Fluid Drag Force Research Articles

Distorted Froude‐scaled flume analysis of large woody debris

AbstractThis paper presents the results of a movable‐boundary, distorted, Froude‐scaled hydraulic model based on Abiaca Creek, a sand‐bedded channel in northern Mississippi. The model was used to examine the geomorphic and hydraulic impact of simplified large woody debris (LWD) elements. The theory of physical scale models is discussed and the method used to construct the LWD test channel is developed. The channel model had bed and banks moulded from 0·8 mm sand, and flow conditions were just below the threshold of motion so that any sediment transport and channel adjustment were the result of the debris element. Dimensions and positions of LWD elements were determined using a debris jam classification model. Elements were attached to a dynamometer to measure element drag forces, and channel adjustment was determined through detailed topographic surveys.The fluid drag force on the elements decreased asymptotically over time as the channel boundary eroded around the elements due to locally increased boundary shear stress. Total time for geomorphic adjustment computed for the prototype channel at the Q2 discharge (discharge occurring once every two years on average) was as short as 45 hours. The size, depth and position of scour holes, bank erosion and bars created by flow acceleration past the elements were found to be related to element length and position within the channel cross‐section. Morphologies created by each debris element in the model channel were comparable with similar jams observed in the prototype channel. Published in 2001 John Wiley & Sons, Ltd.

A model for strain amplification in the actin cytoskeleton of osteocytes due to fluid drag on pericellular matrix

A model is presented that provides a resolution to a fundamental paradox in bone physiology, namely, that the strains applied to whole bone (i.e., tissue level strains) are much smaller (0.04–0.3 percent) than the strains (1–10 percent) that are necessary to cause bone signaling in deformed cell cultures (Rubin and Lanyon, J. Bone Joint Surg. 66A (1984) 397–410; Fritton et al., J. Biomech. 33 (2000) 317–325). The effect of fluid drag forces on the pericellular matrix (PM), its coupling to the intracellular actin cytoskeleton (IAC) and the strain amplification that results from this coupling are examined for the first time. The model leads to two predictions, which could fundamentally change existing views. First, for the loading range 1–20 MPa and frequency range 1–20 Hz, it is, indeed, possible to produce cellular level strains in bone that are up to 100 fold greater than normal tissue level strains (0.04–0.3 percent). Thus, the strain in the cell process membrane due to the loading can be of the same order as the in vitro strains measured in cell culture studies where intracellular biochemical responses are observed for cells on stretched elastic substrates. Second, it demonstrates that in any cellular system, where cells are subject to fluid flow and tethered to more rigid supporting structures, the tensile forces on the cell due to the drag forces on the tethering fibers may be many times greater than the fluid shear force on the cell membrane.

Discrete particle model for sheet flow sediment transport in the nearshore

Fully three‐dimensional discrete particle computer simulations of high‐concentration sheet flow transport in oscillatory flows quantify the effect of fluid acceleration on bed load transport in highly unsteady flows typical of nearshore marine environments. A simple impulse‐momentum approach explains simulation results and forms the basis for adding an acceleration‐related term to widely used energetics sediment transport formulae. Transport predicted by the acceleration term becomes increasingly significant as wave shape approaches the sawtooth profile characteristic of surf zone bores. Simulations integrate F = ma and a corresponding set of equations for the torques for each sphere. Normal and tangential forces between contacting particles are linear functions of the distance between sphere centers and the relative tangential displacement at the contact point, respectively; particle interactions are both inelastic and frictional. Pressure gradient forces generated by the passage of surface gravity waves drive fluid and particle motion in a stack of thin horizontal fluid layers that exchange momentum and exert fluid drag, added mass, and buoyancy forces on particles. Transport properties of the simulated granular‐fluid assemblage are robust to large variations in material properties of the particles. Simulated transport rates agree with available experimental data for unsteady transport of coarse sands; the mode of bed load motion, dispersion of bed load particles, and particle segregation by size and density are qualitatively consistent with available particle‐scale observations of bed load transport of natural particles.

