Plume-like Flow Research Articles

Identification of turbulent structures of inclined negatively buoyant jets with bed effects

Present study aims at exploring turbulent structures of inclined negatively buoyant jets (INBJs) using large eddy simulation (LES). Four types of turbulent structures are observed using the Q criterion and vorticity; namely, shear layer vortices, buoyancy-induced vortices, counter-rotating vortex pair (CRVP), and gravity current structures. The shear layer vortices emerge and develop in the momentum-driven regime of INBJ, where the jet core concentration reduces by nearly 20% due to the small amount of entrainment. Further downstream, where the negative buoyancy dominates the INBJ, the buoyancy-induced vortices form, which are larger and weaker than the momentum-driven structures. The velocity magnitude of the entrained flow also increases by a factor of 4, and the relative concentration decay rises to 70% in the buoyancy-driven region. A CRVP is also detected in the jet cross-sections. The vortex pair may be induced by a secondary flow inside the jet sections due to the jet deflection by gravity. The size of CRVP constantly grows from upstream to downstream, while the vorticity magnitude of it has an increasing-decreasing trend due of the changes in the jet trajectory curvature. A density current forms with big rollers on its edges when the plume-like flow impacts the bottom. Similar to previous studies of density currents, the streaky structure is also observed on the bed. Furthermore, it is found that increasing the bed slope results in bigger and more dispersed turbulent structures, especially in the plume-like regime after the maximum height point.

Determination of the discharge coefficient through vent driven by a non-Boussinesq plume in an emptying filling box

The effect of high buoyancy forces due to density contrast between the fluids on either side of the vent on the discharge coefficient Cd is taken into account in the laboratory. Salt water experiments are conducted at small scale in a large fresh water tank with saline to generate buoyancy force. In non-Boussinesq cases, a larger discharge parameter Γd at the vent may make plume-like flow contract further with a smaller value of the discharge coefficient. A nearly twofold time in the non-dimensional form for draining light fluid fully out of the space is observed comparing with time predicted with a constant value of Cd = 0.6 for simple draining flows. A displacement flow theoretical model with virtual source correction at the initial position of an internal source is developed to reveal a time-dependent movement of the layer interface, and the oscillatory amplitude of the interface overshooting during the transient period is found to depend on the two dimensionless geometrical parameters Λ and Θ.

Nanofluid and nanoemulsion absorbents for the enhancement of CO2 absorption performance

Research on CO 2 capture using nanofluids and nanoemulsions has been actively conducted in recent decades, and numerous studies have achieved improvements in the CO 2 absorption performance of nanofluids and nanoemulsions. In this study, to analyze the enhancement of CO 2 absorption performance achieved by nanofluids and nanoemulsions, experiments are conducted for visualizing the diffusion of CO 2 inside absorbents using the shadowgraph method. The nanofluid absorbents are prepared using SiO 2 solid particles, and the nanoemulsion absorbents are prepared using dodecane as the dispersed phase. The effects of concentration are analyzed through the absorption experiments performed for each concentration. The diffusion coefficient is calculated experimentally and theoretically based on the results of the visualization tests. The absorption of CO 2 begins at the absorbent’s upper interface when the CO 2 gas is stationary or moving. The highest absorption is observed at 0.05 vol% of nanoparticles. The absorption performances of the nanofluid and nanoemulsion absorbents are improved by 23.05% and 26.80%, respectively. As CO 2 is absorbed, the absorbent density changes and the Rayleigh convection becomes prominent, resulting in a plume-like flow. The plume formation and growth stages are subdivided into four stages, and both absorbents are compared. The visualization test results indicate that the hydrodynamic effect is a dominant factor in improving nanofluids’ mass transfer and nanoemulsions. Hydrodynamic effect model. • Enhancement mechanism of CO 2 absorption performance by nanoabsorbents is firstly clarified. • As CO 2 absorption starts, the Rayleigh convection becomes prominent, resulting in a plume-like flow. • The absorption performances of the nanofluid and nanoemulsion absorbents are improved by 23.05% and 26.80%, respectively. • Hydrodynamic-effect model plays an important role in improving the absorption performance.

