Search Results (67)

This study investigates the robustness of Frenet–Serret curvature ($\kappa$) and torsion ($\tau$) estimates derived from noisy discretely-sampled three-dimensional space curves, with emphasis on the comparative performance of cubic spline and cubic Hermite interpolation methods. Accurate estimation of these geometric invariants is essential for reliable analysis of curves arising in signal processing and shape reconstruction; yet, the higher-order derivatives required for their computation exhibit pronounced sensitivity to measurement noise. We examine curves constructed through a Hilbert transform-based parameterization of the form $\mathrm{r}\left(t\right)=\left(X\left(t\right),A\left(t\right)sin\varphi \left(t\right),g\left(t\right)\right),$ where discrete samples are contaminated with additive white Gaussian noise at varying signal-to-noise ratios. Reconstruction is performed using cubic spline interpolation, which ensures global $C^2$ continuity, as well as cubic Hermite spline interpolation, which provides $C^1$ continuity with local tangent control. Frenet frame computations are then applied via regularized finite difference schemes. To characterize noise amplification theoretically, we derive the Curvature Stability Index (CSI) and Torsion Stability Index (TSI) as first-order variance bounds under the delta method. While these indices formalize the derivative-order dependence of noise sensitivity, Monte Carlo simulations reveal that empirical variance exceeds theoretical predictions by factors of ${10}^4$ to ${10}^6$, indicating dominance of nonlinear error propagation. Nevertheless, the indices establish that torsion instability arises fundamentally from third-order derivative structure rather than ground-truth magnitude. Numerical experiments across three geometric regimes constant-invariant helices, variable-curvature helices, and planar curves with identically zero torsion demonstrate that the ratio of the torsion root mean square error to curvature root mean square error consistently ranges from $6.5$ to $9.8$. This disparity persists even in the degenerate planar case, where $\tau \equiv 0$ analytically, confirming that torsion sensitivity is an intrinsic property of the Frenet–Serret formulation. Across all configurations, cubic spline reconstruction yields lower Monte Carlo mean RMSE and reduced empirical variance compared to Hermite spline, providing superior stability for derivative-based invariant estimation. Full article

Tibial torsion significantly influences knee biomechanics, yet its interaction with ACL reconstruction history in elite alpine skiers remains under-investigated. In this cross-sectional observational study, we analyzed 20 elite alpine skiers (7 ACL-reconstructed, 13 non-injured) using a markerless motion capture system during dynamic tasks (Squat, Single-Leg Squat, Lunge). Static tibial torsion was assessed via the Transmalleolar Axis and Thigh–Foot Angle. The results revealed a critical divergence in biomechanical strategies based on tibial alignment (p < 0.05). Skiers with rotational deformity adopted a pattern we describe as a “Stiffness Strategy”, characterized by suppressed knee valgus and hip rotation, but relied on excessive ankle dorsiflexion (39.5°)—a compensatory mechanism that may become limited when constrained by rigid ski boots. In contrast, ACL-reconstructed skiers with normal alignment exhibited what we term an “Instability Strategy”, showing dynamic valgus collapse and persistent asymmetry. These findings suggest that “one-size-fits-all” rehabilitation may be insufficient. We propose that injury prevention protocols may benefit from incorporating anatomical screening, focusing on decoupling mobility for skiers with tibial torsion and enhancing dynamic stability for those with normal alignment. Full article

An integrated nonlinear dynamic model was developed to investigate resonance in a four-cylinder engine mounted on a viscoelastic foundation. A coupled lumped-parameter formulation captures vertical and torsional responses under unbalanced inertial forces, combustion torque, and stochastic base excitation. Time-domain simulations show that at low rotational speeds the vertical displacement reaches transient amplitudes before converging to periodic oscillations, whereas higher excitation speeds reduce steady-state amplitudes. Torsional motion exhibits initial angles near 0.05 rad that decay below 0.01 rad in steady state, with further reduction at higher speeds. Frequency-domain analysis indicates that vibration energy is concentrated in engine-order harmonics between approximately 8 and 50 Hz, while components above 60 Hz are strongly attenuated, yielding a dynamic range exceeding 50 dB. Finite element modal analysis identifies the first four structural modes between 18 Hz and 666 Hz, revealing an increasingly dominant overall translational mode and a localized directional behavior at higher frequencies. A high-dimensional kernel density spectrogram integrates modal and spectral features to map resonance regions. Results indicate that increasing rotational excitation enhances inertial stiffening, systematically reduces displacement amplitudes, and preserves bounded periodic dynamics without instability. Full article

