Airfoil Self-Noise - Investigation with Particle Image Velocimetry

Noise generated aerodynamically by the airflow over a lifting surface is often of concern for applications as diverse as air and ground transportation, heating, ventilation, air-conditioning systems, and wind turbines. The thesis describes the application of advanced optical flow measurements techniques for the visualization and description of the sources of sound on airfoils. These measurement techniques include high-speed stereoscopic and tomographic Particle Image Velocimetry (PIV) together with advanced methods for post-processing to obtain a representation of the aeroacoustic source field. Following this innovative and evolving approach in experimental aeroacoustics, a novel methodology for broadband trailing-edge noise diagnostics by tomographic PIV is proposed. Moreover, on the basis of simultaneous high-speed PIV and acoustic measurements, new and fundamental insights into the mechanism of tonal noise generation due to the interaction of amplified laminar boundary layer instability waves with the trailing edge are presented. Both examples demonstrate the potential impact of advanced PIV methods on present and future research in experimental aeroacoustics.Aerodynamics, Wind Energy, Flight Performance and PropulsionAerospace Engineerin

Coherent Structures at the Serrated Trailing-Edge of a NACA 0012

In the past, different modifications of trailing-edges have been investigated with the aim to reduce noise emissions. One promising solution, which has been shown to reduce the intensity of trailing-edge noise, is the application of so-called sawtooth serrations with different proportions and sizes. Current research aims at applying new tools of flow field interrogation, first of all tomographic PIV, to the problem of trailing-edge noise and at creating links between the phenomenological description of the flow field in terms of coherent structures and noise spectra in order to improve understanding of the noise generating mechanisms. In the framework of this thesis, sawtooth serrations applied to a NACA 0012 airfoil have been investigated and compared to the straight trailing-edge of the unmodified NACA 0012 baseline model. Acoustic far-field, surface pressure and a series of PIV measurements have been performed for this purpose. In a first step, acoustic far-field measurements on a large selection of serrated models have been performed. On one hand, tonal narrowband noise associated to the shedding of vortices from the blunt parts of the trailing-edge poses a large penalty for the reduction in overall sound pressure level. On the other hand, the acoustic measurements have indicated a small reduction in noise level originating from the serrated trailing-edge for frequencies in the broadband spectrum exceeding the tonal range, which is in agreement with Howe’s theory of trailingedge noise. Overall, a decrease in total sound pressure level could not be observed. In a second step, PIV measurements of the boundary layer close to the trailing-edge have been performed in order to identify a subset of coherent structures, which can be linked to the production of noise and in particular to the production of tonal noise. Large coherent structures appearing close to the trailing-edge or in between the serrations have been visualized, dominate in terms of vorticity level and show the shape of horseshoe vortices. Most of these structures show a large spanwise coherence on the order of the distance between two serrations and reappear at the frequency of the tonal noise. This suggests a link between this category of coherent structures and the tonal noise observed in the acoustic spectra.Aerospace Engineerin

Aeroacoustic analysis of a NACA 0015 airfoil with Gurney flap based on time-resolved PIV measurements

The present study investigates the feasibility of high-lift devices noise prediction based on measurements of time-resolved particle image velocimetry (TR-PIV). The model under investigation is a NACA 0015 airfoil with Gurney flap with height of 6% chord length. The velocity fields around and downstream the Gurney flap are measured by PIV and are used for the PIV-based noise predictions. The predictions are assessed via microphone measurements. Since the Gurney flap height is much smaller than the emitted acoustic wavelength, the source of noise can be considered compact and the integral implementation of Curle's analogy based on the unsteady aerodynamic loads can be followed. The results are compared with the simultaneous microphone measurements in terms of time histories and power spectra. The integral formulation of Curle's analogy yields acoustic sound pressure levels in good agreement with the simultaneous microphone measurements for the tonal component. All the calculated far-field noise power spectra reproduce the peak at vortex shedding frequency, which also agrees well with the microphone measurements.AerodynamicsWind Energ

