Effect Of Lift Force Research Articles

Optimization of Photovoltaic Panel Array Configurations to Reduce Lift Force Using Genetic Algorithm and CFD

Aerodynamic lift force acting on the solar structure is important while designing the counterweight for rooftop-mounted solar systems. Due to their unique configuration, the load estimated for solar structures using international building codes can be either higher or lower than the actual. Computational Fluid Dynamics(CFD) simulations haveproven to be an efficient tool for estimating wind loads on solar panels for design purposes and identifying critical design cases. Computational Fluid Dynamics (CFD) simulations usually require high computation power, and slight changes in geometry to find optimum configuration can be time-consuming. An optimization method to minimize lift force effects on solar photovoltaic (PV) arrays installed on rooftops usesthe Computational Fluid Dynamics (CFD)and genetic algorithms proposed in this paper. The tilt angle and pitch between two rows of solar panels were parameterized, and a genetic algorithm was used to search for aconfiguration resulting in minimum wind lift force acting on the solar photovoltaic plant. Only combinations with a performance ratio >80% were considered. Three different rooftopphotovoltaic (PV) plant layout configurations were analyzed in this research. Two rows of photovoltaic (PV) panel arrays wereconsidered for optimization in the 2D domain using ANSYS Fluent. Results showed that the difference in wind-liftforce between optimized configurations against that with maximum lift force configuration for all three cases above was fifty percent.

The dispersion of particles in turbulent semi-circular duct flows

The flow field in a semi-circular duct is simulated by Large Eddy Simulation (LES) and its particle field is simulated by Lagrange particle tracking method. Reynolds number Re b (based on bulk velocity and hydraulic diameter) is 80,000 and Re τ (based on friction velocity and hydraulic diameter) is 3528. Particle diameter d p is chosen as 10, 50, 100, 500 μm corresponding to St as 0.10, 2.43, 9.72, 243.05. The results show that the intensity of the secondary flow near the ceiling is less than that near the floor because the ceiling is curved and able to inhibit the secondary flow. It is found that the difference between the semi-circular duct and the square duct is that the secondary flow in a corner of the semi-circular duct is not symmetrical along the diagonal although they have the same generation mechanism. Regarding the particles, small particles ( d p ≤ 10 μm) are found to uniformly distribute in the duct, while large particles ( d p ≥ 50 μm) preferentially distribute in the corner and floor center. The maximum particles ( d p = 500 μm) fall on the floor quickly and their dispersion mainly depends on the secondary flow near the floor. Particle deposition in the corner depends on particle size due to the effect of secondary flow and gravity. The effect of lift force on particles becomes more significant for 50 and 100 μm particles in comparison with other smaller particles. In the end, the effect of secondary flow is found to be more significant to dominate particle behavior than that of flow fluctuation.

Investigation of air-water two-phase flow characteristics in a 25.4 mm diameter circular pipe

High-quality two-phase flow data are essential for further developing various two-phase flow models and validating two-phase flow codes. The work presented in this paper aims to perform detailed measurements of air-water two-phase flow in a 25.4 mm inner diameter pipe using several techniques developed recently. The gas-phase information is obtained by an integrated method synergizing conductivity probe, high-speed imaging system, and X-ray densitometry system while the Particle Image Velocimetry and Planar Laser-Induced Fluorescence (PIV-PLIF) technique are utilized to measure liquid-phase velocity and turbulence characteristics. Selected conditions in the bubbly, slug, and churn-turbulent flow regimes with the gas and liquid superficial velocities ranging from 0.05 to 4.99 m/s and from 0.10 to 3.88 m/s, respectively, are investigated in this work. The two-phase flow interfacial structures are visualized by both high-speed optical and X-ray images. In fully developed bubbly flows, “wall-peak” void profiles are observed, which is due to the transverse lift force effect. The gas-phase velocities share similar profiles as the liquid-phase velocity but have slightly higher magnitudes due to the buoyancy force effect. In this work, flow development in different flow regimes is investigated as well. As bubbly flows develop along the axial direction, the void profiles may change, and the discrepancy between gas-phase and liquid phase velocities will diminish in the process.