This study presents a high-performance MEMS accelerometer employing a symmetrical differential ‘sandwich’ capacitive structure. An orthogonal rectangular compensation method was integrated with wet anisotropic etching to achieve high structural symmetry. An innovative glass–silicon composite cover plate was adopted, and the upper and lower plates were encapsulated by a sensitive structure via anodic bonding, which effectively reduced the parasitic capacitance. Simulations confirmed sufficient separation between the sensitive-axis (Z-axis) resonant frequency and orthogonal/torsional modes, ensuring low cross-axis coupling. The fabricated device exhibits high sensitivity (0.2216 V/g) and excellent linearity (99.842%) within a 0–8 g range. Furthermore, it demonstrates outstanding noise (7.88 µg/√Hz) and bias-instability (6.39 µg) performance, positioning it competitively against commercial counterparts. The proposed design and process offer a viable technical route for high-precision inertial sensing applications. Full article

Steel girders with corrugated webs are increasingly used in bridge and building structures subjected to cyclic variable loads, where the geometry of the corrugation plays an important role in fatigue performance. This paper investigates the fatigue behaviour and failure mechanisms of full-scale steel girders with sinusoidal corrugated webs subjected to static and cyclic four-point bending. Five simply supported girders were tested: one reference beam under monotonic static loading, two beams under long-term cyclic loading with different load ranges ΔF and numbers of cycles N, and two beams subjected to cyclic loading followed by a static test to failure. The experimental programme focused on the influence of the load range ΔF and the number of cycles N on damage development, stiffness degradation and residual load-bearing capacity, as well as on the interaction between local web instability and global lateral–torsional buckling. The test results show that two main failure mechanisms may occur: (I) local buckling of the corrugated web combined with yielding of the flanges, and (II) a combined mechanism involving local web buckling and lateral–torsional buckling of the girder. For the investigated configurations and within the range of load ranges and numbers of cycles considered, the load range ΔF was found to be the dominant parameter governing fatigue damage, whereas the number of cycles had a secondary influence. The global stiffness of the girders in the elastic range remained almost unchanged until the late stages of loading, and even after pre-fatigue loading, the girders were able to carry a significant portion of their original ultimate load. The results provide experimental data and insight that are relevant for the fatigue assessment and design of steel girders with sinusoidal corrugated webs in bridge and building applications. Full article

Generalised joint hypermobility (GJH) describes an excessive range of joint movement and is associated with increased musculoskeletal injury risk, joint pain, and instability. This study compares lower limb biomechanical characteristics between children with and without GJH. Children aged 5–18 years with GJH (Beighton score ≥ 6/9 pre-puberty, ≥5/9 post-puberty) were age- and sex-matched with controls (Beighton score ≤ 2/9). Biomechanical measures included internal hip rotation, quadriceps (Q) angle, tibial torsion, ankle range of motion (ROM), and foot posture index (FPI). Wilcoxon rank sum test and chi-square were used to assess group differences. Fifty-two participants (median age 11 years, 69% females) included 27 children with GJH and 25 healthy children. Internal hip rotation, Q-angle, ankle ROM and FPI were significantly higher for children with GJH (p < 0.001) than healthy peers. While tibial torsion showed no difference in males, females had greater internal tibial torsion [median difference: right −4° (95%CI:−7,−2), p = 0.002; left −5° (95%CI:−7,−1), p = 0.010]. The largest differences were in ankle ROM [median difference: right 9° (95%CI:7,12); left 9° (95%CI:6,12)]. Children with GJH present different biomechanical measures than non-GJH peers. Further research into the clinical relevance of ROM at the hip, ankle and foot for children with GJH which are movement planes not assessed in Beighton score is warranted. Full article

The drive system of pure electric vehicles is characterized by high transmission efficiency and a rapid torque response, with the centralized drive configuration being the most commonly adopted. To improve the dynamic performance and reliability of such systems, this study investigates the nonlinear dynamic characteristics of high-speed helical gear reducers used in electric vehicles. A coupled bending–torsional–shaft dynamic model is established, in which the time-varying mesh stiffness is calculated using an improved potential energy method. The system responses under varying mesh errors, backlash, and damping ratios are obtained through numerical integration via the variable-step Runge–Kutta method. The results demonstrate that at high input speeds, the helical gear system exhibits complex nonlinear behavior. Small backlash and minor manufacturing errors lead to stable periodic or quasi-periodic motion, whereas increasing these parameters induces dynamic instability. Moreover, enhancing the mesh damping ratio effectively suppresses chaotic responses and improves overall system stability. Full article

This article investigates the structural resistance of thin-walled steel sections classified as Classes 1 to 4 under Eurocode 3. The study focuses on flexural capacity, and takes into consideration the effects of local buckling and the bimoment. Although Class 1 and 2 sections can develop complete plastic resistance, Class 3 sections are limited to elastic behavior prior to local instability. For Class 4 sections, effective width methods are employed to account for the reduction in strength due to early local buckling. Based on Eurocode formulations, these approaches are extended to incorporate the influence of the bimoment, which is significant in thin-walled open sections under non-uniform torsion. A comparative analysis between analytical models and numerical simulations is presented, with an emphasis on how the bimoment alters stress distributions and reduces the effective widths of slender plates. The results underscore the necessity of including these effects in the structural design of thin-walled members, particularly for open profiles subjected to bending and warping. Full article

