Turbulent structures in accelerating boundary layers

The vortical structures in spatially developing turbulent boundary layers subjected to streamwise acceleration are studied. Two cases are examined: one in which the acceleration is insufficient to cause reversion to the laminar state of the initially turbulent flow, and another in which the acceleration is stronger and relaminarization begins to take place; the pressure gradient, however, is not maintained long enough for full reversion to occur. The turbulent statistics show the expected trends: the mean velocity profile deviates significantly from the logarithmic law-of-the-wall and shows, in the strongly accelerated case, a tendency to approach the laminar profile; the turbulent kinetic energy increases less rapidly than the energy of the mean flow. The structure of the inner layer is also significantly altered. In the near-wall region, the streaks become more elongated and show fewer undulations, owing to a significant decrease of the spanwise fluctuations relative to the streamwise ones. The coherent eddies also become fewer, more elongated and aligned in the streamwise direction. Fewer ejections are observed. The vorticity of the coherent quasi-streamwise vortices is approximately the same as in the zero-pressure-gradient region, despite the presence of an additional vortex-stretching mechanism. This may be due to increased dissipation of the thinner, more intense vortices due to this additional vorticity-generating term.