Is Fine-Scale Turbulence Universal?

Most flows of practical interest are turbulent and hence the understanding of these flows is important for both engineering and fundamental reasons. Unfortunately, turbulence remains ``the most important unsolved problem of classical physics''. It consists of a wide range of three-dimensional motions, from large and slow to small and fast. The smallest (and most rapid) motions dissipate the kinetic energy of the flow and determine drag on bodies, dispersion of pollutants and chemical mixing. Unfortunately the very smallness of these motions has, until recently, made them inaccessible to both experiments and computations in flows of practical importance. Predictions of turbulent flows have thus been based on uncertain theories and models of these ``fine scales which are assumed to be the same for all flows i.e. universal. No-one knows if this is true or not. Answering this question requires measurements of a range of flows using techniques capable of resolving their full structure in space as it evolves in time. New techniques developed by the applicants have, for the first time, made the full measurement of these motions possible. Similar advances in computational methods has provided the opportunity for meaningful comparisons with such measurements.In this collaborative initiative between Imperial College London, the University of Cambridge and the University of Southampton, we aim to develop and employ a series of advanced laser diagnostic techniques to measure the time-resolved, three-dimensional features of the fine-scales in a series of `canonical' turbulent shear flows in order to test the hypothesis of universality. Measurements will also be taken at Reynolds numbers accessible to Direct Numerical Simulations especially carried out with this purpose for cross-comparison and validation. The experimental techniques will be based on cinematographic scanning and tomographic Particle Image Velocimetry (PIV) techniques. Regardless of whether the universality hypothesis holds or not, the necessary information to formulate physics based fine-scale models that can account for the multi-scale interactions will be obtained. The data as well as the 3D PIV software will be made available online for researchers in the UK and around the world.

The research will have impacts in the short, medium and long term. In the short term and medium term, beneficiaries are expected to be primarily academics (and other researchers) trying to better understand, measure and model turbulence. They will benefit from the availability of new data that directly compares the fine-scale structure across a range of flows. The analysis of the data will provide new understanding of turbulence. In order to achieve this impact the information will be disseminated through journal publications and through presentations of the latest measurements to the UKTC. Additional beneficiaries will be experimentalists who will benefit from the development of the new techniques: also through publication of journal papers and presentations. In order to maximise the near-term impact, we propose to organise a workshop on fine-scale turbulence at the completion of this project in summer 2014. We aim to collaborate with the convenors of the UKTC to organise this workshop to ensure a wider dissemination. Theoreticians and numerical researchers will benefit from the database of results and the new understanding obtained to further develop models for fine-scales and incorporating these into analytical theories and computational codes. This activity will be facilitated through meetings with interested academics and industrial code-developers and users. Many of these are already in direct contact with the proposers and so will be able to obtain the information through direct interaction. On the experimental side the beneficiaries will be experimentalists who can use the techniques and software for making full time-resolved three-dimensional measurements of fluid flows. This software will be made freely available and downloadable via the internet. The details and availability of this information will be disseminated to researchers via email contacts. In the long term, the much-improved numerical codes and theories can be used in industry for calculating real flows, hence allowing for improvements in design and in technologies. The improved theories and general understanding will have an impact across disciplines in any area where turbulent flow is present. The industrial impact of the proposed work is acknowledged by UK based CD-adapco, one of the leading providers of commercial CFD software worldwide, stating that refining the knowledge of turbulence and the modelling of turbulence will be valuable for a very wide range of engineering applications relevant to the environment, clean energy and transportation. In addition to the more obvious aerospace applications examples are in biological processes (e.g., flow in the lungs), animal locomotion, environmental modelling, dispersion of pollutants, combustion research, climate modelling. This impact is likely to be far-ranging. The experimental measurement and analysis techniques will also, no doubt, find application across a range of fields where researchers need to measure and analyse three-dimensional unsteady phenomena. In particular techniques for analysing large three-dimensional datasets has wide applicability. Furthermore, despite the fundamental nature of the work, Rolls-Royce is also interested in the project and have agreed to participate in our meetings and workshops and provide us with guidance on how the fundamental understanding can be exploited by industry. Apart from contributing to the development of understanding of fine-scale turbulence, the additional benefits of the current project to the UK general public is two-fold. First, the fact that this is first study to present a collaborative initiative to establishing UK as a leader in this area of research. Second, the training of researchers and multiple PhD students in experimental and computational fluid dynamics research will prevent the erosion of expertise in this area in the UK.