Measurement of Profile Drag on an Airplane in Flight by the Momentum Method: Part 2 Page: 4 of 37
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.LTA.C.A. Technical Memorandum ITo. 558 3
b) The Boundary Layer
Under all conditions of flow in confined fluids of slight
viscosity (such as air and water under normal conditions), an
extremely thin layer of the fluid adheres to the walls. From
this layer, which has zero velocity, the flow velocity increases
rapidly until it reaches the velocity of the free or potential
flow. The whole layer, inside of which the velocity increases
from zero to that of the free flow, Prandtl calls the "boundary
lay er. "
Since the boundary layer is always very thin, it is assumed
that there is no pressure drop in it perpendicular to the di-
rection of flow. The static pressure of the boundary layer is
determined rather by the pressure of the limiting potential flow.
In the further treatment we will therefore distinguish between
the regions of potential flow and the boundary layer.
The general course of the flow velocity in the boundary
layer is shown in Figure 52. The flow in the boundary layer can
be either laminar or turbulent. The former is a parallel dis-
place-ment, while the latter is an irregular flow characterized
by a main flow parallel to the wall and a superposed undulatory
motion with components perpendicular to the wall.
When a flow encounters a body, the resulting boundary layer
is at first very thin, Its thickness gradually increases in the
direction of the flow, which is usually laminar at first. At a
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Schrenk, Martin. Measurement of Profile Drag on an Airplane in Flight by the Momentum Method: Part 2, report, March 1930; (https://digital.library.unt.edu/ark:/67531/metadc63671/m1/4/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.