Search Results (3,890)

This study presents a detailed investigation of the temporal evolution of the Nusselt number ($Nu$) in uniformly accelerated and decelerated turbulent pipe flows under a constant heat flux using direct numerical simulations. The influence of different acceleration and deceleration rates on heat transfer is systematically studied, addressing a gap in the previous research. The simulations confirm several key experimental findings, including the presence of three distinct phases in the Nusselt number temporal response—delay, recovery, and quasi-steady phases—as well as the characteristics of thermal structures in unsteady pipe flow. In accelerated flows, the delay in the turbulence response to changes in velocity results in reduced heat transfer, with average $Nu$ values up to $48\%$ lower than those for steady-flow conditions at the same mean Reynolds number. Conversely, decelerated flows exhibit enhanced heat transfer, with average $Nu$ exceeding steady values by up to $42\%$ due to the onset of secondary instabilities that amplify turbulence. To characterize the $Nu$ response across the full range of acceleration and deceleration rates, a new model based on a hyperbolic tangent function is proposed, which provides a more accurate description of the heat transfer response than previous models. The results suggest the potential to design unsteady periodic cycles, combining slow acceleration and rapid deceleration, to enhance heat transfer compared to steady flows. Full article

The Darrieus vertical axis wind turbine (VAWT) is categorized as a lift-based turbomachine. It faces challenges in the low tip speed ratio (TSR) range and requires initial torque for the starting operation. Ongoing efforts are being made to enhance the turbine’s self-starting capability. In this study, Computational Fluid Dynamics (CFD) simulations were utilized to tackle the identified challenge. The Unsteady Reynolds-Averaged Navier–Stokes (URANS) approach was employed, combined with the shear–stress transport (SST) $k-\omega$ turbulence model, to resolve fluid flow equations. The investigation focused on optimizing the placement of auxiliary blades by considering design parameters such as the pitch angle and horizontal and vertical distances. The goal was to increase the turbine efficiency and initial torque in the low-TSR range while minimizing efficiency loss at high-TSR ranges, which is the primary challenge of auxiliary blade installation. Implementing the auxiliary blade successfully extended the rotor’s operational range, shifting the rotor operation’s onset from TSR 1.4 to 0.7. The optimal configuration for installing the auxiliary blade involves a pitch angle of 0°, a horizontal ratio of 0.52, and a vertical ratio of 0.41. To address the ineffectiveness of auxiliary blades at high-TSRs, installing deflectors in various configurations was explored. Introducing a double deflector can significantly enhance the overall efficiency of the conventional Darrieus VAWT and the optimum rotor with the auxiliary blade by 47% and 73% at TSR = 2.5, respectively. Full article

The development of the Modular High-Temperature Gas-Cooled Reactor is a significant milestone in advanced nuclear reactor technology. One of the concerns for the reactor’s safe operation is the effects of a loss-of-flow accident (LOFA) where the coolant circulators are tripped, and forced coolant flow through the core is lost. Depending on the steam generator placement, loop or intracore natural circulation develops to help transfer heat from the core to the reactor cavity, cooling system. This paper investigates the fundamental physical phenomena associated with intracore coolant natural circulation flow in a one-sixth Computational Fluid Dynamics (CFD) model of the Oregon State University High Temperature Test Facility (OSU HTTF) following a loss-of-flow accident transient. This study employs conjugate heat transfer and steady-state flow along with an SST k-ω turbulence model to characterize the phenomenon of core channel-to-channel natural convection. Previous studies have revealed the importance of complex flow distribution in the inlet and outlet plenums with the potential to generate hot coolant jets. For this reason, complete upper and lower plenum volumes are included in the analyzed computational domain. CFD results also include parametric studies performed for a mesh sensitivity analysis, generated using the STAR-CCM+ software. The resulting channel axial velocities and flow directions support the test facility scaling analysis and similarity group distortions calculation. Full article

Based on the RNG k-ε turbulence model and VOF multiphase flow model, a numerical model of horizontal water-entry of the vehicle was established, and the numerical method was verified by experimental results. The cavitation characteristics, fluid resistance, and motion of the vehicle under different conditions were studied during the vehicle’s water-entry process. The results show that the cavitation process can be divided into the cavity development stage, saturation stage, and collapse stage. With the increase in initial velocity and mass of the vehicle, more water vapor will be generated during the water-entry process. The initial velocity of the vehicle had a limited effect on the resistance coefficient. The resistance coefficient in the stable stage remained almost unchanged for vehicles with different masses. Nevertheless, the time interval of the stable stage was shortened, and the resistance coefficient was greater in the gradually increasing stage for the vehicle with a smaller mass. For vehicles with higher initial velocity or smaller mass, the instantaneous velocity decreased faster after it entered the water. The vehicle with a streamlined design was able to reduce the generation of water vapor and decrease fluid resistance and its coefficient, and the vehicle can run farther during the water-entry process. Full article

