14 research outputs found
Experimental investigation of aerofoil tonal noise generation
No full textThe present study investigates the mechanisms associated with tonal noise emission from a NACA 0012 aerofoil at moderate incidence (0°, 1°, 2° and 4° angle of attack) and with Reynolds numbers ranging from 100 000 to 270 000. Simultaneous time-resolved particle image velocimetry (PIV) of the aeroacoustic source region near the trailing edge and acoustic measurements in the far field are performed in order to establish the correspondence between the flow structure and acoustic emissions. Results of these experiments are presented and analysed in view of past research for a number of selected cases. Characteristics of the acoustic emission and principal features of the average flow field agree with data presented in previous studies on the topic. Time-resolved analysis shows that downstream convecting vortical structures, resulting from growing shear layer instabilities, coherently pass the trailing edge at a frequency equal to that of the dominant tone. Therefore, the scattering of the vortical structures and their associated wall pressure fluctuations are identified as tone generating mechanisms for the cases investigated here. Moreover, wavelet analysis of the acoustic pressure and velocity signals near the trailing edge show a similar periodic amplitude modulation which is associated with multiple tonal peaks in the acoustic spectrum. Periodic amplitude modulation of the acoustic pressure and velocity fluctuations on the pressure side are also observed when transition is forced on the suction side, showing that pressure-side events alone can be the cause
Investigation of tonal noise generation on an airfoil with time-resolved PIV
No full textTonal noise generated by airfoils at low to moderate Reynolds numbers has been focus of research since decades. Several explanations of the observed phenomena relate the noise spectrum to the instability of the laminar boundary layer, which has lead to the term laminar boundary layer instability noise. The particular features of boundary layer instability noise have been reported in literature, but remain controversial to date. In the past, studies have been performed relying on hot-wire, Laser Doppler Velocimetry (LDV), phase-locked Particle Image Velocimetry (PIV), and acoustic measurements, as well as numerical simulations and theoretical approaches. In recent years, high-speed PIV has become accepted as a tool for the investigation of aeroacoustic problems since it can provide spatio-temporal information on the structure of the source field. In the present study, far-field microphone measurements are conducted to map the noise emissions of a NACA 0012 airfoil over a range of angles of attack and velocities. The principal features observed in the noise maps agree with findings of previous experiments. For the investigation of the ow structure and source field at the trailing edge, high-speed PIV in conjunction with simultaneous microphone measurements has been performed. The information obtained using this recent measurement technique provides detail on the relation between the acoustic spectrum and the dynamic characteristics of the aeroacoustic source field
Effect of serrated trailing edge on boundary layer instability noise
No full textWings operating at low and moderate Reynolds number such as the ones of UAVs or the blades of small wind turbines and of compressor fans, can be the source of an aeroacoustics phenomenon called laminar boundary layer instability noise. This paper presents an investigation with time-resolved PIV and far field microphones of the dynamical flow phenomena and the acoustic emissions of a NACA 0018 airfoil, with and without serrated trailing edges, operating in a transitional regime (Reynolds number ≈ 3.3 – 4.7 × 105). The measurements are done in an open jet wind tunnel facility with low noise and turbulence intensity. The planar PIV measurements are done on the pressure side of the airfoil in two different planes, parallel and orthogonal to the wing span at the trailing edge region, to capture the flow evolution along the airfoil chord as well as its spanwise arrangement. The results presented here show the effectiveness of the serrated geometries to reduce the noise emitted by this type of flows. Recently published literature outcomes concerning the streamwise evolution of the flow with and without serrations are confirmed. Moreover new insights related to the spanwise coherence of the traveling vortical structures are presented leading to a better understanding of the hydrodynamic and acoustic mechanisms involved in this aeroacoustics phenomenon
A comparative study of turbulent boundary layer wall pressure fluctuations obtained from high-speed tomographic PIV and DNS
No full textTrailing-edge noise diagnostics with low-repetition-rate PIV
No full textA novel approach for trailing-edge noise diagnostics with low-repetition rate Particle Image Velocimetry systems is presented. The approach is based on the application of diffraction theory and implements a recently developed single-snapshot pressure reconstruction in boundary layers using the Vortex-in-cell method in combination with a frozen turbulence assumption to estimate the relevant statistical descriptors of the surface pressure field. The present communication describes the required planar and tomographic Particle Image Velocimetry experiments with a high spatial-dynamic-range system and demonstrates the applicability of the approach by comparison of hydrodynamic and acoustic pressure statistics with available data. The frequency spectra of the hydrodynamic pressure near the surface are found to be sensitive to the model employed for the convective velocity in the application of the frozen turbulence assumption for reduced frequencies f δ*/u∞ > 0.5 (2.3kHz). For lower reduced frequencies, excellent agreement with the reference data is observed
Acoustic Emissions of Semi-Permeable Trailing Edge Serrations
No full textThe trailing edge of a NACA 0018 airfoil is modified through the attachment of serrations with different degrees of permeability. Acoustic beamforming is used to inspect the turbulent boundary layer-trailing edge noise emissions from the unmodified and serrated trailing edges. Different freestream velocities and angles of attack are investigated. The serration permeability is prescribed by having slits cut into the solid surface of the serrations in two different configurations. The results indicate that a certain benefit in noise reduction is obtained from a mixed solid/slitted configuration, while a fully slitted configuration loses most of the noise reduction performance.AerodynamicsAircraft Noise and Climate EffectsWind Energ
Trailing edge serrations: Effect of their flap angle on flow and acoustics
No full textTrailing edge serrations have been proven to work as a passive noise reduction device. Nevertheless, they have also previously been found to increase noise in a particular frequency range, argued in earlier research to be due to the misalignment of the serrations with the direction of the flow in the wake. It emerges as a high-frequency noise increase in a broadband region of the spectrum. This study investigates the effect of serration-flow misalignment on the noise emissions using acoustic beamforming, and finds a correlation with observations made on the flow using particle image velocimetry (PIV). The hydrodynamic source of the noise increase is hereby identified, and a Strouhal number relation for the high-frequency noise increase is proposedAircraft Noise and Climate EffectsWind Energ
On the use of Particle Image Velocimetry to predict trailing edge noise
No full textThe feasibility of aeroacoustic noise predictions based on Particle Image Velocimetry (PIV) measurements is studied. For this purpose, experiments are conducted on a sharp trailing edge (TE) flow developed along a flat plate at free stream velocity of 15m/s. The acoustic emissions were characterized in the NLR Small Anechoic Wind Tunnel (KAT) by means of microphone measurements. The result is used for benchmarking the PIV based noise predictions. PIV measurements were carried in a low-speed wind tunnel of TU Delft with similar properties to that of the KAT facility. Planar PIV measurements, performed at high spatial resolution, characterize the turbulent properties of the boundary layer travelling across the trailing edge. Time resolved Tomographic PIV measurements are obtained over a volume of 19.7mm x 9.1mm x 33.4mm at an acquisition rate of 12kHz. Two different approaches are compared for the prediction of TE noise based on PIV data. The first follows diffraction theory and requires the deduction of the instantaneous surface pressure fluctuations from PIV data. The second one relies on the integral solution of the Lighthill equation obtained through a tailored Green’s function for a semi-infinite half plane. This allows to calculate the scattered noise directly from the velocity data. The predictions based on diffraction theory agree well with the measured far field spectrum up to 4kHz. In contrast, the Green’s function approach yields a significant and systematic overprediction of approximately 12dB.Aerodynamics, Wind Energy & PropulsionAerospace Engineerin
