On the rheoscopic measurement of turbulent decay in wall-bounded flows

Abstract

Quench experiments where the flow passes from a fully turbulent state to a laminar state by an abrupt decrease in the flow Reynolds number ($Re$) have been extensively studied in the literature to quantify the turbulent-laminar transition process in wall-bounded flows. Measurements have been classically made using rheoscopic fluid visualisations, which make turbulent coherent structures easily identifiable, allowing for quantification of the evolution of a turbulent fraction -- the percentage of a given observation window where turbulence is deemed active by the presence of coherent structures, such as streamwise vortices called rolls, and modulations of the streamwise velocity fluctuations called streaks. Decay characteristic times of these structures have therefore been extensively measured. However, owing to the nature of visualization based techniques, only a single decay time is typically extracted, whereas measurements of the velocity field can reveal distinct decay times associated with different velocity or kinetic energy components. As a result, the physical meaning of the decay time inferred from visualization alone is not straightforward. The goal of the present paper is to perform such a comparison quantitatively, using particle image velocimetry (PIV) measurements and rheoscopic fluid visualisations in the same setup: a Couette-Poiseuille experiment. We observe via PIV different characteristic times of decay for streamwise (streaks) and spanwise (rolls) velocity fluctuations. We show that the characteristic time of decay of the turbulent fraction observed by visualisation is close to the decay of the streaks.