Lagrangian Integral Time Scale Research Articles

Effect of crossflow oscillation Strouhal number on circular cylinder wake

The wake flow features and aerodynamic characteristics of a circular cylinder in an oscillating crossflow were experimentally investigated. The study focused on crossflow oscillation Strouhal numbers that were smaller than the natural wake vortex-shedding Strouhal numbers. Crossflow oscillations were generated using a downstream rotating plate method. The flow evolution processes were observed using laser-light sheet assisted smoke flow visualization technique. Wake instability and time-averaged velocities were assessed using a one-component hot-wire anemometer and particle image velocimetry. Pressure distributions on the cylinder's surface were quantified using a linear pressure scanner, obtaining pressure coefficient distributions and drag forces. The wake instability Strouhal number, velocity vectors, streamline patterns, and recirculation bubble geometries in the wake region of the circular cylinder were determined. The wake turbulence properties were analyzed using the triple-decomposition method, including turbulence intensities and Lagrangian integral length and time scales. The results of natural and oscillating crossflows were compared. The wake vortex-shedding Strouhal number was lower than the natural Strouhal number but higher than the crossflow oscillation Strouhal number. It was primarily influenced by the oscillation Strouhal number and the Reynolds number of the crossflow. Critical crossflow Reynolds and oscillation Strouhal numbers were identified, beyond which the wake vortex-shedding Strouhal number reached a constant value. The crossflow oscillation intensity did not significantly affect the wake vortex-shedding behavior. The study provided quantitative descriptions and discussions of recirculation bubble geometries and statistical turbulence properties. Furthermore, the crossflow oscillations led to a substantial reduction in the drag coefficient experienced by the circular cylinder.

EXPERIMENTAL STUDY OF LAGRANGIAN MIXING IN WEAKLY DISSIPATIVE TIDAL CHANNELS

Estuaries are extremely dynamic environments, allowing wildlife to grow in a wide variety of ecosystems because of the interaction of masses of water with different characteristics. In particular, coastal bays and estuaries are characterized by flows driven by hydraulic unbalance such as baroclinic pressure gradients, river inflows and wind stresses, and tidal waves. Here, following a reductionist approach, we examine dispersion processes in a physical model of a tidal channel bounded by an inlet mouth, with tides as the dominant forcing. The presence of a tidal inlet can generate macro-vortices that during a tidal cycle may influence the momentum and mass transport on relatively large distances (Awaji et al. (1980), Awaji (1982), Branyon et al. (2022)). Moreover, tides tend to produce non-monotonic particle velocity correlation leading to possible particle looping trajectories that also reflect on a looping character of the Lagrangian integral time scales, differently from the classical statistically steady or homogeneous turbulence (Enrile et al. (2019)). Our goal is to examine the dispersion regimes by means of two-dimensional velocity measurements at the free surface reproduced in a large-scale physical model as a starting point for Lagrangian analysis. We show how the presence of a tidal inlet generates complex flow patterns depending on the character of the forcing tides. Furthermore, the mixed nature of tides may be crucial to dispersion processes, as it enhances the ability of the flow to transport mass in the direction of the main flow.

Effects of crossflow pulsation intensity on wake properties of a circular cylinder

The effects of the crossflow pulsation intensity on the wake flow properties of a circular-cylinder were experimentally studied in a wind tunnel. A rotating plate actuator technique was used to generate periodic crossflow oscillations. The Reynolds number and oscillation Strouhal number of the crossflow were fixed at 3111 and 0.074, respectively. The crossflow pulsation intensities were varied within the range between 0.035 and 0.074. The wake evolution processes were captured by laser-light-sheet-assisted smoke flow visualization method. The wake instability frequencies and instantaneous oscillating turbulent velocities were detected by a one-component hot-wire anemometer along with a high-speed data acquisition system. The triple-decomposition technique was applied to the instantaneous velocities to consider the effects of the crossflow oscillations on turbulence. The statistical turbulence flow properties such as the Lagrangian integral time and length scales were estimated. The time-averaged flow structures and vorticity contours in the wake were obtained to analyze the wake geometric parameters by averaging instantaneously captured PIV images. The results revealed that at a fixed Reynolds number, the wake instability frequency of a circular cylinder in oscillating crossflow would present a value in-between of the natural wake instability frequency and the crossflow oscillating frequency. The turbulence intensity and Lagrangian integral time and length scales in the cylinder wake of the oscillating crossflow were drastically larger than those in the natural crossflow. The time-averaged geometric parameters of the recirculation bubble in the circular-cylinder wake in oscillating crossflow were significantly smaller than those in the natural crossflow. The crossflow oscillations induced large scale flow motions and hence enhanced entrainment and mixing in the wake. These mechanisms expedited turbulent diffusion of momentum in the wake, and therefore the turbulence intensity and turbulence time and length scales were enlarged and the geometric parameters of recirculation bubble were shortened when compared with those in the natural crossflow.

