Trailing Edge Research Articles

Fluid-Structure Interactions and Aeroacoustic coupling of Airfoil with Flexible Membrane(s)

In this paper, fluid-structure interactions of a NACA 0012 airfoil mounted with short flexible membrane(s) and its coupling effect on airfoil aeroacoustics are presented. A time-domain direct aeroacoustic simulation coupled with structural dynamics is carried out at a low Reynolds number of 50,000 to explore the aeroacoustic-structural interactions. Two different airfoil configurations based on single and dual membranes are analyzed. The membrane deflections and their impact on the flow field are characterized in wavenumber-frequency domain to analyze the structural dynamics due to flow unsteadiness within the laminar boundary layer and the resulting acoustic waves emanating from the airfoil trailing edge. A strong correlation between the membrane displacement and downstream propagating flow is observed for all configurations whereas the correlation is considerably weakened between the membrane displacement and upstream acoustic waves which ultimately results in the airfoil self-noise reduction without affecting the airfoil aerodynamics. The extent of noise reduction for dual membrane airfoil configuration is observed to be considerably higher than the single membrane airfoil configuration which corresponds to a much lower correlation among the upstream propagating acoustic waves and membrane deflection for both the membranes and redistribution of upstream flow energy into different frequencies.

Computational and Experimental Studies of a Low-Noise Aircraft with a Laminar Wing and Engines above the Trailing Edge of the Wing

Computational and Experimental Studies of a Low-Noise Aircraft with a Laminar Wing and Engines above the Trailing Edge of the Wing

Acoustic Shielding and Scattering Effects of a Propeller Mounted Above a Flat Plate

To reduce the overall noise pollution generated by aircraft, design choices often attempt to incorporate acoustic shielding often using aircraft wings to minimise the noise generated by engines and propellers. By mounting a propeller above a wing, increasing the noise shielding, acoustic scattering effects can be introduced which change the directivity and magnitude of the noise generated. Experiments were carried out to investigate the acoustic shielding and scattering effects of a propeller mounted above a flat plate trailing edge. Two propellers were tested at constant rotational rates of 5000 RPM and 7000 RPM respectively. The tests were conducted within an anechoic wind tunnel facility with loading and far-field noise data collected by microphone arrays at a parametric set of propeller locations relative to the flat plate trailing edge. Analysis of the spectral content of the noise along with the tonal content of the signals are presented. The levels of coherence and cross spectrum phase are considered to give greater insights into the levels of noise attenuation due to shielding and scattering effects. Analysis of the acoustic data shows locations with notable levels of noise attenuation for both the tonal and broadband noise content.

An experimental investigation into the influence of installed passively controlled jet flows on wall-pressure fluctuations

Jet-surface interaction(JSI) represents a significant noise problem in the installation of modern ultra-high bypass ratio turbofan engines. The use of chevron nozzles is known to reduce low-frequency jet mixing noise(JM) by increasing the velocity gradient in the jet shear layer. This effect is expected to influence the jet flow in the vicinity of the wing and to modify the jet-surface interaction noise. To clarify the physics of these two(JSI and JM) noise source mechanisms, an extensive experimental investigation has been conducted in the anechoic chamber of the Doak Laboratory at the University of Southampton. Various measurements were carried out on an isolated subsonic jet and an installed beneath a 2D NACA4415 airfoil using different nozzle shapes and passive vortex generator geometries. The wall-pressure field on the airfoil surface was investigated using a set of wall-pressure transducers flush-mounted in the streamwise and spanwise directions on the pressure side of the airfoil. The unsteady wall-pressure data were analysed in both time and frequency domains to assess changes in: 1) surface trailing edge spanwise correlation length, 2) streamwise convection velocity of the hydrodynamic pressure field, and 3) modification of the hydrodynamic pressure field in the vicinity of the surface trailing edge.

