Computational Fluid Dynamics Method Research Articles

Numerical Simulation Analysis of the Correlation between Hydrodynamic Noise of Hydraulic Turbines and Defects in Runner Blades

To investigate the hydrodynamic noise characterization of hydraulic turbines with runner blade defects, this article establishes the intact machine model and three kinds of models with runner blade defects. Using the Computational Fluid Dynamic (CFD) and Computational Acoustic (CA) hybrid simulation computational methods, the hydrodynamic noise field of the hydraulic turbine is numerically simulated, and the results of the acoustic near field and acoustic far field are shown. 1. The double-row leaf grille and the runner are the primary sound source areas of the hydraulic turbine, and the intensity of sound radiation from these areas is positively correlated with the degree of runner blade defects. 2. As the runner blade defects develop, the sound power level (SWL) increases more significantly in the guide vanes near the nose of the spiral case in the double-row leaf grille. The most pronounced increase in the SWL is observed at the defective craters on the runner blades. 3. The frequency of the defective noise signal is primarily concentrated in the low-frequency band. The dominant frequency amplitude associated with runner blade defects increases and rises after the occurrence of defects. Secondary frequency changes are also observed, and the location of these changes varies at different receiving points.

Turbomachines seal flow resistance enhancement and leakage reduction based on flow control method with bow-shaped auxiliary teeth

Improving sealing performance without clearance reduction is important for the economic and safety operation of modern turbomachines. For this purpose, this study proposes the bow-shaped auxiliary teeth to reduce the leakage. One bow-shaped auxiliary tooth is installed after each seal tooth to guide the leakage jet towards the side wall of seal tooth. Then the leakage jet attaches itself to the inner wall of cavity. The Computational Fluid Dynamics method is used to calculate the flow field in the new seal. Results show that flow impingements, flow deflections, and enhanced vena contraction effect increase the leakage flow resistance and reduce the leakage velocity. The application of the bow-shaped auxiliary teeth reduces the leakage flow rate by 43.8–46.3 % compared with the labyrinth seal. Geometry parameters analysis shows that reducing the axial distance between seal tooth and bow-shaped auxiliary tooth, and increasing the central angle of bow-shaped auxiliary tooth are beneficial for leakage reduction. With the increase in the radius of bow-shaped auxiliary tooth, the leakage firstly decreases and then changes a little.

Investigation of the effect of sediment clogging on the hydraulic conductivity of porous asphalt mixes using CFD and DEM methods

This study investigated the impact of sediment clogging on the hydraulic performance of porous asphalt (PA) pavements using a novel numerical approach combining the discrete element method (DEM) and computational fluid dynamics (CFD). In particular, image analysis techniques captured the morphology of real aggregates for the 3D appearance information. Then, the information was used to reproduce aggregates and their gradation in asphalt mixes using DEM, including sediments causing clogging of the PA. The samples in the DEM were transferred to CFD for hydraulic performance simulation. DEM samples were prepared with air voids content of 20% and 25%. Clogging particles with different sizes of 0.5 mm and 0.75 mm were used. The study found that clogging particles can move to deeper layers of the PA pavement but have a high potential to be trapped in upper layers and cause clogging issues in the top 5 mm and middle-to-bottom sections. Due to surface clogging, the air voids content for the 20% porosity PA sample was reduced by approximately 2% on average, whereas up to 7% reduction in air voids content was observed for the 25% PA sample. This led to 37.35% and 42.7% reduction in hydraulic permeability, respectively. The reduction in air voids content below 10 mm from the surface is negligible (0.3–0.7% reduction). Due to the clogging development inside the pore structure, the surface runoff on the top of the sample increases by 8.1% for the 20% air voids asphalt sample case and 37.5% for the 25% voids sample.

Numerical Investigations on Wake Effects of Fish Cages

Abstract A typical fish farm in Norway usually consists of 6 to 12 fish cages, and these fish cages are moored together in a grid shape. Due to wake effects, individual fish cages may experience different flow velocities depending on their positions in the fish farm. Hereby, the drag forces on individual fish cages can be different. The drag forces on the downstream fish cages are difficult to be estimated without knowing the exact flow velocity, which will lead to challenges with the mooring system design. In this study, the computational fluid dynamic (CFD) method combined with a porous media model is employed to investigate the wake effects on the fish farm. The flow field through the fish cages and the drag forces on the downstream fish cages are investigated and discussed.