Lagrangian Stochastic Model for the Motions of Magnetic Footpoints on the Solar Wind Source Surface and the Path Lengths of Boundary-driven Interplanetary Magnetic Field Lines

In this work, we extend Leighton’s diffusion model describing the turbulent mixing of magnetic footpoints on the solar wind source surface. The present Lagrangian stochastic model is based on the spherical Ornstein–Uhlenbeck process with drift that is controlled by the rotation frequency Ω of the Sun, the Lagrangian integral timescale τ L, and the root-mean-square footpoint velocity V rms. The Lagrangian velocity and the positions of magnetic footpoints on the solar wind source surface are obtained from the solutions of a set of stochastic differential equations, which are solved numerically. The spherical diffusion model of Leighton is recovered in the singular Markov limit when the Lagrangian integral timescale tends to zero while keeping the footpoint diffusivity finite. In contrast to the magnetic field lines driven by standard Brownian processes on the solar wind source surface, the interplanetary magnetic field lines are smooth differentiable functions with finite path lengths in our model. The path lengths of the boundary-driven interplanetary magnetic field lines and their probability distributions at 1 au are computed numerically, and their dependency with respect to the controlling parameters is investigated. The path-length distributions are shown to develop a significant skewness as the width of the distributions increases.

Effect of electrostatic forces on the dispersion of like-charged solid particles transported by homogeneous isotropic turbulence

The effect of electrostatic forces on the dispersion of small like-charged inertial particles transported by a homogeneous isotropic turbulent flow is analysed by means of direct numerical simulations coupled with the Lagrangian tracking of charged particles. The results show that particle dispersion decreases for increasing charge. Since turbulent particle dispersion is related to the intensity of particle agitation and to the Lagrangian particle integral time scale, we analyse the effects of electrostatic forces on these two quantities. Particle agitation decreases for increasing particle charge. In fact, particle kinetic energy is not directly modified by electrostatic forces, which are conservative, it is rather the particle entrainment by fluid turbulence that is modified. To support this claim, one shows that the fluid–particle velocity covariance is destructed by electrostatic forces, a destruction can be predicted by drawing an analogy between inter-particle collisions and Coulomb collisions. As expected, electrostatic forces decorrelate particle velocities leading to a decrease of the Lagrangian particle integral time scale. Finally, one shows that the analogy with inter-particle collisions allows us to predict the reduction of particle dispersion.

Modulating flow and mixing characteristics of an inclined jet in crossflow at a large backward inclination angle by acoustic excitation

