ALBERT

Description: Flow structures of an elliptic jet in cross-flow were studied experimentally in a water tunnel using the laser-induced fluorescence technique (LIF), for a range of jet aspect ratio (AR) from 0.3 to 3.0, jet-to-cross-flow velocity ratio (VR) from 1 to 5, and jet Reynolds number from 900 to 5100. The results show that the effects of aspect ratio (or jet exit orientation) are significant only in the near field, and diminish in the far field which depends only on gross jet geometry. For low-aspect-ratio jets, two adjacent counter-rotating vortex pairs (CVP) are initially formed at the sides of the jet column, with the weaker pair subsequently entrained by the stronger pair further downstream. For high-aspect-ratio jets, only one CVP is formed throughout the jet column, but the shear layer develops additional folds along the windward side of the jet. These folds subsequently evolve into smaller scale counter-rotating vortex pairs, which we refer to as windward vortex pairs (WVP). Depending on its sense of rotation, the WVP can evolve into what Haven & Kurosaka (1997) referred to as unsteady kidney vortices or anti-kidney vortices, or, under some circumstances, interconnecting kidney vortices, which have not been reported previously. While Haven & Kurosaka (1997)'s interpretation of the formation of kidney and anti-kidney vortices is topologically feasible, our observation reveals a slightly different formation process. Despite the differences in the near-field flow structures for different jet aspect ratios, the process leading to the formation of the large-scale jet structures (i.e. leading-edge vortices and lee-side vortices) for all cases is similar to that reported by Lim, New & Luo (2001) for a circular jet in cross-flow.

Description: The vortex shedding and wake development of a two-dimensional viscous incompressible flow generated by a circular cylinder which begins its rotation and translation impulsively in a stationary fluid is investigated by a hybrid vortex scheme at a Reynolds number of 1000. The rotational to translational speed ratio a varies from 0 to 6. The method used to calculate the flow can be considered as a combination of the diffusion-vortex method and the vortex-in-cell method. More specifically, the full flow field is divided into two regions: near the body surface the diffusion-vortex method is used to solve the Navier-Stokes equations, while the vortex-in-cell method is used in the exterior inviscid domain. Being more efficient, the present computation scheme is capable of extending the computation to a much larger dimensionless time than those reported in the literature. The time-dependent pressure, shear stress and velocity distributions, the Strouhal number of vortex shedding as well as the mean lift, drag, moment and power coefficients are determined together with the streamline and vorticity flow patterns. When comparison is possible, the present computations are found to compare favourably with published experimental and numerical results. The present results seem to indicate the existence of a critical a value of about 2 when a closed streamline circulating around the cylinder begins to appear. Below this critical a, Karman vortex shedding exists, separation points can be found, the mean lift and drag coefficients and Strouhal number increase almost linearly with a. Above a « 2, the region enclosed by the dividing closed streamline grows in size, Karman vortex shedding ceases, the flow structure, pressure and shear stress distributions around the cylinder tend towards self-similarity with increase a, and lift and drag coefficients approach asymptotic values. The optimum lift to drag ratio occurs at a * 2. The present investigation confirms Prandtl’s postulation of the presence of limiting lift force at high a, and thus the usefulness of the Magnus effect in lift generation is limited. The results show that the present method can be used to calculate not only the global characteristics of the separated flow, but also the precise evolution with time of the fine structure of the flow field. © 1995, Cambridge University Press. All rights reserved.

Description: In the present studies, the effects of the end conditions of a circular cylinder on its wake at a fairly high Reynolds number of Re = 1.57 × 104 were studied. The transverse control cylinder technique (TCCT) was previously reported to be able to induce parallel vortex shedding at Re = 0 (102)In the present work, experimental results showed that the TCCT is still effective in inducing parallel vortex shedding at Re = (104). Initially, before the inclusion of the control cylinders, vortices shed by the main cylinder were curved (all shapes referred to are time-averaged shapes) owing to the influence of the cylinder end conditions. Later, two larger control cylinders of diameter D were included and were located normal and upstream of the main cylinder near its ends to change its end conditions. By manipulating the control distance (the gap between the control cylinders and the main cylinder), different vortex-shedding patterns could be induced. With both control cylinders fixed at the optimum control distance of L1 = L2 = L0 = 1.26D), the main cylinder was induced to shed parallel vortices. For the cases of curved vortex shedding (without control cylinders) and parallel vortex shedding (with control cylinders at the optimum distance of L1 = L2 = L0 = 1.26D), various aerodynamic parameters of the main cylinder were measured and compared. Results showed that the inclusion of the control cylinders speeded up the flow velocity at the ends of the main cylinder and led to a more uniform pressure distribution over the central span of the main cylinder, which finally resulted in parallel vortex shedding. Aerodynamic parameters such as drag coefficient and Strouhal number associated with parallel vortex shedding were found to be larger than their curved shedding counterparts. However, extra caution should be exercised in interpreting their implications as these data were under the influence of additional wind-tunnel blockage caused by the presence of the control cylinders. Preliminary and approximate calculations had shown that blockage effects were likely to be responsible for a significant part in the change in the aerodynamic parameters such as the drag coefficient and Strouhal number when the control cylinders were installed. When the control cylinders were symmetrically placed, but not at the optimum distance (L1 = L2 ≠ L0, the vortex-shedding pattern became curved, and was concave or convex downstream at L1 = L2 〈 L0 or L1 = L2 〉 L0 respectively. When the control cylinders were asymmetrically placed (L1 ≠ L2), oblique vortex shedding was induced, with the oblique vortex slanting in the same way as the straight line joining the centres of the control cylinders. The relation between the Strouhal numbers for parallel and oblique vortex shedding was found to still follow the cosine law. The present work confirms earlier finding by other workers that a non-uniform spanwise base pressure distribution was the cause of spanwise base flow, which led to curved or oblique vortex shedding. © 2005 Cambridge University Press.