Slat Cusp Research Articles

On the Origin of Görtler Vortices in Flow over a Multi-Element Airfoil

The flow characteristics of a 30P30N three-element high-lift airfoil at low Reynolds numbers O104 are examined through three-dimensional simulations using a high-order spectral element method. This study primarily investigates the flow structures of the slat cove and Görtler vortices formed on the upper surface of the main airfoil. Spanwise instability grows exponentially in the slat cove with a constant wavelength, corresponding to that of the subsequently formed Görtler vortices. Görtler number calculations show that curvature-induced centrifugal instability at the slat cusp leads to the subsequent formation of Görtler vortices. Proper orthogonal decomposition (POD) is used to analyze the development of flow structures in the slat cove in different time ranges. At early time, the flow in the slat cove is dominated by shear layers that evolve into spanwise perturbations. These perturbations further evolve into distinct bell-shaped structures close to the slat cusp and are advected to the upper surface of the main airfoil, leading to the formation of Görtler vortices.

Assessment of Slat Extensions and a Cove Filler for Slat Noise Reduction

The slat of a high-lift wing can be a significant noise source during approach and landing. This paper describes an experimental study of the effects of two passive devices (namely, slat extensions and a cove filler) on the aerodynamics and acoustic radiation from a 30P30N model. Test conditions include three effective free-air angles of attack of 5.5, 7.5, and 13 deg, as well as three Reynolds numbers of 1.2, 1.5, and . Steady surface pressure measurements are used to assess the aerodynamics near the midspan section. Unsteady surface pressure and far-field acoustic array measurements are performed to evaluate the near- and far-field pressure fluctuations, respectively. Delay-and-sum beamforming is applied to localize the noise sources and to provide integrated spectra. The slat cusp extensions barely affect the mean distribution, whereas the longest extension achieves up to a 5.4 dB reduction in the overall sound pressure level. The slat-cove filler yields an even higher noise reduction that is lower than the baseline by up to 14 dB. Even though the aerodynamic assessment of the cove filler is incomplete due to the covered pressure ports, the mean distribution over the remaining surface of the model indicates a rather small change compared with the baseline measurements.

Aeroacoustic analysis of slat tones

Experimental measurements were carried out to assess the aeroacoustic characteristics of a 30P30N high-lift device, with particular attention to slat tonal noise. Three different types of slat modifications, namely slat cove filler, serrated slat cusp, and slat finlets have been experimentally examined. The results are presented for an angle of attack of α = 18 at a free-stream velocity of U = 30 m/s, which corresponds to a chord-based Reynolds number of Re = 7 x 10. The unsteady surface pressure near the slat region and far-field noise were made simultaneously to gain a deeper understanding of the slat noise generation mechanisms. The nature of the low-frequency broadband hump and the slat tones were investigated using higher-order statistical approaches for the baseline 30P30N and modified slat configurations. Continuous wavelet transform of the unsteady surface pressure fluctuations along with secondary wavelet transform of the broadband hump and tones were carried out to analyze the intermittent events induced by the tone generating resonant mechanisms. Stochastic analysis of the wavelet coefficient modulus of the surface pressure fluctuations was also carried out to demonstrate the inherent differences of different tonal frequencies. An understanding into the nature of the noise generated from the slat will help design the new generation of quite high-lift devices.

Serrated slat cusp for high-lift device noise reduction

Noise reduction capabilities of slat cusp serration were experimentally assessed and demonstrated for a 30P30N three-element high-lift airfoil fitted with two different types of serrated slat cusps. Aerodynamic characteristics were evaluated with the aid of surface pressure distribution, and insignificant differences were found among all the tested configurations. The unsteady flow characteristics of the slat serrations were examined using near- and far-field measurements to gain a deeper understanding of the noise generation mechanism. Although increased surface pressure fluctuations were demonstrated by the slat serrations, a substantial reduction in the far-field noise was observed. The increase in the near-field energy levels was attributed to the non-propagating hydrodynamic energy field within the slat cove and the main element, whereas the far-field noise reduction was attributed to the elimination of vortex shedding by the slat serration. Further reaffirmation of the observations and hypothesis was provided with higher-order spectral analysis and wavelet analysis.

