172 research outputs found
Velocity and density measurements in forced fountains with negative buoyancy
In fluid mechanics, fountains take place when a source fluid is driven by its own momentum into a surrounding ambient fluid, and it is counterbalanced by buoyancy. These phenomena are largely encountered in nature and human activities. Despite the numerous studies on the subject, few experimental data are available about the internal structure of turbulent fountains. Here, we present a set of laboratory experiments with the aim to (i) get direct velocity and density measurements of fountains in a controlled environment and (ii) obtain insights about the basic physics of the phenomenon. The results concern the characteristics of the mean and turbulent flow: we report the analysis of the turbulent kinetic energy, the velocity skewness, and the Reynolds stresses, including a quadrant analysis of the fluctuating velocities. For some tests, the correlation between density and vertical velocity is investigated for both mean and fluctuating values. We have quantified the momentum transport, which is mainly out-downward at the nozzle axis with peaks at the mean rise height, where also maximum levels of the buoyancy and mass fluxes are present. The ability of acoustic Doppler current profilers to identify the rise height of the fountain and to measure the velocity field is also discussed
Experimental verification of theoretical approaches for radial gravity currents draining from an edge
We present an experimental study of inertial gravity currents (GCs) propagating in a cylindrical wedge under different drainage directions (inward/outward), lock-release (full/partial gatewidth) and geometry (annulus/full cylinder). We investigate the following combinations representative of operational conditions for dam-break flows: (i) inward drainage, annular reservoir, full gate; (ii) outward drainage, full reservoir, full gate; and (iii) outward drainage, full reservoir, partial gate. A single-layer shallow-water (SW) model is used for modelling the first two cases, while a box model interprets the third case; the results of these approximations are referred to as "theoretical". We performed a first series of experiments with water as ambient fluid and brine as intruding fluid, measuring the time evolution of the volume in the reservoir and the velocity profiles in several sections; in a second series, airwas the ambient andwaterwas the intruding fluid. Careful measurements, accompanied by comparisons with the theoretical predictions, were performed for the behaviour of the interface, radial velocity and, most important, the volume decay V(t)/V(0). In general, there is good agreement: the theoretical volume decay is more rapid than the measured one, but the discrepancies are a few percent and the agreement improves as the Reynolds number increases. Velocity measurements show a trend correctly reproduced by the SWmodel, although often a delay is observed and an over- or under-estimation of the peak values. Some experiments were conducted to verify the role of inconsistencies between experimental set-up and model assumptions, considering, for example, the presence or absence of a top lid, wedge angle much less than 2p, suppression of the viscous corner at the centre, reduction of disturbances in the dynamics of the ambient fluid: all these effects resulted in negligible impacts on the overall error. These experiments provide corroboration to the simple models used for capturing radial drainage flows, and also elucidate some effects (like oscillations of the radial flux) that are beyond the resolution of the models. This holds also for partial width lock-release, where axial symmetry is lost
Experimental verification of theoretical approaches for radial gravity currents draining from an edge
We present an experimental study of inertial gravity currents (GCs) propagating in a cylindrical wedge under different drainage directions (inward/outward), lock-release (full/partial gate width) and geometry (annulus/full cylinder). We investigate the following combinations representative of operational conditions for dam-break flows: (i) inward drainage, annular reservoir, full gate; (ii) outward drainage, full reservoir, full gate; and (iii) outward drainage, full reservoir, partial gate. A single-layer shallow-water (SW) model is used for modelling the first two cases, while a box model interprets the third case; the results of these approximations are referred to as âtheoreticalâ. We performed a first series of experiments with water as ambient fluid and brine as intruding fluid, measuring the time evolution of the volume in the reservoir and the velocity profiles in several sections; in a second series, air was the ambient and water was the intruding fluid. Careful measurements, accompanied by comparisons with the theoretical predictions, were performed for the behaviour of the interface, radial velocity and, most important, the volume decay V(t) / V(0). In general, there is good agreement: the theoretical volume decay is more rapid than the measured one, but the discrepancies are a few percent and the agreement improves as the Reynolds number increases. Velocity measurements show a trend correctly reproduced by the SW model, although often a delay is observed and an over- or under-estimation of the peak values. Some experiments were conducted to verify the role of inconsistencies between experimental set-up and model assumptions, considering, for example, the presence or absence of a top lid, wedge angle much less than 2 Ď, suppression of the viscous corner at the centre, reduction of disturbances in the dynamics of the ambient fluid: all these effects resulted in negligible impacts on the overall error. These experiments provide corroboration to the simple models used for capturing radial drainage flows, and also elucidate some effects (like oscillations of the radial flux) that are beyond the resolution of the models. This holds also for partial width lock-release, where axial symmetry is lost
