Computational Fluid Dynamics Simulations Research Articles

Mitigation effect of guide vanes on the vortex-induced vibration of a long-span closed-box girder suspension bridge

This paper investigates the VIV responses and triggering mechanisms of a closed-box girder by wind tunnel experiments and computational fluid dynamics (CFD) simulations. The effects of handrails, gantry rails, and guide vanes (GVs) on the mitigation of VIV are analyzed. The results show that large-scale vortices shed from handrails and gantry rails result in large heaving VIV responses. The torsional VIV occurs due to the alternate vortex shedding in the wake. A handrail with a higher porosity near the upper deck surface is highly effective in suppressing VIV responses, especially for heaving VIV. Gantry rails installed at the medial position of the lower deck surface can mitigate torsional VIV to a large extent. GVs installed inside the gantry rails can mitigate torsional VIV much more significantly than one installed outside. For the girder with high-porosity handrails and GVs installed inside the gantry rails, the large-scale vortices generated by the handrails disappear, and the scale of the vortices induced by the GVs is notably reduced, thereby eliminating the heaving VIVs. Additionally, the vortex scale is reduced in the wake region, while the distance between the girder and vortex shedding increases. This effect significantly reduces torsional VIVs.

Numerical Simulation of the Interaction between a Planar Shock Wave and a Cylindrical Bubble

Three-dimensional (3D) computational fluid dynamics (CFD) simulations have been carried out to investigate the complex interaction of a planar shock wave (Ma = 1.22) with a cylindrical bubble. The unsteady Reynolds-averaged Navier–Stokes (URANS) approach with a level set coupled with volume of fluid (LSVOF) method has been applied in the present study. The predicted velocities of refracted wave, transmitted wave, upstream interface, downstream interface, jet, and vortex filaments are in very good agreement with the experimental data. The predicted non-dimensional bubble and vortex velocities also have great concordance with the experimental data compared with a simple model of shock-induced Rayleigh–Taylor instability (i.e., Richtmyer–Meshkov instability) and other theoretical models. The simulated changes in the bubble shape and size (length and width) against time agree very well with the experimental results. Comprehensive flow analysis has shown the shock–bubble interaction (SBI) process clearly from the onset of bubble compression up to the formation of vortex filaments, especially elucidating the mechanism on the air–jet formation and its development. It is demonstrated for the first time that turbulence is generated at the early phase of the shock cylindrical bubble interaction process, with the maximum turbulence intensity reaching about 20% around the vortex filament regions at the later phase of the interaction process.

Heat transfer coefficient simulation and temperature distribution prediction of wire loops in the Stelmor air‐cooling system

AbstractDuring the Stelmor air‐cooling process, the temperature distribution has a significant impact on the wire loops' final mechanical properties. The temperature distribution during the air‐cooling process is accurately solved by establishing three‐dimensional model and numerical simulations. The heat transfer coefficient at the highly dense region is much smaller than that of wire loops at the low dense region, and changes periodically over time, according to a computational fluid dynamics simulation method. It is also found that the heat transfer coefficient on the cross‐section of the wire loop is very different, with a difference of 70–100 W/m2 K. Finally, the finite difference method is used to calculate the mathematical model of the temperature distribution during the Stelmor air‐cooling process. Comparing the results with the measurement data, the simulation results and measurement data match up well.

Structural response and mechanical properties of the hind wing of the beetle Protaetia brevitarsis.

The folding/unfolding mechanism and collision recovery effect of the beetle's hind wings can provide biomimetic inspiration for the optimization of wing deplorability and the investigation of collision prevention recovery mechanism of new amphibious morphing vehicle. In this study, a method is described to investigate the structural response and mechanical properties of the hind wings of the beetle Protetia brevitarsis under natural conditions. The specially processed test samples were conducted to tensile testing, which facilitates the evaluation of the mechanical properties of specific areas of the hind wing. The micro geometric morphological characteristics of the cross-section of the specimen after tensile fracture were observed by scanning electron microscopy. The three-dimensional morphology of the ventral and dorsal sides of the hind wing was characterized using three-dimensional scanning and reverse modeling methods. The finite element model of the hind wing is developed to investigate the structural deformation and modal response characteristics of its flapping. The uniformly distributed load on the hind wing surface is derived from the lift characteristics obtained from the computational fluid dynamics simulation of flapping wing motion. RESEARCH HIGHLIGHTS: Scanning electron microscope is used to observe the cross-sectional characteristics of the veins and membranes. The material properties of the wing membranes and veins of the hind wings were measured using the tensile testing system. The three-dimensional morphology of the hind wing was characterized using 3D scanning and reverse modeling methods. The finite element model of the hind wing is developed to investigate the structural deformation and modal response characteristics of its flapping.

