Interaction between the free surface and the vortical structures developed in a turbulent flow around a cylinder
23 May 2013-pp 1-4
TL;DR: In this article, the vortical structures developed around the cylinder and their dependence on the distance from the free surface were investigated and the locations of critical points on the immersed surface were determined and the flow rate distribution around cylinder was computed.
Abstract: The study and modeling of flow configuration around a cylinder is a benchmark CFD problem in fluid mechanics; the applications are multiple, from micro-fluidics to hydrology, from fluid-structure interaction to geophysics. The present paper is concerned with a particular motion related to this topic: the interaction between the free surface and the immersed cylinder in an open channel turbulent flow. The study is focused to the investigations of the vortical structures developed around the cylinder and their dependence on the distance from the free surface. Numerical simulations in 2D configurations are performed with VOF code and k-e turbulent models. The calculations of the free surface and the direct flow visualizations disclose a good correlation. The results emphasis the major influence on the flow dynamics of the vorticity transport from the free surface to the cylinder. The locations of the critical points on the immersed surface are determined and the flow rate distribution around cylinder is computed.
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11 Feb 2013TL;DR: This book takes the reader from the fundamentals of computational fluid and solid mechanics to the state-of-the-art in computational FSI methods, special FSI techniques, and solution of real-world problems.
Abstract: Computational Fluid-Structure Interaction: Methods and Applications takes the reader from the fundamentals of computational fluid and solid mechanics to the state-of-the-art in computational FSI methods, special FSI techniques, and solution of real-world problems. Leading experts in the field present the material using a unique approach that combines advanced methods, special techniques, and challenging applications.
446 citations
TL;DR: In this article, the effects of free-surface proximity on the flow field around tidal stream turbines are modelled using actuator disc theory, and the theoretical results are compared to open channel flow experimental results.
Abstract: The effects of free-surface proximity on the flow field around tidal stream turbines are modelled using actuator disc theory. Theoretical results are presented for a blocked configuration of tidal stream turbines such as a linear array that account for the proximity of the free surface and the seabed. The theoretical results are compared to open channel flow experimental results in which the flow field has been simulated using a porous disc and strip. These results are complemented by more detailed measurements of the performance of a model horizontal-axis turbine carried out in a water flume and a wind tunnel. The two sets of experiments represent highly blocked and effectively unblocked cases, respectively. The theoretical model of the effects of free-surface proximity provides a blockage correction for axial induction that can be incorporated in blade element momentum codes. The performance predictions obtained with such a code are in good agreement with the experimental results for C P and C T at low tip-speed ratios. The agreement weakens with increasing tip-speed ratio, as the wake of turbine enters a reversed flow state. A correction following the philosophy of Maskell is applied to C T in this region, which provides a better agreement.
241 citations
TL;DR: In this article, an acoustic Doppler velocimeter and a torque transducer were used to simultaneously measure the three velocity components of the flow at various locations upstream of the turbine and in the wake region and turbine power, respectively.
Abstract: A laboratory experiment was performed to study the dynamically rich interaction of a turbulent open channel flow with a bed-mounted axial-flow hydrokinetic turbine. An acoustic Doppler velocimeter and a torque transducer were used to simultaneously measure at high temporal resolution the three velocity components of the flow at various locations upstream of the turbine and in the wake region and turbine power, respectively. Results show that for sufficiently low frequencies the instantaneous power generated by the turbine is modulated by the turbulent structure of the approach flow. The critical frequency above which the response of the turbine is decoupled from the turbulent flow structure is shown to vary linearly with the angular frequency of the rotor. The measurements elucidate the structure of the turbulent turbine wake, which is shown to persist for at least fifteen rotor diameters downstream of the rotor, and a new approach is proposed to quantify the wake recovery, based on the growth of the largest scale motions in the flow. Spectral analysis is employed to demonstrate the dominant effect of the tip vortices in the energy distribution in the near-wake region and uncover meandering motions.
206 citations
TL;DR: In this paper, it is shown that the instantaneous vorticity flux on either side of this jet is rapidly balanced immediately after the onset of separation from the free surface, and that the jet-like flow can be formed by small-scale breaking of a free-surface wave.
Abstract: Flow past a cylinder beneath a free surface gives rise to fundamental classes of nearwake structure that are distinctly different from the wake of a completely submerged cylinder. A central feature is the generation of a vorticity layer from the free surface due to: localized separation, in the form of small-scale breaking of a free-surface wave; or complete separation from the free surface. This vorticity layer appears adjacent to a layer from the surface of the cylinder, thereby forming a jet-like flow. It is shown that the instantaneous vorticity flux on either side of this jet is rapidly balanced immediately after the onset of separation from the free surface.
147 citations
TL;DR: In this article, a simulation of two-dimensional flow past a cylinder close to a free surface at a Reynolds number of 180 is numerically investigated and the wake behavior for Froude numbers between 0.0 and 0.7 and for gap ratios between 2.1 and 5.0 is examined.
Abstract: Two-dimensional flow past a cylinder close to a free surface at a Reynolds number of 180 is numerically investigated. The wake behaviour for Froude numbers between 0.0 and 0.7 and for gap ratios between 0.1 and 5.0 is examined. For low Froude numbers, where the surface deformation is minimal, the simulations reveal that this problem shares many features in common with flow past a cylinder close to a no-slip wall. This suggests that the flow is largely governed by geometrical constraints in the low-Froude-number limit.At Froude numbers in excess of 0.3–0.4, surface deformation becomes substantial. This can be traced to increases in the local Froude number to unity or higher in the gap between the cylinder and the surface. In turn, this is associated with supercritical to subcritical transitions in the near wake resulting in localized free-surface sharpening and wave breaking. Since surface vorticity is directly related to surface curvature, such high surface deformation results in significant surface vorticity, which can diffuse and then convect into the main flow, altering the development of Strouhal vortices from the top shear layer, affecting wake skewness and suppressing the absolute instability. The variations of parameters such as Strouhal number and formation length are provided for Froude numbers spanning the critical range.At larger Froude numbers, good agreement is obtained with recently published experimental investigations. The previously seen metastable wake states are observed to occur for similar system parameters to the experiments despite the difference in Reynolds numbers by a factor of about 40. The wake state switching appears to be controlled by a feedback loop. Important elements of the feedback loop include the cyclic generation and suppression of the absolute instability of the wake, and the role of surface vorticity and vortices formed from the bottom shear layer in controlling vortex formation from the top shear layer. The proposed mechanism is presented. Shedding ceases at very small gap ratios (–0.2). This behaviour can be explained in terms of the fluid flux through the gap, vorticity diffusion into the surface and opposite-signed surface vorticity from the strong surface deformation.
122 citations