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Large Eddy Simulation (LES) and Ffowcs Williams-Hawkings acoustic analogy are performed to study the effect of trailing-edge blowing on airfoil self-noise. Simulations were conducted using a National Advisory Committee for Aeronautics 0012 airfoil at zero angle of attack and a chord-based Reynolds number of [Formula: see text]. The aerodynamic and aeroacoustic characteristics of the baseline airfoil were thoroughly verified by comparison with previous numerical and experimental data. The noise reduction effects of continuous and local blowing with different blowing ratios and blowing momentum coefficients were compared. A maximum noise reduction of 20 dB was achieved via trailing-edge blowing and the noise reduction mechanisms of the two blowing methods were discussed. The LES results show a pair of recirculation bubbles in the airfoil wake which are suppressed by trailing-edge blowing. As the blowing vortices convect into the wake, they stretch and stabilize the shear flows from airfoil surfaces. Instantaneous vorticity and root mean square velocity fluctuations are also weakened. There is a decrease in the spanwise coherence and an increase in the phase difference, which contribute to noise reduction. It is concluded that the suppression of turbulence fluctuations in the near wake is the main mechanism of noise reduction for airfoil trailing-edge blowing.

The application of a porous coating to a cylinder can significantly reduce the vortex shedding tone when placed in a uniform flow. The mechanism of how this porous media attenuates vortex shedding has been studied more in recent years yet it is still not fully understood. Typical porous materials within a certain range of porosity and airflow resistivity, such as metal foam and porous polyurethane, have been studied extensively; however, the fundamental flow mechanisms responsible for vortex shedding attenuation are very difficult to determine. For example, it is nearly impossible to visualize the internal flow field of porous media with a randomized open-cell internal structure. A Structured Porous Coated Cylinder (SPCC) was designed in recent years to alleviate this internal flow field problem, as the SPCC has clear line of sight along the span and radial direction. SPCC variations have been previously studied and shown to reduce the vortex shedding tone of a bare cylinder in a very similar manner as a randomized porous coated cylinder. In this paper, we present a Tomographic Particle Image Velocimetry study of an SPCC tested in a water tunnel, revealing the previously unseen internal and near-wall flow fields of an SPCC. The flow is visualized in the porous layers, revealing complex interaction between the freestream flow field and the porous structure. Using cross-correlation methods within the flow field, we reveal the entrainment of the flow within the porous layers. Furthermore, implementation of Proper Orthogonal Decomposition shows that vortex shedding occurs within the porous layers.

Acoustic emission of a NACA 0012 aerofoil is investigated over a range of free-stream velocities. Acoustic spectra show a dominant tone and two sets of weaker side tones characterised by different frequency intervals. The frequency of the dominant tones in the acoustic spectra varies with velocity in a ladder-type structure. With increasing Reynolds number, the spectrum becomes progressively more broadband in nature. Through synchronised particle image velocimetry and acoustic measurements, the aeroacoustic noise generation mechanisms, resulting in different spectral characteristics and modulation types, are further investigated. A separation bubble and related significant velocity fluctuations are observed on the pressure side. Pressure side velocity spectra show characteristics similar to the acoustic ones, whereas velocity spectra on the suction side feature broadband characteristics. These findings confirm that noise emission is dominated by pressure side events for the Reynolds number range of this study, i.e. $2 \times 10^{5}$ – $7 \times 10^{5}$ . As the acoustic emission is defined by coherent flow structures, the proper orthogonal decomposition method is adopted to facilitate the understanding of the relation between the complex flow field and acoustic emission. Side tones in the acoustic spectra are attributed to two different modulation mechanisms in the aeroacoustic source region near the trailing edge. By aligning the sound pressure time history and the time coefficients of the dominant modes, the primary modulation of the dominant tone is found to be related to the amplitude modulation of the high-frequency velocity fluctuations associated with the acoustic feedback loop. A secondary modulation is attributed to periodic variation of the separation bubble and, therefore, variation in the roll-up of the shear layer, which results in a modulation of the amplitude of the velocity fluctuations associated with the convecting vortices at the trailing edge.

