Aerofoil Surface Research Articles

Effect of ZrB2 and ZrC and mixing method on Mechanical properties and wear behaviour of Al7075 based composites for aircraft aerofoil surfaces

Developing lightweight materials with notable properties for aerospace applications is a great challenge. The impact of ZrB2 and ZrC reinforcement on the mechanical and wear behaviour of stir-cast Al7075-ZrB2-ZrC composites is investigated with a weight percentage of 2, 4, 6 and 8% ZrB2, and fixed 2% ZrC. The results showed notable improvements with a hardness of 96 HRB, tensile strength of 423MPa and compressive strength of 385MPa; hardness increased by 14%, tensile strength increased by 11% and compressive strength was enhanced by 13% when compared with pure Al7075. The wear behaviour has been analysed by pin-on-disc wear test at room temperature with loads of 10N and 20N against the sliding distance of 400m to 1200m; the wear resistance is improved by 17% and CoF is reduced by 11% compared to pure Al7075. SEM micrographs display the wear mechanism and microstructural changes happened during the wear test. Al7075-ZrB2-ZrC composites are the optimum replacement for aerofoil surfaces because of their remarkable properties.

Post-stall flow control on aerofoils by leading-edge flags

Self-excited oscillations of flags attached at the leading edge of aerofoils have been investigated at post-stall angles of attack at a chord Reynolds number of 100 000. Significant increases in the time-averaged lift coefficient and stall angle have been observed for three aerofoils: one symmetric, one cambered and one with a sharp leading edge. The aerodynamic improvement is due to the periodic formation of vortices caused by the flag oscillations. When the flag is near the aerofoil surface, it is lifted upwards by the induced velocity of the growing vortex. As the flag moves up, the vortex grows in strength and reaches maximum circulation when the flag is furthest from the aerofoil surface and subsequently sheds. Flags with large stiffness exhibit better spatial and temporal coherence of flag oscillations than the compliant flags, resulting in a larger maximum lift coefficient and higher stall angle. For all aerofoils tested, the best lift enhancement with respect to the clean aerofoils is found when the angle of attack is 6° to 10° above the stall angle of the clean aerofoil. High lift is observed when the flags are locked in with the wake instability in a narrow frequency band, depending on the flag mass ratio and length.

Investigation on the Influence of Vortex Generators on Aerofoil

Boundary layer separation is a major cause of concern in the wind turbine and aerospace industries as it adversely affects the performance of aircraft wings and wind turbine blades. The immediate result of flow separation over an aerofoil causes decrease in lift and an increase in drag within the stall range, both of which have a negative impact on the overall performance of aerofoils in the wind turbines. To overcome this problem, vortex generators (VGs) are used for increasing lift force and thus increasing the rotational speed of the blade. They act like small protrusions on wing surfaces which helps in energizing the boundary layers thereby delaying the separation and providing a wide range of attached flow over aerofoil surface. The present research illustrates the comparative analysis on the various shapes of VGs such as rectangular, triangular and gothic by varying the angle of attacks (AoA) from 0-15. The analysis is performed on NACA0012 aerofoil, which is developed using SOLIDWORKS and analysed on ANSYS 2020 R2 software. The values of lift (Cl) and drag (Cd) coefficients with and without VGs are determined using ANSYS fluent and the contours for velocity and pressure are further mapped using CFD Post. The stall angle is evaluated for each of the VG shapes representing the maximum lift that an aerofoil can achieve. Pressure coefficient plots are studied in order to investigate the amount of pressure difference created and its influence AoA at the leading edge of the aerofoil. An overall comparison is made to determine the best VG shape that delivers maximum lift to the aerofoil. The optimum configuration found for VG is gothic shape which illustrates the maximum lift coefficient value, which is highest amongst all the three shapes.

