F. Saltara

Two and Three-Dimensional Simulation of Sound Generated by Flow Around a Circular Cylinder

The objective of this paper is to calculate the far-field sound generated from low mach number flow around a two-dimensional and three-dimensional circular cylinder in the subcritical regime using the Lighthill acoustic analogy. For the two-dimensional case, the timedependent incompressible flow is predicted using unsteady Reynolds-averaged NavierStokes models. For the three-dimensional case, the flow was obtained by solving the filtered Navier-Stokes equations of the Large Eddy Simulation model. As a benchmark, a flow-field with a Reynolds number of 90,000 is employed. The obtained numerical results such as Strouhal number, fluctuating lift and mean drag are compared with experiments. The computed unsteady pressure fluctuations on the cylinder wall are used as a sound source for the acoustic solver. Comparison between the two-dimensional numerical results and the experiment shows that computed acoustic field overpredict the noise amplitude; however, good agreement is obtained if an appropriate correlation length is taken into account. The sound measurements obtained by Revell et al. were carried out with a much longer span cylinder length when compared to the span used for the three-dimensional LES simulations. Therefore, the far-field sound is estimated by two correction methods proposed, respectively, by Kato et al. and Seo & Moon. The aerodynamic and acoustic results obtained by the threedimensional approach agree favorably well with the corresponding experimental data.

Study of Stay Vanes Vortex- Induced Vibrations with different Trailing-Edge Profiles Using CFD

Abstract The 2D flow around 13 similar stay-vane profiles with different trailing edge geometries is investigated to determinate the main characteristics of the excitation forces for each one of them and their respective dynamic behaviors when modeled as a free-oscillating system. The main goal is avoid problems with cracks of hydraulic turbines components. A stay vane profile with a history of cracks was selected as the basis for this work. The commercial finite-volume code FLUENT® was employed in the simulations of the stationary profiles and, then, modified to take into account the transversal motion of elastically mounted profiles with equivalent structural stiffness and damping. The k-ω SST turbulence model is employed in all simulations and a deforming mesh technique used for models with profile motion. The static-model simulations were carried out for each one of the 13 geometries using a constant far field flow velocity value in order to determine the lift force oscillating frequency and amplitude as a function of the geometry. The free-oscillating stay-vane simulations were run with a low mass-damping parameter (m*ζ=0.0072) and a single mean flow velocity value (5m/s). The structural bending stiffness of the stay-vane is defined by the Reduced Velocity parameter (Vr). The dynamic analyses were divided into two sets. The first set of simulations was carried out only for one profile with 2≤Vr≤12. The second set of simulations focused on determining the behavior of each one of the 13 profiles in resonance.

Numerical Simulations of Vortex-Induced Vibration of Flexible Cylinders

The main purpose of this paper is to acquire a better understanding of the hydroelastic interactions, which take place between oscillating flexible cylinders and fluid forces. The cylinders are subjected to currents and shear flow, and the hydrodynamic forces are estimated by CFD tools. This article presents the results of an investigation being carried out at the University of Sao Paulo, in which a discrete vortex method is used to simulate the flow around a flexible cylinder. The calculations are compared with results obtained employing the quasi-steady theory, as proposed by Ferrari [2]. Also, the calculations are compared with experiments of a cantilever flexible cylinder immersed in a current, see Fujarra [6]. The reduced velocity vs. non-dimensional amplitude curve obtained in our calculations is compared with the experimental results. Visualizations of the wake indicate a hybrid mode of vortex shedding along the span. A 2S mode is found in regions of low amplitudes, changing to a 2P mode in the regions of larger amplitudes. The position of the transition of the modes varies with the reduced velocity. Our intention is to apply this model to problems occurring in the offshore industry. In this industry fluids are conveyed from the seabed to the platform through slender structures called risers. These risers are subject to shear and oscillatory flows due to currents and waves, respectively, flows with a very high degree of complexity, with changes of intensity and direction the deeper the water depth. A finite element structural model based on the Euler-Bernoulli beam theory was developed. In order to evaluate the dynamic response, a general equation of motion is solved through a numerical integration scheme in the time domain. The hydrodynamic forces are evaluated in two-dimensional strips. The technique used is the Discrete Vortex Method, which is a Lagrangian numerical scheme to simulate an incompressible and viscous fluid flow. A practical case of marine risers is also presented. In this case the results for various uniform currents acting on a single, flexible cylinder, representing a riser of 120m with 100m under water, are shown. Envelopes of maximum and minimum in-line and transverse displacements are presented. There is also a comparison of a shear flow case between the CFD numerical code with the quasi-steady theory code developed by Ferrari [2].Copyright

