Influence of a cavity on the dynamical behaviour of an airfoil

A new wing design has been the subject of study in the European project VortexCell2050. For several reasons (structural and fuel load) it is desirable to use relatively thick wings. However, thick wings promote flow separation and/or massive vortex shedding, reducing flight performance. The new design airfoil is equipped with a cavity ("vortex cell") in the wing in order to prevent massive flow separation. This thesis aims to obtain insight into the dynamical behaviour of such a wing with a cavity and to explore which numerical methods are suitable for estimating the unsteady forces. In this thesis experiments and computations are presented, using a geometry that was inspired by cavity shapes which are considered in the VortexCell2050 project. For an airfoil with a cavity, oscillations of the shear layer are expected at Strouhal numbers of order unity, based on the width of the cavity opening. For the size of the cavities considered this implies high values, O(10), of the reduced frequency, based on the chord length of the airfoil. In order to conduct experiments in this high reduced frequency range, a new experimental method has been developed. In this experimental setup the airfoil is fixed to the wind tunnel wall and the first acoustic transversal eigenmode of the wind tunnel test section is used to drive an oscillating flow. In the conventional method the airfoil is oscillating. The main fundamental difference between the two methods is the presence of a time dependent uniform pressure gradient, which drives the oscillating flow, in the new method. The results obtained with both methods are equivalent after correcting for an effective buoyancy force induced by this driving pressure gradient. The new method avoids the use of a complex mechanical system to drive the oscillation of the airfoil. The acoustical forcing amplitude is very easy to vary within two orders of magnitude. The method appears to be most suitable for the conduction of experiments at high values of the reduced frequency. The new measurement method is validated by means of experiments on a standard NACA0018 airfoil complemented with two-dimensional Euler simulations. Thereafter two airfoils with slightly different cavity geometries are investigated in the wind tunnel. These experiments consist of measurements of local surface pressures. For the case of an airfoil with a cavity the Euler equations are not suitable. Two-dimensional simulations using the frictionlass flow approximation approach a so-called Batchelor flow, with a uniform rotation in the cavity. This flow is not observed in experiments. For this reason two-dimensional incompressible Navier–Stokes simulations at a Reynolds number, based on the chord length of the airfoil, of 2 · 104 are performed and indicate shear layer oscillations. In order to validate these low Reynolds number numerical results and to gain more insight in the flow physics, flow visualisations are performed in a waterchannel at the same Reynolds numbers as the numerical simulations. The visualisations also show oscillations of the shear layer at the first and second hydrodynamic mode, this is confirmed by hot-wire measurements in the wind tunnel at low Reynolds numbers. The hot-wire measurements also demonstrate that the expected lock-in of the shear layer does occur in a limited range of Reynolds numbers, based on the chord, sufficiently low such that no turbulence is generated, but higher than a critical value. Experiments and two-dimensional Navier–Stokes simulations indicate that for values of the reduced frequency, in the range of 2–10, no significant deviations in the unsteady lift force occur between an airfoil with cavity and the same airfoil without cavity. The cavity does display shear layer oscillations around the expected Strouhal numbers, however, the associated fluctuations in the lift coefficient appear to be neglegible. For the geometries considered the pressure differences over the airfoil are dominated by the added mass of the airfoil

Dynamic measurements on an airfoil using acoustic forcing

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Dynamic measurements on an airfoil using acoustic forcing

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Experimental investigation of Fenestron noise

The noise radiation of an EC135 Fenestron is investigated by means of flight tests. The noise emission was measured with ground microphones during several different maneuvers. These include hover, rearward flight, and forward flight at different combinations of the side slip angle and the airspeed. In hover, a high level of broadband noise is observed. A comparison with available engine noise data suggested that engine noise is not dominant in hover. In rearward flight, the Fenestron may encounter a relatively clean aerodynamic inflow. However, the measurements do not indicate a reduction in noise radiation of the Fenestron. For rearward flight, a significant increase in main rotor noise is observed. Flights at different combinations of the side slip angle and airspeed show that the Fenestron radiates high levels of tonal noise at high-speed flight at negative side slip. The most likely cause for this is a highly disturbed inflow caused by flow separations of the diffusor outlet edge and the sharp trailing edges of the stator blades. Measurements of the tail boom yaw moment and Fenestron drive shaft torque imply that a reverse flow is possible at medium airspeed, while the Fenestron thrust does not reverse sign. Only at higher airspeeds and negative side slip, it is possible to achieve a thrust reversal in addition to a flow reversal

Uncertainty Quantification of Noise Abatement Flight Procedures

Noise abatement flight procedures are usually designed under the assumption that the resulting flight path will be executed by the pilot with very high accuracy. A previous investigation, however, has shown that ven though the flight path is accurately controlled there still is a significant spread in the measured noise metric on the ground. The statistical values obtained in these flight tests are used in this paper to investigate the influence of uncertainty in the position, velocity and wind. The results indicate that the velocity along the flight path plays a dominant role in the resulting statistical properties of the noise metric on the ground. Furthermore it is shown that uncertainty in the velocity has a significant impact on benefit obtained by noise abatement flight procedures. This indicates the need to include uncertainty into the optimisation process. The results of the uncertainty quantification could also be used to compare different flight procedures in terms of robustness

FASTrescue deliverable D3.3c: PAVE database

This document describes the results obtained during the PAVE measurement campaign in 2004 with DLR’s BO105 helicopter. The dataset contains measurements from the main rotor system, including blade pressures,blade bending moments, blade pitch angles and rotorshaft bending moments. Further it contains measurements of various onboard data, such as GPS position, velocity, acceleration, rotor RPM, attitude angles and its time derivatives and time synchronised acoustic measurement on the ground. The availability of blade pressure data makes this data very useful for the validation of CFD codes and the time synchronised acoustic data on the ground makes it very suitable for the development and validation of aeroacoustic tools. Examples of the data are presented together with post processing techniques. A method for the separation of the acoustic time signal into main and tail rotor components in described and demonstrated