Computational Fluid Dynamics Prediction of External Thermal Loads on Film-Cooled Gas Turbine Vanes: A Validation of Reynolds-Averaged Navier–Stokes Transition Models and Scale-Resolving Simulations for the VKI LS-94 Test Case

Given the increasing role of computational fluid dynamics (CFD) simulations in the aerothermal design of gas turbine vanes and blades, their rigorous validation is becoming more and more important. This article exploits an experimental database obtained by the von Karman Institute (VKI) for Fluid Dynamics for the LS-94 test case. This represents a film-cooled transonic turbine vane, investigated in a five-vane linear cascade configuration under engine-like conditions in terms of the Reynolds number and Mach number. The experimental characterization included inlet freestream turbulence measured with hot-wire anemometry, aerodynamic performance assessed with a three-hole pressure probe in the downstream section, and vane convective heat transfer coefficient distribution determined with thin-film thermometers. The test matrix included cases without any film-cooling injection, pressure-side injection, and suction-side injection. The CFD simulations were carried out in Ansys Fluent, considering the impact of mesh sizing and steady-state Reynolds-Averaged Navier-Stokes (RANS) transition modelling, as well as more accurate transient scale-resolving simulations. This work provides insight into the advantages and drawbacks of such approaches for gas turbine hot-gas path designers.

Aerodynamic Performance Evaluation of a Coaxial Octocopter Based on Taguchi Method

Abstract The design and optimization of propellers for unmanned aerial vehicles (UAVs) are essential for optimal performance and high efficiency. This study presents a numerical investigation of the aerodynamic performance of coaxial octocopters using openfoam as flow solver. While the aerodynamic performance is affected by many parameters, the current study focuses on four main parameters: the propeller type, the horizontal and vertical separation distances between the propellers, and the ratio between the rotational speeds of the upper propeller and the lower one. To find the minimum number of simulations to be performed within defined limits, and reduce the number of computational fluid dynamics (CFD) simulations that cause high computational cost, Taguchi method was employed. In this study, average thrusts were calculated for the preliminary design of the octocopter by examining an isolated single propeller and dual- and quad propellers taking their rotation directions into account. The Taguchi design matrix revealed that for all cases investigated, the propeller type is the most dominant design parameter followed by the velocity ratio of the upper propeller to the lower one (nU/nL) and vertical (z/D) and horizontal (ℓ/D) orientation of coaxial propellers. However, it was shown that ℓ/D and z/D may play a significant role in vortex formation and pressure fluctuations which should be considered as design criteria for coaxial octocopters associated with flow attributes. The results showed that the aerodynamic performance parameters are not dependent on all the selected parameters, and demonstrated that the selected propeller designs improved aerodynamic performance.

Postural Change of the Annual Cicada (Tibicen linnei) Helps Facilitate Backward Flight

Cicadas are heavy fliers well known for their life cycles and sound production; however, their flight capabilities have not been extensively investigated. Here, we show for the first time that cicadas appropriate backward flight for additional maneuverability. We studied this flight mode using computational fluid dynamics (CFD) simulations based on three-dimensional reconstructions of high-speed videos captured in a laboratory. Backward flight was characterized by steep body angles, high angles of attack, and high wing upstroke velocities. Wing motion occurred in an inclined stroke plane that was fixed relative to the body. Likewise, the directions of the half-stroke-averaged aerodynamic forces relative to the body (local frame) were constrained in a narrow range (<20°). Despite the drastic difference of approximately 90° in body posture between backward and forward flight in the global frame, the aerodynamic forces in both flight scenarios were maintained in a similar direction relative to the body. The forces relative to the body were also oriented in a similar direction when observed during climbs and turns, although the body orientation and motions were different. Hence, the steep posture appropriated during backward flight was primarily utilized for reorienting both the stroke plane and aerodynamic force in the global frame. A consequence of this reorientation was the reversal of aerodynamic functions of the half strokes in backward flight when compared to forward flight. The downstroke generated propulsive forces, while the upstroke generated vertical forces. For weight support, the upstroke, which typically generates lesser forces in forward flight, is aerodynamically active in backward flight. A leading-edge vortex (LEV) was observed on the forewings during both half strokes. The LEV’s effect, together with the high upstroke velocity, increased the upstroke’s force contribution from 10% of the net forces in forward flight to 50% in backward flight. The findings presented in this study have relevance to the design of micro-aerial vehicles (MAVs), as backward flight is an important characteristic for MAV maneuverability or for taking off from vertical surfaces.

