The aim of this licentiate thesis is to investigate wind-turbines placed in a complex-terrain environment. This is done by studying the flow around small-scale wind-turbine models placed over a landscape model with hills, and by comparing the results with corresponding data obtained over a flat terrain model. The studied flow features include the wind-turbine wake development and the turbine performance under different conditions, the effects from wake interactions, the influence of the ambient turbulence levels and the influence from a complex topography. Wind-tunnel measurements have been performed using particle image velocimetry and hot-wire anemometry to measure the velocity field. Additionally, numerical simulations, based on RANS modelling and actuator-disc techniques, have been made to support the experimental data and to gain further knowledge about the investigated flow cases. The results reveal that the hills promote a downward wake deflection behind the turbines and enhance the wind-turbine wake diffusion. As a consequence of this, and with the flow acceleration introduced by the hills, an improved power performance is seen for turbines exposed to wake-interference effects. A correlation is observed between the turbulence levels present in the flow, and the magnitude to which the hill-induced flow gradients influence the wake: Stronger wake deflections due to the hills are seen when the wind-turbine wake is more diffused. This is for instance the case when the wake of two tandem turbines is studied, or when higher ambient turbulence levels are present in the wind tunnel. A good qualitative agreement is seen when comparing the experimental and numerical results. The simulation results further indicate that the hills give rise to modulations of the wind-turbine wake. It is shown that these modulations can be reasonably captured by means of wake-superposition techniques, given that a wake model with sufficient accuracy is chosen. / Syftet med denna licentiatavhandling är att öka förståelsen om hur vindturbiner påverkas av en omgivande komplex terräng. Huvudsakligen betraktas luftströmningen kring småskaliga vindturbinsmodeller som placerats över en landskapsmodell med kullar. I tillägg görs jämförelser med resultat som erhållits då vindtubinerna placerats över en platt landskapsmodell. De studerade strömningsaspekterna inkluderar vindturbinernas vakutveckling och prestanda under olika förhållanden, inverkan från vakinteraktioner, inflytande från omgivande turbulensnivåer och inverkan från en komplex topografi. Vindtunnelmätningar har utf ̈orts där PIV och varmtrådsanemometri användes för att uppmäta hastighetsfält. I tillägg har numeriska simuleringar utförts baserade på RANS-modellering, där turbinens rotor beskrevs av en porös skiva. Simuleringarna gjordes som komplement till de experimentella mätresultaten för att få en ökad förståelse om de undersökta strömningsfallen. Resultaten från mätningarna och simuleringarna med kullar visar att terrängvariationerna främjar en nedåtgående vakförskjutning bakom turbinerna och ökar vindturbinernas vakdiffusion. Detta, i kombination med luftens acceleration över kullarna, resulterar i att en högre effektprestanda utvinns från en vindturbin vars inströmmande luftflöde störs av vaken från en framförliggande turbin. Vidare observeras kraftigare nedågående vakförskjutningar på grund av det kullriga landskapet då vindturbinsvakarna är mer diffunderade. Detta är exempelvis fallet då vaken bakom två turbiner placerade i en tandemkonfiguration studeras, eller när höga omgivande turbulensnivåer uppmäts i vindtunneln. En bra kvalitativ överensstämmelse kan ses mellan de experimentella och numeriska resultat som uppnås. Resultaten från simuleringarna indikerar dessutom att landskapet med kullar ger upphov till moduleringar av vindturbinens vak. Det visas att dessa moduleringar kan beskrivas någorlunda väl med hjälp av vaksuperpositionsmetoder, givet att en vakmodell med tillräckligt hög noggrannhet väljs. / <p>QC 20180122</p>

Spatial distribution of precipitation in complex terrain can be very uneven and there are many factors to consider when studying it. The goal of this study was such a problem; to analyse the difference in observed annual precipitation in the complex terrain of Seyðisfjörður, a fjord in East-Iceland. The study was carried out in three parts. First, it was investigated if there was a systematic difference between measurement methods using different instruments. Second, the effect of the complex terrain on the spatial distribution of precipitation was investigated, and lastly if this observed difference could be linked to any certain kind of weather regimes. To simplify the analysis, only liquid precipitation was included in the data set.  In Seyðisfjörður there are three divergently located precipitation gauges of three different types and set up. At the Icelandic Meteorological Office in Reykjavík the same type of gauges are co-located with the exact same set up as in Seyðisfjörður. A statistical analysis of those measurements showed that there is a systematic undercatch in tipping bucket gauges compared to weighted capacity and standard accumulation gauges. However, the difference is insignificant in size compared to the observed difference in the complex terrain. The complex terrain was found to highly affect the airflow and therefore the spatial distribution of precipitation, as it almost only rains in synoptic wind directions with an easterly component (measured at a mountain station). To connect events where there was a great difference in precipitation measurements to the synoptic weather situation over the North-Atlantic, a projection connecting the geostrophic and local winds in the fjord was made. It showed that great precipitation as well as when great differences are observed, during two kind of weather regimes. One where a low pressure is travelling along the North-Atlantic storm track to the Norwegian Sea. The other were a low pressure is stationed southwest or west of Iceland in the Irminger Sea and a high pressure stretching up over Scandinavia. Convective precipitation makes up for a minimal part of the total precipitation and is not linked to events with great observed difference. Events with considerable precipitation but little observed difference are fewer and smaller than the events with much great observed precipitation and differences.