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Advisor(s)
Abstract(s)
Uma aeronave V/STOL quando paira junto do solo ou aterra cria um escoamento tridimensional
extremamente complexo entre os jatos, a superfície da própria aeronave e o solo. Devido à
forte interação dos jatos de descolagem com o plano do solo, há a formação de um jato de
parede que flui radialmente desde o ponto de impacto ao longo da superfície do solo e que
interage com escoamentos cruzados causados por vento lateral ou pelo próprio movimento da
aeronave. São várias as consequências que daí podem resultar, tais como: perdas do poder
de descolagem, de sustentação e de potência dos motores devido à re-ingestão dos gases de
escape e diminuição da pressão nas superfícies inferiores das asas e da fuselagem, provocando
uma força de sucção do aparelho em direção ao solo.
O escoamento de jatos incidentes sobre superfícies através de um escoamento cruzado está especialmente
associado a este tipo de aeronaves e tem sido amplamente estudado com diversas
motivações e aplicações práticas. Contudo, a maioria dos estudos realizados consideram altas
alturas de impacto e baixas razões de velocidade entre o jato e o escoamento cruzado. Assim,
no que diz respeito ao problema de ground effect da aeronave V/STOL, os trabalhos já efetuados
não apresentam grande relevância. Devido também à complexidade das novas configurações de
aeronaves V/STOL, como é o caso do F-35B, é importante continuar os estudos já realizados,
nomeadamente a distribuição de pressão que permitirá compreender melhor o fenómeno de
ground effect referido.
Atendendo a que ainda não existem resultados publicados de medições de pressões para o caso
de dois jatos incidentes alinhados com o escoamento cruzado, cuja relevância é precisamente o
F-35B, esta dissertação é dedicada ao estudo experimental da distribuição de pressão de jatos
de impacto através de escoamento cruzado nas partes inferiores da fuselagem e da asa do F-35B
para Vj=U0 iguais a 15, 22:5 e 33:7 e para H=Dm igual a 3. Com o objetivo de complementar
e comparar este estudo, é apresentada também uma simulação numérica deste escoamento.
Através desta são obtidas as distribuições de pressão para as mesmas condições da experiência
realizada, mas também junto ao solo. Para além disto, é mostrado o escoamento médio no
plano vertical de simetria.
Verifica-se maioritariamente a existência de regiões com pressões negativas e nulas. Nas regiões
perto dos jatos estão presentes elevados coeficientes de pressão negativos que dão origem a
uma região de sucção à volta dos jatos de impacto. Através da utilização do esquema numérico
QUICK, apresentamos soluções com um grande nível de precisão numérica, já que este possibilita
a obtenção de soluções independentes da dimensão da malha com um número menor de pontos
do que o exigido por outros esquemas. Nas localizações dos jatos verifica-se a existência das
pressões mais elevadas e as regiões de pressões negativas junto aos jatos de impacto estão
associadas com o núcleo dos ground vortexes. Observa-se a formação de um ground vortex
na região entre os jatos de impacto, fenómeno este que ainda não foi descrito na literatura,
já que era esperado encontrar um escoamento em repuxo nesta zona para baixas alturas de
impacto. Ao compararmos a forma da região de pressões negativas com a forma do ground
vortex verificamos que estas são bastante semelhantes, observando-se a envolvência do jato
incidente como um “cachecol”.
