Jean-Paul Thibault

Investigation of wall normal electromagnetic actuator for seawater flow control

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 ( j  ×  B ) within a seawater boundary layer. Analytical, experimental and computational investigations of EM flow control are presented here. This work is intended to provide understanding of the basic mechanisms involved in turbulence intensity and skin friction reductions as well as in coherent structure extinction. First, the EM actuator and its modes of action are described. This description includes some general remarks on the EM actuator, the set of equations suitable for EM control in seawater and a selection of dimensionless parameters analysed in terms of possible mechanisms of action. Second, some experimental investigations and visualizations of wall-bounded flows under EM actuation are presented: the near-wall vortex around the actuator; the suction zone above the actuat...

Experimental analysis of couplings between electrolysis and hydrodynamics in the context of MHD in seawater

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.

Active Insulation Technique Applied to the Experimental Analysis of a Thermodynamic Control System for Cryogenic Propellant Storage

A technological barrier for long-duration space missions using cryogenic propulsion is the control of the propellant tank self-pressurization (SP). Since the cryogenic propellant submitted to undesired heat load tends to vaporize, the resulting pressure rise must be controlled to prevent storage failure. The thermodynamic vent system (TVS) is one of the possible control strategies. A TVS system has been investigated using on-ground experiments with simulant fluid. Previous experiments performed in the literature have reported difficulties to manage the thermal boundary condition at the tank wall; spurious thermal effects induced by the tank environment spoiled the tank power balance accuracy. This paper proposes to improve the experimental tank power balance, thanks to the combined use of an active insulation technique, a double envelope thermalized by a water loop which yields a net zero heat flux boundary condition and an electrical heating coil delivering a thermal power Pc∈[0−360] W⁠, which accurately sets the tank thermal input. The simulant fluid is the NOVEC1230 fluoroketone, allowing experiments at room temperature T ∈ [40–60] °C. Various SP and TVS experiments are performed with this new and improved apparatus. The proposed active tank insulation technique yields quasi-adiabatic wall condition for all experiments. For TVS control at a given injection temperature, the final equilibrium state depends on heat load and the injection mass flow rate. The cooling dynamics is determined by the tank filling and the injection mass flow rate but does not depend on the heat load Pc⁠.

Influence of Noncondensable Gases on Thermodynamic Control On-Ground Experiments Using a Substitute Fluid

A cryogenic propellant submitted to heat load during long duration space missions tends to vaporize to such an extent that the resulting pressure rise must be controlled to prevent storage failure. The thermodynamic vent system (TVS), one of the possible control strategies, has been investigated using on-ground experiments with NOVEC1230 as substitution fluid. Results obtained for self-pressurization (SP) and TVS control phases have been reported in a previous work. The unexpected inverse thermal stratification observed during these experiments is analyzed in the present work and related to the influence of noncondensable gases. Noncondensable gases, present inside the tank in the form of nitrogen—ten times lighter than the substitution fluid vapor—generate a concentration stratification in the ullage. Assuming the NOVEC1230 remains at saturation in the whole ullage, the density stratification which results from this concentration stratification can explain the observed inverse thermal stratification.

Direct Numerical Simulation of Alternated Spanwise Lorentz Forcing

Spanwise electro-magnetic forcing is used to study turbulence control and drag reduction in a numerical channel flow with a constant mass flow rate and low Reynolds number. The originality of this study comes from the computation of the force field from the geometry of the magnet and the electrode. It is shown that the tilt of the wall-normal component of the vorticity in the spanwise direction characterise the drag reduction caused by alternated spanwise forcing.