The leading-edge slat of high-lift devices is recognized as a dominant source of airframe noise. Realizing effective noise reduction while preserving aerodynamic performance remains a major challenge for high-lift design in civil aircraft. To address this issue, the present study investigates a multi-element airfoil with a very long chord slat (VLCS) through surrogate-based aerodynamic optimization, followed by high-fidelity aeroacoustic analysis and wind tunnel measurements. Compared with a conventional slat, the optimized VLCS exhibits a seamless leading-edge configuration at takeoff and a reduced deflection with an enlarged gap at landing, achieving improvements of 10.4% in takeoff lift-to-drag ratio and 7.8% in landing maximum lift coefficient. Far-field acoustic results indicate that noise reduction at takeoff is primarily associated with the suppression of broadband components, while at landing the dominant low-frequency tonal peaks are effectively attenuated, consistent with modified shear-layer dynamics within the slat cove near the trailing edge. These numerical findings are further confirmed by wind tunnel measurements, showing an overall noise reduction of similar to 1 dB at takeoff and 3 dB at landing. The present results demonstrate the potential of the VLCS for integrated aerodynamic and aeroacoustic optimization of high-lift devices.

Direct numerical simulations of turbulent boundary layers over two types of discontinuous converging and diverging riblets are performed to validate their drag reduction performances and investigate their impacts on turbulence statistics and coherent structures. To suppress the total drag increases in the diverging region of traditional converging and diverging riblets (T-riblets), we design new converging and diverging riblets (N-riblets) with the heights gradually decreasing from the convergence line to the diverging line. Results show that both riblet configurations reduce the skin-friction drag, but the pressure drag is increased. The N-riblets are able to relieve the net drag increase from 7.42% to 0.93%, suggesting their potential in reducing the total drag. Both the discontinuous converging and diverging riblets cause the generation of large-scale secondary flows that originate from the ribbed walls and persist in the downstream wakes over the smooth walls. Their impacts on the time-averaged flow fields and Reynolds stresses are shown to be different between the T- and N-riblets. Moreover, two-point correlations of streamwise velocity fluctuations are calculated to show their modifications on coherent structures, providing more insights to explain the different impacts of T- and N-riblets on the fluid dynamics over the ribbed walls and in the downstream wakes.

Long-horizon prediction of three-dimensional (3D) wall-bounded turbulence with machine-learning methods remains a challenging task, due to the rapid accumulation of autoregressive errors and the substantially computational cost. To address these challenges, we present a hybrid machine-learning framework, in which a channel-time-attention Swin-UNet (CTA-Swin-UNet) and a multi-time-scale fusion correction (MTFC) strategy are developed to predict the turbulent flow fields in a wall-parallel plane, with affordable computational cost. Then, 3D flow fields are reconstructed via a resolvent-based spectral linear stochastic estimation (SLSE), rooting from the predicted planar flow. Results show that the CTA-Swin-UNet outperforms the baseline models (LSTM, FNO and traditional Swin-UNet) in both single-step prediction and autoregressive rollouts, indicating the effectiveness of introducing the CTA module into the Swin-UNet architecture. At the same temporal interval, the CTA-Swin-UNet remains stable for approximately 150 rollout steps, while the baseline models fail within 20 to 50 rollout steps. After introducing the MTFC strategy, a longer horizon upto 300 steps is achieved. Using the resolvent-based SLSE reconstruction further recovers the 3D flow structures and energy spectral distributions from the predicted planar inputs, which demonstrates that the proposed framework provides an effective and computationally efficient approach for long-horizon autoregressive prediction of 3D wall-bounded turbulence.

