Littérature scientifique sur le sujet « Grazing Flow Rig »

This paper deals with a resonant type liner panel with a special surface structure. A typical resonant type liner panel generally consists of a perforated face plate, cells, and a back rigid plate. One of the technical challenges of the acoustic liners applied to the future ultra-high bypass ratio engines is to increase the sound absorption efficiency under grazing conditions because the nacelle, covering of the engine, tends to reduce its length and the lined area. It is known that the sound absorption of the conventional liners tends to deteriorate as grazing flow increases. The authors introduced a special thin acoustically transparent film over the face plate of the acoustic liner. The film, a fine perforated film (FPF), is expected to prevent the interaction of the grazing flow with the opening of the liner face plate. An experimental result with a flow duct rig in JAXA confirmed that the proposed combination of the acoustic liner and the FPF improved the absorption in acoustic energy under grazing conditions, compared with the sole acoustic liner and simple treatment of the FPF.

In this paper, a novel model is proposed for the numerical simulation of noise-attenuating perforated liners. Effusion cooling liners offer the potential of being able to attenuate combustion instabilities in gas turbine engines. However, the acoustic attenuation of a perforated liner is a combination of a number of interacting factors, resulting in the traditional approach of designing perforated combustor liners relying heavily on combustor rig tests. On the other hand, direct computation of thousands of small-scale holes is too expensive to be employed as an engineering design tool. In recognition of this, a novel physical velocity porous media (PVPM) model was recently proposed by the authors as a computationally less demanding approach to represent the acoustic attenuation of perforated liners. The model was previously validated for the normal incidence of a sound wave by comparison with experimental data from impedance tubes. In this paper, the model is further developed for configurations where the noise signal propagates in parallel with the perforated liners, both in the presence and absence of a mean flow. The model is significantly improved and successfully validated within coexisting grazing and bias flow scenarios, with reference to a series of well-recognized experimental data.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
It was studied growth dynamics of Andropogon lateralis, Aristida laevis, Axonopus affinis, Paspalum notatum and Paspalum plicatulum in a natural grassland of the Depressão Central of Rio Grande do Sul, subjected to rotational grazing with heifers, in the spring and summer of 2010/2011. The experimental design was completely randomized with 5 x 2 factorial arrangements, with two grazing intervals, defined by thermal sums of 375 and 750 degree-days, and five species of grasses. Leaf tissue flows, tiller dynamics and stability index of plant population of these grasses were assessed. The studied grasses had a higher leaf growth flow and balance between flows when subjected to grazing intervals of 750 degreedays and higher real efficiency of utilization, tiller appearance rate and site filling when subjected to grazing intervals of 375 degree-days. A. lateralis showed the highest flows of leaf growth, intake and senescence. A. laevis exhibited the lowest real efficiency of utilization. A. affinis and P. notatum had the highest rates of appearance of tillers. P. plicatulum was the only species with stability index of less than one in mean of the period and who had decline in their population of plants. Evaluation periods and/ or seasons influenced the number of senescent leaves per tiller, leaf intake flow, rates of appearance, survival and mortality of tillers and tiller population stability. Summer and, especially the month of December, represented a period of reproductive development of grasses. Intervals between grazing defined in order to respect the natural rhythms of growth of the species tend to optimize the productivity of natural grasslands.
Estudou-se a dinâmica de crescimento de Andropogon lateralis, Aristida laevis, Axonopus affinis, Paspalum notatum e Paspalum plicatulum em uma pastagem natural da Depressão Central do Rio Grande do Sul, submetida a pastoreio rotativo com novilhas, na primavera e verão de 2010/2011. O delineamento experimental foi inteiramente casualizado com arranjo fatorial 2 x 5, sendo dois intervalos entre pastoreios, definidos por somas térmicas de 375 e 750 graus-dia, e cinco espécies de gramíneas. Foram avaliados os fluxos de tecido foliar, a dinâmica de perfilhamento e índice de estabilidade da população de plantas dessas gramíneas. As gramíneas estudadas apresentaram maior fluxo de crescimento foliar e balanço entre os fluxos quando submetidas a intervalos de pastoreio de 750 graus-dia e maior eficiência real de utilização do pasto, taxa de aparecimento e ocupação de sítios quando submetidas a intervalos de pastoreio de 375 graus-dia. A. lateralis apresentou os maiores fluxos de crescimento, consumo e senescência foliar. A. laevis exibiu a menor eficiência real de utilização. A. affinis e P. notatum apresentam as maiores taxas de aparecimento de perfilhos. P. plicatulum foi a única espécie com índice de estabilidade inferior a um, na média do período experimenta e que apresentou declínio em sua população de plantas. Os períodos de avaliação e/ou a estações do ano influenciaram o número de folhas em senescência por perfilho, fluxo de consumo foliar, taxas de aparecimento, sobrevivência e mortalidade de perfilhos e índice de estabilidade da população de perfilhos. O verão e, principalmente o mês de dezembro, foi caracterizado como período de desenvolvimento reprodutivo das gramíneas. Intervalos entre pastoreios definidos de forma a respeitar os ritmos naturais de crescimento das espécies tendem a otimizar a produtividade das pastagens naturais.