On the interaction between ultrasound and contrast agents during Doppler investigations

Knowledge of interaction mechanisms between ultrasound (US) and contrast agents (CA) suspended in blood is important for a correct interpretation of clinical investigation results. Experiments performed in different laboratories have shown that, as a consequence of primary radiation force, CA tend to move away from the US transducer. Accordingly, Doppler spectra produced by particles suspended in moving water turn out to be significantly altered from what is theoretically expected. The purpose of this paper is twofold. First, an original model describing the bubble dynamics as the outcome of the balance between US radiation force and fluid drag force is validated for the case in which bubbles are suspended in blood. The high fluid viscosity is shown to prevent significant bubble deviations from the unperturbed fluid streamlines so that, in large vessels, a residual spectral distortion may exist only at the highest intensity levels permitted by current regulations. Finally, the relative importance and differences between the effect of primary radiation force and streaming mechanisms that, in principle, could lead to similar effects, are discussed. (E-mail: ptortoli@det.unifi.it)

Fluid drag, ball interaction and gyroscopic effects in magnetic fluid grinding

Experimental studies of the magnetic fluid grinding process have previously established that it is an abrasive grinding process. Sliding between the material being ground (for example, a ceramic ball being finished) and the grinding surface occurs because of viscous drag forces and torques acting on the ground material as it is driven through a magnetic liquid. Mechanical models of the process have qualitatively predicted trends in grinding rates resulting from changes in operating conditions, but have not been quantitatively accurate. This paper establishes that the reason is inaccurate modelling, in the previous work, of the fluid drag forces and torques. Improved modelling now gives accurate prediction of the motions in a grinding cell. Interactions between balls travelling in a train and gyroscopic effects are also reported on; they are of secondary importance in influencing slip magnitude at the grinding surface but do provide mechanisms for randomizing ball motions.

Prediction of fluidized bed behaviour in the presence of liquid bridges

The need to understand defluidization due to free surface liquid in gas–solid fluidized bed processes gave rise to a study of the effect of addition of non-volatile liquids with different viscosity and surface tension to a cold model gas–solid fluidized bed. The incremental addition of liquid to a fluidized bed of Geldart group B particles was observed, in general, to result in a transition to group A behaviour and eventually to group C behaviour. The BA and AC boundaries are shown to be associated with the balance between the interparticle liquid bridge forces and the fluid drag force. Using liquid bridge forces calculated from measurements of bridge geometry, the ratios of interparticle force to fluid drag force were found to vary between 0.6 and 1.07 for the AC boundary and between 0.02 and 0.06 for the BA transition boundary. Based on this approach, a practical diagram is developed enabling fluid bed behaviour to be predicted from a knowledge of particle, fluid and liquid properties and the quantity of liquid in the bed.

Doppler spectra from contrast agents crossing an ultrasound field

When contrast agents are injected in a fluid, it is implicitly assumed that they move at the same velocity as the fluid itself. However, a series of in vitro tests performed by using air-filled microbubbles suspended in distilled water, have shown that the Doppler spectrum generated in this case may be notably different from that obtained from non-resonating scatterers. In this paper, we show, through a simple simulation model, that the actual movement of microbubbles may be predicted as the result of the complex balance between two forces: the ultrasound radiation force, which tends to move the particles along the sound beam direction, and the fluid drag force, which tends to move the particles along the fluid stream. The contrast agents turn out to be displaced only during the passage of the ultrasound burst; during the remaining time, they are maintained at the fluid velocity by the drag force. Based on the total particle displacement estimated between consecutive pulses, a series of Doppler spectra corresponding to different intensity levels was computed. This series was shown to be in excellent agreement with the experimental spectra obtained in vitro using Levovist (Schering AG, Berlin, Germany) particles suspended in distilled water flowing at a steady rate.

