Search Results (125)

Periodic fluctuations in pump head are a common unsteady phenomenon in tubular pumps; however, their underlying energy dissipation mechanism remains insufficiently understood. This study conducted transient numerical simulations on a two-blade tubular-flow pump with highly twisted blades operating at the design flow condition. Using entropy production theory, the research quantitatively examined Rotor–Stator Interaction (RSI), vortex development, and hydraulic loss characteristics. The findings reveal that turbulent entropy production is the primary contributor to total energy dissipation, while entropy generated by wall friction is minimal. Although the impeller experiences the greatest absolute energy loss, the fluctuations in entropy production within the guide vane are significantly larger. This indicates that the energy loss represented by the entropy production in the guide vane primarily drives the periodic head fluctuations of the pump. High entropy production is concentrated near both the leading and trailing edges of the guide vane, exhibiting a trough-shaped radial distribution influenced by the leading-edge hub vortex and tip leakage vortex. Additionally, the transient changes in entropy production under RSI are governed by the periodic formation and strengthening of the guide vane passage vortex, along with the shedding and breakdown of the wake vortex. Velocity analysis shows that the circumferential movement of the impeller wake continuously modifies the instantaneous inflow conditions at the guide vane’s leading edge, causing periodic changes in the incidence angle that enhance the passage vortex while weakening the wake vortex. This study provides deep insights into the operational stability of pumps and pumping stations in water transfer projects. Full article

Owing to its inherent flight safety, low takeoff and landing requirements, and favorable economic efficiency, the autogyro UAV, especially its electric and hybrid-electric variants, has become a core platform for low-altitude aviation missions such as transportation, inspection, and surveillance in plateau and offshore regions. However, the low air density and low Reynolds number conditions encountered in plateau regions can induce aerodynamic issues such as premature laminar flow separation, dynamic stall, and increased induced drag, which directly reduce payload capacity and endurance of small electric autogyro UAVs. In offshore environments, strong winds, turbulence, and gust disturbances intensify rotor–wake interactions, cause abrupt variations in aerodynamic loads, and reduce control margins, which severely restricts the mission reliability and flight safety of low-altitude unmanned platforms. These environmental effects collectively degrade rotor performance, including reduced aerodynamic efficiency and insufficient lift generation, and further amplify the energy constraint of electric/hybrid-electric propulsion systems. In response to bottlenecks that restrict the practical application of autogyro UAVs in extreme environments, this paper systematically reviews research progress on lift vector control for autogyro UAV rotors operating under such conditions. First, the typical aerodynamic problems encountered by autogyro UAVs in plateau and offshore environments are summarized, and their underlying physical mechanisms are analyzed from both system-level and local-flow perspectives, with a focus on how environmental factors affect the autorotation stability of unmanned platforms. Subsequently, the development of passive lift vector control technologies is reviewed, with an emphasis on the aerodynamic benefits of passive pitch mechanisms, vortex generators, and blade-tip winglets, as well as their engineering feasibility for small autogyro UAV blades. Active lift vector control technologies are then examined, including air-jet flow control, synthetic jets, and trailing-edge flaps, with discussions of their potential to delay flow separation and stall, enhance rotor aerodynamic efficiency, and an assessment of their adaptability to the energy and structural constraints of unmanned platforms. Finally, a lift vector control strategy suitable for autorotating rotors of autogyro UAVs is proposed, based on careful consideration of energy consumption, structural constraints, and control effectiveness. It provides a reference for aerodynamic optimization and flight control research on electric and hybrid-electric autogyro UAVs operating in extremely low-altitude environments. Full article

