Laminar Plume Research Articles

Flow Parameter Dependence of Laminar Plumes with Buoyancy Reversal from a Line Source

Laminar plumes that under goes buoyancy reversal have just been studied. It is true that fast penetration of the rising fluid will result to strong interaction between the rising plume and the ambient fluid which will lead to a quick halt based on the production of denser fluid that in turn halt their rise height. Thus, the present work have considered some sort of balance between viscous, inertia and buoyancy to have a reasonable plume’s rise height and effective mixing as we observe the behavior of these rising plumes while varying Reynolds number. At initial time interval, plumes were symmetric. There was a sideways flapping and bobbing motion after when the penetrating head became dense and detached. Two regimes of Re dependence fountains height over the range of Re ≤ 200 and the time τn taken to attain that height was recorded. Relations were also drawn that describes the rate of decrease in the fountain’s height from our empirically determined data set. Profiles of temperature and the various velocity components were also determined. Thus, with the quadratic dependence relation assumption, laminar fountains are feasible for Pr = 7 or 11.4, 5 ≤ Re ≤ 200 and 0.5 Fr. The fountains here are independent of Pr but dependent of both Re and Fr.

Study on the development of a laminar buoyant starting plume following a thermal

Direct numerical simulations are conducted to study the dynamics of a laminar forced plume with sudden removal of the momentum source. The pinch-off process dependent on two non-dimensional parameters (Richardson number and injection duration) is analyzed, and the effects of the two parameters on the catching time when the pure plume catches up with the thermal ahead and the corresponding catching height are investigated. Four stages of the flow development, i.e., initial accelerating stage, transition and pinch-off stage, approaching stage, and interaction stage, are observed. It is found that with the Richardson number increasing, the thermal cap ascends to a higher height and the temperature near the heat source becomes lower. The increase of the Richardson number can cause the accumulation of heat beneath the thinnest stem, and the increase of injection duration can accelerate the accumulation of heat in the stem. It is observed that the fresh pure plume may catch up with the thermal ahead. The increase of injection duration can cause the increase of the catching time and catching height, while the increase of the Richardson number leads to the decrease of the catching time for a short injection duration.

Laminar Plumes from a Line Source and Their Possible Flow Behaviours

Numerical investigation on laminar plumes that under goes buoyancy reversal had just been made, fixing both Re & Pr and varying Fr during the simulations. Water density was taken as a quadratic function of temperature. The result showed some side-to-side apping and bobbing motion after the core of the fountain were exposed together with head detachment behaviour. At the fountains core, profiles of vertical velocity component show that decrease in vertical velocity with height is not smoother even at higher Froude numbers as compared to the case in our previous study for (Re = 50). Empirically determined scalings laws for fountain height and time to attain maximum height were obtained. Initially rising plumes were symmetric and takes much longer time to attain a fully developed stage as Fr increases before sufficient dense fluid is produced to halt their rise. Generally, we can conclude that with the quadratic dependence relation assumption, Irrespective of the ow parameters used within the Reynolds number range 50 \(\le\) Re \(\le\) 100 and Prandtl number Pr = 7&11:4 the ow behaviour remains the same. Except for the behaviour in the profiles of vertical velocity that show some slight differences as compared to our earlier investigation, and the Froude number that represents the balance between inertia and buoyancy forces (responsible for the variation in the fountains heights).

