Although the relative importance of airborne transmission of the SARS-CoV-2 virus is controversial, increasing evidence suggests that understanding airflows is important for estimation of the risk of ...contracting COVID-19. The data available so far indicate that indoor transmission of the virus far outstrips outdoor transmission, possibly due to longer exposure times and the decreased turbulence levels (and therefore dispersion) found indoors. In this paper we discuss the role of building ventilation on the possible pathways of airborne particles and examine the fluid mechanics of the processes involved.
The swimming direction of biological or artificial microscale swimmers tends to be randomised over long time-scales by thermal fluctuations. Bacteria use various strategies to bias swimming behaviour ...and achieve directed motion against a flow, maintain alignment with gravity or travel up a chemical gradient. Herein, we explore a purely geometric means of biasing the motion of artificial nanorod swimmers. These artificial swimmers are bimetallic rods, powered by a chemical fuel, which swim on a substrate printed with teardrop-shaped posts. The artificial swimmers are hydrodynamically attracted to the posts, swimming alongside the post perimeter for long times before leaving. The rods experience a higher rate of departure from the higher curvature end of the teardrop shape, thereby introducing a bias into their motion. This bias increases with swimming speed and can be translated into a macroscopic directional motion over long times by using arrays of teardrop-shaped posts aligned along a single direction. This method provides a protocol for concentrating swimmers, sorting swimmers according to different speeds, and could enable artificial swimmers to transport cargo to desired locations.
Boussinesq salt-water laboratory experiments of Rayleigh–Taylor instability (RTI) can achieve mixing efficiencies greater than 0.75 when the unstable interface is confined between two stable ...stratifications. This is much greater than that found when RTI occurs between two homogeneous layers when the mixing efficiency has been found to approach 0.5. Here, the mixing efficiency is defined as the ratio of energy used in mixing compared with the energy available for mixing. If the initial and final states are quiescent then the mixing efficiency can be calculated from experiments by comparison of the corresponding density profiles. Varying the functional form of the confining stratifications has a strong effect on the mixing efficiency. We derive a buoyancy-diffusion model for the rate of growth of the turbulent mixing region,
$\def \xmlpi #1{}\def \mathsfbi #1{\boldsymbol {\mathsf {#1}}}\let \le =\leqslant \let \leq =\leqslant \let \ge =\geqslant \let \geq =\geqslant \def \Pr {\mathit {Pr}}\def \Fr {\mathit {Fr}}\def \Rey {\mathit {Re}}\dot{h} = 2 \sqrt{\alpha A g h}$
(where
$A = A(h)$
is the Atwood number across the mixing region when it extends a height
$h$
,
$g$
is acceleration due to gravity and
$\alpha $
is a constant). This model shows good agreement with experiments when the value of the constant
$\alpha $
is set to 0.07, the value found in experiments of RTI between two homogeneous layers (where the height of the turbulent mixing region increases as
$h =\alpha A g t^2$
, an expression which is equivalent to that derived for
$\dot{h}$
).
Biological systems often involve the self-assembly of basic components into complex and functioning structures. Artificial systems that mimic such processes can provide a well-controlled setting to ...explore the principles involved and also synthesize useful micromachines. Our experiments show that immotile, but active, components self-assemble into two types of structure that exhibit the fundamental forms of motility: translation and rotation. Specifically, micron-scale metallic rods are designed to induce extensile surface flows in the presence of a chemical fuel; these rods interact with each other and pair up to form either a swimmer or a rotor. Such pairs can transition reversibly between these two configurations, leading to kinetics reminiscent of bacterial run-and-tumble motion.
