Influences of Steps on the Hypersonic Boundary-Layer Transition on a Cone

Abstract

The influences of steps on the hypersonic boundary-layer transition on a cone are investigated in a hypersonic quiet wind tunnel, using high-frequency pressure sensors and nano-tracer-based planar laser scattering (NPLS) techniques. The high-frequency surface pressure fluctuations are analyzed using power spectral density analysis, band-pass filtering, and cross-correlation calculation. Quantitative analysis of the NPLS images is carried out based on the grayscale information and time interval. The results demonstrate that the second-mode wave amplitude increases initially and then drops along the streamwise, whereas the characteristic frequency decreases monotonously. However, the second-mode wave amplitude in the 0.4 mm ( $0.5{\delta }_{\mathrm{s}}$ ) and 0.6 mm ( $0.75{\delta }_{\mathrm{s}}$ ) forward-facing step models is constantly rising, and the obvious second-mode wave first appears closer to the downstream than the smooth cone, which suggests that the forward-facing steps could suppress the second-mode disturbances and shifts the transition downstream. Nonetheless, there is only a clear promotion effect when the height of the forward-facing step is 1.6 mm ( $2{\delta }_{\mathrm{s}}$ ). In addition, the backward-facing step could destabilize the boundary layer and shift the transition upstream, and this effect is enhanced by increasing the step height.