On the mathematical theory of plumes

ABSTRACT We reconsider the theory of turbulent plume formation provided by Schmidt (1941a,b) and its integral formulation, particularly that of Morton et al. (1956). A particular issue for the correct formulation of a mathematical theory is whether the plume is taken to have finite or infinite width, and whether the entrainment rate is prescribed or deduced. Fox (1970) showed that the entrainment rate for a plume can be deduced from the governing partial differential equations by the use of integral moment theory, providing one assumes expressions for the velocity and buoyancy profiles, but it is less clear if entrainment needs to be prescribed if the plume is taken to be of infinite width (as might be appropriate for a laminar plume). Here we choose an eddy viscosity model of a plume which differs from those previously used by allowing the eddy viscosity to vanish with the vertical velocity. We then show that for the ordinary differential equations describing the similarity solution for such a plume rising in an unstratified medium, their solution implies that the plume is finite, and that the entrainment rate at the plume edge is a consequence of the model formulation, and does not need to be hypothesised; and we also show that the entrainment coefficient which is thus determined is consistent with values obtained by experiment. We also show that the resulting velocity profiles differ from those found experimentally by their omission of the Gaussian tail, and we suggest that this discrepancy may be resolved in the model by the inclusion of the small molecular kinematic viscosity.

Impact of nonuniform ambient stratification on thermal plume dynamics

Thermal plumes arising from localized buoyancy sources have been studied extensively, mostly focusing on entrainment and mixing in fully developed turbulence. Here, we focus on the primary instabilities of laminar plumes and their transitions as the strength of the buoyancy source is increased. Many prior studies in this transitional regime have reported a myriad of disparate spatiotemporal plume characteristics. We show that this wide variety is tied to various ways an axisymmetric system can undergo symmetry breaking, that the nature of the ambient stratification plays an important role in the symmetry breaking, and this determines the details of the plume spatiotemporal characteristics.

A three-dimensional numerical study of laminar buoyancy-driven columnar vortices

This study deals with the development of a low momentum whirling laminar plume that results from the interaction between a plume and a circulating flow imposed by a cylindrical rotating screen. Numerical simulations were performed by direct simulation of the fully three-dimensional flow governed by the compressible Navier-Stokes and energy equations. The rotating thermal plume forms a columnar vortex governed by the Grashof (GR) and nondimensional rotation velocity (Reynolds number (ReR)), based on the cylindrical domain’s radius and the angular frequency of rotation of the permeable lateral surface. Numerical solutions were obtained in the range 102<Gr<106 and 10<ReR<150. The vortex generated bifurcates into a pair of steady co-rotating vortices within the range 104<Gr<2×105 and 30<ReR<100. The steady state configuration of the flow is described and analysed, showing the evolution of the vorticity contours in cross-sections that start out as circlets, gradually turn into an ellipse and end up bifurcating into a pair of co-rotating vortices. The role of the boundary layer is also described, showing an increase in the distance between the co-rotating vortex pair for a given Gr and ReR numbers but in the absence of a boundary layer. At last, the temporal evolution of the vortex splitting process is compared to measurements of merging of two co-rotating vortices available in the literature, uncovering that the two phenomena take similar timescales to occur.

Characteristics of complex interaction between two oppositely-moving laminar buoyant-plumes within slender vertical-channels

The article presents laminar plumes, cellular structures and spatial-waviness in free convective flow within a vertical channel under anti-symmetric heating, and the subtle flow-features during the transition to anti-asymmetry. We perform numerical simulations for a wide range of cross-sectional Grashof numbers (262.4≤ Grb ≤33025), channel aspect ratio (0.0375≤ψ ≤0.1255) and isothermal boundary conditions (anti-asymmetric heating to anti-symmetric heating). The chosen working fluid is air; hence the Prandtl number (Pr) is fixed at 0.715. Certain modified parameters are defined for the present study, such as the channel Grashof number (Gr) and symmetry ratio (εC), to govern the working condition of the channel. The physics of the interaction between oppositely moving wall plumes is studied thoroughly, and the highly sensitive, spatially-oscillatory nature of the flow near the plume interface is shown with the help of velocity and temperature contours. Analytical expressions for the primary velocity and temperature have been derived for the wall plumes for the developed region. Also, the crucial role of the secondary velocity field on the heat and mass transfer across the plume interface and convection cell formation is highlighted, which fails to find much attention in the available literature. Motivated by the flow-features, a novel vorticity-mapping is performed, and the entire flow domain is divided into two regimes, viz. a highly rotational core region and a unidirectional near-wall region, which are separated and illustrated using limiting streamlines. The rotational, stagnant core region seems to expand as Gr increases, resulting in a drop in pumping performance. The Gr is varied by changing both the ψ as well as the hot wall temperature ΘH. The relative effects of these methods on the channel pumping, and thermal performance are studied and an optimum method of Gr variation is proposed for the channel. Finally, the effect of the degree of opposing buoyancy forces is studied by varying the cold wall temperature to attain thermal boundary conditions ranging from anti-asymmetry to anti-symmetry. As the opposing buoyancy increases, the heat transfer becomes more convection dominant near the colder wall and less convection dominant near the hotter wall. Also, the pumping performance seems to be affected adversely, as εC is increased. A peculiar change in the channel performance is also highlighted during the transition to anti-symmetric heating conditions near εC=0.9.

