Flow Field Forces Research Articles

Analysis of forces on nodules during Coandă-effect-based hydraulic collection

The nodule pickup device is a crucial component of a deep-sea mining system. It is widely perceived that leveraging water flow for the separation and retrieval of nodules is a promising approach. A series of experiments and numerical simulations were conducted to examine the impact of non-dimensional parameters on the force characteristics and flow field of the particle in Coandă-effect-based hydraulic collection. The results showed that the lift coefficient of the particle initially increased before subsequently diminishing from the jet nozzle to the rear. Notably, the lift coefficient α reached its maximum at the convex curved surface between x/d = −0.17 and 0.33. Furthermore, a distinct linear correlation was established between the maximum lift coefficient αmax and Froude number Fr across varying h/d, and an empirical formula for predicting the lift coefficient was developed through data fitting. Upon experimental validation, the prediction exhibited a maximum error of less than 20%. Additionally, numerical simulations revealed that the particle significantly influenced the flow dynamics within the collection area, and the flow field characteristics and the particle forces can be corroborated with each other. The findings not only provided a reliable quantitative tool for assessing the particle force but also facilitated precise predictions of collection performance, aiding in the selection of optimal operational parameters in deep-sea hydraulic collection.

Numerical investigation on designs and performances of multi-dimensional forced convection flow field design of proton exchange membrane fuel cell

The geometric modification of the flow field is validated as an effective measure for improving proton exchange membrane fuel cell (PEMFC) performance and reactant mass transfer ability. In this paper, a parallel flow channel with X-structure connecting two adjacent channels is established. The multi-dimensional forced convection is formed by adding the stepped shape with three connecting flow fields to increase the performance and mass transfer capacity. Six cases based on the flow field of the X shaped channels are proposed to assess the impact of different designs. The results indicate that the X-structure design improves the uniformity of oxygen distribution and drainage capacity. A stepped design with gradually increasing height improves power density and generates a large area of localized gas vertical velocity. The addition of connecting channels results in different flow behaviors at the nodes, such as slight backflow, increased flow velocity, decreased water content, and temperature rise. With a power density improvement of up to 6.9 % compared to the initial flow field, cell performance improves as the order of step height rises and the diamond structure connection channels set. The innovative flow fields are supplied as a reference for future flow field design.

Aerodynamic force characteristics and flow field mechanism of multiple square cylinders in a tandem arrangement

Wind tunnel tests and large eddy simulations were employed to obtain the aerodynamic coefficients and flow fields of two and three tandem square cylinders across various spacing ratios. The spacing ratio L/D, defined as the ratio of the center-to-center spacing between adjacent square cylinders to their width, ranges from 1.2 to 8. By analyzing these results, the aerodynamic force characteristics and their generation mechanism of the three tandem square cylinders at a high Reynolds number (Re = 3.2 × 104) were revealed. By comparing the results of the two and three tandem square cylinders, the effect of adding a third square cylinder behind the two tandem square cylinders on its aerodynamic characteristics was clarified. The results show that unlike the two tandem square cylinders, the three tandem square cylinders exhibit two critical spacing ratios, (L/D)cr1 = 2.5–3 and (L/D)cr2 = 3.5–4, respectively. Based on these two critical spacing ratios, the flow around the three tandem square cylinders is identified as single blunt body, reattachment, and co-shedding regimes. In the single blunt body and reattachment regimes, the addition of a third square cylinder behind significantly alters the flow around the two tandem square cylinders, leading to reduced mean drag coefficients, fluctuating lift coefficients, and Strouhal numbers, along with an increased critical spacing ratio. In the co-shedding regime, this addition has little effect on the flow.

