Search Results (809)

Cavitation represents a critical phenomenon affecting the performance, durability, and acoustic signature of high-speed propellers. This study extends a previous non-cavitating analysis of the same propeller by investigating cavitation onset, evolution and performance change for a 140 mm diameter, three-blade fixed-pitch high-speed propeller. The Rayleigh-Plesset cavitation model, implemented in ANSYS CFX 24.2 within a two-phase Eulerian framework coupled with SST turbulence closure, was used to simulate 72 operational combinations spanning six advance velocities (0–10 m/s) and twelve rotational speeds (300–3600 RPM), with nine representative cases analyzed in detail. Results show that onset thresholds are strongly velocity-dependent: at zero advance velocity (bollard-pull condition, v = 0 m/s) cavitation inception occurs between 1800 and 2000 RPM, whereas at high advance velocity (v = 10 m/s) localized tip-vortex cavitation appears at rotational speeds as low as 300 RPM despite a nominally favourable global cavitation number (σ = 1.96), demonstrating that the global cavitation number alone cannot predict the onset of cavitation. Cavitation consistently initiates at blade tip leading edges, evolving from attached sheet cavitation to supercavitation with vapor fractions exceeding 95% at maximum conditions. At the critical design point, propulsive efficiency reaches η = 38.6% (T = 244.04 N, Q = 12.08 Nm). A direct comparison with the non-cavitating baseline reveals that this effect is regime-dependent: cavitation reduces predicted thrust by 7–11% under bollard pull conditions (v = 0 m/s, 1800–3600 RPM), partially attributable to active-blade-area loss; at moderate advance velocities (v = 2–6 m/s) the two predictions nearly coincide, while at high advance ratio (v = 8–10 m/s) cavitating thrust matches or exceeds the non-cavitating prediction, by up to 88% at v = 10 m/s and 3600 RPM. These findings define indicative operational envelopes, identify blade tip protection as essential for erosion mitigation and provide practical design guidance for high-speed propellers in fast vessels, rescue craft and autonomous surface vehicles. Full article

Objectives: To compare the geometric design, metallurgical properties, mechanical performance, and irrigation dynamics of Reciproc Minima (M20 and M25) and Reciproc Blue R25 instruments. Methods: One hundred and eighty instruments (n = 60/group) were evaluated. Geometry was analyzed using stereomicroscopy, scanning electron microscopy, and 3D surface scanning. Metallurgical characteristics were assessed by energy-dispersive X-ray spectroscopy and differential scanning calorimetry. Mechanical performance tests (n = 10/group) included cyclic fatigue, torsional resistance, bending resistance, buckling resistance, and cutting efficiency. Irrigation dynamics were examined through computational fluid dynamics simulations based on a micro-CT-derived mandibular molar model prepared according to each system and combined with open-ended, side-vented, or double side-vented needles. Data were analyzed using one-way ANOVA or Kruskal–Wallis tests (α = 0.05). Results: The results showed that blade dimensions increased progressively from Minima M20 to Reciproc Blue R25. All instruments had S-shaped cross-sections and non-active tips. Energy-dispersive spectroscopy confirmed near-equiatomic NiTi composition, and similar phase transformation temperatures were observed across groups. Minima M20 showed the highest cyclic fatigue resistance (p < 0.0001), whereas Minima R25 exhibited greater angular deflection (p < 0.0001). Reciproc Blue R25 had the highest buckling resistance and lowest flexibility (p < 0.0001). M25 showed the lowest axial force, indicating the numerically highest cutting efficiency, but it did not differ significantly from Reciproc Blue R25 (p > 0.05). No needle delivered irrigant to working length. The open-ended needle achieved greater apical penetration, particularly with Reciproc Blue R25. Minima M20 generated the highest wall shear stress, and Reciproc Blue R25 the lowest apical pressure. Conclusions: Reciproc Minima and Reciproc Blue R25 showed similar metallurgical characteristics; however, differences in geometric design resulted in distinct mechanical behaviors and irrigation fluid dynamics. These findings suggest that low-taper reciprocating instruments may represent a conservative alternative in anatomically challenging canals, while clinicians should consider the associated differences in mechanical behavior and irrigation dynamics when selecting the most appropriate instrument. Full article

