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 micro-pressure 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.

Keywords:

• supersonic
• visualisation
• backward-facing step
• flow structures
• flow control

Acknowledgements

Great thanks to Prof. Johan Revstedt in LTH of Lund University for his suggestions on the draft.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

Financial support for this project was provided by the National Natural Science Foundation of China [grant number 11172326], [grant number 11302256]; Hunan Provincial Innovation Foundation for Postgraduate [grant number CX2013B002] and Innovation Fund Program for Standout Graduate Students of NUDT [grant number B130103].