Geoscience Reference
In-Depth Information
z
Ω
z
x
y
+
+
x
+
1500mm
Figure 4.9. RPCF apparatus designed and constructed by Tillmark and Alfredsson [1996]: (a) side view; (b) top view.
Table 4.1 provides an extensive list of rotating disk
experimental studies. As can be seen, most disks have
a diameter around 500 mm, with three exceptions with
larger diameters in the range 900-1000 mm. These latter
cases were for studies of the turbulent boundary layer.
The viscous length scale (
It is noteworthy that there are no studies using LDV or
PIV. One reason for this is that the particles required to
seed the flow would tend to stick to the disk surface,
thereby creating stationary cross-flow vortices in the lam-
inar region. Another problem is spatial resolution in the
wall-normal direction. The thinness of the boundary layer
would require very small measurement volumes (for LDV)
or thin laser sheets (for PIV). For a disk experiment in air
it may also be problematic to have particles that follow the
flow accurately since the centrifugal force acting on the
particles close to the disk may be substantial (this would
be less problematic in a water experiment).
For accurate hot-wire measurements, a good calibration
of the anemometer system is necessary. It is usually best to
do the calibration in situ, that is, in the low where the mea-
surements will be taken. In a wind tunnel this is done in the
free stream against a Pitot tube reading, where the Pitot
tube is placed close to the hot wire. Here, the main prob-
lem is to obtain good readings from a Pitot tube at low
velocities. In contrast to measurement of boundary layers
on flat plates or wing profiles in wind tunnels, the rotating
= ν/u τ ,where u τ is the
friction velocity) of the turbulent boundary layer can be
estimated as
= 2
c f
z r = 2
ν
u τ
ν
Re 2 r .
=
(4.3)
c f
=2 u τ /( z r) 2
The skin friction coefficient c f [
is expected
to be a weak function of the Reynolds number, and then
it is clear from equation (4.3) that the larger the radius at a
given Reynolds number, the larger the viscous length scale.
Most studies listed in Table 4.1 were made in air, but
there are also some (e.g., flow visualization studies) that
were made in water. The stability studies used hot-wire
anemometry, whereas for the studies of the turbulent
boundary layer both Pitot tubes and hot wires were used.
]
 
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