Vortex Shedding Noise Research Articles

Experimental and Computational Investigation of Rotor Noise in Hover

The noise magnitude and directivity of a four-bladed 1.108-m-radius rotor in hover were measured at a tip Mach number of 0.42, representative of an electric vertical takeoff and landing vehicle rotor. The tonal noise was filtered from the acoustic spectra, allowing for analysis of the separate contributions of tonal and broadband noise. Accompanying computations of the same rotor and operating conditions were performed using the rotorcraft comprehensive analysis system (RCAS) with a viscous vortex particle method (VVPM) for aerodynamic calculations and PSU-WOPWOP with the semi-empirical Brooks, Pope, and Marcolini (BPM) broadband model for predicting acoustics. Measured broadband and tonal noise contributed similar magnitudes to the overall sound pressure level, with broadband always several decibels greater. This is in contrast to conventional helicopter rotors, which are dominated by tonal noise. Simulated trends of tonal noise matched the measurements, though the higher harmonic components of the acoustic spectra were underpredicted by up to 30 dB. The broadband noise was underpredicted by 10 dB at moderate rotor collectives, as the measured noise contained noise from laminar and turbulent boundary layers, while the model predicted predominantly laminar boundary layer–vortex shedding noise. Comparing the time histories of the measured and simulated rotor thrust revealed that the RCAS-VVPM method does not capture the amplitude of the 4/rev component of the thrust and lacks the higher frequency content present in the measured thrust, indicating a possible source of discrepancy in the tonal acoustic predictions. The results of this study show that methods to predict rotor tonal and broadband noise without the use of lengthy computational fluid dynamics computations must be further developed.

Reduction of Aerodynamic Noise radiated from Rectangular Cylinders in Cross Flow

In the present paper the attention is focused on reduction of noise generated from rectangular cylinders in cross flow. The width-to-height ratios of was 3.0. The tube pitch ratio in the transverse direction was 8.0. We measured the aerodynamic sound radiated from the rectangular cylinders in the range of Reynolds numbers from about 10^3 to 10^4. The several high peaks were formed at the spectrum of SPL. The sound power of vortex shedding noise was proportional to the freestream velocity to the power of six. On the other hand, the strange noise occurred at five times the vortex shedding frequency. We have discussed how to counteract the noise generated by rectangular cylinders.

Control of a circular cylinder flow using attached solid/perforated splitter plates at deflection angles

Control of a circular cylinder flow by rear-attached solid/perforated splitters has been experimentally investigated using Particle Image Velocimetry (PIV) and far-field microphones, respectively. The Reynolds number is Re = 2.7 × 104–6.8 × 104 based on the cylinder diameter D, and the effects of splitter length L (L/D = 0–4.0), porosity σ (0%–22%), and deflection angle α (0°–30°) on noise and flow characteristics are reported. The literature seriously lacks the combination effects of these parameters on bluff-body wakes, especially the flow mechanism on noise variations. Acoustic results show that for a solid splitter, the optimal noise reduction of 18 dB is achieved when L/D = 1.0–1.5, whereas when 2.0 ≤ L/D ≤ 4.0, the noise increases rather than decreases, consistent with the literature. At L/D = 1.0, deflecting and/or perforating the splitter would not further reduce the noise and, on the contrary, weaken the noise control efficiency. However, at L/D = 2.0, a rear-half perforation with σ ≥ 18.2% or a deflection angle of α ≥ 30° individually reduces noise by more than 10 dB compared to the bare cylinder. Combining both methods at L/D = 2.0 eliminates vortex-shedding noise when σ ≥ 11.6% and α ≥ 20°. PIV measurements reveal that noise reduction at L/D = 1.0 results from reduced vortex strength in the flow, while noise increase at L/D = 2.0 is attributed to vortex-splitter trailing edge interaction, which is mitigated by the perforation and/or the deflection of the splitter. The study suggests that, in general, appropriately perforating and deflecting longer splitter plates (L/D ≥ 2.0) effectively reduce bluff-body noise, while solid, shorter splitter plates (0.5 ≤ L/D ≤ 1.5) positioned streamwise achieve significant noise reduction. These insights offer valuable noise control strategies for various engineering applications.

