Blunt Trailing Edge Research Articles

Trailing edge bluntness noise reduction using porous treatments

This paper provides an experimental investigation of the noise generated from a blunt flat plate and the possibility of the control of noise at source using porous trailing edges. The noise generation characteristics have been examined using a flat plate equipped with surface pressure transducers and far-field microphones and was performed in an anechoic open jet wind tunnel. Two types of porous materials with different porosities and permeability constants were tested. Simultaneous near-field and far-field noise measurements, as well as the boundary layer and wake flow measurements, have been carried out to better understand the effects of the porous treatment to the flow-field and the noise generation mechanism of the plate. Results have shown that the use of porous trailing edges can generally lead to the effective suppression of both the tonal noise, due to the vortex shedding from the blunt trailing edge and also the broadband noise of the plate. The level of the noise reduction was also found to be dependent on the porosity and permeability of the porous materials. The directivity patterns of the radiated noise have shown significant noise reduction at the vortex shedding frequencies. The flow measurement results have shown that the use of porous trailing edges leads to the reduction of flow acceleration over the blunt edges, and subsequently the delay and weakening of the vortex shedding. The results presented in this paper provide an impetus for further experimental and numerical studies on the use of porous treatments for the suppression of aerodynamically generated noise at source.

Dynamic hydroelastic response of a surface-piercing strut in waves and ventilated flows

The objective of this work is to study the effects of waves and ventilation on the dynamic hydroelastic response of a surface-piercing strut via towing tank studies. The experimental studies are especially designed to test the hypothesis that flow conditions affect the modal response and hence structural dynamics, which in turn affect the hydrodynamic response through fluid–structure interaction, particularly near regions of mode localization such as frequency coalescence. The results showed that the modal frequencies decrease with increasing submergence, and are higher in fully ventilated flow compared to fully wetted flow. Regular, non-breaking waves lead to simple harmonic oscillations about the mean values at the encountered wave frequency for the slowly varying component of the hydrodynamic loads and tip deformations. The spectral response of the fast fluctuating component of the hydrodynamic loads and tip deformations showed peaks at the modal frequencies and vortex shedding frequencies (off the blunt trailing edge of the strut). Significant dynamic load amplifications and flow-induced vibrations were observed when the second and third modal frequencies coalesced at a submerged aspect ratio of two in fully wetted flow. In fully ventilated flow, the second and third modes separated enough to result in drastically reduced dynamic load fluctuations and flow-induced vibrations. When the submergence decreased, the separation between the modal frequencies increased, which avoided frequency coalescence in both fully wetted and fully ventilated flows. The results suggest that for cases where the wave encountered frequency is well separated from the modal frequencies, the spectral response of the fast fluctuating component of the hydrodynamic loads and tip deformations are governed by the structural response, and not by wave conditions.

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.

The mode B structure of streamwise vortices in the wake of a two-dimensional blunt trailing edge

The structure of streamwise vortices that arise due to secondary instabilities in the wake of a two-dimensional blunt body with a chord-to-thickness ratio of 12.5 was investigated using high-speed stereoscopic particle image velocimetry. Reynolds numbers spanning an order of magnitude from to 25 800 were considered, where is the height of the blunt trailing edge. A modified two-dimensional -criterion ( ) was applied to identify the streamwise vortices. The wavelength of the streamwise vortices, defined as the spanwise distance between adjacent streamwise vortex pairs in the wake, was investigated by applying an autocorrelation algorithm to snapshots of . The most probable wavelength was found to range from to with increasing , and the mean wavelengths increased from to . These wavelength values appeared to increase asymptotically. Visual inspection and cross-correlation analyses based on showed that the streamwise vortices maintain their directions of rotation during primary shedding cycles. The latter analysis was carried out at low because of a large amount of wake distortion and the absence of time-resolved data at high . The characteristics of the streamwise vortex structure found here match those of mode B, which, at similar , dominates the wakes of circular and square cylinders and has also recently been shown to exist in the wake of an elongated blunt body with a larger chord-to-thickness ratio of 46.5 (Gibeau et al., J. Fluid Mech., vol. 846, 2018, pp. 578–604).

