Flow interaction between a streamwise oscillating cylinder and a downstream stationary cylinder

In this paper, we present some experimental results about the physical effects of a cylinder’s streamwise oscillation motion on a downstream one in a tandem arrangement. The upstream cylinder undergoes a controlled simple harmonic oscillation at amplitudes A/d = 0.2–0.8, where d is the cylinder diameter, and the frequency ratio of fe/fsfe/fs = 0–3.0, where fefe is the cylinder oscillation frequency and fsfs is the natural frequency of vortex shedding from a single stationary cylinder. Under these conditions, the vortex shedding is locked to the controlled oscillation motion. Flow visualisation using the planar laser-induced fluorescence and qualitative measurements using hot-wire anemometry reveal three distinct flow regimes behind the downstream cylinder. For fe/fs>(fe/fs)cfe/fs>(fe/fs)c , where (fe/fs)c(fe/fs)c is a critical frequency ratio which depends on A/d and Reynolds number Re, a so-called SA-mode occurs. The upstream oscillating cylinder generates binary vortices symmetrically arranged about the centreline, each containing a pair of counter-rotating vortices, and the downstream cylinder sheds vortices alternately at 0.5fe0.5fe . For 0.7–1.

Three-Dimensionality in the flow of an elastically mounted circular cylinder with two-degree-of-freedom vortex-induced-vibrations

The study numerically investigates the three-dimensionality in the flow and two-degree-of-freedom (2 DOF) vortex-induced-vibrations (VIV) characteristics of an elastically mounted circular cylinder. The cylinder is allowed to vibrate in both streamwise and transverse directions. A low value of mass-ratio with the zero damping coefficient is taken for the simulations. The primary aim is to understand the vortex shedding behind the cylinder and the transition characteristics of the wake-flow from two-dimensional (2D) to three-dimensional (3D). The Reynolds number (Re) is varied from 150 (fully 2D flow) to 1000 (fully 3D flow), which lies inside the laminar range. The reduced velocity is varied which covers all three major VIV branches (Initial Branch (IB), Upper Branch (UB), and the Lower Branch (LB)). The oscillating cylinder sweeps the figure-eight trajectory. Two branches (IB, LB) and three branches (IB, UB, LB) amplitude responses are obtained for the low and high Re values, respectively. The wake behind the cylinder with 2-DOF VIV undergoes the mode-C transition of 2D to 3D flow as opposed to the direct mode-B transition observed for transverse only VIV in the literature. The critical Re range of the 2D to 3D transition for the 2-DOF VIV cylinder at a reduced velocity of 6 is around 250, less than the 1-DOF VIV. Also, this range varies with the variation in and the streamwise to transverse oscillation frequency ratio. A map is proposed for the 2-DOF VIV, highlighting the different modes of transition obtained for combinations of reduced frequency and Re

DYNAMICS OF A CIRCULAR CYLINDER IN CLOSE PROXIMITY TO A WALL

The effect of wall proximity on the mechanism of flow-induced vibration of a circular cylinder mounted in a wind tunnel and free to vibrate with two degrees-of-freedom near a rigid plane boundary were examined at a Reynolds number of 1.86 x 105. Hot-wire anemometry and cylinder-mounted accelerometers were used to characterize the flow-induced vibration of the cylinder. In the near wall region (gap ratios, G/D\u3c 0.4) the system was found to exhibit symptoms of movement-induced vibration resulting from the push-and-draw effect of the cylinder pushing into and out of the fluid immediately upstream and downstream of the cylinder. The variation of the width of the separated wake region with the cylinder motion produces a force in phase with the cylinder velocity, resulting in excitation of the cylinder motion. For G/D \u3e 1.0, the alternate shedding of vortices (the Kármán vortex street) produces a fluctuating lift on the body. The lift force also has an associated drag fluctuation at twice the vortex shedding frequency For 0.4 \u3e G/D\u3e 1.0, the excitation appears to be due to the combined effects of the movement-induced vibration found in the near wall region with the vortex shedding from the cylinder found for the free cylinder when it is far from the wall.. The system exhibited mechanical coupling of the two degrees-of-freedom, so additional tests will be needed to conclusively confirm the findings presented in this thesis

Reduced Order Models for Hydrodynamic Analysis of Pipelines based on Modal Analysis and Machine Learning