Boundary layer ingestion (BLI) can raise propulsive efficiency in civil aviation. Rear-mounted engines, however, introduce complex inlet distortions that include both total-pressure and swirl components, which together challenge transonic ducted fan performance. While recent studies have increasingly recognized the presence of swirl in BLI distortion, its specific coupling mechanisms with compressible flow features—particularly shock wave topology—and its thermodynamic dissipation breakdown under varying rotational speeds remain underexplored. This work aims to isolate and quantify these swirl-driven compressible penalties. Entropy peaks occur at the blade tip and hub. In these regions, local Mach number amplification reshapes the shock topology, which consequently intensifies both shock-boundary-layer interactions and tip leakage. Furthermore, analysis using the Boussinesq number reveals that viscous dissipation dominates the core flow, while thermal dissipation becomes appreciable only in the tip region when swirl is minimal. Reducing the rotational speed to 80% broadens the distortion-affected zones but weakens their local intensity. Incidence-angle analysis further shows that the upstream inlet distortion is not transmitted unchanged to the blade passage: away from the leading-edge influence region it produces strong low-order circumferential incidence peaks, near the leading edge its shape is reorganized, and immediately upstream of the leading edge it is largely smoothed into blade-passing-scale oscillations. Even after this pre-passage attenuation, residual high-incidence sectors can overlap with shock-sensitive spans, destabilizing the tip-region flow and causing substantial additional loss. These findings show that BLI fan penalties are governed not only by the magnitude of inlet swirl distortion, but also by its axial reshaping, spanwise phase shift, and residual alignment with passage shock structures.

The blade pitch angle is a critical geometric parameter that significantly affects the aerodynamic performance of coaxial contra-rotating propellers. In this study, numerical simulations were conducted to systematically investigate the effects of varying the blade pitch angles of the front and rear rotors on the aerodynamic performance of a variable-pitch contra-rotating propeller system. The results demonstrate that as the blade pitch angle increases, both the thrust coefficient and power coefficient of the front and rear rotors exhibit a significant upward trend, while the propulsive efficiency decreases. An increase in the rear rotor’s blade pitch angle slightly reduces the thrust coefficient of the front rotor, whereas an increase in the front rotor’s blade pitch angle significantly enhances the thrust coefficient of the rear rotor. When configuring the blade pitch angles of the front and rear rotors, a larger front blade pitch angle is shown to achieve superior aerodynamic performance.

The axial distance between the upstream and downstream rotors is a critical installation parameter, affecting the aerodynamic and aeroacoustic performances of contra-rotating coaxial rotors. In this study, we investigate the effects of axial distance on the noise characteristics of contra-rotating coaxial rotors with identical and different diameters, by using 20 '' and 16 '' diameter rotors and adjusting the axial distance to the rotor diameter (z/D) within a large range of 0.1-0.6. Results show that the configurations with identical rotor diameter exhibit the best aerodynamic efficiency and lowest noise level at z/D approximate to 0.3-0.4. Installation of a smaller rotor in the upstream results in a significant reduction of the overall sound pressure levels. Details on the noise directivity, features of the tonal and broadband noise, and noise generation mechanisms are discussed with separations of acoustic spectra at different axial distances.

Electric ducted fans (EDFs) are attractive electric propulsion systems owing to their compactness and operational safety. However, reducing noise radiated from EDFs remains challenging. Rim-driven electric ducted fans (RDFs) provide a promising route for noise reduction through tip clearance removal and possible downstream stator elimination. To investigate the noise reduction mechanisms of RDF architectures, we design and compare three configurations: a conventional Baseline ducted fan, a Shafted RDF without tip clearance, and a Shaftless RDF without hub and downstream stators. This controlled comparison enables a systematic assessment of the dominant acoustic trends associated with tip leakage vortex (TLV) dynamics and rotor-stator interaction (RSI), with the flow and acoustic fields predicted using a validated LBM-LES/FW-H framework. The results show that both RDF configurations slightly increase the integrated thrust and torque relative to the Baseline. Acoustically, tip clearance removal in the Shafted RDF suppresses the TLV-related broadband source and reduces the near-wall pressure RMS over the 3–8 BPF range by up to 87.7%, whereas the far-field principal low-order BPF tones and OASPL decrease only moderately. In contrast, the Shaftless RDF suppresses the BPF tonal noise and reduces OASPL by 9.7 dBA on an energy-averaged basis over the 0∘–120∘ observer sector. Duct acoustic modal analysis is further used to examine the RSI noise mechanism, showing that removing the downstream stators suppresses the wake-cutting path that scatters rotor-wake energy into low-order radiating duct modes.

The k−ω model and its variants are widely used in finite volume CFD solvers, as they model wall-bounded turbulence well. However, the singular behavior of ω near the wall prevents it from being applied in numerical schemes based on polynomials, such as the finite element method. The k−ω0 model [Durbin and Yin, AIAA J. 2025], which aims to eliminate the singular behavior of ω, is implemented in discontinuous Galerkin to assess its behavior in polynomial-based finite element frameworks. Additionally, supersonic turbulent boundary layers are tested using the k−ω0 RANS model, demonstrating that the polynomial approximation remains suitable for the k−ω0 model under variable-density conditions. Then, the corresponding adaptive ℓ2−ω DDES model is implemented to assess whether the difference between ω and ω0 affects the hybrid RANS/LES behavior in a finite element framework. Turbulent channel flows and periodic hills are tested with both the k−ω0 RANS model and the adaptive ℓ2−ω DDES model.