Numerical simulation of two coalescing turbulent forced plumes in linearly stratified fluids

A computational fluid dynamic model that can solve the Reynolds-averaged Navier-Stokes equations and the species transport equation is developed to simulate two coalescing turbulent forced plumes, which are released with initial momentum and buoyancy flux into a linearly stable stratified environment. The velocity fields, turbulence structures, and entrainment of two plumes with different source separations and source buoyancy fluxes are analyzed quantitatively, in comparison with a series of physical experiments. An empirical parameterization is proposed to predict the amplification of the maximum rise height of two coalescing forced plumes caused by superposition and mutual entrainment. The maximum values of both turbulent kinetic energy and turbulence dissipation rate decrease monotonically with the increase in source separation of the two turbulent plumes. However, the trajectory of the maximum turbulent viscosity attained in the plume cap region presents two notable enhancements. This variation may be attributed to the turbulence transported from the touching region and the strong mixing around the neutrally buoyant layer between two plumes, while the mixing is caused by the lateral convection and the rebound after overshooting. The plume entrainment coefficient in near vent stems has a positive relationship with the source Richardson number. A transition of flow regimes to plume-like flows would occur when the contribution of initial momentum is important. The entrainment coefficient will decrease in the touching region of two plumes due to mutual entrainment, while the superposition of plumes can lead to distortion of the boundary of plume sectors.

The discharge plume parameter and its implications for an emptying–filling box

The natural ventilation flow driven by an internal buoyant plume in a box involving an upper opening (vent) located at the ceiling (for the outflow) and a large lower opening at the floor (for the inflow) is examined theoretically in a general non-Boussinesq case. Analytical solutions of this emptying–filling box problem allow the characteristics of the flow at the vent to be determined. From these characteristics, a non-dimensional parameter $\unicode[STIX]{x1D6E4}_{d}$ (called the discharge plume parameter) is expressed. This parameter characterizes the initial balance of volume, buoyancy and momentum fluxes in the plume-like flow that forms above the vent. We then note that the value of $\unicode[STIX]{x1D6E4}_{d}$ allows the buoyant fluid layer depth in the box to be estimated, which is a new and interesting result for natural ventilation problems. Following previous experimental results, the decrease of the vent discharge coefficient $C_{d}$ when $\unicode[STIX]{x1D6E4}_{d}$ increases is discussed and a theoretical model based on plume necking is proposed. The emptying–filling box model is then extended for a variable $C_{d}$ (depending on $\unicode[STIX]{x1D6E4}_{d}$). Even though the discharge coefficient may be markedly reduced at high values of $\unicode[STIX]{x1D6E4}_{d}$, our results show that this only affects transients and the steady state of an emptying–filling box for relatively thin buoyant fluid layers.

Supersensitive odor discrimination is controlled in part by initial transient interactions between the most sensitive dorsal olfactory receptors and G-proteins

Pairs of enantiomeric odor ligands are difficult to resolve by instrumental analyses because compounds with mirror-image molecular structures have almost identical physicochemical properties. The olfactory system, however, discriminates (–)-forms of enantiomers from their (+)-forms within seconds. To investigate key olfactory receptors for enantiomer discrimination, we compared behavioral detection and discrimination thresholds of wild-type mice with those of ΔD mice that lack all dorsal olfactory receptors. Surprisingly, wild-type mice displayed an exquisite “supersensitivity” to enantiomeric pairs of wine lactones and carvones in both detection and discrimination tasks using odor plume-like flows in a Y-maze. In contrast, ΔD mice showed >10 10 -fold reductions in enantiomer discrimination sensitivity compared to wild-type mice. ΔD mice detected one or both of the (–)- and (+)-enantiomers over a wide concentration range, but were unable to discriminate them. This “enantiomer odor discrimination paradox” indicates that the most sensitive dorsal receptors play a critical role in hierarchical odor coding for enantiomer identification. In addition, to identify residues responsible for the rapid and robust response of murine olfactory receptor S6 ( mOR-S6 ) via chimeric Gα 15_olf , mutations of the C-terminal helix 8 were analyzed in a heterologous functional expression system. The N-terminal hydrophobic core between helix 8 and TM1−2 of mOR-S6 is important for Gα activation. A point mutation of a helix 8 N-terminal acidic residue eliminated the improved response dynamics via the chimeric Gα 15_olf . This result suggests that an N-terminal acidic residue of helix 8 is responsible for rapid Gα activation. Supersensitive odor discrimination is thus largely governed by signals from the most sensitive dorsal olfactory receptors with the shortest onset latencies, which are controlled in part by initial transient interactions between the receptor C-terminal helix 8 and the Gα C-terminal region.