Numerical simulation of the gas–solid flow in a bed with lateral gas blasting

This paper presents a numerical study of the gas–solid flow in a bed by a Combined Continuum and Discrete Model (CCDM). Numerical experiments are carried out to simulate the motion of 10,000 spherical particles of 4 mm diameter caused by lateral gas blasting into a bed with its thickness equal to the diameter of particles. It is shown that, depending on the gas velocity, the bed can transform from a fixed bed to a fluidised bed or vice versa. Two zones can be identified in such a bed: a stagnant zone in which particles remain in their initial positions, and a mobile zone in which particles can move in various flow patterns. If the gas velocity is in a certain range, the mobile zone is confined in front of the gas inlet, forming the so-called raceway in which particles can circulate. If the gas velocity is higher than a critical value, fluidisation results, with the mobile zone growing by the combined effect of bubble penetration and shearing between moving and static particles until a stable state where the boundary separating the mobile and stagnant zones is unchanged. The dependence of raceway and fluidisation phenomena on gas velocity has been examined in terms of the size and shape of the mobile zone, gas-solid flow patterns and forces acting on individual particles. It is found that large interparticle forces occur along the boundary between the mobile and stagnant zones, whereas large fluid drag forces occur at the roof of a raceway or bubble. The predictions of transition between the static and dynamic states, and the complicated hysteretic behaviour in terms of either bed pressure drop or raceway size are in good agreement with the experimental observations.

A numerical model for simulating mechanical behavior of flexible fibers

A numerical method is developed for simulating the mechanical behavior of flexible fibers. A circular crossed fiber is represented by a number of cylindrical segments linked by spring dash-pot systems. Segments are lined up and bonded to each neighbor. Each bond can be stretched or compressed by changing the bond distance. Bending deflection and twist movement occur, respectively, in the bending and torsion planes. While the bending angle is determined by the positions of two neighboring bonds, a reference twist vector is introduced to record the torsion motion along the segment chain. Fluid drag forces are calculated based on the Stokes’ Law, where a free draining assumption is made. The motion of the fiber is determined by solving the translational and rotational equations of individual segments. Computer simulation has been conducted to verify the single fiber model with elastic theory and excellent agreements have been found between the simulation results and the theory in various situations such as beam deflection under static loads, vibrating cantilevers, and dynamics of helical shaped fibers. Examining orientations of rigid fibers in a viscous shear flow, simulation results suggest that the rotational time is sensitive to the fluid drag torque which is related to the shape of the segments. For highly flexible fibers, the effect of bending deformation on the period of rotation and the rotation orbits is also investigated. This numerical model for single flexible fibers linked by discrete segments provides a framework in the future studies on fibrous assemblies.

A method for simulating magnetically driven arcs in a gas

This paper presents a new, practical method for simulating the behavior and shape of arcs driven by magnetic force in a gas. This simulation method is very useful in designing electric devices that use the magnetically driven arc technique. First, we describe the arc model. In this model, the arc is assumed to be a chain of small rigid cylindrical current elements. Each element receives a Lorentz force from the magnetic field and a fluid drag force from the surrounding gas. Owing to the similarity between an arc and a rigid column, the drag force of the arc is expressed by that of the column. The element moves with a velocity determined by these forces. Consequently, we can obtain the behavior and the shape of the whole arc. Finally, we offer a simulation example. We have obtained an arc that is given a spiral shape by the magnetic field. This simulation result illustrates observations of the arc. © 1998 Scripta Technica, Electr Eng Jpn, 123(2): 24–31, 1998

Fundamental characteristics of wear particle deposition measurement by an improved on-line ferrographic analyzer

An improved on-line ferrographic analyzer and its specific characteristics are presented. This analyzer is very convenient for detecting wear debris in a transparent oil pipe of a machine by using an electromagnet and a photoelectric sensor with a microprocessor. To eliminate the detection error of background colour of the oil sample, the wear index is redefined and can be directly calculated by the microprocessor in this analyzer. This detected value can represent the covered area rate of deposited wear debris and is able to present accurately the debris concentration. Results show that the wear index is slightly decreased by higher viscosity, and that the effects of a flow rate of the oil sample lower than 2 ml min −1 on the wear index are not obvious. A simple theory, accounting for the balance between the magnetic and fluid drag forces on the wear particles, is presented and shows very good agreement with experimental results.

