Bypass transition mechanism in a rough wall channel flow

The combined effect of wall pulsed jets and roughness in a laminar channel flow is investigated to determine whether or not turbulence can be triggered and maintained at low Reynolds numbers. The study is carried out through a direct numerical simulation based on the lattice Boltzmann method. The roughness, mounted on both walls of the channel, consists of transverse square bars spanning the whole width of the channel. Rectangular orifice(s) at the bottom wall act as pulsed jets. The jets are pulsed only once with the second jet activated at the end of the cycle of the first jet. Without activating the jets, the flow in the rough wall channel consists mainly in steady secondary motions in the canopies (spaces between bars) and a skimming two-dimensional laminar flow with some oscillations above the canopies. When the jets are activated, a localized three-dimensional turbulence develops and grows in the channel. Not only it is found that this localized turbulence bears strong similarity to that of a fully rough wall turbulent channel flow, but it appears to be in a “pseudo-” fully rough regime, as observed by the relatively negligible (averaged) viscous drag as compared to the form drag generated by the roughness elements. The physical mechanism which allows this to occur is discussed.