Understanding the mechanism of turbulent skin-friction drag (TSD) generation is of fundamental and practical importance for designing effective drag reduction strategies. However, many previous studies adopted correlation analysis to reveal the causal map between turbulent motions and TSD generation, an approach that is potentially risky as correlation does not necessarily imply causation. In this study, a novel causal inference method called Liang-Kleeman information flow (LKIF) is utilized for the first time to identify the velocity-induced causal structures related to TSD generation in a turbulent channel flow. The statistical properties of the causal structures are comprehensively investigated. The positive and negative causal structures, defined by their signs and respectively associated with an increase and decrease in TSD information entropy, promote and suppress the generation of extreme TSD. Particularly, we find that the underlying physics of causal structures is essentially associated with the processes of streamwise streaks and rolls approaching or receding from the extreme events. Results indicate that the physics-informed LKIF framework can reveal a more explicit and interpretable causal relationship than correlation analysis.

In this study, we investigate the nonlinear noise generated in the wake of an underwater propeller operating under nonuniform behind-hull inflow using large-eddy simulation coupled with the Ffowcs Williams and Hawkings acoustic analogy. The analysis characterizes the extent of the region in which nonlinear noise is influential relative to linear noise under nonuniform inflow, expands the current understanding of the localization of nonlinear noise sources, and interprets the propagation characteristics and spatial distribution of the resulting acoustic field based on the identified source localization. The streamwise evolution of nonlinear noise generation is correlated with energy transmission during azimuthally asymmetric wake destabilization, with the dominant content shifting from the blade-passing frequency tone toward broadband, azimuthally asymmetric radiation. The broadband noise is localized in and related to the short-wave destabilization of the hub vortex, whereas the asymmetry arises from stronger vortex intensity and more vigorous inter-vortex interactions in the upper half of the wake region. In the propeller disk plane, as the observation distance increases, the asymmetry of the noise radiation flips: the louder region alternates between the upper and lower sides of the propeller. This flip occurs because, within a specific localized region, the broadband noise sources in the lower-half wake become stronger than those in the upper half, which is an inversion relative to the distribution of the tonal sources.

The rectangular exhaust nozzles of stealth aircraft typically feature a V-shaped trailing edge (VTE) design. However, no study has focused on the influences of VTEs on the flow and acoustic fields of hot supersonic rectangular jets. In light of this, implicit largeeddy simulations of a rectangular nozzle with and without VTEs are performed at a hot overexpanded condition with a nozzle pressure ratio of 3.0 and a temperature ratio of 3.0. Three VTE nozzles with different deflection angles are studied. Results show that the VTE nozzles are able to prompt the formation of Mach disks, shorten the lengths of potential cores, enhance or suppress the shear-layer mixing in the minor- or major-axis plane, and accelerate the axis switching due to counter-rotating vortex pairs induced along the long-lip sides. The VTE nozzles efficiently suppress or eliminate the screech tone propagating upstream in the minor-axis plane, and the levels of turbulent mixing noise and Mach wave radiation propagating downstream are also substantially reduced, although the broadband noise radiation is amplified in the major-axis plane. The azimuthally integrated overall sound pressure levels show a decrease in the upstream and downstream directions and an increase in the sideline regions. These influences become more significant when the VTEs deflect from inward to outward. The VTE nozzles suffer the penalties of thrust losses, and the total thrust losses arise from the reduced momentum and pressure components for the inward- and outward-deflected VTEs, respectively. The shear-layer oscillation transitions from the flapping mode to a helical mode and then to an irregular pattern, as the VTEs deflect from inward to outward. The wave-number spectra analysis reveals that the VTE nozzles are capable of affecting energy redistribution during wave interactions, leading to the suppression of upstream-propagating guided-jet modes in the fluid-acoustic feedback loop.