We analyse the entrainment and energetics of equal and opposite axisymmetric turbulent air plumes in a vertically confined space at a Rayleigh number of
$1.24\times 10^{7}$
using theory and direct ...numerical simulation. On domains of sufficiently large aspect ratio, the steady state consists of turbulent plumes penetrating an interface between two layers of approximately uniform buoyancy. As described by Baines & Turner (
J. Fluid Mech.
, vol. 37(1), 1969, pp. 51–80), upon penetrating the interface the flow in each plume becomes forced and behaves like a constant-momentum jet, due to a reduction in its mean buoyancy relative to the local environment. To observe the behaviour of the plumes we partition the domain into sub-domains corresponding to each plume. Domains of relatively small aspect ratio produce a single primary mean-flow circulation between the sub-domains that is maintained by entrainment into the plumes. At larger aspect ratios the mean flow between the sub-domains bifurcates, indicating the existence of a secondary circulation within each layer associated with entrainment into the jets. The largest aspect ratios studied here exhibit an additional, tertiary, circulation in the vicinity of the interface. Consistency between independent calculations of an effective entrainment coefficient allows us to identify aspect ratios for which the flow can be modelled using plume theory, under the assumption of a two-layer stratification. To study the flow’s energetics we use a local definition of available potential energy (APE). For plumes with Gaussian velocity and buoyancy profiles, the theory we develop suggests that the kinetic energy dissipation is split equally between the jets and the plumes and, collectively, accounts for almost half of the input of APE at the boundaries. In contrast, 1/4 of the APE dissipation and background potential energy (BPE) production occurs in the jets, with the remaining 3/4 occurring in the plumes. These bulk theoretical predictions agree with observations of BPE production from simulations to within 1 % and form the basis of a similarity solution that models the vertical dependence of APE dissipation and BPE production. Unlike results concerning the dissipation of buoyancy variance and the strength of the circulations described above, the model for the flow’s energetics does not involve an entrainment coefficient.
The concept of a mixing efficiency is widely used to relate the amount of irreversible diabatic mixing in a stratified flow to the amount of energy available to support mixing. This common measure of ...mixing in a flow is based on the change in the background potential energy, which is the minimum gravitational potential energy of the fluid that can be achieved by an adiabatic rearrangement of the instantaneous density field. However, this paper highlights examples of mixing that is primarily ‘buoyancy-driven’ (i.e. energy is released to the flow predominantly from a source of available potential energy) to demonstrate that the mixing efficiency depends not only on the specific characteristics of the turbulence in the region of the flow that is mixing, but also on the density profile in regions remote from where mixing physically occurs. We show that this behaviour is due to the irreversible and direct conversion of available potential energy into background potential energy in those remote regions (a mechanism not previously described). This process (here termed ‘relabelling’) occurs without requiring either a local flow or local mixing, or any other process that affects the internal energy of that fluid. Relabelling is caused by initially available potential energy, associated with identifiable parcels of fluid, becoming dynamically inaccessible to the flow due to mixing elsewhere. These results have wider relevance to characterising mixing in stratified turbulent flows, including those involving an external supply of kinetic energy.
We examine the effect of an indoor-outdoor temperature difference on the transient wind-driven cross-ventilation of a room. Laboratory experiments are performed in a water flume using a reduced-scale ...model room. For solely wind-driven cross-ventilation with no initial temperature difference between the room and the external fluid, the ventilation rate is constant. In experiments, the mean dye concentration decays exponentially, which is expected when the room remains well-mixed. When there is an initial temperature difference but no wind, the buoyancy-driven exchange-ventilation results lie between a model that assumes the room is well-mixed and a new model that assumes no mixing between the incoming flow and the room. When both wind and buoyancy drive the flow, the relative importance of these two effects can be described by a Froude number, Fr. For buoyancy-dominated ventilation (Fr<1), the ventilation rate through the windows can be modelled using an exchange flow with a Fr correction to account for wind effects, which can increase the ventilation rate by up to 40%. For wind-dominated ventilation (Fr>1), a temperature difference slightly reduces the ventilation rate, but only by up to 6%, a change that can be neglected in most applications. Two processes compete to ventilate the room in combined cases: the removal of fluid from a lower layer by flow through the windows and the erosion of an upper layer by entrainment into the jet that crosses the room. The relative rates of these two processes depend on the geometry of the room.
