Riblets
Summary
Scientists have been speculating for many years whether there is any surface having
less drag of a flat plate. The drag of a flat plate is reported in the figure below,
for both laminar (Blasius) and turbulent flow.
Figure 1: Flat plate drag
Experimental studies in the 1970s showed that small grooves (riblets) aligned with
the flow had the property of modifying the near-wall structure of the boundary
layer. In particular, the riblets proved to work as a constraint to the production of
the Reyonlds stresses associated with the growth and eruption of the eddies in the
the low-speed regions of the boundary layers.
Figure 2: common types of riblets
Later research was aimed at investigating the properties of such grooves, by studying the wall boundary conditions and the flow properties at corner regions.
A number of studies of zoologic nature was added to the fluid dynamic problem, by studying the characteristics of fast-swimming sharks and dolphins, from where some ideas were derived.
On of the main practical concerns was (and it is) the amount of drag reduction that can be achieved, and studies were directed to investigating the optimum ratio fin-height/riblet spacing, physical dimensions of the riblets, along with the optimum shape (L- U- V-grooves and others, Fig. 2 above).
Drag Reduction
The skin friction drag reduction data published in the technical literature is variable, but converging to a figure of 8 %, with more conservative values of 5 % to the most optimistic figures of 10-11 %, obtained in laboratory conditions. While these numbers do not seem excessively high, they do lead to enourmous savings.
Take for example a subsonic jet transport, for which the skin friction drag is of the order of 45 % at cruise conditions. If half of the surface could be covered by efficient riblets that provide an 8 % skin friction saving, the total saving would be just less than 4 %, a remarkable amount.
Off-Design Performances
Flow alignment and surface quality are two main concerns, alogn with pressure
gradients, three-dimensional flows and effects of the increased wetted area. The
results are as follows:
No practical effects weere measured on flow mis-alignement up to 15 deg ( 0 deg is a
flow perfectly aligned with the riblet). At higher flow angles, up to 40 deg,
performances deteriorate gradually, and the riblets become ineffective, if not
inappropriate, at such angles. For these reasons some investigators have been
studying three-dimensional riblets, also called compound riblets, that would
be locally optimized to follow the main direction of the flow.
The surface covered with a riblet film may undergo contamination over time, due to
deposition of dust, combustion particulate, atmospheric aggression, etc.
Surface contamination can be a major concern for submerged bodies, such as ships and
submarines. However, there seems to be no effects for periods limited to one day,
whereas in aircraft applications the effects, if any, occur over a much longer time
scale.
Pressure gradients have a minor effect, probably 1-2 % on the total skin friction
drag reduction.
Increase of wetted area is a problem of any riblet geometry (see figure 1 above),
therefore useful configurations are those that, besides stabilizing the boundary
layer, have a limited increase in wetted area. Obviously, the skin friction works
over a larger surface (this is a problem especially with L-grooves.)
Applications are more common in hydrodynamics where the drag reduction
possibilities are larger, in particular on sailing boats. Airfoil applications
showed a drag reduction rate of about 6-8 %, although in some recent experiments a
skin friction drag reduction of 16 % was achieved at an angle of attack of 6 deg.
Skin friction drag for a large commercial aircraft is of the order of 40 % of the
total. This figure is slighty larger for a smaller executive airfract (up to 50
%). Small gains on this numbers translate into major fuel savings and direct
operative costs.
One can easily speculate with the 10 % drag saving given above, but
this is very far from reality. Both Boeing Aircraft and Airbus have tested riblets
for this purpose.
Data reported for a 1/11 scale model of the Airbus A320 at cruise Mach number M = 0.7
was a viscous drag saving of 4.85 %, with about 66 % of the aircraft wetted area
covered by V-riblets (s/h=1).
Application of riblets is generally done using special films, rather than estruding
the grooves directly on the surface. Riblet films have been manifactured by a number
of companies, among them, the 3M company.
The riblets dimensions most widely tested fall in the range 0.02mm – 0.10 mm height,
with optimal h/s ratio of the order 15.
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Flow Mis-Alignment
Surface Contamination
Pressure Gradients
Increase of Wetted Area
Applications
Aircraft
Related Material
Selected References
Specialized references available on request.
Mechanical Eng. Publ. Ltd, London, 1996 (ISBN 0-08529-8917-2)
Ellis Horwood Ltd, Chichester, 1989
(ISBN 0-7458-0753-X)
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