Operating steam turbines under low-flow conditions poses severe threats to structural safety due to flow-separation-induced aeroelastic instability. However, the exact sources of localized unsteady aerodynamic excitations in vortex-dominated regions and their explicit contribution to blade flutter under deep low-flow regimes remain unclear. To address this problem, this paper proposes a quantitative aeroelastic stability assessment method that combines unsteady pressure monitoring within key vortex zones with a one-way fluid–structure interaction (FSI) energy-based framework. Taking the 661 mm last-stage blade of a 350 MW LP steam turbine under 20% THA operating conditions as a representative study, numerical simulations were carried out to capture three-dimensional separated flow structures, isolate dominant unsteady excitation sources via fast Fourier transform (FFT) pressure spectrums, and evaluate traveling-wave flutter across various inter-blade phase angles (IBPAs) for the first three structural modes. The results indicate that flow separation initiates at the rotor root and forms a large-scale recirculation vortex blocking nearly 50% of the spanwise passage. Unsteady pressure monitoring demonstrates that this root recirculation vortex exhibits the maximum pulsation amplitude, serving as the primary source of unsteady aerodynamic excitation. Under fluid–structure coupling, the first three modes all reach their minimum aerodynamic damping coefficients at an IBPA of 45°. When accounting for mechanical damping, the total system damping for the first and second modes becomes negative at 45° IBPA, with the first mode dominating the flutter response. The main novelty of this work lies in revealing the spatial–temporal mechanism by which localized separation vortices govern blade flutter under deep low-flow conditions, providing a precise mapping between localized vortex dynamics and global aeroelastic stability to guide the safe operation of turbine blades. Full article

Dedicated platforms for floating vertical-axis wind turbines (VAWTs) require an in-depth understanding of their strongly unsteady coupled dynamics, yet the effects of platform configuration on motion stability, aerodynamic loading, and wake recovery remain insufficiently clarified. This study develops a high-fidelity aero-hydro-mooring coupled framework base on computational fluid dynamics (CFD) to compare Φ-type floating VAWTs supported by semi-submersible and spar platforms under identical wind–wave excitation. The results show that the semi-submersible configuration, owing to its larger structural scale near the free surface, experiences stronger wave interaction and more pronounced wave-frequency heave, surge, and pitch responses. The spar configuration reduces wave-frequency hydrodynamic excitation because of its deep-draft slender structure, but it is more prone to mean pitch offset and sway–roll–yaw coupling. Mooring responses are governed by mean surge drift, mean pitch inclination, and wave-frequency motions, with the semi-submersible system exhibiting stronger tension fluctuations and the spar system showing a more uneven load distribution among the mooring lines. Under the examined wind–wave condition, the spar configuration exhibits larger fluctuations in instantaneous power, thrust and single-blade torque than the semi-submersible configuration. Wake analysis indicates that the semi-submersible system maintains stronger wake coherence, while the spar system enhances vortex breakdown, turbulent mixing, and velocity-deficit recovery. These findings support platform selection, load control, and array layout optimization for floating VAWTs. Full article

To investigate the dynamic transmission of unsteady hydraulic excitation within pumped-storage units, this study presents a numerical investigation on the three-dimensional shafting vibrations of a pump-turbine based on a one-way transient fluid-structure interaction approach. To resolve the flow field, we adopt the shear stress transport (SST) k-ω turbulence model in conjunction with the Zwart-Gerber-Belamri (ZGB) cavitation model, and the combined numerical approach is first calibrated against experimental measurements. The results indicate that under the 70% partial load, an eccentric helical vortex rope develops inside the draft tube, generating intense low-frequency pressure pulsations that induce chaotic shaft orbits and distinct orbital drift at the turbine guide bearing. Conversely, near the optimal efficiency point (90% load), the vortex rope transitions into a slender, straight conical core, yielding minimum vibration magnitude and exceptional operational stability. At the 100% rated load, the vortex rope expands into a robust straight conical structure extending continuously into the elbow section. Stress analysis reveals that while equivalent stress concentrations consistently occur at the blade root regions, a reduction in the cavitation number at both 90% and 100% loads leads to a counterintuitive decline in blade surface peak stress values. Additionally, stiffness sensitivity analysis demonstrates that the relative change rates of the shaft runout are highly sensitive to the stiffness variations of the turbine guide bearing, where a stiffness reduction triggers a substantial runout growth of approximately 100% along the X- and Y-directions, whereas the variations in the upper and lower guide bearings exert extremely weak impacts. Full article

