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Unsteady aerodynamics training network in airframe components for competitive and environmentally friendly civil transport aircraft.

Final Activity Report Summary - AEROTRANET (Unsteady aerodynamics training network in airframe components for competitive and environmentally friendly civil transport aircraft)

The air flow past a surface recess, such as across an aircraft fuel vent and over panel joints in the bodywork of road vehicles, is often unsteady, producing unwanted vibrations and aerodynamically generated sound. The occurrence of such flow instability in aviation, road and rail transport brought together four academic institutes and four industrial collaborators (Alstom UK, Airbus SAS, Fiat, and Renault) under AEROTRANET, coordinated by Dr. Aldo Rona, to identify instability suppression concepts based on an enhanced knowledge of the flow physics.

To achieve this common objective, AEROTRANET integrated Computational Fluid Dynamics from the Department of Engineering, University of Leicester, UK, Particle Image Velocimetry and passive flow control tests at DIASP, Politecnico di Torino, wind tunnel tests and velocity-pressure correlations at DIMI, Università degli Studi Roma Tre, and adjoint techniques for flow control at the Institut de Mécanique des Fluides de Toulouse (IMFT), in a coordinated trans-European work programme.

The activities involved 20 Early Stage Training (EST) Marie Curie fellows, recruited by the four academic partners, who addressed complementary aspects of this common research topic using different analytical, numerical, and experimental techniques. The fellows were trained at post-graduate level at the partner institutes in a 468.5 man/months programme, using in-house resources as well as collaborations with the Università degli Studi di Roma 'La Sapienza', the Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA), the Italian Ship Model Basin (INSEAN), TU Delft, Politecnico di Milano, NAG (Oxford), HPCx (Edinburgh), CINECA (Bologna), and CASPUR (Rome). A detailed analysis was performed of a cylindrical cavity representative of a wide-body civil aircraft fuel vent at landing, using numerical modelling (University of Leicester), pressure-velocity measurements (Roma Tre) and tomographic Particle Image Velocimetry (TU Delft). Combining these techniques resolved in space and in time the flow instability and the resulting acoustic near-field. The active mixing across the opening creates significant blockage that affects the on-coming boundary layer approaching the enclosure well-upstream of the opening, increasing drag. This interaction and the acoustic radiation are increased at certain speeds at which cavity longitudinal instability modes lock in with acoustic depth modes. A parameter space map indicating the flight and geometry condition for this enhanced resonance is now available to Airbus to steer fuel vent designs away from these conditions.

The flow over a rectangular cavity with a thick inflow boundary layer, representative of that over panel joints in cars and trucks, was found to behave in a rather different way from that of the cylindrical geometry. At the test conditions, the thick inflow prevents the onset of classical Rossiter type instability modes, making this flow acoustically quieter. Particle Image Velocimetry on planes parallel to the cavity floor showed that, above the enclosure, the pattern of streaks coming from the inflow boundary layer breaks down in a more random vorticity distribution. This lack of spanwise coherence is thought to be an additional beneficial effect to reducing cavity noise, as it reduces the spanwise correlation length scale of the acoustic sources, making these less effective. The control of cavity flow was addressed by the adjoint technique at IMFT and the upstream cavity edge was identified as the area most suitable for effective flow control, which was then verified by experiments using a rod in cross-flow at the Politecnico di Torino.