Instrumentation Monthly Test | Measurement | Control
Designing strong and stable seawalls is of utmost importance in times of rising sea levels. However, what happens when a wave hits such a seawall with great force? Scientists so far have been working only with formulas for calculating force and pressure – however, no experimental data has been available.
Scientists of Polytechnic University of Catalunya in Barcelona, Spain, have for the first time developed an experimental approach to determining the forces produced during the impact of a wave.
Pressure and force transducers as well as high-quality measuring amplifiers from HBM have played a pivotal role in this experiment.
The impact of a wave breaking on a vertical wall is a fast and very powerful phenomenon that is very hard to measure. It is important to measure the total force, and its application point, generated by the impact of the wave in order to be able to predict the design loads and to properly design the seawall. The pressure, too, is important in the design of the armour of reinforced concrete.
For these purposes a physical model was built in the Maritime Engineering Laboratory at Polytechnic University of Catalunya (LIM-UPC). The model of the seawall was equipped with 6 P8AP pressure transducers positioned right in the impact zone in order to measure the vertical pressure distribution.
The structure is supported by two Z6C3 beam load cells in order to measure the total force and the momentum generated by the impact of the wave.
P8AP Absolute Pressure Transducer
The P8AP is an absolute pressure transducer based on a strain gauge sensor with a measuring span of 10 bars and an accuracy class 0.3. The P8AP are IP67, that means they are weatherproof but not waterproof and for this reason a box that isolates the sensors from the water is needed.
20 holes were added to the front wall so as to be able to try different patterns of pressure sensors positions. Six holes were plugged with the pressure sensors and the others are plugged with screws in such a manner that the front wall will be waterproof and continuous.
After some tests using various positions of the pressure sensors, the definitive pattern was defined with the six pressure sensors placed on the same vertical and with a distance between each sensor of 25 mm.
The Z6 by HBM is a bending beam load cell with a nominal load of 50 kg and an accuracy of 0.009% of the maximum capacity. The load cells were mechanically fixed to the protecting box of the pressure sensors and fixed at the reticular structure described before.
It is very important that the mechanical connection should be very rigid in order not to absorb any force and affect the measurement. A reticular structure fixed on a super structure of the wave flume (independent from the wave flume itself) is used because in a reticular structure there are just normal forces and the nodes are fixed. The stiffness of the load cells should be much lower than the stiffness of the reticular structure in order to deform itself and to perform the right measure of the force.
The coupling of the two load cells in two different positions is needed in order to obtain time series of the total force, the momentum and the application point of the force.
As the phenomenon is very fast (few milliseconds), HBM’s QuantumX MX840 8-channel amplifier was the best option available on the market with a maximum sample frequency of 19200 Hz for all channels. Both the pressure transducers and the load cells were connected and recorded with the QuantumX controlled with catmanEasy software.
The possibility to connect the six pressure transducers and the two load cells in the same data acquisition system and sample at a very high speed is crucial in order to be able to compare the results obtained from both together.
To verify the results, a comparison between the results of the pressure transducers and the load cells was made, integrating on the vertical section the pressure distribution determined with the P8AP.
The preliminary results show a good agreement between the results of the pressure transducers and the load cells at low sample frequency. Increasing the sample frequency the agreement diminishes. Some sensors are positioned in a wet/dry zone in which the presence of a mixture of air and water during the impact and the geometry of the P8AP can lead to problems during measurement resulting from the compressibility of the air.
In order to solve the problem, the opening of the sensors is to be filled with glycerin and a vacuum is to be generated in order to avoid any vibration of the vacuum membrane and any loss of glycerin.
One of the next steps in the experimental campaign will be to improve the resolution in terms of pressure sensors through the acquisition of 8 more sensors and another QuantumX MX840.
HBM
Founded in Germany in 1950, HBM is today the technology and market leader in the field of test and measurement. HBM’s product range comprises solutions for the entire measurement chain, from virtual to physical testing. The company has production facilities in Germany, USA and China and is represented in over 80 countries worldwide.
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