Fluctuating Pressure Gradients Research Articles

Nearshore sandbar migration predicted by an eddy‐diffusive boundary layer model

We simulated the erosion and accretion of a natural beach using a wave‐resolving eddy‐diffusive model of water and suspended sediment motion in the bottom boundary layer. Nonlinear advection was included in this one‐dimensional (vertical profile) model by assuming that waves propagated almost without change of form. Flows were forced by fluctuating pressure gradients chosen to reproduce the velocity time series measured during the Duck94 field experiment. The cross‐shore flux of suspended sediment beneath each field‐deployed current meter was estimated, and beach erosion (accretion) was calculated from the divergence (convergence) of this flux. Horizontal pressure forces on sediment particles were neglected. The model successfully predicted two bar migration events (one shoreward bar migration and one seaward) but failed to predict a third (seaward migration) event. Simulated seaward sediment transport was due to seaward mean currents. Simulated shoreward sediment transport was due to covariance between wave‐frequency fluctuations in velocity and sediment concentration and was mostly confined to the wave boundary layer. Predicted seaward (shoreward) bar migration was driven by a maximum in the current‐generated (wave‐generated) flux over the sandbar. A wave‐generated downward flux of shoreward momentum into the wave boundary layer contributed to shoreward sediment transport and often had a local maximum over the bar crest. Second‐order nonlinear advection of sediment, mostly representing shoreward advection by the Stokes drift, also often had a local maximum over the bar crest. Together, wave‐generated momentum fluxes and the Stokes drift substantially increased shoreward transport and were essential to predictions of shoreward bar migration.

1602 傾斜管内対向二相流の流動特性

The flow characteristics in counter-current flow in inclined pipe were examined experimentally. The classification of flow characteristics was done by visual observation, pressure gradient and liquid hold-up to characterize the flow regime. As a result, it was found that (1) the pressure gradient fluctuation was the biggest in churn flow, while it was almost constant in stratified flow, (2) the liquid hold-up at the onset of flooding agrees with the Lockhart and Martinelli correlation for the inner diameter 16mm. Furthermore, an experimental correlation to predict the frictional factor multiplier at the onset of flooding was proposed.

Wave Enhancement of Diffusivities within Surficial Sediments

The enhancement of solute diffusivities within coastal surficial sediments as a result of wave action is examined. Fluctuating pressure gradients associated with passing waves cause interstitial water motions leading to enhanced diffusivities through the mechanism of shear dispersion. Wave amplification of diffusivities is likely to be greatest for waves of period ∼10 s, in shallow water, over a bottom of coarse grain. Diffusivity enhancement of hundreds of times molecular diffusivity is achievable. The mechanism is distinct from, but complements, the mechanism of rotational dispersion which has been previously described. Other mechanisms that enhance solute transport within surficial sediments rely on the interaction between wave-driven or steady flow over bottom obstructions such as biogenic structures or sand ripples. It is suggested that while the resulting advective flows may dominate solute transport within the top few 10 s of centimeters of the sediment column, shear dispersion may be more important deeper within the sediment. In any event, in contrast to these other mechanisms, shear dispersion is operative even when the seabed is flat. Application of the theory to sediments of the South Atlantic Bight would suggest that shear dispersion is capable of explaining a major part of the interstitial transport inferred from measurements.

Dissimilarity between the velocity and temperature fields in a perturbed turbulent thermal boundary layer

