1,720,982 research outputs found
Superstatistical distribution of daily precipitation extremes: A worldwide assessment
No full textMaximum annual daily precipitation is a fundamental hydrologic variable that does not attain asymptotic conditions. Thus the classical extreme value theory (i.e., the Fisher-Tippett’s theorem) does not apply and the recurrent use of the Generalized Extreme Value distribution (GEV) to estimate precipitation quantiles for structural-design purposes could be inappropriate. In order to address this issue, we first determine the exact distribution of maximum annual daily precipitation starting from a Markov chain and in a closed analytical form under the hypothesis of stochastic independence. As a second step, we formulate a superstatistics conjecture of daily precipitation, meaning that we assume that the parameters of this exact distribution vary from a year to another according to probability distributions, which is supported by empirical evidence. We test this conjecture using the world GHCN database to perform a worldwide assessment of this superstatistical distribution of daily precipitation extremes. The performances of the superstatistical distribution and the GEV are tested against data using the Kolmogorov-Smirnov statistic. By considering the issue of model’s extrapolation, that is, the evaluation of the estimated model against data not used in calibration, we show that the superstatistical distribution provides more robust estimations than the GEV, which tends to underestimate (7–13%) the quantile associated to the largest cumulative frequency. The superstatistical distribution, on the other hand, tends to overestimate (10–14%) this quantile, which is a safer option for hydraulic design. The parameters of the proposed superstatistical distribution are made available for all 20,561 worldwide sites considered in this work
Investigating the dynamics of bulk snow density in dry and wet conditions using a one-dimensional model
Full text linkThe snowpack is a complicated multiphase mixture with mechanical, hydraulic, and thermal properties highly variable during the year in response to climatic forcings. Bulk density is a macroscopic property of the snowpack used, together with snow depth, to quantify the water stored. In seasonal snowpacks, the bulk density is characterized by a strongly non-linear behaviour due to the occurrence of both dry and wet conditions. In the literature, bulk snow density estimates are obtained principally with multiple regressions, and snowpack models have put the attention principally on the snow depth and snow water equivalent. Here a one-dimensional model for the temporal dynamics of the snowpack, with particular attention to the bulk snow density, has been proposed, accounting for both dry and wet conditions. The model represents the snowpack as a two-constituent mixture: a dry part including ice structure, and air; and a wet part constituted by liquid water. It describes the dynamics of three variables: the depth and density of the dry part and the depth of liquid water. The model has been calibrated and validated against hourly data registered at three SNOTEL stations, western US, with mean values of the Nash–Sutcliffe coefficient ?0.73–0.97 in the validation period.Geoscience & EngineeringCivil Engineering and Geoscience
Numerical characterisation of outboard dynamic-inlet waterjets: propulsive aspects of intake/pump integration
No full textThis study introduces the numerical model of the outboard dynamic-inlet waterjet (ODW), a novel concept in waterjet propulsion. The ODW consists of an axisymmetric nacelle housing an axial-flow pump and offering the unique advantage of being analysed independently of the target vessel. Addressing the gap in current literature, this paper presents a potential design study for the ODW under installed conditions integrating existing models of a hydrodynamic diffuser inlet together with a propulsive pump. Exploiting the symmetry of the propulsor, a single-channel configuration of the pump/intake installation is modelled. Steady Reynolds-averaged Navier–Stokes (RANS) system of equations is solved, closed by the k-ω SST for turbulence and Zwart model for cavitation, respectively. Hydraulic and propulsive maps are generated with progressively increasing far-field velocities at three rotor speeds: 800, 1400, and 2000 rpm. Results reveal that under near nominal conditions, the pump performs similarly to its isolated operations. Integrated with the inlet, it achieves a propulsive efficiency of 0.77 at the highest rotor speed. Conversely, at low advancing speeds, local dynamics near the diffuser throat are found to be a source of possible cavitation issues, obstructing the stream tube and potentially leading to critical conditions
AXIAL-FLOW PUMP/INTAKE FLUID DYNAMIC INTERACTIONS: NUMERICAL PREDICTIONS OF OUTBOARD DYNAMIC-INLET WATERJETS OPERATING UNDER INLET DISTORTION
No full textThe Outboard Dynamic-inlet Waterjet (ODW) has already proved to innovate marine propulsion, outperforming existing technologies during near-nominal operations. Representing the naval equivalent to aero-engines, ODWs operate independently, isolated from the ship. Therefore, as in aviation the propulsive performance strictly depends on the quality of the free-stream, a similar behaviour is expected on ODWs, potentially compromising its mission envelope. With the existing literature discussing the details of nominal operations, the present numerical study aims to address the issues occurring when the system experiences inlet distortion. A structured computational domain is generated around the full annulus geometry, derived by matching an axisymmetric inlet with an axial-flow pump. Unsteady Reynolds-Averaged Navier-Stokes (URANS) equations, coupled with k − w SST and Zwart models for turbulence and cavitation, respectively, are solved to simulate the flow past an ODW operating with a 16◦ free-stream incidence, as pump blades rotate at 1400 r pm. The solution indicates that the entering flow separates on the lower half of the propulsor, breaking into several vortical structures downstream. These vortexes time evolution obstructs the capture stream-tube, deteriorating its distribution. Pump non-uniform inflows affect the incidence on rotor blades, resulting in unbalanced loads and anticipated tip cavitation in the lower right quadrant