The propagation of gravity currents in a circular cross-section channel: experiments and theory
High-Reynolds number gravity currents (GC) in a horizontal channel with
circular/semicircular side walls are investigated by comparing experimental data
and shallow-water (SW) theoretical results. We focus attention on a Boussinesq
system (salt water in fresh water): the denser fluid, occupying part of the depth or
the full depth of the ambient fluid which fills the remaining part of the channel, is
initially at rest in a lock separated by a gate from the downstream channel. Upon the
rapid removal of the gate (âdam breakâ), the denser âcurrentâ begins propagating into
the downstream channel, while a significant adjustment motion propagates upstream
in the lock as a bore or rarefaction wave. Using an experimental channel provided
by a tube of 19 cm diameter and up to 615 cm length, which could be filled to
various levels, we investigated both full-depth and part-depth releases, considered the
various stages of inertial-buoyancy propagation (in particular, the initial âslumpingâ
with constant speed, and the transition to the late self-similar propagation with time
to the power 3=4), and detected the transition to the viscous-buoyancy regime. A first
series of tests is focused on the motion in the lock while a second series of tests
is focused on the evolution of the downstream current. The speed of propagation of
the current in the slumping stage is overpredicted by the theory, by about the same
amount (typically 15 %) as observed in the classical flat bottom case. The length of
transition to viscous regime turns out to be TRe0.h0=x0/U (Re0 D .g0h0/1=2h0= c is
the initial Reynolds number, g0 is the reduced gravity, c is the kinematic viscosity
of the denser fluid, h0 and x0 are the height of the denser current and the length of
the lock, respectively), with the theoretical D3=8 and experimental 0:27
Ascending non-Newtonian long drops in vertical tubes
We report on theoretical and experimental studies describing the buoyancy-driven ascent
of a Taylor long drop in a circular vertical pipe where the descending uid is Newtonian,
and the ascending uid is non-Newtonian yield-shear-thinning and described by the threeparameter
Herschel-Bulkley model, including the Ostwald-deWaele (OdW) model as a
special case for zero yield. Results for the Ellis model are included to provide a more
realistic description of purely shear-thinning behaviour. In all cases, lubrication theory
allows obtaining the velocity pro les and the corresponding integral variables in closed
form, for lock-exchange ow with a zero net ow rate. The energy balance allows deriving
the asymptotic radius of the inner current, corresponding to a stable node of the
di erential equation describing the time evolution of the core radius.
We carried out a series of experiments measuring the rheological properties of the uids,
the speed and the radius of the ascending long drop. For some tests, we measured
the velocity pro le with Ultrasound velocimetry technique. The measured radius of the
ascending current compares fairly well with the asymptotic radius as derived through
the energy balance, and the measured ascent speed shows a good agreement with the
theoretical model. The measured velocity pro les also agree with their theoretical counterparts.
We have also developed dynamic similarity conditions to establish whether laboratory
physical models, limited by availability of real uids with de ned rheological characteristics,
can be representative of real phenomena on a large scale, such as exchanges in
volcanic conduits. The Appendix contains scaling rules for the approximated dynamic
similarity of the physical process analysed; these rules serve as a guide for the design of
experiments reproducing real phenomena
Ascending non-Newtonian long drops in vertical tubes
We report on theoretical and experimental studies describing the buoyancy-driven ascent of a Taylor long drop in a circular vertical pipe where the descending fluid is Newtonian, and the ascending fluid is non-Newtonian yield shear thinning and described by the three-parameter HerschelâBulkley model, including the Ostwaldâde Waele model as a special case for zero yield. Results for the Ellis model are included to provide a more realistic description of purely shear-thinning behaviour. In all cases, lubrication theory allows us to obtain the velocity profiles and the corresponding integral variables in closed form, for lock-exchange flow with a zero net flow rate. The energy balance allows us to derive the asymptotic radius of the inner current, corresponding to a stable node of the differential equation describing the time evolution of the core radius. We carried out a series of experiments measuring the rheological properties of the fluids, the speed and the radius of the ascending long drop. For some tests, we measured the velocity profile with the ultrasound velocimetry technique. The measured radius of the ascending current compares fairly well with the asymptotic radius as derived through the energy balance, and the measured ascent speed shows a good agreement with the theoretical model. The measured velocity profiles also agree with their theoretical counterparts. We have also developed dynamic similarity conditions to establish whether laboratory physical models, limited by the availability of real fluids with defined rheological characteristics, can be representative of real phenomena on a large scale, such as exchanges in volcanic conduits. Appendix B contains scaling rules for the approximated dynamic similarity of the physical process analysed; these rules serve as a guide for the design of experiments reproducing real phenomena