This research activity addresses the development of a test bench for acoustic measurements in a thermally hostile environment, representative of the exhaust in an aircraft engine, for liner characterization purposes. In this regard, a brand-new rig at the Department of Industrial Engineering (DIEF) of the University of Florence (UniFI) was designed and is introduced in detail. The design of this experimental asset, which would be the combination of a high-speed wind tunnel and an acoustic test rig, has been carefully addressed. The rig is presented and its capability to support reliable acoustic tests is preliminarily verified. The impedance of a liner sample is assessed by means of a data reduction process, considering both a 2D and a 3D incoming acoustic field inside the test-section. Results are promising and show the capability of the developed rig to achieve reliable acoustic information in a defined frequency band.

The installation of acoustic liners can represent an effective strategy for noise reduction in aero engines. The performance of an acoustic liner as a sound absorbing device is expressed by the acoustic impedance which mainly depends on its geometry and on the peculiar flow conditions acting on its surface. Several parameters are involved in the estimation of the impedance through a non-trivial dependence; this is even more complex when such devices are installed within a high-speed and high-temperature flow field such the one characteristic of the core nozzle of an aeroengine. Within such a scenario the experimental investigation is fundamental in order to assess the liner performance accurately. The most reliable way to determine the acoustic impedance is through a grazing flow rig which allows to experimentally investigate the acoustic liner performances under different flow conditions by using a controlled acoustic signal as excitation. At the Department of Industrial Engineering in Florence, a novel grazing flow rig for testing liners at flow conditions representative of the Low-Pressure Turbine (LPT) exit, i.e. at high speed and high temperature, has been designed and realized. The present work describes the design of the test rig and the preliminary experimental outcomes achieved for assessing the rig performances. The design of all the main components has been addressed following a comprehensive approach in order to meet the acoustic and aerodynamic requirements and by considering the thermo-structural effects as well. The design process has been supported by the use of Finite Element Analyses, the main results and the design solutions achieved are shown. As the rig is supposed to work under high temperature conditions, the sensors to be used for collecting the acoustic data shall withstand the hot flow as well. Hence, an experimental analysis has been carried out in order to compare the main market solutions available. As a result, the most suitable measurement device was identified. Once the design and the construction of the rig was finished, a preliminary set of tests was performed to prove the eligibility of the rig for the designed purpose. The uniformity of the flow field into the test section has been verified through a preliminary measurement of the flow profile. From the acoustic point of view, the reflection coefficient was measured at the duct ends and the results demonstrated very low reflections proving the effectiveness of the designed anechoic terminations. In addition, a first test on an acoustic liner sample was performed, i.e. the transmission loss was measured at cold flow condition. The overall results demonstrate that the rig is properly designed for the desired purposes.