View Video Presentation: https://doi-org.tudelft.idm.oclc.org/10.2514/6.2021-2226.vidThe mechanisms responsible for vortex shedding reduction, due to the application of a porous coating to a smooth cylinder placed in uniform flow, are still uncertain despite several numerical and experimental studies. To help understand these mechanisms, a transparent Structured Porous Coated Cylinder (SPCC) was manufactured to investigate the internal and near-wall flow fields. The SPCC mimics the more commonly used porous materials such as metal foam and polyurethane, that possess randomized structures preventing a clear line-of-sight along the span and circumference of the porous layers. Tomographic Particle Image Velocimetry was used in a water-tunnel facility to investigate three small regions of an SPCC, on the windward, mid-region and leeward sides. In this paper, previously unseen experimentally obtained internal flow fields of a porous coated cylinder are presented. The following summarize the key observations (1) stagnation from freestream flow to the inner cylinder diameter in the windward region, (2) boundary layer profiles within the porous layer in the circumferential mid-region and (3) bleeding from the porous layer into the wake in the leeward region. These results provide key experimental findings for comparison and validation of numerical simulations of bulk porous media, such as the Darcy-Forscheimer method used in conjunction with the Ffowcs Williams-Hawkings acoustic analogy.

In some situations, a vortex is involved in the inflow of a propeller, e.g. a ground vortex or a vortex shed from the upstream parts of the airframe of the aircraft. In order to study the impact of the vortex on the propeller, the Batchelor vortex is utilised to model the impinging vortex, and its effect on the propeller is calculated by Computational Fluid Dynamics. The loading on the propeller increases due to the impingement of a contra-rotating vortex, and the opposite is true for a co-rotating vortex. However, the torque coefficient of the propeller changes consistently with the thrust coefficient which leads to the efficiency of the propeller being independent of the impinging vortex. With the vortex impinging on the propeller, a strong pressure fluctuation on the blade is observed, especially on the leading edge of the blade. This cyclic pressure also results in a periodic bending moment on the blade, which may have an impact on the structural fatigue of the blade.

By employing a wing-tip vortex impinging onto a propeller, the impact of the propeller on this vortex is studied with the help of Particle Image Velocimetry measurements. The effects of the propeller on the vortex are compared in the flow fields in the vicinity of the blades, i.e. one plane upstream and one plane downstream of the propeller. The meandering of the vortex downstream of the propeller increases by approximately one order of magnitude with respect to that upstream of the propeller. The circulation of the downstream vortex is at the same level as the upstream vortex, and this means the downstream vortex still has the potential to influence the airframe further downstream. As the vortex impinges at different radial positions of the propeller, the stretching effect depends on the local thrust of the propeller, which is different from the case with a vortex impinging on a steady wing. The Proper Orthogonal Decomposition analysis shows that the first and second modes of the flow upstream of the propeller are induced by the blade passing, whilst the third and the fourth modes are induced by the vortex meandering.

Inspired by the silent flight of owl wings, this paper adopts leading edge (LE) serrations for noise reduction of isolated multi-copter rotors. By choosing different serration parameters of height and wavelength, aerodynamic loading and noise emissions of these multi-copter rotors are measured and analyzed at the forward flight condition through a force balance and microphones in an anechoic wind tunnel. Rotors with add-on LE serrations generate slightly lower thrust than the Baseline (BL) rotor at the same rotation speed. In order to assess the noise reduction of different serration designs from the application perspective, a comparison of rotor noise emission is conducted at the same thrust. All serrated rotors considered in this study show lower tonal, broadband, and overall noise levels than the BL rotor. The noise reductions of all serrated rotors are observed to generally follow a scaling law with the Strouhal number in the mid-to-high frequency range. The height-to-wavelength ratio is a potential critical parameter that determines the coupled aerodynamic and aeroacoustic performance of the serrated rotors. The directivity of the rotor noise reduction is also shown through the data of a circular microphone array.

