The role of turbulence in particle-fluid interaction as induced by the outer geometry of catching-type precipitation gauges

This thesis work investigates the particle-fluid interaction of hydrometeors along the terminal part of their fall trajectories, while approaching the collector of catching-type precipitation gauges in windy conditions. Both the turbulence generated by the bluff body aerodynamics of precipitation gauges when impacted by the wind and the free-stream turbulence inherent to the natural wind are addressed to assess their role in precipitation measurements. The bluff body aerodynamics of precipitation gauges induces deviations in the trajectories of the approaching hydrometeors due to the acceleration, updraft and turbulence development upstream and above the collector of the gauge. The resulting wind-induced errors were studied in the literature using different approaches \u2013 field measurement campaigns, numerical simulations and wind tunnel experiments. In this work, the numerical approach based on Computational Fluid Dynamic (CFD) simulation, which reduces, when compared with field observations, the time and resources needed to investigate different configurations by varying the wind speed, type of precipitation and gauge geometry, is employed. A Lagrangian Particle Tracking (LPT) model provides the catch ratios as a function of the particle size and wind speed. The LPT model, already available from the literature, was adapted to simulate the trajectories of water droplets when falling through the atmosphere and approaching the gauge collector by parameterizing liquid particles with spherical shape and using suitable drag coefficient equations. The first part of the work aims to validate the numerical approach against a dedicated, innovative and robust experimental campaign obtained by means of Wind Tunnel (WT) experiments (flow velocity measurements, Particle Image Velocimetry and video tracking of water drops) conducted in the wind tunnel facilities available at DICCA and at Politecnico di Milano (within the PRIN 20154WX5NA project). The video tracking experimental setup allowed to compare observed and simulated trajectories under various wind velocity and drop size conditions, and to validate the Lagrangian Particle Tracking model, here adapted to simulate particles falling at a different vertical velocity than the terminal one. Comparison and validation of numerical simulation results against field-measured data introduce the problem of confronting this simplified approach with the natural atmospheric conditions actually affecting operational instruments in the field. Natural wind fields are indeed characterized by turbulent fluctuations, especially near to the ground where precipitation gauges are located. Dedicated CFD simulations with various turbulence generating solutions, based on imposing specific boundary conditions or inserting suitable obstacles designed to achieve the desired level of free-stream turbulence upstream of the gauge, were performed. Wind tunnel measurements were performed in the DICCA facility using, as a turbulence-generating device, a fixed solid fence with a regular square mesh inserted upstream of a calyx shaped gauge. CFD simulations were performed reproducing the same conditions and results were validated by comparison with WT measurements. The comparison between the uniform and turbulent free-stream conditions showed that the normalized updraft in the upwind part, upstream of the centre of the collector, and the downdraft in the downwind part are less accentuated in the turbulent free-stream configuration than in uniform free-stream conditions. This is ascribable to the energy dissipation induced by turbulent fluctuations. The dissipative effect of the free-stream turbulence also has a damping role on the acceleration of the flow above the collector as demonstrated by CFD results. The overall free-stream turbulence effect on the collection performance of the gauges was quantified by computing and comparing the Collection Efficiency (CE) values in uniform and turbulent free-stream conditions. Results demonstrated that the CE values are higher in turbulent free-stream conditions. The effect of the free-stream turbulence on the collection efficiency of the Hotplate\ua9 snow gauge was investigated, and the literature turbulence intensity level (from 8istad, 2015) impacting on the gauge by was obtained in the simulation by imposing a constant turbulent kinetic energy value as a boundary condition upstream of the gauge. The calculated catch ratios are larger for the free-stream turbulence condition with