Improving the sealing performance of honeycomb seal by drilling double wall-holes

A novel double-wall-hole honeycomb seal (D-WHHCS) is proposed in this paper. D-WHHCS is designed based on a traditional honeycomb seal (HCS) by drilling double holes in the honeycomb sidewalls. The leakage characteristics of D-WHHCS are investigated via the computational fluid dynamics (CFD) method. The sealing performance of D-WHHCS under different seal clearances, wall hole positions, operating pressures and rotating speeds is studied. The sealing mechanism of D-WHHCS is revealed. The results show that D-WHHCS has better sealing performance than HCS under all the considered operating conditions, with a maximum leakage reduction of 3.95%. A fluid concentration effect when the axial cavity number (N) = 14 is observed in D-WHHCS, which causes a 9.16% improvement in the maximum turbulent kinetic energy compared with HCS at the outlet extension. A large vortex in the cavities of D-WHHCS with N = 14 is guided by the jets at the centerlines of the drilling hoes. The average turbulent dissipation in the cavities of D-WHHCS is a maximum increase of 95.30%, even though the vortexes in the cavities with N = 13 are weakened by the counteractive effect of two jets. The leakage reduction performance of D-WHHCS decreases with increasing clearance. An abnormal low-pressure region is produced in the first cavity because the intensive jet covers the first two cavities and suppresses the throttling effect of the first-stage honeycomb cavity. The leakage of D-WHHCS further decreases along with holes closing to rotor surface.

Numerical Investigation of Interventions to Mitigate Heat Stress: A Case Study in Dubai

Urbanization and changes in microclimate have negative impacts on outdoor thermal comfort, making urban design more important. This study aims to improve outdoor thermal comfort in a local climate zone (LCZ) in Dubai using computational fluid dynamics (CFD) methods. This study evaluates cooling interventions, such as vegetation, architectural, and pavement material, using Reynolds-averaged Navier–Stokes (RANS) equations and the SIMPLE scheme. The results show that a combination of cooling interventions affects the average temperature between 4.44 °C and 6.14 °C. Light-colored ground material has a 5.4 °C cooling effect in the LCZ compared with dark-colored materials. The predicted mean vote (PMV) method is used to compare outdoor thermal comfort and the results show that thermal sensation in the LCZ improves from warm to slightly cool. Lastly, the most effective cooling interventions are, in order, shade structures, trees, chimneys, and bushes.

Numerical investigation of inlet-pipe configurations on fluid dynamics and flow-field characteristics inside filling valve

The filling valve requires high efficiency and stability in the working process, which is closely related to the distribution of the flow characteristics inside the valve. In this study, computational fluid dynamics (CFD) methods were present to analyse the pressure, velocity, and other characteristics of the filling valve under different inlet pipe angles and valve openings. The flow characteristics, which are experimentally measured, are consistent with CFD simulation results. Research shows that the increase in the angle of the inlet pipe can improve the uniformity of the flow field inside the valve, produce a better core flow pattern at the valve outlet, and have better fluid stability. Based on the flow characteristics of the valve, to obtain good filling efficiency and stability, a moderate valve opening of 9 mm should be selected, and the flow coefficient can reach 1.08, which is 1.6 times higher than that of 3 mm, and the angle of the inlet pipe should be appropriately increased. This study provides a reference for the use and optimization of filling valves.

MSD-based transient fluid model of liquid circular angular accelerometer for vibration-induced error analysis

This paper investigates the errors induced by linear vibration in the fluidic system of liquid circular angular accelerometers (LCAAs). A two-dimensional mass-spring-damper model is proposed to analyze the pressure difference between both sides of LCAA transducer subjected to linear vibration in both transient and steady states, where the compressibility of fluid is taken into account. The complex flow patterns of fluid are simplified to the positional changes of fluid particles. Springs and dampers are utilized to simulate the forces inside the fluid and between the fluid and the tube wall. Simulations are carried out based on the linear acceleration step excitation to explore the influence of model parameters, LCAA structure parameters and fluid parameters. The proposed model is validated by comparing the characteristics of the simulated pressure difference with their corresponding theoretical values, and is further verified by the comparison with the results from computational fluid dynamics method. Furthermore, the pressure difference under linear acceleration excitations in various forms is analyzed. The results indicate that the abrupt changes in linear acceleration are the primary cause of pressure difference oscillation, and the compressibility of fluid leads to increased pressure difference.

