Modelação da nuvem de retardante químico: optimização no combate aos fogos florestais

Doutoramento em Ciências Aplicadas ao AmbienteO número de fogos e a área ardida em Portugal têm aumentado nos últimos anos. As causas para este facto são diversas. Importa, pois, conhecê-las para as poder eliminar. Esta filosofia preventiva encontra-se hierarquicamente acima de uma atitude remediativa como o combate aos incêndios florestais. Contudo, não existindo uma eficiência plena na prevenção, deve o Estado munir-se das ferramentas necessárias para um combate eficaz deste problema que surge todos os verões. Os meios aéreos são um exemplo por excelência de eficácia, uma vez que possibilitam a descarga de milhares de litros de água ou retardantes químicos sobre, ou a jusante, da frente de fogo. A sua velocidade, aliada à possibilidade em aceder mesmo a sítios remotos para meios terrestres, torna-os ainda mais valiosos nos meses quentes. Porém, a sua eficácia encontra-se associada à construção de uma linha contínua de retardante, com a largura e concentração adequada. Caso tal não se verifique, estar-se-á a desperdiçar em parte, esta ferramenta. Ainda hoje não se conhece toda a física da nuvem, especialmente a atomização/deformação e dispersão da mesma na atmosfera, pelo que a forma de aplicar o retardante não está optimizada. Exceptuando o sistema MAFFS, a aplicação de retardantes químicos ou água faz-se pela abertura do aerotanque, ficando o líquido sujeito à acção da gravidade. Ao sair do tanque, as forças aerodinâmicas atomizam o fluido, formando gotas milimétricas ou menores. Estas gotas, por sua vez, vão cair até atingirem o solo ou a vegetação. Durante este percurso, a nuvem de gotas é dispersada, sofre ainda evaporação e, em menor extensão, troca de calor com o ar atmosférico. Importa referir que são diversas as variáveis operacionais que condicionam a mancha de retardante no solo. Dentro destas, as mais influentes são (1) a velocidade de voo (2) altura de voo, (3) caudal de saída de retardante, (4) direcção de voo e (5) propriedades físicas do retardante, nomeadamente viscosidade e elasticidade. Estas variáveis devem ser ajustadas de forma a optimizar a descarga de retardante numa situação concreta, interessando analisar a mancha necessária para parar ou diminuir a frente de fogo e variáveis ambientais como o vento, temperatura e humidade atmosférica. De forma a integrar o efeito de todas as variáveis, foram realizadas simulações que utilizam estes parâmetros operacionais como inputs. Foram usados dois modelos distintos - RAM e FLUENT - com propósitos e alcances diferentes. O trabalho desenvolvido permitiu concluir acerca da aplicabilidade de códigos CFDs para modelar a nuvem de retardante. Estas simulações descrevem várias propriedades associadas à nuvem, nomeadamente (i) campo de velocidades, (ii) turbulência, (iii) humidade, (iv) temperatura, (v) densidade, (vi) trajectória das gotas de retardante e (vii) pressão. As simulações comprovam que os aerotanques mais modernos, que controlam e debitam um caudal constante, são mais eficientes, por produzirem uma mancha igualmente mais uniforme segundo a sua longitudinal. O MAFFS é um sistema especial que possui esta mesma vantagem, existindo ainda outros exemplos comerciais com libertação gravítica do fluido. Os sistemas mais recentes são, contudo, mais caros e de utilização limitada na Europa. As simulações efectuadas permitem inferir acerca do efeito das principais variáveis operacionais: • No que se refere à altura de voo, verifica-se uma correlação directa com o aumento da dispersão transversal e uma consequente diminuição das concentrações de retardante. Para largadas a alturas inferiores a 30 metros, existem riscos acrescidos de o fluído atingir o solo antes de ser completamente atomizado, pondo em perigo vidas e bens em terra. Este valor mínimo de segurança deve ser ajustado tendo em conta o caudal, a viscosidade e a elasticidade do retardante, bem como o aerotanque em causa; • A trajectória de voo é definida pela posição e orientação da mancha de retardante necessárias a jusante da frente de fogo. Contudo, o ângulo entre a trajectória do avião e a direcção do vento atmosférico condiciona a dispersão da nuvem, especialmente das gotas mais finas; • A variável mais influente na mancha de retardante é dada pelo cociente entre o caudal de retardante à saída do aerotanque e a velocidade do avião, ou seja, a quantidade de líquido debitado por unidade de tempo e de espaço linear percorrido pelo avião; • As propriedades físicas do fluido influenciam directamente o comportamento da nuvem. A atomização do líquido é inversamente proporcional à visco-elasticidade do retardante, o que afecta a distribuição granulométrica das gotas. A fase final do revestimento do combustível é dependente da visco-elasticidade e também da tensão superficial do retardante; • Diversos outros parâmetros operacionais influenciam o comportamento da nuvem e, portanto, a mancha de retardante, como as condições atmosféricas (vento, humidade e temperatura) e a proximidade à frente de fogo; A