Comprehensive evaluation of two-group interfacial area transport equation and new intergroup transfer model

Interfacial Area Transport Equation (IATE) is developed to dynamically capture the flow structure change and predict Interfacial area concentration (IAC) for various flow regimes. Some recent experimental results show developing flows with intensive bubble coalescence at bubbly to slug transition in small pipes. The drastic IAC decreasing due to bubble coalescence under such conditions has not been well studied in previous IATE benchmark efforts, therefore it may not be captured by the current constitutive models for IATE. In this work, a comprehensive evaluation of the state-of-the-art two-group IATE models is performed using the small pipe data collected at bubbly and slug flow with a focus on transition. The evaluation result shows unsatisfactory performance in predicting the developing flow with intensive bubble coalescence at bubbly to slug transition. The reason is the underestimation of intergroup transfer from group I bubbles to group II bubbles. To improve the IATE performance, a new intergroup wake-entrainment model is developed to enhance the prediction capability for the intergroup void and IAC transfer. The new model incorporates a transition function derived based on the log-normal bubble size distribution and the lift force effect on bubble movement for group I bubbles, which allows for a smooth transition from bubbly to slug flow. The new intergroup transfer model significantly improves the IATE prediction capability in the 25.4 mm ID pipe and its applicability on other types of flow channels including 50.8 mm ID pipe and 200 × 10 mm duct is also verified.

Numerical simulation of local bubble size distribution in bubble columns operated at heterogeneous regime

This work investigates the effect of lift force, turbulent dispersion force, bubble breakup models, bubble coalescence models, and the parameters in the class method for solving PBEs on the simulated local bubble size distribution (BSD) at various radial positions (r/R = 0.0, 0.27, 0.58 and 0.8), and the local Sauter mean diameter (Sd) in bubble columns. The results suggest that the local BSD and Sd are slightly affected by the radial positions but greatly influenced by the lift force and the parameters in the class method (e.g. the minimum bubble diameter and maximum bubble diameter). The prediction of BSD could be promoted by including the lift force. The local Sd increases with the increase of the minimum bubble diameter and the maximum bubble diameter.

Observation of particles behavior near the wall using a narrow channel

Particle behavior on flat plate in narrow flow channel was experimentally investigated. As the test particle, polystyrene particle, glass particle, TiO2 and SiO2 powder were used and compared differences in behaviors for each particle. Number density of particles was measured using the microscope camera. Relatively large particles were removed from the wall surface to upper layer of the flow by the effect of lift force. However, the small and high density particles such as the TiO2 powder, could not remove from the wall surface. In addition, the difference in particle velocity for each particle diameter in a steady flow was measured using polystyrene at near the wall surface and center of narrow flow channel. At near the wall surface, large particles trended to move faster than small particles. At center of narrow flow channel, particle velocity was constant regardless of particle diameter. At near the wall surface, large particles was affected by the velocity distribution by shear layer and move faster than small particles.

Effect of lift force and hydrodynamic torque on fluidisation of non-spherical particles

The aim of many industrial processes is to manipulate solid particle aggregates within gas suspensions. Prime examples of such processes include fluidised bed reactors, cyclone separators, and dust collectors. In recent years, fluidised bed reactors have been used in the gasification of biomass particles. When fluidised, these particles are subject to various hydrodynamic forces such as drag, lift and torque due to interactions with the fluid. Computational approaches, which can be used to replicate laboratory and industrial scale processes, offer a crucial method for the study of reactor design and for the formulation of optimal operating procedures. Until now, many computer models have assumed particles to be spherical whereas, in reality, biomass feedstocks typically consist of non-spherical particles. While lift and torque are of minimal importance for spherical particles, non-spherical particles experience varying lift force and torque conditions, depending on particle orientation relative to the direction of the fluid velocity. In this study, we present a numerical investigation on the effect of different lift force and torque correlations on fluidised spherocylindrical particles. We find that lift force has a significant influence on particle velocities parallel to the direction of gravity. On the other hand, particle orientation is dependent on hydrodynamic torque. Results from this numerical study provide new insight with regards to the dynamics of non-spherical particles that can be of paramount importance for industrial processes involving non-spherical particles.