Dissimilarity between the streamwise velocity and temperature fields is investigated in a perturbed turbulent thermal boundary layer using direct numerical simulation. Perturbations are provided into a turbulent boundary layer by uniform blowing or suction from a spanwise slot. The skin-friction coefficient and Stanton number are significantly changed due to blowing and suction. The streamwise evolution of the Stanton number is very different from that of the skin-friction coefficient because the pressure gradient induced by blowing or suction plays an important role in the momentum transport. Before the slot, the mean streamwise velocity changes smoothly, as compared to the mean temperature, due to the pressure gradient, and a similar behavior is also observed in the variations of the streamwise velocity and temperature fluctuations. Above the slot, the change in the mean streamwise velocity is smaller than that in the mean temperature, and after the slot the mean streamwise velocity recovers slowly to the unperturbed flow as compared to the mean temperature. By investigating each term in the mean streamwise velocity and temperature budgets, it is concluded that the mean pressure gradient is the main source of dissimilarity between the mean streamwise velocity and temperature fields. The budgets for the variances of the streamwise velocity and temperature fluctuations show that the difference between the production terms in both budget equations, which results from the existence of the mean pressure gradient, is the primary source of dissimilarity between the streamwise velocity and temperature fields above the slot, whereas downstream of the slot, the velocity pressure-gradient term (i.e., pressure gradient fluctuation) is another important source of dissimilarity. Dissimilarity between the streamwise velocity and temperature fluctuations caused by blowing persists longer in the streamwise direction than that by suction.

Fluid particle accelerations in fully developed turbulence.

The motion of fluid particles as they are pushed along erratic trajectories by fluctuating pressure gradients is fundamental to transport and mixing in turbulence. It is essential in cloud formation and atmospheric transport, processes in stirred chemical reactors and combustion systems, and in the industrial production of nanoparticles. The concept of particle trajectories has been used successfully to describe mixing and transport in turbulence, but issues of fundamental importance remain unresolved. One such issue is the Heisenberg-Yaglom prediction of fluid particle accelerations, based on the 1941 scaling theory of Kolmogorov. Here we report acceleration measurements using a detector adapted from high-energy physics to track particles in a laboratory water flow at Reynolds numbers up to 63,000. We find that, within experimental errors, Kolmogorov scaling of the acceleration variance is attained at high Reynolds numbers. Our data indicate that the acceleration is an extremely intermittent variable--particles are observed with accelerations of up to 1,500 times the acceleration of gravity (equivalent to 40 times the root mean square acceleration). We find that the acceleration data reflect the anisotropy of the large-scale flow at all Reynolds numbers studied.

Studies of wispy-annular flow using transient pressure gradient and optical measurements

Measurements of the characteristics of annular two-phase flow at high mass fluxes are reported which confirm the existence and demonstrate the behaviour of core structures (wisps) in such flows. Measurements were made of pressure gradient fluctuations and of optical obscuration. These demonstrated that the core structures had frequencies typically of the order of 5–8 Hz and that they travelled at velocities approaching those of the gas.

Monitoring a shelf edge current using bottom pressures or coastal sea-level data

Low-frequency fluctuations in ocean currents can be monitored by their effects on water level and bottom pressure gradients due to geostrophy. A predominantly barotropic current along the Norwegian continental slope is studied to show how routine monitoring of coastal sea-level, air pressure and coastal hydrography can be used to monitor fluctuations of an important source of salt and heat to arctic regions. These data are also needed as forcing boundary conditions for running models of the adjacent shelf seas. The first attempt showed a promising validation to monthly average transports measured 1300 km upstream. The evidence suggests that changes in the slope current are established on the time scale of a few days along the entire continental slope, apparently forced by sea-level changes in the northeast Atlantic. Correcting for atmospheric pressure and the buoyancy of the coastal water near the tide gauge at Bodø, we obtain five-day running average coastal sea-level fluctuations that correlate to within 15% of observed current and bottom pressure gradient fluctuations 300 km away. The extension of the derived relation for monitoring this flow over large distances, with a time resolution of 5 days is demonstrated. Improved quality of the measurements is needed to reduce the inherent sources of noise. Part of the noise is due to mesoscale variability in the coastal current. This can be reduced by averaging over longer times. The method fails at higher time resolutions due to atmospheric variability, basin modes and other processes that are unaccounted for in the simple geostrophic model.

Bubble detection with horizontal pressure gradient measurements in gas-fluidised beds

Horizontal pressure gradient fluctuations caused by a bubble passage in the neighbourhood of the probe are simulated using Davidson's pressure field around a circular bubble. The salient features of these fluctuations are studied with regard to their sensitivity to the obliquity and randomness of the bubble traversing path. The utility aspects of horizontal pressure differential measurements over vertical pressure differentials are highlighted and a qualitative comparison between the simulated and those experimentally obtained from a two-dimensional bed, showing strong similarities are presented.

