Bluff-body drag reduction using feedback control

Recent experiments have shown that active, open-loop control of the flow behind an axisymmetric bluff body can achieve roughly a 10% reduction in pressure drag with actuation in the form of a pulsed jet around the circumference of the back face of the body. Although the effect has yet to be fully explained, it appears that the jet can behave as a virtual spoiler that inhibits turbulent transport and entrainment (splitter plates behind bluff bodies are known to reduce pressure-drag via the inhibition of entrainment, but with the penalty of splitter-plate friction-drag). This effect shows significant promise and the open-loop control investigations form part of a related research programme funded directly by Ferrari S.p.A., who intend to introduce pulsed-jet forcing for drag reduction on one of their production models for 2012. A closely-related effect has also been identified in the control of turbulent separation from a backward-facing step. There are two logical extensions to this work: the first is developing an understanding of the virtual-spoiler effect which is likely to have more general application to the automobile, marine and wind-engineering sectors. The second is the introduction of physically-based feedback control for drag reduction of bluff bodies. Feedback control is likely to provide even greater reductions in drag without increasing the complexity of the hardware, allows the actuator power to be minimised, increasing the net energy gain and can be designed to automatically adapt to changing flow conditions. These form the two key objectives of this proposal. Our ultimate vision is that automotive vehicle drag-reductions of 10-15% (corresponding to fuel-savings of around 5-8% at motorway speeds) would be achievable, reducing the impact of road transportation of global CO2 emissions.

The economic costs of not responding to the challenges of climate change have received considerable recent publicity, most notably in the Stern report. In a comprehensive and wide-ranging assessment, Stern has proposed that the reduction in greenhouse gas emissions is an economic imperative. The principal conclusions are stark: the benefits of strong and early action far outweigh the economic costs of not acting, and the overall costs and risks of climate change will be equivalent to losing at least 5% of global GDP each year, now and forever. If a wider range of risks and impacts is taken into account, the prospect is even bleaker. More positively, the report outlines the way forward suggesting that the investment that takes place in the next 10-20 years will have a profound effect on the climate in the second half of this century and in the next. There are three elements of policy: one relates to the pricing of carbon, another to the promote energy efficiency and individual awareness. The third, and the one most relevant to investment in science and technology (and therefore highly relevant to this proposal) is to support innovation and the deployment of low-carbon technologies . Within the transport sector, Stern clearly identifies the automotive sector as the dominant source of CO2 emissions. The overall CO2 contribution to the automotive sector (shown here as 76%, approximately) is very close to the OECD data and DOE data provided on p.3 of the Case for Support. Therefore the case for very significant impact in transport emissions by development of the ideas and technology stemming from the work of this proposal is undeniable. A drag reduction of 10% is, we believe, achievable and is one of an number of other technologies also identified by Stern. To reduce the environmental impact of transport on a global scale, technological advances are needed in areas ranging from fluid mechanics to fuel cycles and lighter materials. Improvements in operational practices and changes in societal attitudes will also play crucial roles. This proposal aims to contribute to the large-scale effort needed by focussing in detail on some technological improvements in fluid mechanics that may play an important role. Its aim is to further understanding of how fluid flows contribute to carbon emissions from road transport, and to investigate ways of manipulating these flows in order to reduce emissions.