arXiv subjects

Pietro Salizzoni

Publications and source records attributed to Pietro Salizzoni.

8 recordsLinked to original sources

arXiv2026

Gravity current fronts advancing along a heated wall

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 \Lambda_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 \Lambda_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 \Lambda_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.

physics.flu-dyn↗

arXiv2026

Experimental study on gravity currents flowing on heated walls

We present an experimental study on steady gravity currents advancing along a heated wall. 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 turbulent vertical and horizontal 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 \textnormal{CO}$_2$ mass, enthalpy, and buoyancy, performed at different distances from the source. Three wall-heating conditions are considered: an adiabatic case, a moderately heated case, and a strongly heated case. 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. Although wall heating influences the vertical shear, the Brunt-Vaisala frequency, and both shear and buoyancy production of turbulent kinetic energy within the stably-stratified region characterized by an almost constant vertical gradient of streamwise velocity, neither the gradient Richardson number nor the flux Richardson number exhibits a clear trend in this region with the imposed wall heat flux.

physics.flu-dyn↗

arXiv2023

Fluid Tunnel Research for Challenges of Urban Climate

Experimental investigations using wind and water tunnels have long been a staple of fluid mechanics research for a large number of applications. These experiments often single out a specific physical process to be investigated, while studies involving multiscale and multi-physics processes are rare due to the difficulty and complexity in the experimental setup. In the era of climate change, there is an increasing interest in innovative experimental studies in which fluid (wind and water) tunnels are employed for modelling multiscale, multi-physics phenomena of the urban climate. High-quality fluid tunnel measurements of urban-physics related phenomena are also much needed to facilitate the development and validation of advanced multi-physics numerical models. As a repository of knowledge in modelling these urban processes, we cover fundamentals, recommendations and guidelines for experimental design, recent advances and outlook on eight selected research areas, including (i) thermal buoyancy effects of urban airflows, (ii) aerodynamic and thermal effects of vegetation, (iii) radiative and convective heat fluxes over urban materials, (iv) influence of thermal stratification on land-atmosphere interactions, (v) pollutant dispersion, (vi) indoor and outdoor natural ventilation, (vii) wind thermal comfort, and (viii) urban winds over complex urban sites. Further, three main challenges, i.e., modelling of multi-physics, modelling of anthropogenic processes, and combined use of fluid tunnels, scaled outdoor and field measurements for urban climate studies, are discussed.

physics.flu-dyn↗

arXiv2022

Simple photochemical modelling of NOX pollution in a street canyon

To predict pollutant concentration in urban areas, it is crucial to take into account the chemical transformations of reactive pollutants in operational dispersion models. In this work, we derive and discuss different NO-NO2-O3 chemical street canyon models with increasing complexity and we analytically evaluate their applicability in different urban contexts. We then evaluate the performance of the models in predicting NO2 concentration at different locations within an urban district by comparing their predictions with measurements acquired in a field campaign. The results are in line with analytical speculations and give indications as to which model to use according to the conditions of the urban street canyon. In courtyards with limited ventilation and without direct emissions, the performance of the photostationary model is satisfactory. On the other hand, the application of a non-photostationary model significantly improves the predictions in urban canyons with direct vehicular emissions. The applicability of the proposed models in operational tools at the city scale is finally discussed.

physics.ao-ph↗

arXiv2022

High resolution wind-tunnel investigation about the effect of street trees on pollutant concentration and street canyon ventilation

Greening cities is a key solution to improve the urban microclimate and mitigate the impact of climate change. However, the effect of tree planting on pollutant dispersion in streets is still a debated topic. To shed light on this issue, we present a wind-tunnel experiment aimed at investigating the effect of trees on street canyon ventilation. An idealized urban district was simulated by an array of blocks, and two rows of model trees were arranged at the sides of a street canyon oriented perpendicularly with respect to the wind direction. Reduced scale trees were chosen to mimic a realistic shape and aerodynamic behaviour. Three different spacings between the trees were considered. A passive scalar was injected from a line source placed at ground level and concentration measurements were performed in the whole canyon. Results show that the presence of trees alters the concentration pattern in the street with a progressive shift from a nearly two-dimensional to a three-dimensional field depending on tree density. Despite the significant change of the concentration field induced by trees, the average level of pollution in the street, and thus the overall ventilation efficiency, does not show a specific trend with the density of trees.

