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For models with large numerical simulation costs, such as high-speed trains, using as few samples as possible to construct a high-precision surrogate model during aerodynamic multi-objective optimization is critical to improving optimization efficiency. This study proposes a sequential infill criterion (SIC) appropriate for the Kriging surrogate model to address this issue. Three multi-objective functions are employed to test the feasibility of constructing a surrogate model based on SIC, and the SIC surrogate model then performs multi-objective aerodynamic optimizations on the high-speed train. The findings indicate that the expected improvement infill criterion (EIC) in the first stage can enhance the global prediction accuracy of the SIC. An infill criterion based on EIC that fuses gradient information (PGEIC) in the second stage is proposed to seek samples in the Pareto front. The PGEIC surrogate model achieves the lowest generational distance and prediction error. The performance of EIC for global search, EIC for Pareto front search, and infill criterion for Pareto front search using only gradient information is poor. The final PGEIC–SIC surrogate model of train aerodynamics has less than 1% prediction error for the three optimization objectives. The optimal solution reduces the aerodynamic drag force of the head car and the aerodynamic drag and lift force of the tail car by 4.15%, 3.21%, and 3.56%, respectively, compared with the original model. Furthermore, sensitivity analysis of key parameters revealed that the nose height v1, cab window height v3, and lower contour line have a greater impact on aerodynamic forces. Moreover, the nose and cab window heights of the optimal model have been reduced, and the lower contour line is concave. Correspondingly, the streamlined shape appears more rounded and slender. [ABSTRACT FROM AUTHOR]

Multiphase Richtmyer–Meshkov instability (RMI) is often accompanied by a dispersed phase of particles, where the evolution of the mix zone width (MZW) is a significant issue. The Stokes number (S t) is a key dimensionless parameter for particle-containing multiphase flows because it represents the ability of particles to follow the fluid. However, our theoretical analysis and numerical simulation indicate that the Stokes number is not the only dominant parameter for the evolution of multiphase RMI. This study uses the derivation of particle and fluid momentum equations to demonstrate the inability of the Stokes number to predict MZW evolution, that is, even at the same Stokes number, increasing the particle density or the radius leads to completely different MZW evolution trends. This study proposes a novel dimensionless number, S d , to measure the effect of drag on the fluid owing to the particles. S d is the ratio of the relaxation time of the fluid velocity affected by the particle force to the characteristic time of the shock wave. We developed theoretical models of MZW at different S d values. Subsequently, a set of multiphase RMI numerical simulations on uniformly distributed particles with different S t and S d values was conducted. The numerical results verify the theoretical predictions and effectiveness of the proposed dimensionless number. The phase diagram containing different simulation cases demonstrates that the Stokes number cannot be used to predict MZW and must be combined with S d to determine its evolution. [ABSTRACT FROM AUTHOR]

High-throughput droplet incubation is an integral part of various lab-on-chip platforms. Packing droplets efficiently before sending them to the incubation region is essential. For this purpose, different oil extraction mechanisms have been used. Here, we propose one such oil extraction structure and study the effect of channel parameters on droplet clustering, especially the channel height relative to droplet size. Droplets relatively smaller than channel dimensions follow continuous flow dynamics. Hence, simple hydrodynamic resistance modeling flow inside the main and side channels could estimate their flow features. When the droplet diameter exceeds the channel height, its dynamics are observed to be deviating drastically from simple isolated droplet motion and hydrodynamic resistance network model. Though accurate, Eulerian–Lagrangian formulation for modeling two-component fluid flow becomes computationally expensive when dealing with many droplets. Hence, along with hydrodynamics modeling, we adopt a simplified hindrance model to incorporate the effects of modified drag force and emulsion viscosity. This model matches well with experimental results and reduces the design time. Finally, we built an efficient oil extraction and droplet incubation platform for high throughput (200–400 droplets/s) for incubating a large number of cell-encapsulated droplets (7 × 105–8.4 × 105) for a significant amount of time (30–70 min). This study offers insights into droplet dynamics in the oil extraction region, which can be further helpful in building an efficient lab-on-chip platform. [ABSTRACT FROM AUTHOR]