Background: Non-missile penetrating traumatic brain injury (pTBI) is a rare but life-threatening condition. In self-inflicted cases involving the frontal sinus, a rigorous preoperative diagnostic workup is crucial to assess foreign body fragmentation, trajectory, and potential vascular compromise. The complexities of such cases demand a highly structured approach to prevent severe secondary brain injury and long-term infectious sequelae. Case Presentation: A 36-year-old male with schizophrenia presented after attempting suicide by stabbing a kitchen knife through his frontal sinus. Multimodal diagnostic imaging was immediately employed. Skull radiographs and computed tomography (CT) precisely delineated the blade traversing the frontal sinus and entering the anterior cranial fossa. Crucially, given the proximity to the skull base, digital subtraction angiography (DSA) was proactively performed, which confirmed the absence of major cerebrovascular injury and safely guided the surgical strategy. Based on these precise imaging findings, a bifrontal craniotomy was performed. During the procedure, the main blade was extracted, and a retained broken metallic tip—recognized upon close intraoperative inspection of the extracted blade—was successfully retrieved from the intracranial compartment. Postoperative wound infections (with causative pathogens isolated as Serratia odorifera and coagulase-negative Staphylococci) were effectively eradicated with targeted antibiotics. The patient achieved a favorable neurological recovery. Conclusions: Multimodal neuroimaging, particularly the combination of CT and DSA, can be highly beneficial in the diagnostic workup of complex pTBI. Precise preoperative imaging helps guide the optimal surgical approach, ensuring complete foreign body removal and minimizing severe secondary complications. Early intervention, guided by standardized imaging and followed by aggressive medical management, can yield positive outcomes even in severely morbid presentations. Full article

For unshrouded turbine blades, the high-speed tip clearance leakage flow induced by the pressure gradient on the pressure and suction surface sides significantly increases the thermal load in the turbine blade tip regions. Film cooling technology is an effective measure for reducing the external heat transfer temperature at the blade tips. Novel cooling strategies for the turbine blade tip are introduced to reduce adiabatic wall temperature. Here, the spatial distribution of film cooling effectiveness and the associated flow physics of the rib-slot and trapezoidal-slot tip configurations are investigated numerically and experimentally under transonic conditions. Film cooling effectiveness on the tip is quantified using the pressure-sensitive paint technique. Density ratios of 1.5 and 2.0 are considered, with tip clearances set at 0.7% and 1.5% of the blade height and the cascade exit Mach number set at 1.05. Both the trapezoidal-slot and rib-slot tip cooling approaches are capable of establishing complete film coverage over the tip surface. The rib-slot configuration delivers markedly higher effectiveness values over the mid-chord and trailing-edge regions relative to the trapezoidal-slot design. Conversely, the trapezoidal-slot scheme improves tip aerodynamic performance compared with the rib-slot arrangement, with the benefit being most pronounced in the larger clearance setting. Full article

With the rapid development of renewable energy, pumped storage power plants have taken on critical functions such as frequency regulation and grid stabilization. Consequently, higher demands are placed on their core component—the pump-turbine—requiring further improvements in efficiency, extended service life, and reduced cavitation to ensure reliable and stable operation. Tip leakage flow (TLF) is a complex three-dimensional flow structure in pump turbines, as well as in other turbomachinery. In particular, it generates tip leakage vortex (TLV), which leads to severe damage of pump turbines in pumped storage power plants, such as dramatic efficiency decreases. It originates from the clearance between the blade tip and the casing. The pressure difference between the two sides of the blade drives fluid from the pressure side through the tip gap into the suction side. The process produces a distinct shear layer, leakage jet, and secondary vortex structures. In turbomachinery, performance degradation and structural failure often arise from unsteady flow features. One critical case is vortex breakdown (VB) caused by the TLV. This paper reviews unsteady mechanisms linked to vortex breakdown, including Kelvin-Helmholtz (KH) instability, cavitation, and, finally, the paper discusses geometric modulation to improve system efficiency and reduce cavitation. These factors act both as signals and as triggers of instability. KH structures, vortex breakdown, and cavitation modes together define the instability of tip leakage flows. Geometric and boundary conditions serve as tuning knobs for system sensitivity. Full article