Turbulent spots in the flame of a diffusion torch

Jet flame in modes of manageable instability development inside the source of jet formation is one of the methods for controlling the space-time and thermodynamic parameters of the torch. When a system of vortices flows into a free atmosphere, their interaction with the mixing layer and the flame front is accompanied by the appearance of a number of effects. The introduction of artificial perturbations into the flow allowed initiating the onset of a laminarturbulent transition in the tube, which led to the formation of vortex structures in the jet stream. Hot-wire anemometric measurements of the velocity pulsations in the isothermal flow has shown that the disturbance introduced by the controlled valve is an acoustic precursor and a large-scale vortex formation. In this work, we investigated the effect of a system of turbulent spots on a laminar diffusion plume. Based on the two-point space-time temperature correlation dependences, data characterizing the rate of propagation of thermal perturbations during burning of H2/CO2 and C3H8/CO2 fuel mixtures were obtained.

Steady laminar plume generated from a heated line in polymer solutions

From the theoretical viewpoint, governing equations are derived for a steady laminar plume generated from a heated line in polymer solutions, in which similarity variables and the single polymer chain model are introduced. The resolved solutions imply that polymers promote the velocity in the centerline-near region but suppress that in the region far from the centerline. The equivalent effect of polymers is understood as producing two space-dependent source terms, which can explain the interaction between polymers and fluid flow from the viewpoint of energy transport. There exists a critical Weissenberg number (Wi) beyond which the promotion effect in the centerline-near region disappears, which results from the competition of stretching and relaxation of the polymer chain. Meanwhile, the corresponding modification of similarity velocity and heat transport are illustrated and validated numerically by single plume flow in polymer solutions. This work thus may contribute to the understanding of the polymer effect on single plume flow and further the heat transport enhancement mechanism in the bulk flow of turbulent Rayleigh-Bénard convection with polymers.

Near-infrared measurement of axisymmetric temperature field formed by free convection from a 1-mm-diameter heating sphere in water

This paper presents a method of reconstructing transient, three-dimensional axisymmetric temperature fields in water formed by laminar free convection from a small heating sphere. This method is based on the temperature dependence of the water absorption coefficient at a near-infrared wavelength of 1150 nm. The horizontal line profiles of the absorbance at this wavelength, obtained using a unidirectional transmission imaging technique, were approximated with multi-Gaussian functions to robustly and efficiently apply the inverse Abel transform. In the experiment, a 1-mm-diameter steel sphere in water was heated by electromagnetic induction. The reconstruction results show the formation of spherical temperature distributions within the thermal conduction regime, followed by the occurrence of upward free convection, leading to a laminar plume. The size and shape of the plume depended on the heat generation rate in the sphere. These results were analyzed and verified through a comparison with the results of a numerical simulation on the temperature and flow fields.

