The UNSI Project

Summary

The European UNSI (Unsteady Viscous Flow in the Context of Fluid-Structure Interaction) project involved 15 European partners in the period from January 1998 to September 2000. It was undertaken as a major industrial research project in the former DGXII, Brite-Euram, Directorate within the fourth European Community Research Framework Programme.

For many decades, robust methods for structural analysis and linearized unsteady aerodynamics have been coupled and used by the aircraft industry to computationally clear a new design for flutter. Thus, within the domain of Computational Structural Mechanics (CSM), complex structural models are available which are coupled with linear potential aerodynamics.

However, to account for “non-conventional” and complex flows such as flows with separating boundary layers at subsonic, transonic or supersonic speed, more sophisticated CFD tools are required with improved physical capabilities. The treatment of complex unsteady separated and/ or transonic flow phenomena demand CFD tools with good predictive accuracy. Therefore, it proves necessary to overcome still existing problems in CFD, like turbulence modelling for unsteady flows, mesh movement and adaptation aspects as well as problems in sophisticated coupling strategies.

Moreover, in order to support an enhancement of CFD, comprehensive measurements of unsteady 2D and 3D flows are necessary to provide reliable values of global flow field variables as well as of boundary layer and turbulence quantities. These results are seen to be an absolute necessity for a proper validation. The incentive of this approach is that a direct cross-fertilization between experimentalists and theoreticians will then take place.

To summarize the objectives of the UNSI project, four major goals must to be considered:

• 1. Improvements of computational fluid dynamics methods with respect to time-accurate unsteady flow behaviour.

• 2. A comprehensive calibration of CFD methods by carrying out measurements and computations in parallel to realize cross-fertilization and to improve turbulence modelling aspects particularly for unsteady flows.

• 3. Extension and/or development of coupling procedures to account for sophisticated flow analysing tools (up to Navier-Stokes methods) coupled to structural mechanics.

• 4. Demonstration of the capabilities to predictive unsteady aeroelastic phenomena, including flow with separation and/or flutter.

Hence, a coupled aeroelastic simulation tool has two basic ingredients: (1) A means of computing the aerodynamic forces over the whole aircraft at each instance of time, and (2) a means of calculating the deformation of the aircraft due to these forces. To reach the goal of a simulation tool for complicated flow cases, first of all the CFD methods have to be improved. Methods ranging from non-linear potential methods and Euler equations with boundary layer coupling, to full Navier-Stokes solvers have to be improved and often adapted. Moreover, the coupling between flow and structure has to be revisited.