This study presents a climatology of convectively-generated strong wind gusts (SWG) occurred in Southern Brazil in the 2005-2015 period. The selection of these events was based on surface hourly data from the operational network of automated weather stations (AWSs) operated by Brazil’s National Meteorological Institute (INMET, in portuguese) and geostacionary meteorological satellite imagery. The time series of the atmospheric variables recorded by the AWSs during the SWGs events were evaluated aiming at detecting cold pools and mesohighs. Proximity soundings and Climate Forecast System Reanalysis (CFSR) and Climate Forecast System Version 2 (CFSv2) reanalysis data were used to assess the atmospheric environment during the occurence of the wind gust events. It was evalueted whether the atmospheric conditions highlight environments that discriminates the occurence of the SWG of those less intense. Moreover, a comparision was conducted among atmospheric profiles generated from CFSR/CFSv2 reanalysis and profiles obtained from operational soundings taken in Southern Brazil in the 1996-2015 period. The results showed that INMET’s AWS were able to sample convectively-driven cold pools and mesohighs following the wind gusts. The highest frequency of SWGs was in the spring and summer months. Most SWGs were detected from mid-afternoon to overnight hours. The western portion of Southern Brazil displayed the largest frequency of SWGs. The median value of pressure variations following the SWG +4,6 hPa, with extreme values (95% percentile) reaching +8,2 hPa. The median value for temperature variations was -6,5 C, with extremes values below -13,0 C. Overall, the results showed that the atmospheric parameters demonstrate some discrimination between SWGs and weaker wind gusts. The Downdraft Convective Available Potential Energy (DCAPE) and the Derecho Composite Parameter (DCP) were the atmospheric parameters that better discriminate the atmospheric environment favorable to the occurence of SWGs. The comparision between atmospheric profiles generated from CFSR/CFSv2 data and observed soundings showed that the reanalysis reproduce well the thermodynamic parameters, but significantly underestimates the kinematic parameters. / Neste trabalho é apresentada uma climatologia de rajadas (RAJ) de vento convectivas intensas ocorridas na região sul do Brasil entre 2005 e 2015. A seleção destes eventos foi feita com base nos dados horários da rede operacional de estações meteorológicas automáticas (EMAs) de superfície do Instituto Nacional de Meteorologia (INMET) e imagens de satélites meteorológicos geoestacionários. As séries temporais das variáveis atmosféricas registradas pelas EMAs durante os eventos de RAJ foram avaliadas para detectar piscinas de ar frio e mesoaltas. O ambiente atmosférico durante a ocorrência das RAJ foi analisando utilizando-se perfis atmosféricos extraídos de sondagens de proximidade e de dados de reanálise do Climate Forecast System Reanalysis (CFSR) e Climate Forecast System Version 2 (CFSv2). Avaliou-se se as condições atmosféricas ressaltam ambientes que discriminem a ocorrência das rajadas convectivas mais intensas daquelas menos intensas. Foi conduzida também uma comparação entre os perfis atmosféricos extraídos da reanálise CFSR/CFSv2 e os perfis obtidos de sondagens operacionais realizadas no sul do Brasil para o período entre 1996 e 2015. Os resultados mostraram que as EMAs-INMET conseguiram amostrar as piscinas de ar frio e mesoaltas que acompanham as rajadas convectivas. As rajadas intensas ocorreram com mais frequência na primavera e verão, e predominantemente entre o final da tarde e a madrugada. Em geral, houve uma ligeira tendência para uma maior ocorrência de RAJ nas EMAs-INMET do setor oeste da Região Sul. A mediana das variações de pressão acompanhando a RAJ foi de +4,6 hPa, com valores mais extremos (percentil 95%) atingindo +8,2 hPa. A mediana das variações de temperatura foi -6,5 C, com valores extremos abaixo de -13,0 C. De maniera geral, os parâmetros atmosféricos conseguiram demonstrar alguma discriminação entre a classe de RAJ intensas e as rajadas mais fracas. O Downdraft Convective Available Potential Energy (DCAPE) e o Derecho Composite Parameter (DCP) foram os parâmtros que melhor discriminaram ambientes atmosféricos favoráveis à ocorrência de rajadas de vento intensas. A comparação entre reanálise CFSR/CFSv2 e sondagens observadas mostrou que a reanálise reproduziu bem os parâmetros termodinâmicos, mas subestimou significativamente os parâmtros cinemáticos.

