Wave generator

The new wave generator is inspired by a pneumatic tide generator used by HR Wallingford (HRW) (Wilkie and Young, 1952) which is capable of producing a pro-totype tide of 12.5 hours in 7.5 minutes (Bazin, 2008). In order to achieve this, the generator draws a controlled volume of water from a wave basin into a tank, and releases it over time. An air pump linked to the tank provides a constant suction, and a separate motor controlled valve regulates the air pressure within the tank.

Tides, being of longer period than tsunami require a slower exchange of water be-tween the basin and the tank. So in order to produce waves of similar scaled period to tsunami a faster control mechanism was necessary. The new generator is also re-quired to produce waves in a flume, rather than a wave basin, and so the necessary dimensions and geometry are totally re-designed. Aside from these alterations, the principles behind the new generator are essentially the same as that of Wilkie and Young (1952).

The tsunami generator is operated by varying the pressure inside the tank, by actu-ation of a computer-controlled and motorised valve. A constant suction is supplied

by the air pump. When the control valve (CV) is closed, water is drawn up into the tank from the flume, and released back to the flume when the valve is opened. A second security valve (SV) is present to prevent the water level from reaching the top of the tank and being drawn into the air pump. This scenario would damage the pump, and as such the SV is always set slightly open to allow regulation of the internal vacuum and prevent water being drawn up too far inside the tank. It is occasionally necessary to alter the SV position, particularly if varying the water depth in the flume greatly, but on the whole and for the duration of these tests the SV remained in the same position throughout. This is clearly desirable as the SV position alters the performance of the generator. The released water forms the positive part of the wave, and the draw-up stage produces the negative trough in the flume. The experimental set-up can be seen in Figure 3.5.

Figure 3.5: Schematic diagram of the flume at HRW and new pneumatic wave generator system. Vertical scale emphasised. Not to scale.

The operation of the motorised valve is controlled in a user interface based in

Lab-View software, which has been developed by HRW staff. The valve is able to open between zero and 42^◦ and this can be varied manually, or from a pre-defined time-series loaded onto the control computer. A summary of the design process and calculations which were done prior to the manufacture of this generator can be found in Rossetto et al. (2011) and Charvet (2012). The generator tank is 1.15m wide, 1.8m high and 4.8m long and fully removable from the wave flume so that normal paddle generators can be reinstated when the generator is not in use.

3.3.2 Flume

The flume used for the experiments described by this chapter is shown in Figure 3.6. It is located in the Froude Modelling Hall at HR Wallingford, Oxfordshire, UK.

The length of the flume is 45 metres by 1.2 metres wide and it is constructed of reinforced concrete with two viewing windows (3.3 by 1.7m). The effective length of the flume is further reduced by some 12.2 meters by the pneumatic generator installed in front of the insitu paddle generators and a sump created behind a 13.7m stretch of 1:20 fixed bathymetry. A flat area of beach for conducting structural tests provides a constant depth propagation region of 15.2m, with some 28 metres in total for propagation. The beach area is chosen to coincide with the location of one of the viewing windows so that video data can be collected.

The flume is filled with fresh water which at the time of the experiments (Au-tumn/Winter 2008) is on average 8^◦ Celsius. This water is filtered, but contained traces of organic debris and other non organic particles originating from its source in the adjacent River Thames. Though generally invisible to the eye, this debris can affect some instruments. In particular the velocity readings are found to be particularly susceptible, as the probes are frequently contaminated with fine algae fibres which clog the mechanisms periodically.

The control room can be seen in Figure 3.6 spanning between the adapted flume and that adjacent to it and the view is looking towards the beach inundation area.

At the far end of the flume is a sump, where inundating water is collected and measured. The water is returned to the main flume via a sump pump, which is used between tests to retain fluid volumes in the system.

Figure 3.6: 45m wave flume in Froude Modelling Hall at HW Wallingford, photographed from the top of the wave generator.

Throughout this work, various quantities are measured. As stated earlier, the ex-perimental work was undertaken in conjunction with that of Charvet (2012) and as such many of the measurements made in that study are highly relevant to this thesis. The different types of data observed are described separately. Most of the data are captured using a 64 channel data acquisition system enabling simultaneous recordings of all quantities to a single file on a computer. Other measurements, in particular the video recording require a separate recording system located away from the main control office, or manual measurements in the case of sump volumes.