Bluff bodies have extensive implementation in engineering. For instance, marine risers, jumpers, umbilicals, and bundle flowlines are the examples of the circular bluff bodies which are used in the field of offshore engineering. They can be designed both to be fixed or flexible supported and to be a single or tandem configurations. The subsea structures are subjected to the external hydrodynamic loads such as cyclic loads, vibrations, high pressure, etc. For example, one of the commonly observed damaging flow features associated with hydrodynamics of the flexible supported bluff bodies is vortex shedding. Therefore, investigation of the instantaneous flow structures and the hydrodynamic forces acting on the bluff bodies at the operational conditions need to be performed to prevent the degradation mechanisms and increase service life of the subsea structures. Nowadays, the modern techniques are used in order to analyse the flow field data with high efficiency. For example, neural networks (NNs) are trained with massive experimental or numerical simulation data to predict the spatial-temporal evolution of the dominant coherent structures of the flow field and structural behaviour can be considered as an alternative to the conventional computational fluid dynamics (CFD) simulations. In the present thesis, numerical investigations of the flow around cylindrical bluff bodies in the upper transition Reynolds number regime ( = 3.6 ∙ 10^6 ) are performed. Two pipeline operational conditions are considered. First one is tandem configuration of the two stationary pipelines subjected to steady flow. Second one is a pipeline undergoing the vortex-induced-vibrations (VIV) subjected to a steady current. Two-dimensional (2D) Unsteady Reynolds-Averaged-Navier-Stokes (URANS) equations combined with the standard k−w SST turbulence model are solved.  The open source CFD toolbox OpenFOAM v2012 is employed to perform the simulations. The Reduced Order Models (ROMs) which can provide a low-dimensional representation of the simulation data with reduced computational time and cost are designed. Dynamic mode decomposition (DMD) and proper orthogonal decomposition (POD) techniques are implemented for the first and second cases, respectively. In addition, further development of the ROMs for the VIV cylinder case is done by implementing the long short-term neural network (LSTM-NN). The neural network based model allows to make the predictions of the dominant hydrodynamic characteristics of the flow around the cylindrical bluff bodies subjected to a high Reynolds number flow at a future time instances with a reduced computational cost

Hydrodynamic Forces acting on Two Flexible Free-hanging Cantilevers in Tandem Configurations due to Cross-flows

The experimental study has been performed on two flexible free-hanging circular cantilevers in tandem configurations subjected to uniform cross-flows. The experiment was intended to investigate the time-dependent forces characteristics acting on the cylinders due to VortexInduced Vibration (VIV) phenomenon. The tests cylinders have free bottom-end conditions and can freely oscillate. The motions of the cylinders are evaluated as a bidirectional motion, in-line and transverse to the flow. Each cylinder has a length-to-diameter ratio of 34.4 with a low mass ratio of about 1.24. Based on cylinder’s diameter and free-stream flow velocities, the Reynolds number varied from 10,800 to 37,800. For examining Wake Induced Vibration (WIV) on the induced forces characteristics, five different gaps between the cylinders were employed. New various findings indicated that the dynamics of the present two free-hanging cantilever cylinders in tandem configurations are unique and definitely different to those other tandem configurations of either, two stationary cylinders or a transverse-only motion downstream cylinder lies behind a stationary upstream one

Investigation on the Interaction of an Impinging Jet with Cylinder Wakes

Jet impingement cooling is a widely used cooling method due to the high heat transfer rates associated with it. Research for improving heat transfer rates for this cooling is still being carried out due to its broad application in various fields like gas turbine blade cooling, electronic component cooling, and paper drying. The unsteady jet oscillation effectively enhances the stagnation region and the time-averaged heat transfer rates. It is shown that a novel passive jet oscillation technique can be achieved using the vortices periodically shed from a cylinder placed upstream in a channel with an initial crossflow. Preliminary CFD results prove the hypothesis of jet oscillation induced by the cylinder vortices and that the lateral jet oscillation is an efficient method for uniform distribution of heat transfer. The statistical analysis concluded jet oscillation is most sensitive to cylinder vortex strength. A frequency spectral analysis is performed to classify oscillating and non-oscillating cases. Finally, unsteady numerical and experimental research is carried out to determine the effect of cylinder-jet distance, cylinder diameter, and velocity ratio on jet oscillation and heat transfer rate. The range of cylinder-jet distance and velocity ratio tested are S/d = 2 – 4 and VR = 4 – 12, respectively. The flow interaction mechanism leading the jet oscillation is analyzed using TKE, vorticity, and velocity contours in time. The flow feature analysis concluded the cylinder wakes deformed the jet core inducing lateral and angular oscillations. The heat transfer results showed the Nusselt number is proportional to the velocity ratio for oscillating jet cases. The non-oscillating jet enhances the heat transfer rate by 94% in the wall jet region due to crossflow interaction. And the optimum oscillating jet case improved the stagnation region Nusselt number by 19%