Modelling of unclosed nonlinear terms in a pdf closure for turbulent flames

Turbulent combustion modelling based on the probability density function (pdf) approach has proven to offer certain significant advantages over other solution methods. In the pdf formulation, important nonlinear terms appearing in the mathematical description of the flow are closed. The most prominent examples are in the case of the joint velocity-scalar pdf, the effects of chemical reactions and mean pressure gradient. Established models used in Monte Carlo particle pdf modelling to approximate the unclosed terms in the pdf equation are briefly reviewed. The models for the molecular mixing of scalars, viscous dissipation of momentum, and fluctuating pressure gradient perform well in constant density flows. However, modifications of these models are necessary if variable-density combusting flows are to be modelled accurately. This problem is here examined with particular reference to the flamelet regime of combustion, where the effects of combustion-induced acceleration of a fluid may be quantified. Simple approximations which result in a stochastic model for the effect of volume generation in flamelets are proposed.

Computations of steady-state and transient premixed turbulent flames using pdf methods

Premixed propagating turbulent flames are modeled using a one-point, single time, joint velocity-composition probability density function (pdf) closure. The pdf evolution equation is solved using a Monte Carlo method. The unclosed terms in the pdf equation are modeled using a modified version of the binomial Langevin model for scalar mixing of Valiño and Dopazo, and the Haworth and Pope (HP) and Lagrangian Speziale-Sarkar-Gatski (LSSG) models for the viscous dissipation of velocity and the fluctuating pressure gradient. The source terms for the presumed one-step chemical reaction are extracted from the rate of fuel consumption in laminar premixed hydrocarbon flames, computed using a detailed chemical kinetic mechanism. Steady-state and transient solutions are obtained for planar turbulent methane-air and propane-air flames. The transient solution method features a coupling with a Finite Volume (FV) code to obtain the mean pressure field. The results are compared with the burning velocity measurements of Abdel-Gayed et al. and with velocity measurements obtained in freely propagating propane-air flames by Videto and Santavicca. The effects of different upstream turbulence fields, chemical source terms (different fuels and strained/unstrained laminar flames) and the influence of the velocity statistics models (HP and LSSG) are assessed.

Wall-vorticity flux dynamics in a two-dimensional turbulent boundary layer

Four high-frequency-response pressure transducers with 10 viscous units resolution each have been used to obtain simultaneously the fluctuating pressure gradients at the wall of a zero-pressure-gradient boundary layer and then to compute the vorticity flux away from the wall. Since the viscous force on an element of incompressible fluid is determined by the local vorticity gradients, understanding of their dynamical characteristics is essential in identifying the turbulent structure. Extremely high and low amplitudes of both vorticity gradients have been observed which contribute significantly to their statistics although they have low probability of appearance. The r.m.s. of the vorticity flux when scaled with inner wall variables depends very strongly on the Reynolds number, indicating a breakdown of this type of scaling. The application of a small threshold to the data indicated two preferential directions of the vorticity flux vector. An attempt has been made to identify these high- and low-amplitude signals with physical phenomena associated with bursting-sweep processes. Vortical structures carrying bipolar vorticity are the dominant wall structures which are associated with the violent events characterized by large fluctuations of vorticity flux.

Unsteady flow of power-law fluids through circular pipes

This paper is concerned with the flow of power-law fluids through circular pipes under impulsively started conditions both for constant and superposed fluctuating pressure gradients. Flows with purely periodic pressure gradients started from rest have also been considered.