Investigating condensation from humid air under mixed convection regime

A wide range of industrial applications rely heavily on heat transfer during vapor condensation from a mixture of water vapor and noncondensable gases (NCG). For a vertically-mounted condenser plate, the vapor-diffusion boundary-layer thickness is influenced by the interplay of the thermogravitational and forced flow fields, eliciting classical mixed convection scenario. This thickness in turn dictates the condensation heat and mass transfer rates. While condensation in presence of NCG under free and forced convection scenarios are well-characterized in the literature, its counterpart in the mixed convection regime is relatively uncharted. Herein, condensation from an upward stream of humid air over a vertically mounted mild steel condenser surface is characterized under different flow velocities. The free-stream flow is thus directed opposite to the thermogravitational flow induced next to the plate. We observe that with increasing the magnitude of the upward flow velocity of the free stream, the condensation heat transfer coefficient (CHTC) initially decreases until it reaches a minimum at 0.4 m/s, beyond which the CHTC rises again with the flow velocity. Using the Nusselt analogy for mixed convection for the relevant flow regimes we substantiate our experimental findings and extend the observation for predicting condensation behavior under different experimental ambient conditions.

Numerical Study of the Force Characteristics and Flow Field Patterns of a Cylinder in the Internal Solitary Wave

The density of the ocean is unevenly distributed along the depth direction, showing a stratified structure. When there is an external disturbance, large-scale internal solitary waves are easily generated. The internal solitary waves are bounded by the intermediate pycnocline, and the currents in the upper and lower layers will flow in opposite directions. This generates strong shear forces that threaten the safety of marine structures. In this paper, the flow field distribution characteristics of a cylinder under the action of internal solitary waves at different scales are analyzed as a research object. The whole cylinder is discretized into 40 regions, and the horizontal force applied to each section of the cylinder is extracted. The force characteristics of the cylinder are analyzed. It is concluded that the pressure is the main factor determining the magnitude of the total combined force. In addition, the paper extracts the main flow structures from the modal decomposition point of view and explains the reasons affecting the force behavior of the cylinder.

Optothermal Microparticle Oscillator Induced by Marangoni and Thermal Convection.

The light-fueled microparticle oscillator, exemplifying sustained driving in a static light source, potentially holds applications in fundamental physics, cellular manipulation, fluid dynamics, and various other soft-matter systems. The challenges of photodamage due to laser focusing on particles and the control of the oscillation direction have always been two major issues for microparticle oscillators. Here, we present an optical-thermal method for achieving a 3D microparticle oscillator with a fixed direction by employing laser heating of the gold film surface. First, the microparticle oscillation without direction limitation is studied. The photothermal conversion originates from the laser heating of a gold film. The oscillation mechanism is the coordination of the forces exerted on the particles, including the thermal convective force, thermophoresis force, and gravity. Subsequently, the additional Marangoni convection force, generated by the temperature gradient on the surface of a microbubble, is utilized to control the oscillation direction of the microparticle. Finally, a dual-channel oscillation mode is achieved by utilizing two microbubbles. During the oscillation process, the microparticle is influenced by flow field forces and temperature gradient force, completely avoiding optical damage to the oscillating microparticle.

Ceiling effect of flapping wing rotorcrafts to enable energy-efficient perching

Flapping wing rotorcrafts (FWRs) combine both the motion characteristics of flapping and rotary wings, exhibiting high aerodynamic efficiency at low Reynolds numbers. In this paper, the ceiling effect of FWRs has been studied through numerical and experimental methods to further investigate the aerodynamic performance of FWRs operating under a ceiling and to explore the feasibility of enhancing the flight efficiency of FWRs via ceiling-effect-based perching locomotion. Based on the momentum theory and blade element methods, a theoretical model is first established to predict the additional thrust generated by the FWR operating under the ceiling. Additionally, to uncover the detailed aerodynamic mechanisms of FWRs' ceiling effect, the computational fluid dynamics (CFD) simulations were conducted to analyze the changes in force production and flow field around the FWR at 75–115 mm distances from the ceiling. Furthermore, experimental methods were employed to validate the theoretical model and CFD simulation. The results demonstrate a continuous increase in the thrust from 19.18 to 22.15 gf as the ceiling distance decreases, while the total energy consumption remains relatively constant. Leveraging the ceiling effect, the tested FWR could achieve an additional lift force of up to 9.5% at 75 mm ceiling height with a 33 Hz flapping frequency. Finally, a ceiling perching demonstration was conducted to validate the feasibility of achieving FWRs' energy-efficient locomotion based on ceiling effects. Our study highlights the positive influence of ceiling effect on FWRs, showing a promising way to further improve the flight efficiency of FWRs.

