Lower Drag Force Research Articles

Computational hydrodynamics of a typical 3-fin surfboard setup

Surfing is becoming increasingly popular, and the economic market for surfing products is growing. The performance of the surfboards and the fins has evolved continuously through the experience and innovative spirit of surfers and shapers. However, provided performance characteristics are only subjective and of descriptive nature. Therefore, we apply and analyze objective performance indicators as (1) lift and drag coefficient for each fin and the entire configuration and (2) lift-to-drag ratio for the entire fin configuration.This numerical study investigated a commercial 3-fin configuration, mounted into a rectangular simulation region, considering the flow at several different angles of attack between 0°and 45°. RANS and URANS simulations were performed with the SST k−ω turbulence model at an inflow velocity of 5 m/s.Simulation results showed that the fins have an optimal range of attack angles where they produce the most lift. The lift forces generate a turning moment which forces the board to be turned in direction to the shore favoring the controllability and stability of the surfboard. The higher the angle of attack, the higher is the drag coefficient. Lift-to-drag ratio values showed that for speeding up small attack angles with low drag forces are preferable. Furthermore, unsteady effects as flow separation combined with vortex shedding occur at high angles of attack above 20°which can only be resolved by URANS simulations. These unsteady effects have high negative influences on stability due to high fluctuation amplitudes for lift and drag forces.In summary, this study presents, to the best of the authors’ knowledge, for the first time, steady and unsteady forces on surfboard fins and discusses their potential influences on the surfer’s controllability and stability of the surfboard during typical surfing maneuvers.

Population Balance Model Simulation of the Particle Effect on Flow Hydrodynamics in Slurry Beds

AbstractComputational fluid dynamics (CFD) simulation can provide in‐depth knowledge on hydrodynamics in slurry‐bed reactors. The accuracy of CFD simulation is highly depending on the description of the interactions between gas phase and slurry phase with the drag model. Conventional drag models were usually developed for gas‐liquid systems without considering the presence of particles in slurry‐bed reactors. A modified drag model is proposed by taking into account the effect of particles. Simulation results proved the accuracy of the model in predicting hydrodynamics in the slurry bed. A higher particle concentration can increase the gas velocity and decrease the gas holdup due to the lower drag force between gas phase and slurry phase. The bubble size is predicted to be larger at higher particle concentrations due to the increased liquid eddy length scale. Considering the particle effect is critical for accurately simulating slurry‐bed reactors.

Mathematical Modeling Of The Hydrodynamic Forces On A Low Aspect-ratio Curved Otter Board

As an important accessory of single trawl, otter board has the effect of providing net horizontal expansion. Hydrodynamic performance of otter board directly affects net horizontal expansion, affects further the catching and fishing efficiency[1-2].For improving the fishing efficiency and technology level and solving the world energy crisis in some way, carrying out the research of trawl door hydrodynamic performance in order to design and optimize new energy-efficient with high expansion and low drag force otter board becomes a new researchers’ focus [2].This paper selects a low aspect-ratio curved otter board as research object, presents a method based on the flume experiment results and for mathematical modeling of the hydrodynamic forces on the otter board. These forces have been found as a function of angles of attack and slip. The coefficients are parameterized for smoothing and computational performance.A method is summarized for extending to the evaluation of arbitrary shape otter board and outside the normal operating.A verified by experiment results computational efficient model of the steady state hydrodynamic forces is finally proposed, suitable for trawl control system design and analysis.

Leaky Flow through Simplified Physical Models of Bristled Wings of Tiny Insects during Clap and Fling

In contrast to larger flight-capable insects such as hawk moths and fruit flies, miniature flying insects such as thrips show the obligatory use of wing–wing interaction via “clap and fling” during the end of upstroke and start of downstroke. Although fling can augment lift generated during flapping flight at chord-based Reynolds number (Re) of 10 or lower, large drag forces are necessary to clap and fling the wings. In this context, bristles observed in the wings of most tiny insects have been shown to lower drag force generated in clap and fling. However, the fluid dynamic mechanism underlying drag reduction by bristled wings and the impact of bristles on lift generated via clap and fling remain unclear. We used a dynamically scaled robotic model to examine the forces and flow structures generated during clap and fling of: three bristled wing pairs with varying inter-bristle spacing, and a geometrically equivalent solid wing pair. In contrast to the solid wing pair, reverse flow through the gaps between the bristles was observed throughout clap and fling, resulting in: (a) drag reduction; and (b) weaker and diffuse leading edge vortices that lowered lift. Shear layers were formed around the bristles when interacting bristled wing pairs underwent clap and fling motion. These shear layers lowered leakiness of flow through the bristles and minimized loss of lift in bristled wings. Compared to the solid wing, peak drag coefficients were reduced by 50–90% in bristled wings. In contrast, peak lift coefficients of bristled wings were only reduced by 35–60% from those of the solid wing. Our results suggest that the bristled wings can provide unique aerodynamic benefits via increasing lift to drag ratio during clap and fling for Re between 5 and 15.

