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Every aerodynamic optimization is proceeded by a parameterization of the studied aerial object, and due to its influence on the final optimization process, careful attention should be made in choosing the appropriate parameterization method. An aerodynamic optimization of a morphing trailing edge is performed using a free-form deformation parameterization technique with the purpose of examining the influence of the initial conditions of the parameterization on the optimization results, namely on the number of control points. High-fidelity gradient-based optimization using the discrete adjoint method is established by the coupling of OpenFOAM and Python within the DAFoam optimization framework. The results indicate that the number of control points has a considerable effect on the optimization process, in particular on the convergence, objective function value, and on the deformation feasibility. [ABSTRACT FROM AUTHOR]

In the present study, helicopter rotor blades with trailing edge flaps are studied to analyze their aerodynamic and performance characteristics for hover conditions. The effect of different characteristics of trailing edge flaps, i.e. flap cord, flap span, the radial location of the flap along the blade and flap deflection angle are inspected. Preliminary calculations are conducted using the properties of BO105 rotor. In order to examine the parameters of the rotor blade, an in-house aerodynamic code is developed by applying the Blade Element Momentum Theory formulations with tip loss effects and XFoil program is used to find 2-D airfoil characteristics. Consequently, effects of several parameters of trailing edge flap concept are demonstrated in several figures where variation of the Flap authority (FA) and the figure of merit authority (FMA) are examined. Flap authority is more affected by the change of Sp, Cr, Md, d parameters in cases where the thrust coefficient is low. It is directly proportional to the change of each of the flap parameters with the change of the FA value. The FMA increases in low-loaded blades when the flaps approach the root, while it decreases when the flaps are positioned towards the blade tip. [ABSTRACT FROM AUTHOR]

The Mach stem height is an important parameter in the Mach reflection of steady supersonic flow. Various experimental, numerical, and theoretical works have been conducted to study this parameter in the past. However, much of the established work focuses around a single set of trailing edge heights. Here, we perform a study to show the dependence of Mach stem height on the trailing edge height for a wider range of geometry. Through numerical simulation for a set of trailing edge heights, we found that the normalized Mach stem height is almost linear with respect to the normalized wedge trailing edge height. The parameter used for normalization can be either the inlet height or the length of the lower wedge surface. The observation of this linear trend is justified through a simplified analysis, which leads to an expression of the Mach stem height that linearly depends on the trailing edge height. The present study extends our knowledge about how the geometry affects the Mach stem height, and provides a basis for future work to elaborate analytical models for Mach stem height. [ABSTRACT FROM AUTHOR]

In the present paper, large eddy simulations are performed to study two different mechanisms of Fan/OGV broadband noise: airfoil self-noise and turbulence interaction noise. Firstly, the current study focuses on the prediction of airfoil self-noise from a thin plate with a sharp trailing edge and a chord-based Reynolds number of the order of 106. The boundary layer is tripped to trigger transition to turbulence, and a parameter study is performed to study the influence of the near-wall modeling, grid topology and refinement in the near-wall and wake regions, the spanwise domain extent, and the tripping method. Empirical and analytical models, as well as available DNS data are used for validation purposes. Secondly, the interaction noise from a thin plate impinged by an incoming synthetic turbulent flow is studied. For both cases, far-field acoustic spectra are compared to Amiet's models for leading and trailing edge noise showing a good agreement. [ABSTRACT FROM AUTHOR]

Aims: A flow-control technique by integrating a rotating cylinder onto an airfoil of NACA 4418 is proposed. Methods: Both numerical and experimental studies are performed to investigate its aerodynamic performance. Results: It is found that a rotating cylinder mounted in the 0.45 of chord length on the airfoil gets satisfactory effects in improving the lift of an airfoil. Conclusion: The improvements cover from angles of attack 0°–18°, which provide a wider operation range than that when the rotating cylinder placed in leading edge or trailing edge as proposed by most of the former researches. The study also suggests that the speed ratio is relative to the increase of negative pressure in the upper surface and the protrusion height ratio is relative to the influence area of flow field, which indicates that lift coefficient is increased with the increment of the speed ratio and protrusion height ratio. [ABSTRACT FROM AUTHOR]

Highlights • Bubbles move behind the impeller blade due to pressure distribution. • Most bubbles in an unbaffled stirred vessel are fragmented in the trailing vortices. • The fragmented bubbles are transported by the trailing vortices, the core of which traps the bubble. • Smaller and more complicated bubbles are generated with increasing the impeller rotation rate. • More bubbles are agglomerated near the shaft with increasing the impeller rotation rate due to pressure distribution. Abstract The present study investigated the mechanism of small bubble breakup in an unbaffled vessel stirred by a 4-blade paddle impeller. Bubbles were injected underneath the rotating blades. Numerical simulation and experimental observations were performed to investigate the related phenomena. The Volume of Fluid (VOF) method was utilized to simulate a gas-liquid multiphase flow. The injected bubbles were broken behind the impeller blade, where they were transported due to a decreasing pressure gradient caused by the impeller rotation. At the behind of the blade, bubbles were elongated and fragmented due to the trailing vortices developing from the rear surface of blade. The fragmented bubbles were entrapped at the cores of trailing vortices, and then floated up in the space where the trailing vortices became weak. The size of the broken bubbles was dependent on the impeller rotation rate. [ABSTRACT FROM AUTHOR]

In this study, the far-field noise from a pitching NACA 0012 airfoil was measured at a Reynolds number of 6.6 × 104. The pitching motion was in sinusoidal functions with a mean incident angle of 0°. Cases with the pitching amplitude varying from 7.5° to 15° and frequency from 3 to 8 Hz were tested, corresponding to the reduced frequency from 0.094 to 0.25. A microphone was placed in the far-field and the particle image velocimetry technique was utilized to study the flow structures near the trailing edge. The short-time Fourier transformation results of the noise signals revealed that a high-level narrow-band noise hump occurred at a specific angle of attack in a pitching cycle. At the corresponding moment, a coherent vortex street convecting on the airfoil surface was observed, and the vortex shedding frequency was in good agreement with the central frequency of the noise hump. The occurrence of the noise humps was attributed to the laminar boundary layer separation. In one pitching period, the moment when the narrow-band noise hump occurs is independent from the pitching amplitude and it is delayed as the pitching frequency increases. Larger pitching frequency or amplitude results in lower peak level of the noise humps. [ABSTRACT FROM AUTHOR]

