Abstract
Improving the fuel efficiency of aircraft has become an important task. Not only do airlines benefit from decreasing their direct operational costs by saving increasingly expensive fuel, but also the environmental aspect has gained growing interest, and regulators are presently looking at limiting greenhouse gas emissions. To date, the actual optimizations applied to new airplanes have been limited to reduction of weight, enhanced wing-profile shaping, higher surface quality and engine improvement, but it is thought that there is little potential left in these technology fields. Laminar flow control (LFC) provides a total drag reduction potential of up to 16% if 40% laminar flow can be maintained on the swept-back wings and tail planes of a current airliner.
The consideration of a corresponding three-dimensional boundary layer is stringently required in order to capture the features of the inherent crossflow instability. Owing to surface roughness, steady crossflow vortices (CFVs) typically prevail and cause early transition to turbulence if LFC is not applied. Detailed studies on the transition process have been carried out in the last decades and revealed the secondary-instability mechanism of (steady and travelling) CFVs. Several control methods have been developed and proposed since:
1. Distributed roughness elements method (DRE, Saric & al.). A one-time excitation of closely spaced steady CFVs generates a (secondarily) more stable flow scenario and thus delays transition. The concept of upstream flow deformation (UFD, Kloker & al.) pursues the same goal, by roughness, local blowing/suction, or Plasma actuators, in case in repeated fashion downstream (distributed flow deformation, DFD).
2. Means to weaken the basic crossflow and thus to diminish primary instability. Suction is one possibility, in practice not through ideal, spanwise slits but through discrete micro holes or slots. But unstable CFV modes can be generated by them jeopardizing the benefit of (ideal homogeneous) suction. Especially designed suction panels may combine suction with method #1, called formative suction. Alternatively, a volume force against the basic crossflow may be applied by a spanwise row of Plasma actuators.
3. Strong localized (“pinpoint”) suction to suppress secondary instability caused by CFVs.
On the other hand, laminar flow may be compromised by too large discrete roughness or oversuction. Knowledge on the “effective” roughness height or local suction strength, causing instant transition, is essential for successful LFC.
Fundamental investigations on these topics, based on direct numerical simulations (DNS) with a high-order in-house code, are presented and discussed.
FRIEDERICH, T., KLOKER, M.J. (2012) Control of the secondary cross-flow instability using localized suction. J. Fluid Mech. 706, 470-495. MESSING, R., KLOKER, M.J. (2010) Investigation of suction for laminar flow control of three-dimensional boundary layers. J. Fluid Mech. 658, 117-147. BONFIGLI, G., KLOKER, M.J. (2007) Secondary instability of crossflow vortices: validation of secondary instability theory by DNS. J. Fluid Mech. 583, 229-272. WASSERMANN, P., KLOKER, M.J. (2002) Mechanisms and passive control of crossflow-vortex-induced transition in a three- dimensional boundary layer. J. Fluid Mech. 456, 49-84.