Turbulent ‘stopping plumes’ and plume pinch-off in uniform surroundings

Observations of turbulent convection in the environment are of variously sustained plume-like flows or intermittent thermal-like flows. At different times of the day the prevailing conditions may change and consequently the observed flow regimes may change. Understanding the link between these flows is of practical importance meteorologically, and here we focus our interest upon plume-like regimes that break up to form thermal-like regimes. It has been shown that when a plume rises from a boundary with low conductivity, such as arable land, the inability to maintain a rapid enough supply of buoyancy to the plume source can result in the turbulent base of the plume separating and rising away from the source. This plume ‘pinch-off’ marks the onset of the intermittent thermal-like behavior. The dynamics of turbulent plumes in a uniform environment are explored in order to investigate the phenomenon of plume pinch-off. The special case of a turbulent plume having its source completely removed, a ‘stopping plume’, is considered in particular. The effects of forcing a plume to pinch-off, by rapidly reducing the source buoyancy flux to zero, are shown experimentally. We release saline solution into a tank filled with fresh water generating downward propagating steady turbulent plumes. By rapidly closing the plume nozzle, the plumes are forced to pinch-off. The plumes are then observed to detach from the source and descend into the ambient. The unsteady buoyant region produced after pinch-off, cannot be described by the power-law behavior of either classical plumes or thermals, and so the terminology ‘stopping plume’ (analogous to a ‘starting plume’) is adopted for this type of flow. The propagation of the stopping plume is shown to be approximately linearly dependent on time, and we speculate therefore that the closure of the nozzle introduces some vorticity into the ambient, that may roll up to form a vortex ring dominating the dynamics of the base of a stopping plume.

Simulation on thermal stratification and de-stratification in liquefied gas tanks

The phenomena of thermal stratification and de-stratification in liquefied gas tanks with heat leak through the side wall were researched numerically. The calculating model developed on ANSYS FLUENT 12.0 was verified through the experimental results from literature. The flow behaviors of the liquid and the temperature distributions at stratification and de-stratification stages were illustrated and the interaction between them was analyzed. When the liquid is stratified, the flow field presents a semi-circulation near the liquid surface with a plume-like flow under it and there exists a cold core under the surface which resisting the warmer circulating flow. The de-stratification of the stratified liquid begins when the circulating loops close with the resistance reduced because of the disappearance of the cold core, and once the circulation forms, it extends down to make the thermal stratifications disappear layer by layer from up to down.

Impinging axisymmetric turbulent fountains

The behavior of a turbulent fountain that impinges perpendicularly on a horizontal plate a distance H from the fountain origin in a quiescent environment of uniform density is investigated. Images from light induced fluorescence visualization of the flow provide a clear picture of the internal structure of the vertical fountain, impinging radial outflow and the subsequent plumelike flow. For source-plate separations H∕Lm0⩽1.5, where Lm0 is the source jet length, and source Froude number Fr0>10, the impingement of the fountain results in a buoyant radial wall jet that spreads horizontally before separating from the plate at a radius Rsp. The re-entrainment into the fountain and wall jet of buoyant fluid released on separation leads to the formation of a toroidal vortex of outer radius ∼Rsp. The correlation Rsp∕H=0.03Rirad−0.75, deduced from theoretical scalings and the results of our experiments, successfully relates the maximum nondimensional radial spread to the Richardson number Rirad that characterizes the source of the buoyant radial wall jet. The transient development of the flow and influence of the buoyant plume above the impingement zone are discussed.

On Boussinesq and non-Boussinesq starting forced plumes

The characteristics of Boussinesq and non-Boussinesq starting forced plumes were investigated in this study. Two distinct periods in the transient plume penetration are identified, namely, the period of flow development (PFD) and period of developed flow (PDF). Similarity solutions are developed in PDF by incorporating the behaviour of an isolated buoyant vortex ring and recent laboratory results on the trailing forced plume, and the temporal variation of the penetration rate is derived during the different phases of jet-like, transitional and plume-like flow. To verify the similarity solutions, experiments were conducted on vertical starting forced plumes using combined particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) with refractive index matching. The discharge Reynolds number was varied from 3773 to 7403 and the range of excess densities ( are initial plume and ambient density, respectively) from 2.77% to 25.07%. The experimental results revealed distinct differences between plumes having an initial density difference of larger or smaller than 15% due to the non-Boussinesq effects. Thus, the value of 15% was employed as an approximate criterion to divide the plumes into Boussinesq versus non-Boussinesq cases. The measured penetration rates and the mean centreline axial velocity of the Boussinesq starting forced plumes agreed well with the analytical predictions at the fully developed stage. However, the behaviour was substantially more complex for the non-Boussinesq plumes. In the transient records, it was noted that the time scales for the penetration of the starting plumes and the velocity development in the trailing forced plume were similar, but the time scale for the Gaussian profile to become self-similar was somewhat longer.

