Analysis of added mass in cavitating flow

The paper addresses a theoretical study of the added mass effect in cavitating flow.The cavitation is considered to induce a strong time–space variation of the fluid density at the interface between an inviscid fluid and a three-degree-of-freedom rigid section. The coupled problem is then simplified to a Laplace equation written for the pressure with a boundary condition at the fluid–structure interface depending on the acceleration, the velocity of the structure and on the rate of change of flow density. It is shown that contrary to the homogeneous flow, the added mass operator is not symmetrical and depends on the flow through fluid density variation. The added mass coefficients decrease as the cavitation increases which should induce an increase of the natural structural frequencies. The model shows also an added damping operator related to the rate of change of flow density. Added damping coefficients are found to be positive or negative according to the rate of change of the fluid density, indicating the possibility of instability development between flexible structures and unsteady cavitating flows

An experimental study of boundary-layer transition induced vibrations on a hydrofoil

This paper aims at characterizing experimentally laminar to turbulent transition induced vibrations. Here, the transition is known to be triggered by a Laminar Separation Bubble that results from a laminar separation of the boundary-layer flow on a hydrofoil. In this study we consider two NACA66312 (Mod) laminar hydrofoils at low angles of incidence (mostly 2° and 4°) and Reynolds numbers ranging from Re=450 000 to 1 200 000, in order to get transitional regimes. The first hydrofoil, made of steel (E=2.1×1011 Pa), is referred to as the rigid hydrofoil, although it is seen to vibrate under the action of the LSB. To better understand the possible interaction between the flow and the foil vibrations, vibration measurements are repeated using a flexible hydrofoil (E=3×109 Pa) of same geometry (under zero loading) and in close configurations. The experiments are carried out at the French Naval Academy Research Institute (IRENav, France). Wall pressure and flow velocity measurements enable a characterization of the laminar separation bubble and the identification of a vortex shedding at a given frequency. It is hence shown that the boundary-layer transition induces important foil vibrations, whose characteristics in terms of frequency and amplitude depend on the vortex shedding frequency, and can be coupled with natural frequencies of the hydrofoils

Analysis of added mass in cavitating flow

The paper addresses a theoretical study of the added mass effect in cavitating flow.The cavitation is considered to induce a strong time–space variation of the fluid density at the interface between an inviscid fluid and a three-degree-of-freedom rigid section. The coupled problem is then simplified to a Laplace equation written for the pressure with a boundary condition at the fluid–structure interface depending on the acceleration, the velocity of the structure and on the rate of change of flow density. It is shown that contrary to the homogeneous flow, the added mass operator is not symmetrical and depends on the flow through fluid density variation. The added mass coefficients decrease as the cavitation increases which should induce an increase of the natural structural frequencies. The model shows also an added damping operator related to the rate of change of flow density. Added damping coefficients are found to be positive or negative according to the rate of change of the fluid density, indicating the possibility of instability development between flexible structures and unsteady cavitating flows

Nonlinear disturbance evolution in a two-dimensional boundary layer along an elastic plate and induced radiated sound

The interaction between a boundary-layer flow and an elastic plate is addressed by direct numerical simulation, taking into account the full coupling between the fluid flow and the flexible wall. The convectively unstable flow state is harmonically forced and two-dimensional nonlinearly saturated wavelike disturbances are computed along archetype-plates with respect to stiffness and natural frequencies. In the aim of determining the low-Mach-number radiated sound for the system, the simulation data are used to solve the Lighthill’s equation in terms of a Green’s function in the wavenumber-frequency space. Different degrees of fluid-structure coupling are implemented in the radiated sound model and the resulting acoustic pressure levels are compared. The sound radiation levels are shown to be increased in the presence of flexible walls with however significant differences in the radiated pressure levels for different coupling assumptionsThe authors gratefully acknowledge Thales Underwater Systems and DCNS for their financial support to this work

Cavity induced vibration of flexible hydrofoils

The objective of this work is to investigate the influence of cavity-induced vibrations on the dynamic response and stability of a NACA66 hydrofoil at 8° angle of attack at Re=750 000 via combined experimental measurements and numerical simulations. The rectangular, cantilevered hydrofoil is assumed to be rigid in the chordwise direction, while the spanwise bending and twisting deformations are represented using a two-degrees-of-freedom structural model. The multiphase flow is modeled with an incompressible, unsteady Reynolds Averaged Navier–Stokes solver with the k–ω Shear Stress Transport (SST) turbulence closure model, while the phase evolutions are modeled with a mass-transport equation based cavitation model. The numerical predictions are compared with experimental measurements across a range of cavitation numbers for a rigid and a flexible hydrofoil with the same undeformed geometries. The results showed that foil flexibility can lead to: (1) focusing – locking – of the frequency content of the vibrations to the nearest sub-harmonics of the foil׳s wetted natural frequencies, and (2) broadening of the frequency content of the vibrations in the unstable cavitation regime, where amplifications are observed in the sub-harmonics of the foil natural frequencies. Cavitation was also observed to cause frequency modulation, as the fluid density, and hence fluid induced (inertial, damping, and disturbing) forces fluctuated with unsteady cavitation.The authors gratefully acknowledge Ms. Kelly Cooper (program manager) and the Office of Naval Research (ONR), for their financial support through Grant nos. N00014-11-1-0833 and N0014-12-C-0585, as well as ONR Global and Dr. Woei-Min Lin (program manager) through grant no. N62909-12-1-7076

URANSE simulation of an active variable-pitch cross-flow Darrieus tidal turbine: Sinusoidal pitch function investigation

