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National Advisory Committee for Aeronautics, Technical Notes - Skin Friction of Incompressible Turbulent Boundary Layers Under Adverse Pressure Gradients

Experimental data for skin friction of turbulent boundary layers
under adverse pressure gradients from several sources are presented in
graphical form. Data obtained by the momentumrbalance method are shown
to follow a trend opposite to that of data dbtained.by hot-wire and
heat—transfer methods. The momentum equation and total-pressure-
measuring techniques are discussed in relation to skin-friction
calculations.

The conclusion is reached that momentum method is of questionable
value in regions of adverse gradient and should not be relied.upon
because the sensitivity to small measuring errors and to deviations from
assumed flow conditions in test channels is too great. A new integral
energy parameter is introduced and its relation to skin-friction data
is demonstrated on the basis of available material. A new momentum
thickness which includes the turbulent momentum contribution, is also
introduced, and it is shown that this thickness may be as much as
7% percent lower than the conventional momentum thickness, near the
turbulent separation point.

The present knowledge concerning the development of turbulent
boundary layers under adverse pressure gradients and approaching
separation is far from adequate. In particular, the skin friction
under adverse pressure gradients cannot be determined with reasonable
accuracy because of the lack of enough reliable experimental and
theoretical information. OnLy through positive boundary-layer—control
devices, such as suction slots, suction areas, vortex generators, and
so forth, can the design engineer be reasonably assured that the boundary
layer will develop in a desired manner.

The present analysis was prepared at the NACA Lewis laboratory to
assemble and to review in a critical manner the available information
concerning the skin-friction trend of an incompressible turbulent
boundary layer in regions of adverse pressure gradients in order to
show the conflicting data obtained by different experimental methods
and to bring out the inportant characteristics and parameters affecting
skin—friction development.