respect to the uniform one for all characteristic sizes of snowflakes. Consequently, the same effect was observed in the calculated CE values. In addition, in order to introduce a realistic level of turbulence at the gauge collector elevation in the simulation, wind speed measurements obtained from a 3D ultrasonic anemometer in the Nafferton Farm site (UK), recorded at high frequency (20 Hz) and at the gauge elevation, were analysed to calculate the free-stream turbulence intensity values for various wind speeds. This was used to perform a CFD simulation on a chimney shaped gauge and to calculate its effect on the collection performance. To better reproduce the decay of the turbulence intensity in space and its effect on the gauge, Large Eddy Simulations (LES) were also performed in both uniform and turbulent free-stream conditions while simulating the trajectories of solid precipitation particles, which are more sensitive than raindrops to the turbulent fluctuations. Results, in terms of the catch ratio for each characteristic size of snowflakes, show a different behaviour when compared to the uniform conditions. A larger free-stream turbulence intensity induces a more pronounced undercatch for small size particles (less than 2 mm) with respect to the uniform case, while the undercatch is reduced for larger particles. This is due to the greater aptitude of the small size particles to follow the turbulent velocity fluctuations, while larger particles are more inertial, and to the reduced velocity components that particles cross in turbulent free-stream conditions near the gauge body. The obtained CE values are higher in turbulent free-stream conditions, confirming the observations already obtained for the airflow features, where a potential overestimation of the undercatch obtained in uniform free-stream conditions was hypothesized. Based on the CFD results and on the validation provided by wind tunnel observations it is possible to conclude that accounting for the free-stream airflow turbulence in the simulation is required to avoid underestimation of the collection efficiency of precipitation gauges. A turbulent free-stream is indeed the natural atmospheric condition of the wind impacting on operational precipitation gauges in the field. This work demonstrates that numerical derivation of correction curves for use in precipitation measurements as proposed hitherto in the literature is affected by a systematic overestimation of the wind-induced error due to the simplifying assumption of uniform free-stream conditions. Finally, in order to achieve results that can be used in an operational context, suitable Collection Efficiency (CE) curves and the associated adjustment curves, which directly provide the expected undercatch as a function of the wind speed and the measured precipitation intensity, were derived for two sample measurement instruments. The first one is best suited for rainfall measurements and is characterised by the common cylindrical shape of traditional catching type gauges, therefore a numerical formulation of the CE curves as a function of rainfall intensity is proposed. The second one, the Hotplate\ua9 gauge, is best suited for snowfall measurements and is characterised by an innovative measuring principle implying a dedicated geometry of the sensor. In this case, the numerically derived CE curves are expressed as a function of snowfall intensity. For the typical cylindrical gauge, the residual dependency of the CE curves on the rainfall intensity was investigated in order to obtain a single CE expression as a function of both the rainfall intensity and wind speed. The parameters of the Particle Size Distribution (PSD) for various classes of the RI were derived by literature data from the Italian territory. Then the variation of the PSD parameters as a function of the RI was obtained, and subsequently also the parameters of the sigmoidal curves, used to fit the numerical CE values, were parametrized with the RI. As a result, easy to use adjustment curves as a function of both the measured rainfall intensity and wind speed were derived. In the case of the Hotplate\ua9 snow gauge, the shape of the CE curves differs from the typical sigmoidal one due to its complex geometry. At low wind speed, the aerodynamic response of the gauge is predominant and CE values decrease with increasing the wind speed up to a wind threshold value beyond which the geometrical effect on the collection performance starts to be relevant and the CE increases. At very high wind speeds the geometrical contribution prevails and the CE becomes even larger than one