This article describes a 2D CFD simulation implementation of a crossflow tidal turbine, the blades of which have their pitch modified during revolution. Unsteady flow around the turbine is computed with an URANSE method, using the solver ANSYS-CFX. Spatial and temporal discretizations have been studied. The pitch motion of the blades is obtained through mesh deformation, and the main rotation is implemented through sliding boundaries, with general grid interface model. The turbulence model used is kx SST. Langtry Menter transition model was tried but showed high discrepancies with experimental results. Five experimental cases were used to assess the accuracy of the simulation. It provided accurate computed forces for a wide range of tip speed ratios, and proved to be suitable for exploratory simulations. Harmonic pitch control was thus implemented for a tip speed ratio of 5, close to an operational value for a crossflow turbine. First, second and third harmonics pitch function were tested. It was shown that an improvement of more than 50% could be achieved with the second harmonics, with a large reduction in thrust. The flow inside the turbine and close to the blade was examined so that the case of performance improvement due to pitch control could be clearly understood. It was observed that turbine efficiency improvement requires a very slight recirculation and an angle of attack decrease on the upstream part of the turbine, and an angle of attack increase on the downstream part. The flow deceleration through the turbine was found to be a primary factor in pitch function as well. Moreover the hydrodynamic torque and thus the energy required to control the pitch were found to be insignificant

Tonal Noise Control Versus Performances on NACA0015 Hydrofoil

Strong correlation between boundary layer excitation and hydrofoil eigenmode occurs for specific angles of attack and Reynolds numbers, resulting in strong tonal noise emissions under certain circumstances. This study aims to analyze the performance of vibration control methods such as truncated trailing edge or boundary layer triggering. The study explores the conditions for hydro elastic trailing edge vibrations on a hydrofoil clamped in a hydrodynamic tunnel, with tests performed for Reynolds numbers up to 1.2 × 10⁶ and various angles of attack up to 10°. Experiments were conducted on a clamped 0.1 m chord NACA0015 aluminum hydrofoil, with measurements taken for both vibrations and hydrodynamic components such as lift, drag, and moment. The goal of the study is to closely understand the impact of vibration control on hydrofoil efficiency. This could be very beneficial for those who are working on optimizing profile design. The study first presents the experimental setup, followed by an analysis of the hydrofoil’s vibratory response and a presentation of the effectiveness of control solutions. Then, the performance of these solutions is discussed followed by a conclusion

Concepts, Modélisation et Commandes des Hydroliennes

Face au problème de la gestion des déchets nucléaire et aux émissions de gaz à effet de serre, les énergies renouvelables occupent une place avancée parmi les énergies d’avenir grâce à leur faible impact sur l’environnement ; d’autant plus que ces énergies jouent un rôle important dans la lutte contre le changement climatique et dans le développement économique de certains pays. Ces atouts, alliés à des technologies de plus en plus performantes, favorisent le développement des énergies renouvelables mais de manière encore très inégale selon le type de ressources considérées. Une de ces énergies renouvelables, l’énergie hydrolienne, suscite depuis quelques années un intérêt particulier du fait de ses nombreux avantages. En effet, la force et la vitesse des courants de marée, phénomène prédictible, peuvent être connues des décennies à l’avance. Pour une hydrolienne placée à un endroit donné, il est donc possible, par opposition aux autres énergies renouvelables, dépendant des conditions météorologiques, de connaître à tout moment quelle sera, au premier ordre, la puissance extractible par les gestionnaires de réseaux d’énergie afin d’alimenter ses consommateurs. De plus, les pays d’Europe de l’Ouest et en particulier le Royaume Uni et la France possèdent de nombreux sites près des cotes ou cette énergie est exploitable dans de bonnes conditions économiques. Le but de chapitre est la présentation succincte des principaux concepts hydroliens puis de donner des éléments quant à la modélisation d’un concept de base, et enfin introduire des éléments de contrôle/command

An Experimental Investigation of Partial Cavitation on a Two - Dimensional Hydrofoil

An investigation of leading edge partial cavitation was performed including the cavitation inception conditions, the cavitation patterns together with cavity length measurements. The investigation was enhanced by wall-pressure measurements using an instrumented hydrofoil equipped with seventeen wall-pressure transducers. Several experimental results are presented in the paper. A peak of pressure fluctuations was recorded at the cavity closure. The peak magnitude was seen to increase with the cavity length. For cavity lengths that did not exceed about half the foil chord, the peak originated from pressure fluctuations at a Strouhal number based on the cavity length close to 0.2. For longer cavities, the cavity began to pulsate at a lower frequency with the cavity length varying from the vicinity of the leading edge up to the trailing edge. The reason for such a phenomena is discussed

Influence of the Cavitation Model on the Simulation of Cloud Cavitation on 2D Foil Section

For numerical simulations of cavitating flows, many physical models are currently used. One approach is the void fraction transport equation-based model including source terms for vaporization and condensation processes. Various source terms have been proposed by different researchers. However, they have been tested only in different flow configurations, which make direct comparisons between the results difficult. A comparative study, based on the expression of the source terms as a function of the pressure, is presented in the present paper. This analytical approach demonstrates a large resemblance between the models, and it also clarifies the influence of the model parameters on the vaporization and condensation terms and, therefore, on the cavity shape and behavior. Some of the models were also tested using a 2D CFD code in configurations of cavitation on two-dimensional foil sections. Void fraction distributions and frequency of the cavity oscillations were compared to existing experimental measurements. These numerical results confirm the analytical study