This paper presents new unsteady surface pressure measurements on a NACA 0012 airfoil placed in grid turbulence at multiple angles of attack (0-10 deg). A new analysis framework approach was used to study the airfoil pressure jump. Root-mean-square pressure levels indicate that the pressure response was contained to the first 15-25% of the chord for all angles of attack. The pressure jump spectra compared reasonably well with Amiet's theory at low wave numbers; however, differences observed at higher wave numbers were attributed to flow distortion. The framework parameters show that the spectral levels were reasonably balanced across the airfoil but the coherence and phase between the suction and pressure sides showed significant changes. At zero angle of attack, the airfoil's local pressure response broke down when the airfoil thickness was greater than one-third of the streamwise convective wavelength. At angles of attack, the phase response was explained in terms of the distortion of eddies as they encounter the leading edge and are subsequently convected by the flow on each side of the airfoil. These results provide new insights into turbulence-airfoil interaction physics and can be used to improve turbulence distortion and acoustic models.

This paper presents the design of a new, novel experimental rotor rig, called the Blade Fluctuating Surface Pressure (BFSP) rig. The BFSP rig is designed to measure the fluctuating surface pressure (FSP) using the remote microphone method, at various locations on the surface of a small diameter, two-bladed rotor, representative of rotors found on small unmanned aerial systems (sUAS). FSP data have been obtained across a range of blade collective pitch angles of 0 degrees, 6 degrees and 12 degrees rotational rates of 2000, 3000, 4000 and 5000 RPM, and varying edgewise freestream velocities from static to 10 and 15 m/s. The pressure taps on the blade surface have been arranged so that the pressure jump spectra across the rotor blade at three chord-normalised positions of x/c = 1/16, x/c = 1/8 and x/c= 1/4 are measured. These spectra show the presence of shaft rate and blade passage frequency (BPF) integer and half-integer harmonic tones under the static and edgewise freestream conditions tested. These tones of the BPF are moderately coherent and nearly out-of-phase at static freestream conditions and increase in coherence further with the introduction of edgewise freestream flow. These highly coherent, nearly out-of-phase tones as well as coherent out-of-phase broadband content are observed at a chord-normalised position of x/c= 1/16 between normalised frequencies of 0.5-6 f /BPF, with a double peaked shape in phase difference plots across all freestreams. At a flow speed of U(infinity)8 = 10m/s the coherence strengthens and expands to 0.5-6 f/BPF whilst at U-infinity 8 = 15m/s it reaches toward 9 f/BPF. Under these edgewise freestream conditions, these coherent tones are close to being exactly out-of-phase between 0.5-3 f/BPF at x/c = 1/8 and x/c= 1/4. By calculating the pressure jump spectra, the extraneous noise is assumed to be mostly cancelled, leaving only the aerodynamic loading sources of FSP. This assumption means that the BPF tones measured under static freestream conditions are most likely due to aeroelastic blade vibration.

Many infection risk models rely on the well-mixed assumption, neglecting variations in aerosol concentration and particle size distribution. Additionally, these models lack real-time aerosol data integration, limiting their ability to assess infection risk and safe occupancy time dynamically. While CFD-based models provide spatiotemporal aerosol distribution, they often use predefined emission rates that do not account for particle size effects. This study presents a novel radius-resolved infection risk model that estimates infection probability and maximum occupancy time based on measured aerosol concentrations. The model directly utilizes the pathogen concentration derived from in-situ aerosol measurements to estimate real-time infection risk at the sensor location. Furthermore, CFD simulations incorporating the particle size distribution of exhaled aerosols in a stale air classroom environment are applied to generate spatiotemporal infection risk and safe occupancy time maps. The effects of the infected occupant's position, particle radius, activity level, and age on infection risk are investigated. Results highlight significant spatial variability, with our model estimating infection risk 20% lower than the well-mixed model at the sensor but over three times higher near the infector, underscoring the limitations of well-mixed models. Maximum occupancy time maps reveal that, in certain locations, nearly half of the occupants remain uninfected after 25 min, whereas, in other locations, only three occupants remain uninfected. This demonstrates that infection probability is an inadequate safety metric, while maximum safe occupancy time is more reliable. The developed model can serve as an estimator of infection risk and safe occupancy time for adaptive ventilation and filtration strategies.