The internal flow of a semi-open axial flow fan is highly three-dimensional and unsteady due to the absence of a confined passage. The evolution of complex vortical structures, such as the tip leakage vortex (TLV) and corner separation vortex (CSV), remains poorly understood. This study used high-resolution particle image velocimetry (PIV) to conduct multi-region, multi-view measurements of the flow field in a semi-open fan for an outdoor air conditioning unit. The generation, development, and breakdown of the TLV were analyzed, revealing transient nonuniform flow and wake evolution. Dynamic mode decomposition (DMD) was applied to extract dominant frequencies and spatial modes. The results show that the TLV has a dominant frequency of 98.5 Hz (2.19 times the rotational frequency), accounting for 88.5% of the total energy, and exhibits periodic shedding and asymmetric breakdown. The CSV dominates at 16.44 Hz, slightly above blade rotation, and interacts with the TLV. In the wake region, the dominant frequency is 248.45 Hz, arising from the nonlinear superposition of TLV harmonics, the CSV frequency, and the blade passing frequency. This study provides an experimental basis and a low-dimensional coherent structure model for internal flow diagnostics and the structural optimization of semi-open axial flow fans. Full article

Loading fluctuations cause structural responses such as deformations and vibrations on the propeller. Structural response of propellers results in vibrations on the shaft system or even the hull. Considering the demand for structural safety, the correlation between structural response of propellers and inflow conditions is numerically studied in the present paper. The interaction between the propeller and turbulence structures and vortex shedding from upstream structures is considered. Loading fluctuations on the propeller blade are obtained by a turbulence model of improved delayed detached eddy simulations (IDDESs). The deformations and vibrations of propeller blades fixed at their roots are captured considering fluid–structure interaction. Results show that the loading fluctuations and vibrations on the propeller contain tonal components occurring at harmonics of shaft frequency and broadband components. Inhomogeneous inflow amplifies pressure fluctuations as a product of space frequency and shaft frequency (SF). Inhomogeneous inflow also results in more intense fluctuations of velocity in the tip vortex at SF and blade wake at blade passing frequency and encounter frequency. As a result of loading fluctuations, the vibration of the blade is a superposition of excited vibrations and natural vibrations. Inhomogeneous inflow amplifies the vibrations at the encounter frequency. Resonance of the blade can be observed when the excited frequency approaches the first natural frequency. Full article

Vertical axis wind turbines (VAWTs) have regained attention for distributed, urban, and floating offshore applications, yet the literature remains fragmented across competing rotor concepts and modelling traditions. This review consolidates the principal archetypes—Savonius, H-Darrieus, troposkein Darrieus, helical Darrieus, and Savonius–Darrieus hybrids—through five governing parameters: drag-versus-lift-driven operating principle, tip speed ratio $\mathsf{\lambda }=\mathsf{\omega }\mathrm{R}/{\mathrm{V}}_{\infty }$ (0.6–1.2 for Savonius; 2.5–5.0 for Darrieus), solidity $\mathsf{\sigma }=\mathrm{N}\mathrm{c}/\mathrm{R}$ (0.1–0.4), chord-based Reynolds number ${\mathrm{R}}_{\mathrm{e}\_\mathrm{c}}$ (${10}^5–{10}^6$), and peak power coefficient ${\mathrm{C}}_{\mathrm{p}\_\mathrm{m}\mathrm{a}\mathrm{x}}$ (0.15–0.25 for Savonius; 0.35–0.45 for optimized H-Darrieus). Off-design performance is dominated by unsteady mechanisms that quasi-steady streamtube models cannot resolve—leading edge vortex shedding, dynamic stall hysteresis, blade–wake interaction, and flow-curvature-induced virtual camber—each examined for its contribution to the instantaneous torque ${\mathrm{C}}_{\mathrm{T}}\left(\mathsf{\theta }\right)$ and the cycle-averaged ${\mathrm{C}}_{\mathrm{p}}$. Turbulence closures are benchmarked against phase-locked PIV and torque measurements: $\mathrm{k}–\mathsf{\omega }\mathrm{S}\mathrm{S}\mathrm{T}$ URANS captures peak-region ${\mathrm{C}}_{\mathrm{p}}$ to within $\pm 5–10\%$ but over-predicts torque below ${\mathsf{\lambda }}_{\mathrm{o}\mathrm{p}\mathrm{t}}$; the $\mathsf{\gamma }–{\mathrm{R}}_{\mathrm{e}\_\mathsf{\theta }}$ transition SST model reduces this error to $\pm 3–5\%$; DES, DDES, and LES reach $\pm 2–3\%$ at one to two orders of magnitude higher cost. Best practice computational fluid dynamics (CFD) guidelines are consolidated: domain extents of 15D upstream, 10D downstream, and 20D lateral; rotating sub-domain ${\mathrm{D}}_{\mathrm{r}\mathrm{o}\mathrm{t}}$ $\approx 1.5\mathrm{D}$; ${\mathrm{y}}^{+}\le 1$; $\Delta \mathsf{\theta }\le 0.1°$; and 20–30 revolutions before sampling. Performance enhancement strategies (variable pitch, guide vanes, helical twist, and hybridization) are reviewed quantitatively, with reported ${\mathrm{C}}_{\mathrm{p}}$ gains of $5–30\%$. Four research priorities are identified: (i) transition-sensitive turbulence closures validated below ${\mathrm{R}}_{\mathrm{e}\_\mathrm{c}}$ = $5\times {10}^5$; (ii) coupled aero-hydro-servo-elastic models for floating offshore VAWTs; (iii) machine-learning-augmented turbulence modelling—including physics-informed neural networks (PINNs) and neural-network-corrected RANS closures—to improve unsteady flow prediction at sub-LES cost; and (iv) integrated aeroacoustic–aeroelastic frameworks for urban and building-integrated deployment. Full article

