Observation of Cross-Flow Instability Mode over a Yawed Cylinder at Mach 2

摘要

Low-speed, subsonic, three-dimensional (3-D) boundary layers are destabilized by the so-called cross-flow (CF) instability, which belongs to the basic class of inflection-point-type mechanisms. Linear-stability analyses show that the CF instability is susceptible to both a stationary mode and a time-dependent mode. Previous studies have shown that, under certain circumstances, the time-dependent (traveling) mode can be more unstable than the stationary mode. Using the naphthalene sublimation technique, striations on the surface resulting from the stationary CF mode have been visualized and are seen to be almost parallel to the external flow direction. In addition, it has been observed that the stationary mode in quiet environments has a hypersensitivity to surface roughness of the model, whereas the traveling mode is prone to dominant appearance in the presence of larger freestream disturbances. It is, therefore, plausibly expected that the stationary mode should dominate the 3-D boundary-layer transition in the low disturbance environments that typify cruise conditions. However, in apparent contradiction to this, experimental observations on a yawed circular cylinder have revealed that the traveling mode is dominant even under low freestream disturbance conditions, a result also anticipated from linear-stability theory. Surprisingly, stationary vortices rather than traveling modes on the surface of disks spinning in still air (another application that replicates the important physical phenomena for swept-wing flow) appear to dominate without exception. Because it has been shown that curvature favors the traveling mode over the stationary mode, these differing observations from previous experiments on the CF instability might be spawned by differing surface curvatures of the various models employed; however, this issue remains elusive. A comprehensive assessment that includes the previous summary and an exhaustive review of three-dimensional boundary-layer transition may be found in Saric et al..