This paper summarises an approach to generating controllable pressure gradients within an open-jet configuration for aeroacoustics research. A novel open-jet pressure gradient test rig has been designed for the UNSW Anechoic Wind Tunnel with the help of RANS simulations. A range of pressure gradients is created by adjusting the inclination angle of the top plate to change the cross-sectional area along the streamwise direction gradually. The test model mounting point located on the bottom plate adjacent to partially opened side walls to allow far-field noise measurements. A comprehensive characterisation of flow quality, pressure gradient parameters and acoustic data quality has been carried out using Particle Imaging Velocimetry (PIV), surface pressure taps, and a microphone array. The test rig produces near-uniform pressure gradient flows at the model mounting point, with momentum Reynolds numbers (Red) Re d ) ranging from 3014 to 11853 and Clauser's pressure gradient parameters (/3) /3 ) from-0.24 to 1.66. The pressure gradients generated by this facility are approximately linear, approaching the model mounting point, and the boundary layer profiles compare favourably with those from conventional hard-walled enclosed pressure gradient wind tunnel facilities. Measurements of airfoil trailing-edge noise from this test rig compare well with classical semi-empirical model predictions. Simultaneous acoustic and flow measurements on square finite wall-mounted cylinder showcase the capability of this facility for coupled acoustic-flow diagnosis.

This paper presents the far-field noise radiated by a flat plate with sinusoidal leading edge (LE) serrations immersed in isotropic and anisotropic turbulence. Nine serration patterns have been tested, four with varying wavelength (normalised by the span, b) in the range lambda/b = 0.10 - 0.29 of constant amplitude h, (normalised by chord, c) h/c = 0.06 and five with varying amplitude in the range h/c = 0.03 - 0.17 of constant wavelength lambda/b = 0.24. The serrated LEs were tested in grid-generated turbulence and the wake of a cylinder positioned at various distances upstream of the LE at chord-based Reynolds numbers from Re-c = 1.97 x 10(5) to 6.91 x 10(5). The serrations displayed substantially lower noise levels than the unserrated, straight-edge case at 1 < St(c) < 20 in isotropic turbulence and at 3 < St(c) < 50 in anisotropic turbulence, where St(c) is the chord-based Strouhal number. In both inflows, the greatest noise reduction was achieved by the smallest wavelength serration of lambda/b = 0.10 (among those with varying wavelength), and by the largest amplitude serration of h/c = 0.17 (among those with varying amplitude). For both optimal cases, the average noise reduction achieved in isotropic turbulence was between 4-6 dB over the chord-based Strouhal number range 1 < St(c) < 20 when the Reynolds number was varied from 1.97 x 10(5) to Re-c <= 6.91 x 10(5). In anisotropic turbulence, a gradual noise reduction as a function of Stc occurred over 1 < St(c) < 10 and a constant reduction of 8-9 dB was measured at Stc > 10. At St(c) > 20, a slight decrease in noise reduction was observed for 2.96 x 10(5) <= Re-c <= 6.91 x 10(5), suggesting that self-noise dominates in this region, reducing the effectiveness of the serrations. Varying the distance between the cylinder and the LE affects noise reduction in a similar way to changing the Reynolds number, with the major differences occurring at the vortex shedding frequency and at high frequencies of St(c) > 10. This is attributed to small changes in the flow anisotropy, with only a 16% and 18% difference in the root-mean-square (rms) ratio of u(rms)/v(rms) and u(rms)/w(rms) respectively when the cylinder is closest to the LE and when it is further away.

