Search Results (886)

This study investigates the under-explored contribution of internal radial stress to macroscopic gravitational potentials. We utilize a novel, precision-damped torsion balance apparatus designed to isolate and measure gravitational perturbations induced by radial stress within rapidly rotating macroscopic masses, rigorously controlling for acoustic, thermal, and electromagnetic variables. Our findings document an anisotropic mechanical influence along the plane of rotation that exceeds standard weak-field gravitational predictions. The measured apparatus potential exhibits a kinematic scaling (∝ω⁴) consistent with a linear torsion spring responding to an underlying quadratic (ω²) driving force. While active Herzan leveling, counter-rotating geometry, and precision balancing successfully eliminated bulk frame-drag and mechanical vibration, this atmospheric baseline study concedes that the persistent ω² force may still be masked by complex, non-linear fluid dynamic asymmetries. Consequently, this paper establishes a ‘Stage 1’ methodological baseline, mapping the absolute limits of atmospheric testing and defining the engineering prerequisites for future high-vacuum gravitational extractions. Isolating this residual force in a future high-vacuum environment is imperative. Should such testing confirm the signal is gravitationally sourced, this anisotropy suggests a potential coupling between the internal stress-energy tensor and the local spacetime metric that is not fully accounted for in standard linear approximations. If this phenomenological stress-metric coupling can be isolated and shown to scale macroscopically, it could theoretically provide a non-linear mechanism for observed rotational gravitational symmetry breaking. The speculative astrophysical implications of such a coupling are also discussed. Full article

Vortices and dynamic stall play a critical role in the performance of vertical-axis wind turbines, often leading to significant energy losses. Active and passive control strategies can be employed to delay the dynamic stall, particularly at low tip-speed ratios below 0.5. In this study, three-dimensional Computational Fluid Dynamics simulations of a Darrieus H turbine equipped with a NACA 0018 airfoil are performed using a sinusoidal pitch control method. A vorticity analysis framework is developed to evaluate vortex transport, growth, and detachment, enabling a rigorous assessment approach. The simulations, conducted at 8 m/s wind speed, are validated against experimental data, showing less than 4% deviation. The analysis examines the correlation between vorticity dynamics and turbine performance across both the span and chord of the airfoil. At a tip-speed ratio of 0.5, where dynamic stall is dominant, the pitch control delays flow separation and increases performance by 238%. At a tip-speed ratio of 1.4, performance improves by 57%, although the flow shifts toward a drag-dominated regime in which vortex detachment plays a reduced role. This demonstrates that quantitative vorticity analysis remains effective under variable pitch and can identify stages of the dynamic stall, including imminent vortex separation, offering a basis for optimizing vertical-axis wind turbines. Full article

Gas dispersion and liquid circulation are critical factors governing the performance of gas–liquid stirred tanks, yet the coupled effects of operating conditions and impeller geometry on hydrodynamic behavior remain insufficiently understood. In this study, a Euler–Euler two-fluid model coupled with the standard k–ε turbulence model is employed to investigate gas dispersion, drag-coefficient distribution, power consumption, and mixing dead zones in a Rushton turbine stirred tank. The numerical model is validated against published experimental data before examining the influences of impeller rotational speed, gas injection velocity, blade height, and blade length. The results reveal that high drag coefficients are concentrated along the impeller discharge path, vortex-core regions and near-wall annular structures, indicating zones of reduced gas–liquid slip and strong interphase momentum exchange. Increasing the stirring speed from 400 to 800 rpm reduces the fluid and liquid dead-zone fractions from 93.31% to 54.27% and from 24.64% to 12.08%, respectively, but increases the power from 8.78 to 77.00 W. By contrast, increasing the gas-injection velocity from 5 to 13 m/s decreases the power draw but expands the fluid dead-zone fraction from 50.94% to 94.54%, indicating a marked deterioration in effective liquid circulation associated with gas accumulation near the impeller. Blade height mainly adjusts local shear and gas-holdup uniformity, whereas blade length is the dominant geometric parameter for dead-zone suppression, although its benefit becomes marginal at the largest length because of the severe power penalty. These findings clarify the coupled effects of gas dispersion, liquid circulation, drag-coefficient distribution, and power demand, and provide guidance for the design and operation of gas–liquid stirred tanks. Full article

We explore the analogy between the asymptotic (high Reynolds number) scaling of two canonical wall-bounded turbulent flows, namely zero-pressure-gradient (ZPG) and pipe flows. We find that the two flows can be characterised using similar scaling laws that relate the streamwise turbulence intensity to friction: the product of the dimensionless drag and the square root of a dimensionless boundary-layer thickness for ZPG flow plays the role of the streamwise turbulence intensity for pipe flow, and the two flows are linked through a Reynolds-number-dependent correction term derived from the logarithmic mean-velocity profile. As a consequence, the squared product scales as the friction factor. Establishing a common, friction-based turbulence intensity scaling for ZPG and pipe flows is of interest both for fundamental studies of canonical wall-bounded flows and for the specification of the turbulence intensity in computational fluid dynamics simulations. Full article