título conclusivo, sugerem-se aerotanques que debitem um caudal constante, apontam-se os cuidados a ter com a altura da largada e alerta-se para a necessidade de ajustar as propriedades físicas do fluido às circunstâncias do incêndio em questão. Não se tendo esgotado o presente estudo, recomenda-se a sua continuação ao nível da simulação e da aferição dos modelos através de ensaios à escala real.Both area and number of forest fires have grown during the last years. There are several causes that explain this fact. It is important to know these causes in order to avoid them. Prevention is more desirable than the remediation of fire fighting. However, the efficiency of the prevention is below 100 % and, thus, there is the need of wildfire fighting tools. Aerial means are an excellent example of efficiency as they can drop thousands litres of water or chemical retardants above or at the front of the fire line. Aircraft velocities and its possibility to access remote places for terrestrial means, make them even more important in an integrated system. However, their efficiency is dependent on the continuity of the crop pattern with both cross range and concentrations needed. If these levels of retardant are not achieved, its efficiency can be low. The physics of the retardant cloud is not yet known in detail, especially the atomization/deformation of the bulk liquid, and thus the application of the retardant is not optimized. Except in the MAFFS, the aerial application of retardants begins by the liquid exit promoted by gravity. As the liquid leaves the air tanker, this fluid is subject to aerodynamic forces that atomize the retardant into droplets. These particles fall in the atmosphere till they reach the ground or the vegetation. During the cloud trajectory, droplets are dispersed, evaporated and exchange heat with the surrounding air. There are several operational variables that influence the retardant pattern at the ground. The most important are: (1) airplane velocity, (2) airplane height, (3) retardant flow-rate, (4) direction between airplane and wind and (5) physical properties of the retardant fluid, mainly the viscosity and elasticity. These parameters can and should be adjusted in order to optimize the aerial application of retardants in function of the retardant pattern needed to slow down the fire and the environmental conditions like wind, humidity and air temperature. In order to integrate all of the influencing variables, simulations were made, by using these variables as model inputs. Two models were used – RAM and FLUENT with different purposes. The developed work allowed verifying that CFDs are applicable in order to describe the cloud behaviour. Performed simulations compute several cloud properties such as: (1) wind field, (2) turbulence, (3) humidity, (4) temperature, (5) density, (6) droplets trajectory and (7) pressure field. The simulations preclude that the most modern air tankers that control the flow-rate are more efficient because they produce a more constant retardant pattern along its longitudinal. MAFFS is an example of such technology, coexisting with others where the retardant is released by the gravity force. These new systems are, however, more expensive and rarely used in Europe.The developed simulations allow the following conclusions: • In what concerns the flight height, there is an increase of the lateral dispersion of retardant and a decrease in the retardant pattern maximum concentrations. For drops below 30 meters, there is a considerable risk of injury for people on the ground because the fluid can attain the soil before a complete atomization. This minimum value must be adjusted considering the fluid properties and the retardant flow rate exiting the air tanker and its geometry; • Although the airplane trajectory is defined by the desired fire wall, the angle between the airplane and the wind influences the dispersion of the cloud, especially of the smaller droplets; • The most important parameter affecting the retardant pattern is the ratio between the flow-rate and the airplane velocity, which is the amount of retardant released by unit of time and space; • The retardant properties do affect the cloud behaviour and thus the efficiency. The atomization is inversely proportional to the viscoelasticity. The third phase of the retardant coating is dependent of the viscosity and also on the superficial tension of the chemical retardant. • There are other parameters that influence the retardant cloud at a less extent, like the atmospheric conditions (temperature, humidity and stability) and the proximity of the fire front. The main conclusions that arise from this work are (1) the recommendation of the use of the most modern air tankers with constant flow-rate, (2) care with low height drops, (3) and the adjustment of the retardant physical properties with the real characteristics of the fire being analyzed. The study presented is not terminated and, thus, it is recommended its continuation at the level of the models validation extent based on more experimental work

