Vortex Ejection Research Articles

Inducing Deep Sweeps and Vortex Ejections on Patterned Membrane Surfaces to Mitigate Surface Fouling.

Patterned membrane surfaces offer a hydrodynamic approach to mitigating concentration polarization and subsequent surface fouling. However, when subjected to steady crossflow conditions, surface patterns promote particle accumulation in the recirculation zones of cavity-like spaces. In order to resolve this issue, we numerically subject a two-dimensional, patterned membrane surface to a rapidly pulsed crossflow. When combined with cavity-like spaces, such as the valleys of membrane surface patterns, a rapidly pulsed flow generates mixing mechanisms (i.e., the deep sweep and the vortex ejection) and disrupts recirculation zones. In only four pulses, we demonstrate the ability of these mechanisms to remove over half of the particles trapped in recirculation zones via massless particle tracking studies (i.e., numerical integration of the simulated velocity field). The results of this work suggest that when combined with a rapidly pulsed inlet flow, patterned membrane surfaces can not only alleviate concentration polarization and the surface fouling that follows but also reduce the need for traditional cleaning methods that require operational downtime and often involve the use of abrasive chemical agents.

Hydrodynamic interactions with coral topography and its impact on larval settlement

Many benthic larvae rely on ambient flow and turbulence for their dispersal to settlement sites. After reaching the seafloor, larvae must prevent predation as well as overcome flow forces which act to dislodge them in order to achieve successful settlement. Most oceanic benthic habitats are topographically complex and are characterized by a combination of tidally driven currents and wave-driven oscillatory flows, which can exert substantial forces along settlement surfaces. In this study, computational fluid dynamics was used to numerically model both wave-dominated and unidirectional flows over surfaces of varying topography designed to mimic the surface roughness of corals. Near-surface hydrodynamic parameters, including velocities, turbulence statistics, and surface shear stresses, were computed, along with forces on simulated larvae settled on surfaces. It was found that widely spaced surface roughness (characterized as a k-type roughness with roughness width/height > 1) yielded 50–100% higher surface shear stress than tightly spaced roughness (characterized as d-type with width/height < 1), owing to eddy vortex ejection from k-type topographies. Results also found that a larva experiences up to two orders of magnitude greater mean fluid forces when settled on top of a roughness element rather than between roughness elements. Maximum drag and lift forces on a larva were two to three times greater in wave conditions compared to unidirectional flows. Our findings demonstrate that larvae have a higher probability to remain attached to surfaces when in less wavy environments and sheltered within coral crevices of d-type roughness.

A Numerical Study of the Bifurcation of Rectangular Synthetic Jets

The present paper explores the mechanism of vortex formation and its bifurcation process for a rectangular orifice synthetic jet. The orifice aspect ratio is equal to two (7 x 14 mm2) with hydraulic diameter 9.33 mm and 3 mm thickness. A 3-D numerical simulation using Large-eddy -simulation (LES) model on Fluent software is performed to investigate the vortex formation and its bifurcation process. The diaphragm oscillates at amplitude of 1.6 mm and 2.6mm compromising a volumetric ejection of 1500mm3 and 2500mm3 respectively through orifice at 1 Hz of actuation frequency. The geometry of the cavity and the diaphragm displacement profile as well as the volumetric ejection is kept identical in both the experimental study and the numerical simulations. It has been observed that vortex ring bifurcates in both the cases and the simulation results are in good agreement with the experimental observations. In both the bifurcation cases, the vortex ring undergoes axial switching followed by bifurcation in the minor axis plane. Further the bifurcated rings moves at away from each other in the same plane at different divergence angle depending on the vortex ejection velocity during forward stroke of the diaphragm and its actuation frequency.

Rapidly Pulsed Pumping Accelerates Remediation in A Vertical Circulation Well Model

One factor that slows groundwater remediation is the sequestration of contaminant in dead-end pores, that is, pores that are not flushed through by flow through the aquifer. Furthermore, rebound of apparently remediated aquifers can occur as a result of the eventual release of the contaminant trapped in these dead-end pores. Since the operational costs generally outweigh the capital costs of a remediation project, reduction of the duration of treatment should reduce the overall cost of the average remediation. It has been shown that a rapidly pulsed flow can increase the mixing between dead-end and well-connected pores through computational fluid dynamics models with idealized pore geometry and column tests. A rapidly pulsed flow induces a deep sweep upon a sudden increase in velocity and a vortex ejection upon a sudden decrease in velocity that substantially accelerates the remediation of contaminant from these dead-end pores. To examine rapidly pulsed pumping in a more realistic configuration, a model vertical circulation well was constructed. The porous medium was well-sorted crushed glass to minimize sorption. Removal of a fluorescent dye, which represents a dissolved contaminant, under a rapidly pulsed flow was compared to a steady flow. The modeled well revealed accelerated removal of dissolved contaminants under a rapidly pulsed flow.

