Vortex shedding modes around oscillating non-uniform double heave plates

Multiple co-axial heave plates of uniform geometry are attached to offshore platforms for inducing damping and added mass. These effects generally decrease the magnitude of the dynamic response of the platform under applied environmental excitation forces. When spacing between heave plates is decreased the damping and added mass performance are altered due to their strong vortex interaction. A new non-uniform plate configuration is proposed that may create different hydrodynamic characteristics. The modes of vortex shedding around plate edges in a non-uniform arrangement under forced harmonic oscillation are investigated via the CFD method. Furthermore, a new simplified formula for the total theoretical added mass of a general non-uniform double disk is proposed when it is in the zone which vortex interaction take places. The results show that a non-uniform double disk has better hydrodynamic performance as compared to the uniform double configuration for a given spacing. It is also observed that the mode and direction of the vortex shedding are different for uniform and non-uniform arrangements in a given spacing and KC value.

Numerical investigation of vortex-induced vibration for two tandem circular cylinders with different diameters

Vortex-induced vibration of two tandem circular cylinders with different diameters under low Reynolds number (Re = 200) is investigated numerically by solving the uncompressible two-dimensional Navier–Stokes equations. The arbitrary Lagrange–Euler method is used to simulate the movement of mesh. Effects of diameters and gap ratios are considered. Numerical results show that diameter ratios and gap ratios have little effect on maximum vibration amplitude. Four different vortex shedding modes are detected in this study.

Three-Dimensional Direct Numerical Simulations of Flows Past an Inclined Cylinder Near a Plane Boundary

Flows past an inclined cylinder in the vicinity of a plane boundary are numerically investigated using direct numerical simulations. Parametric studies are carried out at the normal Reynolds number of 500, a fixed gap ratio of 0.8 and five inclination angles (α) ranging from 0° to 60° with an increment of 15°. Two distinct vortex-shedding modes are observed: parallel (α ≤ 15°) and oblique (α ≥ 30°) vortex shedding modes. The occurrence of the oblique vortex shedding is accompanied by the base pressure gradient along the cylinder span and the resultant axial flows near the cylinder’s base. The drag and lift coefficients decrease from the parallel mode to the oblique mode, owing to the intensified three-dimensionality of the wake flows and the phase difference in the vortex-shedding along the span. The Independent Principle (IP) is valid in predicting the hydrodynamic forces and the wake patterns when α ≤ 15°, and IP might produce unacceptable errors when α ≥ 30°. Compared to the mean drag force, the fluctuating lift force is more sensitive to the inclination angle. The IP validity range is substantially smaller than that for flows past a wall-free cylinder.

Interaction Between IL and CF VIV: On the Importance of Orbital Direction

This paper discusses results from an experiment where forces on a rigid cylinder are measured during prescribed oscillations both in-line with and transverse to a constant flow. Two “figure of eight” oscillation patterns with identical shape but opposite orbital direction, relative to the flow, have been tested at a Reynolds number of 24000. Results show that the hydrodynamic force acting on the cylinder is significantly different for the two orbital directions. The force in phase with velocity, which represents the energy transfer between the fluid and the cylinder, has opposite sign and different magnitude for the two orbital directions. Flow visualization by particle image velocimetry (PIV) reveals that the two orbits leads to different vortex shedding modes. Hydrodynamic forces at multiples of the oscillation frequency, known as higher harmonics, are seen for both orbital directions. Comparison with pure in-line and pure transverse oscillations indicates that the higher harmonics are related to oscillations in in-line direction. A three-dimensional Large Eddy Simulation numerical simulation with equivalent experiment parameters has been conducted. It is very encouraging to see a good agreement between numerical results and observations with respect to global forces, vortex shedding modes and hydrodynamic co-efficients.

POD ANALYSIS OF THREE-DIMENSIONAL HARMONICALLY FORCED WAKE FLOW OF A CIRCULAR CYLINDER

A 3D numerical simulation of a circular cylinder wake is presented in this paper. The cylinder is harmonically forced in the stream-wise direction. The objective of the present work is to investigate the effect of the oscillation amplitude on the secondary transition of the wake. The frequency of the lift force is then linked to the form of the vortex shedding mode. The relation between these vortex shedding modes using POD analysis of the transverse velocity and the unsteady lift coefficient of 3D simulation is in good agreement with the 2D model. Results show that the 3D spanwise effect, which can change the wake structure, is suppressed at Re = 200 by streamwise oscillation of the cylinder. Thus the 2D analysis can effectively model the temporal instability of the wake flow.

Near-Wake Flow Structures of a Corner-Chamfered Square Cylinder at Higher Harmonic Excitations

In this paper, the results of a flow visualization study on the flow structures around a chamfered square cylinder are presented. Square cylinders having side dimension Bo with corner chamfering ‘b’ are used such that b/Bo ratio assumes values 0, 0.1, 0.2 and 0.3. Corners of the square cylinder are equally cut by a measure ‘b’ both in the stream-wise and cross stream directions. Flow over these cylinders are visualized in a water channel. All the studies correspond to a Reynolds number value of 2100 (based on Bo). Results are taken for two situations (a) cylinders are stationary and (b) cylinders are oscillated at frequency ‘fe’. The main objective of this study is to investigate the near-wake flow structures around the cylinders at harmonic and higher harmonic excitations. Experiments were conducted for fe/fs= 1.0, 1.5, 2.0, 2.5 and 3.0 where fs is the vortex shedding frequency from the stationary cylinder for each b/Bo ratio. Peak-to-peak amplitude of excitation is kept at 1B in all cases. In this investigation, the main focus is on investigating the vortex shedding modes, mechanisms and the number of vortices shed per shear layer as the cylinder completes one oscillatory cycle as a function of fe/fs ratio.

Evolution of unstable system.

Scenario of appearance and development of instability in problem of a flow around a solid sphere at rest is discussed. The scenario was created by solutions to the multimoment hydrodynamics equations, which were applied to investigate the unstable phenomena. These solutions allow interpreting Stokes flow, periodic pulsations of the recirculating zone in the wake behind the sphere, the phenomenon of vortex shedding observed experimentally. In accordance with the scenario, system loses its stability when entropy outflow through surface confining the system cannot be compensated by entropy produced within the system. The system does not find a new stable position after losing its stability, that is, the system remains further unstable. As Reynolds number grows, one unstable flow regime is replaced by another. The replacement is governed tendency of the system to discover fastest path to depart from the state of statistical equilibrium. This striving, however, does not lead the system to disintegration. Periodically, reverse solutions to the multimoment hydrodynamics equations change the nature of evolution and guide the unstable system in a highly unlikely direction. In case of unstable system, unlikely path meets the direction of approaching the state of statistical equilibrium. Such behavior of the system contradicts the scenario created by solutions to the classic hydrodynamics equations. Unstable solutions to the classic hydrodynamics equations are not fairly prolonged along time to interpret experiment. Stable solutions satisfactorily reproduce all observed stable medium states. As Reynolds number grows one stable solution is replaced by another. They are, however, incapable of reproducing any of unstable regimes recorded experimentally. In particular, stable solutions to the classic hydrodynamics equations cannot put anything in correspondence to any of observed vortex shedding modes. In accordance with our interpretation, the reason for this isthe classic hydrodynamics equations themselves.