We present an experimental study on steady gravity currents advancing along a heated wall. The study aims to assess how the structure and dynamics of a wall-heated current are modified with respect to the adiabatic-wall case. The current is generated by a mixture of air and carbon dioxide continuously supplied at the channel inlet. To have a complete point-wise characterization of the flow, simultaneous high-frequency measurements of two velocity components, CO _2 concentration, and temperature are performed. An experimental protocol is presented to reconstruct the local fluid density and to estimate vertical and horizontal turbulent fluxes of CO _2 , temperature, and buoyancy. The reliability of both the flow measurements and of the estimate of convective heat flux exchanged at the wall is assessed through integral balances of CO _2 mass, enthalpy, and buoyancy, performed at different distances from the source. In the heated experiments, a convectively unstable boundary layer forms near the wall, capped by a stably stratified region. The influence of this condition on the first- and second-order flow statistics profiles is examined. With respect to the adiabatic case, the floor heating induces a rise in the wall drag, a reduction in the maximum flow velocity, and a thickening of the gravity current region characterized by a nearly constant velocity. In contrast, the vertical height of the mixing zone, characterized by an almost-constant vertical gradient of the streamwise velocity, does not exhibit a clear dependence on heating intensity.

The propagation of sustained Boussinesq gravity-current fronts along a heated wall is examined through laboratory experiments. The gravity currents are generated by continuously supplying a mixture of air and carbon dioxide at the inlet of a rectangular channel, while uniform wall heating is provided by resistive fabrics. In addition to the Froude number, defined either at the source (Fr_s) or at the current front (Fr), we show that the front velocity is governed by Λ_s, the ratio of the two buoyancy fluxes per unit area driving the flow: that generated by wall heating and that supplied at the inlet. As wall heating increases (i.e., as Λ_s increases), the current front propagates more slowly. This slowdown results from the direct heating of the current, which reduces the buoyancy of its head, and from the interaction between the front and the thermal plumes generated by natural convection downstream. In the most strongly heated experiments, the front eventually stops at a distance from the source that is inversely proportional to Λ_s. In these cases, the buoyancy of the head changes sign, and the head evolves into a positively buoyant plume whose vertical extent continues to increase. The experimental results are compared with predictions from a lumped analytical model describing the propagation and eventual arrest of the current.

This work experimentally investigates the natural ventilation of an emptying filling box under stochastic forcing in an opposing wind. A point buoyancy source on the floor generates a turbulent plume that rises towards the ceiling, producing a buoyant layer. The resulting stack effect induces a volume flux exiting through the high-level opening, equal to that entering through the low-level opening. The buoyancy-induced pressure is reduced by an opposing wind. If the wind velocity exceeds a critical threshold, two different states may occur: either the stratification is preserved although the buoyant layer thickens, or a well-mixed regime takes place. We find that if wind undergoes stochastic fluctuations, a noise-induced phenomenon occurs: when the stratification persists, the height of the interface fluctuates around a mean value significantly lower than that exhibited in the constant-wind scenario. The deviation from the constant-wind equilibrium increases as the mean wind velocity and the noise intensity grow. Moreover, the transition to the well-mixed regime may occur for wind velocity lower than the critical threshold under constant wind conditions. These results confirm the predictions given by a theoretical model, where the system is forced by stochastic wind fluctuations modelled with an Ornstein-Uhlenbeck process. This modelling choice is supported by the experimental characterisation of the pressure field. Lastly, we provide an approximate analytical solution for the system's mean behaviour.

The accidental release of hazardous airborne pollutants on industrial sites creates risks associated with the exceedance of toxicity or explosivity limits. Capturing these risks requires predicting higher-order statistics of concentration fluctuations at various distances from the source. This challenge, already complex in atmospheric boundary layers, is further complicated by the typical built environment of industrial sites. To address this, we conducted wind-tunnel experiments on the dispersion of a passive scalar from a localized ground-level source within a reduced-scale model of an industrial site. The experiments measured the velocity and concentration fields, while varying the geometry of an upstream building simulating typical complex industrial structures.A key focus of our investigation is the one-point passive scalar concentration PDF, whose experimental realizations were systematically compared to three analytical models: the gamma, two-parameter Weibull and lognormal distributions. The gamma distribution generally provides the best predictions, although the lognormal model performs better within the building wake near the source. While the main discrepancies between theoretical distributions and experimental data consistently occur at low concentration values, all three distributions accurately predict the 95th and 99th concentration percentiles. Thus, peak and hazardous concentration levels can be reliably estimated even without fully capturing the complete concentration distribution.

