Numerical modelling of atmospheric flows over complex sites with special regard to the forest canopy

The present thesis describes the work carried out using the OpenFOAM solver with a Reynolds-Averaged Navier Stokes (RANS) approach to investigate the wind flow at complex sites for wind-energy exploitation. Toward this objective, several physical effects such as buoyancy, forest canopies, Coriolis forces, stratification as well as humidity have been implemented in the model to improve wind-field predictions. First, the wind flow in an urban environment and, more precisely, a university campus is investigated. A stationary logarithmic profile for the wind velocity at the inlet is prescribed. Despite the assumption of a flat terrain, which is a drastic simplification of the real ground, the study shows how a simple canopy model improves the prediction of the flow at the site. The simulation is validated with long term measurements from a network of six stations. Secondly, results from a rural case in the Swabian Alb in Southern Germany, characterized by a forested escarpment, are presented. The model is adapted to atmospheric boundary layer (ABL) flows and a computational domain with a ground conforming to the site orography is built. To get more realistic boundary conditions and to avoid the assumption of logarithmic profiles, the solver is coupled with a numerical weather prediction (NWP) model. The coupling is performed using a one-way approach, i.e the coarse weather model provides input to the OpenFOAM solver through the lateral boundary conditions of the computational domain. Simulations with and without forest are compared. The results with a canopy model clearly show at the lower levels a flow deceleration and an increase in turbulence intensities by a factor of four, when compared to results without forest. The study reveals again the important impact of the forest on the wind-field, especially at turbine-relevant heights. Finally, the transient approach (unsteady RANS) is tested by using time-dependent boundary conditions. The accuracy of the coupling is evaluated by validating the simulation results against measurements from a tall meteorological tower as well as an unmanned aircraft system. Adopting a transient approach leads to an excellent agreement of the model. The thesis shows that an unsteady RANS based solver, which accounts for first-order relevant physics, can be valuable for a wind resource assessment at low computational cost compared to detached-eddy (DES) or large-eddy (LES) simulations

Comparison of CFD Simulation to UAS Measurements for Wind Flows in Complex Terrain: Application to the WINSENT Test Site

This investigation presents a modelling strategy for wind-energy studies in complex terrains using computational fluid dynamics (CFD). A model, based on an unsteady Reynolds Averaged Navier-Stokes (URANS) approach with a modified version of the standard k-ε model, is applied. A validation study based on the Leipzig experiment shows the ability of the model to simulate atmospheric boundary layer characteristics such as the Coriolis force and shallow boundary layer. By combining the results of the model and a design of experiments (DoE) method, we could determine the degree to which the slope, the leaf area index, and the forest height of an escarpment have an effect on the horizontal velocity, the flow inclination angle, and the turbulent kinetic energy at critical positions. The DoE study shows that the primary contributor at a turbine-relevant height is the slope of the escarpment. In the second step, the method is extended to the WINSENT test site. The model is compared with measurements from an unmanned aircraft system (UAS). We show the potential of the methodology and the satisfactory results of our model in depicting some interesting flow features. The results indicate that the wakes with high turbulence levels downstream of the escarpment are likely to impact the rotor blade of future wind turbines

A Two-Day Case Study: Comparison of Turbulence Data from an Unmanned Aircraft System with a Model Chain for Complex Terrain

The airborne measurement platform MASC-3 (Multi-Purpose Airborne Sensor Carrier) is used for measurements over a forested escarpment in the Swabian Alps to evaluate the wind field. Data from flight legs between 20 and 200 m above the ground on two consecutive days with uphill (westerly) flow in September 2018 are analyzed. In the lowest 140 m above the ground a speed-up is found with increased turbulence and changes in wind direction directly over the escarpment, whereas in the lowest 20 to 50 m above the ground a deceleration of the flow is measured. Additionally, simulation results from a numerical model chain based on the Weather Research and Forecasting (WRF) model and an OpenFOAM (Open Source Field Operation and Manipulation) model, developed for complex terrain, are compared to the data captured by MASC-3. The models and measurements compare well for the mean wind speed and inclination angle

Fiabilité de la mesure de vitesse débitante à l'aval d'une singularité en réseau d'assainissement

When managing a sewer network, the water discharge has to be measured in real time at several locations in the network. In locations where a simple stage discharge relationship cannot be applied due to an influence of the backwater curve, this measurement is to be performed using sensors that measure the bulk velocity locally in order to estimate the discharge. The aim of this work is to quantify the errors made when such measurement is performed downstream from two types of singularities: a bend and a confluence, which are commonly encountered in sewer networks. Two types of sensors are tested: a time-of-flight flow meters and a acoustic Doppler flowmeter. Both technologies are first detailed along with their possible sources of errors when used in the vicinity of singularities. The second section then introduces the numerical calculation of all flow configurations tested downstream both kinds of singularities. The third section shows the results in terms of error of discharge estimation as a function of the location with regards to the singularities, the type of singularity and the flow conditions. This permits to finally make recommendations for the use of such sensors in a real sewer network.Les gestionnaires de réseau d'assainissement urbain sont amenés à équiper leurs réseaux de points de mesures débimétriques. Lorsqu'une relation permanente permettant de relier localement la hauteur d'eau et le débit n'existe pas, il est nécessaire de mesurer la vitesse moyenne de l'écoulement afin d'en déduire le débit localement. L'objectif de cet article est de quantifier les erreurs faites lorsqu'une mesure de vitesse est effectuée à l'aval d'une singularité pour des situations réalistes de collecteurs d'assainissement urbain. La première section de cet article présente les deux types de capteurs (cordes et Doppler) en détaillant leur mode d'échantillonnage spatial ainsi que les sources d'erreur associées. La deuxième section présente la démarche de modélisation numérique ainsi que les domaines de calcul associés à l'aval respectivement d'une confluence et d'un coude. Enfin la troisième partie présente les résultats en termes de réponse des capteurs à ces champs de vitesse et discute les erreurs pour les différentes configurations considérées

Projet COACHS - Rapport finalisé

Le projet avait pour principal objectif de contribuer au déploiement de systèmes d'instrumentation intégrée permettant une surveillance en continu et en temps réel des rejets des réseaux d'assainissement au milieu naturel. L'instrumentation intégrée signifiait ici la prise en compte de la chaîne métrologique complète, de la qualification d'un site de mesure et du choix d'une méthode au calcul d'incertitudes et à la valorisation des données, du choix des capteurs à leur implantation. La mise en synergie de recherches en mécanique des fluides numérique et de travaux expérimentaux a permis de : proposer des méthodes de mesure du débit dans un écoulement canalisé et de détermination du débit déversé au niveau des postes de refoulement ; développer une méthodologie d'évaluation des débits conserÎs et déversés au niveau des déversoirs d'orage ; montrer sur des exemples comment, pour un site donné, des recommandations peuvent être formulées pour la mise en place d'une instrumentation intégrée au niveau des déversoirs d'orage ; préciser l'influence d'une singularité (coude ou confluence) sur l'écoulement en aval et ses répercussions sur le positionnement d'un débitmètre à l'aval d'une telle singularité