Non-linear vibration analysis of oleo pneumatic landing gear at touchdown impact

In this paper, nonlinear vibration analysis of a typical aircraft oleo pneumatic landing gear has been done. Mathematical model of the main landing gear is developed, and dynamic equations have been written incorporating the effect of lift force, friction force to study the landing gear behaviour at touchdown condition. The nonlinear effect of stiffness, damping coefficient properties are considered in the analysis. The displacement, velocity, acceleration values are obtained on different landing velocities by numerical simulations using MATLAB/Simulink. Using the same parameters of the aircraft, landing gear and tire, the dynamic analysis was also done in ABAQUS. The results obtained by the nonlinear vibration analysis using the developed model in MATLAB/Simulink have close agreement with the results obtained from ABAQUS. On the basis of the presented model, dynamic simulations of landing for large transport aircraft were performed for different sink velocities. The developed model is also helpful to fine tune the stiffness and damping properties of landing gear in the design stage itself to reduce the replacement / repair cost and increase the life of landing gear assembly.

Theoretical study of moving magnetic beads on an inclined plane and its application in the ratchet separation technique

For first time, motion of a magnetic bead ascending an inclined surface is investigated. The translational and rotational velocities of magnetic beads traveling on an inclined plane in the creeping flow regime are studied. The governing equations considering lift force and magnetic torque are obtained. Rolling and slipping cases are studied in detail. It is shown that the lift force effect is critical for large values of sedimentation Reynolds number (Res) and negligible for small values of Res. This method is applicable for neutrally buoyant and heavy magnetic bead motion. Practical application of this study is implemented in the ratchet configuration for separation of magnetic beads with different sizes. This is applicable for novel applications such as drug delivery, magnetic tweezers, and magnetic actuated stiffness testing systems which require accurate magnetic bead sizes for accurate function.

3D CFD simulation of flashing flows in a converging-diverging nozzle

Flashing of initially sub-cooled water in a vertical circular converging-diverging nozzle is simulated with two-fluid model incorporating drag and non-drag forces. Phase change is assumed to be induced only by interphase heat transfer while pressure jump across the interface is ignored. The reliability of CFD simulations is confirmed by comparison with experimental data and one-dimensional results basing on a drift-flux model. Satisfactory prediction of mass flow rate and cross-section averaged parameters is achieved. The predicted radial distribution of void fraction is however much more flat than the measured one. The discrepancy is found to be caused by the effect of lift force and the reversal of relative velocity. Cases characterised with large pressure-undershoot exhibit significant pressure non-equilibrium effect in form of a pressure recovery process. Sensitivity tests on bubble number density, heat transfer coefficient as well as nucleation are performed.

Validation of a flight model for predicting debris trajectory from the explosion of an ammunition storage magazine

In this study, a debris throw model which considers the effects of debris shape and speed on the aerodynamic coefficients was employed to predict the flight trajectory of debris generated from an internal explosion of ammunition storage magazine. In order to validate the accuracy of the employed model, a series of launching tests were conducted by using three projectile blocks with different shapes and material properties. Fourteen tests with the actual flight trajectories recorded were conducted. Good comparisons between the experimental results and the numerical predictions based on the employed flight model were obtained. In addition, a parametric study was carried out to investigate the influence of the drag and the lift coefficients on the flight trajectories of debris with different aspect ratios. Results illustrate that the drag force always has a significant effect on the flight trajectory, the lift force effects is only significant for the rectangular and irregular blocks.