Structured porous blunt trailing edge with uniform and non-uniform parameters for vortex shedding noise reduction

Aiming at reducing the blunt trailing edge noise of flat plate, three-dimensional structured porous media with uniform and non-uniform porosities are designed. Acoustics and flow measurement using far-field microphone and particle image velocimetry (PIV) respectively have been implemented in the anechoic wind tunnel to investigate the mechanisms of noise reduction and flow manipulation. The chord-based inflow Reynolds numbers range from 4.8 × 105 to 1.5 × 106. The present work reveals that the regularized structured porous media could provide an effective noise reduction for the blunt trailing edge of flat plate similar to conventional randomized porous media (e.g. metal foam). When the uniform porosity is larger than 0.73, the tonal noise is suppressed significantly and the overall sound pressure level is decreased by 10 dB at least. However, some unexpected additional noise in the mid-to-high frequencies (1500–9000 Hz) occurs, which is associated with primary flow interactions inside the pore structures (cavity resonance). Further, four non-uniform porosity cases are studied to explore the optimization of porosity distribution. Not only is the tonal noise significantly reduced but also the additional noise is lower than that of uniform cases, which indicates that non-uniform porosity has more potential advantages for noise reduction. It is interestingly discovered that for the non-uniform porous trailing edge, the volume-average porosity and distribution trend play important roles respectively in suppressing the tonal noise and the additional mid-to-high-frequency noise. The PIV results demonstrate that 3D printed structured porous media can significantly weaken the vortex intensity of the shear layer and delay the generation of vortex shedding downstream, which are the critical underlying mechanisms for noise reduction. The porosity of uniform cases and the volume-average porosity of non-uniform cases are closely associated with the vortex shedding and the high-values benefit of suppression. And the analysis of wake velocity spectrum indicates that the additional noise is not correlated to the wake flow, but the cavity resonance might be the main reason.

Relationship between vortex shedding noise and remotely-sensed surface pressure fluctuations of a structured porous-coated cylinder

Tonal noise suppression of a cylinder placed in uniform flow has been achieved, to some extent, by coating it with a structured porous material as a form of passive flow and noise control. A previously studied structured porous-coated cylinder is investigated in an anechoic wind tunnel to determine the relationship between the far-field vortex shedding noise and the pressure recorded on the outer porous surface. To date, no experimental studies have been conducted on the surface pressure of any type of porous-coated cylinder. Acoustic measurements are obtained using an equispaced microphone arc array and simultaneously unsteady surface pressure fluctuations are obtained around the cylinder mid-span circumference using remote-sensing techniques. By obtaining simultaneous time-dependent signals, more light is shed on the underlying noise-reduction mechanism of the structured porous-coated cylinder. In this paper, strong relationships between surface pressures and acoustic signals are revealed at the vortex shedding frequency. A spatio-temporal relationship between surface pressure and vortex shedding phenomena is also presented that helps explain the role of the structured porous media in passive flow and noise control.

Spanwise Coherence Measurements of Porous Coated Cylinders in Uniform Flow

A porous coated cylinder (PCC) generates weaker vortex shedding noise than a smooth cylinder placed in uniform flow. A key understanding of how the porous media relates to vortex shedding reduction is linked to the spanwise coherence length of the PCC at the near-wall region. To date, few studies estimate the PCC spanwise coherence to be five-to-six outer diameters in length, yet a more comprehensive study is needed. This paper presents an experimental investigation conducted in an anechoic wind tunnel using a set of polyurethane PCCs tested with varying porosity, thickness and at several Reynolds numbers. Spanwise coherence is estimated at various circumferential angles using (1) flush-mounted microphones distributed along the bare cylinder span, beneath the porous layer, to record surface pressure fluctuations, and (2) two hot-wire anemometry probes in the near-wall flow field, where one probe remains in place and the other is shifted along the span. A transfer function of the porous layer is estimated based on the surface pressure and velocity fluctuations. The results presented here give some new visions of the spatial structure of vortex shedding caused by porous materials, noise generating mechanisms and a fitting formula of spanwise coherence length of PCCs.