We present an experimental study to assess the influence of realistic traffic conditions on the ventilation required to confine the smoke produced by a tunnel fire. The experiments are performed on a reduced- scale tunnel, with hot smoke modeled by a buoyant helium-air release. The tunnel is ventilated by two extraction vents placed on both sides of the source to confine the smoke, and a longitudinal flow is induced by a longitudinal pressure gradient. The traffic condition is simulated by arranging cubic blocks, representing vehicles, of two different sizes inside the tunnel. For each block size and various tunnel configurations (different damper geometries and positions, and the presence or absence of vertical barriers downstream of the vents), experiments are performed with and without longitudinal flow. The presence of vehicles can lead to outcomes that differ from the well-established results in the literature obtained in their absence, i.e., in an empty tunnel. Specifically, the performance of rectangular dampers becomes equivalent to that of squared-centered dampers unless vertical barriers are used. Furthermore, when barriers are employed, the geometry and position of the dampers become irrelevant to ventilation efficiency. Obstacles primarily affect ventilation efficiency when they interact with the buoyant smoke and change its stratification regime.

Knowledge on both the mean field and fluctuations of concentration is necessary to estimate risks linked to pollutant exposure on industrial sites. While time-averaged concentrations provide meaningful information for chronic risk exposure, local exceedance of high concentration values potentially trigger chemical reactions or exceed harmful limits for living organisms. Information on threshold exceeding is thus a key parameter for toxicity assessment and accident management, hence the focus of this work on concentration fluctuations.Several models for the probability distribution functions (PDF) of pollutant concentrations have been proposed in the literature. Several authors agree on the fact that the gamma distribution provides a reliable model for the one-point concentration PDF in case of localised releases of pollutants in atmospheric boundary layers (Cassiani et al., 2020). However, the accuracy of the gamma distribution was still not properly investigated within domain characterised by a complex geometry. This is indeed the aim of this experimental study, focusing on the one-point concentration PDF due to a localised release of pollutant within a group of buildings. Furthermore, we aim at verifying models predicting average times of exceeding concentration thresholds (e.g. Bertagni et al., 2020).Wind and concentration fields were characterised on the small-scale model of an idealised industrial site in a wind tunnel reproducing the atmospheric boundary layer. Higher order statistics were computed from concentration time series collected with a fast flame ionisation detector of frequency 400 Hz. In addition to the work presented hereby, this experimental dataset could be used to validate numerical dispersion models in future studies. Best agreement between the experimental one-point concentration PDF and the gamma distribution are observed in the mid- and far-field. In contrast, the gamma distribution induces a systematic underestimation of concentration fluctuations in the near-field. Notably the Gamma distribution does not capture the occurrence of high intensity peaks measured sporadically, especially in recirculating regions in building wakes.Mean threshold exceeding times are computed assuming a gamma distribution and hence show best correlation with experimental data in the mid- and far-field. Frequency of threshold exceeding show less accurate results for higher limits. Summarising, the gamma distribution is shown to be a reliable model for the one-point concentration PDF in the mid- and far-field, but exhibits poor correlation in the near-field due to the presence of recirculation zones and intense meandering motion of the pollutant plume. The model for mean times of threshold exceeding, relying on the assumption of a gamma distribution, show a similar behaviour. ReferencesBertagni, M. B., Marro, M., Salizzoni, P., & Camporeale, C. (2020). Level-crossing statistics of a passive scalar dispersed in a neutral boundary layer. Atmospheric Environment, 230, 117518. https://doi.org/10.1016/j.atmosenv.2020.117518Cassiani, M., Bertagni, M. B., Marro, M., & Salizzoni, P. (2020). Concentration Fluctuations from Localized Atmospheric Releases. Boundary-Layer Meteorology, 177(2), Article 2. https://doi.org/10.1007/s10546-020-00547-4

