FOAMWAKE

The FOAMWAKE project is carried out by a consortium consisting of an academic partner (Laboratoire de Mécanique des Fluides et Acoustique UMR 5509), two institutional partners (IFREMER and ONERA), and a major company (SIREHNA, a subsidiary of Naval Group). The objective of this project is to characterize the two-phase bubbly wake generated behind a cylinder piercing the air-water surface. Specifically, the work of this project contributes to the characterization of the physical phenomena that govern the generation, propagation, and persistence of this two-phase wake. These phenomena are currently underexplored, poorly understood, and even less modelled. A second line of work aims to characterize the perception of this wake by optronic sensors. Indeed, the bubbles generated by the wake have very different optical properties from those of the background seastate, which can lead to the detection and recognition of the semi-immersed craft that generated the wake. The military purpose of this project is the understanding and control of the optical indiscretion generated by the foam wake behind a body piercing the free surface (for example, a submarine mast operating at periscopic immersion) and, reciprocally, the improvement of the detection capabilities of these phenomena by optical sensors. Civil applications include, among others, maritime security or the phenomenon of air entrainment downstream of hydraulic structures, the modelling of which is necessary in hydroelectric applications, but often very empirical. The objective of the project is to understand the physical phenomena that control bubble formation, the size distributions of these bubbles, their propagation in the wake of the body, and their persistence in the form of foam. To achieve this goal, the following tasks will be carried out: 1) Conducting measurements downstream of a fixed cylinder piercing a flow at speeds up to 5 m/s. This experimental facility, which already exists at LMFA, is small in size (length 2.3 m) but extensively available. The test facility will help understand the generation of bubbles and their propagation in the wake near the body, particularly through measurements by optical phase detection probes providing access to bubble size and velocity distributions. The persistence of the wake cannot be studied using this test facility. 2) Conducting tests in seawater in a larger-scale towing tank (IFREMER). These tests will complement the previous ones, in an unconfined environment. In addition to providing information on a scale closer to the application, they will allow the study of the persistence of the foam wake downstream of the mast. This test facility will also provide access to the free-surface shape representative of that generated by a mast in an infinite medium. These tests will validate the numerical models used or developed in task 3, or models developed subsequently. This task will be carried out in collaboration with LMFA, and will also include measurements by optical sensors by ONERA. 3) Conducting numerical simulations by Naval Group replicating the tests carried out in the previous two tasks, with two turbulence modelling approaches (RANS and LES). These simulations will not aim to track the bubbles but to characterize the global flow topology and turbulent structures and thus bridge the gap between the observed bubble propagation in the experiments and turbulence. These simulations will be complemented by direct numerical simulations with Basilisk (LMFA) to focus on a smaller scale and on air entrainment mechanisms, by accurately tracking the interface. The final aim of the experiments and simulations we propose will be to develop an air entrainment model for this configuration, in order to clarify how the smallest air inclusions responsible for the persistence of the wake are generated.