High-speed PIV analysis of trailing edge aeroacoustics

Tonal noise generated by airfoils observed at low to moderate Reynolds numbers is related to laminar boundary layer instabilities, which has lead to the term laminar boundary layer instability noise. The particular features of the acoustic spectrum have been discussed and a number of theories have been proposed in literature over the past 50 years. Previous research suggests that the appearance of tonal noise is related to a feedback between the acoustic waves scattered at the trailing edge and the receptive part of the boundary layer [4]. Reported studies have been performed on the basis of hot-wire anemometry, laser doppler velocimetry, phase-locked PIV, acoustic measurements, numerical simulations or theoretical models. In recent years, PIV has become an alternative for the investigation of aeroacoustic sources. In particular the aeroacoustic sources for trailing edge noise have been investigated by Schröder et al. [10] using time-resolved PIV and Shannon and Morris [11] based on phase-locked PIV. Nakano et al. [8] focussed on the subject of laminar boundary layer instability noise on an airfoil and related their occurrence on the pressure side to noise emissions based on a correlation based technique. In the present study, planar high-speed PIV is performed simultaneously with acoustic far-field measurements. This combination allows to associate features of the acoustic emissions to events in the source field near the trailing edge for a better understanding of the tonal noise generation on an airfoil. In the past a wide range of Reynolds numbers has been investigated for different airfoil models, most notably the NACA0012 which is also selected for the present study. In particular, it is found that a periodic amplitude modulation of rapidly growing instabilities on the pressure side of the airfoil is responsible for the occurrence of multiple tones for the present configuration.Aerodynamics, Wind Energy and PropulsionAerospace Engineerin

Investigation of aeroacoustics and flow dynamics of a NACA 0015 airfoil with a Gurney flap using TR-PIV

The present study employs simultaneous planar TR-PIV and microphone measurements to obtain the flow dynamics and aeroacoustic causality correlation associated with a Gurney flap of various sizes in case of low Mach and high Reynolds number flows. The objectives are to investigate the secondary shedding mode for the case of a turbulent boundary layer and to understand the mechanism of noise generation by identifying structures that are highly correlated with far field pressure fluctuations. The instantaneous velocity and vorticity fields show the flapping motion of the wake and the coherent vortex shedding process. The tonal peaks are clearly audible and correspond to the vortex shedding frequency. The PSD of the flow fluctuations and acoustic spectra did not indicate a secondary mode of shedding in case of turbulent boundary layer. The Strouhal numbers of the vortex shedding are found to be close to that of a bluff body in a flow. Causality correlation between pressure fluctuations in the far-field and the near field fluctuations indicates that the vertical velocity in the wake of the model is highly correlated with the far-field pressure fluctuations. This study provides an example of the potential of the causality correlation technique in identifying flow structures/regions highly correlated with noise in case of complex high-lift devices, making it possible to design flaps with lower acoustic emissions.AerodynamicsWind Energ

Aeroacoustic analysis of an airfoil with Gurney flap based on time-resolved particle image velocimetry measurements

Particle image velocimetry for the experimental assessment of trailing edge noise sources has become focus of research in recent years. The present study investigates the feasibility of the noise prediction for high-lift devices based on time-resolved particle image velocimetry (PIV). The model under investigation is a NACA 0015 airfoil with a Gurney flap with a height of 6% of the chord length. The velocity fields around and downstream of the Gurney flap were measured by PIV and used to compute the corresponding pressure fields by solving the Poisson equation for incompressible flows. The reconstructed pressure fluctuations on the airfoil surface constitute the source term for Curle's aeroacoustic analogy, which was employed in both the distributed and compact formulation to estimate the noise emission from PIV. The results of the two formulations are compared with the simultaneous far-field microphone measurements in the temporal and spectral domains. Both formulations of Curle's analogy yield acoustic sound pressure levels in good agreement with the simultaneous microphone measurements for the tonal component. The estimated far-field sound power spectra (SPL) from the PIV measurements reproduce the peak at the vortex shedding frequency, which also agrees well with the acoustic measurements.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.AerodynamicsWind Energ

High-speed PIV analysis of trailing edge aeroacoustics

Tonal noise generated by airfoils observed at low to moderate Reynolds numbers is related to laminar boundary layer instabilities, which has lead to the term laminar boundary layer instability noise. The particular features of the acoustic spectrum have been discussed and a number of theories have been proposed in literature over the past 50 years. Previous research suggests that the appearance of tonal noise is related to a feedback between the acoustic waves scattered at the trailing edge and the receptive part of the boundary layer [4]. Reported studies have been performed on the basis of hot-wire anemometry, laser doppler velocimetry, phase-locked PIV, acoustic measurements, numerical simulations or theoretical models. In recent years, PIV has become an alternative for the investigation of aeroacoustic sources. In particular the aeroacoustic sources for trailing edge noise have been investigated by Schröder et al. [10] using time-resolved PIV and Shannon and Morris [11] based on phase-locked PIV. Nakano et al. [8] focussed on the subject of laminar boundary layer instability noise on an airfoil and related their occurrence on the pressure side to noise emissions based on a correlation based technique. In the present study, planar high-speed PIV is performed simultaneously with acoustic far-field measurements. This combination allows to associate features of the acoustic emissions to events in the source field near the trailing edge for a better understanding of the tonal noise generation on an airfoil. In the past a wide range of Reynolds numbers has been investigated for different airfoil models, most notably the NACA0012 which is also selected for the present study. In particular, it is found that a periodic amplitude modulation of rapidly growing instabilities on the pressure side of the airfoil is responsible for the occurrence of multiple tones for the present configuration