Electromagnetic (EM) flow control of boundary layer refers to the use of 'wall-flush' electrodes (j, current density) and 'sub-surface' magnets (B, magnetic induction) used in combination to create local Lorentz body forces (j × B). In the present application the working fluid is seawater. Close to the boundary wall, these j × B forces can act directly on velocity and vorticity. In this paper, the characterization of a wall-normal EM actuator (i.e. j × B forces are mainly wall-normal above the central axis of the actuator) is considered. An idealized inertial and integral approach leads to the definition of characteristic EM numbers in term of velocity, time, acceleration and length-scales. These numbers are useful in introducing an EM parameter similar to the Froude number. Furthermore, two asymptotic EM flow regimes, which depend on flow velocity and on EM forces intensity, are also discussed.

ElectroMagnetic (EM) Flow Control deals with the concept of using in combination "wall-flush" electrodes (j, DC current supply) and "sub-surface" magnets (B, magnetic induction origin) to create directly local body forces (jxB) within a seawater boundary layer. Analytical, experimental and computational investigations of EM Flow Control are presented here. This work is intended to understand the basic mechanisms involved in turbulence intensity and skin friction reductions as well as in coherent structure extinction. First, EM actuator and its modes of action are described. This description includes: some general remarks on EM actuator; the set of equations suitable to EM control in seawater; and a selection of dimensionless parameters analysed in term of possible mechanisms of action. Second, some experimental investigations and visualizations of wall bounded flows under EM actuation are presented: near wall vortex around the actuator; suction zone above the actuator; wall jets around the actuator; boundary layers "suction -blowing". 1 PACS numbers: 47.27.Rc, 47.65.+a † LEGI is a joint laboratory

The work presented is entirely devoted to the coupling between electrochemistry and hydrodynamics. It takes place in the magnetohydrodynamics (MHD) of a seawater context for direct propulsion or flow control. The experimental measurements reported were carried out in the small seawater tunnel of our laboratory using seawater with a NaCl concentration of 35 g l-1. The first aspect of our study is the possible effect of electrolysis micro-bubbles on the flow, for instance, leading to a possible modification of the turbulent boundary layer or the effect of the flow on bubble characteristics. The second aspect is the effect of the flow conditions on the electrode working conditions, for instance the effective electroactive species and the electrode potential evolution. The main conclusions are that the electrolysis micro-bubbles do not affect the flow in the domain considered, but, in contrast, flow conditions act on bubble size and distribution. On the other hand, the competition between electroactive species (i.e. anodic reactions) is entirely controlled by the flow. Which means, from a practical point of view, that there is no opportunity to select an anode material that enables the selection of oxygen evolution instead of chlorine in the conditions prescribed by seawater MHD applications.