Regeneration mechanisms of near-wall turbulence structures

Abstract

Direct numerical simulations of a highly constrained plane Couette flow are employed to study the dynamics of the structures found in the near-wall region of turbulent flows. Starting from a fully developed turbulent flow, the dimensions of the computational domain are reduced to near the minimum values which will sustain turbulence. A remarkably well-defined, quasi-cyclic and spatially organized process of regeneration of near-wall structures is observed. This process is composed of three distinct phases: formation of streaks by streamwise vortices, breakdown of the streaks, and regeneration of the streamwise vortices. Each phase sets the stage for the next, and these processes are analysed in detail. The most novel results concern vortex regeneration, which is found to be a direct result of the breakdown of streaks that were originally formed by the vortices, and particular emphasis is placed on this process. The spanwise width of the computational domain corresponds closely to the typically observed spanwise spacing of near-wall streaks. When the width of the domain is further reduced, turbulence is no longer sustained. It is suggested that the observed spacing arises because the time scales of streak formation, breakdown and vortex regeneration become mismatched when the streak spacing is too small, and the regeneration cycle at that scale is broken.

Lower branch equilibria in Couette flow: the emergence of canonical states for arbitrary shear flows

“…Investigation of the results towards the end of the curves did not suggest the development of a singularity in the solution, and the failure to converge is probably associated with the wave's critical layer becoming multi-valued in y at a fixed z. The upper limits for ↵ are consistent with computationally determined minimum box sizes required for sustained turbulence in plane Couette flow (Hamilton, Kim & Waleffe 1995). At lower values of ↵ the curves all approach the origin and, though convergence for small ↵ was not attained, the curves are expected to enter following the ⇢ ⇠ ↵ 5/6 law established in Hall & Sherwin (2010).…”

Transient growth of perturbations in Stokes oscillatory flows

“…As for steady flows, three-dimensional fluctuations are necessary to observe a self-sustaining wall turbulence in subcritical regimes. Indeed, as a common feature of the near-wall turbulence, the development of streamwise low-speed streaks is observed at the end of the transition to turbulence (Jimènez & Moin 1991;Hamilton, Kim & Waleffe 1995). The amplification is fast; the whole transition can be realised within one deceleration phase.…”

A self-sustaining process model of inertial layer dynamics in high Reynolds number turbulent wall flows

“…For example, it has been argued that LSMs and VLSMs arise from the spontaneous organization of attached and/or detached hairpin vortices and hairpin vortex packets [14]. Here, we investigate a complementary possibility, namely that certain large-scale quasi-coherent flow structures in the outer region of turbulent wall flows arise directly from a self-sustaining, multiple space and timescale process, in loose analogy with the way in which near-wall streaks and vortices have been argued to arise from a single-scale (uniformly space-filling) SSP [1]. It should be emphasized that a similar thesis has been advanced recently by Hwang and co-workers [11,12,16], but here we focus specifically on developing a semi-analytical, mechanistic description that has the potential to explain the formation and maintenance of uniform momentum zones (UMZs) and interlaced 'vortical fissures' (VFs), arguably the primal coherent structures in the outer part of turbulent wall flows at extreme Reynolds number.…”