FLOW FEATURES IN A FULLY DEVELOPED RIBBED DUCT FLOW AS A RESULT OF LES

摘要

The present contribution concerns the simulation of a turbulent flow in a stationary square section ribbed duct by means of LES method. The pitch distance of the ribs was fixed to p/h=10, together with a high blockage value of 0.3 h/D, and a Reynolds number (defined with bulk velocity (U) and hydraulic diameter (D)) of 40000. These parameters were chosen to reproduce the experiments of Casarsa (2003). An attempt of topology description is proposed, using concepts of streamsurface, vortex core detection, wall streamline and bifurcation lines. Furthermore, knowledge on flow topology derived from these concepts is qualitatively compared with isosurface of second scalar invariant of the velocity gradient tensor (Q) used on the averaged and/or instantaneous velocity fields. Differences between a 2D ribbed channel and the present flow, where sidewall effect are dominants, are highlighted. Especially, the existence of a strong secondary flow on the windward side of the rib is revealed by the visualization of streamsurfaces, wall streamlines and bifurcation lines. Vorticity dominated regions are detected and extracted using isosurfaces of the (Q). Thanks to well-positioned streamsurfaces and a vortex core detection method, the effect of the lateral walls on the wake of the rib is put in evidence. Apart of this analysis of the average velocity field, a sequence of instantaneous images displaying creation of coherent structures on the leading edge of the rib is presented. These spanwise structures are shaped in Lambda (Λ) type vortices by a mechanism similar than the one already described in Dubief & Delcayre (2000), showing the capabilities of the present approach to reproduce averaged and instantaneous features of complex flows.