Antoine Ducoin

Numerical Modeling of Unsteady Cavitating Flows around a Stationary Hydrofoil

The objective of this paper is to evaluate the predictive capability of three popular transport equation-based cavitation models for the simulations of partial sheet cavitation and unsteady sheet/cloud cavitating flows around a stationary NACA66 hydrofoil. The 2D calculations are performed by solving the Reynolds-averaged Navier-Stokes equation using the CFD solver CFX with the k-ω SST turbulence model. The local compressibility effect is considered using a local density correction for the turbulent eddy viscosity. The calculations are validated with experiments conducted in a cavitation tunnel at the French Naval Academy. The hydrofoil has a fixed angle of attack of α=6° with a Reynolds number of Re = 750,000 at different cavitation numbers σ. Without the density modification, over-prediction of the turbulent viscosity near the cavity closure reduces the cavity length and modifies the cavity shedding characteristics. The results show that it is important to capture both the mean and fluctuating values of the hydrodynamic coefficients because (1) the high amplitude of the fluctuations is critical to capturing the extremes of the loads to ensure structural safety and (2) the need to capture the frequency of the fluctuations, to avoid unwanted noise, vibrations, and accelerated fatigue issues.

Physical and numerical investigation of cavitating flows around a pitching hydrofoil

The objective of this paper is to investigate cavitating flows around a pitching hydrofoil via combined physical and numerical studies. The aims are to (1) improve the understanding of the interplay between unsteady cavitating flow, hydrofoil motion, and hydrodynamic performance, (2) quantify the influence of pitching rate on subcavitating and cavitating responses, and (3) quantify the influence of cavitation on the hydrodynamic load coefficients and surrounding flow structures. Results are presented for a NACA66 hydrofoil undergoing controlled, slow (α=6∘/s) and fast (α=63∘/s) pitching motions from α = 0° to α = 15° and back to α = 0° for both subcavitating and cavitating conditions at a moderate Reynolds number of Re = 750 000. The experimental studies were conducted in a cavitation tunnel at the French Naval Academy, France. The numerical simulations are performed by solving the incompressible, multiphase Unsteady Reynolds-Averaged Navier-Stokes Equations via the commercial code CFX using a transport...

Hydroelastic Responses of a Flexible Hydrofoil in Turbulent, Cavitating Flow

The objective of this work is to develop and validate a robust method to simulate the hydroelastic responses of flexible hydrofoil in turbulent, cavitating flow. A two degrees-of-freedom (2-DOF) model is used to simulate the plunging and pitching motion at the foil tip due to bending and twisting deformation of a 3-D cantilevered hydrofoil. The 2-DOF model is loosely coupled with the commercial computational fluid dynamics (CFD) solver STAR-CCM+ to efficiently simulate the fluid-structure interaction (FSI) responses of a cantilevered, rectangular hydrofoil. The numerical predictions are compared with experimental measurements for cases with and without cavitation. The experimental studies were conducted in the cavitation tunnel at the French Naval Academy (IRENav), France. Only quasi-steady cases with Reynolds number (Re) of 750,000 are shown in this paper. In general, the numerical results agree well with the experimental measurements and observations. The results show that elastic deformation of the POM polyacetate (flexible) hydrofoil lead to increases in the angle of attack, which resulted in higher lift and drag coefficients, lower lift to drag ratio, and longer cavities compared to the stainless steel (rigid) hydrofoil. Whereas only stable cavitation cases are considered in this paper, significant interaction effects were observed during experiments for cases with unstable cavitation due to interations between the foil natural frequencies and the cavity shedding frequencies. Transient analysis of the FSI responses of 3-D elastic hydrofoils in turbulent, cavitating flow is currently under work.Copyright

A Numerical Study of Cavitation Induced Vibration

The present work deals with an original study of the dynamics of an elastic structure immerged in an unsteady partial cavitating flow. The latter corresponds to the case of a leading edge attached cavity that exhibits periodical oscillations. The elastic structure is a cantilevered rectangular hydrofoil made of polyacetal plastic material (E = 3GPa). The computational fluid dynamics is based on a 2D unsteady single fluid model of cavitation with a barotropic law and a k – e – RNG modified turbulent model. The computational structure dynamics is carried out using a 3D finite element code. The fluid structure coupling is based on a chained weak coupling algorithm for which the 2D unsteady local fluid loading is computed on a rigid hydrofoil, then interpolated on the 3D deformable hydrofoil to compute the structural dynamics. The results are compared to the experiment ones carried out in the hydrodynamic tunnel of the research institute at the French Naval Academy for flow conditions close to the numerical ones. It is shown that in spite of a weak coupling algorithm, the forced vibration due to the periodical behaviour of the unsteady partial cavity is rather well predicted by the computation and compared favourably with the experiments. However, the experiments reveal that cavitation influences the natural modal response of the elastic structure in a more complex fluid structure interaction process.Copyright

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

Numerical Investigation of Deformable Hydrofoils in Steady Flows

The work is developed within the general frame of marine structure design. The study presents a numerical investigation of the hydro elastic behaviour of a deformable lifting body in a uniform steady flow. The fluid is considered as inviscid. The structure problem is solved by a finite element method and the flow problem is solved by a finite volume method using two commercial codes. Both problems are coupled through an iterative algorithm based on the exchange of boundary conditions at the flow-structure interface. The study is conducted on a cambered rectangular hydrofoil mounted in a hydrodynamic tunnel simulating the experiment that is currently performed in our laboratory. Results obtained from the fluid-structure computations including the deformation together with the hydrodynamic coefficients are presented. The influence of the fluid-structure coupling has been highlighted through comparisons with “non-coupled” simulations. Depending on the flow conditions, the twist of the hydrofoil together with the hydrodynamic loading are observed through the coupled simulation.Copyright