Publications

Accurate prediction of the performance of a vertical-axis wind turbine (VAWT) using Computational Fluid Dynamics (CFD) simulation requires a domain size that is large enough to minimize the effects of blockage and uncertainties in the boundary conditions on the results. It also requires the employment of a sufficiently fine azimuthal increment (dθ) combined with a grid size at which essential flow characteristics can be accurately resolved. The current study systematically investigates the effect of the domain size and azimuthal increment on the performance of a 2-bladed VAWT operating at a moderate tip speed ratio of 4.5 using 2-dimensional and 2.5-dimensional simulations with the unsteady Reynolds-averaged Navier-Stokes (URANS). The grid dependence of the results is studied using three systematically refined grids. The turbine has a low solidity of 0.12 and a swept area of 1 m2. Refining dθ from 10.0° to 0.5° results in a significant (≈43%) increase in the predicted power coefficient (CP) while the effect is negligible (≈0.25%) with further refinement from 0.5° to 0.05° at the given λ. Furthermore, a distance from the turbine center to the domain inlet and outlet of 10D (D: diameter of turbine) each, a domain width of 20D and a diameter of the rotating core of 1.5D are found to be safe choices to minimize the effects of blockage and uncertainty in the boundary conditions on the results.

In the present study the relative importance of several model parameters related to RANS simulation of urban wind flow was investigated, using a district of Livorno city (Italy) as a case study for which earlier wind tunnel tests were performed. CFD simulations were performed at the same reduced scale (1:300) and for the same inflow wind direction α = 240° as the wind tunnel tests. The impact of three model parameters was investigated: the level of geometrical simplification (two levels of detail), inflow conditions (two profiles) and turbulence model(four turbulence models). Four validation metrics were used to quantify the impact of each considered parameter on calculated mean wind velocities near the ground: Fractional Bias (FB), Normalized Mean Square Error (NMSE), correlation coefficient (R), and the fraction of data within a factor of 1.3 (FAC1.3). The geometrical simplifications were shown to have a much larger effect on the results than inflow conditions and turbulence modeling approach, both in terms of correlation (R) and FAC1.3.

Spoked wheels are commonly used in cycling races and their aerodynamic performance is a critical factor in the overall cycling performance, as the wheels can be responsible for about 10% of the total cyclist-bicycle drag. Although several computational fluid dynamics (CFD) simulations have been carried out for wheel aerodynamics in the past decades, it is still not clear to which extent the outcome of such simulations is sensitive to the computational parameters set by the user. The present paper aims at defining a framework for CFD simulations of an isolated spoked wheel by a systematic sensitivity analysis focused on the computational grid, wheel rotation modeling and turbulence modeling. The results show: (i) a high sensitivity to the wheel surface grid, y+ (<4) and far-field growth rate (≤1.15); (ii) the wheel rotational approaches with moving reference frame (MRF) and hybrid MRF-RW (RW ​= ​rotating wall approach) can provide a satisfactory agreement with wind tunnel data available in the literature (+9.7% and −2.1% deviations, respectively); (iii) k-ω SST, γ-SST or realizable k-ε are suitable as turbulence models. This work is intended to stimulate the accurate and reliable application of CFD for the assessment and optimization of wheel aerodynamics.

The convective heat and moisture transfer (H&MT) at the windward wall of a 2D (infinitely long) square-shaped building is evaluated after a rain shower. The H&MT in the wall is modelled with a Heat, Air and Moisture model which is coupled with CFD to obtain detailed information on the convective transfer at the exterior surface. A significant spatial variation of the convective transfer coefficients (CTCs) is found over the wall, showing high drying rates near the roof top. The temporal variation of the CTCs is limited for this specific case. The coupled approach, using CFD, shows an added value compared to the use of constant (spatial and temporal) CTCs.

Vertical axis wind turbines (VAWTs) are promising candidates for wind energy harvesting in the urban environment. However, their aerodynamic performance still falls behind of their horizontal axis counterparts. This could be associated with the comparatively small research they have received in the past decades as well as their complex unsteady aerodynamics. Computational Fluid Dynamics (CFD) has been widely used to evaluate and improve the aerodynamic performance of VAWTs. An extensive literature study reveals that the 2D unsteady Reynolds-Averaged Navier-Stokes (URANS) approach has been used in the majority of CFD studies on VAWTs while the sensitivity of the CFD results to the choice of the turbulence model has not yet been comprehensively investigated in the literature. Therefore, the current study intends to evaluate the aerodynamic performance of three different VAWTs, calculated using 2D URANS with seven different one- to four-equation turbulence models, namely Spalart-Allmaras, RNG and realizable k-e, k-ω SST, kωSSTi (with the intermittency model), k-kl-ω and transition SST. The results are also compared against inviscid simulations. The turbines are selected based on the availability of the experimental data for the comparison in addition due to their geometrical and operational differences which would help to further generalize the conclusions. A comparative analysis of the turbine power performance and wake velocities for the turbines indicates that the 3-equation k-kl-ω and the 4-equation transition SST models are the two best-performing models while the inviscid modeling and the e-based models exhibit the worst performance for CP predictions in comparison to the experiment. The findings support more accurate CFD simulations of VAWTs.

Full-scale measurements have shown that variations in wind-driven rainfall (WDR) distribution over small-scale topographic features such as small hills and valleys can be quite large. Therefore, these variations should be taken into account in e.g. catchment hydrology, runoff and erosion studies and in the design of rainfall monitoring networks. Full-scale measurements however are expensive and time-consuming. A few researchers have attempted numerical modeling but model validation is rather scarce. This paper presents 2D Computational Fluid Dynamics simulations to determine the wind-driven rainfall (WDR) distribution over a small isolated hill and in a valley. Validation is performed by comparing the numerical results with fullscale measurements. It is shown that the CFD simulations can provide a good indication of the WDR distribution over these types of small-scale topography.