Effect of the Guide Vane on the Hydraulic Stability of a Low-head, Large-discharge Industrial Hydraulic Turbine

The hydraulic turbine has been used extensively in the field of energy conservation. For turbines that have low heads and large discharge, improving recovery efficiency and stability is crucial due to their significant hydraulic impact. This paper provides a detailed analysis of the correlation between the influence of radial guide vanes on the stability of low-head, large-discharge turbines focusing on hydraulic performance and energy dissipation before and after the implementation of guide vanes. Moreover, in this paper, two types of turbines, with and without guide vanes, were designed considering the desulfurization scenario. Hydraulic efficiency, radial force, and internal flow field mechanics were numerically studied, and validated through experiments. The results reveal that the working range of the hydraulic turbine could be widened and the energy recovery efficiency improved by a maximum of 3.11% in the small flow rate under the action of guide vanes. Furthermore, it results in a substantial reduction in the radial force of the impeller. Subsequently, the variation in entropy production of different components under full flow rate conditions was compared between the models with and without guide vanes. The total energy consumption decreases sharply under overall working conditions due to the flow control ability of guide vanes affecting the flow state. The entropy production rate of the impeller remains the largest regardless of the presence of a guide vane in the turbine. The vortices inside the guide vanes increase obviously with the flow rate increase.

The effects of abrasive moving speed and motion mode on the thinning mechanism of SiC in three-body contact

Three-body contact is the main contact type in polishing process and leads to a different thinning mechanism than the two-body contact. Molecular dynamics simulation is employed to investigate the thinning mechanism of 3C-SiC substrate in three-body contact. The thinning mechanisms of 3C-SiC under different moving speeds and motion modes of diamond abrasive are compared. Through the analysis of force, temperature, potential energy, stress distribution and atomic flow field, the causes of different thinning mechanisms are explained. It is found that the influence of moving speed is mainly reflected in the temperature rise of substrate when the motion mode of abrasives is the same. The changes of motion mode will significantly alter the stress distribution, which is closely related to the damage depth and atomic flow field. When the feed speed is the same, increasing the self-rotation speed of abrasives can reduce concentration zone of hydrostatic stress and then reduce the damage depth. The self-rotation of abrasives will also change the distribution of von Mises stress, resulting in the different displacement directions of 3C-SiC atoms. Dislocations are more easily generated when the displacement direction of SiC atoms is the same. The moving speed of abrasive is found to have little effect on the thinning mechanism, while the motion mode of abrasive will significantly change the thinning mechanism.

Drag force coefficient and flow field variations of net with different levels of biological fouling under large-eddy simulation

Drag force coefficient and flow field variations of net with different levels of biological fouling under large-eddy simulation

Experimental investigation of the flow control over an airfoil with owl-inspired trailing-edge modification: On the material, length, and spacing sensitivity

We experimentally investigate the effect of material, length, and spacing of trailing-edge extensions on controlling the flow over an airfoil based on our recent experimental work. Force measurements and flow field quantifications were carried out to investigate the aerodynamic performance and flow structures in the wake of an airfoil and, thus, to reveal differences in control effectiveness and mechanisms. Moreover, multi-scale proper orthogonal decomposition and spectral proper orthogonal decomposition are employed to extract coherent flow structures in the flow field. The results indicate that the owl feather can improve the aerodynamic performance, while artificial materials lead to decreased lift-to-drag ratio. However, nylon has optimal adaptability and robustness in controlling turbulent fluctuations, including Reynolds stress and turbulent kinetic energy at different angles of attack (AOAs). The length sensitivity is highly associated with the AOA, i.e., the optimal length increases with the increase in AOA. In addition, the spacing sensitivity correlates with the Reynolds number (Re), i.e., the optimal spacing decreases with higher Re at high AOA. These differences root in the competition effect between the increasing adverse pressure gradient and the interference on regular vortex shedding. It is concluded that nylon with mediate length (L = 0.2D) and relatively large spacing (S = 0.5B) is recommended for wake control and noise attenuation of the S833 airfoil.