This study investigates the turbulent dispersion of pollutants in the wake of a two-dimensional square obstacle. Utilizing Laser Doppler Anemometry and Particle Image Velocimetry, we characterized the flow dynamics, identifying a recirculation zone downstream of the obstacle, marked by high shear and increased turbulent viscosity, and playing a crucial role in turbulent momentum exchange. We evaluated the turbulence kinetic energy budget, estimating its dissipation rate, and found traditional isotropy and Taylor hypothesis methods inadequate within the wake region. Furthermore, we explored pollutant dispersion from a linear source located downstream the obstacle. Analysis of mean concentration and variance revealed that the log-normal distribution is most effective for modelling concentrations within the recirculating region, while the Gamma distribution suits areas outside it. Testing various closure models for turbulent mass fluxes highlighted the limitations of the Simplified Gradient Diffusion Hypothesis model, favouring more complex closure models for longitudinal trends, though these still faced challenges with intensity estimation. The Simplified Gradient Diffusion Hypothesis model proved robust for vertical mass fluxes, with satisfactory results in turbulent diffusivity and turbulent Schmidt number calculations. The experimental results serve as a benchmark for validating numerical simulations and assessing the accuracy of closure models typically employed in pollutant dispersion modelling.

Inserting vegetation within the urban environment mitigates the urban heat island effect, the flooding risk, and improves air quality. However, its aerodynamic effect has remarkable impact on the pollutant transport and, consequently, on human health comfort. Indeed, the presence of vegetation within an urban canyon leads to non-trivial patterns of pollutant concentration and mass fluxes, as a consequence of complex mean and turbulent velocity fields. In addition to the vegetation density, the flow structure within canyons is influenced by their geometry and by the wind direction. The aim of the present study is to experimentally investigate the velocity field within a canyon, varying the vegetation density and the wind direction. We measured flow velocity statistics within an indefinitely long street canyon, with unit height-to-width ratio, subject to a neutrally stratified boundary layer modeled in the wind tunnel of École Centrale de Lyon. The aerodynamic impact of vegetation was reproduced by inserting plastic miniatures of trees along the two long sides of the canyon. We considered an empty canyon and a vegetated canyon, whose longitudinal axes are oriented with angles of 0°, 30°, and 60° with respect to the external wind flow. Results reveal that when the canyon is inclined with respect to the external wind direction the mean flow follows a complex helicoidal structure. The presence of trees decreases significantly the mean longitudinal velocity and weakens the transversal circulation in the inclined canyon. The dampening effect of the mean longitudinal flow is more marked increasing the inclination angle of the canyon. Turbulent fluctuations are enhanced above the tree crowns, mostly when the wind blows parallel to the canyon axis. On the contrary, turbulent fluctuations decreases at tree trunk and crown levels, in particular when the canyon is inclined of 60° with respect to the external wind direction. Spectra of the velocity signal show that the presence of trees induces an evident shift of the energy peak towards high frequencies. The collected data constitute a step forward to understand and modeling the urban microclimate.

The aim of this study is to experimentally investigate the dynamics of a heavy gas release and highlight its main differences compared to that of a passive gas. To achieve this, we examine the vertical emission of both heavy and passive gases from an elevated source, situated within a turbulent boundary layer. The wind tunnel experiment is designed to simulate a realistic release of oxygen at T = - 40 ^∘ C, following a similar setup to the study performed by Schatzmann et al. (Atmos Environ 27:1105–1116, 1993). The release is characterised by means of simultaneous velocity and concentration measurements along vertical and lateral profiles at various downwind distances from the source. The data were used to estimate the turbulent mass flows, the turbulent Schmidt number, and the high-order statistics of the scalar field. Furthermore, we conduct an in-depth analysis of the production and dissipation terms of the concentration variance. These are subsequently used to estimate the typical time scale for turbulent mixing using two different micro-mixing models that parametrize the effects of molecular diffusion. These findings are crucial for improving the accuracy of operational dispersion models that simulate localised releases of dense gases in the atmosphere.