Precipitation Measurement Instruments: Calibration, Accuracy and Performance

Though ranking high among the relevant environmental variables (due to the well-known significant interactions with the everyday human life and economic activities), atmospheric precipitation is not yet measured operationally with neither the degree of accuracy that would meet the most demanding applications nor any rigorous standardization framework [...

Wind tunnel validation of the aerodynamic performance of rain gauges simulated using a CFD approach.

Wind is recognized as the primary cause for the undercatch of solid and liquid precipitation as experienced by catching type gauges. The airflow pattern above the collector, modified by the presence of the gauge body, influences the particle trajectories and reduces the collection of precipitation. Windshields are employed in the field to reduce the impact of wind. As an alternative, measured data are corrected in post-processing using correction functions derived from field data or numerical simulations. Aerodynamic rain gauges have been also developed, with their outer shape designed to reduce the aerodynamic impact of the gauge body on the surrounding airflow. In a previous work, CFD simulations of aerodynamic gauges were performed and the performance of different shapes were compared. The aim of this work is to validate the airflow pattern around the gaugeas predicted by improved CFD simulations by performing wind tunnel tests both in laminar and turbulent base-flow conditions. The airflow in the proximity of the gauge was simulated using the Unsteady Reynolds Average Navier-Stokes (URANS) equations approach. Advantages of the URANS method include the possibility of describing accurate time-varying patterns of the turbulent air velocity field while maintaining acceptable computational requirements. The simulations were performed under two different turbulence conditions in order to assess the role of the base-flow turbulence on the calculated flow pattern. In the first case, the free stream velocity profile is assumed steady and uniform. Under these conditions the time varying pattern of the airflow around the rain gauge collector is due to the instrument aero-dynamics alone. The second case includes a free-stream turbulence intensity approximately equal to 13%, generated by introducing a fixed solid fence upstream the gauge. Validation of the CFD results was provided by realizing the same airflow conditions in the DICCA wind tunnel and measuring the air velocity components in different fixed positions around the collector of the gauge. Results are presented in comparative terms, based on the time-averaged air velocity, the amplitude of the oscillating components and the turbulent kinetic energy

WITHDRAWN: Experimental evidence of the wind-induced bias of precipitation gauges using Particle Image Velocimetry and particle tracking in the wind tunnel

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Thermo-fluid dynamic simulation of the Hotplate precipitation gauge.

The present study addresses the aerodynamic response of the recently developed "Hotplate" liquid/solid precipitation gauge when exposed to the wind. The Hotplate gauge employs two heated thin plates to provide a reliable method of precipitation measurement. The measuring principle is based on an algorithm to associate the latent heat needed to evaporate the snow, or the rain, falling on the instrument and the precipitation rate. However, the presence of the instrument body immersed in a wind field is expected to induce significant deformations of the airflow pattern near the gauge, with an impact on the associated catching efficiency. Indeed, the fall trajectories of the hydrometeors when approaching the gauge can be deviated away from the collecting plate resulting, in general, in some underestimation of the precipitation rate. After an initial analysis of real-world "Hotplate" measurements from a field test site located in Marshall, CO (USA) and the comparison with more traditional measurements obtained from a co-located, shielded reference gauge, the role of wind-induced errors is highlighted. The main approach used in this work is based on the numerical simulation of the airflow field around the gauge, using Computational Fluid Dynamics (CFD) to identify areas where the wind-induced updraft, local acceleration and turbulence are significant. The performed CFD airflow simulations use the URANS SST k - \u3c9 modelling scheme, and are the first modelling step to quantify the associated undercatch. These will be possibly coupled in future developments with particle tracking models to derive suitable correction curves for operational purposes. Due to the specific measurement principle exploited by the "Hotplate" gauge, which measures the heat flux needed to evaporate the collected water amount under a constant plate surface temperature, thermo-fluid dynamic simulations are addressed as well. Dedicated tests have been performed in the wind tunnel facility available at DICCA, University of Genoa to validate simulation results. Results indicate that the presence of wind is a relevant source of systematic bias when using the "Hotplate" gauge for the measurement of precipitation, and its effect must be corrected by adopting suitable correction curves as a function of the wind velocity. The magnitude of the correction can be derived from numerical thermo-fluid dynamic simulations and an assessment of the airflow patterns developing around the gauge at various wind velocity regimes is provided in this work. Wind tunnel tests allowed for a substantial validation of the numerical results, and possible improvements of the model are highlighted and proposed for future developments

Laboratory assessment of two catching type drop-counting rain gauges.

This study reports the results of laboratory tests performed to assess the performance of three drop counting rain gauges of the catching type , and to propose suitable correction so as to make them compliant with the specifications of the World Meteorological Organisation (WMO) at one minute time resolution for Rainfall Intensity (RI) measurements. The tests were limited to the steady state conditions, with known and constant flow rates provided to the instrument at various reference intensities for a sufficient period of time, in order to compare the measures provided by the gauge with the reference figures (which is known as dynamic calibration). The instruments investigated are manufactured by Ogawa Seiki Co. Ltd (Japan) and the Chilbolton RAL (UK). They are designed as high-sensitivity drop counter type rain gauges. Using a suitable correction algorithm, based on calibration curves as obtained from the tests performed in the laboratory, it is possible to improve the accuracy of the three instruments and to obtain results that are fully compatible with the WMO required measurement uncertainty provided in the CIMO guide (WMO, Pub. No 8), although only within the acceptable measurement ranges. The laboratory tests were performed under known and constant flow rates in closely controlled conditions, according to the recommended procedures developed during the WMO Laboratory Intercomparison of RI gauges and recommended by WMO. The performance in the field may be lower than those observed in the laboratory, due to errors induced by the atmospheric conditions, installation, status of maintenance, etc

