Vortex Shedding and Modal Behavior of a Circular Cylinder Equipped with Flexible Flaps

When a cylinder is subject to a flow, vortices will be shed that can lead to strong tonal noise. The modification of the cylinder with soft, flexible flaps made of silicone rubber has been shown to affect the vortex shedding cycle in a way that the Strouhal number associated with the vortex shedding suddenly jumps to a higher value at a certain Reynolds number. In the present study, the effect of the flexible flaps on the vortex shedding is further examined by subsequently reducing the number of flaps and additionally shortening their length. Acoustic measurements and camera recordings of the flap motion, performed in an aeroacoustic wind tunnel, suggest that the sudden jump of the Reynolds number is caused by the movement of the outer flaps. A comparison with the eigenfrequencies obtained from a numerical modal analysis of the different flap rings revealed that the cause of the Strouhal number jump is most likely a lock-in of the natural vortex shedding cycle with the next higher eigenfrequency of the outer flaps

Reducing aerofoil-turbulence interaction noise through chordwise-varying porosity

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IRLS based inverse methods tailored to volumetric acoustic source mapping

Planar microphone arrays are of common use in acoustic source identification methods, as well as the use of planar calculation grids. Indeed, the assumption is that the planar grid contains all sources of interest. However, this assumption may not be true in several applications and hence return misleading results. One tentative to overcome this issue is to consider three-dimensional surface adhering on the target. Unfortunately, also this choice may not be enough to obtain accurate results in challenging applications like aeroacoustic source mapping, since noise sources are not necessarily located on the surface of the target. This paper aims to analyze the issues and the benefits arising when the calculation grid turns into a volume. Two inverse methods based on Iterative Re-weighted Least Squares (IRLS) and Bayesian Regularization (BR) are formulated: Equivalent Source Method (ESM-IRLS) and Covariance Matrix Fitting (CMF-IRLS). Even though these methods are based on concepts already known in literature, the focus of this paper is on theoretical and algorithmic aspects that make them able to produce accurate volumetric acoustic maps. The methods proposed are applied both on a simulated and an experimental test case. The former is reported to highlight the difference between standard surface mapping and volumetric mapping. The latter reports an application on an airfoil in an open jet. A comparison with the CLEAN-SC approach is reported in both cases to show the performance of the proposed methods with respect to a well-known state of the art algorithm

Aeroacoustic noise reduction by application of end plates on wall-mounted finite airfoils

One known method to reduce vortex shedding from the tip of a blade is the use of end plates or winglets. Although the aerodynamic impact of such end plates has been investigated in the past, no studies exist on the effect of such end plates on the far-field noise. The aeroacoustic noise reduction of three different end-plate geometries is experimentally investigated. The end plates are applied to the free end of a wall-mounted symmetric NACA 0012 airfoil and a cambered NACA 4412 airfoil with an aspect ratio of 2 and natural boundary layer transition. Microphone array measurements are taken in the aeroacoustic open-jet wind tunnel at BTU Cottbus-Senftenberg for chord-based Reynolds numbers between 75,000 and 225,000 and angles of attack from 0∘ to 30∘. The obtained acoustic spectra show a broad frequency hump for the airfoil base configurations at higher angles of attack that is attributed to tip noise. Hot-wire measurements taken for one configuration show that the application of an end plate diffuses the vorticity at the tip. The aeroacoustic noise contribution of the tip can be reduced when the endplates are applied. This reduction is most effective for higher angles of attack, when the tip vortex is the dominant sound source.TU Berlin, Open-Access-Mittel – 202

IRLS based inverse methods tailored to volumetric acoustic source mapping

Planar microphone arrays are of common use in acoustic source identification methods, as well as the use of planar calculation grids. Indeed, the assumption is that the planar grid contains all sources of interest. However, this assumption may not be true in several applications and hence return misleading results. One tentative to overcome this issue is to consider three-dimensional surface adhering on the target. Unfortunately, also this choice may not be enough to obtain accurate results in challenging applications like aeroacoustic source mapping, since noise sources are not necessarily located on the surface of the target. This paper aims to analyze the issues and the benefits arising when the calculation grid turns into a volume. Two inverse methods based on Iterative Re-weighted Least Squares (IRLS) and Bayesian Regularization (BR) are formulated: Equivalent Source Method (ESM-IRLS) and Covariance Matrix Fitting (CMF-IRLS). Even though these methods are based on concepts already known in literature, the focus of this paper is on theoretical and algorithmic aspects that make them able to produce accurate volumetric acoustic maps. The methods proposed are applied both on a simulated and an experimental test case. The former is reported to highlight the difference between standard surface mapping and volumetric mapping. The latter reports an application on an airfoil in an open jet. A comparison with the CLEAN-SC approach is reported in both cases to show the performance of the proposed methods with respect to a well-known state of the art algorithm. (C) 2020 Elsevier Ltd. All rights reserved

Surface curvature effects on the tonal noise of a wall-mounted finite airfoil

This paper is concerned with the influence of camber on the noise of a wall-mounted finite airfoil with natural boundary layer transition. Tonal noise measurements taken in an aeroacoustic wind tunnel are presented for airfoils with aspect ratio of 2, NACAxx12 profile and camber between 0 and 6% at 40% chord. The results show camber is an important parameter that determines the operating conditions for which acoustic tone generation occurs and the number and intensity of the tones produced. Airfoils with 0%-2% camber have an acoustic signature that is dominated by a high amplitude primary tone, whereas the spectra of airfoils with higher camber of 4%-6% feature a more pronounced side tone structure. Tonal noise production does not collapse with lift coefficient, demonstrating that the local flow conditions influence the noise source. Tonal noise production is explained in terms of changes to mean flow topology, namely the location of flow separation, which is linked to tonal noise generation. Scaling of airfoil tonal noise is found to vary with angle of attack and pressure gradient. Empirical scaling laws for the primary tone frequency dependence on velocity are also derived for the cambered airfoils

Unsteady surface pressure measurements on trailing edge serrations based on digital MEMS microphones

The physics of turbulent flow reaching the airfoil trailing edge is fundamental to the understanding of the Turbulent Boundary Layer (TBL) noise production mechanism. The experimental validation of semi-empirical and computational models requires advances in current experimental techniques and instrumentation to allow accurate determination of turbulent structures. This research investigates the applications of digital MEMS microphones embedded for the experimental determination of unsteady wall pressure in a 3D printed NACA0012 airfoil at chord-based Reynolds number 200,000<Re<700,000. The development of this experimental technique brings challenges regarding the construction since sensors and components must be miniaturized in order to maximize the sensor spatial resolution. This paper discusses the implementation of this novel wall pressure transducer. Results indicate that the proposed approach can be effectively applied to the experimental determination of near-wall turbulence structures and other flow features, which is a major step toward improved TBL noise prediction