AERODYNAMICS ASSESSMENT USING CFD FOR A LOW DRAG SHELL ECO-MARATHON CAR

Having a small car running with low power can be achieved by reducing the aerodynamics drag, rolling resistance and mechanical frictions between the moving parts. The Shell Eco-Marathon competition held around the world with events in Europe, USA and Asia shows every year new techniques and ideas to reduce the power needed to drive the car. The record of over 3400 km on the equivalent of a single litre of fuel is an indication of how car can run efficiently. The problem with these low drag cars is the driver perception about the shape of the car. Although the tear drop shape is known as having the minimum drag, practically this shape cannot be used due to size and packaging limitations in addition to the safety issue. In this work, a low drag concept car is proposed using initial CAD design. The concept car is examined using a commercial CFD software by simulating the airflow around car. The spatial distribution of the pressure and velocity vectors are utilized to improve the car shape and to achieve a low drag force coefficient while keeping the down force at its minimum value. By changing the car front, underneath and rear shapes, it was possible to reduce the drag coefficient from 0.430 for the baseline to 0.127 for the final design, while meeting the competition regulations.

Wind-Tunnel Investigation of the Aerodynamic Performance of Surface-Modification Cables

Abstract The wind-induced vibration of stay cables of cable-stayed bridges, which includes rain-wind-induced vibration (RWIV) and dry galloping (DG), has been studied for a considerable amount of time. In general, mechanical dampers or surface modification are applied to suppress the vibration. In particular, several types of surface-modification cable, including indentation, longitudinally parallel protuberance, helical fillet, and U-shaped grooving, have been developed. Recently, a new type of aerodynamically stable cable with spiral protuberances was developed. It was confirmed that the cable has a low drag force coefficient, like an indented cable, and that it prevented the formation of water rivulets on the cable surface. In this study, the stability for RWIV of this cable was investigated with various flow angles and protuberance dimensions in a wind-tunnel test. It was found that the spiral protuberance cable is aerodynamically stable against both RWIV and DG for all test wind angles. The effects of the protuberance dimensions were also clarified.

Computational Fluid Dynamics (CFD) Study on a Hybrid Airship Design

The aerodynamic lift and drag performance is one of the important considerations for hybrid airship configuration design. In conjunction with this, simulation study of aerodynamic characteristics can certainly benefit the process of deriving the best possible configuration for hybrid airship design. The aim of this study is to investigate the trend of aerodynamic lift and drag performance for an airship design in different velocities, altitudes and design fineness ratio using the Star CCM+ analysis tool. The airship model applied in this case study is an approximate model of the Atlant-100 airship. It is found that the airship model with low design fineness ratio typically generates much better aerodynamic lifting force in comparison to those with high design fineness ratio. On the other hand, while the range of estimated drag coefficient values is found to be rather insignificantly different, the presence of effects from the design fineness ratio is still evident. Generally, high design fineness ratio for the airship model seems to produce much lower drag force.

The Influence of Building Packing Densities on Flow Adjustment and City Breathability in Urban-like Geometries