Effects of decongestion on nasal cavity air conditioning efficiency: a CFD cohort study

Decongestion reduces blood flow in the nasal turbinates, enlarging the airway lumen. Although the enlarged airspace reduces the trans-nasal inspiratory pressure drop, symptoms of nasal obstruction may relate to nasal cavity air-conditioning. Thus, it is necessary to quantify the efficiency of nasal cavity conditioning of the inhaled air. This study quantifies both overall and regional nasal air-conditioning in a cohort of 10 healthy subjects using computational fluid dynamics simulations before and after nasal decongestion. The 3D virtual geometry model was segmented from magnetic resonance images (MRI). Each subject was under two MRI acquisitions before and after the decongestion condition. The effects of decongestion on nasal cavity air conditioning efficiency were modelled at two inspiratory flowrates: 15 and 30 L min−1 to represent restful and light exercise conditions. Results show inhaled air was both heated and humidified up to 90% of alveolar conditions at the posterior septum. The air-conditioning efficiency of the nasal cavity remained nearly constant between nostril and posterior septum but dropped significantly after posterior septum. In summary, nasal cavity decongestion not only reduces inhaled air added heat by 23% and added moisture content by 19%, but also reduces the air-conditioning efficiency by 35% on average.

A Combined Computational Fluid Dynamics and Thermal–Hydraulic Modeling Approach for Improving the Thermal Performance of Corrugated Tank Transformers: A Comparative Study

In this study, we present a complete analysis of the thermal behavior of an oil-filled distribution transformer using two different approaches: numerically, by applying computational fluid dynamics (CFD) simulations; and analytically, by using thermal–hydraulic modelling (THM). The THM method becomes challenging when the corrugated transformer tank wall structures are complex, and therefore CFD simulations are required to provide in-depth details of the winding oil thermal–hydraulic behavior and to generate parameters for improving the THM calculation. The numerical and analytical thermal performance results were then compared with heat-run measurements of a case study transformer and the accuracy of both approaches on different thermal performance parameters was validated. This study has certain reference value for improving the thermal performance and operation efficiency of distribution transformers, and eventually aims to provide engineers with an effective tool to design the most efficient and reliable distribution transformers with corrugated tank walls for various applications.

Aerodynamic Analysis of Variable Camber-Morphing Airfoils with Substantial Camber Deflections

In recent years, morphing wings have become not only a concept, but an aerodynamic solution for the aviation industry to take a step forward toward future technologies. However, continuously morphing airfoils became an interesting answer to provide green energy solutions. In this paper, the authors conducted experimental research on a continuously camber-morphing airfoil using the Particle Image Velocimetry (PIV) and Computational Fluid Dynamics (CFD) methods. The main objective of this work was to research a variety of morphing airfoils with different camber deflections. An average velocity distribution and turbulence distribution were compared and are discussed. The two-dimensional PIV results were compared to the CFD simulations to validate the numerical method’s accuracy and obtain the aerodynamic coefficient’s trends. A further comparison revealed that morphing airfoils have better aerodynamic performance than conventional airfoils for very low camber deflections and create substantial amounts of drag for significant camber deflections.