Previous investigations revealed that the backward-inclined jet in crossflow exhibits seemingly lower transverse velocity and turbulence intensity at inclination angles higher than a critical value about 25°, thus resulting in significantly low jet fluid dispersion properties. In this article, the acoustic excitations were applied to a backward-inclined jet in crossflow at an inclination angle higher than the critical value to examine experimentally whether the flow and dispersion characteristics could be improved or not. The flow in the test section of an open-loop wind tunnel was used as the crossflow. A profiled nozzle assembly with a loudspeaker installed in the lower part of it was used to generate the pulsating jet. The instantaneous and time-averaged smoke flow patterns were obtained by the laser-light-sheet assisted flow visualization method. The binary edge detection method was employed to the long-exposure smoke flow images to measure the jet spread width. The turbulence intensities, as well as the Lagrangian integral turbulence time and length scales, were obtained using a hotwire anemometer. The jet-fluid dispersion characteristics were examined using the tracer-gas detection technique. The results revealed that in the domain of jet pulsation intensity and excitation Strouhal number, the pulsed backward-inclined jet in crossflow presented three characteristic flow modes: synchronized oscillating jet, transition, and synchronized shear-layer vortices. The synchronized oscillating jet exhibited violent transverse oscillations due to the high axial jet pulsations induced by the acoustic excitation. The transverse jet width and jet penetration height were enlarged. The turbulence intensities were significantly increased, and the turbulence time and length scales were decreased, therefore led to a significant enhancement in the jet-fluid dispersions and mixing capabilities.

An Overview of the Lagrangian Dispersion Modeling of Heavy Particles in Homogeneous Isotropic Turbulence and Considerations on Related LES Simulations

Particle tracking is a competitive technique widely used in two-phase flows and best suited to simulate the dispersion of heavy particles in the atmosphere. Most Lagrangian models in the statistical approach to turbulence are based either on the eddy interaction model (EIM) and the Monte-Carlo method or on random walk models (RWMs) making use of Markov chains and a Langevin equation. In the present work, both discontinuous and continuous random walk techniques are used to model the dispersion of heavy spherical particles in homogeneous isotropic stationary turbulence (HIST). Their efficiency to predict particle long time dispersion, mean-square velocity and Lagrangian integral time scales are discussed. Computation results with zero and no-zero mean drift velocity are reported; they are intended to quantify the inertia, gravity, crossing-trajectory and continuity effects controlling the dispersion. The calculations concern dense monodisperse spheres in air, the particle Stokes number ranging from 0.007 to 4. Due to the weaknesses of such models, a more sophisticated matrix method will also be explored, able to simulate the true fluid turbulence experienced by the particle for long time dispersion studies. Computer evolution and performance since allowed to develop, instead of Reynold-Averaged Navier-Stokes (RANS)-based studies, large eddy simulation (LES) and direct numerical simulation (DNS) of turbulence coupled to Generalized Langevin Models. A short review on the progress of the Lagrangian simulations based on large eddy simulation (LES) will therefore be provided too, highlighting preferential concentration. The theoretical framework for the fluid time correlation functions along the heavy particle path is that suggested by Wang and Stock.

Near-Surface Transport Properties and Lagrangian Statistics during Two Contrasting Years in the Adriatic Sea

This paper describes the near-surface transport properties and Lagrangian statistics in the Adriatic semi-enclosed basin using synthetic drifters. Lagrangian transport models were used to simulate synthetic trajectories from the mean flow fields obtained by the Massachusetts Institute of Technology general circulation model (MITgcm), implemented in the Adriatic from October 2006 until December 2008. In particular, the surface circulation properties in two contrasting years (2007 had a mild winter and cold fall, while 2008 had a normal winter and hot summer) are compared here. In addition, the Lagrangian statistics for the entire Adriatic Basin after removing the Eulerian mean circulation for numerical particles were calculated. The results indicate that the numerical particles were slower in this simulation when compared with the real drifters. This is because of the reduced energetic flow field generated by the MIT general circulation model during the selected years. The numerical results showed that the balanced effects of the wind-driven recirculation in the northernmost area(which would be a sea response to the Bora wind field) and the Po River discharge cause the residence times to be similar during the two selected years (182 and 185 days in 2007 and 2008, respectively). Furthermore, the mean angular momentum, diffusivity, and Lagrangian velocity covariance values are smaller than in the real drifter observations, while the maximum Lagrangian integral time scale is the same.