The rapid increase in the penetration of renewable energy has imposed more stringent requirements on the regulation capacity and response speed of Francis turbines in modern power grids. Vortex-induced energy loss significantly constrains the energy performance and hydraulic stability of giant Francis turbines. However, the formation mechanisms of vortex-induced hydraulic loss near the operating boundary remain insufficiently understood. Based on numerical simulations and parameter validation under 30 representative operating conditions, three 50% rated load conditions located near the operating boundary were strategically selected for detailed investigation. By integrating rigid vorticity analysis with entropy production theory, the vortex dynamics and hydraulic loss characteristics were systematically quantified and visualized. The results indicate that entropy production rates caused by turbulent dissipation and wall shear constitute the primary components of hydraulic loss, among which entropy production rate caused by turbulent dissipation (EPRT) is more sensitive to variations in external operating conditions and dominates both the magnitude and spatial distribution of energy dissipation. Distinct loss evolution patterns are observed in the runner and the draft tube. Recirculation and separation flows along the blade surfaces alter the normal blade loading distribution in the runner. In the draft tube, hydraulic loss is mainly governed by the energy dissipation associated with the interaction between the main flow region and the reverse flow region, while the intensity of hydraulic loss is not directly related to the specific vortex morphology. Overall, shear vorticity remains the key mechanism responsible for the increase in EPRT. This study provides theoretical insights and practical evidence for understanding the mechanisms of vortex-induced energy loss in giant Francis turbines and for quantitatively evaluating the distribution and evolution of hydraulic loss. Full article

Forced resonance induced by rotor–stator interaction (RSI) is a primary driver of high-cycle fatigue (HCF) failure in aero-engine blisks. To overcome the inability of traditional non-contact excitation methods to replicate authentic three-dimensional aerodynamic forces and the predictive biases of pure numerical approaches regarding complex flow excitation energy, this study investigates the forced resonance characteristics of a rotating blisk using a novel aerodynamic excitation system through integrated numerical and experimental approaches. First, a one-way fluid–structure interaction (FSI) framework, coupling the Nonlinear Harmonic (NLH) method with Finite Element Analysis (FEA), was established to efficiently reconstruct the unsteady aerodynamic loads on blade surfaces. The analysis reveals an excitation mechanism dominated by the upstream propagation of the downstream potential field, based on which the numerical resonance response was predicted. In addition, investigating rotor–stator axial clearance as a key variable indicates that there is a strictly monotonically decreasing dependence of the aerodynamic excitation magnitude on the rotor–stator axial clearance. However, the spatial patterns of the primary first-order harmonic excitation remain relatively insensitive to changes in the rotor–stator axial clearance. Finally, by leveraging these excitation characteristics, broadband aero-resonance of the first three modes was successfully induced within the 2600 Hz frequency range under experimental conditions. This validates both the effectiveness of the experimental apparatus and the fidelity of the numerical model. This research not only clarifies the excitation mechanism under vortex generator-induced RSI but also provides a novel testing platform and theoretical framework for rotating modal analysis in advanced propulsion systems. Full article