Aerodynamic Performance Improvement on NACA 4415 Airfoil by Using Cavity

This study uses a numerical method to analyze the cavity's use on the airfoil's trailing edge and the aerodynamic effects it generates. The type of airfoil used is NACA 4415. The variations in the Reynolds number examined in this study are 2×105 and 3×105. The governing equation is the Reynolds Averaged Navier-Stokes paired with the k-ε turbulence model. This study concludes that the cavity can increase Cl in the airfoil but cannot delay the stall. The increase in Cd is also a negative effect of using a cavity in the airfoil. The cavity can increase Cl by increasing the pressure on the lower side near the trailing edge. Meanwhile, the cavity increases Cd because it creates a separation of the fluid flow, forming a vortex when viewed in a streamlined form of fluid flow.

Design and Shape Monitoring of a Morphing Wing Trailing Edge

The morphing wing trailing edge is an attractive aviation structure due to its shape-adaptive ability, which can effectively improve the aerodynamic performance of an aircraft throughout the whole flight. In this paper, a mechanical solution for a variable camber trailing edge (VCTE) based on a multi-block rotating rib is proposed. Parametric optimizations are conducted to achieve the smooth and continuous deformation of the morphing rib. A prototype is designed according to the optimized results. In addition, the deformations of the trailing edge are monitored via an indirect method using a fiber Bragg grating (FBG) sensor beam. Finally, ground tests are performed to investigate the morphing capacity of the VCTE and the shape monitoring ability of the proposed method. Our results indicate that a maximum deflection range from 5° upward to 15° downward can be obtained for the VCTE and the indirect sensing system can satisfactorily monitor the deformation of the trailing edge.

Effect of surfaces roughness of a staggered tube bank in cross flow with air on heat transfer and pressure drop

Many options are devoted to enhancing heat transfer alongside lowering the pressure drop inside the tube banks in cross flow with air. Among these, is attaching splitter plates (SPs) to the trailing edges of the tubes as extended surfaces. That already proved from literature its effective use. Another proposed option is to increase the surface's roughness, with limited available data from literature. Thus, the present research was established to study the effect of using different surface roughness of a staggered tube-bank in cross flow with air on heat transferred and pressure drop besides the comparison with installing SPs. To this aim, a full 3D CFD model is developed to study the two mentioned options without any symmetrical boundary conditions. The study extended to include the effect of Remax changes from 5000 to 100,000. While three surface relative roughness; ks/D = 0 (smooth), ks/D = 0.01, and ks/D = 0.02 are investigated. The results confirmed that increasing heat transfer surface roughness helps to augment the total heat transferred. While a limited pressure drop increase is reported compared to installing SPs. Combining both; increasing surface roughness, and, adding SPs, led to multiplying the total heat transfer rate.

Flexible blades to improve Darrieus turbine performance and reduce cost

Low cost “sailwing” aerofoils of flexible nylon mounted between a rigid leading edge of circular cross section and a rigid flat trailing edge were investigated in the 1970s and 1980s. Wind tunnel measurements of lift and drag of these aerofoils over the full range of angles of attack were published, but no predictions were made for a complete wind turbine incorporating them. Another paper stated that a sailwing turbine with aerofoils incorporating pivoting aerodynamic profile leading edges was found to exert considerable starting torque. In the present paper, published lift and drag data have been used to predict the performance of a small Darrieus hydrokinetic or wind turbine with sailwing aerofoils, using a standard multiple streamtube model over the full range of tip-speed ratios (TSRs) from 0.5 to 3. To provide a fair comparison, this method is also used to predict the performance of the turbine with symmetrical NACA0018 and cambered Gö420 blade profiles. At all examined TSRs, average torque and power of the turbine with flexible blades are higher than the turbines with rigid blades. For instance, at TSR of 2.5, the flexible blade turbine achieved 55 % and 165 % higher average torque than the turbines with symmetrical and cambered blades, respectively. This finding is attributed to the fact that the flexible blade camber reverses with reversing incidence, producing high lift coefficients at low incidence. Drag is higher for the flexible aerofoil, probably due to separation at the circular cross section leading edge, and measures are suggested to improve this. The flexible aerofoils are very cheap and simple to fabricate, making them an attractive option for affordable hydrokinetic turbines.