With the continuous development of the shipping market, bow thrusters have become more important for ship maneuvering. Therefore, the performance of bow thrusters is studied in this paper. In order to obtain an unsteady performance of the bow thruster under different ship speed conditions, the SST k-ω turbulence model is adopted to predict the hydrodynamics of the bow thruster. With the ship’s speed increasing gradually, the variation characteristics of hydrodynamic coefficients and the flow field distribution at key positions are analyzed. The results show that with an increase in ship speed to three knots, the thrust coefficient and torque coefficient of the bow thruster decrease by 2.69~4.07% and 2.34~3.08%. In addition, the blade vibration amplitude intensifies. In the departure direction, the propeller load is more susceptible to being influenced and decreases by an additional 2.34~4.16% compared with that in the berthing direction. Meanwhile, it is found that the velocity distribution is asymmetrical. The inlet velocity at the bow side is faster, which results in the maximum peak pressure being about three times the minimum peak pressure. In addition, the pressure’s nonuniformity in the tunnel increases gradually with the increase in ship speed. Compared with the pressure distribution in the berthing direction, the pressure distribution before and after the propeller is more uniform, which is consistent with the results of hydrodynamic change and velocity distribution. The research in this paper has a certain reference significance for understanding the hydrodynamic performance of bow thrust operation. Full article

Two enigmatic gamma-ray features in the galactic central region, known as Fermi Bubbles (FBs), were found from Fermi-LAT data. An energy release, (e.g., by tidal disruption events in the Galactic Center, GC), generates a cavity with a shock that expands into the local ambient medium of the galactic halo. A decade or so ago, a phenomenological model of the FBs was suggested as a result of routine star disruptions by the supermassive black hole in the GC which might provide enough energy for large-scale structures, like the FBs. In 2020, analytical and numerical models of the FBs as a process of routine tidal disruption of stars near the GC were developed; these disruption events can provide enough cumulative energy to form and maintain large-scale structures like the FBs. The disruption events are expected to be ${10}^{-4}\sim {10}^{-5}{\mathrm{yr}}^{-1}$, providing an average power of energy release from the GC into the halo of $\dot{\mathcal{E}}\sim 3\times {10}^{41}$ erg ${\mathrm{s}}^{-1}$, which is needed to support the FBs. Analysis of the evolution of superbubbles in exponentially stratified disks concluded that the FB envelope would be destroyed by the Rayleigh–Taylor (RT) instabilities at late stages. The shell is composed of swept-up gas of the bubble, whose thickness is much thinner in comparison to the size of the envelope. We assume that hydrodynamic turbulence is excited in the FB envelope by the RT instability. In this case, the universal energy spectrum of turbulence may be developed in the inertial range of wavenumbers of fluctuations (the Kolmogorov–Obukhov spectrum). From our model we suppose the power of the FBs is transformed partly into the energy of hydrodynamic turbulence in the envelope. If so, hydrodynamic turbulence may generate MHD fluctuations, which accelerate cosmic rays there and generate gamma-ray and radio emission from the FBs. We hope that this model may interpret the observed nonthermal emission from the bubbles. Full article

The advances in research on the explosion load characteristics of the fuel–air mixture in vented chambers are reviewed herein. The vented explosion loads are classified into three typical types based on this comprehensive literature research. These models are the accumulation load model, attenuation load model, and interval jump load model. The characteristics of the three different typical vented explosion load models are analyzed using Fluidy-Ventex. The research results show that overpressure is largely determined by methane concentrations and vented pressure. The turbulent strength increased from the original 0.0001 J/kg to 1.73 J/kg, which was an increase of 17,300 times, after venting in the case of a 10.5 v/v methane concentration and 0.3 kPa vented pressure. When the vented pressure increased to 7.3 kPa, the turbulent strength increased to 62.2 J/kg, and the overpressure peak correspondingly increased from 69 kPa to 125 kPa. In the case of the interval jump load model, the explosion overpressure peak tends to ascend when the intensity of the fluid disturbance rises due to the venting pressure increasing at a constant initial gas concentration. When the venting pressure reaches tens of kPa, the pressure differential increases sharply on both sides of the relief port, and a large amount of combustible gas is released. Therefore, there is an insufficient amount of indoor combustible gas, severe combustion is difficult to maintain, and the explosion load mode becomes the attenuation load model. Full article