Understanding the unsteady aerodynamic behavior of quadrotor unmanned aerial vehicle (UAV) is critical for improving flight stability, control, and performance, particularly in complex operational environments. In closely spaced multirotor configurations, coherent tip vortices shed from each blade convect downstream and form helical vortex streets that interact with subsequent blades and neighboring rotors. These interactions induce rapid fluctuations in local inflow velocity and effective angle of attack, resulting in transient lift variations, increased vibratory loads, and elevated acoustic emissions. This study presents a comprehensive computational investigation of quadrotor rotor interactions and wake dynamics using a large-eddy simulation (LES). Detailed analyses reveal that the formation and evolution of tip vortices and blade–vortex interaction phenomena significantly influence lift fluctuations and aerodynamic loading. The simulations capture transient wake structures and their effects on neighboring rotors, highlighting unsteady aerodynamic mechanisms that are not adequately predicted by conventional RANS or URANS approaches. Parametric studies examining vortex-street offset distance demonstrate the sensitivity of wake-induced instabilities to design and operational parameters. The results provide new physical insights into multirotor wake dynamics and establish the LES as a predictive framework for quantifying unsteady aerodynamic loading in quadrotor drones. The findings provide insights into the complex flow physics of multirotor systems, offering guidance for more accurate modeling, rotorcraft design optimization, and the development of control strategies that mitigate adverse unsteady aerodynamic effects. This study provides new insights into rotor–vortex-street interactions, with applications to multirotor UAVs, by isolating multi-vortex coupling effects and quantifying the influence of horizontal vortex spacing on unsteady aerodynamic loading, complementing existing high-fidelity LES research. Full article

The trailing-edge design of a wind turbine airfoil is critical for balancing the aerodynamic performance and structural robustness of a wind turbine blade. In this paper, the S809 airfoil and its blunt trailing-edge variant, the S809-100 airfoil, are taken as the research objects. The aerodynamic and flow-field characteristics of both airfoils are analyzed by computational fluid dynamics, which is validated by U.S. National Renewable Energy Laboratory experiments and wind tunnel particle image velocimetry. The results show that the S809-100 airfoil achieves a higher lift coefficient across the entire angle of attack ($\alpha$) range 0–18°, with a superior lift-to-drag ratio within 8–12°. Three distinct states of aerodynamic response are identified for both airfoils, based on time series and spectral features of lift and drag coefficients, and flow-field structures: steady convergence state, periodic fluctuation state, and irregular fluctuation state. The two airfoils differ significantly in aerodynamic response transition with respect to $\alpha$: for the S809 airfoil, the aerodynamic response remains in a steady convergence state up to $\alpha =16°$ before shifting to a periodic fluctuation state, while for the S809-100 airfoil, it exhibits a periodic fluctuation state from $\alpha =0°$ and transitions to an irregular fluctuation state beyond $\alpha =14.2°$. This difference stems from trailing-edge thickening, which induces flow unsteadiness in the S809-100 airfoil. This shift in the aerodynamic response from the periodic fluctuation state to the irregular fluctuation state is attributed to the transition from single-frequency large-scale vortex shedding to a multi-scale vortex interaction, confirmed via spectral and flow-field analyses. This study focuses on the correlated flow structures of wind turbine airfoils and deepens the understanding of unsteady aerodynamic responses; the combined analysis of enhanced aerodynamic performance and induced unsteady fluctuation due to trailing-edge thickening offers a valuable reference for wind turbine blade design. Full article

High-speed, low-pressure turbines in geared turbofans operate at transonic exit Mach numbers and low Reynolds numbers. Engine-relevant data remain scarce. The SPLEEN C1 linear cascade was investigated at $M_{\mathrm{out}}=0.70–0.95$ and $R{e}_{\mathrm{out}}=\mathrm{65,000}–\mathrm{120,000}$ under steady inlet flow. Experiments were combined with 2D RANS and MISES, including transition modeling and inlet-turbulence decay calibrated to measurements. Results are consistent with conventional LPT behavior: loss decreased with increasing Mach and Reynolds numbers, except when shocks interacted with the blade boundary layer ($M\approx 0.95$). Profile loss dropped by $23\%$ from $M=0.70$ to $0.95$ at $Re=\mathrm{70,000}$, as well as by $19\%$ at $M=0.80$ when open separation is suppressed. Secondary loss decreased by up to $25\%$ at $Re=\mathrm{70,000}$ and showed weak sensitivity to the Reynolds number. A coupled loss model predicted profile loss with a root-mean square error of 4.7%. Secondary-loss modeling reproduced global trends: separating endwall dissipation from mixing kept errors within $\pm 10\%$ for most cases, but accuracy degraded near the shock–boundary layer interaction case and at the highest Reynolds number. Mixing dominated endwall loss (∼75%), with the passage vortex contributing ∼50% (±10%) of the mixing component. Full article

