Search Results (783)

Manipulating complex light fields through highly anisotropic scattering medium (HASM) remains a fundamental challenge due to the intricate underlying physics and broad application potential. We introduce a unified theoretical and experimental framework for generating and controlling arbitrarily polarized curved caustic beams using an extended polarization transfer matrix (EPTM) for the first time, enabling intuitive polarization transformation through HASM. The EPTM is experimentally measured via a four-step phase-shifting technique, and its submatrices are independently modulated with tailored caustic phase profiles. This strategy facilitates the creation of diverse high-resolution caustic beams, including Gaussian and vortex types with tunable energy distribution, polarization states, and vorticity. The achievement of polarization transformation through HASM by our approach offers versatile manipulation over optical field properties such as multiple high-resolution caustic beams, angular momentum flux, and polarization, paving the way for enhanced functionality in advanced optical systems. Full article

The global concern regarding the reduction of carbon emissions has led to the development of hydrogen as a clean, carbon-free fuel for combustion systems. The present work combines Particle Image Velocimetry flow field measurements and Reynolds-Averaged Navier–Stokes numerical simulations to investigate the reactive flow downstream of a newly developed flame holder as part of a hydrogen-fueled afterburner system. The obtained numerical results are in reasonable agreement, for a RANS simulation, with the PIV measured data. According to the results presented in this article, it can be seen that ignition occurs, the flame is attached to the flame holder, and vortices develop downstream of the flame holder. These vortices facilitate the mixing of hydrogen with the flue gas coming from the gas generator. The recirculation zone generated by the flame holder in the flow measures approximately 100 mm, with the peak negative velocity reaching around 10 m/s. Downstream of the recirculation zone, the far-field free stream velocity on the centerline reaches 20 m/s. Outside the recirculation region, in the radial direction, the free stream is accelerated to an experimentally measured value of approximately 40 m/s, at 20 mm downstream from the flame holder, and 35 m/s at 100 mm downstream of the flame holder. The information gathered thus far will aid further investigation of the presented hydrogen-fueled afterburner system. Full article

Increasingly slender bridge decks are prone to wind-induced damage, where the complex interactions between the incoming wind, deck, and adjacent wake flows play a deciding role. However, the unsteady wake dynamics at small but realistic angles of attack and their compact reduced-order representation remain insufficiently understood. The unsteady wakes subject to angle of attack from 3° to 5° are investigated via Koopman analysis with the Dynamic Mode Decomposition (DMD), aiming to construct accurate reduced-order models for largely repeated canonical cases, while preserving physical and phenomenological fidelity. Instantaneous velocity and vorticity fields reveal a clear separation-reattachment cycle: leading edge separation bubbles form and migrate upstream at drag peaks, then collapse and reattach at drag valleys. Shear layers roll up into dual vortices that pair, merge with Kelvin–Helmholtz-type shear-layer instabilities, and alternately shed from the deck’s upper and lower surfaces, driving oscillatory wake deflection and attendant drag and lift fluctuations. DMD identifies four dominant modes that together account for over 90% of the turbulent kinetic energy: time averaged base flow, the fundamental vortex shedding mode, and two higher frequency shear-layer modes. Adequate truncation reduces data dimensionality by an order of magnitude while keeping the normalized error below 6%. The results demonstrate that a DMD-based reduced-order model built on Unsteady Reynolds Averaged Navier–Stokes (URANS) data can faithfully preserve both large-scale separation topology and fine-scale vortical structures across small angles of attack, providing a compact and accurate representation of bridge-deck wakes for repeated canonical configurations. Full article

This study investigated the influence of spatial orientation on bead morphology and molten pool dynamics during cold metal transfer wire arc additive manufacturing (CMT-WAAM). Experiments in horizontal, transverse, vertical-down, and vertical-up orientations under varying wire feed speeds revealed that increasing the feed rate improved bead uniformity and reduced defects in horizontal deposition, while gravity-induced asymmetry dominated non-horizontal orientations. Transverse cladding produced tilted, uneven beads with reduced penetration; vertical-down enhanced lateral spreading but resulted in the shallowest weld depth; vertical-up limited spreading, yielding narrow beads with higher reinforcement. Optimal cladding quality was achieved at a wire feed speed of 6.7 m/min for the first layer, with a reduced heat input applied for subsequent layers to minimize residual stress and deformation. Numerical simulations further elucidated transient temperature and flow fields. Heat accumulation and dissipation varied with orientation and layer sequence: horizontal deposition formed deep, symmetric pools; transverse deposition generated asymmetric vortices and uneven solidification; vertical-up deposition caused upward counterflow with restricted spreading; vertical-down promoted rapid spreading and faster solidification. A detailed comparison between simulated and experimental temperature distributions and cross-sectional profiles demonstrated excellent agreement, thereby validating the accuracy and predictive capability of the developed model. This integrated experimental-numerical approach provided a comprehensive understanding of orientation-dependent molten pool behavior and offered a robust framework for optimizing process parameters, enhancing dimensional accuracy, and controlling defects in CMT additive manufacturing. Full article