Large-eddy simulations and modal reconstruction of laminar transonic buffet

Transonic buffet refers to the self-sustained periodic motion of shock waves observed in transonic flows over wings and can limit the flight envelope of aircraft. Based on the boundary layer characteristics at the shock foot, buffet has been classified as laminar or turbulent and the mechanisms underlying the two have been proposed to be different (Dandois et al., J. Fluid Mech., vol. 18, 2018, pp. 156–178). The effect of various flow parameters (freestream Mach and Reynolds numbers and sweep and incidence angles) on laminar transonic buffet on an infinite wing (Dassault Aviation's supercritical V2C aerofoil) is reported here by performing large-eddy simulations (LES) for a wide range of parameters. A spectral proper orthogonal decomposition identified the presence of a low-frequency mode associated with buffet and high-frequency wake modes related to vortex shedding. A flow reconstruction based only on the former shows periodic boundary-layer separation and reattachment accompanying shock wave motion. A modal reconstruction based only on the wake mode suggests that the separation bubble breathing phenomenon reported by Dandois et al. is due to this mode. Together, these results indicate that the physical mechanisms governing laminar and turbulent buffet are the same. Buffet was also simulated at zero incidence. Shock waves appear on both aerofoil surfaces and oscillate out of phase with each other indicating the occurrence of a Type I buffet (Giannelis et al., Aerosp. Sci. Technol., vol. 18, 2018, pp. 89–101) on a supercritical aerofoil. These results suggest that the mechanisms underlying different buffet types are the same.

Development of mullite-alumina ceramic shells for precision investment casting of single-crystal high-pressure turbine blades

Alumina-mullite ceramic shell moulds were developed to cast single-crystal hollow high-pressure turbine blades (SX-HPTB). Mullite and fine alumina powders were used as fillers. Two colloidal silica binders were used to bind the filler particles. One colloidal silica binder (binder A) had no polymer and the second colloidal silica binder (binder B) had 8 wt % polymer. Eight slurry compositions with fine alumina substitution (0, 10, 30 and 50 wt %) were prepared. Silica cores were used to achieve hollow castings. The effect of alumina substitution and binders on the slurry characteristics, flexural strength of green and fired samples, air permeability and self-load sag resistance was studied. It was observed that with an increase in alumina percentage, the test specimens showed increased viscosity, pick-up weight, density, green strength and fired residual strength. In addition, the slurries prepared from binder B possessed larger values of viscosity and pick-up weight compared to the slurries prepared from binder A. The residual strength of fired test specimens containing binder A was higher than those containing binder B. The test specimens with 30 wt % fine alumina substitution showed the highest self-load sag resistance w.r.t. each test temperature. Also, the air permeability of the fired samples was higher than the green (de-waxed) samples. The casting of hollow HPTB components was performed at two temperatures (1525 °C and 1550 °C) using nickel-based superalloy CMSX4. The HPTB components, cast at 1550 oC from ceramic shell mould having binder B and 30 wt % fine alumina substitution, resulted in an acceptable aerofoil profile and surface roughness.

Quadrupole noise generated from a low-speed aerofoil in near- and full-stall conditions

In this paper, direct numerical simulations are performed for low-speed flows past a NACA0012 aerofoil at high incidence angles. The aim is to investigate the significance of quadrupole noise generated due to separated shear layers, in comparison to dipole noise emanating from the aerofoil surface. The two different noise components (dipole and quadrupole) are calculated by using the Ffowcs Williams & Hawkings method in two different approaches: one with a solid surface and another with a permeable surface. The quadrupole noise is then estimated approximately by taking the relative difference between the two. The current study provides detailed comparisons between the quadrupole and dipole noise components at various observer locations in a wide range of frequencies. The comparisons are also made in terms of Mach number scaling, which differs significantly from theoretical predictions and changes rapidly with frequency. Additionally, pre-, near- and full-stall conditions are cross-examined, which reveals significant differences in the quadrupole contributions, including changes in the major source locations and frequencies. It is found that the inclusion of the quadrupole sources gives rise to the predicted noise power level at all frequencies (varying between 2 and 10 dB for an observer above the aerofoil) compared to the dipole-only case. The quadrupole contribution is far from negligible even at the low subsonic speeds (Mach 0.3 and 0.4) when aerofoil stall occurs.