Large-eddy simulations of complex aerodynamic flows over multi-element iced airfoils

Large-eddy simulations (LESs) of flows over two types of iced airfoils with three multi-elements are performed to investigate the aerodynamic characteristics and complex interactions between flows generated from slat, main, and flap elements. The two iced airfoils are considered under supercooled large droplet (SLD) and non-SLD conditions. A good agreement of the mean properties between our numerical and previous experimental data demonstrates that our LES method provides an accurate solution of the complex flows around iced airfoils., whereas it is not for unsteady Reynolds-averaged Navier–Stokes (URANS) data that is simulated independently. For the iced airfoils under the SLD and non-SLD conditions, the aerodynamic degradation is found compared to that of a clean airfoil because the separation bubbles (SBs) induced by ice accretion change shear layer (SL) trajectory shed from the slat cusp, leading to a severe reduction in mass flow. Furthermore, we show that the flow interactions near the slat gap play a crucial role in determining the flow characteristics on main and flap elements (e.g., flow separation). Although strong flow interactions are observed for the non-SLD case because of the presence of upwind horn-shaped ice, the smaller gap distance of the SLD case leads to a larger lift loss. The unsteady features of SBs on the upper surfaces of the slat and main elements under the non-SLD condition are characterized by the power spectral density (PSD) of the pressure fluctuations with multiple peaks at low and high frequencies.

Flow characteristics of slat cove fillers

Experimental measurements using Particle Image Velocimetry were carried out to understand the flow characteristics of a 30P30N high-lift airfoil with and without slat cove fillers. The tests were carried out for the 30P30N airfoil with a retracted chord of c=0.35, at angles of attack of α=6∘ and 12∘, and for a chord-based Reynolds number of Rec=7.0×105. The wall pressure fluctuation results show that the use of slat cove fillers eliminates the slat tonal noise component. The results of the mean flow fields such as the normalized mean velocity, Reynolds stress components, and turbulent kinetic energy are presented for the baseline, half-slat cove filler, and slat cove filler configurations. The velocity contour results with streamlines showed a recirculation region within the slat cavity. The use of the half-slat cove filler reduced the size of the recirculation region and the use of the slat cove filler eliminated the recirculation region. The mean velocity and turbulent kinetic energy profiles at the slat wake showed insignificant difference between the three tested configurations. The Particle Image Velocimetry results were further analyzed using Proper Orthogonal Decomposition. The results showed that the first two eigenmodes of the vorticity with the highest energy were contained within the slat shear layer and vortex shedding emanating from the slat cusp for the baseline case. The energy levels were distributed over the slat shear layer for the half-slat cove filler and slat cove filler configurations as the vortex shedding is suppressed by the use of the slat cove fillers.

Three-Dimensional Simulation Analysis of the Influence of Slat Structural Parameters on Slat Aerodynamic Noise

Slat aerodynamic noise is one of important components of the frame noise. Adjusting the parameters of the slat structure can suppress the slat noise radiation effectively. Based on the typical multi-element airfoil 30P30N, firstly, by means of adjusting the parameters of slat structure, such as changing relative positions between slat and main airfoil and closing slat gap etc., a new slat profile is obtained; Then, based on the DDES simulation method, the characteristics of vortex distribution is analyzed for the new airfoil structure model, and the directivity and SPL of the far field noise are also investigated; Finally, through comparison analysis of the parameters, the physics of the generation of slat aerodynamic noise is discovered, and effects of slat parameters on the features of slat aerodynamic noise are also obtained. The results of the study show that the vortex strength of the flow around the slat has close relation to the source of slat noise. By means of adjusting slat position parameters and morphing of the slat's trailing edge, the vortex strength of the flow close to the slat cusp is reduced effectively, and further largely reducing the slat noise radiation, which can be achieved with preserving high lift coefficient of the airfoil.