Nernst effect, seebeck effect, and vortex dynamics in the mixed state of superconductors

We demonstrate that in the presence of pinning a simple relation exists between Nernst and Seebeck coefficients and the resistivity tensor, based on the vortex equation of motion and the two-fluid model. Thus the combination of the electric and thermoelectric transport experiments can be used to test the basic models for the vortex dynamics in superconductors. Then we show how two different vortex dynamics models can be subjected to these tests. We find that the vortex dynamics model without various normal fluid drag forces is consistent with those experiments, and that the alternative model with those drag forces is not.

A Method of Simulating Magnetically Driven Arcs

This paper presents a new, practical method of simulating the behavior of arcs driven by magnetic force in a gas. This simulation method is very useful in designing electric devices using magnetically driven arc technique. First, the authors describe the arc model. In this model, the arc is assumed to be a chain of small rigid cylindrical current elements. Each element receives Lorentz`s force from the magnetic field and a fluid drag force from the surrounding gas. Owing to the similarity between an arc and a rigid column, the drag force of the arc is expressed by that of the column. The element moves with the velocity determined by these forces. Consequently, the authors can obtain the behavior and the shape of the whole arc. Finally, simulation example is shown.

Reduction in Added Fluid Torque for Aqua-Robot Arm.

The motion of a robot arm subrerged in water is affected by added mass, fluid drag force and flow-induced vibration. The study, thus, deals with a technique for reducing these fluid influences on arm motion by ejecting air bubbles from the surface of the arm. Experiments are carried out using a two-degree-of freedom and two-joint manipulator. By ejecting air bubbles, we show experimentally a reduction of added fluid torque by examining the effects of arm acceleration, air ejecting pressure, and pattern on torque reduction. The following major results are obtained:(1)added fluid force to the robot arm acceleration in water is reduced by a certain pattern of air bubble ejection and air ejecting pressure and the reduction reaches a maximum of about 45% and (2)the relationship of added fluid torque reduction with air ejection pattern, air ejecting pressure or arm acceralation is experimentally clarified.

A method of simulating magnetically driven arcs [in switchgear

This paper presents a new, practical method of simulating the behavior of arcs driven by magnetic forces in a gas. This simulation method is very useful in designing electric devices using magnetically driven arc techniques. First, the authors describe the arc model. In this model, the arc is assumed to be a chain of small rigid cylindrical current elements. Each element receives Lorentz's force from the magnetic field and a fluid drag force from the surrounding gas. Owing to the similarity between an arc and a rigid column, the drag force of the arc is expressed by that of the column. The element moves with the velocity determined by these forces. Consequently, one can obtain the behavior and the shape of the whole arc. Finally, a simulation example is shown.

Neural Network Control for Fluid Drag Force Reduction in an Aqua-Robot Arm by Ejecting Air Bubbles.

This paper deals with experimental examination of the feasibility of a neural network control scheme for reducing fluid drag force, which is developed to select the patterns of air-bubble injection from the surface of the 2 link-2 degree of freedom aqua-robot manipulator arm to result in reducing the fluid drag force acting on the manipulator arm. Two neural networks are implemented in the control system of the manipulator immersed in water ; one is for the inverse dynamics of the manipulator between the state variables and fluid drag force, and the other is for selection of an air-bubble injection pattern to minimize the fluid drag force in accordance with the manipulator motion. This neural network control scheme achieves a 30% reduction in fluid drag force for a high-speed motion during manipulator arm operation.