Investigation of Sand Ripple Dynamics with Combined Particle Image and Tracking Velocimetry

AbstractAccurately assessing the response of sediments to oscillatory flows requires high-resolution fluid velocity and sediment transport measurements at the fluid–sediment interface. Fluid and sediment grain velocities were measured simultaneously with combined particle image and tracking velocimetry under oscillatory flows over movable sand ripples. Three high-speed cameras equipped with varying optical filters were used to distinguish between fluorescent fluid tracers and the grains, from which the fluid and grain velocities were determined, respectively. Individual grains were tracked during transport to determine velocities and trajectories. Sediment grains were first mobilized by a vortex impacting the bed during flow reversal and suspended into the water column just prior to vortex ejection from the ripple crest, similar to previous observations. During phases of maximum flow velocity, additional grains were mobilized by the shear stress and were subsequently suspended. The flow reversed and similar observations were made in the opposite direction. Consequently, four peaks in suspended sediment concentration were observed throughout the flow cycle, consistent with previous observations. However, some previous researchers attributed peaks in suspended sediment concentration occurring during phases of maximum flow velocity to sediment-laden vortices that were shed from adjacent ripples. The measured sediment grain velocities were of similar magnitude and phase to the near-bed fluid velocities when the grains were being advected with the flow. Measurements of suspended sediment concentration agreed well with semiempirical formulations having an average root-mean-square deviation of approximately 4 × 10−5m3m−3. Predictions of settling velocity also compared well with the laboratory estimates, agreeing to within 90%.

Acceleration of groundwater remediation by deep sweeps and vortex ejections induced by rapidly pulsed pumping

AbstractOne key limiting factor to groundwater remediation is contaminant sequestered in pores whose contents do not mix well with the bulk flow. Mixing between well‐connected (pores whose volume is flushed as water flows through the aquifer) and poorly connected pores (pores whose volume does not exchange readily when water flows through the aquifer) is of primary concern. Under steady flow, contaminants are effectively trapped in the poorly connected pores and are transferred only by molecular diffusion. This slow mixing process between pore types is a bottleneck to remediation. We present a novel rapidly pulsed pumping method that increases the mixing between these pore types. We do it in the context of pump‐and‐treat remediation because it is the most common remediation practice. In rapidly pulsed pumping, the increase in flow causes a deep sweep, which pushes the flow into poorly connected pores and sweeps out sequestered contaminants. The decrease in flow causes a vortex ejection, which causes the vortex within the poorly connected pore to emerge with contaminant. These actions are modeled with computational fluid mechanics to elucidate the individual mechanisms and determine how they function and interact. Cleanup of single and multiple poorly connected pore systems were simulated and show the acceleration possible. This technique can decrease the time and cost needed to remediate contaminated aquifers, which in the United States has been estimated to exceed $1 trillion. Since our rapidly pulsed pumping method enhances mixing between well‐connected and poorly connected pores, it can be applied to other remediation schemes such as in situ methods.

On the vortex ejection in a chamber with a swirling air flow

The longitudinal (axial) velocities of the air in a vortex chamber of diameter 0.393 m were experimentally investigated at different values of the tangential and bottom blows. On the basis of the data obtained, the back (adjoint) flow of air blown from the atmosphere through the outlet of the chamber (vortex ejection) was calculated. The dependence of this flow on the relative radius of the outlet of the chamber, the Rossby number, and the value of the bottom blow was determined.

