Jacques Andre Astolfi

An experimental study of unsteady partial cavitation

Unsteady partial cavitation can cause damage to hydraulic machinery and understanding it requires knowledge of the basic physics involved. This paper presents the main results of a research program based on wall-pressure measurements aimed at studying unsteadiness in partial cavitation. Several features have been pointed out. For cavity lengths that did not exceed half the foil chord the cavity was stated to be stable. At the cavity closure a peak of pressure fluctuations was recorded originating from local cavity unsteadiness in the closure region at a frequency depending on the cavity length. Conversely, cavities larger than half the foil chord were stated to be unstable. They were characterized by a cavity growth/destabilization cycle settled at a frequency lower than the previous ones. During cavity growth, the closure region fluctuated more and pressure fluctuations traveling in the cavity wake were detected

A joint experimental and numerical study of mechanisms associated to instability of partial cavitation on two-dimensional hydrofoil

The present work was carried out in the scope of a numerical-experimental collaborative research program, whose main objective is to understand the mechanisms of instabilities in partial cavitating flow. Experiments were conducted in the configuration of a rectangular foil located in a cavitation tunnel. Partial cavitation was investigated by multipoint wall-pressure measurements together with lift and drag measurements and numerical videos. The computations were conducted on two-dimensional hydrofoil section and are based on a single fluid model of cavitation: the liquid/vapor mixture is considered as a homogeneous fluid whose composition is regulated by a barotropic state law. The algorithm of resolution is derived from the SIMPLE approach, modified to take into account the high compressibility of the medium. Several physical features were pointed out by this joint approach. Particularly two distinct cavity self-oscillation dynamics characterized by two different frequencies (dynamics 1 and dynamics 2) ...

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.

Numerical Prediction of Cavitating Flow on a Two-Dimensional Symmetrical Hydrofoil and Comparison to Experiments

This paper presents comparisons between two-dimensional (2D) CFD simulations and experimental investigations of the cavitating flow around a symmetrical 2D hydrofoil. This configuration was proposed as a test case in the “Workshop on physical models and CFD tools for computation of cavitating flows” at the 5th International Symposium on cavitation, which was held in Osaka in November 2003. The calculations were carried out in the ENSTA laboratory (Palaiseau, France), and the experimental visualizations and measurements were performed in the IRENav cavitation tunnel (Brest, France). The calculations are based on a single-fluid approach of the cavitating flow: the liquid/vapor mixture is treated as a homogeneous fluid whose density is controlled by a barotropic state law. Results presented in the paper focus on cavitation inception, the shape and the general behavior of the sheet cavity, lift and drag forces without and with cavitation, wall pressure signals around the foil, and the frequency of the oscillations in the case of unsteady sheet cavitation. The ability of the numerical model to predict successively the noncavitating flow field, nearly steady sheet cavitation, unsteady cloud cavitation, and finally nearly supercavitating flow is discussed. It is shown that the unsteady features of the flow are correctly predicted by the model, while some subtle arrangements of the two-phase flow during the condensation process are not reproduced. A comparison between the peer numerical results obtained by several authors in the same flow configuration is also performed. Not only the cavitation model and the turbulence model, but also the numerical treatment of the equations, are found to have a strong influence on the results.

A model for tip vortex roll-up in the near field region of three-dimensional foils and the prediction of cavitation onset

Abstract A research program known as “Action Concertee Cavitation” initiated in France in 1991 was aimed at investigating Tip Vortex Cavitation (TVC) in various experimental facilities operating over a large range of Reynolds numbers. An original method to analyze the numerous data collected during the program was developed. From this method, a correlation model of the roll-up process close to the tip (near field region) of elliptical loading foils is proposed. It is based on the experimental evidence of a linear relationship, during the roll-up process, between a and (Γt) 0.5 , where a is the local vortex core radius, Γ the local vortex intensity and t the convection time. The core radius and the vortex intensity are modeled with power laws in such a way the linear relationship is satisfied. With the hypothesis of an axisymmetric vortex, the minimum of the pressure coefficient on the vortex path is computed. It agrees well with the experimental critical cavitation number obtained for lift coefficients ranging from 0.2 to 0.6 and for Reynolds numbers ranging from 4×10 5 to 6×10 6 . Moreover, the model shows that the actual vortex diffusion appears to be faster than that predicted by laminar flow hypothesis. This can be explained by an apparent viscosity of about two orders of magnitude larger than the molecular viscosity during the roll-up process. Finally, as the model is based on a limited number of parameters to describe a complex phenomenon, it indicates also some important trends which should be examined by those seeking to mitigate the occurrence of cavitation on lifting surfaces as designers of pumps, propellers and other fluid machinery.

