Grids have been used as a means of generating nearly isotropic turbulence for about fifty years. Even so, there does not appear to be a single document which gives adequate and simple rules for the design of such devices in an airflow installation. This paper attempts to fill this gap by means of a synthesis of experimental data with simple analyses, such that useful design guidelines are derived. Pressure losses, turbulence intensities, spectra, correlation functions and length scales are all examined. The present results are found to agree well with other data published in the literature.

The generation of nearly isotropic turbulence by means of grids

This paper presents an experimental study on the aeroacoustics of a flat plate rig with a highly instrumented serrated trailing-edge. The role of near-field flow properties, namely, surface pressure fluctuations and spanwise coherence, in the noise suppression capability of serration is not properly understood. The results from this test rig aim to provide additional insight into the effects of the serration on the hydrodynamic field (flow field) and the scattering of the pressure waves along the trailing-edge. Despite its unconventional size, beamforming results showed a significant reduction of far-field noise over a broadband frequency range. The associated flow field is characterized by mean and spectral analyses of static and dynamic surface pressure measurements as well as hot-wire measurements. The mean pressure coefficient results and the boundary layer velocity profiles over the serrated trailing-edge showed minute differences compared to the baseline straight trailing-edge. However, the frequency-dependent energy content of the unsteady surface pressure fluctuations demonstrates an elevated energy region around the serration edges at low frequencies. Although there is an increase in the energy content of the low frequency pressure fluctuations on the serrated trailing-edge, a significant phase difference of the pressure waves is observed, which may be indicative of destructive interference. The temporal studies regarding the unsteady surface pressure fluctuations corroborate the presence of quasi-periodic large scale structures emanating from the serration edges.

On the aeroacoustic characterization of a robust trailing-edge serration

A detailed experimental study on the aerodynamic performance and noise emission of airfoils and fan blades with permeable leading edges under disturbed inflow conditions was performed. The airfoils...

https://arc.aiaa.org/doi/pdf/10.2514/1.J060396

Permeable Leading Edges for Airfoil and Fan Noise Reduction in Disturbed Inflow

Trailing-edge noise is the dominant contributor to the noise generated by aircraft and wind turbines. Serrations on the trailing edge play a crucial role in suppressing the aerodynamic noise of an airfoil, and bionic airfoil sections are confirmed to be rewarding to sound suppression. However, how these characteristics affect the noise emission is still unknown. In this study, the bio-inspired oblique trailing-edge serrations are embedded within the trailing edge of the airfoil with unique cross section of the owl wing, which differs from the previous design. The noise reduction mechanism of coupled airfoils with innovative asymmetric and conventional trailing-edge serrations are explored at a low Mach number. Numerical results show that the largest lift-to-drag ratio with 17.69 and the smallest sound pressure level with 15.72 dB for the airfoil with bio-inspired oblique serrations are obtained among the investigated airfoils. An additional noise reduction of 3.68 dB can be achieved by using innovative asymmetric serrations. Moreover, the widespread large-scale disordered vorticities triggered by smooth airfoil on the pressure side are detached into the smaller-scale vortices triggered by coupled airfoil. The spanwise correlation reflecting the noise emission is significantly decreased. Distinguishingly, more turbulent kinetic energy and pressure fluctuations are emitted in the longitudinal direction on account of the intense collision of the airflow and the gap flow generated by conventional serrations. It is expected that this in-depth study of sound suppression will serve as an essential guide for airfoil design and noise control for micro-aircraft and fluid machinery coping with disturbing areas. 

Aeroacoustic investigation of asymmetric oblique trailing-edge serrations enlighted by owl wings

Aeroacoustic Performance of Propellers in Turbulent Flow

An experimental study was conducted to investigate the effect of the bevel angle on the radiated noise and the associated flow-field over the trailing-edge. The bevel angle of the trailing-edge was adjusted over a broad range of angles to demonstrate the aeroacoustic properties of the attached and separated boundary layers. The far-field noise results obtained from the beamforming array were assessed in conjunction with the associated flow-field. Far-field measurement results show an increased level of radiated noise for a range of bevel angles where the flow remains attached compared to the baseline flat-plate case without a bevel angle. Near-field measurements are presented in terms of pressure coefficients, unsteady surface pressure results, turbulence properties of the boundary layer, and corresponding spectral properties of these quantities. The near-field measurement results indicate that introducing a bevel angle yields a region of favorable pressure gradient on the flat-plate and bevel, which accelerates the flow and reduces the size of structures in the boundary layer up until the mid-bevel region, which then decelerates toward the trailing-edge. This change increases the energy content of surface pressure fluctuations as well as the energy content of the velocity field over the trailing-edge and wake, which, in turn, results in an increased radiated far-field noise as supported by beamforming results. Furthermore, at a sufficiently high bevel angle, where pronounced flow separation occurs, the lack of interaction between trailing-edge and shear layer leads to a significant reduction in the radiated far-field noise compared to that of the baseline flat-plate.