Downward flame spread over combined copper foil and PMMA sheet in opposed forced flow field

Downward flame spread over combined copper foil and PMMA sheet in opposed forced flow field

Effect of relative humidity and dust moisture content on filtration performance of bag filter

Bag filters are commonly used to remove fine particulate matter from industrial flue gases. Rapid formation of dense dust cakes is a major issue in bag filters as it necessitates more frequent cleaning, leading to higher operating costs. The rate of cake resistance formation is related to the interparticle forces, which vary with the wetting state of the particle surface. To reduce the cake resistance, moisture is introduced via two methods: regulating the ambient relative humidity (RH) and dust moisture content. The results show that the introduction of moisture affects the cake thickness, mass loading, and particle size distribution on the filter bag, which are critical factors in determining cake resistance. Higher RH and conditions far from the critical dust moisture content (1.44%) contribute to low-resistance filter cakes. In addition, a non-uniform filter cake structure on the surface of the filter bag was investigated, and a correlation between the filter bag height and the filter cake structure parameters was developed. We introduced interparticle forces into computational fluid dynamics coupled with discrete phase model (CFD-DPM) simulations to simulate the wet particles. Particle agglomerates and flow field forces are attributed to the non-uniform particle distributions on the filter bag. These findings provide insights into reducing cake resistance and routes to control increasing resistance over time, as well as a more accurate prediction of cake resistance.

Formation Mechanism of Three-Layer Solidified Structure in Commercial Purity Al During Solidification With a Forced Flow Field Induced by Pulsed Magnetic Field

Formation Mechanism of Three-Layer Solidified Structure in Commercial Purity Al During Solidification With a Forced Flow Field Induced by Pulsed Magnetic Field

Prediction of Cavitation Performance over the Pump-Jet Propulsor Using Computational Fluid Dynamics and Hybrid Deep Learning Method

The cavitation performance of an oblique flow field is different from that under a pure axial flow field. This study analyzed the hydrodynamic performance, bearing force, and tip clearance flow field under different rotating speeds and different cavitation numbers in an oblique flow field. Furthermore, this study proposed a hybrid deep learning model CNN-Bi-LSTM to quickly and accurately predict the bearing force of a pump-jet propulsor (PJP), which will solve the problem of time-consuming calculation and consumption of considerable computing resources in traditional computational fluid dynamics. The Shear–Stress–Transport model and Reynolds-averaged Navier–Stokes equations were utilized to procure the training and testing datasets. The training and testing datasets were reasonably divided in the ratio of 7:3. The results show that the propulsion efficiency decreased more obviously under higher rotating speed conditions, with a maximum decrease of up to 13.59%. The small cavitation numbers 1.4721 and high oblique angle significantly impacted the efficiency reduction; the maximum efficiency loss exceeded 20%. Thus, a small cavitation number 1.4721 is extremely detrimental to the propulsion efficiency of the PJP due to the large cavitation area. Moreover, the intensity of the tip clearance vortex continuously increased with the rotating speed. The CNN-Bi-LSTM deep model successfully predicted the phase difference and trend change of the propulsor bearing force under different conditions. The prediction difference was large at the crest and trough of the bearing force, but it is within the acceptable error range.

Inlet Flow Distortion in an Advanced Civil Transport Boundary Layer Ingesting Engine Installation

AbstractThis article presents first-of-a-kind measurements, and complementary computations, of the flow through the propulsion system of a boundary layer ingesting, twin-engine advanced civil transport aircraft configuration. The experiments were carried out in the NASA Langley 14- by 22-foot Subsonic Tunnel, using a 1:11 scale model of the D8 “double-bubble” aircraft with electric ducted fans providing propulsive power. Overall force and moment measurements and flow field surveys at the inlet and nozzle exit planes were obtained. The computations were carried out with the NASA OVERFLOW code. The measurements and computations were conducted for a range of aircraft angles of attack and propulsor powers representing operating points during the aircraft mission. Velocity and pressure distributions at the propulsor inlet and exit, and integral inlet distortion metrics, are presented to quantify the flow nonuniformity due to boundary layer ingestion. The distorted inflow exhibits qualitative and quantitative changes over the mission, from a unidirectional stratified stagnation pressure at cruise to a streamwise vortex structure at climb conditions. The computations capture these flow features and reveal the interactions between airframe and propulsor that create these three-dimensional flow variations.