Reconciling Gaussian plume and Computational Fluid Dynamics models of particulate dispersion

Abstract Computational Fluid Dynamics (CFD) is increasingly used to model particulate dispersion in situations where Gaussian Dispersion models are inappropriate or inaccurate. However, there is evidence which indicates that many CFD models under-predict lateral plume spread. This paper aims to address this by implementing a strategy which incorporates wind direction variability into CFD models using a formulation which is also used in the UK-ADMS plume spread module. In the present work, a series of CFD simulations are run at various wind angles. The outputs from these simulations are weighted using a Gaussian probability density function and combined to produce a plume. The standard k − e model has been employed to solve the RANS equations of the flow field for stable, neutral and unstable atmospheric stabilities, coupled with the Lagrangian Particle tracking model to model dispersion. By comparing the CFD accretion profiles to UK-ADMS dry deposition results, it is observed that the proposed modelling methodology produces lateral spreading of the plume which is comparable to that obtained using UK-ADMS. However, the Lagrangian integral time scale constant, c L , which governs the influence turbulence has on the dispersion, must also be modified to bring absolute values of accretion rates in line with those observed in UK-ADMS.

Directional change of tracer trajectories in rotating Rayleigh-Bénard convection.

The angle of directional change of tracer trajectories in rotating Rayleigh-Bénard convection is studied as a function of the time increment τ between two instants of time along the trajectories, both experimentally and with direct numerical simulations. Our aim is to explore the geometrical characterization of flow structures in turbulent convection in a wide range of timescales and how it is affected by background rotation. We find that probability density functions (PDFs) of the angle of directional change θ(t,τ) show similar behavior as found in homogeneous isotropic turbulence, up to the timescale of the large-scale coherent flow structures. The scaling of the averaged (over particles and time) angle of directional change Θ(τ)=〈|θ(t,τ)|〉 with τ shows a transition from the ballistic regime [Θ(τ)∼τ^{c} with c=1] for τ≲τ_{η}, with τ_{η} the Kolmogorov timescale, to a scaling with smaller exponent c for τ_{η}≲τ≲T_{L}, with T_{L} the Lagrangian integral timescale. This scaling exponent is approximately constant in the weakly rotating regime (Rossby number Ro≳2.5) and is decreasing for increasing rotation rates when Ro≲2.5. We show that this trend in the scaling exponent is related with the large-scale coherent structures in the flow; the large-scale circulation for Ro≳2.5 and vertically aligned vortices emerging from the boundary layers (BLs) near the top and bottom plates and penetrating into the bulk for Ro≲2.5. In the viscous BLs, the PDFs of θ(t,τ) and scaling properties of Θ(τ) are in general different from those measured in the bulk and depend on the type of boundary layer, in particular whether the BL is of Prandtl-Blasius type (Ro≳2.5) or of Ekman type (Ro≲2.5). When it is of Ekman type, a stronger dynamic coupling exists between the BL and the bulk of the flow, resulting in similar scaling exponents in BL and bulk.

Statistical steady state in turbulent droplet condensation

Motivated by systems in which droplets grow and shrink in a turbulence-driven supersaturation field, we investigate the problem of turbulent condensation in a general manner. Using direct numerical simulations, we show that the turbulent fluctuations of the supersaturation field offer different conditions for the growth of droplets which evolve in time due to turbulent transport and mixing. Based on this, we propose a Lagrangian stochastic model for condensation and evaporation of small droplets in turbulent flows. It consists of a set of stochastic integro-differential equations for the joint evolution of the squared radius and the supersaturation along the droplet trajectories. The model has two parameters fixed by the total amount of water and the thermodynamic properties, as well as the Lagrangian integral time scale of the turbulent supersaturation. The model reproduces very well the droplet size distributions obtained from direct numerical simulations and their time evolution. A noticeable result is that, after a stage where the squared radius simply diffuses, the system converges exponentially fast to a statistical steady state independent of the initial conditions. The main mechanism involved in this convergence is a loss of memory induced by a significant number of droplets undergoing a complete evaporation before growing again. The statistical steady state is characterized by an exponential tail in the droplet mass distribution. These results reconcile those of earlier numerical studies, once these various regimes are considered.

Target Lagrangian kinematic simulation for particle-laden flows.