The hydroelastic response of a surface-piercing hydrofoil in multi-phase flows. Part 1. Passive hydroelasticity

Compliant lift-generating surfaces have widespread applications as marine propellers, hydrofoils and control surfaces, and the fluid–structure interactions (FSI) of such systems have important effects upon their performance and stability. Multi-phase flows like cavitation and ventilation alter the hydrodynamic and hydroelastic behaviours of lifting surfaces in ways that are not yet completely understood. This paper describes experiments on one rigid and two flexible variants of a vertical surface-piercing hydrofoil in wetted, ventilating and cavitating conditions. Tests were conducted in a towing tank and a free-surface cavitation channel. This work, which is Part 1 of a two-part series, examines the passive, or flow-induced, fluid–structure interactions of the hydrofoils. Four characteristic flow regimes are described: fully wetted, partially ventilated, partially cavitating and fully ventilated. Hydroelastic coupling is shown to increase the hydrodynamic lift and yawing moments across all four flow regimes by augmenting the effective angle of attack. The effective angle of attack, which was derived using a beam model to account for the effect of spanwise twisting deflections, effectively collapses the hydrodynamic load coefficients for the three hydrofoils. A generalized cavitation parameter, using the effective angle of attack, is used to collapse the lift and moment coefficients for all trials at a single immersed aspect ratio, smoothly bridging the four distinct flow regimes. None of the hydrofoils approached the static divergence condition, which occurs when the hydrodynamic stiffness negates the structural stiffness, but theory and experiments both show that ventilation increases the divergence speed by reducing the hydrodynamic twisting moment about the elastic axis. Coherent vortex shedding from the blunt trailing edge of the hydrofoil causes vortex-induced vibration at an approximately constant Strouhal number of 0.275 (based on the trailing edge thickness), and leads to amplified response at lock-in, when the vortex-shedding frequency approaches one of the resonant modal frequencies of the coupled fluid–structure system.

Effects of non-uniform permeability on vortex shedding and noise control of blunt trailing edge

In this paper, the characteristics of the vortex shedding and noise of the blunt trailing edge with different porous permeable segment are studied by numerical simulation. The permeability distribution is adjusted through continuously changing the viscous drag of porous region to achieve uniformity and non-uniformity. The simulation results firstly show that the porous trailing edge can stabilize the free shear layer in a certain range of viscous drag coefficient to suppress the vortex shedding and reduce the overall noise level significantly. The optimized non-uniform permeability can further reduce the noise by 3dB than that of uniform permeability for porous blunt trailing edge. The discrete strip-shaped porous blunt trailing edge is proposed for more practical engineering application. It is found that the noise level is increased by 2 dB rather than optimal function distribution. However, the discrete strip porous trailing edge still has significant noise reduction capacity with respect to solid blunt trailing edge. The three-dimensional simulation demonstrates further that the non-uniform permeable trailing edge behaves more potential than uniform permeable trailing edge and reveals the crucial spanwise synergy effects by porous treatment.

FSI analysis of francis-99 hydrofoil employing SBES model to adequately predict vortex shedding

The added effects from the fluid on a structure submerged in water significantly affect its dynamic response. Since the hydraulic turbine runner is geometrically complex and involves complicated flow phenomena, the research on simple hydrofoil offers a unique opportunity to investigate added effects and mutual interaction of the elastic structure and vortical flow. For this purpose, the fluid structure interaction of Francis-99 hydrofoil was analysed using the Stress Blended Eddy Simulation (SBES). Advantage of this hybrid RANS-LES turbulence model over RANS models is shown by its enhanced ability to represent vortex shedding. The results of modal sensitivity analysis showed, that fillets of the fixed hydrofoil have negligible influence on the natural frequencies of the hydrofoil and therefore the simplified geometry was used. The modal analysis of fully fixed hydrofoil both in the air and submerged in water were carried out to investigate the added mass effect. Moreover, the hydrodynamic damping for various flow velocities was also investigated for the first bending mode. Overall results are complemented by sensitivity analysis of time step size and mesh for both structural and fluid domains. The results showed that the computed damping ratio above the lock-in and vortex shedding frequency at lock-in are largely underestimated. Therefore, the geometry with blunt trailing edge was additionally tested.

