Significant changes in the abundances of stratospheric ozone were first observed in the Antarctic but now extend over much of the globe. Measurements have established that chlorine chemistry due to human releases of chlorofluorocarbons is responsible for the annual ozone decrease popularly known as the "ozone hole". Significant ozone changes have also been observed in the Arctic and at mid-latitudes. Measurements of ozone changes throughout the world will be discussed, observations of related chemicals will be presented, and the current state of knowledge of present and future ozone depletion will be summarized. New research results relating to the causes of mid-latitude ozone depletion will also be presented, and projections of when the Antarctic ozone hole may 'close' will be described.
[4INV.02] Nonlinear stability and saturation in 3-D boundary layers.
William Saric (Arizona State University)
The important recent progress in three-dimensional boundary-layer transition is reviewed. We focus on the crossflow instability that leads to nonlinear saturation and transition on swept wings with pressure gradients. Following a brief overview of swept-wing instability mechanisms and the crossflow problem, a summary of the important findings of the 1990s is given. The discussion is presented from the experimental viewpoint and where appropriate, relevant comparisons with CFD are drawn. The recent research conducted at ASU is described in more detail in order to underscore the latest developments concerning nonlinear effects and transition control. In particular, it is shown that the introduction of micron sized roughness organizes the unstable modes in harmonics (up to nine) and it is possible to isolate single-mode growth in order to provide a data base for computations. The measurements show a clear nonlinear distortion of the mean flow and saturation of the stationary structure. Comparisons with nonlinear PSE calculations show outstanding agreement in the growth rate and amplitude saturation when curvature is taken into account. It is also shown that certain roughness spacings inhibit the growth of the most unstable modes and transition can be moved beyond the smooth case.