Due to modern comfort demands and global warming, heating, ventilation, and air conditioning (HVAC) systems are widely used in many homes and buildings. However, HVAC based on the Vapor Compression System (VCS) is a major energy consumer, accounting for 20–50% of a building’s energy consumption and responsible for 29% of the world’s CO₂ emissions. Dew-point evaporative coolers offer a sustainable alternative yet face challenges, e.g., dew point and wet bulb effectiveness. Given the above, dew point evaporative cooling systems may find a place to dethrone conventional air conditioning systems. This research aims to design a dew point evaporative cooler system with better performance in terms of dew point and wet bulb effectiveness. In terms of methodology, a heat exchanger as part of a counter-flow dew point cooling system was designed and analyzed using COMSOL simulations under different representative climatic, geometric, and dimensional conditions, taking into account turbulent flow. Next, our model was compared with other cooling systems. The results show that our model performs similarly to other cooling systems, with an error of around 6.89% in the output temperature at low relative humidity (0–21%). In comparison, our system is more sensitive to humidity in the climate, whereas heat pumps can operate in high humidity. The average dew point and wet bulb effectiveness were also higher than reported in the literature, at 91.38% and 147.84%, respectively. In addition, there are some potential limitations of the simulations in terms of the assumptions made about atmospheric conditions. For this reason, the results cannot be generalized but must be considered as a starting point for future research and technology development projects. Full article

Pressure-Retarded Osmosis (PRO) is an osmotically driven membrane-based process that has recently garnered significant attention from researchers due to its potential for clean energy harvesting from salinity gradients. The complex interactions between mixed-mode channel flows and osmotic fluxes in real PRO membrane modules necessitate high-fidelity modeling approaches. In this work, an efficient CFD framework is developed for the 3D simulation of osmotically driven membrane processes. This approach is based on a two-way coupling between a CFD solver, which captures external concentration polarization (ECP) effects, and an analytical representation of internal concentration polarization (ICP). Consequently, the osmotic water flux and reverse salt flux (RSF) can be accurately determined, accounting for all CP effects without any limitations on the geometrical complexity of the membrane chamber or its flow mode/regime. The proposed model is validated against experimental data, showing good agreement across various PRO case studies. Additionally, the model’s flexibility to simulate other types of osmotically driven processes such as forward osmosis (FO) is examined. Thus, the contributions of ECP and ICP effects in local osmotic pressure drop along the membrane chamber are comprehensively compared for FO and PRO modes. Finally, the capability of the CFD model to simulate a lab-scale PRO module is demonstrated across a range of Reynolds numbers from low-speed laminar up to turbulent flow regimes. Full article

In this study, we investigate a yaw control strategy in a two-turbine wind farm with 3.5 MW turbines, aiming to optimize power management. The wind farm is equipped with a nacelle-mounted multi-plane LiDAR system for wind speed measurements. Using an analytical model and integrating LiDAR and SCADA data, we estimate wake effects and power output. Our results show a 2% power gain achieved through optimal yaw control over a year-long assessment. The wind predominantly blows from the southwest, perpendicular to the turbine alignment. The optimal yaw and power gain depend on wind conditions, with higher turbulence intensity and wind speed leading to reduced gains. The power gain follows a bell curve across the range of wind inflow angles, peaking at 1.7% with a corresponding optimal yaw of 17 degrees at an inflow angle of 12 degrees. Further experiments are recommended to refine the estimates and enhance the performance of wind farms through optimized yaw control strategies, ultimately contributing to the advancement of sustainable energy generation. Full article

This paper utilizes the waste bunker of a waste incineration plant as the analysis model. It analyzes the airflow characteristics under various unloading door opening states and the air flow velocity through CFD simulation. The simulation analysis results show that when one unloading door is opened, it is recommended to adjust the opening amplitude or set an air outlet to optimize the airflow distribution. If two unloading doors are opened, it is advised to prioritize the two middle unloading doors (M4, M5) or the two rightmost doors (M7, M8). Furthermore, the exhaust port located relatively far from the unloading door should be closed to reduce the turbulence of the airflow. When all unloading doors are opened, the air flow velocity at the unloading door needs to be increased to achieve an efficient exhaust effect and prevent negative pressure problems at low speed. The results offer theoretical support for odor control technologies and provide valuable design recommendations for air outlets and unloading doors in municipal waste incineration plants. Additionally, this study proposes optimization strategies and effective solutions for addressing odor pollutant diffusion in waste incineration facilities. Full article