City breathability refers to the air exchange process between the flows above and within urban canopy layers (UCL) and that of in-canopy flow, measuring the potential of wind to remove and dilute pollutants, heat and other scalars in a city. Bulk flow parameters such as in-canopy velocity (Uc) and exchange velocity (UE) have been applied to evaluate the city breathability. Both wind tunnel experiments and computational fluid dynamics (CFD) simulations were used to study the flow adjustment and the variation of city breathability through urban-like models with different building packing densities.We experimentally studied some 25-row and 15-column aligned cubic building arrays (the building width B=72mm and building heights H=B) in a closed-circuit boundary layer wind tunnel. Effect of building packing densities (λp=λf=0.11, 0.25, 0.44) on flow adjustment and drag force of each buildings were measured. Wind tunnel data show that wind speed decreases quickly through building arrays due to strong building drag. The first upstream building induces the strongest flow resistance. The flow adjustment length varies slightly with building packing densities. Larger building packing density produces lower drag force by individual buildings and attains smaller velocity in urban canopy layers, which causes weaker city breathability capacity.In CFD simulations, we performed seven test cases with various building packing densities of λp=λf=0.0625, 0.11, 0.25, 0.36, 0.44 and 0.56. In the cases of λp=λf=0.11, 0.25, 0.44, the simulated profiles of velocity and drag force agree with experiment data well. We computed Uc and UE, which represent horizontal and vertical ventilation capacity respectively. The inlet velocity at 2.5 times building height in the upstream free flow is defined as the reference velocity Uref. Results show that UE/Uref changes slightly (1.1% to 0.7%) but Uc/Uref significantly decreases from 0.4 to 0.1 as building packing densities rise from 0.0625 to 0.56. Although UE is induced by both mean flows and turbulent momentum flux across the top surface of urban canopy, vertical turbulent diffusion is found to contribute mostly to UE.

Numerical Study of Hydrodynamic Forces for AFM Operations in Liquid.

For advanced atomic force microscopy (AFM) investigation of chemical surface modifications or very soft organic sample surfaces, the AFM probe tip needs to be operated in a liquid environment because any attractive or repulsive forces influenced by the measurement environment could obscure molecular forces. Due to fluid properties, the mechanical behavior of the AFM cantilever is influenced by the hydrodynamic drag force due to viscous friction with the liquid. This study provides a numerical model based on computational fluid dynamics (CFD) and investigates the hydrodynamic drag forces for different cantilever geometries and varying fluid conditions for Peakforce Tapping (PFT) in liquids. The developed model was verified by comparing the predicted values with published results of other researchers and the findings confirmed that drag force dependence on tip speed is essentially linear in nature. We observed that triangular cantilever geometry provides significant lower drag forces than rectangular geometry and that short cantilever offers reduced flow resistance. The influence of different liquids such as ultrapure water or an ethanol-water mixture as well as a temperature induced variation of the drag force could be demonstrated. The acting forces are lowest in ultrapure water, whereas with increasing ethanol concentrations the drag forces increase.

Enhanced Nanoparticle Removal Using Surfactants

ABSTRACTNanoparticles are used in chemical mechanical planarization for semiconductor manufacturing as well as in other precision manufacturing operations. Particles used in processing need to be removed from surfaces in order to enhance yields. Nanoparticles are difficult to remove from surfaces during cleaning due to the high van der Waals attractive forces between particles and surfaces relative to the low fluid drag forces that are used for typical removal methods. Ionic surfactant molecules can adsorb on particles and surfaces to create an electrostatic repulsion between particles and surfaces as well as provide a steric barrier to mitigate adsorption and adhesion. The effectiveness of the surfactant in enhancing particle removal is related to surfactant properties, and it can be correlated with and modeled relative to the critical micelle concentration of the surfactant. The general approach for modeling will be discussed, and the model will be compared with particle removal data.

Rolling resistance and propulsion efficiency of manual and power-assisted wheelchairs

Rolling resistance is one of the main forces resisting wheelchair propulsion and thus affecting stress exerted on the upper limbs. The present study investigates the differences in rolling resistance, propulsion efficiency and energy expenditure required by the user during power-assisted and manual propulsion. Different tire pressures (50%, 75%, 100%) and two different levels of motor assistance were tested. Drag force, energy expenditure and propulsion efficiency were measured in 10 able-bodied individuals under different experimental settings on a treadmill. Results showed that drag force levels were significantly higher in the 50%, compared to the 75% and 100% inflation conditions. In terms of wheelchair type, the manual wheelchair displayed significantly lower drag force values than the power-assisted one. The use of extra-power-assisted wheelchair appeared to be significantly superior to conventional power-assisted and manual wheelchairs concerning both propulsion efficiency and energy expenditure required by the user. Overall, the results of the study suggest that the use of power-assisted wheelchair was more efficient and required less energy input by the user, depending on the motor assistance provided.

