A turbulent cylinder wake with cylinder corners modified or a neighbouring cylinder present.

摘要

This thesis presents experimental studies of a turbulent cylinder wake in the presence of a neighboring cylinder. The effect of the corners of a square cylinder on the wake is also investigated. The thesis includes four topics. Firstly, flow structures, Strouhal numbers and their downstream evolutions in the wake of two staggered circular cylinders are investigated at Re = 7,000 using hotwire, flow visualization and particle image velocimetry (PIV) techniques. Four distinct flow structures are identified at x/d = 10, i.e., two single-street modes (S-I and S-II) and two twin-street modes (T-I and T-II), based on vortex shedding frequencies, flow topology and their downstream evolution. Mode S-I is further divided as two different categories, i.e., S-Ia and S-Ib, in view of their different strength of the vortices. The distinct flow structures are arised from different initial conditions. Secondly, heat and momentum transport associated with the four flow structures was studied at x/d = 10 and 20 ( Re = 7,000). Free-stream fluid is almost equally entrained from either side into the wake in Modes S-Ia and T-II, but largely entrained from the downstream cylinder side in Modes S-II and T-I. The entrainment motion in Mode S-Ib is very weak due to the very weak vortex strength. Vortices decay considerably more rapidly in the twin-street modes, under vigorous interactions between the streets, than in the single-street modes. Thirdly, the near wake of square cylinders with different corner radii was experimentally studied based on PIV, laser Doppler anemometry (LDA) and hotwire measurements at Re = 2,600. A conditional sampling technique was developed to obtain the phase-averaged PIV data in order to characterize quantitatively the effect of corner radii on the near-wake flow structure. Finally, the wake of asymmetric bluff bodies was measured using PIV, LDA, load-cell, hotwire and flow visualization techniques at Re = 2,600∼8,500. It is found that, while the asymmetric cross-section of the cylinder causes the wake centreline to shift towards the sharp corner side of the bluff body, the wake remains globally symmetric about the shifted centreline and generates a non-zero mean lift on the cylinder.