Abstract Double-suction centrifugal pumps are widely used in irrigation projects along the Yellow River due to their high capacity and high reliability. Sediment erosion of the impeller and the corresponding pump performance degradation are key issues since the double-suction centrifugal pump operates in sediment-laden water. Experiments and numerical method were used to study the erosion wear of the impeller in a double-suction pump in the Jingtai Yellow River Irrigation Project(JYRIP). The effects of particle size, particle shape and particle concentration on the pump performance were investigated. Observations during operation combined with scanning electron microscope(SEM) were used to verify the erosion mechanisms at different positions of the impeller blade. Regarding the numerical method, the Eulerian-Lagrangian approach combined with the Mclaury model was employed to predict the sand particles erosion. Numerical results for the erosion distribution and locations of severe erosion wear were consistent with the experimental observations. There was a transition of erosion wear from impacting to cutting from the leading edge(LE) to the trailing edge(TE) of the blade surface. Erosion wear damage was found to be mainly distributed near the LE and the TE. Larger, angular particles and a higher particle concentration caused more severe erosion damage and increased the pump performance reduction rate. Moreover, the effect of particle shape on the performance was less than the effect of particle size and particle concentration. These results provide a reference for optimization of the design of anti-wear blades of the double- suction pump and can help predict the performance of the pump with sediment-laden water. Highlights • Experiments and numerical prediction of sediment erosion for JYRIP. • Erosion forms transition from impacting to cutting across the blade surface. • Pump performance decrease with increase in particle size and concentration. • Particle shape has little effect on pump performance in the JYRIP. [ABSTRACT FROM AUTHOR]

Abstract Experimental studies were conducted at an anechoic chamber to investigate the effectiveness of quarter flat tip, half flat tip and rectangular serrations on reducing propeller trailing edge noise. The experiments were conducted under four rotating speeds and the noise generated by the propeller was measured at four different positions. The thrust force produced by each propeller was also measured at each rotating speed. The performance of the quarter flat tip serrated propeller was poor at all rotating speeds and at all positions. The performance of the half flat tip serrated propeller was good at all rotating speeds and at all positions. It obtained overall noise reduction up to 5.8 dBA. For rectangular serrated propeller, its performance was only good at 2000 rpm at all positions and it obtained overall noise reduction up to 3.1 dBA. By overall, half flat tip serrated propeller had the best performance in term of noise attenuation compared to other two types of serrated propellers and its loss of thrust force was also 16.8% lower than that of rectangular serrated propeller. [ABSTRACT FROM AUTHOR]

Abstract In the present paper, the effects of three different pre-swirl ducts on propulsion performance of a series 60 ship in comparison to the case with no duct are explored experimentally by self-propulsion test measurements and CFD analysis. To this accomplishment, Mewis duct, Becker twisted fins and unconventional half circular ducts are considered. For investigating the propulsion performance, thrust and torque coefficients, efficiency components of the intended propulsion system, wake and pressure distributions are evaluated. Based on our tests, the highest thrust and torque coefficients are achieved at lower advance ratios for Becker twisted fins, while; Mewis ducts provided the highest value of thrust and torque coefficients at higher advance ratios. Maximum value for propeller behind hull efficiency, relative rotative efficiency and propulsive efficiency are obtained with respect to unconventional half circular duct, Mewis duct, and the case with no pre-swirl duct. Based on the wake distribution analysis, local deformation on the upstream wake induced by pre-swirl ducts is detected. Moreover, it is found that the vortex shedding from trailing edges of the propeller blades has led to a defection on the downstream wake field. [ABSTRACT FROM AUTHOR]

Highlights • Modifying the blade pressure side (EPS profile) of a centrifugal pump. • Pressure pulsations are captured and compared. • Jet wake flow pattern is modulated and more uniform flow field is obtained. Abstract In the present work, the effects of modifying the blade pressure side (EPS profile) on unsteady pressure pulsations and flow structures in a low specific speed centrifugal pump are carried out by experimental and numerical methods. Results are compared to the original trailing edge (OTE profile). Unsteady pressure signals are captured at twenty measuring points at flow rate of 0–1.6Q d. It is observed that the pump head of the EPS profile is improved for all the concerned working conditions. Pressure amplitudes at the blade passing frequency are compared and discussed in detail. It is found that the EPS profile contributes to pressure pulsation reduction obviously. For all the measured flow rates, pressure amplitudes are attenuated evidently at major measuring positions, especially at high flow rates. As for the mean pressure amplitude of twenty measuring points, pressure amplitude is reduced more than 20% at the nominal flow rate using the EPS profile. From relative velocity distribution, it is found that the uniformity of flow field at the blade outlet region would be improved significantly by the EPS profile. Besides, the corresponding vorticity magnitude at the blade outlet would be reduced compared to the OTE profile. The combined effects contribute to the reduction of pressure amplitude using the EPS profile. [ABSTRACT FROM AUTHOR]

The aerodynamic and aeroacoustic behaviors of wavy trailing edge modified flatback wind turbine blade have been investigated through numerical simulation. Previous studies have demonstrated aerodynamic and aeroacoustic benefits of the span-wise wavy trailing edge modification on the flatback trailing edge. The previous two-dimensional airfoil test cases have been extended to a complete set of design parametric study. Aerodynamic and aeroacoustic characteristics of the parametric study have been discussed, relying on the wave size, including recently added cases. Furthermore, in the current study, the wavy trailing edge modification has been applied to the Sandia flatback blade called "SNL100-03FB" and tested in rotating blade conditions. Several variations of the wavy trailing edge design have been tested for a range of wind speeds using an overset computation domain. The numerical simulation employs the in-house developed Navier–Stokes solver, OVERTURNS, as well as Graphics Processing Unit (GPU)-accelerated solver, GPURANS3D. Both are hybrid Reynolds Averaged Navier Stokes (RANS)/Large Eddy Simulation (LES) simulation; delayed detached eddy simulation has been used with the Spalart–Allmaras turbulent model and modified γ − R e θ − S A laminar–turbulent transition model. [ABSTRACT FROM AUTHOR]

