Experiments on drag-reducing surfaces and their

optimization with an adjustable geometry
Authors: BECHERT D. W. , BRUSE M. , HAGE W. , VAN DER HOEVEN J. G. T. , HOPPE G. 
Published: July 2002

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%.
The results

of the present investigation, however, demonstrate a considerable improvement
over

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.
Previous

experiments in wind tunnels had to contend with very small riblet dimensions
which

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
mm. This

increased size of the surface structures enables test surfaces to be

manufactured with

conventional mechanical methods, and it also enables us to build test surfaces
with

adjustable geometry. In addition, the Berlin oil channel has a novel shear

stress balance

with an unprecedented accuracy of ±0.3%. This latter feature is
a

prerequisite for a systematic experimental optimization.In the present investigation, surfaces with longitudinal ribs and additional

slits are

studied. The experiments cover a fairly large range of parameters so that
the drag

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,
i.e. with

triangular and semi-circular grooves, have been carried out. These measurements
were

necessary in order to establish the connection between our oil channel
data and

previous data from wind tunnels. As was previously established, we found
a drag

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
continuously

during the experiment. It turned out, that slits in the surface did not

contribute to the

drag reduction. Nevertheless, these investigations show how perforated
surfaces (e.g.

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
be

determined, i.e. half of the lateral rib spacing. For this configuration,
we

found an

8.7% skin-friction reduction. By carefully eliminating deleterious effects

(caused by

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

out. The

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
by the

present experiments.(iv) For technological applications of riblets, e.g. on long-range

commercial aircraft,

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
exhibits

better durability and enables previously developed manufacturing methods
for plastic

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.