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Technologies to IMProve Airframe Noise

Final Report Summary - TIMPAN (Technologies to IMProve Airframe Noise)

During the TIMPAN project, advanced low noise design concepts were investigated and noise tested on a 1/4 scaled main landing gear model in the German-Dutch wind tunnel.

Landing gear related airframe noise is one of the dominant aircraft noise components at approach. Due to the advances in aircraft engine noise reduction, airframe noise became a major noise component during approach and landing. For wide body aircraft in particular the dominant airframe noise sources are the landing gears followed by aerodynamic noise originating from deployed high-lift devices. It therefore is essential to particularly reduce landing gear noise.

A variety of gear configurations were tested including a new side-stay design, different modifications of bogie inclination, wheel spacing, bogie fairings with different flow transparency, leg-door configurations and brake fairings. The acquired far-field noise data were compared against the results from a landing gear noise prediction model, transposed to full scale flight conditions and compared against the full scale test data obtained for the previously accomplished SILENCER project advanced A340 style 4-wheel main landing gear. An optimal combination of tested gear modifications led to a noise reduction of up to 8 dB(A) in terms of overall A-weighted noise levels relative to the SILENCER reference gear configuration.

When air is blown from a slot directly upstream of a flap, the flow over the flap can bear large adverse pressure gradients without separation. This effect is used to design high-lift airfoils with low momentum coefficients of blowing. For experimental assessment of these airfoils a rectangular wing with an aspect ratio of 4.3 was built. The flow around the model in a low speed wind tunnel is analysed using pressure measurements, oil flow pictures and particle image velocimetry. For Reynolds numbers of about Re= 1x10^6 the dimensionless momentum coefficient of the jet and the angle of attack of the airfoil are varied. Numerical simulations of the three-dimensional flow around the circulation control airfoil in the wind tunnel are compared to the experimental data. Good agreement is observed in terms of pressure distributions and wall streamlines.

Numerical simulations of the flow around profiles using circulation control were conducted to find favourable geometries with low momentum coefficients. As a starting point for two-dimensional investigations of circulation control a modern transonic airfoil was chosen. The flow around profiles utilising trailing edge blowing is simulated by solving the Reynolds-averaged Navier- Stokes equations using the DLR hybrid unstructured flow solver TAU, which is based on a finite volume scheme. The numerical investigations using TAU simulate the flow around the profile at free stream conditions. Free stream values were chosen according to the standard atmosphere (i.e. 0 m for take-off and landing but 10 500 m for cruise flight) for a Reynolds number that represents the wing of a 260-seat passenger aircraft. The total pressure and total temperature of the pressurised air used for circulation control were chosen corresponding to the flow conditions in the exhaust of a modern shrouded high-bypass engine.

The experimental investigations were conducted in the low speed wind tunnel of the Technische Universitaet Braunschweig, which is a closed-return atmospheric tunnel with a 1.3 m x 1.3 m closed test section. An opening angle of gamma = 0.2 degrees of the floor and ceiling of the test section compensates for the boundary layer growth. In the measurement section a maximum speed of 55 m/s can be achieved. A heat exchanger in the settling chamber allows constant flow temperature. The wind tunnel model with circulation control was investigated at a free stream velocity of v(inf.) = 50 m/s, which results in a Mach number of Ma(inf.) = 0.15 and a Reynolds number of Re = 1 x 10^6.

The flow around the wing utilising circulation control was simulated by solving the Reynolds-averaged Navier-Stokes equations, using the DLR hybrid unstructured flow solver TAU again. To increase the accuracy of the three-dimensional (3D) simulations, low speed preconditioning was used. The viscous walls of the test section were also simulated to obtain realistic results for the flow around the wind tunnel model. For efficient flow computations the chimera technique was employed with a local grid defined around the airfoil. The chimera technique allows using an already attained solution as a restart for a following simulation with a different angle of attack. Using the chimera technique to gradually increase the angle of attack, the flow in the vicinity of maximum lift can be simulated correctly.

The experimental investigation of an airfoil with circulation control using an internally blown high-lift flap yields high normal force coefficients at comparably low momentum coefficients. Pressure distributions along the airfoil, wake probing, PIV measurements and oil flow pictures are obtained to establish a data set useful for validating numerical simulation methods. First numerical simulations show a good agreement with the measured data, even if a simple one equation turbulence model without curvature correction is used. More numerical simulations have to be conducted to see if the same good agreement can be achieved for maximum lift and if the momentum coefficient and the angle of attack for which separation starts can be predicted as well.

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