Study on Flow Characteristics of Integrated Natural Circulation Reactor Under Inclined Conditions

In order to study the flow characteristics of an integrated natural circulation reactor under inclined conditions, a typical three-dimensional analysis model of natural circulation was established. The natural circulation under inclined conditions was numerically simulated using the computational fluid dynamics method, and the velocity and temperature distribution characteristics of heat exchangers and mixed-flow channels with different inclined angles were analyzed. The results show that as the inclination angle of the heat exchanger flow rate increases, there is a spatial migration phenomenon corresponding to the direction of the average flow rate. A large inclination angle will lead to a serious deterioration of the natural circulation capacity in the lower channel. Under inclined conditions, there is a phenomenon of temperature stratification in the mixed-flow channel.

Numerically investigating the effect of using nanofluids on the thermal performance and coefficient of performance of a U-tube deep borehole ground source heat pump

In this study, a deep-borehole ground source heat pump was simulated using computational fluid dynamics (CFD) method. The thermal performance and coefficient of performance (COP) of the vertical ground source heat pump simulated with accuracy using CFD for water and 4 different nanofluids as working fluids were investigated. Two nanofluids contained silver and copper metallic nanoparticles and the other two nanofluids contained copper and aluminum oxide nanoparticles. For each of these nanofluids as well as water, the heat exchange rate, total exchanged heat, outlet temperature and COP were calculated and the results were compared together. The results showed that using nanofluids leads to increased exchanged heat. Nanofluids based on aluminum oxide, copper oxide, copper and silver nanoparticles resulted in 4.83, 5.76, 6.9, 7.69 percent increase in total exchanged heat, respectively. In addition to increased exchanged heat, using nanofluids increased the outlet temperature. Also, the results showed that the maximum and minimum increase in COP were 36.5% and 5.2% which were related to using silver and aluminum oxide nanoparticle-based nanofluids, respectively.

Effect of combined surge and pitch motion on the aerodynamic performance of floating offshore wind turbine

Floating offshore wind turbines (FOWTs) can encounter variations in the phase difference and amplitude of combined surge-pitch platform motion. The phase difference between surge and pitch movement can result in the superposition or cancellation of wind turbine motion, potentially leading to significant changes in its aerodynamic characteristics. Herein, a wind turbine model was developed using the computational fluid dynamics (CFD) method to investigate the aerodynamic performance of the wind turbine. The findings revealed significant variations in the aerodynamics characteristics of the wind turbine when subjected to surge and pitch motions. When the rotor moves forward with a large relative velocity, dynamic stall can be triggered and induce the violent fluctuation of aerodynamic load. Moreover, vortex ring state (VRS) may occur when the wind turbine moves backward, leading to the recirculation of tip and root vortices and possibly causing negative thrust and torque. The trend of the drag force acting on the tower exhibits an almost complete opposition to that of the rotor thrust with the change of phase difference in the combined surge-pitch motion. It is also noted that the tower lift fluctuates asymmetrically, and as the platform motion amplitude increases, the tower lift gradually increases towards one side.

Modeling techniques in nanofinishing processes - a review

With the growing demand of high surface finish components, the need of nanofinishing techniques arises immensely. Generally, finishing experiments are performed to analyse the surface finish, temperature rise, and material removal. Using modeling techniques, one can analyse the same without carrying out experiments for different nanofinishing processes. These techniques help in finding different unexpected situations before initiating any finishing process. Using the methods of computational fluid dynamics (CFD), finite element method (FEM), molecular dynamics simulation, and mechanistic techniques it becomes easy to predict material removal, heat generation using the results of velocity, pressure, stress, and temperature. Modeling techniques offer fluid flow behavior, workpiece behavior visualizations under the effect of forces, cost-effectiveness solutions, and identification of problematic issues. In this article, a comprehensive review of different modeling techniques is presented for better understanding and visualization of the nanofinishing process. This review paper attempts to provide an informative literature survey on the research being done to simplify the understanding of modeling techniques and which modeling technique should be selected along with suitable modeling parameters considered by researchers for a particular nanofinishing process.

Study on the influence of ship waves on the dynamic response of floating tidal current power generation device

To study the influence of ship traveling waves on the hydrodynamic performance between the carrier platform and the turbine, the computational fluid dynamics method and the DFBI motion module of STAR CCM+ software were adopted. Under the condition of the coupling of the ship traveling wave and wave current generated at different speeds, the overall motion response of the power plant is simulated. What’s more, the influence of different wave conditions on the heave, pitch, and roll of the carrier platform and the energy efficiency of the turbine is analyzed. The results show that the ship speed has little effect on the carrier heave motion, but the maximum value of the carrier pitch motion increases with the increase in the ship speed. The value of turbine energy acquisition efficiency always decreases with decreasing lateral spacing and increases with decreasing speed. This paper has a certain reference value for the engineering application of floating tidal current power generation equipment.

