|Title:||Turbulent structures in the wake of circular cylinders|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Cylinders -- Fluid dynamics
Turbulence -- Measurement
|Department:||Department of Mechanical Engineering|
|Pages:||xxvii, 183 leaves : ill. ; 30 cm|
|Abstract:||This thesis presents experimental studies of turbulent structures in the wake of circular cylinders in the cross flow. The objective of the studies is to gain, through experimental investigation, the knowledge to improve our understanding of turbulent structures in the wake of single cylinder and two tandem cylinders. The phase-averaged technique is the major analysis method to extract the coherent and incoherent turbulent structures in the cylinder wake and investigate the three-dimensional vorticity due to the Reynolds number effect. Three topics are studied in this thesis. Firstly, the flow structures, momentum and heat transport in the wake of two tandem circular cylinders have been experimentally investigated. Measurements were conducted at x/d = 10, 20 and 30 at a Reynolds number of 7000 using a three-wire (one X-wire plus a cold wire) probe, in conjunction with an X-wire. The upstream cylinder was electrically slightly heated. The flow behind two tandem cylinders is conventionally divided into three regimes based on whether the shear layers separated from the upstream cylinder overshoot or reattach on the downstream cylinder before forming a vortex street, or form vortices between the cylinders. The present investigation however uncovers two remarkably different flow structures in the reattachment regime, depending on whether the shear layers from the upstream cylinder reattach on the after-body or fore-body of the downstream cylinder. As such, four cylinder center-to-center spacing ratios, i.e. L/d = 1.3, 2.5, 4.0 and 6.0, were examined, each representing one distinct flow structure. The phase-averaged sectional streamlines and vorticity contours display a single vortex street, irrespective of different regimes. However, the detailed flow structure, in particular the vortex strength, depends upon L/d. Furthermore, the evolution of vortices is distinct from one to another. The cross-stream distributions of the Reynolds stresses and heat fluxes vary at the same x/d between the regimes. So do the coherent contributions to the Reynolds stresses and heat fluxes. The results are connected to different initial conditions for the four flow categories. The momentum and heat transport characteristics of the flow are summarized for each category. Following the investigation of the flow topology of the two tandem circular cylinders, three-dimensional vorticity in a turbulent cylinder wake is examined. A circular cylinder with a diameter of 12.7mm was used to generate the wake flow. Measurements were conducted at x/d = 10, 20 and 40 at a Reynolds number of 2500. The three components of the vorticity vector in the intermediate region of a turbulent cylinder wake were measured simultaneously using a multi hot-wire probe. This probe has an improved spatial resolution compared with those reported in the literature. The behaviour of the instantaneous velocity and vorticity signals is examined. Both coherent and incoherent vorticity fields are investigated using a phase-averaged technique. The iso-contours of the phase-averaged longitudinal and lateral vorticity variances, <wx2> and <wy2>, wrap around the spanwise structures of opposite sign and run through the saddle point along the diverging separatrix. The observation conforms to the previous reports of the occurrence of the longitudinal structures based on flow visualizations and numerical simulations. The magnitude of these contours is about the same as that of the maximum coherent spanwise vorticity at the vortex center, indicating that the strength of the longitudinal structures is comparable to that of the spanwise vortices. Furthermore, <wx2> and <wy2> exhibit maximum concentration away from the vortex center, probably because of a combined effect of the large-scale spanwise vortices and the intermediate-scale longitudinal structures. Coherent structures contribute about 36% to the spanwise vorticity variance at x/d = 10. The contribution decreases rapidly to about 5% at x/d = 40. The present results suggest that vorticity largely reside in relatively small-scale structures. Finally, the Reynolds number effects on three-dimensional vorticity in a turbulent wake are studied. When Reynolds number, Re, varies from 103 to 104, there is a large change in the turbulent near-wake dynamics (e.g. the base pressure coefficient, fluctuating lift coefficient and vortex formation length) of a circular cylinder, which has previously been connected to the generation of small-scale Kelvin-Helmholtz vortices. Using the same multi-wire vorticity probe as that used in three-dimensional vorticity in a turbulent cylinder wake, all three components of vorticity were simultaneously measured in the intermediate region of a turbulent circular-cylinder wake at Re = 2,500, 5,000 and 10,000, respectively. It is observed that the root-mean-square values of the three vorticity components increase with Re, especially the streamwise component, which shows a large jump from Re = 5 x 103 to Re = 104. At Re = 2.5 x 103, the maximum phase-averaged spanwise vorticity variance, <wz2>*, normalized by d and U﹢, is twice as large as its counterpart of the streamwise component, <wx2>*, or the lateral component, <wy2>*. However, at Re = 104, the maximum <wz2>* is merely 55% larger than the maximum <wx2>* or 47% larger than the maximum <wy2>*. The observation is consistent with the perception that the three-dimensionality of the flow is enhanced at higher Re due to the occurrence of Kelvin-Helmholtz vortices. The effect of Re on vorticity signals, spectra, coherent and incoherent vorticity fields are also examined. Seven publications, including 4 refereed journal papers and 3 refereed conference proceedings, have been produced out of this work.|
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