Simulation of Transient Temperature and Clearance after Shutdown of Aeroengine Based on CFD and FEA Coupled Models

It is crucial to comprehend the heat soak phenomenon, which may result in a significant temperature increase after the shutdown followed by a gradual decrease. This could bring potential risks for the engine including oil coking. The temperature change of engine components dictates the clearance after shutdown, while startup strategies are primarily based on this. A simulation strategy, utilizing computational fluid dynamics (CFD) and finite element analysis (FEA) coupled models, is suggested to investigate the transient temperature and clearance after shutdown. The maximum temperature deviation between the simulation result and experimental data are less than 6%. Flow parameters, including velocity and mass flow rate obtained from the CFD result, were applied as boundaries of the FEA model. Based on the FEA model, transient temperature calculations were also conducted for 20 hours after shutdown. The results indicate that the FEA model demonstrates good agreement with the CFD simulation, with a maximum deviation of less than 5% and at only 0.2% of the simulation time. After the engine shuts down, the stator’s temperature change rate is faster than that of the rotor due to better cooling conditions and relatively small heat capacity. Consequently, the seal clearance increases in the initial period after shutdown and then decreases to a minimum value. The nondimensional minimum clearance can be 0.8 times the cold state value at the location of the high-pressure turbine seal.

Environmental-Sensing and adaptive optimization of wave energy converter based on deep reinforcement learning and computational fluid dynamics

This paper introduces a novel coupled model for real-time control of the point absorber wave energy converter (WEC) using parallelized deep reinforcement learning (DRL), where the WEC is situated within a numerical wave tank (NWT) built with the method of computational fluid dynamics (CFD). An in-house solver is developed to couple with the DRL and CFD to solve the interaction between WEC and the fluid environment. Validations on wave generation, wave-floater interaction, and power take-off (PTO) unit are carried out. Then, neglecting the detailed model for the PTO technologies, the DRL-based strategy dynamically adjusts the PTO force as a function of the wave features and floater motion. Based on the interaction data, the model-free DRL is outstanding in adaptability and robustness. Simulation results reveal that DRL control improves the wave energy absorption in irregular wave environments, resulting in improvement of 107.5 % compared to the resistive control, with better device protection performance than the model predictive control (MPC). An additional analysis of model-free characteristics of DRL demonstrates the optimization ability independent of floater modeling. This work is the first in-depth study of DRL control of WECs in CFD simulation, providing a more accurate simulation and an optimization process closer to the reality.

Treatment for middle cerebral artery bifurcation aneurysms: in silico comparison of the novel Contour device and conventional flow-diverters.

Endovascular treatment has become the standard therapy for cerebral aneurysms, while the effective treatment for middle cerebral artery (MCA) bifurcation aneurysms remains a challenge. Current flow-diverting techniques with endovascular coils cover the aneurysm orifice as well as adjacent vessel branches, which may lead to branch occlusion. Novel endovascular flow disruptors, such as the Contour device (Cerus Endovascular), are of great potential to eliminate the risk of branch occlusion. However, there is a lack of valid comparison between novel flow disruptors and conventional (intraluminal) flow-diverters. In this study, two in silico MCA bifurcation aneurysm models were treated by specific Contour devices and flow-diverters using fast-deployment algorithms. Computational fluid dynamic simulations were used to examine the performance and efficiency of deployed devices. Hemodynamic parameters, including aneurysm inflow and wall shear stress, were compared among each Contour device, conventional flow-diverter, and untreated condition. Our results show that the placement of devices can effectively reduce the risk of aneurysm rupture, while the deployment of a Contour device causes more flow reduction than using flow-diverters (e.g. Silk Vista Baby). Besides, the Contour device presents the flow diversion capability of targeting the aneurysm neck without occluding the daughter vessel. In summary, the in silico aneurysm models presented in this study can serve as a powerful pre-planning tool for testing new treatment techniques, optimising device deployment, and predicting the performance in patient-specific aneurysm cases. Contour device is proved to be an effective treatment of MCA bifurcation aneurysms with less daughter vessel occlusion.

