The Compliant Wall
The complaint wall is a relatively recent idea to reduce the drag by using flexible
coatings on the aero- hydrodynamic surfaces. The idea of flexible skins
comes from observations on swimming dolphins, and more generally on animal
propulsion.
The compliant wall must interact with the boundary layer and influence
its development, laminar or turbulent. If this has to be true, the wavelength of the
skin flexibility must be of the same order of magnitude of the boundary layer
thickness, while the amplitude must be of the same order as the viscous sublayer.
The idea is that, when the surface is affected by fluid flow around it, it will start
an interaction surface-boundary layer such that the boundary layer remains attached
for a longer length and the drag of the system will be reduced.
The problem, however, is not so easy. The properties of the compliant surface must be
well understood, else the effect is … the opposite (that is a compliant surface
that triggers boundary layer separation).
Some energy considerations are necessary:
In the case of a rigid surface, the drag produced serves to
dissipate propulsion power into the fluid (by means of of viscosity, radiation, etc.)
In the case of a passive compliant surface part of
this energy goes into the surface itself and is dissipated through internal damping.
With an active compliant surface power would be required to
activate the surface-boundary layer interaction (for example, heating of the coating
to activate its compliant properties).
Therefore, the fundamental question is whether the net energy balance is positive or
negative.
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