[Display omitted] • Novel sBF method for scalable, cost-effective slippery surface was developed. • Nonzero interfacial slip was confirmed by direct velocity field measurements. • Significant drag reduction was observed under turbulent flow condition. • Remarkable anti-biofouling was shown in real-sea tests over 50 days. The maritime industry has been seeking efficient solutions to combat hydrodynamic friction and biofouling, which increase operational costs and environmental issues. To address these concerns, we developed a novel sprayable breath figure (sBF) method to create a scalable and cost-effective lubricant-infused surface (LIS) for reducing frictional drag forces on marine vehicles. The proposed sBF method facilitated the rapid production of multilayered porous polymer films with micron-scale spherical cavities. It can be applied to large and curved surfaces, thus overcoming the limitations of traditional breath figure methods. Further surface treatment of oxygen plasma etching followed by polydimethylsiloxane (PDMS) brush grafting was employed to optimize the interfacial slip and lubricant retention of the slippery surface coating. The PDMS brush-grafted slippery surfaces demonstrated significant drag reduction under turbulent flow conditions. This was confirmed by direct velocity field measurements, showing nonzero slip velocity unlike conventional no-slip surfaces. The same surfaces also exhibited remarkable anti-biofouling performance, severely resisting marine biofouling in real-sea field tests over 50 days. The proposed sBF method was successfully applied to large-area and curved submerged bodies, and achieved a noticeable drag reduction under high-speed turbulent flows. This study is a critical step toward the practical implementation of this technology in the marine industry. [ABSTRACT FROM AUTHOR]

• The coupling between the elastic spike and hypersonic flow is solved by a new FSI algorithm. • The drag reduction by spike is affected by the spike geometry sizes and materials. • The spike deformation, pressure distribution, aerodynamic drag and lift forces are revealed. • Larger spike diameters, shorter spikes, and spikes with larger elastic moduli and lower densities are preferred for drag reduction. In this study, a new fluid structure interaction (FSI) algorithm is developed for the aerodynamic-elasticity problem, aiming to address the coupling problem between compressible fluids and deformable solid structures. To verify the feasibility of the proposed model, a two-dimensional cantilever panel case under shock waves was conducted. Our numerical results present good consistency with the data from literatures. Furthermore, the validated FSI algorithm is applied on an elastic spike under compressible hypersonic flow. The complicated interaction between the fluid and solid structure under various spike geometry sizes and materials were revealed in respect of the spike deformation, pressure, temperature, and flow fields distribution. The main findings show that the spike deformation largely affects pressure and temperature on the blunt, the aerodynamic drag and lift force, indicating the necessity of FSI. For the spike diameters and lengths, it is found that larger spike diameters and shorter spikes benefits for deformation reduction, thereby minimizing fluctuations in aerodynamic drag force. For spike materials, spikes with larger elastic moduli and lower densities are preferred for the drag reduction. Thus, the proposed FSI model can accurately predict flow fields around and deformations for elastic spikes and offer guidance for drag reduction design in aerospace engineering. [Display omitted] [ABSTRACT FROM AUTHOR]

Agricultural tractors are improving their capacity, speed, and performance. However, the characteristic form of the agricultural tractors is not known in terms of aerodynamics. This is required in modeling transportational duties and their impacts on energy and environment, especially considering tractor-trailer couples. In this work, this characteristic form solely investigated apart from the couple to base a foundation. The geometry was adapted from a commercial model. The model was simplified greatly to isolate numerous parameters yielding a generic shape. Only geometrical features as parameters were selected as the nose shape as the leading surface and the windshield angle. Scaled models were tested in a wind tunnel. Drag coefficients independent from Reynolds number, drag forces, pressure distributions on the symmetry planes and pressure coefficients were obtained. An extrapolation was made in order to predict drag force related fuel consumption and CO 2 emission for a full-scale tractor on-road transportation scenario based on experimentally obtained drag coefficient. It is understood that changes in tractor front surface topology and wind shield angle can lead to drag changes up to 3%. A 0.72 value of drag coefficient may be assumed for the generic agricultural tractor form. Based on this value, an on-road agricultural tractor cruising with 70 km h−1 is predicted to have a drag sourced fuel consumption of 3.9 kg h−1 and CO 2 emission of 10.19 kg h−1. Tractor trailer couples and their platoons are of interest in terms of research and computational simulations that may utilize present results as validator. • On-road transportation by agricultural tractors is emphasized and evaluated. • Generic agricultural tractor form and its aerodynamic aspects are investigated experimentally for the first time. • Dimensionless drag that can be used for extrapolations for the transportation scenarios is presented. • Experimental pressure distribution at the symmetry plane of the agriculture tractor is provided. • Fuel consumption and carbon dioxide emission due to drag are approximated. [ABSTRACT FROM AUTHOR]