On-bottom stability of subsea lightweight pipeline (LWP) on sand soil surface

This paper discusses the on-bottom stability of subsea lightweight pipeline (LWP) on sand soil seabed subjected to a combined load of wave and current. Hydrodynamic loads were applied on an LWP to test the on-bottom stability of the pipe. The tests of LWP were carried out with different pipe outer diameter (OD) and different pipe submerged weight. The lateral movement of pipe experiences three characteristic periods based on the lateral displacement of pipe: (1) slightly shaking with sand scour; (2) crossover sand breakouts; (3) laterally moving within a displacement of 2.0 × OD or the pipe will continue to lateral movement after the lateral displacement of 2.0 × OD. A lateral displacement of 2.0 × OD is selected as the displacement criteria for the on-bottom stability of LWP because the pipe will be continuously laterally moving in case the lateral displacement of pipe is larger than 2.0 × OD for LWP, which is different from the on-bottom stability of a rigid steel pipe. Based on the test results, an empirical correlation was established to describe pipeline's lateral on-bottom stability. Three-dimensional ABAQUS dynamic finite element simulations of an LWP on flat seabed are carried out. In accordance with the experimental observations, a modified correlation suggested in DNV-RP-F109 code is used for simulating the lateral resistance of LWP on sand soil surface. The effects of lift force on the pipe when the pipe contacts the seabed were considered. A MathCAD worksheet based on the generalised lateral stability method in DNV-RP-F109 was used to assess the pipeline on-bottom stability. The comparison results of MathCAD and finite element analysis (FEA) showed that the FEA simulation gives a good agreement with test results when the lateral pipe displacement is less than 2.0 × OD. The modified FEA model can be used for the on-bottom stability analysis of LWP to reduce conservatism.

A computational fluid dynamics study on oil-in-water dispersion in vertical pipe flows

A three-dimensional steady-state mathematical model for an oil-in-water dispersed flow was developed to study the effects of lift force on the dispersed phase behaviour in a vertical pipe. The Euler/Lagrange multiphase scheme implemented in CFD code ANSYS Fluent was used to investigate the interaction between the oil droplets (dispersed phase) and water (continuous phase). The flow field of the continuous phase was resolved by solving the Reynolds-averaged Navier–Stokes conservation equations with the k–ɛ turbulence model and standard wall function. The motion of oil droplets was obtained from the resolved flow field of the continuous phase as such that a two-way coupling was enforced to account for the influence of oil droplets on the water flow. Simulations were performed to model published experimental observation. The computational domain was a vertical pipe (3.2m in length and 38mm in diameter). Inter-phase forces (gravitational, drag, buoyancy and shear-lift) were taken into consideration where the shear-lift force is of our primary focus. Three different mixture velocities (1.5, 2.0, 2.5m/s) were simulated with oil fraction ranging from 0 to 0.5. Uniform droplet diameters of 0.5–6mm were tested. Here, droplet breakup and coalescence were not modelled. Comparison with experimental results was given in terms of the ratio of pressure gradient. The simulations show that the shear-lift force plays an important role in the overall force balance acting on the droplets and influences the radial distribution of the oil droplets (i.e. droplets tend to concentrate at the core of the pipe rather than near the wall). Oil droplets diameters are found to depend on the mixture velocity for the case under consideration.

High-throughput, single-stream microparticle focusing using a microchannel with asymmetric sharp corners

A new microfluidic device for fast and high-throughput particle focusing is reported. The particle focusing is based on the combination of inertial lift force effect and centrifugal force effect generated in a microchannel with a series of repeated asymmetric sharp corners on one side of the channel wall. The inertial lift force induces two focused particle streams in the microchannel, and the centrifugal force generated at the sharp corner structures tends to drive the particles laterally away from the corner. With the use of a series of repeated asymmetric sharp corner structures, a single and highly focused particle stream was achieved near the straight channel wall at a wide range of flow rate. In comparison with other hydrodynamic particle focusing methods, this method is less sensitive to the flow rate and can work at a higher flow rate (up to 700 μL/min) and Reynolds number (Re = 129.5). With its simple structure and operation, and high throughput, this method can be potentially used in particle focusing processes in a variety of lab-on-a-chip applications.