•A room undergoing buoyancy-driven exchange ventilation may be stratified.•Stratification in a room can affect ventilation rate.•Temperature effects can reduce the rate of wind-driven cross-ventilation.•Wind can increase the rate of buoyancy-driven exchange ventilation.
In this study, we evaluated several simple natural ventilation models of cross ventilation and single-sided ventilation with data measured in a full-scale field study in London. In the field study, ...the ventilation rate in a naturally ventilated office was measured using a tracer gas technique with CO2. Internal temperatures were measured using a vertical temperature array. The external temperature, wind speed and direction were measured at a nearby weather station. In addition, a 1:200 scale model of the urban area within 300 m of the test room was built in a wind tunnel to measure the pressure coefficients. The ventilation models were evaluated with input data from two sources. Wind data from a nearby airport and pressure coefficients from the literature were used, as is common practice. Alternatively, wind data measured at the local weather station and the pressure coefficients measured from wind tunnel experiments were used. The results showed that, regardless of the input data sources, the cross-ventilation model in general gives reasonable predictions. For single-sided ventilation, several empirical models were evaluated and poor predictions were obtained using the models. We discuss ways in which models of natural ventilation might be improved in the future.
•A full-scale field study in a naturally ventilated office room in London.•Ventilation rates of both cross-ventilation and single-sided ventilation were measured.•Wind-tunnel experiments on a reduced-scale model of the building and surroundings, measuring pressure coefficients.•Measured local wind data and pressure coefficients were compared with commonly used models.•Evaluation of simple analytical ventilation models were conducted.
Abstract
How do active galactic nuclei with low optical luminosities produce powerful radio emission? Recent studies of active galactic nuclei with moderate radio and low optical luminosities ...(Fanaroff & Riley class I, FR I) searching for broad nuclear emission lines in polarized light, as predicted by some active galactic nucleus unification models, have found heterogeneous results. These models typically consist of a central engine surrounded by a torus of discrete dusty clouds. These clouds would absorb and scatter optical emission, blocking broad nuclear emission lines, and reradiate in mid-infrared. Some scattered broad-line emission may be observable, depending on geometry, which would be polarized. We present a wide-band infrared spectroscopic analysis of 10 nearby FR I radio galaxies to determine whether there is significant emission from a dusty obscuring structure. We used Markov Chain Monte Carlo algorithms to decompose Spitzer/IRS spectra of our sample. We constrained the wide-band behavior of our models with photometry from the Two Micron All Sky Survey, Spitzer/IRAC, Spitzer/MIPS, and Herschel/SPIRE. We find that one galaxy is best fit by a clumpy torus and three others show some thermal mid-infrared component. This suggests that in those three there is likely some obscuring dust structure that is inconsistent with our torus models and there must be some source of photons heating the dust. We conclude that 40% of our FR I radio galaxies show evidence of obscuring dusty material, possibly some other form of hidden broad-line nucleus, but only 10% favor the clumpy torus model specifically.
Experiments were conducted in an UK inter‐city train carriage with the aim of evaluating the risk of infection to the SARS‐CoV‐2 virus via airborne transmission. The experiments included in‐service ...CO2 measurements and the measurement of salt aerosol concentrations released within the carriage. Computational fluid dynamics simulations of the carriage airflow were also used to visualise the airflow patterns, and the efficacy of the HVAC filter material was tested in a laboratory. Assuming an infectious person is present, the risk of infection for a 1‐h train journey was estimated to be 6 times lower than for a full day in a well‐ventilated office, or 10–12 times lower than a full day in a poorly ventilated office. While the absolute risk for a typical journey is likely low, in the case where a particularly infectious individual is on‐board, there is the potential for a number of secondary infections to occur during a 1‐h journey. Every effort should therefore be made to minimize the risk of airborne infection within these carriages. Recommendations are also given for the use of CO2 sensors for the evaluation of the risk of airborne transmission on train carriages.
PDF