Swirling flow induced by jets and plumes

The interactions of the slow flow induced by the entrainment of axisymmetric jets and plumes with the surrounding geometry may result in the appearance of azimuthal swirling motion. This swirling flow evolves as it approaches the jet (or plume) as a result of the action of viscous forces on the solid surfaces bounding the fluid domain. When the initial size of the jet or plume a is much smaller than the characteristic radial distance $$R_\infty $$ at which the swirl is generated, the evolution of the flow leads to a self-similar description with weak swirling motion, valid at intermediate radial distances R such that $$a \ll R \ll R_\infty $$ and axial distances L from the source such that $$a \ll L \ll R_\infty $$ . In the present investigation of both laminar and turbulent jets and plumes, it is found that the circulation $$\varGamma $$ is described by a self-similar solution of the second kind, with the exponent $$\lambda $$ in the radial decay rate $$\varGamma \propto (R/R_\infty )^\lambda $$ obtained as an eigenvalue. The resulting azimuthal velocity distributions can find application in mathematical formulations of jet and plume problems involving interactions with ambient swirl, relevant in studies of dust devils and fire whirls.

From a steady plume to periodic puffs during confined carbon dioxide dissolution

In this paper, the stability of a laminar plume due to solutal convection is addressed from experimental, numerical and theoretical points of view. A topless vertical tube containing water is put in a pressure cell filled with carbon dioxide ( $\text{CO}_{2}$ ). The diffusion of $\text{CO}_{2}$ at the free surface creates a thin layer of heavy fluid underneath the surface. This unstable density gradient generates a steady laminar plume which goes downward through the entire tube. A quasi-steady flow settles in the tube, filling gradually the bottom of the tube with heavy fluid. During this laminar regime, the velocity of the plume slowly decreases due to the build-up of the background density gradient. Surprisingly, despite the decrease of the Reynolds number, the laminar plume suddenly destabilises via a varicose mode into periodic pulsed puffs after an onset time which depends on the height of the tube and on the solutal Rayleigh number $Ra$ . This periodic regime is followed by an aperiodic regime, which lasts until the complete saturation of the solution. The observed destabilisation is explained as a result of the interplay between the feedback of the global recirculating flow and the progressive density stratification of the background fluid. The wavelength, frequency, onset time and phase velocity of the instability are explored using particle image velocimetry (PIV) measurements over two decades of Rayleigh number. The characteristics of the instability appear to be almost independent of the Bond number but strongly dependent on the solutal Rayleigh number and the aspect ratio. The phase velocity is very close to the fluid velocity of the plume before the instability, which has been predicted in various works to scale as $Ra^{2/3}(\ln \,Ra)^{1/3}$ . The wavelength is close to 4.5 times the radius of the cylinder (independent of aspect ratio, Bond number and Rayleigh number) such that the frequency scales as the phase velocity. The onset time, which is proportional to the height of the cylinder, scales as $Ra^{-0.55}$ and depends on the Bond number. A simplified model inspired from Lorenz’ waterwheel is proposed to explain the destabilisation process after partial fill-up of the cylinder. Although very qualitative, the model captures the key features of the experimental observations.

Characterization of a Laminar Plasma Plume Based on Dielectric-Barrier Discharge at Atmospheric Pressure

With argon used as working gas, a laminar plasma plume with a fairly large volume is flushed out of a dielectric-barrier discharge device into the ambient air at atmospheric pressure. Discharge characteristics are investigated through electrical and optical methods. Results indicate that discharge power, based on Lissajous plot, increases with increasing the peak voltage. High-speed imaging indicates that the formation mechanism of the plasma plume corresponds to a series of microdischarge filaments leaving outlet of the gas duct, whose probable lifetime is about several milliseconds.

