Inflatable weir hydraulics

Tagwi, Dayton (2015-03)

Thesis (MEng)--Stellenbosch University, 2015.


ENGLISH ABSTRACT: General objective of the study This thesis aims to evaluate the hydraulics of an inflatable weir in its fully inflated position to the almost fully deflated position using different diameter circular weirs with varying discharges, by considering the change in the weir radius and the dynamic pressures on the weir. In the evaluation, three cylindrical weirs were installed in a 2m wide flume and tested over various discharges. Methodology The three weirs, one with a 300mm diameter, another with a 250mm diameter, and the last one with a 100mm diameter, were used to determine the effects of over flow water on the weir as seen in the different stages of the normal operation of an inflatable weir. Simulation involved measurement of the upstream and downstream water levels with the weir height involved at stable over flow conditions. Measurement of pressure variations was done on the weir faces with different water inflow rates to the test flume with three pressure sensors installed on each weir at 0°, 11.25° and 22.5° from the crest to the downstream. Additionally a single 0.15m radius weir was tested for pressures 67.5°, 78.75° and 90° from weir crest. Water level variation on the downstream of the weir was created by means of a variable tail gate to observe its effects. Results of the investigation The effects of upstream arches, stage, radius of curvature, discharge, pressure, energy losses over the weir and the downstream hydraulic jump were investigated in the inflation and deflation of the inflatable weir. The findings were as follows: ►Based on literature by Chanson and Montes (1998), Shabanlou et al. (2013), Schmockeret al. (2011) and Bahzad et al. (2010), upstream arches have insignificant influence onthe performance of the inflatable weir. There is rather reduced afflux due to the shape ofthe upstream of the weir from the Bernoulli’s equation. This shape of the upstream of theweir also contributes to the transport of sediments Gebhardt et al. (2012). ►Investigation of the discharge coefficient and factors influencing showed that: oAs the weir radius is reduced during the deflation, the unit discharge over each weirincreased with increase in head above the crest. oDischarge coefficient of the inflatable weir increases with the increase in head aboveweir crest, and the discharge coefficient is inversely proportional to the radius ofcurvature of the weir. ►Investigation of pressures on the downstream face of the weir models showed that: oThe negative (suction) pressure acting on the downstream face of the weir becomesincreasingly negative with increase in H/R values. oPoint of separation of nappe was seen with pulsations of pressure of the recordpressure. Generally, energy dissipation over the weir decreases with the decrease in the weir radius and the jump is more stable with the smallest circular weir and can be more accurately determined in the case of a small weir. Conclusions and Recommendations The inflatable weir has a high discharge at its fully inflated position. Its hydraulic performance is largely influenced by inflow head and is inversely proportional to the radius of curvature. Nappe pulsation as seen in the nappe vibrations can cause the vibration of weir. Future research on inflatable weirs should aim to monitor the negative pressure on measuring pressures further down the face of the weir because larger negative pressures are expected to develop after 90˚ as with this study.

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