Nonlinear resonances in a laminar wall jet: ejection of dipolar vortices

Nonlinear mechanisms leading to the ejection of dipolar vortices from a laminar wall jet are being investigated using highly accurate Navier–Stokes simulations. With a set of well-defined numerical experiments for a forced Glauert wall jet, the nonlinear resonant interaction between the large-amplitude harmonic disturbance and a small-amplitude wave packet is systematically explored using two-dimensional simulations. Generated by a small-amplitude pulse, the wave packet experiences rapid resonant growth in the subharmonic part of its spectrum resulting in vortex mergings and, ultimately, the ejection of a pair of counter-rotating vortices from the wall jet. This two-dimensional subharmonic instability, if not mitigated by competing three-dimensional instabilities, can lead to the detachment of the entire wall jet from the surface. As shown using three-dimensional direct numerical simulations, vortex ejection still occurs in a forced transitional wall jet if the two-dimensional wave packet can reach a large amplitude level upstream of the region of three-dimensional turbulent breakdown. Movies are available with the online version of the paper.

Laboratory experiments of fine-scale mixing and mass transport within a coral canopy

Laboratory experiments obtained fine scale measurements of turbulent shear stresses and rates of mixing and mass transfer over a nonliving bed of the coral, Porites compressa, the dominant species found in Kaneohe Bay, Hawaii. A reef canopy was placed in a recirculating wave-current flume and flow was generated that simulated the flow characteristics of the reef flat of Kaneohe Bay. Turbulence and velocity structure under both unidirectional and wave-dominated currents were measured using a two-dimensional laser Doppler anemometer. Mass transport measurements were made using a planar laser-induced fluorescence technique in which the scalar transport of Rhodamine 6G dye, fluxed from the surfaces of the coral, was quantified. Results show that the action of surface waves, interacting with the structure of the reef, can increase instantaneous shear and mixing up to six times compared to that of unidirectional currents. Maximum shear and mass transport events coincided with flow separation within the wave-current boundary layer and the ejection of vortices into the flow. Wave action also acted to increase the vertical flux of water from within the coral structure. The combined effects of increased turbulent stress and fluid exchange from the interior of the canopy increased mass flux due to wave action 2.3±0.5 times that measured for comparable unidirectional currents.

Detailed measurements of velocities and suspended sand concentrations over full‐scale ripples in regular oscillatory flow

The knowledge and modeling of wave‐induced sand transport over rippled beds still has significant shortcomings, which is partly related to a lack of measurements of the detailed processes from controlled laboratory experiments. We have carried out new measurements of the detailed time‐dependent velocity and suspended sand concentration field around vortex ripples for regular oscillatory flow conditions. The fact that the ripples were mobile and the flow conditions were full‐scale makes these measurements unique. We made velocity measurements for 14 different flows and concentration measurements for three of these flows. The velocity and concentration field above ripples are dominated by the generation and ejection of vortices on the ripple flanks around the time of flow reversal. Vortex formation results in near‐ripple flow reversals ahead of free‐stream reversals and velocity maxima near the ripple crest that are much higher than the free‐stream maxima. Asymmetry in the free stream produces steady circulation cells with dominant offshore mean flow up the ripple lee slope, balanced by weaker onshore streaming up the ripple stoss slope as well as higher up in the flow. The time‐ and bed‐averaged horizontal velocity profile comprises an offshore streaming near the bed and an onshore drift higher up in the flow. The vortices are responsible for three main concentration peaks: one just after on‐offshore flow reversal associated with the passage of a sand‐laden vortex followed by two smaller peaks due to advected suspension clouds generated by vortex action at the neighboring onshore ripples. The sand flux field measured for one typical asymmetric flow condition is dominated by an offshore flux associated with the suspended sand cloud generated by vortex shedding from the ripple's lee slope around the time of on‐offshore flow reversal. The net (time‐averaged) current‐related and wave‐related horizontal sand fluxes are generally offshore directed and mostly contained within 1.5 ripple heights above the ripple crest. The wave‐related suspended transport component is larger, but the contribution of the current‐related suspended sand transport cannot be neglected. In addition to the measured offshore net transport of suspended sand, there is an onshore‐directed transport very close to the ripple surface. The total net transport is in the offshore direction for this specific asymmetric flow condition.