Self-consistent modeling of laminar electrohydrodynamic plumes from ultra-sharp needles in cyclohexane

This paper presents a self-consistent model of electrohydrodynamic (EHD) laminar plumes produced by electron injection from ultra-sharp needle tips in cyclohexane. Since the density of electrons injected into the liquid is well described by the Fowler-Nordheim field emission theory, the injection law is not assumed. Furthermore, the generation of electrons in cyclohexane and their conversion into negative ions is included in the analysis. Detailed steady-state characteristics of EHD plumes under weak injection and space-charge limited injection are studied. It is found that the plume characteristics far from both electrodes and under weak injection can be accurately described with an asymptotic simplified solution proposed by Vazquez et al. [“Dynamics of electrohydrodynamic laminar plumes: Scaling analysis and integral model,” Phys. Fluids 12, 2809 (2000)] when the correct longitudinal electric field distribution and liquid velocity radial profile are used as input. However, this asymptotic solution deviates from the self-consistently calculated plume parameters under space-charge limited injection since it neglects the radial variations of the electric field produced by a high-density charged core. In addition, no significant differences in the model estimates of the plume are found when the simulations are obtained either with the finite element method or with a diffusion-free particle method. It is shown that the model also enables the calculation of the current-voltage characteristic of EHD laminar plumes produced by electron field emission, with good agreement with measured values reported in the literature.

Global stability of buoyant jets and plumes

The linear global stability of laminar buoyant jets and plumes is investigated under the low-Mach-number approximation. For Richardson numbers in the range $10^{-4}\leqslant Ri\leqslant 10^{3}$ and density ratios $S=\unicode[STIX]{x1D70C}_{\infty }/\unicode[STIX]{x1D70C}_{jet}$ between 1.05 and 7, only axisymmetric perturbations are found to exhibit global instability, consistent with experimental observations in helium jets. By varying the Richardson number over seven decades, the effects of buoyancy on the base flow and on the instability dynamics are characterised, and distinct behaviour is observed in the low-$Ri$ (jet) and in the high-$Ri$ (plume) regimes. A sensitivity analysis indicates that different physical mechanisms are responsible for the global instability dynamics in both regimes. In buoyant jets at low Richardson number, the baroclinic torque enhances the basic shear instability, whereas buoyancy provides the dominant instability mechanism in plumes at high Richardson number. The onset of axisymmetric global instability in both regimes is consistent with the presence of absolute instability. While absolute instability also occurs for helical perturbations, it appears to be too weak or too localised to give rise to a global instability.

Dynamics of a single buoyant plume in a FENE-P fluid

The dynamics of a single laminar buoyant plume in a FENE-P (finitely extensible nonlinear elastic-Peterlin) fluid has been investigated by performing a series of direct simulations over a range of Weissenberg numbers. Examination of this flow has given insight into the heat transfer reduction phenomenon observed recently in more complex Rayleigh-Benard turbulence. The simulations, which were performed with a Rayleigh number of 2.53×106 and a maximal polymeric extension of L = 100, show that the wall heat flux is reduced by about 28% at a Weissenberg number of 20, which is in reasonable agreement with the results obtained in Rayleigh-Benard turbulence. In addition, the global flow kinetic energy was reduced by about one order of magnitude.

Orientation and food search behaviour of a deep sea lobster in turbulent versus laminar odour plumes

New Zealand scampi (Metanephrops challengeri) is a commercially important deep-water lobster species that is caught by bottom trawling on areas of muddy seafloor on the continental shelf below 300 m. Efforts are being made to develop lower impact potting methods to harvest scampi, however, they can only be caught when out of their burrows and searching for food. This emergent food searching behaviour appears to be associated with periods of higher tidal flow. Such water flow will increase turbulence along the sea floor, which has been observed to improve the efficiency of chemosensory food searching in some lobster species. Consequently, this study examined the food search behaviour of scampi in response to odours from two types of bait (mackerel and mussel) in both turbulent and laminar flows. Scampi were more efficient at foraging in the turbulent flow than in the laminar flow, using shorter search paths in response to both types of bait. Scampi in the turbulent flow reached the mussel bait 44% faster and with lower mean heading angles than in laminar flow. However, there was no difference between the flow regimes for the mackerel bait. The pattern of orientation behaviour was similar under both flow regimes, suggesting that the scampi were using the same orientation strategy, but it was more accurate in turbulent flows. The results show that the foraging efficiency of scampi improves in turbulent conditions and that this may explain their increased emergent behaviour during periods of higher tidal flows.