An Experimental Study on Rotor Aerodynamic Noise Control Based on Active Flap Control

Reducing rotor aerodynamic noise is an important challenge in helicopter design. Active flap control (AFC) on rotors is an effective noise reduction method. It changes the segment airfoil shape, aerodynamic load distribution, and the wake path of the rotor flow by adding trailing edge flaps (TEFs). Although AFC noise reduction control is easily simulated, the relevant experiments have not been widely conducted due to test technical problems and limited financial support. The acoustic characteristics of the AFC-equipped rotor, such as the placement of TEFs for noise reduction and whether multiple winglets can provide a better effect than single winglets, have not been verified in previous experiments. In this work, an AFC-equipped rotor with two TEFs was designed, and its acoustic properties were tested in the FL-17 acoustic wind tunnel with microphone arrays in the far field. The results showed that the noise reduction effect of AFC was closely related to the control frequency and phase. Increasing the control phase could move the reduction region toward the azimuth-decreasing region for far-field noise. The noise reduction in a single outboard TEF was better than that in a single inboard TEF, while the dual-TEF model performed better. In this experiment, the average noise reduction in the observation point at the lower front of the rotor could be more than 3 dB, and the maximum noise reduction could be 6.2 dB.

Numerical study on the mechanism of fluid energy transfer in an axial flow pump impeller under the rotating coordinate system

It is a necessary condition to obtain the fluid movement law and energy transfer and loss mechanism in the impeller of the axial pump for achieving an efficient and accurate design of the axial flow pump. Based on the shear stress transport k-ω turbulence model, a three-dimensional unsteady numerical simulation of the whole flow field of an axial flow pump was presented at different flow rates. Combined with the Bernoulli equation of relative motion, the flow field structure in the impeller under design condition was studied quantitatively in the rotating coordinate system. The fluid movement law and energy transfer and loss mechanism in the impeller of the axial flow pump was described in detail. In the relative coordinate system, the mechanical energy of the fluid on the same flow surface conserves. The dynamic energy is continuously transformed into pressure energy from the leading edge to the trailing edge and the dynamic energy is continuously transformed into pressure energy from the leading edge to the trailing edge. The energy conversion is mainly completed in the front half of the blade. The friction loss and the mixing loss are the basic sources of losses in the impeller flow passage. Most hydraulic losses of impeller flow passage are caused by friction and the hydraulic losses near the trailing edge are dominated by mixing loss. This research has certain reference significance for further understanding the flow field structure in the impeller of the axial flow pump, improving its design theory and method, and then realizing its efficient and accurate design of the axial flow pump.

Numerical and Experimental Investigations of Axial Flow Fan with Bionic Forked Trailing Edge

To improve the performance of the aerodynamic properties and reduce the aerodynamic noise of an axial flow fan in the outdoor unit of an air conditioner, this study proposed a bionic forked trailing-edge structure inspired by the forked fish caudal fin and implemented by modifying the trailing edge of the prototype fan. The effect of the bionic forked trailing edge on the aerodynamic and aeroacoustic performance was investigated experimentally, and detailed analyses of the blade load and internal vortex structures were performed based on large-eddy simulations (LES). It is shown that the bionic forked trailing edge could effectively adjust the blade load distribution, reduce the pressure difference between the pressure side and suction side near the trailing edge of the blade tip region, and weaken the intensity and influence range of the inlet vortex (IV) and the tip leakage vortex (TLV). The discrete noise caused by the vortices in the rotor tip area was also reduced, particularly at the blade passing frequency (BPF) and its harmonic frequency. The experimental results confirmed the existence of an optimal bionic forked trailing-edge structure, resulting in the maximum power-saving rate γ of 7.5% and the reduction of 0.3 ~ 0.8 dB of aerodynamic noise, with an included angle θt of 13.5°. The detailed analysis of the internal vortex structures provides a good reference for the efficiency improvement and noise reduction of axial flow fans.

