Large Eddy Simulations and Experiments of Flow Around Finite-Height Cylinders

Abstract

The paper presents experimental and numerical results for the flow around a surface-mounted circular cylinder at the two height-to-diameter ratios of 2.5 and 5. The Reynolds number based on approach flow velocity and cylinder diameter is 43,000 and 22,000 for these two cases and the boundary layer of the approach flow has a thickness of about 10% of the cylinder height. The experiments comprise both flow visualizations with dye and laser Doppler velocimeter measurements of all mean velocity and fluctuation components. The numerical study is performed by an elaborate large eddy simulation on a staggered Cartesian grid using the immersed boundary method. The instantaneous flow behaviour including the shedding is analysed with information based on animations. For the long cylinder alternating shedding is found to occur over most of the height while for the shorter cylinder the shedding is observed mainly near the ground where it is also mostly alternating but intermittently also symmetrical. The mean-flow behaviour is analysed with the aid of streamlines and contour plots of mean-velocity and fluctuation components Flow Turbulence Combust (2010) 84:239-275 in various planes and a detailed comparison of LES and LDV results is provided, showing generally good agreement. The LES with very fine resolution near the free end allow a detailed study of the complex flow in this region with owl-face topology on the end wall previously observed in experiments. Behind the cylinder, the longitudinal recirculation region, the downstream development of tip vortices and the emergence of trailing vortices further downstream are analysed. The sum of the results, together with those from previous studies that were reviewed extensively, provides a comprehensive picture of the very complex flow behaviour.

Performance enhancement of downstream vertical-axis wind turbines

“…These accelerations have been shown to extend significantly downstream in the flow around cylinders of similar aspect ratio to the VAWTs in this study. 21 Additionally, accelerations near the rotor are also observed in numerous experimental and numerical studies of a single VAWT 20,22,23 and in arrays of VAWT. 12 It is important to note in the single turbine cases, the magnitude and extent of these accelerations do not necessarily reflect the regions of acceleration expected around full-scale VAWTs due to blockage effects in experimental and modeled conditions.…”

Forced Convection Heat Transfer from a Finite-Height Cylinder

“…The size of the domain is 7.5h × 2h × 2.8h in streamwise, wall-normal and spanwise directions, respectively. The cross-section corresponds to the one used in the experiments [22] against which the flow calculations were validated. The body-fitted grid consists of about 50 million cells in total; it is the same as the one used by Palau-Salvador et al [21] and is shown in two cuts in Fig.…”

“…The stretching factor is 3%. Note that in the previous study [22] a different numerical method was employed, namely, a cartesian staggered grid was used and the presence of the cylinder was modelled using an immersed boundary method.…”

“…There are many previous works with LES applied to the flow around finite-height cylinders. For a review see [22]. More recent studies not covered in that review are the experiments of Rostamy et al [27] or the numerical studies of Frederich and Thiele [8] and Krajnovic [17].…”

“…The configuration of the present study corresponds to one of the cases investigated by Palau-Salvador et al [22] using experiments and large eddy simulations. These authors studied the flow around cylinders of different h/D ratios (where h is the height and D the diameter of the cylinder) at Reynolds numbers based on the free-stream velocity and the cylinder diameter between 20000-50000.…”