HyReS (HydralabIV)

HyRes (Hydraulic Response of Structures ) is a Joint Research Activity within HYDRALAB IV and addresses two of its focus areas: ‘Water & Structures’ and ‘Water & Ice’. The response of coastal and offshore structures to wave loading is a significant issue in their design. At high latitudes, dynamic ice-structure interaction also causes high loads to act on fixed or floating moored offshore structures and harbor facilities. This JRA aims at enhancing the quality of the services provided by hydraulic infrastructures for the testing of structures by improving techniques on wave generation and measurements on the interaction of water and structures or floating ice.

FZK is concerned with three research tasks within HyReS and carried out experiments for the generation of focused waves and tsunamis as part of its contribution:

1.     Wave propagation: All existing techniques for the generation of focused wave packets or freak waves in a laboratory flume require a constant water depth up to the focal point and many of them cannot deal with wave reflection. In order to overcome these problems a new method shall be elaborated within this task to generate a specific wave sequence (e.g. a focused wave train) at a predefined location in the wave flume over variable water depth and for cases with highly reflective structures. For the generation of focused waves or other deterministic wave sequences a Self Correcting Method (SCM) by means of a few iteration steps has been developed and tested in a Numerical Wave Tank (NWT) and eventually validated in the Large Wave Flume (Großer Wellenkanal, GWK). The SCM was proven to work in presence of variable water depth, wave reflection and the combination of both with very good results.  The validity of the algorithm to produce both non breaking and breaking focused waves both over constant and variable water depth was eventually demonstrated in experiments in GWK.  Finally, the SCM was used to generate a tsunami recorded at Pago Pago harbour (Samoa), the test was developed in the NWT and the wave time series was accurately reproduced at the predefined location in GWK.

2.    Assessment of structural response of fixed structures: The main objective of this task is to explore techniques for measuring breaking wave impacts with high spatial and temporal resolution in order to gain more insight into the spatial pressure distribution during these highly impulsive events and increase the reliability of load predictions based on spatial integration of pressures. After a comprehensive review of available measurement techniques, tactile pressure sensors were identified to be most appropriate to meet the desired requirements. The tactile sensor technique consists of a kind of very thin flexible pad divided into several sensing cells or ‘sensels’ and has been well established for quasi-static measurements in dry environments especially in medical and automobile research. The application in water, especially for highly dynamic measurements like breaking wave impact, is very challenging and was systematically elaborated within the project for the first time. This includes the development and testing of a methodology for an accurate calibration for dynamic loads as well as the application of the sensors in large scale tests in GWK, where methods were developed to make them water prove and to provide a vacuum over the pads, which is essential to obtain quantitatively reliable measurements.  The tactile sensors could be shown to work successfully in two experiments in GWK about breaking wave impact on a 1:3 slope and on a parapet on top of a vertical wall.

3.     Optimization of wave time series: The hydraulic response of structures is usually investigated by design sea states (DSS), which usually consist of 1000 waves or more for statistical relevance even if eventually only a few waves are of importance. To increase the efficiency of hydraulic test the objective of this task is to reduce the test duration significantly by using Substitue Sea States (SSS) where only the relevant waves for the particular problem under investigation are generated. Hence, the first step towards SSS is to define the structures and the relevant responses involved in wave-sea bed-structure interaction generally tested in wave tanks. Once the structures and responses are determined the most relevant waves or wave sequences within a normal design sea state have to be identified in order to put them together into a SSS, which must be finally generated in a flume or basin and validated by comparison with results from conventional testing. FZK was focused on breaking wave impacts on vertical walls and piles seeking for a direct correlation between the impact loads and the incoming waves, which could finally not be obtained. However, the next step, i.e. the generation of artificial, yet realistic wave sequences, could be solved by using the SCM as described above, but still there is a lot of future research on this topic required before the SSS become an established method in laboratory testing.

Publikationen

Ramachandran, K., Roldan Genzalez, R., Oumeraci, H., Schimmels, S., Kudella, M., Van Doorslaer, K., Van Doorslaer, K., De Rouck, J., Versluys, T., and Trouw, K. (2012): ‘Loading of vertical walls by overtopping bores using pressure and force sensors - A Large scale model study’., ICCE 2012, Santander, Spain.

Ramachandran, K., Roldan Genzalez, R., Schimmels, S., Oumeraci, H., Van Doorslaer, K. (2012): ‘Impact loads on a vertical wall due to overtopping bore – Large scale physical model experiments’. , Coastlab 2012, Ghent, Belgium.

Ramachandran, K., Schimmels, S., Kudella, M., van Doorslaer, K., De Rouck, J., Versluys, T., Trouw, K. (2012): Measuring wave impacts in large scale tests, using both pressure and force sensors., 33rd International Conference on Coastal Engineering (ICCE), Santander, Spain.

Ramachandran, K., Schimmels, S. (2013): Measuring Pressure Distributions on Coastal Structures, 9. FZK Kolloquium “Modellierung im Seebau und Küsteningenieurwesen”, Forschungszentrum Küste, Hannover

H. Fernández, S. Schimmels, V. Sriram (2013): Focused wave generation by means of a self correcting method., International Conference on Offshore and Polar Engineering, ISOPE 2013. Anchorage, USA

Ramachandran, K., Schimmels, S., Stagonas, D., Müller, G. (2013): Measuring Wave Impact on Coastal Structures with High Spatial and Temporal Resolution – Tactile Pressure Sensors a Novel Approach, 35th IAHR World Congress, Chengdu, China

Schimmels, S.; Sriram, V.; Didenkulova, I.; Fernández, H. (2014): On the generation of Tsunami in a large scale wave flume., Proceedings of the 34th International Conference on Coastal Engineering (ICCE 2014), Seoul, Korea.

Fernández, H.; Sriram, V.; Schimmels, S.; Budzik, M.; Oumeraci, H. (2014): Focused wave generation in laboratory flumes over uneven bottom., Proceedings of the 34th International Conference on Coastal Engineering (ICCE 2014), Seoul, Korea.

Fernández, H.; Sriram, V.; Schimmels, S.; Oumeraci, H. (2014): Extreme wave generation using self correcting method — Revisited. , Coastal Engineering 93, 15–31.