Abstract The need for faster and more accurate manufacturing methods for composite parts continues to grow. Co-curing composite structures can decrease manufacturing time by eliminating secondary operations such as grinding, jigging, bonding, and fastening while creating lighter and more accurate parts. As a demonstrator for co-curing techniques, a six-meter carbon fiber wing for a high-altitude and high-speed dynamically soaring unmanned aerial vehicle (UAV) was designed and manufactured in one cure cycle. Two wing-skin molds were created using low density tooling board, with the mold geometry directly machined into the material, reducing tool manufacturing time and cost. An aluminum insert was used to create a trailing edge cavity while maintaining a simple parting line of the wing tool. Three removable forms made of polystyrene foam inside of the wing cavity were used to position six internal webs and, after curing and removal of the forms, resulted in a hollow wing with internal webs. The resulting wings showed some defects in the wing skins but overall produced structurally sound parts. The method shows great potential for creating complex composite parts using only a single cure cycle with little finishing work and no secondary bonding, resulting in high precision at a relatively low cost. [ABSTRACT FROM AUTHOR]

Abstract The effects of cropping, anhedral, Gurney flaplike strips and winglets on the vortex flow and aerodynamic properties of a reverse delta wing (RDW) were investigated at Re = 3.81 × 105. The cropping led to a minor change in the RDW vortex flow property and aerodynamics while the anhedral produced inferior aerodynamics but strengthened vortices compared to the baseline wing. The Gurney flaplike strips increased both the lift and drag and vortex strength. Meanwhile, the winglets kept the vortex flow properties mainly unchanged. The lift force predicted based on the vortex strength was found to be within 4% to 14% difference compared to the force balance data, which suggests that although the outboard location of the RDW trailing vortices appeared to imply their irrelevance to the lift production, their strength, however, represented a significant portion of the lift. Finally, the lift-induced drag computed based on the vorticity flowfield was also found to account for 17–27% of the total drag. The present measurements will serve as a comparison for next-phase ground effect study. [ABSTRACT FROM AUTHOR]

Larger and more flexible wind turbine blades are currently being manufactured. Those highly flexible blades suffer from loading of aeroelastic nature which increases the fatigue damage. Smart blade concepts are being developed to reduce the aerodynamic loading. The state of the art favors the discrete deformable trailing edge concept. Many authors have reported adequate performance of this type of actuators in reducing the blade vibrations. However, the question of whether the actuator can maintain its authority under strong external loading remains still answered. To solve this question, actuator models that include the loading produced by the blade vibration are required. In this article, a smart morphing trailing edge model is presented that includes the inertial forces produced by the blade dynamics. The model is applied to a commercial actuator and the influence of its parameters is analyzed. Finally, a simple estimation of the inertial loading produced by a 35-m wind turbine blade at the flutter instability condition is analyzed to understand the design requirements of this type of systems. [ABSTRACT FROM AUTHOR]

Blades are crucial parts of aero-engines. Their manufacturing is difficult because of their thinness, complex profile and stringent requirements. Electrochemical machining (ECM) is an important approach for manufacturing blades, but it is difficult to machine the leading and trailing edges. This article adopts cross-structural cathodes in ECM to solve the open electric field space problem. The key difficulty in the design of cross-structural cathodes is the position of the crossed-point. The height and the offset of the crossed-point are optimized using the ANSYS software. The simulation results show that the position of the crossed-point obviously affects the machining accuracy, along the leading and trailing edges of the blade. A pair of cross-structural cathodes with one uniform section is designed, and a series of corresponding ECM experiments are carried out. The experiments demonstrate that the ECM process is stable, the profiles are sleek and the machining dimensional error is reduced to 0.08 mm at the leading and trailing edges of the blade when using the new cathode structure. [ABSTRACT FROM AUTHOR]

A theoretical model is proposed in the current work to shed light on the noise-reduction mechanisms of trailing-edge serrations, which have been shown to suppress noise generated at the trailing edge due to the scattering of boundary layer pressure fluctuations. The current analytical model, which is developed by incorporating Fourier series expansions and the Wiener-Hopf method, can quickly predict noise reductions due to various types of trailing-edge serrations at low Mach numbers. The present model is validated by comparing its predictions with relevant published theoretical and experimental results. Moreover, from the Winer-Hopf analysis, an effective method based on Fourier series expansion is proposed to approximately evaluate the noise-reduction performance of different serrated geometries under various working conditions. The current work shows that trailing-edge serrations reduce noise by producing scattered waves with spanwise modes which could be evanescent, and explains why some serrated geometries are more effective than others in suppressing noise. The efficient noise prediction capability of the proposed model makes it very suitable to be used in the trailing-edge designs and evaluations of low-noise aircraft components such as wings and aero-engine fan, compressor and turbine cascades. [ABSTRACT FROM AUTHOR]

Unsteady flow characteristic and film cooling effect in the blade trailing edge cutback region are numerically investigated using DES (detached eddy simulation) method at three blowing ratios, four lip thickness ratios and three rib geometries. With the experimental data, reliability and accuracy of the numerical methods are validated. The results indicate that, compared with RANS (Reynolds-averaged Navier-Stokes) equations solutions, the DES method has a superior accuracy in predicting the film cooling effectiveness on trailing-edge cutback. The lip thickness has a pronounced effect on shedding vortex scale downstream the slot. As the lip thickness ratio decreases, the film cooling effect on trailing edge cutback is improved. Compared with the line ribs, the arrangement of pin fin ribs enhances the uniformity of coolant coverage on trailing edge cutback along lateral direction. However, with different types of rib, the film cooling effectiveness on the trailing edge cutback is mainly determined by the stable cooling flow structures and mixing effect between mainstream and coolant. The development of vortex system downstream the slot is significantly affected by the blowing ratio, which in turn influences the film cooling effect on trailing edge cutback. [ABSTRACT FROM AUTHOR]