Keyword Index Vol. 74 (2001)

High resolution measurements of suspended sand concentrations have been made using a multi-transducer acoustic back scatter sensor over both steep and flat bedforms under low energy swell conditions with weak currents present on a micro-tidal beach in the southwest of England. Similar measurements were made over steep bedforms in a large-scale laboratory wave basin using a wave record simulated from field data. A detailed interpretation of the suspension process associated with steep vortex type ripples was made possible with knowledge of the position of the acoustic sensors relative to the bedform geometry.Analysis of time-averaged, wave ensemble-averaged suspended sediment concentrations, and velocity-concentration phase angles indicates that the bedforms are an important control on the suspension patterns produced by wave-induced flows. Steep asymmetric ripples under shoaling waves produces greater amounts of suspension than low steepness ripples due to the effects of vortex ejection associated with the steep ripples.Over rippled beds, we find evidence for a significant phase couplined between the resuspended sediment and the bedforms in the near bed region (<10 cm elevation); at higher elevations the coupling appears to be much less significant. This means that suspended sand transport rates in the near bed region, computed from fast response sensors, are highly sensitive to the positioning of the sensors, both horizontally and vertically, relative to the sea bed.

On the effect of nonuniform seeding on particle dispersion in two-dimensional mixing layers

The effect of nonuniform seeding on the dispersion of fluid elements and heavy particles has been investigated in two-dimensional, incompressible mixing layers. The cross-stream dispersion of fluid elements can be enhanced using nonuniform seeding with most particles near the saddle point because of the greater lateral extent of streamlines in this region, though the increase in dispersion compared to a uniform seeding occurs only after a few vortex turnover times. Compared to fluid elements, additional mechanisms—ejection from vortex cores, separatrix crossing, and the effect of the initial particle velocity—must be considered in the analysis of nonuniform seeding on heavy particle dispersion. The influences of these additional mechanisms are first investigated in a Stuart vortex. With increasing response time, vortex ejection and separatrix crossing shift the streamwise position maximizing lateral transport towards the vortex core. While changes in the initial particle velocity increase/decrease displacement, lateral dispersion may still be enhanced by appropriate nonuniform seeding of particles near the saddle point. Numerical simulations of the incompressible Navier–Stokes equations are then used to study cross-stream dispersion in a temporally evolving two-dimensional mixing layer. Stokes numbers St in the calculations were 0.05, 1, 10, and 100 where St is defined as the ratio of the particle response time to the time scale formed using the vorticity thickness of the initial mean flow. Particles were initially distributed nonuniformly at the interface between the two streams or along a line parallel to the interface. Simulation results show that the seeding location maximizing lateral dispersion is both time and Stokes number dependent, with larger increases in dispersion for the interface seeding. For Stokes numbers of order unity cross-stream dispersion exhibits a weak dependence on initial position since particles are efficiently ejected from the vortex core with subsequent motion confined to the nearby region outside the separatrix in one of the freestreams. Simulation results also show that substantial increases in particle dispersion can be obtained using nonuniform seedings relative to that obtained from an initially uniform distribution.

Vertical and horizontal structure is suspended sand concentrations and wave-induced fluxes over bedforms

High resolution measurements of suspended sand concentrations have been made using a multi-transducer acoustic back scatter sensor over both steep and flat bedforms under low energy swell conditions with weak currents present on a micro-tidal beach in the southwest of England. Similar measurements were made over steep bedforms in a large-scale laboratory wave basin using a wave record simulated from field data. A detailed interpretation of the suspension process associated with steep vortex type ripples was made possible with knowledge of the position of the acoustic sensors relative to the bedform geometry. Analysis of time-averaged, wave ensemble-averaged suspended sediment concentrations, and velocity-concentration phase angles indicates that the bedforms are an important control on the suspension patterns produced by wave-induced flows. Steep asymmetric ripples under shoaling waves produces greater amounts of suspension than low steepness ripples due to the effects of vortex ejection associated with the steep ripples. Over rippled beds, we find evidence for a significant phase couplined between the resuspended sediment and the bedforms in the near bed region (<10 cm elevation); at higher elevations the coupling appears to be much less significant. This means that suspended sand transport rates in the near bed region, computed from fast response sensors, are highly sensitive to the positioning of the sensors, both horizontally and vertically, relative to the sea bed.