Optimal strategy of the asymmetric wave energy converter survival in extreme waves

Enhancing the survival performance of wave energy converters (WECs) in extreme wave conditions is crucial, and reducing wave loads is a key aspect of this. Placing the device underwater has been recognized as a beneficial strategy, yet the determination of the optimal submerged depth and the effects of varying wave conditions remain ambiguous. To address this, the study numerically analyzes the total forces in both horizontal and vertical directions, along with their harmonic components, across different wave configurations. A computational fluid dynamics method is employed to investigate a triangular-baffle bottom-shaped oscillating floater, which is known for its high energy conversion efficiency. The findings indicate that submerging the device to a depth equivalent to half the actual focused amplitude (1/2Ab) is the most effective strategy in the given sea state, offering superior wave force reduction vertically and robust performance horizontally. The analysis of harmonics reveals the significant contribution of high-order components to the total wave forces. Additionally, the study examines the impact of focused wave amplitudes and peak frequencies, showing that although force reductions are lessened in more extreme conditions, the optimal submerged depth of 1/2Ab still yields near 30% reduction in total vertical force and 22% in total horizontal force. This research provides theoretical insight that can guide the enhancement of WECs' survival capabilities in practical engineering applications.

Inversion of electron densities in plasma wakes of hypersonic targets

Focusing on the plasma wakes of hypersonic targets (PWHT), this work provides a method for the inversion of electron densities in the PWHT by integrating electromagnetic radiation theory and three-wave coupling theory (TWCT). Utilizing the Computational Fluid Dynamics (CFD) method, taking a blunt cone as an example, the spatial distribution characteristics of electron densities in the plasma wake are simulated and analyzed in detail. The Zakharov system of equations (ZSE) is applied to the PWHT for the discussion about the electromagnetic radiation generated by plasma parametric instability. Combining the wavenumber spectrum of electromagnetic radiation and the TWCT, an inverse method of the electron densities of plasma wakes is proposed and applied to a blunt cone model. The results indicate that the inversion error rate is effectively held below 1.37 %. Furthermore, reasons for errors are elucidated through the utilization of the wavenumber spectrum structure. The proposed method may contribute to the development of hypersonic wake diagnostic techniques, and offer new insights and directions for wind tunnel experiments, shock tube experiments, as well as research on hypersonic target detection.

Numerical simulation on in-vessel molten corium behavior with external vessel cooling using smoothed particle hydrodynamics

The in-vessel retention through external reactor vessel cooling (IVR-ERVC) strategy is a key management strategy for early termination of a nuclear severe accident that can threaten the integrity of the reactor vessel. To simulate the physical phenomena of the molten corium, the smoothed particle hydrodynamic (SPH) method is utilized in this study. The SPH method is a Lagrangian computational fluid dynamic (CFD) method that can simulate multi-fluid stratification, turbulence, natural circulation, radiative heat transfer, thermal ablation, and crust formation. To address the external vessel cooling, it is coupled with a conventional 1-D nuclear system analysis method. The 1-D system analysis code can calculate the two-phase natural circulation of cooling water and the convective heat transfer on the external reactor vessel wall. These two simulation codes exchange the temperature and heat flux of the reactor vessel outer wall. This study numerically simulated the IVR-ERVC strategy for a Korean high-power reactor and compared it with the traditional lumped parameter method (LPM). Unlike LPM, this study provides localized detailed data about the thermal hydraulic behavior of molten corium and visualization of phenomena in the IVR-ERVC strategy. This enhances our understanding of the phenomena in IVR-ERVC strategy and introduces new perspectives.