Studies of the mass composition of cosmic rays and proton-proton interaction cross-sections at ultra-high energies with the Pierre Auger Observatory

In this work, we present an estimate of the cosmic-ray mass composition from the distributions of the depth of the shower maximum (Xmax) measured by the fluorescence detector of the Pierre Auger Observatory. We discuss the sensitivity of the mass composition measurements to the uncertainties in the properties of the hadronic interactions, particularly in the predictions of the particle interaction cross-sections. For this purpose, we adjust the fractions of cosmic-ray mass groups to fit the data with Xmax distributions from air shower simulations. We modify the proton-proton cross-sections at ultra-high energies, and the corresponding air shower simulations with rescaled nucleus-air cross-sections are obtained via Glauber theory. We compare the energy-dependent composition of ultra-high-energy cosmic rays obtained for the different extrapolations of the proton-proton cross-sections from low-energy accelerator data

Study of downward Terrestrial Gamma-ray Flashes with the surface detector of the Pierre Auger Observatory

The surface detector (SD) of the Pierre Auger Observatory, consisting of 1660 water-Cherenkov detectors (WCDs), covers 3000 km2 in the Argentinian pampa. Thanks to the high efficiency of WCDs in detecting gamma rays, it represents a unique instrument for studying downward Terrestrial Gamma-ray Flashes (TGFs) over a large area. Peculiar events, likely related to downward TGFs, were detected at the Auger Observatory. Their experimental signature and time evolution are very different from those of a shower produced by an ultrahigh-energy cosmic ray. They happen in coincidence with low thunderclouds and lightning, and their large deposited energy at the ground is compatible with that of a standard downward TGF with the source a few kilometers above the ground. A new trigger algorithm to increase the TGF-like event statistics was installed in the whole array. The study of the performance of the new trigger system during the lightning season is ongoing and will provide a handle to develop improved algorithms to implement in the Auger upgraded electronic boards. The available data sample, even if small, can give important clues about the TGF production models, in particular, the shape of WCD signals. Moreover, the SD allows us to observe more than one point in the TGF beam, providing information on the emission angle

Combined fit to the spectrum and composition data measured by the Pierre Auger Observatory including magnetic horizon effects

The measurements by the Pierre Auger Observatory of the energy spectrum and mass composition of cosmic rays can be interpreted assuming the presence of two extragalactic source populations, one dominating the flux at energies above a few EeV and the other below. To fit the data ignoring magnetic field effects, the high-energy population needs to accelerate a mixture of nuclei with very hard spectra, at odds with the approximate E$^{-2}$ shape expected from diffusive shock acceleration. The presence of turbulent extragalactic magnetic fields in the region between the closest sources and the Earth can significantly modify the observed CR spectrum with respect to that emitted by the sources, reducing the flux of low-rigidity particles that reach the Earth. We here take into account this magnetic horizon effect in the combined fit of the spectrum and shower depth distributions, exploring the possibility that a spectrum for the high-energy population sources with a shape closer to E$^{-2}$ be able to explain the observations

The dynamic range of the upgraded surface-detector stations of AugerPrime

The detection of ultra-high-energy cosmic rays by means of giant detector arrays is often limited by the saturation of the recorded signals near the impact point of the shower core at the ground, where the particle density dramatically increases. The saturation affects in particular the highest energy events, worsening the systematic uncertainties in the reconstruction of the shower characteristics. The upgrade of the Pierre Auger Observatory, called AugerPrime, includes the installation of an 1-inch Small PhotoMultiplier Tube (SPMT) inside each water-Cherenkov station (WCD) of the surface detector array. The SPMT allows an unambiguous measurement of signals down to about 250m from the shower core, thus reducing the number of events featuring a saturated station to a negligible level. In addition, a 3.8m2 plastic scintillator (Scintillator Surface Detector, SSD) is installed on top of each WCD. The SSD is designed to match the WCD (with SPMT) dynamic range, providing a complementary measurement of the shower components up to the highest energies. In this work, the design and performances of the upgraded AugerPrime surface-detector stations in the extended dynamic range are described, highlighting the accuracy of the measurements. A first analysis employing the unsaturated signals in the event reconstruction is also presented

Investigating multiple elves and halos above strong lightning with the fluorescence detectors of the Pierre Auger Observatory

ELVES are being studied since 2013 with the twenty-four FD Telescopes of the Pierre Auger Observatory, in the province of Mendoza (Argentina), the world’s largest facility for the study of ultra-high energy cosmic rays. This study exploits a dedicated trigger and extended readout. Since December 2020, this trigger has been extended to the three High levation Auger Telescopes (HEAT), which observe the night sky at elevation angles between 30 and 60 degrees, allowing a study of ELVES from closer lightning. The high time resolution of the Auger telescopes allows us to upgrade reconstruction algorithms and to do detailed studies on multiple ELVES. The origin of multiple elves can be studied by analyzing the time difference and the amplitude ratio between flashes and comparing them with the properties of radio signals detected by the ENTLN lightning network since 2018. A fraction of multi-ELVES can also be interpreted as halos following ELVES. Halos are disc-shaped light transients emitted at 70-80 km altitudes, appearing at the center of the ELVES rings, due to the rearrangement of electric charges at the base of the ionosphere after a strong lightning event