To resolve the fatigue fracture occurring at the tip trailing edge of thin-walled single-crystal turbine rotor blades for a specific aero-engine, this paper derives the relationship between the single-crystal constitutive stiffness matrix and crystal orientation, and clarifies the influence mechanism of crystal orientation on blade-vibration characteristics. The influence rules and degrees of uncertain parameters on the vibration characteristics of thin-walled and thickened-profile blades are comparatively investigated. The results reveal that blade dynamic frequency is strongly affected by both blade profile thickness and crystal orientation, and blade profile optimization can effectively weaken the effects of crystal orientation and blade thickness on the dynamic frequency of critical modes. Based on the above results, the hazardous resonance overlooked in conventional engineering approaches is detected using uncertainty-based vibration analysis. The corresponding failure mechanism is illustrated, the resonant speed range is determined, and the risk of bending–torsion coupled flutter is detected. The outcomes provide valuable guidance for the frequency tuning design of turbine blades. Full article

Small unmanned aerial vehicles (UAVs) are widely used in civil and military fields, and their noise problem has always been the industry’s focus. Compared with a traditional propeller fan, a ducted fan offers higher aerodynamic efficiency, lower aerodynamic noise, and greater safety. It has become the key power component of small UAVs. However, due to the rigid restriction on tip clearance, the traditional integral ducted fan is prone to generating a tip leakage vortex, which produces high-intensity aerodynamic noise and significantly reduces propulsion efficiency. To address the above key problem restricting the quiet flight of small UAVs, this paper designs a segmented ducted fan (SDF). It preliminarily explores the influence of the segmented clearance on the fan’s flow field structure and acoustic radiation characteristics. Specifically, the k-ω SST (shear stress transport) turbulence model and the broadband noise source model were used to establish a computational fluid dynamics model, and the effects of fan speed (20,000–40,000 rpm) and duct spacing (0–20 mm) on its aeroacoustic characteristics were systematically studied. The results showed that the SDF’s acoustic power level maximum (APL_max) was significantly higher than that of the traditional integral structure, especially at high speed. At 40,000 rpm, increasing the duct spacing to 20 mm resulted in a sudden increase in APL_max to 194.5 dB, 61.3 dB higher than that of the integral type. Its essence was derived from the three-stage chain amplification mechanism: (1) strong tip leakage vortex induced by geometric clearance; (2) broadband noise caused by vortex impacting the duct wall; (3) resonant coupling of leakage vortex harmonic frequency and duct cavity standing wave. Based on this, a collaborative noise reduction path was proposed: compressing the spacing to ≤10 mm to suppress the intensity of leakage vortex, designing the periodicity of failure vortex combined with the serrated blade tip/inner wall rubber strip, and blocking the acoustic cavity resonance with non-uniform wall stiffness or 8–10 kHz Helmholtz resonator, providing a solution for the low-noise design of UAV propulsion system. Unfortunately, our study cannot currently resolve transient characteristics; only time-averaged velocity/pressure flow-field contours and total acoustic power distribution are obtained for qualitative analysis of macroscopic noise variation laws and flow-sound correlation. Full article