Hydrodynamics of Fractal Aggregates with Radially Varying Permeability

Hydrodynamic properties of fractal aggregates with radially varying permeability are investigated in terms of two parameters: radius of equivalent solid sphere experiencing the same drag as the aggregate (Ω*radius of aggregate) and fluid collection efficiency (η) of the aggregate. Resistance to the fluid flow through the aggregate is predicted to increase with increasing fractal dimension, while the fluid collection efficiency is expected to decrease. It is shown that a reasonable estimate of the fluid drag force on a fractal aggregate may be obtained by assigning a constant volume-averaged porosity to the aggregate and using any of the expressions available in the literature for aggregates with uniform permeability. The two hydrodynamic parameters, Ω and η, are used to modify the existing expressions for interactions between solid spheres to account for the porous nature of aggregates and thus calculate the collision rate kernels for interacting aggregates. The ratio of hydrodynamic radius of an aggregate to its radius of gyration predicted by the proposed model was in reasonable agreement with an experimental value reported in the literature.

The motion of sediment‐water mixtures during intense bedload transport: computer simulations

ABSTRACTA new model, which couples fluid and particle dynamics, has been developed to study the motion of the sediment‐water mixture during intense bedload transport, including the velocity profiles of both sediment and water, the roughness length of an upper plane bed and the thickness of moving sediment layers. Standard mixing length theory is used to model the motion of water above the boundary between the overlying water and the sediment‐water mixture. The turbulent flow within the moving sediment layers is described by a shear stress model, in which the effective viscosity of the flowing water is proportional to the velocity difference between the fluid and the sediment. The particle dynamics method, in which the equations of motion of each of many particles are solved directly, is applied to model the movement of sediment particles. The particle‐fluid interaction is expressed by a velocity‐squared fluid drag force exerted on each sediment particle. Both computer simulation results and theoretical analysis have shown that the velocities of both sediment and fluid during intense sediment transport decrease exponentially with depth in the top layers of a fast‐moving sediment—water mixture. The thickness of the moving sediment layers, obtained from the computer simulation results, is proportional to the shear stress, which agrees with previous experimental observations.

Dusty Plasma Studies in the Gaseous Electronics Conference Reference Cell.

Particle “dust” in processing plasmas is of critical concern to the semiconductor industry because of the threat particles pose to device yield. A number of important investigations into the formation, growth, charging, transport and consequences of particulate dust in plasmas have been made using the Gaseous Electronics Conference Reference Cell as the reactor test-bed. The greatest amount of work to date has been directed toward a better understanding of the role that electrostatic, ion drag, neutral fluid drag and gravitational forces play in governing the dynamic behavior of particle cloud motion. This has been accomplished by using laser light scattering (LLS) techniques to track the motion of suspended particle clouds in rf discharges. Also, statistical correlation’s in the fluctuation of scattered laser light intensity [dynamic laser light scattering (DLLS)] can be used to determine information about particle size, motion, and growth dynamics. These results are reviewed, along with recent work demonstrating that charged dust particles in a plasma can form a strongly coupled Coulomb liquid or solid. New results from DLSS experiments performed in the Reference Cell are presented that show process-induced dust particles confined in an electrostatic trap exhibit low-frequency oscillatory motion consistent with charge density wave (CDW) motion predicted for strongly coupled Coulomb liquids.

Comparison of Experimental and Simulated Grain Flows

Fully three-dimensional computer simulations of identical spheres flowing in an inclined glass-walled channel only slightly wider than a particle diameter successfully reproduce profiles of mean velocity, bulk density, and particle rotations as well as profiles of fluctuating quantities measured from high-speed motion pictures of physical experiments. All simulation parameters are measured experimentally. Both full simulations of the geometry of the physical experiments and simulations using periodic boundary conditions in the downchute direction are used to gather micromechanical information. For these collision-dominated flows, quantitative predictions of the simulations are relatively insensitive to details of the particle-interaction model and particularly the particle stiffness, but are relatively sensitive to extraneous fluid drag forces and the chute geometry.