Finger Velocity Research Articles

The influence of diffusive convection on sedimentation from buoyant plumes

Diffusive convection driven by the differential diffusion of density altering fluid properties may enhance the scavenging of particles from natural buoyant plumes. For single-phase (fluid–solute–heat) systems this phenomenon has been extensively studied because salt fingering generated at the oceanic thermocline is a major mechanism of salt transport in the oceans. However, the influence of this process on particle laden plumes, for example, fluvial plumes in lakes and estuaries volcanic clouds and seafloor hydrothermal plumes is largely unknown. In this paper, we present direct experimental measurements of the interfacial particle flux at the plume base which can be applied to predict particle scavenging from natural buoyant plumes. Particle flux is measured using a light attenuation technique employing a chain of photodiodes which average concentration over a large number of fingers. The results are in good general agreement with earlier studies based on finger velocity. Flux measurements cover a wide range of conditions from those where diffusive convection dominates to those where settling and diffusive convection are of a similar magnitude. For very small particles double diffusive (salt finger) theory is applicable to two component particulate systems as suggested by earlier studies [Green, T., 1987. The importance of double diffusion to the settling of suspended material. Sedimentology 34, 319–331]. Two component diffusive convection theory is extended to three components in order to predict particle scavenging from marine fluvial plumes which involve the diffusion of sediment, salt and heat. For larger particles which settle significantly the flux can be approximated by adding the double-diffusive and settling fluxes. A theory to predict particle transport through the lower layer and sedimentation at the bed is developed based on the observation of strong convection below the plume. Application of our theory and experimental results indicates that while double diffusion may significantly influence the longitudinal distribution and vertical sorting of deposits from lacustrine plumes, the diffusive convection process is generally insignificant in marine plumes. Observational evidence for lacustrine double diffusion based on water column measurements is presented.

Failure of cerebellar patients to time finger opening precisely causes ball high-low inaccuracy in overarm throws.

We investigated the idea that the cerebellum is required for precise timing of fast skilled arm movements by studying one situation where timing precision is required, namely finger opening in overarm throwing. Specifically, we tested the hypothesis that in overarm throws made by cerebellar patients, ball high-low inaccuracy is due to disordered timing of finger opening. Six cerebellar patients and six matched control subjects were instructed to throw tennis balls at three different speeds from a seated position while angular positions in three dimensions of five arm segments were recorded at 1,000 Hz with the search-coil technique. Cerebellar patients threw more slowly than controls, were markedly less accurate, had more variable hand trajectories, and showed increased variability in the timing, amplitude, and velocity of finger opening. Ball high-low inaccuracy was not related to variability in the height or direction of the hand trajectory or to variability in finger amplitude or velocity. Instead, the cause was variable timing of finger opening and thereby ball release occurring on a flattened arc hand trajectory. The ranges of finger opening times and ball release times (timing windows) for 95% of the throws were on average four to five times longer for cerebellar patients; e.g., across subjects mean ball release timing windows for throws made under the medium-speed instruction were 11 ms for controls and 55 ms for cerebellar patients. This increased timing variability could not be explained by disorder in control of force at the fingers. Because finger opening in throwing is likely controlled by a central command, the results implicate the cerebellum in timing the central command that initiates finger opening in this fast skilled multijoint arm movement.

Centrifuge Modeling of Unstable Infiltration and Solute Transport

Unstable flow can result in the formation of fingers during infiltration into dry soil. Centrifuge modeling is a potentially useful tool to study the relationship between finger size, spacing, and velocity. It can also be used to investigate solute transport in such fingers. Physical properties of the fingers are obtained for three tests conducted at elevated acceleration levels. A fourth test was conducted at 1g. The physical parameters compare well with theoretical predictions. To assess solute transport in fingers, a known concentration of solute was introduced after the fingers had formed. The resulting breakthrough curves were analyzed using the two-region model as well as the advection dispersion equation with appropriate initial and boundary conditions. Although the two-region model is physically more plausible, it was found to match the extensive tailing observed in the breakthrough curves only marginally better than the advection-dispersion equation.

Effect of anisotropy on viscous fingering patterns of polymer solutions in linear Hele-Shaw cells.

Viscous fingering patterns of hydroxypropyl methyl cellulose (HPMC) solutions were investigated by forcing air in linear geometry isotropic and anisotropic Hele-Shaw cells as functions of HPMC concentration and injection pressure of air. The resulting patterns depended on the HPMC concentration and the cell anisotropy. The characteristic quantities of pattern growth, such as the finger velocity and the finger width, were evaluated. The finger velocity in the anisotropic linear cell was faster than that in the isotropic one, and the finger velocity and the finger width were well correlated with the pattern morphological changes. (c) 1999 American Institute of Physics.