The target Lagrangian kinematic simulation method was motivated as a stochastic Lagrangian particle model that better synthesizes turbulence structure, relative to stochastic separated flow models. By this method, the trajectories of particles are constructed according to synthetic turbulent-like fields, which conform to a target Lagrangian integral timescale. In addition to recovering the expected Lagrangian properties of fluid tracers, this method is shown to reproduce the crossing trajectories and continuity effects, in agreement with an experimental benchmark.

Statistical properties of the surface velocity field in the northern Gulf of Mexico sampled by GLAD drifters

AbstractThe Grand LAgrangian Deployment (GLAD) used multiscale sampling and GPS technology to observe time series of drifter positions with initial drifter separation of O(100 m) to O(10 km), and nominal 5 min sampling, during the summer and fall of 2012 in the northern Gulf of Mexico. Histograms of the velocity field and its statistical parameters are non‐Gaussian; most are multimodal. The dominant periods for the surface velocity field are 1–2 days due to inertial oscillations, tides, and the sea breeze; 5–6 days due to wind forcing and submesoscale eddies; 9–10 days and two weeks or longer periods due to wind forcing and mesoscale variability, including the period of eddy rotation. The temporal e‐folding scales of a fitted drifter velocity autocorrelation function are bimodal with time scales, 0.25–0.50 days and 0.9–1.4 days, and are the same order as the temporal e‐folding scales of observed winds from nearby moored National Data Buoy Center stations. The Lagrangian integral time scales increase from coastal values of 8 h to offshore values of approximately 2 days with peak values of 3–4 days. The velocity variance is large, , the surface velocity statistics are more anisotropic, and increased dispersion is observed at flow bifurcations. Horizontal diffusivity estimates are in coastal regions with weaker flow to in flow bifurcations, a strong jet, and during the passage of Hurricane Isaac. The Gulf of Mexico surface velocity statistics sampled by the GLAD drifters are a strong function of the feature sampled, topography, and wind forcing.

Global characteristics of coherent vortices from surface drifter trajectories

AbstractAn algorithm is developed that can automatically identify loopers in Lagrangian trajectory data, i.e., looping trajectories that complete at least two orbits, in a significant update to Griffa et al. (2008). This algorithm is applied to the Global Drifter Program data set, and over 15,000 looping trajectory segments are identified worldwide. While two third of these segments are 14–39 days long, some persist for hundreds of days; the longest looper in the record persisted for 287 days. The paths taken by the vortices at the center of these looper trajectory segments can be calculated from these data. The Lagrangian integral time scale can also be estimated for the looper segments, and is generally very close to the orbital period—a value several times larger than the integral time scales characterizing nonloopers. Fundamental time‐mean quantities such as total kinetic energy and velocity are shown to be significantly different between loopers and nonloopers. These results suggest that a careful approach to the data might require separately calculating means of the nonloopers and loopers, and only afterward combining the weighted results for an overall time‐mean picture. While many of the loopers with large radii orbit vortices identified in altimeter‐derived eddy census data, many with smaller radii do not match vortices resolved in altimetry. The data from this study are available at http://www.aoml.noaa.gov/phod/loopers/.

Comparing isopycnal eddy diffusivities in the Southern Ocean with predictions from linear theory

We show that the potential vorticity diffusivity predicted by linear stability analysis (LSA), is the same as a linearized version of Lagrangian cross-stream isopycnal diffusivity. Both can be written in terms of the same expression − the product of the eddy kinetic energy (EKE) and the integral time scale that involves the Lagrangian decay scale γ or the growth rate ωi of the most unstable wave, and a frequency that is related to the difference of the mean flow speed and real part of the phase speed of the unstable waves.Diffusivities from LSA are compared to Lagrangian isopycnal eddy diffusivities estimated from more than 700,000 numerical particles in the Southern Ocean of an eddying model. They show different spatial dependency. LSA predicts eddy diffusivities that are enhanced at the steering level where the mean flow speed equals the phase speed of the unstable waves. In contrast, Lagrangian diffusivities exhibit no clear steering level maxima, but are instead surface intensified in many places. The differences between the Lagrangian and diffusivities from LSA can be understood because EKE predicted from LSA differs from the simulated one, and because the estimated decay scale γ is on average about 4 times larger than the largest linear growth rate. The diagnosed Lagrangian integral time scale has maxima at the depth where the mean flow speed equals the phase speed of the most unstable wave, but the diffusivity maxima are shifted towards the surface because the simulated EKE decreases rapidly with depth. Possibilities for a simple parameterization for the diffusivity are discussed.