Effect of lift force on the aerodynamics of dust grains in the protoplanetary disk

We newly introduce lift force into the aerodynamics of dust grains in the protoplanetary disk. Although many authors have so far investigated the effects of the drag force, gravitational force and electric force on the dust grains, the lift force has never been considered as a force exerted on the dust grains in the gas disk. If the grains are spinning and moving in the fluid, then the lift force is exerted on them. We show in this paper that the dust grains can be continuously spinning due to the frequent collisions so that the lift force continues to be exerted on them, which is valid in a certain parameter space where the grain size is larger than ~ 1 m and where the distance from the central star is larger than 1 AU for the minimum mass solar nebula. In addition, we estimate the effects of the force on the grain motion and obtain the result that the mean relative velocity between the grains due to the lift force is comparable to the gas velocity in the Kepler rotational frame when the Stokes number and lift-drag ratio are both ~ 1. This estimation is performed under the assumptions of the steady state and the isotropic spin angular momentum. We also estimate the mean relative velocity when the grains keep spinning and conclude that the lift force marginally affects the mean relative velocity in the minimum mass solar nebula. If there is a grain-concentrated part in the disk, the relative velocity due to the lift force may dominate there because of high collision rate.

유동가시화를 이용한 혼-타의 간극유동 차단장치 효과에 관한 실험적 검증

Recently, rudder erosion due to cavitation has been frequently reported on a semi-spade rudder of a high-speed large ship. This problem raises economic and safety issues when operating ships. The semi-spade rudders have a gap between the horn/pintle and the movable wing part. Due to this gap, a discontinuous surface, cavitation phenomenon arises and results in unresolved problems such as rudder erosion. In this study, we made a rudder model for 2-D experiments using the NACA0020 and also manufactured gap flow blocking devices to insert to the gap of the model. In order to study the gap flow characteristics at various rudder deflection angles(<TEX>$5^{\circ}$</TEX>, <TEX>$10^{\circ}$</TEX>, <TEX>$35^{\circ}$</TEX>) and the effect of the gap flow blocking devices, we carried out the velocity measurements using PIV(Particle Image Velocimetry) techniques and cavitation observation using high speed camera in Seoul National University cavitation tunnel. To observe the gap cavitation on a semi-spade rudder, we slowly lowered the inside pressure of the cavitation tunnel until cavitation occurred near the gap and then captured it using high-speed camera with the frame rate of 4300 fps(frame per second). During this procedure, cavitation numbers and the generated location were recorded, and these experimental data were compared with CFD results calculated by commercial code, Fluent. When we use gap flow blocking device to block the gap, it showed a different flow character compared with previous observation without the device. With the device blocking the gap, the flow velocity increases on the suction side, while it decreases on the pressure side. Therefore, we can conclude that the gap flow blocking device results in a high lift-force effect. And we can also observe that the cavitation inception is delayed.

3차원 비선형 조파문제 해석을 위한 수치해법 연구

For free surface flow problem, a high-order spectral/boundary element method is adapted as an efficient numerical tool. This method is one of the most efficient numerical methods by which the nonlinear gravity waves can be simulated and hydrodynamic forces also can be calculated in time domain. In this method, the velocity potential is expressed as the sum of surface potential and body potential. Then, surface potential is solved by using the high-order spectral method and body potential is solved by using the high-order boundary element method. Using the combination of these two methods, the free surface flow problems of a submerged moving body are solved in time domain. In the present study, lifting surface theory is added to the former work to include effects of lift force. Therefore, a new formulation for the basic mathematical theory is introduced to contain the lift body in calculation.