Natural ventilation is key for reducing energy demand and ensuring indoor air quality. We study the deterministic and stochastic dynamics of a naturally ventilated system where a room with a steady buoyancy source connects to an unforced atrium (an arrangement representative of many real buildings). The buoyant fluid accumulates at the ceiling of both spaces. A stratification arises, inducing the stack effect which drives the ventilation of the system. We consider the occurrence of an opposing wind. Firstly, a constant wind is imposed, and the steady states are analysed for different atrium geometries. It is known that when no external wind occurs the atrium may either enhance or worsen the ventilation of the room depending on its geometry. We find that for any geometry the atrium enhances the ventilation for a wind velocity large enough. Secondly, stochastic fluctuations are introduced in the wind velocity. These induce a ‘noise-induced transition’: the mean position of the interface between warm and cold layers lies below that observed under constant wind. This phenomenon was already known for single-room systems; however, we find that the room-atrium system is less sensitive to this effect and to variations in the coefficient of variation of wind velocity.

Most studies on the aerodynamic impact of trees on street canyon ventilation focus on external winds perpendicular to the street axis, as these are crucial for dispersing ground-level pollutants like vehicle emissions. Other wind directions, despite their importance for urban pollutant dispersion, receive less attention. Non-perpendicular winds introduce an advective component along the canyon, which is strongly influenced by tree presence and density, significantly shaping pollutant spread across downwind streets. To address this gap, we conducted velocity measurements in a reduced-scale street canyon aligned with the external flow, varying the canyon's height-to-width ratio and tree density. The aerodynamic effects of trees were simulated by placing plastic tree miniatures along the canyon's lateral walls. Our results show that trees within a square canyon reduce the mean velocity by 80% and decrease velocity fluctuations by 15% compared to an empty canyon. At the rooftop, the interaction between the external flow and tree crowns leads to a 30% increase in velocity fluctuations. As the aspect ratio decreases (i.e., for wider canyons), a vegetation-free corridor between the rows of trees results in a 25% decrease in mean longitudinal velocity and a 30% increase in velocity fluctuations compared to the empty large canyon. This induces a street ventilation more efficient with respect to the vegetated square canyon, especially when pollutants are emitted at pedestrian level. This study provides valuable experimental data for evaluating the spatially-averaged mean longitudinal velocity in urban canyons, which can be incorporated into analytical and numerical models for better urban air quality management.

We study the deterministic and stochastic dynamics of a naturally ventilated system composed of a room and a taller atrium. In the room, a point buoyancy source on the floor generates a turbulent plume which rises towards the ceiling. At the ceiling, the buoyant fluid accumulates, inducing a stratification. The room is connected to the atrium through an opening at the room ceiling level. Therefore, the room buoyant fluid flows through it and rises towards the ceiling of the atrium. The stack effect caused by the accumulation of buoyant fluid in both the environments drives the natural ventilation of the system. The performance of natural ventilation can be assessed by measuring the flow rate of fresh air entering the room due to the stack effect. Several external factors, such as wind, can influence this flow rate. A reduction (increment) in the flow rate is described as worsening (enhancement). We consider the occurrence of a wind opposing the stack effect. Firstly, a constant wind is considered and the corresponding deterministic dynamics is focused on. We study the steady states for different geometries of the atrium, and the transient before the steady state is reached. It is well-known that when no external wind occurs the atrium may either enhance or worsen the ventilation of the room depending on its geometry. Our main finding is that for any atrium geometry the atrium enhances the ventilation, provided that the wind velocity is large enough. However, during the transient, the atrium worsens the ventilation. Secondly, a random component is added to the wind velocity. This stochastic forcing entails a so-called ‘noise-induced transition' of the system dynamics, namely a structural change of the time-averaged behaviour: the warm-cold layers air in the room fluctuates around a mean value lower than the elevation exhibited in case of constant wind. We investigate the effects of the stochastic forcing on the system for increasing coefficient of variation of wind velocity, finding that the room-atrium system is less sensitive to stochastic forcing compared to a single room.