physics.ao-ph↗

arXiv2021

Connecting Large-Scale Velocity and Temperature Bursts with Small-Scale Intermittency in Stratified Turbulence

Non-Gaussian statistics of large-scale fields are routinely observed in data from atmospheric and oceanic campaigns and global models. Recent direct numerical simulations (DNSs) showed that large-scale intermittency in stably stratified flows is due to the emergence of sporadic, extreme events in the form of bursts in the vertical velocity and the temperature. This phenomenon results from the interplay between waves and turbulent motions, affecting mixing. We provide evidence of the enhancement of the classical small-scale (or internal) intermittency due to the emergence of large-scale drafts, connecting large- and small-scale bursts. To this aim we analyze a large set of DNSs of the stably stratified Boussinesq equations over a wide range of values of the Froude number ($Fr\approx 0.01-1$). The variation of the buoyancy field kurtosis with $Fr$ is similar to (though with smaller values than) the kurtosis of the vertical velocity, both showing a non-monotonic trend. We present a mechanism for the generation of extreme vertical drafts and vorticity enhancements which follows from the exact equations for field gradients.

physics.flu-dyn↗

arXiv2019

Experimental investigation of vertical turbulent transport of a passive scalar in a boundary layer: statistics and visibility graph analysis

The dynamics of a passive scalar plume in a turbulent boundary layer is experimentally investigated via vertical turbulent transport time-series. Data are acquired in a rough-wall turbulent boundary layer that develops in a recirculating wind tunnel set-up. Two source sizes in an elevated position are considered in order to investigate the influence of the emission conditions on the plume dynamics. The analysis is focused on the effects of the meandering motion and the relative dispersion. First, classical statistics are investigated. We found that (in accordance with previous studies) the meandering motion is the main responsible for differences in the variance and intermittency, as well as the kurtosis and power spectral density, between the two source sizes. On the contrary, the mean and the skewness are slightly affected by the emission conditions. To characterize the temporal structure of the turbulent transport series, the visibility algorithm is exploited to carry out a complex network-based analysis. Two network metrics -- the average peak occurrence and the assortativity coefficient -- are analysed, as they can capture the temporal occurrence of extreme events and their relative intensity in the series. The effects of the meandering motion and the relative dispersion of the plume are discussed in the view of the network metrics, revealing that a stronger meandering motion is associated with higher values of both the average peak occurrence and the assortativity coefficient. The network-based analysis advances the level of information of classical statistics, by characterizing the impact of the emission conditions on the temporal structure of the signals in terms of extreme events and their relative intensity. In this way, complex networks provide -- through the evaluation of network metrics -- an effective tool for time-series analysis of experimental data.

physics.flu-dyn↗

arXiv2016

Turbulent transport and entrainment in jets and plumes: a DNS study

We present a new DNS data set for a statistically axisymmetric turbulent jet, plume and forced plume in a domain of size $40 r_0 \times 40 r_0 \times 60 r_0$, where $r_0$ is the source diameter. The data set provides evidence of the validity of the Priestley and Ball entrainment model in unstratified environments (excluding the region near the source), which is corroborated further by the Wang and Law and Ezzamel \emph{et al.} experimental data sets, the latter being corrected for a small but influential co-flow that affected the statistics. We show that the turbulence in the core region of the jet and the plume are practically indistinguishable, although the invariants of the anisotropy tensor reveal a significant change in the turbulence near the plume edge. The DNS data indicates that the turbulent Prandtl number is about 0.7 for both jets and plumes. For plumes, this value is a result of the difference in the ratio of the radial turbulent transport of radial momentum and buoyancy. For jets however, the value originates from a different spread of the buoyancy and velocity profiles, in spite of the fact that the ratio of radial turbulent transport terms is approximately unity. The DNS data does not show any evidence of similarity drift associated with gradual variations in the ratio of buoyancy profile to velocity profile widths.

physics.flu-dyn↗