Sheathless hydrodynamic positioning of buoyant drops and bubbles inside microchannels

Particles, bubbles, and drops carried by a fluid in a confined environment such as a pipe can be subjected to hydrodynamic lift forces, i.e., forces that are perpendicular to the direction of the flow. We investigated the positioning effect of lift forces acting on buoyant drops and bubbles suspended in a carrier fluid and flowing in a horizontal microchannel. We report experiments on drops of water in fluorocarbon liquid, and on bubbles of nitrogen in hydrocarbon liquid and silicone oil, inside microchannels with widths on the order of 0.1-1 mm. Despite their buoyancy, drops and bubbles could travel without contacting with the walls of channels; the most important parameters for reaching this flow regime in our experiments were the viscosity and the velocity of the carrier fluid, and the sizes of drops and bubbles. The dependencies of the transverse position of drops and bubbles on these parameters were investigated. At steady state, the trajectories of drops and bubbles approached the center of the channel for drops and bubbles almost as large as the channel, carried by rapidly flowing viscous liquids; among our experiments, these flow conditions were characterized by larger capillary numbers and smaller Reynolds numbers. Analytical models of lift forces developed for the flow of drops much smaller than the width of the channel failed to predict their transverse position, while computational fluid dynamic simulations of the experiments agreed better with the experimental measurements. The degrees of success of these predictions indicate the importance of confinement on generating strong hydrodynamic lift forces. We conclude that, inside microfluidic channels, it is possible to support and position buoyant drops and bubbles simply by flowing a single-stream (i.e., "sheathless") carrier liquid that has appropriate velocity and hydrodynamic properties.

On the role of the lateral lift force in poly-dispersed bubbly flows

An extensive experimental database comprising air–water as well as steam-water upwards vertical pipe flows for a pressure up to 6.5 MPa was used to investigate the effect of the lateral lift force on turbulent poly-dispersed flows with medium or high gas volume fraction. It was clearly shown that the lift force plays an important role also in such flows. Several effects such as bubble coalescence and breakup as well as fast rising large bubbles which push small bubbles towards the pipe wall superpose the effect of the lift force but can be separated from this effect. The critical bubble diameter, at which the lift force changes its sign, predicted by using Tomiyama’s correlation agrees well with experimental data obtained for turbulent air–water and steam-water flows with medium and high void fraction and a broad spectrum of bubbles sizes. The values for this critical bubble diameter are confirmed by the experimental data within the frame of the uncertainty of the data. Consequences of the action of the lateral lift force on flow structures in different flow situations are discussed. From the investigations it can be concluded that the lift force including the bubble size dependent change of its sign should be considered in a proper numerical 2D or 3D-simulation on flows in which bubbles in the range of several millimeters are present.

Effect of thermophoresis and other parameters on the particle deposition on a tilted surface

In this study a modified version of v2-f turbulence model ( φ– α), is applied to simulate a non-isothermal air-flow. The φ- α model and a two-phase Eulerian approach complement each other to predict the rate of particle deposition on a tilted surface. The φ– α model can accurately calculate the normal fluctuations, which mainly represent the non-isotropic nature of turbulence regime near the wall. The Eulerian model was modified considering the most important mechanism in the particle deposition rate when compared to the experimental data. The model performance is examined by comparing the rate of particle deposition on a vertical surface with the experimental data in a turbulent channel flow available in the literature. The effects of lift force, turbophoretic force, thermophoreric force, electrostatic force, gravitational force and Brownian/turbulent diffusion were examined on the particle deposition rate. The results show that, using the φ– α model predicts the rate of deposition with reasonable accuracy. The results of modified particle model are in good agreement with the experimental data. This study highlights the paramount effect of thermophoretic force on the particle deposition rate and clearly shows that when the temperature difference exceeds a certain limit, the electrostatic force has insignificant effect on the particle deposition rate. Furthermore, it is indicated that even at small temperature differences, the effect of tilt angle on the particle deposition rate for intermediate-size particles is negligible.