Effect of trailing-edge bevel on the aeroacoustics of a flat-plate

A preliminary version of this paper was presented at the AIAA Aviation 2020 forum (Paper 2020-2525).This paper provides an insight into the grid generated turbulence for aeroacoustic studies. Sever...

Grid Generated Turbulence for Aeroacoustic Facility

This article presents a comprehensive mapping of wall pressure fluctuations over an airfoil under three different inflow conditions to shed light on some basic assumptions taken granted for the recent aeroacoustic and aerodynamics experimental studies and in the noise prediction models. Unsteady and steady pressure measurements were performed over a heavily instrumented airfoil which was exposed to smooth inflow, grid generated turbulent inflow, and a smooth inflow with a tripping tape over the airfoil to explore the unsteady response of the airfoil for a broad range of angles of attack, 0◦ {less than or equal to} ᵯC; {less than or equal to} 20◦. The results are presented in terms of non-dimensional pressure coefficient, root mean square non-dimensional pressure coefficient, frequency-energy content pattern map at isolated frequencies for the entire airfoil,and spectra of frequency-energy content at selected transducer locations. The results show that the unsteady airfoil response patterns for the tripped boundary layer and turbulence ingestion cases show a dramatic difference compared to the airfoil response patterns of the smooth inflow conditions. The response patterns differ across angles of attack, frequency, and between both sides of the airfoil. The results suggest a three region pattern for smooth inflow case, a two region pattern for tripped boundary layer case, and a two region pattern for the turbulence ingestion case. Moreover, the results indicate that the presence of tripping may provide a flow with necessary statistical characteristics for the experimental rigs representing the full-scale application; however, it may misrepresent the frequency-dependent nature of the boundary layer.

/pdf/experimental-investigation-on-the-unsteady-surface-pressure-3efospf8.pdf

Experimental Investigation on the Unsteady Surface Pressure Fluctuation Patterns over an Airfoil

Airfoil turbulence interaction noise and the flow field up to and over the porous leading edge is experimentally studied. The porous leading edges were of the same base triply periodic minimal surface structure with varying porosity to enable us to understand how the porosity, permeability, and pore size affect the generated turbulence interaction noise. The turbulent flow was generated by means of a passive turbulence grid that does not affect the normal background noise of the wind tunnel. Far-field noise results were obtained from a polar microphone array to assess the directivity of the sound as well as the narrowband frequency contributions. Far-field noise results demonstrate that increasing porosity reduces the turbulence interaction noise over low-to-mid frequencies, with a penalty of a high frequency noise increase. Flow measurement results indicate hydrodynamic penetration of the flow into the porous structure at the leading edge. Furthermore, the two-point correlation analysis of the velocity fluctuations approaching the leading edge shows that the turbulent structures approaching the solid leading edge appear to deform into more two-dimensional structures, whereas in the case of the porous leading edge, the turbulent structures appear to retain a strong spanwise coherence up to the point of hydrodynamic penetration.

/pdf/the-effect-of-leading-edge-porosity-on-airfoil-turbulence-1d92ss0i.pdf

The effect of leading edge porosity on airfoil turbulence interaction noise.

/pdf/effect-of-trailing-edge-bevel-angle-on-the-sound-generation-1gy7vm0sr0.pdf

Luke Bowen

Papers

Effect of trailing-edge bevel on the aeroacoustics of a flat-plate

Grid Generated Turbulence for Aeroacoustic Facility

Experimental Investigation on the Unsteady Surface Pressure Fluctuation Patterns over an Airfoil

The effect of leading edge porosity on airfoil turbulence interaction noise.

Effect of Trailing-Edge Bevel Angle on the Sound Generation of a Flat Plate