The aeroacoustic wind tunnel at Southern University of Science and Technology (SUSTech) is a new facility commissioned for studies of sound generation mechanisms, noise reduction methods, and aerodynamic designs. The wind tunnel is operated at low Mach numbers and low-to-moderate Reynolds numbers. This study details the design methodology for wind-tunnel major components. The validation tests of the flow quality are performed by pitot-tube and hot-wire measurements, and those of the aeroacoustic performance are conducted by far-field microphone measurements. The aerodynamic and acoustic performance is compared with established data, and the results show that the wind tunnel has a high quality of flow field and low background noise levels. A benchmark test of airfoil tonal noise is conducted by microphone and Particle Image Velocimetry (PIV) measurements. The observed tones exhibit consistent features with existing experimental observations and the mechanisms are explained with the help of flow field near the trailing edge.

During the ground operation of aircraft, the interaction between the propulsor-induced flow field and the ground may lead to the generation of ground vortices. Utilizing numerical approaches, the source of vorticity entering ground vortices is investigated. The results show that the production of wall-parallel components of vorticity has a strong contribution from the wall-parallel components of the pressure gradient on the wall, which is generated by the action of the propulsor. This mechanism is a supplementation for the vorticity transported from the far-field boundary layer, which has been assumed the main vorticity source in a number of previous publications. Furthermore, the quantitative prediction of the occurrence of ground vortices is performed from the numerical results. As the distance of the propeller form the ground decreases, and as the thrust of the propeller increases, ground vortices are generated from the ground and enter the propeller. In addition, the vortices which exist near the ground but does not enter the propeller plane are observed and visualized by three-dimensional data. Graphical abstract: [Figure not available: see fulltext.]

Cylindrical bodies in uniform flows can be coated with a porous medium as a passive flow and noise control method in an effort to reduce the acoustic effects of vortex shedding. To date, the employed open-cell porous materials typically possess a randomized internal structure. This paper presents the design and validation of a novel 3-D printed structured porous coated cylinder that has significant flexibility, in that the porosity and pores per inch of the porous coating can be modified independently and relatively easily. The performance of the structured porous coating design is compared against porous polyurethane and metal foam with the same coating dimensions and similar pores per inch and porosity via an experimental acoustic investigation, revealing strong similarity in the passive noise control performance of each material type. A numerical comparison illustrates the similarities of the wake structure of the 3-D printed porous coated cylinder to an equivalent Darcy&ndash, Forchheimer model simulation that represents a randomized internal porous structure. The performance similarities of these different porous material types indicate that a structured porous geometry can be used to understand the internal flow behavior of the porous medium responsible for reducing the cylinder vortex shedding tone that is otherwise extremely difficult or impossible with typical randomized porous structures. Moreover, significant potential exists for the porous structure to be further optimized or smartly tailored by architectural design for different control purposes, coating geometries and dimensions, and working conditions.

Porous coated cylinders have been shown to reduce the vortex shedding tone and broadband noise of a bare cylinder placed in uniform flow within specific Reynolds number regimes. The processes by which the vortex shedding and thus tone suppression take place are still uncertain despite numerous numerical and experimental studies. It is understood that adding a porous medium to a bare cylinder will have an influence on the Reynolds number of cylinder, yet the increase of outer diameter alone and the influences of surface roughness are insufficient to explain the changes in the shedding tone magnitude and frequency that are observed by many. Investigating the internal flow field of a porous coated cylinder could lead to a deeper understanding of the flow processes that result in the tonal noise reduction. This has not been achieved to date, as commonly used materials such as metal foam and polyurethane possess randomized porous structures, which make investigating the internal flow field nearly impossible without affecting the structure itself. This paper presents a preliminary analysis of the internal and external flow fields of two structured porous coated cylinders. The cylinders were manufactured using solid transparent materials that possess direct lines of sight through the pores in the axial and spanwise directions. Such structured porous coated cylinders have been previously successful in reducing the typical vortex shedding tone. Tomographic and 2-D planar Particle Image Velocimetry (PIV) were used in a water-tunnel facility to visualize the internal and external flow fields. To date only the 2-D planar PIV results have been post-processed that reveal differences in the wake for the two different cylinder types such as recirculation of flow around the pores. Vorticity flow structures are observed to vary along the cylinder span in the same pattern as the porous structure and streamlines at the windward cylinder side reveal the entry of flow into the porous medium.

