Investigation of turbulence-surface interaction noise mechanisms and their reduction using porous materials

The interaction of an airfoil with incident turbulence is an important source of aerodynamic noise in numerous applications, such as turbofan engines, cooling systems for automotive and construction industries, high-lift devices on aircraft wings, and landing gear systems. In these instances, turbulence is generally produced by elements that are installed upstream of the wing profile and generate inflow distortions. A possible strategy for the reduction of turbulence-interaction noise, also referred to as leading-edge noise, is represented by the integration of porous media in the structure of the airfoil. However, the physical mechanisms involved in this noise mitigation technique remain unclear. The present thesis aims to elucidate these phenomena and, particularly, how porosity affects the incoming turbulence characteristics in the immediate vicinity of the surface. This problem has been addressed from different perspectives, namely from the technological, experimental, and analytical ones. An innovative design for a porous NACA-0024 profile fitted with melamine foam is proposed. The noise reduction performance achieved with such a porous treatment is evaluated through a novel version of the generalized inverse beamforming (GIBF) implemented with an improved regularization technique. The algorithm is first applied to different experimental benchmark datasets in order to evaluate its ability to reconstruct distributed aeroacoustic sources and to assess its accuracy and variability in different conditions. Results indicate that the implemented method provides an enhanced representation of the distributed noise-source regions and higher performance in terms of accuracy and variability if compared with other common beamforming techniques. GIBF is then employed together with far-field microphone measurements to characterize the leading-edge noise radiated by solid and porous NACA-0024 profiles immersed in the wake of an upstream cylindrical rod at different free-stream velocities. A noise reduction of up to 2dB is found for frequencies around the vortex-shedding peak, with a trend that is independent of the Reynolds number, whereas significant noise regeneration is observed at higher frequencies, most probably due to surface roughness. Subsequently, the flow-field alterations due to porosity in the stagnation region of the airfoils are investigated by means of mean-wall pressure, hot-wire anemometry, and particle image velocimetry measurements. The porous treatment mostly preserves the integrity of the NACA-0024 profile’s shape but yields a wider opening of the jet flow that increases the drag force. Moreover, porosity allows for damping of the velocity fluctuations near the surface and has limited influence on the upstream mean-flow field. In particular, the upwash component of the root-mean-square of the velocity fluctuations turns out to be significantly attenuated in a porous airfoil in contrast to a solid one, resulting in a strong decrease of the turbulent kinetic energy in the stagnation region. The present effect is more pronounced for higher Reynolds numbers. The mean spanwise vorticity close to the body appears also to be mitigated by the porous treatment. Furthermore, the comparison between the power spectral densities of the incident turbulent velocities demonstrates that porosity has an effect mainly on the low-frequency range of the turbulent-velocity spectrum, with a spatial extent up to about two leading-edge radii from the stagnation point. In addition, the vortex-shedding frequency peak in the power spectrum of the streamwise velocity fluctuations close to the airfoil surface is found to be suppressed by porosity. The present results show analogies with the outcomes of the aeroacoustic analysis, highlighting the important role played by the attenuated turbulence distortion due to the porous treatment of the airfoil in the corresponding noise reduction. An analytical model based on the rapid distortion theory (RDT) to predict the turbulent flow around a porous cylinder is formulated with the aim of improving the understanding of the effect of porosity on turbulence distortion and interpreting the experimental results. The porous treatment, characterized by a constant static permeability, is modeled as a varying impedance boundary condition applied to the potential component of the velocity that accounts for Darcy’s flow within the body. The RDT implementation is first validated through comparisons with published velocity measurements in the stagnation region of an impermeable cylinder placed downstream of a turbulence grid. Afterwards, the impact of porosity on the velocity field is investigated through the analysis of the one-dimensional velocity spectra at different locations near the body and the velocity variance along the stagnation streamline. The porous surface affects the incoming turbulence distortion near the cylinder by reducing the blocking effect of the body and by altering the vorticity deformation caused by the mean flow. The former leads to an attenuation of the one-dimensional velocity spectrum in the low-frequency range, whereas the latter results in an amplification of the high-frequency components. This trend is found to be strongly dependent on the turbulence scale and influences the evolution of the velocity fluctuations in the stagnation region. The porous RDT model is finally adapted to calculate the turbulence distortion in the vicinity of the porous NACA- 0024 profile leading edge. The satisfactory agreement between predictions and experimental results suggests that the present methodology can improve the understanding of the physical mechanisms involved in the airfoil-turbulence interaction noise reduction through porosity and can be instrumental in designing such passive noise-mitigation treatments.Novel Aerospace MaterialsWind Energ