On the influence of porous coating thickness and permeability on passive flow and noise control of cylinders

Aeroacoustic studies of porous cylinder coatings, which reduce the vortex shedding noise of cylinders in uniform flow, have received increased attention. To further understand how porous coatings suppress the vortex shedding tone, a numerical and experimental investigation of porous coated cylinders at a Reynolds number of 1.27×105 is presented herein. Two parameters were varied: (1) the ratio of the porous thickness to the bare cylinder diameter, and (2) the airflow resistivity of the porous media. Numerical simulations were performed using the large-eddy simulation approach with the Smagorinsky sub-grid length-scale model, and the Ffowcs-Williams and Hawkings acoustic analogy was used to predict the acoustic far-field pressure field. A numerical investigation of the internal flow field within the porous layer is presented, which provides a better understanding of the influence of the aforementioned parameters on the development of boundary layers within the porous media and the passive flow and noise control capability. Experiments were performed in an aeroacoustic open-jet wind tunnel to validate the acoustic and external flow field results, which typically showed very good agreement with the numerical results. For porous coatings with low airflow resistivity, the thickness of the porous coating, relative to the inner diameter, has a strong influence on passive flow and noise control. Vortex shedding originates from the inner diameter. Conversely, porous coatings with a higher airflow resistivity are less influenced by their thickness, and vortex shedding originates from the outer diameter (porous surface). The complex coupling of these parameters leads to the conclusion that the optimal selection of the thickness and airflow resistivity is typically dependent on the Reynolds number.

Experimental investigation of the laminar boundary layer vortex-shedding noise by an airfoil within a closed-vein wind tunnel

This study concerns the experimental characterization of trailing edge noise, the understanding of which is crucial for mitigating acoustic pollution across major industries. An aeroacoustic experiment is carried out using a closed-vein wind tunnel to investigate the laminar boundary layer vortex-shedding (LBL-VS) noise of a symmetric NACA0021 airfoil in low Reynolds number flows (Re ≤ 163,500). Steady aerodynamic and acoustic measurements are performed, with numerous conditions covered (flow velocity from 10 m/s to 24.5 m/s, airfoil incidence from −10° to 10°). The aerodynamic results reveal that, in the pre-stall regime, the airfoil’ suction side exhibits both a laminar separation bubble (LSB) and a trailing edge detached flow – which both make LBL-VS noise likely to occur. The acoustic results reveal that, when at low speed and moderate incidence, the airfoil emits one to two tones, which can be both attributed to LBL-VS noise. In particular, their respective frequency is seen to scale as the 0.8th power of the flow velocity, whereas varying linearly with the incidence. At higher speeds, these two tones vanish to the profit of other, more intense tonal emissions, whose frequency does not scale with the velocity nor the incidence. These tones are attributed to resonance effects coming from a retroaction of the reverberant environment onto the LBL-VS noise emission, which then locks-on to some of the duct resonant frequencies via an acoustic feedback loop. Revealing indirectly the presence of the pre-existing LBL-VS noise, these resonant tones emerge only when the flow velocity and incidence obey specific conditions, namely a roughly linear relationship.

Laminar Separation Bubble Noise on a Propeller Operating at Low Reynolds Numbers

This paper explains the presence and relevance of noise caused by a laminar separation bubble (LSB) on a propeller operating at a low Reynolds number. Microphone measurements of a propeller with both clean and forced boundary-layer transition blades are carried out in an anechoic wind tunnel by varying the propeller advance ratio from 0 to 0.6, corresponding to a tip Reynolds number ranging from to . The flow behavior on the blade surface and around the propeller is investigated with oil-flow visualizations and particle image velocimetry. At and 0.6, vortex shedding from the LSB causes high-frequency noise that appears as a hump in the far-field noise spectra. Forcing the location of the boundary-layer transition suppresses the LSB and, consequently, the hump, reducing the noise emission of about 5 and 10 dB at and 0.6, respectively. The fact that the hump is caused by LSB vortex shedding noise is further assessed by using a semi-empirical noise model; it shows that the hump is constituted by tones of different amplitudes and frequencies, emitted at different spanwise sections along the blade.