Active control of vortex shedding from a blunt trailing edge using oscillating piezoelectric flaps

Active control of vortex shedding from a blunt trailing edge is achieved experimentally using piezoelectric actuators. The system can suppress and amplify the vortex shedding pattern in the wake, as well as force near-wake symmetry. An application of closed-loop control is also demonstrated.

Numerical investigation into the effect of the trailing edge shape on added mass and hydrodynamic damping for a hydrofoil

Engineering applications prove that the modification of the trailing edge shape of a blade can effectively reduce the amplitude of the vortex-induced vibration, but the mechanism behind it is still not fully understood. In this investigation, unsteady CFD and two-way FSI numerical simulation methods are performed on three-dimensional (3D) NACA 0009 hydrofoils with blunt and Donaldson trailing edges. The vortex shedding formation process, the added mass and the hydrodynamic damping ratio at different flow velocities are investigated. The relative deviations in the vortex shedding frequency, the natural frequency in water, and the hydrodynamic damping ratio obtained by simulation are within 12.88%, 4.62% and 8.82%, respectively, compared with the experimental results. As the flow velocity increases, the added mass for blunt and Donaldson trailing edge hydrofoils remains almost constant and gradually decreases, respectively. The hydrodynamic damping ratio of the hydrofoil increases linearly as the flow velocity increases with both blunt and Donaldson trailing edges involving a slope of 0.0032 and 0.005, respectively. At a flow velocity of 20 m/s, the maximum amplitude is reduced to 57% after Donaldson trailing edge modification. The results show that the hydrodynamic damping ratio of the Donaldson trailing edge is significantly increased when the flow velocity is greater than 20 m/s. The reason for this is the collision between the upper and lower trailing edge vortexes, which leads to the faster dissipation of vortex intensity.

Design and Analysis of Rotor/Wing Flap for Canard Rotor/Wing Aircraft

In the present study, a kind of blunt trailing edge airfoil with its flap is designed for the purpose of reducing drags of blunt leading and trailing edge airfoils which are used for rotor/wing of CRW. The aerodynamic characteristics of designed flaps and elliptic airfoil which has the same thickness are numerically investigated based on the Reynolds-averaged Navier–Stokes equations and the SST k-ω two-equation turbulence model. The comparisons of aerodynamic force and moment coefficients show that, the lift characteristic is improved for designed flaps, but the drag will not be reduced if the flap has great bending trailing edge, and small bending trailing edge design is suitable for this kind of flap, and it will reduce the drag and improve the longitudinal stability characteristics.

Numerical Investigation of Added Mass and Hydrodynamic Damping on a Blunt Trailing Edge Hydrofoil

Added mass and hydrodynamic damping play significant roles in fluid-structure interaction (FSI) in hydraulic turbines. Added mass can reduce natural frequencies, while hydrodynamic damping could result in a higher amplitude decay speed of the vibration. In order to quantify the added mass and hydrodynamic damping of a three-dimensional (3D) NACA 0009 hydrofoil with a blunt trailing edge, a two-way FSI simulation method was employed. The effects of grid scale, time-step, turbulence model, exciting force, and numerical damping on the calculation accuracy of the two-way FSI numerical simulation were analyzed in great detail through comparison with the previously published experimental data. Hydraulic force was obtained by using a transitional shear stress transport model at the flow region of the Reynolds number ReL = 0.2 × 106–2 × 106. The vortex shedding frequency, the natural frequency of the first-order bending mode in water, and the hydrodynamic damping ratio obtained from the numerical simulations agree well with the experimental data, with maximum deviations in 6.12%, 4.53%, and 8.82%, respectively. As the flow velocity increases, the natural frequency may not significantly change, while the added mass coefficient gradually increases, considering the effect of added stiffness. Above the first-order bending mode lock-in region, the results indicate that the first-order bending mode hydrodynamic damping ratio increases linearly with velocity. The present numerical achievements offer a higher level of accuracy for predicting the added mass and hydrodynamic damping characteristics of a hydrofoil.