Mixing regimes for the flow of dense fluid down slopes into stratified environments

Downslope flows into density-stratified environments have been observed to have the character of detraining gravity currents on small slopes, and of entraining plumes on steep slopes. In this paper, observations of flows on slopes of intermediate (20) steepness are described, and their mixing properties quantified. Both gravity-current-like and plume-like flows are observed, and an observational boundary between these two types is identified. Theoretical models for the bulk properties of these flows are presented, and their predictions are compared with the observations. A theoretical criterion is derived for the limit of applicability of the gravity-current model in terms of the Buoyancy number, the bottom slope and the bottom drag coefficient. This provides a criterion for the boundary between the plume-like and gravity-current-like flows, which is consistent with the observations. These results have implications for the modelling of downslope flows in nature, and indicate where the appropriate dynamical model may change from one type to the other.

Anorogenic Gross Spitzkoppe granite stock in central western Namibia: Part I. Petrology and geochemistry

The Gross Spitzkoppe is a small (~30 km 2 ) anorogenic granite stock that was emplaced into the Damara Orogenic Belt of central western Namibia in response to mantle plume activity and early Cretaceous rifting of western Gondwanaland. The epizonal Gross Spitzkoppe stock (GSS), penecontemporaneous mafic and felsic dikes in the country rocks, and synplutonic mafic dikes and mafic magmatic inclusions within the GSS indicate bimodal magmatism. The GSS consists of three zonally arranged granitic units: medium-grained biotite granite at the margins, coarse-grained biotite granite, and porphyritic granite at the center of the stock. Late-stage silicic dikes and pegmatites cut the granites of the stock. All the granites are mildly peraluminous, topaz-bearing, high-silica monzogranites that contain extremely Fe-rich biotite (siderophyllite-annite) as the only primary mafic silicate. Common accessory minerals include: topaz, fluorite, magnetite, zircon, monazite, thorite, ilmenite, columbite, and niobian rutile. Miarolitic cavities and pegmatite pockets within the stock suggest that fluid saturation was attained. Rectilinear structures of biotite-rich schlieren along the marginal parts of the stock suggest that magma flow was parallel to the walls of the chamber, whereas turbulent or plume-like flows are indicated by curved, circular, and ladder-dike schlieren. Geochemical variation among the granites can be attributed to fractional crystallization of major and accessory minerals and by accumulation of halogen complexes with rare-earth and high-field-strength elements. The negative, partially irregular, correlation between whole-rock contents of F and Cl suggests that Cl was partially lost to a vapor phase during magmatic degassing. The granites of the Gross Spitzkoppe show clear A-type and within-plate granite characteristics, corresponding to the actual tectonic setting. The granites are interpreted to have formed by partial crustal remelting related to mafic underplating in a continental rift environment

Suppression of the Karman Vortex Street due to Positive Buoyancy. Cause and Effect of Fluid Type on Thermo-Fluid Pattern.

The cause and effect of fluid type on thermo-fluid pattern of the suppression of the isothermal Karman vortex street to a modified vortex street and a plume-like flow due to positive buoyancy in a circular cylinder wake with an upward freestream of mercury, air and water are made clear by solving numerically of the two-dimensional, laminar, time-dependent continuity equation, Navier-Stokes equations with the buoyancy term, and energy equation. For any type of fluid, the cause of the suppression is an extinction of wake vorticity ; shear layers increases and the wake vorticity decreases due to positive buoyancy. Behavior of the suppression is different each other in mercury, air and water, beacuse an increase of the wall vorticity and a decrease of the wake vorticity are different in those fluid.

Experimental study of natural convection heat transfer in a semicircular enclosure

An experimental study of natural convection heat transfer in a differentially heated semicircular enclosure was carried out. The flat surface was heated and the radial surface was cooled isothermally. The effects of angle of enclosure inclination on the heat transfer across semicircular regions of several radii were measured for Rayleigh numbers RaR ranging from 6.72 × 106 to 2.33 × 108, using water as the working fluid. The angle of inclination varied from −90 degrees to 90 degrees with radii R of 50, 40, and 30 mm. The flow patterns were sketched from the results of a visualization experiment using aluminum powder. The temperature measurements in the enclosure were carried out using liquid crystals and thermocouples. The results indicate that different flow patterns were encountered as the angle of inclination varied, and the heat transfer rate was largely dependent on the flow pattern. In particular, enhanced heat transfer rates can be obtained when plume-like flow occurs along both hot and cold walls in the case of an upward-facing hot wall. Heat transfer for the inclined enclosure can be predicted using the equation for a vertical enclosure presented in this paper. © 1998 Scripta Technica, Inc. Heat Trans Jpn Res, 26(2): 131–142, 1997