Direct numerical simulation studies of Lagrangian intermittency in turbulence

Lag-averaged Lagrangian statistics from direct numerical simulations over a range of Reynolds numbers are analyzed to test the predictions of the Lagrangian Refined Similarity Hypothesis (LRSH). The analysis uses the Lagrangian integral time scale to scale the lag since it is the natural time scale to reveal trends and scaling with Reynolds number. Both the velocity difference and the dissipation rate probability density functions (PDFs) collapse across inertial sub-range and diffusive scales for approximately the same values of the scaled lag, and in the zero lag limit are independent of the lag and depend only on the Reynolds number. These findings are consistent with the LRSH. The velocity difference PDFs are characterized by stretched exponential tails, while the dissipation rate PDFs for small lags have a log normal core with power law tails at both large and small values of the dissipation rate. The velocity structure functions show inertial sub-range similarity scaling with Reynolds number which extends to smaller scales with increasing Reynolds number. Estimates of the scaling exponents obtained are consistent with those from previous studies. They tend to saturate at a value of about two for high order moments. Non-dimensional acceleration moments show a striking power law dependence on Reynolds number from which novel estimates of the scaling exponents have been determined. Similarity scaling is much more elusive to demonstrate in the dissipation rate moments. The data are consistent with, but do not confirm, the Oboukhov relationship connecting velocity structure functions and dissipation rate moments on inertial sub-range scales.

Drifter-derived estimates of lateral eddy diffusivity in the World Ocean with emphasis on the Indian Ocean and problems of parameterisation

The Global Drifter Program data set, collected in the period 1979–2011, is divided in drogue-on and drogue-off subsets which are then used to develop 5°×5° bin estimates of lateral eddy diffusivity in the World Ocean's mixed and surface layers, respectively, by means of a modification of the Davis (1987. Journal of Marine Research, 430 (45), 635–666.; 1991. Deep Sea Research. 38, 432 (1), S531–S571.) approach. The analysis is geographically focused on the Indian Ocean, the region where such calculations have been lacking. As the Indian Ocean circulation is strongly determined by monsoonal winds, an effort was made to calculate the diffusivity relative to monsoon-driven mean currents. The drifter-derived estimates of the asymptotic lateral diffusivity at large time lags K∞, were found to be below 1×104m2s−1 almost everywhere in the World Ocean, while the maximum diffusivity Kmax, equivalent to integrating the Lagrangian velocity autocorrelation to the first zero crossing, exceeds K∞ in strong ocean currents by a factor of 2–4. At mid latitudes (20°≤|ϕ|≤50°), the estimates of K∞ in the mixed and surface layers are shown to follow the frozen field regime parameterisation K∞~EKE×Lbc, where EKE is the eddy kinetic energy and Lbc is the baroclinic Rossby deformation radius. In contrast, at low latitudes (|ϕ|<29°) in the surface layer, the fixed float regime parameterisation Kmax,K∞=EKE×T0, where T0=1.9 days and 1.3 days is the typical value of the Lagrangian integral timescale in the cases of Kmax and K∞, respectively, is preferable.

Time persistence of floating-particle clusters in free-surface turbulence

We study the dispersion of light particles floating on a flat shear-free surface of an open channel in which the flow is turbulent. This configuration mimics the motion of buoyant matter (e.g., phytoplankton, pollutants, or nutrients) in water bodies when surface waves and ripples are smooth or absent. We perform direct numerical simulation of turbulence coupled with Lagrangian particle tracking, considering different values of the shear Reynolds number (Re_{τ}=171 and 509) and of the Stokes number (0.06<St<1 in viscous units). Results show that particle buoyancy induces clusters that evolve towards a long-term fractal distribution in a time much longer than the Lagrangian integral fluid time scale, indicating that such clusters overlive the surface turbulent structures which produced them. We quantify cluster dynamics, crucial when modeling dispersion in free-surface flow turbulence, via the time evolution of the cluster correlation dimension.

