Laminar to turbulent transition in a finite length square duct subjected to inlet disturbance

Abstract

Laminar to turbulent flow transition in a finite length square duct has been carried out by imposing novel spatiotemporal finite amplitude inlet disturbance on the laminar flow. The present direct numerical simulation study demonstrates the effect of inlet disturbance on laminar to turbulent transition. A laminar flow in a finite length square duct is considered at bulk Reynolds number Re = 2260 and Re = 1540, to which a novel spatiotemporal disturbance is introduced through a narrow banded region at the inlet of the square duct. The puff (transition) and slug (turbulent) flow dynamics indicate the laminar to turbulent transition in a square duct. Disturbance introduced at Re = 2260 laminar flow propagates downstream, giving puff and slug flow phenomena similar to pipe flows. However, at Re = 1540, inlet disturbance shows only a puff-like structure. The four vortex mean secondary flow is observed in a puff region, while the conventional eight vortex is observed in the slug region. The coherent structures of the transition (puff) flow show the presence of dual-type hairpin structures. The turbulent kinetic energy spectrum indicates conventional −5/3 spectra for slug flow and −2 energy spectra for puff flow. Thus, in this paper, it is shown that the puff and slug characteristics of laminar to turbulent transition in a square duct are similar to that of a circular duct. It is also shown that the novel inlet disturbance through a narrow banded region captures the dynamics of laminar to turbulent transition in a square duct.