1. The HIFiRE program is organized as a collection of up to ten research projects, each of which has a unique subset of goals regarding aerodynamics, propulsion, navigation, control, materials, and others. Aerothermodynamics, including boundary-layer transition, is a primary goal of the HIFiRE-1 and -5 tests. HIFiRE-1, a cone-cylinder-flare geometry, examined natural and roughness-induced transition.2-4Tests in the BAM6QT showed a significant transition delay under quiet flow, reinforcing the necessity of quiet-flow testing as a component of transition studies.5
2. Many factors influence transition, including Reynolds number, surface roughness, and atmospheric conditions such as freestream noise level.6,9The noise level of a flow can be defined as the ratio of root-meansquare (rms) pitot stagnation pressure to mean pitot stagnation pressure. Atmospheric levels are typically < 0.05%.10Such low levels are regarded as "quiet". Accurate wind-tunnel testing requires matching as many flight parameters as possible, but achieving this low noise level in a wind tunnel has been particularly difficult.11Conventional tunnels have noise levels of 1-3%. Reference 12 contains a thorough discussion of the influence of tunnel noise on boundary-layer transition. The higher noise levels in a conventional groundtest facility have been shown to lead to early transition as well as to changes in the parametric trends of transition. Thus, ground tests in a quiet tunnel are necessary in order to obtain more accurate transition estimates.13
3. Remarkable images of boundary-layerinstability and transition along the model centerline were obtained with Filtered Rayleigh Scattering in the centerline plane (Figure 4 in Ref. 16). Prior to transition, traveling waves of density fluctuations with a wavelength 4-5 times the boundary layer thickness were imaged. Waves with such a long wavelength are unlikely to have been second-mode waves. At higher Re nearer to transition onset, the fluctuation wavelength decreased to twice the boundary layer thickness, which agrees with the measurements in Tunnel B as well as theoretical predictions for second-mode waves.
4. The same technique was used to image the boundary layer in a plane perpendicular to the model axis (Figure 5 in Ref. 16). The centerline ballooning of the boundary layer predicted by the computations of Kimmel et al. was observed.18As Re was increased to 6.5-8.0 · 106/m, vortical structures were visible on either side of the centerline bulge. At yet higher Re = 10.5 · 106/m, flow near the centerline was late-transitional, and vortices had begun to develop further from the centerline.
5. Computational analysis of the HIFiRE-5 was conducted for selected wind tunnel test cases as well as the proposed flight conditions.24,25Among the cases tested were two simulating the HIFiRE-5 at 0 and 4° angle of attack in the BAM6QT near the maximum quiet pressure achieved at that time (970 kPa in December 2008). Unsurprisingly, the computations predict many of the same interesting flow features encountered in the experiments discussed above.