PIV measurements and CFD simulations of a hydrofoil at lock-in

As part of an ongoing investigation into the mitigation of vortex induced vibrations of hydrofoils, a combined experimental and numerical study of the fluid-structure interactions and wake of a hydrofoil at lock-in has been conducted at the Waterpower laboratory of the Norwegian University of Science and Technology. The hydrofoil has a blunt trailing edge and Von Karman vortex shedding induces a lock-in effect at a chord based Reynolds number of about 2,7·106. The present paper presents the initial measurements of vortex shedding frequencies going through lock-in, along with CFD simulations at lock-off conditions as well as some empirical estimates of vortex shedding. Experimentally the hydrofoil wake was studied in detail using particle image velocimetry (PIV). Hydrofoil vibration frequencies were measured by both a strain gauge positioned near the trailing edge of the foil as well as by a laser doppler vibrometer (LD-V). Numerically the phenomena was simulated using ANSYS CFX. Several different turbulence models was tested, from the two-equation standard k − ϵ model to the scale adaptive SST-SAS model, with considerably different results. It is observed that the vibrations induced at lock-in considerably shifts and reduces the hydrofoil wake velocity deficit. Further, the CFD results suggest that the driving parameter influencing the shedding frequency is the cross flow separation distance at the trailing edge.

Boat-tail effects on the global wake dynamics of a flat-backed body with rectangular section

A three-dimensional body of rectangular section with a blunt trailing edge is studied for various angles of after-body boat-tailing. The Reynolds number of the flow is Re=4.0×105 and the body is in the vicinity of the ground. It is found that the wake dynamics is strongly dependent on the coupling of top and bottom boat-tail angles, triggering either long-time bi-stability between two mirror static asymmetric states or an anti-symmetric periodic mode, both leading to large cross-flow force fluctuations. The transition is shown to depend on the aspect ratio of the vertical base and not on the flow orientations imposed by the coupling of angles. Within the limitation of the aspect ratio for which the static states persist and dominate the wake dynamics, as boat-tails are installed at the after-body, they generate a vertical base pressure gradient component on which the static states adapt according to the mechanism proposed by Bonnavion and Cadot (2018).

Effect of Tailing-Edge Thickness on Aerodynamic Noise for Wind Turbine Airfoil

The influence of wind turbine airfoil trailing edge thickness on aerodynamics and aerodynamic noise characteristics was studied using the computational fluid dynamics (CFD)/ Ffowcs Williams–Hawkings (FW–H) method in the present work. First, the airfoil of a DU97-W-300-flatback airfoil was chosen as the research object, and numerical method validation was performed. Three kinds of turbulence calculation methods (unsteady Reynolds average Navier-Stokes (URANS), detached eddy simulation (DES), and large eddy simulation (LES)) were investigated in detail, and three sets of grid scales were used to study the impact of the airfoil on the aerodynamic noise. Secondly, the airfoil trailing edge thickness was changed, and the impact of trailing edge thickness on aerodynamics and aerodynamic noise was investigated. Results show that three kinds of turbulence calculation methods yield the same sound pressure frequency, and the magnitude of the sound pressure level (SPL) corresponding to the mean frequency is almost the same. The calculation of the SPL of the peak value and the experimental results can match well with each other, but the calculated core frequency is slightly lower than the experimental frequency. The results of URANS and DES are closer to each other with a changing trend of SPL, and the consequences of the DES calculation are closer to the experimental results. From the comparison of two airfoils, the blunt trailing edge (BTE) airfoil has higher lift and drag coefficients than the original airfoil. The basic frequency of lift coefficients of the BTE airfoil is less than that of the original airfoil. It is demonstrated that the trailing vortex shedding frequency of the original airfoil is higher than that of the BTE airfoil. At a small angle of attack (AOA), the distribution of SPL for the original airfoil exhibits low frequency characteristics, while, at high AOA, the wide frequency characteristic is presented. For the BTE airfoil, the distribution of SPL exhibits low frequency characteristics for the range of the AOA. The maximum AOA of SPL is 4° and the minimum AOA of SPL is 15°, while, for the original airfoil, the maximum AOA of SPL is 19°, and the minimum AOA is 8°. For most AOAs, the SPL of the BTE airfoil is larger than that of the original airfoil.

