Ensemble-averaged measurements in the turbulent near wake of two side-by-side square cylinders

Abstract

The ensemble-averaged characteristics of the turbulent near-wake flow around two side-by-side identical square cylinders at a Reynolds number ≈23100 have been studied using a two-component laser-Doppler velocimeter system. The work focuses on a single case with a gap/diameter ratio of 2, for which the resulting individual vortex streets are coupled so as to yield a flow predominantly symmetric about the line midway between the two cylinders. Data sorting or conditioning according to phase was performed with the aid of pressure signals taken from taps on a sidewall of each cylinder. The two-cylinder results are compared in detail to results from a previous study of the one-cylinder case. Vortex structures shed on the side towards the flow centreline, termed inner structures, are distinguished from those shed on the free-stream side, termed outer structures, and the differences between the features associated with the two different structures are examined. The circulation associated with outer structures evolves downstream in a manner similar to that observed in the one-cylinder case, but the circulation of the inner structures is found to decrease dramatically downstream. This not only gives support to previous theoretical predictions but also reconciles these with previously apparently conflicting experimental observations. Information regarding vortex structure motion and the relevant length and time scales is obtained. Differences between momentum and vorticity transport, particularly across the flow centreline are pointed out, and effective turbulent vorticity fluxes are defined. Similarities in local flow topologies in one- and two-cylinder cases are discussed, and the role of local velocity-gradient invariants and their relationship to critical points and turbulence statistics are examined.