Experimental modal validation of a semi-analytical method for assessing dynamic behaviour of flood gates

Abstract

A new semi-analytical model is developed to predict the dynamic response of slender gates in combination with an overhang, which are subjected to standing waves. Still little is known for these types of gates for this specific forcing type. The semi-analytical model is still in its development phase and must be validated on its performance. This thesis aims for validation of the modal calculations performed by the Semi-Analytical model for submerged gates, which include fluid structure interaction. Several experiments were executed in the wave flume at the faculty of Civil Engineering at the Technical University of Delft. Dry hammer tests, wet hammer tests and wave tests were executed for different water levels. Different plates were investigated in the experiments: Solid Plate and Reinforced Plate. A measurement plan was designed to obtain reliable results from the measurement devices (strain gauges and accelerometers). The experimental data was subjected to an Experimental Modal Analysis algorithm. The Frequency Domain Decomposition turned out to be most suitable for this situation. The mode shapes that were found were subjected to a Modal Assurance Criterion (MAC), in order to compare them with the semi-analytical prediction. The results from the wet modal analysis showed that the Solid Plate had good correspondence with the prediction of the semi-analytical model. The mode shapes turned out to have a high MAC values, while the natural frequencies showed small relative errors for the modes under consideration. The Reinforced Plate was less accurate. In the wave experiments the first three modes were found, which were also believed to have the highest energy input. Considerable high MAC values assured that the identified modes were indeed the same as the modes of the semi-analytical prediction. The natural frequencies showed larger errors for especially the first mode (approx. 30%). It was observed that several measurement errors might have influenced the results of the Reinforced Plate. After finalizing the experiments for this plate, I discovered that the stiffeners and front plates came loose from the U-frame. The dynamic quantities of the Reinforced Plate were therefore adjusted during the experiments. The datasets of the experiments that were most trustworthy were selected for the different analyses. The Regular Wave Impact experiments showed good correspondence and where therefore assumed to be correct. The Solid Plate was concluded to behave as predicted by the semi-analytical model. High correlation between the predicted and identified modes and small errors in the natural frequencies were observed. The data from the Reinforced Plate showed that the high energy modes were identified for the wave experiments. Modal shapes had high correlation between predicted and identified ones, while the natural frequencies had somewhat large errors. It was observed that small natural frequency errors for the input modes resulted in relatively small errors for the calculated modes. Further validation of the model should focus on the prediction of maxima and time series of the response. The step from modal analysis to a time series is a final step in the semi-analytical model.