Numerical Simulations of the Flow Around Flexible Cylinders

In the offshore industry fluids are conveyed from the seabed to the platform through slender structures named risers. These risers are subject to shear and oscillatory flows due to currents and waves respectively, flows with a very high degree of complexity, with changes of intensity and direction the deeper the water depth. The main purpose of this work is to investigate the hydroelastic interactions which take place between flexible cylinders and fluid forces. The cylinders are subject to uniform flow, and the hydrodynamic forces are estimated by CFD, in a quasi three-dimensional fashion. This article presents the results of an investigation being carried out at the University of Sao Paulo and sponsored by the Brazilian Oil Company Petrobras. In this research a discrete vortex method is used to simulate the flow around a flexible cylinder. A description of this method can be found at Yamamoto et al. (OMAE 2001). A finite element structural model based on the Euler-Bernoulli beam theory was developed. In order to evaluate the dynamic response, a general equation of motion is solved through a numerical integration scheme in the time domain. The hydrodynamic forces are evaluated in two-dimensional strips. The technique used is the Discrete Vortex Method, which is a Lagrangian numerical scheme to simulate an incompressible and viscous fluid flow. The calculations are compared with experiments of a cantilever flexible cylinder immersed in a current, see Fujarra [6]. The reduced velocity vs. non-dimensional amplitude curve obtained in our calculations is compared with the experimental results. Visualizations of the wake indicate a hybrid mode of vortex shedding along the span. A 2S mode is found in regions of low amplitudes, changing to a 2P mode in the regions o larger amplitudes. The position of the transition of the modes varies with the reduced velocity. A practical case of marine risers is also presented. In this case the results for various uniform currents acting on a single, flexible cylinder, representing a riser of 120m with 100m under water, are shown. Envelopes of maximum and minimum in-line and transverse displacements are presented.Copyright

Comparative analysis of turbulence models for slat noise sources calculations employing structured meshes

Results are confronted against experimental results and simulations with quasi-laminar SST model. It was shown that the proposed Limited Numerical Scales model does not correctly couples unresolved and resolved scales, resulting in large vortices recirculating in the slat cove. With the large energy stimulation coupling, results match computations with quasilaminar approach and approximates experimental results. The need for three-dimensional simulations in order to capture noise generation mechanisms is then reinforced.

Investigation of the Flow Around Two Circular Cylinders Employing the Spectral Element Method With Modal Decomposition

Flow induced vibrations play an important role in the process of fatigue in cylindrical elements of offshore platforms. One of the main sources of vibration is due to vortex shedding. Such vortices are originated from the separation of the boundary layer and the low-pressure regions cause the hydrodynamic forces to oscillate. This kind of vibration is known in literature as vortex-induced vibration (VIV), and it has some points that are not well understood yet. When a circular cross section is considered, the point of separation changes with the Reynolds number, increasing the complexity of the phenomena. Additionally, when two bluff bodies are disposed near each other, the vortices shed by one of them can reach the other, interfering with the vortex formation close to the later, and constituting another source of vibration. This work is an effort to provide a better comprehension of the vortex-induced vibration phenomena in a flow around groups of cylinders. In this paper, a detailed computational study of the flow around two rigid cylinders in a tandem arrangement is carried out. The spectral/hp element method is used in the simulations, with a modal decomposition in the span direction. The three-dimensional structures in the wake are analysed, and the level of synchronization of the flow along the span is verified for a gap of five diameters between the cylinders.Copyright