Experimental study of natural convection heat transfer in a semicircular enclosure

An experimental study of natural convection heat transfer in a differentially heated semicircular enclosure was carried out. The flat surface was heated and the radial surface was cooled isothermally. The effects of angle of enclosure inclination on the heat transfer across semicircular regions of several radii were measured for Rayleigh numbers RaR ranging from 6.72 × 106 to 2.33 × 108, using water as the working fluid. The angle of inclination varied from −90 degrees to 90 degrees with radii R of 50, 40, and 30 mm. The flow patterns were sketched from the results of a visualization experiment using aluminum powder. The temperature measurements in the enclosure were carried out using liquid crystals and thermocouples. The results indicate that different flow patterns were encountered as the angle of inclination varied, and the heat transfer rate was largely dependent on the flow pattern. In particular, enhanced heat transfer rates can be obtained when plume-like flow occurs along both hot and cold walls in the case of an upward-facing hot wall. Heat transfer for the inclined enclosure can be predicted using the equation for a vertical enclosure presented in this paper. © 1998 Scripta Technica, Inc. Heat Trans Jpn Res, 26(2): 131–142, 1997

Application of laser holographic interferometry to temperature measurements in buoyant air jets

HE present work is concerned with the temperature measurements on a heated, round, air jet-like or plumelike flow using a real-time holographic interferometry. Buoyant jets arid plumes occur in many environmental problems, from the discharge of thermal effluents into lakes or oceans, to the emission of hot gas in the atmosphere through chimneys. The study of jet- and plume-like flows has been a subject of investigation for almost a half-decade. In most previous investigations of buoyant air jets and plumes, temperatures were measured using intrusive apparatus, for instance, thermocouples1 and the two-wire probes.2 However, temperature distributions typically encountered in these kinds of flows are spatially and temporally variant and require measurements with high spatial and temporal resolution. Recognition of this need has resulted in a considerable effort toward development of a wide range of diagnostic techniques. Owing to current significant progress in the laser optical diagnostics, nonintrusive temperature measurements can be made by many techniques such as holographic interferometry, MachZehnder interferometry, light-scattering thermometry,3-5 and so forth. Among these nonintrusive techniques available for the experimental study of air jets and plumes, holographic interfefometry has the capability for less expensive, in situ, planewise temperature measurements with high spatial and temporal resolution. It is this technique that is employed in the present work.

Temperature measurements in a turbulent round plume

The basic objective of this paper is to present and analyze some experimental information regarding the structure of the mean and fluctuating temperature field in a turbulent, vertical round water plume issuing into non-stratified, non-moving ambient water. The focus of this study is the transition of plume-like flows to fully developed self-preserved plumes. One interesting result of this paper is that self preservation of the turbulent field is accomplished at approximately twice the normalized axial distance required for self-preservation of the mean temperature field. It is found that the normalized centerline intensity of temperature fluctuations has an asymptotic value of 44% in the fully developed region.

Plane turbulent buoyant jets. Part 2. Turbulence structure

The turbulence structure of a plane vertical buoyant jet in the transition state from jet-like to plume-like growth is the object of this investigation. The ambient fluid is of uniform density and motionless except for the flow induced by the jet. An analysis of the turbulence energy equation reveals that the production of turbulent energy by the buoyancy forces relative to the production by the shear stress increases as the jet Richardson number increases, and becomes constant for a plume-like buoyant jet. A systematic set of experiments was carried out to examine the turbulence structure for a wide range of initial Richardson numbers, extending from a value appropriate to a jet-like flow (very close to zero) to that appropriate for a plume-like flow (approximately 0·6). Fast-response thermistors and a laser-Doppler velocimeter were used to measure the buoyant jet's temperature and velocity respectively. The temperature and velocity data were recorded magnetically in digital form and subsequently processed to extract both mean and fluctuating values. The turbulence intensity and the probability density distribution of the temperature and velocity fluctuations, the maximum and minimum temperature, the intermittency, and the frequency of crossing of the hot/cold and the cold/hot interface of a buoyant jet were investigated. It was determined that the intensity of temperature and velocity fluctuations increases with increasing Richardson number. An explanation is suggested for the large-scale vortices observed in a plume.