Prediction of fingering in porous media

Immiscible displacement, involving two fluids in a porous medium, can be unstable and fingered under certain conditions. In this paper, the original linear instability criterion of Chuoke et al. [1959] is generalized, considering wettability of two immiscible fluids to the porous medium. This is then used to predict 24 specific flow and porous medium conditions for the onset of wetting front instability in the subsurface. Wetting front instability is shown to be a function of the driving fluid wettability to the medium, differences in density and viscosity of the fluids, the magnitude of the interfacial tension, and the direction of flow with respect to gravity. Scenarios of water and nonaqueous‐phase liquid infiltration into the vadose zone are examined to predict preferential flow and contamination of groundwater. The mechanisms of finger formation, propagation, and persistence in the vadose zone are reviewed, and the existing equations for calculating the size, the number and velocity of fingers are simplified for field applications. The analyses indicate that fingers initiate and propagate according to spatial and temporal distribution of the dynamic breakthrough (water‐ or air‐entry) pressures in the porous medium. The predicted finger size and velocity are in close agreement with the experimental results.

Finger Behavior of a Shear Thinning Fluid in a Hele-Shaw Cell

We make a theoretical study of the behavior of a simple fluid displacing a shear thinning fluid confined in a Hele-Shaw cell. To study the Saffman-Taylor instability when the displaced fluid is non Newtonian we face the problem of having a field which is non laplacian. By means of an hodographic transformation we are able to solve the problem in the case of weak shear thinning while taking into account the non laplacian character of the equation. Our results predict that the finger width decreases towards zero for small values of the surface tension parameter which is inversely proportional to the finger velocity.

Transfer function analysis of dynamic cerebral autoregulation in humans.

To test the hypothesis that spontaneous changes in cerebral blood flow are primarily induced by changes in arterial pressure and that cerebral autoregulation is a frequency-dependent phenomenon, we measured mean arterial pressure in the finger and mean blood flow velocity in the middle cerebral artery (VMCA) during supine rest and acute hypotension induced by thigh cuff deflation in 10 healthy subjects. Transfer function gain, phase, and coherence function between changes in arterial pressure and VMCA were estimated using the Welch method. The impulse response function, calculated as the inverse Fourier transform of this transfer function, enabled the calculation of transient changes in VMCA during acute hypotension, which was compared with the directly measured change in VMCA during thigh cuff deflation. Beat-to-beat changes in VMCA occurred simultaneously with changes in arterial pressure, and the autospectrum of VMCA showed characteristics similar to arterial pressure. Transfer gain increased substantially with increasing frequency from 0.07 to 0.20 Hz in association with a gradual decrease in phase. The coherence function was > 0.5 in the frequency range of 0.07-0.30 Hz and < 0.5 at < 0.07 Hz. Furthermore, the predicted change in VMCA was similar to the measured VMCA during thigh cuff deflation. These data suggest that spontaneous changes in VMCA that occur at the frequency range of 0.07-0.30 Hz are related strongly to changes in arterial pressure and, furthermore, that short-term regulation of cerebral blood flow in response to changes in arterial pressure can be modeled by a transfer function with the quality of a high-pass filter in the frequency range of 0.07-0.30 Hz.

Scaling unstable infiltration in the vadose zone

This technical note examines the feasibility of physically modelling unstable water flow in the form of fingers in the vadose zone. A scaling analysis is performed based on published expressions for the width and velocity of fingers, as well as the distance between fingers. Particular reference is given to the use of a geotechnical centrifuge as an experimental apparatus for modelling this phenomenon. Initial results are presented for 1g and 10g fingers from a preliminary experimental program undertaken to explore the analysis. These results agree with the scaling analysis presented and indicate that finger scaling using centrifugal modelling techniques is possible.

Closed‐form solution for finger width in sandy soils at different water contents

Wetting front instability and gravity‐driven fingers can occur during infiltration in sandy soil. These soils overlay many major aquifers. Finger width and velocity are important parameters in predicting the risk of groundwater pollution in these soils. In previous studies, closed‐form solutions were developed for air dry soil. In uniformly wet soils, where the fingers are much wider, predictive finger width equations have failed so far. In this paper we simplify further the Parlange‐Hill closed‐form solution for dry soil and show that, by taking hysteresis into account, it applies to fingers emanating from a point source in a sandy test soil at two widely different moisture contents.

