Airborne particles are a major route for transmission of COVID-19 and many other infectious diseases. When a person talks, sings, coughs, or sneezes, nasal and throat secretions are spewed into the air. After a short initial fragmentation stage, the expelled material is mostly composed of spherical particles of different sizes. While the dynamics of the largest droplets are dominated by gravitational effects, the smaller aerosol particles, mostly transported by means of hydrodynamic drag, form clouds that can remain afloat for long times. In subsaturated air environments, the dependence of pathogen-laden particle dispersion on their size is complicated due to evaporation of the aqueous fraction. Particle dynamics can significantly change when ambient conditions favor rapid evaporation rates that result in a transition from buoyancy-to-drag dominated dispersion regimes. To investigate the effect of particle size and evaporation on pathogen-laden cloud evolution, a direct numerical simulation of a mild cough was coupled with an evaporative Lagrangian particle advection model. The results suggest that while the dispersion of cough particles in the tails of the size distribution are unlikely to be disrupted by evaporative effects, preferential aerosol diameters (30–40 μ m) may exhibit significant increases in the residence time and horizontal range under typical ambient conditions. Using estimations of the viral concentration in the spewed fluid and the number of ejected particles in a typical respiratory event, we obtained a map of viral load per volume of air at the end of the cough and the number of virus copies per inhalation in the emitter vicinity.

A pattern recognition technique has been applied to simultaneously sampled multipoint hot-wire anemometry data obtained in the far wake of a circular cylinder. Data from both the streamwise fluctuating velocity field and the temperature field have been analysed employing a computer code that uses a correlation approach to automatically detect and ensemble average flow patterns and patterns for mean-square fluctuations. Statistical tests then allow the significance and contribution to the turbulence intensity of the detected structures to be evaluated. This procedure has been used to infer the three-dimensional topology of the double-roller eddies previously identified in the far-wake region and to relate these to the motions responsible for entrainment. It appears that the two types of motion are not independent, but are linked together, forming parts of horseshoe vortex structures which account for at least 40% of the total turbulence energy. These structures originate near the centre of the flow, may extend across the centreline and typically occur in groups of about three. The resulting picture of the flow dynamics is related to the conclusions drawn from similar data by other workers and a possible regeneration mechanism is presented. The addition to the code of a fine-scale activity indicator, the choice of which is discussed in some detail, has allowed the relationship between these energetic large-scale motions and smaller eddies to be investigated. It seems that the most intense fine-scale activity is associated with the vortical cores of the double-roller eddies. It is shown that this observation is consistent with the concepts of ‘isotropy’ and ‘spotiness’ of the dissipative scales. It also suggests that the horseshoe vortices loose energy both to their own secondary instabilities and to smaller scales resulting from the breakup of other highly strained large eddies.

An experimental study to identify the structures present in a jet in crossflow has been carried out at a jet-to-crossflow velocity ratio U/Ucf = 3.8 and Reynolds number Re = UcfD/v = 6600. The hot-wire velocity data measured with a rake of eight X-wires at x/D = 5 and 15 and flow visualizations using planar laser-induced fluorescence (PLIF) confirm that the well-established pair of counter-rotating vortices is a feature of the mean field and that the upright, tornado-like or Fric's vortices that are shed to the leeward side of the jet are connected to the jet flow at the core. The counter-rotating vortex pair is strongly modulated by a coherent velocity field that, in fact, is as important as the mean velocity field. Three different structures – folded vortex rings, horseshoe vortices and handle-type structures – contribute to this coherent field. The new handle-like structures identified in the current study link the boundary layer vorticity with the counter-rotating vortex pair through the upright tornado-like vortices. They are responsible for the modulation and meandering of the counter-rotating vortex pair observed both in video recordings of visualizations and in the instantaneous velocity field. These results corroborate that the genesis of the dominant counter-rotating vortex pair strongly depends on the high pressure gradients that develop in the region near the jet exit, both inside and outside the nozzle.

Simultaneous velocity and temperature measurements were made with rakes of sensors that sliced a slightly heated turbulent wake in the spanwise direction, at different lateral positions 150 diameters downstream of the cylinder. A pattern recognition analysis of hotter-to-colder transitions was performed on temperature data measured at the mean velocity half-width. The velocity data from the different ‘slices’ was then conditionally averaged based on the identified temperature events. This procedure yielded the topology of the average three-dimensional large-scale structure which was visualized with iso-surfaces of negative values of the second eigenvector of [S2+Ω2]. The results indicate that the average structure of the velocity fluctuations (using a triple decomposition of the velocity field) is found to be a shear-aligned ring-shaped vortex. This vortex ring has strong outward lateral velocities in its symmetry plane which are like Grant's mixing jets. The mixing jet region extends outside the ring-like vortex and is bounded by two foci separated in the spanwise direction and an upstream saddle point. The two foci correspond to what has been previously identified in the literature as the double rollers.The ring vortex extracts energy from the mean flow by stretching in the mixing jet region just upstream of the ring boundary. The production of the small-scale (incoherent) turbulence by the coherent field and one-component energy dissipation rate occur just downstream of the saddle point within the mixing jet region. Incoherent turbulence energy is extracted from the mean flow just outside the mixing jet region, but within the core of the structure. These processes are highly three-dimensional with a spanwise extent equal to the mean velocity half-width.When a double decomposition is used, the coherent structure is found to be a tube-shaped vortex with a spanwise extent of about 2.5l0. The double roller motions are integral to this vortex in spite of its shape. Spatial averages of the coherent velocity field indicate that the mixing jet region causes a deficit of mean streamwise momentum, while the region outside the foci of the double rollers has a relatively small excess of streamwise momentum.

A pattern-recognition procedure designed to extract footprints of organized structures from turbulent signals is developed and used to analyse the large-eddy organization of several turbulent wake flows. The pattern-recognition technique is intended to be a general-purpose analytical tool that makes no use of specific flow characteristics, and that can be implemented as a computer code independent of the types of signals to be processed. The technique is applied to analyse the wake generated by a single cylinder at downstream positions ranging from x/D = 10 to x/D = 220. Also the structural features of the wakes behind a rotating cylinder, two cylinders of unequal diameters and two cylinders of equal diameter, one rotating, are examined at x/D = 140. In the near wake the large-scale motions detected are Kármán vortices, whose periodic activity persists up to 60 diameters. Further downstream the most significant coherent structures detected are single and double rollers with shear-aligned vorticity, whose dimensions and velocity intensities are properly scaled by the half-width of the wake and the local r.m.s. values, respectively. The similarities observed in the organized motions identified in the different wakes at x/D = 140, suggest that the roller organization may be an intrinsic characteristic of fully developed turbulent plane wake flows, irrespective of initial conditions.