Emptying filling boxes – free turbulent versus laminar porous media plumes

We examine the transient evolution of a negatively buoyant, laminar plume in an emptying filling box containing a uniform porous medium. In the long time limit, $\unicode[STIX]{x1D70F}\rightarrow \infty$, the box is partitioned into two uniform layers of different densities. However, the approach towards steady state is characterized by a lower contaminated layer that is continuously stratified. The presence of this continuous stratification poses non-trivial analytical challenges; we nonetheless demonstrate that it is possible to derive meaningful bounds on the range of possible solutions, particularly in the limit of large $\unicode[STIX]{x1D707}$, where $\unicode[STIX]{x1D707}$ represents the ratio of the draining to filling time scales. The validity of our approach is confirmed by drawing comparisons against the free turbulent plume case where, unlike with porous media plumes, an analytical solution that accounts for the time-variable continuous stratification of the lower layer is available (Baines &amp; Turner, J. Fluid Mech., vol. 37, 1969, pp. 51–80; Germeles, J. Fluid Mech., vol. 71, 1975, pp. 601–623). A separate component of our study considers time-variable forcing where the laminar plume source strength changes abruptly with time. When the source is turned on and off with a half-period, $\unicode[STIX]{x0394}\unicode[STIX]{x1D70F}$, the depth and reduced gravity of the contaminated layer oscillate between two extrema after the first few cycles. Different behaviour is seen when the source is merely turned up or down. For instance, a change of the source reduced gravity leads to a permanent change of interface depth, which is a qualitative point of difference from the free turbulent plume case.

Numerical simulations of a line plume impinging on a ceiling in cold fresh water

Laminar plumes from a line source of warm water at the base of a shallow, homogeneous body of cold water (below the temperature of maximum density) were simulated by a computational model. The plume water undergoes buoyancy reversal as it mixes with the cold ambient. If this occurs before the plume has reached the ceiling of the domain, the plume flaps from side to side. Otherwise, it spreads along the ceiling and then sinks, with a vortex enclosed between the rising plume and the sinking flow. Some of the dense, mixed water from the sinking flow is re-entrained into the rising plume, while the rest flows outwards along the floor. However, with high source temperatures, a sufficient volume of warm water eventually builds up to also form a positively buoyant gravity current along the ceiling.

Odor source localization in complex visual environments by fruit flies.

Flying insects routinely forage in complex and cluttered sensory environments. Their search for a food or a pheromone source typically begins with a whiff of odor, which triggers a flight response, eventually bringing the insect near the odor source. However, pinpointing the precise location of an odor source requires use of both visual and olfactory modalities, aided by odor plumes. Here, we investigated odor-tracking behavior in fruit flies (Drosophila melanogaster) presented with low- or high-contrast visual landmarks, either paired with or separate from an attractive odor cue. These experiments were conducted either in a gentle air stream which generated laminar odor plumes or in still air in which odor dissipates uniformly in all directions. Trajectories of flies revealed several novel features of their odor-tracking behavior in addition to those previously documented. First, in both moving and still air, odor-seeking flies rely on the co-occurrence of visual landmarks with olfactory cues to guide them to odorant objects. Second, flies abruptly decelerate upon encountering an odor plume, thereafter steering towards the nearest visual objects that had no inherent salience in the absence of odor. Thus, interception of an attractive odor increases their salience to nearby high-contrast visual landmarks. Third, flies adopt distinct odor-tracking strategies during flight in moving versus still air. Whereas they weave in and out of plumes towards an odor source in airflow, their approach is more incremental in still air. Both strategies are robust and flexible, and enable flies to reliably find odor sources under diverse visual and airflow environments.