High-speed jet-in-crossflow (JICF) configurations are central to several aerospace applications, including turbine-blade film cooling, thrust vectoring, and fuel or hydrogen injection in combusting or reacting flows. This study employs high-fidelity direct numerical simulations (DNS) to investigate the dynamics of a supersonic jet (Mach 3.73) interacting with a subsonic crossflow (Mach 0.8) at low Reynolds numbers. Three momentum-flux ratios (J = 2.8, 5.6, and 10.2) are considered, capturing a broad range of jet–crossflow interaction regimes. Turbulent inflow conditions are generated using the Dynamic Multiscale Approach (DMA), ensuring physically consistent boundary-layer turbulence and accurate representation of jet–crossflow interactions. Modal decomposition via proper orthogonal decomposition (POD) and spectral POD (SPOD) is used to identify the dominant spatial and spectral features of the flow. Across the three configurations, near-wall mean shear enhances small-scale turbulence, while increasing J intensifies jet penetration and vortex dynamics, producing broadband spectral gains. Downstream of the jet injection, the spectra broadly preserve the expected standard pressure and velocity scaling across the frequency range, except at high frequencies. POD reveals coherent vortical structures associated with shear-layer roll-up, jet flapping, and counter-rotating vortex pair (CVP) formation, with increasing spatial organization at higher momentum ratios. Further, POD reveals a shift in dominant structures: shear-layer roll-up governs the leading mode at high J, whereas CVP and jet–wall interactions dominate at lower J. Spectral POD identifies global plume oscillations whose Strouhal number rises with J, reflecting a transition from slow, wall-controlled flapping to faster, jet-dominated dynamics. Overall, the results demonstrate that the momentum-flux ratio (J) regulates not only jet penetration and mixing but also the hierarchy and characteristic frequencies of coherent vortical, thermal, and pressure and acoustic structures. The predominance of shear-layer roll-up over counter-rotating vortex pair (CVP) dynamics at high J, the systematic upward shift of plume-oscillation frequencies, and the strong analogy with low-frequency shock–boundary-layer interaction (SBLI) dynamics collectively provide new mechanistic insight into the unsteady behavior of supersonic jet-in-crossflow flows. Full article

Wind turbines operate in harsh environments where leading-edge blade erosion from particulates like sand, rain, and insects is prevalent, significantly degrading aerodynamic performance and reducing power output. To counteract this, this study proposes a novel flow-control method using detached micro-cylinders placed upstream of the leading edge of eroded S809 (a wind turbine blade profile) airfoils. The approach is inspired by the concept of symmetry recovery in disturbed flows, where strategically introduced perturbations can restore balance to an asymmetric separation pattern. The aerodynamic performance of the S809 airfoil was numerically investigated under three leading-edge erosion depths (0.2%, 0.5%, and 1% of chord length, *c*) with a fixed micro-cylinder diameter of 1% *c* positioned at fifteen different locations. Findings reveal that the strategic placement of micro-cylinders ahead of the leading edge or on the pressure side markedly enhances the aerodynamic efficiency of airfoils with 0.2% and 0.5% erosion, achieving a maximum improvement of 148.7% in the lift-to-drag ratio (L/D) difference function for the 0.5% eroded airfoil. This performance recovery is interpreted as a partial restoration of flow symmetry, disrupted by erosion-induced separation. The interaction between the cylinder wake and the spill-over stall vortex originating from the erosion groove was identified as the primary mechanism, injecting high-energy fluid into the boundary layer to suppress flow separation. This study systematically parametrizes the effect of erosion depth and cylinder placement, offering new insights for mitigating erosion-induced performance loss through controlled asymmetry introduction. Full article