Semi-empirical calibration of remote microphone probes using Bayesian inference

The empirical calibration of remote microphone probes (RMP), used to acquire wall-pressure fluctuations, can introduce spurious resonance into the sensor transfer function due to the difference in the pressure field inside the calibrator geometry over multiple calibration steps. Such spurious resonance subsequently propagates into the unsteady-pressure data at which the calibration is applied, hindering the accuracy of the measurements. Current post-processing methods for tackling these issues are often manual and strongly dependent on the operator's expertise. In this study, we propose an original semi-empirical calibration method to remove spurious resonance in a less operator-reliant manner. The approach is based on fitting an existing analytical fluid-dynamical model for the propagation of pressure waves in the probe to the empirical calibration data using Bayesian inference. The proposed method is successfully applied to three datasets, from a simple probe recessed behind a pinhole to a more complex branching RMP. For all the configurations, spurious resonance is eliminated from the transfer function with a strongly reduced impact of the operator intervention while retaining the resonant features that are characteristic of the RMP. The affected frequency bands are then replaced using the underlying physical model. In this way, the detrimental impact of spurious resonance is removed from the measured wall-pressure spectra. Furthermore, the RMP parameters retrieved by the fit can also be used as inputs to corrective models, specifically to account for averaging effects due to the probe sensing area or for the impact of grazing flow or temperature variations on the transfer function.Wind Energ

Development of the SmartAnswer Demonstrator: a Didactic Wind Tunnel for Aeroacoustic Applications^∗

A small didactic wind tunnel demonstrator has been designed and manufactured at the von Karman Institute for Fluid Dynamics to illustrate the physical principles at stake in flow-induced noise generation, offer an audible perception of the effectiveness of noise-mitigation strategies, and serve as a practical test bench for aeroacoustic education and research. Seven mitigation technologies are embedded in a single facility, which addresses the noise generation by an airfoil, noise propagation in a duct, and noise transmission through a flexible panel. A challenging objective of this facility was to offer a perceptible impression of various aeroacoustic noise mechanisms at low flow speeds and a live assessment of the effectiveness of the noise-reduction technologies. Different approaches combining multiple microphones, advanced signal-processing techniques, and real-time audio feedback have been implemented to this end. The results establish that the demonstrator enables a clear perception of the effectiveness of the noise-mitigation technologies. The facility is also suitable for fast and inexpensive preliminary investigations of future noise-reduction concepts, taking advantage of rapid prototyping techniques.Wind EnergyNovel Aerospace Material

Experimental and Analytical Investigation of the Distortion of Turbulence Interacting with a Porous Airfoil

View Video Presentation: https://doi-org.tudelft.idm.oclc.org/10.2514/6.2021-2289.vidA possible strategy for the reduction of the aeroacoustic noise generated by turbulence interacting with a wing profile, also referred to as leading-edge noise, is represented by the implementation of a porous medium in the structure of the airfoil. However, the physical mechanisms involved in this noise mitigation technique remain unclear. The present work aims at elucidating these phenomena and particularly how the porosity affects the incoming turbulence characteristics in the immediate vicinity of the surface. A porous NACA-0024 profile integrated with melamine foam has been compared with a solid baseline, both airfoils being in turn subjected to the turbulence shed by an upstream cylindrical rod. The mean wall-pressure distribution along the airfoils shows that the implementation of the porous material mostly preserves the integrity of the NACA-0024 profile’s shape. Results of hot-wire anemometry indicate that the porous design proposed in this study allows for damping of the velocity fluctuations and has a limited influence on the upstream mean flow field. Specifically, the upwash component of the root-mean-square of the velocity fluctuations turns out to be significantly attenuated in the porous case in contrast to the solid one. Furthermore, the comparison between the power spectral densities of the incident turbulent velocities demonstrates that the porosity has an effect mainly on the low-frequency range of the turbulent velocity power spectrum. This evidence is in line with the results of the acoustic beamforming measurements, which exhibit a noise abatement in an analogous frequency range. On the basis of these observations, an interpretation of the phenomena occurring in the turbulence-interaction noise reduction due to a porous treatment of the airfoil is finally given with reference to the theoretical inputs of the rapid distortion theory.Wind Energ