Wind-tunnel measurements of the airflow pattern above the collector of different shielded and unshielded precipitation gauges.

Wind is the first environmental source of precipitation undercatch for catching-type precipitation gauge. This work presents an aerodynamic investigation on different precipitation gauge geometries and on a wind shield by means of wind tunnel tests. Experiments have been jointly performed by University of Genoa, DICCA, and Politecnico di Milano within the Italian project PRIN 20154WX5NA \u201cReconciling precipitation with runoff: the role of understated measurement biases in the modelling of hydrological processes\u201d. The airflow, around precipitation gauges, was measured employing two different experimental techniques: a traversing system equipped with \u201cCobra\u201d multi hole pressure probes and the Particles Image Velocimetry PIV. Cobra probes allow to measure the three components of the local flow velocity in the measuring points, while PIV technique provides two-dimensional velocity fields on the investigated planes. The airflow velocity and direction were investigated for different wind speed values and different precipitation gauge geometries: the \u201cchimney\u201d, the \u201ccylindrical\u201d and the \u201cinverted conical\u201d shapes. The effect of a traditional Single Alter windshield was also assessed on the cylindrical shape. These experiments allow to detect qualitatively and quantitatively the main features of the flow, speed-up and updraft, above the collector which influence the particle trajectories and their collection. Results confirm the dependency of the airflow disturbance on the gauge geometry, especially in terms of maximum local velocity and distribution of the upward and downward components of the vertical velocity. PIV velocity fields and Cobra velocity profiles show the expected attenuation of the flow velocity above a gauge located inside the windshield due to the break of the flow induced by the shield slats. The experimental campaign provided a wide dataset suitable for the validation of numerical Computational Fluid Dynamics simulations. This work is propaedeutic to the quantification of the precipitation undercatch and the elaboration of correction curves to obtain the actual precipitation in windy conditions

Calibration Uncertainty of Non-Catching Precipitation Gauges

Precipitation is among the most important meteorological variables for, e.g., meteorological, hydrological, water management and climate studies. In recent years, non-catching precipitation gauges are increasingly adopted in meteorological networks. Despite such growing diffusion, calibration procedures and associated uncertainty budget are not yet standardized or prescribed in best practice documents and standards. This paper reports a metrological study aimed at proposing calibration procedures and completing the uncertainty budgets, to make non-catching precipitation gauge measurements traceable to primary standards. The study is based on the preliminary characterization of different rain drop generators, specifically developed for the investigation. Characterization of different models of non-catching rain gauges is also included

Parameterization of the Collection Eciency of a Cylindrical Catching-Type Rain Gauge Based on Rainfall Intensity

Despite the numerous contributions available in the literature about the wind-induced bias of rainfall intensity measurements, adjustments based on collection eciency curves are rarely applied operationally to rain records obtained from catching-type rain gauges. The many influencing variables involved and the variability of the results of field experiments do not facilitate the widespread application of adjustment algorithms. In this paper, a Lagrangian particle tracking model is applied to the results of computational fluid dynamic simulations of the airflow field surrounding a rain gauge to derive a simple formulation of the collection eciency curves as a function of wind speed.A new parameterization of the influence of rainfall intensity is proposed. The methodology was applied to a cylindrical gauge, which has the typical outer shape of tipping-bucket rain gauges, as a representative specimen of most operational measurement instruments. The wind velocity is the only ancillary variable required to calculate the adjustment, together with the measured rainfall intensity. Since wind is commonly measured by operational weather stations, its use adds no relevant burden to the cost of meteo-hydrological networks