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Department of Mechanical Engineering
Stanford University
Abstract-
A non-equilibrium boundary layer can be defined as a wall-attached shear
layer having one or more additional length scales beyond the normal inner
and outer length scales. If the additional length scale has a different
dependence on Reynolds number than either of the conventional scales, then
the turbulence will be Reynolds number dependent. We have investigated
this using a laboratory scale subsonic wind tunnel mounted in a large
pressure vessel, allowing at least a 25:1 range of Reynolds number with
fixed geometry. A custom LDA system was developed to insure adequate
spatial resolution at the highest Reynolds numbers. Flat plate boundary
layer measurements acquired as a baseline allowed development of new
scaling laws which collapse all available fully resolved Reynolds stress
data. Our main interest is in non-equilibrium boundary layers recovering
from regions of strong adverse pressure gradient including one case with a
separation bubble. The boundary layers recover by developing a
stress-equilibrium layer which is in equilibrium with the local wall shear
stress. All Reynolds stress components collapse to the flat plate scaling
in this layer. The part of the boundary layer outside of the
stress-equilibrium layer exhibits distinct Reynolds number effects. The
implications for modern modeling techniques will be discussed.
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