Abstract In this paper, we validate three-dimensional numerical models of subduction against a laboratory experiment. The design of our numerical models follows that of the laboratory, and employs a single subducting plate fixed at its trailing edge without an overriding plate. Rheology and buoyancy are not thermally dependent, but importantly the model has a free surface boundary. All the models include a sticky air layer, which is an approximation to a free surface boundary condition, as opposed to a free-slip condition that unrealistically binds the plate to the upper surface. We use a parallel, Python, particle-in-cell, finite-element scheme, and present 8 different models that differ in their viscosity at the interfaces of the subducting plate. We consider harmonic and arithmetic averaging of the viscosity together with a plate composite rheology associated with arithmetic averaging, which similarly to the harmonic averaging results in two lubricated sub-layers at the plate lower and upper interfaces. Despite showing a similar subduction-induced poloidal and toroidal flow in the mantle, the agreement of the models with the experiment is overall ordinary. The rates at which the slab tip sinks and the plate hinge retreats are approaching the laboratory counterparts to within 7–18%, at best, when using the composite plate models. Errors in the characterization of the laboratory parameters can account for 5% of such differences. As the composite plate sub-layers are made stiffer, the plate hinge also begins to depart from the steady retreat observed in the laboratory experiment. All numerical models exhibit a secondary Rayleigh-Taylor instability along the plate edge that is not observed in the laboratory experiment. The development of this instability is further delayed when the numerical spatial resolution is increased, or when the composite plate sub-layers are made stiffer. This benchmark exercise shows that 1) the glucose surface tension in the laboratory, not taken into account in the numerical modelling, is sufficient to prevent the flank instability during the retreat, and 2) numerical models are usually free from it due to their use of stiffer plate sub-layers and/or their display of a limited amount of retreat compared to this study. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We consider the asymptotic structure of a steady developed viscous thin film passing the sharp trailing edge of a horizontally aligned flat plate under the weak action of gravity acting vertically and surface tension. The surprisingly rich details of the flow in the immediate vicinity of the trailing edge are elucidated both analytically and numerically. As a central innovation, we demonstrate how streamline curvature serves to regularise the edge singularity apparent on larger scales via generic viscous–inviscid interaction. This is shown to be provoked by weak disturbances of accordingly strong exponential downstream growth, which we trace from the virtual origin of the flow towards the trailing edge. They represent a prototype of the precursor to free interaction in the most general sense, which, interestingly, has not attracted due attention previously. Moreover, we delineate how an increased effect of gravity involves marginally choked flow at the edge. [ABSTRACT FROM AUTHOR]

Direct numerical simulations were conducted to uncover physical aspects of a transverse sonic jet injected into a supersonic cross-flow at a Mach number of 2.7. Simulations were carried out for two different jet-to-cross-flow momentum flux ratios (J) of 2.3 and 5.5. It is identified that collision shock waves behind the jet induce a herringbone separation bubble in the near-wall jet wake and a reattachment valley is formed and embayed by the herringbone recirculation zone. The recirculating flow in the jet leeward separation bubble forms a primary trailing counter-rotating vortex pair (TCVP) close to the wall surface. Analysis on streamlines passing the separation region shows that the wing of the herringbone separation bubble serves as a micro-ramp vortex generator and streamlines acquire angular momentum downstream to form a secondary surface TCVP in the reattachment valley. Herringbone separation wings disappear in the far field due to the cross-interaction of lateral supersonic flow and the expansion flow in the reattachment valley, which also leads to the vanishing of the secondary TCVP. A three-dimensional schematic of surface trailing wakes is presented and explains the formation mechanisms of the surface TCVPs. [ABSTRACT FROM AUTHOR]

Abstract An adaptive RBF observer-sliding mode controller is designed for the vibration suppression of a two-dimensional aeroelastic system, using a single trailing-edge control surface. The prototypical aeroelastic model describes the plunge and pitch motion of a wing, including cubic nonlinear structural stiffness and unsteady aerodynamics. The unsteady aerodynamics are modeled with an approximation to Theodorsen's theory. It's assumed that only the pitch angle is measured and the remaining state variables needed for full state feedback are estimated by the designed observer. A neural-network is employed to approximate the nonlinear dynamics of the observer system, and then a sliding surface is put forward on the estimation space. The finite-time reachability of the predesigned sliding surface is guaranteed by the designed sliding mode control law. The effectiveness of the proposed strategy is finally demonstrated by simulation results. [ABSTRACT FROM AUTHOR]

The present work focuses on the alleviation of Blade Vortex Interaction (BVI) noise annoyance through a control methodology generating high-frequency aerodynamic BVI counter-actions. The low-power requirements make the Micro-Trailing Edge Effectors (MiTEs) particularly suited for this kind of application. The controller layout is set by observing the BVI scenario while the actuation law is efficiently synthesized through a process based on an analytical unsteady sectional aerodynamic formulation. The validation of the proposed control methodology is carried out through numerical investigations of a realistic helicopter main rotor in flight descent, obtained using computational tools for potential-flow aerodynamic and aeroacoustic analyses based on boundary element method solutions. In order to capture the aerodynamic influence of MiTEs through potential-flow simulations, the MiTEs are replaced by trailing edge plain flaps which provide equivalent aerodynamic responses. Results concerning the proposed controller capability to alleviate high-frequency blade loads and subsequent emitted noise from BVI events are presented and discussed. [ABSTRACT FROM AUTHOR]

