A critical-layer framework for turbulent pipe flow

TLDR

The study presents a model-based description of the scaling and radial location of turbulent fluctuations in pipe flow to illuminate the scaling behaviour of very large scale motions. The model treats the nonlinear Reynolds stress as an unknown forcing, yielding a linear relation between velocity response and forcing, examines propagating modes without assuming small perturbations, and identifies the optimal forcing shape that produces dominant modes observed in turbulent pipe flow. The analysis reveals critical‑layer‑like behaviour with a wall layer scaling as \(R^{+1/2}\) and a critical layer scaling as \(R^{+2/3}\), consistent with wall‑turbulence scaling and showing viscosity effects extend well beyond the wall.

Abstract

A model-based description of the scaling and radial location of turbulent fluctuations in turbulent pipe flow is presented and used to illuminate the scaling behaviour of the very large scale motions. The model is derived by treating the nonlinearity in the perturbation equation (involving the Reynolds stress) as an unknown forcing, yielding a linear relationship between the velocity field response and this nonlinearity. We do not assume small perturbations. We examine propagating modes, permitting comparison of our results to experimental data, and identify the steady component of the velocity field that varies only in the wall-normal direction as the turbulent mean profile. The "optimal" forcing shape, that gives the largest velocity response, is assumed to lead to modes that will be dominant and hence observed in turbulent pipe flow. An investigation of the most amplified velocity response at a given wavenumber-frequency combination reveals critical layer-like behaviour reminiscent of the neutrally stable solutions of the Orr-Sommerfeld equation in linearly unstable flow. Two distinct regions in the flow where the influence of viscosity becomes important can be identified, namely a wall layer that scales with $R^{+1/2}$ and a critical layer, where the propagation velocity is equal to the local mean velocity, that scales with $R^{+2/3}$ in pipe flow. This framework appears to be consistent with several scaling results in wall turbulence and reveals a mechanism by which the effects of viscosity can extend well beyond the immediate vicinity of the wall.