Coordination between the transport and the grasp components during prehension movements.

In this study, the possible influence of the transport on the grasp component of prehension movements was investigated. The first phase of the transport (acceleration phase) and of the grasp (finger aperture phase) kinematics were studied under conditions of visual and non-visual object presentation (prehension experiment). In the non-visual condition, object size was estimated by haptics and object position was estimated by proprioception. Eight subjects were required to reach and grasp three objects of different size located at two distances. An additional experiment (matching experiment) was carried out to control the scaling of object size in two conditions. The results showed that in the matching experiment size estimation for large objects was similar in the two conditions, whereas small stimuli were underestimated in the haptic condition. In the prehension experiment, maximal finger aperture and velocity of finger aperture were greater in the non-visual than in the visual condition, and the difference was greater for small than for large stimuli. Moreover, in both conditions, finger opening was larger for prehension movements directed to the far than to the near objects, but only for smaller stimuli. Hand trajectory variability increased in the non-visual condition and with the distance, whereas finger opening variability was only affected by the non-visual condition. For smaller stimuli, increased finger opening with distance was positively correlated with the increase in wrist variability in the visual condition, but not in the non-visual condition. Furthermore, increased finger opening between visual and non-visual conditions was correlated with the increase in wrist variability, for smaller objects at the near object location.(ABSTRACT TRUNCATED AT 250 WORDS)

The characteristics of arm movements executed in unusual force environments.

Human subjects pointed at stationary visual targets without sight of their arm while the force environment was varied by applying weight or spring loads to the hand. The path travelled by the finger, pointing accuracy, and the shape of the finger velocity profile remained invariant across all force environments after a single practice trial. However, the magnitude and duration of the velocity profile depended consistently on the presence and size of a weight load. In contrast, velocity was not affected by spring loads. An analysis of movement dynamics in our study indicated that inertial and gravitational load components were compensated by separate mechanisms, the former employing time- and the latter magnitude scaling of muscle force profiles. The presence of such separate mechanisms led us to predict little problems for movement dynamics in weightlessness, which was indeed confirmed in a study on pointing movements aboard the KC-135 aircraft.

Wetting front instability in unsaturated porous media: A three-dimensional study in initially dry sand

Three-dimensional fingers caused by gravity driven, wetting front instability in unsaturated porous media were explored through laboratory experimentation. Two sets of experiments were conducted using initially dry sand in large 30 cm diameter columns to guide analytical development for finger velocity and diameter. The first set consisted of ponding water on a two-layer sand system with a fine sand overlying a coarse sand. Here, a complicated pattern of interaction among fingers was found to occur. In the second set, water was applied directly to the coarse layer as 2 cm diameter area sources, enabling systematic study of individual finger structure. Based on dimensional analysis and the experimental results, general relationships were found for finger velocity and cross-sectional area as a function of both the flux through the finger and hydraulic properties of the coarse layer represented by the sorptivity, saturated conductivity, and initial and saturated moisture contents. Unlike the two-dimensional case, air entrapment was an important factor in explaining the experimental results.

Wetting front instability: 1. Theoretical discussion and dimensional analysis

Highly nonuniform moisture and solute transport in sandy soils has long been noted. A mechanism for nonuniform transport is an instability in the infiltration flow itself, or wetting front instability. When a wetting front becomes unstable, “fingers” form and move down through the vadose zone to the phreatic surface bypassing much of the unsaturated porous medium. In this paper a theoretical framework for wetting‐front‐instability experimentation is developed through classical dimensional analysis. Relationships between system parameters and initial/boundary conditions and unstable flow field behavior, as characterized by finger width and finger velocity, are derived. The scaling theory of E. E. Miller and R. D. Miller (1956) is used to generalize these relationships to similar porous media with different mean grain sizes and show the effects of coarseness and fluid properties. Other formulations for finger width, derived through linear stability analysis, are examined to arrive at possible functional forms not yielded by dimensional analysis.

Coordination of three-joint digit movements for rapid finger-thumb grasp.