• The PMP has an advantage in simultaneous control of flow drag and noise. • The PMP reduces the pressure drag coefficient more than 20%. • The PMP suppresses both the tonal and broadband noise components. • The PPI is an important parameter affecting pressure drag and noise level. Pressure drag and noise level are two key parameters related to flow past a bluff body, thus various active and passive methods have been developed to reduce the drag and noise. The paper presents a passive method of simultaneously reducing the pressure drag and aerodynamic noise associated with flow past a smooth circular cylinder by placing a porous material plate (PMP) downstream the cylinder. Multi-points instantaneous wall pressure and far-field acoustic pressure have been measured in an anechoic wind-tunnel facility. The experimental results confirm that the PMP is effective to reduce both the pressure drag and the noise level, showing an attractive advantage compared with the impermeable splitter plate and porous coating. Parametric studies reveal effects of the pores per inch (PPI) of PMP, PMP-cylinder spacing and incoming flow velocity on the pressure drag and noise level. [ABSTRACT FROM AUTHOR]

The underwater configuration analysis of a towing rope is crucial for estimating the rope tension and the state of the towed body. In the case where the rope inclination angle significantly increases, the partial hydrodynamic force coefficients of a rope segment should display apparent variations with the local inclination angle. However, constant drag coefficients ignoring the angle-dependent drag effect are widely used in numerical studies. To address the importance of the angle-dependent force coefficients for analyzing the underwater configuration of a towing rope, Large Eddy Simulations are carried out to obtain the drag forces of a cylindrical rope segment with varying inclination angles and Reynolds numbers. To model the rope deformation, a tensile experiment is also conducted to establish the nonlinear tension-strain relationship of the fibre rope. Parametric studies are performed to investigate the difference in the results calculated by both constant and angle-dependent hydrodynamic force coefficients. It was found that the application of constant drag coefficients leads to significant underestimation of the rope tension near the towed body, especially when the rope experiences large inclination angles. Therefore, the present study highlights the importance of using the angle-dependent hydrodynamic coefficients to predict the configuration and tension of towing ropes. • The underwater configuration of a towing rope is analyzed based on the drag coefficient validated by the inclination angle. • Large Eddy Simulations are carried out to obtain the drag coefficients at various inclination angles and Reynolds numbers. • Experiment is performed to establish the tension-elongation relationship of the synthetic fibre rope. • The rope tension predicted by the angle-dependent coefficients may be much larger than the one by constant coefficients. [ABSTRACT FROM AUTHOR]

• Numerical simulation was used to study the effect of surface hydrophobic of fine particles on hydrocyclone separation. • Increasing the contact angle of the particles can effectively reduce drag force and greatly improve the separation efficiency. • The total separation efficiency increased from 69.6 % to 76.71 %, and the cut size decreased from 22.4 μm to 17.8 μm. Hydrocyclones are devices that can efficiently separate and classify mixtures of multiphase systems based on density and grain size differences under centrifugal sedimentation. The cut size, denoted as d 50 , indicates the separation performance of the device, with lower values indicating higher separation limits. To reduce the cut size, researchers have tried different methods, such as modifying the hydrocyclone structure or increasing the feed pressure. However, the contact angle of the fine particles has a significant effect on their drag force, which affects the separation efficiency and accuracy. In this study, we explored the effect of surface wettability on the separation process and proposed an interface regulation method to reduce the cut size. Our experimental results indicate that by changing the contact angle from 10° to 87°, the total separation efficiency of hydrophobic glass beads increased from 69.6 % to 76.71 %, and the cut size decreased from 22.4 μm to 17.8 μm under the same operating parameters. Our work is of great theoretical interest and potential engineering applications for improving the efficiency and accuracy of hydrocyclone separation of fine particles. [ABSTRACT FROM AUTHOR]