A test-rig and an airfoil blade have been designed for the UNSW Anechoic Tunnel (UAT) to measure the flow and noise statistics in tip leakage flows under adverse pressure gradient. The baseline flow (without blade) in the rig has been fully characterised through mean wall pressure and boundary layer velocity and turbulence measurements. The test rig was then used to study the tip clearance noise radiation and mean wall pressure under zero and adverse pressure gradient conditions. Flow-field simulations using RANS approach that show good agreement with the measurements are also presented. The baseline measurements show that the test rig is capable of generating pressure gradient parameter (beta) up to approximately 1.11 on the endwall. The mean wall pressure and noise radiation from a tip leakage flow were measured for three pressure gradient parameters (beta = 0, 0.51 and 1.11), three geometric angles of attack (alpha(g) = 0 degrees, 6 degrees and 12 degrees) and chord-based Reynolds number of approximately 350,000. For each measurement, 53 clearance heights between 1mm and 101mm were considered. It is shown that the adverse pressure gradient increases the overall wall pressure under the tip without significantly modifying the overall pressure distribution, except for very small clearances at the largest angle of attack configuration where a low pressure zone forms near the leading-edge for adverse pressure gradient inflow conditions. The effect of pressure gradient on the tip clearance noise is a strong function of tip clearance height. For small clearances, adverse pressure gradient results in lower noise levels, while for larger clearances it results in an increase in noise levels due to a stronger tip vortex associated noise source.

Noise reduction results are presented for a set of leading edge serrations on a flat plate airfoil in isotropic turbulence, obtained from integrated beamforming results. Triangular, sinusoidal, and tangent serration profiles have been investigated, as well as a modified sinusoid which consists of a sinusoid at the tip and a tangent curve at the root. The serrations have a non-dimensional amplitude and wavelength of h/c = 0.065 and 7/At between 0.7 and 15 respectively, where h the serration amplitude, c is the chord length, 7 is the serration wavelength and At is the spanwise integral length scale of the incoming flow. All serrations show a reduction in noise level compared to the straight leading edge reference case. Between 2 and 3 dB noise reduction was observed for all cases at a Strouhal number based on h, Sth, with value 0.1. Above Sth = 0.3, leading edge noise reduction increases with frequency and peaks at a serration wavelength of /At = 1, a trend that holds for all shape profiles investigated. In this higher frequency range, subtle variations in shape profile significantly vary the noise reduction qualities. The modified sinusoidal profile outperforms all others up to Sth = 1, as long as the serration wavelength is shorter than 7/At = 4. At higher frequencies, the modified sinusoidal serrations have a similar noise reduction to the triangular and tangent curve serrations, which reaches a maximum of dB for serrations with 7/At = 4. Overall sound pressure level results show the noise reduction this high-frequency regime is independent of the Reynolds number. Fluctuating pressure measurements at the roots of the sinusoidal serration are used to extend previously published models of the optimal serration wavelength. For the current data set, the optimal serration wavelength approaches 7/At = 2.7 at high Reynolds numbers. Furthermore, a reinterpretation of historical data, combined with the present data set, shows the optimal serration wavelength depends upon the ratio of serration amplitude to streamwise turbulent length scale h/Ax.

Modal decomposition techniques are widely employed to analyze turbulent flows by identifying coherent structures, which are often linked to key flow features, such as unsteady loading and pressure fluctuations. Modal decomposition offers a powerful framework for uncovering these structures and interpreting their role in flow dynamics. The most popular methods include POD (proper orthogonal decomposition), SPOD (spectral proper orthogonal decomposition), and DMD (dynamic mode decomposition). In this paper, we explore the flow interaction with tandem cylinders arranged in a bi-stable configuration. The velocity field, unsteady surface pressure, and far-field acoustic pressure of a tandem cylinder configuration are measured simultaneously using a high-speed PIV system, a remote microphone technique, and a far-field microphone, respectively. We utilize POD, SPOD and kEDMD (kernelized extended DMD) to identify coherent structures in the vorticity field. These three methods are compared to relate the vertical structures associated with the key flow regimes to surface and far-field pressure measurements. Results show that POD and SPOD offer insight into the important frequencies in an efficient manner, whilst the versatility of DMD-based methods offers further clarity into the dynamics of the system via the Koopman modes.

