HRT

Abstract

We are investigating the characteristics of localized disturbances in the flow through a straight duct with a square cross-section. For this purpose, we have performed pseudo-spectral DNS in streamwise periodic domains with a length of 100 duct half-widths. By imposing a body force locally and for a finite temporal interval (similar to the work in a pipe by Willis & Kerswell, Phys. Rev. Lett. 98/100, 2007/2008), a 'puff' is generated when the bulk Reynolds number (based upon the half width) is chosen within the range of approximately 800 to 1000; for lower values the disturbance decays, whereas for higher values the puff expands and ends up filling the entire periodic domain. We have followed the evolution of equilibrium puffs for several thousand bulk time units, enabling us to compute the statistics of the perturbed flow. Phase-locked averaging has been performed in order to reveal the internal structure of the puffs.