Effect of a porous coating on the vortex shedding noise of a cylinder in turbulent flow

The effect of a porous coating on the vortex shedding noise of a circular cylinder under disturbed inflow conditions was examined experimentally. The measurements were performed on a cylinder coated with an open-porous foam as well as on a bare reference cylinder in a small aeroacoustic wind tunnel using grid-generated turbulence. The measurements were performed at Reynolds numbers based on outer cylinder diameter from 1.2·104 to 4.2·104. In laminar flow, the porous coating leads to a narrowing of the vortex shedding tonal noise peak as well as to a slight shift towards lower Strouhal numbers in this Reynolds number range. In turbulent flow, the present results show the clear trend that this shift towards lower Strouhal numbers is even more pronounced. In addition, the vortex shedding peak level decreases for the porous coated cylinder, an effect that, on average, is also stronger when the inflow is turbulent.

Nonflat-Plate Transition-Serrated Trailing Edge for Airfoil Self-Noise Reduction

Trailing-edge noise is the primary source of airfoil self-noise. Nonflat-plate serrated trailing edge can suppress the broadband trailing-edge noise at high frequencies, while the overall noise reduction is compromised by low-frequency narrowband bluntness-induced noise at the sawtooth root. In this paper, a nonflat-plate transition-serrated trailing edge is proposed to reduce the bluntness-induced noise. The configuration uses a wedge-shaped transition structure within the sawtooth gap to eliminate the bluntness noise. Firstly, the appropriate parameters of the transition structure are determined by numerical simulation. Then, optimal configuration is employed for test verification. Finally, the noise reduction mechanism of the configuration with the best noise reduction is analyzed. Simulation results indicate that the configuration can accelerate the fragmentation of large-scale spanwise vortices at sawtooth root, weakening the vortex shedding noise. Moreover, compared with the nonflat-plate serrated trailing edge, it forms counter-rotating oblique vortices at a position further forward within the sawtooth gap, promoting transverse flow, thereby reducing broadband noise over most frequencies. Experimental results show that the configuration with an appropriate transition structure can afford the highest reduction of 27.2 dB for the bluntness-induced noise and 5.7 dB for the trailing-edge noise.

Trailing-edge noise reduction potential of a locally applied shallow dimpled surface

There is a strong need for airframe noise reduction methods that preserve the aerodynamic performance of lift-generating airframe components. Inspired by the drag reduction studies of van Nesselrooij et al. (2016) and Spalart et al. (2019), the current study explored the trailing-edge noise reduction potential of shallow dimples applied near the trailing edge of a NACA0012 airfoil (approximately the latter 18% of the chord). The effect of shallow dimples on the aerodynamic coefficients is studied by Reynolds Averaged Navier–Stokes (RANS). To investigate the noise reduction credentials of dimples, high-fidelity Overset-Large Eddy Simulations are carried out for a chord based Reynolds number Rec of 4.2 ×105 at zero angle of attack. Near-field analysis revealed that this local application of dimples eliminated the tonal vortex shedding noise, which is otherwise present in the reference configuration without dimples. Furthermore, the application of shallow dimples resulted in the breakdown of the span-wise coherence in the low-to-mid frequency range and a reduction in the convective velocity of the stream-wise convecting eddies. These effects have a favourable impact on the trailing edge noise reduction with a modest 1.8% increase in the overall drag (less than 1 drag count). The far-field sound characteristics revealed that the dimple treatment resulted in the reduction of overall sound pressure levels in the downstream radiation direction (≈ 5.5 dB) and an increase in the upstream radiation direction (≈ 3.5 dB). In a nutshell, this manuscript argues for the inclusion of shallow dimples to the list of potential candidates for passive noise reduction methods, such as serrations, porous materials, and finlets.