In this work, the effect of multiple transverse jets on the turbulent boundary layer developing over a flat plate is experimentally investigated for aeroacoustic purposes. A single line of jet nozzles with different spanwise spacings is located parallel to the trailing-edge of the plate, at approximately 30 jet diameters upstream of the trailing-edge. The axes of the jet nozzles have an inclination of 15° with respect to the streamwise direction. Two values of the jet velocity ratio (r = ujet/u∞) are considered, r = 1 and r = 2. The simultaneous measurement of streamwise velocity and surface pressure fluctuations is performed with hot-wire anemometry and flush-mounted microphones, respectively. The mean velocity profiles show that the low inclination angle of the multiple jets prevents the formation of adverse pressure gradients, and therefore, the multiple jets injection does not lead to flow separation, at least at the range of downstream locations under investigation. From the velocity measurements, the jets merge downstream of the jet nozzles and form a layer of jet fluid characterized by a low energy content. The estimates of the far-field noise show that jets injection at a velocity ratio of r = 1 leads to noise attenuation over the whole range of frequencies under analysis. At a velocity ratio of r = 2, jets injection enables to gain a larger noise reduction than at r = 1 at low frequencies, but the estimated far-field noise is expected to increase at high frequencies. [ABSTRACT FROM AUTHOR]

Trailing-edge mixing flows associated with coolant injection are complex, in particular at transonic flows, and result in significant aerodynamics losses. The objective of this paper is to evaluate the impacts of hole injection near the suction side throat on shock wave control and aerodynamic losses. A series of tests and calculations on effects of hole injection on the suction-side throat of a high-pressure turbine vane cascade with and without trailing-edge injection were conducted. Wake traverses with a five-hole probe and tests of pressure distributions on the turbine profile were taken for total injection mass flow ratios of 0% and 1.2% under test Mach numbers of 0.7, 0.78, and 0.87. Meantime, numerical predictions are carried out for exit isentropic Mach numbers of 0.7, 0.78, 0.87, and 1.1 and hole-injection mass flow ratios of 0%, 0.17%, 0.3%, and 0.89%. Numerical predictions show a reasonable agreement with the experimental data, and wake total pressure losses and flow angles as well as pressure distributions on the turbine profile were compared to calculations without hole injection, indicating a significant effect of hole injection on the profile wake development and its blockage effect on the shock-wave flow in the vane cascade passage. At subsonic flows, the hole injection on the suction side throat thickens the suction-side boundary layer, and increases the flow mixing, thus causing increased wake losses and flow angles. At transonic flows, while the trailing-edge injection reduces the strength of the shock wave at the trailing-edge pressure side, the hole injection on the suction side throat alters the local pressure fields, and then tends to enhance the shock-wave at the trailing-edge pressure-side; however, it seems to reduce the strength of the shock-wave at the trailing-edge suction side. [ABSTRACT FROM AUTHOR]

The results of unsteady-flow simulations and experiments are discussed to investigate active noise-reduction effects on the stator–rotor interaction in a single-stage low-speed compressor with nonuniform trailing edge blowing. It is found that for the investigated type of stator–rotor interaction noise, nonuniform trailing edge blowing has beneficial noise-reducing effects. The overall aim is to demonstrate that nonuniform trailing edge blowing can compensate momentum loss and reduce the axial thrust on rotor blades. The results illustrate how nonuniform trailing edge blowing influences the sound pressure level of the blade-passing frequencies and results in active noise reduction effects. The study was conducted using a trailing edge blowing system, a four-hole dynamic flow-field measurement system, and phase lock technology. The results obtained show that nonuniform trailing edge blowing leads to substantial noise-reduction effects, lowering sound levels by more than 10 dB with about 5‰ of inlet mass flow. [ABSTRACT FROM AUTHOR]

During engine operation, fan casing abradable liners are worn by the blade tip, resulting in the formation of trenches. This paper describes the influence of these trenches on the fan blade tip aerodynamics. A detailed understanding of the fan tip flow features for cropped and trenched clearances is first developed. A parametric model is then used to model trenches in the casing above the blade tip and varying blade tip positions. It is shown that increasing clearance via a trench reduces performance by less than increasing clearance through cropping the blade tip. A response surface method is then used to generate a model that can predict fan efficiency for a given set of clearance and trench parameters. This model can be used to influence fan blade design and understand engine performance degradation in service. It is shown that an efficiency benefit can be achieved by increasing the amount of tip rubbing, leading to a greater portion of the tip clearance sat within the trench. It is shown that the efficiency sensitivity to clearance is biased toward the leading edge (LE) for cropped tips and the trailing edge (TE) for trenches. [ABSTRACT FROM AUTHOR]

The impact of porous surfaces on the near-wall turbulent structures and the generated trailing-edge noise is analyzed for several trailing-edge shapes of finite thickness using a high resolution large-eddy simulation (LES)/computational aeroacoustics (CAA) method. The porous surface of the trailing edge is defined by the porosity and the viscous permeability determined by the solution of a turbulent flat plate boundary layer at a Reynolds number 1280 based on the displacement thickness in the inflow cross section. The volume-averaged approach for the homogeneous porous medium shows that the porous impedance scales linearly with the porosity and exponentially with the mean structure size of a porous medium. The drag induced by the porous surface changes the friction velocity and the permeability Reynolds number Re K which determines the porous impedance R s scaled by Re K − 2 / 3 . The trailing-edge noise is analyzed for three solid and three porous trailing edges. The effect of a finite span is investigated by the spanwise correlation model based on the measured coherence distribution. The acoustic prediction shows a good agreement with measurements of the broadband spectrum and the strong tone generated by a finite trailing-edge thickness. The pressure gradient inside the porous media is redistributed by the Darcy drag defined by the viscous permeability and the porosity. The mean pressure increases in the upstream direction inside the porous medium such that the flow acceleration involved in the acoustic generation is reduced inside the porous medium. The noise reduction by a porous medium reaches 11 dB for the trailing-edge shape which possesses a sharp corner for the solid surface. The porous surface applied to a semi-circular trailing edge achieves a 4 dB noise reduction. The directivity pattern for individual components of the acoustic spectrum shows that the massive noise reduction is determined at the tone. Enhanced wave diffraction by the thick flat plate changes the directivity pattern in the high frequency range. [ABSTRACT FROM AUTHOR]