Bulk volumetric liquid water content in a seasonal snowpack: Modeling its dynamics in different climatic conditions
No full textEnhanced identification of coherent structures in the flow evolution of a pitching wing
No full textThe present work investigates the three-dimensional dynamics which affects the evolution of dynamic stall over a pitching wing. The phenomenon is traced over a finite section of a spanwise periodically-extended infinite blade at Reynolds chord number of Rec = 1.35· 105 and a reduced pitching frequency of k = 0.1. The geometry is obtained from NACA 0012 aerofoil unit chord extrusion. Modal analysis is conducted on the computed flow field through Spectral Proper Orthogonal Decomposition. SPOD inputs are gathered via Computational Fluid Dynamics results employing Delayed Detached Eddy Simulations with k− ω SST closure. The three-dimensional database is converted to a two-dimensional spatial distribution by exploiting the spanwise homogeneity of the system. In particular, the SPOD filter role is addressed by decomposing the velocity field. Thus, three different filter sizes are chosen to investigate the response of the modal set corresponding to the two dominant pairs. The results prove that stronger filtering actions are less effective over low-harmonic contents, already featuring high correlation levels around a single dominant frequency. Conversely, higher harmonics responses are more dependent on the filter dimension. The latter, in fact, enforces a temporal constraint that may introduce detrimental effects on the energetic optimality of the spatial modes, thus compromising the possibility to draw a direct connection with physical evolutions
Numerical simulations of pump/intake interactions under non-uniform incoming streamtubes for Outboard Dynamic-inlet Waterjets
No full textThis study presents a numerical investigation of off-design performance in an Outboard Dynamic-Inlet Waterjet (ODW) operating under distorted inflow conditions. Leveraging recent advances in marine propulsion, the ODW is modeled independently from the ship as an axisymmetric nacelle housing a propulsive pump. Expanding on existing research focused on nominal ODW conditions, this paper explores a free-stream incidence angle of 16◦ using a full annular model. The model couples an axisymmetric inlet with an axial-flow pump rotating counterclockwise at 1400 rpm. Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulations, conducted over five impeller cycles, account for turbulence and cavitation using the k-ω SST and Zwart models, respectively. The results reveal the development of a vortex in the lower-right quadrant of the intake, reducing pressure on that side and promoting cavitation on the blades advancing through the right. Additionally, flow separation on the nacelle's outer walls near the stagnation point leads to a cavitation cloud symmetrically covering a 114◦ sector around the propulsor mid-plane
Future perspectives of run-of-the-river hydropower and the impact of glaciers’ shrinkage: The case of Italian Alps
No full textWe assess the impacts of nine climate-change scenarios on the hydrological regime and on hydropower production of forty-two glacierized basins across the Italian Alps, assumed exemplary of similar systems in other glacierized contexts. Each of these basins includes one (or more) hydropower plant, here treated as a run-of-the-river system. We implemented a semi-distributed hydrologic model that divides each basin in elevation bands and reconstructs orographic effects on both precipitation and temperature. The nine climate-change scenarios quantify the individual and combined effects of an increase in temperature and a change in liquid-solid phase partition. The simulation horizon is 2016–2065. Thus, we avoided long-term scenarios and worked at short-medium range to maximize the relevance of this work for decision makers. Our results predict a decline of about −30% in average summer runoff across all basins compared to present. Because most of this decrease in runoff occurs during high-flow periods when the run-of-the-river capacity of these plants is exceeded, this result translates into a median decrease of about −3% in hydropower production for run-of-the-river systems through 2065, across all the basins and all scenarios. The predominant cause of this decline is glacier shrinkage, whereas different temperature or precipitation trends plays a marginal role. Run-of-the-river hydropower production in basins where the current glacier coverage is less than 10% of total area is particularly robust to climate change
Numerical Assessment of a Two-Phase Model for Propulsive Pump Performance Prediction
Full text linkThe present work provides a detailed numerical investigation of a turbopump for waterjet applications in cavitating conditions. In particular, the study focuses on the complexities of cavitation modelling, serving as a pivotal reference for future computational research, especially in off-design hydro-jet scenarios, and it aims to extend current model assessments of the existing methods, by disputing their standard formulations. Thus, a computational domain of a single rotor-stator blade passage is solved using steady-state Reynolds-Averaged Navier–Stokes equations, coupled with one-, two-, and four-equation turbulence models, and compared with available measurements, encompassing both nominal and thrust breakdown conditions. Through grid dependency analysis, a medium refinement with the Shear Stress Transport turbulence model is chosen as the optimal configuration, reducing either computational time and relative error in breakdown efficiency to 1%. This arrangement is coupled with a systematic study of the Zwart cavitation model parameters through multipliers ranging from (Formula presented.) to (Formula presented.). Results reveal that properly tuning these values allows for a more accurate reconstruction of the initial phases of cavitation up to breakdown. Notably, increasing the nucleation radius reduces the difference between the estimated head rise and experimental values near breakdown, reducing the maximum error by 4%. This variation constrains vapour concentration, promoting cavitation volume extension in the passage. A similar observation occurs when modifying the condensation coefficient, whereas altering the vaporization coefficient yields opposite effects
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