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Record-breaking simulation of aircraft wake turbulence

Wake turbulence in the air of the kind formed when an aircraft takes off or lands. (Credit: NASA Langley Research Center)

Dangerous vortexes form when an aircraft takes off or lands. A
combination of optimised numerical methods, a supercomputer and highly
developed visualisation methods revealed how this process generates
primary and secondary vortexes, and how the secondary vortexes cause
the primary ones to decay.

Post-docs Philippe Chatelain and Michael Bergdorf, doctoral student
Diego Rossinelli, Petros Koumoutsakos, Professor at the Institute of
Computational Science of ETH Zurich, and Alessandro Curioni and Wanda
Andreoni of the Department of Computational Sciences at the IBM Zurich
Research Laboratory in Rüschlikon explain that they have succeeded in a
“record-breaking simulation” of wake turbulence formed by the vortexes
that occur when an aircraft lands or takes off.

This involved the scientists using high-resolution calculations to
simulate, with an accuracy two to three orders of magnitude higher than
previously, the vortexes generated when an aircraft takes off or lands.
Alessandro Curioni explains that the simulation also visibly revealed
how the wake turbulence vortexes become unstable. He says this in turn
allows a better understanding of the underlying physics and is thus
useful in aircraft construction.

Modelling the air flow

The flow pattern was modelled by dividing the air up into very small
elementary cells which were then simulated as particles independent of
one another. Instead of studying the behaviour of several million
particles as previously, the Zurich researchers have now succeeded with
up to 6 billion particles. The scientists published their results in
“Computer Methods in Applied Mechanics and Engineering”.

The vortex wakes generated by aircraft when they land or take off
are dangerous: they can cause a following aircraft to crash. The
special high-resolution simulation has now enabled the researchers to
visualise for the first time how very small vortexes interact. This
allowed them to achieve a large and previously unattained value for
what is known as the Reynolds number.

Petros Koumoutsakos explains that the larger this value, the more
realistic is the simulation. He says this is because the number
characterises the aircraft’s vortex wake and is proportional to the
aircraft’s speed and wingspan. On the other hand the number of
computational elements needed to simulate this turbulence increases as
the square of the Reynolds number, thus limiting the possibility of
simulating it by calculation. Koumoutsakos says that although the new
simulation has allowed a closer approach to reality than ever before,
the joint aim of the ETH Zurich researchers and IBM scientists is to be
able to simulate even more realistic flight conditions.

Reducing environmental pollution

For their simulation the researchers used a supercomputer provided
by IBM with 16,000 processors – corresponding to about 10,000 modern
laptops. Through aircraft design adapted to it, the study can
contribute to reducing the formation of vortexes and accelerating their
self-destruction. Koumoutsakos says: “That is of the greatest
importance for flight safety, noise reduction and air pollution.”
Michael Bergdorf explains that “The structure of the vortexes can have
a considerable effect on fuel consumption.” He says that a large
proportion of aircraft noise is both generated by and carried by the
vortexes.

Increasing flight frequency

Air traffic worldwide is growing by about five percent per year.
Passenger numbers are forecast to double by 2025. This will lead to
bottlenecks at airports. However, if a more exact knowledge of the
generation of vortexes and their effect enables aircraft to be designed
so that they form fewer vortexes and the latter destroy themselves more
rapidly, this could also allow the prescribed separation distances
between takeoff and landing times to be shortened, thus increasing
flight frequency.

Journal reference: Chatelain, P. et al.: Billion vortex particle
direct numerical simulations of aircraft wakes, Computer Methods in
Applied Mechanics and Engineering 197, 1296-1304 (2008),
doi:10.1016/j.cma.2007.11.016.

ETH Zurich (2008, March 11). Record-breaking Simulation Of Aircraft Wake Turbulence. ScienceDaily. Retrieved March 12, 2008, from http://www.sciencedaily.com­ /releases/2008/03/080307104712.htm