Innovative coatings for reducing flow-induced cylinder noise by altering the sound diffraction

The aerodynamic noise radiated by the flow past a cylinder in the subcritical regime can be modeled by a quadrupolar sound source placed at the onset position of the vortex-shedding instability that is scattered by the surface with a dipolar directivity. When the cylinder is coated with a porous material, the intensity of the shed vortices is greatly reduced, determining a downstream shift of the instability-outbreak location. Consequently, sound diffraction is less efficient, and noise is mitigated. In this paper, an innovative design approach for a flow-permeable coating based on a further enhancement of such an effect is proposed. The results of phased-microphone-array measurements show that, once the leeward part of the cover is integrated with components that make the flow within the porous medium more streamlined, the quadrupolar source associated with the vortex-shedding onset is displaced more downstream, yielding additional noise attenuation of up to 10 dB with respect to a uniform coating. Furthermore, the same noise-control mechanism based on the weakening of the sound scattering can be exploited when these components are connected to the bare cylinder without the porous cover. In this case, the mitigation of overall sound pressure levels is comparable to that induced by the coated configurations due to the lack of noise increase produced by the inner flow interacting within the pores of the material. Remarkable sound reductions of up to 10 dB and a potential drag-force decrease are achieved with this approach, which paves the way for disruptive and more optimized noise-attenuation solutions.Wind EnergyGroup Garcia Espallarga

Experimental investigation of turbulent coherent structures interacting with a porous airfoil

Abstract: The flow field on solid and porous airfoils subjected to turbulence shed by an upstream cylindrical rod and the corresponding far-field noise radiations are studied through particle image velocimetry (PIV) and microphone measurements, respectively. Three different Reynolds numbers based on the rod diameter are considered in a range between 2.7 × 10 4 and 5.4 × 10 4, and two porous airfoil models are tested to analyze the influence of the design elements of the permeable treatment. A standard proper orthogonal decomposition (POD) algorithm is employed to band filter the different length scales that characterize the turbulent flow, making it feasible to determine which turbulence scales are affected by porosity. The aeroacoustic results indicate that the porous treatment of the wing profile leads to a noise reduction at low frequencies and a noise regeneration at high frequencies due to surface roughness. The investigation on the flow field shows that the main effect of porosity is to mitigate the turbulent kinetic energy in the stagnation region, attenuating the distortion of turbulence interacting with the airfoil surface. The application of the POD algorithm indicates that this effect acts mainly on the largest scales of turbulence. Graphic abstract: [Figure not available: see fulltext.]. 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.Wind Energ

Experimental investigation of airfoil turbulence-impingement noise reduction using porous treatment

The present work aims at evaluating the effectiveness of the use of porous materials for reducing airfoil turbulence-impingement noise. To pursue this objective, a porous NACA-0024 profile filled with melamine foam has been designed for comparison with a solid model. The porous media constituting the airfoil has been fully characterized in order to determine the parameters that describe the material. The two profiles have been tested in a rod airfoil configuration in the anechoic chamber of von Karman Institute for Fluid Dynamics. A free-stream velocity of 30 m/s (corresponding to a chord-based Reynolds number of 3.2·105) and an angle of attack of 0◦ have been considered for the tests. Hot-wire anemometry measurements have been performed with the aim of characterizing the boundary layers around the two airfoils and investigating the eventual nose mitigation mechanisms that occur due to the porous treatment. Moreover, the static pressure distributions along the two surfaces have been studied to find correlations with the velocity profiles. The results show that the use of a porous treatment leads to a decrease of turbulence-impingement noise in the low-frequency range and to an increase in the high-frequency one, mostly due to surface roughness noise. The application of an inverse beamforming technique has lead to a qualitative comparison of noise distribution maps at different one-third octave band frequencies for the two cases. Furthermore, the pressure values of each map point within a rectangular region surrounding the leading edge have been summed in order to retrieve the integrated one-third octave band spectra.Wind Energ

On the Aerodynamic-Noise Sources in a Circular Cylinder Coated with Porous Materials

