Electromagnetic flow control for drag reduction and separation prevention

The flow of an electrically conducting fluid like sea-water can be
controlled by electromagnetic forces, i.e.~Lorentz-forces. These forces
may be generated by an appropriate chosen arrangement of permanent
magnets and electrodes. A strip like geometry as shown in fig. 1
produces a Lorentz-force with a streamwise component only. In a first
approximation this force is independent of the spanwise coordinate z and
decays exponentially with increasing wall distance y [1]. The successful
application of a surface parallel Lorentz force in streamwise direction
to control the flow around a cylinder has been demonstrated in
[2]. We consider here the action of such a force on a flat plate
boundary layer up to Re=9*10^5 and the flow around a NACA-0017-like
hydrofoil with Re<8*10^4. The experiments were carried out in a
saltwater facility and accompanied by flow visualizations in an open
channel with sodium hydroxide as the working fluid.

>From the boundary layer equations with the Lorentz-force term one gains
a characteristic parameter Z (Tsinober--Shtern parameter [1]) describing
the ratio of electromagnetic to viscous forces. If this parameter equals
one, the boundary layer equations have a solution with an exponential
flow profile similar to the asymptotic suction profile. A considerable
transition delay and therefore drastic drag reduction should be expected
from such a boundary layer [3], because the exponential profile has
proven to be much more stable than the Blasius one. Force balance
measurements on a flat plate show indeed the reduction of total drag by
more than 80\% (see fig. 2). However, this drag reduction is due to the
momentum added to the flow by the body force. The skin friction on the
contrary is even slightly increased, as can be concluded from the
velocity profiles in fig. 2. The reason for this behavior
lies in the real force distribution and probably the high turbulence level
of the environment. It is ...