Subtraction of test mass angular noise in the LISA Technology Package interferometer

We present recent sensitivity measurements of the LISA Technology Package interferometer with articulated mirrors as test masses, actuated by piezo-electric transducers. The required longitudinal displacement resolution of 9 pm/sqrt[Hz] above 3 mHz has been demonstrated with an angular noise that corresponds to the expected in on-orbit operation. The excess noise contribution of this test mass jitter onto the sensitive displacement readout was completely subtracted by fitting the angular interferometric data streams to the longitudinal displacement measurement. Thus, this cross-coupling constitutes no limitation to the required performance of the LISA Technology Package interferometry.Comment: Applied Physics B - Lasers and Optics (2008

Noise of a model helicopter rotor due to ingestion of turbulence

A theoretical and experimental investigation of the noise of a model helicoper rotor due to ingestion of turbulence was conducted. Experiments were performed with a 0.76 m dia, articulated model rotor for a range of inflow turbulence and rotor operating conditions. Inflow turbulence levels varied from approximately 2 to 19 percent and tip Mach number was varied from 0.3 to 0.52. Test conditions included ingestion of a atmospheric turbulence in outdoor hover as well as ingestion of grid generated isotropic turbulence in the wind tunnel airstream. In wind tunnel testing, both forward flight and vertical ascent (climb) were simulated. Far field noise spectra and directivity were measured in addition to incident turbulence intensities, length scales, and spectra. Results indicate that ingestion of atmospheric turbulence is the dominant helicopter rotor hover noise mechanism at the moderate to high frequencies which determine perceived noise level

Reduction of blade-vortex interaction noise using higher harmonic pitch control

An acoustics test using an aeroelastically scaled rotor was conducted to examine the effectiveness of higher harmonic blade pitch control for the reduction of impulsive blade-vortex interaction (BVI) noise. A four-bladed, 110 in. diameter, articulated rotor model was tested in a heavy gas (Freon-12) medium in Langley's Transonic Dynamics Tunnel. Noise and vibration measurements were made for a range of matched flight conditions, where prescribed (open-loop) higher harmonic pitch was superimposed on the normal (baseline) collective and cyclic trim pitch. For the inflow-microphone noise measurements, advantage was taken of the reverberance in the hard walled tunnel by using a sound power determination approach. Initial findings from on-line data processing for three of the test microphones are reported for a 4/rev (4P) collective pitch control for a range of input amplitudes and phases. By comparing these results to corresponding baseline (no control) conditions, significant noise reductions (4 to 5 dB) were found for low-speed descent conditions, where helicopter BVI noise is most intense. For other rotor flight conditions, the overall noise was found to increase. All cases show increased vibration levels

Aeroacoustics of a coaxial rotor in level flight

The aeroacoustic characteristics of a coaxial system with teetering rotors in level forward °ight are com- pared to those of an equivalent articulated single rotor with the same solidity. A lifting line representation of the blade aerodynamics is coupled to Brown's Vorticity Transport Model to simulate the aerodynam- ics of the rotor systems. The acoustic ¯eld is determined using the Ffowcs Williams-Hawkings equation. Acoustic analysis shows that the principal contribution to noise radiated by both the coaxial and equivalent single rotor systems is at the fundamental blade passage frequency, but that the coaxial rotor generates higher sound pressure levels (by 10 dB for the evaluated con¯gurations) than the equivalent single rotor at all °ight speeds. The sources of blade vortex interaction (BVI) noise are investigated and the principal BVI events are identi¯ed. For the coaxial rotor, the most intense impulsive noise is seen to be generated by the inter-rotor BVI on the advancing side of the lower rotor. The impulsive noise that is generated by blade vortex interactions for the equivalent single rotor reduces in amplitude as the strength of BVI events on the rotor decreases with forward speed. Conversely, the BVI noise of the coaxial rotor intensi¯es with increasing °ight speed due to the increasing strength of the interaction between the wake of the upper rotor and the blades of the lower rotor. The impulsive noise due to BVI for the coaxial rotor is found to be higher by 20{35 dB compared to the equivalent single rotor. The overall and impulsive noise characteristics of the coaxial system are found to be weakly sensitive to changes in rotor separation and the relative phasing of the rotors

Effect of rotor stiffness and lift offset on the aeroacoustics of a coaxial rotor in level flight

The acoustic characteristics of a twin contra-rotating coaxial rotor configuration with significant flapwise stiffness are investigated in steady forward flight. The Vorticity Transport Model is used to simulate the aerodynamics of the rotor system and the acoustic field is determined using the Ffowcs Williams-Hawkings equation implemented using the Farassat-1A formulation. Increasing the hub stiffness alters the strengths of the blade vortex interactions, particularly those between the upper and lower rotors, and affects the intensity and directivity of the blade vortex interaction noise produced by the system. The inter-rotor blade vortex interaction on the advancing side of the lower rotor is the principal source of the most intensively focused noise that is generated by a conventionally articulated coaxial rotor system. For stiffened coaxial rotors, this particular inter-rotor blade vortex interaction is weakened as a result of a broad redistribution in lateral loading, yielding a reduction in the intensity of the noise that is produced by this interaction. The spanwise distribution of loading on the rotors of a stiffened coaxial system can be modified further by altering the lateral partition of lift (or lift offset). It is shown that decreasing the lift offset has the effect of counteracting the redistribution of loading due to flapwise stiffness and hence increases the blade vortex interaction noise as well as the power consumed by the rotor. Conversely, a reduction in both the power consumption and the blade vortex interaction noise is observed if the lift offset is increased, with the maximum benefit of lift offset being achieved at high speed. The computational results suggest that the noise from the dominant inter-rotor blade vortex interaction can be ameliorated through the use of lift offset control on stiffened coaxial systems, to the extent that the noise produced by this interaction can be made to be comparable to that produced by the other, weaker interactions between the two rotors of the system

Statistical skull models from 3D X-ray images

We present 2 statistical models of the skull and mandible built upon an elastic registration method of 3D meshes. The aim of this work is to relate degrees of freedom of skull anatomy, as static relations are of main interest for anthropology and legal medicine. Statistical models can effectively provide reconstructions together with statistical precision. In our applications, patient-specific meshes of the skull and the mandible are high-density meshes, extracted from 3D CT scans. All our patient-specific meshes are registrated in a subject-shared reference system using our 3D-to-3D elastic matching algorithm. Registration is based upon the minimization of a distance between the high density mesh and a shared low density mesh, defined on the vertexes, in a multi resolution approach. A Principal Component analysis is performed on the normalised registrated data to build a statistical linear model of the skull and mandible shape variation. The accuracy of the reconstruction is under the millimetre in the shape space (after rigid registration). Reconstruction errors for Scan data of tests individuals are below registration noise. To take in count the articulated aspect of the skull in our model, Kernel Principal Component Analysis is applied, extracting a non-linear parameter associated with mandible position, therefore building a statistical articulated 3D model of the skull.Comment: Proceedings of the Second International Conference on Reconstruction of Soft Facial Parts RSFP'200