LIF measurement of the diluting effect of surface waves on turbulent buoyant plumes.

In this paper, the diluting effect of surface waves on a buoyant plume has been measured using a Laser Induced Fluorescence (LIF) technique. The resulting time-averaged, full field concentration maps have allowed quantification of enhanced mixing due to surface waves as well as measurement of other plume parameters

Scaling for turbulent viscosity of buoyant plumes in stratified fluids : PIV measurement with implications for submarine hydrothermal plume turbulence

© The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part I: Oceanographic Research Papers 129 (2017): 89-98, doi:10.1016/j.dsr.2017.10.006.Time-resolved particle image velocimetry (PIV) has been used to measure instantaneous twodimensional velocity vector fields of laboratory-generated turbulent buoyant plumes in linearly stratified saltwater over extended periods of time. From PIV-measured time-series flow data, characteristics of plume mean flow and turbulence have been quantified. To be specific, maximum plume penetration scaling and entrainment coefficient determined from the mean flow agree well with the theory based on the entrainment hypothesis for buoyant plumes in stratified fluids. Besides the well-known persistent entrainment along the plume stem (i.e., the ‘plumestem’ entrainment), the mean plume velocity field shows persistent entrainment along the outer edge of the plume cap (i.e., the ‘plume-cap’ entrainment), thereby confirming predictions from previous numerical simulation studies. To our knowledge, the present PIV investigation provides the first measured flow field data in the plume cap region. As to measured plume turbulence, both the turbulent kinetic energy field and the turbulence dissipation rate field attain their maximum close to the source, while the turbulent viscosity field reaches its maximum within the plume cap region; the results also show that maximum turbulent viscosity scales as νt,max = 0.030 (B/N)1/2, where B is source buoyancy flux and N is ambient buoyancy frequency. These PIV data combined with previously published numerical simulation results have implications for understanding the roles of hydrothermal plume turbulence, i.e. plume turbulence within the cap region causes the ‘plume-cap’ entrainment that plays an equally important role as the ‘plume-stem’ entrainment in supplying the final volume flux at the plume spreading level.Part of this work was financially supported by the National Natural Science Foundation of China and Natural Science Foundation of Zhejiang Province under respective Project no. 11672267 and LR16E090001 to ZH. HJ was supported by a National Science Foundation Grant NSF OCE-1038055 through the RIDGE2000 program and an internal funding from WHOI

Two-phase velocity measurement in a particle-laden jet

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Zonation of positively buoyant jets interacting with the water-free surface quantified by physical and numerical modelling

ABSTRACT: The evolution of positively buoyant jets was studied with non-intrusive techniques-Particle Image Velocimetry (PIV) and Laser Induce Fluorescence (LIF)-by analyzing four physical tests in their four characteristic zones: momentum dominant zone (jet-like), momentum to buoyancy transition zone (jet to plume), buoyancy dominant zone (plume-like), and lateral dispersion dominant zone. Four configurations were tested modifying the momentum and the buoyancy of the effluent through variations of flow discharge and the thermal gradient with the receiving water body, respectively. The physical model results were used to evaluate the performance of numerical models to describe such flows. Furthermore, a new method to delimitate the four characteristic zones of positively buoyant jets interacting with the water-free surface was proposed using the angle (?) shaped by the tangent of the centerline trajectory and the longitudinal axis. Physical model results showed that the dispersion of mass (concentrations) was always greater than the dispersion of energy (velocity) during the evolution of positively buoyant jets. The semiempirical models (CORJET and VISJET) underestimated the trajectory and overestimated the dilution of positively buoyant jets close to the impact zone with the water-free surface. The computational fluid dynamics (CFD) model (Open Field Operation And Manipulation model (OpenFOAM) is able to reproduce the behavior of positively buoyant jets for all the proposed zones according to the physical resultsFunding: The work described in this paper is part of a research project financed by the VI National Plan (2008–2012) for Research in Science and Technological Innovation of the Spanish Government (VERTIZE CTM2012-32538)

Spatial and temporal resolution of fluid flows: LDRD final report

This report describes a Laboratory Directed Research and Development (LDRD) activity to develop a diagnostic technique for simultaneous temporal and spatial resolution of fluid flows. The goal is to obtain two orders of magnitude resolution in two spatial dimensions and time simultaneously. The approach used in this study is to scale up Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) to acquire meter-size images at up to 200 frames/sec. Experiments were conducted in buoyant, fully turbulent, non-reacting and reacting plumes with a base diameter of one meter. The PIV results were successful in the ambient gas for all flows, and in the plume for non-reacting helium and reacting methane, but not reacting hydrogen. No PIV was obtained in the hot combustion product region as the seed particles chosen vaporized. Weak signals prevented PLIF in the helium. However, in reacting methane flows, PLIF images speculated to be from Poly-Aromatic-Hydrocarbons were obtained which mark the flame sheets. The results were unexpected and very insightful. A natural fluorescence from the seed particle vapor was also noted in the hydrogen tests

