Wake characteristics of a screen cylinder and a screen shrouded cylinder

This thesis presented experimental results on the wake characteristics of a screen cylinder (of 67% porosity) as well as that of a circular cylinder enclosed by the screen cylinder, or the so-called shrouded cylinder. The objective of the studies was to gain an understanding on turbulent structures and vortex shedding mechanism behind such cylinders through experimental investigations. The wake of a solid cylinder was also studied to provide benchmark for the screen cylinder and the shrouded cylinder. Phase-averaged analysis was the major technique used to extract the coherent turbulent structures in the cylinder wakes and to quantify their contributions to the Reynolds stresses. The wavelet multiresolution technique was also used to educe the turbulent structures of various scales. Flow visualization using a smoke wire was conducted to further illustrate the vortex formation mechanisms in the wakes of the screen cylinder and solid cylinder. Four topics were studied in this thesis.

Firstly, the statistical characteristics of the wake parameters behind a screen cylinder and a solid cylinder were discussed. Based on the streamwise evolution of these parameters, vortex formation mechanisms in the two wakes were proposed. These results were supported by flow visualisations using a smoke wire in a low speed wind tunnel. Secondly, analyses on the flow structures and momentum transport were further carried out. Both studies were conducted in the range of x/d = 560 at a Reynolds number of 7000 using X-type hotwire probes. The two studies uncovered two different regions behind the screen cylinder; the growth region and decay region of vortices. The first region revealed an increasing size of the vortex structures, where the shear layer vortices resulting from Kelvin-Helmholtz instabilities amalgamated as evolving in the streamwise direction. The second region involved a gradual decay of the fully-formed vortices. The transition location between the two regions depended upon the change of the wake profile of the root-mean-square of the lateral velocity component, from a twin-peak profile of the developing shear layer to a single peak profile. The latter was an indication of the occurrence of a single vortex street. The formation of the large-scale vortices was delayed until this location and the formation length was extended significantly, i.e. up to about 40d downstream in the present study. The phase-averaged sectional streamlines and the vorticity contours displayed distinct vortex structures from that of Kármán vortices behind a solid cylinder. The transport of vorticity and momentum were summarized for the two cylinder wakes through the proposed conceptual models of vortex formation and decay processes.

Following the investigation of the coherent flow structures behind the screen cylinder, turbulent structures of various scales were examined. The wavelet multiresolution analysis was used for this purpose to examine the velocity and vorticity characteristics in the streamwise direction. A one-dimensional vorticity probe consisting of an X-probe straddled by a pair of parallel hot wires was used to measure the velocity fluctuations which were later used to calculate the spanwise vorticity. The wavelet results conform the phase-averaged analysis which reflected different formation mechanism and decay of the organized structures behind a screen cylinder compared to that behind a solid cylinder. At x/d=10, the dominant contributors to the Reynolds stresses came from the intermediate-scale structures in the screen cylinder wake, whereas this was the large-scale structures in the solid cylinder wake. The dominant roles of the large-scale structures in contributing to the Reynolds stresses was perceived after x/d=10 in the screen cylinder wake. The wake vorticity was significantly dominated by the intermediate-scale structures and had the smallest values at the large-scale structures, indicating that vorticity also mostly resided in the intermediate-scale structures for the screen cylinder wake.

Finally, the screen shroud effect on the wake characteristics and vortex shedding of a circular cylinder was studied. A circular cylinder with a diameter of 12.7 mm was enclosed by a screen cylinder of 26 mm diameter, yielding an outer-to-inner diameter ratio of 2.0. Measurements were conducted at x/d = 5, 10, 20 and 40 at a Reynolds number of 7000, based on the inner cylinder diameter d. The coherent fields were investigated using the phase-averaged technique. It was found that the shrouded cylinder impaired vortex shedding in the near-wake where large-scale vortices only formed approximately 10d downstream. These vortices were less energetic than that in the solid cylinder wake and decayed at a comparable rate in the two wakes after initially observed. It is conjectured that the above effects of the shroud result in the reduction of vortex-induced vibration of a bare cylinder.