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This work presents the design and the calibration of a test rig specially developed to measure the in-plane forces transferred between the blade platforms through the under-platform damper and their relative displacement. This device is composed of two distinct parts each one representing a platform. One is static and accommodates the load cells which measure the forces in two perpendicular directions; the other produces the in-plane motion, actuated by two piezoelectric stacks. The device reproduces any in-plane relative displacement between two adjacent platforms and measures both the relative motion between platforms and the forces they reciprocally transmit. The damper, placed between the two platform simulators, is loaded by thin wires pulled by dead weights, a way to apply the equivalent of the centrifugal force. The mechanical features of the rig are described and discussed with their influence on the measurements. An example application is given. Tests aim at assessing the role of “outer” measured parameters (such as frequency and amplitude of platform-to-platform relative displacement, damper external load (simulating the in-service centrifugal load), damper geometry) on the shape and area of the hysteresis cycle and therefore the damper real and imaginary stiffness components. It is found that equal values for the supposedly governing “outer” parameters may lead to a multiplicity of markedly different hysteresis cycles. The same happens if platform-to-platform force is considered rather than displacement. It is shown how the system evolves through the many possible equilibrium conditions. It is also shown how the forces between damper and underplatforms are calculated. It is suggested that the measurement of platform-to-platform hysteresis cycles is an effective way to synthetically approach the problem of elastic coupling and energy dissipation between adjacent blades, while detailed knowledge of forces exchanged between the underplatform and damper contact surfaces will be a valuable tool toward the better knowledge of damper micromechanics, perhaps opening a better way to finding damper geometries capable of reducing the scatter of hysteresis cycle shape and area. Two dampers are investigated, at this stage, in order to assess the dependence of the above said behavior on the damper geometry. Results show that dampers exhibit multiple behaviors under the same input conditions. They may be alarming because they show that the damper-platforms system always converges to the solution with the lowest hysteresis area, a fact which deserves of course deeper investigations.

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RESUMO: Visando à obtenção de informações técnicas que auxiliem aprimorar a eficiência hidráulica de filtros de areia nos processos de filtragem e retrolavagem, buscou-se avaliar o potencial de utilização de uma bancada didática de túnel de vento como método de visualização e caracterização das linhas de fluxo geradas pela interação da geometria de um modelo de dreno (crepina) com diferentes velocidades de escoamento. Os ensaios foram realizados, instalando-se um modelo comercial de dreno, do tipo cilíndrico, em um módulo experimental construído para ser acoplado a um túnel de vento vertical. Simularam-se as três vazões de escoamento de ar fornecidas pelo módulo, obtendo-se registros fotográficos com uma câmara de alta velocidade, cujas imagens foram tratadas para destacar a disposição das linhas de fluxo e os caminhos preferenciais de escoamento para os sentidos de escoamento de filtragem e retrolavagem. Correlacionou-se, por similitude, o intervalo de operação do túnel de vento com valores de vazão praticados em processos de filtração com água, em filtros de areia comerciais, com 40; 60 e 100 cm de diâmetro. Os resultados obtidos validaram a metodologia proposta, permitindo analisar o efeito da geometria da crepina nas linhas de fluxo experimentais, tanto no modo de filtragem quanto no de retrolavagem. PALAVRAS-CHAVE:irrigação localizada, filtração, retrolavagem. APPLICATION OF A VERTICAL WIND TUNNEL TO EVALUATE FLOW LINES GENERATED BY SAND FILTERS UNDERDRAINSABSTRACT: Looking for technical information to improve the hydraulic efficiency in sand filters during the filtering and backwash processes, this project had the objective to evaluate a teaching wind tunnel as a reliable method to visualize and characterize flow lines distribution generated by the interaction of a specific model of underdrain with different flow velocities. The experiment was performed by installing a commercial underdrain model, cylinder type, in an experimental section specially constructed to be coupled to a vertical wind tunnel. Three flow rate values were simulated in the tunnel, obtaining high-speed photographs. The obtained images were treated to highlight the flow lines distribution and preferred flow pathways for the filtration and backwash processes. The wind tunnel operating range was correlated by similarity with values of water flow rate in filtration processes carried out in commercial sand filters with diameters of 40, 60 and 100 cm. The obtained results validated the proposed methodology, allowing the visualization and analysis of the underdrain geometry effects on the generated flow lines, for both filtration and backwash processes.KEYWORDS: localized irrigation, filtration, backwash. INTRODUÇÃOTendo em vista que a irrigação no Brasil utiliza, na maioria das vezes, águas superficiais provenientes de rios, lagos e açudes, que não se encontram em condições adequadas devido à presença de contaminações químicas, físicas ou biológicas, a caracterização da qualidade da água deve ser abordada como prioridade na definição dos par...

This work presents the development of a test rig capable of measuring the forces transferred between the blade platforms through the under-platform damper. This test rig is composed of two distinct parts each one representing a platform. The static part contains the load cells, which measure the forces in two perpendicular directions; the moving part controlled using two piezoelectric actuators reproduces any in-plane relative displacement between two adjacent platforms. In this scheme, the damper is placed between these two platforms and loaded by dead weights that reproduce the effects of centrifugal force. The hysteresis cycle, of the damper system, is obtained using the measured forces and the imposed displacement. In addition, two laser beams can be used to measure the damper displacement and its tilt angle, which allows validating dynamic models of the damper. Moreover, the test rig is designed to allow heating the specimens up to temperatures which are normally found in real operation. Finally, the test rig provides necessary variables to study the damper performance and to evaluate some contact models used to simulate under-platform dampers.