ENSEMBLE-AVERAGED MEASUREMENTS IN THE TURBULENT NEAR WAKE OF 2 SIDE-BY-SIDE SQUARE CYLINDERS

The ensemble-averaged characteristics of the turbulent near-wake flow around two side-by-side identical square cylinders at a Reynolds numbe r approximate to 23100 have been studied using a two-component laser-D oppler velocimeter system. The work focuses on a single case with a ga p/diameter ratio of 2, for which the resulting individual vortex stree ts are coupled so as to yield a flow predominantly symmetric about the line midway between the two cylinders. Data sorting or conditioning a ccording to phase was performed with the aid of pressure signals taken from taps on a sidewall of each cylinder. The two-cylinder results ar e compared in detail to results from a previous study of the one-cylin der case. Vortex structures shed on the side towards the flow centreli ne, 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 examine d. The circulation associated with outer structures evolves downstream in a manner similar to that observed in the one-cylinder case, but th e circulation of the inner structures is found to decrease dramaticall y downstream. This not only gives support to previous theoretical pred ictions but also reconciles these with previously apparently conflicti ng experimental observations. Information regarding vortex structure m otion and the relevant length and time scales is obtained. Differences between momentum and vorticity transport, particularly across the flo w centreline are pointed out, and effective turbulent vorticity fluxes are defined. Similarities in local flow topologies in one-and two-cyl inder cases are discussed, and the role of local velocity-gradient inv ariants and their relationship to critical points and turbulence stati stics are examined.