Preliminary Design Guidelines for Considering the Vibration and Noise of Low-Speed Axial Fans Due to Profile Vortex Shedding

This paper presents a critical overview on worst-case design scenarios for which low-speed axial flow fans may exhibit an increased risk of blade resonance due to profile vortex shedding. To set up a design example, a circular-arc-cambered plate of 8% relative curvature is investigated in twofold approaches of blade mechanics and aerodynamics. For these purposes, the frequency of the first bending mode of a plate of arbitrary circular camber is expressed by modeling the fan blade as a cantilever beam. Furthermore, an iterative blade design method is developed for checking the risky scenarios for which spanwise and spatially coherent shed vortices, stimulating pronounced vibration and noise, may occur. Coupling these two approaches, cases for vortex-induced blade resonance are set up. Opposing this basis, design guidelines are elaborated upon for avoiding such resonance. Based on the approach presented herein, guidelines are also developed for moderating the annoyance due to the vortex shedding noise.

Passive Control of Vortex Shedding Noise of Circular Cylinders by Passive Air-Jet Blowing

AbstractPassive air-jet blowing is an effective yet simple technique to control flow-induced noise due to vortex shedding of bluff bodies. The current study investigates the effect of the specific ...

CHARACTERIZATION OF TRAILING EDGE BROADBAND NOISE FROM WIND TURBINE BLADES

Wind turbine noise is a critical issue for siting and its operation in offshore and terrestrial conditions. In this work, we analysed trailing edge bluntness vortex shedding noise source for a land based turbine of size 2MW and blade span of 38m using modified BPM noise solver. A regression approach has been implemented to predict the shape function in terms of thickness to chord ratio of aerofoils used for blade. For trailing edge height of 1 % chord, computations for sound power level were done at wind speed of 8m/s, 17 RPM, and showed that present regression approach predicts the noise peak of 78dBA at f ~ 10 kHz. These results were also validated using experiment data from GE 1.5sle, Siemens 2.3MW turbines with blade lengths of 78 -101m and agreed within 2 % at very high frequencies, f > 5kHz. In addition, results from present approach agreed with original BPM and modified BPM by Wei et al at high frequencies, f ~ 10kHz where the bluntness noise becomes predominant. The slope of noise curves from present approach, and modified BPM methods are lower when compared with original BPM and show sound level coincidence with peak Strouhal number of ~ 3.3.

Numerical study on the airfoil self-noise of three owl-based wings with the trailing-edge serrations

Previous publications have summarized that three special morphological structures of owl wing could reduce aerodynamic noise under low Reynolds number flows effectively. However, the coupling noise-reduction mechanism of bionic airfoil with trailing-edge serrations is poorly understood. Furthermore, while the bionic airfoil extracted from natural owl wing shows remarkable noise-reduction characteristics, the shape of the owl-based airfoils reconstructed by different researchers has some differences, which leads to diversity in the potential noise-reduction mechanisms. In this article, three kinds of owl-based airfoils with trailing-edge serrations are investigated to reveal the potential noise-reduction mechanisms, and a clean airfoil based on barn owl is utilized as a reference to make a comparison. The instantaneous flow field and sound field around the three-dimensional serrated airfoils are simulated by using incompressible large eddy simulation coupled with the FW-H equation. The results of unsteady flow field show that the flow field of Owl B exhibits stronger and wider-scale turbulent velocity fluctuation than that of other airfoils, which may be the potential reason for the greater noise generation of Owl B. The scale and magnitude of alternating mean convective velocity distribution dominates the noise-reduction effect of trailing-edge serrations. The noise-reduction characteristic of Owl C outperforms that of Barn owl, which suggests that the trailing-edge serrations can suppress vortex shedding noise of flow field effectively. The trailing-edge serrations mainly suppress the low-frequency noise of the airfoil. The trailing-edge serration can suppress turbulent noise by weakening pressure fluctuation.