Investigating the dynamic response of pepper leaves at different growth stages under droplet impact is of great significance for optimizing spray deposition processes. In this study, high-speed imaging combined with mechanical modeling was employed to systematically analyze the inclination angle variation and bending deformation behavior of pepper leaves at the seedling, flowering, and fruiting stages under droplet impact. The temporal evolution of petiole inclination angle and leaf blade inclination angle under different impact velocities and impact positions was investigated, and the energy transfer mechanism of bending energy during droplet impact was quantitatively analyzed. The results showed that pepper leaves exhibited a typical damped oscillation response after droplet impact. As the droplet impact velocity increased, the degree of leaf deformation increased significantly and the vibration duration was prolonged markedly. When the impact velocity increased from 0.89 to 1.53 m s⁻¹, the maximum vertical displacement (Δh_max) near the leaf tip increased from 0.8 to 1.9 mm at the seedling stage, from 0.6 to 3.1 mm at the flowering stage, and from 1.2 to 4.5 mm at the fruiting stage. Along the leaf length direction, both the maximum inclination angle variation and vertical displacement gradually increased from the near-petiole region toward the near-tip region because of reduced local stiffness and weaker structural constraints. In contrast, the maximum inclination angle variation generally decreased with increasing growth stage, mainly due to the increase in leaf mass and enhanced structural support. Based on the bending energy model, the conversion of droplet kinetic energy into leaf structural deformation energy was further analyzed. The results demonstrated that the bending energy of leaves increased significantly with increasing droplet impact velocity. When the droplet kinetic energy increased from 3.4 μJ to 8.0 μJ, the maximum bending energy increased from 0.063 μJ to 0.21 μJ. Among different regions, the near-tip area exhibited the highest bending energy response because of its higher flexibility and lower bending stiffness. These findings provide new insights into the interaction mechanism between droplets and flexible leaves and offer a theoretical basis for improving spray deposition efficiency and optimizing spraying strategies for pepper plants at different growth stages. Full article

Non-periodic leading-edge wear near the impeller tip is investigated with respect to the aerodynamic performance, steady flow organization, and near-stall unsteady evolution of a centrifugal compressor. A full-annulus three-dimensional impeller–vaned-diffuser model is established for a baseline configuration (O-M) and a non-periodically worn configuration (W-M). The two configurations are compared in terms of performance characteristics, near-tip pressure coefficient, static pressure, entropy, relative Mach number, three-dimensional vortical structures, and pressure fluctuation signals. The W-M generally produces a lower total pressure ratio than the O-M, with a maximum reduction of approximately 0.7%. Nevertheless, the isentropic efficiency is slightly improved over the main operating range, with a peak increase of about 0.6%, and the near-stall flow rate shifts toward a lower value. Pressure coefficient distributions at 95% span show that leading-edge wear weakens both the pressure-side pressure peak and the suction-side suction peak of the worn blades, redistributing the near-tip loading from a highly leading-edge-concentrated form to a broader chordwise distribution. The steady flow fields indicate that wear does not eliminate local low-pressure or high-entropy regions; rather, it reorganizes their circumferential arrangement, converting originally synchronized low-pressure zones, high-entropy bands, and high-speed shear layers into a non-uniform pattern with alternating strong and weak passages. Near-stall unsteady results further reveal that pressure cells, high-entropy zones, and large-scale vortical structures in the O-M exhibit clear cross-passage propagation, whereas the corresponding disturbances in the W-M remain predominantly localized, dispersed, and asynchronous. These results demonstrate that, for the wear location and blade-to-blade distribution considered here, non-periodic leading-edge wear affects stability primarily by weakening the circumferentially coherent amplification of disturbances, rather than by simply reducing all local loss sources. Full article