Long-span cable-supported bridges, such as cable-stayed and suspension bridges, are highly sensitive to wind-induced effects due to their flexibility, low damping, and relatively light weight. Aerodynamic analysis is therefore essential in their design and safety assessment. This study examines the aeroelastic stability of the Oued Dib cable-stayed bridge in Mila, Algeria, with emphasis on vortex shedding, galloping, torsional divergence, and classical flutter. A finite element modal analysis was carried out on a three-dimensional model to identify natural frequencies and mode shapes. A two-dimensional deck section was then analyzed using Computational Fluid Dynamics (CFD) under a steady wind flow of U = 20 m/s and varying angles of attack (AoA) from −10° to +10°. The simulations employed a RANS k-ω SST turbulence model with a wall function of Y+ = 30. The results provided detailed airflow patterns around the deck and enabled the evaluation of static aerodynamic coefficients—drag (C_D), lift (C_L), and moment (C_M)—as functions of AoA. Finally, the bridge’s aeroelastic performance was assessed against the four instabilities. The findings indicate that the Oued Dib Bridge remains stable under the design wind conditions, although fatigue due to vortex shedding requires further consideration. Full article

Topoisomerase 1 (Top1) removes transcription-related helical torsions and thus plays an important role in preventing genome instability instigated by the formation of non-canonical DNA secondary structures. The genetically tractable Saccharomyces cerevisiae model proved effective in defining the critical function of Top1 to prevent recombination and chromosomal rearrangement at G4-forming genomic loci and studying the human cancer-associated Top1 mutants through the expression of analogous yeast mutants. We previously showed that cleavage-defective Top1 mutants strongly elevate the rate of recombination at G4 DNA, which involves binding to G4 DNA and interaction with the protein nucleolin (Nsr1 in yeast). Here, we further explored the mechanism of genome instability induced by the yeast Top1Y740* mutant, analogous to the human Top1W765Stop mutant conferring resistance to CPT. We show that yTop1Y740* elevates duplications as well as recombination specifically at G4-forming sequences. Interestingly, SUMOylation of yTop1Y740*, which does not affect the G4 DNA-binding or Nsr1-interaction by this mutant, is necessary for such elevated G4-specific genome instability. Many tumors with mutations at the C-terminal residues of Top1, particularly W765, have significantly high G4-associated mutations, underscoring the importance of further investigation into how SUMOylation affects the function of these Top1 mutants at G4 DNA. Full article

Nonlinear wind-induced vibrations and coupled static–dynamic instabilities pose significant challenges for long-span suspension bridges, especially under large-amplitude and high-angle-of-attack conditions. However, existing studies have yet to fully capture the mechanisms behind large-amplitude torsional flutter. To address this, wind tunnel experiments were performed on H-shaped bluff sections and closed box girders using a high-precision five-component piezoelectric balance combined with a custom support system. Complementing these experiments, a finite element time-domain simulation framework was developed, incorporating experimentally derived nonlinear flutter derivatives. Validation was achieved through aeroelastic testing of a 1:110-scale model of the original Tacoma Narrows Bridge and corresponding numerical simulations. The results revealed Hopf bifurcation phenomena in H-shaped bluff sections, indicated by amplitude-dependent flutter derivatives and equivalent damping coefficients. The simulation results showed less than a 10% deviation from experimental and historical wind speed–amplitude data, confirming the model’s accuracy. Failure analysis identified suspenders as the critical failure components in the Tacoma collapse. This work develops a comprehensive performance-based design framework that improves the safety, robustness, and resilience of long-span suspension bridges against complex nonlinear aerodynamic effects while enabling cost-effective, targeted reinforcement strategies to advance modern bridge engineering. Full article

Curved-spoke wheels have been proposed as an effective way to overcome stair-like obstacles with smooth, rotation-only motion. However, when the wheel’s contact point shifts, discontinuous changes in its radius of curvature cause abrupt drops in the robot’s linear speed, often leading to reduced payload stability and slip. As a result, maintaining reliable stair climbing becomes more difficult. At higher speeds, these sudden changes become stronger, further reducing dynamic stability. To address these issues, we propose a passive Compliant Spiral Torsional Suspension (C-STS) attached to the wheel’s drive axis. Through camera-based marker tracking, we analyzed wheel trajectories under various stiffness and speed conditions. In particular, we define the deceleration caused by the velocity drop during contact transitions as our dynamic stability metric and demonstrate that the C-STS significantly reduces this deceleration across low-, medium-, and high-speed climbing, based on comparisons both with and without the suspension. It also raises the average velocity, likely due to a brief release of stored elastic energy, and lowers the net torque requirement. Our findings show that the proposed C-STS greatly improves dynamic stability and suggest its potential for enhancing stair-climbing performance in curved-wheel-based robotic systems. Furthermore, our approach may extend to other reconfigurable wheels facing similar instabilities. Full article