Dynamics of the large-scale structures and associated noise emission in airfoil slats

We investigate the slat narrowband peak noise generating mechanism. Unsteady flow data were generated by a lattice-Boltzmann-based commercial code for four configurations, accounting for variations of the airfoil angle of attack and slat overlap. Comparison with experimental results indicates that the aspects of the flow field relevant for the generation of the narrowband peaks were accurately captured. Frequency-domain proper orthogonal decomposition (POD) is applied to identify dominant large-scale structures in the frequency range dominated by the peaks. The combined use of the two POD metrics, namely, the turbulent kinetic energy in the turbulent flow region and the acoustic pressure in the far field, demonstrated that the structures most correlated with the noise resemble spanwise coherent Kelvin–Helmholtz vortices which dominate the slat cove only at the frequency of the narrowband peaks. Time evolution of the structures educed using the acoustic pressure correlation provides detailed evidence of the hydrodynamic and acoustic steps of a Rossiter-like feedback mechanism between the slat cusp and trailing edge. The combined analysis of results for the different slat configurations provides an explanation for the effect of the slat configuration on the amplitude of the narrowband peaks observed in previous studies, particularly the influence of the main-element suction peak.

Aeroacoustic simulation of the high-lift airfoil using hybrid reynolds averaged Navier-Stokes/high-order implicit large eddy simulation method

A hybrid RANS/HILES method (HRILES) is developed by combining the RANS-SST model and high order implicit large eddy simulation method (HILES) and employed with the Ffowcs Williams-Hawkings (FW-H) equation to predict the slat noise of 30P30N high-lift airfoil. Comparison has been made between the HRILES method and the traditional DDES based on the full-turbulence model SST by simulating the single cylinder case with <inline-formula><tex-math id="M2">\begin{document}$Re_{{d}}=4.3\times10^4$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20190777_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="20-20190777_M2.png"/></alternatives></inline-formula>. The HRILES method is able to predict the transition phenomenon and the small-scale separation bubble in the sub-critical wake region while the DDES can't and get a better mean wall pressure distribution than DDES. The amplitude and frequency spectrum of the far-field sound pressure level are in good agreement with the experimental data. In the simulation of 30P30N high-lift airfoil, the famous IDDES model is also used for comparison, both results are compared with experimental measurements. The computational mesh is provided by Japan Aerospace Exploration Agency (JAXA) in the Workshop on Benchmark problems for Airframe Noise Computations (BANC). The HRILES method obtains quantitative agreement with experimental data in terms of mean wall pressure coefficient, frequency spectrum of pressure fluctuations on the slat surface, and the mean flow statistics in the slat cusp shear layer. The IDDES model slightly underestimate the suction effect on the upper surface of the slat, and delays the instabilities in the slat cusp shear layer. The near-field noise spectra are compared with measurements obtained in JAXA low-speed Wind Tunnel. Narrow band peaks present are well recovered by both methods, while IDDES model overestimate the broadband noise. Far-field noise directivity results of every components, filtered in the band [256Hz–10KHz], are compared with each other, and the slat cove is confirmed to dominate the sound noise levels. The slat and flap noises show a typical dipole distribution, while the main wing noise's directivity is not apparent. Computational results show that the HRILES method, as one kind of generalized Hybrid RANS/LES method, HRILES can smoothly switch between the SST model and the HILES method. HRILES has the high-resolution simulation capability of the HILES in the LES region, and can reduce the requirements of the HILES method on the near-wall grid distribution by using the SST model in the inner boundary layer. As a result, the HRILES method has advantages in simulations at high reynolds numbers and aeroacoustic problems. Further research will be carried out in the applications at higher reynolds number flows with complex geometry in the future.

Wake/shear layer interaction for low-Reynolds-number flow over multi-element airfoil

Time-resolved particle image velocimetry (TR-PIV) and hydrogen bubble visualization are employed to study the effects of Reynolds number on the wake/shear layer interactions over multi-element airfoil (30P30N). The Reynolds number based on the stowed chord length (Rec) ranges from 9.3 × 103 to 3.05 × 104. According to the variation of dominated flow structures, a critical Rec interval from 1.27 × 104 to 1.38 × 104 is found, which is novel for the low-Reynolds-number flow over multi-element airfoil. The slat wakes can be divided into two types by this critical interval. When Rec is smaller than this critical interval, no roll-up occurs to the shear layer of slat cusp. Gortler vortices generated by a virtual curved wall dominate the slat wake. When Rec is larger than this critical interval, roll-ups occur to the shear layer of slat cusp, which is similar to the cases at high Reynolds number (Rec ~ 106). These roll-ups and their evolution result in the co-existence of spanwise vortices and streamwise vortices in the slat wake. Different kinds of slat wake result in different kinds of wake/shear layer interactions above the main element. The flow physics behind these complex interactions, especially the novel flow structures and their evolution, is analyzed in detail to contribute to the fundamental research of wake/shear layer interactions. When Gortler vortices dominate the slat wake, they could trigger streaky structures within the leading-edge separated shear layer of the main element. When spanwise vortices and streamwise vortices co-exist in the slat wake, novel spanwise “double secondary vortices” are triggered above the main element by the spanwise vortices of slat cusp shear layer.