Computation of Aerodynamic and Acoustic Characteristics of NACA0012 Airfoil Using the Zonal RANS–IDDES Approach

The presented results of computations of the aerodynamic and acoustic characteristics of an isolated symmetric NACA0012 airfoil with a blunt trailing edge in a subsonic homogeneous viscous perfect gas flow at zero angle of attack are obtained using the MP5 modified finite-difference scheme proposed in this study. The computations are accomplished within the zonal RANS–IDDES approach engaging the basic RANS Spalart–Allmaras model. A synthetic turbulence generator with a three-dimensional source is used to create turbulent content at the entrance to the IDDES domain. A series of model problems are considered, whose solutions allowed tuning the parameters of the proposed difference scheme and solving some methodological problems related to processing the simulation results.

A flow separation model for hydrofoil, propeller and duct sections with blunt trailing edges

The panel method does not apply to hydrofoils, propellers and ducts with blunt trailing edges due to the flow separation downstream. In this paper, a model is proposed to represent the flow separation with an extension, and a low-order panel method coupled with a boundary layer solver is used. The criteria of zero lift and zero moment are adopted to determine the end of the extension zone, and flow separation criteria are used to determine the starting points on either side of the section. The model is applied to hydrofoil, bare duct and ducted propeller sections with blunt trailing edges. The pressure distributions and skin frictions along the hydrofoils and ducts correlate well with those from the Reynolds-averaged Navier–Stokes method. The thrust and torque of the propeller agree much better with experimental measurements when the extension is determined from this model rather than choosing random locations. This model requires much less computational effort while preserving high accuracy, and thus can be used reliably in designing and analysing hydrofoils and propeller ducts with blunt trailing edges.

Modal analysis of the laminar boundary layer instability and tonal noise of an airfoil at Reynolds number 150,000

A direct numerical simulation of the flow field around a controlled-diffusion airfoil within an anechoic wind-tunnel at [Formula: see text] incidence and a high Reynolds number of [Formula: see text] is performed for the first time using a lattice Boltzmann method. The simulation compares favorably with experimental measurements of wall-pressure, wake statistics, and far-field sound. The simulation noticeably captures experimentally observed high-amplitude acoustic tones that rise above a broadband hump. Both noise components are related to a breathing of a recirculation bubble formed around 65–70% of the chord, and to Kelvin-Helmholtz instabilities in the separated shear layer that yield rollers that break down into turbulent vortices whose diffraction at the trailing edge produces a strong dipole acoustic field. A wavelet analysis of the wall-pressure signals combined with some flow visualization has shown that the flow statistics are dominated by intense events caused by intermittent, large and intense bursting rollers. Several modal analyses of these events are performed on both the wall-pressure fluctuations and the span averaged flow field in order to analyse the boundary layer instability which triggers the typical sharp tones over a broadband hump in airfoil noise. A suction side Kelvin-Helmholtz instability is observed to be coupled with a pressure side vortex shedding induced by the sudden transition to turbulence and the blunt trailing edge.

Unstable wake dynamics of rectangular flat-backed bluff bodies with inclination and ground proximity