As modern wind turbines continuously upscale, capturing the complex aeroelastic interactions and wake interference in tandem configurations becomes crucial for structural safety and wind farm efficiency. In this study, a high-fidelity, two-way coupled fluid-structure interaction framework is developed by integrating an 8-degree-of-freedom structural dynamics model with Large Eddy Simulation. A comprehensive set of 12 load cases is evaluated to systematically investigate the effects of inflow conditions, uniform and Atmospheric Boundary Layer, downstream structural flexibility, and tandem spacing, including 0D, 2D, and 4D. The results reveal that Atmospheric Boundary Layer inflow acts as a strong perturbation, triggering premature breakdown of helical tip vortices within a short axial distance ( ) and driving large-scale wake meandering that accelerates far-wake velocity recovery. Quantitatively, the downstream flexible rotor operates as an intense turbulence generator, superimposing significant "added turbulence" onto the incoming wake. This wake impingement, combined with ambient turbulence, drastically amplifies structural fatigue: the tower base overturning moment damage equivalent load surges by 147.3% under ABL conditions compared to uniform inflow, and escalates by an additional 32.5% as tandem spacing increases to 4D. Furthermore, blade aeroelasticity consistently superimposes severe fatigue penalties, increasing the isolated turbine's DEL by nearly 49.5% under uniform flow. These findings quantitatively highlight the critical necessity of employing Fluid-Structure Interactions simulations to accurately predict unsteady aerodynamic loads and optimize the design of modern large-scale wind farms.

This study examines the propagation characteristics and sources of linear noise generated by an underwater propeller, employing a tailored formulation of the Ffowcs Williams-Hawkings equation. The simplified formulation enhances efficiency of the noise calculations by assuming uniform propagation times for noise sources located at different points on the blade surface. Comparisons of the predicted noise under uniform and nonuniform inflow conditions show that azimuthally varying inflow significantly amplifies the overall sound pressure level across most emission angles. Notably, the noise directivity and decay rates exhibit substantial changes, particularly within the propeller disk plane. These variations are primarily attributed to the amplification and dominant contribution of blade passing frequency (BPF) loading noise. An equivalent emission point acoustic model, derived from the simplified formulation, offers intuitive insights into the propagation behavior and source distribution of the loading noise. In the near field, the noise radiation is determined by variations in both radiation distance and blade forces. The predominant BPF tone arises from elements associated with different harmonics of the blade force, with interference among these elements causing azimuthally asymmetric noise radiation under the nonuniform inflow condition. In the far field, the noise spectrum is governed solely by the force derivatives, and the force fluctuations under nonuniform inflow only amplify the BPF tones, as interblade interference suppresses components at other frequencies. Regarding the noise source distribution, the near-field BPF tone and far-field broadband noise under the uniform inflow primarily originate from the root of the blade suction side and the blade trailing edge, respectively. Under the nonuniform inflow condition, the dominant BPF tone in both near and far fields is driven by the interaction of azimuthally varying inflow with the leading edges of the blades.

The bevelled nozzle is a promising noise control approach and has been tested to suppress the noise levels in supersonic circular jets, but not in rectangular jets so far. In this study, implicit large-eddy simulations are performed to analyse the noise control of supersonic rectangular jets with single- and double-bevelled nozzles. Three nozzle pressure ratios ( $NPR = 2.3$ , 3.0 and 5.0) are considered to form two over-expanded cold jets and one under-expanded cold jet, exhausted from a baseline convergent–divergent rectangular nozzle with an aspect ratio of 2.0. Results show that, with the increase of $NPR$ , the oscillation of the jet plume is switched from a symmetrical mode to a flapping mode (preferential in the minor-axis plane), then to a helical mode, together with a reduction of the screech frequency. The amplitude of the screech tone is the strongest in the flapping jet, and the turbulent mixing noise is the most prominent in the helically oscillating jet. The single-bevelled nozzle induces asymmetric shock-cell structures and deflects the jet plumes, and the double-bevelled nozzle primarily enables the enhancement of the shear-layer mixing and shortens the lengths of the jet potential cores. With the bevelled nozzles, the gross thrusts of the baseline nozzle are increased by $0.05 \sim 7.38$ %. Details on the characteristics of far-field noise in the jets with/without the bevel cuts and their noise control mechanisms are discussed using the Ffowcs Williams–Hawkings acoustic analogy, dynamic mode decomposition and spatio-temporal Fourier transformation. Results suggest that the noise control has a close relationship with the destruction of well-organized coherent structures and the suppression of upstream-propagating guided-jet modes, which interrupt the feedback mechanism accounting for the generation of screech tones in the supersonic rectangular jets.