This study presents an extended Blade Element Momentum Theory (BEMT) framework for predicting the hover performance of an EC135-class light-twin helicopter rotor while quantifying the impact of sectional aerodynamic uncertainty on global rotor metrics. Because the proprietary EC135 airfoils are not publicly available, a reproducible surrogate blade based on the ONERA OA213 and OA209 airfoils is adopted. The airfoil substitution is explicitly treated as an epistemic modelling assumption, and its effect on rotor-level hover predictions is assessed through a dedicated geometric comparison and bounded sensitivity analysis. The classical BEMT formulation is enhanced with Prandtl tip-loss corrections, Mach-dependent sectional aerodynamics, and an iterative non-uniform inflow model. Aerodynamic coefficients are obtained from Gaussian Process (GP) surrogate models trained on XFOIL-generated databases and calibrated using cross-validation techniques. The calibrated GP models are coupled with the rotor solver and their predictive uncertainty is propagated through Monte Carlo simulations. For the nominal hover trim condition, the rotor was trimmed to $C_T=0.00607$, while the model predicted a power coefficient of 0.00041 and a figure of merit of 0.819. The propagated GP/XFOIL-conditioned uncertainty yields a 95% confidence interval of 0.8074–0.8285 for the figure of merit, indicating limited sensitivity of rotor performance to sectional aerodynamic uncertainty. The influence of compressibility and tip-loss effects is also quantified. In a separate Caradonna–Tung solver-verification case, the thrust-coefficient error is reduced from 32.57% to 7.78% when finite-aspect-ratio corrections are included. The proposed framework provides a fast, reproducible, and uncertainty-aware approach for helicopter rotor hover analysis suitable for preliminary design and performance assessment. Full article

This paper presents a comprehensive three-dimensional numerical investigation of a novel vertical-axis wind turbine (VAWT) characterised by a unique aerodynamic profile and a passive blade-pitch control mechanism. Unlike conventional fixed-geometry designs, the proposed turbine utilizes rectangular blades mounted on horizontal axes via articulated bearings, allowing them to rotate freely up to 90 degrees, constrained by a vertical pin-and-belt system. This configuration ensures that blades on the power-stroke side hit the vertical stopper to capture maximum wind energy, while blades on the return-stroke side open up to 90 degrees to significantly reduce aerodynamic drag. This dynamic adjustment enables the turbine to operate efficiently in low-wind conditions (3–5 m/s) while maintaining enhanced torque stability. To ensure numerical reliability, a rigorous grid independence study was performed, and the computational domain was configured to eliminate wall interference effects. The aerodynamic performance was analyzed using COMSOL Multiphysics v6.2 by solving the Reynolds-averaged Navier–Stokes (RANS) equations. Four turbulence models—SST, k–ε, k–ω, and RNG—were evaluated, with the SST model demonstrating the highest fidelity in capturing flow separation and wake structures under adverse pressure gradients. This study establishes the turbine’s performance benchmarks, including the power coefficient (C_p) versus tip speed ratio (TSR) curves. The numerical results were validated against laboratory experimental data, with excellent agreement (relative error < 5%). The findings identify the optimal geometric parameters and tangential velocity distributions that distinguish this configuration (Patent FAP 20240465) from traditional VAWTs. Finally, the successful implementation of a 2 kW prototype confirms the model’s accuracy and highlights the turbine’s potential as a stable and efficient solution for sustainable urban energy harvesting. Full article

