UNSflow: A low-order, open-source solver for problems that involve unsteady and nonlinear fluid dynamics

This project aims to derive and develop new theoretical and low-cost numerical methods for analysing general unsteady aerofoil and wing flows that may exhibit nonlinearities such as flow separation and vortex shedding. The solvers implemented through these methods will be made available (open-source) to the public, academia and industry through the UNSflow project.

Unsteady fluid dynamics is ubiquitous in modern aerospace research problems such as aerodynamic optimisation of wind-energy harvesting devices, design of flapping wing fliers, use of flapping foils for propulsion/high-lift, and design of aircraft with flexible wings (such as HALE - High-Altitude Long Endurance, or futuristic aircraft with large aspect ratios). Reducing emissions of pollutants and greenhouse gases, which is the prime motivation behind many of these problems, can only be accomplished by a mix of renewable strategies and incremental improvements. The flow physics in these problems exhibits significant nonlinearities arising from flow separation and vortex shedding which cannot be adequately represented by closed-form theoretical formulations. Though computational fluid dynamics (CFD) and experimental methods have contributed much to the understanding of unsteady flow features, they are unsuitable for use in preliminary design and optimisation because of time and cost considerations. This project aims to develop low-cost, physics-based models for unsteady aerodynamics based on the discrete-vortex method, which will enable fast simulations of medium-fidelity, and provide a simple framework for parametric studies, design optimisation, real-time simulation and interdisciplinary studies (by coupling with other solvers).

UNSflow intends to be a new class of low-cost solvers that sacrifice an acceptable level of accuracy in fluid simulations for a tremendous speedup in simulation time, while being fully physics-based and retaining the fundamental flow quantities. The guiding philosophy in development of the solver is to retain only the physics which are significant in the flow regimes of specific applications. They are hence not an alternative to high-fidelity CFD and experiments, which will still be needed in the final phases of industrial production, but for fewer ideas/concepts. In effect, this will lead to reduced time and cost in the design cycle, and perhaps even a better solution in the long run, because more exploration of the design space will be possible. This research also intends to support the activities of teachers, students and hobbyists who may not have access to CFD software and computing resources. Potential applications for this class of users include design of ornithopters, quadcopters, and home-made wind-energy harvesting devices.

The research to be carried out in this project is fundamental in nature and underpins several applied problems. It is intended to derive new theoretical and numerical tools to study general unsteady flows with intermittent separation and reattachment. It will assist the principal investigator's research group in its research on applied problems such as aerodynamic optimisation, dynamic stall alleviation, flapping-wing design and wind-energy harvesting. The research will also be useful to other research groups working on unsteady flows (for both fundamental and applied research), as a preliminary design/analysis tool for various applications, and student projects.

The current dependency on fossil fuels and associated climate change is the most urgent threat of our times, and is a global problem with environmental, socio-economic and geo-political ramifications. As the rate of global energy consumption is only expected to increase, novel, interdisciplinary and international collaborations are necessary, involving leaders from diverse fields such as scientists, politicians and investors. Replacement of coal, oil and gas in the generation of electric power is unlikely to be achieved through any one technology, and instead requires a mix of renewable strategies. The research proposed herein, by developing novel low-order models for unsteady fluid problems, aims to tackle this problem on two fronts - by developing methods that enable higher efficiencies in aerial transport, and by enabling the design and optimisation of wind/water energy harvesting devices.

Unsteady flow physics are prevalent in many forms of modern aerial transport. High-Altitude Long Endurance (HALE) aircraft are intended to be systems powered by renewable energy, used for observations, communications, reconnaissance and even for providing internet access in remote areas. These aircraft have long, flexible wings which undergo large deformations, fluid-structure interaction, and require a complete rethink of control laws and systems. Helicopter efficiency is highly influenced by the dynamic stall phenomenon, which involves flow separation and vortex shedding, and is a classical problem in unsteady aerodynamics. Unmanned aircraft which undergo sharp manoeuvres, and novel concept such as micro-air vehicles based on flapping wings, also require unsteady aerodynamics modelling. Even in conventional aircraft wings, the trend is increasing aspect ratio and flexibility with the use of the composites. The transport industry has historically been conservative in adopting innovations and the integration of any novelty undergoes a lengthy and expensive process. Advances in modelling and simulation and the availability of multi-fidelity tools for unsteady aerodynamics could mitigate this cost significantly.

Wind energy harvesters of all configurations, such as horizontal-axis, vertical-axis and piezoelectric flapping-foil generators (which harvest energy using the phenomenon of aerodynamic flutter) involve unsteady aerodynamic phenomena which affect their performance. Optimisation of these harvesters, and using them optimally in various scenarios (urban, farmland, etc.) requires parametric studies spanning a large number of design variables for which the current research provides functionality.

Finally, this research and its outputs will have academic and entrepreneurial/innovation impact. As an open-source tool that facilitates simple coupling with other solvers to tackle multidisciplinary problems, UNSflow will be (and is being) employed by academics for fundamental studies on vortex flow structures and flow separation, and for teaching. It is worth noting that significant inventions and advances in technology have arisen from private innovators and entrepreneurs, hobbyists and small industries. These groups typically lack access to CFD tools, supercomputers and experimental facilities, and will hence benefit from models that could be used to test new concepts and designs. This research aims to develop new theoretical and numerical methods in unsteady fluid dynamics, incorporate these in software, and make them freely available to the public.