The Ph.D. thesis deals with extreme wind gust analysis over the area of the Czech Republic. The first part of the thesis deals with processing of wind measurements, in particular maximum wind gusts measurements. Analysis of high-frequency wind measurement using 3-D sonic anemometer on the Kopisty station is included. Homogenization of the highest daily wind gusts was performed. Descriptive statistical analysis of measured wind gust values was performed. The following part of the thesis describes statistical theory of extreme values and discusses its applicability to wind gust data. Some theoretical findings were obtained. Numerous numerical experiments were performed focused on evaluation of proposed method. In the last part of the thesis station measurements were processed using the proposed methods and a model of dependence between extreme and mean wind climate was derived. The model was applied to the map of mean wind climate calculated earlier on the Institute of Atmospheric Physics and thus a map of extreme wind climate was obtained. The accuracy of this map was estimated. The map was compared with other maps of extreme wind calculated by other authors earlier.

This dissertation broadly seeks to understand the effect different kinematic parameters, external forces, and dynamic wing conformation have on the fluid dynamics of flapping flight. The primary motivation is to better grasp the fundamental fluid phenomena driving efficient flapping flight in the Reynolds number regime of birds, bats, and man made fliers of similar scale. The CFD solver (GenIDLEST) used is a Navier-Stokes solver in a finite volume formulation on non-staggered structured multiblock meshes. It has the capability for both body-fitted moving grid simulations and Immersed Boundary Method (IBM) for simulating complex bodies moving within a fluid. To that purpose we investigate the response of a rigid flapping thin surface planar wing in forward flight, at Re=10,000, subjected to frontal gusts. Gusts are a common ecological hazard for flapping fliers, especially in crowded environments. Among the various temporal and spatial scales of gust possible, we look at the phasing and duration of very large spatial scale gusts and their impact on the unsteady fluid dynamics of flapping within a single flapping cycle. The gust is characterized by a step function with time scale much smaller than the flapping time period. Having the advantage of prescribing the motion, as well as the timing and duration of the gust, this allowed the observation of the effect of angle of attack (AOA) and wing rotation on the evolution of the Leading Edge Vortex (LEV) and, hence the instantaneous lift and thrust profiles, by varying the parameters. During the downstroke, frontal gusts accelerated the flow development resulting in early separation of existing LEVs and formation of new ones on the wing surface which influenced the force generation by increasing the lift and thrust. These phenomena underscored the importance of the unsteady vortex structures as the primary force generators in flapping flight.The effect of the gust is observed to be diminished when it occurs during rapid supination of the wing. Unlike the influence of the vortices during the downstroke, the upstroke primarily reacted to effective AOA changes. A key characteristic of the kinematics of fliers in nature is stroke deviation. We investigate this phenomenon using a similar framework as above on a rigid thin surface flat-plate flapping wing in forward flight. Stroke deviation happens due to a variety of factors including wing flexion, wing lateral translation, and wing area change and here we investigate the different stroke deviation trajectories. Various trajectories were analyzed to assess the different capabilities that such kinematics might offer. The instantaneous lift and thrust profiles were observed to be influenced by a combination of the Leading Edge Vortex (LEV) and the Trailing Edge Vortex (TEV) structures existing in the flow at any given time. As an index of the cost of performance across all cases, the power requirements for the different cases, based on the fluid torques, are analyzed. Anti-clockwise figure-of-eight-cycle deviation is shown to be very complex with high power costs while having better performance. The clockwise elliptic-cycle held promise in being utilized as a viable stroke deviation trajectory for forward flight over the base non stroke deviation case. Armed with insight gained from these simple flapping structures, we are able to conduct the analysis of the flapping flight data obtained on a fruit bat. Understanding the full complexity of bat flight and the ways in which bat flight differs from that of other vertebrate flight requires attention to the intricate functional mechanics and architecture of the wings and the resulting unsteady transient mechanisms of the flow around the wings. We extract the detailed kinematic motion of the bat wing from the recorded data and then simulate the bat wing motion in the CFD framework for a range of Reynolds numbers. The Strouhal number calculated from the data is high indicating that the flow physics is dominated by the oscillatory motion. From the data the bat exhibits fine control of its mechanics by actively varying wing camber, wing area, torsional rotation of the wing, forward and backward translational sweep of the wing, and wing conformation to dictate the fluid dynamics. As is common in flapping flight, the primary force generation is through the attached unsteady vortices on the wing surface. This force output is modulated by the bat through varying wing camber and the wing area. Proper orthogonal decomposition of the wing kinematics is undertaken to compile a simpler set of kinematic modes that can approximate the original motion used by the fruit bat. These modes are then analyzed based on aerodynamic performance and power cost for more efficient flight. Understanding the physics of these modes will help us use them as prescribed kinematics for mechanical flappers as well as improve upon them from nature. / Ph. D.