At their hovering phase of flight, V/STOL aircrafts create a three dimensional flowfield between jets, airframe surface and ground. The impingement of each downward-directed jet on the ground results in the formation of a wall jet which flows radially from the impinging point along the ground surface. Ground effect may occur and change the lift forces on the aircraft, cause reingestion of exhaust gases into the engine intake and raise fuselage skin temperatures. A V/STOL aircraft operating in ground vicinity with front wind or small forward movement may result in enhanced under pressures in the aft part of the aircraft causing a suction down force and a change of the pitching movement towards the ground. Experiments on the aerodynamics of jets through a crossflow have mostly been reported for large impingement heights, for low velocity ratios between the jet and the crossflow and the focus of most studies are the velocity distribution on the flow field. These works have only peripheral relevance to the VSTOL F-35B ground effect problem, being the pressure distribution an important parameter to analyse this phenomenon. For the new generation of V/STOL aircrafts, such as F-35B, no relevant studies can be found, because the impinging jets are aligned with the crossflow and this geometry has not yet been considered. An experimental study was done to provide information to the pressure measurement along the bottom of the fuselage and the wing of the vectored-thrust F-35B fighter configuration in the transition-speed range. The experiments were carried out for Vj=U0 = 15, 22:5 and 33:7, and for H=Dm = 3. To complete this investigation and compare with the experimental results, a numerical simulation for the same impingement height and velocity ratios was also performed. Furthermore, it is presented the pressure measurement next to the ground and the predicted mean vertical velocity component distribution along the vertical plane of simmetry. The existence of regions with negative and null pressures are predominants. Near the impingement jets there are large negative pressure coefficients, that are responsible for the suckdown phenomenon. High level of numerical accuracy was obtained by grid refinement and the use of the high order method QUICK. On the impingement jets locations we can see the regions with high static pressure and the low pressures which are associated with the core of the ground vortexes. We can observe the formation of a ground vortex in the region between the impingement jets, a fact that has not been reported in the literature and which is surprising at the first sight, because the expected was to find an upwash fountain flow for lower impingement heights. Comparing the shapes of the negative pressure regions with the ground vortex we can verify that they are very similar, with the crossflow wrapping around the impinging point like a scarf.
At their hovering phase of flight, V/STOL aircrafts create a three dimensional flowfield between jets, airframe surface and ground. The impingement of each downward-directed jet on the ground results in the formation of a wall jet which flows radially from the impinging point along the ground surface. Ground effect may occur and change the lift forces on the aircraft, cause reingestion of exhaust gases into the engine intake and raise fuselage skin temperatures. A V/STOL aircraft operating in ground vicinity with front wind or small forward movement may result in enhanced under pressures in the aft part of the aircraft causing a suction down force and a change of the pitching movement towards the ground. Experiments on the aerodynamics of jets through a crossflow have mostly been reported for large impingement heights, for low velocity ratios between the jet and the crossflow and the focus of most studies are the velocity distribution on the flow field. These works have only peripheral relevance to the VSTOL F-35B ground effect problem, being the pressure distribution an important parameter to analyse this phenomenon. For the new generation of V/STOL aircrafts, such as F-35B, no relevant studies can be found, because the impinging jets are aligned with the crossflow and this geometry has not yet been considered. An experimental study was done to provide information to the pressure measurement along the bottom of the fuselage and the wing of the vectored-thrust F-35B fighter configuration in the transition-speed range. The experiments were carried out for Vj=U0 = 15, 22:5 and 33:7, and for H=Dm = 3. To complete this investigation and compare with the experimental results, a numerical simulation for the same impingement height and velocity ratios was also performed. Furthermore, it is presented the pressure measurement next to the ground and the predicted mean vertical velocity component distribution along the vertical plane of simmetry. The existence of regions with negative and null pressures are predominants. Near the impingement jets there are large negative pressure coefficients, that are responsible for the suckdown phenomenon. High level of numerical accuracy was obtained by grid refinement and the use of the high order method QUICK. On the impingement jets locations we can see the regions with high static pressure and the low pressures which are associated with the core of the ground vortexes. We can observe the formation of a ground vortex in the region between the impingement jets, a fact that has not been reported in the literature and which is surprising at the first sight, because the expected was to find an upwash fountain flow for lower impingement heights. Comparing the shapes of the negative pressure regions with the ground vortex we can verify that they are very similar, with the crossflow wrapping around the impinging point like a scarf.
Description
Keywords
Distribuição de Pressão Escoamento Cruzado F-35b Jatos de Impacto Vórtice de Solo e V/Stol