Inspired by owl wings, trailing edge (TE) serrations are adopted for noise reduction of multi-copter rotors. By choosing different parameters of serrations, i.e. the height to wave-length ratio, retrofitted manners of add-on and cut-in, as well as the radial range, aerodynamic and acoustic performance of isolated multi-copter rotors are compared at the forward flight condition. Rotors with add-on serrations generate higher thrust than the Baseline (BL) rotor at the same rotational speed, and the opposite trend is found for the rotors with cut-in serrations. The add-on models considered in this study have lower power consumption than the cut-in and BL models while generating the same thrust. In order to compare the noise reduction of different designs from the application perspective, analysis of Overall Sound Pressure Level (OASPL) at the same thrust is conducted. Serrated models considered in this study have lower OASPL than the BL model for most cases, which is contributed by both the broadband and tonal noise reductions. In addition, the model with half-span serrations achieves approximately the same OASPL reduction as that with full-span serrations.

This paper presents a numerical study on the flow and noise control of circular cylinders with non-uniform porous coating over the circumference in the subcritical flow regime ( R e = 4.7 × 10 4 ). A numerical method is employed that combines three-dimensional large eddy simulation (LES) with Ffowcs Williams-Hawkings (FW-H) acoustic analogy, and is validated by comparing with available experimental and numerical data of flow over circular cylinders. The numerical results show that applying high-porosity material in the separation zone is most effective on reducing aerodynamic drag and noise. This partial high-porosity coating enhances the slip velocity at the porous-fluid interface, stabilizes shear layers detached from the cylinder, and hence suppresses vortex shedding, as well as raising the base pressure on the leeward side and reducing drag. A high-porosity coating with a 60 ∘ circumferential coverage of the separation zone is found to be a cost-effective configuration in controlling both drag and noise. Varying the porosity on the leeward side of a cylinder, however, shows slight differences on flow and acoustic characteristics. This study is expected to provide some useful information for porous coated cylinders aimed at flow and noise control towards practical applications with constrains on coating area.

The aerodynamic and aeroacoustic performances of air over porous-coated cylinders are studied numerically with a new porous-fluid coupling method, which is based on k-ω turbulence model and implemented in the in-house compressible flow solver, aerodynamic and aeroacoustic investigation simulator (AAISIM). The turbulent boundary layer of the porous-fluid interface, which was commonly overlooked, is considered in the AAISIM. The permeable Ffowcs Williams and Hawkings (PFW-H) acoustic analogy is implemented and proved to be more accurate than the Ffowcs Williams and Hawkings (FW-H) method for porous-coated cylinder case. The porous coatings with pores per inch (PPI) ranging from 5 to 80 are considered in the present study. Lift and drag forces are collected from the inner cylinder wall surface, porous matrix, and the porous-fluid interface. Instead of monotonically increasing or decreasing, both the root-mean-square (RMS) lift coefficient and the aeolian noise achieve minimum values for the 10 PPI case, and the maximum total average drag coefficient is obtained for the 20 PPI case. The dead-zone length of the porous-coated cylinder is elongated than that of the solid case. The recirculating bubbles shift away from the inner cylinder surface to the interface. The turbulent kinetic energy increases near the porous-fluid interface as the number of PPI becomes larger.

