Experimental investigation of laminar turbulent intermittency in pipe flow

Abstract

In shear flows, turbulence first occurs in the form of localized structures (puffs/spots) surrounded by laminar fluid. We here investigate such spatially intermittent flows in a pipe experiment showing that turbulent puffs have a well-defined interaction distance, which sets their minimum spacing as well as the maximum observable turbulent fraction. Two methodologies are employed. Starting from a laminar flow, puffs are first created by locally injecting a jet of fluid through the pipe wall. When the perturbation is applied periodically at low frequencies, as expected, a regular sequence of puffs is observed where the puff spacing is given by the ratio of the mean flow speed to the perturbation frequency. At large frequencies however puffs are found to interact and annihilate each other. Varying the perturbation frequency, an interaction distance is determined which sets the highest possible turbulence fraction. This enables us to establish an upper bound for the friction factor in the transitional regime, which provides a well-defined link between the Blasius and the Hagen-Poiseuille friction laws. In the second set of experiments, the Reynolds number is reduced suddenly from fully turbulent to the intermittent regime. The resulting flow reorganizes itself to a sequence of constant size puffs which, unlike in Couette and Taylor-Couette flow are randomly spaced. The minimum distance between the turbulent patches is identical to the puff interaction length. The puff interaction length is found to be in agreement with the wavelength of regular stripe and spiral patterns in plane Couette and Taylor-Couette flow.

The rise of fully turbulent flow

“…8,9 The sudden appearance of localised turbulent patches in an otherwise quiescent flow was first observed by Osborne Reynolds for pipe flow 1 and has since been found to be the starting point of turbulence in most shear flows. 2,4,[10][11][12][13][14][15] Curiously, in this regime it is impossible to maintain turbulence over extended regions as it automatically 16,17 reduces to patches of characteristic size, called puffs in pipe flow (see Fig. 1a).…”

“…1a) and cannot form larger clusters. 17,18 At larger flow rates, the situation is fundamentally different: once triggered, turbulence aggressively expands and eliminates all laminar motion (Fig. 1b).…”

“…Customised connectors made from perspex allowed an accurate fit of the pipe segments. A specially made pipe inlet consisting of several meshes and a smooth convergence from a 100mm wide section to the 10mm pipe was used to avoid inlet disturbances and eddie formation (see Samanta et al 17 for details). In this way the water flow could be held laminar for R in excess of 8000.…”

Study of the instability of the Poiseuille flow using a thermodynamic formalism

“…We observe a stable train of localized wave packets, similar to that reported by Jimenez (24) in an L = 8π channel flow for Re < 5,000. A localized wave packet (puff) has a characteristic length scale of 20 and resembles those in the 3D pipe flow (26,27).…”

Extraction of 3D vortex structures from a turbulent puff in a pipe using two-color illumination and flakes

“…Visualization using flakes is a classical but feasible alternative to velocimetry-based approaches, and has been used to observe various flow structures that are subject to distortion (e.g., Park et al 1981;Dominguez-Lerma et al 1985;Bandyopadhyay 1986;Samanta et al 2011). Using this method, local shear flows can be grasped directly as brightness information.…”