Vortex shedding noise and flow mode analysis of cylinder with full/partial porous coating

In this paper, the characteristics of vortex shedding and noise of the circular cylinder with full and partial porous coating are studied numerically to learn the optimal structure design of coating structure. Mode analysis methods including Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) are employed to identify the potential mechanism of flow modulation by porous coating. The present results show: 1) When the thickness of full porous coating is within a reasonable range (the relative thickness is 0.2 to 0.8), the vortex shedding and noise is impacted significantly. For the partial porous coating, the noise reduction can be achieved up to 7.1 dB when 1/4 of the cylinder surface is coated. 2) Change of circumferential positions/range of coating have different effects. The frequency of the vortex shedding can be reduced obviously when the position before separation point (θ = ±45°∼±90°) is coated. And coating around the separation point (θ = ±90°∼±135°) has effect on the attenuating the amplitude of vortex shedding. 3) The POD mode analysis reveals that porous coating increases the dominant modes energy associated with lift and decreases the modes energy ratio associated with drag, which is the main reason for the reduction of cylinder drag. 4) The DMD analysis indicates that porous coating can improve the stability of the flow field which consequently reduces the flow-induced noise.

Vortex Shedding Noise from Finite, Wall-Mounted, Circular Cylinders Modified with Porous Material

The aerodynamic noise generated by a circular cylinder in a crossflow is a classical problem in aeroacoustics, which can be found in various practical applications, such as parts of the landing gear of airplanes, train pantographs, and antennas of vehicles. The noise is especially disturbing because it contains strong tonal contributions due to the regular vortex shedding at the cylinder. If the cylinders are not two-dimensional, but finite and wall-mounted at one end and free at the other, additional flow phenomena occur at both the tip and the wall junction, which can lead to additional noise contributions. One method to reduce the aerodynamic noise from such cylinders is to cover them with a flow-permeable, porous material. The present paper describes an experimental study on the effect of porous covers on the reduction of noise generated by wall-mounted, finite, circular cylinders. Thereby, special focus was put on the identification of both the optimum position and the optimum extent of the porous cover regarding a maximum noise reduction. The experiments included both measurements of the far field noise as well as flow measurements in the wake of the cylinders. They were conducted in an aeroacoustic open jet wind tunnel at subsonic flow speeds. It was found that a placement of porous material at the wall end is more efficient in reducing Kármán vortex shedding noise than a placement of porous material at the free end.

Vortex Induced Aerodynamic Noise Generation from Wind Turbine Blades

An important aerodynamic noise source from lifting surface occurs from trailing edge of an aerofoil as found in wind turbine blades. In this work, semi-empirical method proposed by Brookes, Pope, Marcolini is applied to evaluate trailing edge bluntness vortex shedding noise source. For low Mach number flows (0.1884) and moderate to high chord Reynolds number, 4.73 × 105 - 3.35 × 106, change in sound power level was assessed for trailing edge thicknesses in terms of 0.1%, 0.5% and 1% chord lengths at wind speeds of 8 m/s, 10 m/s. For overall change of trailing edge thickness from 0.1% to 1% chord, an increase in noise levels up to 50 dB was found at low frequencies, while a decrease up to 30 dB was found between mid-band to high frequencies of spectra.

Experimental investigation on noise emissions of an airfoil with non-flat plate trailing edge serrations

This paper presents an experimental study on the aeroacoustic characteristics of a NACA 0018 with nonflat plate-type serrated trailing edge at Reynolds numbers of 500000 in an open-jet wind tunnel. The experimental results show that noise reduction can be achieved in the moderate frequency range for nonflat plate type triangular serrated trailing edges, but at the cost of producing narrowband tone noise. The level of the vortex shedding noise declines when serration length 2 h decreases. It is imperative that the vortex shedding tone noise be completely suppressed when a novel slanting root serrated trailing edge is adopted. Meanwhile, the ability of broadband noise reduction is maintained.