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Computational analyses

Computational utilisation strategies

The main purpose of unsteady aerodynamic modelling is the fast, accurate prediction of transient flow fields in the immediate vicinity of an aircraft, as well as the resulting aerodynamic forces acting on that aircraft.

There are three different types of unsteadiness which, in reality, often occur together:

• Motion-induced unsteadiness caused by movement of the geometry over which the air is flowing – for example flutter, manoeuvring loads.
• Externally-induced unsteadiness caused by interferences in the incident flow – for example tail buffet, gusts, wake vortices.
• Flow unsteadiness – the flow itself is no longer steady but fluctuates periodically over time or chaotically – for example, shock-buffet.

The transonic range of Mach numbers – which is problematic regarding flutter and load analysis – calls for the greatest modelling effort due to the complexity of the prevailing flow phenomena: when strong compression waves occur, a computational CFD model of the Euler-boundary layer interaction type is required. Based on today’s knowledge, consistent modelling of the boundary separation that occurs in addition to this can only be performed by solving Reynolds-averaged Navier-Stokes (RANS) equations at precise times, although this requires great computational effort, some five to six orders of magnitude above the potential-theoretical model, such as the Doublet-Lattice Method (DLM), which is accurate enough in the compressible subsonic Mach number range.

More information on this can be found here:

• Linearised RANS solver
• Correction procedures for linear methods