Turbulence Control Group
Academic staff: Dr Duncan Lockerby; Dr Yongmann Chung; Prof Peter Thomas; Dr Petr Denissenko; Dr Chris Davies (Cardiff University)
Post-doctoral researcher: Dr Faisal Baig
PhD students: Carlos Duque; Nick Pearce; Dana Elam (supervised by Dr Y. Chung); Edward Hurst (supervised by Dr Y. Chung).
If current rates of global air travel are to be sustained, a drastic reduction in aircraft fuel consumption and fuel emissions is required; the Advisory Council for Aeronautical Research in Europe (ACARE) has set an ambitious goal of 50% reduction in CO2 emissions per passenger/km by the year 2020. The ACARE targets can only be achieved with technical advancements in a number of areas: one key area is aircraft aerodynamics.
The main contributor to aerodynamic drag, and thus fuel emissions, is fine-scale turbulence that exists very near to the aircraft’s surface during cruise. Our aims in the Turbulence Control Group are :
a) to create novel computational and experimental techniques to study these near-wall flow structures which are responsible for high drag in turbulent boundary layers, and
b) using these tools and facilities, develop flow-control technologies capable of majorly reducing drag on passenger jet aircraft.
An efficient computational model for near-wall turbulence
The major challenge in modelling turbulence at aircraft scales is the extremely fine spatial and temporal scales over which the critical flow structures exist. Furthermore, even if these scales could be resolved in simulation, there is the added problem of distinguishing the important flow structures from turbulent noise.
At Warwick we have developed a reduced-order model (Lockerby & Carpenter, 2005, AIAA. J. 43) that allows the study of the basic near-wall flow structures (streaks and vortices) within a linear Navier-Stokes simulation. Despite turbulence being characteristically nonlinear, such a linear model is appropriate for the early stages of turbulent structure (streak) development, where our interest principally lies. There are a number of advantages to this approach over full Direct Numerical Simulation (DNS). First, the model provides a ‘cleaned-up’ and simplified view of the near-wall structures, which is potentially more informative and insightful than fully-resolved simulations (see Figure, right). Secondly, it is numerically very efficient and allows the study of extremely high Reynolds numbers; simulations at Re = 10,000 (based on displacement thickness) have been performed, which is representative of flows over passenger jet aircraft — far beyond the reach of DNS.
Passive flow-control actuators
If an actuator is to be used on an aircraft, it must save more energy (in reduced drag) than it requires for its operation. Unfortunately, many powered devices that have been developed for the drag-reduction application are unlikely to satisfy this fundamental net-saving requirement. At Warwick, in a project funded jointly by Airbus and the EPSRC, we are investigating the potential of using new types of non-powered (passive) devices for turbulent drag reduction. One idea seeks to exploit Helmholtz resonance – the flow phenomenon of singing bottles – to produce micro-scale jet flows in response to turbulent noise. We are also investigating means by which the drag-reducing capabilities of riblets (surface ridges, such as on shark skin) can be amplified using these Helmholtz resonators.
In a two-dimensional turbulent boundary layer, such as that over a flat plate, near-wall flow structures are reasonably well understood. However, there is much less known about these structures in a three-dimensional context, such as on the wing of a passenger jet aircraft. This gap in our understanding needs to be addressed before we can seriously develop methods for controlling and characterising turbulence on swept wings.
Funded by a three-year EPSRC grant (GR/T27358/01), we at Warwick have designed and constructed a water channel capable of studying the near-wall turbulent structures in a three-dimensional boundary layer (see Figure, bottom right). This purpose-built facility allows us to change the level of pressure gradient, and hence cross flow three-dimensionality, in a manner that has previously not been possible. The flow visualisation technique to be employed will be similar to that used by Kline et al. (1969) in two-dimensional boundary layers (see figure, middle right).
Wireless flow control (SWIFT)
The characteristics of near-wall turbulence dictate that any aircraft drag-reduction system requires large arrays of actuators and sensors – even if only for monitoring purposes. Existing wiring interconnectivity in aircraft is already highly complex (as shown in figure, below), and so the addition of vast arrays of additional actuating and sensor elements would require a step change in complexity with major cost and weight penalties. In collaboration with the University of Sheffield and Queen’s University Belfast, and funded under an EPSRC/Airbus grant, we are investigating the possibility of controlling and monitoring flow-control devices using wireless technology. The Scalable Wirelessly Interconnected Flow-Control Technology (SWIFT) project is an integrated investigation of: flow-control systems with minimum energy consumption; a wireless aircraft nervous system to reduce wiring weight and complexity; and performance/health monitoring to meet safety demands and minimise Direct Operating Costs. The collaboration will deliver, amongst other outputs, a physical demonstrator of wireless supervision and control of a novel drag-reduction technique.
A320 wiring loom. Taken from: www.lia-tech.com.
The reduced-order streak model: (a) side, (b) plan and (c) front-view contours of streak velocity perturbation. (Large image)
Sublayer streaks: Hydrogen-bubble flow visualizations of horizontal slices of a turbulent boundary layer, Kline et al. (1969)
The cross-flow water channel