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Unsteady aircraft aerodynamics

Diagram explaining the term unsteady flow

When it comes to the aeroelasticity of fixed-wing aircraft, the main purpose of unsteady aerodynamic modelling involves the fast, accurate forecasting of transient flow fields in the immediate vicinity of an aircraft and the resulting aerodynamic forces acting on that aircraft. Here, as can be seen in the image above, we confine ourselves to the low-frequency portion or, in the case of periodic flows over time, the portion of the generally turbulent flow that does not disappear after phase averaging, because high-frequency, turbulent fluctuations due to the natural low-pass behaviour of a large aircraft structure are not trivial or are only noticeable indirectly in the form of so-called eddy viscosity.

Below are three different types of unsteadiness that, 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.

Transonic flow – below Mach 1 – is critical for the aeroelasticity of rigid-wing aircraft because of transonic dip in the flutter stability boundary or overall higher changing aerodynamic loads. The Institute of Aeroelasticity has paid special attention to this circumstance in the unsteady aerodynamic modelling conducted with wind tunnel experiments and computational work for years.

Computational analyses

• Linearised RANS solver
• Correction procedures for linear methods

Wind tunnel experiments

• Unsteady aerodynamic forces on the rudder
• Interference between wing and engine
• Arrow-shaped wing with high Reynolds numbers
• Shock-buffet and aerodynamic resonance
• Excitation of a flexible wing by gusts
• Laminar wing flutter