An alternative method for measuring velocities in open-channel flows: perfomance evaluation of a Pitot tube compared to an acoustic meter

Abstract

Hydrometric measurements undertaken in channels with high velocities are conditioned to the particularities of the flow, which is often characterized by instantaneous fluctuations and disturbances on the free surface. In such cases, the uncertainties associated with velocity fluctuations exceed the precision offered by the instruments that are employed in conventional techniques. A reasonable accuracy of the results is therefore sufficient to accomplish the objective of the measurements. The use of devices based on Pitot's principle in fast open-channel flows could be an effective alternative to conventional velocity meters. This study aimed to develop a Pitot tube in its simplest configuration and evaluate its performance in a laboratory channel at velocities ranging from 0.2 to 2.0 m/s. The uncertainties in the static and total head readings were propagated to the output velocities, showing that the device built has the potential for measurements over 1.2 m/s, but it is not recommended for low velocities (<0.6 m/s). The results were compared to those taken using an Acoustic Doppler Velocimeter (ADV). The instantaneous velocity readings indicated uncertainties of the same order of magnitude in both instruments. The differences between the mean velocities measured by the Pitot tube and the ADV were restricted to an agreement range of 15%, which is expected to be gradually reduced with further increase in flow velocity. The results showed the similar performances of both devices regarding the higher velocity estimates. Therefore, velocity meters should be developed to employ Pitot devices as an alternative method in high-velocity open-channel flows.Keywords: Hydrometry; Open channel flow; Differential head meters; Pitot tube. RESUMOAplicações de hidrometria em canais com altas velocidades estão sujeitas às condições do fluxo, que geralmente é caracterizado por flutuações instantâneas e perturbações na superfície livre. Nesses casos, as incertezas associadas às flutuações de velocidade superam a precisão oferecida pelos instrumentos empregados nas técnicas convencionais. Uma exatidão razoável dos resultados é, portanto, suficiente para atender ao objetivo das medições. O uso de dispositivos baseados no princípio de Pitot em escoamentos rápidos pode ser uma alternativa eficaz ao uso dos medidores de velocidade convencionais. Este estudo teve por objetivo confeccionar um tubo de Pitot em sua configuração mais simples e avaliar seu desempenho em um canal experimental para velocidades variando no intervalo de 0,2 a 2,0 m/s. As incertezas nas leituras de carga estática e de carga total foram propagadas para as velocidades estimadas, mostrando que o dispositivo construído tem potencial para medições acima de 1,2 m/s, mas não é recomendado para baixas velocidades (<0,6 m/s). Os resultados foram comparados com um Velocímetro Acústico de Efeito Doppler (ADV). As leituras de velocidade instantânea indicaram incertezas de mesma magnitude em ambos os instrumentos. As diferenças entre as velocidades médias estimadas co...

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