, it is apparent that supersonic turbulent flow over a BFS (STF-BFS) has a larger expansion angle and a shorter recirculation region, and its redeveloped boundary layer increases at a smaller obliquity while the angle of reattachment shock is the same for the supersonic laminar flow over a BFS (SLF-BFS). With regard to time-evolution features, the K-H vortices in the SLF-BFS suffers from shearing, expansion, reattachment and three-dimensional effects while in the STF-BFS large-scale structures are affected by the incline and distortion at the reattachment point due to expansion, viscosity and reverse-pressure. Studies on local regions indicate that in the SLF-BFS, the emergence of compression waves which distinctly converge into a reattachment shock is due to the local convective Mach number and the inducement of K-H vortices in the free shear layer. Nevertheless, in the STF-BFS, compression waves and K-H vortices are barely evident, and the formation of a reattachment shock is related to the wall compressive effect. As a typical flow with simple geometrical boundaries, flow over a backward facing step (BFS) contains many complicated structures, including separation and reattachment. Separated/reattaching flows often occur in natural environments such as air movement in valleys or over fences as well as in many man-made facilities such as airfoils with an attack angle, reentry aircrafts, diffusers, and turbo engines. However, generally speaking, separation results in negative effects on these systems, e.g. reducing manipulation of aircraft, increasing heat loss from walls, creating noise and dynamic structural loads. However, in the research field of supersonic flows, the BFS is always adopted as a typical configuration for ignition in a scramjet, where the recirculation region has an important role in stabilizing the firing of the engine. Steps on the surface of supersonic/hypersonic aircrafts make flow fields complex, and consequently appropriate research is propitious for optimizing the dynamic design of aircraft. In the past several decades, a lot of experimental and numerical work has been done on BFS flows, focusing on the flow mechanism, controlling effect and unsteadiness. To study flow...

At Mach number 3.4, visualisation experiments of flow over backward-facing step (BFS) with or without roughness band attached on upstream wall are carried out via traditional schlieren and newly developed nano-tracer-based planar laser scattering (NPLS). The time-averaged flow characteristic of the reattachment region and the instantaneous rich structures of the redeveloping boundary layer in the steamwise-normal plane are both revealed. Additionally, top views in the different planes (y/h = 0.67, 1.00, 1.33, 1.67, 2.00) are imaged with a resolution of 0.064 mm/pixel. By contrasting the NPLS images at different times, the unsteady evolution characteristic of the coherent vortices in the redeveloping boundary layer was discussed. Static wall pressure is measured by a micropressure scanning system. The incipient formation positions are pointed out statistically. Without roughness, the longitudinal structures with scales of 1.0h and 1.2h form later and distribute in a longer region compared to that with roughness. Fractal analysis is applied and the averaged fractal dimensions of the overall and sectional flow structures are calculated. If roughness is adopted, the fractal dimension will be larger and the turning point in the sectional fractal dimensions is earlier. However, the dimension tends to be one coincided value in the farther downstream.

In a Mach 3.8 wind tunnel, both instantaneous and time-averaged flow structures of different scales around a blunt double-cone with or without supersonic film cooling were visualized via nano-tracer planar laser scattering (NPLS), which has a high spatiotemporal resolution. Three experimental cases with different injection mass flux rates were carried out. Many typical flow structures were clearly shown, such as shock waves, expansion fans, shear layers, mixing layers, and turbulent boundary layers. The analysis of two NPLS images with an interval of 5 μs revealed the temporal evolution characteristics of flow structures. With matched pressures, the laminar length of the mixing layer was longer than that in the case with a larger mass flux rate, but the full covered region was shorter. Structures like K—H (Kelvin—Helmholtz) vortices were clearly seen in both flows. Without injection, the flow was similar to the supersonic flow over a backward-facing step, and the structures were relatively simpler, and there was a longer laminar region. Large scale structures such as hairpin vortices were visualized. In addition, the results were compared in part with the schlieren images captured by others under similar conditions.