The key issue for developing energy-saving high-speed trains is to optimize aerodynamic performance. Therefore, the strategy of installing shields beneath bogies (SBBs) is proposed to improve the aerodynamic performance of the train further. A high-speed train Case0 without SBB and two high-speed trains Case1 and Case2 with different SBBs are established. Numerical simulations are carried out using the IDDES method to analyze the differences in the aerodynamic characteristics of the three trains. According to the findings, the largest error in surface pressure between the wind tunnel test and numerical simulation is roughly 3.6%. The aerodynamic resistance of the head, middle, and tail car of Case1 is reduced by 29.7%, 7.3%, and −7.2% in comparison to Case0, respectively, and that of Case2 train is reduced by 56.7%, 32.1%, and 17.1%. The aerodynamic resistance of the whole Case1 and Case2 is reduced by 12.2% and 37.7%, respectively, with Case2 having a more significant drag reduction effect. The vortex shedding range of Case0 is approximately 1.35 times that of Case2. Moreover, the presence and type of the SBB will affect the flow field around and behind Bogie6, resulting in a 9° difference in the wake downwash angle between Case2 and Case0. A comprehensive analysis of the surface pressure and aerodynamic forces of the train, the pressure, velocity, turbulence kinetic energy, and vortex structure of the flow field reveals that Case 2 exhibits lower aerodynamic resistance and better characteristics in the flow field. The study's findings can serve as data support and reference for the research and development of next-generation high-speed trains and installing equipment on railways. • The shields beneath bogies (SBBs) are installed to improve the aerodynamic performance of the high-speed train further. • The drag force reduction effects of different forms of SBB on the carriages and the whole train are studied. • A comprehensive comparison is carried out on the train with different SBBs from the aerodynamic force and flow field. • The findings can support the development of next-generation high-speed trains and installing equipment on railways. [ABSTRACT FROM AUTHOR]

This study evaluated the individual drag force model (iDFM), a subgrid-scale model for simulating tsunami inundation in coastal urban areas. The iDFM was then applied to characterize Typhoon Haiyan-induced storm surge inundations in 2013. Using a post-typhoon survey dataset containing inundation depths at different survey points with inundation limits, the performance of iDFM was validated, and the results confirmed that iDFM accurately estimates the storm surge inundation depths but overestimates the limit of inundation. In addition, the results revealed that the iDFM flow velocity was a dominant factor, and the inclusion of buildings in the simulated affected the arrival of the leading edge of inundation. Furthermore, the reduction in flow velocity was sensitive to the subgrid-scale parameters, and the spatial distribution pattern of buildings located in the inundated area affected the relative difference in the maximum flow velocity with the regional roughness model. The inundation characteristics of the Typhoon Haiyan storm surge models were compared with a historical tsunami event. The inundation characteristics were similar and model differences were observed mainly in the fluid velocity and momentum flux rather than the inundation depth. However, the magnitudes of relative differences observed in Tacloban were larger for fluid velocity and momentum flux, respectively, than in the tsunami event. The effect of building drag on advection was also substantial in Tacloban due to the differences in the inflow mass and momentum fluxes. • This study applys a subgrid-scale model of the tsunami inundation in a coastal urban area, called the individual drag force model (iDFM), to Typhoon Haiyan-induced storm surge inundations in 2013. The accuracy of iDFM is validated in comparison with the post-survey dataset, measuring inundation depths at survey points and the limit of inundation. In addition, the characteristics of the modeled inundation flow by the iDFM are examined by comparing them with the characteristics of the regional roughness model corresponding to the land use category. Moreover, the similarities and differences in the inundation characteristics are compared with a historical tsunami event in the authors' previous publication. [ABSTRACT FROM AUTHOR]

