Wings of modern aircraft are thin and streamlined, thus ensuring maximum aerodynamic efficiency. From a structural viewpoint a thick wing would be more efficient in carrying the load. The tendency of increasing aircraft size shifts the design balance towards giving more weight to structural considerations. As a result, improving the aerodynamics of thick wings is essential for further progress in aviation.
Trapping vortices is a technology for preventing vortex shedding and reducing drag in flows past bluff bodies. Large vortices forming in high-speed flows past bluff bodies tend to be shed downstream, with new vortices forming in their stead.This leads to an increase in drag and unsteady loads on the body, and produces an unsteady wake. If the vortex is kept near the body at all times it is called trapped.
A trapped vortex could be just a steady separation eddy above an aerofoil at high angle of attack, but the use of a vortex cell helps. Practical implementation of the trapped-vortex idea is difficult, since the trapped vortex needs to be almost steady in the sense that it should remain in the close vicinity of the body. Stabilising a trapped vortex was a major challenge for the project.
VortexCell2050 aimed at combining the trapped-vortex technology with active flow control. The specific major objectives of the project were:
- To develop a software tool for designing a thick airfoil with a trapped vortex assuming that the flow is stable, apart from small-scale turbulence.
- To develop a methodology and software tools for designing a system of stabilisation of such a flow.
- To design and estimate the performance of an airfoil with a trapped vortex and stabilisation system for High-Altitude Long Endurance unmanned aircraft.
The project work, aimed at advancing the vortex-cell technology, can be grouped in two large parts.
- The first part, a highly ambitious but also quite risky line of research was aimed at testing, at the end of the project, a wing with a vortex cell serving as a prototype for a practical application, namely, a High-Altitude Long-Endurance (HALE) aircraft. This required performing series of fundamental experimental studies of the properties of flows with trapped vortices, in order to provide the necessary data for the design tools, which also needed to be developed, and required experiments simply aimed at gaining experience in working with vortex cells with active control. The high degree of risk in this line of research was due to its sequential nature, such that each following step in the plan could be done only after the previous step has been successfully finished, while, as is common in scientific research, success could not be guaranteed for any of the steps. Indeed, the final HALE airfoil that was tested has a system of control not utilising a feedback algorithm, contrary to the initial plan.
- The second part of the work had two goals: to mitigate, where possible, the risk in the first part, and to use the chance of exploring, within a favourable context of a common goal, various alternative avenues of research on trapped vortices.
The results obtained generally meet the objectives specified at the start of the project, and provide the solution to the problem initially addressed, namely:
- A software tool for designing a flow past a thick airfoil with a trapped vortex assuming that this flow is stable, apart from small-scale turbulence, was developed.
- A methodology and software tools for designing a system of stabilisation of such a flow were developed.
- An airfoil with a trapped vortex and a stabilisation system for the High-Altitude Long Endurance aircraft was designed, built, tested and its performance was estimated.
The results obtained in the project provide a significant step forward as compared to the state of the art. In particular, the development of the software tool for optimising the shape of the vortex cell, the significant body of data on three-dimensional effects collected, and the significant body of data obtained on the actively controlled flows past airfoils should be distinguished.
- The software tool developed in the project optimises the vortex cell shape for the case of zero mean flow rate of the stabilisation system, as it was initially preconceived, while the developed control scheme requires a non-zero mean flow rate. Therefore, further developments should either generalise the optimisation tool to non-zero mean flow rate or achieve control with zero mean flow rate.
- The developed scheme of active control should be classified as an open-loop scheme, rather than the initially envisaged closed-loop scheme. However, the numerical results obtained but not tested in experiment suggest that there are significantly more efficient control schemes. Further work should be concentrated on unsteady and feedback control schemes.
- The performance of thick airfoil with a vortex cell and control system designed and tested was observed to be better than the performance of the thick airfoil with control system but without the vortex cell, but only in a certain range of the angle of attack, and, more importantly, only when the flow past an airfoil with a vortex cell was in the more favourable branch of the hysteresis loop of the flow regime. While methods of attaining the favourable branch were identified numerically, further experimental research should take care that the corresponding provisions are made in the design.