Author:
LODAHL C. R.,SUMER B. M.,FREDSØE J.
Abstract
This work concerns the combined oscillatory flow and current in
a circular, smooth
pipe. The study comprises wall shear stress measurements, and laser-Doppler-anemometer
velocity and turbulence measurements. Three kinds of pipes were used,
with diameters D=19 cm, 9 cm, and 1.1 cm, enabling the influence
of the parameter
R/δ to be studied in the investigation (R/δ
ranging
from about 3 to 53), where R is the
radius of the pipe, and δ is the Stokes layer thickness. The ranges
of the two other
parameters of the combined flow processes, namely the current Reynolds
number,
Rec,
and the oscillatory-flow boundary-layer (i.e. the wave–boundary layer)
Reynolds
number, Rew, are:
Rec=0−1.6×105, and
Rew=0−7×106. The transition
to turbulence
in the combined flow case occurs at a current Reynolds number larger than
the
conventional value, ca. 2×103, depending on
Rew, and R/δ. A turbulent
current can
be laminarized by superimposing an oscillatory flow. The overall average
value of the
wall shear stress (the mean wall shear stress) may retain its steady-current
value, it may
decrease, or it may increase, depending on the flow regime. The increase
(which can be
as much as a factor of 4) occurs when the combined flow is in the wave-dominated
regime, while the oscillatory-flow component of the flow is in the turbulent
regime. The
component of the wall shear stress oscillating around the mean wall shear
stress can
also increase with respect to its oscillatory-flow-alone value. For this
to occur, the
originally laminar oscillatory boundary layer needs to become a fully developed
turbulent boundary layer, when a turbulent current is superimposed. This
increase can
be as much as O(3–4). The velocity profiles across
the cross-section of the pipe change
near the wall when an oscillatory flow is superimposed on a current, in
agreement with
the results of the wall shear stress measurements. The period-averaged
turbulence
profiles across the cross-section of the pipe behave differently for different
flow regimes.
When the two components of the flow are equally significant, the turbulence
profile
appears to be different from those corresponding to the fundamental cases;
the level
of turbulence increases (only slightly) with respect to those experienced
in the
fundamental cases.
Publisher
Cambridge University Press (CUP)
Subject
Mechanical Engineering,Mechanics of Materials,Condensed Matter Physics
Cited by
122 articles.
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