The paper investigates experimentally the global wake dynamics of a simplified three-dimensional ground vehicle at a Reynolds number of $Re\simeq 4.0\times 10^{5}$. The after-body has a blunt rectangular trailing edge leading to a massive flow separation. Both the inclination (yaw and pitch angles) and the distance to the ground (ground clearance) are accurately adjustable. Two different aspect ratios of the rectangular base are considered; wider than it is tall (minor axis perpendicular to the ground) and taller than it is wide (major axis perpendicular to the ground). Measurements of the spatial distribution of the pressure at the base and velocity fields in the wake are used as topological indicators of the flow. Sensitivity analyses of the base pressure gradient expressed in polar form (modulus and phase) varying ground clearance, yaw and pitch are performed. Above a critical ground clearance and whatever the inclination is, the modulus is always found to be large due to the permanent static symmetry-breaking instability, and slightly smaller when aligned with the minor axis of the base rather than when aligned with the major axis. The instability can be characterized with a unique wake mode, quantified by this modulus (asymmetry strength) and a phase (wake orientation) which is the key ingredient of the global wake dynamics. An additional deep rear cavity that suppresses the static instability allows a basic flow to be characterized. It is shown that both the inclination and the ground clearance constrain the phase dynamics of the unstable wake in such way that the component of the pressure gradient aligned with the minor axis of the rectangular base equals that of the basic flow. Meanwhile, the other component related to the major axis adjusts to preserve the large modulus imposed by the instability. In most cases, the dynamics explores only two possible opposite values of the component along the major axis. Their respective probability depends on the geometrical environment of the wake: base shape, body inclination, ground proximity and body supports. An expression for the lateral force coefficients taking into account the wake instability is proposed.

Aerodynamic noise reduction by plasma actuators for a flat plate with blunt trailing edge

An experimental study of active control of the vortex shedding narrowband tonal noise radiated near the blunt trailing edge of a flat plate with elliptical leading edge was performed using three different configurations of the single dielectric barrier discharge (DBD) plasma actuators. These devices can produce electric winds in the tangential, downward and spanwise directions, respectively, near the blunt trailing edge. Acoustics and flow measurements were carried out simultaneously at Reynolds numbers between 0.75 × 105 and 4 × 105, based on the flat plate chord length, inside an aeroacoustic facility. The range of alternating-current (AC) input voltages to these plasma actuators was relatively low at < 5 kV. The “tangential” plasma actuator is not very effective in the suppression of vortex shedding tonal noise (maximum 1–2 dB reduction), although the spatial distribution of the wake coherent modes calculated from the proper orthogonal decomposition becomes more compact than that produced by the baseline plasma off case, resulting in a shift of the tone frequency to a higher value. The “downward” plasma actuator can suppress the vortex shedding noise almost completely at the tone frequency (about 15 dB reduction at input voltage of 4.2 kV). The mechanism is related to the induced plasma jet acting as a virtual barrier to inhibit the interaction between the upper and lower separating shear layers, and to delay the formation of the vortex shedding. The “spanwise” plasma actuator, which can project array of streamwise vortices into the wake and compartmentalise the vortex shedding across the span, demonstrated a more superior tonal noise reduction capability at low input voltage (about 12 dB reduction at 3.0 kV). It is found that the plasma-induced jet magnitudes between 9 and 10% and 7% of the freestream velocity for the downward and spanwise plasma actuators, respectively, are already sufficient to achieve an effective reduction of the vortex shedding tonal noise.

Trailing-edge flow and noise control using porous treatments

This paper is concerned with the application of porous treatments as a means of flow and aerodynamic noise reduction. An extensive experimental investigation is undertaken to study the effects of flow interaction with porous media, in particular in the context of the manipulation of flow over blunt trailing edges and attenuation of vortex shedding. Comprehensive boundary layer and wake measurements have been carried out for a long flat plate with solid and porous blunt trailing edges. Unsteady velocity and surface pressure measurements have also been performed to gain an in-depth understanding of the changes to the energy–frequency content and coherence of the boundary layer and wake structures as a result of the flow interaction with a porous treatment. Results have shown that permeable treatments can effectively delay the vortex shedding and stabilize the flow over the blunt edge via mechanisms involving flow penetration into the porous medium and discharge into the near-wake region. It has also been shown that the porous treatment can effectively destroy the spanwise coherence of the boundary layer structures and suppress the velocity and pressure coherence, particularly at the vortex shedding frequency. The flow–porous scrubbing and its effects on the near-wall and large coherent structures have also been studied. The emergence of a quasi-periodic recirculating flow field inside highly permeable surface treatments has also been investigated. Finally, the paper has identified several important mechanisms concerning the application of porous treatments for aerodynamic and aeroacoustic purposes, which can help more effective and tailored designs for specific applications.