Direct numerical simulation of wall turbulence subjected to an unfavorable pressure gradient
Résumé:
The main objective of this work is to analyze the effects of a moderate adverse pressure gradient on the dynamics of turbulent boundary layer flows. For that purpose, a direct numerical simulation (DNS) of the turbulent boundary layer (TBL) subjected to a moderate adverse pressure gradient (APG) out of equilibrium has been performed using the open- source code Incompact3d up to a Reynolds number of 8000 based on momentum thickness. A large database resolved in time and space was collected and used to analyze the turbulence statistics. Special attention has been paid to the existence and evolution of the outer peak of Reynolds stresses observed in APG wall-bounded flows. Different velocity scalings have been investigated and tested against the numerical results. The velocity scale based on the shear stress is shown to scale all the Reynolds stresses profiles for different Reynolds numbers, indicating that all Reynolds stresses are associated with a single dynamics of turbulent structures.
The large-scale coherent structures of the streamwise velocity fluctuations have been investigated using two-point spatial correlation. A comparison with a zero pressure gradient case at an equivalent Reynolds number allows us to further investigate the effect of the pressure gradient on the size and inclination of attached coherent structures. A deeper investigation of the coherent structures was also performed, where each structure was detected separately based on a thresholding method to distinguish between the effects of large and small scales and to better understand the mechanisms controlling the dynamics of these structures. The contribution of large-scale motions (LSM) on the Reynolds stresses comparing with ZPG case was also analyzed.