Flow turbulence characteristics and mass transport in the near wake region of an aquaculture cage net panel

Cage-based aquaculture has been growing rapidly in recent years. In some locations, cagebased aquaculture has resulted in the clustering of large quantities of cages in fish farms located in inland lakes or reservoirs and coastal embayments or fjords, significantly affecting flow and mass transport in the surrounding waters. Existing studies have focused primarily on the macro-scale flow blockage effects of fish cages, and the complex wake flow and associated near-field mass transport in the presence of the cages remain largely unclear. As a first step toward resolving this knowledge gap, this study employed the combined Particle Image Velocimetry and Planar Laser Induced Fluorescence (PIV-PLIF) flow imaging technique to measure turbulence characteristics and associated mass transport in the near wake of a steady current through an aquaculture cage net panel in parametric flume experiments. In the near-wake region, defined as ~3M (mesh size) downstream of the net, the flow turbulence was observed to be highly inhomogeneous and anisotropic in nature. Further downstream, the turbulent intensity followed a power-law decay after the turbulence production region, albeit with a decay exponent much smaller than reported values for analogous grid-generated turbulence. Overall, the presence of the net panel slightly enhanced the lateral spreading of the scalar plume, but the lateral distribution of the scalar concentration, concentration fluctuation and transverse turbulent scalar flux exhibited self-similarity from the near-wake region where the flow was still strongly inhomogeneous. The apparent turbulent diffusivity estimated from the gross plume parameters was found to be in reasonable agreement with the Taylor diffusivity calculated as the product of the transverse velocity fluctuation and integral length scale, even when the plume development was still transitioning from a turbulent-convective to turbulent-diffusive regime. The findings of this study provide references to the near-field scalar transport of fish cages, which has important implications in the assessment of the environmental impacts and environmental carrying capacity of cage-based aquaculture

Formation-breakdown cycle of turbulent jets in a rotating fluid

Results of comprehensive particle image velocimetry measurements investigating the dynamics of turbulent jets in a rotating fluid are presented. It is observed that background system rotation induces a time-periodic formation–breakdown cycle of the jets. The flow dynamics associated with this process is studied in detail. It is found that the frequency of the cycle increases linearly with the background rotation rate. The data show that the onset of the breakdown phase and of the reformation phase of the cycle can be characterized in terms of a local Rossby number employing an internal velocity and a length scale of the jet. The critical values for this local Rossby number, for onset of breakdown and reformation, scale linearly with a global Rossby number based on the flow conditions at the source. The analysis of the experimental data suggests centrifugal instability as the potential origin of the formation–breakdown cycle

Turbulent entrainment in flows induced by distributed buoyancy sources

Free shear and wall-bounded buoyancy-driven turbulent flows occur in both natural environments and industrial situations. In this thesis, to better understand the entrainment process within these flows, experiments and theory have been used to investigate point and distributed buoyancy sources and, in particular, the effect of a bounding vertical wall on these flows. A free shear flow was first investigated by performing velocity and scalar edge measurements on an axisymmetric plume created by a continuous point source release of buoyancy. By conditionally sampling the velocity measurements based on the presence of both eddies and plume fluid, engulfment, whereby large pockets of ambient are engulfed in to the plume, was shown to be a dominant turbulent entrainment process. To isolate the effect of a wall on a turbulent buoyancy-driven flow, a line plume distant from all vertical boundaries and a wall plume, adjacent to a vertical wall were also studied. Simultaneous velocity and buoyancy field measurements were performed and a reduction in the net entrainment, and entrainment coefficient, for a wall plume were found. This reduction was investigated by considering an energy decomposition of the entrainment coefficient where the relative contributions of turbulent production, buoyancy and viscous terms were calculated. The reduced entrainment was also investigated by considering the statistics of the turbulent interface. Finally, simultaneous velocity and buoyancy field measurements on a vertically distributed buoyant plume were performed by forcing relatively dense fluid through a very low porosity plate. A reduced entrainment coefficient, compared to that of a wall plume, was observed. In order to model the ventilation of a room with a heated or cooled wall the flow was then enclosed within a mechanically ventilated model room. The evolving and steady-state ambient stratification was measured using dye-attenuation with an LED-light bank for varying buoyancy fluxes and ventilation flow rates.PhD sponsored and partly funded by Aru

Detailed Measurements of Fire-Induced Mixing Phenomena

This study successfully validated the use of salt-water analog modeling as an effective diagnostic tool for predicting fire-induced flows. A technique was developed for taking measurements using combined Planar Laser-Induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV), and results were analyzed with respect to smoke filling as well as transient ceiling layer dynamics, and turbulent mixing intensity. Data was shown to be in good agreement with theory, further validating the salt-water analogy as a tool for diagnostics, prediction, and scaling of fire phenomena