In fluid analysis of stent models with a braided structure, conventional modeling using Finite Element Method (FEM) boundaries requires extremely fine fluid mesh resolution. The objective of this study is to propose a grid size design strategy for FEM–Discrete Element Method (DEM) coupled analysis. In the proposed method, the fluid is modeled using FEM, while the stent is modeled using a continuous arrangement of DEM particles. The volume-force-based coupling method eliminates the need for node sharing between the FEM and DEM, thereby reducing the modeling workload. In this study, we derived grid sizes based on flow analysis around a single strand and verified the flow analysis around a 3D braided stent model. The results showed that the proposed method reduced the number of fluid grids by approximately 44% compared to conventional methods. In this case, the maximum errors in velocity and pressure were 0.0064 m/s and 17.07 Pa, respectively, and high correlations of 0.9 or higher were obtained for both distributions. Furthermore, the maximum relative error in the drag coefficient was 4.332%. This study provides guidelines for a fluid grid size design method that enables the reduction in computational cost and modeling burden in fluid flow analysis around stents. Full article

This study investigates the aerodynamic and thermal impacts of tire sidewall cooling fins on a passenger vehicle using high-fidelity computational fluid dynamics (CFD) simulations. Continuous heat accumulation from tire rotation and road friction can degrade structural durability. To address this thermal challenge, a parametric study was conducted on the DrivAer notchback vehicle model across various fin angles from −67.5° to 67.5°. The results revealed a distinct design space that offers simultaneous aero-thermal improvements. Specifically, the 22.5° fin configuration demonstrates a dual-benefit performance, achieving a 3.79% net reduction in overall vehicle drag alongside a 17.36% increase in the average heat transfer coefficient (HTC). Conversely, the −22.5° configuration yields the maximum cooling enhancement with a 30.49% increase in average HTC but incurs a 2.52% drag penalty. Microdrag and Turbulent Kinetic Energy (TKE) analyses successfully explain the underlying fluid mechanisms governing these trade-offs. These findings provide practical design guidelines for flow control on rotating wheels, showing that tire sidewall geometries can enhance full-vehicle aerodynamic efficiency and tire thermal reliability. Full article

With the continuous increase in horizontal displacement during unconventional oil and gas drilling, drilling operations are commonly plagued by high frictional resistance, severe pipe sticking issues and difficulties in directional drilling. Conventional single-component water-based drilling fluid lubricants fail to satisfy multifunctional demands, including high-temperature resistance, salt tolerance and long-term lubrication. In this work, a novel microsphere lubricant, RB-Si, with high-temperature and high-salinity resistance was developed by combining the merits of solid and liquid lubricants. The product was synthesized using myristic acid and triethanolamine as raw materials via co-reaction with boric acid and nano-silica. Laboratory tests reveal that after aging at 180 °C, the lubrication coefficient reduction rates of based mud, saturated salinity-based mud and a high-density (2.0 g/cm³) drilling fluid gel system containing 1.0 wt% RB-Si reached 90.1%, 83.3% and 62.8%, respectively. Meanwhile, RB-Si can effectively plug the micropores in the filter cake, reduce fluid loss, inhibit shale hydration and swelling, and exhibit excellent compatibility with drilling fluids. RB-Si rapidly adsorbs onto the surfaces of metallic drill strings and formation rocks to construct a durable and high-strength lubricating film, accompanied by the rolling friction of the incorporated microspheres, thereby reducing frictional drag between the drill string/casing and the drill string/borehole wall. This lubricant effectively mitigates high frictional resistance under prolonged friction conditions, and is expected to provide technical support for long-horizontal-well drilling. Full article

Improving motor efficiency by reducing frictional losses in the journal bearings used in many rotating machines is critical for advancing sustainable mechanical systems, especially automobiles. This study investigates the application of microbubbles, which are known to reduce frictional drag in fluids. The incorporation of microbubbles into lubricants offers an environmentally friendly friction-reduction method that avoids chemical additives. In this work, microbubbles were generated within lubricating oil and applied to a journal bearing. Experimental measurements of friction torque during shaft rotation demonstrated that lubricating oil containing microbubbles yielded lower torque than oil without microbubbles. These results indicate that microbubbles effectively reduce friction in journal bearings. Furthermore, a transparent quartz glass bearing and acrylic shaft were used to directly observe the dynamics of microbubbles flowing within the journal bearing clearance to elucidate the underlying frictional torque reduction mechanism. Full article