Pressure-sensitive paint measurements on the cavity with passive control devices under transonic flow

Abstract In this study, the pressure-sensitive paint (PSP) technique, specifically the mesoporous-particle-based PSP, was employed to compare rectangular cavities with varying length-to-depth ratios (L/D) and different trailing edge shapes under transonic conditions. By utilizing PSP, comprehensive and quantitative pressure data were obtained, enabling the simultaneous observation of surface flow field distribution. The results obtained using PSP were found to be consistent with those obtained from conventional pressure sensors. The study revealed that the pressure distribution within the cavities changed with increasing L/D, and cavities with different trailing edge shapes demonstrated a reduction in pressure at the bottom region. Furthermore, the comparison of results obtained through the oil flow method corroborated the PSP findings, indicating that a beveled or sawtooth-shaped trailing edge of the cavity induced air flow deflection, effectively disrupting the upstream shear flow structure and altering the pressure distribution at the cavityʼs bottom.

Tail-shake risks assessment & mitigation by wind-tunnel tests on air-intake installation on a heavy-weight H/C configuration

AbstractIn this work, the key role of the upper-deck design including engine installation as a potential source of tail-shake is at focus. The work is based on a Wind-Tunnel Test (WTT) campaign performed at the Airbus Helicopters’ Marignane wind-tunnel facilities on a high-fidelity minibody fuselage at scale 1:3.5 representing a generic heavy-helicopter upper deck.Two different engine intake installations for a Power Unit (PU) have been investigated; in a first configuration, the air intake is implemented at the pylon-fairing trailing edge. The second configuration consists in positioning two air intakes on each side of the pylon fairing, close to the maximum cross-section location. Different measurement methods to evaluate aerodynamic interactions and wake sources are proposed: flow-separation assessments from surface oil flow visualisations, time-resolved Particle Image Velocimetry (PIV) measurements and unsteady skin-pressure measurements at the cowlings. Tail-shake-related indicators are then proposed. Basically, a configuration that produces strong vortices characterised by a broadband spectral signature is believed to gather all the conditions for tail-shake to emerge.The flow over the clean configuration is first analysed for various combinations of angle-of-attack and sideslip, highlighting four different areas of flow separation at the cowlings. The complex flow topology around the upper deck is then assessed, which includes a spectral analysis of the flow in the PIV planes. The influence of the air intakes (operating or not) is then evaluated. When located at the pylon-fairing trailing edge and operating, the air intake has a spectacular impact on the flow-field topology. It is responsible for the generation of an intense broadband wake interacting with the pylon-fairing lip vortices, which is believed to be a potential source of tail-shake. The second air-intake configuration is also not favourable, as it requires enlarging the pylon fairing by 100mm, which causes an intense wake similarly to a blunt body. At last, a mitigation mean is proposed for the first configuration. It demonstrates a significant reduction of the wake intensity and broadband signature at the source.

Aerodynamic Optimization of Trailing-Edge-Serrations for a Wind Turbine Blade Using Taguchi Modified Additive Model

For the rotor, achieving relatively high aerodynamic performance in specific wind conditions is a long-term goal. Inspired by the remarkable flight characteristics of owls, an optimal trailing edge serration design is investigated and proposed for a wind turbine rotor blade. Fluid flow interaction with the proposed serrations is explored for different wind conditions. The result is supported by subsequent validation with three-dimensional numerical tools. The present work employs a statistical-numerical method to predict and optimize the shape of the serrations for maximum aerodynamic improvement. The optimal combination is found using the Taguchi method with three factors: Amplitude, wavelength, and serration thickness. The viability of the solution on an application is assessed using the Weibull distribution of wind in three selected regions. Results show that the presence of serration is capable of improving the annual power generation in all the investigated cities by up to 12%. The rated speed is also shifted from 10 m/s to 8 m/s for most configurations. Additionally, all configurations show similar trends for the instantaneous torque, where an increase is observed in pre-rated speed, whereas a decrease is noticed in the post-rated speed region. A look at the flow field pattern for the optimal design in comparison with the clean blade shows that the modified blade is able to generate more lift in the pre-stall region, while for the post-stall region, early separation and increased wake dominate the flow.

