VortexCell2050 - Fundamentals of Actively Controlled Flows with Trapped Vortices
Overview
Background & policy context:
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.
Objectives:
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.
Methodology:
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.
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