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The investigation is concentrated on two important quantities – the Strouhal number and the mean base suction coefficient, both measured at the mid-span position. Reynolds numbers from about 50 to 4 × 104 were investigated. Different aspect ratios, at low blockage ratios, were achieved by varying the distance between circular end plates (end plate diameter ratios between 10 and 30). It was not possible, by using these end plates in uniform flow and at very large aspect ratios, to produce parallel shedding all over the laminar shedding regime. However, parallel shedding at around mid-span was observed throughout this regime in cases when there was a slight but symmetrical increase in the free-stream velocity towards both ends of the cylinder. At higher Re, the results at different aspect ratios were compared with those of a ‘quasi-infinite cylinder’ and the required aspect ratio to reach conditions independent of this parameter, within the experimental uncertainties, are given. For instance, aspect ratios as large as L/D = 60–70 were needed in the range Re ≈ 4 × 103–104. With the smallest relative end plate diameter and for aspect ratios smaller than 7, a bi-stable flow switching between regular vortex shedding and ‘irregular flow’ was found at intermediate Reynolds number ranges in the subcritical regime (Re ≈ 2 × 103).

Direct numerical simulations of two-dimensional (2D) and 3-D unsteady flow around a square cylinder for moderate Reynolds numbers (Re=150–500) are performed, employing an implicit fractional step method finite-volume code with second-order accuracy in space and time. The simulations, which are carried out with a blockage ratio of 5.6%, indicate a transition from 2-D to 3-D shedding flow between Re=150 and Re=200. Both spanwise instability modes, A and B, are present in the wake transitional process, similar to the flow around a circular cylinder. However, seemingly in contrast to a circular cylinder, the transitional flow around a square cylinder exhibits a phenomenon of distinct low-frequency force pulsations (Re=200–300). For 3-D simulations, the Strouhal number and the mean drag coefficient are in general agreement with existing experiments. Between Re=300 and 500, an increase in the spanwise coupling of fluctuating forces is indicated. The influence of the spanwise aspect ratio using periodic boundary conditions, a finer grid, and a finer time step is also investigated.

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Large eddy simulation of flow past a rigid prism of a square cross section with one side facing the oncoming flow at Re=2.2×104 is performed. An incompressible code is used employing an implicit fractional step method finite volume with second-order accuracy in space and time. Three different subgrid scale models: the Smagorinsky, the standard dynamic, and a dynamic one-equation model, are applied. The influence of finer grid, shorter time step, and larger computational spanwise dimension is investigated. Some global quantities, such as the Strouhal number and the mean and rms values of lift and drag, are computed. A scheme for correcting the global results for blockage effects is presented. By comparison with experiments, the results produced by the dynamic one-equation one give better agreement with experiments than the other two subgrid models. [S0098-2202(00)01001-4]

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SUMMARYFlows over two tandem cylinders were analysed using the newly developed collocated unstructured computational uid dynamics (CUCFD) code, which is capable of handling complex geometries. A Reynolds number of 100, based on cylinder diameter, was used to ensure that the ow remained laminar. The validity of the code was tested through comparisons with benchmark solutions for ow in a lid-friven cavity and ow around a single cylinder. For the tandem cylinder ow, also mesh convergence was demonstrated, to within a couple of percent for the RMS lift coe cient.The mean and uctuating lift and drag coe cients were recorded for centre-to-centre cylinder spacings between 2 and 10 diameters. A critical cylinder spacing was found between 3.75 and 4 diameters. The uctuating forces jumped appreciably at the critical spacing. It was found that there exists only one reattachment and one separation point on the downstream cylinder for spacings greater than the critical spacing.The mean and the uctuating surface pressure distributions were compared as a function of the cylinder spacing. The mean and the uctuating pressures were signiÿcantly di erent between the upstream and the downstream cylinders. These pressures also di ered with the cylinder spacing.

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