Sequential Data Assimilation of Turbulent Flow and Pressure Fields over Aerofoil

The present study concentrates on the global velocity and pressure fields recovery over a NACA0012 aerofoil using the adjoint-based sequential data assimilation (DA) approach. Planar particle image velocimetry (PIV) measurement is conducted in the water channel to capture the time-resolved velocity field above the aerofoil, and the velocity field data are then used for DA. The attack angle is fixed at , and the Reynolds number, based on the inflow velocity and chord length , is . Conventional pressure determination methods that use either integration or a solution of the Poisson equation are applied to compute the instantaneous pressure field from the PIV data. Dynamic detached-eddy simulation (DDES) and Reynolds-averaged Navier–Stokes (RANS) simulations using the shear strain transport model are also performed for comparison. The conventional pressure determination methods yield large errors due to the error accumulation and boundary effects, whereas the RANS simulation seriously mispredicts the flow separation, and the DDES also fails due to low mesh resolution. By solving the primary-adjoint equation system and updating the correction for the governing equations at time steps for which observations exist, DA accurately reproduces the instantaneous flowfield in the observational region, which substantially improves the prediction of pressure distribution on the aerofoil surface and the flow in the wake region. Using the observational data in half of the region, DA also improves the flow and pressure prediction in the other half region. The present DA method is therefore able to reproduce the global fields from limited measurement data.

Acoustic-roughness receptivity in subsonic boundary-layer flows over aerofoils

The generation of a viscous–inviscid instability through scattering of an acoustic wave by localised and distributed roughness on the upper surface of a NACA 0012 aerofoil is studied with a time-harmonic compressible adjoint linearised Navier–Stokes approach. This extends previous work by the authors dedicated to flat plate geometries. The key advancement lies in the modelling of the inviscid acoustic field external to the aerofoil boundary layer, requiring a numerical solution of the convected Helmholtz equation in a non-uniform inviscid field to determine the unsteady pressure field on the curved aerofoil surface. This externally imposed acoustic pressure field subsequently drives the acoustic boundary layer, which fundamentally determines the amplitudes of acoustic-roughness receptivity. A study of receptivity in the presence of Gaussian-shaped roughness and sinusoidally distributed roughness at Mach number $M_\infty =0.4$ and Strouhal numbers $\mathcal {S} \approx \{46,69,115\}$ shows the effects of various parameters, most notably angle of attack, angle of incidence of the externally imposed plane acoustic wave and geometry of surface roughness; the latter is varied from viewpoint of its placement on the aerofoil surface and its wavelength. The parametric study suggests that non-parallel effects are quite substantial and that considerable differences arise when using parallel flow theory to estimate the optimal width of Gaussian-shaped roughness elements to provoke the greatest response. Furthermore, receptivity amplitudes for distributed roughness are observed to be generally higher for lower angles of attack, i.e. for less adverse pressure gradients. It is also shown that the boundary layer is more receptive to upstream-travelling acoustic waves.

Computational analysis to enhance the compressible flow over an aerofoil surface

PurposeSince the inception of aerospace engineering, reducing drag is of eternal importance. Over the years, researchers have been trying to improve the aerodynamics of National Advisory Committee for Aeronautics (NACA) aerofoils in many ways. It is proved that smooth-surfaced NACA 0012 aerofoil produces more drag in compressible flow. Recent research on shark-skin pattern warrants a feasible solution to many fluid-engineering problems. Several attempts were made by many researchers to implement the idea of shark skin in the form of coatings, texture and more. However, those ideas are at greater risk when it comes to wing maintenance. The purpose of this paper is to implement a relatively larger biomimetic pattern which would make way for easy maintenance of patterned wings with improved performance.Design/methodology/approachIn this paper, two biomimetic aerofoils are designed by optimizing the surface pattern of shark skin and are tested at different angles of attack in the computational flow domain.FindingsThe results of the biomimetic aerofoils prove that viscous and total drag can be reduced up to 33.08% and 3.68%, respectively, at high subsonic speed when validated against a NACA 0012 aerofoil. With the ample effectiveness of patched shark-skin pattern, biomimetic aerofoil generates as high as 10.42% lift than NACA 0012.Originality/valueIn this study, a feasible shark-skin pattern is constructed for NACA 0012 in a transonic flow regime. Computational results achieved using the theoretical model agree with experimental data.

The evolution of darker wings in seabirds in relation to temperature-dependent flight efficiency.