GAS-LIQUID SLUG FLOW:

This contribution begins with a discussion of the occurrence and characteristics of slug flow. Slug flow results in high liquid velocities and hence high pressure gradients and large fluctuations in pressure gradient. However, it does provide a means of transporting large amounts of liquid. The most prevalent model of slug flow is the cell model in which a unit cell encompassing the slug and its associated stratified region is set up. To close this model, various relationships are required (slug velocity, void fraction in the slug, slug frequency etc.) and available relationships of this type are discussed. The mixing zone at the slug front is a particularly important one and is discussed in detail. Evaluation of the statistical characteristics of slug flow can be extremely useful in dealing with design problems (for example the maximum length of a slug which influences the separation equipment) and such distributions are discussed. Finally, the two-equation model for gas-liquid slug flow is presented.

Two novel applications of bootstrap currents: Snakes and jitter stabilization

Both neoclassical theory and certain turbulence theories of particle transport in tokamaks predict the existence of bootstrap (i.e., pressure-driven) currents. Two new applications of this form of noninductive current are considered in this work. In the first, an earlier model of the nonlinearly saturated m=1 tearing mode is extended to include the stabilizing effect of a bootstrap current inside the island. This is used to explain several observed features of the so-called ‘‘snake’’ reported in the Joint European Torus (JET) [R. D. Gill, A. W. Edwards, D. Pasini, and A. Weller, Nucl. Fusion 32, 723 (1992)]. The second application involves an alternating current (ac) form of bootstrap current, produced by pressure-gradient fluctuations. It is suggested that a time-dependent (in the plasma frame), radio-frequency (rf) power source can be used to produce localized pressure fluctuations of suitable frequency and amplitude to implement the dynamic stabilization method for suppressing gross modes in tokamaks suggested in a recent paper [A. Thyagaraja, R. D. Hazeltine, and A. Y. Aydemir, Phys. Fluids B 4, 2733 (1992)]. This method works by ‘‘detuning’’ the resonant layer by rapid current/shear fluctuations. Estimates made for the power source requirements both for small machines such as COMPASS and for larger machines like JET suggest that the method could be practically feasible. This ‘‘jitter’’ (i.e., dynamic) stabilization method could provide a useful form of active instability control to avoid both gross/disruptive and fine-scale/transportive instabilities, which may set severe operating/safety constraints in the reactor regime. The results are also capable, in principle, of throwing considerable light on the local properties of current generation and diffusion in tokamaks, which may be enhanced by turbulence, as has been suggested recently by several researchers.

Hydrodynamics of a flowing gas‐solids suspension in a tube at high angles of inclination

AbstractA study was made of pneumatic transport of particles of sand suspended in air in a 31.8 mm diameter Plexiglas tube inclined at either 72 or 90 degrees with respect to the horizontal plane. The solids holdup, pressure gradient and pressure drop fluctuations over a 4.55 m test section were measured as functions of the air velocity and the solids flux. Previously observed sudden changes in slope of plots of solids holdup versus solids flux at constant gas velocity in a vertical tube (Mok et al., 1989), which were identified as transition points between dilute and dense phases, were confirmed and extended to inclined tubes. Breaks were also found in plots of pressure drop fluctuations versus solids flux at constant gas velocity.

Rotational dispersion in porous media due to fluctuating flows

We suggest that dispersion in a saturated porous medium subject to a fluctuating pressure gradient can occur as a result of the processes of shear (Taylor) dispersion and what we will label rotational dispersion. In contrast to shear dispersion, rotational dispersion does not rely on molecular diffusion to be effective but requires that the direction of the pressure gradient rotates with time. Such rotational gradients are ubiquitous in nature, occurring whenever a pressure wave propagates across the surface of a porous medium such as soil or marine sediments. The efficiency and character of rotational dispersion is investigated using Monte Carlo simulations of the dispersal of clouds of particles through a highly idealized porous medium. These simulations demonstrate that rotational dispersion behaves as a diffusive process and that it can be many times more effective than molecular diffusion or shear dispersion as a transport mechanism. The results of the theory were tested experimentally using a wave tank with a bed of sand as the porous medium. These experiments demonstrate that passing waves can greatly enhance solute transfer between the bed and the overlying water. Furthermore, the measured increases in solute transfer rates are quantitatively consistent with the predictions obtained from the theory of rotational dispersion presented herein.

