Visualisation on supersonic flow over backward-facing step with or without roughness

Abstract

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.

Low-frequency unsteadiness mechanisms in shock wave/turbulent boundary layer interactions over a backward-facing step

“…These Görtler-like vortices were also reported in the experimental visualization via nano-tracer-based planar laser scattering (Zhu et al. 2015). In addition, small unsteady shedding vortices along the shear layer were identified by Chen et al.…”

“…The wavelength of these vortices is equal to two or three times the boundary layer thickness for a wide range of Mach numbers. These Görtler-like vortices were also reported in the experimental visualization via nano-tracer-based planar laser scattering (Zhu et al 2015). In addition, small unsteady shedding vortices along the shear layer were identified by Chen et al (2012) using the same visualization techniques.…”

Dynamics of a supersonic transitional flow over a backward-facing step

“…Behind the reattachment point, the distance between the sonic line (white dashed line) and wall decreases with the streamwise distance as a result of the increasing velocity gradient near the wall, which indicates the evolution towards a fully turbulent boundary layer. The reattachment length L r = 3.6h is much smaller than the typical values reported for incompressible cases (e.g., 19.8h [11]) due to the more energetic mixing of the shear layer with the increasing Reynolds number. This interpretation is in accordance with the finding that the reattachment length is reduced with the Reynolds number in the transitional regime [30].…”

“…However, the convection of the K-H vortices is faster in the supersonic flow, which makes it more likely for the high-speed part of the upstream vortices to reach the slow part of the following vortices and to promote the mixing of the shear layer. Existing experimental results have confirmed the behavior of the vortex pairing in the supersonic 103904-8 laminar flow [19]. However, if the separation length is too short, like in supersonic turbulent flow, there is not enough resident time for K-H vortices to grow and pair within the shear layer.…”

“…Therefore, it is reasonable to conjecture that a different mechanism may contribute to the transition process in the supersonic case. Yi et al [18,19] inferred from their flow visualization that the unsteadiness of the separation system depends on compressibility and K-H vortices of the shear layer rather than the restriction effects of the wall or the reattachment events. Additionally, a broadband oscillation dynamics was also found in recent experiments with low-level turbulent inflow using particle image velocimetry (PIV) [20].…”