This study investigates the passive control of backward-facing step flows using rigid and elastic cantilevered plates, with emphasis on flow reattachment, pressure recovery, and fluid–structure interaction effects. The numerical methodology was first validated against an experimental benchmark, yielding a reattachment-length prediction within 1.28% of the measured value. Following validation, simulations were performed using the unsteady Reynolds-averaged Navier–Stokes equations coupled with the Spalart–Allmaras turbulence model. Rigid plate configurations were examined at momentum-thickness-based Reynolds numbers of $R{e}_{\theta }=500$, 1000, and 5000, while two-way fluid–structure interaction simulations were conducted at $R{e}_{\theta }=5000$ to evaluate the influence of structural stiffness. For the baseline configuration, the non-dimensional reattachment length and base drag coefficient remained within the ranges of $x_r/h=6.18$–$6.34$ and $c_B=0.199$–$0.205$, respectively. The most effective rigid configuration, $L/h=2.5$, reduced the reattachment length from $x_r/h=6.34$ to $x_r/h=5.57$ and the base drag coefficient from $c_B=0.205$ to $c_B=0.175$ at $R{e}_{\theta }=5000$, corresponding to reductions of approximately 12% and 15%, respectively. The fluid–structure interaction simulations showed that plate stiffness strongly influences flow-control effectiveness. The stiffest elastic configuration, with $E=2\times {10}^9$ Pa, achieved $x_r/h=6.14$ and $c_B=0.196$, whereas more flexible plates exhibited larger deformation and reduced aerodynamic benefit. Overall, the results demonstrate that cantilevered plates provide an effective passive flow-control strategy for backward-facing step flows. Rigid plates deliver the greatest aerodynamic improvement, while elastic plates require sufficient structural stiffness to maintain favorable pressure recovery and flow-reattachment characteristics. Full article

Unmanned Air Vehicles (UAVs) are becoming increasingly popular and widely used in a variety of industries such as agriculture, construction, delivery, surveillance, rescue operations, mapping, wildlife tracking and many more. With the advancements in technology, UAVs are becoming more autonomous and able to perform tasks with minimal human intervention, rendering their use indispensable for military and law enforcement purposes. In terms of control surfaces, V-tail configurations are commonly used on UAVs due to their advantages in control and stability performance, as well as their ability to reduce drag and improve overall efficiency. However, research on V-tail design and sizing is limited, particularly for Class I mini-UAVs. The objective of this paper is to identify a methodology for the Conceptual and Preliminary sizing and design of a V-tail of a Class I Mini UAV (NATO classification). The methodology follows the design of a V-tail from the characteristics of the conventional tail of the UAV. Once the characteristics of the conventional tail are extracted, V-tail geometric characteristics are computed. The stability derivatives of the V-tail are then calculated. The methodology for the analytical aerodynamic characteristics and stability derivatives is a combination of two existing methodologies: one methodology for V-tail stability and control derivatives, which refers to the Preliminary or Detailed Design of an aircraft, and one methodology for a conventional tail design, which refers to the Conceptual and Preliminary design of an aircraft. With this combination, a V-tail Preliminary design methodology was achieved. Furthermore, the aerodynamic characteristics and stability derivatives of the designed V-tail were verified by Low Fidelity Aerodynamics simulation, and then by High Fidelity Aerodynamics by means of Computational Fluid Dynamics (CFD). Full article

In this study, we numerically investigated the effect of friction pad geometry on drag torque reduction in the disengaged state of a wet clutch used in an integrated mechanical limited-slip differential (mLSD)-disconnector module. A paper-based friction pad was considered, and three-dimensional computational fluid dynamics (CFD) simulations were performed for a periodic sector using ANSYS Fluent 2021 R1 under oil-filled single-phase conditions. The numerical approach was validated against previously reported numerical results, showing an average deviation in dimensionless drag torque of less than 0.5%. The effects of friction pad geometry on lubricant flow behavior, wall shear stress, and drag torque were then evaluated. Based on these results, full factorial combination analysis and piecewise cubic Hermite interpolating polynomial (PCHIP) methods were applied to derive an optimized groove geometry that reduces drag torque while limiting the reduction in friction pad area. The selected PCHIP-based geometry reduced drag torque by approximately 4.10% relative to the reference geometry while limiting the friction pad area loss to within 2.50%. These results show that drag torque can be reduced by forming an effective oil discharge path through appropriate groove geometry design, providing a practical CFD-based optimization approach for improving wet clutch efficiency while maintaining the effective friction pad area. Full article