The influence of the velocity ratio (VR) between the jet and main flow on the wake structure and aerodynamic loss of the trailing edge jet is studied using particle image velocimetry and numerical simulations. Three different velocity ratios, namely, VR = 0.5, 1.0, 1.5, are chosen for this comparative study. The Reynolds number (Reh) based on the slot height (h) and the mainstream velocity (U0) are 3380. Results show that the influence of jet on wake structure is significant such that the wake region shrinks and the turbulent kinetic energy is enhanced as the velocity ratio increases. The distribution area of strong vorticity is enlarged with increasing velocity ratio. By using proper orthogonal decomposition and fast Fourier transfer analysis, the variation of velocity ratio demonstrates significant impact on vortex shedding and turbulent kinetic energy. The aerodynamic loss coefficient is nearly constant between VR = 0.5 and 1.0, but increases by 3.25 % as the velocity ratio increases from 1.0 to 1.5. [ABSTRACT FROM AUTHOR]

This paper compares several approaches for the prediction of the noise emitted by a NACA0012 airfoil at 0 ∘ and 4 ∘ of angle of attack and a Reynolds number R e = 1.5 × 10 6 . Amiet's semi-analytical model for trailing-edge noise is combined with two-dimensional Reynolds-Averaged Navier-Stokes (RANS) computations. The wall-pressure spectrum, which constitutes the cornerstone of Amiet's model, is obtained by processing the boundary layer data extracted from the simulations. The specific contribution of the paper is a comparison of two families of prediction methods: semi-empirical wall-pressure models that are fitted to experimental databases, and statistical approaches based on the integration of the Poisson equation across the boundary layer profile. The semi-empirical models that were calibrated on airfoil databases provide better predictions with experiments, while the models based on flat-plate boundary layer data fail to reproduce the measured spectra. Considering the statistical approach, it was shown to predict the general spectral features, but with an overall under-prediction of about 3 dB. It can be concluded from this study that the statistical approach proves indeed more robust than semi-empirical models when the latter were not precisely calibrated for the flow under consideration. Further improvements of the statistical approach are suggested for future work. [ABSTRACT FROM AUTHOR]

The trailing edge of a NACA 0018 airfoil is modified through the attachment of serrations with different degrees of permeability. Acoustic beamforming is used to inspect the turbulent boundary layer-trailing edge noise emissions from the unmodified and serrated trailing edges. Different freestream velocities and angles of attack are investigated. The serration permeability is prescribed by having slits cut into the solid surface of the serrations in two different configurations. The results indicate that a certain benefit in noise reduction is obtained from a mixed solid/slitted configuration, while a fully slitted configuration loses most of the noise reduction performance. [ABSTRACT FROM AUTHOR]

This study focuses on a passive noise abatement technique in a small contra-rotating fan, aiming at reducing the interaction noise between the two rotors through porous trailing edge (TE) treatment to the forward rotor. A preliminary design with fixed perforation parameters is experimentally investigated, and 6-7 dB overall noise reduction is achieved compared with baseline design under the same aerodynamic output. A three-dimensional (3D), full-wheel, unsteady-flow numerical simulation of the acoustic design is carried out to better understand the noise reduction mechanism. Comparisons of monitored unsteady forces acting on both the forward and the aft rotor between baseline and perforated fan indicate that such treatment reduces all the unsteady forces. Thus, it can be concluded that the noise reduction would be due to not only the mitigation of viscous wake of forward rotor before impinging upon the downstream blades but also the reduction of the response of the upstream rotor to the potential flow interaction with the downstream rotor. Furthermore, a parametric study in a selected range is conducted to minimize the adverse effect of aerodynamic unloading due to TE perforation and to improve the acoustic benefit. The parameters in the parametric study include perforation ratio, aperture diameter, and perforation distribution. Trends are deducted from this, and it is recommended that there exists an optimal perforation ratio; the smallest possible aperture diameter and the decreasing perforation ratio distribution away from the blade TE should be selected in consideration of both aerodynamic and acoustic effects. [ABSTRACT FROM AUTHOR]

This paper investigates the effects of finite flat porous extensions to semi-infinite impermeable flat plates in an attempt to control trailing-edge noise through bioinspired adaptations. Specifically the problem of sound generated by a gust convecting in uniform mean steady flow scattering off the trailing edge and the permeable-impermeable junction is considered. This set-up supposes that any realistic trailingedge adaptation to a blade would be sufficiently small so that the turbulent boundary layer encapsulates both the porous edge and the permeable-impermeable junction, and therefore the interaction of acoustics generated at these two discontinuous boundaries is important. The acoustic problem is tackled analytically through use of the Wiener-Hopf method. A two-dimensional matrix Wiener-Hopf problem arises due to the two interaction points (the trailing edge and the permeable-impermeable junction). This paper discusses a new iterative method for solving this matrix Wiener-Hopf equation which extends to further two-dimensional problems, in particular those involving analytic terms that exponentially grow in the upper or lower half-planes. This method is an extension of the commonly used 'pole removal' technique and avoids the need for full matrix factorisation. Convergence of this iterative method to an exact solution is shown to be particularly fast when terms neglected in the second step are formally smaller than all other terms retained. The new method is validated by comparing the iterative solutions for acoustic scattering by a finite impermeable plate against a known solution (obtained in terms of Mathieu functions). The final acoustic solution highlights the effects of the permeable-impermeable junction on the generated noise, in particular how this junction affects the far-field noise generated by high-frequency gusts by creating an interference to typical trailing-edge scattering. This effect results in partially porous plates predicting a lower noise reduction than fully porous plates when compared to fully impermeable plates. [ABSTRACT FROM AUTHOR]