Numerical investigation of clocking effect on irreversible energy loss in volute for centrifugal pump with vaned diffuser based on entropy generation method

In order to study temporal and spatial variation of energy loss and reveal influence rule of clocking effect on energy loss and flow field in centrifugal pump with vaned diffuser, numerical simulations were carried out. Numerical simulations were validated by experiments. Local equilibrium of turbulence generation and dissipation was verified to state feasibility of energy loss methods. Hydraulic loss in each component was assessed, which shows that variation of hydraulic loss in volute is more significant due to clocking effect than that in other components. Vortical structure in volute varies greater under different guide vane positions than it in other components. The results show that positive and negative flow angle appears interlaced in tongue region with high velocity gradient, especially when diffuser is mounted at θd1. Since a clearance presents between tongue and vane trailing edge at θd1, a small vortex forms. Which promotes the blocking effect of the large-scale vortex at volute outlet pipe. Therefore, high velocity gradient appears at small vortex region and volute outlet pipe region. The shear strain rate at the tongue and volute outlet pipe is more significant than that at other diffuser mounted positions. Energy loss at these regions increases. This situation can be improved by a more suitable diffuser mounted position θd4, where the clearance and small vortex vanishes, the shear strain rate, velocity gradient, and energy loss decrease.

Thrust Force for Drone Propeller with Normal and Serrated Trailing Edge

The drone becomes more recognized in the civilian sector; the drone's popularity becomes increases as time goes by. Nevertheless, despite the excitement of flying drones, several types of issues occur caused by the drone. In some circumstances, the aeroacoustics noise is a big concern, and quiet drone propellers would be more environmentally friendly to the surrounding area. Moreover, the noise from the drone can be a nuisance for the surrounding population and animals. Therefore, a solution needs to be proposed to reduce the sound level produced by the drone so that drone can be piloted in a surrounding area without breaking any noise level limit set by the government. Hence, the propeller's serrated trailing edge type is the proposed solution to this problem. The serrated trailing edge propeller can reduce several drone noise decibels based on past research. Thus, an investigation is conducted to study the thrust force between the normal propeller and the serrated propeller. The aerodynamic performance of the serrated propeller is analysed using computational fluid dynamic simulation and compared to that of the normal propeller. Ansys Fluent 2021 is used to solve the dependable RNG k-epsilon turbulence model. The thrust force, thrust coefficient, and lift coefficient operating on both propellers were all simulated. The results obtained by the transient approach for propellers have been validated by earlier experimental studies.

Aeroacoustic airfoil shape optimization enhanced by autoencoders

Aeroacoustic noise is a major concern in wind turbine design that can be minimized by optimizing the airfoils that shape the rotating blades. To this end, we present a framework for airfoil shape optimization to reduce the trailing edge noise for the design of wind turbine blades. Far-field noise is evaluated using Amiet’s theory coupled with the TNO-Blake model to calculate the wall pressure spectrum and fast turn-around XFOIL simulations to evaluate the boundary layer parameters. The computational framework is first validated using a NACA0012 airfoil at 0° angle of attack. Particle swarm optimization is used to find the optimized airfoil configuration. The multi-objective optimization minimizes the A-weighted overall sound pressure level at various angles of attack, while ensuring enough lift and minimum drag. Furthermore, traditional shape optimization techniques show difficulties to converge to the optimum, when too many shape parameters are included in the optimization. For this reason, we compare classic parameterization methods to define the airfoil geometry (i.e., CST) to a machine learning method (i.e., a variational autoencoder). We observe that variational autoencoders can represent a wide variety of geometries, with only four encoded variables, leading to efficient optimizations, which result in improved optimal shapes. When compared to the baseline geometry, a NACA0012, the autoencoder-based optimized airfoil reduces by 3% (1.75 dBA) the overall sound pressure level (with decreased noise across the entire frequency range), while maintaining favorable aerodynamic properties in terms of lift and drag.