Air pollution in cities, intensified by vehicular traffic emissions and reduced ventilation, poses a significant health risk. Obstacles, like solid or vegetation barriers, are being considered as strategies to reduce pollution exposure for pedestrians and nearby residents in street canyons. This study utilises wind tunnel experiments, to simulate a typical urban canyon with street intersections on both sides, in a 1:200 scale and an H/W=0.5, positioned perpendicular to the free stream wind flow. A passive scalar representing a vehicular pollutant is released along the length of the street canyon. Concentration measurements inside the canyon are performed to determine the effect of parked cars, boundary walls, hedges and trees on pollutant concentration exposure for pedestrians on the sidewalk. Results show that one circulating vortex is generated within the canyon, driving the pollutant to accumulate along the leeward (upwind) wall. Tightly parked cars, boundary walls and hedges, placed along the sidewalk near the leeward wall, can reduce pedestrian pollutant exposure by 15 %, 23 % and 11 % respectively along this sidewalk. Attributes such as obstacle height, surface roughness and porosity play a key role in their performance. However, trees, when placed in the same area, increase pedestrian pollutant exposure by 51 % and 17 % under dense and sparse tree arrangements, respectively. While a broader analysis that considers the variability of vegetation attributes (e.g., porosity, stand density) is desirable, this study remains crucial for validating numerical simulations and suggesting optimal urban measures to reduce pollution exposure for citizens.

We present an experimental investigation of the turbulent entrainment in non-Boussinesq steady plumes, focusing on three helium releases issued from an axisymmetric source, and with increasing Reynolds number. Two-dimensional instantaneous velocity fields, measured using particle image velocimetry (PIV), are exploited to compute first- and second-order velocity statistics focusing on the near field, i.e., up to a distance of a few tens of source radii. Flow visualizations and velocity statistics profiles are investigated to describe the flow transition from a quasilaminar zone, governed by Rayleigh-Taylor instability, to a more distinctly turbulent region. The vertical evolution of the integral fluxes, the Richardson number, and the entrainment coefficient are presented enlightening the influence of an increasing Reynolds number at the source. The plume characteristic density is reconstructed based on the assumption, sustained by recent literature data, of a null-divergence flow. Making use of the entrainment decomposition, we investigate the contribution of the different physical mechanisms involved in the mixing process at varying distances from the source, with a focus on the effect of the local density ratio. Our results show that the variations of the entrainment coefficient in the turbulent region are primarily affected by the near-field generation of turbulent kinetic energy and by a rising contribution of buoyancy effects. Both features do not exhibit a clear dependence on local variations of the density ratio.

We investigate the concentration fluctuations of passive scalar plumes emitted from small, localised (point-like) steady sources in a neutrally stratified turbulent boundary layer over a rough wall. The study utilises high-resolution large-eddy simulations for sources of varying sizes and heights. The numerical results, which show good agreement with wind-tunnel studies, are used to estimate statistical indicators of the concentration field, including spectra and moments up to the fourth order. These allow us to elucidate the mechanisms responsible for the production, transport and dissipation of concentration fluctuations, with a focus on the very near field, where the skewness is found to have negative values - an aspect not previously highlighted. The gamma probability density function is confirmed to be a robust model for the one-point concentration at sufficiently large distances from the source. However, for ground-level releases in a well-defined area around the plume centreline, the Gaussian distribution is found to be a better statistical model. As recently demonstrated by laboratory results, for elevated releases, the peak and shape of the pre-multiplied scalar spectra are confirmed to be independent of the crosswind location for a given downwind distance. Using a stochastic model and theoretical arguments, we demonstrate that this is due to the concentration spectra being directly shaped by the transverse and vertical velocity components governing the meandering of the plume. Finally, we investigate the intermittency factor, i.e. the probability of non-zero concentration, and analyse its variability depending on the thresholds adopted for its definition.