The supersonic wake of a circular cylinder in Mach 3 flow was studied through spectral proper orthogonal decomposition (SPOD) of high-speed focussing schlieren datasets. A wavenumber decomposition of the SPOD eigenvectors was found to be an effective tool for isolating imaging artefacts from the flow features, resulting in a clearer interpretation of the SPOD modes. The cylinder wake consists of both symmetric and antisymmetric instabilities, with the former being the dominant type. The free shear layers that form after the flow separates from the cylinder surface radiate strong Mach waves that interact with the recompression shocks to release significant disturbances into the wake. The wake shows a bimodal vortex shedding behaviour with a purely hydrodynamic instability mode around a Strouhal number of 0.2 and an aeroacoustic instability mode around Strouhal number of 0.42. The hydrodynamic mode, which is presumably the same as the incompressible case, is weaker and decays rapidly as the wake accelerates due to increasing compressibility. The aeroacoustic mode is the dominant shedding mode and persists farther into the wake because of an indirect energy input received through free-stream acoustic waves. A simple aeroacoustic feedback model based on an interaction between downstream propagating shear-layer instabilities and upstream propagating acoustic waves within the recirculation region is shown to accurately predict the shedding frequency. Based on this model, the vortex shedding in supersonic flows over a circular cylinder occurs at a universal Strouhal number (based on approach free-stream velocity and feedback path length) of approximately 0.3.

The analytical model to predict the wall-pressure spectrum under a turbulent boundary layer depends on two flow parameters: the streamwise velocity gradient and the correlation of the wall-normal velocity in the wall-normal direction. As the turbulence within a boundary layer is highly anisotropic, its effects are taken into account through modification of the velocity correlation term. This paper compares the suitability of existing anisotropic models to predict such wall-pressure spectra, specifically on their performance in the wavenumber domain. Here, the boundary layer is represented by a channel flow with Re-tau = 1000, which was simulated using wall-resolved, large-eddy simulation (LES). The wall-pressure fluctuation data from the channel flow LES were used to compute the wall-pressure spectrum, while the time and space-resolved flow data were used to compute the velocity fluctuation spectra. These LES results were validated against published direct numerical simulation results and were subsequently used as the reference results when compared against existing isotropic and anisotropic models. This study highlights the differences in the wall-pressure and velocity correlation spectra, where the highly anisotropic spectra computed from the LES flow data were compared against those predicted by the analytical models. This study also suggests that the current anisotropic models require further improvements to accurately replicate the wall-pressure spectrum in the wavenumber domain.

This paper presents a numerical framework to study the interaction of isotropic turbulence with airfoils. Specifically, the developed numerical framework is used to investigate the distortion of the turbulent structures interacting with an airfoil’s leading edge. For turbulence modeling, Large Eddy Simulation (LES) is used. The isotropic turbulent inflow for the Computational Fluid Dynamics (CFD) simulations is synthetically generated using the turbulent digital filter method. The case studied with the numerical framework is a NACA 0012 airfoil with a chord-based Reynolds number of Rec=2×105 and an angle of attack of α=5°. The numerical simulation results are compared to high-speed particle image velocimetry (PIV) measurements performed for a NACA 0012 airfoil at the equivalent chord-based Reynolds number and turbulent inflow conditions. The CFD results of the numerical framework compare well with the experiments in terms of velocity spectra and RMS values upstream of the airfoil’s leading edge. The spectra and correlations of the velocity field generated by the turbulent digital filter demonstrate its ability to generate isotropic turbulent inflow. Instantaneous velocity fields show that the airfoil suppresses large-scale turbulent structures of the incoming turbulent flow. The size of the incoming turbulent structures decreases as they approach the leading edge due to the presence of the airfoil. The pre-multiplied spectra of the different velocity components show that downstream of the airfoil’s leading edge, the turbulent structures are stretched in the streamwise direction. The streamwise turbulent integral length scales and the velocity RMS values upstream of the airfoil’s leading edge indicate that the velocity components most affected by distortion are the streamwise and wall-normal components.