A deep learning computational fluid dynamics solver for simulating liquid hydrogen jets

Modeling and simulating the sudden depressurization of liquids inside nozzles is a significant challenge because of the plethora of the associated complex phenomena. This pressure drop together with the rapid phase change of the liquid is important characteristics of flash boiling. Computational fluid dynamics (CFD) multiscale simulations of flashing jets usually deploy additional models for modeling heat and mass transfer with long computational times. Intermediate steps such as volumetric meshing in mesh-based methods can also significantly increase the computational cost. This paper aims at providing academia and industry with a modeling tool to simulate and investigate the complex multi-facet phenomenon of flash-boiling atomization deploying a machine-learning method that could save thousand Central Processing Unit hours offering instantaneous CFD predictions. The presented machine-learning CFD method completely replaces the traditional CFD simulations workflow and requires little simulation expertise from the end-user. Notably, this is a novel model that couples for the first time the thermodynamic non-equilibrium with convolutional neural networks to simulate flashing liquid hydrogen jets thousand times faster than the standalone CFD solver. The accuracy of the novel approach is evaluated, demonstrating adequate accuracy compared to different unseen simulations and experiments. This work offers the groundwork for further accelerating CFD predictions in multiphase flow problems and could significantly improve testing flash-boiling scenarios in various industrial settings.

Detachment dynamics of sessile droplets triggered by interaction forces between oil and wall in a microchannel

Understanding the detachment dynamics of droplets adhered to reservoir walls holds significant importance for the residual oil displacement process in high water-cut oilfields. Existing studies demonstrated that increasing mainstream shear by adjusting displacement flow and weakening wall adhesion by increasing contact angle can enhance droplet detachment. However, the complex physical and chemical oil–wall interactions, a crucial reservoir feature, cannot be solely relied on the macroscopic representation through contact angle. A deviation in understanding the process of crude oil droplet detachment would be resulted due to the change of wall adhesion. Considering the intricate physical and chemical interactions between oil and walls, in this paper, we employed the extended Derjaguin–Landau–Verwey–Overbeek (EDLVO) theory to establish an oil–wall interaction forces system, and coupling computational fluid dynamics method to further explore the detachment dynamics of sessile oil droplets in a microchannel under varying EDLVO forces. The findings showed that (1) by increasing the Capillary number, the droplets gradually occur in four typical dynamic states: static, sliding, detachment, and pinch-off. (2) Static droplets are more prone to experience sliding behavior when influenced by EDLVO forces. (3) For droplets undergoing detachment and pinch-off, EDLVO forces inhibit entrainment behavior. These results contribute to an enhanced comprehension of droplet detachment dynamics in reservoirs, offering fresh insights for enhanced oil recovery strategies.

Numerical simulation of optimizing the swing curve of a 3DOF biomimetic pectoral fin in drag mode

In response to the optimization problem of the motion law of the pectoral fins of biomimetic robotic fish, this paper uses computational fluid dynamics (CFD) methods to achieve the CFD optimization of two types of rigid biomimetic pectoral fins with three degrees of freedom (DOF): elliptical and 8-shaped swing curves. By analyzing the hydrodynamic characteristic curve of the pectoral fins, pressure distribution upstream and downstream of the fin surface, velocity vector of the surrounding flow field during pectoral fin swing, and three-dimensional vortex structure, it is found that when the pectoral fins are pushed forward in an elliptical swing curve, the resistance peak value decreases after two optimizations. However, at the same time, a certain amount of thrust is lost. When the pectoral fin advances in an 8-shaped swinging curve, after two optimizations, the pectoral fins resistance decreases while maintaining an enormous thrust, comprehensively improving the propulsive effect of the pectoral fin on the robotic fish. By substituting the optimized 8-shaped pattern into the robotic fish model, it is found that the flapping angle of 35° has better drag reduction performance, and the average swimming speed of the robotic fish is the fastest at this angle.

Numerical investigations of the restriction effects on a ship navigating in pack-ice channel

This paper adopts Computational Fluid Dynamics (CFD) method and Discrete Element Method (DEM) to simulate a ship navigating in pack-ice channel considering the effects of channel restriction. First, from the simulations of the ship sailing in open-water channel by CFD, it is found that the effects of channel width on water resistance is more pronounced than the impacts by level-ice thickness. Then, based on the stable and converged flow field obtained by CFD simulations, the CFD-DEM coupling method is applied to reveal the effects of channel width and level ice thickness on ship-ice-water interactions and ice resistances at different ship speeds. The results indicate that as the channel width decreases, the accumulation of pack ice in the areas of bow and midship intensifies. The ice resistances on the bow and midship increase notably, and the lateral ice forces exhibit pronounced asymmetric and random characteristics. In extremely narrow pack-ice channels, lower ship speed results in lager ice resistance and lateral ice force on the ship. In addition, as the level ice thickness decreases, the pack ice is more easily squeezed below the level ice, resulting in the decreases of ice resistance and lateral ice force.