Interplanetary Propagation Behavior of the Fast Coronal Mass Ejection on 23 July 2012

The fast coronal mass ejection (CME) from 23 July 2012 raised attention due to its extremely short transit time from Sun to 1 AU of less than 21 h. In-situ data from STEREO-A revealed the arrival of a fast forward shock with a speed of more than 2200 km s$^{-1}$ followed by a magnetic structure moving with almost 1900 km s$^{-1}$. We investigate the propagation behavior of the CME shock and magnetic structure with the aim to reproduce the short transit time and high impact speed as derived from in-situ data. We carefully measure the 3D kinematics of the CME using the graduated cylindrical shell model, and obtain a maximum speed of 2580$\pm$280 km s$^{-1}$ for the CME shock and of 2270$\pm$420 km s$^{-1}$ for its magnetic structure. Based on the 3D kinematics, the drag-based model (DBM) reproduces the observational data reasonably well. To successfully simulate the CME shock, we find that the ambient flow speed should be of average value close to the slow solar wind speed (450 km s$^{-1}$), and the initial shock speed at a distance of 30 $R_{\odot}$ should not exceed $\approx$2300 km s$^{-1}$, otherwise it would arrive much too early at STEREO-A. The model results indicate that an extremely low aerodynamic drag force is exerted on the shock, smaller by one order of magnitude compared to the average. As a consequence, the CME hardly decelerates in interplanetary space and maintains its high initial speed. The low aerodynamic drag can only be reproduced when reducing the density of the ambient solar wind flow, in which the massive CME propagates, to $\rho_{\rm sw}$=1-2 cm$^{-3}$ at the distance of 1 AU. This result is consistent with the preconditioning of interplanetary space owing to a previous CME.

Coronal mass ejections: a driver of severe space weather

Coronal mass ejections: a driver of severe space weather

나선형 미세채널 내부에 형성되는 딘와류 이용한 미세입자 분리소자

This paper presents a microparticle separator using a spiral microchannel. A particle separator based on the dean vortex was designed, fabricated, and characterized. Two different spiral microchannels were fabricated. Width and initial radius of rotation in the spiral microchannel were fixed to <TEX>$300{\mu}m$</TEX> and 1.75 mm, respectively. Two different depths of the microchannels were designed at <TEX>$50{\mu}m$</TEX> and <TEX>$80{\mu}m$</TEX>. In this experimental study, the equilibrium position of microparticles was monitored by using fluorescent microbeads. In the case of a low dean number (<1.0), lift force and dean drag force were similar, indicating that microbeads were distributed to almost all areas across microchannels. However, in the case of a high dean number (>1.0), dean drag force rather than lift force was dominant, indicating that microbeads moved toward the inner wall of the spiral microchannel.

Formation of pre-coating dynamic membrane on mesh filter by cross-flow filtration of PAC–water suspension in a bioreactor: experimental and modeling

This study presents a closed-loop process for forming pre-coated dynamic membrane used in membrane bioreactor. Powder-activated carbon (PAC) particles are suspended by using axial-flow agitator whilst simultaneously deposited upon the filter medium by cross-flow filtration. The revolution speed of agitator is varied in the range of 150–450 rpm. Computational fluid dynamic is used to calculate velocity and shear rate adjacent to membrane surface. The force balance of the particles considers the driving force of dynamic membrane formation, drag force due to negative pressure-driven of suction pump, lift force due to mechanical mixing, and adhesive force due to particle–particle interactions. The effect of two major terms including drag and lift forces on the porosity of dynamic membrane is systematically investigated. The lower drag force results in the higher porosity of dynamic membrane. The results also show that the denser dynamic membrane is formed when the lift force becomes lower. The porosity of dynamic membrane is ranged from 0.38 to 0.46 while the thickness of dynamic membrane is altered between 100 and 500 μm. A dimensionless immobilized parameter is derived to predict the formation of pre-coating dynamic membrane. The results obtained from modeling show that dynamic membranes formed at higher values of immobilization parameter have a more cohesive structure. The results obtained from the experiments and the model reveal that the thickness of 400μm of PAC layer is considered as a stability threshold thereafter the structure of dynamic membrane becomes loose and unstable. The rejection capacity of dynamic membrane is also evaluated at different thicknesses by using Formazin solution. The results reveal that the turbidity of filtrate decreases with increasing the thickness of dynamic membrane. The results display that rejection capacity of pre-coating dynamic membrane is comparable to that of microfiltration membrane.