Hybrid joining mechanism of rivet plug oscillating laser welding for dual-phase steel and magnesium alloy

A novel hybrid joining method of rivet plug oscillating laser welding (RPOLW) was explored for joining DP590 dual-phase steel to AZ31B magnesium alloy. Microstructure, mechanical properties, and fracture mode of joints were analyzed, and force field, temperature, and flow field of molten pool were simulated. The results showed that the RPOLW joint presented the characteristic of asymmetrical double molten pool. The load-bearing areas of the joint were categorized into the metallurgical joining area and the riveting joining area. Meanwhile, the metallurgical joining area was the core load-bearing area of the joint, which was composed of a mixed-phase zone and an Al-rich zone. The tensile-shear load of RPOLW joint reached 2.2 kN at the optimal parameter, which was 36% higher than that of riveting joint. The heat accumulation in left molten pool resulted in a greater penetration depth of this side, the mixed-phase zone and Al-rich zone of the left molten pool were more evenly distributed. However, Al-rich zone was embedded in mixed-phase zone in the right molten pool, which was caused by the migration of the flow-field center. Consequently, the fracture position of joint was transferred to the mixed-phase zone in left molten pool.

Formation Mechanism of "Fractured Columnar Dendrite" in Commercial Purity Al During Solidification with a Forced Flow Field Induced by Pulsed Magnetic Field

Formation Mechanism of "Fractured Columnar Dendrite" in Commercial Purity Al During Solidification with a Forced Flow Field Induced by Pulsed Magnetic Field

Numerical Simulation of Viscous Flows Around a Surface Combatant Model at Different Drift Angles Using Overset Grids

Viscous flows around ship hull is of great complexity, and when the ship is advancing with drift angles, the flow field can be more complicated. In this paper, the viscous flow field around an obliquely towed surface combatant DTMB 5512 is computed using the unsteady Reynolds-averaged Navier–Stokes (URANS) method. The numerical simulations are carried out by the in-house CFD solver naoe-FOAM-SJTU, which is developed on the open source platform OpenFOAM. To refine local grids and simulate dynamic ship motions, the overset grid approach is applied. Grid convergence study is first performed at straight-ahead towed condition with three sets of grid number and the results show monotonically convergence. Six different drift angles for the DTMB 5512 model at Froude number of 0.28 are simulated and the predicted hydrodynamic forces and flow field are presented. During the simulation, the ship hull is free to trim and sinkage at a free surface environment. As for straight-ahead towed condition, the total resistance coefficient and flow field, i.e., wave pattern and wake region, between this work and the experiment are compared. Both force coefficients and flow field show good agreement with the available experimental data. For oblique towed conditions, the lateral force, yaw moment and the derived hydrodynamic derivatives are also presented and compared with the experimental results. The variation of wave pattern and wake region at different drift angles are presented and analyzed. The results show that the current approach can be an effective tool to predict the viscous flows and hydrodynamic loads for oblique towed ships.

A Stable Dendritic Growth with Forced Convection: A Test of Theory Using Enthalpy-Based Modeling Methods

The theory of stable dendritic growth within a forced convective flow field is tested against the enthalpy method for a single-component nickel melt. The growth rate of dendritic tips and their tip diameter are plotted as functions of the melt undercooling using the theoretical model (stability criterion and undercooling balance condition) and computer simulations. The theory and computations are in good agreement for a broad range of fluid velocities. In addition, the dendrite tip diameter decreases, and its tip velocity increases with increasing fluid velocity.