Design and commissioning of experiments for supersonic ORC nozzles in linear cascade configuration

In organic Rankine cycle (ORC) power systems, the turbo-expander involves the major technical challenges as the demand for compactness and flexibility of operation couples with severe compressibility and non-ideal gas effects. For these reasons the design of ORC turbines heavily relies on advanced aerodynamic models, whose validation is crucial but is still limited due to a lack of experimental data. To fill this gap, an experimental campaign on supersonic ORC nozzle cascades has been launched at Politecnico di Milano. This paper describes the conception and set-up of a novel class of experiments on linear cascades representative of stator nozzles of axial/radial ORC turbines, and aims at serving as a reference for future experimental investigations on ORC cascades. The paper also discusses the technical challenges of performing measurements on supersonic flows for a vapor at thermodynamic conditions close to saturation. Moreover, the paper reviews all steps required to design the campaign, discussing the blade design and the numerical simulations performed to assess the flow configuration in the linear cascade, with emphasis on the periodicity of the flow. The main objective of the experiments is to investigate the flow phenomena occurring in the blade trailing edge region and to retrieve the total pressure losses through the cascade with a relatively high spatial resolution, aiming at providing a benchmark for numerical simulations. To this end, the set of measurement techniques includes supersonic total pressure probes and wall pressure taps, as well as thermo-couples and schlieren visualizations. The main result of this preliminary work is the implementation of a relatively simple methodology to carry out experiments in blade cascade operated with organic vapors and aimed at evaluating cascade losses, without resorting to specifically calibrated instrumentation.

A Review of Cooling Studies on Gas Turbine Rotor Blades with Rotation

Increases in power density and thermal efficiency of a highly efficient gas turbine engine motivate an ever-mounting turbine entry temperature. The combined metallurgical and cooling advancements ensure the structural integrity of a gas turbine rotor blade that spins at high rotor speeds in a gas stream with temperatures above the melting point of the blade material. The cooling performances promoted by a variety of heat transfer enhancement methods typical of the coolant channels of the leading edge, the mid-chord region, and the trailing edge of a gas turbine rotor blade are reviewed. The manifested rotational effects on the aerothermal performances of impinging jets and swirl chambers for leading-edge cooling, multi-pass ribbed, dimpled, and/or wavy channels over the mid-chord region, as well as pin fin and latticework narrow ducts in the trailing edge of a gas turbine rotor blade, are summarized and cross-examined. Research orientations for future cooling studies aimed at preventing the development of hot spots in a gas turbine rotor blade are recommended.

Study on the power extraction performance of a flapping airfoil power generator using jet flow method

Flapping airfoil power generator (FAPG) is becoming popular due to high-power extraction with low noise. However, the effect of a combined jet of suction and blowing on the power extraction performance of the flapping airfoil is not fully explored. The present study presents a flapping airfoil model with a compound jet of suction and blowing, and the jet parameters were optimized using the Taguchi method and CFD approach for the maximum power extraction efficiency of the airfoil. It was found that the power extraction efficiency of the flapping airfoil can be improved by applying the compound jet to the trailing edge of the airfoil. Compared to the baseline NACA0015 airfoil, the optimized jet parameters lead to the maximum improvement in efficiency by 38.14%. The analysis of flow field characteristics of the flapping airfoil reveals that the optimal jet parameters can not only enhance the strength of the vortex on airfoil but also improve the vortex motion mechanism, which results in a greater lift of airfoil and better synergy between the lift and the plunging velocity.