Coating a circular cylinder with porous materials constitutes an effective passive strategy for reducing the flow-induced noise linked to the vortex shedding. Despite the number of investigations in the last decade, the noise-mitigation mechanisms associated with this technique remain unclear. The present research aims to clarify the role played by the alterations in the flow field due to porosity in the aerodynamic-sound attenuation of a cylinder coated with metal foam. Far-field acoustic and particle-image-velocimetry (PIV) measurements were performed at the Delft University of Technology for Reynolds numbers ranging in the subcritical regime. The aeroacoustic results show that a significant tonal and broadband suppression could be achieved with the porous treatment of the body. For the coated cylinder, the dominant sources do not appear to be distributed over the surface but rather are situated several diameters downstream of it, with a lower amplitude. The PIV data reveal that the main effect of the coating is to stabilize the cylinder wake, which results in an elongation of the vortex-formation length and a decrease in the turbulence kinetic energy. In particular, the position where the vortex shedding starts corresponds to the region of the dominant noise sources. The conclusions drawn in this study potentially provide an insightful indication for the design of more effective sound-control solutions.Wind EnergyNovel Aerospace Material

Jet-installation noise reduction with flow-permeable materials

This paper investigates the application of flow-permeable materials as a solution for reducing jet-installation noise. Experiments are carried out with a flat plate placed in the near field of a single-stream subsonic jet. The flat plate is modular and the solid surface near the trailing edge can be replaced with different flow-permeable inserts, such as a metal foam and a perforated plate structure. The time-averaged jet flow field is characterized through planar PIV measurements at three different velocities (Ma=0.3, Ma=0.5 and Ma=0.8, where Ma is the acoustic Mach number), whereas the acoustic far-field is measured with a microphone arc-array. Acoustic measurements confirm that installation effects cause significant noise increase, up to 17 dB for the lowest jet velocity, particularly at low and mid frequencies (i.e. St<0.7, with the Strouhal number based on the jet diameter and velocity), and mostly in the upstream direction of the jet. By replacing the solid trailing edge with the metal foam, noise abatement of up to 9 dB is achieved at the spectral peak for Ma=0.3 and a polar angle θ=40∘, with an overall reduction in the entire frequency range where jet-installation noise is dominant. The perforated plate provides lower noise reduction than the metal foam (7 dB at the spectral peak for Ma=0.3 and θ=40∘), and it is less effective at low frequencies. This is related to the values of permeability and form coefficient of the materials, which are the major parameters controlling the pressure balance across the trailing edge and, consequently, the noise generated by the plate. However, despite having a high permeability, the plate with the metal-foam trailing edge still has a distinct noise production at mid frequencies (St≈0.43 for Ma=0.3). Based on the analyses of different treated surface lengths, it is conjectured that the solid-permeable junction in the plate acts as a new scattering region, and thus its position also affects the far-field noise, which is in line with analytical predictions in the literature. Nonetheless, both types of inserts provide significant noise reduction and are potential solutions for the problem of jet-installation noise.Wind EnergyNovel Aerospace Material

Effect of porosity on Curle's dipolar sources on an aerofoil in turbulent flow

Integrating a porous material into the structure of an aerofoil constitutes a promising passive strategy for mitigating the noise from turbulence–body interactions that has been extensively explored in the past few decades. When a compact permeable body is considered in the aeroacoustic analogy derived by Curle to predict this noise source, a dipole associated with the non-zero unsteady Reynolds stresses appears on the surface in addition to the dipole linked to the pressure fluctuations. Nevertheless, the relative contribution of this source to the far-field noise radiated by a porous wing profile has not been clarified yet. The purpose of the current research work is twofold. On the one hand, it investigates the impact of porosity on the surface-pressure fluctuations of a thick aerofoil immersed in the wake of an upstream circular rod at a Mach number of 0.09. On the other hand, it quantifies the relevance of the Reynolds-stresses term on the surface as a sound-generation mechanism. The results from large-eddy simulations show that the porous treatment of the wing profile yields an attenuation of the unsteady-pressure peak, which is localised in the low-frequency range of the spectrum and is induced by the milder distortion of the incoming vortices. However, porosity is ineffective in breaking the spanwise coherence or in-phase behaviour of the surface-pressure fluctuations at the vortex-shedding frequency. The Reynolds-stresses term is found to be considerable in the stagnation region of the aerofoil, where the transpiration velocity is larger, and partly correlated with the unsteady surface pressure, suggesting constructive interference between the two terms. This results in a non-negligible contribution of this term to the far-field acoustic pressure emitted by the porous wing profile for observation angles near the stagnation streamline. The conclusions drawn in the present study eventually provide valuable insight into the design of innovative and efficient passive strategies to mitigate surface–turbulence interaction noise in industrial applications.Wind Energ