Seabirds have evolved numerous adaptations that allow them to thrive under hostile conditions. Many seabirds share similar colour patterns, often with dark wings, suggesting that their coloration might be adaptive. Interestingly, these darker wings become hotter when birds fly under high solar irradiance, and previous studies on aerofoils have provided evidence that aerofoil surface heating can affect the ratio between lift and drag, i.e. flight efficiency. However, whether this effect benefits birds remains unknown. Here, we first used phylogenetic analyses to show that strictly oceanic seabirds with a higher glide performance (optimized by reduced sink rates, i.e. the altitude lost over time) have evolved darker wings, potentially as an additional adaptation to improve flight. Using wind tunnel experiments, we then showed that radiative heating of bird wings indeed improves their flight efficiency. These results illustrate that seabirds may have evolved wing pigmentation in part through selection for flight performance under extreme ocean conditions. We suggest that other bird clades, particularly long-distance migrants, might also benefit from this effect and therefore might show similar evolutionary trajectories. These findings may also serve as a guide for bioinspired innovations in aerospace and aviation, especially in low-speed regimes.

Unsteady simulations of migration and deposition of fly-ash particles in the first-stage turbine of an aero-engine

AbstractParticulate deposits in aero-engine turbines change the profile of blades, increase the blade surface roughness and block internal cooling channels and film cooling holes, which generally leads to the degradation of aerodynamic and cooling performance. To reveal particle deposition effects in the turbine, unsteady simulations were performed by investigating the migration patterns and deposition characteristics of the particle contaminant in a one-stage, high-pressure turbine of an aero-engine. Two typical operating conditions of the aero-engine, i.e. high-temperature take-off and economic cruise, were discussed, and the effects of particle size on the migration and deposition of fly-ash particles were demonstrated. A critical velocity model was applied to predict particle deposition. Comparisons between the stator and rotor were made by presenting the concentration and trajectory of the particles and the resulting deposition patterns on the aerofoil surfaces. Results show that the migration and deposition of the particles in the stator passage is dominated by the flow characteristics of fluid and the property of particles. In the subsequential rotor passage, in addition to these factors, particles are also affected by the stator–rotor interaction and the interference between rotors. With higher inlet temperature and larger diameter of the particle, the quantity of deposits increases and the deposition is distributed mainly on the Pressure Side (PS) and the Leading Edge (LE) of the aerofoil.

Leading Edge Blowing to Mimic and Enhance the Serration Effects for Aerofoil

Leading edge serration is now a well-established and effective passive control device for the reduction of turbulence–leading edge interaction noise, and for the suppression of boundary layer separation at high angle of attack. It is envisaged that leading edge blowing could produce the same mechanisms as those produced by a serrated leading edge to enhance the aeroacoustics and aerodynamic performances of aerofoil. Aeroacoustically, injection of mass airflow from the leading edge (against the incoming turbulent flow) can be an effective mechanism to decrease the turbulence intensity, and/or alter the stagnation point. According to classical theory on the aerofoil leading edge noise, there is a potential for the leading edge blowing to reduce the level of turbulence–leading edge interaction noise radiation. Aerodynamically, after the mixing between the injected air and the incoming flow, a shear instability is likely to be triggered owing to the different flow directions. The resulting vortical flow will then propagate along the main flow direction across the aerofoil surface. These vortical flows generated indirectly owing to the leading edge blowing could also be effective to mitigate boundary layer separation at high angle of attack. The objectives of this paper are to validate these hypotheses, and combine the serration and blowing together on the leading edge to harvest further improvement on the aeroacoustics and aerodynamic performances. Results presented in this paper strongly indicate that leading edge blowing, which is an active flow control method, can indeed mimic and even enhance the bio-inspired leading edge serration effectively.

Surface flow modification of aerofoils for automotive racing car applications

An aerofoil commonly used in aerospace engineering to produce lift is also employed in the motor sport industry to produce downforce for improving traction during cornering. This paper investigates aerofoil surface modification through ‘golf ball dimpling’, used to reduce flow separation behind a golf ball. The studies of other researchers have shown that this type of design can have a positive effect on improving aerofoil performance. However, no optimization information of dimple sizing is given in literature. Therefore, three types of dimpling sized at 5, 10 and 15 mm are applied to the surface of a NACA 6615 wing at 25% chord length from the leading edge in this study using Computational Fluid Dynamics (CFD) as an initial design process. Then a physical model, made through 3D printing additive manufacturing (AM), is tested at angles of attack (AoA) ranging from [Formula: see text] to [Formula: see text] and wind speed up to 30 m/s in a subsonic wind tunnel. Experimental and CFD results show that the smallest dimple size provides the most significant increase on lift to drag ratio at high AoA above [Formula: see text]. This ratio increases further with the wind speed, indicating that a high AoA wing favors down force to improve drag reduction performance.