[Display omitted] • Operando observation system is constructed to make in situ test on lab-on-a-chip. • Image processing method is developed based on automatic object detection. • It is first-time visualization of dynamic deposition process of submicron particle. • The growth site and direction of particle chain are investigated systematically. • This work confirms the effect of drag force on ultimate height of particle chain. Submicron particle is a harmful air pollutant due to size effect, gathering characteristic and biological activity, which not only causes a serious threat to human health and public sanitary security, but also is difficult to capture with lowest removal efficiency. Here, Operando investigation system is constructed to explore deposition characteristics of submicron particles in electric-flow coupled fields through combining microfluidic technology for forming micro electric field on the lab-on-a-chip with micro-visualization technology for capturing dynamic deposition process. Besides, based on object detection and deep learning, an image processing method is developed which can realize automatic object detection and rapid extraction of required information. Results show that particle chain grows preferentially from the convex position to prove the influence of electrode surface roughness on the growth site. The global electric field has a positive effect on keeping particle chain growing up at upright direction, and the growth process of particle chain is proposed: forming growth site stage, grow fast stage and fluctuation stage. Particle velocity in the airflow direction presents a decrease trend during the process particle migrates towards to electrode surface, which proves the direct influence of boundary layer effect on the ultimate height of particle chain. [ABSTRACT FROM AUTHOR]

• A falling particle receiver thermal model is implemented in MATLAB. • The model is validated against CFD and is suitable for system-level analysis. • The model is applied to assess performances of different receiver configurations. • Receiver thermal efficiency increases with number of stages. • Recirculation only marginally improves receiver performances. Falling particle receivers are one of the most promising new generation solar tower technologies. They have the advantage of being directly heated, thus relaxing flux limitations associated with conventional receivers, and being able to achieve very high temperatures (>1000 °C) without degradation. The goal of this work is the development of a thermal model able to describe the behaviour of a falling particle receiver under different operating conditions, to provide the value of its thermal efficiency within reasonable computational time so that the model can be applied for optimization of solar tower plants at system-level. The thermal model was developed starting from models described in literature, upgrading them with various features (e.g., 2D/3D discretization, drag force effect, etc.) to describe more in detail the heat transfer phenomena occurring during receiver operation. The obtained model results were verified against CFD showing better agreement than the original model. Furthermore, the thermal model was employed in a case study with the goal of optimizing the receiver size for a given solar field and performing off-design and annual performance analyses. Results showed that the adoption of multiple stages increases performance compared to a free-falling particle receiver both under nominal and off-design conditions, and the higher the number of stages the better is the performance. A free-falling particle receiver showed a yearly thermal efficiency of 72.0 %, while the five stages one achieved 75.3 %. In addition, recirculation was investigated as part load strategy for improving performance of the free-falling receiver, but the benefits in terms of performance were limited. [ABSTRACT FROM AUTHOR]

When flexible structures deflect under unidirectional flow or sway under oscillatory flow, their hydrodynamic drag forces are reduced. The flow-induced motion of a structure is governed by the balance between hydrodynamic forces and restoring forces due to stiffness (a ratio known as the Cauchy number, C a). Movement of and drag on structures under unidirectional (current) and oscillatory (wave) flow have often been studied separately, but such flows frequently coexist. In this study, a simple numerical model of a slender flexible plate, which can represent a seagrass blade, was used to investigate how the posture of and drag on the plate in oscillatory flow was impacted by collinear currents moving in the same (following) and opposite (opposing) directions, relative to the wave propagation. The rate of work done against hydrodynamic drag forces defines the rate of wave energy dissipation, E D , w. The velocity acting on the blade was modeled as if the blade existed in a canopy of blades. For conditions in which blade motion was observed, added currents influenced E D , w in two ways. When the ratio of the time-averaged velocity within the canopy, U 1 , to the wave velocity amplitude, U w , was less than 0.33 ± 0.05 , E D , w was reduced relative to the pure wave condition, which was attributed to current-induced deflection. However, when U 1 / U w exceeded 0.33 ± 0.05 , E D , w increased. While the blade increasingly deflected with increasing current, higher currents also limited unsteady wave-induced reconfiguration, increasing the relative motion between the blade and fluid and therefore increasing drag. While C a can predict the drag reduction associated with weak current, it cannot predict the increase in drag for U 1 / U w > 0.33. Finally, the model was used to explore the influence of current on wave amplitude evolution across a seagrass meadow. Relative to pure wave conditions, opposing currents enhanced wave damping, but following currents had little impact on wave damping. [ABSTRACT FROM AUTHOR]