Numerical and experimental investigation of natural flow-induced vibrations of flexible hydrofoils

The objective of this work is to present combined numerical and experimental studies of natural flow-induced vibrations of flexible hydrofoils. The focus is on identifying the dependence of the foil’s vibration frequencies and damping characteristics on the inflow velocity, angle of attack, and solid-to-fluid added mass ratio. Experimental results are shown for a cantilevered polyacetate (POM) hydrofoil tested in the cavitation tunnel at the French Naval Academy Research Institute (IRENav). The foil is observed to primarily behave as a chordwise rigid body and undergoes spanwise bending and twisting deformations, and the flow is observed to be effectively two-dimensional (2D) because of the strong lift retention at the free tip caused by a small gap with a thickness less than the wall boundary layer. Hence, the viscous fluid-structure interaction (FSI) model is formulated by coupling a 2D unsteady Reynolds-averaged Navier-Stokes (URANS) model with a two degree-of-freedom (2-DOF) model representing the spanwise tip bending and twisting deformations. Good agreements were observed between viscous FSI predictions and experimental measurements of natural flow-induced vibrations in fully turbulent and attached flow conditions. The foil vibrations were found to be dominated by the natural frequencies in absence of large scale vortex shedding due to flow separation. The natural frequencies and fluid damping coefficients were found to vary with velocity, angle of attack, and solid-to-fluid added mass ratio. In addition, the numerical results showed that the in-water to in-air natural frequency ratios decreased rapidly, and the fluid damping coefficients increased rapidly, as the solid-to-fluid added mass ratio decreases. Uncoupled mode (UM) linear potential theory was found to significantly over-predict the fluid damping for cases of lightweight flexible hydrofoils, and this over-prediction increased with higher velocity and lower solid-to-fluid added mass ratio.

An Experimental Study of Boundary-Layer Transition Induced Vibrations on a Hydrofoil

In this paper, we investigate through an experimental approach the laminar to turbulent transition in the boundary-layer flow along a hydrofoil at a Reynolds number of 7.5 × 105 , together with the vibrations of the hydrofoil induced by the transition. The latter is caused by a Laminar Separation Bubble (LSB) resulting from a laminar separation of the boundary-layer. The experiments, conducted in the hydrodynamic tunnel of the Research Institute of the French Naval Academy, are based on wall pressure and flow velocity measurements along a rigid hydrofoil, which enable a characterization of the Laminar Separation Bubble and the identification of a vortex shedding at a given frequency. Vibrations measurements are then carried out on a flexible hydrofoil in the same operating conditions. The results indicate that the boundary-layer transition induces important vibrations, whose characteristics in terms of frequency and amplitude depend on the vortex shedding frequency, and can be coupled with natural frequencies.Copyright

Fluid Structure Interaction Analysis on a Transient Pitching Hydrofoil

In this paper, the structural behavior of a deformable hydrofoil in forced pitching motion is analyzed through an experimental approach. The experimental study is based on the measurement in a hydrodynamic tunnel of the foil displacement obtained with a video camera. Tip section displacement is compared to the hydrodynamic loading obtained on a rigid hydrofoil using wall pressure measurement. The structural response appears to be strongly linked to hydrodynamic phenomena such as laminar to turbulent transition and leading edge vortex shedding. The influence of pitching velocity is discussed. Finally, the paper presents displacement measurements in cavitating flows.Copyright

Noise generation in an unstable boundary layer flow over a flexible wall

The boundary‐layer flow, for instance along a sonar dome, gives rise to hydrodynamic noise due to the pressure fluctuations. The prediction of the resulting self‐noise received by the sonar antenna is based on models, which in general take only partially into account the flexibility of the dome wall. The present work readdresses the problem of hydrodynamic noise, considering the geometrically simplified model of a two‐dimensional unstable boundary‐layer flow along an elastic plate with clamped ends. The incompressible Navier‐Stokes equations are fully coupled to the elastic plate model and the system is numerically solved for various plate materials. The unstable flow dynamics is analyzed with respect to the wall properties. The Fourier‐transformed stress tensor is then used in the framework of Lighthills analogy to determine the generated radiative sound, emphasizing the effect of wall‐flexibility. This work is supported by Thales Underwater System and DCNS.