Direct observation of subtropical mode water circulation in the western North Atlantic Ocean

Eighteen Degree Water (EDW) is the dominant subtropical mode water of the North Atlantic subtropical gyre and is hypothesized as an interannual reservoir of anomalous heat, nutrients and CO2. Although isolated beneath the stratified upper-ocean at the end of each winter, EDW may re-emerge in subsequent years to influence mixed layer properties and consequently air–sea interaction and primary productivity. Here we report on recent quasi-Lagrangian measurements of EDW circulation and stratification in the western subtropical gyre using an array of acoustically-tracked, isotherm-following, bobbing profiling floats programmed to track and intensively sample the vertically homogenized EDW layer and directly measure velocity on the 18.5°C isothermal surface.The majority of the CLIVAR Mode Water Dynamics Experiment (CLIMODE) bobbers drifted within the subtropical gyre for 2.5–3.5 years, many exhibiting complex looping patterns indicative of an energetic eddy field. Bobber-derived Lagrangian integral time and length scales (3 days, 68km) associated with motion on 18.5°C were consistent with previous measurements in the Gulf Stream extension region and fall between previous estimates at the ocean surface and thermocline depth. Several bobbers provided evidence of long-lived submesoscale coherent vortices associated with substantial EDW thickness. While the relative importance of such vortices remains to be determined, our observations indicate that these features can have a profound effect on EDW distribution. EDW thickness (defined using a vertical temperature gradient criterion) exhibits seasonal changes in opposition to a layer bounded by the 17°C and 19°C isotherms. In particular, EDW thickness is generally greatest in winter (as a result of buoyancy-forced convection), while the 17°–19°C layer is thickest in summer consistent with seasonal Ekman pumping. Contrary to previous hypotheses, the bobber data suggest that a substantial fraction of subducted EDW is isolated from the atmosphere for periods of less than 24 months. Seasonal-to-biennial re-emergence (principally within the recirculation region south of the Gulf Stream) appears to be a common scenario which should be considered when assessing the climatic and biogeochemical consequences of EDW.

Flow and mixing characteristics of swirling double-concentric jets subject to acoustic excitation

Characteristic flow modes, flow evolution processes, jet spread width, turbulence properties, and dispersion characteristics of swirling double-concentric jets were studied experimentally. Jet pulsations were induced by means of acoustic excitation. Streak pictures of smoke flow patterns, illuminated by a laser-light sheet, were recorded by a high-speed digital camera. A hot-wire anemometer was used to digitize instantaneous velocity instabilities in the flow. Jet spread width was obtained through a binary edge identification technique. Tracer-gas concentrations were measured for information on jet dispersions. Two characteristic flow patterns were observed: (1) synchronized vortex rings appeared in the low excitation intensity regime (the excitation intensity less than one) and (2) synchronized puffing turbulent jets appeared in the high excitation intensity regime (the excitation intensity greater than one). In the high excitation intensity regime, the “suction back” phenomenon occurred and therefore induced in-tube mixing. The jet spread width and turbulent fluctuation intensity exhibited particularly large values in the high excitation intensity regime at the excitation Strouhal numbers smaller than 0.85. At the excitation Strouhal numbers >0.85, the high-frequency effect caused significant decay of jet breakup and dispersion—the jet spread width and fluctuation intensity decreased sharply and may, at very high Strouhal numbers, asymptotically approach values almost the same as the values associated with unexcited jets. Exciting the jets at the high excitation intensity regime, the effects of puffing motion and in-tube mixing caused breakup of the jet in the near field and therefore resulted in a small Lagrangian integral time and small length scales of fluctuating eddies. This effect, in turn, caused drastic dispersion of the central jet fluids. It is possible that the excited jets can attain 90 % more improvements than the unexcited jets. We provide a domain regarding excitation intensity and Strouhal number to facilitate identification of characteristic flow modes.