The evolution of a coherent structure in a cylindrical wake was studied through observing its energy contribution to the flow field. Analysis using the proper orthogonal decomposition on the PIV data measured at two Reynolds numbers (Re) of 3840 and 9440 was performed. The coherent structure was identified by checking the Fourier power spectrum for each temporal mode coefficient and selecting those whose peak magnitudes were greater than the smallest magnitude of the identified harmonic frequency family as the large-scale organized motions. The energy contribution by the coherent structure is significantly dependent on Re. The evolution of the energy contribution by the coherent structure exhibits a monotonously decaying trend when moving downstream. The coherent structure primarily contains the Kármán vortices in the near wake. The contribution weight of the secondary vortices gradually increases, along with the streamwise distance, except in the very upstream subregions for the case of Re = 9440. The energy contribution by the secondary vortices immediately behind the cylinder (x/d = 0.5–5.5) was 30% for Re = 9440, in comparison with <1% for Re = 3840, but decayed rapidly to the value of <10% in the downstream subranges. Full article

A time-stepping, lifting-line, computationally efficient tool for preliminary design applications is developed to predict the unsteady aerodynamic loads of rotors operating in hovering and axial flight. The velocity field induced by wake vorticity is computed using a free-wake vortex-lattice model, while sectional aerodynamic loads are evaluated through the application of Küssner and Schwarz’s airfoil theory. The vorticity released by the trailing edge is related to the distribution of bound circulation and is convected downstream to form the vortex-lattice wake. The local bound circulation is determined by applying the Kutta–Joukowski theorem for unsteady flows. The proposed unsteady aerodynamic solver is successfully validated by comparison with both experimental data available in the literature and numerical results obtained by a three-dimensional boundary element method computational tool for potential flow. It does not apply to rotors in edgewise flight conditions and when compressibility effects are not negligible. Full article

The inherent residual swirl at the furnace outlet of tangentially fired boilers induces severe thermal deviations on high-temperature heating surfaces. To reveal the evolutionary patterns of thermal deviation during peak regulation operation, this study employed a numerical method to analyze the thermal deviation characteristics of high-temperature heating surfaces in a 350 MW tangentially fired boiler under 12 operating conditions (covering 100%, 72%, 50%, and 28% loads). As the load decreases, the average absolute vorticity at the platen bottom plane also decreases. However, its decreasing rate is pretty slow compared with that of the load, indicating that the velocity deviation increases relatively. In addition, the flow field becomes more sensitive to the mill group operation mode. Correspondingly, the thermal deviations of the division platen superheater, rear platen superheater, finishing reheater, and finishing superheater all show an upward trend. When the load decreases from 100% to 28%, the average heat absorption deviations of the four heating surfaces increase to 3.48, 2.06, 1.27, and 1.99 times their original values, respectively. A comparison of the internal operating conditions under four loads shows that activating the lower-layer burners helps to reduce thermal deviation. This study proposes indicators for characterizing and analyzing the thermal deviation of high-temperature heating surfaces and provides general suggestions for the operation of tangentially fired boilers. Full article

This study presents a computational–experimental assessment of two industrial centrifugal fans used in cement production, focusing on aerodynamic optimization and energy efficiency validation. The first case concerns a Farin Kiln Filter Fan initially constrained by existing inlet duct geometry, which caused vortex formation, flow asymmetry, and a pressure loss exceeding 15%. CFD analyses identified major inlet vortices and asymmetric splitter loading, guiding a redesigned configuration with an expanded fan body (1982–2520 mm), an increased outlet width (1808–1858 mm), and a vortex breaker to stabilize inlet flow. CFD simulations indicated a flow rate of 601,241 m³/h, static pressure of 2200 Pa, and total pressure of 2580 Pa, achieving an 83% efficiency. Field validation confirmed a 34.4% reduction in shaft power, 30% decrease in torque, and 4% gain in efficiency, corresponding to 449 MWh/year energy savings and 180 t CO₂/year emission reduction, assuming 8000 operational hours. The second case involves an Induced Draft (ID) Fan designed for 441,643 m³/h flow at 990 rpm. Transient CFD simulations using the SST k–ω model captured rotor–stator interaction and confirmed the effectiveness of the design revisions in suppressing swirl and flow separation. The optimized design achieved 8653 Pa static pressure, 9203 Pa total pressure, and 83% efficiency under design conditions. Field measurements showed a 26.2% drop in shaft power and 19.6% improvement in efficiency, yielding 2527 MWh/year energy savings and an estimated 1011 t CO₂/year emission reduction. Overall, the CFD-guided redesign framework demonstrated strong alignment between simulations and field measurements, highlighting the method’s practical relevance for improving fan performance and energy sustainability in industrial systems. Full article