An experimental investigation of the wake flow structures, surface pressure fluctuations, and noise production of a square finite-wall-mounted cylinder with an aspect ratio of 2.4 is presented. The cylinder was immersed in flows with favorable, near-zero, and adverse pressure gradients at a Reynolds number of 48,000, based on cylinder width. Far-field noise, unsteady surface pressure, and particle image velocimetry measurements were taken simultaneously using the open-jet pressure-gradient test rig in the UNSW Anechoic Wind Tunnel. Favorable and adverse pressure gradients were found to enhance and suppress the cylinder tonal noise, respectively. A favorable pressure gradient reduced the size of the recirculation region in the wake, which intensified the free-end downwash and suppressed the junction upwash. The intensities of the cylinder surface pressure fluctuations were slightly increased at the primary and secondary shedding Strouhal numbers (based on cylinder width) of approximately 0.1 and 0.2. Conversely, the recirculation region expanded under an adverse pressure gradient, with the downwash weakened and the upwash enhanced. The peaks in the surface pressure spectra were noticeably attenuated at the primary shedding Strouhal number and completely suppressed at the secondary Strouhal number. Wake flow structures correlated with the surface pressure fluctuations and far-field noise were identified to understand the enhancement or suppression mechanisms of the surface pressure fluctuations and the associated shedding regimes.

Spanwise-varying porosity is experimentally investigated as a method to reduce trailing-edge bluntness vortex shedding noise. This class of edge porosity is introduced via flat-plate extensions to the trailing edge. A reference nonporous plate and three different series of porous plates are tested, where each series includes uniform, optimized spanwise-varying, and random spanwise-varying porous plates. In addition, the plate porosity is characterized by a non-dimensional number that links the perforation geometry to theoretical changes in acoustic scaling on flow speed. Acoustic beamforming measurements of the nonporous edge indicate tonal bluntness-induced vortex shedding at an edge-thickness-based Strouhal number of 0.168 to 0.187. The porous edge extensions can reduce this tonal peak by up to 14 dB, functionally eliminating it from the acoustic spectrum. In general, the higher the non-dimensional parameter of the plates, the better the noise attenuation of the bluntness peak. The beamforming results further shed light on the difference in the sound created by the different hole spacings, where at a select velocity, the uniform and randomly-spaced porosity are the most effective at tonal noise reduction. Lastly, the presence of porosity at the trailing edge increases excess roughness noise at high frequencies; however, the optimized spanwise spaced holes increase this noise the least.

This paper provides a description of the important noise sources for low-Mach number ducted propellers. It presents an analysis of various noise source strengths using generic acoustic models from the literature. The paper shows that for low-Mach number applications, unsteady noise sources are of major importance. The duct expansion ratio is shown to reduce tonal noise from steady loading because as the expansion ratio increases, the rotor thrust is reduced. Haystacking noise, created by multiple cuts of the same eddy by subsequent rotor blades, is analysed using an analytical approach. The time and length scales within the ducted propeller are shown to significantly affect the intensity of the noise created by this interaction. The relationship between rotor thrust, duct geometry, turbulence distortion and noise is found to be an important research gap that needs to be addressed by future research programs.

Leading-edge noise is a complex phenomenon that occurs when a turbulent fluid encounters a solid object, and is a notable concern in various engineering applications. This study enhances a mathematical leading-edge noise model (Hales et al., J. Fluid Mech., vol. 970, 2023, A29) for anisotropic flow and porous boundaries. The model has two key components. First, we adjust the velocity spectrum to account for the possibility of anisotropy in the flow. This paper rigorously introduces a third dimension for the turbulence spectrum that preserves the turbulence kinetic energy and mathematical definitions for integral length scales. Second, we adapt the fully analytical acoustic transfer function to account for different boundaries by implementing convective impedance boundary conditions when formulating the gust-diffraction problem. This problem is then solved using the Wiener-Hopf technique. We discuss important aspects of this method, including the factorisation of a non-trivial scalar kernel function and the application of suitable edge conditions for the problem. Each modification is inspired by experimental leading-edge noise data using a series of different porous leading edges and anisotropic turbulence generated by a cylinder upstream of the edge. Experimental data demonstrate the interplay between anisotropy and leading-edge modifications while achieving the characteristic mid-frequency noise reduction expected from porous leading edges. Our model is adapted to best fit the trends of the data via a tailored impedance function, leading to good agreement with all datasets across an extended frequency range. This tailored function is used to successfully validate the model against other datasets from a different set of experiments.

