Friction Drag Reduction by Air Injection for Naval Hydrodynamics – F-DRAINH

The study of the coupling between liquid phase and gas phase will be treated with a two-way coupling approach.
The configuration of the study is the boundary layer developping along a flat horizontal plate, with favorable effect of gravity for gas injection (injection at the top). Depending on the nature of the flow: bubbles or cavity, two-phase couplings are effective at different scales. Both configurations (bubble layer and air cavity) will be addressed in the project.
Experimental tests will be conducted in the cavitation tunnel of IRENav. For that purpose, it requires the development of original visualisation methods (PIV/PTV), suited to two-phase flows with high spatial resolution. These tests will serve as support for the validation of numerical tools developed in the project.
Numerical calculations are performed by IMFT and LML with complementary approaches. The numerical tool developed by IMFT (Jadim) is well suited to the bubble layer study for low void fractions. The numerical framework is based on the eulerian calculation of the liquid flow, taking care of the coupling between the bubbles and the liquid flow for all the scales of the turbulence, the bubbles being tracked by a lagrangian approach.
Different numerical tools based on homogeneous fluid model are tested and developed by LML. The ability of these tools can be extended from cavitating flows to ventilated flows. We propose in this study to test different levels of complexity of numerical models (2D, 3D), different approaches to the resolution of the turbulence (RANS, LES) for the prediction of drag and instabilities due to air cavities.

For the bubble drag reduction, the part of bubble induced compressibility effect and the part of bubble-turbulence interaction effect will be investigated. In particular, the dispersion of the bubbles by the large scales of the liquid turbulence, the bubble induced change in the turbulence distribution of the liquid in the very near wall region and the bubble induced change in the spatial distribution of the wall friction will be considered. The influence of both the bubble size and the void fraction on the near wall turbulence and on the wall shear stress will be analyzed.
For air cavity drag reduction, air injection induced modifications in the physical characteristic of the gas-liquid mixture (density, viscosity) and their consequence on both the viscous drag and the stability of the cavity will be investigated. The approach will be based on a homogeneous flow assumption. In particular, compressibility effects will be analyzed. A particular attention will be paid to the 2D and 3D study of the closure region of unsteady cavities. To achieve this, experimental and numerical methods, developed for the analysis of unsteady cavitating flows, will be adapted to the analysis of this ventilated flow. The change in the viscous drag and stability of the cavity due the variation of the external flow conditions (direction, intensity and fluctuations) will be examined.

This project brings together expertise in numerical and experimental approaches. It will allow different communities working on two-phase flows to share working methods. The analysis of the physical interactions between the liquid and gas phases will enable to better understand the mechanisms responsible for drag reduction by air injection, the influence parameters and progress towards modeling . The overall long-term objective is to develop numerical tools to help optimization of drag reduction by air injection in operating conditions (hull’s design and injection conditions must be carefully checked).

Numerical study of effect of the afterbody on steady state ventilated cavities, M. Adama Maiga, S. Fuzier, O. Coutier Delgosha, 8th International Conference on multiphase flow, ICMF 2013, Jeju, Korea, May 26-31, 2013

The present project is linked to the military and civilian context of active flow control. It aims at studying the reduction in viscous drag by air injection for naval hydrodynamic applications. Air injection inside a boundary layer developing along ship hulls makes possible to decrease the local friction coefficient by more than 80%. It is then expected to increase the velocity of surface ships and reduce their fuel consumption. Actually the physical mechanisms are not completely understood and for a better control and generalization of the process, modelling the gas-liquid phase interactions is required.
This project intends to develop numerical and modelling tools to predict air injection drag reduction, based on the collaboration between three partners: GIP French Naval Academy/IRENav, IMFT and LML with both numerical and experimental methods. In this project, the mechanisms that govern air injection drag reduction will be analyzed in details in the case of a two-phase flow developing along an horizontal flat plate including different states of the flow: bubbly flow and air cavity, both steady and unsteady cavities. Systematic tests of influent parameters will be conducted.
For the bubble drag reduction, the part of bubble induced compressibility effect and the part of bubble-turbulence interaction effect will be investigated. In particular, the dispersion of the bubbles by the large scales of the liquid turbulence, the bubble induced change in the turbulence distribution of the liquid in the very near wall region and the bubble induced change in the spatial distribution of the wall friction will be considered. For that purpose, it requires the development of original experimental methods, suited to two-phase flows. The numerical framework is based on the eulerian calculation of the liquid flow, taking care of the coupling between the bubbles and the liquid flow for all the scales of the turbulence, the bubbles being tracked by a lagrangian approach. The influence of both the bubble size and the void fraction on the near wall turbulence and on the wall shear stress will be analyzed.
For air cavity drag reduction, air injection induced modifications in the physical characteristic of the gas-liquid mixture (density, viscosity) and their consequence on both the viscous drag and the stability of the cavity will be investigated. The approach will be based on a homogeneous flow assumption. In particular, compressibility effects will be analyzed. A particular attention will be paid to the 2D and 3D study of the closure region of unsteady cavities. To achieve this, experimental and numerical methods, developed for the analysis of unsteady cavitating flows, will be adapted to the analysis of this ventilated flow. The change in the viscous drag and stability of the cavity due the variation of the external flow conditions (direction, intensity and fluctuations) will be examined.
As a summary, the analyses of the physical mechanisms responsible for air injection drag reduction will lead to a discrimination of the different numerical tools, according to their ability to predict the viscous drag, as a function of the gas-liquid coupling level and the turbulence scale resolution. Systematic tests of influent parameters and the development of models will be useful for the hulls’ design of eco and fast ship.