Understanding the transient flow phenomena accompanying projectile discharge is essential for improving the safety, efficiency, and predictability of small-scale ballistic systems. Despite extensive numerical studies on muzzle flows and shock formation, experimental visualization and quantitative data on the coupling between pressure waves, jet structures, and projectile motion remain limited. This work addresses this gap by employing high-speed schlieren imaging and schlieren image velocimetry (SIV) to investigate the near-field aerodynamics of an airsoft-type projectile propelled by a CO₂ jet. Three optical configurations were analyzed—a Toepler single-mirror system, a Z-type without knife edge, and a Z-type with knife edge—to assess their sensitivity and suitability for resolving acoustic and turbulent features. The measured velocity of concentric pressure waves (≈355 m/s) agrees with the theoretical local speed of sound, validating the optical calibration. Projectile tracking yielded a mean speed of 71 ± 1.6 m/s, with drag and kinetic energy analyses confirming significant near-muzzle deceleration due to jet–projectile interaction. The SIV analysis provided additional insight into the convection velocity of coherent jet structures (≈75 m/s), tangent velocity fluctuations (±0.8 m/s), and vorticity distribution along the jet boundary. The results demonstrate that even compact schlieren setups, when coupled with quantitative image analysis, can capture the essential dynamics of unsteady compressible flows, providing a foundation for future diagnostic development and modeling of projectile–jet interactions. Full article

The hydrodynamic performance of a rudder behind a propeller is critical for determining vessel maneuvering stability. During navigation, the coupled effects of the oblique flow angle (β) and the rudder angle (δ) significantly alter the wake velocity field and vortex patterns aft of the rudder. However, the synergistic control mechanism of these two variables requires further quantitative investigation. This study employs the RANS method with the SST k-ε turbulence model to numerically simulate flow under advance coefficients (J) ranging from 0.3 to 0.9, oblique flow angles (β) from 0° to 15°, and rudder angles (δ) from 0° to 35°. Hydrodynamic coefficients, including the lift coefficient, drag coefficient, and lift-to-drag ratio, were calculated for the rudder. The evolution of the horizontal velocity and vortex fields was captured, with the model validated through localized flow field visualization. The results reveal that when β ≤ 3°, δ is the dominant factor influencing rudder hydrodynamics. Conversely, when β ≥ 9°, β becomes the primary regulating factor. The coupling effect induces significant asymmetry in the velocity distribution across the rudder surfaces and pronounced flow separation on the windward side, generating a complex vortex system (including primary and secondary vortices) on the leeward side. This research elucidates the coupled control mechanism of oblique flow and rudder angle, providing insights for enhancing steering margins and a quantitative foundation for optimizing rudder profiles in challenging sea environments characterized by high oblique flow and large rudder angles. Full article

The synthesis of high-performance adhesives imposes stringent requirements on the design of stirred reactors: simultaneous achievement of efficient mixing and minimal energy dissipation in highly viscous media remains the principal challenge. In this study, computational fluid dynamics (CFD) was employed to solve the Navier–Stokes equations for the high-viscosity epoxy system and numerically simulating the flow fields of four representative reactor configurations across a prescribed range of rotational speeds. Specifically, the four representative reactor configurations were (i) single-serrated shaft, (ii) eccentric single-serrated shaft, (iii) uniaxial single-blade paddle combined with a single-serrated dual-axis assembly, and (iv) biaxial single-blade paddle coupled with a single-serrated triaxial assembly. The mixing performance was quantitatively assessed by systematically comparing the evolution of mixing speed, vorticity fields, restricted power consumption, and mixing time across a range of rotational speeds. The results demonstrated that the synergistic deployment of an eccentric impeller and a differential-speed single-propeller shaft effectively disrupted the axisymmetric flow pattern, compressed the chaotic isolation zones, and intensified both axial exchange and global chaotic mixing. Among the configurations examined, the uniaxial single-propeller–single-serrated biaxial arrangement reduced the mixing time by 13.43% and cut the specific energy consumption by approximately 58.32%, thereby attaining markedly higher energy efficiency. This research will provide guidance for the study of efficient mixing of adhesives. Full article