Modeling effects of interphase transport coefficients on biomass pyrolysis in fluidized beds

This study numerically characterizes the effects of interphase transport coefficients on the simulation of biomass pyrolysis in fluidized-bed reactors. Numerical modeling of sub-grid structures can affect the evolution of interphase transport coefficients and influence the predictive capability of coarse-grid computational fluid dynamics (CFD) models in simulating fluidized-bed reactors. In this study, a multi-fluid model that solves mass, momentum, energy and species conservations was coupled with chemical reactions to simulate a laboratory-scale biomass fast pyrolysis reactor. Different formulations of drag and heat transfer coefficients were employed. Comparisons between the simulated and experimental results show that the drag coefficient model considering detailed sub-grid structures predicted lower drag forces and performed better than the homogeneity-based drag correlation models. Lower drag forces on solid biomass particles resulted in lower solid biomass outflux, higher gas velocities, and shorter tar residence time, all resulting in higher tar yields. On the other hand, heat transfer correlations had less effect on the temperature distributions and final product yields. These findings indicate that when coarse-grid CFD is used to simulate biomass fast pyrolysis in fluidized-bed reactors, effects of sub-grid structures need to be taken into account in the formulations of drag coefficients.

Numerical Analysis of Turbulent Fluid Flow and Drag Coefficient for Optimizing the AUV Hull Design

Autonomous Underwater Vehicles (AUVs) are robots able to perform tasks without human intervention (remote operators). Research and development of this class of vehicles has growing, due to the excellent characteristics of the AUVs to operate in different situations. Therefore, this study aims to analyze turbulent single fluid flow over different geometric configurations of an AUV hull, in order to obtain test geometry that generates lower drag force, which reduces the energy consumption of the vehicle, thereby increasing their autonomy during operation. In the numerical analysis was used ANSYS-CFX&#174 11.0 software, which is a powerful tool for solving problems involving fluid mechanics. Results of the velocity (vectors and streamlines), pressure distribution and drag coefficient are showed and analyzed. Optimum hull geometry was found. Lastly, a relationship between the geometric parameters analyzed and the drag coefficient was obtained.

1187 EFFECT OF DIMPLE PATTERN ON THE FLOW CHARACTERISTICS OF AIRCRAFT WING

In order to obtain high efficiency aircraft wings and to improve the energy efficiency in the fields of eco-friendly transportation, the flow characteristics of the aircraft wing were studied with the change of lift-to-drag ratio(L/D) through the CFD analysis. The design process was focused on generating the high lift force and low drag force as the lift-to-drag ratio was increased. In this paper, various dimple patterns, in this case circular-shaped dimple pattern, circular-shaped dimple pattern having embossed conical structure in it, and the dimple pattern having the satellite dimple ahead of it, were numerically designed. Each dimple distance and its position were changed as the artificial conditions. The numerical analyses were conducted by using a commercial code, ANSYS CFX. Numerical result depending on the lift-to-drag ratio distribution was graphically depicted for various dimple patterns.

익형 표면의 딤플 형상변화에 따른 유동특성 연구

In order to have the high efficiency of aircraft wing and to improve the energy efficiency in field of eco-friendly transportation, the performance characteristics of the aircraft wing were studied with the change of lift to drag ratio through<BR>the CFD analysis. The design process was focused on generating the high lift force and low drag force as the lift to drag ratio was increased. In this paper, various dimple patterns were numerically designed to investigate the flow characteristics.<BR>Hexagon-and circle-shaped dimples, dimple distance and position were changed as the artificial conditions. The numerical analyses were conducted by using the commercial code, ANSYS CFX. Numerical results dependent on the turbulence intensity<BR>and lift to drag ratio distribution were graphically depicted for various dimple patterns.

Combined organic–inorganic fouling of forward osmosis hollow fiber membranes

This research focused on combined organic–inorganic fouling and cleaning studies of forward osmosis (FO) membranes. Various organic/inorganic model foulants such as sodium alginate, bovine serum albumin (BSA) and silica nanoparticles were applied to polyamide-polyethersulfone FO hollow fiber membranes fabricated in our laboratory. In order to understand all possible interactions, experiments were performed with a single foulant as well as combinations of foulants. Experimental results suggested that the degree of FO membrane fouling could be promoted by synergistic effect of organic foulants, the presence of divalent cations, low cross-flow velocity and high permeation drag force. The water flux of fouled FO hollow fibers could be fully restored by simple physical cleaning. It was also found that hydrodynamic regime played an important role in combined organic–inorganic fouling of FO membranes.