Experimental investigations of unsteady cavitating flows in a hydrofoil tip leakage region with different gap sizes are conducted to highlight the development of gap cavitation. The experiments were taken in a closed cavitation tunnel, during which high-speed camera had been used to capture the cavitation patterns. A new visualization method based on image processing was developed to capture time-dependent cavitation patterns. The results show that the visualization method can effectively capture the cavitation patterns in the tip region, including both the attached cavity in the gap and the tip leakage vortex (TLV) cavity near the trailing edge. Moreover, with the decrease of cavitation number, the TLV cavity develops from a rapid onset-growth-collapse process to a continuous process, and extends both upstream and downstream. The attached cavity in the gap develops gradually stretching beyond the gap and combines with the vortex cavity to form the triangle cavitating region. Furthermore, the influences of gap size on the cavitation are also discussed. The gap size has a great influence on the loss across the gap, and hence the locations of the inception attached cavity. Besides, inception locations and extending direction of the TLV cavity with different gap sizes also differ. The TLV in the case with = 0.061 is more likely to be jet-like compared with that in the case with = 0.024, and the gap size has a great influence on the TLV strength. [ABSTRACT FROM AUTHOR]

With the flourishing development of the subway construction, more and more attentions are paid to the subway tunnel ventilation. The piston wind caused by the running trains can not only improve the tunnel environment, but also strengthen the power of the tunnel ventilation. The majority of the current researches are based on similar experiments or CFD simulations, yet most of which lack universality. It takes long time to finish the research cycles and it is difficult to generalize the conclusions based on these researches. This paper is aimed to put forward a universal prediction formula to estimate the ventilation effect of piston wind. In this paper, a uniformly accelerated Bernoulli theoretical model is established firstly, which is validated by the experimental data and numerical simulations. Secondly, a passing through model is built with the verified theoretical method and the influence of the key parameters on the ventilation rate are analyzed. Thirdly, the grey correlation analysis is conducted to study each influencing factors, meanwhile, a general prediction formula on the ventilation rate is proposed, which is applicable to the most subway tunnels in China. Finally, the effects of the trailing distance between multi-trains on the tunnel ventilation is discussed. [ABSTRACT FROM AUTHOR]

A numerical study has been carried out to elucidate the effects of rounding the sharp edges on flow-induced forces on a square cylinder immersed in a laminar cross flow. Rounding reduces both the upstream stagnation pressure and the downstream base pressure. Consequently, competition between these two pressure reductions yields the minimum drag on the cylinder when its edges are partially rounded. It was also found that the leading-edge rounding is mainly responsible for the topological change thus the drag reduction, while the trailing- edge rounding alone just enhances lift fluctuation. However, trailing-edge rounding plays a role of stabilizing the flow when all of the four edges are rounded. [ABSTRACT FROM AUTHOR]

Trailing edge modification is one of the most effective ways to achieve camber variations. Usual flaps and aileron implement this concept and allow facing the different needs related to take-off, landing, and maneuver operations. The extension of this idea to meet other necessities, less dramatic in terms of geometry change yet useful a lot to increase the aircraft performance, moves toward the so-called morphing architectures, a compact version of the formers and inserted within the frame of the smart structures’ design philosophy. Mechanic (whether compliant or kinematic), actuation and sensor systems, together with all the other devices necessary for its proper working, are embedded into the body envelope. After the successful experiences, gained inside the SARISTU (SmARt Intelligent Aircraft STrUctures) project where an adaptive trailing edge was developed with the aim of compensating the weight variations in a medium-size commercial aircraft (for instance, occurring during cruise), the team herein exploits the defined architecture in the wing of a typical airfoil, used on high-altitude long-endurance aircraft such as the Global Hawk. Among the peculiarities of this kind of aerial vehicle, there is the long endurance, in turn, associated with a massive fuel storage (approximately around 50% of the total weight). A segmented, finger-like, rib layout is considered to physically implement the transition from the baseline airfoil to the target configurations. This article deals with an extensive estimation of the possible benefits related to the implementation of this device on that class of planes. Parametric aerodynamic analyses are performed to evaluate the effects of different architectural layouts (in-plane geometry extension) and different shape envelopes (namely, the rotation boundaries). Finally, the expected improvements in the global high-altitude long-endurance aircraft performance are evaluated, following the implementation of the referred morphing device. [ABSTRACT FROM AUTHOR]

A time-domain hybrid approach for aerodynamic noise prediction is developed based on a discrete vortex model (DVM) for the unsteady incompressible flow simulation and the acoustic perturbation equations (APE) for the acoustical field computation. The aim is to assess the applicability of the present DVM-APE method to the problems where sound is generated by the large-scale coherent flow structures. The hybrid DVM-APE approach is employed to predict the vortex-shedding noise from the blunt trailing edge of a flat plate. Simulations are implemented on flat plates with different thicknesses in a certain range of low Mach numbers, in order to identify the scaling dependence of the vortex-shedding noise on the freestream speed as well as the plate thickness. Acoustical directivity patterns at different Helmholtz numbers are presented, and agreements are achieved when compared with previous studies. A comparison of the sound pressure level spectrum between the present DVM-APE simulation and the published experimental results is also presented, showing good agreements for both the peak frequencies and the sound pressure levels. [ABSTRACT FROM AUTHOR]

A quantitative characterization of the effects obtained by high frequency–low amplitude trailing edge actuation is presented. Particle image velocimetry, pressure and aerodynamic forces measurements are carried out on a wing prototype equipped with shape memory alloys and trailing edge piezoelectric-actuators, allowing simultaneously high deformations (bending) in low frequency and higher-frequency vibrations. The effects of this hybrid morphing on the forces have been quantified and an optimal actuation range has been identified, able to increase lift and decrease drag. The present study focuses more specifically on the effects of the higher-frequency vibrations of the trailing edge region. This actuation allows manipulation of the wake turbulent structures. It has been shown that specific frequency and amplitude ranges achieved by the piezoelectric actuators are able to produce a breakdown of larger coherent eddies by means of upscale energy transfer from smaller-scale eddies in the near wake. It results a thinning of the shear layers and the wake’s width, associated to reduction of the form drag, as well as a reduction of predominant frequency peaks of the shear-layer instability. These effects have been shown by means of frequency domain analysis and Proper Orthogonal Decomposition. [ABSTRACT FROM AUTHOR]