Human thumb and index finger kinematics were examined for multiple repetitions of a simple grasp task as a means to evaluate motor planning and execution of these important hand movements. Subjects generated a rapid (approximately 90-ms duration) pinch movement of the index finger and thumb from an open-hand position. Approximately 400 repetitions were obtained from four naive subjects. The two-dimensional trajectory of the fingertip and the angular positions of the metacarpophalangeal (MP) and proximal interphalangeal (PIP) joints of the index finger were recorded along with the angular position of the thumb interphalangeal joint (TH). Individual joint angular positions were transduced using planar electrogoniometers of an exoskeletal linkage design. Except for consistent single-peaked joint angle and digit trajectory velocity profiles, most kinematic features of the grasp varied considerably across trials, including fingertip spatial position at contact, specific finger paths, finger and thumb path distances, finger and thumb peak tangential velocities, and 5) individual joint rotation magnitudes and peak velocities. However, this kinematic variability was not random. Variable TH angular positioning was paralleled by complementary two-dimensional variations in the finger path. These fingertip adjustments were accomplished by actively controlled, reciprocal angular positioning of the MP and PIP joints. Specifically, with natural reductions in thumb flexion, MP flexion was greater while PIP flexion was reduced and vice versa. These adjustments acted to minimize variations in the point contact of the finger on the thumb and yielded a robust and seemingly natural preference for finger-thumb contact at the more distal surfaces of the digits. The kinematic variability was not due to the finger and thumb movements being controlled independently of digit contact. The variable appositional movements of the finger and thumb and the associated contact force were generated as a single action. This was indicated by an absence of kinematic or force adjustments after contact, smooth digit trajectories with a single peak in their tangential velocities, and finger-thumb contact that consistently occurred well after peak velocity. Likewise, because the variability in the kinematics of the grasp was systematic, it apparently was not due simply to sloppiness or noise in motor execution.(ABSTRACT TRUNCATED AT 400 WORDS)

The spectral signature of salt fingers

The spectra of either temperature, salinity or vertical velocity which might be measured by a sensor moving through a field of salt fingers are considered. A theoretical calculation is presented which attempts to incorporate the deviations from squareness and perfect orientation of the salt fingers. The resulting spectra rise to a peak close to ω = (√2) kv and decrease rapidly for larger ω, where ω is the temporal frequency of the spectrum, k the dominant wave number of the salt fingers and ν the horizontal velocity of the sensor. A spectrum of vertical velocity of laboratory-generated salt fingers is shown to be in close agreement with the theoretical prediction.

On the structure of salt fingers

The structure of fingers in heat-solute system is investigated in the laboratory. In particular, the velocity of the fluid in the fingers and the horizontal and vertical length scales of the fingers, which form at the interface between two layers with the upper layer having an excess of the heat and solute but still less dense than the lower layer, are measured and related to the temperature and solute concentration differences across the interface. Dimensional analysis is used to predict the way the details of the finger motions depend upon the properties of the well-mixed layers on either side of the interface. The experimental results are found to be in agreement with these predictions. The laboratory data are then extrapolated in order to provide estimates for the size and velocity of salt fingers in the ocean, using data appropriate to the layered microstructure found beneath the Mediterranean outflow by Howe and Tait (1970).

Leaching of a Surface Layer of Sodium Chloride into Tile Drains in a Sand‐Tank Model

AbstractA series of experiments was conducted to study the leaching of NaCl through tile drains from a surface layer of salinized sand in a sand‐tank model. The effects of two recharge rates, three salinities in the salted‐sand layer, and three drain spacings were evaluated. Each experiment was conducted with a steady‐state, arch‐shaped water table maintained by recharge from a battery of capillary tubes that gave a uniform infiltration over the model. All the salt was leached from the model. The salt was dyed blue so that the leaching front could be photographed. The salinity of the tank solution and of the drainage water was measured at regular intervals. Intense fingering developed in all experiments in which a salinized layer was used. Fingering caused a rapid and erratic redistribution of salt within the model and an erratic variation in the salinity of the drainage water until about 1 pore volume had drained. The higher the initial sand salinity, the more intense was the fingering, the longer were the fingers, and the higher were the finger velocities. The behavior of the fingering transition zone was controlled by the interaction between the density gradients and the hydraulic gradient; this latter depends on the recharge rate and the drain spacing. The salinity of the outflow drainage water rose quickly to a peak and then declined almost as quickly for a short period, after which it reduced more gradually. After about 1 pore volume had drained, the salinity of the drainage water declined exponentially in all salinized experiments. Experimental curves fitted closely to an exponential relationship except initially and where fingering resulted in erratic variations.

The tapping velocity of fingers in forest workers with Raynaud's phenomenon

The tapping velocity of fingers in forest workers with Raynaud's phenomenon