Direct numerical simulations of turbulent flow in a conical diffuser

Abstract

Direct numerical simulations (DNS) were carried out to investigate the turbulence statistics and coherent structures in a fluid flow through a conical diffuser, called an Azad diffuser, for three opening angles of 2°, 4°, and 8°. The adverse pressure gradient affected the axisymmetric flow such that the streamwise component of the turbulence intensity was significantly increased in the outer region of the diffuser. The maximum Reynolds shear stress increased as the opening angle increased. The premultiplied energy spectra showed that the most energetic wall-normal height and its wavelength increased when the opening angle increased. Time-resolved instantaneous flow fields and two-point correlation functions provided evidence for the streamwise merging of low-speed streaking structures. The outer vortical structures shifted toward the diffuser axis as the flow passed the diffuser throat. The swirling motions of the individual hairpins in the outer region were stronger in the diffuser flow than in the straight pipe flow.