This work develops a tool to efficiently estimate the diffractive loads on a quadrotor uncrewed aerial vehicle (UAV) immediately following a nearby explosion. Existing models in the literature that predict the time history of wind velocity and the overpressure at a single distance from the blast are extended to model a moving blast wave that passes over the vehicle. The time-varying diffractive loads (i.e., due to the blast-induced pressure differential) are first modeled for a single sphere in a blast wave and then for a quadrotor approximated as a series of spheres connected by rods—one sphere for each of the four motors and one sphere for the central body. The overpressure and wind velocity models are compared with computational fluid dynamics (CFD) data. To illustrate the computational approach, a representative quadrotor model is perturbed by a blast from an initial hover flight condition in simulation. The rigid body dynamics are simulated over a short duration (ninety milliseconds) to determine the UAV’s state immediately after the explosion has concluded. The vehicle state history is predicted under the assumption of diffractive loads with a quadratic drag model and constant hover thrust. Full article

Steel is the most used metallic material in the world and the demand for new improved high-quality steel products due to modern performance requirements has been increased. Steel is produced in small and big bulks in accordance with the necessities inside the secondary refining process such as Electric Arc Furnaces (EAF), Basic Oxygen Furnaces and AOD converters where pig-iron or scrap steel are mixed and recycled. Here steel is foundry, and additional elements like carbon and other ferrous-alloys or metals are added to obtain the chemical composition required; then, oxidation and reduction eliminate some undesirable inclusions. Moreover, these lances also blow gases towards the AOD converter to generate twirlings that move the melting steel inside. Therefore, it is so important to understand hydrodynamic behavior to improve industrial practices. In this work, different fine meshes were solved to guarantee the good concordance and fidelity of the results regarding the fluid flow dynamic in the melting bath inside an AOD converter under the influence of a blowing system. The results refer to the finest mesh which had the best fit and lower fluctuations. Then, this work focuses on the simulation of a configuration of seven nozzles disposed beside the first conic inverted wall, which are used to blow a mix of argon and oxygen to promote the movement of the melting steel inside and drag those undesirable materials up towards the slag. Initially in this work an approaching analysis was done by comparing the penetration of the gas blown to the melting bath. After verification a hydrodynamical analysis is done by comparing vertical and horizontal cut views of velocity fields and streamlines, contributing to the understanding of hydrodynamic behavior. Furthermore, a kinetic energy analysis was also done to support the profiting and dissipation of the energy inside the fluid. Full article

In the process of offshore natural gas extraction, natural gas hydrates tend to form within the wellbore. This secondary hydrate formation can potentially cause severe blockages. Current prediction methods primarily rely on temperature–pressure curves, which often overlook the critical effects of high-velocity fluid flow, particularly the impact and drag forces acting on the hydrates. To address this limitation, this study proposes a novel risk prediction model that innovatively decomposes the hydrate-induced wellbore blockage into three distinct stages: implantation, scour, and fracture. Each stage is mathematically evaluated using a dedicated analytical model: the impulse equation for implantation, the negative pressure suction equation for scour, and the hydrate fracture toughness equation for fracture. A region is deemed at risk of hydrate blockage only when all three stage conditions are simultaneously satisfied. Sensitivity analysis focusing on four key parameters—hydrate particle size, temperature, gas flow rate, and impact angle—revealed that increasing either the hydrate particle size during nucleation or the extraction temperature significantly reduces the risk of secondary hydrate blockage. Moreover, a typical case study demonstrated that the application of this three-stage model considerably narrows and refines the predicted risk area compared to traditional thermodynamic models. These results provide a solid theoretical foundation for accurately predicting secondary hydrate blockage risks and offer targeted strategies for flow assurance and mitigation in critical wellbore sections. Full article

To investigate the coupled evolution of cavity morphology, hydrodynamic characteristics, and motion behavior during the wide-speed-range acceleration of an underwater hypervelocity vehicle, a numerical framework for supercavitating multiphase flow was established by coupling the Improved Delayed Detached Eddy Simulation (IDDES) turbulence model, the Schnerr–Sauer cavitation model, and the Volume of Fluid (VOF) method. Combined with the overset mesh technique and the DFBI six-degree-of-freedom model, the multiphase flow and motion characteristics during acceleration were systematically studied. The results show that the ventilated cavity strongly compresses the natural cavity, leading to a complex gas–vapor–liquid three-phase coexistence structure in the mid-body conical section and stern region, with the ventilated cavity eventually becoming dominant. The drag coefficient exhibits a three-stage evolution associated with cavity development over the conical section, cylindrical section, and the final formation of a supercavity. Once the vehicle is enveloped by the supercavity, pressure drag becomes dominant. Ventilation timing significantly affects supercavity formation and flow stability. Low-speed ventilation reduces drag earlier but prolongs the three-phase coexistence period and cavity formation process, whereas high-speed ventilation promotes the rapid formation of a stable supercavity. The supercavity formation time reaches 0.5 s under ventilation at 30 m/s, which is more than twice the value for ventilation at 70 m/s. Full article