In order to alleviate rotor noise of multi-copters, a wavy rotor is modified from the baseline (BL) rotor by shifting every other cross section toward the trailing edge. Utilizing balance and far-field microphones, aerodynamic and acoustic performance of the wavy rotor is analysed. The aerodynamic performance of the wavy rotor is approximately the same as the BL rotor in terms of available thrust and power loading. Reduction of tonal noise in the middle-frequency range is observed for high RPM (Revolutions Per Minute) values, which is consistent with the trend of thrust spectra. Broadband noise decrease in the high frequency range is noticed for all RPM values. The noise reduction in the circumferential range corresponding to the advancing blade is larger than that of the retreating blade. The decrease of overall sound pressure level (OASPL) of the wavy rotor with respect to the BL rotor is about 1.4 ~ 2 dB.

The interaction between a propeller and its self-induced vortices originating on the ground is investigated in a scaled experiment. The velocity distribution in the flow field in two different planes containing the self-induced vortices is measured by particle image velocimetry (PIV). These planes are a wall–parallel plane in close proximity to the ground and a wall–normal plane just upstream of the propeller. Based on the visualization of the flow field in these two planes, the occurrence of ground vortices and its domain boundary are analysed. The elevation of the propeller from the ground and the thrust of the propeller are two parameters that determine the occurrence of ground vortices. The main features of the propeller inflow in the presence of the ground vortices are highlighted. Moreover, the analysis of the non-uniform inflow in the azimuthal direction shows that with increasing the propeller thrust coefficient and decreasing the elevation of the propeller above the ground, the variation of the inflow angle of the blade increases. Graphical Abstract: [Figure not available: see fulltext.]

The aerodynamic operating conditions of a propeller can include complex situations where vorticity from sources upstream can enter the propeller plane. In general, when the vorticity enters in a concentrated form of a vortex, the interaction between the vortex and blade is referred to as blade–vortex interaction or BVI. The interaction may affect the propeller performance as well as its noise production. In the present paper, investigations of the interaction of a wing tip vortex generated by a lifting surface upstream of the rotor plane and an eight-bladed propeller are reported. Utilizing two ends of an upstream wing with non-symmetrical airfoil, the rotation of the incoming vortex could be made to co-rotate or to contra-rotate with the propeller. The ensuing velocity fields were quantified with the help of particle image velocimetry (PIV), and the propeller performance was evaluated with the help of a rotating shaft balance (RSB) mounted on the propeller shaft. The results describe the displacement of the vortex core, as it moves through the rotor plane as well as the positive effect on the thrust and torque of the contra-rotating vortex and the opposite of it in the case of the co-rotating vortex. The current research could be applied to analyse the influence of the incoming vortex on the propeller, e.g., ground vortex, tip vortex shed from a control surface, etc.

During the ground operation of propeller aircraft, the interaction between the ground and the flow field produced by the propeller may lead to the generation of a vortex originating on the ground. Such ground induced vortex results in non-uniform inflow into the propeller disk. Furthermore, the vortex may raise debris from the ground that may damage the blades, decreasing their life time. For this phenomenon, the flow field near the ground is of interest because it provides information on the strength of the vortex before it is being ingested by the propeller. To obtain insight into the origins and the development of the ground vortex, velocity measurements have been performed in the flow field produced by a scaled propeller model using a three-component PIV system on a plane parallel to the ground and in the proximity of it. The velocities in the test plane suggest that a horseshoe vortex is formed above the ground which is built to help understand the flow topology of ground vortices. The vortex/no-vortex domain boundary which is relied upon the presence of node and saddle points in the time averaged flow is presented. The effect of varying the thrust coefficient of the propeller has been assessed under headwind conditions. The averaged strength of the maximum wall normal vortex is compared at different thrust coefficients. The results show that average levels of vortex strength increases up to a maximum and then decreases to achieve a plateau. The pressure under the propeller is measured and the mean value of pressure gradient magnitude keeps increasing as thrust coefficient increases. Vortex wandering is observed at all test conditions in both the lateral and the longitudinal directions for all test conditions. The influence of the ground induced non-uniform inflow upon the propeller performances is found to be negligible.