A coupled CFD-DEM model was adopted to investigate the floating ice accumulation mechanism and its disturbance to the flow field in the pump house of coastal nuclear power plants in cold regions. Based on numerical simulations, the motion, accumulation, and flow interaction characteristics of floating ice under various release positions and heights were analyzed. The results indicate that the release height significantly governs the accumulation morphology and hydraulic response. The release height critically determines ice accumulation patterns and hydraulic responses. For inlet scenarios, lower heights induce a dense, wedge-shaped accumulation at the coarse trash rack, increasing thickness by 57.69% and shifting the accumulation 38.16% inlet-ward compared to higher releases. Conversely, higher releases enhance dispersion, expanding disturbances to the central pump house and intensifying flow heterogeneity. In bottom release cases, lower heights form wall-adhering accumulations, while higher releases cause ice to rise into mid-upper layers, thereby markedly intensifying local vortices (peak intensity 79.68, approximately 300% higher). Spatial release locations induce 2.7–4.8-fold variations in flow disturbance intensity across monitoring points. These findings clarify the combined impact of the release height and location on the ice accumulation and flow field dynamics, offering critical insights for the anti-ice design and flow safety assessment of pump houses. Full article

This paper presents a roughness surface model for Lagrangian simulations that interacts with both temperature and vorticity fields. The chosen problem is the uniform flow around a rough circular cylinder heated with constant temperature under mixed convection. The methodology used is the Temperature Particles Method (TPM), in which both vorticity and temperature fields are discretized in particles to simulate the real flow in a purely Lagrangian form. The simulation is computationally extensive due to the application of the Biot–Savart law for the two fields and the calculation of buoyancy forces, which is alleviated by the use of parallel programming with OpenMP. The simulation of roughness effects for both fields is obtained using a Large Eddy Simulation (LES) model for vorticity, based on the second-order velocity structure function, which is correlated with the thermal diffusivity through the turbulent Prandtl number. In general, the results indicate that roughness increases the drag coefficient, while an increase in the Richardson number reduces this coefficient. Full article

This study systematically investigates the velocity characteristics and coupling mechanisms of tunnel flow fields under the interactions of natural wind, traffic wind, mechanical ventilation, and structural factors (such as transverse passages and relative positions between vehicles and fans). Using CFD simulations combined with turbulence model analyses, the flow behaviors under different coupling scenarios are explored. The results show that: (1) Under natural wind conditions, transverse passages act as key pressure boundaries, reshaping the longitudinal wind speed distribution into a segmented structure of “disturbance zones (near passages) and stable zones (mid-regions)”, with disturbances near passages showing “amplitude enhancement and range contraction” as natural wind speed increases. (2) The coupling of natural wind and traffic wind (induced by moving vehicles) generates complex turbulent structures; vehicle motion forms typical flow patterns including stagnation zones, high-speed bypass flows, and wake vortices, while natural wind modulates the wake structure through momentum exchange, affecting pollutant dispersion. (3) When natural wind, traffic wind, and mechanical ventilation are coupled, the flow field is dominated by momentum superposition and competition; adjusting fan output can regulate coupling ranges and turbulence intensity, balancing energy efficiency and safety. (4) The relative positions of vehicles and fans significantly affect flow stability: forward positioning leads to synergistic momentum superposition with high stability, while reverse positioning induces strong turbulence, compressing jet effectiveness and increasing energy dissipation. This study reveals the intrinsic laws of tunnel flow field evolution under multi-factor coupling, providing theoretical support for optimizing tunnel ventilation system design and dynamic operation strategies. Full article

The ODYSEA (Ocean DYnamics and Surface Exchange with the Atmosphere) mission will provide simultaneous two-dimensional measurements of currents and winds for the first time. According to the ODYSEA radar concept, with a high incidence angle, current noise is primarily driven by backscattered power, which is triggered by wind speed. Therefore, random noise will affect the quality of observations. In low wind conditions, the absence of surface roughness increases the noise level considerably, to the point where the measurement becomes unusable, as the error can exceed 3 m/s at 5 km posting compared to mean current amplitudes of tens of cm/s. Winds higher than 7.5 m/s enable current measurements at 5 km posting with an RMS accuracy below 50 cm/s, but derivatives of currents will amplify noise, hampering the understanding of ocean dynamics and the interaction between the ocean and the atmosphere. In this context, this study shows the advantages and limitations of using noise-reduction algorithms. A convolutional neural network, a UNet inspired by the work of the SWOT (Surface Water and Ocean Topography) mission, is trained and tested on simulated radial velocities that are representative of the global ocean. The results are compared with those of classical smoothing: an Adaptive Gaussian Smoother whose filtering transfer function is optimized based on local wind speed (e.g., more smoothing in regions of low wind). The UNet outperforms the kernel smoother everywhere with our simulated dataset, especially in low wind conditions (SNR << 1) where the smoother essentially removes all velocities whereas the UNet mitigates random noise while preserving most of the signal of interest. Error is reduced by a factor of 30 and structures down to 30 km are reconstructed accurately. The UNet also enables the reconstruction of the main eddies and fronts in the relative vorticity field. It shows good robustness and stability in new scenarios. Full article