The presence of vegetation within urban canyons leads to non-trivial patterns of the concentration of airborne pollutants, as a result of the complex structure of the velocity field. To investigate the relationship between concentration, velocity fields and vegetation density, we have performed wind-tunnel experiments in a reduced-scale street canyon, oriented perpendicular to the external wind flow, within which we placed a steady ground-level line source of a passive tracer. The aerodynamic behavior of vegetation was reproduced by inserting plastic miniatures of trees along the two long sides of the canyon, according to three different densities. The canyon ventilation was investigated by acquiring one-point simultaneous statistics of concentration and velocity over a dense grid of points within the canyon. The results show that the presence of trees hinders the upward mean vertical velocity at the rooftop, causes a reduction of the turbulent kinetic energy inside the canyon, and reduces the energy content of the large scales. The scalar concentration is conversely characterized by an enhanced level of turbulent fluctuations, whose magnitude is not dampened increasing the tree density. Within the canyon, high tree density inhibits turbulent mass fluxes, which are instead enhanced at roof level, where the mean component of the scalar flux is however hindered. A statistical analysis of concentration time series reveals that the lognormal distribution is suitable to model concentration fluctuations and extreme events, in dispersing plumes emitted by a linear source.

<p>Concentration fluctuations from continuous small sources in a neutral boundary layer were investigated by mean of high-resolution Large Eddy Simulation (LES). The data set includes concentration moments up to the fourth order and shows the range of validity of the Gamma probability density function (PDF) model for the concentration fluctuations and the transition to a  Gaussian PDF for ground sources. We also investigated systematically the off-center line peaks in the concentration variance showing that they are persistent for ground level sources while they disappear according to theoretical arguments for elevated sources. The analysis includes a thorough investigation of the distribution of the most energetic components in the frequency domain by using spectral analysis of the LES results and a stochastic model based on simple theoretical arguments. This analysis supports the picture that the peak in concentration variance frequency distribution is related to both the plume dispersion geometry and the turbulent flow. The results also confirm the recent literature findings that for an elevated source the peak in the concentration variance frequency distribution is almost independent from the crosswind location for a given downwind distance from the source. To our knowledge no previous LES or wind tunnel study had the completeness of the current study.</p>

Drafting is a strategy in marathon running races that reduces the drag force acting on a designated runner. Drafting involves a formation of athletes, called pacers, running in front of (and sometimes behind) the designated runner. The present experiments evaluated complex formations involving up to seven pacers with the aim of enhancing the performances of an elite marathoner. Wind tunnel measurements with 1/10 scale articulated runner manikins were carried out. First, simple formations with one or two pacers were examined and the results were compared with previous investigations. Second, the runner formations of the Nike Breaking2 and INEOS 1:59 Challenge events whose goal was to break the 2 h marathon barrier were studied. The main finding was the identification of three complex formations of six and seven pacers that allow a drag-force reduction of about 60% with respect to a solo runner and an estimated time saving of 262 s. These results point to how it may be possible to run the fastest marathon ever.

Drag coefficient vs Reynolds number for a smooth and a rough cylinder

We investigate the ventilation conditions required to control the propagation of smoke, produced by a tunnel fire, in the presence of two inertial forcings: a transverse extraction system and a longitudinal flow. For that purpose, we performed a series of experiments in a reduced-scale tunnel, using a mixture of air and helium to simulate the release of hot smoke during a fire. Experiments were designed to focus on the ventilation flows that allow the buoyant release to be confined between two adjacent extraction vents. Different source conditions, in terms of density and velocity of the buoyant release, were analysed along with different vent configurations. Experiments allowed us to quantify the increase of the extraction velocity needed to confine the buoyant smoke, overcoming the effect of an imposed longitudinal velocity. Vents with a rectangular shape, and spanning over the whole tunnel width, provide the best performance. Finally, we studied the stratification conditions of the flow, individuating four regimes. Interestingly, when the stratification conditions fade out, as both the longitudinal flow and vertical extraction flows increase, the flow dynamics becomes almost independent of the forcing induced by the presence of buoyant smoke, which eventually acts as a passive scalar transported by the flow.