Previous research has established that surfaces with tiny ribs
(riblets) aligned in the
streamwise direction can reduce the turbulent wall-shear stress below that
of a smooth
surface. Typical skin-friction reductions have been found to be about 5%.
of the present investigation, however, demonstrate a considerable improvement
this value. This improvement is achieved by a systematic experimental optimization
which has been guided by theoretical concepts.
A key feature of our experiments is the utilization of an oil channel.
experiments in wind tunnels had to contend with very small riblet dimensions
typically had a lateral rib spacing of about 0.5 mm or less. By contrast,
in our oil
channel, the ribs can have a lateral spacing of between about 2 and 10
increased size of the surface structures enables test surfaces to be
conventional mechanical methods, and it also enables us to build test surfaces
adjustable geometry. In addition, the Berlin oil channel has a novel shear
with an unprecedented accuracy of ±0.3%. This latter feature is
prerequisite for a systematic experimental optimization.
In the present investigation, surfaces with longitudinal ribs and additional
studied. The experiments cover a fairly large range of parameters so that
reduction potential of a surface with ribs and/or slits is
worked out conclusively. A
large parameter range is made possible because of the adjustability of
the surfaces as
well as the automatic operation of the oil channel. In particular, the
following tests were run:
(i) Shear stress measurements with conventional riblet configurations,
triangular and semi-circular grooves, have been carried out. These measurements
necessary in order to establish the connection between our oil channel
previous data from wind tunnels. As was previously established, we found
reduction of about 5%.
(ii) An adjustable surface with longitudinal blade ribs and with slits
was built and
tested. Both groove depth and slit width could be varied separately and
during the experiment. It turned out, that slits in the surface did not
contribute to the
drag reduction. Nevertheless, these investigations show how perforated
for boundary-layer control) can be designed for minimal parasitic drag.
On the other
hand, with closed slits, an optimal groove depth for the rib surface could
determined, i.e. half of the lateral rib spacing. For this configuration,
8.7% skin-friction reduction. By carefully eliminating deleterious effects
little gaps, etc.), the skin-friction reduction could be improved to a
record value of 9.9%.
(iii) A quantitative comparison between theory and experiment was carried
theory is based on the assumption that riblets impede the fluctuating turbulent
crossflow near the wall. In this way, momentum transfer and
shear stress are reduced.
The simplified theoretical model proposed by Luchini (1992) is supported
(iv) For technological applications of riblets, e.g. on long-range
the above thin-blade ribs are not practical. Therefore, we have devised
a surface that
combines a significantly improved performance (8.2 %) with a geometry which
better durability and enables previously developed manufacturing methods
riblet film production to be used. Our riblet geometry exhibits
trapezoidal grooves with
wedge-like ribs. The flat floor of the trapezoidal grooves permits an undistorted
visibility through the transparent riblet film which is essential
for crack inspection on aircraft.