On the near field behavior of inclined negatively buoyant jets

et al. 2020

Water

We present a study of wave overtopping of barriers. The phenomenon of the wave overtopping over emerged structures is reproduced both numerically and experimentally. The numerical simulations are carried out by a numerical scheme for three-dimensional free-surface flows, which is based on the solution of the Navier–Stokes equations in a novel integral form on a time-dependent coordinate system. In the adopted numerical scheme, a novel wet–dry technique, based on the exact solution of the Riemann problem over the dry bed, is proposed. The experimental tests are carried out by adopting a nonintrusive and continuous-in-space image-analysis technique, which is able to properly identify the free surface even in very shallow waters or breaking waves. A comparison between numerical and experimental results, for several wave and water-depth conditions, is shown.

Section: Three-dimensional Validation Testmentioning

confidence: 99%

Numerical and Experimental Investigation of Wave Overtopping of Barriers

Cannata

Tamburrino

et al. 2020

Water

“…The method is robust to high spatial velocity gradients and appearance/disappearance of particles. A detailed description of the dissimilarity measurement algorithm, with an evaluation of the performance compared to the classical crosscorrelation used in the PIV technique, is reported by Falchi et al (2006), whereas the tracking procedure is described by Besalduch et al (2013Besalduch et al ( , 2014.…”

Section: Experimental Set-up and Proceduresmentioning

confidence: 99%

On the Flow Past an Array of Two-Dimensional Street Canyons Between Slender Buildings

Badas

Boundary-Layer Meteorol

Garau

et al. 2019

The flow above idealized, two-dimensional series of parallelepipedal buildings is examined with the aim of investigating how the building width (W) to height (H) aspect ratio affects the turbulence in the roughness sublayer and the ventilation of the underlying street canyons. We compare the case of buildings with a squared section (ARB = W/H = 1.0) with a configuration with slender buildings (ARB = 0.1) both in the case of unit canyon width (D) to height (H) aspect ratio (ARC = D/H = 1) and in the case of ARC = 2. The former corresponds to skimming flow and the latter to wake-interference regime. Measurements are performed in a water channel, measuring velocity on a vertical mid-plane using a particle image velocimetry technique. The mean flow, its second-order turbulence statistics, the exchange fluxes, and the integral time scales investigated, with results showing that slender buildings enhance turbulence production and yield longer integral time scales in the region just above the building roof. Namely, in the skimming-flow and wakeinterference regimes, the maximum vertical velocity variance is more than doubled and increased by 50%, respectively. The combined analysis of the turbulence production fields and the snapshots of the flow during sweep and ejection events demonstrate that the shear layer between the canyon and the external flow is significantly more unstable with slender buildings, mainly because the damping effect of the vertical velocity fluctuations from the flat roof of the upwind building is substantially missing. Consequently, a larger (downstream) portion of the interface is prone to the direct interaction of the external flow structures. The higher turbulence intensity promotes the ventilation at the canyon interface, which is increased by a factor of two in the skimming-flow regime and a factor of 1.26 in the wake-interference regime. In summary, the present experiments show that the effect of the reduced building aspect ratio is particularly significant when the urban canopy consists of narrow canyons. The result is of interest since narrow street canyons are typically bounded by slender buildings in the urban texture of the old European city centres.

“…Querzoli 2011, Di Bernardino et al 2015 -a); see Ferrari (2017) for a review. The Feature Tracking Velocimetry technique, described in (Besalduch et al, 2013(Besalduch et al, , 2014, is based on: (1) the identification of the particle images on the frames by means of the Harris corner detection algorithm (Harris and Stephens 1988); (2) the interrogation windows comparison, for which the windows are centred on the particles and comparisons are made by measuring the dissimilarity between the windows of successive frames; (3) the evaluation of the particles displacement between two successive frames is defined as the displacement minimizing the dissimilarity between interrogation windows. The dissimilarity was determined by the Lorentzian robust estimation (Falchi et al, 2006).…”

Section: Laboratory Experimentsmentioning

confidence: 99%

Air exchange in urban canyons with variable building width: a numerical LES approach

Garau

Badas

et al. 2019

IJEP

The aim of this work is to gain further insight into the role played by the building aspect ratio (ARB, i.e. the ratio of the building width, WB, to the building height, H) and its influence on street canyon flow. We carried out a series of Large Eddy Simulations with arrays of obstacles with different widths, ranging from 0.5 to 2.0, and two canyon aspect ratios (ARC, i.e. the ratio of the canyon width, W, to H) ARC = 0.5 and ARC = 1.0. Experimental data was obtained in a water channel for the corresponding configuration and used to validate numerical simulations. Results were found strongly dependent on the building aspect ratios, with two distinct behaviors identified with respect to the canyon aspect ratio. The residence time decreases with decreasing ARB, irrespective of the canyon aspect ratio, suggesting that ARB and ARC can be optimized to guarantee an optimal street canyon ventilation in urban planning.