Carlos Levi

A Two-Dimensional Numerical Investigation of the Hysteresis Effect on Vortex Induced Vibration on an Elastically Mounted Rigid Cylinder

The hysteresis effect on the vortex induced vibration (VIV) on a circular cylinder is investigated by the numerical solution of the two-dimensional Reynolds averaged Navier-Stokes equations. An upwind and total variation diminishing (TVD) conservative scheme is used to solve the governing equations written in curvilinear coordinates and the k-e turbulence model is used to simulate the turbulent flow in the wake of the body. The cylinder is supported by a spring and a damper and free to vibrate in the transverse direction. In previous work, numerical results for the amplitude of oscillation and vortex shedding frequency were compared to experimental data obtained from the literature to validate the code for VIV simulations. In the present work, results of practical interest are presented for the power absorbed by the system, phase angle, amplitude, frequency, and lift coefficient. The numerical results indicate that the hysteresis effect is observed only when the frequency of vortex shedding gets closer to the natural frequency of the structure in air.

PREDICTION OF VORTEX-INDUCED VIBRATION OF LONG FLEXIBLE CYLINDERS MODELED BY A COUPLED NONLINEAR OSCILLATOR: INTEGRAL TRANSFORM SOLUTION *

The Generalized Integral Transform Technique (GITT) was applied to predict dynamic response of Vortex-Induced Vibration (VIV) of a long flexible cylinder. A nonlinear wake oscillator model was used to represent the cross-flow force acting on the cylinder, leading to a coupled system of second-order Partial Differential Equations (PDEs) in temporal variable. The GITT approach was used to transform the system of PDEs to a system of Ordinary Differential Equations (ODEs), which was numerically solved by using the Adams-Moulton and Gear method (DIVPAG) developed by the International Mathematics and Statistics Library (IMSL). Numerical results were presented for comparison to those given by the finite difference method and experimental results, allowing a critical evaluation of the technique performance. The influence of variation of mean axial tension induced by elongation of flexible cylinder was evaluated, which was shown to be not negligible in numerical simulation of VIV of a long flexible cylinder.

Validation of a finite difference method for the simulation of vortex-induced vibrations on a circular cylinder

Abstract The Karman Vortex Street generated by a circular cylinder is investigated by the numerical solution of the compressible Navier–Stokes equations in the incompressible Mach number range (Mach Chorin, 1968 ) is fairly good while convergence time is very much better. The investigation suggests that the compressible Navier–Stokes equations may be used as an efficient alternative to study incompressible flows as well. Mach numbers just below 0.3 are enough to simulate incompressible flow behavior and at the same time do not cause numerical ill-conditioning in the solution. In addition, some relevant features of the vortices generated and carried by the wake of the cylinder could be fairly well captured.

Numerical Simulation of Roll Damping of a FPSO

The viscous flow problem of roll damping of a FPSO is investigated by means of numerical solution of the unsteady two-dimensional Navier-Stokes equations. The finite volume method using unstructured grid is used to solve the integral form of the governing equations. The cross section of the FPSO hull with an initial roll displacement is left free to oscillate in roll, heave and sway in an initially still fluid. The numerical simulation provides a realistic picture of the physics of the phenomenon, capturing the vortex formation around the bilge keel. The numerical results are compared with experimental data showing a fairly good qualitative and quantitative agreement of the motion damping.Copyright

A Numerical Investigation of Vortex Induced Vibration on an Elastically Mounted Rigid Cylinder

The Vortex-Induced Vibration on an elastically mounted circular cylinder is investigated by the numerical solution of the two-dimensional Reynolds Averaged Navier-Stokes equations and results are compared with experimental data. The upwind TVD scheme of Roe – Sweby is used to solve the governing equations and the k-e turbulence model is used to simulate the turbulent flow in the wake of the cylinder. The cylinder is laterally supported by a spring and a damper and is free to oscillate in the transverse direction. Results for the lift coefficient amplitude, displacement amplitude, frequency, phase angle, and power absorbed by the system are presented and compared to experimental data. The code was tested for the fixed cylinder case, and for the moving cylinder. The comparison with experimental data obtained from the literature showed the good quality of the numerical results and validated the code for simulations of vortex-induced vibration.Copyright

An experimental study of the spanwise correlation of vortex shedding in the towing tank

