Understanding and Predicting Turbulence and its Effects in Fluid Flows by Modelling and Simulation

Understanding and Predicting Turbulence and its Effects in Fluid Flows by Modelling and Simulation

Research on modelling and simulating turbulent flows spans a broad range of topics, from fundamental studies on the physics of turbulence and its computational characterisation, to the application of complete algorithms to complex engineering flows – see Research overview.  Alongside research, educating students in fluid mechanics and users of complex codes for turbulent flows on the intricacies of turbulence and on how turbulence is represented within CFD codes is a crucially important pillar within an academic environment.  The book below aims to contribute to this goal.

The emphasis of the research is on the simulation of flows via LES (Large Eddy Simulation), DNS (Direct Numerical Simulation) and hybrid LES-RANS methods, with the RANS (Reynolds-averaged Navier Stokes) component used to resolve the near-wall regions in circumstances in which wall-resolved LES is too costly. After many years of research on RANS modelling, up to about 2004, studies in which RANS forms the sole modelling framework, form a minority activity.

Some research continues, however, on improvements to second-moment closure for separated flows. Simulation is pursued, on the one hand, in order to gain insight into complex turbulence phenomena in a variety of generic flows and, on the other hand, to identify the capabilities and limitations of RANS-based techniques, in particular, when applied to complex, industrially-relevant flows.

In recent years, research has shifted progressively towards more fundamental topics relevant to the control of near-wall flows, either to avoid separation, with pulsed synthetic jets, or to achieve drag reduction.  These topics are especially pertinent to civil aviation under bthe general heading of "green technologies". In the area of drag reduction, particular attention has focused on turbulence mechanisms and benefits arising from the imposition of tranverse oscillatory wall motion (an "active" method) and the use of wavy surfaces (a "passive" method).   As part of this research area, gaining insight into the influence of large-scale coherent turbulent structures in the outer part of the near-wall layer on the near-wall region, and thus on the drag-reduction effectiveness, is the subject of ongoing efforts.

Much of the recent research on flow and turbulence control has involved the use of DNS performed on large national (UK) computer facilities - HECTOR and ARCHER, in particular - with simulations run on 5000-10000 CPUs in parallel, generating many terabytes of data.  In addition, DNS databases generated at extremely high cost elsewhere (e.g. in Spain and the USA) on high-Reynolds-number flows in channels are being exploited in an effort to understand key fundamental turbulence mechanisms that impact on friction drag and thus on the ability of active and passive control strategies to reduce the drag.