Experimental Investigations of Roughness Effects on Transition on Blunt Spherical Capsule Shapes

Numerical Investigation of Roughness Effects on Transition on Spherical Capsules

“…It features a similar R a value as the surface with the standard IR coating but has a roughness structure that is well defined, reproducible, and also amenable to numerical studies. The roughness patch triggered laminar-turbulent transition in recent experiments in the HLB at unit Reynolds numbers that are similar to those required in the case of the standard IR coating, but the onset of transition depends on the position of the capsule model in the wind-tunnel test section [16]. If the capsule model with increased roughness height due to the patch is positioned such that its stagnation point region is located closer to the centerline of the test section, transition is observed for Re∕l ⪆ 15 × 10 6 ∕m.…”

“…To investigate the effect of micron-sized roughness on the capsule boundary layer, DNS of the supersonic flow around the HLB capsule and the corresponding hemisphere are performed considering finite patches of distributed roughness. The roughness patch geometry for these simulations is deduced from capsule experiments in the HLB [16] and will be quantified subsequently. The generic roughness mimics the Nextel Velvet Coating that allows for infrared surface temperature measurements and showed to have a distinct influence on transition [8].…”

“…V, the results of DNS are presented that attempt to replicate the conditions of the HLB capsule experiment with the specifically tailored roughness patch. The experimental results for the HLB and TAMU capsules among others are discussed in further detail in a companion paper by Radespiel et al [16].…”

“…Numerical studies on modal disturbance growth in the wake of discrete roughness elements at unit Reynolds number conditions of the experiment showed that roughness element heights well above the mean roughness of the IR coating are required for the onset of a noteworthy modal disturbance growth in the wake flow [17,18]. For further studies of the distributed surface-roughness effects, a specifically designed micron-sized surface-roughness patch of 20 mm × 20 mm was fabricated and placed at the center of the capsule forebody [16], that is, at some distance downstream of the stagnation point region. The patch consists of uniformly spaced rectangular micron-sized roughness elements.…”

“…Additional experimental studies [8,15,16] on laminar-turbulent transition in the boundary layer of a blunt Apollo-like capsule were performed at Mach 5.9 in the Hypersonic Ludwieg tube at the Technische Universität Braunschweig (HLB) in the unit Reynolds number range of Re∕l ≈ 6 × 10 6 ∕m to Re∕l ≈ 20 × 10 6 ∕m. Infrared (IR) thermography was used to monitor laminar and transitional surface heating.…”