Torsional vibrations pose a serious challenge in drilling operations and can lead to effects such as stick-slip phenomena, tool wear, and reduced drilling efficiency. While previous research has been conducted on torsional vibrations, there is a notable gap in comparative studies that assess the scalability of downscale models to real-world drilling conditions. This study fills this gap by systematically comparing torsional vibrations in downscale and upscale drill strings under different torque conditions at three different depths, shedding light on scaling effects in drilling vibrations. Numerical simulation was carried out taking into account non-linear interactions, damping effects, and torque variations. The laboratory set-up was for a well length of 15 m and was geometrically scaled to represent an upscale well of 450 m. Certain operational parameters such as rotation speed, torque, density, and friction coefficients were modified to keep realistic dynamic behavior, and all models were run at an identical speed of rotation to enforce consistency. The results show that both the upscale and downscale models exhibited stick-slip behavior, but differences in vibration intensity and stabilization trends point out how scaling affects torsional dynamics. Notably, the upscale bit first faced higher torsional oscillation than the set rotation speed after overcoming stick-slip before stabilizing, whereas the downscale bit went through prolonged stick-slip instability before synchronization. This study enhances the understanding of scaling effects in torsional drilling vibrations, offering a foundation for optimizing experimental setups and improving predictive modeling in drilling operations. Full article

Background/Objectives: The diagnostic guidelines for pediatric patellofemoral instability (PFI) remain incomplete. PFI remains a challenging issue as it affects the biomechanics of the knee joint, triggers anterior knee pain, and is linked to the development of early-onset osteoarthritis. The diagnostic process is complicated by numerous anatomical factors that must be considered. This review aims to consolidate current knowledge presented in the literature on radiological diagnostics for PFI in pediatric populations, with the application of all imaging techniques—including ultrasonography (US), magnetic resonance imaging (MRI), computed tomography (CT), and radiography (RTG)—which enable the evaluation of anatomical risk factors critical for the diagnosis, prevention, and treatment of PFI. Methods: A search of the PubMed/MEDLINE database was conducted to identify relevant studies from 1975 to 2024. The search terms were as follows: (patellar or patella) and (instability or displacement or dislocation) and (diagnostic or diagnosis or imaging or radiographic). A total of 2743 articles were retrieved, which were screened to yield 29 studies for further review. These studies were then divided into seven categories regarding the diagnostic methods: risk factors, tibial tubercle trochlear groove (TT-TG)/tibial tubercle posterior cruciate ligament (TT-PCL), MPFL injury and cartilage damage, patella and trochlear dysplasia, torsional abnormalities, coronal plane alignment, and genetics. Results: The methods presented statistically significant differences, with those most commonly used for the diagnosis of patella dislocation being TT-TG index, MPFL rapture, and trochlear dysplasia. Conclusions: In summary, multiple diagnostic tools, including MRI, CT, X-ray, and physical examination, are available for the assessment of PFI, each contributing to treatment decisions. Although MRI remains the primary diagnostic tool, further research is needed to establish more precise diagnostic criteria. Full article

The double-layer (DL) cable-supported photovoltaic (PV) module system is an emerging type of structure that has garnered significant attention in recent years due to its large span, strong terrain adaptability, and economic advantages. As it is a flexible structure supported by cables, wind-induced vibrations can lead to structural instability or even component damage, posing a serious threat to the safety of PV power plants. Determining the wind-induced vibration characteristics of the DL cable-supported PV module system is the foundation for ensuring its structural safety. In this study, based on wind tunnel tests performed on an aeroelastic model, a typical DL cable-supported PV module system used in a real engineering project was examined. The wind-induced displacement and torsional vibration characteristics of the model were compared and analyzed under different wind speeds. The shading effects of the PV array were also studied, and the impact of different wind angles and the initial tilt angles of PV modules on the wind-induced vibration characteristics was revealed. The results show that the greatest displacement vibration response occurs in the vertical direction; in comparison, displacements in other directions are smaller. Wind-induced torsional vibrations are negligible and can be ignored compared to displacement vibrations. The wind-induced vibration of the first row in the cable-supported PV array is significantly greater than that of the subsequent rows, and the shading effect is obvious. In the same row, the displacement vibration of modules at the center span is greater than at the sides. Different wind angles and initial PV module tilt angles affect the wind-induced vibration characteristics. When the wind direction is perpendicular to the cables and wind suction occurs, the wind-induced vibration is maximal. Within a limited range, the larger the initial tilt angle of the PV module, the greater the wind-induced vibration. Full article