Riblet
Riblets are a surface geometry that reduce the frictional resistance on surfaces subject to turbulent flow . This surface geometry consists of fine ribs that have a very sharp point. The longitudinal axis of the ribs lies in the direction of flow. The name is borrowed from the English word for ripple .
Such riblets have been found on the scales of fast swimming sharks .
background
In fluid mechanics , which is a sub-area of physics, a distinction is made between two different types of flow: the laminar and the turbulent flow .
Most of the currents that occur in technical applications are turbulent. As a rule, they have higher friction losses than laminar flows, which can be explained by the transport of momentum across the main flow direction in the (hydrodynamic) boundary layer . The flow directly on the surface overflowed is dominated by vortex structures that transport energy-rich fluid to the wall surface and thereby cause increased wall friction.
The fine grooves on the shark scales (and on the artificial shark skin) hinder the transverse movements of the eddies in the turbulent flow. In this way, wall friction can be reduced by up to 10% in laboratory experiments, and resistance reductions of approx. 8% are achieved in technical applications.
Since the grooves on this drag-reducing surface are tiny, they are not "perceived" as rough by the current . However, this property only works in a certain speed range, so the groove surface must be specially designed for each application. An existing creased film cannot simply be used for any application.
scope of application
In order to effectively reduce the drag of a vehicle with a grooved surface, the proportion of wall friction in the flow drag must be large. This proportion is 100% for pipelines, but less for vehicles such as airplanes, ships and submarines. In motor vehicles, turbulence dominates the overall aerodynamic drag.
A surface of this kind is created either by gluing on appropriately ribbed riblet foils or by varnishing (with embossing of the structure before the varnish is cured with UV light ).
The use of small ribs has been studied and optimized since the early 1980s with the aim of reducing the fuel consumption of long-haul aircraft. The dimensions of these fine grooves, which are arranged parallel to the direction of flow, depend on the speed and viscosity of the medium. For commercial aircraft, the distances are between 30 µm and 60 µm.