Slat noise suppression using upstream mass injection

A new approach, mass injection near the slat cusp, is proposed for the suppression of slat noise. The mechanism and efficiency of the slat noise suppression using upstream mass injection are numerically studied with hybrid Reynolds-averaged Navier-Stokes simulation/Large eddy simulation (RANS/LES) of a three-component high-lift model. Ffowcs-Williams and Hawking (FW-H) integration is employed to obtain far-field acoustic characteristics. Results show that with injection, aerodynamic capability of the high-lift devices is sustained. In addition, a considerable noise reduction is observed in the downward direction. With detailed analysis of the time-averaged and instantaneous flow fields, two mechanisms are identified as the reason accounting for the noise suppression: upstream mass injection significantly reduces the scale and strength of the vortices stemming from the cusp; the shear layer is lifted up, which relieves the shear layer impingement on the slat lower surface.

Aeroacoustic Effects of High-Lift Wing Slat Track and Cut-Out System

Effects of the slat track and cut-out on noise propagation were investigated using compact source models for a high-lift wing. Two different geometries of the high-lift devices, with and without the slat track and cut-out, were investigated. Convection effects were also considered with the mean flows obtained from precursor simulations of corresponding high-lift configurations. Major noise sources in the slat cove region were modelled with equivalent compact dipoles placed along the slat trailing edge and cusp. Results show that sound propagation is modified by the add-on components. The acoustic waves are amplified significantly and radiate more energy towards the ground due to the interaction between propagating waves and the slat track and cut-out. Although the cancellation caused by interference of the radiated and reflected waves happens in both of these configurations especially in the downwards direction, it is more prominent once sound waves pass the slat track and cut-out. Wave cancellation is also significant when the wavelength is comparable to the size of the barrier such as the gap between the slat and the main element. It is also noticed that higher frequencies radiate large proportion of energy towards ground for the sources at slat cusp.

Unsteady Flow Analysis of a Multi-Element Airfoil Using Lattice Boltzmann Method

High-lift devices employed onmodern aircraft are significant contributors to overall airframe noise. In this paper, a lattice Boltzmannmethodwith a very large eddy simulation approach is applied to computationally investigate the aerodynamic and aeroacoustic behavior of the flow around a generic high-lift configuration (three-element airfoil) at low Mach number. Three-dimensional time-dependent nearly incompressible simulations were conducted at different angles of attack to capture the instantaneous andmeanflowfields around the airfoil, previously predicted by Navier–Stokes studies. The computed mean flow results showed good agreement with existing experimental and numerical data, which include the pressure distributions around the elemental surfaces and the time-averagedmean flowfield within the slat cove. As a major objective of the present study, the unsteady flow simulations were used to capture the slat cove unsteadiness, a source of both broadband and narrowband noise. In particular, the effect of angle of attack on the shear layer emanating from the slat cusp, slat trailing-edge vortex shedding, convection, and reattachment of vortical structures near the slat gap were explored by the present simulations. Consequently, the acoustic implications of such complex unsteady flow phenomenon within the slat cove were explained and discussed in detail.

Effect of Three-Dimensional Shear-Layer Structures on Slat Cove Unsteadiness

Numerical simulations are used to investigate the local and global dynamics of large-scale, three-dimensional vorticity structures within the free shear layer originating from the slat cusp of a multielement airfoil configuration. Results indicate that accounting for the local three-dimensionality of flow fluctuations leads to substantially improved agreement between the computed unsteady near-field solution and wind tunnel measurements based on particle image velocimetry. Analysis of simulation data indicates the potential significance of high intensity turbulent fluctuations near the reattachment location along the slat lower surface toward the generation of broadband slat noise. The computed acoustic characteristics, in terms of the frequency spectrum and spatial distribution within short distances from the slat, resemble the previously reported, subscale measurements of slat noise.