The noise generation and flow characteristics of a forced-transitioned NACA 0012 airfoil with a micro-tube porous trailing edge and its solid counterpart have been numerically investigated. The near-field flow dynamics and far-field noise predictions are obtained using compressible large-eddy simulations and the Ffowcs-William and Hawkings acoustic analogy, respectively. The computed far-field noise levels agree with experimental data, showing that the micro-tube structure reduces low-frequency trailing-edge noise without noticeably altering the noise directivity but induces additional high-frequency noise along the direction perpendicular to the airfoil chord. An in-depth analysis of the trailing-edge flow and surface pressure fields reveals that the noise reduction is largely linked to the ‘cross-jet-like’ flow permeation through the micro-tube structures. The interaction between the flow permeation and the boundary layer turbulence leads to a reduction in the strength of the Reynolds-stress source term in the pressure-side flow field and a reduction in the magnitude of the source terms in Amiet’s trailing-edge noise model. Spectral proper orthogonal decomposition is performed to identify flow structures that are spatially and temporally coherent within the micro-tube geometries and to link the flow features to the attenuation in noise scattering efficiency. The effect of the micro-tube structure on the wavenumber-frequency spectra of the trailing-edge surface pressure field is quantitatively analysed. Additional high-energy regions exist in the wavenumber-frequency spectra of the porous trailing edge, resulting from the turbulent eddies that permeate through the micro-tubes. Further, the high-frequency noise increase mechanism is identified as the acoustic dipole noise arising from unsteady force fluctuations.

This paper examines and compares methods of separating tonal and broadband components of the noise generated by small rotors as commonly found on small unmanned aerial systems (sUAS). Time synchronous analysis (TSA) methods with varying averaging algorithms, such as ensemble averaging (EA), exponential weighted moving averaging (EWMA) and Kalman filter averaging (KFA), are compared against themselves and against a cross-correlation-based method. The decomposition methods are used on noise measurements of a small isolated rotor under static operation and edgewise flight conditions at 10 m/s in the UNSW anechoic wind tunnel (UAT). The best method for isolating tones is TSA-EWMA and for isolating the broadband spectrum of noise is the cross-correlation method, based on the spectral reconstruction of the experimental data.

Noise reduction results are presented for a set of leading edge serrations on a flat plate airfoil in isotropic turbulence, obtained from integrated beamforming results. Triangular, sinusoidal, and tangent serration profiles are investigated, as well as a modified sinusoid which is a sinusoid at the tip and tangent curves at the root. The serrations have a non-dimensional amplitude and wavelength of h/c(0) = 0.065 and lambda/Lambda(t) between 1.1 and 9.1 respectively, where h is the serration amplitude, c(0) is the chord length, lambda is the serration wavelength and Lambda(t) is the transverse integral length scale of the incoming flow. All serrations show a reduction in noise compared to the flat leading edge reference case. All cases show a local peak in the noise reduction at a serration amplitude based on the Strouhal number of St(h) = 0.1, the magnitude of which is largely invariant to serration profile or wavelength at a value of between 2 and 3 dB. Above St(h) = 0.2, the leading edge noise reduction increases with frequency and is larger for shorter wavelengths, a trend that holds for all shape profiles investigated in the current work. In this higher frequency range, the shape profile significantly impacts the noise reduction qualities of the serration. The modified sinusoidal profile outperforms all others in the current study up to St(h) = 1, whereby its noise reduction becomes identical to those of the triangular and tangent curve serrations, which for serrations of lambda/Lambda(t) = 4, reaches a maximum of 9 dB.