Aerodynamics, computers and the environment.

Abstract

We are faced with global warming and dramatic increases in the world aircraft fleet. Computing power continues to inexorably rise, and machines are now powerful enough to make new technological break-throughs in the aerospace industry. This Theme Issue seeks to explore how computers should be used in future and how they may impact critical problems in aviation and its impact on the environment. However, the work also has wider relevance to the general fields of transport and energy. The environmental impact of aircraft with respect to emissions, including noise, is an area of critical importance. In many instances, aerodynamic performance and noise are intrinsically linked through turbulence. Go to any major international conference on turbulence, and one would be hard pressed to find many delegates who could agree on a definition of what turbulence actually is. As the Nobel prize winner Richard Feynman said ‘Turbulence is the last great unsolved problem in classical physics’. This makes the mathematical modelling of turbulence challenging. Equations complete enough to virtually exactly describe turbulent flow—the Navier–Stokes equations—have been available for over a century. Until recently, the standard practice in computational fluid dynamics (CFD) is to solve a time-averaged version of the Navier–Stokes equations, using a simplified model to represent the turbulence—the Reynolds-averaged Navier–Stokes (RANS) approach. However, computing powers have increased to the point where one can seriously consider the near-direct solution1 (NDS) of these equations for practically relevant flows. Thus, armed with high-performance computing (HPC), modern computer graphics and analysis tools we can now, for practically relevant systems, unlock turbulence's secrets and thus intelligently manipulate the turbulent flow field, to improve performance and so address global environmental challenges. This situation was not unforeseen. Chapman (see [1]), director of aeronautics at NASA, proposed, using generally well-founded scientific arguments, that when computers reached 1014 FLOPS (floating point operations per second) we could perform NDS that would begin to rival aerodynamic tests. Modern HPC provision now exceeds Chapman's expectations reaching petascale with exascale computing due around 2018. Hence, now the ability to directly predict turbulence, for complex engineering systems, without recourse to accuracy reducing assumptions is close at hand. Computer-processing speeds have increased by a factor of around one million in the past 25 years (see Jameson [2]), and continued improvements are expected. Currently, flow physics insights from NDS are allowing the improvement of reduced-order mathematical models for design. In addition, simulations that potentially offer greater accuracy than tremendously expensive rig tests are now emerging. A notable shoot from the emerging era is work of Morton et al. [3] (US Air force Laboratory), who performed an NDS variant for an F/A-18 fighter configuration. Tail buffet was explored, and successful comparison made with real flight data. There are many other examples. Thus, after waiting with eager expectation, for several decades, Chapman's prophecy is coming closer to fulfilment. This Theme Issue explores what is needed to complete the fulfilment, how will things gradually change as the fulfilment is approached, what might the new era look like and when it will arrive. Obviously, what is meant by fulfilment is a complex thing, because aerospace systems involve a wide range of components with very different flows and degrees of coupling/dependence with other components. Hence, different flows will come to fruition at vastly different times. The Theme Issue comprises 12 papers exploring the abovementioned theme. The papers typically look at the status of computational modelling around 2030–2035.