Application of Machine Learning Models in Coaxial Bioreactors: Classification and Torque Prediction

Coaxial bioreactors are known for effectively dispersing gas inside non-Newtonian fluids. However, due to their design complexity, many aspects of their design and function, including the relationship between hydrodynamics and bioreactor efficiency, remain unexplored. Nowadays, various numerical models, such as computational fluid dynamics (CFD) and artificial intelligence models, provide exceptional opportunities to investigate the performance of coaxial bioreactors. For the first time, this study applied various machine learning models, both classifiers and regressors, to predict the torque generated by a coaxial bioreactor. In this regard, 500 CFD simulations at different aeration rates, central impeller speeds, anchor impeller speeds, and rotating modes were conducted. The results obtained from the CFD simulations were used to train and test the machine learning models. Careful feature scaling and k-fold cross-validation were performed to enhance all models’ performance and prevent overfitting. A key finding of the study was the importance of selecting the right features for the model. It turns out that just by knowing the speed of the central impeller and the torque generated by the coaxial bioreactor, the rotating mode can be labelled with perfect accuracy using k-nearest neighbors (kNN) or support vector machine models. Moreover, regression models, including multi-layer perceptron, kNN, and random forest, were examined to predict the torque of the coaxial impellers. The results showed that the random forest model outperformed all other models. Finally, the feature importance analysis indicated that the rotating mode was the most significant parameter in determining the torque value.

A Physically Motivated Heat Source Model for Laser Beam Welding

In this contribution, we present a physically motivated heat source model for the numerical modeling of laser beam welding processes. Since the calibration of existing heat source models, such as the conic or Goldak model, is difficult, the representation of the heat source using so-called Lamé curves has been established, relying on prior Computational Fluid Dynamics (CFD) simulations. Lamé curves, which describe the melting isotherm, are used in a subsequent finite-element (FE) simulation to define a moving Dirichlet boundary condition, which prescribes a constant temperature in the melt pool. As an alternative to this approach, we developed a physically motivated heat source model, which prescribes the heat input as a body load directly. The new model also relies on prior CFD simulations to identify the melting isotherm. We demonstrate numerical results of the new heat source model on boundary-value problems from the field of laser beam welding and compare it with the prior CFD simulation and the results of the Lamé curve model and experimental data.

Tidal turbine hydrofoil design and optimization based on deep learning

The optimal design of hydrofoils is critical to improve the hydrodynamic performance of the tidal turbine. However, the global optimization of hydrofoils is limited by the high dimensionality of the design space, which requires extensive computational fluid dynamics simulations. This paper proposes an interactive framework for hydrofoil design and optimization based on deep learning. Generative adversarial networks are used to parameterize the hydrofoil design, which automatically learns representations from existing hydrofoils and controls new hydrofoil generation using fewer variables to reduce optimization dimensions. Moreover, the surrogate model based on convolutional neural networks is constructed, which realizes the mapping of hydrofoil design and operating parameters to hydrodynamic performance parameters. The framework can generate a large number of smooth and realistic hydrofoils with three design variables and quickly predict the performance, enabling effective optimization design of hydrofoils. The results show that the optimized hydrofoil shapes have larger lift-to-drag ratios than those of the common hydrofoils. Furthermore, the optimized hydrofoil is applied to the design of 3D horizontal axis tidal turbine blades. The simulation results show that the framework is effective and stable, which can facilitate the design of tidal turbine rotors and provide hydrofoils with higher power coefficients.

Analysis and Prediction of Equivalent Diameter of Air Bubbles Rising in Water

The equivalent diameter of rising bubbles in liquids is an important parameter that has been investigated for decades by researchers for different purposes. Bubble diameter plays important role in quantifying oil and gas leaks in subsea leak analysis, since it allows the prediction of the magnitude of leaks in seabed petroleum wells and other structures through images obtained by underwater vehicles at great depths. Most studies available in the literature on the subject focus on investigating air bubbles in water; therefore, they were used as the main guide of the experimental apparatus described in this article. Several tests were conducted with air bubble chain in tap water, whose flow rate ranged from 21.1 mL/min to 234.4 mL/min, whereas the bubble equivalent diameter ranged from 4.1 mm to 8.2 mm. In addition, computational fluid dynamics simulations were carried out for comparison purposes; they were validated as potential tools to help designing an automated subsea gas leakage monitoring system based on image analysis algorithms. The herein proposed model could be both analytically and experimentally validated, based on comparisons to findings reported by other authors. This procedure enabled gathering evidence about the most efficient analytical predictions available in the literature for the herein addressed scenario. The results in the present study are consistent to those recorded in the main related articles.