Computational fluid dynamics modelling to design and optimise power kites for renewable power generation

Power kites provide the potential rewards of obtaining the disused energy supply from high altitude wind. This paper aims to provide a design of power kite and optimise the potential for renewable power generation. The power kite was modelled using computational fluid dynamics to study its characteristics. The numerical modelling results were compared against the wind tunnel experimental study and two 3D printed power kites. The design was optimised using several variables, of which include aerofoil choice, surface roughness, wind speed and operating parameters. The results suggest that operating the kites at minimum 15 m horizontal separation is favourable, with the trailing kite operating below the leading, removing the potential for this kite to operate in the wake turbulence of the first. This paper presents relevant, applicable data which can be used for predicting the performance, and potentially optimising further power kite design.

On the universal trends in the noise reduction due to wavy leading edges in aerofoil–vortex interaction

Existing studies suggest that wavy leading edges (WLEs) offer substantial reduction of broadband noise generated by an aerofoil undergoing upstream vortical disturbances. In this context, there are two universal trends in the frequency spectra of the noise reduction which have been observed and reported to date: (i) no significant reduction at low frequencies followed by (ii) a rapid growth of the noise reduction that persists in the medium-to-high frequency range. These trends are known to be insensitive to the aerofoil type and flow condition used. This paper aims to provide comprehensive understandings as to how these universal trends are formed and what the major drivers are. The current work is based on very-high-resolution numerical simulations of a semi-infinite flat-plate aerofoil impinged by a prescribed divergence-free vortex in an inviscid base flow at zero incidence angle, continued from recent work by the authors (Turner & Kim, J. Fluid Mech., vol. 811, 2017, pp. 582–611). One of the most significant findings in the current work is that the noise source distribution on the aerofoil surface becomes entirely two-dimensional (highly non-uniform in the spanwise direction as well as streamwise) at high frequencies when the WLE is involved. Also, the sources downstream of the LE make crucial contributions to creating the universal trends across all frequencies. These findings contradict the conventional LE-focused one-dimensional source analysis that has widely been accepted for all frequencies. The current study suggests that the universal trends in the noise-reduction spectra can be properly understood by taking the downstream source contributions into account, in terms of both magnitude and phase variations. After including the downstream sources, it is shown in this paper that the first universal trend is due to the conservation of total (surface integrated) source energy at low frequencies. The surface-integrated source magnitude that decreases faster with the WLE correlates very well with the noise-reduction spectrum at medium frequencies. In the meantime, the high-frequency noise reduction is driven almost entirely by destructive phase interference that increases rapidly and consistently with frequency, explaining the second universal trend.

The steady aerodynamics of aerofoils with porosity gradients.

This theoretical study determines the aerodynamic loads on an aerofoil with a prescribed porosity distribution in a steady incompressible flow. A Darcy porosity condition on the aerofoil surface furnishes a Fredholm integral equation for the pressure distribution, which is solved exactly and generally as a Riemann-Hilbert problem provided that the porosity distribution is Hölder-continuous. The Hölder condition includes as a subset any continuously differentiable porosity distributions that may be of practical interest. This formal restriction on the analysis is examined by a class of differentiable porosity distributions that approach a piecewise, discontinuous function in a certain parametric limit. The Hölder-continuous solution is verified in this limit against analytical results for partially porous aerofoils in the literature. Finally, a comparison made between the new theoretical predictions and experimental measurements of SD7003 aerofoils presented in the literature. Results from this analysis may be integrated into a theoretical framework to optimize turbulence noise suppression with minimal impact to aerodynamic performance.

