MagnetoHydroDynamics (MHD) in sea water was first considered as direct propulsion concept. Applying Lorentz forces in selected domains of a seawater boundary layer is now considered as a very promising concept to obtain significant drag reduction, turbulence intensity reduction, boundary layer separation prevention. Basically two configurations can be identified depending if the Lorentz forces field is acting mainly perpendicular or parallel to the wall. The concept uses periodic arrangements of permanent magnets and electrodes (both wall flash mounted). Depending on the electrodes power supply, the forces can be permanent or periodic. Anyhow the forces are directly and locally acting in the flow (close to the wall).
The mechanism which is effectively allowing that local (in space
and time) and relatively moderate Lorentz forces can strongly change the
behaviour of a turbulent boundary layer is not clearly understood.
Consequently we decided to analyse both JxB forces (additional term in the
Navier Stokes equation) and curl JxB (additional term in the vorticity
equation) distributions in the flow. Our aim is to understand what is the
best place and time to act on the turbulent boundary layer and what is the
best spatio-temporal reference frame to use for the modelling. An attempt
is also made to achieve a better understanding of the drag reduction
mechanism through the near wall vorticity dynamics by a simplified model.
We presently concentrate our contribution on the analysis of
idealised and/or asymptotic situations in order to select the pertinent
scales and non dimensional parameters. Our object is to identify one or few
specific situations which could be analysed in the same way experimentally
and numerically. These methods are complementary used for model validation
and understanding of the basic acting mode of the Lorentz forces on the
boundary layer structures and dynamics.