This paper proposes a modified TNO model for the prediction of aerofoil trailing-edge noise for wind turbine applications. The capabilities of the current modified model and four variants of the TNO model are analysed through a comprehensive study which includes 10 aerofoils and involves two different wind tunnels. The Reynolds numbers considered are between 1.13 and 3.41 million, and the effective angles of attack are between −2.20° and 13.58°. The merit of a model is assessed by comparing two aspects of the numerically predicted and the experimentally measured sound pressure level spectra: the sound pressure level difference between two different aerofoils at similar lift coefficients within a certain frequency range (referred to as the delta noise); and the closeness in terms of spectral magnitude and shape of the predicted and measured sound pressure level spectra. The current modified model is developed by deriving new formulations for the computation of the wall pressure fluctuation spectrum. This is achieved by using the approximate ratio of the normal Reynolds stress components for an anisotropic flow over a flat plate to estimate the vertical Reynolds stress component, and by introducing new stretching factors to take the effects of turbulent flow anisotropy into account. Compared with the four TNO model variants tested, the current modified model has strong delta noise prediction ability, and is able to predict sound pressure level spectra that are more consistent and closer to measurements for the vast majority of aerofoils and flow conditions tested in the two wind tunnels. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The broadband noise generated by the scattering of turbulent flow at the trailing edge of a NACA 0018 airfoil with trailing edge serrations is investigated, varying both the airfoil angle of attack and serration flap angle. Acoustic emissions from the trailing edge are measured using a microphone array. The noise level is observed to be higher than that of the airfoil without serrations at frequencies beyond a crossover value. The latter is found to scale with a characteristic Strouhal number based upon the boundary layer thickness and the freestream velocity. A satisfactory collapse of the results under varying angles of attack and freestream velocities is observed. The modifications of the hydrodynamic behavior and the noise increase are linked by high-speed observations conducted with particle image velocimetry. An increase in the energy of turbulent fluctuations is also observed at the expected crossover frequency. The dominant cause of the increased noise is thereby identified at the pressure side edge of the serrations at a given flap angle. [ABSTRACT FROM AUTHOR]

Different products, such as adhesives, pharmaceutical patches, batteries, and fuel cell membranes, require coating discrete patches onto moving substrates. For coating rectangular patches, intermittent slot die coating is the preferred method. The patches can be obtained by rapidly starting and stopping the flow out of the coating die. Controlling the flow start-up and shutdown to produce sharp and uniform leading and trailing edges of each patch is challenging. Different ways to control the liquid feeding are used to optimize the process. Even if the start-up and shutdown of the feeding system are well designed, the transient flow in the coating bead contributes to the formation of nonuniform leading and trailing edges of coated patches. In this work, we analyze how the operating conditions, die geometry, and liquid properties affect the coating bead breakup process and the trailing edge configuration. The process is directly related to the contact line dynamics. The results show that the uniformity of the trailing edge of each coated patch can be improved by changing the die shoulder angle and wetting characteristics of the die surface. [ABSTRACT FROM AUTHOR]

The mechanisms responsible for swishing and thumping noises generated by wind turbines are unclear and the existence of which have significantly affected the perception of wind energy by the community. To better understand the nature of this noise source, this study, for the first time, investigates the correlation between the potential noise generation mechanisms in wind farms and the characteristics of the perceived noise reported by residents in the vicinity of the farms in survey data. Published reports and measurements show that in addition to the perceived noise near the turbines, the thumping noise, in general, is perceived far downstream of the turbines. Normal swish perceived in a short distance from a wind turbine, especially in the cross-wind directions, can be explained by the convective amplification and directivity of the trailing edge noise. As will be discussed in this article, there exists strong evidence that the dominant mechanism of wind farm noise is associated with amplitude modulation of the aerodynamic noise by the eddies generated when the turbine blade partially stalls or due to an interaction with the turbine wake. This hypothesis is primarily based on the low frequency characteristics of the stall and also the distance and direction of the noise propagation. Moreover, it is hypothesised that the wake supplements this effect as it results in refraction and modulation of the emitted noise. [ABSTRACT FROM AUTHOR]

The structure of a trailing vortex from a wing undergoing small amplitude, low frequency heaving motion is investigated using space-time representations determined from stereo particle image velocimetry. The evolution of the vortex shows large fluctuations of axial velocity deficit and circulation during the oscillation cycle. Correspondingly, large variations of swirl ratio occur and onset of pronounced azimuthal vorticity arises. At a given cross-section of the vortex, the pattern of azimuthal vorticity moves around its axis in an ordered fashion as both it and the pattern of velocity defect increase in magnitude and scale. When the swirl ratio attains its minimum value during the oscillation cycle, and this value lies below the theoretically established critical threshold for amplification of azimuthal modes, the magnitude and scale of the pattern of azimuthal vorticity is maximized. Subsequent increase of the swirl ratio yields attenuation of the azimuthal vorticity. Onset of pronounced azimuthal vorticity when the swirl ratio decreases involves rapid amplification, then disruption, of axial vorticity fluctuation. [ABSTRACT FROM AUTHOR]

Guide vanes in Francis turbine faces frequent erosion due to its operation in sand laden water, which demands focus on design, manufacturing, operation and maintenance of turbines. These vanes are designed on high precision with standard hydrofoils for efficient conversion of pressure to kinetic energy. The erosion on surfaces, upper and lower ends and leading and trailing edges of guide vanes induces cross flow at clearance gaps, tip leakage at trailing edges and high friction on the surfaces, increasing hydraulic losses associated with flow. Review on post erosion hydraulic and mechanical effect on guide vanes and its vicinity are the main objectives of this work. It has been assisted by flow around vanes, field observation of erosion, possible operational problem and method of laboratory estimation of erosion. The vulnerable locations, erosion severity, possible post erosion problems, research gap and possible measures to comprehend them have been focused, which will be useful in further research activities. [ABSTRACT FROM AUTHOR]