The Very Long Chord Slat (VLCS) has emerged as a promising high-lift device configuration for commercial jetliners. This study focuses on the parametric shape design of two-dimensional high-lift devices incorporating VLCS, with particular emphasis on slat contour and position modifications. A surrogate-based optimization (SBO) framework, integrating Kriging metamodeling and the Non-dominated Sorting Genetic Algorithm, was employed to enhance the lift and drag performance at targeted angles of attack (AOA). The results reveal distinct Pareto fronts, demonstrating that a 25

Resolvent-based modelling and estimation is critically dependent on the nonlinear forcing input and hence understanding its role in the flow response is of great significance. This study quantifies the nonlinear forcing input in the resolvent formulation and investigates its characteristics for compressible turbulent boundary layers at Mach number 5.86 and friction Reynolds number 420 subject to adiabatic- and cold-wall conditions. Results show that, with the addition of the eddy viscosity to the resolvent operator, the cross-spectral density (CSD) of the forcing tends to exhibit a spatially uncorrelated distribution, which suggests that the spatial cross-coherence may be neglected and makes the modelling of the forcing input potentially easier. Aiming to quantify the different importance of each forcing component in generating turbulent fluctuations, contributions of the eddy-viscosity-corrected forcing to the flow responses are investigated through reduced-order analysis and matrix decomposition. The streamwise motions are almost insensitive to the temperature-related forcing, and can be oppositely influenced by the wall-normal and spanwise forcing components. By retaining only the diagonal components in the CSD of the forcing input, the assumption of forcing decorrelation in space and among components is also examined in the input–output framework. It is found that this simplified input is able to capture the dominant turbulence features and the local forcing is observed to cause inner-layer responses. That is, present results suggest adequate modelling of the CSD of the forcing can be achieved retaining only its diagonal components. On the basis of the current findings, the forcing input in the resolvent-based framework is thus modelled, with the wall-normal dependence and amplitude ratio between forcing components designed for compressible turbulent boundary layers. Through an algebraic Lyapunov equation, improved estimations of the statistical spectral densities of velocity and temperature fluctuations are finally obtained, in contrast to the results by simply assuming the forcing CSD to be an identity matrix.

High-order methods, such as the discontinuous Galerkin method, have gained increasing attention in the computational fluid dynamics community because they are expected to deliver high-accuracy results on complex flows and geometries. However, in realistic high Reynolds number flows, the significant computational cost and lack of robustness are unavoidable concerns when compared to low-order methods. The present paper develops a detached eddy simulation model based on the k-g equations, where the near-wall scaling of the new transport variables suits the polynomial nature of the discontinuous Galerkin method. The hybrid modeling framework of "l(2) x 1/T" mimics traditional Smagorinsky and dynamic subgrid models in the eddy-resolving regions, and provides seamless hybrid modeling of the unresolved scales in coarse-mesh simulations of wall turbulence and separated flows. Robustness is achieved by both proper near-wall scaling of turbulence variables and the scale-dependent hybrid modeling of unresolved turbulence. For the latter, no extra cutoff filtering, overintegration, or additional viscosity is needed regardless of the mesh resolution. Our model can perform both traditional delayed detached eddy simulation and wall-modeled large eddy simulation for high Reynolds number turbulent flows like its predecessor, the adaptive l(2)-w model [Yin and Durbin, Int. J. Heat Fluid Flow 62, 499 (2016)]. By implementing the method in a spectral element discontinuous Galerkin solver, the proposed model is tested in a transonic airfoil with shock-turbulent boundary layer interaction.

In this study, we introduce a deep generative model, named Multi-Species Generative Adversarial Network (MS-GAN), which is developed to extract the low-dimensional manifold of three-dimensional multi-species surfaces. In the development of MS-GAN, we extend the free-form deformation by incorporating principal component analysis to increase the non-linear deformation ability while maintaining geometric smoothness. The implicit information of multiple baselines is embedded in the feature extraction layers, to enhance the diversity and parameterization of multi-species dataset. Furthermore, Wasserstein GAN with a gradient penalty is used to ensure the stability and convergence of the training networks. Two experiments, ruled surfaces and propeller blade surfaces, are performed to demonstrate the advantages and superiorities of MS-GAN.