Chemistry Reference
In-Depth Information
As a numerical example consider the parameters D= 10
-3
s
-1
, Q = 0.015 cm
2
s
-1
, and 3
max
= 40 mN m
-1
which is not unreasonable for a slick. This gives
a ''slick width'' according to (7) of # 150 m.
In both examples above we have assumed that there is no motion in the
film, so that it can be considered to act like a ''rigid'' plate. In the next case
study we shall see that even in a steady state situation there may be a pat-
tern of motion in the film that change the total viscous drag.
2.3. Accumulation of a surface film by flow under surface
obstruction
It is commonly observed that an obstruction on the surface of water flow-
ing gently in an open channel (with free passage underneath the obstruc-
tion) causes a spontaneous surface film to accumulate on the upstream
side. Although this is not the way a surface slick is generated, there is per-
haps something to be learned here.
A very interesting controlled experiment was described by Mockros and
Krone (1968). Ordinary tap water was recirculated in a flume 113 cm wide
8 cm deep and 4 m long. When a shallow obstruction was placed across
the full width of the channel, a surface film was observed growing. The
upstream front of the film was easily visible by a small ripple, the so-
called Reynolds ridge (Harper and Dixon 1974). The growth followed a
power law first at a fast rate, and then at a reduced rate (Figure 3).
Fig. 3.
Typical growth curve for the surface film (taken from Mockros and Krone
1968)
