Various ways were tried recently to decrease the friction drag of a
body in a flow; they all employ influencing the boundary layer. One
of them consists in keeping the boundary layer Laminar by suction;
promising tests have been carried out. Since for large Reynolds
numbers the friction drag of the laminar boundary layer is much
lower than that of the turbulent boundary layer, a considerable
saving in drag results from keeping the boundary layer laminar,
even with the blower power required for suction taken into account.
The boundary layer is kept laminar by suction in two ways: first,
by reduction of the thickness of the boundary layer and second, by
the fact that the suction changes the form of the velocity
distribution so that it becomes more stable, in a manner similar to
the change by a pressure drop. There by the critical Reynolds
number of the boundary layer (USigma*/V) (sub crit) becomes
considerably higher than for the case without suction. This latter
circumstance takes full effect only if continuous suction is
applied which one might visualize realized through a porous wall.
Thus the suction quantities required for keeping the boundary layer
laminar become so small that the suction must be regarded as a very
promising auxiliary means for drag reduction.
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