Enhancing Axial Flow Fan Performance in Air-Cooled Condensers: Tip Vortex Manipulators and Comparative Analysis of Numerical Simulation and Experimental Testing

Abstract Direct dry-cooled power plants typically operate numerous large-diameter axial flow fans in air-cooled condensers (ACCs) to facilitate the condensation of steam in the plant's thermodynamic cycle. Enhanced fan efficiency may, at scale, lead to significant parasitic power reductions and subsequent increases in plant power output. The performance of these fans may be increased by adding blade-tip modifications, which act to control blade-tip leakage flow. Previous studies have shown that manipulating the tip leakage vortex (TLV) increases the blade-to-air momentum transfer and stabilizes the air flow structures as it traverses the fan blade. This paper takes a step toward the development of improved tip vortex manipulators for an ACC fan. Three-dimensional computational fluid dynamics (CFD) simulations, and experimental testing of the modified and unmodified fan within an ISO test facility are performed. The results are then utilized to assess the accuracy of the simulation model, visualize the manipulated flow structures at the blade tip, and ultimately quantify the effect of the endplate designs on fan performance. Excellent correlation between numerical and test results for the unmodified fan is observed, and the benefit of blade-tip modification on fan performance is again confirmed experimentally. The simulation model, however, fails to predict improvements in fan performance under modification, and simulated tip leakage flows are investigated for clarification. It is hypothesized that deflection of tip endplates under operation account for differences between simulation and experiment, and that fluid–structure interaction analyses could potentially resolve discrepancies.

A Semi-Analytical Prediction Model for Fluid-Elastic Instability of a Normal Triangular Tube Array Under Transverse Flow Excitation

The heat exchange tubes in the steam generator are susceptible to vibration caused by fluid flow, which can lead to damage to both the tubes and their support structures due to collisions. To enhance the predictive accuracy and cost effectiveness of fluid-elastic instability mitigation, multiple models have been created to circumvent its occurrence. In this research, a model has been developed to predict fluid-elastic instability in tube arrays by integrating Hassan’s time-domain-solving model with a parameter acquisition method using computational fluid dynamics (CFD) simulations. By utilizing CFD methods, a comprehensive set of tube-in-channel model parameters were acquired. This method eliminates the requirement of empirical parameters obtained through experiments. The acquired parameters were integrated into the time-domain, tube-in-channel model. This model predicts fluid-elastic stability for a single flexible tube or a bundle of seven tubes within a rigid tube array, accounting for fluid forces in the lift direction. The stability map accurately represents the stiffness effect of flow-induced vibration, agreeing with experimental results and highlighting that the model may effectively utilize parameters obtained from CFD simulations. The combination of the time-domain-solving model and the CFD-based parameter acquisition method has been shown to produce a reliable model.

Influence of building spatial patterns on wind environment and air pollution dispersion inside an industrial park based on CFD simulation.

The "spatial pattern-wind environment-air pollution" within building clusters is closely interconnected, where different spatial pattern parameters may have varying degrees of impact on the wind environment and pollutant dispersion. Due to the complex spatial structure within industrial parks, this complexity may lead to the accumulation and retention of air pollutants within the parks. Therefore, to alleviate the air pollution situation in industrial parks in China and achieve the circular transformation and construction of parks, this study takes Hefei Circular Economy Demonstration Park as the research object. The microscale Fluent model in computational fluid dynamics (CFD) is used to finely simulate the wind flow field and the diffusion process of pollutants within the park. The study analyzes the triad relationship and influence mechanism of "spatial pattern-wind environment-air pollution" within the park and studies the influence of different spatial pattern parameters on the migration and diffusion of pollutants. The results show a significant negative correlation between the content of pollutants and wind speed inside the industrial park. The better the wind conditions, the higher the air quality. The spatial morphology parameters of the building complex are the main influences on the condition of its internal wind environment. Building coverage ratio and degree of enclosure have a significant negative correlation with wind conditions. Maintaining them near 0.23 and 0.37, respectively, is favorable to the quality of the surrounding environment. Moreover, the average height of the building is positively correlated with the wind environment condition. The rate of transport and dissipation of pollutants gradually increases as the average building height reaches 16 m. Therefore, a reasonable building planning strategy and arrangement layout can effectively improve the wind environment condition inside the park, thus alleviating the pollutant retention situation. The obtained results serve as a theoretical foundation for optimizing morphological structure design within urban industrial parks.