Turbulent structures in accelerating boundary layers

The vortical structures in spatially developing turbulent boundary layers s ubjected to streamwise acceleration are studied. Two cases are examined: on e in which the acceleration is insufficient to cause reversion to the lamin ar state of the initially turbulent flow, and another in which the accelera tion is stronger and relaminarization begins to take place; the pressure gr adient, however, is not maintained long enough for full reversion to occur. The turbulent statistics show the expected trends: the mean velocity profi le deviates significantly from the logarithmic law-of-the-wall and shows, i n the strongly accelerated case, a tendency to approach the laminar profile ; the turbulent kinetic energy increases less rapidly than the energy of th e 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, mor e elongated and aligned in the streamwise direction. Fewer ejections are ob served. The vorticity of the coherent quasi-streamwise vortices is approxim ately the same as in the zero-pressure-gradient region, despite the presenc e of an additional vortex-stretching mechanism. This may be due to increase d dissipation of the thinner, more intense vortices due to this additional vorticity-generating term.