Spatio-temporal evolution of coherent vortices in wall turbulence with streamwise curvature

Effects of wall curvature on the turbulent structure in curved channel flow are studied by means of direct numerical simulation (DNS). Four dfferent radii of curvature, delta/R-c = 0.013, 0.05, 0.1 and 0.2, are examined. The DNS results show reduction of turbulence intensities on the convex side and enhancement of radial turbulence intensity in the centre region of channel with the increase in curvature. Analyses on the flux Richardson number and the Reynolds shear stress clarify that such turbulence modulation is attributed to the extra production of the Reynolds stress caused by the centrifugal force. In addition, a spatio-temporal linear stochastic estimation ( LSE) is applied to the computed velocity field to extract the development of turbulence-producing eddies affected by centrifugal force. The development of turbulence-producing motion on the convex side is associated with a large-scale strong roll-cell working against the centrifugal stabilizing effect. The spatio-temporal LSE also reveals that on the convex side strong outward and inward motions are induced by the roll-cell before the second- and fourth-quadrant motions are developed.