Onset of Darcy-B\ue9nard convection under throughflow of a shear-thinning fluid
We present an investigation on the onset of Darcy-B\ue9nard instability in a two-dimensional porous medium saturated with a non-Newtonian fluid and heated from below in the presence of a uniform horizontal pressure gradient. The fluid is taken to be of power-law nature with constant rheological index and temperature-dependent consistency index. A two-dimensional linear stability analysis in the vertical plane yields the critical wavenumber and the generalised critical Rayleigh number as functions of dimensionless problem parameters, with a non-monotonic dependence from and with maxima/minima at given values of , a parameter representing the effects of consistency index variations due to temperature. A series of experiments are conducted in a Hele-Shaw cell of aspect ratio to provide a verification of the theory. Xanthan Gum mixtures (nominal concentration from 0.10 % to 0.20 %) are employed as working fluids with a parameter range and. The experimental critical wavenumber corresponding to incipient instability of the convective cells is derived via image analysis for different values of the imposed horizontal velocity. Theoretical results for critical wavenumber favourably compare with experiments, systematically underestimating their experimental counterparts by 10 % at most. The discrepancy between experiments and theory is more relevant for the critical Rayleigh number, with theory overestimating the experiments by a maximum factor less than two. Discrepancies are attributable to a combination of factors: nonlinear phenomena, possible subcritical bifurcations, and unaccounted-for disturbing effects such as approximations in the rheological model, wall slip, ageing and degradation of the fluid properties
Onset of Darcy--BĂŠnard convection under throughflow of a shear-thinning fluid
We present an investigation on the onset of Darcy--B'enard instability in a two--dimensional porous medium saturated with a non--Newtonian fluid and heated from below in presence of a uniform horizontal pressure gradient. The fluid is taken to be of power--law nature with constant rheological index and temperature dependent consistency index . A two--dimensional linear stability analysis in the vertical plane yields the critical wave number and the generalised critical Rayleigh number as functions of dimensionless problem parameters, with a non monotonic dependence from and with maxima/minima at given values of , a parameter representing the effects of consistency index variations due to temperature.
A series of experiments are conducted in a Hele-Shaw cell of aspect ratio to provide a verification of the theory. Xanthan Gum mixtures {(nominal concentration from 0.10% to 0.20%)} are employed as working fluids with a parameter range and . The experimental critical wave number corresponding to incipient instability of the convective cells is derived via image analysis for different values of the imposed horizontal velocity. Theoretical results for critical wave number favourably compare with experiments, systematically underestimating their experimental counterparts by at most. The discrepancy between experiments and theory is more relevant for the critical Rayleigh number, with theory overestimating the experiments by a maximum factor less than two. Discrepancies are {attributable} to a {combination of factors: nonlinear phenomena, possible subcritical bifurcations, and unaccounted-for disturbing effects such as approximations in the rheological model, wall slip, ageing and degradation of the fluid properties.
Replication, Pathogenesis and Transmission of Pandemic (H1N1) 2009 Virus in Non-Immune Pigs
The declaration of the human influenza A pandemic (H1N1) 2009 (H1N1/09) raised important questions, including origin and host range [1,2]. Two of the three pandemics in the last century resulted in the spread of virus to pigs (H1N1, 1918; H3N2, 1968) with subsequent independent establishment and evolution within swine worldwide [3]. A key public and veterinary health consideration in the context of the evolving pandemic is whether the H1N1/09 virus could become established in pig populations [4]. We performed an infection and transmission study in pigs with A/California/07/09. In combination, clinical, pathological, modified influenza A matrix gene real time RT-PCR and viral genomic analyses have shown that infection results in the induction of clinical signs, viral pathogenesis restricted to the respiratory tract, infection dynamics consistent with endemic strains of influenza A in pigs, virus transmissibility between pigs and virus-host adaptation events. Our results demonstrate that extant H1N1/09 is fully capable of becoming established in global pig populations. We also show the roles of viral receptor specificity in both transmission and tissue tropism. Remarkably, following direct inoculation of pigs with virus quasispecies differing by amino acid substitutions in the haemagglutinin receptor-binding site, only virus with aspartic acid at position 225 (225D) was detected in nasal secretions of contact infected pigs. In contrast, in lower respiratory tract samples from directly inoculated pigs, with clearly demonstrable pulmonary pathology, there was apparent selection of a virus variant with glycine (225G). These findings provide potential clues to the existence and biological significance of viral receptor-binding variants with 225D and 225G during the 1918 pandemic [5]
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