Although there are many correlation studies dedicated to the vortex shedding of cylinders in wind tunnels, the experiments conducted in water have not been carried out well before, especially for the flexible cylinder. In order to investigate this area, the present experiment was conducted in a towing tank and both the rigid and flexible cylinders were used. Various results are obtained including the correlation coefficients, correlation lengths and probability distribution, which exhibit several valuable features. Firstly, the correlation coefficients of the flexible cylinder are more irregular than that of the rigid cylinder. Secondly, the correlation lengths have visual peaks at Re ≈ 5.0 ∼ 6.0 × 103 for both the rigid and flexible cylinders, and a sharp decrease after this region should involve the inception of vortex dislocations. In addition, the analysis of root mean square and probability density function of the phase drift gives more insights of the vortex shedding along the spanwise different cross-sections.

Behavior of a Mono-Column Structure (MonoBr) in Waves

MonoBr is the name of a concept of a mono-column structure with a moon-pool developed by PETROBRAS to operate in deep water. A set of tests has been carried out at LabOceano / COPPE / UFRJ to analyze its behavior in waves. Different configurations of the moon-pool entrance have been tested. The main objective of the measurements carried out is to determine the influence of different restrictions on the behavior of the vertical motion of the structure. Results of these measurements are presented and discussed in the paper.Copyright

A Numerical Investigation of the Hysteresis Effect on Vortex Induced Vibration on an Elastically Mounted Rigid Cylinder

The hysteresis effect on the vortex induced vibration (VIV) on a circular cylinder is investigated by the numerical solution of the Reynolds average Navier-Stokes equations. An upwind and Total Variation Diminishing (TVD) conservative scheme is used to solve the governing equations written in curvilinear coordinates and the k-e turbulence model is used to simulate the turbulent flow in the wake of the body. The cylinder is supported by a spring and a damper and free to vibrate in the transverse direction. In previous work, numerical results for the amplitude of oscillation and vortex shedding frequency were compared to experimental data obtained from the literature to validate the code for VIV simulations. In the present work, results of practical interest are presented for the power absorbed by the system, phase angle, amplitude, frequency, and lift coefficient. The numerical results indicate that the hysteresis effect is observed only when the frequency of vortex shedding gets closer to the natural frequency of the structure in air.Copyright

Chapter 27 – Calculation of the flow around ship hulls using a parallel CFD code

Publisher Summary This chapter discusses calculation of the flow around ship hulls using a parallel computational fluid dynamics (CFD) code. A free surface is used for simulating the flow around ship hulls. The working environment with the specific compilation options is used to assess their efficiency. Several configurations of the 4-nodes cluster are compared in terms of speed. The free surface incompressible turbulent flow around the hull is investigated by the numerical solution of the unsteady Navier–Stokes equations for slightly compressible flows, and the results are compared with different numerical and experimental results. The “beam” and “warming” implicit factorized scheme is used to solve the governing equations. Large eddy simulation is used together with the Smagorinsky subgrid scale model to simulate the turbulent flow. The free surface is treated through the re-meshing technique, using an algebraic routine for accompanying the displacements. The complexity and high sensitivity of such a physical flow phenomenon require an accurate and robust numerical model. The proposed numerical solution is able to provide a good picture of the real physics of the turbulent flow. The numerical results compare fairly well with experimental results from various sources.

Numerical Simulations of Regular Waves in a Hydrodynamic Laboratory Basin

Hydrodynamic behavior of offshore floating structures is of fundamental practical importance to engineers and designers. Physical modelling of offshore structures in hydrodynamic laboratory is a common practice in this field. Due to the increasing in the computer power and the development of the numerical algorithms, the use of the numerical wave tanks (NWT) have become a complementary tool to the model tests. The knowledge of the drawbacks of numerical model is an important issue for engineers and researchers, especially in the models whose governing equations do not have exact solution. This work presents the application of numerical model to simulate the generation and propagation of regular waves in the LabOcean Basin (LabOceano/ COPPE/UFRJ) using the commercial code Ansys-CFX. A cross-section of the offshore basin have been used. Waves have been generated by flap type wavemaker. Period wave in the range from 1.75 to 3.00 seconds have been simulated. For all tests, the analytical wave steepness is smaller than 0.017. Numerical results have been compared with Stokes wave theory and experimental data obtained in the offshore basin. Both the behavior of the free surface and the reflection coefficient have been evaluated. Good agreement was found for the wave profile, mean wave height and mean wave period. Divergences between numerical and experimental results were found evaluating the reflection parameters.Copyright