Turbulent pulsations of the microelectrode limiting diffusion current

Flat mounted microelectrodes, for which the longitudinal length,L, is small in comparison with the diffusion entrance region are used to measure wall shear stress fluctuations. However, microelectrode turbulent noise depends both on wall shear stress fluctuations (i.e. longitudinal velocity pulsations) and on normal pressure gradient fluctuations (i.e. normal velocity pulsations). The relative value of these two factors depends on the microelectrode length. The influence of normal and longitudinal velocity pulsations on the microelectrode turbulent noise are the same order of magnitude if the lengthL is approximately 10 or 20 times the thickness of the viscous sublayer. Consequently it is possible to determine statistical characteristics of normal and longitudinal velocity fluctuations by using a number of microelectrodes of different lengths in the flow direction.

Mechanisms of nocturnal exchange between the rain forest and the atmosphere

We have observed a number of events that lead to nocturnal turbulent mixing between the Amazon rain forest and the atmosphere. Two mechanisms that weaken the stable nocturnal thermal inversion at canopy top are identified with nocturnal release of carbon dioxide and concurrent heat flux into and moisture flux out of the Amazon forest canopy. In the first mechanism, abrupt onset of cloudiness interrupts nocturnal radiative cooling, leading to a period with lowered Richardson number and enhanced heat, moisture, and carbon dioxide fluxes. Significant exchange between the forest canopy and the atmosphere can also occur even on totally clear nights. On these nights, occasional nocturnal increases in the wind speed above the canopy stimulate vertical oscillations in the canopy layer that are both amplitude and frequency modulated during the event. A simple model is presented to support the postulate that these oscillations result from a resonant interaction, hypothesized to be a consequence of a vertically fluctuating pressure gradient force operating on a deepening stable layer near canopy top. Turbulent diffusion associated with increased winds deepens and weakens the stable layer at canopy top, lowering the buoyancy frequency in the canopy. Eventually, periods of resonance occur. Model results indicate that the recovery of the layer takes approximately twice as long as the period of enhanced wind that produced the event.

PDF calculations of piloted turbulent nonpremixed flames of methane

A velocity-composition joint pdf transport equation has been solved by the Monte Carlo method to calculate the structure of pilot-stabilized turbulent nonpremixed flames of methane. Three components of velocity and a conserved scalar, namely, mixture fraction, ξ are jointly represented in the pdf. A new model is used for turbulent frequency. Turbulent dissipation and the fluctuating pressure gradient terms are conditionally modeled. Two simple models for thermochemistry are used. In one, density is a piecewise function of ξ, and in the other, density is obtained from calculations of a laminar counterflow diffusion flame of methane with a stretch rate, a = 100 s −1. Calculations are compared with the corresponding experimental measurements performed on a number of flames ranging from flames with low mixing rates to ones close to extinction. The velocity, turbulence, and mixing fields are predicted with reasonable accuracy down to x D j ∼ 30 . The agreement with experiments is less satisfactory at x D j = 50 . This will improve with the use of better pdf models which have recently been developed [27, 30, 31]. The reactive scalars are very well predicted only for flames which are far from extinction. This is expected considering the simplicity of the thermochemical model. This paper is a first step towards implementing realistic chemistry in the code, and hence predicting the observed finite rate kinetic effects in these flames.

Doppler diagnosis of partial vertebral/subclavian steals convertible to full steals with physiologic maneuvers.

Subclavian steal syndrome has been classically described as a unidirectional phenomenon in which retrograde flow occurs in the ipsilateral vertebral artery in cases of high-grade proximal subclavian stenosis. Pulsed Doppler examination in cases of subclavian stenosis shows that partial steals with to-and-fro flow do occur. A model based on fluctuating pressure gradients to explain the partial steal phenomenon is discussed. The usefulness of physiologic maneuvers such as ipsilateral arm exercise and postocclusive hyperemia in converting partial steals to full steals is demonstrated. In cases in which surgical bypass to correct partial steals is carried out, a relative steal can be demonstrated by Doppler examination before and after physiologic maneuvers.