Low-level jets (LLJs) are local wind speed maximums in the atmospheric boundary layer. In the Baltic Sea, LLJs are frequently occurring in spring and summer. It is an important phenomena to consider for wind energy parks, and changes in turbulence during the jets can effect the efficiency of said parks. In this study, the effect that offshore LLJs have on turbulence intensity (TI) is analysed and the goal is tounderstand if TI significantly changes as the jets form, and if the changes aredifferent above and below the core. The theory of shear sheltering predicts that turbulence decreases below the core of a LLJ, and it has been experimentally tested previously with various results. However, turbulence characteristics above the core ofa LLJ has not been studied before. LiDAR measurements of wind speed and TI profiles, up to 300 m, from the island of Östergarnsholm in the Baltic Sea are used. The measurements are from the period 2016-2020 and are limited to a sector with unobstructed line-of-sight to the ocean. Complete LLJ-events, which includes non-LLJ profiles before and after the actual jets, are analysed. The LLJs are found to appear in low TI conditions related to stable stratification. Mean TI increases with 38 - 47% above the core as the jets appear, and then returns to approximately the initial values after the jets disappear. Below the core, mean TI instead decreases with 14 - 19% during the jets, which is compatible with the theory of shear sheltering. For future studies it is recommend to choose a location with larger unobstructed line-of-sight to the ocean, further optimise the LLJ-finding algorithm and also analyse other turbulent quantities. / Vindmaxima på låg höj (LLJ, för eng. Low-level jets) är lokala vindhastighetsmaximum i det atmosfäriska gränsskiktet. I Östersjön är LLJs vanliga, framförallt på våren och sommaren. Det är ett viktigt fenomen att beakta för vindkraftsparker, och turbulensförändringar i samband med LLJs kan påverka effektiviteten av vindkraftverk. I denna studie analyseras effekten som LLJs över havet har på turbulensintensiteten (TI) och målet är att förstå om TI förändras närströmmarna bildas, och om förändringarna är olika ovan och under kärnan. En teori förutspår att turbulens minskar under kärnan i en LLJ, s.k. skjuvningsblockering (eng. Shear sheltering), och den har testats tidigare med varierande resultat. Turbulens ovanför kärnan i en LLJ har dock inte studerats tidigare. LiDAR-mätningar av vindhastighets och TI-profiler, upp till 300 m, vid ön Östergarnsholm i Östersjön används. Mätningarna är från perioden 2016-2020 och är begränsade till en sektor med fri siktlinje mot havet. Kompletta LLJ-event, vilket inkluderar icke-LLJ-profiler före och efter själva strömmen, analyseras. Resultatet visar att LLJs förekommer vid låga TI-förhållanden relaterade till stabil skiktning. Medel TI ökar med 38 - 47% överkärnan när strömmarna dyker upp och återgår sedan till ungefär de ursprungligavärdena efter att strömmarna försvunnit. Under kärnan minskar medel TI istället med 14 - 19% i samband med strömmarna, vilket är förenligt med skjuvningsblockerings-teorin. För framtida studier är det rekommenderat att välja en plats med större fri siktlinje till havet, ytterligare optimera identifikationen av kompletta LLJ-event och även analysera andra variabler för att karakterisera turbulensen.

Shear sheltering is defined as the effect of the mean flow velocity profile in a boundary layer on the turbulence caused by an imposed gust. In aeroacoustic applications turbulent boundary layers interacting with blade trailing edges or roughness elements are an important source of sound, and the effect of shear sheltering on these noise sources has not been studied in detail. Since the surface pressure spectrum below the boundary layer is the primary driver of trailing edge and roughness noise, this thesis considers the effect that shear sheltering has on the surface pressure spectrum below a boundary layer. This study presents a model of the incoming turbulence as a vortex sheet at a specified height above the surface and shows, using canonical boundary layers and approximations to numerical results, how the mean flow velocity profile can be manipulated to alter the surface pressure spectrum and hence the associated trailing edge noise. The results from this model demonstrate that different mean velocity profiles drive significant changes in the unsteady characteristics of the flow. The surface pressure fluctuations results also suggest that boundary layers where the shear in the mean velocity profile is significant can be beneficial for the reduction of trailing edge noise at particular frequencies. / Includes bibliography. / Thesis (M.S.)--Florida Atlantic University, 2020. / FAU Electronic Theses and Dissertations Collection

Trailing edge noise control is an important problem associated mainly with wind turbines. As turbulence in the air flows over a wind turbine blade, it impacts the trailing edge and scatters, producing noise. Traditional methods of noise control involve modifying the physical trailing edge, or the scattering efficiency. Recently, inspired by the downy covering of owl feathers, researchers developed treatments that can be applied to the trailing edge to significantly reduce trailing edge noise. It was hypothesized that the noise reduction was due to manipulating the incoming turbulence, rather than the physical trailing edge itself, representing a new method of noise control. However, only acoustic measurements were reported, meaning the associated flow physics were still unknown. This thesis describes a comprehensive wall jet experiment to measure the flow effects near the bio-inspired treatments, termed “finlets” and “rails,” and relate those flow effects to the noise reduction. This was done using far-field microphones, a single hot-wire probe, and surface pressure fluctuation microphones. The far-field noise results showed that each treatment successfully reduced the noise, by up to 7 dB in some cases. The surface pressure measurements showed that the spanwise coherence was slightly reduced when the treatments were applied to the trailing edge. The velocity measurements clearly established the presence of a shear layer near the top of the treatments. As a whole, the dataset led to the shear-sheltering hypothesis: the bio-inspired treatments are effective based on reducing the spanwise pressure correlation and by sheltering the trailing edge from turbulent structures with the shear layer they create. / Master of Science / This thesis describes a project aimed at developing a technology inspired by the silent flight of owls, with the end goal of using this technology to reduce the noise generated by wind turbines. Specifically, the phenomenon known as "trailing edge noise" is the primary source of wind turbine noise, and is the noise source of interest here. It occurs when air turbulence (which can be thought of as unsteady air fluctuations) crashes into the rear (trailing) edge of wind turbine blades, scattering and producing noise. Typically, methods of reducing this noise source involve changing the shape of the trailing edge; this may not always be practical for existing wind turbines. Recently, inspired by the downy covering of owl feathers, researchers developed treatments that can be applied directly to the trailing edge, significantly reducing trailing edge noise. This bio-inspired concept was verified with numerous acoustic measurements. Based on those measurements, researchers hypothesized that the noise reduction was achieved by manipulating the incoming turbulence before it scattered off the trailing edge, rather than by changing the existing wind turbine blade, representing a new method of trailing edge noise control. However, as only acoustic measurements (not flow measurements) were reported, the changes in turbulence could not be examined. With the above motivation in mind, this thesis describes a comprehensive wind tunnel experiment to measure the changes in the aerodynamics and turbulence near the bio-inspired treatments, and relate those changes to the reduction in trailing edge noise. This was done using a hot-wire probe to measure the aerodynamics, as well as microphones to measure the radiated noise and surface pressure fluctuations. As a whole, the experimental results led to the shear-sheltering hypothesis: the bio-inspired treatments are effective based on the creation of a shear layer (a thin region between areas with different air speeds) which shelters the trailing edge from some turbulence, as well as by de-correlating surface pressure fluctuations along the trailing edge.