Cycloidal propellers are known for their omnidirectional vectored thrust, enabling smooth transitions between hovering and forward flight, making them ideal for unmanned aerial vehicles (UAVs) and electric vertical take-off and landing (eVTOL) aircraft. However, cycloidal propellers tend to have lower hovering efficiency than screw propellers at the unmanned aerial vehicle (UAV) scale. Adding end plates to the blade tips can improve hovering efficiency by suppressing blade tip vortices. But the impact of these end plates have not been thoroughly studied. This paper aims to seek the designs with enhanced hovering efficiency and develop design guidelines for cycloidal propellers with end plates. Comprehensive force measurement experiments are performed on designs with and without end plates, and designs with rotating and static end plates. Complementary high-fidelity numerical analysis is performed to gain deeper insights into the complex 3D flow structures and the role of end plates in suppressing induced power losses. Our study reveals that end plates can effectively suppress the efficiency degradation typically associated with low aspect ratio blades. We demonstrate that even with a blade aspect ratio of 1.5, a cycloidal propeller equipped with end plates can achieve high hovering efficiency, thereby establishing a new design guideline for lightweight, high-performance propulsion systems. The designs with stationary end plates are superior to those with rotating end plates because rotation introduces additional torque caused by the friction force. Designs featuring thick end plates (${\overline{t}}_e=0.056$) outperform those with thin end plates (${\overline{t}}_e=0.004$), as the rounded edges can eliminate end plate vortices. A comprehensive parametric study is conducted, evaluating blade chord-to-radius ratios from 0.26 to 0.65, aspect ratios from 0.5 to 3.0, pitching amplitudes from 10° to 50°, as well as end plate configurations (stationary vs. rotating, and thin vs. thick). From this parameter space, the best design was identified as featuring stationary thick end plates (${\overline{t}}_e=0.056$), a chord-to-radius ratio of 0.65, and a large pitching amplitude of 40 degrees. It achieves a hovering efficiency of 0.72 with a blade aspect ratio of 3, which is comparable to that of sub-scale rotors with similar Reynolds number. In contrast, for the cases without end plates, the highest hovering efficiency is lower than 0.6. 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

Small-scale vertical-axis wind turbines (VAWTs) are increasingly essential for the “blue economy,” providing autonomous power to remote coastal communities, offshore platforms, and marine industries. However, the design of efficient Darrieus-type rotors is complicated by complex unsteady aerodynamics, particularly the phenomenon of dynamic stall. This study aims to establish and validate a cost-effective yet accurate mathematical modeling approach for simulating unsteady turbulent flow around a Darrieus H-rotor to support practical engineering applications. The research methodology integrates computational fluid dynamics (CFD) with physical experiments in a hydrodynamic channel. The numerical model utilizes the unsteady Reynolds-averaged Navier–Stokes (URANS) equations closed with the Strain-Adaptive Linear Spalart–Allmaras (SALSA) turbulence model, chosen for its efficiency in capturing flow separation. The system of initial equations was being devised relatively to an arbitrary curvilinear coordinate system. The pressure and velocity fields have been coordinated using the artificial compressibility method adapted to calculate non-stationary problems. Experimental verification was conducted in the GT-400 hydrodynamic tube using a three-bladed H-rotor model, where flow structures were visualized via the colored jet method at tip speed ratios λ ranging from 2 to 5 and Reynolds number 1470. The findings reveal that dynamic stall occurs over a significant portion of the blade trajectory, characterized by vortex generation at the leading edge and subsequent advection along the chord. Qualitative comparison demonstrates a high degree of correlation between the calculated vortex dynamics and physical flow spectra. These results confirm that the URANS-SALSA approach provides a rational compromise between computational cost and physical accuracy. Full article

Ducted propulsion fans are widely recognized for their ability to enhance aerodynamic efficiency and operational safety by utilizing a surrounding shroud to contain the flow and mitigate blade tip losses. However, maximizing thrust and optimizing internal flow dynamics remain critical challenges in further improving their aerodynamic performance. This study investigates the aerodynamic characteristics of a ducted propulsion fan configured with a secondary air intake channel designed to enhance mass flow ingestion. Utilizing Reynolds-Averaged Navier–Stokes (RANS) simulations coupled with the Shear Stress Transport (SST) k-omega turbulence model, the internal flow dynamics and aerodynamic efficiency of configurations both with and without the secondary air intake channel are examined. The secondary air intake, strategically located adjacent to the rotor blade tip, increases the mass flow rate and, consequently, enhances thrust. Physically, this configuration successfully reinjects bypass flow to mitigate tip leakage vortices, significantly reducing the low-velocity wake regions adjacent to the rotor tip. Several configurations were evaluated by systematically varying the intake channel’s position, curvature, and the dimensions of its inlet and outlet ports under static conditions at 6000 rpm. Numerical results demonstrate that the optimal design improves thrust by an additional 2.2% compared to the baseline ducted fan without the auxiliary intake port due to the mitigated tip vortices and stabilized flow field. Full article