Unsteadiness in hypersonic leading-edge separation

Abstract

Hypersonic leading-edge separation is studied toward understanding the varying shock-related unsteadiness with freestream Reynolds number (\(1.66 \times 10^5 \le \text{Re}_D \le 5.85 \times 10^5\)) in the newly constructed hypersonic Ludwieg tunnel (HLT) at a freestream design Mach number of \(M_\infty =6.0\). An axisymmetric flat-face cylinder of base body diameter \(D=35\) mm is fitted with axial protrusions of different fineness (\(d/D=0.1,0.2,0.26,0.34\) at \(L/D=1.4\)) and slenderness (\(L/D=0.7,1,1.4,1.9\) at \(d/D=0.2\)) ratio to induce a wide range of leading-edge separation intensities. Qualitative and quantitative assessments are made using schlieren imaging, planar laser Rayleigh scattering, and unsteady pressure measurements. A well-known to-and-fro shock motion called pulsation and a flapping shock-shear layer oscillation is observed as \(\text{Re}_D\) changes. A shorter protrusion length (\(L/D=0.7\)), associated with pulsation, produces a pressure loading four orders higher than the cases with longer protrusion lengths associated with flapping. There exists a critical separation length (\(L/D \ge 1.4\)) beyond which the separated shear layer trips to turbulence and introduces fluctuations in the recirculation region as \(\text{Re}_D\) increases. The intensity of the separated turbulent shear layer is dampened by an order of magnitude, provided the reattachment angle is shallow by increasing the fineness ratio to \(d/D \ge 0.34\). There also exists a set of critical geometrical parameters (\(L/D=1, d/D=0.2\)) for which the unsteady modes (pulsation and flapping) switch between themselves during successive runs, probably due to upstream fluctuations. From the modal analysis of the Rayleigh scattering images, the first four dominant modes that drive flapping are identified as translatory flapping, sinuous flapping, and large and small-scale shedding.