Variability of suspended sand concentrations, transport and eddy diffusivity under non-breaking waves on the shoreface

Suspended sand concentration profiles and current speed measurements were made in 1.6 ± 0.4m water depth on the seaward side of a bar beyond the break point at Stanhope Lane beach, Prince Edward Island, Canada, during a 4 day period. Two mild storm events occurred and 7 min-long bursts of 4.4 Hz data were recorded approximately every hour. Concentration profiles, with a resolution of 5 mm and 7.5 mm, were obtained using a 3 MHz acoustic backscatter (ABS) instrument. Some data on bed form dimensions were obtained by divers between storm events. Multiple bed echoes from the ABS also provide further information on the ripple heights but no direct observations were available at times of higher waves. During the 4 day period the local position of the bed decreased by about 8 cm, erosion occurring in two short (⋟6 h) periods, one during each storm event. Mean currents at this location were weak (<0.1 m s −1). The resuspension coefficient γ 0, calculated from the concentration at 2 cm above the bed, decreased as the wave height increased (in the break-off range) supporting the observations of Vincent et al. [(1991) Marine Geology, 96, 1–18]. Large inter-burst variability was observed in the concentration profiles and in the eddy diffusivity and suspended transport fluxes computed from these profiles. This variability was due to the short length of record (⋟7min) relative to wave groups and to the location of the ABS relative to bed forms; to obtain consistent concentration and transport fluxes it is necessary to average many bursts over a time scale that is long compared to both groupiness and bed form mobility. Except at the beginning and end of events when wave conditions were changing rapidly, averaging over groups of bursts with similar wave conditions produced eddy diffusivity profiles characterized by a linear ɛ s gradient of (20 ±2.5) cm s −1 from the sea bed to 20 ± 5cm, beyond which ɛ s slowly decreased, and a suspended sand transport which was all below 20 cm and dominated by a jet-like shoreward flow in the lowest 3–4 cm above the sea bed. Bursts obtained when conditions were in, or close to, the equilibrium range of bed forms showed concentration profiles which were partly exponential (hence ɛ s was constant with height) and transport profiles with more structure. Sensitivity tests indicate that concentrations and eddy diffusivities may be underestimated by about 20% by using the modal size for the sand in suspension rather than a distribution of sizes. Towards the end of the second storm a “low suspension event” of ⋟6 h duration occurred and suspended sand concentrations about 5–10 cm decreased by 1–2 orders of magnitude while the near bed concentrations remained approximately constant, suggesting that the bed forms responsible for vortex ejection of sand higher into the water column had been wiped out and the sea bed had become (temporarily) flat. Wave heights did not change significantly but wave period increased from 4.8 to 6.2 s, decreasing the maximum bed shear stress. The low suspension period may have a transient response of the sea bed to changed hydrodynamic forcing prior to the subsequent growth of bed forms with a different wavelength.

Flow and mass transfer characteristics in wavy channels for oscillatory flow

A study of fluid flow and mass transfer for oscillatory flow has been carried out in wavy channels, which consist of two sinusoidal wavy plates. The experiments were carried out under the following conditions: 5 × 10 2 < Re p < 10 4, 2.1 × 10 −2 < St p < 8.6 × 10 −2. The flow was neither dominated by viscous forces nor quasi-steady in this experiment. The recirculation vortex is formed within the furrow during the acceleration phase and is ejected from the wall just after flow reversal. The mass transfer rate strongly depends on Re p but the effect of St p is insignificant. The wavy channels yield a large mass transfer enhancement as compared with the corresponding straight channel. This enhancement is attributed to the vortex ejection observed.

WAVE INDUCED VELOCITIES CLOSE TO A RIPPLED BED

The results are reported of velocity measurements in oscillatory flow over rippled beds. Velocities were measured with a laserdoppler anemometer in both an oscillating tray rig and an oscillatory flow U-tube. Both self-formed and artificial ripples were examined. The results obtained in the two test rigs were compared and it was concluded that the flow fields obtained in the two cases were dynamically similar. Measurements of the flow field clearly showed the formation, growth and ejection of vortices, as well as a strong surge of fluid over the ripples during and after flow reversal.

Instability in Hypersonic Flow about Blunt Bodies

Studies of the flow field associated with hypersonic helium flow over blunt bodies with surface cavities have shown a very violent instability to be connected with these cavities. The mechanism of this instability appears to involve the formation within the cavity of vortices which are subsequently ejected. Cavity shapes more conducive to vortex formation are associated with the least stable flow. Experiments using the hydraulic analogy for compressible gas flow have clarified the nature of these unstable flows. It is suggested that the violent action of such vortices may explain some of the deeper pits observed in many meteorites. The continuous ejection of vortices during atmospheric passage would ablate material from the cavities, whence the depth of any cavities initially present on a meteoric body would increase during passage through the earth's atmosphere.