Understanding the dispersion of floating objects and ocean properties at the ocean surface is crucial for various applications, including oil spill management, debris tracking and search and rescue operations. While mesoscale turbulence has been recognized as a primary driver of dispersion, the role of submesoscale processes is poorly understood. This study investigates the largely unexplored mechanism of dispersion by refracted wave fields. In situ observations demonstrate significantly faster and distinct dispersion patterns for objects influenced by wind, waves and currents compared to those solely driven by ocean currents. Numerical simulations of wave fields refracted by ocean eddies corroborate these findings, revealing diffusivities that exceed those of turbulent diffusion at scales up to 10 km during energetic sea states. Our results highlight the importance of ocean waves in dispersing surface material, suggesting that refracted wave fields may play a significant role in submesoscale spreading. As atmospheric forcing at the ocean surface will only strengthen due to anthropogenic contributions, additional research into wave refraction is necessary. This requires concurrent high-resolution measurements of wind, waves and currents to inform the revisions of large-scale coupled models to better include the submesoscale physics. Full article

In response to the national “dual-carbon” strategic goals, a dual-mode ocean energy generation device was researched. First, improvements were made to the power generation system and mechanical structure of an oscillating buoy with a radius of 2.5 m, designing a power generation device that utilizes both wave energy and tidal energy. Subsequently, wave generation was carried out using the Realizable k-ε turbulence model, and the buoy’s strength was analyzed using the Ansys Fluent simulation platform with finite element analysis, with simulation results indicating a rational design. Then, the Froude–Krylov hypothesis method and the oscillating buoy body decomposition method were used to calculate the combined force in the vertical direction of the buoy, and combined with other forces, the maximum wave force on the buoy was determined. It was verified that the buoyancy of the buoy met the requirements of the hydraulic system. Finally, the hydraulic power generation system model was simulated and analyzed using AMEsim, yielding a power generation of 55.2284 kW. Using a formula, the maximum power generation of a single buoy was calculated to be approximately 15.5 kW, and the ideal maximum power generation of the entire device was able to reach 101.7284 kW. Full article

This paper aims to discuss the internal flow and cavitation characteristics of petal bionic nozzle holes under different injection pressures to improve the atomization effect of methanol. The FLUENT (v2022 R1) software is used for simulation. The Schnerr-Sauer cavitation model in the Mixture multiphase flow model is adopted, considering the evaporation and condensation processes of methanol fuel to accurately simulate cavitation and internal flow performance. The new nozzle hole is compared with the ordinary circular nozzle hole for analysis to ensure research reliability. The results show that the cavitation of the petal bionic nozzle hole mainly occurs at the outlet, which can enhance the atomization effect. In terms of turbulent kinetic energy, the internal turbulent kinetic energy of the petal bionic nozzle hole is greater under the same pressure. At 1 MPa, its outlet turbulent kinetic energy is 38.37 m²/s², which is about 2.3 times that of the ordinary circular nozzle hole. When the injection pressure is from 0.2 MPa to 1 MPa, the maximum temperature of the ordinary circular nozzle hole increases by about 33.4%, while that of the petal bionic nozzle hole only increases by 12.3%. The intensity of internal convection and vortex is significantly reduced. The outlet velocity and turbulent kinetic energy distribution of the petal bionic nozzle hole are more uniform. In general, the internal flow performance of the petal bionic nozzle hole is more stable, which is beneficial to the collision and fragmentation of droplets and has better uniformity of droplet distribution. It has a positive effect on improving the atomization effect of methanol injection in the intake port of methanol-diesel dual-fuel engines. Full article

In this paper, we used meteorological data to simulate vertical profiles of optical turbulence at the Special Astrophysical Observatory (SAO) (Russia, 43°40′19″ N 41°26′23″ E, 2100 m a.s.l.), site of the 6 m Big Telescope Alt-azimuthal. For the first time, the vertical profiles of optical turbulence are calculated for the SAO using ERA-5 reanalysis data. These profiles are corrected using DIMM measurements as well as estimations of atmospheric boundary layer heights. We may note that the method basically reconstructs the most important features of the shape of the measured profile under clear sky. Atmospheric turbulent layers were identified, and the strength of optical turbulence in these layers was estimated. The model hourly values of seeing corresponding to the obtained vertical profiles range from 0.40 to 3.40 arc sec; the values of the isoplanatic angle vary in the range from 1.00 to 3.00 arc sec (at $\lambda$ = 500 nm). The calculated median of seeing is close to 1.21 arc sec. These estimations are close to the measured median of seeing (1.21 arc sec). Full article