On the receptivity of aerofoil tonal noise: an adjoint analysis

For moderate-to-high Reynolds numbers, aerofoils are known to produce substantial levels of acoustic radiation, known as tonal noise, which arises from a complex interplay between laminar boundary-layer instabilities, trailing-edge acoustic scattering and upstream receptivity of the boundary layers on both aerofoil surfaces. The resulting acoustic spectrum is commonly characterised by distinct equally spaced peaks encompassing the frequency range of convectively amplified instability waves in the pressure-surface boundary layer. In this work, we assess the receptivity and sensitivity of the flow by means of global stability theory and adjoint methods which are discussed in light of the spatial structure of the adjoint global modes, as well as the wavemaker region. It is found that for the frequency range corresponding to acoustic tones the direct global modes capture the growth of instability waves on the suction surface and the near wake together with acoustic radiation into the far field. Conversely, it is shown that the corresponding adjoint global modes, which capture the most receptive region in the flow to external perturbations, have compact spatial support in the pressure surface boundary layer, upstream of the separated flow region. Furthermore, we find that the relative spatial amplitude of the adjoint modes is higher for those modes whose real frequencies correspond to the acoustic peaks. Finally, analysis of the wavemaker region points at the pressure surface near 30 % of the chord as the preferred zone for the placement of actuators for flow control of tonal noise.

The importance of the unsteady Kutta condition when modelling gust–aerofoil interaction

The Kutta condition is applied to aerofoils with sharp trailing edges to allow for viscous effects to be considered within a simplified system of equations that are inviscid. This paper discusses in detail the inclusion of an unsteady Kutta condition at a sharp trailing edge during gust–aerofoil interaction and illustrates how the analytic solution for the far-field noise generated by this interaction changes if the unsteady Kutta condition is neglected, or more precisely, if the unsteady pressure is permitted to be singular at the trailing edge. The analytic solution, both with and without the unsteady Kutta condition, is compared with numerical results that have no imposed unsteady Kutta condition. Importantly the results agree well only when the unsteady Kutta condition is neglected in the analytic solution. This paper highlights where the far-field acoustics are most affected by neglecting the unsteady Kutta condition for a variety of singularities that can occur in the unsteady pressure at the trailing edge and shows that results permitting different behaviour in the unsteady surface pressure at the trailing edge could give significantly different far-field noise predictions, even though the surface pressure elsewhere along the aerofoil surface agrees with benchmark solutions.

Experimental investigation of flowfield over an iced aerofoil

ABSTRACTWind-tunnel measurements were used to study the characteristics of the unsteady separation bubbles on a NACA 0015 aerofoil with simulated two-dimensional leading-edge glaze ice accretions. The unsteadiness present in the iced-aerofoil flowfield was determined using measurements of the time-dependent aerofoil surface pressure distribution at Reynolds number of 1.0 × 106. Additionally, the unsteady flow features were investigated through the power spectrum of the stream-wise velocity fluctuations using a hot-wire anemometry. The results showed that the highest value of root-mean-square fluctuation of the surface pressure consistently occurred upstream of the mean shear-layer reattachment location. Spectral analysis of stream-wise velocity fluctuations near reattachment location revealed evidence of the regular frequency at Strouhal numbers of 0.5-0.63. Moreover, the low-frequency oscillations associated with shear-layer flapping was also identified in the wake velocity spectra on the order of 10 Hz that resulted in Strouhal numbers of 0.0186-021.

DETERMINATION OF SUITABLE ELECTROLYTE FOR MAXIMUM MRR AND THE BEST SURFACE FINISH FOR MACHINING TITANIUM ALLOYS BY ECM

The rapid developments in recent years of the advanced technological industries like aerospace, nuclear power etc. has been accompanied by an increasing use of difficult-to-machine like titanium and nimonic. Some Aerospace industries manufacture R-29B engine which is fitted into MIG-27M aircraft. In this engine, six out of eleven stages of compressor rotor blades and compressor stator blades upto fifth stage are made of titanium alloys. A great deal of problems is encountered in machining these blades conventionally because of some inherent mechanical properties.These problems are multiplied due to the complexity of the blade aerofoil surface. All these problems result in frequent resharpening of tools and cams, higher machining times due to lower speed, feed & depth of cuts, higher inventory of tools and incorporation of an intermediate belt grinding process to give required shape, size and finish to the blade. One of the methods of conveniently machining them is by electro chemical machining. (ECM). In this paper, several experiments have been carried out to find out suitable electrolyte(s) among four electrolytes, namely Sodium Chloride (NaCl), Sodium Nitrate (NaNa3), Potassium Bromide (KBr) & Sodium Sulphate (Na2So4) for machining titanium alloys by ECM. Experiments have been conducted by using ANOVA”to optimize the results.